[Federal Register Volume 89, Number 73 (Monday, April 15, 2024)]
[Proposed Rules]
[Pages 26620-26701]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2024-07397]



[[Page 26619]]

Vol. 89

Monday,

No. 73

April 15, 2024

Part IV





Environmental Protection Agency





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40 CFR Part 50





Review of the Secondary National Ambient Air Quality Standards for 
Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter; Proposed 
Rule

  Federal Register / Vol. 89 , No. 73 / Monday, April 15, 2024 / 
Proposed Rules  

[[Page 26620]]


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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2014-0128; FRL-5788-02-OAR]
RIN 2060-AS35


Review of the Secondary National Ambient Air Quality Standards 
for Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: Based on the Environmental Protection Agency's (EPA's) review 
of the air quality criteria and national ambient air quality standards 
(NAAQS) for oxides of nitrogen (N oxides), oxides of sulfur 
(SOX), and particulate matter (PM), the Environmental 
Protection Agency (EPA) proposes to revise the existing secondary 
sulfur dioxide (SO2) standard to an annual average, averaged 
over three consecutive years, with a level within the range from 10 to 
15 parts per billion (ppb). Additionally, the Agency proposes to retain 
the existing secondary standards for N oxides and PM, without revision. 
The EPA also proposes revisions to the data handling requirements for 
the proposed secondary SO2 NAAQS.

DATES: Comments must be received on or before June 14, 2024.
    Public Hearings: The EPA will hold a virtual public hearing on this 
proposed rule. This hearing will be announced in a separate Federal 
Register notice that provides details, including specific dates, times, 
and contact information for these hearings.

ADDRESSES: You may submit comments, identified by Docket ID No. EPA-HQ-
OAR-2014-0128, by any of the following means:
     Federal eRulemaking Portal: https://www.regulations.gov/ 
(our preferred method). Follow the online instructions for submitting 
comments.
     Email: [email protected]. Include the Docket ID No. 
EPA-HQ-OAR-2014-0128 in the subject line of the message.
     Mail: U.S. Environmental Protection Agency, EPA Docket 
Center, Air and Radiation Docket, Mail Code 28221T, 1200 Pennsylvania 
Avenue NW, Washington, DC 20460.
     Hand Delivery or Courier (by scheduled appointment only): 
EPA Docket Center, WJC West Building, Room 3334, 1301 Constitution 
Avenue NW, Washington, DC 20004. The Docket Center's hours of 
operations are 8:30 a.m.-4:30 p.m., Monday-Friday (except Federal 
Holidays).
    Instructions: All submissions received must include the Docket ID 
No. for this document. Comments received may be posted without change 
to https://www.regulations.gov, including any personal information 
provided. For detailed instructions on sending comments and additional 
information on the rulemaking process, see the SUPPLEMENTARY 
INFORMATION section of this document.

FOR FURTHER INFORMATION CONTACT: Ms. Ginger Tennant, Health and 
Environmental Impacts Division, Office of Air Quality Planning and 
Standards, U.S. Environmental Protection Agency, Mail Code C539-04, 
Research Triangle Park, NC 27711; telephone: (919) 541-4072; email: 
[email protected].

SUPPLEMENTARY INFORMATION:

General Information

Preparing Comments for the EPA

    Follow the online instructions for submitting comments. Once 
submitted to the Federal eRulemaking Portal, comments cannot be edited 
or withdrawn. The EPA may publish any comment received to its public 
docket. Do not submit electronically any information you consider to be 
Confidential Business Information (CBI) or other information whose 
disclosure is restricted by statute. Multimedia submissions (audio, 
video, etc.) must be accompanied by a written submission. The written 
comment is considered the official comment and should include 
discussion of all points you wish to make. The EPA will generally not 
consider comments or comment contents located outside of the primary 
submission (i.e., on the web, the cloud, or other file sharing system). 
For additional submission methods, the full EPA public comment policy, 
information about CBI or multimedia submissions, and general guidance 
on making effective comments, please visit https://www.epa.gov/dockets/commenting-epa-dockets.
    When submitting comments, remember to:
     Identify the action by docket number and other identifying 
information (subject heading, Federal Register date and page number).
     Explain why you agree or disagree, suggest alternatives, 
and substitute language for your requested changes.
     Describe any assumptions and provide any technical 
information and/or data that you used.
     Provide specific examples to illustrate your concerns and 
suggest alternatives.
     Explain your views as clearly as possible, avoiding the 
use of profanity or personal threats.
     Make sure to submit your comments by the comment period 
deadline identified.

Availability of Information Related to This Action

    All documents in the dockets pertaining to this action are listed 
on the www.regulations.gov website. This includes documents in the 
docket for the proposed decision (Docket ID No. EPA-HQ-OAR-2014-0128) 
and a separate docket, established for the Integrated Science 
Assessment (ISA) (Docket ID No. EPA-HQ-ORD-2013-0620) that has been 
incorporated by reference into the docket for this proposed decision. 
Although listed in the index, some information is not publicly 
available, e.g., CBI or other information whose disclosure is 
restricted by statute. Certain other material, such as copyrighted 
material, is not placed on the internet and may be viewed with prior 
arrangement with the EPA Docket Center. Additionally, a number of the 
documents that are relevant to this proposed decision are available 
through the EPA's website at https://www.epa.gov/naaqs/. These 
documents include the Integrated Science Assessment for Oxides of 
Nitrogen, Oxides of Sulfur and Particulate Matter Ecological Criteria 
(U.S. EPA, 2020a), available at https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=349473, and the Policy Assessment for the Review 
of the Secondary National Ambient Air Quality Standards for Oxides of 
Nitrogen, Oxides of Sulfur, and Particulate Matter, (U.S. EPA, 2024), 
available at https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards.

Table of Contents

    The following topics are discussed in this preamble:

Executive Summary
I. Background
    A. Legislative Requirements
    B. Related Control Programs
    C. History of the Secondary Standards for N Oxides, 
SOX and PM
    1. N Oxides
    2. SOX
    3. Related Actions Addressing Acid Deposition
    4. Most Recent Review of the Secondary Standards for N Oxides 
and SOX
    5. PM
    D. Current Review
II. Rationale for Proposed Decisions
    A. Introduction
    1. Basis for Existing Secondary Standards

[[Page 26621]]

    2. Prior Review of Deposition-Related Effects
    3. General Approach for This Review
    B. Air Quality and Deposition
    1. Sources, Emissions and Atmospheric Processes Affecting 
SOX, N Oxides and PM
    2. Recent Trends in Emissions, Concentrations and Deposition
    3. Relationships Between Concentrations and Deposition
    C. Welfare Effects Evidence
    1. Nature of Effects
    a. Direct Effects of SOX and N Oxides
    b. Acid Deposition-Related Ecological Effects
    c. Nitrogen Enrichment and Associated Ecological Effects
    d. Other Deposition-Related Effects
    2. Public Welfare Implications
    3. Exposure Conditions and Deposition-Related Metrics
    a. Acidification and Nitrogen Enrichment in Aquatic Ecosystems
    b. Deposition-Related Effects in Terrestrial Ecosystems
    c. Direct Effects of N Oxides, SOX and PM in Ambient 
Air
    D. Quantitative Exposure and Risk Assessment for Aquatic 
Acidification
    1. Key Design Aspects
    2. Key Limitations and Uncertainties
    3. Summary of Results
    E. Proposed Conclusions
    1. Evidence and Exposure/Risk-Based Considerations in the Policy 
Assessment
    a. Direct Effects on Biota
    b. Evidence of Ecosystem Effects of S and N Deposition
    c. Sulfur Deposition and SOX
    d. Nitrogen Deposition and N Oxides and PM
    2. CASAC Advice and Public Comments
    3. Administrator's Proposed Conclusions
    F. Proposed Decision on the Secondary Standards
III. Interpretation of the Secondary SO2 Standard
    A. Background
    B. Interpretation of the Secondary SO2 Standard
IV. Ambient Air Monitoring Network for SO2
V. Clean Air Act Implementation Requirements for Proposed Secondary 
SO2 Standard
    A. Designation of Areas
    B. Section 110(a)(1) and (2) Infrastructure SIP Requirements
    C. Prevention of Significant Deterioration and Nonattainment New 
Source Review Programs for the Proposed Secondary SO2 
Standard
    D. Alternative PSD Compliance Demonstration Approach for the 
Proposed Secondary SO2 Standard
    E. Transportation Conformity Program
    F. General Conformity Program
VI. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review and 
Executive Order 13563: Improving Regulation and Regulatory Review
    B. Paperwork Reduction Act (PRA)
    C. Regulatory Flexibility Act (RFA)
    D. Unfunded Mandates Reform Act (UMRA)
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With 
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From 
Environmental Health Risks and Safety Risks
    H. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use
    I. National Technology Transfer and Advancement Act (NTTAA)
    J. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations and Executive Order 14096: Revitalizing Our Nation's 
Commitment to Environmental Justice for All
References

Executive Summary

    This document presents the Administrator's proposed decisions in 
the current review of the secondary NAAQS for SOX, N oxides, 
and PM. The existing secondary standards are: for SO2, 0.5 
ppm as a 3-hour average not to be exceeded more than once in a year; 
for NO2, 53 ppb as an annual average; for PM2.5, 
15.0 [micro]g/m\3\ as the 3-year average of annual averages, and 35 
[micro]g/m\3\ as the 3-year average of annual 98th percentile 24-hour 
averages; and, for PM10, 150 [micro]g/m\3\ as a 24-hour 
average, not to be exceeded more than once per year on average over 
three years. Sections 108 and 109 of the Clean Air Act (CAA, the Act) 
require the EPA to periodically review the air quality criteria--the 
science upon which the standards are based--and the standards 
themselves. Under section 109(b)(2) of the Act, a secondary standard 
must ``specify a level of air quality the attainment and maintenance of 
which in the judgment of the Administrator, based on such criteria, is 
requisite to protect the public welfare from any known or anticipated 
adverse effects associated with the presence of [the] pollutant in the 
ambient air.'' In conducting this review of the secondary 
SOX, N oxides, and PM NAAQS, the EPA has carefully evaluated 
the currently available scientific literature on the ecological effects 
of SOX, N oxides, and PM,\1\ focusing particularly on the 
new literature available since the conclusion of the previous reviews 
in 2012 and 2013, respectively, as described in the Integrated Science 
Assessment (ISA). The ecological effects addressed in this review 
include direct effects of N oxides and SOX, and PM loading, 
on vegetation surfaces, as well as ecological effects related to 
atmospheric deposition of S and N compounds in sensitive ecosystems.
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    \1\ The ecological effects of PM that are the focus of this 
action were not considered in EPA's recently completed 
reconsideration of the primary and secondary NAAQS for PM. In the 
review of the PM secondary standards completed in 2020, and 
reconsidered more recently, the EPA considered effects on visibility 
and climate and materials damage, but did not consider the 
ecological effects that are addressed here (89 FR 16202, March 6, 
2024).
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    Sulfur oxides and N oxides, their transformation products (which 
include particulate compounds), and N- and S-containing components of 
PM in ambient air can contribute to atmospheric deposition of S and N 
compounds. Emissions of SOX, N oxides, PM and PM precursors 
have declined dramatically over the past two decades, continuing a 
longer-term trend. In response to the reductions in S- and N-containing 
compounds, levels of S and N deposition have also been reduced, 
although the declining trend in N deposition in the last decade has 
slowed and, in some areas, reversed, due to increasing ammonia 
emissions.
    The Administrator's proposed decision in this review is to revise 
the existing secondary SO2 standard and to retain the 
existing secondary standards for N oxides and PM. In this document, the 
EPA summarizes the background and rationale for the Administrator's 
proposed decisions in this review. The EPA solicits comment on the 
proposed decisions described here and on a number of alternate options, 
and requests commenters also provide the rationales supporting the 
views articulated in submitted comments.
    The Administrator's proposed decisions are based on his 
consideration of the characterization of the available scientific 
evidence in the ISA, quantitative and policy analyses presented in the 
Policy Assessment (PA), and advice from the Clean Air Scientific 
Advisory Committee (CASAC).\2\ In conveying its advice in this review, 
the CASAC provided consensus advice that the existing SO2 
and NO2 secondary standards were adequate to protect against 
direct effects of S and N oxides on plants and lichens. With regard to 
deposition-related effects and SO2, the majority of CASAC 
recommended an annual secondary standard of 10-15 ppb, and the minority 
recommended a secondary standard identical to the existing primary 
standard. In consideration of deposition-related effects and the 
NO2 and PM2.5 secondary standards, the

[[Page 26622]]

CASAC majority recommended revision of the levels of the existing 
annual NO2 and PM2.5 secondary standards, and the 
minority recommended adopting secondary standards identical to the 
existing annual NO2 and PM2.5 primary standards.
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    \2\ Over the course of this review, the EPA developed planning 
documents, an ISA and a PA, drafts of which were made available for 
public comment and reviewed by the CASAC Oxides of Nitrogen, Oxides 
of Sulfide and Particulate Matter Secondary NAAQS Panel (https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards).
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    Based on his consideration of the ecological effects evidence in 
the ISA, the evaluations and quantitative information in the PA, 
including the quantitative REA for aquatic acidification, and advice 
from the CASAC, the Administrator is proposing that the current short-
term secondary SO2 standard is not requisite to protect the 
public welfare from known or anticipated adverse effects associated 
with the presence of SOX in ambient air, including 
particularly deposition-related effects, and that it should be revised 
to also provide such protection against effects related to deposition 
of sulfur (S) compounds to ecosystems. Specifically, the EPA is 
proposing to revise the existing standard to be an annual average 
standard, averaged over three years, with a level within the range from 
10 to 15 parts per billion (ppb) based on the Administrator's proposed 
judgment that a standard in this range would provide protection for 
both direct effects on vegetation surfaces and ecosystem deposition-
related effects. The EPA solicits comments on this proposal, including 
the averaging time, form and range of levels for the revised standard. 
The EPA also solicits comments on a number of alternative options for a 
new secondary SO2 standard. The EPA solicits comment on 
setting the level for a new annual average standard (averaged over 
three years) in the range from 5 to 10 ppb, and on revising the 
existing secondary standard to be identical to the existing primary 
standard in all respects. Further, the EPA solicits comments on 
retaining the existing 3-hour standard, in addition to establishing a 
new annual secondary standard.
    The Administrator is also proposing to retain the secondary 
nitrogen dioxide (NO2) and PM standards, without revision. 
With regard to the secondary NO2 standard, the Administrator 
finds that the evidence related to the N oxides, NO2 and 
nitrogen oxide (NO), does not clearly call into question the adequacy 
of protection provided by the existing standard for direct effects or 
for deposition-related effects (in light of the currently diminished 
role of N oxides in total N deposition, particularly in areas with 
highest deposition), such that revision is not warranted. The EPA 
solicits comment on the proposed decision to retain the existing 
secondary NO2 standard, without revision, and also on the 
alternative approach of revising the form of the existing standard to a 
3-year average and the level to a value within the range from 35 to 40 
ppb.
    With regard to PM, the Administrator proposes to conclude that the 
current evidence does not call into question the adequacy of the 
existing PM standards with regard to direct effects and deposition of 
pollutants other than S and N compounds. Further, he judges that 
protection of sensitive ecosystems from S deposition is more 
effectively achieved through a revised SO2 standard than a 
PM standard, and that a revised PM standard is not warranted to provide 
public welfare protection against adverse effects related to S or N 
deposition. The Administrator additionally proposes to conclude that 
PM2.5 is not an appropriate indicator for a secondary 
standard intended to provide protection of the public welfare from 
adverse effects related to S or N deposition. Thus, based on 
consideration of the PA analyses and conclusions, and consideration of 
CASAC advice, the Administrator proposes to conclude that no change to 
the annual PM2.5 secondary standard is warranted and he 
proposes to retain the existing PM2.5 secondary standard, 
without revision. The EPA solicits comment on the proposed decision. 
Additionally, in recognizing that there may be alternate views with 
regard to whether and to what extent a standard with a PM2.5 
indicator might be expected to provide control of N deposition, and in 
light of the rationale provided by the CASAC minority, the EPA also 
solicits comment on the alternative approach of revising the secondary 
PM2.5 (with PM2.5 referring to particles with a 
nominal mean aerodynamic diameter less than or equal to 2.5 
micrometers) annual standard to a level of 12 micrograms per meter 
cubed ([micro]g/m\3\). With regard to other PM secondary standards, 
based on evaluations and conclusions of the PA, including consideration 
of recommendations from the CASAC, the Administrator proposes to retain 
the existing 24-hour secondary PM2.5 standard, without 
revision. Further, based on the lack of evidence calling into question 
the adequacy of the secondary PM10 standards for protection 
of ecological effects, he also proposes to retain the secondary 
PM10 standards without revision.
    This document additionally includes proposed revisions related to 
implementation of the proposed secondary SO2 annual 
standard. Specifically, the EPA is proposing revisions to the data 
handling requirements in appendix T of 40 CFR part 50 to include 
specifications needed for the proposed new annual average standard. 
This document also describes the SO2 monitoring network and 
its adequacy for surveillance for the proposed annual standard. Lastly, 
the document discusses implementation processes pertinent to 
implementation of the proposed new standard.

I. Background

A. Legislative Requirements

    Two sections of the CAA govern the establishment and revision of 
the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to 
identify and list certain air pollutants and then to issue air quality 
criteria for those pollutants. The Administrator is to list those 
pollutants ``emissions of which, in his judgment, cause or contribute 
to air pollution which may reasonably be anticipated to endanger public 
health or welfare''; ``the presence of which in the ambient air results 
from numerous or diverse mobile or stationary sources''; and for which 
he ``plans to issue air quality criteria. . . .'' (42 U.S.C. 
7408(a)(1)). Air quality criteria are intended to ``accurately reflect 
the latest scientific knowledge useful in indicating the kind and 
extent of all identifiable effects on public health or welfare which 
may be expected from the presence of [a] pollutant in the ambient air. 
. . .'' 42 U.S.C. 7408(a)(2).
    Section 109 of the Act (42 U.S.C. 7409) directs the Administrator 
to propose and promulgate ``primary'' and ``secondary'' NAAQS for 
pollutants for which air quality criteria are issued [42 U.S.C. 
7409(a)]. Under section 109(b)(2), a secondary standard must ``specify 
a level of air quality the attainment and maintenance of which, in the 
judgment of the Administrator, based on such criteria, is requisite to 
protect the public welfare from any known or anticipated adverse 
effects associated with the presence of [the] pollutant in the ambient 
air.'' \3\
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    \3\ Under CAA section 302(h) (42 U.S.C. 7602(h)), effects on 
welfare include, but are not limited to, ``effects on soils, water, 
crops, vegetation, manmade materials, animals, wildlife, weather, 
visibility, and climate, damage to and deterioration of property, 
and hazards to transportation, as well as effects on economic values 
and on personal comfort and well-being.''
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    In setting primary and secondary standards that are ``requisite'' 
to protect public health and welfare, respectively, as provided in 
section 109(b), the EPA's task is to establish standards that are 
neither more nor less stringent than necessary. In so doing, the EPA 
may not consider the costs of implementing the standards. See 
generally, Whitman v.

[[Page 26623]]

American Trucking Ass'ns, 531 U.S. 457, 465-472, 475-76 (2001). 
Likewise, ``[a]ttainability and technological feasibility are not 
relevant considerations in the promulgation of national ambient air 
quality standards'' (American Petroleum Institute v. Costle, 665 F.2d 
1176, 1185 [D.C. Cir. 1981]). However, courts have clarified that in 
deciding how to revise the NAAQS in the context of considering standard 
levels within the range of reasonable values supported by the air 
quality criteria and judgments of the Administrator, EPA may consider 
``relative proximity to peak background . . . concentrations'' as a 
factor (American Trucking Ass'ns, v. EPA, 283 F.3d 355, 379 [D.C. Cir. 
2002]).
    Section 109(d)(1) of the Act requires periodic review and, if 
appropriate, revision of existing air quality criteria to reflect 
advances in scientific knowledge on the effects of the pollutant on 
public health and welfare. Under the same provision, the EPA is also to 
periodically review and, if appropriate, revise the NAAQS, based on the 
revised air quality criteria.\4\
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    \4\ This section of the Act requires the Administrator to 
complete these reviews and make any revisions that may be 
appropriate ``at five-year intervals.''
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    Section 109(d)(2) addresses the appointment and advisory functions 
of an independent scientific review committee. Section 109(d)(2)(A) 
requires the Administrator to appoint this committee, which is to be 
composed of ``seven members including at least one member of the 
National Academy of Sciences, one physician, and one person 
representing State air pollution control agencies.'' Section 
109(d)(2)(B) provides that the independent scientific review committee 
``shall complete a review of the criteria . . . and the national 
primary and secondary ambient air quality standards . . . and shall 
recommend to the Administrator any new . . . standards and revisions of 
existing criteria and standards as may be appropriate. . . .'' Since 
the early 1980s, this independent review function has been performed by 
the CASAC of the EPA's Science Advisory Board.
    Section 109(b)(2) specifies that ``[a]ny national secondary ambient 
air quality standard prescribed under subsection (a) shall specify a 
level of air quality the attainment and maintenance of which in the 
judgment of the Administrator, based on such criteria, is requisite to 
protect the public welfare from any known or anticipated adverse 
effects associated with the presence of such air pollutant in the 
ambient air.'' Consistent with this statutory direction, EPA has always 
understood the goal of the NAAQS is to identify a requisite level of 
air quality, and the means of achieving a specific level of air quality 
is to set a standard expressed as a concentration of a pollutant in the 
air, such as in terms of parts per million (ppm), parts per billion 
(ppb), or micrograms per cubic meter ([mu]g/m\3\). Thus, while 
deposition-related effects are included within the ``adverse effects 
associated with the presence of such air pollutant in the ambient 
air,'' EPA has never found a standard that quantifies atmospheric 
deposition onto surfaces to constitute a national secondary ambient air 
quality standard.

B. Related Control Programs

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once the EPA has 
established them. Under CAA sections 110 and part D, subparts 1, 5, and 
6 for nitrogen and sulfur oxides, and subparts 1, 4, and 6 for PM, and 
related provisions and regulations, States are to submit, for the EPA's 
approval, State implementation plans (SIPs) that provide for the 
attainment and maintenance of such standards through control programs 
directed to sources of the pollutants involved. The States, in 
conjunction with the EPA, also administer the prevention of significant 
deterioration of air quality program that covers these pollutants. See 
42 U.S.C. 7470-7479. In addition, Federal programs provide for or 
result in nationwide reductions in emissions of N oxides, 
SOX, PM and other air pollutants under title II of the Act, 
42 U.S.C. 7521-7574, which involves controls for motor vehicles, 
nonroad engines and equipment, and the new source performance standards 
under section 111 of the Act, 42 U.S.C. 7411.

C. History of the Secondary Standards for N Oxides, SOX and PM

    Secondary NAAQS were first established for N oxides, SOX 
and PM in 1971 (36 FR 8186, April 30, 1971). Since that time, the EPA 
has periodically reviewed the air quality criteria and secondary 
standards for these pollutants, with the most recent reviews that 
considered the evidence for ecological effects of these pollutants 
being completed in 2012 and 2013 (77 FR 20218, April 3, 2012; 78 FR 
3086, January 15, 2013). The subsections below summarize key 
proceedings from the initial standard setting in 1971 to the last 
reviews in 2012-2013.\5\
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    \5\ Since the late 1970s, each review of the air quality 
criteria and standards has generally involved the development of an 
Air Quality Criteria Document or ISA and a Staff Paper or staff 
Policy Assessment, which is often accompanied by or includes a 
quantitative exposure or risk assessment, prior to the regulatory 
decision-making phase.
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1. N Oxides
    The EPA first promulgated NAAQS for N oxides in April 1971 after 
reviewing the relevant science on the public health and welfare effects 
in the 1971 Air Quality Criteria for Nitrogen Oxides (air quality 
criteria document or AQCD).\6\ With regard to welfare effects, the 1971 
AQCD described effects of NO2 on vegetation and corrosion of 
electrical components linked to particulate nitrate (U.S. EPA, 1971). 
The primary and secondary standards were both set at 0.053 parts per 
million (ppm) NO2 as an annual average (36 FR 8186, April 
30, 1971). In 1982, the EPA published an updated AQCD (U.S. EPA, 
1982a). Based on the 1982 AQCD, the EPA proposed to retain the existing 
standards in February 1984 (49 FR 6866, February 23, 1984). After 
considering public comments, the EPA published the final decision to 
retain these standards in June 1985 (50 FR 25532, June 19, 1985).
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    \6\ In reviews initiated prior to 2007, the AQCD provided the 
scientific foundation (i.e., the air quality criteria) for the 
NAAQS. Since that time, the ISA has replaced the AQCD.
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    The EPA began a second review of the primary and secondary 
standards for oxides of nitrogen in 1987 (52 FR 27580, July 22, 1987). 
In November 1991, the EPA released an updated draft AQCD for CASAC and 
public review and comment (56 FR 59285, November 25, 1991). The CASAC 
reviewed the draft document at a meeting held on July 1, 1993, and 
concluded in a closure letter to the Administrator that the document 
provided ``an adequate basis'' for EPA's decision-making in the review 
(Wolff, 1993). The final AQCD was released later in 1993 (U.S. EPA, 
1993). Based on the 1993 AQCD, the EPA's Office of Air Quality Planning 
and Standards (OAQPS) prepared a Staff Paper,\7\ drafts of which were 
reviewed by the CASAC (Wolff, 1995; U.S. EPA, 1995a). In October 1995, 
the EPA proposed not to revise the secondary NO2 NAAQS (60 
FR 52874; October 11, 1995). After consideration of the comments 
received on the proposal, the Administrator finalized the decision not 
to revise the

[[Page 26624]]

NO2 NAAQS (61 FR 52852; October 8, 1996). The subsequent 
(and most recent) review of the N oxides secondary standard was a joint 
review with the secondary standard for SOX, which was 
completed in 2012 (see subsection 4 below).
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    \7\ Prior to reviews initiated in 2007, the Staff Paper 
summarized and integrated key studies and the scientific evidence, 
and from the 1990s onward, it also assessed potential exposures and 
associated risk. The Staff Paper also presented the EPA staff's 
considerations and conclusions regarding the adequacy of existing 
NAAQS and, when appropriate, the potential alternative standards 
that could be supported by the evidence and information. More recent 
reviews present this information in the Policy Assessment.
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2. SOX
    The EPA first promulgated secondary NAAQS for sulfur oxides in 
April 1971 based on the scientific evidence evaluated in the 1969 AQCD 
(U.S. DHEW, 1969a [1969 AQCD]; 36 FR 8186, April 30, 1971). These 
standards, which were established on the basis of evidence of adverse 
effects on vegetation, included an annual arithmetic mean standard, set 
at 0.02 ppm SO2,\8\ and a 3-hour average standard set at 0.5 
ppm SO2, not to be exceeded more than once per year. In 
1973, based on information indicating there to be insufficient data to 
support the finding of a study in the 1969 AQCD concerning vegetation 
injury associated with SO2 exposure over the growing season, 
rather than from short-term peak concentrations, the EPA proposed to 
revoke the annual mean secondary standard (38 FR 11355, May 7, 1973). 
Based on consideration of public comments and external scientific 
review, the EPA released a revised chapter of the AQCD and published 
its final decision to revoke the annual mean secondary standard (U.S. 
EPA, 1973; 38 FR 25678, September 14, 1973). At that time, the EPA 
additionally noted that injury to vegetation was the only type of 
SO2 welfare effect for which the evidence base supported a 
quantitative relationship, stating that although data were not 
available at that time to establish a quantitative relationship between 
SO2 concentrations and other public welfare effects, 
including effects on materials, visibility, soils, and water, the 
SO2 primary standards and the 3-hour secondary standard may 
to some extent mitigate such effects. The EPA also stated it was not 
clear that any such effects, if occurring below the current standards, 
were adverse to the public welfare (38 FR 25679, September 14, 1973).
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    \8\ Established with the annual standard as a guide to be used 
in assessing implementation plans to achieve the annual standard was 
a maximum 24-hour average concentration not to be exceeded more than 
once per year (36 FR 8187, April 30, 1971).
---------------------------------------------------------------------------

    In 1979, the EPA announced initiation of a concurrent review of the 
air quality criteria for oxides of sulfur and PM and plans for 
development of a combined AQCD for these pollutants (44 FR 56730, 
October 2, 1979). The EPA subsequently released three drafts of a 
combined AQCD for CASAC review and public comment. In these reviews, 
and in guidance provided at the August 20-22, 1980, public meeting of 
the CASAC on the first draft AQCD, the CASAC concluded that acidic 
deposition was a topic of extreme scientific complexity because of the 
difficulty in establishing firm quantitative relationships among 
emissions of relevant pollutants, formation of acidic wet and dry 
deposition products, and effects on terrestrial and aquatic ecosystems 
(53 FR 14935, April 26, 1988). The CASAC also noted that a fundamental 
problem of addressing acid deposition in a criteria document is that 
acid deposition is produced by several different criteria pollutants: 
oxides of sulfur, oxides of nitrogen, and the fine particulate fraction 
of suspended particles (U.S. EPA, 1982b, pp. 125-126). The CASAC also 
felt that any document on this subject should address both wet and dry 
deposition, since dry deposition was believed to account for a 
substantial portion of the total acid deposition problem (53 FR 14936, 
April 26, 1988; Lippman, 1987). For these reasons, CASAC recommended 
that, in addition to including a summary discussion of acid deposition 
in the final AQCD, a separate, comprehensive document on acid 
deposition be prepared prior to any consideration of using the NAAQS as 
a regulatory mechanism for the control of acid deposition.
    Following CASAC closure on the AQCD for oxides of sulfur in 
December 1981, the EPA released a final AQCD (U.S. EPA, 1982b), and the 
EPA's OAQPS prepared a Staff Paper that was released in November 1982 
(U.S. EPA, 1982c). The issue of acidic deposition was not, however, 
assessed directly in the OAQPS Staff Paper because the EPA followed the 
guidance given by the CASAC, subsequently preparing the following 
documents to address acid deposition: The Acidic Deposition Phenomenon 
and Its Effects: Critical Assessment Review Papers, Volumes I and II 
(U.S. EPA, 1984a, b) and The Acidic Deposition Phenomenon and Its 
Effects: Critical Assessment Document (U.S. EPA, 1985) (53 FR 14935-36, 
April 26, 1988). Although these documents were not considered criteria 
documents and had not undergone CASAC review, they represented the most 
comprehensive summary of scientific information relevant to acid 
deposition completed by the EPA at that point.
    In April 1988, the EPA proposed not to revise the existing 
secondary standards for SO2 (53 FR 14926, April 26, 1988). 
This proposed decision with regard to the secondary SO2 
NAAQS was due to the Administrator's conclusions that: (1) based upon 
the then-current scientific understanding of the acid deposition 
problem, it would be premature and unwise to prescribe any regulatory 
control program at that time; and (2) when the fundamental scientific 
uncertainties had been decreased through ongoing research efforts, the 
EPA would draft and support an appropriate set of control measures (53 
FR 14926, April 26, 1988). This review of the secondary standard for 
SOX was concluded in 1993, subsequent to the Clean Air Act 
Amendments of 1990 (see section I.C.3). The EPA decided not to revise 
the secondary standard, concluding that revisions to the standard to 
address acidic deposition and related SO2 welfare effects 
were not appropriate at that time (58 FR 21351, April 21, 1993). In 
describing the decision, the EPA recognized the significant reductions 
in SO2 emissions, ambient air SO2 concentrations, 
and ultimately deposition expected to result from implementation of the 
title IV program, which was expected to significantly decrease the 
acidification of water bodies and damage to forest ecosystems and to 
permit much of the existing damage to be reversed with time (58 FR 
21357, April 21, 1993). While recognizing that further action might be 
needed to address acidic deposition in the longer term, the EPA judged 
it prudent to await the results of the studies and research programs 
then underway, including those assessing the comparative merits of 
secondary standards, acidic deposition standards and other approaches 
to controlling acidic deposition and related effects, and then to 
determine whether additional control measures should be adopted or 
recommended to Congress (58 FR 21358, April 21, 1993).
3. Related Actions Addressing Acid Deposition
    In 1980, Congress created the National Acid Precipitation 
Assessment Program (NAPAP). During the 10-year course of this program, 
the program issued a series of reports, including a final report in 
1990 (NAPAP, 1991). On November 15, 1990, Amendments to the CAA were 
passed by Congress and signed into law by the President. In title IV of 
these Amendments, Congress included a statement of findings including 
the following:

    (1) the presence of acidic compounds and their precursors in the 
atmosphere and in deposition from the atmosphere represents a threat 
to natural resources, ecosystems, materials, visibility, and public 
health; . . . (3) the problem of acid deposition is of national and 
international significance; . . .

[[Page 26625]]

(5) current and future generations of Americans will be adversely 
affected by delaying measures to remedy the problem[.]

    The goal of title IV was to reduce emissions of SO2 by 
10 million tons and N oxides emissions by 2 million tons from 1980 
emission levels in order to achieve reductions over broad geographic 
regions/areas. In envisioning that further action might be necessary in 
the long term, Congress included section 404 of the 1990 Amendments. 
This section requires the EPA to conduct a study on the feasibility and 
effectiveness of an acid deposition standard or standards to protect 
``sensitive and critically sensitive aquatic and terrestrial 
resources'' and at the conclusion of the study, submit a report to 
Congress. Five years later, the EPA submitted to Congress its report 
titled Acid Deposition Standard Feasibility Study: Report to Congress 
(U.S. EPA, 1995b) in fulfillment of this requirement. The Report to 
Congress concluded that establishing acid deposition standards for Sand 
N deposition might at some point in the future be technically feasible 
although appropriate deposition loads for these acidifying chemicals 
could not be defined with reasonable certainty at that time.
    The 1990 Amendments also added new language to sections of the CAA 
pertaining to ecosystem effects of criteria pollutants, such as acid 
deposition. For example, a new section 108(g) was inserted, stating 
that ``[t]he Administrator may assess the risks to ecosystems from 
exposure to criteria air pollutants (as identified by the Administrator 
in the Administrator's sole discretion).'' The definition of welfare in 
CAA section 302(h) was expanded to indicate that welfare effects 
include those listed therein, ``whether caused by transformation, 
conversion, or combination with other air pollutants.'' Additionally, 
in response to legislative initiatives such as the 1990 Amendments, the 
EPA and other Federal agencies continued research on the causes and 
effects of acidic deposition and related welfare effects of 
SO2 and implemented an enhanced monitoring program to track 
progress (58 FR 21357, April 21, 1993).
4. Most Recent Review of the Secondary Standards for N Oxides and 
SOX
    In December 2005, the EPA initiated a joint review \9\ of the air 
quality criteria for oxides of nitrogen and sulfur and the secondary 
NAAQS for NO2 and SO2 (70 FR 73236, December 9, 
2005).\10\ The review focused on the evaluation of the protection 
provided by the secondary standards for oxides of nitrogen and oxides 
of sulfur for two general types of effects: (1) direct effects on 
vegetation of exposure to gaseous oxides of nitrogen and sulfur, which 
are the type of effects that the existing NO2 and 
SO2 secondary standards were developed to protect against, 
and (2) effects associated with the deposition of oxides of nitrogen 
and sulfur to sensitive aquatic and terrestrial ecosystems (77 FR 
20218, April 3, 2012).
---------------------------------------------------------------------------

    \9\ Although the EPA has historically adopted separate secondary 
standards for oxides of nitrogen and oxides of sulfur, the EPA 
conducted a joint review of these standards because oxides of 
nitrogen and sulfur and their associated transformation products are 
linked from an atmospheric chemistry perspective, as well as from an 
environmental effects perspective. The joint review was also 
responsive to the National Research Council (NRC) recommendation for 
the EPA to consider multiple pollutants, as appropriate, in forming 
the scientific basis for the NAAQS (NRC, 2004).
    \10\ The review was conducted under a schedule specified by 
consent decree entered into by the EPA with the Center for 
Biological Diversity and four other plaintiffs. The schedule, which 
was revised on October 22, 2009, provided that the EPA sign notices 
of proposed and final rulemaking concerning its review of the oxides 
of nitrogen and oxides of sulfur NAAQS no later than July 12, 2011, 
and March 20, 2012, respectively.
---------------------------------------------------------------------------

    The Integrated Review Plan (IRP) for the review was released in 
December 2007, after review of a draft IRP by the public and CASAC (72 
FR 57570, October 10, 2007; Russell, 2007; U.S. EPA, 2007). The first 
and second drafts of the ISA were released in December 2007 and August 
2008, respectively, for the CASAC and public review (72 FR 72719, 
December 21, 2007; 73 FR 10243, February 26, 2008; Russell and 
Henderson, 2008; 73 FR 46908, August 12, 2008; 73 FR 53242, September 
15, 2008; Russell and Samet, 2008a). The final ISA (referred to as 2008 
ISA below) was released in December 2008 (73 FR 75716, December 12, 
2008; U.S. EPA, 2008a). Based on the scientific information in the ISA, 
the EPA planned and developed a quantitative Risk and Exposure 
Assessment (REA),\11\ two drafts of which were made available for 
public comment and reviewed by the CASAC (73 FR 10243, February 26, 
2008; 73 FR 50965, August 29, 2008; Russell and Samet, 2008b; 73 FR 
53242, September 15, 2008; 74 FR 28698, June 17, 2009; Russell and 
Samet, 2009). The final REA was released in September 2009 (U.S. EPA, 
2009a; 74 FR 48543; September 23, 2009).
---------------------------------------------------------------------------

    \11\ Although the REA for the 2012 review was presented in its 
own separate document, the REA for a NAAQS review may be presented 
in its own separate document or as one or more appendices in the PA 
(e.g., U.S. EPA 2020b, 2020c, and PA for current review [U.S. EPA, 
2024]).
---------------------------------------------------------------------------

    Drawing on the information in the final REA and ISA, the EPA OAQPS 
prepared a PA, two drafts of which were made available for public 
comment and review by the CASAC (75 FR 10479, March 8, 2010; 75 FR 
11877, March 12, 2010; Russell and Samet, 2010b; 75 FR 57463, September 
21, 2010; 75 FR 65480, October 25, 2010; Russell and Samet, 2010a). The 
final PA was released in January 2011 (U.S. EPA, 2011). Based on 
additional discussion subsequent to release of the final PA, the CASAC 
provided additional advice and recommendations on the multipollutant, 
deposition-based standard described in the 2011 PA (76 FR 4109, January 
24, 2011; 76 FR 16768, March 25, 2011; Russell and Samet, 2011).
    For the purpose of protection against the direct effects on 
vegetation of exposure to gaseous oxides of nitrogen and sulfur, the PA 
concluded that consideration should be given to retaining the current 
standards. With respect to the effects associated with the deposition 
of oxides of nitrogen and oxides of sulfur to sensitive aquatic and 
terrestrial ecosystems, the 2011 PA focused on the acidifying effects 
of nitrogen and sulfur deposition on sensitive aquatic ecosystems. 
Based on the information in the ISA, the assessments in the REA, and 
the CASAC advice, the 2011 PA concluded that consideration should be 
given to a new multipollutant standard intended to address deposition-
related effects.
    On August 1, 2011, the EPA published a proposed decision to retain 
the existing annual average NO2 and 3-hour average 
SO2 secondary standards, recognizing the protection they 
provided from direct effects on vegetation (76 FR 46084, August 1, 
2011). Further, after considering the multipollutant approach to 
establishing secondary standards that was described in the 2011 PA, the 
Administrator proposed not to set such a new multipollutant secondary 
standard in light of a number of uncertainties. Additionally, the 
Administrator proposed to revise the secondary standards by adding 
secondary NO2 and SO2 standards identical to the 
1-hour primary NO2 and SO2 standards, both set in 
2010, noting that these new primary standards \12\ would result in 
reductions in oxides of nitrogen and sulfur that would likely reduce 
nitrogen and sulfur deposition to sensitive

[[Page 26626]]

ecosystems (76 FR 46084, August 1, 2011). After consideration of public 
comments, the final decision in the review was to retain the existing 
standards to address the direct effects on vegetation of exposure to 
gaseous oxides of nitrogen and sulfur and to not set additional 
standards particular to effects associated with deposition of oxides of 
nitrogen and sulfur on sensitive aquatic and terrestrial ecosystems at 
that time (77 FR 20218, April 3, 2012).
---------------------------------------------------------------------------

    \12\ The 1-hour primary standards set in 2010 included the 
NO2 standard of 100 ppb, as the 98th percentile of 1-hour 
daily maximum concentrations, averaged over three years, and the 
SO2 standard of 75 ppb, as the 99th percentile of 1-hour 
daily maximum concentrations, averaged over three years (75 FR 6474, 
February 9, 2010; 75 FR 35520, June 22, 2010).
---------------------------------------------------------------------------

    The EPA's 2012 decision was challenged by the Center for Biological 
Diversity and other environmental groups. The petitioners argued that 
having decided that the existing standards were not adequate to protect 
against adverse public welfare effects such as damage to sensitive 
ecosystems, the Administrator was required to identify the requisite 
level of protection for the public welfare and to issue a NAAQS to 
achieve and maintain that level of protection. The District of Columbia 
Circuit (D.C. Circuit) disagreed, finding that the EPA acted 
appropriately in not setting a secondary standard given the EPA's 
conclusions that ``the available information was insufficient to permit 
a reasoned judgment about whether any proposed standard would be 
`requisite to protect the public welfare . . .'.'' \13\ In reaching 
this decision, the court noted that the EPA had ``explained in great 
detail'' the profound uncertainties associated with setting a secondary 
NAAQS to protect against aquatic acidification.\14\
---------------------------------------------------------------------------

    \13\ Center for Biological Diversity, et al. v. EPA, 749 F.3d 
1079, 1087 (2014).
    \14\ Id. at 1088.
---------------------------------------------------------------------------

5. PM
    The EPA first established a secondary standard for PM in 1971 (36 
FR 8186, April 30, 1971), based on the original AQCD, which described 
the evidence as to effects of PM on visibility, materials, light 
absorption, and vegetation (U.S. DHEW, 1969b). To provide protection 
generally from visibility effects and materials damage, the secondary 
standard was set at 150 [micro]g/m\3\, as a 24-hour average, from total 
suspended particles (TSP), not to be exceeded more than once per year 
(36 FR 8187; April 30, 1971).\15\
---------------------------------------------------------------------------

    \15\ Additionally, a guide to be used in assessing 
implementation plans to achieve the 24-hour standard was set at 60 
[micro]g/m\3\, as an annual geometric mean (36 FR 8187; April 30, 
1971).
---------------------------------------------------------------------------

    In October 1979, the EPA announced the first periodic review of the 
air quality criteria and NAAQS for PM (44 FR 56730, October 2, 1979). 
As summarized in subsection 2 above, the EPA developed a new AQCD for 
PM and SOX, drafts of which were reviewed by the CASAC (U.S. 
EPA, 1982b). Subsequently, the EPA OAQPS developed a Staff Paper (U.S. 
EPA, 1982d), two drafts of which were reviewed by the CASAC 
(Friedlander, 1982). Further, the EPA OAQPS prepared an Addendum to the 
1982 Staff Paper, which also received CASAC review (Lippman, 1986; U.S. 
EPA, 1986). After consideration of public comments on a proposed 
decision, the final decision in that review revised the indicator for 
PM NAAQS from TSP to particulate matter with mass median diameter of 10 
microns (PM10) (49 FR 10408, March 20, 1984; 52 FR 24634, 
July 1, 1987). With an indicator of PM10, two secondary 
standards were established to be the same as the primary standards. A 
24-hour secondary standard was set at 150 [micro]g/m\3\, with the form 
of one expected exceedance per year, on average over three years. 
Additionally, an annual secondary standard was set at 50 [micro]g/m\3\, 
with a form of annual arithmetic mean, averaged over three years (52 FR 
24634, July 1, 1987).
    In April 1994, the EPA initiated the second periodic review of the 
air quality criteria and NAAQS for PM. In developing the AQCD, the 
Agency made available three external review drafts for public and CASAC 
review; the final AQCD was released in 1996 (U.S. EPA, 1996). The EPA's 
OAQPS prepared a Staff Paper that was released in November 1997, after 
CASAC and public review of two drafts (U.S. EPA, 1996; Wolff, 1996). 
Revisions to the PM standards were proposed in 1996, and in 1997 the 
EPA promulgated final revisions (61 FR 65738; December 13, 1996; 62 FR 
38652, July 18, 1997). With the 1997 decision, the EPA added new 
standards, using PM2.5 as the indicator for fine particles. 
The new secondary standards were set equal to the primary standards, in 
all respects, as follows: (1) an annual standard with a level of 15.0 
[micro]g/m\3\, based on the 3-year average of annual arithmetic mean 
PM2.5 concentrations from single or multiple community-
oriented monitors; \16\ and (2) a 24-hour standard with a level of 65 
[micro]g/m\3\, based on the 3-year average of the 98th percentile of 
24-hour PM2.5 concentrations at each monitor within an area. 
Further, the EPA retained the annual PM10 standard, without 
revision, and revised the form of the 24-hour PM10 standard 
to be based on the 99th percentile of 24-hour PM10 
concentrations at each monitor in an area.
---------------------------------------------------------------------------

    \16\ The 1997 annual PM2.5 standard was compared with 
measurements made at the community-oriented monitoring site 
recording the highest concentration or, if specific constraints were 
met, measurements from multiple community-oriented monitoring sites 
could be averaged (i.e., ``spatial averaging''). In the last review 
(completed in 2012) the EPA replaced the term ``community-oriented'' 
monitor with the term ``area-wide'' monitor. Area-wide monitors are 
those sited at the neighborhood scale or larger, as well as those 
monitors sited at micro- or middle-scales that are representative of 
many such locations in the same core-based statistical area (CBSA) 
(78 FR 3236, January 15, 2013).
---------------------------------------------------------------------------

    Following promulgation of the 1997 p.m. NAAQS, several parties 
filed petitions for review, raising a broad range of issues. In May 
1999, the U.S. Court of Appeals for the D.C. Circuit upheld the EPA's 
decision to establish fine particle standards, (American Trucking 
Ass'ns, Inc. v. EPA, 175 F. 3d 1027, 1055-56 [D.C. Cir. 1999]). The 
D.C. Circuit also found ``ample support'' for the EPA's decision to 
regulate coarse particle pollution, but vacated the 1997 
PM10 standards, concluding that the EPA had not provided a 
reasonable explanation justifying use of PM10 as an 
indicator for coarse particles (Id at 1054-55). Pursuant to the D.C. 
Circuit's decision, the EPA removed the vacated 1997 PM10 
standards, leaving the pre-existing 1987 PM10 standards in 
place (65 FR 80776, December 22, 2000). The D.C. Circuit also upheld 
the EPA's determination not to establish more stringent secondary 
standards for fine particles to address effects on visibility (Id at 
1027). The D.C. Circuit also addressed more general issues related to 
the NAAQS, including issues related to the consideration of costs in 
setting NAAQS and the EPA's approach to establishing the levels of 
NAAQS.
    In October 1997, the EPA initiated the third periodic review of the 
air quality criteria and NAAQS for PM (62 FR 55201, October 23, 1997). 
After the CASAC and public review of several drafts of the AQCD, the 
EPA released the final AQCD in October 2004 (U.S. EPA, 2004a, b). The 
EPA's OAQPS finalized the Staff Paper in December 2005 (U.S. EPA, 
2005). On December 20, 2005, the EPA announced its proposed decision to 
revise the NAAQS for PM and solicited public comment on a broad range 
of options (71 FR 2620, January 17, 2006). On September 21, 2006, the 
EPA announced its final decisions to revise the PM NAAQS to provide 
increased protection of public health and welfare (71 FR 61144, October 
17, 2006). Revisions to the secondary standards were identical to those 
for the primary standards, with the decision describing the protection 
provided specifically for visibility and non-visibility related welfare 
effects (71 FR 61203-61210, October 17, 2006). With regard to the 
standards for fine particles, the EPA revised the level of

[[Page 26627]]

the 24-hour PM2.5 standards to 35 [micro]g/m\3\, retained 
the level of the annual PM2.5 standards at 15.0 [micro]g/
m\3\, and revised the form of the annual PM2.5 standards by 
narrowing the constraints on the optional use of spatial averaging. 
With regard to the standards for PM10, the EPA retained the 
24-hour standards, with levels at 150 [micro]g/m\3\, and revoked the 
annual standards.
    Several parties filed petitions for review of the EPA's 2006 p.m. 
NAAQS decision. One of these petitions raised the issue of setting the 
secondary PM2.5 standards identical to the primary 
standards. On February 24, 2009, the D.C. Circuit issued its opinion in 
American Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. Cir. 2009) 
and remanded the standards to the EPA because the Agency failed to 
adequately explain why setting the secondary PM standards identical to 
the primary standards provided the required protection for public 
welfare, including protection from visibility impairment (Id. at 528-
32). The EPA responded to the court's remands as part of the subsequent 
review of the PM NAAQS, which was initiated in 2007.
    In June 2007, the EPA initiated the fourth periodic review of the 
air quality criteria and the PM NAAQS (72 FR 35462, June 28, 2007). 
Based on the NAAQS review process, as revised in 2008 and again in 
2009, the EPA held science/policy issue workshops on the primary and 
secondary PM NAAQS (72 FR 34003, June 20, 2007; 72 FR 34005, June 20, 
2007), and prepared and released the planning and assessment documents 
that are part of the review process (i.e., IRP [U.S. EPA, 2008b], ISA 
[U.S. EPA, 2009b], REA planning document for welfare [U.S. EPA, 2009c], 
and an urban-focused visibility assessment [U.S. EPA, 2010], and PA 
[U.S. EPA, 2011]). In June 2012, the EPA announced its proposed 
decision to revise the NAAQS for PM (77 FR 38890, June 29, 2012). In 
December 2012, the EPA announced its final decisions to revise the 
primary and secondary PM2.5 annual standards (78 FR 3086, 
January 15, 2013). With regard to the secondary standards, the EPA 
retained the 24-hour PM2.5 and PM10 standards, 
with a revision to the form of the 24-hour PM2.5, to 
eliminate the option for spatial averaging (78 FR 3086, January 15, 
2013). Petitioners challenged the EPA's final rule. On judicial review, 
the revised standards and monitoring requirements were upheld in all 
respects (National Association of Manufacturers v. EPA, 750 F.3d 921, 
[D.C. Cir. 2014]).
    The subsequent review of the PM secondary standards, completed in 
2020, and its subsequent reconsideration, focused on consideration of 
protection provided from visibility effects, materials damage, and 
climate effects (85 FR 82684, December 18, 2020; 89 FR 16202, March 6, 
2024). Those effects--visibility effects, materials damage and climate 
effects--are not addressed in this review. The evidence for ecological 
effects of PM is addressed in the review of the air quality criteria 
and standards described in the PA for this review.

D. Current Review

    In August 2013, the EPA issued a call for information in the 
Federal Register for information related to the newly initiated review 
of the air quality criteria for oxides of sulfur and oxides of nitrogen 
and announced a public workshop to discuss policy-relevant scientific 
information to inform the review (78 FR 53452, August 29, 2013). Based 
in part on the information received in response to the call for 
information, the EPA developed a draft IRP, which was made available 
for consultation with the CASAC and for public comment (80 FR 69220, 
November 9, 2015). Comments from the CASAC and the public on the draft 
IRP were considered in preparing the final IRP (Diez Roux and 
Fernandez, 2016; U.S. EPA, 2017). In developing the final IRP, the EPA 
expanded the review to also include review of the criteria and 
standards related to ecological effects of PM in recognition of 
linkages between these pollutants (oxides of nitrogen, oxides of sulfur 
and PM) with respect to atmospheric transformation of N and S oxides 
into particulate compounds, deposition of N and S compounds and 
associated ecological effects (U.S. EPA, 2017). Addressing the 
pollutants together enables a comprehensive consideration of the nature 
and interactions of the pollutants, which is important for ensuring 
thorough evaluation of the scientific information relevant to 
ecological effects of N and S deposition.
    In March 2017, the EPA released the first external review draft of 
the Integrated Science Assessment (ISA) for Oxides of Nitrogen, Oxides 
of Sulfur, and Particulate Matter Ecological Criteria (82 FR 15702, 
March 30, 2017), which was then reviewed by the CASAC at a public 
meeting on May 24-25, 2017 (82 FR 15701, March 30, 2017) and August 31, 
2017 (82 FR 35200, July 28, 2017; Diez Roux and Fernandez, 2017). With 
consideration of comments from the CASAC and the public, the EPA 
released a second external review draft (83 FR 29786, June 26, 2018), 
which the CASAC reviewed at public meetings on September 5-6, 2018 (83 
FR 2018; July 9, 2018) and April 27, 2020 (85 FR 16093, March 30, 2020; 
Cox, Kendall, and Fernandez, 2020a).\17\ The EPA released the final ISA 
in October 2020 (85 FR 66327, October 19, 2020; U.S. EPA, 2020a). In 
planning for quantitative aquatic acidification exposure/risk analyses 
for consideration in the PA, the EPA solicited public comment and 
consulted with the CASAC (83 FR 31755, July 9, 2018; Cox, Kendall, and 
Fernandez, 2020b; U.S. EPA, 2018; 83 FR 42497, August 22, 2018).
---------------------------------------------------------------------------

    \17\ A change in CASAC membership contributed to an extended 
time period between the two public meetings.
---------------------------------------------------------------------------

    The draft PA, including the REA for aquatic acidification as an 
appendix, was completed in May 2023 and was made available for review 
by the CASAC and for public comment (88 FR 34852, May 31, 2023). The 
CASAC review was conducted at public meetings held on June 28-29, 2023 
(88 FR 17572, March 23, 2023), and September 5-6, 2023 (88 FR 45414, 
July 17, 2023). The CASAC conveyed advice on the standards and comments 
on the draft PA in its September 27, 2023, letter to the Administrator 
(Sheppard, 2023). The final PA was completed in January 2024 (89 FR 
2223, January 12, 2024).
    Materials upon which this proposed decision is based, including the 
documents described above, are available to the public in the docket 
for this review.\18\ The timeline for the remainder of this review is 
governed by a consent decree that requires the EPA to sign a notice of 
proposed decision by April 9, 2024, and a final decision notice by 
December 10, 2024 (Center for Biological Diversity v. Regan [No. 4:22-
cv-02285-HSG (N.D. Cal.]).
---------------------------------------------------------------------------

    \18\ The docket for this review, EPA-HQ-OAR-2014-0128, has 
incorporated the ISA docket (EPA-HQ-ORD-2013-0620) by reference. 
Both are publicly accessible at www.regulations.gov.
---------------------------------------------------------------------------

II. Rationale for Proposed Decisions

    This section presents the rationale for the Administrator's 
proposed decisions in the review of the secondary standards for the 
ecological effects of SOX, N oxides and PM. This rationale 
is based on a thorough review of the full evidence base, including the 
scientific information available since the last review of the secondary 
standards for N oxides and SOX, which is generally published 
between January 2008 and May 2017 (and considered in the ISA), as well 
as more recent studies identified during peer review or by public

[[Page 26628]]

comments (ISA, section IS.1.2),\19\ integrated with the information and 
conclusions from previous assessments and presented in the ISA, on 
ecological effects associated with SOX, N oxides and PM and 
pertaining to their presence in ambient air. The Administrator's 
rationale also takes into account: (1) the PA evaluation of the policy-
relevant information in the ISA and presentation of quantitative 
analyses of air quality, exposure and aquatic acidification risks; (2) 
CASAC advice and recommendations, as reflected in discussions of drafts 
of the ISA and PA at public meetings and in the CASAC's letters to the 
Administrator; and (3) public comments received during the development 
of these documents.
---------------------------------------------------------------------------

    \19\ In addition to the review's opening ``Call for 
Information'' (78 FR 53452, August 29, 2013), multiple search 
methodologies were applied to identify relevant scientific findings 
that have emerged since the 2008 ISA. Search techniques for the 
current ISA identified and evaluated studies and reports that have 
undergone scientific peer review and were published or accepted for 
publication between January 2008 (providing some overlap with the 
cutoff date for the last ISA) and May 2017. Studies published after 
the literature cutoff date for this ISA were also considered if they 
were submitted in response to the Call for Information or identified 
in subsequent phases of ISA development, particularly to the extent 
that they provide new information that affects key scientific 
conclusions. References that are cited in the ISA, the references 
that were considered for inclusion but not cited, and electronic 
links to bibliographic information and abstracts can be found at: 
https://hero.epa.gov/hero/index.cfm/project/page/project_id/2965 
(ISA, section IS.1.2).
---------------------------------------------------------------------------

    In presenting the rationale for the Administrator's proposed 
decisions and their foundations, section II.A provides background on 
the general approach in this review, including a summary of the basis 
for the existing standards (section II.A.1), a summary of the prior 
review of the SOX and N oxides standards for deposition-
related effects (section II.A.2) and the general approach for the 
current review (section II.A.3).
    Section II.B summarizes air quality information and analyses 
relating S and N deposition to concentrations of SOX, N 
oxides and PM. Section II.C summarizes the currently available 
ecological effects evidence as summarized in the ISA, focusing on 
consideration of key policy-relevant aspects. Section II.D summarizes 
the exposure and risk information for this review, drawing on the 
quantitative analyses of aquatic acidification risk, presented in the 
PA. Section II.E presents the Administrator's proposed conclusions on 
the current standards and potential alternatives (section II.E.3), 
drawing on both evidence-based and exposure/risk-based considerations 
from the PA (section II.E.1) and advice from the CASAC (section 
II.E.2).

A. Introduction

    As is the case for all such reviews, this review is based, most 
fundamentally, on using the Agency's assessments of the current 
scientific evidence and associated quantitative analyses to inform the 
Administrator's judgment regarding secondary standards for 
SOX, N oxides and PM that are requisite to protect the 
public welfare from known or anticipated adverse effects associated 
with that pollutant's presence in the ambient air. The EPA's 
assessments are primarily documented in the ISA and PA, both of which 
have received CASAC review and public comment (82 FR 15702, March 30, 
2017; 82 FR 15701, March 30, 2018; 83 FR 29786; June 26, 2018; 83 FR 
31755, July 9, 2018; 85 FR 16093; March 20, 2020; 88 FR 34852, May 31, 
2023; 88 FR 17572, March 23, 2023; 88 FR 45414, July 17, 2023). In 
bridging the gap between the scientific assessments of the ISA and the 
judgments required of the Administrator in his decisions on the current 
standard, the PA evaluates policy implications of the assessment of the 
current evidence in the ISA and the quantitative exposure and risk 
analyses and information documented in the PA. In evaluating the public 
welfare protection afforded by the current standard, the four basic 
elements of the NAAQS (indicator, averaging time, level, and form) are 
considered collectively.\20\
---------------------------------------------------------------------------

    \20\ The indicator defines the chemical species or mixture to be 
measured in the ambient air for the purpose of determining whether 
an area attains the standard. The averaging time defines the period 
over which air quality measurements are to be averaged or otherwise 
analyzed. The form of a standard defines the air quality statistic 
that is to be compared to the level of the standard in determining 
whether an area attains the standard. For example, the form of the 
annual NAAQS for fine particulate matter (PM2.5) is the 
average of annual mean concentrations for three consecutive years, 
while the form of the 3-hour secondary NAAQS for SO2 is 
the second highest 3-hour average in a year. The level of the 
standard defines the air quality concentration used for that 
purpose.
---------------------------------------------------------------------------

    The Agency's approach in its review of secondary standards is 
consistent with the requirements of the provisions of the CAA related 
to the review of NAAQS and with how the EPA and the courts have 
historically interpreted the CAA. These provisions require the 
Administrator to establish secondary standards that, in the 
Administrator's judgment, are requisite (i.e., neither more nor less 
stringent than necessary) to protect the public welfare from known or 
anticipated adverse effects associated with the presence of the 
pollutant in the ambient air. In so doing, the Administrator considers 
advice from the CASAC and public comment. This approach is based on a 
recognition that the available welfare effects evidence generally 
reflects a range of effects that include ambient air exposure 
circumstances for which scientists generally agree that effects are 
likely to occur as well as lower levels at which the likelihood and 
magnitude of response become increasingly uncertain. The CAA does not 
require that standards be set at a zero-risk level, but rather at a 
level that reduces risk sufficiently so as to protect the public 
welfare from known or anticipated adverse effects.
    The Agency's decisions on the adequacy of the current secondary 
standards and, as appropriate, on any potential alternative standards 
considered in a review, are largely public welfare policy judgments 
made by the Administrator based on the Administrator's informed 
assessment of what constitutes requisite protection against adverse 
effects to public welfare. A public welfare policy decision draws upon 
scientific information and analyses about welfare effects, exposure and 
risks, as well as judgments about the appropriate response to the range 
of uncertainties that are inherent in the scientific evidence and 
analyses. The ultimate determination as to what level of damage to 
ecosystems and the services provided by those ecosystems is adverse to 
public welfare is not wholly a scientific question, although it is 
informed by scientific studies linking ecosystem damage to losses in 
ecosystem services and information on the value of those losses of 
ecosystem services. In reaching such decisions, the Administrator seeks 
to establish standards that are neither more nor less stringent than 
necessary for this purpose.
    Thus, in general, conclusions reached by the Administrator in 
secondary NAAQS reviews on the amount of public welfare protection from 
the presence of the pollutant(s) in ambient air that is appropriate to 
be afforded by a secondary standard take into account a number of 
considerations. Among these considerations are the nature and degree of 
effects of the pollutant, including the Administrator's judgments on 
what constitutes an adverse effect to the public welfare as well as the 
strengths and limitations of the available and relevant information, 
with its associated uncertainties. Across reviews, it is generally 
recognized that such judgments should neither overstate nor understate 
the strengths and limitations of the evidence and information nor the 
appropriate inferences to be drawn as to risks to public welfare, and 
that the choice of

[[Page 26629]]

the appropriate level of protection is a public welfare policy judgment 
entrusted to the Administrator under the CAA taking into account both 
the available evidence and the uncertainties (80 FR 65404-05, October 
26, 2015). Thus, the Administrator's final decisions in such reviews 
draw upon the scientific information and analyses about welfare 
effects, environmental exposures and risks, and associated public 
welfare significance, as well as judgments about how to consider the 
range and magnitude of uncertainties that are inherent in the 
scientific evidence and quantitative analyses.
1. Basis for Existing Secondary Standards
    In the last review of the secondary standards for SOX 
and N oxides, completed in 2012, the EPA retained the existing 3-hour 
SO2 standard, with its level of 0.5 ppm, and the annual 
NO2 standard, with its level of 0.053 ppm (77 FR 20218, 
April 3, 2012). Both of these secondary standards were established in 
1971 (36 FR 8186, April 30, 1971). The basis for both the existing 
SO2 and NO2 secondary standard is to provide 
protection to the public welfare related to direct effects on 
vegetation (U.S. DHEW, 1969a; U.S. EPA, 1971).
    The welfare effects evidence for SOX in previous reviews 
indicates a relationship between short- and long-term SO2 
exposures and foliar damage to cultivated plants, reductions in 
productivity, species richness, and diversity (U.S. DHEW, 1969a; U.S. 
EPA, 1982c; U.S. EPA, 2008). At the time the standard was set, 
concentrations of SO2 in the ambient air were also 
associated with other welfare effects, including effects on materials 
and visibility related to sulfate, a particulate transformation product 
of SO2 (U.S. DHEW, 1969a). However, the available data were 
not sufficient to establish a quantitative relationship between 
specific SO2 concentrations and such effects (38 FR 25679, 
September 14, 1973). Accordingly, direct effects of SOX in 
ambient air on vegetation is the basis for the existing secondary 
standard for SOX. Effects on materials and visibility (which 
relate to particles in air, including sulfates) have more recently been 
considered in the PM secondary NAAQS reviews (e.g., 85 FR 82684, 
December 18, 2020).
    The welfare effects evidence for N oxides in previous reviews 
includes foliar injury, leaf drop, and reduced yield of some crops 
(U.S. EPA, 1971; U.S. EPA, 1982c; U.S. EPA, 1993; U.S. EPA, 2008a). 
Since it was established in 1971, the secondary standard for N oxides 
has been reviewed three times, in 1985, 1996, and 2012 (50 FR 25532, 
June 19, 1985; 61 FR 52852; October 8, 1996; 77 FR 20218, April 3, 
2012). Although those reviews identified additional effects related to 
N deposition, they all have concluded that the existing NO2 
secondary standard provided adequate protection related to the 
``direct'' effects of airborne N oxides on vegetation on which the 
standard is based).
    In the last review of the secondary PM standards with regard to 
protection from ecological effects, completed in 2013, the EPA retained 
the 24-hour PM2.5 standard, with its level of 35 [micro]g/
m\3\, and the 24-hour PM10 standard, with its level of 150 
[micro]g/m\3\ (78 FR 3228, January 15, 2013). With regard to the annual 
PM2.5 standard, the EPA retained the averaging time and 
level, set at 15 [micro]g/m\3\, while revising the form to remove the 
option for spatial averaging consistent with this change to the primary 
annual PM2.5 standard (78 FR 3225, January 15, 2013). The 
2013 review considered the PM standards with regard to protection for 
an array of effects that include effects on visibility, materials 
damage, and climate effects, as well as ecological effects, and the EPA 
concluded that those standards provided protection for ecological 
effects (e.g., 78 FR 3225-3226, 3228, January 15, 2013). In reaching 
this conclusion, it was noted that the PA for the review explicitly 
excluded discussion of the effects associated with deposited PM 
components of N oxides and SOX and their transformation 
products, which were being addressed in the joint review of the 
secondary NO2 and SO2 NAAQS (78 FR 3202, January 
15, 2013). The ecological effects of PM considered include direct 
effects on plant foliage as well as effects of the ecosystem loading of 
PM constituents such as metals or organic compounds (2009 ISA, section 
2.5.3). For all of these effects, the 2013 decision recognized an 
absence of information that would support any different standards and 
concluded the existing standards, with the revision to the form of the 
annual PM2.5 standard, provided the requisite protection (78 
FR 3086, January 15, 2013).
2. Prior Review of Deposition-Related Effects
    In the 2012 review of the NO2 and SO2 
secondary standards, the EPA recognized that a significant increase in 
understanding of the effects of N oxides and SOX had 
occurred since the prior secondary standards reviews for those 
pollutants (77 FR 20236, April 3, 2012). Considering the extensive 
evidence available at that time, the Agency concluded that the most 
significant risks of adverse effects of N oxides and SOX to 
public welfare were those related to deposition of N and S compounds to 
both terrestrial and aquatic ecosystems (77 FR 20236, April 3, 2012). 
Accordingly, in addition to evaluating the protection provided by the 
secondary standards for N oxides and SOX from effects 
associated with the airborne pollutants, the 2012 review also included 
extensive analyses of the welfare effects associated with nitrogen and 
sulfur deposition to sensitive aquatic and terrestrial ecosystems (77 
FR 20218, April 3, 2012).
    Based on the available evidence, the risks of atmospheric 
deposition analyzed in the 2009 REA related to two categories of 
ecosystem effects: acidification and nutrient enrichment (U.S. EPA, 
2009a). The analyses included assessment of risks of both types of 
effects in both terrestrial and aquatic ecosystems. While the available 
evidence supported conclusions regarding the role of atmospheric 
deposition of S and N compounds in acidification and nutrient 
enrichment of aquatic and terrestrial ecosystems, there was variation 
in the strength of the evidence and of the information supporting 
multiple quantitative linkages between pollutants in ambient air and 
ecosystem responses and potential public welfare implications.
    While there is extensive evidence of deleterious effects of 
excessive nitrogen loadings to terrestrial and aquatic ecosystems, 
consideration of the nutrient enrichment-related effects of atmospheric 
N and S deposition with regard to identification of options to provide 
protection for deposition-related effects was limited by several 
factors. For example, the co-stressors affecting forests, including 
other air pollutants such as ozone, and limiting factors such as 
moisture and other nutrients, confound the assessment of marginal 
changes in any one stressor or nutrient in a forest ecosystem, limiting 
the information on the effects of changes in N deposition on 
forestlands and other terrestrial ecosystems (2011 PA, section 6.3.2). 
Further, only a fraction of the deposited N was reported to be taken up 
by the forests, with most of the N retained in the soils, such that 
forest management practices can significantly affect the nitrogen 
cycling within a forest ecosystem (2008 ISA section 3.3.2.1 and Annex 
C, section C.6.3). Factors affecting consideration of aquatic 
eutrophication effects included the appreciable contributions of non-
atmospheric sources to waterbody nutrient loading, which affected our 
attribution of specific effects to

[[Page 26630]]

atmospheric sources of N, and limitations in the ability of the 
available data and models to characterize incremental adverse impacts 
of atmospheric N deposition (2011 PA, section 6.3.2).
    The linkages between terrestrial acidification and atmospheric 
deposition of N and S compounds were also limited by the sparseness of 
available data for identifying appropriate assessment levels for 
terrestrial acidification indicators and uncertainties with regard to 
empirical case studies in the ISA (e.g., the potential for other 
stressors to confound relationships between deposition and terrestrial 
acidification effects). However, the evidence in the 2008 ISA and the 
REA analyses of aquatic acidification provided strong support to the 
evidence for a relationship between atmospheric deposition of N and S 
compounds and loss of acid neutralizing capacity (ANC) in sensitive 
ecosystems, with associated aquatic acidification effects.
    In light of the evidence and findings of these analyses and advice 
from the CASAC, the PA concluded it was appropriate to place greatest 
confidence in findings related to the aquatic acidification-related 
effects of N oxides and SOX relative to other deposition-
related effects. Therefore, the PA focused on aquatic acidification 
effects from deposition of N and S compounds in identifying policy 
options for providing public welfare protection from deposition-related 
effects of N oxides and SOX, concluding that the available 
information and assessments were only sufficient at that time to 
support development of a standard to address aquatic acidification. 
Consistent with this, the PA concluded it was appropriate to consider a 
secondary standard in the form of an aquatic acidification index (AAI) 
and identified a range of AAI values (which correspond to minimum ANC 
levels) for consideration (2011 PA, section 7.6.2). Conceptually, the 
AAI is an index that uses the results of ecosystem and air quality 
modeling to estimate waterbody ANC. The standard level for an AAI-based 
standard was conceptually envisioned to be a national minimum target 
ANC for waterbodies in the ecoregions of the U.S. for which data were 
considered adequate for these purposes (2011 PA, section 7.6.2).
    While the NAAQS have historically been set in terms of an ambient 
air concentration, an AAI-based standard was envisioned to have a 
single value established for the AAI, but the concentrations of 
SOX and N oxides would be specific to each ecoregion, taking 
into account variation in several factors that influence waterbody ANC, 
and consequently could vary across the U.S. The factors, specific to 
each ecoregion (``F factors''), which it was envisioned would be 
established as part of the standard, include: surface water runoff 
rates and so-called ``transference ratios,'' which are factors applied 
to back-calculate or estimate the concentrations of SOX and 
N oxides corresponding to target deposition values that would meet the 
AAI-based standard level, which is also the target minimum ANC (2011 
PA, Chapter 7).\21\ The ecoregion-specific values for these factors 
would be specified based on then available data and simulations of the 
Community Multiscale Air Quality (CMAQ) model, and codified as part of 
such a standard. As part of the standard, these factors would be 
reviewed in the context of each periodic review of the NAAQS.
---------------------------------------------------------------------------

    \21\ These were among the ecoregion-specific factors that 
comprised the parameters F1 through F4 in the AAI equation (2011 PA, 
p. 7-37). The parameter F2 represented the ecoregion-specific 
estimate of acidifying deposition associated with reduced forms of 
nitrogen, NHX (2011 PA, p. 7-28 and ES-8 to ES-9). The 
2011 PA suggested that this factor could be specified based on a 
2005 CMAQ model simulation over 12-km grid cells or monitoring might 
involve the use of monitoring data for NHX applied in dry 
deposition modeling. It was recognized that appreciable spatial 
variability, as well as overall uncertainty, were associated with 
this factor.
---------------------------------------------------------------------------

    After consideration of the PA conclusions, the Administrator 
concluded that while the conceptual basis for the AAI was supported by 
the available scientific information, there were limitations in the 
available relevant data and uncertainties associated with specifying 
the elements of the AAI, specifically those based on modeled factors, 
that posed obstacles to establishing such a standard under the CAA. It 
was recognized that the general structure of an AAI-based standard 
addressed the potential for contributions to acid deposition from both 
N oxides and SOX and quantitatively described linkages 
between ambient air concentrations, deposition, and aquatic 
acidification, considering variations in factors affecting these 
linkages across the country. However, the Administrator judged that the 
limitations and uncertainties in the available information were too 
great to support establishment of a new standard that could be 
concluded to provide the requisite protection for such effects under 
the Act (77 FR 20218, April 3, 2012). These uncertainties generally 
related to the quantification of the various elements of the standard 
(the ``F factors''), and their representativeness at an ecoregion 
scale. These uncertainties and the complexities in this approach were 
recognized to be unique to the 2012 review of the NAAQS for N and S 
oxides and were concluded to preclude the characterization and degree 
of protectiveness that would be afforded by an AAI-based standard, 
within the ranges of levels and forms identified in the PA, and the 
representativeness of F factors in the AAI equation described in the 
2011 PA (77 FR 20261, April 3, 2012).

. . . the Administrator recognizes that characterization of the 
uncertainties in the AAI equation as a whole represents a unique 
challenge in this review primarily as a result of the complexity in 
the structure of an AAI based standard. In this case, the very 
nature of some of the uncertainties is fundamentally different than 
uncertainties that have been relevant in other NAAQS reviews. She 
notes, for example, some of the uncertainties uniquely associated 
with the quantification of various elements of the AAI result from 
limitations in the extent to which ecological and atmospheric 
models, which have not been used to define other NAAQS, have been 
evaluated. Another important type of uncertainty relates to 
limitations in the extent to which the representativeness of various 
factors can be determined at an ecoregion scale, which has not been 
a consideration in other NAAQS.'' [77 FR 20261, April 3, 2012]

    The Administrator concluded that while the existing secondary 
standards were not adequate to provide protection against potentially 
adverse deposition-related effects associated with N oxides and 
SOX, it was not appropriate under section 109 to set any new 
or additional standards at that time to address effects associated with 
deposition of N and S compounds on sensitive aquatic and terrestrial 
ecosystems (77 FR 20262-20263, April 3, 2012).
3. General Approach for This Review
    As is the case for all NAAQS reviews, this secondary standards 
review uses the Agency's assessment of the current scientific evidence 
and associated quantitative analyses as a foundation to inform the 
Administrator's judgments regarding secondary standards that are 
requisite to protect the public welfare from known or anticipated 
adverse effects. The approach for this review of the secondary 
SOX, N oxides, and PM standards builds on the last reviews 
of those pollutants, including the substantial assessments and 
evaluations performed over the course of those reviews, and considering 
the more recent scientific information and air quality data now 
available to inform understanding of the key policy-relevant issues in 
the current review.

[[Page 26631]]

    This review of the secondary standards for SOX, N 
oxides, and PM assesses the protection provided by the standards from 
two categories of effects: direct effects of the airborne pollutants 
and indirect effects of the associated S- and N-containing compounds 
(in gaseous and particulate form) deposited in ecosystems. In so doing, 
the review draws on the currently available evidence as assessed in the 
ISA (and prior assessments) and quantitative exposure, risk, and air 
quality information in the PA, including the REA for aquatic 
acidification.
    With regard to direct effects, we draw on the currently available 
evidence as assessed in the ISA, including the determinations regarding 
the causal nature of relationships between the airborne pollutants and 
ecological effects, which focus most prominently on vegetation, and 
quantitative exposure and air quality information. Based on this 
information, we consider the policy implications, most specifically 
whether the evidence supports the retention or revision of the current 
NO2 and SO2 secondary standards. With regard to 
the effects of PM, we take a similar approach, based on the evidence 
presented in the current ISA and conclusions from the review of the PM 
NAAQS concluded in 2013 (in which ecological effects were last 
considered) to assess the effectiveness of the current PM standard to 
protect against these types of impacts.
    With regard to deposition-related effects, we consider the evidence 
for the array of effects identified in the ISA (and summarized in 
section II.B below), including both terrestrial and aquatic effects; 
and the limitations in the evidence and associated uncertainties; as 
well as the public welfare implications of such effects. The overall 
approach takes into account the nature of the welfare effects and the 
exposure conditions associated with effects in identifying S and N 
deposition levels appropriate to consider in the context of public 
welfare protection. To identify and evaluate metrics relevant to air 
quality standards (and their elements), we have assessed relationships 
developed from air quality measurements near pollutant sources and 
deposition estimates nearby and in downwind ecoregions. In so doing, 
the available quantitative information both on deposition and effects, 
and on ambient air concentrations and deposition, has been assessed 
with regard to the existence of linkages between SOX, N 
oxides, and PM in ambient air and deposition-related effects. These 
assessments then inform judgments on the likelihood of occurrence of 
deposition-related effects under air quality that meets the existing 
standards for these pollutants, or potential alternatives.
    In considering the information on deposition and effects, we 
recognize that the impacts from the dramatically higher deposition 
rates of the past century can affect how ecosystems and biota respond 
to more recent, lower deposition rates, complicating interpretation of 
impacts related to more recent, lower deposition levels. This 
complexity is illustrated by findings of studies that compared soil 
chemistry across 15-30-year intervals (1984-2001 and 1967-1997) and 
reported that although atmospheric deposition in the Northeast declined 
across those intervals, soil acidity increased (ISA, Appendix 4, 
section 4.6.1). As noted in the ISA, ``[i]n areas where N and S 
deposition has decreased, chemical recovery must first create physical 
and chemical conditions favorable for growth, survival, and 
reproduction'' (ISA, Appendix 4, section 4.6.1). Thus, the extent to 
which S and N compounds (once deposited) are retained in soil matrices 
(with potential effects on soil chemistry) influences the dynamics of 
the response of the various environmental pathways to changes in air 
quality, in addition to the influences of emissions, ambient air 
concentrations and associated deposition.
    The two-pronged approach to this review's consideration of 
deposition-related effects based on the available information in the 
ISA (summarized in section II.C and II.D below) includes the 
consideration of deposition levels that may be associated with 
ecological effects of potential concern. In this step, we consider and 
strive to focus on effects for which the evidence is most robust with 
regard to established quantitative relationships between deposition and 
ecosystem effects. The information for terrestrial ecosystems is 
derived primarily from analysis of the evidence presented in the ISA. 
For aquatic ecosystems, primary focus is given to effects related to 
aquatic acidification, for which we have conducted quantitative risk 
and exposure analyses based on available modeling applications that 
relate acid deposition and acid buffering capability in U.S. 
waterbodies, as summarized in section II.D below (PA, section 5.1 and 
Appendix 5A).
    In parallel fashion to identification of deposition levels for 
consideration, air quality and deposition analyses have been employed 
to inform an understanding of relationships between ambient air 
concentrations near pollutant sources in terms of metrics relevant to 
air quality standards (and their elements) and ecosystem deposition 
estimates. As described in section II.B below, several different types 
of analyses have been performed in this review for this purpose. 
Interpretation of findings from these analyses, in combination with the 
identified deposition levels of interest, and related policy judgments 
regarding limitations and associated uncertainties of the underlying 
information, inform the Administrator's proposed conclusions on the 
extent to which existing standards, or potential alternative standards, 
might be expected to provide protection from these levels.
    In summary, our approach to evaluating the standards with regard to 
protection from ecological effects related to ecosystem deposition of N 
and S compounds (presented in the sections that follow) involves 
multiple components: (1) review of the scientific evidence to identify 
the ecological effects associated with the three pollutants, both those 
related to direct pollutant contact and to ecosystem deposition; (2) 
assessment of the evidence and characterization of the REA results to 
identify deposition levels related to categories of ecosystem effects; 
(3) analysis of relationships between ambient air concentrations of the 
three pollutants and deposition of N and S compounds to understand key 
aspects of these relationships that can inform the Administrator's 
decisions on policy options for ambient air standards to protect 
against air concentrations associated with direct effects and with 
deposition-related effects that are judged adverse to the public 
welfare. As is described in sections II.B and II.E, for two of the 
pollutants, N oxides and PM, relating ambient air concentrations to 
deposition (of N compounds) is particularly complex because N 
deposition also results from an additional air pollutant that is not 
controlled by NAAQS for N oxides and PM. Thus, separate from the 
evaluation of standards for SOX, the evaluation for N oxides 
and PM also considers current information (e.g., spatial and temporal 
trends) related to the additional air pollutant, ammonia 
(NH3), that contributes to N deposition and to PM components 
that do not contribute to N deposition. Evaluation of all of this 
information, together, is considered by the Administrator in reaching 
his proposed decision, as summarized in section II.E.

B. Air Quality and Deposition

    The three criteria pollutants that are the focus of this review 
(SOX, N oxides, and PM) include both gases and

[[Page 26632]]

particles. Both their physical state and chemical properties, as well 
as other factors, influence their deposition as N- or S-containing 
compounds. The complex pathway from emissions of these pollutants and 
their precursors to eventual deposition varies by pollutant and is 
influenced by a series of atmospheric processes and chemical 
transformations that occur at multiple spatial and temporal scales (PA, 
Chapters 2 and 6).
    A complication in the consideration of the influence of these 
criteria pollutants on N deposition (and associated ecological effects) 
is posed by the contribution of other, non-criteria, pollutants in 
ambient air, specifically NH3. As summarized below, although 
there is a decreasing temporal trend in emissions of N oxides, the 
coincident increasing trend in NH3 emissions has reduced the 
influence of N oxides on N deposition (PA, sections 6.2.1, 6.4.2 and 
7.2.3.3). Variability and temporal changes in the composition of PM, 
including with regard to N- (and S-) containing compounds, is another 
factor affecting decisions in this review (as discussed in sections 
II.1.d(3)) and II.3 below).
    This section includes a brief summary of the major emission sources 
of SOX, N oxides, and PM (section II.B.1). This is followed 
by a description of how those emissions are transported and transformed 
within the atmosphere to eventually contribute to S and N deposition 
(section II.B.1). Available information on current levels of emissions 
and air concentrations of these three pollutants across the U.S. and 
their trends is summarized in section II.B.2, accompanied by a 
description of estimated deposition levels across the U.S. and how they 
have changed over the past two decades. Finally, while many of the 
ecological effects examined in this review are associated with 
deposition of N and S, the NAAQS are set in terms of pollutant 
concentrations. To that end, section II.B.3 discusses the findings of 
analyses performed to relate ambient air concentrations of the relevant 
pollutants and S or N deposition, over a range of conditions (e.g., 
pollutant, region, time period), and summarizes key observations that 
may inform the Administrator's judgments in this review.
1. Sources, Emissions and Atmospheric Processes Affecting 
SOX, N Oxides and PM
    Sulfur dioxide is one of a small group of highly reactive gases 
collectively known as SOX. Sulfur dioxide is generally 
present at higher concentrations in the ambient air than the other 
gaseous SOX species (ISA, Appendix 2, section 2.1) and, as a 
result, SO2 is the indicator for the existing NAAQS for 
SOX. The main anthropogenic source of SO2 
emissions is fossil fuel combustion (PA, section 2.2.2). Based on the 
2020 National Emissions Inventory (NEI), the top three emission sources 
of SO2 in the U.S. are: coal-fired electrical generating 
units (48% of total), industrial processes (27%), and other stationary 
source fuel combustion (9%).
    Once emitted to the atmosphere, the atmospheric lifetime of 
SO2 is typically less than 1-2 days; it can either remain in 
the gas phase or be oxidized to form sulfate particles 
(SO42-). Modeling studies suggest that oxidation 
accounts for more than half of SO2 removal on a national 
basis (PA, section 2.1.1). The rate of SO2 oxidation 
accelerates with greater availability of oxidants. Oxidants are 
generally depleted near source stacks, so that more SO2 is 
oxidized to SO42- in cleaner air downwind of 
SOX sources (2008 ISA, section 2.6.3.1). The atmospheric 
lifetime of SO42- particles is longer, ranging 
from 2 to 10 days. As SO42- particles are 
generally within the fine particle size range, they are a component of 
PM2.5 (PA, section 2.1.1). The spatial distribution of both 
SO2 and SO42- deposition reflects the 
distribution of SOX emissions (i.e., most S deposition is in 
the eastern U.S.; PA, section 2.5.3) and wind patterns. Precipitation 
variability also modulates the spatial distribution of S wet 
deposition. In sum, both SO2, and the 
SO42- particles converted from SO2, 
contribute to S deposition but do so over different time and geographic 
scales, with dry deposition of SO2 typically occurring near 
the source, and wet deposition of sulfate particles being more regional 
in nature.
    The term N oxides refers to all forms of oxidized nitrogen 
compounds, including nitric oxide (NO), NO2, nitric acid 
(HNO3), and particulate nitrate 
(NO3-). Most N oxides enter the atmosphere as 
either NO or NO2, which are collectively referred to as 
NOX (PA, section 2.1.2). Anthropogenic sources account for 
the majority of NOX emissions in the U.S., per the 2020 NEI, 
with highway vehicles (26% of total), stationary fuel combustion which 
includes electric generating units (25%), and non-road mobile sources 
(19%) identified as the largest contributors to total emissions. Other 
anthropogenic NOX sources include agricultural field 
burning, prescribed fires, and various industrial processes such as 
cement manufacturing and oil and gas production (PA, section 2.2.1).
    Once emitted into the atmosphere, NOX can deposit to the 
surface or be chemically converted to other gaseous N oxides, including 
HNO3, as well as to particulate NO3-. 
Unlike particulate SO42-, which exists almost 
entirely in the fine particle range, NO3- 
particles may occur either in the fine or coarse size range, such that 
not all particulate NO3- is a component of 
PM2.5. Each form of oxidized N is removed from the 
atmosphere at different rates by both dry and wet deposition. As a 
general rule, the gas phase species tend to have shorter atmospheric 
lifetimes, either dry depositing (e.g., as HNO3) or quickly 
converting to particulate NO3-. Particulate 
NO3- is more efficiently removed by precipitation 
(wet deposition) and has a similar atmospheric lifetime as particulate 
SO42- (2-10 days).
    In addition to N oxides, there is another category of nitrogen 
pollutants, referred to as reduced nitrogen, which is distinct from N 
oxides but also contributes to nitrogen deposition. The most common 
form of reduced N in the air is ammonia gas (NH3). Sources 
of NH3 emissions include livestock waste (49% of total in 
2020 NEI), fertilizer application (33%) and aggregate fires (11%). 
Ammonia tends to dry deposit near sources (PA, section 2.1.3). It can 
also be converted to particle form, as ammonium (NH4\+\), 
which can be transported farther distances and is most efficiently 
removed by precipitation (PA, section 2.1.3). Ammonia, unlike N oxides 
or PM2.5, is not a criteria pollutant and is not directly 
regulated under CAA section 109.
    In sum, particulate matter is both emitted to the atmosphere and 
can be formed in the atmosphere from precursor chemical gases (such as 
is the case for NOX and SOX). The components of 
PM2.5 mass that contribute to S and N deposition are 
secondary products formed in the atmosphere after being emitted (e.g., 
particulate sulfate, particulate NO3-, 
NH4\+\). There are other components of PM2.5 mass 
that do not contribute to S and N deposition, e.g., black carbon, 
organic carbon, dust (PA, section 2.4.3).
2. Recent Trends in Emissions, Concentrations, and Deposition
    Emissions of SOX, oxides of N, and PM have declined 
dramatically over the past two decades, continuing a longer-term trend 
(PA, section 2.2). NEI data indicate an 87% decrease in total 
SO2 emissions between 2002 and 2022, including reductions of 
91% in emissions from electricity generating units and 96% in emissions 
from mobile

[[Page 26633]]

sources. Total anthropogenic NOX emissions have also trended 
downward across the U.S. between 2002 and 2022 at only slightly smaller 
percentages than SO2. Nationwide estimates indicate a 70% 
decrease in anthropogenic NOX emissions over this time 
period, driven in part by large emission reductions in the highway 
vehicle sector (84%) and from stationary fuel combustion (68%) (PA, 
section 2.2.1). In contrast with these declining 20-year trends in 
NOX and SOX emissions, the annual rate of 
NH3 emissions has increased by over 20 percent since 2002 
(PA, section 2.2.3). The two largest contributors are emissions from 
livestock waste and fertilizer application, which have increased by 11% 
and 44%, respectively, from 2002 to 2022. These trends in emissions 
have had ramifications for N deposition patterns across the U.S., as 
described further below.
    As expected, the large reductions in SOX and 
NOX emissions have resulted in substantially lower ambient 
air concentrations in recent years relative to what was observed in 
previous periods. The State and Local Air Monitoring Stations (SLAMS) 
network supports the implementation of the NAAQS. In 2021, all ambient 
monitoring sites with valid SO2 design values (n=333) \22\ 
are less than the level of the existing secondary standard (500 ppb) 
\23\ and more than 75 percent of the sites have design values less than 
20 ppb (PA, section 2.4.2). These values reflect a downward trend over 
the past two decades with median 3-hour secondary SO2 values 
down substantially from 2000 levels (from ~50 ppb to ~10 ppb).
---------------------------------------------------------------------------

    \22\ A design value is a statistic that summarizes the air 
quality data for a given area in terms of the indicator, averaging 
time, and form of the standard. Design values can be compared to the 
level of the standard and are typically used to designate areas as 
meeting or not meeting the standard and assess progress towards 
meeting the NAAQS. Design values are computed and published annually 
by EPA (https://www.epa.gov/air-trends/air-quality-designvalues).
    \23\ The existing secondary standard for SO2 is 0.5 
ppm (500 ppb), as a 3-hour average, not to be exceeded more than 
once per year.
---------------------------------------------------------------------------

    Similar trends are evident in the data for the primary 
SO2 standard (annual 99th percentile of 1-hour daily maximum 
concentrations, averaged over 3 years with a level of 75 ppb). In the 
2019-2021 period, the maximum design value for the primary 
SO2 standard was 376 ppb at a monitoring site near an 
industrial park in southeast Missouri. It is important to note that 
peak and mean SO2 concentrations are higher at source-
oriented sites than monitoring locations that are not source-oriented. 
Additionally, it is not uncommon for there to be high SO2 
values in areas with recurring volcanic eruptions (e.g., Hawaii). In 
the mid-1990s, the median value of all sites with valid 1-hour 
SO2 design values often exceeded 75 ppb (PA, Figure 2-26). 
Since then, the entire distribution of values (including source-
oriented sites) has continued to decline such that the median value 
across the network of sites is now between 5 and 10 ppb (PA, Figure 2-
26). The EPA also evaluated trends in annual average SO2 
data from 2000-2021 and observed improving trends of similar magnitude 
with the longer-term (annual) averaging time. It is important to note 
that both peak and mean SO2 concentrations are higher at 
source-oriented sites than monitoring locations that are not source-
oriented.
    Regarding NO2, design values at all 399 sites with valid 
secondary NO2 design values (annual average concentrations) 
in 2021 are less than the 53 ppb level of the existing secondary 
standard,\24\ and the majority of sites (98 percent) have design values 
that are less than 20 ppb. In 2021, the maximum was 30 ppb,\25\ and the 
median was 7 ppb. As with SO2, the more recent 
NO2 design values also reflect a downward trend over the 
past two decades. Median annual NO2 design values across the 
U.S. decreased by ~50% between 2000 and 2021 (15 ppb to 7 ppb).
---------------------------------------------------------------------------

    \24\ Sites in the contiguous U.S. have met the existing 
NO2 secondary standard since around 1991 (PA, Figure 2-
22).
    \25\ The maximum annual average NO2 concentrations 
has been at, slightly above or slightly below 30 ppb since about 
2008, with the highest 3-year average value just above 30 ppb (PA, 
Figures 2-22 and 7-9).
---------------------------------------------------------------------------

    Likewise, the median of the annual average PM2.5 
concentrations decreased substantially from 2000 to 2021 (from 12.8 
[micro]g/m\3\ to 8 [micro]g/m\3\). The median of the annual 98th 
percentile 24-hour PM2.5 concentrations at the more than 
1000 sites monitored also decreased, from 32 [micro]g/m\3\ in 2000 to 
21 [micro]g/m\3\ in 2021. Although both the annual average and 98th 
percentile 24-hour PM2.5 concentrations decreased steadily 
from the early 2000s until 2016, these values have fluctuated in recent 
years due to large-scale wildfire events (PA, section 2.4.3; U.S. EPA, 
2023, Figures 23 and 24).
    These emission reductions and subsequent downward trends in air 
concentrations have also contributed to a nationwide decrease in N and 
S deposition (PA, sections 2.5.3 and 6.2.1). Total S deposition and N 
deposition declined by 68% and 15%, respectively, calculated as a 
nationwide, three-year average between 2000-2002 and 2019-2021 (PA, 
section 6.2.1). The trend in S deposition is more robust than for N 
because of the offsetting influence of increasing emissions of reduced 
forms of nitrogen over the same timeframe. The largest reductions in 
total S and N deposition are seen in regions downwind of point sources 
and transportation corridors related to emission reductions from 
electricity generating units and mobile sources.
3. Relationships Between Concentrations and Deposition
    As the NAAQS are set in terms of pollutant concentrations, analyses 
in the PA evaluated relationships between criteria pollutant 
concentrations in ambient air and ecosystem deposition across the U.S. 
We examined these relationships over a range of conditions (e.g., 
pollutant, region, time period), and considered deposition both near 
sources and at distance (allowing for pollutant transport and 
associated transformation). The findings of these analyses, described 
in detail in Chapter 6 and Appendix 6A of the PA, have informed 
consideration of indicators and levels for potential secondary 
standards based on consideration of deposition-related effects (PA, 
Chapter 7).
    As is evident from the air quality-deposition analyses, relating 
ecosystem deposition to ambient air concentrations is not 
straightforward. Deposition rates vary across ecosystems nationally, 
and there is not a simple one-to-one relationship between ambient air 
concentrations of any one indicator and S or N deposition. As discussed 
above, the atmospheric processes that lead from pollutant emissions 
loading to eventual deposition to the earth's surface are complex. 
Multiple chemicals, both gaseous and particulate, from multiple types 
of sources contribute to S and N deposition. Further, both criteria 
pollutants and non-criteria pollutants contribute to N deposition. 
There are also multiple deposition pathways (i.e., dry deposition and 
wet deposition) that can influence the spatial and temporal scales at 
which deposition occurs, which vary by pollutant and pollutant phase.
    In light of these challenges, the PA employed five different 
approaches for considering relationships between S and N deposition 
rates and ambient air concentrations. First, as part of a ``real-world 
experiment,'' the PA analyses leveraged the recent downward trends in 
NOX and SOX emissions and corresponding air 
quality concentrations as well as the trends in deposition estimates 
(TDep or total deposition) to examine the correlation between

[[Page 26634]]

observed decreases in emissions and concentration and observed changes 
in deposition over the past two decades (PA, section 6.2.1). The TDep 
estimates used in these analyses are based on a hybrid approach that 
involves a fusion of measured and modeled values, where measured values 
are given more weight at the monitoring locations and modeled data are 
used to fill in spatial gaps and provide information on chemical 
species that are not measured by routine monitoring networks (Schwede 
and Lear, 2014).\26\ For the second approach, we assessed how air 
quality concentrations and associated deposition levels are related 
within a chemical-transport model (CMAQ \27\) both nationally and then 
at certain Class I areas \28\ (PA, section 6.2.2.1) where additional 
monitoring data are collected as part of the Clean Air Status and 
Trends Network (CASTNET) and the Interagency Monitoring of Protected 
Visual Environments (IMPROVE) networks. As a third approach, we 
analyzed the relationships across a limited number of monitoring 
locations (in Class I areas) where both air quality data (CASTNET and 
IMPROVE) and wet deposition of S and N was measured to evaluate the 
associations between concentrations and deposition at a local scale 
(PA, section 6.2.2.2 and 6.2.2.3). The fourth approach also considered 
the local associations between the two terms at the local scale but did 
so using a broader set of ambient air concentration measurements (i.e., 
all valid SO2, NO2, and PM2.5 
measurements at SLAMS across the U.S.) and a hybrid set of deposition 
estimates (TDep) (PA, section 6.2.3).
---------------------------------------------------------------------------

    \26\ Other than the estimates associated with the CMAQ analysis 
(second approach referenced above), the deposition estimates used in 
these analyses are those provided by the National Atmospheric 
Deposition Program, TDep Science Committee. One of the outputs of 
this effort are annual datasets of total deposition estimates in the 
U.S., which are referred to as the TDep datasets (technical updates 
available from NADP, 2021; ISA, appendix 2, section 2.6).
    \27\ The CMAQ is a state of the science photochemical air 
quality model that relies on scientific first principles to simulate 
the concentration of airborne gases and particles and the deposition 
of these pollutants back to Earth's surface under user-prescribed 
scenarios. See https://www.epa.gov/cmaq for more detail.
    \28\ Areas designated as Class I include all international 
parks, national wilderness areas which exceed 5,000 acres in size, 
national memorial parks which exceed 5,000 acres in size, and 
national parks which exceed 6,000 acres in size, provided the park 
or wilderness area was in existence on August 7, 1977. Other areas 
may also be Class I if designated as Class I consistent with the 
CAA.
---------------------------------------------------------------------------

    Finally, in recognition of the fact that air quality at upwind 
locations can also influence downwind deposition, the fifth approach 
used a trajectory model (HYSPLIT--The Hybrid Single-Particle Lagrangian 
Integrated Trajectory model) to identify upwind areas where emissions 
might be expected to influence deposition at downwind ecoregions (PA, 
section 6.2.4 and Appendix 6A).\29\ Once those potential zones of 
influence were established, we evaluated the relationships between air 
quality metrics for the three pollutants \30\ at sites within those 
zones with deposition estimates in the downwind ecoregion, as 3-year 
averages for five periods: 2001-2003, 2006-2008, 2010-2012, 2014-2016 
and 2018-2020. The metrics, Ecoregion Air Quality Metrics (EAQMs), 
include a weighted-average (EAQM-weighted) and a maximum metric (EAQM-
max). The EAQM-max is the maximum metric value among the sites linked 
to the downwind ecoregion and, for the EAQM-weighted, the value of each 
site linked to the downwind ecoregion was weighted by how often the 
forward HYSPLIT trajectory crossed into the ecoregion, i.e., sites with 
more frequent trajectory intersections with the ecoregion are weighted 
higher (PA, section 6.2.4.1).
---------------------------------------------------------------------------

    \29\ Upwind sites of influence were identified for all 84 
ecoregions (level III categorization) in the contiguous U.S.
    \30\ For SO2, there were two sets of metrics: one 
based on an annual average and one based on the 2nd highest 3-hour 
maximum concentration in the year. Both the NO2 and 
PM2.5 metrics are annual averages. For relating to 3-year 
average deposition, all are averaged across three years.
---------------------------------------------------------------------------

    As with any assessment, there are uncertainties and limitations 
associated with the analyses summarized above. These are more fully 
discussed in the PA (PA, sections 6.3 and 6.4). The evaluation of 
measured air quality concentrations (SO2, NO2, 
and PM2.5) and TDep estimates of deposition at all SLAMS 
(generally composed of sites that use either a Federal Reference Method 
[FRM] or a Federal Equivalence Method [FEM]) is a robust analysis 
(i.e., large number of monitors distributed across the U.S.) and 
particularly relevant given that compliance with the current standards 
(both primary and secondary) is judged using design value metrics based 
on measurements at the current SO2, NO2 and 
PM2.5 monitors. However, these site-based comparisons do not 
account for deposition associated with the transport of pollutants 
emitted some distance upwind. Each of the other analyses completed to 
bolster this analysis have their own limitations ranging from model 
uncertainty to limited geographical scope.
    The full set of quantitative results of the characterization of air 
quality and deposition relationships are discussed more thoroughly in 
Chapter 6 and Appendix 6A of the PA. In combination, these analyses 
supported the PA conclusion of a strong association between 
SO2 and S deposition. Regarding N oxides and PM, however, 
the results, and associated information, indicated more variable 
relationships between NO2 concentrations and N deposition, 
and PM2.5 concentrations with either S or N deposition.
    For SO2, annual monitored SO2 concentrations, 
at existing monitors within the SLAMS network, averaged over 3 years at 
the national scale were highly correlated to S deposition estimates in 
the TDep dataset at the local scale (correlation coefficient of 
0.70),\31\ especially in the earlier periods of the record and across 
the eastern U.S. (PA, section 6.2.3). This association was confirmed by 
the relationships between SO2 annual values at the 
identified upwind sites of influence and S deposition estimates from 
TDep in downwind ecoregions, especially in those locations where the 
annual average SO2 concentrations are greater than 5 ppb 
(PA, section 6.2.4.2). Finally, we note that the observed declines in 
national levels of S deposition over the past two decades have occurred 
during a period in which emissions of SO2 have also declined 
sharply (PA, sections 6.2.1 and 6.4.1).
---------------------------------------------------------------------------

    \31\ The correlation coefficients are based on Spearman's rank 
correlation coefficient. These coefficients are generally used to 
assess how well the relationship between two variables can be 
described via a monotonic function. The term ``r value'' is 
sometimes used as shorthand for this correlation coefficient. Higher 
values indicate that the two variables are highly associated with 
one another (can range from 1.0 to -1.0).
---------------------------------------------------------------------------

    Analyses in the PA also investigated relationships between S 
deposition and air quality metrics other than the current indicator 
species (SO2) in a limited number of circumstances. For 
example, an evaluation of the association between 
SO42- and total S deposition across 27 Class I 
areas where data for both parameters were available, concluded that the 
correlations between particle sulfate and total sulfate (i.e., 
SO2 + SO42-) with total S deposition 
(correlation coefficients of 0.55 and 0.61, respectively) was lower 
than what was exhibited for SO2 and S deposition at the 
SLAMS (PA, section 6.2.2). The analyses also concluded that there was 
poor correlation (correlation coefficient of 0.33) between 
PM2.5 mass, as measured at IMPROVE sites, with total S 
deposition estimates for those sites (PA, sections 2.3.3 and 6.2.2.3). 
While these analyses are based on data at a relatively limited number 
of sites, as compared to the SLAMS network, the

[[Page 26635]]

results suggest that there are no clear advantages to considering 
PM2.5 mass, particle sulfate, or total sulfate as an 
indicator for a secondary NAAQS, over using SO2.
    Both NO2 and certain components of PM2.5 can 
contribute to N deposition. As is the case for SO2 and S 
deposition, there are multiple pathways for N deposition (dry and wet), 
and multiple scales of N deposition (local and regional). However, 
there are some additional complications in the consideration of how air 
quality concentrations (i.e., NO2 and PM2.5 mass) 
are associated with eventual N deposition. First, not all N deposition 
is caused by the criteria pollutants (PA, Chapter 2 and section 6.1.1). 
Ammonia emissions also lead to N deposition, especially through dry 
deposition at local scales. Second, only certain components of 
PM2.5 mass contribute to N deposition (i.e., 
NO3- and NH4\+\). As a result of these 
two factors, the association between NO2 concentrations and 
N deposition, and PM2.5 concentrations and N deposition is 
less robust than what is observed for SO2. Our multi-faceted 
approach to evaluating these relationships confirmed this expectation. 
For example, when comparing NO2 observations at SLAMS across 
the U.S. against the N deposition estimates from TDep, there are weaker 
associations than what is observed in the similar SO2 
comparisons (PA, section 6.4.2). There is little correlation for N 
deposition with concentrations of NO2, as evidenced by a 
Spearman's correlation coefficient of 0.38, compared to 0.70 for 
SO2 and S deposition (PA, Table 6-6 and Table 6-4). Further, 
the trajectory-based analyses of the relationships between 
NO2 annual values in the identified upwind zones of 
influence and N deposition estimates from TDep in downwind ecoregions 
indicate negative correlations (PA, Table 6-10). These negative 
correlations are observed for both the EAQM-weighed and EAQM-max 
values. This relative lack of association was confirmed by considering 
national trends over the past 20 years, where sharp declines in 
NO2 emissions and concentrations are linked in time with 
sharp declines in oxidized N deposition (PA, Table 6-2), but not 
associated with recent trends in total or reduced atmospheric N 
deposition. Since 2010, NO2 concentrations have continued to 
drop while N deposition has remained steady (PA, section 6.2.1). As 
noted for S deposition and S compound metrics above, the PA also 
investigated relationships between N deposition and air quality metrics 
other than the current indicator species (NO2). Across the 
27 Class I areas where collocated data were available, the PA evaluated 
the relationships between several air quality parameters (e.g., nitric 
acid, particulate NO3-, and NH4\+\) 
and, as for S deposition and S compound metrics, the PA concluded there 
were no clear advantages over the consideration of NO2 or 
PM2.5 mass. In sum, the evidence suggests that 
NO2 would be a weak indicator of total atmospheric N 
deposition, especially in areas where ammonia is prevalent or where 
PM2.5 mass is dominated by species other than 
NO3- or NH4\+\ (PA, section 6.4.2).

C. Welfare Effects Evidence

    The information summarized here is based on our scientific 
assessment of the welfare effects evidence available in this review; 
this assessment is documented in the ISA \32\ and its policy 
implications are further discussed in the PA (and summarized in section 
II.E.1 below). More than 3,000 studies are newly available since the 
last review and considered in the ISA.\33\ While expanding the evidence 
for some effect categories, studies on acid deposition, a key group of 
effects from the last review, are largely consistent with the evidence 
that was previously available. The subsections below briefly summarize 
the following aspects of the evidence: the nature of welfare effects of 
S oxides, N oxides and PM (section II.C.1); the potential public 
welfare implications (section II.C.2); and exposure concentrations and 
deposition-related metrics (section II.C.3).
---------------------------------------------------------------------------

    \32\ The ISA builds on evidence and conclusions from previous 
assessments, focusing on synthesizing and integrating the newly 
available evidence (ISA, section IS.1.1). Past assessments are cited 
when providing further details not repeated in newer assessments.
    \33\ The study count and citations are available on the project 
page for the ISA on the Health & Environmental Research Online 
(HERO) website documents these studies (https://heronet.epa.gov/heronet/index.cfm/project/page/project_id/2965).
---------------------------------------------------------------------------

1. Nature of Effects
    This welfare effects evidence base available in the current review 
includes decades of extensive research on the ecological effects oxides 
of nitrogen, oxides of sulfur and PM. In the sections below we 
summarize the nature of the direct effects of gas-phase exposure to 
oxides of nitrogen and sulfur (section II.C.1.a), acid deposition-
related ecological effects (section II.C.1.b), N enrichment and 
associated effects (section II.C.1.c), and other effects (section 
II.C.1.d).
a. Direct Effects of SOX and N Oxides
    There is a well-established body of scientific evidence that has 
shown that acute and chronic exposures to oxides of N and S, such as 
SO2, NO2, NO, HNO3 and peroxyacetyl 
nitrate (PAN) in the air, are associated with negative effects on 
vegetation. Such scientific evidence, as was available in 1971, was the 
basis for the current secondary NAAQS for oxides of sulfur and oxides 
of nitrogen. The current scientific evidence continues to demonstrate 
such effects, with the ISA specifically concluding that the evidence is 
sufficient to infer a causal relationship between gas-phase 
SO2 and injury to vegetation (ISA, Appendix 3, section 
3.6.1), and between gas-phase NO, NO2 and PAN and injury to 
vegetation (ISA, Appendix 3, section 3.6.2). The ISA additionally 
concluded the evidence to be sufficient to infer a causal relationship 
between exposure to HNO3 and changes to vegetation, noting 
that experimental exposure can damage leaf cuticle of tree seedlings 
and HNO3 concentrations have been reported to have 
contributed to declines in lichen species in the Los Angeles basin 
(ISA, Appendix 3, section 3.6.3).
    Specifically for SOX, high concentrations in the first 
half of the twentieth century have been blamed for severe damage to 
plant foliage that occurred near large ore smelters during that time 
(ISA, Appendix 3, section 3.2). In addition to foliar injury, which is 
usually a rapid response, SO2 exposures have also been 
documented to reduce plant photosynthesis and growth. The appearance of 
foliar injury can vary significantly among species and growth 
conditions (which affect stomatal conductance). For lichens, damage 
from SO2 exposure has been observed to include reduction in 
metabolic functions that are vital for growth and survival (e.g., 
decreases in photosynthesis and respiration), damage to cellular 
integrity (e.g., leakage of electrolytes), and structural changes (ISA, 
Appendix 3, section 3.2; Belnap et al., 1993; Farmer et al., 1992, 
Hutchinson et al., 1996).
    Although there is evidence of plant injury associated with 
SO2 exposures dating back more than a century (ISA, Appendix 
3, section 3.2), as exposures have declined in the U.S., some studies 
in the eastern U.S. have reported increased growth in some 
SO2-sensitive tree species (e.g., Thomas et al., 2013). 
Although the authors attributed the growth response to reductions in 
SO2-associated acid deposition, and related recovery from 
soil acidification, the relative roles of different pathways are

[[Page 26636]]

unclear as a historical deposition record was not available (ISA, 
Appendix 3, section 3.2). Other researchers have suggested that the 
observed response was related to the fact that the trees were growing 
on a limestone outcrop that could be well buffered from soil 
acidification (Schaberg et al., 2014). This seems to suggest a somewhat 
faster recovery than might be expected from deposition-related soil 
acidification, which may indicate a relatively greater role for changes 
in ambient air concentrations of SO2, in combination with 
changes in other gases, than was previously understood (ISA, Appendix 
3, section 3.2 and Appendix 5, section 5.2.1.3).
    The evidence base evaluated in the 1993 Air Quality Criteria 
Document for Oxides of N included evidence of phytotoxic effects of NO, 
NO2, and PAN on plants through decreasing photosynthesis and 
induction of visible foliar injury (U.S. EPA, 1993 [1993 AQCD]). The 
1993 AQCD additionally concluded that concentrations of NO, 
NO2, and PAN in the atmosphere were rarely high enough to 
have phytotoxic effects on vegetation. Little new information is 
available since that time on these phytotoxic effects at concentrations 
currently observed in the U.S. (ISA, Appendix 3, section 3.3).
    The evidence indicates that HNO3 had a role in observed 
declines in lichen species in the 1970s in the Los Angeles basin (ISA, 
Appendix 3, section 3.3). A 2008 resampling of areas shown to be 
impacted in the past by HNO3 found community shifts, 
declines in the most pollutant-sensitive lichen species, and increases 
in abundance of nitrogen-tolerant lichen species compared to 1976-1977, 
indicating that these lichen communities have not recovered and had 
experienced additional changes (ISA, Appendix 3, section 3.4). The 
recently available evidence on this topic also included a study of six 
lichen species that reported changes in physiology and functioning 
including decreased chlorophyll content and chlorophyll fluorescence, 
decreased photosynthesis and respiration, and increased electrolyte 
leakage from HNO3 exposures for 2-11 weeks (daily peak 
levels near 50 ppb) in controlled chambers. (ISA, Appendix 3, section 
3.4).
b. Acid Deposition-Related Ecological Effects
    The connection between SOX and N oxide emissions to 
ambient air, atmospheric deposition of N and/or S, and the 
acidification of acid-sensitive soils and surface waters is well 
documented with many decades of evidence, particularly in the eastern 
U.S. (ISA, section IS.5; Appendix 8, section 8.1). In the atmosphere, 
SOX and N oxides undergo reactions to form various acidic 
compounds that are removed from the atmosphere through deposition. 
Acidifying deposition can affect biogeochemical processes in soils, 
with ramifications for terrestrial biota and for the chemistry and 
biological functioning of associated surface waters (ISA, Appendix 7, 
section 7.1). These effects depend on the magnitude and rate of 
deposition, as well as multiple biogeochemical processes that occur in 
soils and waterbodies.
    Soil acidification is influenced by the deposition of inorganic 
acids (HNO3 and H2SO4), and by 
chemical and biological processes. When NO3-, or 
SO42- leach from soils to surface waters, an 
equivalent number of positive cations, or countercharge, are also 
transported. If the countercharge is provided by a base cation (e.g., 
calcium, [Ca\2+\], magnesium [Mg\2+\], sodium [Na\+\], or potassium 
[K\+\]), rather than hydrogen ions (H\+\), the leachate is neutralized, 
but the soil becomes more acidic from the hydrogen ions left behind and 
the base saturation of the soil is reduced by the loss of the base 
cation. Depending on the relative rates of soil processes that 
contribute to the soil pools of H\+\ and base cations, such as 
weathering, continued SO42- or 
NO3- leaching can deplete the soil base cation 
pool, which contributes to increased acidity of the leaching soil 
water, and by connection, the surface water. Accordingly, the ability 
of a watershed to neutralize acidic deposition is determined by a 
variety of biogeophysical factors including weathering rates, bedrock 
composition, vegetation and microbial processes, physical and chemical 
characteristics of soils, and hydrology (ISA Appendix 4, section 4.3).
(1) Freshwater Ecosystems
    As was the case in the last review, the body of evidence available 
in this review, including that newly available, is sufficient to infer 
a causal relationship between N and S deposition and the alteration of 
freshwater biogeochemistry (ISA, section IS.6.1). Additionally, based 
on the previously available evidence, the current body of evidence is 
also sufficient to conclude that a causal relationship exists between 
acidifying deposition and changes in biota, including physiological 
impairment and alteration of species richness, community composition, 
and biodiversity in freshwater ecosystems (ISA, section IS.6.3).
    The effects of acid deposition on aquatic systems depend largely 
upon the ability of the system to neutralize additional acidic inputs 
from the environment, whether from the atmosphere or from surface 
inputs. There is a large amount of variability between freshwater 
systems in this regard, which reflects their underlying geology as well 
as their history of acidic inputs. Accordingly, different freshwater 
systems (e.g., in different geographic regions) respond differently to 
similar amounts of acid deposition. The main factor in determining 
sensitivity is the underlying geology of an area and its ability to 
provide soil base cations through weathering to buffer acidic inputs 
(ISA, Appendix 8, section 8.5.1). As noted in the ISA, ``[g]eologic 
formations having low base cation supply, due mainly to low soil and 
bedrock weathering, generally underlie the watersheds of acid-sensitive 
lakes and streams'' (ISA, Appendix 8, p. 8-58).
    Longstanding evidence has well characterized the changes in 
biogeochemical processes and water chemistry caused by N and S 
deposition to surface waters and their watersheds and the ramifications 
for biological functioning of freshwater ecosystems (ISA, Appendix 8, 
section 8.1). The 2020 ISA found that the newly available scientific 
research ``reflects incremental improvements in scientific knowledge of 
aquatic biological effects and indicators of acidification as compared 
with knowledge summarized in the 2008 ISA'' (ISA, Appendix 8, p. 8-80). 
Previously and newly available studies ``indicate that aquatic 
organisms in sensitive ecosystems have been affected by acidification 
at virtually all trophic levels and that these responses have been well 
characterized for several decades'' (ISA, Appendix 8, p. 8-80). For 
example, information reported in the previous 2008 ISA ``showed 
consistent and coherent evidence for effects on aquatic biota, 
especially algae, benthic invertebrates, and fish that are most clearly 
linked to chemical indicators of acidification'' (ISA, Appendix 8, p. 
8-80). These indicators are surface water pH, base cation ratios, ANC, 
and inorganic aluminum (Al) concentration (ISA, Appendix 8, Table 8-9).
    The effects of waterbody acidification on fish species are 
especially well understood in the scientific literature, and many 
species (e.g., brown and brook trout and Atlantic salmon) have been 
documented to have experienced adverse effects from acidification (ISA, 
Appendix 8, section 8.3). Among these species, the earliest lifestages 
are most

[[Page 26637]]

sensitive to acidic conditions. Many effects of acidic surface waters 
on fish, particularly effects on gill function or structure, relate to 
the combination of low pH and elevated dissolved Al (ISA, Appendix 8, 
section 8.3.6.1). In general, biological effects in aquatic ecosystems 
are primarily attributable to low pH and high inorganic aluminum 
concentration (ISA, p. ES-14). Waterbody pH largely controls the 
bioavailability of Al, which is toxic to fish, and aluminum 
mobilization is largely confined to waters with a pH below about 5.5, 
which the ISA describes as corresponding to an ANC in the range of 
about 10 to 30 [micro]eq/L in low to moderate DOC waters of the 
Northeast (ISA, Appendix 7, section 7.1.2.6 and Appendix 8, section 
8.6.4).
    The parameter ANC is an indicator of the buffering capacity of 
natural waters against acidification. Although ANC does not directly 
affect biota, it is an indicator of acidification that relates to pH 
and aluminum levels (ISA, p. ES-14), or watershed characteristics like 
base cation weathering (BCw) rate (ISA, Appendix 8, sections 8.1 and 
8.3.6.3). Accordingly, ANC is commonly used to describe the potential 
sensitivity of a freshwater system to acidification-related effects. It 
can be measured in water samples and is also often estimated for use in 
water quality modeling, as is done in the aquatic acidification risk 
assessment for this review, as summarized in section II.D below. Water 
quality models are generally better at estimating ANC than at 
estimating other indicators of acidification-related risk, such as pH. 
Acid neutralizing capacity is estimated as the molar sum of strong base 
cations minus the molar sum of strong acid anions, specifically 
including SO42- and NO3- 
(e.g., Driscoll et al., 1994). Thus, values below zero indicate a 
deficit in the ability to buffer acidic inputs, and increasing values 
above zero represent increasing buffering capability for acidic inputs. 
Further, across waterbodies within impacted areas of Shenandoah 
National Park streams and Adirondack Mountain lakes, a positive 
relationship has often been observed between ANC and number of fish 
species, at least for the ANC range from about zero to 50 [micro]eq/L 
(ISA, Appendix 7, section 7.1.2.6; Cosby et al., 2006; Sullivan et al., 
2006, Bulger et al., 1999).
    Values of ANC can also be influenced by high concentrations of 
naturally occurring organic acids, which can reduce bioavailability of 
Al, buffering effects usually associated with low pH and high total Al 
concentrations (Waller et al., 2012; ISA, Appendix 8, section 8.3.6.4); 
in waters where that occurs, ANC may not be a good indicator of risk to 
biota.
    In addition to acidity of surface waters quantified over weeks or 
months, waterbodies can also experience spikes in acidity in response 
to episodic precipitation or rapid snowmelt events. In these events 
(hours-days), a surge or pulse of drainage water, containing acidic 
compounds, is routed through upper soil horizons rather than the deeper 
soil horizons that would usually provide buffering for acidic compounds 
(ISA, Appendix 7, section 7.1). While some streams and lakes may have 
chronic or base flow chemistry that provides suitable conditions for 
aquatic biota, they may experience occasional acidic episodes with the 
potential for deleterious consequences to sensitive biota (ISA, 
Appendix 8, section 8.5). For example, in some impacted northeastern 
waterbodies, ANC levels may dip below zero for hours to days or weeks 
in response to such events, while waterbodies labeled chronically 
acidic have ANC levels below zero throughout the year (ISA, Appendix 7, 
section 7.1.1.2; Driscoll et al., 2001). Accordingly, headwater streams 
in both the eastern and western U.S. tend to be more sensitive to such 
episodes due to their smaller watersheds and, in the east, their 
underlying geology (ISA, Appendix 8, section 8.5.1).
    National survey data dating back to the early 1980s through 2004, 
that were available for the 2008 ISA, indicated acidifying deposition 
had acidified surface waters in the southwestern Adirondacks, New 
England uplands, eastern portion of the upper Midwest, forested Mid-
Atlantic highlands, and Mid-Atlantic coastal plain (2008 ISA, section 
4.2.2.3; ISA, Appendix 8, section 8.5.1). For example, a survey of 
waterbodies in the Adirondacks in 1984-1987 found 27% of streams to 
have ANC values below zero, with a minimum value of -134 
microequivalents per liter ([mu]eq/L) (Sullivan et al., 2006). Values 
of ANC below 20 [mu]eq/L in Shenandoah stream sites were associated 
with fewer fish of sensitive species compared to sites with higher ANC 
(Bulger et al., 1999). A more recent study of two groups of Adirondack 
lakes for which water quality data were available from 1982 and 1992, 
respectively, reported significant increases in ANC in the large 
majority of those lakes, with the magnitude of the increases varying 
across the lakes (Driscoll et al., 2016; ISA, Appendix 7, section 
7.1.3.1). As described in the ISA, ``[a]cidic waters were mostly 
restricted to northern New York, New England, the Appalachian Mountain 
chain, upper Midwest, and Florida'' (ISA, Appendix 8, p. 8-60). Despite 
the appreciable reductions in acidifying deposition that have occurred 
in the U.S. since the 1960s and 1970s, aquatic ecosystems across the 
U.S. are still experiencing effects from historical contributions of N 
and S (ISA, Appendix 8, section 8.6).
(2) Terrestrial Ecosystems
    There is longstanding evidence that changes in soil biogeochemical 
processes caused by acidifying deposition of N and S to terrestrial 
systems are linked to changes in terrestrial biota, with associated 
impacts on ecosystem characteristics. The currently available evidence, 
including that newly available in this review, supports and strengthens 
this understanding (ISA, Appendix 5, section 5.1). Consistent with 
conclusions in the last review, the current body of evidence is 
sufficient to infer a causal relationship between acidifying deposition 
and alterations of biogeochemistry in terrestrial ecosystems. 
Additionally, and consistent with conclusions in the last review, the 
current body of evidence is sufficient to infer a causal relationship 
between acidifying N and S deposition and the alteration of the 
physiology and growth of terrestrial organisms and the productivity of 
terrestrial ecosystems. The current body of evidence is also sufficient 
to conclude that a causal relationship exists between acidifying N and 
S deposition and alterations of species richness, community 
composition, and biodiversity in terrestrial ecosystems (2008 ISA, 
sections 4.2.1.1 and 4.2.1.2; 2020 ISA, Appendix 4, section 4.1 and 
Appendix 5, sections 5.7.1 and 5.7.2).
    Deposition of acidifying compounds to acid-sensitive soils can 
cause soil acidification, increased mobilization of Al from soil to 
drainage water, and deplete the pool of exchangeable base cations in 
the soil (ISA, Appendix 5, section 5.2 and Appendix 4, sections 4.3.4 
and 4.3.5). The physiological effects of acidification on terrestrial 
biota include slower growth and increased mortality among sensitive 
plant species, which are generally attributable to physiological 
impairment caused by Al toxicity (related to increased availability of 
inorganic Al in soil water) and a reduced ability of plant roots to 
take up base cations (ISA, Appendix 4, section 4.3 and Appendix 5, 
section 5.2). The U.S. tree species most studied with regard to effects 
of acid deposition are red spruce and sugar maple, although there is 
also evidence for other tree species such as flowering

[[Page 26638]]

dogwood (ISA, Appendix 5, section 5.2.1).
    The physiological effects of acidifying deposition on terrestrial 
biota can also result in changes in species composition whereby 
sensitive species are replaced by more tolerant species, or the 
sensitive species that were dominant in the community become a 
minority. For example, increasing soil cation availability (as in 
Ca\2+\ addition or gradient experiments) was associated with greater 
growth and seedling colonization for sugar maple while American beech 
was more prevalent on soils with lower levels of base cations where 
sugar maple is less often found (ISA, Appendix 5, section 5.2.1.3.1; 
Duchesne and Ouimet, 2009). In a study of understory species 
composition, soil acid-base chemistry was found to be a predictor of 
understory species composition (ISA, Appendix 5, section 5.2.2.1). 
Additionally, limited evidence, including a recent S addition study and 
agricultural soil gradient study, indicated that soil acid-base 
chemistry predicted and was correlated with diversity and composition 
of soil bacteria, fungi, and nematodes (ISA, Appendix 5, section 
5.2.4.1).
    In addition to Ca\2+\ addition experiments, the recently available 
evidence also includes addition or gradient studies evaluating 
relationships between soil chemistry indicators of acidification (e.g., 
soil pH, base cation to aluminum (Bc:Al) ratio, base saturation, and 
Al) and ecosystem biological endpoints, including physiological and 
community responses of trees and other vegetation, lichens, soil biota, 
and fauna (ISA, Appendix 5, Tables 5-2 and 5-6). Further, the 2020 ISA 
reports on several large observational studies evaluating statistical 
associations between tree growth or survival, as assessed at monitoring 
sites across the U.S. and estimates of average deposition of S or N 
compounds at those sites over time periods on the order of 10 years 
(ISA, Appendix 5, section 5.5.2 and Appendix 6, section.6.2.3.1; Dietze 
and Moorcroft, 2011; Thomas et al., 2010; Horn et al., 2018). Negative 
associations were observed for survival and growth in several species 
or species groups with S deposition metrics; positive and negative 
associations were reported with N deposition (PA, sections 5.3.2.3 and 
5.3.4 and Appendix 5B).
    Although there has been no systematic national survey of U.S. 
terrestrial ecosystem soils, the forest ecosystems considered the most 
sensitive to terrestrial acidification from atmospheric deposition 
include forests of the Adirondack Mountains of New York, Green 
Mountains of Vermont, White Mountains of New Hampshire, the Allegheny 
Plateau of Pennsylvania, and mountain top and ridge forest ecosystems 
in the southern Appalachians (2008 ISA, Appendix 3, section 3.2.4.2; 
ISA, Appendix 5, section 5.3). Underlying geology is the principal 
factor governing the sensitivity of both terrestrial and aquatic 
ecosystems to acidification from S and N deposition. Geologic 
formations with low base cation supply (e.g., sandstone, quartzite), 
due mainly to low weathering rates, generally underlie these acid 
sensitive watersheds. Other factors also contribute to the overall 
sensitivity of an area to acidifying nitrogen and sulfur deposition, 
including topography, soil chemistry, land use, and hydrology (ISA, 
Appendix 5, section 5.3). For example, ``[a]cid-sensitive ecosystems 
are mostly located in upland mountainous terrain in the eastern and 
western U.S. and are underlain by bedrock that is resistant to 
weathering, such as granite or quartzite sandstone'' (ISA, Appendix 7, 
p. 7-45). Further, as well documented in the evidence, biogeochemical 
sensitivity to deposition-driven acidification (and eutrophication [see 
section 4.3 below]) is the result of historical loading, geologic/soil 
conditions (e.g., mineral weathering and S adsorption), and 
nonanthropogenic sources of N and S loading to the system (ISA, 
Appendix 7, section 7.1.5).
    Recently available evidence includes some studies describing early 
stages of recovery from soil acidification in some eastern forests. For 
example, studies at the Hubbard Brook Experimental Forest in New 
Hampshire reported indications of acidification recovery in soil 
solution measurements across the period from 1984 to 2011 (ISA, 
Appendix 4, section 4.6.1; Fuss et al., 2015). Another study of 27 
sites in eastern Canada and the northeastern U.S. found reductions in 
wet deposition SO42- were associated with 
increases in soil base saturation and decreases in exchangeable Al 
(ISA, Appendix 4, section 4.6.1; Lawrence et al., 2015). Recent 
modeling analyses indicate extended timeframes for recovery are likely, 
as well as delays or lags related to accumulated pools of S in forest 
soils (ISA, Appendix 4, section 4.6.1).
c. Nitrogen Enrichment and Associated Ecological Effects
    The numerous ecosystem types that occur across the U.S. have a 
broad range of sensitivity to N enrichment. Organisms in their natural 
environments are commonly adapted to the nutrient availability in those 
environments. Historically, N has been the primary limiting nutrient 
for plants in many ecosystems. In such ecosystems, when the limiting 
nutrient, N, becomes more available, whether from atmospheric 
deposition, runoff, or episodic events, the subset of plant species 
able to most effectively use the higher nitrogen levels may out-compete 
other species, leading to a shift in the community composition that may 
be dominated by a smaller number of species, i.e., a community with 
lower diversity (ISA, sections IS.6.1.1.2, IS.6.2.1.1 and IS.7.1.1, 
Appendix 6, section 6.2.4 and Appendix 7, section 7.2.6.6). Thus, 
change in the availability of nitrogen in nitrogen-limited systems can 
affect growth and productivity, with ramifications on relative 
abundance of different species of vegetation, and potentially further 
and broader ramifications on ecosystem processes, structure, and 
function.
    Both N oxides and reduced forms of nitrogen (NHX) can 
contribute to N enrichment. In addition to atmospheric deposition, 
other sources of S and N can play relatively greater or lesser roles in 
contributing to S and N inputs, depending on location. For example, 
many waterbodies receive appreciable amounts of N from agricultural 
runoff and municipal or industrial wastewater discharges. For many 
terrestrial and freshwater ecosystems, sources of N other than 
atmospheric deposition, including fertilizer and waste treatment, 
contribute to ecosystem total N with contributions that can be larger 
than that from atmospheric deposition (ISA Appendix 7, sections 7.1 and 
7.2). Additionally, the impacts of historic deposition in both aquatic 
and terrestrial ecosystems pose complications to discerning the 
potential effects of more recent lower deposition rates.
(1) Aquatic and Wetland Ecosystems
    Nitrogen additions, including from atmospheric deposition, to 
freshwater, estuarine and near-coastal ecosystems can contribute to 
eutrophication, which typically begins with nutrient-stimulated rapid 
algal growth developing into an algal bloom that can, depending on 
various site-specific factors, be followed by anoxic conditions 
associated with the algal die-off (ISA, ES.5.2). Decomposition of the 
plant biomass from the subsequent algal die-off contributes to reduced 
waterbody oxygen, which in turn can affect higher-trophic-level 
species, e.g., contributing to fish mortality (ISA, p. ES-18).
    The extensive body of evidence in this area is sufficient to infer 
causal relationships between N deposition and

[[Page 26639]]

the alteration of biogeochemistry in freshwater, estuarine and near-
coastal marine systems (ISA, Appendix 7, sections 7.1 and 7.2). 
Further, consistent with findings in the last review, the current body 
of evidence is sufficient to infer a causal relationship between N 
deposition and changes in biota, including altered growth and 
productivity, species richness, community composition, and biodiversity 
due to N enrichment in freshwater ecosystems (ISA, Appendix 9, section 
9.1). The body of evidence is sufficient to infer a causal relationship 
between N deposition and changes in biota, including altered growth, 
total primary production, total algal community biomass, species 
richness, community composition, and biodiversity due to N enrichment 
in estuarine environments (ISA, Appendix 10, section 10.1).
    The impact of N additions on wetlands, and whether they may serve 
as a source, sink, or transformer of atmospherically deposited N, is 
extremely variable and depends on the type of wetland and other 
factors, such as physiography, and local hydrology, as well as climate 
(ISA, section IS.8.1 and Appendix 11, section 11.1). Studies generally 
show N enrichment to decrease the ability of wetlands to retain and 
store N, which may diminish the wetland ecosystem service of improving 
water quality (ISA, section IS.8.1). Consistent with the evidence 
available in the last review, the current body of evidence is 
sufficient to infer a causal relationship between N deposition and the 
alteration of biogeochemical cycling in wetlands. Newly available 
evidence regarding N inputs and plant physiology expands the evidence 
base related to species diversity. The currently available evidence, 
including that newly available, is sufficient to infer a causal 
relationship between N deposition and the alteration of growth and 
productivity, species physiology, species richness, community 
composition, and biodiversity in wetlands (ISA, Appendix 11, section 
11.10).
    The relative contribution of atmospheric deposition to total 
wetland N loading varies with wetland type, with bogs receiving the 
greatest contribution and accordingly being most vulnerable to nutrient 
enrichment effects of N deposition (ISA, Appendix 11, section 11.1). 
For example, bogs, which receive 70-100% of hydrological input from 
rainfall, are more sensitive to N deposition than fens (55-83% as 
rainfall), which are more sensitive than coastal wetlands (10-20% as 
rainfall) (ISA, Appendix 11, section 11.10). For freshwater fens, 
marshes, and swamps, inputs from ground and surface water are often of 
similar order of magnitude as that from precipitation, while estuarine 
and coastal wetlands receive water from multiple sources, with 
precipitation being among the smaller of those sources (ISA, Appendix 
11, section 11.1).
    Nitrogen loading and other factors contribute to nutrient 
enrichment, which contributes to eutrophication. Such nitrogen-driven 
eutrophication alters freshwater biogeochemistry and can impact 
physiology, survival, and biodiversity of sensitive aquatic biota. 
Evidence newly available in this review provides insights regarding N 
enrichment and its impacts in several types of aquatic systems, 
including freshwater streams and lakes, estuarine and near-coastal 
systems, and wetlands. With regard to freshwaters, for example, studies 
published since the 2008 ISA augment the evidence base for high-
elevation waterbodies where the main N source is atmospheric 
deposition. Recent evidence continues to indicate that N limitation is 
common in oligotrophic waters in the western U.S., with shifts in 
nutrient limitation, from N limitation, to between N and phosphorus (P) 
limitation, or to P limitation, reported in some alpine lake studies 
(ISA, Appendix 9, section 9.1.1.3). Small inputs of N in such water 
bodies have been reported to increase nutrient availability or alter 
the balance of N and P, with the potential to stimulate growth of 
primary producers and contribute to changes in species richness, 
community composition, and diversity.
    Another type of N loading effect in other types of freshwater lakes 
includes a role in the composition of freshwater algal blooms and their 
toxicity (ISA, Appendix 9, section 9.2.6.1). Information in this 
review, including studies in Lake Erie, indicates that growth of some 
harmful algal species, including those that produce microcystin, are 
favored by increased availability of N and its availability in 
dissolved inorganic form (ISA, Appendix 9, p. 9-28; Davis et al., 2015; 
Gobler et al., 2016).
    The relative contribution of N deposition to total N loading varies 
among waterbodies. For example, atmospheric deposition is generally 
considered to be the main source of N inputs to most headwater stream, 
high-elevation lake, and low-order stream watersheds that are far from 
the influence of other N sources like agricultural runoff and 
wastewater effluent (ISA, section ES5.2). In other fresh waterbodies, 
however, agricultural practices and point source discharges have been 
estimated to be larger contributors to total N loading (ISA, Appendix 
7, section 7.1.1.1). Since the 2008 ISA, several long-term monitoring 
studies in the Appalachian Mountains, the Adirondacks, and the Rocky 
Mountains have reported temporal patterns of declines in surface water 
NO3- concentration corresponding to declines in 
atmospheric N deposition (ISA, Appendix 9, section 9.1.1.2). Declines 
in basin wide NO3- concentrations have also been 
reported for the nontidal Potomac River watershed and have been 
attributed to declines in atmospheric N deposition (ISA, Appendix 7, 
section 7.1.5.1).
    Nutrient inputs to coastal and estuarine waters are important 
influences on the health of these waterbodies. Continued inputs of N, 
the most common limiting nutrient in estuarine and coastal systems, 
have resulted in N over-enrichment and subsequent alterations to the 
nutrient balance in these systems (ISA, Appendix 10, p. 10-6). For 
example, the rate of N delivery to coastal waters is strongly 
correlated to changes in primary production and phytoplankton biomass 
(ISA, Appendix 10, section 10.1.3). Algal blooms and associated die-
offs can contribute to hypoxic conditions (most common during summer 
months), which can contribute to fish kills and associated reductions 
in marine populations (ISA, Appendix 10). Further, the prevalence and 
health of submerged aquatic vegetation (SAV), which is important 
habitat for many aquatic species, has been identified as a biological 
indicator for N enrichment in estuarine waters (ISA, Appendix 10, 
section 10.2.5). Previously available evidence indicated the role of N 
loading in SAV declines in multiple U.S. estuaries through increased 
production of macroalgae or other algae, which reduce sunlight 
penetration into shallow waters where SAV is found (ISA, Appendix 10, 
section 10.2.3). Newly available studies have reported findings of 
increased SAV populations in two tributaries of the Chesapeake Bay 
corresponding to reduction in total N loading from all sources since 
1990 (ISA, Appendix 10, section 10.2.5). The newly available studies 
also identify other factors threatening SAV, including increasing 
temperature related to climate change (ISA, Appendix 10, section 
10.2.5).
    The degree to which N enrichment and associated ecosystem impacts 
are driven by atmospheric N deposition varies greatly and is largely 
unique to the specific ecosystem. Analyses based on data across two to 
three decades

[[Page 26640]]

extending from the 1990s through about 2010 estimate that most of the 
analyzed estuaries receive 15-40% of their N inputs from atmospheric 
sources (ISA, section ES 5.2; ISA, Appendix 7, section 7.2.1), though 
for specific estuaries contributions can vary more widely. In areas 
along the West Coast, N sources may include coastal upwelling from 
oceanic waters, as well as transport from watersheds. Common N inputs 
to estuaries include those associated with freshwater inflows 
transporting N from agriculture, urban, and wastewater sources, in 
addition to atmospheric deposition across the watershed (ISA, section 
IS 2.2.2; ISA, Appendix 7, section 7.2.1).
    There are estimates of atmospheric N loading to estuaries available 
from several recent modeling studies (ISA, Table 7-9). One analysis of 
estuaries along the Atlantic Coast and the Gulf of Mexico, which 
estimated that 62-81% of N delivered to the eastern U.S. coastal zone 
is anthropogenic in source, also reported that atmospheric N deposition 
to freshwater that is subsequently transported to estuaries represents 
17-21% of the total N loading into the coastal zone (McCrackin et al., 
2013; Moore et al., 2011). In the Gulf of Mexico, 26% of the N 
transported to the Gulf in the Mississippi/Atchafalaya River basin was 
estimated to be contributed from atmospheric deposition (which may 
include volatilized losses from natural, urban, and agricultural 
sources) (Robertson and Saad, 2013). Another modeling analysis 
identified atmospheric deposition to watersheds as the dominant source 
of N to the estuaries of the Connecticut, Kennebec, and Penobscot 
rivers. For the entire Northeast and mid-Atlantic coastal region, 
however, it was the third largest source (20%), following agriculture 
(37%) and sewage and population-related sources (28%) (ISA, Appendix 7, 
section 7.2.1). Estimates for West Coast estuaries indicate much 
smaller contribution from atmospheric deposition. For example, analyses 
for Yaquina Bay, Oregon, estimated direct deposition to contribute only 
0.03% of N inputs; estimated N input to the watershed from N fixing red 
alder (Alnus rubra) trees was a much larger (8%) source (ISA, Appendix 
7, section 7.2.1; Brown and Ozretich, 2009).
    Evidence in coastal waters has recognized a role of nutrient 
enrichment in acidification of some coastal waters (ISA, Appendix 10, 
section 10.5). More specifically, nutrient-driven algal blooms may 
contribute to ocean acidification, possibly through increased 
decomposition, which lowers dissolved oxygen levels in the water column 
and contributes to lower pH. Such nutrient-enhanced acidification can 
also be exacerbated by warming (associated with increased microbial 
respiration) and changes in buffering capacity (alkalinity) of 
freshwater inputs (ISA, Appendix 10, section 10.5).
(2) Terrestrial Ecosystems
    It is long established that N enrichment of terrestrial ecosystems 
increases plant productivity (ISA, Appendix 6, section 6.1). Building 
on this, the currently available evidence, including evidence that is 
longstanding, is sufficient to infer a causal relationship between N 
deposition and the alteration of the physiology and growth of 
terrestrial organisms and the productivity of terrestrial ecosystems 
(ISA, Appendix 5, section 5.2 and Appendix 6, section 6.2). Responsive 
ecosystems include those that are N limited and/or contain species that 
have evolved in nutrient-poor environments. In these ecosystems the N-
enrichment changes in plant physiology and growth rates vary among 
species, with species that are adapted to low N supply being readily 
outcompeted by species that require more N. In this manner, the 
relative representation of different vegetation species may be altered, 
and some species may be eliminated altogether, such that community 
composition is changed and species diversity declines (ISA, Appendix 6, 
sections 6.3.2 and 6.3.8). The currently available evidence in this 
area is sufficient to infer a causal relationship between N deposition 
and the alteration of species richness, community composition, and 
biodiversity in terrestrial ecosystems (ISA, section IS.5.3 and 
Appendix 6, section 6.3).
    Previously available evidence described the role of N deposition in 
changing soil carbon and N pools and fluxes, as well as altering plant 
and microbial growth and physiology in an array of terrestrial 
ecosystems (ISA, Appendix 6, section 6.2.1). Nitrogen availability is 
broadly limiting for productivity in many terrestrial ecosystems (ISA, 
Appendix 6, section 6.2.1). Accordingly, N additions contribute to 
increased productivity and can alter biodiversity. Eutrophication, one 
of the mechanisms by which increased productivity and changes in 
biodiversity associated with N addition to terrestrial ecosystems can 
occur, comprises multiple effects that include changes to the 
physiology of individual organisms, alteration of the relative growth 
and abundance of various species, transformation of relationships 
between species, and indirect effects on availability of essential 
resources other than N, such as light, water, and nutrients (ISA, 
Appendix 6, section 6.2.1).
    The currently available evidence for the terrestrial ecosystem 
effects of N enrichment, including eutrophication, includes studies in 
a wide array of systems, including forests (tropical, temperate, and 
boreal), grasslands, arid and semi-arid scrublands, and tundra (PA, 
section 4.1; ISA, Appendix 6). The organisms affected include trees, 
herbs and shrubs, and lichen, as well as fungal, microbial, and 
arthropod communities. Lichen communities, which have important roles 
in hydrologic cycling, nutrient cycling, and as sources of food and 
habitat for other species, are also affected by atmospheric N (PA, 
section 4.1; ISA, Appendix 6). The recently available studies on the 
biological effects of added N in terrestrial ecosystems include 
investigations of plant and microbial physiology, long-term ecosystem-
scale N addition experiments, regional and continental-scale monitoring 
studies, and syntheses.
    The previously available evidence included N addition studies in 
the U.S. and N deposition gradient studies in Europe that reported 
associations of N deposition with reduced species richness and altered 
community composition for grassland plants, forest understory plants, 
and mycorrhizal fungi (soil fungi that have a symbiotic relationship 
with plant roots) (ISA, Appendix 6, section 6.3). New evidence for 
forest communities in this review indicates that N deposition alters 
the physiology and growth of overstory trees, and that N deposition has 
the potential to change the community composition of forests (ISA, 
Appendix 6, section 6.6). Recent studies on forest trees include 
analyses of long-term forest inventory data collected from across the 
U.S. and Europe (ISA, Appendix 6, section 6.2.3.1). The recent evidence 
also includes findings of variation in forest understory and non-forest 
plant communities with atmospheric N deposition gradients in the U.S. 
and in Europe. For example, gradient studies in Europe have found 
higher N deposition to be associated with forest understory plant 
communities with more nutrient-demanding and shade-tolerant plant 
species (ISA, Appendix 6, section 6.3.3.2). A recent gradient study in 
the U.S. found associations between herb and shrub species richness and 
N deposition, that were highly dependent on soil pH (ISA, Appendix 6, 
section 6.3.3.2).
    Recent evidence includes associations of variation in lichen 
community

[[Page 26641]]

composition with N deposition gradients in the U.S. and Europe, (ISA, 
Appendix 6, section 6.3.7; Table 6-23). Differences in lichen community 
composition have been attributed to atmospheric N pollution in forests 
throughout the West Coast, in the Rocky Mountains, and in southeastern 
Alaska. Differences in epiphytic lichen growth or physiology have been 
observed along atmospheric N deposition gradients in the highly 
impacted area of southern California, and also in more remote locations 
such as Wyoming and southeastern Alaska (ISA, Appendix 6, section 
6.3.7). Historical deposition may play a role in observational studies 
of N deposition effects, complicating the disentangling of responses 
that may be related to more recent N loading.
    Newly available findings from N addition experiments expand on the 
understanding of mechanisms linking changes in plant and microbial 
community composition to increased N availability. Such experiments in 
arid and semi-arid environments indicate that competition for resources 
such as water may exacerbate the effects of N addition on diversity 
(ISA, Appendix 6, section 6.2.6). The newly available evidence includes 
studies in arid and semiarid ecosystems, particularly in southern 
California, that have reported changes in plant community composition, 
in the context of a long history of significant N deposition, with 
fewer observations of plant species loss or changes in plant diversity 
(ISA, Appendix 6, section 6.3.6).
    Nitrogen limitation in grasslands and the dominance by fast-growing 
species that can shift in abundance rapidly (in contrast to forest 
trees) contribute to an increased sensitivity of grassland ecosystems 
to N inputs (ISA, Appendix 6, section 6.3.6). Studies in southern 
California coastal sage scrub communities, including studies of the 
long-term history of N deposition, which was appreciably greater in the 
past than recent rates, indicate impacts on community composition and 
species richness in these ecosystems (ISA, Appendix 6, sections 6.2.6 
and 6.3.6). In summary, the ability of atmospheric N deposition to 
override the natural spatial heterogeneity in N availability in arid 
ecosystems, such as the Mojave Desert and California coastal sage scrub 
ecosystems in southern California, makes these ecosystems sensitive to 
N deposition (ISA, Appendix 6, section 6.3.8).
    The current evidence includes relatively few studies of N 
enrichment recovery in terrestrial ecosystems. Among N addition studies 
assessing responses after cessation of additions, it has been observed 
that soil nitrate and ammonium concentrations recovered to levels 
observed in untreated controls within 1 to 3 years of the cessation of 
additions, but soil processes such as N mineralization and litter 
decomposition were slower to recover (ISA, Appendix 6, section 6.3.2; 
Stevens, 2016). A range of recovery times have been reported for 
mycorrhizal community composition and abundance from a few years in 
some systems to as long as 28 or 48 years in others (ISA, Appendix 6, 
section 6.3.2; Stevens, 2016; Emmett et al., 1998; Strengbom et al., 
2001). An N addition study in the midwestern U.S. observed that plant 
physiological processes recovered in less than 2 years, although 
grassland communities were slower to recover and still differed from 
controls 20 years after the cessation of N additions (ISA, Appendix 6, 
section 6.3.2; Isbell et al., 2013).
d. Other Deposition-Related Effects
    Additional categories of effects for which the current evidence is 
sufficient to infer causal relationships with deposition of S or N 
compounds or PM include changes in mercury methylation processes in 
freshwater ecosystems, changes in aquatic biota due to sulfide 
phytotoxicity, and ecological effects from PM deposition (ISA, Table 
IS-1). The current evidence, including that newly available in this 
review, is sufficient to infer a causal relationship between S 
deposition and the alteration of mercury methylation in surface water, 
sediment, and soils in wetland and freshwater ecosystems. The process 
of mercury methylation is influenced in part by surface water 
SO42- concentrations, as well as the presence of 
mercury. Accordingly, in waterbodies where mercury is present, S 
deposition, particularly that associated with SOX, has a 
role in production of methylmercury, which contributes to methylmercury 
accumulation in fish (ISA, Appendix 12, section 12.8). Newly available 
evidence has improved our scientific understanding of the types of 
organisms involved in the methylation process, as well as the 
environments in which they are found, and factors that influence the 
process, such as oxygen content, temperature, pH, and carbon supply, 
which themselves vary temporally, seasonally, and geographically (ISA, 
Appendix 12, section 12.3). The currently available evidence is also 
sufficient to infer a new causal relationship between S deposition and 
changes in biota due to sulfide phytotoxicity, including alteration of 
growth and productivity, species physiology, species richness, 
community composition, and biodiversity in wetland and freshwater 
ecosystems (ISA, section IS.9). Sulfur deposition can contribute to 
sulfide and associated phytotoxicity in freshwater wetlands and lakes, 
with the potential to contribute to effects on plant community 
composition in freshwater wetlands (ISA, Appendix 12, section 12.2.3).
    With regard to PM deposition, the currently available evidence is 
sufficient to infer a likely causal relationship between deposition of 
PM and a variety of effects on individual organisms and ecosystems 
(ISA, Appendix 15, section 15.1). Particulate matter includes a 
heterogeneous mixture of particles differing in origin, size, and 
chemical composition. In addition to N and S and their transformation 
products, other PM components, such as trace metals and organic 
compounds, when deposited to ecosystems, may affect biota. Material 
deposited onto leaf surfaces can alter leaf processes and PM components 
deposited to soils and waterbodies may be taken up into biota, with the 
potential for effects on biological and ecosystem processes. Studies 
involving ambient air PM, however, have generally involved conditions 
that would not be expected to meet the current secondary standards for 
PM. Further, although in some limited cases, effects have been 
attributed to particle size (e.g., soiling of leaves by large coarse 
particles near industrial facilities or unpaved roads), ecological 
effects of PM have been largely attributed more to its chemical 
components, such as trace metals, which can be toxic in large amounts 
(ISA, Appendix 15, sections 15.2 and 15.3.1). The evidence largely 
comes from studies involving areas experiencing elevated concentrations 
of PM, such as near industrial areas or historically polluted cities 
(ISA, Appendix 15, section 15.4).
2. Public Welfare Implications
    The public welfare implications of the evidence regarding S and N 
related welfare effects are dependent on the type and severity of the 
effects, as well as the extent of the effect at a particular biological 
or ecological level of organization or spatial scale. We discuss such 
factors here in light of judgments and conclusions regarding effects on 
the public welfare that have been made in NAAQS reviews.
    As provided in section 109(b)(2) of the CAA, the secondary standard 
is to ``specify a level of air quality the attainment and maintenance 
of which in the judgment of the Administrator . . . is requisite to 
protect the public welfare from any known or anticipated adverse 
effects associated with the presence of

[[Page 26642]]

such air pollutant in the ambient air.'' The secondary standard is not 
meant to protect against all known or anticipated welfare effects 
related to oxides of N and S, and particulate matter, but rather those 
that are judged to be adverse to the public welfare, and a bright-line 
determination of adversity is not required in judging what is requisite 
(78 FR 3212, January 15, 2013; 80 FR 65376, October 26, 2015; see also 
73 FR 16496, March 27, 2008). Thus, the level of protection from known 
or anticipated adverse effects to public welfare that is requisite for 
the secondary standard is a public welfare policy judgment made by the 
Administrator. The Administrator's judgment regarding the available 
information and adequacy of protection provided by an existing standard 
is generally informed by considerations in prior reviews and associated 
conclusions.
    The categories of effects identified in the CAA to be included 
among welfare effects are quite diverse, and among these categories, 
any single category includes many different types of effects that are 
of broadly varying specificity and level of resolution. For example, 
effects on vegetation and effects on animals are categories identified 
in CAA section 302(h), and the ISA recognizes effects of N and S 
deposition at the organism, population, community, and ecosystem level, 
as summarized in section II.C.1 above (ISA, sections IS.5 to IS.9). As 
noted in the last review of the secondary NAAQS for NOX and 
SOX, while the CAA section 302(h) lists a number of welfare 
effects, ``these effects do not define public welfare in and of 
themselves'' (77 FR 20232, April 3, 2012).
    The significance of each type of effect with regard to potential 
effects on the public welfare depends on the type and severity of 
effects, as well as the extent of such effects on the affected 
environmental entity, and on the societal use of the affected entity 
and the entity's significance to the public welfare. Such factors have 
been considered in the context of judgments and conclusions made in 
some prior reviews regarding public welfare effects. For example, in 
the context of secondary NAAQS decisions for ozone (O3), 
judgments regarding public welfare significance have given particular 
attention to effects in areas with special Federal protections (such as 
Class I areas), and lands set aside by States, Tribes and public 
interest groups to provide similar benefits to the public welfare (73 
FR 16496, March 27, 2008; 80 FR 65292, October 26, 2015).\34\ In the 
2015 O3 NAAQS review, the EPA recognized the ``clear public 
interest in and value of maintaining these areas in a condition that 
does not impair their intended use and the fact that many of these 
lands contain O3-sensitive species'' (73 FR 16496, March 27, 
2008).
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    \34\ For example, the fundamental purpose of parks in the 
National Park System ``is to conserve the scenery, natural and 
historic objects, and wildlife in the System units and to provide 
for the enjoyment of the scenery, natural and historic objects, and 
wildlife in such manner and by such means as will leave them 
unimpaired for the enjoyment of future generations'' (54 U.S.C. 
100101). Additionally, the Wilderness Act of 1964 defines designated 
``wilderness areas'' in part as areas ``protected and managed so as 
to preserve [their] natural conditions'' and requires that these 
areas ``shall be administered for the use and enjoyment of the 
American people in such manner as will leave them unimpaired for 
future use and enjoyment as wilderness, and so as to provide for the 
protection of these areas, [and] the preservation of their 
wilderness character . . .'' (16 U.S.C. 1131 (a) and (c)). Other 
lands that benefit the public welfare include national forests which 
are managed for multiple uses including sustained yield management 
in accordance with land management plans (see 16 U.S.C. 1600(1)-(3); 
16 U.S.C. 1601(d)(1)).
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    Judgments regarding effects on the public welfare can depend on the 
intended use for, or service (and value) of, the affected vegetation, 
ecological receptors, ecosystems and resources and the significance of 
that use to the public welfare (73 FR 16496, March 27, 2008: 80 FR 
65377, October 26, 2015). Uses or services provided by areas that have 
been afforded special protection can flow in part or entirely from the 
vegetation that grows there or other natural resources. Ecosystem 
services range from those directly related to the natural functioning 
of the ecosystem to ecosystem uses for human recreation or profit, such 
as through the production of lumber or fuel (Constanza et al., 2017; 
ISA, section IS.13). The spatial, temporal, and social dimensions of 
public welfare impacts are also influenced by the type of service 
affected. For example, a national park can provide direct recreational 
services to the thousands of visitors that come each year, but also 
provide an indirect value to the millions who may not visit but receive 
satisfaction from knowing it exists and is preserved for the future (80 
FR 65377, October 26, 2015).
    In the last review of the secondary NAAQS for NOX and 
SOX, ecosystem services were discussed as a method of 
assessing the magnitude and significance to the public of resources 
affected by ambient air concentrations of oxides of nitrogen and sulfur 
and associated deposition in sensitive ecosystems (77 FR 20232, April 
3, 2012). That review recognized that although there is no specific 
definition of adversity to public welfare, one paradigm might involve 
ascribing public welfare significance to disruptions in ecosystem 
structure and function. The concept of considering the extent to which 
a pollutant effect will contribute to such disruptions has been used 
broadly by the EPA in considering effects. An evaluation of adversity 
to public welfare might also consider the likelihood, type, magnitude, 
and spatial scale of the effect, as well as the potential for recovery 
and any uncertainties relating to these considerations (77 FR 20218, 
April 3, 2012).
    The types of effects on aquatic and terrestrial ecosystems 
discussed in section II.C.1 above differ with regard to aspects 
important to judging their public welfare significance. For example, in 
the case of effects on timber harvest, such judgments may consider 
aspects such as the heavy management of silviculture in the U.S., while 
judgments for other categories of effects may generally relate to 
considerations regarding natural areas, including specifically those 
areas that are not managed for harvest. For example, effects on tree 
growth and survival have the potential to be significant to the public 
welfare through impacts in Class I and other areas given special 
protection in their natural/existing state, although they differ in how 
they might be significant.
    In this context, it may be important to consider that S and N 
deposition-related effects, such as changes in growth and survival of 
plant and animal species, could, depending on severity, extent, and 
other factors, lead to effects on a larger scale including changes in 
overall productivity and altered community composition (ISA, section 
IS.2.2.1 and Appendices 5, 6, 8, 9, and 10). Further, effects on 
individual species could contribute to impacts on community composition 
through effects on growth and reproductive success of sensitive species 
in the community, with varying impacts to the system through many 
factors including changes to competitive interactions (ISA, section 
IS.5.2 and Appendix 6, section 6.3.2).
    In acid-impacted surface waters, acidification primarily affects 
the diversity and abundance of fish and other aquatic life, and the 
ecosystem services derived from these organisms. (2011 PA, section 
4.4.5). In addition to other types of services, fresh surface waters 
support several cultural services, such as aesthetic, recreational, and 
educational services. The type of service that is likely to be most 
widely and significantly affected by aquatic

[[Page 26643]]

acidification is recreational fishing. Multiple studies have documented 
the economic benefits of recreational fishing. Freshwater rivers and 
lakes of the northeastern United States, surface waters that have been 
most affected by acidification, are not a major source of commercially 
raised or caught fish; they are, however, a source of food for some 
recreational and subsistence fishers and for other consumers (2009 REA, 
section 4.2.1.3). It is not known if and how consumption patterns of 
these fishers may have been affected by the historical impacts of 
surface water acidification in the affected systems. Non-use services, 
which include existence (protection and preservation with no 
expectation of direct use) and bequest values, are arguably a 
significant source of benefits from reduced acidification (Banzhaf et 
al., 2006). Since the 2012 review, additional approaches and methods 
have been applied to estimate the potential effects of aquatic 
acidification on uses and services of affected aquatic ecosystems; with 
regard to economic impacts, however, ``for many regions and specific 
services, poorly characterized dose-response between deposition, 
ecological effect, and services are the greatest challenge in 
developing specific data on the economic benefits of emission 
reductions'' (ISA, Appendix 14, p. 14-23).
    Nitrogen loading in aquatic ecosystems, particularly large 
estuarine and coastal water bodies, has and continues to pose risks to 
the services provided by those ecosystems, with clear implications to 
the public welfare (2011 PA, section 4.4.2; ISA, Appendix 14, section 
14.3.2). For example, the large estuaries of the eastern U.S. are an 
important source of fish and shellfish production, capable of 
supporting large stocks of resident commercial species and serving as 
breeding grounds and interim habitat for several migratory species 
(2009 REA, section 5.2.1.3). These estuaries also provide an important 
and substantial variety of cultural ecosystem services, including 
water-based recreational and aesthetic services. And as noted for fresh 
waters above, these systems have non-use benefits to the public (2011 
PA, section 4.4.5). Studies reviewed in the ISA have explored both 
enumeration of the number of ecosystem services that may be affected by 
N loading, and the pathways by which this may occur, as well as 
approaches to valuation of such impacts. A finding of one such analysis 
was that ``better quantitative relationships need to be established 
between N and the effects on ecosystems at smaller scales, including a 
better understanding of how N shortages can affect certain 
populations'' (ISA, Appendix 14, sections 14.5 and 14.6). The relative 
contribution of atmospheric deposition to total N loading varies widely 
among estuaries, however, and has declined in more recent years (ISA, 
Appendix 10, section 10.10.1).
    A complication to consideration of public welfare implications that 
is specific to N deposition in terrestrial systems is its potential to 
increase growth and yield of plants, that may be agricultural and 
forest crops. Such increased growth and yield may be judged and valued 
differently than changes in growth of other species. As noted in 
section II.C.1 above, enrichment in natural ecosystems can, by 
increasing growth of N limited plant species, change competitive 
advantages of species in a community, with associated impacts on the 
composition of the ecosystem's plant community. The public welfare 
implications of such effects may vary depending on their severity, 
prevalence, or magnitude, such as with only those rising to a 
particular severity (e.g., with associated significant impact on key 
ecosystem functions or other services), magnitude or prevalence 
considered of public welfare significance. Impacts on some of these 
characteristics (e.g., forest or forest community composition) may be 
considered of greater public welfare significance when occurring in 
Class I or other protected areas, due to the value that the public 
places on such areas. In considering such services in past reviews for 
secondary standards for other pollutants (e.g., O3), the 
Agency has given particular attention to effects in natural ecosystems, 
indicating that a protective standard, based on consideration of 
effects in natural ecosystems in areas afforded special protection, 
would also ``provide a level of protection for other vegetation that is 
used by the public and potentially affected by O3 including 
timber, produce grown for consumption and horticultural plants used for 
landscaping'' (80 FR 65403, October 26, 2015).
    Although more sensitive effects are described with increasingly 
greater frequency in the evidence base of effects related to ecosystem 
deposition of N and S compounds, the available information does not yet 
provide a framework that can specifically tie various magnitudes or 
prevalences of changes in a biological or ecological indicator (e.g., 
lichen abundance or community composition \35\) to broader effects on 
the public welfare. The ISA finds that while there is an improved 
understanding from information available in this review of the number 
of pathways by which N and S deposition may affect ecosystem services, 
most of these relationships remain to be quantified (ISA, Appendix 14, 
section 14.6).\36\ This gap creates uncertainties when considering the 
public welfare implications of some biological or geochemical responses 
to ecosystem acidification or N enrichment, and accordingly complicates 
judgments on the potential for public welfare significance. That 
notwithstanding, while shifts in species abundance or composition of 
various ecological communities may not be easily judged with regard to 
public welfare significance, at some level, such changes, especially if 
occurring broadly in specially protected areas, where the public can be 
expected to place high value, might reasonably be concluded to impact 
the public welfare. An additional complexity in the current review with 
regard to assessment of effects associated with existing deposition 
rates is that the current, much-improved air quality and associated 
reduced deposition is within the context of a longer history that 
included appreciably greater deposition in the middle of the last 
century, the environmental impacts of which may remain, affecting 
ecosystem responses.
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    \35\ As recognized in section II.C.1.c above, lichen communities 
have important roles in ecosystem function, such as in hydrologic 
cycling, nutrient cycling, and as sources of food and habitat for 
other species (ISA, appendix 6).
    \36\ While ``there is evidence that N and S emissions/deposition 
have a range of effects on U.S. ecosystem services and their social 
value'' and ``there are some economic studies that demonstrate such 
effects in broad terms,'' ``it remains methodologically difficult to 
derive economic costs and benefits associated with specific 
regulatory decisions/standards'' (ISA, appendix 14, pp. 14-23 to 14-
24).
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    In summary, several considerations are recognized as important to 
judgments on the public welfare significance of the array of welfare 
effects at different exposure conditions. These include uncertainties 
and limitations that must be taken into account regarding the magnitude 
of key effects that might be concluded to be adverse to ecosystem 
health and associated services. Additionally, there are numerous 
locations vulnerable to public welfare impacts from S or N deposition-
related effects on terrestrial and aquatic ecosystems and their 
associated services. Other important considerations include the 
exposure circumstances that may elicit effects and

[[Page 26644]]

the potential for the significance of the effects to vary in specific 
situations due to differences in sensitivity of the exposed species, 
the severity and associated significance of the observed or predicted 
effect, the role that the species plays in the ecosystem, the intended 
use of the affected species and its associated ecosystem and services, 
the presence of other co-occurring predisposing or mitigating factors, 
and associated uncertainties and limitations.
3. Exposure Conditions and Deposition-Related Metrics
    The ecological effects identified in section II.C.1 above vary 
widely with regard to the extent and level of detail of the available 
information that describes the exposure circumstances that may elicit 
them. The information for direct effects of SOX, N oxides 
and PM in ambient air is somewhat more straight-forward to consider as 
it is generally in terms of concentrations in air. For deposition-
related effects, the information may be about S and N compounds in soil 
or water or may be for metrics intended to represent atmospheric 
deposition of those compounds. For the latter, as recognized in section 
II.A.3 above, we face the challenge of relating that information to 
patterns of ambient air concentrations.
    With regard to the more complex consideration of deposition-related 
effects such as ecosystem acidification and N enrichment, there is also 
wide variation in the extent and level of detail of the evidence 
available to describe the ecosystem characteristics (e.g., physical, 
chemical, and geological characteristics, as well as atmospheric 
deposition history) that influence the degree to which deposition of N 
and S associated with the oxides of S and N and PM in ambient air 
elicit ecological effects. One reason for this relates to the 
contribution of many decades of uncontrolled atmospheric deposition 
before the establishment of NAAQS for PM, oxides of S and oxides of N 
(in 1971), followed by the subsequent decades of continued deposition 
as standards were implemented and updated. The impacts of this 
deposition history remain in soils of many parts of the U.S. today 
(e.g., in the Northeast and portions of the Appalachian Mountains in 
both hardwood and coniferous forests, as well as areas in and near the 
Los Angeles Basin), with recent signs of recovery in some areas (ISA, 
Appendix 4, section 4.6.1; 2008 ISA, section 3.2.1.1). This backdrop 
and associated site-specific characteristics are among the challenges 
faced in identifying deposition targets that might be expected to 
provide protection going forward against the array of effects for which 
we have evidence of occurrence in sensitive ecosystems as a result of 
the deposition of the past.
    Critical loads (CLs) are frequently used in studies that 
investigate associations between various chemical, biological, 
ecological and ecosystem characteristics and a variety of N or S 
deposition-related metrics. The term critical load, which in general 
terms refers to an amount (or a rate of addition) of a pollutant to an 
ecosystem that is estimated to be at (or just below) that which would 
result in an ecological effect of interest, has multiple 
interpretations and applications (ISA, p. IS-14). The dynamic nature of 
ecosystem pollutant processing and the broad array of factors that 
influence it adds complications to critical load identification and 
interpretation. Time is an important dimension, which is sometimes 
unstated, (e.g., in empirical or observational analyses) and is 
sometimes explicit (e.g., in steady-state or dynamic modeling analyses) 
(ISA, section IS.2.2.4). Further, this variety in meanings stems in 
part from differing judgments and associated identifications regarding 
the ecological effect (both type and level of severity) on which the 
critical load focuses, and judgment of its significance or meaning.
    Studies, based on which CLs are often identified, vary widely with 
regard to the specific ecosystem characteristics being evaluated (or 
judged), as well as the benchmarks selected for judging them, such as 
the deposition-related metrics, their scope, method of estimation and 
time period. The specific details of these various factors influence 
the strengths and limitations, and associated uncertainty, of using 
critical load information from such studies for different applications. 
The summary that follows is intended to reach beyond individual 
critical loads developed over a variety of studies and ecosystems and 
consider the underlying study findings with regard to key aspects of 
the environmental conditions and ecological characteristics studied. A 
more quantitative variation of this is the methodology developed for 
the aquatic acidification REA in this review, presented in the PA and 
summarized in section II.D below. In those analyses, the concept of a 
critical load is employed with steady-state modeling that relates 
deposition to waterbody acid neutralizing capacity.
    While recognizing the inherent connections between watersheds and 
waterbodies, such as lakes and streams, the organization of this 
section recognizes the more established state of the information, 
tools, and data for aquatic ecosystems for characterizing relationships 
between atmospheric deposition and acidification and/or nutrient 
enrichment effects under air quality associated with the current 
standards (PA, Chapter 5). Further, we recognize the generally greater 
role of atmospheric deposition in waterbodies impacted by aquatic 
acidification compared to its role in eutrophication-related impacts of 
surface waters, particularly rivers and estuaries in and downstream of 
populated watersheds, to which direct discharges have also long 
contributed, as recognized in section II.C.1.c(1) above (ISA, Appendix 
13, section 13.1.3.1; ISA, Appendix 7, section 7.1.1.1; 2008 ISA, 
section 3.2). Therefore, with regard to deposition-related effects, we 
focus first on the quantitative information for aquatic ecosystem 
effects in sections II.C.3.a. Section II.C.3.b discusses the available 
evidence regarding relationships between deposition-related exposures 
and the occurrence and severity of effects on trees and understory 
communities in terrestrial ecosystems. Section II.C.3.c discusses the 
currently available information related to consideration of exposure 
concentrations associated with other welfare effects of nitrogen and 
sulfur oxides and PM in ambient air.
a. Acidification and Nitrogen Enrichment in Aquatic Ecosystems
    Prior to the peak in S deposition levels that occurred in the 1970s 
and early 1980s, surface water sulfate concentrations were increasing 
in response to the extremely high S deposition of the preceding years. 
Subsequently, and especially more recently, surface water sulfate 
concentrations have generally decreased, particularly in the Northeast 
(Robinson et al., 2008; ISA, section 7.1.5.1.4). Some studies of long-
term projections in some waterbodies (e.g., in the Blue Ridge Mountains 
region in Virginia), however, continue to indicate little or slow 
reduction in acidic ions, even as emissions have declined. This is an 
example of the competing role of changes in S adsorption on soils and 
the release of historically deposited S from soils into surface 
water,\37\ which some modeling has suggested will delay chemical 
recovery in those water bodies (ISA, Appendix 7, sections 7.1.2.2 and 
7.1.5.1).
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    \37\ Some modeling studies in some areas have indicated the 
potential for a lagged response even as emissions and deposition 
decline; this lag reflects a reduction in soil absorption of 
SO4-2 and leaching of previously accumulated S 
from watersheds (ISA, appendix 7, section 7.1.2.2).

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[[Page 26645]]

    In the 2012 review of the oxides of N and S, quantitative analyses 
relating deposition in recent times (e.g., since 2000) to ecosystem 
acidification, and particularly aquatic acidification, were generally 
considered to be less uncertain, and the ability of those analyses to 
inform NAAQS policy judgments more robust, than analyses related to 
deposition and ecosystem nutrient enrichment or eutrophication (2011 
PA). While quantitative assessment approaches for aquatic 
eutrophication as a result of total N loading are also well 
established, and the evidence base regarding atmospheric deposition and 
nutrient enrichment has expanded since the 2012 review, the 
significance of non-air N loading to rivers, estuaries and coastal 
waters (as recognized in section II.C.1 above) continues to complicate 
the assessment of nutrient enrichment-related risks specifically 
related to atmospheric N deposition. Accordingly, the REA analyses 
developed in this review focus on aquatic acidification. The REA and 
its findings regarding deposition rates associated with different 
levels of aquatic acidification risk are summarized in section II.D 
below. Thus, the paragraphs below focus on the available quantitative 
information regarding atmospheric deposition and N enrichment in 
aquatic ecosystems.
    There are several other categories of effects to aquatic ecosystems 
from deposition of nitrogen and sulfur for which there is significant 
scientific evidence, based on which the ISA has made determinations of 
causality; these include N enrichment in various types of aquatic 
systems, including freshwater streams and lakes, estuarine and near-
coastal systems, and wetlands, as described in section II.C.1 
above.\38\ Separate quantitative analyses have not been performed in 
this review for N enrichment-related effects in these waterbodies in 
recognition of a number of factors, including modeling and assessment 
complexities, and site- or waterbody-specific data requirements, as 
well as, in some cases, issues of apportionment of atmospheric sources 
separate from other influential sources. Rather, we have reviewed the 
evidence as characterized in the ISA with regard to deposition level 
estimates that studies have related to various degrees of different 
effects with associated differences in potential for or clarity in 
public welfare significance. The overview provided here draws on the 
summary in the PA (PA, section 5.2).
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    \38\ Two other categories of effects assessed in the ISA (and 
for which causal determinations are made) are mercury methylation 
and sulfide toxicity (ISA, appendix 12). While the evidence was 
sufficient to support causal determinations between S deposition and 
these effects, quantitative information to support quantitative 
analysis in this review.
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    The eutrophication of wetlands and other aquatic systems is 
primarily associated with nitrogen inputs whether from deposition or 
other sources. The ranges of deposition associated with these effects 
is very broad and ranges from levels on the order of a few kg N/ha-yr 
for impacts to diatom communities in high elevation lakes to over 500 
kg N/ha-yr for some effects of interest in some wetland N addition 
studies. While the information available on these types of impacts is 
sufficient for causal determinations, it is often very localized and 
less informative for other uses, such as for the purpose of 
quantitative assessment relating deposition to waterbody response at an 
array of U.S. locations. Accordingly, in this review, this information 
was considered from a more descriptive perspective in characterizing 
conditions reported in the evidence as associated with various effects 
described in section II.C.1 above.
    There is also considerable information available for estuaries and 
coastal systems. The relationship between N loading and algal blooms, 
and associated water quality impacts, has led to numerous water quality 
modeling projects, over the past few decades, that have quantified 
eutrophication processes in multiple estuaries, near coastal marine 
ecosystems and large river systems, to relate N loading to various 
water quality indicators to inform water quality management decision-
making in multiple estuaries, including Chesapeake Bay, Narraganset 
Bay, Tampa Bay, Neuse River Estuary and Waquoit Bay (ISA, Appendix 7, 
section 7.2). These projects often use indicators of nutrient 
enrichment, such as chlorophyll a, dissolved oxygen, and reduced 
abundance of submerged aquatic vegetation, among others (ISA, section 
IS.7.3 and Appendix 10, section 10.6). For these estuaries, the 
available information regarding atmospheric deposition and the 
establishment of associated target loads varies across the various 
estuaries (ISA, Appendix 7, Table 7-9).
    While a focus is identification of total N loading targets for 
purposes of attaining water quality standards for such indicators, the 
modeling work also includes apportionment of sources, which vary by 
system. The assignment of targets to different source types (e.g., 
groundwater, surface water runoff, and atmospheric deposition) in 
different waterbodies and watersheds also varies for both practical and 
policy reasons. Further, during the multi-decade time period across 
which these activities have occurred, atmospheric deposition of N in 
coastal areas has declined. In general, however, atmospheric deposition 
targets for N for the large systems summarized above have been on the 
order of 10 kg/ha-yr, with some somewhat lower and some somewhat 
higher.
    The establishment of target N loads to surface waterbodies is in 
many areas related to implementation of the total maximum daily load 
(TMDL) requirements of section 303(d) of the Clean Water Act (CWA).\39\ 
Nutrient load allocation and reduction activities in some large 
estuaries predate development of CWA 303(d) TMDLs. The multiple 
Chesapeake Bay Agreements signed by the U.S. EPA, District of Columbia, 
and States of Virginia, Maryland, and Pennsylvania first established 
the voluntary government partnership that directs and manages bay 
cleanup efforts and subsequently included commitments for reduction of 
N and phosphorus loading to the bay. Efforts prior to 2000 focused 
largely on point-source discharges, with slower progress for nonpoint-
source reductions via strategies such as adoption of better 
agricultural practices, reduction of atmospheric N deposition, 
enhancement of wetlands and other nutrient sinks, and control of urban 
sprawl (2008 ISA, section 3.3.8.3). Studies since 2000 estimate 
atmospheric deposition as a major N source in the overall N budget for 
Chesapeake Bay \40\ (ISA, section 7.2.1; Howarth, 2008; Boyer et al., 
2002). The TMDL established for Chesapeake Bay in 2010, under 
requirements of section 303(d) of the Clean Water Act, included a 
loading allocation for atmospheric deposition of N directly to tidal 
waters, which was projected to be achieved by 2020 based on air quality 
progress under existing Clean Air Act regulations and programs (U.S. 
EPA, 2010).\41\
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    \39\ Under the CWA, section 303(d), every two years, states and 
other jurisdictions are required to list impaired waterbodies not 
meeting water quality standards. For waterbodies on the list, a TMDL 
must be developed that identifies the maximum amount of pollutant a 
waterbody can receive and still meet water quality standards, e.g., 
standards for dissolved oxygen and chlorophyll a (which are 
indicators of eutrophication).
    \40\ For example, a 2011 analysis estimated atmospheric 
deposition to the Chesapeake Bay watershed to account for 
approximately 25% of total N inputs to the estuary (ISA, Appendix 7, 
section 7.2.1).
    \41\ As recognized on the EPA web page describing this activity, 
the TMDL, formally established in December 2010 ``is designed to 
ensure that all pollution control measures needed to fully restore 
the Bay and its tidal rivers are in place by 2025.'' The website 
also indicates that '' EPA expects practices in place by 2017 to 
meet 60 percent of the necessary reductions,'' and for some areas to 
recover before others, but for it to take years after 2025 for the 
Bay and its tributaries to fully recover (https://www.epa.gov/chesapeake-bay-tmdl/frequent-questions-about-chesapeake-bay-tmdl).

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[[Page 26646]]

    Jurisdictions for other U.S. estuaries have also developed TMDLs to 
address nutrient loading causing eutrophication. For example, 
atmospheric deposition in 2000 was identified as the third largest 
source of N loading to Narragansett Bay via the watershed and directly 
to the Bay, at 20% of the total (ISA, Appendix 7, section 7.2.1). 
Similarly, atmospheric deposition was estimated to account for 
approximately a third of N input to several small- to medium-sized 
estuaries of southern New England, with the percentage varying widely 
for individual estuaries (ISA, Appendix 7, section 7.2.1; Latimer and 
Charpentier, 2010).\42\ Another modeling study in the Waquoit Bay 
estuaries in Cape Cod, Massachusetts, using data since 1990, estimated 
atmospheric deposition to have decreased by about 41% while wastewater 
inputs increased 80%, with a net result that total loads were concluded 
to not have changed over that time period (ISA, Appendix 7, section 
7.2.1). Another well-studied estuarine system is Tampa Bay, for which a 
2013 study estimated atmospheric sources to account for more than 70% 
of total N loading based on 2002 data (ISA, Appendix 7, section 7.2.1). 
The TMDL for Tampa Bay allocates 11.8 kg/ha-yr N loading to atmospheric 
deposition (ISA, Appendix 16, section 16.4.2; Janicki Environmental, 
2013). The Neuse River Estuary is another for which modeling work has 
investigated the role of N loading from multiple sources on nutrient 
enrichment \43\ and associated water quality indicators, including 
chlorophyll a (ISA, Appendix 10, section 10.2).
---------------------------------------------------------------------------

    \42\ For example, across the 74 estuaries in the 3-state coastal 
region studied, N from atmospheric deposition to estuary watersheds 
was generally estimated to account for less than 25% of total N 
inputs, while estimates for a few small estuaries in CT were higher 
than 51% (but below 75%) (Latimer and Charpentier, 2010).
    \43\ One evaluation of progress in achieving mandated N 
reductions in the Neuse River Basin in NC found that flow-normalized 
N loading from NO3- decreased beginning in the 
1992-1996 period (ISA Appendix 10, section 10.2; Lebo et al., 2012).
---------------------------------------------------------------------------

    Nitrogen loading to estuaries has also been considered specifically 
with regard to impacts on submerged aquatic vegetation. For example, 
eelgrass coverage was estimated to be markedly reduced in shallow New 
England estuaries with N loading at or above 100 kg N/ha-yr (ISA, 
Appendix 10, section 10.2.5). Another study estimated loading rates 
above 50 kg/ha-yr as a threshold at which habitat extent may be 
impacted (ISA, Appendix 10, section 10.2.5; Latimer and Rego, 2010). 
Factors that influence the impact of N loading on submerged vegetation 
include flushing and drainage in estuaries (ISA, Appendix 10, section 
10.6).
b. Deposition-Related Effects in Terrestrial Ecosystems
    The evidence of atmospheric deposition contributing to 
acidification and N enrichment-related effects in terrestrial systems 
is strong, as evidenced by the causal determinations made in the ISA 
and summarized in section II.C.1.b(2) and II.C.1.c(2) above. 
Accordingly, the subsections below describe the available information 
in this review, including that available in the last review, regarding 
quantitative relationships between atmospheric deposition rates and 
specific terrestrial effects of interest.
    The terrestrial analyses in the 2012 review included a critical 
load-based quantitative modeling analysis focused on BC:Al ratio in 
soil (the benchmarks for which are based on laboratory responses rather 
than field measurements) for terrestrial acidification and a 
qualitative characterization of nutrient enrichment (2009 REA). The 
more qualitative approach taken for nutrient enrichment in the 2012 
review involved describing deposition ranges identified from 
observational or modeling research as associated with potential 
effects/changes in species, communities, and ecosystems, with 
recognition of uncertainties associated with quantitative analysis of 
these depositional effects (2011 PA, section 3.2.3). In this review, 
rather than performing new quantitative analyses focused on terrestrial 
ecosystems, we draw on analyses in the 2009 REA, and on more recent 
published studies recognized in the ISA that provide information 
pertaining to deposition levels associated with effects related to 
terrestrial acidification and N enrichment. This approach considers the 
available studies with investigation into various assessment 
approaches.
    The subsections below discuss the available studies of deposition 
and risk to trees (section II.C.3.b(1)) and to herbs, shrubs, and 
lichens in section II.C.3.b(2). Since the 2012 combined review of the 
secondary NAAQS for N oxides and SOX, in addition to 
publications of analyses that apply steady-state (and dynamic) modeling 
to predict future soil acidity conditions in various regions of the 
U.S. under differing atmospheric loading scenarios (ISA, Appendix 4, 
section 4.6.2), several publications have analyzed large datasets from 
field assessments of tree growth and survival, as well as understory 
plant community richness, with estimates of atmospheric N and/or S 
deposition (ISA, Appendix 6, section 6.5). These latter studies 
investigate the existence of associations of variations in plant 
community or individual measures (e.g., species richness, growth, 
survival) with a metric for deposition during an overlapping time 
period, generally of a decade or two in duration. Soil acidification 
modeling and observational studies, as well as experimental addition 
studies, each with their various design features and associated 
strengths and limitations (as noted immediately below), inform 
consideration of N and S deposition levels of interest in the review.
    In general, observational or gradient studies differ from the 
chemical mass balance modeling approach in a number of ways that are 
relevant to their consideration and use for our purposes in this 
review. One difference of note is the extent to which their findings 
reflect or take into account the ecosystem impacts of historical 
deposition. Observational studies are describing variation in 
indicators in the current context (with any ecosystem impacts, 
including stores of deposited chemicals that remain from historical 
loading). Historical loading, and its associated impacts, can also 
contribute to effects analyzed with estimates of more recent deposition 
in observational studies. Mass balance modeling, in the steady-state 
mode that is commonly used for estimating critical loads for 
acidification risk, does not usually address the complication of 
historical deposition impacts that can play a significant role in 
timing of system recovery. In this type of modeling, timelines of the 
various processes are not addressed. While this provides a simple 
approach that may facilitate consideration unrelated to recovery 
timelines, it cannot address the potential for changes in influential 
factors that may occur over time with different or changed deposition 
patterns.
    For example, in considering the potential for terrestrial ecosystem 
impacts associated with different levels of deposition, the simple mass 
balance models common for estimating critical acid loads related to 
BC:Al ratio are often run for the steady state case. Accordingly, the 
underlying assumption is that while historic deposition, and the 
various ways it may affect soil chemistry into the future (e.g., 
through the stores of historically deposited sulfur), may affect time 
to reach steady state (e.g., as the system processes the

[[Page 26647]]

past loadings), it would not be expected to affect the steady state 
solution (i.e., the estimated critical load for the specified soil 
acidification indicator target value). The complexities associated with 
site-specific aspects of ecosystem recovery from historic depositional 
loading (which contribute uncertainties to interpretation of steady-
state solutions) become evident through application of dynamic models.
    Observational studies, on the other hand, due to their focus on an 
existing set of conditions, are inherently affected by the potential 
influence of historical deposition and any past or remaining 
deposition-related impacts on soil chemistry and/or biota, in addition 
to other environmental factors. The extent of the influence of 
historical deposition (and its ramifications) on the associations 
reported in these studies with metrics quantifying more recent 
deposition is generally not known. Where patterns of spatial variation 
in recent deposition are similar to those for historic deposition, 
there may be potential for such influence. This is an uncertainty 
associated with interpretation of the observational studies as to the 
deposition levels that may be contributing to the observed variation in 
plant or plant community responses. Thus, while observational studies 
contribute to the evidence base on the potential for N/S deposition to 
contribute to ecosystem effects (and thus are important evidence in the 
ISA determinations regarding causality), their uncertainties (and 
underlying assumptions) differ from those of modeling analyses, and 
they may be somewhat less informative with regard to identification of 
specific N and S deposition levels that may elicit ecosystem impacts of 
interest. Both types of studies, as well as N addition experiments, 
which are not generally confounded by exposure changes beyond those 
assessed (yet may have other limitations) have been considered, with 
key findings summarized below.
(1) Deposition and Risks to Trees
    The available quantitative information regarding S and N deposition 
and effects on trees including modeling applications, both from the 
2012 review and from studies, is described in the ISA. Steady-state 
modeling analysis performed in the 2009 REA estimated annual amounts of 
acid deposition at or below which one of three BC:Al target values 
would be met in a 24-state area in which the acid-sensitive species, 
red spruce and sugar maple, occur. A range of acid deposition was 
estimated for each of the three target values. Recent estimates of 
total S and N deposition in regions of the U.S. appear to meet all but 
the most restrictive of these targets, for which the uncertainty is 
greatest (e.g., ISA, Appendix 2, sections 2.6 and 2.7). Uncertainties 
associated with these analyses include those associated with the 
limited dataset of laboratory-generated data on which the BC:Al target 
values are based (PA, section 5.3.2).There are also uncertainties in 
the steady-state modeling parameters, most prominently those related to 
base cation weathering and acid neutralizing capacity (2009 REA, 
section 4.3.9). More recent publications have employed a new approach 
to estimating these parameters, including the weathering parameter 
(BCw), which reduced the uncertainty associated with this parameter 
(e.g., Phelan et al., 2014; McDonnell et al., 2012; ISA, Appendix 4, 
sections 4.6.2.1 and 4.8.4 and Appendix 5, section 5.4).
    Experimental addition studies of S, or S plus N have been performed 
in eastern locations and focused on a small set of species, including 
sugar maple, aspen, white spruce, yellow poplar, and black cherry; 
these studies generally have not reported growth effects (PA, Appendix 
5B, section 5B.3.1; ISA, Appendix 6, sections 6.3.4, 6.3.5 and Table 6-
21). A study involving both S and N additions greater than 20 kg/ha-yr 
for each substance reported increased growth rate for sugar maple but 
not for the second species (Bethers et al., 2009), while another study 
of similar dosing of S and N reported reduced growth in three species 
after 10 years that resolved in two of the species after 22 years 
(Jensen et al., 2014). In both situations, background deposition 
contributions were also appreciable, e.g., greater than 6 kg N/ha-yr 
(PA, Appendix 5B, Table 5B-1). Uncertainties associated with these 
analyses include the extent to which the studies reflect steady-state 
conditions. Given the variability in the durations across these studies 
and the relatively short durations for some (e.g., less than five 
years), it might be expected that steady-state conditions have not been 
reached, such that the S/N loading is within the buffering capacity of 
the soils. With regard to N addition, the available studies have 
reported mixed results for growth and survival for several eastern 
species including oaks, spruce, maples and pines (PA, Table 5B-1; 
Magill et al., 2004; McNulty et al., 2005; Pregitzer et al., 2008; 
Wallace et al., 2007). Some studies have suggested that this variation 
in responses is related to the dominant mycorrhizal association of the 
species (e.g., Thomas et al., 2010). It is not clear the extent to 
which such findings may be influenced by species-specific sensitivities 
or soils and trees already impacted by historic deposition, or other 
environmental factors.
    With regard to S deposition, the two large observational studies 
that analyzed growth and/or survival measurements in tree species at 
sites in the eastern U.S. or across the country report negative 
associations of tree survival and growth with the S deposition metric 
for nearly half the species individually and negative associations of 
tree survival for 9 of the 10 species' functional type groupings 
(Dietze and Moorcroft, 2011; Horn et al., 2018 \44\). Interestingly, 
survival for the same 9 species groups was also negatively associated 
with long-term average ozone (Dietze and Moorcroft, 2011). The S 
deposition metrics for the two studies were mean annual average 
deposition estimates for total S or sulfate (wet deposition) during 
different, but overlapping, time periods of roughly 10-year durations. 
The full range of average SO42- deposition 
estimated for the 1994-2005 time period assessed by Dietze and 
Moorcroft (2011) for the eastern U.S. study area was 4 to 30 kg S 
ha-1yr-1. The second study covered the more 
recent time period (2000-2013) and 71 species distributed across the 
U.S. To draw on this study with regard to S deposition levels of 
interest, the distribution of S deposition estimates for each species 
were considered in the PA (Appendix 5B, section 5B.2 and Attachments 2A 
and 2B). In this national-scale analysis, the range of median S 
deposition for sites of those species for which negative associations 
with growth or survival were reported, was 5 to 12 kg S 
ha-1yr-1, with few exceptions (Horn et al., 
2018).\45\
---------------------------------------------------------------------------

    \44\ The study by Horn et al. (2018) constrained the S analyses 
to preclude a positive association with S.
    \45\ This range is for median S deposition estimates (based on 
measurement interval average, occurring within 2000-2013) of 
nonwestern species with negative associations with growth or 
survival ranged (Horn et al., 2018).
---------------------------------------------------------------------------

    Regarding N deposition, the three large observational studies that 
analyzed growth and/or survival measurements in tree species at sites 
in the northeastern or eastern U.S., or across the country, report 
associations of tree survival and growth with several N deposition 
metrics (Dietze and Moorcroft, 2011; Thomas et al., 2010; Horn et al., 
2018). Estimates of average N deposition across the full set of sites 
analyzed by Thomas et al. (2010) in 19 states in the northeastern 
quadrant of the U.S. ranged from 3 to 11 kg N/ha-yr for the period 
2000-2004. The N deposition metrics for these three studies were mean

[[Page 26648]]

annual average deposition estimates for total N or nitrate (wet 
deposition) during different, but overlapping, time periods that varied 
from 5 to more than 10 years. The full range of average NO3 
deposition estimated for the 1994-2005 time period assessed by Dietze 
and Moorcroft (2011) for the eastern U.S. study area was 6 to 16 kg N 
ha-1yr-1. Median N deposition estimated 
(measurement interval average [falling within 2000-13]) at sites of 
nonwestern species for which associations with growth or survival were 
negative (either over full range or at median for species) ranged from 
7 to 12 kg N ha-1yr-1 (Horn et al., 2018).
    In considering what can be drawn from these studies with regard to 
deposition levels of potential interest for tree species effects, such 
as the ranges identified above, a number of uncertainties are 
recognized. For example, several factors were not accounted for that 
have potential to influence tree growth and survival. Although 
O3 was analyzed in one of the three studies, soil 
characteristics and other factors with potential to impact tree growth 
and survival (other than climate) were not assessed, contributing 
uncertainty to their interpretations. Further, differences in findings 
for the various species (or species' groups) may relate to differences 
in geographic distribution of sampling locations, which may contribute 
to differences in ranges of deposition history, geochemistry etc.
    Additionally, as noted above, the extent to which associations 
reflect the influence of historical deposition patterns and associated 
impact is unknown. There is a general similarity in findings among the 
studies, particularly of Horn et al. (2018) and Dietze and Moorcroft 
(2011), even though the time period and estimation approach for S and N 
deposition differ (PA, Appendix 5B, Table 5B-6). The extent to which 
the differences in growth or survival across sites with different 
deposition estimates are influenced by historically higher deposition 
(e.g., versus the magnitude of the average over the measurement 
interval) is unknown. Given the role of deposition in causing soil 
conditions that affect tree growth and survival, and a general 
similarity of spatial variation of recent deposition to historic 
deposition, an uncertainty associated with quantitative interpretation 
of these studies is the extent to which the similarity in the two 
studies' finding may indicate the two different metrics to both be 
reflecting geographic variation in impacts stemming from historic 
deposition. Although the spatial deposition patterns across the full 
time period are somewhat similar, the magnitudes of S and N deposition 
in the U.S. have changed appreciably over the time period covered by 
these studies (e.g., PA, Appendix 5B, Figures 5B-9 through 5B-12).\46\ 
The appreciable differences in deposition magnitude across the time 
periods also contribute uncertainty to interpretations related to 
specific magnitudes of deposition associated with patterns of tree 
growth and survival. There are few studies of recovery in historically 
impacted areas in the ISA that might address such uncertainties (e.g., 
ISA, section IS.4.1, IS.5.1, IS.11.2).
---------------------------------------------------------------------------

    \46\ Sulfur deposition in the U.S. across the full period of 
these studies (1994-2013) generally exhibited a consistent pattern 
of appreciable declines, with study plots, particularly in the East, 
having experienced decades of much higher S deposition in the past. 
Similarly, N deposition during the combined time period of the 
studies (1994-2013) has also changed, with many areas experiencing 
declines and a few areas experiencing deposition increases for some 
N species and in total N (PA, section 6.2.1).
---------------------------------------------------------------------------

(2) Deposition Studies of Herbs, Shrubs and Lichens
    The available studies that may inform our understanding of exposure 
conditions, including N deposition levels, of potential risk to herb, 
shrub and lichen communities include observational or gradient studies 
and experimental addition conducted in different parts of the U.S. 
Among the studies of plant communities are observational studies of 
herbaceous species richness at sites in a multi-state study area and of 
grassland or coastal sage scrub communities in southern California, and 
experimental addition experiments in several western herb or shrub 
ecosystems. The experimental addition studies indicate effects on 
community composition associated with annual N additions of 10 kg N/ha-
yr (in the context of background deposition on the order of 6 kg N/ha-
yr) and higher (PA, section 5.3.3.1). Experiments involving additions 
of 5 kg N/ha-yr variously reported no response or increased cover for 
one species (in context of background deposition estimated at 5 kg N/
ha-yr). The landscape-level analysis of coastal sage scrub community 
history in southern California observed a greater likelihood of 
recovery of sites with relatively low invasion of exotic invasive 
grasses when the N deposition metric level was below 11 kg N/ha-yr. 
Lastly, the multi-state analysis of herbaceous species richness 
reported a negative association with N deposition metric values above 
8.7 kg N/ha-yr at open-canopy sites and above 6.5 kg/ha-yr and low pH 
sites. In forested sites, negative associations were found above 11.6 
kg N/ha-yr in sites with acidic soil pH at or above 4.5 (PA, section 
5.3.3).
    Limitations and associated uncertainties vary between the two types 
of studies (experimental addition and observational). Both are limited 
with regard to consideration of the impacts of long-term deposition. 
While there are some experimental addition studies lasting more than 20 
years, many are for fewer than 10 years. Additionally, such studies are 
necessarily limited with regard to the number and diversity of species 
and ecosystems that can be analyzed. In the case of observational 
studies, decades of S and N deposition--and elevated levels of airborne 
pollutants, including ozone and nitrogen oxides, in the U.S.--have 
affected the ecosystems studied; and these studies generally have not 
accounted for the influence of historical deposition on the 
associations observed with more recent deposition metrics. Further, 
given that observational studies occur in real time, there is 
uncertainty associated with characterization, including quantification, 
of the particular exposure conditions that may be eliciting patterns of 
ecosystem metrics observed.
    The few studies of lichen species diversity and deposition-related 
metrics, while contributing to the evidence that relates deposition, 
including acidic deposition in eastern locations, to relative abundance 
of different lichen species, are more limited with regard to the extent 
that they inform an understanding of specific exposure conditions in 
terms of deposition levels that may elicit specific responses. A number 
of factors limit such interpretations of the currently available 
studies (PA, section 5.3.3.2). These factors include uncertainties 
related to the methods employed to represent N deposition, the 
potential role of other unaccounted-for environmental factors 
(including O3, SO2, S deposition and historical 
air quality and associated deposition), and uncertainty concerning the 
independence of any effect of deposition levels from residual effects 
of past patterns of deposition. We additionally note the information on 
exposure conditions associated with effects on lichen species of oxides 
of N such as HNO3 in section II.C.3.c below.
c. Direct Effects of N Oxides, SOX and PM in Ambient Air
    The evidence related to exposure conditions for other effects of 
SOX, N oxides and PM in ambient air includes concentrations 
of SO2 and NO2 associated with effects on plants,

[[Page 26649]]

concentrations of NO2 and HNO3 associated with 
effects on plants and lichens, and quite high concentrations of PM that 
affect plant photosynthesis. The PM effects described in the evidence 
are nearly all related to deposition. With regard to oxides of N and S, 
we note that some effects described may be related to dry deposition of 
SO2 and HNO3 onto plant and lichen surfaces. 
These exposure pathways would be captured in observational studies and 
could also be captured in some fumigation experiments.
    With regard to SO2, the evidence comes from an array of 
studies, primarily field studies for the higher concentrations 
associated with visible foliar injury and laboratory studies for other 
effects. With regard to oxides of N, the evidence indicates that 
effects on plants and lichens occur at much lower exposures to 
HNO3 (than to NO2). The laboratory and field 
studies of oxides of N vary regarding their limitations; field studies 
are limited with regard to identification of threshold exposures for 
the reported effects, and uncertainties associated with controlled 
experiments include whether the conditions under which the observed 
effects occur would be expected in the field. With regard to the 
latter, the elevated concentrations of NO2 and 
HNO3 in the Los Angeles area in the 1970s-90s are well 
documented as is the decline of lichen species in the Los Angeles Basin 
during that time (PA, section 5.4.2).\47\ Such an assessment of lichen 
species and elevated concentrations of N oxides is not available for 
other locations across the U.S.
---------------------------------------------------------------------------

    \47\ For example, concentrations of HNO3 reported in 
forested areas of California in the 1980s ranged up to 33 ug/m\3\, 
and annual average NO2 concentrations in the Los Angeles 
area ranged from 0.078 ppm in 1979 to 0.053 ppm in the early 1990s 
(PA, section 5.4.2). Ambient air concentrations of HNO3 
in the Los Angeles metropolitan area have declined markedly, as 
shown in Figure 2-23 of the PA, which compares concentrations at 
CASTNET monitoring sites between 2019 and 1996 (PA, section 2.4.1).
---------------------------------------------------------------------------

    Ecological effects associated with SOX in ambient air 
include effects on vegetation, such as foliar injury, depressed 
photosynthesis, and reduced growth or yield. Within the recently 
available information are observational studies reporting increased 
tree growth in association with reductions in SO2 emissions. 
These studies, however, do not generally report the SO2 
concentrations in ambient air or account for the influence of changes 
in concentrations of co-occurring pollutants such as ozone (ISA, 
Appendix 3, section 3.2). The available data for direct effects are 
drawn from experimental studies or observational studies in areas near 
sources, with the most studied effect being visible foliar injury to 
various trees and crops (ISA, Appendix 3, section 3.2; 1982 AQCD, 
section 8.3). With regard to foliar injury, the current ISA states 
there to be ``no clear evidence of acute foliar injury below the level 
of the current standard'' (ISA, p. IS-37). Few studies report yield 
effects from acute exposures, with the available ones reporting 
relatively high concentrations, such as multiple hours with 
concentrations above 1 ppm (1982 AQCD, section 8.3). Effects have also 
been reported on photosynthesis and other functions in a few lichen 
species groups, although recovery of these functions was observed from 
short, multi-hour exposures to concentrations below about 1 ppm (ISA, 
Appendix 3, section 3.2).
    The direct welfare effects of N oxides in ambient air include 
effects on plants and lichens. For plants, studies reported in the ISA 
did not report effects on photosynthesis and growth resulting from 
exposures of NO2 concentrations below 0.1 ppm (ISA, Appendix 
3, section 3.3). The information is more limited regarding exposures to 
other oxides of N.
    The evidence for HNO3 includes controlled exposure 
studies describing foliar effects on several tree species. Nitric acid 
has also been found to deposit on and bind to the leaf or needle 
surfaces. Studies of ponderosa pine, white fir, California black oak 
and canyon live oak involving continuous chamber exposure over a month 
to 24-hour average HNO3 concentrations generally ranging 
from 10 to 18 [mu]g/m\3\ (moderate treatment) or 18 to 42 [mu]g/m\3\ 
(high treatment), with the average of the highest 10% of concentrations 
generally ranging from 18 to 42 [mu]g/m\3\ (30-60 [mu]g/m\3\ peak) or 
89 to 155 [mu]g/m\3\ (95-160 [mu]g/m\3\ peak), resulted in damage to 
foliar surfaces of the 1 to 2-year old plants (ISA, Appendix 3, section 
3.4; Padgett et al., 2009). The moderate treatment reflects exposure 
concentrations observed during some summer periods in the Los Angeles 
Basin in the mid-1980s, including a high HNO3 concentration 
of 33 ug/m\3\ in August 1986 (Padgett et al., 2009; Bytnerowicz and 
Fenn, 1996), when annual average NO2 concentrations in the 
Basin ranged up to 0.058 ppm (U.S. EPA, 1987).
    In addition to the observational studies of lichen communities in 
the Los Angeles Basin impacted by ambient air concentrations from the 
70s, the available evidence for lichens includes a recent laboratory 
study, involving daily HNO3 exposures for 18 to 78 days, 
with daily peaks near 50 ppb (~75 [mu]g/m\3\) that reported decreased 
photosynthesis, among other effects (ISA, Appendix 6, section 6.2.3.3; 
Riddell et al., 2012). Based on studies extending back to the 1980s, 
HNO3 has been suspected to have had an important role in the 
dramatic declines of lichen communities that occurred in the Los 
Angeles basin (ISA, Appendix 3, section 3.4; Nash and Sigal, 1999; 
Riddell et al., 2008; Riddell et al., 2012). For example, lichen 
transplanted from clean air habitats to analogous habitats in the Los 
Angeles basin in 1985-86 were affected in a few weeks by mortality and 
appreciable accumulation of H\+\ and NO3- (ISA, 
Appendix 3, section 3.4; Boonpragob et al., 1989). The Los Angeles 
metropolitan area experienced NO2 concentrations well in 
excess of the NO2 secondary standard during this period. For 
example, annual average NO2 concentrations in Los Angeles 
ranged up to 0.078 ppm in 1979 and remained above the standard level of 
0.053 ppm into the early 1990s (PA, Appendix 5B, section 5B.4.1). Over 
the last several decades, the magnitude of both dry deposition of 
HNO3 and annual average HNO3 concentration in 
this area, and nationally, and the spatial extent of high deposition 
rates and concentrations have dramatically declined (PA, Figure 2-23; 
ISA, Appendix 2, Figure 2-60). The evidence indicates NO2, 
and particularly HNO3, as ``the main agent of decline of 
lichen in the Los Angeles basin'' (ISA, Appendix 3, p. 3-15), thus 
indicating a role for the elevated concentrations of nitrogen oxides 
documented during the 1970s to 1990s (and likely also occurring 
earlier). More recent studies indicate variation in eutrophic lichen 
abundance to be associated with variation in N deposition metrics (ISA, 
Appendix 6, section 6.2.3.3). The extent to which these associations 
are influenced by residual impacts of historic air quality is unclear 
and the extent to which similar atmospheric conditions and ecological 
relationships exist in other locations in the U.S. is uncertain.
    Little information is available on welfare effects of airborne PM 
at concentrations commonly occurring in the U.S. today, and the 
available information does not indicate effects to occur under such 
conditions. The extent to which quantitative information is available 
for airborne PM concentrations associated with ecological effects 
varies for the different types of effects. The concentrations at which 
PM has been reported to affect vegetation (e.g., through effects on 
leaf surfaces, which may affect function, or through effects

[[Page 26650]]

on gas exchange processes) are generally higher than those associated 
with conditions meeting the current standards and may be focused on 
specific particulate chemicals rather than on the mixture of chemicals 
in PM occurring in ambient air (ISA, Appendix 15, sections 15.4.3 and 
15.4.6). Studies involving ambient air PM have generally involved 
conditions that are much higher than those common to the U.S. today 
(ISA, Appendix 15, sections 15.4.3 and 15.4.4).

D. Quantitative Exposure/Risk Assessment for Aquatic Acidification

    Our consideration of the scientific evidence available in the 
current review is informed by results from quantitative analyses of 
estimated acidic deposition and associated risk of aquatic 
acidification (PA, section 5.1 and Appendix 5A). These REA analyses, 
like those in the last review, make use of well-established modeling 
tools and assessment approaches for this endpoint. Other categories of 
effects of S and N deposition have been the subject of quantitative 
analyses, both in the last review (e.g., terrestrial acidification) and 
in other contexts (e.g., eutrophication of large rivers and estuaries), 
each with associated complexities and specificity (section II.C.3 
above). The PA while focusing the new analyses on aquatic acidification 
risks, as summarized here, also draws on findings of available analyses 
for the other categories of effects (e.g., as summarized in section 
II.C.3 above).
    The REA analyses, summarized here and presented in detail in 
Appendix 5A of the PA, have focused on ANC as an indicator of aquatic 
acidification risk (PA, section 5.1 and Appendix 5A). This focus is 
consistent with such analyses performed in the 2012 review and with the 
longstanding evidence that continues to demonstrate a causal 
relationship between S and N deposition and alteration of freshwater 
biogeochemistry and between acidifying S and N deposition and changes 
in biota, including physiological impairment and alteration of species 
richness, community composition, and biodiversity in freshwater 
ecosystems (ISA, Table ES-1), as summarized in section II.C.1 above.
    Section II.D.1 summarizes key aspects of the assessment design, 
including the conceptual approach and tools, indicator reference or 
benchmark concentrations, the assessment scales, study areas and 
waterbodies analyzed, and exposure and risk metrics derived. Key 
limitations and uncertainties associated with the assessment are 
identified in section II.D.2 and the exposure and risk estimates are 
summarized in section II.D.3. An overarching focus of these analyses is 
characterization of aquatic acidification risk in sensitive ecoregions 
associated with different deposition conditions.
1. Key Design Aspects
    The REA for this review entailed a multi-scale analysis of 
waterbodies in the contiguous U.S. that assessed waterbody-specific 
aquatic acidification at three spatial scales: national, ecoregion, and 
case study area (PA, Appendix 5A). The assessment involved evaluation 
of deposition and water quality response at the waterbody site level, 
which has been summarized at the national, ecoregion, and case study 
level. The national-scale analysis involved waterbody sites across the 
U.S. for which relevant data were available.\48\ The ecoregion-scale 
analysis focused on waterbodies in a set of 25 ecoregions generally 
characterized as acid-sensitive; and the more localized case study-
scale analysis focused on waterbodies in five case study areas across 
the U.S., within each of which were Class I areas.
---------------------------------------------------------------------------

    \48\ The national-scale analysis focused on the contiguous U.S. 
as there are insufficient data available for Hawaii, Alaska, and the 
territories. Of the four hierarchical levels of ecoregion 
categorization, the REA utilized level III which divides the 
contiguous U.S. into 84 ecoregions (Omernik and Griffith, 2014). The 
69 of these 84 ecoregions in which there was at least one site with 
sufficient data comprised the national scale.
---------------------------------------------------------------------------

    The impact of acidifying S or N deposition estimated for five 
different time periods (2001-03, 2006-08, 2010-12, 2014-16 and 2018-20) 
was evaluated using a CL approach that relied on waterbody location-
specific CL estimates derived for other applications and available in 
the National Critical Loads Database (NCLD) \49\ (PA, Appendix 5A). The 
CL estimates used in the assessment were largely based on steady-state 
modeling, and the modeling applications focused on ANC, producing CL 
estimates (acidifying deposition in terms of kg/ha-yr or meq/m\2\-yr 
[milliequivalents per square meter per year] for S and N compounds) for 
different ANC concentrations. Of the 84 ecoregions in the contiguous 
U.S., 64 have at least one waterbody site with a CL estimate (PA, 
Appendix 5A). Given its common use in categorizing waterbody 
sensitivity, ANC was used as the indicator of acidification risk in 
this assessment (PA, section 5.1.2.2). Deposition estimates, as 3-year 
averages of annual TDep estimates, were compared to the CL estimates 
for three different ANC benchmark concentrations (targets or 
thresholds), in recognition of the watershed variability and associated 
uncertainties, as an approach for characterizing aquatic acidification 
risk (PA, section 5.1).
---------------------------------------------------------------------------

    \49\ The NCLD is comprised of CLs calculated from several common 
models: (1) steady-state mass-balance models such as the Steady-
State Water Chemistry (SSWC), (2) dynamic models such as Model of 
Acidification of Groundwater In Catchments (MAGIC) (Cosby et al., 
1985) or Photosynthesis EvapoTranspiration Biogeochemical model 
(PnET-BGC) (Zhou et. al., 2015) run out to year 2100 or 3000 to 
model steady-state conditions and (3) regional regression models 
that use results from dynamic models to extrapolate to other 
waterbodies (McDonnell et. al., 2012; Sullivan et al., 2012a). Data 
and CL estimates in the NCLD are generally focused on waterbodies 
impacted by deposition-driven acidification and are described in 
documentation for the database version (PA, section 5.1.2.3; Lynch 
et al., 2022).
---------------------------------------------------------------------------

    The available evidence and scientific judgments were considered in 
identifying the three ANC benchmark concentrations: 20 [mu]eq/L, 30 
[mu]eq/L, 50 [mu]eq/L (PA, section 5.1.2.2). Selection of these 
benchmark ANC concentrations reflects several considerations. For 
example, most aquatic CL studies conducted in the U.S. since 2010 use 
an ANC of 20 and/or 50 [mu]eq/L, because 20 [mu]eq/L has been suggested 
to provide protection for a ``natural'' or ``historical'' \50\ range of 
ANC, and 50 [mu]eq/L to provide greater protection, particularly from 
episodic acidification events \51\ (Dupont et al., 2005; Fakhraei et 
al., 2014; Lawrence et al., 2015; Lynch et al., 2022; McDonnell et al., 
2012, 2014; Sullivan et al., 2012a, 2012b). For example, levels below 
20 [mu]eq/L have been associated with fish species reductions in some 
sensitive waterbodies of the Shenandoah and Adirondack Mountains. 
Levels of ANC ranging from 30 to 40 [mu]eq/L have been reported to 
provide sufficient buffering to withstand acidic inputs associated with 
episodic springtime rain or snowmelt events. An ANC value of 50 [mu]eq/
L has often been cited in the literature as a target for many areas, 
and in the 2012 review, ANC values at or above 50 [mu]eq/L were 
described as providing an additional level of protection although with 
increasingly greater uncertainty for values at/above

[[Page 26651]]

75 [mu]eq/L \52\ (2011 PA, pp. 7-47 to 7-48). In the western U.S., 
lakes and streams vulnerable to deposition-driven aquatic acidification 
are often found in the mountains where surface water ANC levels are 
naturally low and typically vary between 0 and 30 [mu]eq/L (Williams 
and Labou, 2017; Shaw et al., 2014). For these reasons, this 
assessment, also develops results for an ANC threshold of 50 [mu]eq/L 
for sites in the East and 20 [mu]eq/L for sites in the West (denoted as 
``50/20'' [mu]eq/L).\53\ Thus, the set of benchmark concentrations used 
in this REA include ANC concentrations that are naturally occurring in 
many areas and also include concentrations that, depending on watershed 
characteristics, may provide additional buffering in times of episodic 
acidification events.
---------------------------------------------------------------------------

    \50\ For example, dynamic modeling simulations in acid-sensitive 
streams of the southern Blue Ridge Mountains have predicted all 
streams to have pre-industrial time ANC levels above 20 [mu]eq/L, 
while also predicting more than a third of the streams to have pre-
industrial ANC levels below 50 [mu]eq/L (Sullivan et al., 2011).
    \51\ As noted in section II.C.1 above, events such as spring 
snowmelt and heavy rain events can contribute to episodic 
acidification events. For example, in some impacted northeastern 
waterbodies, particularly headwater streams, ANC levels may dip 
below zero for hours to days or weeks in response to such events, 
while waterbodies labeled chronically acidic have ANC levels below 
zero throughout the year (ISA, Appendix 6, section 6.1.1.1; Driscoll 
et al., 2001).
    \52\ In considering higher ANC levels (e.g., up to 80 [mu]eq/L 
and higher), it was also recognized that many waterbodies, 
particularly in acid-sensitive regions of contiguous never had an 
ANC that high and would never reach an ANC that high naturally 
(Williams and Labou 2017; Shaw et al., 2014; PA, section 5.1.2.2). 
Additionally, in conveying its advice in the 2012 review, the CASAC 
expressed its view that ``[l]evels of 50 [mu]eq/L and higher would 
provide additional protection, but the Panel has less confidence in 
the significance of the incremental benefits as the level increases 
above 50 [mu]eq/L'' (Russell and Samet, 2010a; pp. 15-16).
    \53\ This approach is also used in multiple studies and the NCLD 
(PA, section 5.1.2.2).
---------------------------------------------------------------------------

    Since acidification of waterbodies is controlled by local factors 
such as geology, hydrology, and other landscape factors, aquatic CLs 
for acidification were determined at the waterbody level (based on 
site-specific data) and then summarized at the national, ecoregion, and 
case study level. National-scale analyses were performed using two 
approaches: one considering acid deposition of N and S compounds 
combined and one for S deposition only. Findings from these analyses 
indicated that across the five different time periods analyzed, the 
percent of waterbodies exceeding their CLs was similar for the two 
approaches (PA, Appendix 5A, sections 5A.1.6.2 and 5A.2.1). Thus, to 
facilitate interpretation of the results, further analysis of the 
results focused on the findings for S only deposition.
    Critical load estimates for specific waterbody sites across the 
contiguous U.S. were drawn from the NCLD (version 3.2.1) \54\ for 
comparison to total deposition estimates in the same locations for the 
five time periods. Comparisons were only performed for sites at which 
CL estimates were greater than zero, indicating that achievement of the 
associated target ANC would be feasible.\55\ The results of these 
analyses are summarized with regard to the spatial extent and severity 
of deposition-related acidification effects and the protection from 
these effects associated with a range of annual S deposition.
---------------------------------------------------------------------------

    \54\ A waterbody is represented as a single CL value. In many 
cases, a waterbody has more than one CL value calculated for it 
because different studies determined a value for the same waterbody. 
When more than one CL exists, the CL from the most recent study was 
selected, while the CL values were averaged when the publications 
are from the same timeframe (PA, appendix 5A, section 5A.1.5).
    \55\ Critical load estimates are estimates of the S deposition 
rate at which a particular waterbody site is estimated to be able to 
achieve a specified ANC level. A CL estimate at or below zero would 
indicate that no S deposition estimate would provide for such a 
result.
---------------------------------------------------------------------------

    The ecoregion-scale analyses focused on 25 ecoregions,\56\ 18 in 
the East and 7 in the West. Ecoregions are areas of similarity 
regarding patterns in vegetation, aquatic, and terrestrial ecosystem 
components. The 25 ecoregions in this analysis each had more than 50 
waterbody sites (or locations) for which a prior modeling application 
had developed a CL estimate, which was available in the NCLD (PA, 
section 5A.2.2.2). Although a total of 32 ecoregions had more than 50 
CL sites,\57\ four in the West were excluded as having very low 
deposition that resulted in no CL exceedances across the complete 20-
year analysis period. An additional three ecoregions (i.e., 
Southeastern Plains, Southern Coastal Plain, and Atlantic Coastal Pine 
Barrens) were excluded as they are known to have naturally acidic 
surface waters, and the low CL estimates for these ecoregions (and 
resulting CL exceedances) are likely driven by natural acidity linked 
to high levels of dissolved organic carbon, hydrology, and natural 
biogeochemical processes rather than atmospheric deposition (2008 ISA, 
section 3.2.4.2; Baker et al., 1991; Herlihy et al., 1991).
---------------------------------------------------------------------------

    \56\ The ecoregion classification scheme used to group waterbody 
sites into ecoregions is based on that described in Omernik (1987), 
which classifies regions through the analysis of the patterns and 
the composition of biotic and abiotic characteristics that affect or 
reflect differences in ecosystem quality and integrity (e.g., 
geology, physiography, vegetation, climate, soils, land use, 
wildlife, and hydrology).
    \57\ In light of the size of the level III ecoregions, 50 was 
identified as an appropriate minimum number of CL sites within an 
ecoregion to include it in the analysis.
---------------------------------------------------------------------------

    The case study scale represents the smallest scale at which CLs and 
their comparison to deposition estimates were summarized and is 
intended to give some insight into potential local impacts of aquatic 
acidification. Five case study areas across the U.S. were examined: 
Shenandoah Valley Area, White Mountain National Forest, Northern 
Minnesota, Sierra Nevada Mountains, and Rocky Mountain National Park 
(details presented in PA, section 5.1.3.3 and Appendix 5A, section 
5A.2.1). These areas include a number of national parks and forests 
that vary in their sensitivity to acidification, but represent high 
value or protected ecosystems, such as Class 1 areas, wilderness, and 
national forests (PA, Appendix 5A, section 5A.2.1). The most well 
studied of these, the Shenandoah Valley Area case study, includes the 
Class I area, Shenandoah National Park, and waterbodies in each of 
three ecoregions. The number of waterbody sites with CLs available in 
the NCLD for the Shenandoah study area (4,977 sites) is nearly an order 
of magnitude greater than the total for the four other areas combined 
(524 sites).
    The analyses at different scales differed in how results were 
summarized and evaluated. For example, at the national scale, 
percentages of water bodies with deposition estimates exceeding their 
CLs were reported for each of the five time periods for which 
deposition was assessed. From the case study scale analyses, we focused 
primarily on the distribution of CL estimates in each study area. In so 
doing, the CLs for each case study area were characterized in terms of 
the average, and two lower percentiles (e.g., the 30th percentile CL, 
which is the value below 70% of the CL estimates for that study area, 
and the 10th percentile).
    In the ecoregion-scale analyses, percentages of waterbody sites per 
ecoregion that exceeded their estimated CLs and percentages of 
waterbody sites that fell at or below them--for each of the three ANC 
thresholds--were summarized by ecoregion for each of the five time 
periods: 2001-2003, 2006-2008, 2010-2012, 2014-2016 and 2018-2020 (PA, 
section 5.1.3.2 and Appendix 5A, section 5A.2.2). Percentages of 
waterbody sites that did not exceed their estimated CLs were described 
as achieving the associated ANC threshold (or target). These results 
were then considered in two ways. The first is based on a binning of 
this dataset of ecoregion-time period combinations and percentages by 
ecoregion median deposition levels (at/below 5 kg/ha-yr, at/below 6 kg/
ha-yr, etc). The second approach involved summarizing ecoregion-
specific trends in percentage of waterbodies per ecoregion estimated to 
achieve the three threshold or target ANC values (or estimated to 
exceed the associated CLs).
2. Key Limitations and Uncertainties
    We have characterized the nature and magnitude of associated 
uncertainties and their impact on the REA estimates

[[Page 26652]]

based primarily on a mainly qualitative approach, informed by several 
quantitative sensitive analyses (PA, Appendix 5A, section 5A.3). The 
mainly qualitative approach used to characterize uncertainty here and 
in quantitative analyses in other NAAQS reviews is described by World 
Health Organization (WHO, 2008). Briefly, with this approach, we have 
identified key aspects of the assessment approach that may contribute 
to uncertainty in the conclusions and provided the rationale for their 
inclusion. Then, we characterized the magnitude and direction of the 
influence on the assessment for each of these identified sources of 
uncertainty. Consistent with the WHO (2008) guidance, we scaled the 
overall impact of the uncertainty by considering the degree of 
uncertainty as implied by the relationship between the source of 
uncertainty and the exposure and risk estimates. A qualitative 
characterization of low, moderate, and high was assigned to the 
magnitude of influence and knowledge base uncertainty descriptors, 
using quantitative observations relating to understanding the 
uncertainty, where possible. Where the magnitude of uncertainty was 
rated low, it was judged that large changes within the source of 
uncertainty would have only a small effect on the assessment results 
(e.g., an impact of few percentage points upwards to a factor of two). 
A designation of medium implies that a change within the source of 
uncertainty would likely have a moderate (or proportional) effect on 
the results (e.g., a factor of two or more). A characterization of high 
implies that a change in the source would have a large effect on 
results (e.g., an order of magnitude). We also included the direction 
of influence, whether the source of uncertainty was judged to 
potentially over-estimate (``over''), under-estimate (``under''), or 
have an unknown impact to exposure/risk estimates.
    Two types of quantitative analyses informed our understanding of 
the variability and uncertainty associated with the CL estimates 
developed in this assessment and support the overall uncertainty 
characterization. The first type of analysis is a sensitivity analysis 
using Monte Carlo techniques to quantify CL estimate uncertainty 
associated with several model inputs, and the second is an analysis of 
the variation in CL estimates among the three primary modeling 
approaches on which the CLs used in this assessment were based.
    As overarching observations regarding uncertainty associated with 
this REA, we note two overarching aspects of the assessment. The first 
relates to interpretation of specific thresholds of ANC, and the second 
to our understanding of the biogeochemical linkages between deposition 
of S and N compounds and waterbody ANC, and the associated estimation 
of CLs. While ANC is an established indicator of aquatic acidification 
risk, there is uncertainty in our understanding of relationships 
between ANC and risk to native biota, particularly in waterbodies in 
geologic regions prone to waterbody acidity. Such uncertainties relate 
to the varying influences of site-specific factors other than ANC, such 
as soil type. Uncertainty associated with our understanding of the 
biogeochemical linkages between deposition and ANC and the 
determination of steady-state CLs is difficult to characterize and 
assess. Uncertainty in CL estimates is associated with parameters used 
in the steady-state CL models. While the Steady-State Water Chemistry 
(SSWC) and other CL models are well conceived and based on a 
substantial amount of research and applications available in the peer-
reviewed literature, there is uncertainty associated with the 
availability of the necessary data to support certain model components.
    The strength of the CL estimates and the exceedance calculation 
rely on the ability of models to estimate the catchment-average base-
cation supply (i.e., input of base cations from weathering of bedrock 
and soils and air), runoff, and surface water chemistry. Key parameters 
in this modeling include estimates of the catchment-average base-cation 
supply (i.e., input of base cations from weathering of bedrock and 
soils and air), runoff, and surface water chemistry. The uncertainty 
associated with runoff and surface water parameters relates to 
availability of measurements; however, the ability to accurately 
estimate the catchment supply of base cations to a water body is still 
difficult and uncertain (PA, Appendix 5A, section 5A.3). This area of 
uncertainty is important because the catchment supply of base cations 
from the weathering of bedrock and soils is the factor with the 
greatest influence on the CL calculation and has the largest 
uncertainty (Li and McNulty, 2007). For example, the well-established 
models generally rely on input or simulated values for BCw rate, a 
parameter the ISA notes to be ``one of the most influential yet 
difficult to estimate parameters in the calculation of critical acid 
loads of N and S deposition for protection against terrestrial 
acidification'' (ISA, section IS.14.2.2.1). Obtaining accurate 
estimates of weathering rates is difficult because weathering is a 
process that occurs over very long periods of time and the estimates on 
an ecosystem's ability to buffer acid deposition rely on accurate 
estimates of weathering. Although the approach to estimate base-cation 
supply for the national case study (e.g., F-factor approach) has been 
widely published and analyzed in Canada and Europe, and has been 
applied in the U.S. (e.g., Dupont et al., 2005 and others), the 
uncertainty in this estimate is unclear and could be large in some 
cases.
    In light of the significant contribution of this input to the CL 
estimates, a quantitative uncertainty analysis of CL estimates based on 
state-steady CL modeling was performed (PA, Appendix 5A, section 
5A.3.1). This analysis, involving many model simulations for the more 
than 14,000 waterbodies, drawing on Monte Carlo sampling, provided a 
description of the uncertainty around the CL estimate in terms of the 
confidence interval for each waterbody mean result. The size of the 
confidence interval for S CL estimates ranged from 0.1 kg S/ha-yr at 
the 5th percentile to 5.3 kg S/ha-yr at the 95th percentile. Smaller 
confidence intervals were associated with CLs determined with long-term 
water quality data and low variability in runoff measurements. 
Estimates of CL determined by one or very few water quality 
measurements, and in areas where runoff is quite variable (e.g., the 
western U.S.), had larger confidence intervals, indicating greater 
uncertainty. Critical load estimates with the lowest uncertainty were 
for waterbody sites in the eastern U.S., particularly along the 
Appalachian Mountains, in the Upper Midwest, and in the Rocky 
Mountains, which are areas for which there are relatively larger site-
specific datasets (e.g., for water quality parameters). Greater 
uncertainty is associated with CLs in the Midwest and South and along 
the CA to WA coast. This uncertainty in the Midwest is associated with 
most of the CLs in waterbodies in this area being based on one or a few 
water quality measurements, while the high uncertainty for sites along 
the CA and WA coasts relates to variability in runoff values. On 
average, the size of the confidence interval for the vast majority of 
CLs (those based on the widely used steady-state water chemistry model) 
was 7.68 meq S/m\2\-yr or 1.3 kg S/ha-yr, giving a confidence interval 
of 3.84 meq/m\2\-yr or 0.65 kg S/ha-yr. While a 
comprehensive analysis of uncertainty had not been completed for these

[[Page 26653]]

estimates prior to this assessment, judgment by EPA experts suggested 
the uncertainty for combined N and S CLs to be on average about 0.5 kg/ha-yr (3.125 meq/m\2\-yr), which is generally consistent 
with the range of uncertainty determined from this quantitative 
uncertainty analysis (PA, Appendix 5A, section 5A.3).
    At the ecoregion scale, 51 ecoregions had sufficient data to 
calculate the 5th to 95th percentile (PA, Appendix 5A, Table 5A-56). 
Smaller confidence intervals around the mean CL (i.e., lower 
uncertainty CLs) were associated with ecoregions in the Appalachian 
Mountains (e.g., Northern Appalachian and Atlantic Maritime Highlands, 
Blue Ridge, Northern Lakes and Forests, and North Central Appalachians) 
and Rockies (e.g., Sierra Nevada, Southern Rockies, and Idaho 
Batholith). Ecoregions with more uncertain CLs included the 
Northeastern Coastal Zone, Cascades, Coast Range, Interior Plateau, and 
Klamath Mountains/California High North Coast Range.
    Although the vast majority of CLs in this assessment were based on 
the SSWC model, an analysis was conducted to understand differences in 
the CLs calculated with the different methods. There are three main CL 
approaches all based on the watershed mass-balance approach where acid-
base inputs are balanced. The three approaches include: (1) SSWC model 
and F-Factor that is based on quantitative relationships to water 
chemistry (Dupont et al., 2005; Scheffe et al., 2014; Lynch et al., 
2022), (2) Statistical Regression Model that extrapolated weathering 
rates across the landscape using water quality or landscape factors 
(Sullivan et al., 2012b; McDonnell et al., 2014), and (3) Dynamic 
Models (Model of Acidification of Groundwater In Catchments [MAGIC)] or 
Photosynthesis EvapoTranspiration Biogeochemcial model [Pnet-BGC]). 
Critical load values were compared between these models to determine 
model biases. Results from the comparison between different CL methods 
that were used to calculate the critical loads in the NCLD are 
summarized in PA Appendix 5A, section 5A.3.1, for lakes in New England 
and the Adirondacks and streams in the Appalachian Mountains. Overall, 
good agreement was found between the three methods used to calculate 
CLs, indicating there was not a systematic bias between the methods and 
that they should produce comparable results when used together as they 
were in these analyses (PA, Appendix 5A, section 5A.3).
3. Summary of Results
    The findings from the aquatic acidification REA are summarized in 
terms of S deposition due to the finding of a negligible additional 
influence of N deposition compared to S deposition on acidification in 
this assessment \58\ (Appendix 5A, section 5A.2.1). As summarized more 
fully below, the analyses of five case study areas, including the 
acidification-impacted Shenandoah Valley area, indicate that with 
annual average S deposition below 12 and 10 kg/ha yr, the average 
waterbody in each area (average as to acid-sensitivity) would be 
estimated to achieve the ANC benchmarks of 20 and 50 [micro]eq/L, 
respectively. Seventy percent of waterbodies in each area would be 
estimated to achieve these benchmarks with deposition below 10 and 7 
kg/ha-yr, respectively. At the ecoregion-scale, the results from the 
analysis of 25 ecoregions, dominated by acid-sensitive waterbodies, 
indicate acid buffering capabilities to have improved substantially 
over the past 20 years, and particularly between the first and second 
decades of the period. By the 2010-2012 period, the percentages of 
waterbodies achieving the three ANC benchmarks in all 25 ecoregions 
exceeded 80%, 80% and 70% (for 20, 30 and 50 [micro]eq/L, 
respectively). By the subsequent analysis period (2014-2016), these 
percentages were 90%, 80% and 80%. The ecoregion median annual average 
deposition in all 25 ecoregions was below 8 kg/ha-yr for 2010-2012 and 
below 5 kg/ha-yr for 2014-2016. An alternate approach to analyzing 
these estimates (for the 25 ecoregions across all five time periods) 
suggested that the three ANC benchmarks could be met in more than 80%, 
80% and 70% of waterbodies per ecoregion in all ecoregions and time 
periods for which annual average ecoregion median deposition is 
estimated to be at or below 7 kg/ha-yr.
---------------------------------------------------------------------------

    \58\ More specifically, the percentage of waterbodies across the 
contiguous U.S. estimated to exceed a CL for combined total S and N 
are very similar or just slightly higher (e.g., by 1-2%) than S only 
percentages of the waterbodies estimated to not meet the ANC 
benchmarks. This indicates that most of the N deposition entering 
the watershed is retained within the watershed and/or converted to 
gaseous N (PA, Appendix 5A, section 5A.2.1).
---------------------------------------------------------------------------

    Between the three-year period of 2000-2002, which was the analysis 
year for the 2009 REA, and 2018-2020, the latest period considered in 
the REA for this review, national average sulfur deposition has 
declined appreciably across the U.S. This decline in deposition is 
reflected in the very different aquatic acidification impact estimates 
for the two periods. Unlike the findings for 2000-2002 in the 2009 REA, 
in the national-scale analysis of the current REA, few waterbody sites 
are estimated to be receiving deposition in excess of their CLs for 
relevant ANC targets under recent deposition levels. While recognizing 
inherent limitations and associated uncertainties of any such analysis, 
the national-scale assessment performed as part of the current review 
indicates that under deposition scenarios for the 2018-2020 time 
period, the percentage of waterbodies nationwide that might not be able 
to maintain an ANC of 50 [micro]eq/L is less than 5% (table 1; PA, 
Table 5-1).

Table 1--Percentage of Waterbodies Nationally for Which Annual Average S Deposition During the Five Time Periods
         Assessed Exceed the Waterbody CL (for CLs Greater Than 0) for Each of the Specified ANC Targets
----------------------------------------------------------------------------------------------------------------
        ANC ([micro]eq/L)          2001-2003 (%)   2006-2008 (%)   2010-2012 (%)   2014-2016 (%)   2018-2020 (%)
----------------------------------------------------------------------------------------------------------------
20..............................              22              16               5               3               1
30..............................              25              19               7               4               2
50..............................              28              24              11               6               4
50/20 *.........................              28              23              10               6               4
----------------------------------------------------------------------------------------------------------------
* This combination refers to the use of a target of 50 [micro]eq/L in eastern ecoregions and 20 [micro]eq/L in
  western ecoregions.


[[Page 26654]]

    The case study analyses provide estimates of S deposition (with 
associated uncertainties) that might be expected to allow these 
geographically diverse locations to meet the three ANC targets (20, 30 
and 50 [micro]eq/L). Focusing on the three eastern case studies, the CL 
modeling indicates that at an annual average S deposition of 9-10 kg/
ha-yr, the sites in these areas, on average,\59\ might be expected to 
achieve an ANC at or above 50 [micro]eq/L. At an annual average S 
deposition of about 6-9 kg/ha-yr, 70% of the sites in the areas are 
estimated to achieve an ANC at or above 20 [micro]eq/L and at about 5-8 
kg S/ha-yr, 70% are estimated to achieve an ANC at or above 30 
[micro]eq/L. Lower S deposition values are estimated to achieve higher 
ANC across more sites. Across the three eastern areas, the CL estimates 
for each ANC target are lowest for the White Mountains National Forest 
study area, and highest for the Shenandoah Valley study area.
---------------------------------------------------------------------------

    \59\ The term ``average'' here refers to the average CL 
estimated for the specified ANC across all sites with CL estimates 
in each case study area (PA, Table 5-6).
---------------------------------------------------------------------------

    The ecoregion-level analyses of ANC levels and deposition estimates 
for the five periods from 2001-2003 through 2018-2020 illustrate the 
spatial variability and magnitude of the findings for several target 
ANC levels (50, 30 and 20 [micro]eq/L) and the temporal changes across 
the 20-year period, as described in the PA, section 5.1.3.2. For 
example, during the two most recent 3-year periods, the ecoregion 
median S deposition estimates in 2014-16 were below 5 kg/ha-yr in all 
25 ecoregions in the analysis and the estimates for 2018-20 were all 
below 4 kg/ha-yr (table 2). Although the ecoregion S deposition 
estimates in the 18 eastern ecoregions analyzed were all below 5 kg/ha-
yr in the two most recent time periods (2014-16 and 2018-20), the full 
dataset of five time periods ranges from below 2 up to nearly 18 kg/ha-
yr.

     Table 2--Summary of Ecoregion Medians Derived as Median of S Deposition Estimates at CL Sites Within an
                                                    Ecoregion
----------------------------------------------------------------------------------------------------------------
                                                     Ecoregion median * total sulfur deposition (kg S/ha-yr)
                                                ----------------------------------------------------------------
                                                   2001-03      2006-08      2010-12      2014-16      2018-20
----------------------------------------------------------------------------------------------------------------
All 25 Ecoregions:
    Minimum....................................         1.18         1.22         1.02         1.08         0.62
    Maximum....................................        17.27        14.44         7.25         4.58         3.88
    Median.....................................         7.77         6.50         3.71         2.32         1.73
18 Eastern Ecoregions:
    Minimum....................................         4.01         3.10         2.34         1.88         1.31
    Maximum....................................        17.27        14.44         7.25         4.58         3.88
    Median.....................................        11.08         9.36         4.76         2.97         2.04
7 Western Ecoregions:
    Minimum....................................         1.18         1.22         1.02         1.08         0.62
    Maximum....................................         1.94         1.83         1.47         1.56         1.19
    Median.....................................         1.40         1.52         1.29         1.17         0.87
----------------------------------------------------------------------------------------------------------------
* The ecoregion medians for which descriptive statistics are presented here are medians of the deposition
  estimates across each ecoregion's waterbody sites with CL estimates.

    The ecoregion-scale results (e.g., percentage of waterbodies per 
ecoregion estimated to achieve the various ANC targets, or 
alternatively to exceed the associated CLs) for the 18 eastern and 7 
western ecoregions are summarized in two ways. One approach, summarized 
further below, is framed by the temporal trends in deposition, and the 
second approach is in terms of ecoregion-time period combinations, 
using ecoregion deposition estimates (medians of deposition estimates 
at waterbodies with CLs in each ecoregion) as the organizing parameter. 
For example, table 3 presents the percentages of waterbody sites per 
ecoregion estimated to achieve the three ANC target levels, summarized 
by bins for different magnitudes of ecoregion median annual average S 
deposition (regardless of the 3-year period in which it occurred). For 
the 18 eastern ecoregions and five time periods, there are 90 
ecoregion-time period combinations, and for each of these, there are 
waterbody percentages for each of the three ANC targets. In table 3, 
the combinations are binned by the ecoregion median S deposition 
associated with that ecoregion and time period. As can be seen from 
this table, fewer than half of the eastern ecoregion-time period 
combinations had an S deposition estimate at or below 4 kg/ha-yr (table 
2).\60\ Table 3 indicates that lower levels of S deposition are 
associated with improved ANC values and greater percentages of 
waterbodies expected to reach ANC targets. Across the ecoregion-time 
period dataset of CL exceedances for the three ANC targets for all 90 
eastern ecoregion-time period combinations (for which ecoregion median 
S deposition was at or below 18 kg/ha-yr), 73% of the combinations had 
at least 90% of waterbodies per ecoregion estimated to achieve ANC at 
or above 20 [micro]eq/L, and 60% had at least 90% of the waterbodies 
estimated to achieve ANC at or above 50 [micro]eq/L (table 3). For S 
deposition estimates at or below 9 kg/ha-yr (approximately three 
quarters of the combinations), at least 90% of all waterbodies per 
ecoregion were estimated to achieve ANC at or above 20, 30 and 50 
[micro]eq/L in 87%, 81% and 72% of combinations, respectively. For S 
deposition estimates at or below 5 kg S/ha-yr (the lowest ecoregion 
median deposition bin that includes at least half of the full dataset), 
these values are 96%, 92% and 82% of combinations. For the 75 western 
ecoregion-time period combinations, all of which had an S deposition 
estimate below 4 kg/ha-yr, at least 90% of waterbodies per ecoregion 
were estimated to achieve an ANC at or above 50 [micro]eg/L (table 3; 
PA, Table 5-5).
---------------------------------------------------------------------------

    \60\ The ecoregion median S deposition in all seven of the 
western ecoregions in all five time periods were at or below 2 kg/
ha-yr (PA, Table 5-4).

[[Page 26655]]



 Table 3--Percentage of Ecoregion-Time Periods Combinations With at Least 90, 85, 80, 75 and 70% of Waterbodies Estimated To Achieve an ANC At/Above the
                     ANC Targets of 20, 30 and 50 [micro]eq/L as a Function of Annual Average S Deposition for 18 Eastern Ecoregions
                                                         [90 Ecoregion-time period combinations]
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                    Number of                     % Waterbodies per ecoregion-time period meeting specified ANC target
Total sulfur deposition (kg S/ha-  ecoregion- ----------------------------------------------------------------------------------------------------------
          yr) at/below:               time
                                     periods    90%    85%    80%    75%     70%    90%    85%    80%    75%     70%    90%    85%    80%    75%    70%
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                  ANC target of 20 [micro]eq/L
                                        ANC target of 30 [micro]eq/L
                                        ANC target of 50 [micro]eq/L
--------------------------------------------------------------------------------------------------------------------------------------------------------
2................................          10    100    100    100    100    100     100    100    100    100    100     100    100    100    100    100
3................................          29    100    100    100    100    100     100    100    100    100    100      97    100    100    100    100
4................................          41    100    100    100    100    100      95    100    100    100    100      93     98    100    100    100
5................................          51     96     98    100    100    100      92     98    100    100    100      82     94     96     98    100
6................................          59     93     98    100    100    100      88     98    100    100    100      78     93     97     98    100
7................................          63     92     98    100    100    100      87     97    100    100    100      78     92     95     98    100
8................................          67     87     94    100    100    100      82     91     99    100    100      73     87     93     96    100
9................................          69     87     94    100    100    100      81     91     99    100    100      72     87     93     96    100
10...............................          73     85     92     99     99     99      78     89     97     99     99      70     85     92     95     99
11...............................          76     83     91     97     99     99      76     88     96     99     99      68     83     91     95     99
12...............................          79     81     89     95     96     97      73     86     94     96     96      66     81     89     92     96
13...............................          81     80     88     95     96     98      73     85     94     96     96      65     80     88     93     96
14...............................          84     77     86     93     95     96      70     83     92     94     95      63     79     86     90     94
15...............................          86     76     84     91     93     95      69     81     90     92     93      62     77     84     88     92
16...............................          88     75     83     90     92     94      68     81     89     91     92      61     76     83     88     91
17...............................          88     75     83     90     92     94      68     81     89     91     92      61     76     83     88     91
18...............................          90     73     81     88     90     92      67     79     87     89     90      60     74     81     86     89
--------------------------------------------------------------------------------------------------------------------------------------------------------

    Given the decreasing temporal trend in S deposition across all 
ecoregions, we also analyzed the aquatic acidification results at the 
ecoregion scale across the 20 years represented by the five time 
periods (2001-03, 2006-08, 2010-12, 2014-16, 2018-20) from a temporal 
perspective. With regard to percentages of waterbodies per ecoregion 
estimated to achieve the three ANC targets, an appreciable improvement 
is observed for the latter three time periods compared to the initial 
two time periods (e.g., PA, Figure 5-13). By the 2010-2012 time period, 
more than 70% of waterbodies in all 25 ecoregions are estimated to 
achieve an ANC at or above 50 [micro]eq/L and at least 85% are able to 
achieve an ANC at or above 20 [micro]eq/L (Figure 1; PA, Table 7-2). By 
the 2014-2016 period, the percentages are 85% and nearly 90%, 
respectively. The median deposition for the CL sites in each of the 18 
eastern ecoregions during the latter three time periods range from 1.3 
kg S/h-yr to 7.3 kg S/h-yr and with each reduction in S deposition in 
each subsequent time period, more waterbodies in each of the eastern 
ecoregions are estimated to be able to achieve the ANC targets. Nearly 
90% of the 18 eastern ecoregions are estimated to have at least 90% of 
their waterbodies achieving an ANC of 20 [micro]eq/L in the 2010-12 
period and achieving an ANC of 50 [micro]eq/L in the 2014-16 period. 
When the 7 western ecoregions are included in a summary based on ANC 
targets of 20 [micro]eq/L for the West and 50 [micro]eq/L for the 
East,\61\ over 70% of the full set of ecoregions are estimated to have 
at least 90% of their waterbodies achieving the ANC targets by the 
2010-12 period. More than 90% of the ecoregions are estimated to have 
at least 90% of their waterbodies achieving the ANC targets by the 
2014-16 period (Figure 1; PA, Table 7-2).
---------------------------------------------------------------------------

    \61\ This combination of targets recognizes the naturally and 
typically low ANC levels observed in western waterbodies while also 
including a higher target for the East (as described in the PA, 
section 5.1.2.2).
---------------------------------------------------------------------------

BILLING CODE 6560-50-P

[[Page 26656]]

[GRAPHIC] [TIFF OMITTED] TP15AP24.000


[[Page 26657]]


BILLING CODE 6560-50-C

E. Proposed Conclusions

    In reaching his proposed decision on the current secondary 
standards for SOX, N oxides and PM (presented in section 
II.E.3), the Administrator has taken into account policy-relevant, 
evidence-based and air quality-, exposure- and risk-based 
considerations discussed in the PA (summarized in section II.E.1), as 
well as advice from the CASAC, and public comment on the standard 
received thus far in the review (section II.E.2). In general, the role 
of the PA is to help ``bridge the gap'' between the Agency's assessment 
of the current evidence and quantitative analyses of air quality, 
exposure, and risk, and the judgments required of the Administrator in 
determining whether it is appropriate to retain or revise the NAAQS. 
Evidence-based considerations draw upon the EPA's integrated assessment 
of the scientific evidence presented in the ISA (summarized in section 
II.C above) to address key policy-relevant questions in the review. 
Similarly, the air quality-, exposure- and risk-based considerations 
draw upon our assessment of air quality, exposure, and associated risk 
(summarized in section II.D above).
    This approach to reviewing the secondary standards is consistent 
with requirements of the provisions of the CAA related to the review of 
the NAAQS and with how the EPA and the courts have historically 
interpreted the CAA. As discussed in section I.A above, these 
provisions require the Administrator to establish secondary standards 
that, in the Administrator's judgment, are requisite (i.e., neither 
more nor less stringent than necessary) to protect the public welfare 
from known or anticipated adverse effects associated with the presence 
of the pollutant in the ambient air. Consistent with the Agency's 
approach across all NAAQS reviews, the EPA's approach to informing 
these judgments is based on a recognition that the available welfare 
effects evidence generally reflects a continuum that includes ambient 
air-related exposures for which scientists generally agree that effects 
are likely to occur through lower levels at which the likelihood and 
magnitude of response become increasingly uncertain. The CAA does not 
require the Administrator to establish secondary standards at a zero-
risk level, but rather at levels that reduces risk sufficiently so as 
to protect the public welfare from known or anticipated adverse 
effects.
    The proposed decision on the secondary standards for 
SOX, N oxides and PM described below is a public welfare 
policy judgment by the Administrator that draws upon the scientific 
evidence for welfare effects, quantitative analyses of air quality, 
exposure, and risks, as available, and judgments about how to consider 
the uncertainties and limitations that are inherent in the scientific 
evidence and quantitative analyses. The four basic elements of the 
NAAQS (i.e., indicator, averaging time, form, and level) have been 
considered collectively in evaluating the public welfare protection 
afforded by the current standards. The Administrator's final decision 
will additionally consider public comments received on this proposed 
decision.
1. Evidence- and Exposure/Risk-Based Considerations in the Policy 
Assessment
    The PA presents an evaluation of the evidence and quantitative 
analyses of air quality, exposure and potential risk related to 
ecological effects of SOX, N oxides and PM. These ecological 
effects include both direct effects of the three criteria pollutants on 
biota, and ecological effects of ecosystem deposition of N and S 
associated with these pollutants. Because the role of the PA is to 
provide the broadest array of options for consideration consistent with 
the scientific information, the PA presents multiple policy options for 
consideration by the Administrator in this review of the secondary 
NAAQS for SOX, N oxides and PM. These options, which are 
only briefly summarized here, are discussed in detail in section 7.4 of 
the PA, including with regard to the varying strength of support 
provided for each by the current evidence and quantitative analyses. 
For SOX, the PA options identified include adoption of an 
annual average SO2 standard, averaged over three years, with 
a level within the range extending below 15 ppb and down to 5 ppb. 
Based on consideration of the available air quality analyses indicating 
that such an annual standard could be expected to also provide 
appropriate control for short-term concentrations, the PA recognizes 
options that would either revise the existing 3-hour secondary standard 
to an annual standard or augment it with an annual standard.
    For N oxides and PM2.5, the PA recognizes options for 
retention of the existing standards, without revision, and also options 
for revision, although with recognition of appreciable associated 
uncertainty. For N oxides, the PA recognizes the options of retaining 
the existing secondary NO2 standard (with its annual average 
concentration of 53 ppb) or revising the existing standard level to 
within a range below 53 ppb to as low as 40-35 ppb, in combination with 
consideration of a form that entails averaging the annual average 
across three consecutive years. With regard to PM, the PA recognizes 
options of either retaining the existing suite of secondary standards 
or revising the current annual secondary PM2.5 standard 
level to within a range below 15 [micro]g/m\3\ to as low as 12 
[micro]g/m\3\.
    The PA additionally considered the potential for establishment of a 
revised secondary standard or suite of standards with alternate 
indicator(s) that might target specific chemicals that deposit N and S 
(e.g., particulate NO3-, 
SO42-, NH4\+\), but recognized there 
to be a number of associated uncertainties and complications that 
include uncertainties in relationships between concentrations near 
sources and in areas of deposition. Based on the currently available 
data and analyses, the PA did not find there to be advantages or 
benefits to these alternate indicators over those for the established 
indicators, while also noting that establishing a standard based on one 
or more of these indicators would require the establishment of new or 
updated regulatory monitoring networks and measurement methods that 
would require additional time and resources (PA, sections 7.2 and 7.4).
    The PA additionally recognizes that, as is the case in NAAQS 
reviews in general, decisions by the Administrator on the adequacy of 
existing standards or the appropriateness of new or revised standards 
will depend on a variety of factors, including science policy judgments 
and public welfare policy judgments. These factors include public 
welfare policy judgments concerning the appropriate benchmarks on which 
to place weight, as well as judgments on the public welfare 
significance of the effects that have been observed at the exposures 
evaluated in the welfare effects evidence. The factors relevant to 
judging the adequacy of the standard also include the interpretation 
of, and decisions as to the weight to place on, different aspects of 
the quantitative REA and air quality-deposition information and 
analyses, and associated uncertainties. Thus, the Administrator's 
conclusions regarding the secondary standards for SOX, N 
oxides and PM will depend in part on public welfare policy judgments, 
science policy judgments regarding aspects of the evidence and 
exposure/risk estimates, and judgments about the level of public 
welfare protection that is requisite under the Clean Air Act.
    The subsections below summarize key considerations from the PA. 
These focus first on consideration of the evidence, as evaluated in the 
ISA (and supported by the prior ISA and AQCDs), including

[[Page 26658]]

that newly available in this review, and the extent to which it alters 
the EPA's overall conclusions regarding ecological effects of 
SOX, N oxides and PM, both regarding direct effects on biota 
and regarding ecological effects of ecosystem deposition of N and S 
compounds. The PA also considers the available information related to 
the general approach or framework in which to evaluate public welfare 
protection of the standard. Additionally, the PA considers the 
currently available quantitative information regarding environmental 
exposures likely to occur in areas of the U.S. where the standards are 
met. In so doing, the PA considers associated limitations and 
uncertainties, and the significance of these exposures with regard to 
the potential for effects, their potential severity and any associated 
public welfare implications. The PA also considers judgments about the 
uncertainties inherent in the scientific evidence and quantitative 
analyses that are integral to consideration of whether the currently 
available information supports or calls into question the adequacy of 
the current secondary standards.
a. Direct Effects on Biota
    In considering the currently available evidence and quantitative 
information pertaining to ecological effects of SOX, N 
oxides and PM in ambient air, other than those associated with 
ecosystem deposition of S and N, the PA focused on several aspects. 
These include the extent to which the newly available information 
alters our scientific understanding of the ecological effects of 
SOX, N oxides and PM in ambient air; the extent to which the 
currently available information indicates the potential for exposures 
associated with ecological effects under air quality meeting the 
existing standards and whether such effects might be of sufficient 
magnitude, severity, extent and/or frequency such that they might 
reasonably be judged to be adverse to public welfare; and to what 
extent important uncertainties identified in past reviews have been 
reduced and/or whether new uncertainties emerged. These considerations 
are summarized below, first for SOX, followed by N oxides 
and then PM.
(1) Sulfur Oxides
    As summarized in section II.C.1 above, the previously available 
evidence base describes the direct effects of SOX in ambient 
air on vegetation and very little of the currently available 
information is newly available in this review. Among the gaseous 
SOX--which include SO, SO2, SO3, and 
S2O--only SO2 is present in the lower troposphere 
at concentrations relevant for environmental considerations (ISA, 
Appendix 2, section 2.1). Sulfate is the prominent S oxide present in 
the particulate phase. The available evidence, largely comprising 
studies focused on SO2, documents the effects of 
SO2 on vegetation, including foliar injury, depressed 
photosynthesis and reduced growth or yield (ISA, Appendix 3, section 
3.2). The newer studies continue to support the determination that the 
evidence is sufficient to infer a causal relationship between gas-phase 
SO2 and injury to vegetation (ISA, section 3.6.1).
    In general, direct effects on plants occur at SO2 
exposures higher than a 3-hour average concentration of 0.5 ppm (500 
ppb). The evidence derives from a combination of laboratory studies and 
observational studies. With regard to the sensitive effect of foliar 
injury, the current ISA finds ``no clear evidence of acute foliar 
injury below the level of the current standard'' (ISA, p. IS-37). 
Further, the ``limited new research since 2008 adds more evidence that 
SO2 can have acute negative effects on vegetation but does 
not change conclusions from the 2008 ISA regarding . . . the 
SO2 levels producing these effects'' (ISA, p. IS-37).
    Uncertainties associated with the current information are generally 
similar to those existing at the time of the last review. In large 
part, these uncertainties relate to limitations of experimental studies 
in reflecting the natural environment and limitations of observational 
studies in untangling effects of SO2 from those of other 
pollutants that may have influenced the analyzed effects. Regardless of 
these uncertainties, the evidence indicates effects are generally 
associated with air concentrations and durations not expected to occur 
when the existing standard is met (PA, section 7.1.1; ISA, Appendix 2, 
section 2.1)
(2) Nitrogen Oxides
    The currently available information on direct effects of gaseous N 
oxides in ambient air is composed predominantly of studies of 
NO2 and HNO3, and also of PAN, with regard to 
effects on plants and lichens (as summarized in section II.C.1 above). 
The very few studies newly available in this review do not alter our 
prior understanding of effects of these N oxides, which include visible 
foliar injury and effects on photosynthesis and growth at exposures 
considered high relative to current levels in ambient air (ISA, section 
3.3). Thus, as in the last review, the ISA again concludes that the 
body of evidence is sufficient to infer a causal relationship between 
gas-phase NO, NO2, and PAN and injury to vegetation (ISA, 
section IS.4.2).
    With regard to NO2 ambient air exposure concentrations, 
the newly available information does not alter prior conclusions 
regarding exposure conditions associated with visible injury and 
effects on plant photosynthesis or growth. The 1993 AQCD for N oxides 
concluded that concentrations of NO, NO2, and PAN in the 
atmosphere are rarely high enough to have phytotoxic effects on 
vegetation, and since that document, very little new research has been 
performed on these phytotoxic effects at concentrations currently 
observed in the U.S. (ISA, Appendix 3, sections 3.3 and 3.6.2; U.S. 
EPA, 1993). Further, there is ``little evidence in recent years to 
suggest that PAN poses a significant risk to vegetation in the U.S.'' 
(ISA, Appendix 3, p. 3-13).
    Regarding another N oxide compound, HNO3, in ambient 
air, the previously available evidence included experimental studies of 
leaf cuticle damage in tree seedlings, a finding confirmed in a more 
recent study, and also studies of effects on lichens. Effects of 
HNO3 may be related to vapor exposures or direct contact via 
deposition (PA, section 7.1.2; ISA, Appendix 3, section 3.4). The 
evidence also includes studies of effects related to historic 
conditions in the Los Angeles Basin that indicate N oxides, and 
particularly HNO3, to be ``the main agent of decline of 
lichen in the Los Angeles [B]asin'' (ISA, Appendix 3, p. 3-15). A 
reassessment in 2008 found that lichen communities have not recovered 
from the damage evident in the 1970s, although the extent to which this 
reflects residual impacts of earlier effects is unknown (PA, section 
7.1.2; ISA, Appendix 3, section 3.4). The newer studies continue to 
support the findings of the 2008 ISA, such that as in the last review, 
the ISA again concludes ``the body of evidence is sufficient to infer a 
causal relationship between gas-phase HNO3 and changes to 
vegetation'' (ISA, section 4.3).
    The recently available information for HNO3 includes 
effects on tree foliage under controlled 12-hour exposures to 50 ppb 
HNO3 (approximately 75 [micro]g/m\3\) and in longer, 32- or 
33-day exposures in which peak HNO3 concentrations for the 
``moderate'' treatment (30-60 [micro]g/m3) encompassed the range 
reported in summers during the 1980s in the Los Angeles Basin (ISA, 
Appendix 3, section 3.4). During that period, NO2 
concentrations in the Basin ranged up to 0.058 ppm, exceeding the 
secondary standard (PA, section 5.4.2; U.S. EPA,

[[Page 26659]]

1987). Effects on lichen photosynthesis have been reported from daily 
6.5-hour varying exposures with peaks near 50 ppb (~75 [micro]g/m\3\) 
lasting longer than 18 days (ISA, Appendix 6, section 6.2.3.3; Riddell 
et al., 2012).
    In considering the potential for concentrations of N oxides, 
including HNO3, that are associated with ecological effects 
to occur under air quality conditions meeting the current 
NO2 standard, the PA noted that air quality at all ambient 
air monitoring sites in the contiguous U.S. has met the existing 
secondary NO2 standard since around 1991 (PA, Figure 2-22). 
In considering the potential for HNO3 concentrations of a 
magnitude sufficient to pose risk of effects to occur under conditions 
that meet the current NO2 secondary standard, the PA also 
considered the magnitude of NO2 concentrations in the Los 
Angeles Basin. During the 1970s to 1990s, the Los Angeles metropolitan 
area experienced NO2 concentrations in excess of the 
NO2 secondary standard (e.g., annual average concentrations 
up to 0.078 ppm in 1979 and above 0.053 ppm into the early 1990s). At 
the time of the 2008 reassessment mentioned above, which reported that 
impacts documented on lichen communities in the 1970s still remained, 
NO2 concentrations were well below the standard (PA, section 
7.1.2; ISA, Appendix 3, section 3.4), although the extent to which this 
finding relates to a lag in recovery or concurrent air pollutant 
concentrations is unknown. The PA notes that the risk of 
HNO3 effects to lichens may be from both direct and 
deposition-related exposure related to direct contact of the chemical 
to the lichen surfaces (PA, section 7.1.2).
    In summary, the currently available information is somewhat limited 
with regard to the extent to which it informs conclusions on the 
potential for ambient air exposures associated with ecological effects 
under air quality meeting the existing NO2 secondary 
standard. More recent studies also indicate variation in eutrophic 
lichen abundance to be associated with variation in metrics 
representing N deposition, although the extent to which these 
associations are influenced by residual impacts of the historic air 
quality is unclear (ISA, Appendix 6, section 6.2.3.3; PA, sections 
5.3.3.2 and 7.1.2). While new uncertainties have not emerged, 
uncertainties remain in our interpretation of the evidence, including 
those related to limitations and associated uncertainties of the 
various study types. A key uncertainty affecting interpretation of 
studies of historic conditions in the LA Basin relates to the extent to 
which other air pollutants or local conditions may have contributed to 
the observations. With regard to the risk posed by N oxides, and 
particularly HNO3, the evidence summarized in the ISA 
indicates the potential for effects on lichen species related to air 
quality occurring during periods when the current secondary standard 
was not met. The evidence is limited, however, in support of 
conclusions of effects under conditions meeting the current standard 
(PA, section 7.1.2).
(3) Particulate Matter
    As summarized in section II.C above, the evidence for ecological 
effects of PM is consistent with that available in the last review. The 
causal determinations with regard to ecological effects of PM in the 
2013 p.m. review (2009 p.m. ISA) and in this review (2020 ISA) focused 
on effects associated with PM loading (e.g., to leaf surfaces), rather 
than direct effects of PM suspended in ambient air. In this review, as 
in the last one, the ecological effects evidence was found to be 
sufficient to conclude there is likely to exist a causal relationship 
between deposition of PM and a variety of effects on individual 
organisms and ecosystems (ISA, Appendix 15; 2012 p.m. ISA, section 
9.4).
    With regard to direct effects of PM in ambient air, the available 
information indicates effects occurring only at ambient air 
concentrations well in excess of the existing secondary standards. 
While some uncertainties remain, new uncertainties have not emerged 
since the last review. In summary, little information is available on 
effects of PM under generally lower PM concentrations in ambient air 
likely to occur under conditions meeting the current secondary 
standards, and the limited available information does not indicate 
effects to occur under those conditions (PA, section 7.1.3).
b. Evidence of Ecosystem Effects of S and N Deposition
    The evidence base of ecological effects related to atmospheric 
deposition of N and S compounds has expanded since the last review, as 
summarized above, and continues to be strong in documenting roles of 
SOX, N oxides and PM (including N and S compounds) in 
aquatic acidification, nutrient enrichment and other effects, as 
summarized in section II.C.1 above. A long-standing evidence base 
documents the array of effects of both acidic deposition in aquatic and 
terrestrial ecosystems and ecosystem N enrichment. The evidence for 
acid deposition effects, extending back many decades, has accrued in 
part through study of ecosystem acidification that has resulted from 
many decades of acidifying deposition (ISA, section ES.5.1 and Appendix 
4, section 4.6). As noted in section II.C and II.D above, both S and N 
compounds have contributed to ecosystem acidification, with relative 
contributions varying with emissions, air concentrations, and 
atmospheric chemistry, among other factors. Ecological effects have 
been documented comprehensively in waterbodies of the Adirondack and 
Appalachian Mountains, and in forests of the Northeast, at the organism 
to ecosystem scale. With regard to N enrichment, research on its 
effects in estuaries and large river systems across the U.S. extends 
back at least four decades, and there is longstanding evidence of 
effects in estuaries along the East and Gulf Coasts of the U.S., as 
summarized in more detail in Chapters 4 and 5 of the PA (ISA, Appendix 
7, section 7.2.9; 2008 ISA, section 3.3.2.4; Officer et al., 1984). 
Additionally, the evidence base on the effects of N enrichment in 
terrestrial ecosystems, primarily in grassland and forested ecosystems, 
includes evidence that was available in the last review (e.g., 2008 
ISA, sections 3.3.3 and 3.3.5).
    Some uncertainties associated with the evidence available in the 
2012 review remain, and some additional important uncertainties have 
been identified. In addition to uncertainties related to the specific 
air quality circumstances associated with effects (e.g., magnitude, 
duration, and frequency of concentrations associated with effects), 
there are also uncertainties associated with the effects of N and S 
deposition expected under changing environmental circumstances. Such 
uncertainties include atmospheric loading that has declined since 2000, 
with associated changes to soil and waterbody biogeochemistry and 
meteorological changes associated with changing climate (ISA, section 
IS.12; PA section, 7.2.1). The PA also recognizes important 
uncertainties associated with the various assessment approaches 
employed by different study types (PA, sections 5.3 and 7.2.1). For 
example, uncertainties associated with observational studies include 
uncertainty regarding the potential influence of historical deposition 
on species distribution, richness, and community composition observed 
in recent times (ISA, section IS.14.2.1). Additionally, there are 
uncertainties contributed by variation in physical, chemical, and 
ecological responses to N and S deposition, and by the potential 
influence of unaccounted-for stressors

[[Page 26660]]

on response measures. Uncertainties associated with addition 
experiments \62\ include, among others, those related to the potential 
for effects to occur over longer periods than those assessed in those 
studies (PA, section 5.3.4.1).
---------------------------------------------------------------------------

    \62\ Addition experiments generally refers to field experiments 
where compounds (e.g., NO3- or 
SO42- in acidification experiments) are added 
(generally annually) to the soil of large forested (for tree 
studies) areas and the tree measurements (e.g., growth rate) are 
compared to those in an untreated or control area.
---------------------------------------------------------------------------

    Lastly, studies reporting atmospheric deposition rates associated 
with effects involve authors' judgments regarding the magnitude of 
responses considered to be effects, and may also be limited by a lack 
of clarity as to references or baselines from which responses are 
assessed and with regard to judgments associated with reference or 
baseline conditions. As noted in the ISA, ``[t]he majority of studies 
that evaluate terrestrial N CLs for N enrichment effects are based on 
observed response of a biological receptor to N deposition (or N 
addition as a proxy for deposition), without a known soil chemistry 
threshold that causes the biological effect'' (ISA, p. IS-113).\63\ 
Variability in physical, chemical, and ecological characteristics of 
ecosystems also contribute uncertainty to such judgments (PA, section 
7.2.1).
---------------------------------------------------------------------------

    \63\ In describing critical loads developed from observational 
studies (or empirical data), the ISA recognizes distinctions from 
other studies, as seen in the following excerpt (ISA, p. IS-113). 
The majority of studies that evaluate terrestrial N CLs for N 
enrichment effects are based on observed response of a biological 
receptor to N deposition (or N addition as a proxy for deposition), 
without a known soil chemistry threshold that causes the biological 
effect. In contrast, CLs for acidification are typically based on 
the deposition amount that gives rise to a soil chemical indicator 
value which is known to cause an adverse biological effect. The link 
between soil chemical indicator and biological effect is based on 
empirical evidence (Appendix 5). The relationship between deposition 
and the biogeochemistry that causes effects on soil chemistry is 
typically modeled (Appendix 4; section IS.14.2).
---------------------------------------------------------------------------

    In sum, a wealth of scientific evidence, spanning many decades, 
demonstrates effects of acidifying deposition associated with N and S 
compounds in aquatic and terrestrial ecosystems (ISA, sections ES.5.1, 
IS.5.1, IS.5.3, IS.6.1 and IS.6.3; 2008 ISA, section 3.2; U.S. EPA, 
1982b, Chapter 7). This evidence base supports conclusions also reached 
in the 2008 ISA (for the review completed in 2012) of causal 
relationships between N and S deposition and alteration of soil and 
aquatic biogeochemistry, alteration of the physiology and growth of 
terrestrial organisms and of associated productivity, changes in 
aquatic biota, including physiological impairment, and alteration of 
species richness, community composition, and biodiversity in both 
aquatic and terrestrial ecosystems (ISA, Table ES-1). Similarly, a 
robust evidence base demonstrates effects of N enrichment in both 
estuarine and freshwater ecosystems, supporting conclusions also 
reached in the last review of a causal relationship between N 
deposition and changes in biota, including altered growth and 
productivity, and alteration of species richness, community composition 
and biodiversity due to N enrichment (ISA, sections ES.5.2, IS.6, and 
IS.7, and Table ES-1). Additional effects of N deposition in wetlands, 
also recognized in the last review, include alteration of 
biogeochemical cycling, growth, productivity, species physiology, 
species richness, community composition, and biodiversity (ISA, Table 
ES-1).
    In terrestrial ecosystems, as in the last review, the now expanded 
evidence base supports determination of a causal relationship between N 
deposition and alteration of species richness, community composition, 
and biodiversity (ISA, Table ES-1). The ISA additionally determines 
there to be a causal relationship for alteration of the physiology and 
growth of terrestrial organisms and associated productivity, a category 
of effects not included in the 2008 ISA (ISA, Table ES-1). Other 
evidence of effects causally associated with S deposition in wetland 
and freshwater ecosystems includes that related to chemical 
transformation and associated toxicity, most specifically alteration of 
mercury methylation, which was also recognized in the last review. The 
other category of effects, not included in the last review, is related 
to sulfide phytotoxicity and its associated effects in wetland and 
freshwater ecosystems (ISA, Table ES-1).
    Thus, while an array of effects is associated with S and N 
deposition, information important for quantitative analysis varies 
across the array. For some categories of effects (e.g., sulfide 
phytotoxicity) the information is limited and/or quite variable across 
locations with regard to environmental levels relating to effects, thus 
hindering analysis. For others, information is limited and/or quite 
variable, with regard to its linkages to the criteria pollutants. The 
information with clearest implications to NAAQS decisions pertains to 
SOX and S deposition-related ecosystem acidification. While 
the information regarding effects associated with N loading to 
ecosystems is extensive, information to support quantitative analysis 
to inform NAAQS decisions regarding N oxides and PM is not clear, with 
multiple complicating factors. Such factors include contributions from 
other, non-criteria pollutants, such as NH3. As noted in 
section II.D above, the role of N deposition in aquatic acidification 
is considered in the REA. With regard to other N deposition-related 
effects of N oxides and PM, the information does not provide effective 
support for such analysis, independent of effects from other (non-
criteria) pollutants, or, in some cases, from other (non-air) sources.
c. Sulfur Deposition and SOX
    Evidence- and exposure/risk-based considerations discussed in the 
PA pertaining to S deposition and SOX in ambient air are 
summarized in the subsections below. These considerations reflect 
discussion in the PA, which draws on the available welfare effects 
evidence described in the current ISA, the 2008 NOX/
SOX ISA, the 2009 p.m. ISA, and past AQCDs, as well as 
information available from quantitative analyses (summarized in 
Chapters 5 and 6 of the PA), both analyses recently developed and those 
available from the 2009 REA.
    In considering potential public welfare protection from S 
deposition-related effects in aquatic ecosystems (in light of the 
aquatic acidification REA results summarized in II.D above), the PA 
notes as an initial matter, the integral role of watersheds in aquatic 
ecosystem health (e.g., ISA, Appendix 8, section 8.1 and Appendix 16, 
section 16.4.2) and the effects of acidic deposition on forested areas 
in the watersheds that are distinct from effects in water bodies (e.g., 
reduced tree growth and survival). Further, as discussed in section 
II.C.2 above, there are an array of benefits of watershed forested 
areas to the public, including such ecosystem services as silviculture, 
drinking water supply protection, recreational uses. In light of these 
public benefits, the PA recognizes the public welfare implications of 
various effects of acidifying deposition on the natural resources in 
these areas, with the public welfare significance dependent on the 
severity and extent of such effects. Given the more extensive 
quantitative analyses for aquatic acidification in this review, the PA 
discusses the public welfare implications of S deposition-related 
effects in aquatic ecosystems with an eye toward their prominence for 
decision-making in this review (PA, sections 4.5 and 7.2.2.2). In so 
doing, the PA judges that in focusing on public welfare protection from 
aquatic acidification-related effects will provide protection from 
watershed soils, and

[[Page 26661]]

accordingly, for associated watershed resources.
    The PA notes that, as also recognized in the 2012 review, aquatic 
ecosystems provide a number of services important to the public 
welfare, ranging from recreational and commercial fisheries to 
recreational activities engaged in by the public (77 FR 20232, April 3, 
2012). Because aquatic acidification primarily affects the diversity 
and abundance of aquatic biota, it also affects the ecosystem services 
that are derived from the fish and other aquatic life found in these 
surface waters (PA, section 4.5; ISA, Appendix 14, section 14.3.1). 
Fresh surface waters support several cultural services, such as 
aesthetic and educational services; the type of service that is likely 
to be most widely and significantly affected by aquatic acidification 
is recreational fishing, with associated economic and other benefits. 
Other potentially affected services include provision of food for some 
recreational and subsistence fishers and for other consumers, as well 
as non-use services, including existence (protection and preservation 
with no expectation of direct use) and bequest values (PA, section 
4.5).
    The PA recognizes that some level of S deposition and associated 
risk of aquatic acidification, including those associated with past 
decades of acidifying deposition in the Northeast, can impact the 
public welfare and thus might reasonably be judged adverse to the 
public welfare. Depending on magnitude and the associated impacts, 
there are many locations in which S deposition and associated aquatic 
acidification can adversely affect the public welfare. For example, 
there is evidence in some waterbodies that aquatic acidification 
resulting in reduced acid buffering capacity can adversely affect 
waterbodies and associated fisheries, which in addition to any 
commercial ramifications can have ramifications on recreational 
enjoyment of affected areas (PA, sections 5.1.1 and 4.5). The evidence 
is less clear as to what level of risk to an aquatic system, in terms 
of estimates for achieving various ANC targets across sites within an 
ecoregion, might be judged of public welfare significance.
    In other secondary NAAQS reviews, the EPA's consideration of the 
public welfare significance of the associated effects has recognized a 
particular importance of Class I areas and other similarly protected 
areas. Accordingly, we note that waterbodies that have been most 
affected by acidic deposition are in the eastern U.S., including in 
several Class I areas and other national and state parks and forests 
(PA, section 5.1.2.1).\64\ Two waterbodies in such areas were included 
as case studies in the aquatic acidification REA: Shenandoah Valley 
Area and Rocky Mountain National Park (PA, section 5.1.3.3). While 
assuring continued improvement of affected waterbodies throughout the 
U.S. (e.g., through lower S deposition than the levels of the past) may 
reasonably be considered to be of public welfare importance, it is 
particularly important in Class I and similarly protected areas. In 
this review, in considering the potential public welfare significance 
of aquatic acidification effects of differing levels of S deposition, 
the PA summarizes the REA ecoregion-scale results in terms of 
percentages of ecoregions in which differing percentages of waterbodies 
are estimated to achieve the three acid buffering capacity targets. The 
PA summarized results in this way to inform identification of S 
deposition estimates in the context of potential policy options.
---------------------------------------------------------------------------

    \64\ A comparison of Figures 4-4 and 5-6 of the PA indicates 
multiple Class I areas in ecoregions considered acid sensitive.
---------------------------------------------------------------------------

    The first subsection below (II.E.1.c(1)) focuses on the aquatic 
acidification REA analyses (summarized in section II.D above), 
considering first their use of ANC as the indicator of acidification 
risk, then evaluating the exposure/risk estimates as to what they 
indicate about acidification risks in freshwater streams and lakes of 
the contiguous U.S. for S deposition rates estimated to have occurred 
over the past two decades (much of which is newly assessed in this 
review),\65\ and lastly identifying important uncertainties associated 
with the estimates. Section II.E.1.c(2) considers the evidence and 
quantitative exposure/risk information from a public welfare protection 
perspective, focusing first on what might be indicated regarding 
deposition conditions under which waterbodies in acid-sensitive 
ecoregions might be expected to achieve acid buffering capacity of 
interest and what the available information indicates pertaining to the 
consideration of public welfare protection from S deposition related 
effects in aquatic ecosystems. Lastly, section II.E.1.c(2) considers 
what the published quantitative information regarding S deposition and 
terrestrial acidification indicates regarding deposition levels of 
potential concern, along with associated uncertainties in this 
information. Section II.E.1.c(3) then summarizes considerations in 
relating SOX air quality metrics to deposition of S 
compounds.
---------------------------------------------------------------------------

    \65\ Aquatic acidification risk analyses in the last review 
considered deposition estimates for 2002 and 2006 derived from CMAQ 
modeling, 2002 emissions estimates (2009 REA, Appendix 1).
---------------------------------------------------------------------------

(1) Quantitative Information for Ecosystem Risks Associated With S 
Deposition
    As in the last review, we give primary attention to the 
quantitative assessment of aquatic acidification (including 
particularly that attributable to S deposition). While noting the 
uncertainties associated with results of the aquatic acidification REA, 
as summarized in section II.D.2 above, the PA recognized these results 
to be informative to the identification of S deposition levels 
associated with potential for aquatic acidification effects of concern, 
as summarized below. This assessment of quantitative linkages between S 
deposition and potential for aquatic acidification is one component of 
the approach implemented in the PA for informing judgments on the 
likelihood of occurrence of such effects under differing air quality 
conditions.
    Although the approaches and tools for assessing aquatic 
acidification have often been applied for S and N deposition in 
combination, the approach taken in the REA for this review is focused 
on S deposition. This focus is supported by analyses in the PA 
indicating the relatively greater contribution of S deposition than N 
deposition to aquatic acidification risk under the more recent air 
quality conditions that are the focus of this review (PA, Appendix 5A). 
As summarized in section II.D above, the aquatic acidification 
assessment has relied on well-established site-specific water quality 
modeling applications with a widely recognized indicator of aquatic 
acidification, ANC.
    Quantitative tools are also available for the assessment of 
terrestrial acidification related to S deposition, as they were in the 
last review, and the findings from these analyses presented in the 2009 
REA have been considered in this review in the context of more recently 
available evidence (PA, section 5.3.2.1; 2009 REA, section 4.3).\66\ In 
the last review, analyses that related estimated atmospheric deposition 
of acidic N and S compounds (for early 2000s time period) to 
terrestrial effects, or indicators of terrestrial ecosystem risk, were 
generally considered to be more uncertain than conceptually similar 
modeling analyses for aquatic

[[Page 26662]]

ecosystems. For example, the 2009 REA concluded that ``aquatic 
acidification is clearly the targeted effect area with the highest 
level of confidence'' (2009 REA, section 7.5; 2011 PA, section 1.3). 
Additionally, the PA for this review notes that quantitative tools and 
approaches are not well developed for other ecological effects 
associated with atmospheric deposition of S compounds, such as mercury 
methylation and sulfide toxicity in aquatic systems (PA, sections 
4.2.3.1 and 4.2.3.2).
---------------------------------------------------------------------------

    \66\ Given findings from the 2009 REA that aquatic acidification 
provided a more sensitive measure for use in assessing deposition 
related to ecosystem acidification, and consideration of recent 
information not likely to result in a different finding, the REA for 
the current review focused on aquatic acidification.
---------------------------------------------------------------------------

    As described in sections II.C and II.D above, ANC is an indicator 
of susceptibility or risk of acidification-related effects in 
waterbodies, with lower levels indicating relatively higher potential 
for acidification and related waterbody effects. The support for this 
relationship is strongest in aquatic systems low in organic material, 
and the evidence comes predominantly from historically affected 
waterbodies in the eastern U.S. (e.g., in the Adirondack Mountains) and 
Canada. In waterbodies with relatively higher levels of dissolved 
organic material, the presence of organic acid anions contributes to 
reduced pH, but also creates complexes with dissolved aluminum that 
protect resident biota against aluminum toxicity (ISA, Appendix 8, 
section 8.3.6.2; PA, section 7.2.2.1). Accordingly, biota in such 
systems tolerate lower ANC values (and pH) than biota in waterbodies 
with low dissolved organic carbon. Thus, while the evidence generally 
supports the use of ANC as an acidification indicator and for purposes 
of judging a potential for ecosystem acidification effects generally, 
the relationship with risk differs depending on the presence of 
naturally occurring organic acids, which also affects the 
responsiveness of ANC to acidifying deposition in these areas. For 
these reasons, in some areas, ANC is less well supported as an 
indicator for acidic deposition-related effects (and waterbodies are 
less responsive to changes in acidic deposition) due to dissolved 
organic material; these areas include the Middle Atlantic Coastal 
Plain, Southern Coastal Plains, and Atlantic Coastal Pine Barrens 
ecoregions (PA, section 5.1.2.2).
    The PA considers the available evidence to provide strong support 
for use of ANC for purposes of making judgments regarding risk to 
aquatic biota in streams impacted by acidifying deposition, and for 
consideration of the set of targets analyzed in the aquatic 
acidification REA: 20, 30, and 50 [mu]eq/L (PA, section 5.1). There is 
longstanding evidence of an array of impacts on aquatic biota and 
species richness reported in surface waters with ANC values below zero 
and in some historically impacted waterbodies with ANC values below 20 
[mu]eq/L (PA, section 5.1.2.2). The severity of impacts is greatest at 
the lowest ANC levels. This evidence derives primarily from lakes and 
streams of the Adirondack Mountains and areas along the Appalachian 
Mountains. As recognized in the 2012 review, in addition to providing 
protection during base flow situations, ANC is a water quality 
characteristic that affords protection against the likelihood of 
decreased pH from episodic events in impacted watersheds. For example, 
some waterbodies with ANC below 20 [mu]eq/L have been associated with 
increased probability of low pH events, that, depending on other 
factors as noted above, may have potential for reduced survival or loss 
of fitness of sensitive biota or lifestages (2008 ISA, section 
5.1.2.1). In general, the higher the ANC level above zero, the lower 
the risk presented by episodic acidity. In summarizing and considering 
the acidification risk estimates for the different scales of analysis 
(national, ecoregion and case study) and using the water quality 
modeling-based CLs derived for three different ANC targets (20, 30 and 
50 [mu]eq/L), the PA recognizes both the differing risk that might be 
ascribed to the different ANC targets, as well as the variation in ANC 
response across waterbodies that may be reasonable to expect with 
differences in geology, history of acidifying deposition, and in 
patterns of S deposition. Further, the PA recognizes limitations and 
uncertainties in the use of ANC as an indicator for model-based risk 
assessments as noted above (PA, section 7.2.2.1).
    The REA national-scale analysis of more than 13,000 waterbody sites 
in 69 ecoregions demonstrated an appreciable reduction in risk over the 
20-year period of analysis (PA, section 5.1.3). For the 2001-2003 
period, more than 20% of waterbodies analyzed nationally were estimated 
to be unable to achieve an ANC of 20 [mu]eq/L or greater based on S 
deposition estimates (table 1 above). This percentage declines 
significantly by the 2010-2012 period, and by the 2018-20 period, only 
1% and 4% of waterbodies analyzed nationally were estimated to be 
unable to achieve or exceed ANC targets of 20 [mu]eq/L and 50 [mu]eq/L, 
respectively (table 1). The 25 ecoregions included in the ecoregion-
scale analyses (i.e., 18 in the East and 7 in the West in which there 
are at least 50 waterbody sites with CL estimates) are dominated by 
ecoregions categorized as acid sensitive (PA, Table 5A-5) and excludes 
the three ecoregions identified above as having natural acidity related 
to organic acids \67\ (PA, section 5.1.2.1). The ecoregion-scale 
results across the 20-year period reflect the results at the national 
scale, but the percentages of waterbodies not able to meet the ANC 
targets are higher than the national percentages due to the dominance 
of the acid-sensitive ecoregions among the 25 analyzed in the 
ecoregion-scale analysis. Specifically, in the most affected ecoregion 
(Central Appalachians), more than 50% of waterbodies were estimated to 
be unable to achieve an ANC of 20 [mu]eq/L or greater based on S 
deposition estimates for the 2001-2003 period; the percentage was close 
to 60% for an ANC target of 50 [mu]eq/L (Figure 1 above, and PA, Figure 
5-13). By the 2018-2020 period, less than 10% of waterbodies in any of 
the 25 ecoregions (and less than 5% in all but one) were estimated to 
be unable to achieve an ANC of 20 [mu]eq/L and less than 15% of 
waterbodies in the most affected ecoregion were estimated to be unable 
to achieve an ANC of 50 [mu]eq/L (Figure 1 above and PA, Figure 5-13).
---------------------------------------------------------------------------

    \67\ The natural acidity contributes to a reduced responsiveness 
to changes in acidic deposition.
---------------------------------------------------------------------------

    The PA recognizes uncertainty associated with two overarching 
aspects of the aquatic acidification REA (PA, section 5.1.4 and 
Appendix 5A, section 5A.3). The first relates to interpretation of 
specific thresholds or benchmark concentrations of ANC with regard to 
aquatic acidification risk. The second relates to our understanding of 
the biogeochemical linkages between deposition of S and N compounds and 
waterbody ANC (which is reflected in the modeling employed), and the 
associated estimation of CLs. With regard to interpretation of ANC 
thresholds, while ANC is an established indicator of aquatic 
acidification risk, there is uncertainty in our understanding of 
relationships between ANC and risk to native biota, particularly in 
waterbodies in geologic regions prone to waterbody acidity. Such 
uncertainties relate to a number of factors, including the varying 
influences of site-specific factors other than ANC. These other site-
specific factors include prevalence of organic acids in the watershed, 
as well as historical loading to watershed soils that can influence 
acidity of episodic high-flow events (PA, sections 5.1.4 and 7.2.2.1 
and Appendix 5A, section 5A.3). There are also uncertainties associated 
with the estimates of S deposition used in the analyses of CL 
exceedances, including those for the national- and ecoregion-scale 
analyses (PA, section 6.3.1, Table

[[Page 26663]]

6-13). Consideration of such uncertainties informs the weighing of the 
findings of the quantitative analyses. For example, there is more 
uncertainty associated with CLs in areas that are less well studied. 
Thus, the PA suggests that it is appropriate to put greater emphasis on 
the more well studied areas and/or less emphasis on estimates for the 
tails of the distributions (e.g., upper/lower percentiles) of waterbody 
exceedances within an ecoregion or case study area. This information 
additionally informs interpretation of the potential risk associated 
with estimates for the different ANC targets.
    With regard to estimation of CLs for the different ANC targets, 
associated uncertainties, generally related to parameters used in the 
steady-state CL models, are difficult to characterize and assess. Such 
uncertainties contribute uncertainty to estimation of the ANC levels 
that individual waterbodies might be expected to achieve under 
different rates of S deposition. While the water quality models used 
for estimating aquatic acidification CL are well conceived and based on 
a substantial amount of research and applications available in the 
peer-reviewed literature, there is uncertainty associated with the 
availability of the necessary data to support certain model components 
(PA, Appendix 5A, section 5A.3). For example, as recognized in section 
II.D.2 above, the data to support the site-specific model inputs for 
some areas are more limited than others, with associated greater 
uncertainties (PA, sections 4.2.1.3 and 5.1.4).
    Most particularly, the strength of the CL estimates and the 
exceedance calculation rely on the ability of models to estimate the 
catchment-average base-cation supply (i.e., input of base cations from 
weathering of bedrock and soils and air), runoff, and surface water 
chemistry. The uncertainty associated with runoff and surface water 
parameters relates to measurement availability, which varies among 
waterbodies. The model input associated with estimating base cation 
catchment supply is the base cation weathering rate, which the ISA 
recognizes as ``one of the most influential yet difficult to estimate 
parameters in the calculation of critical acid loads of N and S 
deposition for protection against terrestrial acidification'' (ISA, 
section IS.14.2.2.1; Li and McNulty, 2007). Although the approach to 
estimate base-cation supply in the REA (e.g., F-factor approach) has 
been widely published and analyzed in Canada and Europe, and has been 
applied in the U.S. (e.g., Dupont et al., 2005 and others), the 
magnitude of uncertainty in this estimate is unclear and could be large 
in some cases. The REA's quantitative analysis of uncertainty in CL 
estimates indicates lower uncertainty associated with CLs estimated for 
sites with more extensive and longer-term water quality datasets and 
relatively low variability in the runoff measurements, such as CLs for 
waterbody sites in the eastern U.S., particularly along the Appalachian 
Mountains, in the Upper Midwest, and in the Rocky Mountains. The 
analysis found greater uncertainty associated with CLs estimated for 
sites in the Midwest and South and along the CA to WA coast (PA, 
Appendix 5A, section 5A.3.1).
(2) General Approach for Considering Public Welfare Protection
    In discussing key considerations in judging public welfare 
protection from S deposition in the context of the review of the 
secondary standard for SOX, the PA first focused on the 
results of the aquatic acidification REA as to what they indicated 
about deposition conditions under which waterbodies in sensitive 
ecoregions might be expected to achieve ANC levels of interest. In so 
doing, the PA focused on the results of the aquatic acidification REA 
at three scales: national-scale, ecoregion-scale and the more localized 
case study-scale, giving particular focus to the ecoregion and case-
study analyses, which use the waterbody-specific comparisons of 
estimated deposition and waterbody CLs to provide ecoregion wide and 
cross-ecoregion summaries of estimated waterbody responses to ecoregion 
estimates of deposition. The PA also considered the extent to which 
waterbodies in each ecoregion analyzed were estimated to achieve ANC 
levels at or above each of the three targets in recognition of the 
variation in ANC response reasonably expected across waterbodies in an 
ecoregion based on both differences in watersheds that can affect 
sensitivity to S deposition and with different spatial or geographic 
patterns of S deposition.
    At the national scale, as summarized in section II.E.1.c(1) above, 
unlike the case for the 2000-2002 period, few waterbodies are estimated 
to be receiving deposition in excess of their CLs for the three ANC 
targets under recent deposition estimates. For example, for S 
deposition estimates for the most recent time period (2018-2020), only 
4% of waterbodies nationally were estimated to exceed CLs for an ANC of 
50 [mu]eq/L and 1% for an ANC of 20 [mu]eq/L (table 1 above). In this 
time period (2018-2020), median estimates of deposition in all of the 
69 ecoregions that are represented in these national-scale percentages 
(ecoregions with at least one site with a CL estimate) are at or below 
approximately 4 kg S/ha-yr (PA, Tables 5A-15 and 5A-11).
    Based on the array of CL-based analyses, the PA provides a general 
sense of the ANC values that waterbodies in sensitive regions across 
the continental U.S. may be able to achieve, including for areas 
heavily affected by a long history of acidifying deposition, such as 
waterbodies in Shenandoah Valley. In the case study for that well 
studied area (4,977 sites distributed across three ecoregions), 90% of 
waterbody sites are estimated to be able to achieve an ANC at or above 
20 [mu]eq/L (focusing on S deposition only) with S deposition of 7.1 
kg/ha-yr and 70% with S deposition of 9.4 kg/ha-yr (PA, section 
5.1.3.3). For an ANC target at or above 50 [mu]eq/L in the Shenandoah 
Valley case study, the corresponding deposition estimates are 4.1 and 
6.3 kg/ha-yr (PA, Table 5-6). For the other case study areas (White 
Mountain National Forest, Northern Minnesota, Sierra Nevada Mountains 
and Rocky Mountain National Park), there are appreciably fewer 
waterbody sites for which modeling has been performed to estimate CLs, 
and accordingly greater uncertainty. Yet, the case study area averages 
of waterbody CLs for achieving ANC at or above each of the three 
targets (20, 30 or 50 [mu]eq/L) is quite similar across the five case 
studies, ranging from 9.4 kg/ha-yr for an ANC of 50 [mu]eq/L in 
Shenandoah Valley Area to 12 kg/ha-yr for an ANC of 20 [mu]eq/L in both 
Shenandoah and Sierra Nevada Mountains case study areas (PA, Table 5-
6).
    Findings from the ecoregion-scale analyses of 25 ecoregions (18 
East and 7 West), nearly all of which are considered acid sensitive, 
indicated ranges of deposition estimates associated with high 
percentages of waterbodies estimated to achieve the three ANC targets 
that are similar to the case study results immediately above. This was 
the case when considering the ecoregion-scale analysis results in both 
of the two ways they were presented: (1) in terms of ecoregion median 
deposition regardless of time period or ecoregion (ecoregion-time 
period combinations), and (2) in terms of temporal trends in S 
deposition and waterbody percentages achieving ANC targets.
    For example, in the ecoregion-time period combinations 
presentation, at least 90% of waterbody sites in 87% of the eastern 
ecoregion-time period combinations are estimated to be able to achieve 
an ANC at or above 20 [mu]eq/L with ecoregion median S deposition at

[[Page 26664]]

or below 9 kg/ha-yr and in 96% of those combinations for S deposition 
at or below 5 kg/ha-yr (table 4 below). This summary contrasts with, 
and indicates appreciably greater acid buffering capacity than, the 
estimates for S deposition at or below 18 kg/ha-yr (table 4 below). 
Further, 70% of waterbody sites in all 18 eastern ecoregions are 
estimated to achieve an ANC at or above 50 [mu]eq/L with ecoregion 
median S deposition at or below 9 kg/ha-yr. Although fewer ecoregion-
time period combinations are associated with still lower S deposition 
estimates, contributing to increased uncertainty, we also note that for 
the lowest bin that is composed of at least half of the full eastern 
ecoregion dataset (51 ecoregion-time periods with S deposition 
estimates at or below 5 kg/ha-yr), 90% of waterbodies per ecoregion 
were estimated to achieve an ANC at or above 20 [mu]eq/L in 96% of the 
combinations and at or above 50 [mu]eq/L in 82% of the combinations 
(table 4 below).
    In total, the ecoregion-time periods presentation indicates the 
likelihood of appreciably more waterbodies achieving the acid buffering 
capacity targets among the combinations with ecoregion median 
deposition at or below 9 kg/ha-yr (and for the bins for lower values) 
in eastern ecoregions compared to the estimates of waterbodies 
achieving acid buffering targets based on the full dataset that 
includes deposition estimates up to 18 kg/ha-yr. More specifically, 
this reflects an appreciably greater percentage of waterbodies in more 
ecoregions achieving ANC at or above 20 [mu]eq/L, at or above 30 
[mu]eq/L, and at or above 50 [mu]eq/L (table 4 below), with ecoregion 
median deposition levels at or below 9 kg/ha-yr. Additionally, these 
percentages increase across the bins for the lower deposition 
estimates, although they are also based on smaller proportions of the 
supporting dataset (i.e., fewer ecoregion-time period combinations in 
each subsequently lower deposition bin).

Table 4--Summary of the Eastern Ecoregion and Time Period Combinations Achieving Different ANC Targets With Estimated S Deposition at or Below Different
                                                                         Values
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                       % of Eastern ecoregion-time period combinations ** with at least  90%, 80% or 70%
                                                                                         waterbodies per ecoregion achieving ANC target
                                                            % of      ----------------------------------------------------------------------------------
             S deposition  (kg/ha-yr) *                 combinations      >=90% of waterbodies        >=80% of waterbodies        >=70% of waterbodies
                                                          included    ----------------------------------------------------------------------------------
                                                                          20       30       50        20       30       50        20       30       50
--------------------------------------------------------------------------------------------------------------------------------------------------------
ANC ([micro]eq/L) at/below:
    <=18............................................             100        73       67       60        88       87       81        92       90       89
    <=13............................................              90        80       73       65        95       94       88        98       96       96
    <=11............................................              84        83       76       68        97       96       91        99       99       99
    <=9.............................................              77        87       81       72       100       99       93       100      100      100
    <=7.............................................              70        92       87       78       100      100       95       100      100      100
    <=6.............................................              66        93       88       78       100      100       97       100      100      100
    <=5.............................................              57        96       92       82       100      100       96       100      100      100
--------------------------------------------------------------------------------------------------------------------------------------------------------
* These values are ecoregion median estimates across all waterbody sites in an ecoregion with a CL estimate.
** These percentages are from the more extensive presentation of results in PA, Table 5-5.

    The PA observes that estimates from the temporal trend perspective 
similarly indicate appreciable percentages of waterbodies per ecoregion 
being estimated to achieve the acid buffering capacity targets with 
ecoregion median deposition below a range of approximately 8 to 5 kg/
ha-yr. For example, during the latter half of the 20-year period 
analyzed (i.e., by the 2010-2012 period), by which time all 25 
ecoregions are estimated to have more than 70% of waterbodies able to 
achieve an ANC at or above 50 [mu]eq/L (and at least 85% able to 
achieve an ANC at or above 20 [mu]eq/L), median deposition in 95% of 
the ecoregions was below 8 kg S/ha-yr, ranging from 1.3 to 7.3 kg S/ha-
yr (PA, Table 7-2 and Figure 7-1). As shown in table 5 below, with each 
reduction in S deposition in each subsequent time period, more 
waterbodies in each of the eastern ecoregions are estimated to be able 
to achieve the ANC targets. Nearly 90% of the 18 eastern ecoregions are 
estimated to have at least 90% of their waterbodies achieving an ANC of 
20 [mu]eq/L in the 2010-12 period and achieving an ANC of 50 [mu]eq/L 
in the 2014-16 period. When the 7 western ecoregions are included in a 
summary based on ANC targets of 20 [mu]eq/L for the West and 50 [mu]eq/
L for the East,\68\ over 70% of the full set of ecoregions are 
estimated to have at least 90% of their waterbodies achieving the ANC 
targets by the 2010-12 period (table 5). By the 2014-2016 and 2018-2020 
periods, 24 of the 25 ecoregions were estimated to have more than 90% 
of waterbodies able to achieve an ANC at/above 50 [mu]eq/L, and median 
S deposition in all 25 ecoregions was below 5 kg/ha-yr (table 5).
---------------------------------------------------------------------------

    \68\ This combination of targets recognizes the naturally and 
typically low ANC levels observed in western waterbodies while also 
including a higher target for the East, as described in section 
5.1.2.2 of the PA.

                       Table 5--Ecoregions Estimated To Have Different Percentages of Waterbodies Achieving Different ANC Targets for the Five Deposition Periods Analyzed
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                              % (n) of ecoregions with specified percentage of waterbodies per ecoregion achieving specified ANC
                                                            ------------------------------------------------------------------------------------------------------------------------------------
                                                                                   ANC: 20 [mu]eq/L                                    30 [mu]eq/L                        50 [mu]eq/L
                                                            ------------------------------------------------------------------------------------------------------------------------------------
                        Time period                             Ecoregion median S        Percent of waterbodies  per          Percent of waterbodies  per        Percent of waterbodies  per
                                                              deposition (kg/ha-yr)                ecoregion                            ecoregion                          ecoregion
                                                            ------------------------------------------------------------------------------------------------------------------------------------
                                                                 Min          Max         90%         80%         70%         90%         80%         70%         90%         80%         70%
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                       East
                                                                                                       Of 18 Eastern Ecoregions
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
2001-03....................................................          4.0       17.3      39% (7)    67% (12)    72% (13)     28% (5)    61% (11)    72% (13)     22% (4)     50% (9)    72% (13)
2006-08....................................................          3.1       14.4      44% (8)    72% (13)    89% (16)     33% (6)    72% (13)    78% (14)     33% (6)    67% (12)    72% (13)

[[Page 26665]]

 
2010-12....................................................          2.3        7.3     89% (16)   100% (18)   100% (18)    83% (15)   100% (18)   100% (18)    61% (11)    89% (16)   100% (18)
2014-16....................................................          1.9        4.6     94% (17)   100% (18)   100% (18)    94% (17)   100% (18)   100% (18)    89% (16)   100% (18)   100% (18)
2018-20....................................................          1.3        3.9    100% (18)   100% (18)   100% (18)    94% (17)   100% (18)   100% (18)    94% (17)   100% (18)   100% (18)
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                       All
                                                                                                  Of 25 Ecoregions (18 East, 7 West)
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
2001-03....................................................          1.2       17.3     56% (14)    76% (19)    80% (20)    48% (12)    72% (18)    80% (20)    44% (11)    64% (16)    80% (20)
2006-08....................................................          1.2       14.4     60% (15)    80% (20)    92% (23)    52% (13)    80% (20)    84% (21)    52% (13)    76% (19)    80% (20)
2010-12....................................................          1.0        7.3     92% (23)   100% (25)   100% (25)    88% (22)   100% (25)   100% (25)    72% (18)    92% (23)   100% (25)
2014-16....................................................          1.1        4.6     96% (24)   100% (25)   100% (25)    96% (24)   100% (25)   100% (25)    92% (23)   100% (25)   100% (25)
2018-20....................................................         0.62        3.9    100% (25)   100% (25)   100% (25)    96% (24)   100% (25)   100% (25)    96% (24)   100% (25)   100% (25)
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Note: Estimates for ANC of 50 [mu]eq/L (East) and 20 [mu]eq/L (West) are identical to those for 50 in all 25 ecoregions.

    The temporal trends in percentage of waterbodies estimated to 
achieve the target ANC levels for each of the 25 individual ecoregions 
document a large difference between the time periods prior to 2010 and 
subsequent time periods (Figure 1 above; PA, Figure 7-1). For the S 
deposition estimated for the 2010-2012 time period, more than 70% of 
waterbodies are estimated to be able to achieve an ANC of 50 [mu]eq/L 
in all 25 ecoregions (Figure 1, left panel), and 85% to 100% of 
waterbodies in all ecoregions are estimated to be able to achieve an 
ANC of 20 [mu]eq/L (Figure 1, right panel).
    Given the dependency of the ANC estimates on S deposition 
estimates, this distinction between the period prior to 2010 and the 
subsequent decade is also seen in the ecoregion deposition estimates 
for the 25 REA ecoregions (Figure 2; PA, Figure 7-2). The distribution 
of deposition estimates at waterbody sites assessed in each ecoregion, 
and particularly the pattern for the higher percentile sites in each 
ecoregion, illustrates the deposition estimates that are driving the 
REA estimates. For example, among the 25 East and West ecoregions 
during the two periods prior to 2010, the medians of the ecoregion 90th 
percentile deposition estimates ranged from approximately 14 to 17 kg/
ha-yr, with maximum values above 20 kg/ha-yr (Figure 2). This contrasts 
with the deposition estimates during the 2010-2020 period when, among 
all 25 ecoregions, the medians of the ecoregion 90th percentile 
deposition estimates ranged from approximately 2 to 5 kg/ha-yr, with 
all ecoregion 90th percentile estimates below 8 kg/ha-yr (Figure 2). 
The contrast is less sharp for the ecoregion medians, as the median is 
a statistic less influenced by changes in the magnitude of values at 
the upper end of the distribution (Figure 2).
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    Thus, in considering identification of S deposition levels that may 
be associated with a desired level of ecosystem protection for an 
SOX

[[Page 26667]]

standard, the PA took note of the increased percentages of waterbodies 
estimated to achieve more protective ANC levels across the five time 
periods. The pattern of estimated improving water quality over the 20-
year study period is paralleled by the pattern of declining deposition, 
which is more obvious in the upper percentiles (than the median) of the 
distribution of values per ecoregion (Figure 2). This pattern indicates 
appreciable difference between the first and second decades of the 
period in terms of S deposition (at upper percentiles as well as at the 
median of sites within the 25 ecoregions) and associated aquatic 
acidification risk. The ecoregion with the highest S deposition in the 
latter decade (2010-2020) had 90th percentile estimates ranging from 
approximately 8 kg/ha-yr to just below 5 kg/ha-yr (and median estimates 
with a very similar range) across this decade (Figure 2). As noted 
immediately above, the risk estimates associated with the deposition 
estimates of this decade indicate generally high percentages of 
waterbodies per ecoregion as able to achieve or exceed the three ANC 
targets. Similarly, the ecoregion-time period binning summary also 
indicates generally high percentages of waterbodies achieving ANC 
targets for ecoregion median S deposition at or below about 8 or 9 kg/
ha-yr (table 4). Lastly, the case study CL estimates also indicate 
appreciable portions of the case study areas that might be expected to 
attain the three ANC targets with deposition below 9 kg/ha-yr. Thus, in 
light of these observations, the PA describes S deposition, on an 
areawide basis, that falls below approximately 10-5 kg/ha-yr, or 8-5 
kg/ha-yr (differing slightly depending on the supporting analysis), as 
being associated with the potential to achieve acid buffering capacity 
levels of interest in an appreciable portion of sensitive areas.
    In considering what the quantitative information regarding S 
deposition and terrestrial acidification indicates regarding deposition 
levels of relatively greater and lesser concern for potential 
acidification-related effects (and the associated uncertainties), the 
PA considers soil chemistry modeling analyses (both those described in 
published studies and an analysis performed in the 2009 REA), studies 
involving experimental additions of S compounds to defined forestry 
plots, and observational studies of potential relationships between 
terrestrial biota assessments and metrics for S deposition (PA, section 
5.3). With regard to soil chemistry modeling analyses performed in the 
last review, the PA notes that the resulting estimates of acidic 
deposition CLs for three values of the soil acidification indicator, 
BC:Al ratio, indicated a range generally well above the CL estimates 
associated with achieving various ANC targets in the aquatic 
acidification analyses discussed above. The soil acidification CLs were 
also above all of the ecoregion estimates (across the five time periods 
from 2001 through 2020) considered in the aquatic acidification 
analyses (PA, Table 5-7). Thus, the PA concluded that these soil 
acidification modeling findings indicate that a focus on aquatic 
acidification might reasonably be expected to also provide protection 
from soil acidification effects on terrestrial biota. With regard to 
studies involving S additions to experimental forested areas, the PA 
notes that although the number of tree species that have been included 
in such experiments is somewhat limited, the more widely recognized 
sensitive species (based on field observations) have been included in 
such studies. Across these studies, the PA observes that effects on the 
trees analyzed have not been reported with S additions below 20 kg/ha-
yr (which is in addition to the atmospheric deposition occurring during 
the experiment).
    The PA also considers the recently available quantitative 
information regarding S deposition and terrestrial acidification drawn 
from two observational studies that report associations of tree growth 
and/or survival metrics with various air quality or S deposition 
metrics, providing support to conclusions regarding the role of acidic 
S deposition on tree health in the U.S., most particularly in regions 
of the eastern U.S. (PA, section 5.3.2.3 and Appendix 5B, section 
5B.3.2). The metrics used in the two largest studies include site-
specific estimates of average SO42- deposition 
and of average total S deposition over the interval between tree 
measurements, generally on the order of 10 years (Dietze and Moorcroft, 
2011; Horn et al., 2018). In the study that used 
SO42- as the indicator of acidic S deposition, 
and for which the study area was the eastern half of the contiguous 
U.S., site-specific average SO42- deposition 
(1994-2005) ranged from a minimum of 4 kg/ha-yr to a maximum of 30 kg/
ha-yr (Dietze and Moorcroft, 2011). Review of the study area for this 
study and a map indicating geographic patterns of deposition during the 
period of the deposition data indicate the lowest deposition areas to 
be west of the Mississippi River, northern New England (e.g., Maine) 
and southern Georgia and Florida (in which S deposition in the 2000-
2002 period is estimated to fall below 8 kg/ha-yr), and the highest 
deposition areas to be a large area extending from New York through the 
Ohio River valley (PA, Appendix 5B, Figures 5B-1 and 5B-11). In the 
second study, deposition at the sites with species for which growth or 
survival was negatively associated with S deposition ranged from a 
minimum below 5 kg/ha-yr to a site maximum above 40 kg/ha-yr, with 
medians for these species generally ranging from around 5 to 12 kg S/
ha-yr (Appendix 5B, section 5B.3.2.3; Horn et al., 2018).
    In considering these study observations, the PA notes the history 
of appreciable acidic deposition in the eastern U.S., with its 
associated impacts on soil chemistry, that has the potential to be 
exerting a legacy influence on tree growth and survival more recently 
(PA, section 5.3.2 and Appendix 5B). Further, the PA notes that at a 
national-scale, the geographic deposition patterns (e.g., locations of 
relatively greater versus relatively lesser deposition) more recently 
appear to be somewhat similar to those of several decades ago (e.g., 
PA, sections 2.5.4 and 6.2.1). This similarity in patterns is 
recognized to have the potential to influence findings of observational 
studies that assess associations between variation in tree growth and 
survival with variation in levels of a metric for recent deposition at 
the tree locations, and to contribute uncertainty with regard to 
interpretation of these studies as to a specific magnitude of 
deposition that might be expected to elicit specific tree responses, 
such as those for which associations have been found. The PA notes 
that, as recognized in the study by Dietze and Moorcroft (2011), which 
grouped species into plant functional groups, acidification impacts on 
tree mortality result from cumulative long-term deposition, and 
patterns reported by their study should be interpreted with the 
knowledge that acidification impacts on tree mortality result from 
cumulative long-term deposition (PA, section 5.3.1 and Appendix 5B).
(3) Relating Air Quality Metrics to S Deposition
    In considering what the available information and air quality 
analyses indicate regarding relationships between air quality metrics 
and S deposition, the PA evaluated trends over the past two decades as 
well as a series of analyses of relationships between S deposition and 
ambient air concentrations of SO2 (in terms of 3-year 
averages of the existing SO2 standard and as an annual 
average), and between S deposition and ambient air

[[Page 26668]]

concentrations of other S compounds (e.g., SO42- 
or summed SO42- and SO2) at 27 Class I 
area sites, as summarized in section II.B above.\69\ With regard to 
indicators other than SO2, lower correlations were observed 
for collocated total S deposition estimates with indicators of 
atmospheric S-containing pollutants (particulate 
SO42- and the sum of S in SO2 and 
particulate SO42- in 27 Class I areas than 
between S deposition and annual average SO2 concentrations 
(averaged over three years) at SLAMS monitors (PA, Figures 6-27 and 6-
31 and Table 6-4). Thus, while the data at the Class I area sites 
(collocated CASTNET and IMPROVE network sites) provide information for 
S compounds other than SO2, the analyses based on data from 
SLAMS are considered particularly relevant given that those sites are 
primarily in areas of higher SO2 concentrations near 
emissions sources and collect FRM/FEM measurements for evaluating 
ambient air concentrations relative to the existing NAAQS. Information 
from these monitoring sites is useful in understanding how changes in 
SO2 emissions, reflected in ambient air concentrations, may 
relate to changes in deposition and, correspondingly, what secondary 
standard options might best relate to ambient air concentrations such 
that deposition in areas of interest is maintained at or below range of 
levels identified above (PA, section 7.2.2.3).
---------------------------------------------------------------------------

    \69\ The air quality metrics include one based on the current 
secondary SO2 NAAQS, which is the second highest 3-hour 
daily maximum in a year, as well as an annual average SO2 
air quality metric (averaged over three years). In light of the many 
factors contributing variability to S deposition, the analyses focus 
on a 3-year average of all of the air quality and deposition metrics 
and include multiple years of data, generally on the order of 20 
years and covering a period of declining concentrations and 
deposition. Of the two air quality metrics analyzed, the PA focused 
primarily on the annual average of SO2 concentrations, 
averaged over 3 years, given the focus on control of long-term S 
deposition and the greater stability of the metric (PA, section 
7.2.2.3).
---------------------------------------------------------------------------

    Together the air quality and deposition data and analyses in the PA 
indicate a significant association of S deposition with SO2 
concentrations, with statistically significant correlation coefficients 
ranging from approximately 0.50 up to 0.70 from the trajectory-based 
and SLAMS analyses for the five 3-year time periods (during 2001-2020) 
across all ecoregions. Higher correlations were observed for dry S 
deposition and at sites in the eastern U.S. (PA, section 7.2.2.3). As 
summarized in section II.B above, S deposition is generally higher in 
the east and dry S deposition is generally higher near SO2 
emissions sources. In considering the two types of analyses, relating 
concentrations to deposition either nearby or in downwind areas, the PA 
notes that a strength of the analyses for concentrations and deposition 
estimates at SLAMS locations is the capturing of near-source 
deposition, while a strength of the trajectory-based analyses is 
accounting for the role of transport and transformation in contributing 
to downwind deposition.
    While recognizing the significant correlations between 
SO2 concentrations and S deposition, the PA additionally 
took note of the variability in, and uncertainty associated with 
relationships between SO2 concentrations at SLAMS monitors 
and nearby and/or downwind S deposition. The variability relates to the 
complexity of the atmospheric chemistry, pollutant transport, and 
deposition processes (PA, sections 2.1.1 and 2.5). The uncertainty in 
these relationships relates to a number of factors, including 
uncertainty in our estimates of S deposition (PA, section 2.5.2) and 
spatial distribution of monitor sites, including the representation of 
significant SO2 emissions sources, as well as elements of 
the trajectory-based analysis, e.g., inclusion criteria for identifying 
monitoring sites of influence (PA, section 6.3 and Table 6-13). The PA 
concluded that it is unclear how much and in what way each of these 
various uncertainties in the data and analyses, and the inherent 
variability of the physical and chemical processes involved, might 
impact the conclusions concerning ambient air SO2 
concentrations related to S deposition estimates at different scales 
(PA, section 7.2.2.3). In light of such uncertainty and variability, 
the REA aquatic acidification analyses and discussion of S deposition 
levels focused on statistics for deposition estimates representing 
large areas (e.g., at the ecoregion median and 75th or 90th percentile, 
and case study area average or 70th and 90th percentile CLs). While 
uncertainty may be greater for relating concentrations to higher points 
on the distribution of deposition in an ecoregion, the PA recognized 
that it is the higher deposition estimates, if focused on individual 
waterbodies, that will contribute most to aquatic acidification risk, 
and additionally observed that the distribution of S deposition 
estimates within ecoregions has collapsed in the more recent years of 
the 20-year analysis period, with 90th percentile estimates falling 
much close to the medians than in the first decade of the period 
(Figure 2 above; PA, Figure 7-2).
    In light of the declining trend in S deposition and the 
corresponding REA estimates of increasing ANC in sensitive ecoregions 
(as discussed above), the PA considered the annual average 
SO2 concentration at SLAMS across five time periods from 
2000-2020. In so doing, the PA focused on the most recent time periods 
analyzed (i.e., since 2010) when the REA indicated appreciably improved 
levels of acid buffering capability in the waterbodies of the 25 
analyzed ecoregions (when ANC targets were met or exceeded in a high 
percentage of water bodies across a high percentage of ecoregions). 
This information indicates that during the most recent time periods (in 
which ecoregion median S deposition estimates for the 25 REA ecoregions 
were below 10 kg/ha-yr), the highest 3-year average annual 
SO2 concentrations were generally somewhat above 10 ppb 
(with some exceptions during the 2019-2021 period), and 95% of the 
concentrations in each of the three most recent periods are just at or 
below 5 ppb (PA, Figure 7-5, left panel). The distributions of annual 
average SO2 concentrations exhibit a similar pattern of 
concentrations to that for the 3-year averages, suggesting there to be 
little year-to-year variability in this metric (PA, Figure 7-5).
    In identifying levels for consideration for a potential annual 
average SO2 standard, the PA also gives attention to the 
SO2 concentrations at monitoring sites of influence 
identified in the trajectory-based analyses across different ranges of 
downwind ecoregion S deposition estimates. In the dataset for all 84 
ecoregions in the contiguous U.S., the maximum annual average 
SO2 concentrations, averaged over three years, at sites of 
influence to downwind ecoregions with median S deposition ranging below 
9 kg S/ha-yr to 6 kg/ha-yr,\70\ were all below 15 ppb, and 75% of the 
monitor sites of influence concentrations were at or below 10 ppb (PA, 
Figure 7-3).\71\ In the subset of data for the 25 REA ecoregions with 
their upwind monitors, for the bin that includes deposition below 9 
down to 6 kg/ha-yr, the concentrations for the metric based on maximum 
concentration at upwind sites of influence (EAQM-max) range as high as 
15 ppb, with more than half below 10 ppb (PA, Figure 7-4, left panel). 
The EAQM-max concentrations associated with ecoregion median deposition 
in the

[[Page 26669]]

lowest bins (S deposition below 6 kg/ha-yr) were all below 10 ppb. This 
pattern suggests that when the highest EAQM-max concentration is 
somewhat below 15 ppb and down to 10 ppb, the ecoregion median 
deposition is below 9 kg/ha-yr and the 90th percentile deposition is 
below 13 kg/ha-yr. When the highest EAQM-max concentration is at 
approximately 11 ppb, or 10 ppb, both the median and 90th percentile 
deposition are below 9 kg/ha-yr (PA, Figure 7-4).
---------------------------------------------------------------------------

    \70\ The bin for ``<9-6 kg/ha-yr'' is discussed here as it is 
the bin closest to the deposition target range of 10 or 8 to 5 kg/
ha-yr identified above.
    \71\ Figure 7-3 of the PA presents the pairs of median 
deposition estimates and associated upwind sites of influence EAQM-
max SO2 concentrations from the trajectory-based analysis 
in section 6.2.4 of the PA (specifically, the combined datasets 
presented in PA, Figures 6-40 and 6-41).
---------------------------------------------------------------------------

    The PA additionally discusses limitations associated with relating 
individual monitor SO2 concentrations to S deposition in the 
context of the two metrics employed in the trajectory-based analyses. 
Between these metrics, somewhat stronger correlations were found for 
the annual average SO2 weighted EAQM (which provides for 
proportional weighting of air concentrations from locations projected 
to contribute more heavily to a particular ecoregion), compared to the 
EAQM-max, particularly for the first two to three time periods of the 
20-year period. This difference is related to the extent to which 
monitor concentrations can be indicative of atmospheric loading. The 
weighted EAQM is intended to more closely represent the atmospheric 
loading for the locations (and associated sources) of the contributing 
(sites of influence) monitors than a single contributing monitor can. 
However, the weighted metric is not directly translatable to a standard 
level (which is an upper limit on concentrations in individual 
locations). Conversely, unweighted concentrations (even from the 
maximum contributing monitor) are limited in the extent to which they 
can reflect atmospheric loading due to a number of factors, including 
monitor and source distribution and magnitude of emissions. The lower 
correlations observed between deposition and the maximum EAQM in areas 
of lower concentrations are an indication of this complexity. Across a 
broad enough range in deposition (e.g., as occurring in the earlier 
time periods and in the East), a rough correlation is observed, which 
breaks down across smaller ranges in deposition, as evidenced by the 
much lower correlations for the more recent period with its much lower 
magnitude of deposition and much smaller range in deposition (PA, 
section 7.2.2.3).
    In its consideration of the trajectory-based analyses to identify a 
range of annual average SO2 EAQM-max concentrations that may 
be associated with an ecoregion median S deposition range from 5 to 10 
kg S/ha-yr, the PA recognizes several important considerations. First, 
monitor concentrations of SO2 can vary substantially across 
the U.S., reflecting the distribution of sources, and other factors 
such as meteorology. This complicates consideration of how the EAQM-
max, the maximum contributing monitor identified in the trajectory-
based analysis (summarized in section II.B above and described more 
fully in section 6.2.4 of the PA) relates to S deposition levels in 
downwind ecosystems. Another consideration is the substantial scatter 
in the relationship between S deposition estimates and measured 
SO2 concentrations with ecoregion median S deposition values 
below 5 kg/ha-yr. This scatter in the relationship between measured 
SO2 concentration and S deposition estimates at these lower 
deposition levels contributes increased uncertainty to conclusions 
regarding potential secondary standard SO2 metric levels 
intended to relate to ecoregion median deposition levels at or below 5 
kg/ha-yr (PA, section 7.2.2.3).
    With regard to consideration of relationships between S deposition 
and PM2.5, poor correlations were observed for total S 
deposition estimates with PM2.5 at the 27 Class I area sites 
(r=0.33, PA, Figure 6-31), with not much stronger correlations for 
ecoregion S deposition estimates with PM2.5 at upwind sites 
of influence from the trajectory-based analysis (r=0.22 and 0.48, PA, 
Table 6-12). While the correlations in the trajectory-based analyses 
for deposition in eastern ecoregions were much higher (r=0.83 and 
0.90), the coefficients were negative for deposition in western 
ecoregions. The PA concluded that the preponderance of western sites in 
the Class I area dataset (20 of the 27 sites) may be an influence on 
the low correlation observed for that dataset. Given that the analyses 
involving total S deposition and ambient air 
SO42- concentrations are at remote locations 
(Class I areas), distant from sources of SO2 emissions, and 
that that relationship is not stronger than that for SO2 at 
the SLAMS, which are near sources monitoring SO2 (the source 
for atmospheric SO42-), the PA found that the 
analyses did not indicate an advantage for an indicator based on 
SO42- measurements (or 
SO42- and SO2 combined), such as is 
currently collected at CASTNET sites, or PM2.5 mass over 
options for a potential annual average standard metric focused on 
SO2 concentrations (based on FRM/FEMs).\72\
---------------------------------------------------------------------------

    \72\ It is also of note that use of SO42- 
measurements, alone or in combination with SO2 
concentrations, as an indicator of a new standard would entail 
development of sample collection and analysis FRM/FEMs and of a 
surveillance network.
---------------------------------------------------------------------------

d. Nitrogen Deposition and N Oxides and PM
    The subsections below summarize the evidence and exposure/risk-
based considerations of the PA pertaining to N deposition and 
concentrations of N oxides and PM in ambient air. These considerations 
draw on the available welfare effects evidence described in the current 
ISA (as well as prior ISAs and AQCDs), and discussed in Chapters 4, 5 
and 6 of the PA. The focus of these considerations is primarily on N 
deposition and effects other than aquatic acidification (PA, sections 
4.3, 5.2 and 5.3). As recognized in section II.D above, the PA finds S 
deposition to be the dominant influence on aquatic acidification risk 
in the 20-year period analyzed (2001-2020), based on the finding that 
the inclusion of acidic N deposition to the aquatic acidification risk 
analyses did not appreciably change patterns and percentages of 
waterbodies estimated to exceed CLs for the three ANC targets (PA, 
section 5.1.2.4).
    In considering potential public welfare protection from N 
deposition-related effects (in light of the evidence summarized in 
sections II.C.1 and II.C.3 above), the PA recognizes that the effects 
of N deposition in both aquatic and terrestrial ecosystems have 
potential public welfare implications (PA, section 7.2.3.2). For 
example, in the case of eutrophication in large estuaries and coastal 
waters of the eastern U.S., the public welfare significance of effects 
related to decades of N loading is illustrated by the broad State, 
local and national government engagement in activities aimed at 
assessing and reducing the loading (PA, section 5.2.3). This 
significance relates both to the severity of the effects and the wide-
ranging public uses dependent on these waters. These waterbodies are 
important sources of fish and shellfish production, capable of 
supporting large stocks of resident commercial species and serving as 
breeding grounds and interim habitat for several migratory species, and 
also provide an important and substantial variety of cultural ecosystem 
services, including water-based recreational and aesthetic services, as 
well as non-use benefits to the public. The impacts of eutrophication 
relate to the consequence of the rapid and appreciable algal growth it 
fuels. Decomposition of the plant biomass from the subsequent algal 
die-off contributes to reduced waterbody oxygen which, among other 
things, in turn contributes to fish

[[Page 26670]]

mortality, and changes in aquatic habitat related to changes in 
resident plant and animal species (PA, section 4.3; ISA, Appendix 7).
    The relative contribution of atmospheric deposition to total N 
loading, however, varies widely among estuaries and has declined in 
more recent years, contributing a complexity to considerations in this 
review. While such complications may not affect smaller, more isolated 
fresh waterbodies for which N loading is primarily from atmospheric 
deposition, the evidence with regard to public welfare significance of 
any small deposition-related effects in these systems is less clear and 
well established. For example, the public welfare implications of 
relatively subtle effects of N enrichment in aquatic systems, such as 
shifts in phytoplankton species communities in remote alpine lakes, are 
not clear. Additionally, the public welfare implications of 
HNO3 effects on lichens (which might be considered to be 
``direct'' effects or the result of deposition onto plant surfaces) are 
also not clear and might depend on the extent to which they impact 
whole communities, other biota, or ecosystem structure and function 
(PA, section 7.2.3.2).
    With regard to N enrichment in terrestrial ecosystems, the 
associated effects may vary with regard to public welfare implications. 
As noted above with regard to impacts of aquatic acidification, the PA 
recognizes that some level of N deposition and associated effects on 
terrestrial ecosystems can impact the public welfare and thus might 
reasonably be judged adverse to the public welfare. Depending on 
magnitude and the associated impacts, there are situations in which N 
deposition and associated nutrient enrichment-related impacts might 
reasonably be concluded to be significant to the public welfare, such 
as N deposition that alters forest ecosystem community structures in 
ways that appreciably affect use and enjoyment of those areas by the 
public (PA, section 7.2.3.2).
    A complication to consideration of public welfare implications that 
is specific to N deposition in terrestrial systems is its potential to 
increase growth and yield of agricultural and forest crops (including 
timber), which may be judged and valued differently than changes in 
growth of some species in natural ecosystems. Nitrogen enrichment in 
natural ecosystems can, by increasing growth of N limited plant 
species, change competitive advantages of species in a community, with 
associated impacts on the composition of the ecosystem's plant 
community. The public welfare implications of such effects may vary 
depending on their severity, prevalence or magnitude, such as with only 
those rising to a particular severity (e.g., with associated 
significant impact on key ecosystem functions or other services), 
magnitude or prevalence considered of public welfare significance (PA, 
section 7.2.3.2).
(1) Quantitative Information for Ecosystem Risks Associated With N 
Deposition
    The PA considers the available information regarding air quality 
and atmospheric deposition and risk or likelihood of occurrence of 
ecosystem effects under differing conditions. In so doing, the PA notes 
the varying directionality of some of the N enrichment-related effects 
in terrestrial ecosystems, such that some effects can, in particular 
ecosystems and for particular species seem beneficial (e.g., to growth 
or survival of those species), although in a multispecies system, 
effects are more complex with potential for alteration of community 
composition. The information is also considered with regard to the key 
limitations and associated uncertainties of this evidence.
    Beginning with the appreciable evidence base documenting 
assessments of N loading to waterbodies across the U.S., the PA notes 
the waterbody-specific nature of such responses and the relative role 
played by atmospheric deposition, among other N sources. For example, 
the relative contribution to such loading from atmospheric deposition 
compared to other sources (e.g., agricultural runoff and wastewater 
discharges) varies among waterbody types and locations, which can be a 
complicating factor in quantitative analyses. Additionally, 
characteristics of resident biota populations and other environmental 
factors are influential in waterbody responses to N loading, e.g., 
temperature, organic microbial community structure, and aquatic habitat 
type, among others (ISA, Appendix 7). Based on identification of 
eutrophication as a factor in impacts on important fisheries in some 
estuaries across the U.S., multiple government and nongovernment 
organizations have engaged in research and water quality management 
activities over the past several decades in large and small estuaries 
and coastal waters across the U.S. These activities have generally 
involved quantitative modeling of relationships between N loading and 
water quality parameters such as dissolved oxygen (ISA, Appendix 7, 
section 7.2). This research documents both the impacts of N enrichment 
in these waterbodies and the relationships between effects on waterbody 
biota, ecosystem processes and functions, and N loading (PA, section 
5.2.3). The evidence base recognizes N loading to have contributions 
from multiple types of sources to these large waterbodies and their 
associated watersheds, including surface and ground water discharges, 
as well as atmospheric deposition. Accordingly, loading targets or 
reduction targets identified for these systems have generally been 
identified in light of policy and management considerations related to 
the different source types, as discussed further in section II.E.1.d(2) 
below.
    Focused assessments in freshwater lakes, including alpine lakes, 
where atmospheric deposition may be the dominant or only source of N 
loading, also provide evidence linking N loading with seemingly subtle 
changes (PA, section 5.2.2). Such seemingly subtle changes include 
whether P or N is the nutrient limiting phytoplankton growth (and 
productivity) and shifts in phytoplankton community composition, for 
which public welfare implications are less clear (PA, section 7.2.3.1). 
An additional type of aquatic ecosystem effect recognized in the 
available evidence for N loading, particularly to freshwaters, relates 
to an increase in the toxicity of the organic material released by 
algae that is associated with harmful algal blooms (ISA, Appendix 9, 
section 9.2.6.1). Information available in this review indicates that 
growth of some harmful algal species, including those that produce 
microcystin (one of the chemicals associated with harmful blooms), are 
favored by increased availability of N and its availability in 
dissolved inorganic form (ISA, Appendix 9, p. 9-28). Although this is 
an active research area, few if any datasets are currently available 
that quantitatively relate N loading to risk of harmful blooms, 
including those that may distinguish roles for different deposition 
components such as deposition of oxidized N or of particulate reduced N 
distinguished from that of N loading via dry deposition of reduced N.
    With regard to terrestrial ecosystems and effects on trees and 
other plants, the PA recognizes the complexity, referenced above, that 
poses challenges to approaches for simulating terrestrial ecosystem 
responses to N deposition across areas diverse in geography, geology, 
native vegetation, deposition history, and site-specific aspects of 
other environmental characteristics. In its consideration of the 
different types of quantitative analysis, the PA recognizes

[[Page 26671]]

limitations particular to each, and associated uncertainties. 
Uncertainties associated with the soil acidification modeling analyses 
in the last review include those associated with the limited dataset of 
laboratory-generated data on which the BC:Al targets are based, as well 
as the steady-state modeling parameters, most prominently those related 
to base cation weathering and acid-neutralizing capacity (PA, section 
5.3.4.1). Uncertainties associated with experimental addition analyses 
include the extent to which the studies reflect steady-state 
conditions, as well as a lack of information regarding historic 
deposition at the study locations (PA, section 5.3.4.1). Several 
aspects of observational or gradient studies of tree growth and 
survival (or of species richness for herbs, shrubs and lichens) 
contribute uncertainties to identification of deposition levels of 
potential concern for tree species effects, including unaccounted-for 
factors with potential influence on tree growth and survival (e.g., 
ozone and soil characteristics), as well as the extent to which 
associations may reflect the influence of historical deposition 
patterns and associated impact. Thus, while the evidence is robust as 
to the ability for N loading from deposition to contribute to changes 
in plant growth and survival and associated alterations in terrestrial 
plant communities, a variety of factors, including the history of 
deposition and variability of response across the landscape, complicate 
our ability to quantitatively relate specific N deposition rates, 
associated with various air quality conditions, to N enrichment-related 
risks of harm to forests and other plant communities in areas across 
the U.S. (PA, section 5.3.4).
(2) General Approach for Considering Public Welfare Protection
    As an initial matter, the PA notes that the effects of 
acidification on plant growth and survival, at the individual level, 
are generally directionally harmful, including reduced growth and 
survival. In contrast, the effects of N enrichment can, in particular 
ecosystems and for particular species, be beneficial or harmful (e.g., 
to growth or survival of those species). Accordingly, there is added 
complexity to risk management policy decisions for this category of 
effects, including the lack of established risk management targets or 
objectives, particularly in light of historical deposition and its 
associated effects that have influenced the current status of 
terrestrial ecosystems and their biota, structure, and function.
    Further, the PA recognizes the contribution to N deposition of 
atmospheric pollutants other than the criteria pollutants N oxides and 
PM, most significantly the contribution of NH3 (PA, section 
6.2.1). This contribution has increased since the last reviews of the 
NO2 and PM secondary standards, as seen in Figures 6-17, 6-
18 and 6-19 of the PA, reflecting increases in NH3 emissions 
over that time period. These trends of increased NH3 
emissions and reduced N deposition coincide with decreasing trends in N 
oxides emissions and associated contributions of oxidized N to total N 
deposition (PA, Figures 6-3 and 6-19). The TDep estimates of different 
types of N being deposited at the 92 CASTNET sites indicate that since 
about 2015, reduced N compounds comprise a greater proportion of total 
N deposition than do oxidized compounds, with reduced N in recent years 
generally accounting for more than 50% of total N deposition (PA, 
Figure 6-19). Further, dry deposition of NH3 as a percentage 
of total N deposition at CASTNET sites ranges up to a maximum of 65% at 
the highest site in 2021 (PA, Figure 6-19). The 75th percentile for 
these sites is greater than 30% (i.e., at 25% of the CASTNET sites, 
more than 30% of N deposition is from dry deposition of 
NH3). This is a noteworthy value given that these sites are 
generally in the West, with few in the areas of highest NH3 
emissions where the percentage would be expected to be higher still 
(PA, Figures 6-20 and 2-9).
    In light of the contrasting temporal trends for emissions of 
oxidized and reduced N compounds, the PA observes that the influence of 
ambient air concentrations of N oxides and PM on N deposition appears 
to have declined over the past 20 years, complicating consideration of 
the protection from N deposition-related effects that can be provided 
by secondary NAAQS for these pollutants. Thus, the PA finds that 
NH3, which is not a criteria pollutant, and its contribution 
to total N deposition, particularly in parts of the U.S. where N 
deposition is highest, is a complicating factor in considering policy 
options related to NAAQS for addressing ecological effects related to N 
deposition (e.g., PA, Figure 6-18 and 6-13).
    In considering what the currently available quantitative 
information regarding terrestrial ecosystem responses to N deposition 
indicates about levels of N deposition that may be associated with 
increased concern for adverse effects, the PA focuses first on the 
evidence for effects of N deposition on trees derived from both 
experimental addition studies and observational studies of potential 
relationships between tree growth and survival and metrics for N 
deposition. With regard to the information available from experimental 
addition tree studies, the PA recognizes study limitations and 
associated uncertainties, while noting that the lowest forest N 
addition that elicited effects was 15 kg N/ha-yr over a 14-year period 
occurring from 1988-2002 (PA, sections 5.3.2 and 7.2.3.2 and Appendix 
5B, Table 5B-1; McNulty et al., 2005). Based on the estimates from the 
array of observational studies, the PA finds that N deposition with a 
range of 7 to 12 kg/ha-yr, on a large area basis, may be a reasonable 
characterization of conditions for which statistical associations have 
been reported for terrestrial effects, such as reduced tree growth and 
survival and species richness of herbs and shrubs (PA, sections 5.3.4 
and 7.2.3.2).
    With regard to observational or gradient studies of N deposition 
and tree growth and survival (or mortality), the PA gave particular 
attention to three recently available studies that used the U.S. Forest 
Service dataset of standardized measurements at sites across the U.S. 
(Dietze and Moorcroft, 2011; Thomas et al., 2010; Horn et al., 2018). 
These studies cover overlapping areas of the U.S. (PA, Appendix 5B, 
Figure 5B-1) and report associations of tree growth and/or survival 
metrics with various N deposition metrics for three different time 
periods. These studies provide support to conclusions regarding a role 
for N deposition in affecting tree health in the U.S., most 
particularly in regions of the eastern U.S., where confidence in the 
study associations is greatest (PA, section 5.3.2.3 and Appendix 5B, 
section 5B.3.2). In considering information from these studies, the PA 
notes the history of N deposition in the eastern U.S. and the 
similarity between geographic patterns of historical deposition and 
more recent deposition patterns in the U.S., which may influence the 
findings of observational studies, contributing an uncertainty to 
estimates of a specific magnitude of deposition rate that might be 
expected to elicit specific tree responses, such as increased or 
decreased growth or survival (PA, sections 5.3.2 and 7.2.3.2 and 
Appendix 5B). The largest study, which included 71 species with ranges 
across the U.S., reported associations of tree survival and growth with 
N deposition that varied from positive to negative across the range of 
deposition at the measurement plots for some species, and also varied 
among species (PA,

[[Page 26672]]

Appendix 5B, section 5B.3.2.3; Horn et al., 2018). The median 
deposition values across the sample sites for species with significant 
positive or negative associations generally ranged from 7 to 12 kg N/
ha-yr (PA, section 5.3.2 and Appendix B, section 5B.3.2.3). Among the 
species for which the association varied from negative to positive 
across deposition levels, this is the range for those species for which 
the association was negative at the median deposition value (PA, 
section 5.3.4).\73\
---------------------------------------------------------------------------

    \73\ This also excluded species for which sample sites were 
limited to the western U.S. based on recognition by the study 
authors of greater uncertainty in the west (Horn et al., 2018).
---------------------------------------------------------------------------

    With regard to studies of herb and shrub community response, the PA 
notes a number of recently available studies report on addition 
experiments (PA, section 5.3.3.1 and Appendix 5B, section 5B.3.1). The 
lowest rate of N addition, in an addition study, for which community 
effects have been reported include 10 kg N/ha-yr. With an addition of 
10 kg N/ha-yr over a 10-year period, grassland species numbers 
declined; in a subset of plots for which additions then ceased, 
relative species numbers increased, converging with controls after 13 
years (PA, Appendix 5B, Table 5B-7; Clark and Tilman, 2008). Recent 
gradient studies of coastal sage scrub in southern California have 
indicated N deposition above 10 or 11 kg/ha-year to be associated with 
increased risk of conversion to non-native grasslands or reduced 
species richness (PA, Appendix 5B; section 5B.3.2; Cox et al., 2014; 
Fenn et al., 2010). A larger observational study of herb and shrub 
species richness in open- and closed-canopy communities using a 
database of site assessments conducted over a 23-year period and 
average N deposition estimates for a 26-year period, reported 
significant influence of soil pH on the relationship between species 
richness and N deposition metric. A negative association was observed 
for acidic (pH 4.5) forested sites with N deposition estimates above 
11.6 kg N/ha-yr and for low pH open canopy sites (woods, shrubs and 
grasses) with N deposition estimates above 6.5 kg N/ha-yr (PA, section 
5.3.3.1). Lastly, the PA notes the observational studies that have 
analyzed variation in lichen community composition in relation to 
indicators of N deposition (PA, section 5.3.3.2 and Appendix 5B, 
section 5B.4.2). In addition to limitations with regard to 
interpretation, uncertainties associated with these studies include 
alternate methods for utilizing N deposition estimates as well as the 
potential influence of unaccounted-for environmental factors, e.g., 
ozone, SO2, and historical air quality and associated 
deposition (PA, section 5.3.3.2).
    With regard to the evidence for effects of N deposition in aquatic 
ecosystems, we recognize several different types of information and 
evidence. This information includes the observational studies utilizing 
statistical modeling to estimate critical loads, such as those related 
to subtle shifts in the composition of phytoplankton species 
communities in western lakes. This also includes the four to five 
decades of research on the impacts and causes of eutrophication in 
large rivers and estuaries. In considering this diverse evidence base, 
we take note of the robust evidence base on N loading and 
eutrophication, with its potentially significant impacts on submerged 
aquatic vegetation and fish species, particularly in large river 
systems, estuaries, and coastal systems.
    As noted above, the public attention, including government 
expenditures, that has been given to N loading and eutrophication in 
several estuarine and coastal systems are indicative of the recognized 
public welfare implications of related impacts. In large aquatic 
systems across the U.S., the relationship between N loading and algal 
blooms, and associated water quality impacts (both short- and longer-
term), has led to numerous water quality modeling projects to inform 
water quality management decision-making in multiple estuaries, 
including Chesapeake Bay, Narraganset Bay, Tampa Bay, Neuse River 
Estuary and Waquoit Bay (ISA, Appendix 7, section 7.2). These projects 
often use indicators of nutrient enrichment, such as chlorophyll a, 
dissolved oxygen, and abundance of submerged aquatic vegetation, among 
others (ISA, section IS.7.3 and Appendix 10, section 10.6). For these 
estuaries, the available information regarding atmospheric deposition 
and the establishment of associated target loads varies across the 
various estuaries (ISA, Appendix 7, Table 7-9). Further, in many cases 
atmospheric loading has decreased since the initial modeling analyses.
    As summarized in section II.C.3 above, analyses in multiple East 
Coast estuaries--including Chesapeake Bay, Tampa Bay, Neuse River 
Estuary and Waquoit Bay--have considered atmospheric deposition as a 
source of N loading (ISA, Appendix 7, section 7.2.1). Total estuary 
loading or loading reductions were established in TMDLs developed under 
the Clean Water Act for these estuaries. Levels identified for 
allocation of atmospheric N loading in the first three of these 
estuaries were 6.1, 11.8 and 6.9 kg/ha-yr, respectively, and 
atmospheric loading estimated to be occurring in the fourth was below 5 
kg/ha-yr (PA, section 7.3). This information, combined with the 
information from terrestrial studies summarized above, led to the PA 
identifying 7-12 kg/ha-yr as an appropriate N deposition range on which 
to focus in considering policy options (PA, section 7.2.3.2).
(3) Relating Air Quality Metrics to N Deposition Associated With N 
Oxides and PM
    In exploring how well various air quality metrics relate to N 
deposition, the analyses in Chapter 6 of the PA examine the 
relationships between air concentrations, in terms of various air 
quality metrics (including design values for the current standards), 
and N deposition in areas near or downwind from the ambient air 
monitoring sites. The PA finds the analyses utilizing data from 
monitors using FRM/FEM to collect ambient air concentration data for 
evaluation with the NAAQS (e.g., to identify violations) to be 
particularly relevant given that the current standards are judged using 
design values derived from FRM/FEM measurements at existing SLAMS (PA, 
section 7.2.3.3). Given their role in monitoring for compliance with 
the NAAQS, most or many of these monitors are located in areas of 
relatively higher pollutant concentrations, such as near large sources 
of NO2 or PM. Accordingly, the PA notes that information 
from these monitoring sites can help inform how changes in 
NO2 and/or PM emissions, reflected in ambient air 
concentrations, relate to changes in deposition and, correspondingly, 
what secondary standard options might best regulate ambient air 
concentrations such that deposition in sensitive ecosystems of interest 
is maintained at or below levels of potential concern.
    In considering the information and findings of these analyses 
regarding relationships between N deposition and N oxides and PM in 
ambient air, the PA considers the current forms and averaging times of 
the secondary PM and NO2 NAAQS. For N oxides, the current 
secondary standard is the annual average of NO2, and that 
for PM is the average of three consecutive years of annual averages. As 
in the assessments of S deposition and air quality metrics, the 
quantitative air quality and N deposition analyses in the PA focus on 
3-year average metrics (e.g., annual average NO2 and N 
deposition, averaged over three years) and include multiple time 
periods of data to better

[[Page 26673]]

assess more typical relationships. For consistency and simplicity, most 
of these air quality-deposition analyses focus on the five 3-year 
periods also used for S deposition and SOX: 2001-03, 2006-
08, 2010-12, 2014-16 and 2018-20.
    As an initial matter, the PA notes that relationships between N 
deposition and NO2 and PM air quality are affected by 
NH3 emissions and non-N-containing components of PM (PA, 
section 6.4.2). The PA further notes that the influence of these 
factors on the relationships has varied across the 20-year evaluation 
period and varies across different regions of the U.S. (PA, section 
6.2.1). Both of these factors are recognized to influence relationships 
between total N deposition and NO2 and PM air quality 
metrics.
    For total N deposition estimated for grid cells with collocated 
SLAMS monitors, the correlations with annual average NO2 
concentrations, averaged over three years, are generally low across all 
sites and particularly in the East (e.g., correlation coefficients 
below 0.4), although somewhat higher for sites in the West (PA, Table 
6-6). This likely reflects the relatively greater role of 
NH3 in N deposition in the East, which for purposes of the 
analyses in this PA extends across the Midwest (PA, section 6.4.2). For 
N deposition and NO2 at upwind monitoring sites of 
influence, the correlation between estimates of total N deposition (wet 
plus dry) in eastern ecoregions and annual average NO2 
concentrations at monitor sites of influence (identified via 
trajectory-based modeling) for the five periods from 2001-2020 is low 
to moderate (correlation coefficients below 0.4, with the exception of 
one for EAQM-weighted in 2001-03 at 0.6), with the earlier part of the 
20-year period, when NO2 concentrations were higher and 
NH3 emissions were lower (as indicated by Figures 6-6 and 6-
5 of the PA) having relatively higher correlation than the later part 
(PA, Figures 6-6 and 6-4). The correlation is negative or near zero for 
the western ecoregions (PA, section 6.2.4).
    The reductions in NO2 emissions over the past 20 years 
have been accompanied by a reduction in deposition of oxidized N (PA, 
section 6.2.1). However, increases in NH3 emissions, 
particularly in the latter 10 years of the period analyzed (2010-2020), 
have modified the prior declining trend in total N deposition. That is, 
coincident with the decreasing trends in NO2 emissions and 
in deposition of oxidized N in the past 10 years there is a trend of 
increased NH3 and increased deposition of reduced N 
(NH3 and NH4\+\), most particularly in areas of 
the Midwest, Texas, Florida and North Carolina (PA, Figures 6-16 and 6-
17). The PA finds this to indicate that, while in the earlier years of 
the assessment period controls on NO2 emissions may have 
resulted in reductions in deposition of oxidized N, in more recent 
years they have much less influence on total N deposition (PA, sections 
6.2.1 and 6.4). In terms of ecoregion median statistics, the PA 
observes the decreasing trend in ecoregion median total N deposition 
across the period from 2001 through 2012, while taking note that from 
2012 onward total N deposition increases, most particularly in 
ecoregions where most of the total deposition is from reduced N (PA, 
Figure 7-6).
    The PA also considers the impact of increasing deposition of 
reduced N on the 20-year trend in total N deposition as illustrated by 
TDep estimates at the 92 CASTNET sites. At these sites, the PA observes 
that the median percentage of total N deposition comprised by oxidized 
N species, which is driven predominantly by N oxides, has declined from 
more than 70% to less than 45% (PA, Figure 6-19). Based on examination 
of the trends for components of reduced N deposition, the PA notes that 
the greatest influence on the parallel increase in N deposition 
percentage composed of reduced N is the increasing role of 
NH3 dry deposition. The percentage of total N deposition at 
the CASTNET sites that is from NH3 has increased, from a 
median below 10% in 2000 to a median somewhat above 25% in 2021 (PA, 
Figure 6-19).
    Recognizing limitations in the extent to which CASTNET sites can 
provide information representative of the U.S. as a whole, the PA also 
analyzed TDep estimates for the most recent period (2018-2020) with 
regard to the percent of total N deposition represented by reduced N 
across the U.S. In areas with ecoregion median total N deposition above 
9 kg/ha-yr (PA, Figure 7-7, upper panel), the ecoregion median 
percentage of total N deposition composed of reduced N is greater than 
60% (PA, Figure 7-7, lower panel). The PA further notes that recent 
(2019-2021) TDep estimates across individual TDep grid cells similarly 
show that areas of the U.S. where total N deposition is highest, and 
where it is greater than potential N deposition targets identified in 
section 7.2.3.2 of the PA, are also the areas with the greatest 
deposition of NH3 (PA, Figure 7-8), comprising more than 30% 
of total N deposition. That is, the PA finds that NH3 driven 
deposition is greatest in regions of the U.S. where total deposition is 
greatest (PA, section 7.2.3.3).
    Turning to PM2.5, the PA notes that, as with 
NO2 concentrations, the correlation for ecoregion median N 
deposition and PM2.5 concentrations at upwind sites of 
influence is better for eastern ecoregions than western ecoregions, for 
which there is no correlation at all (PA, section 6.2.4). For total N 
deposition and PM2.5 concentrations at SLAMS, a low to 
moderate correlation is observed, also slightly higher at eastern 
versus western sites (PA, section 6.2.3). In considering the two 
factors mentioned above (NH3 emissions and non-N containing 
components of PM), the PA notes that some NH3 transforms to 
NH4\+\, which is a component of PM2.5, while also 
noting that, in the areas of greatest N deposition, the portion 
represented by deposition of gaseous NH3 generally exceeds 
30%. Additionally, while NH3 emissions have been increasing 
over the past 20 years, the proportion of PM2.5 that is 
composed of N compounds has declined. The median percentage of 
PM2.5 comprised by N compounds has declined from about 25% 
in 2006-2008 to about 17% in 2020-2022 and the highest percentage 
across sites declined from over 50% to 30% (PA, section 6.4.2 and 
Figure 6-56). Further, this percentages varies regionally, with sites 
in the nine southeast states having less than 10% of PM2.5 
mass composed of N compounds (PA, Figure 6-56).
    In summary, the PA concludes that in recent years, NH3, 
which is not a criteria pollutant, contributes appreciably to total N 
deposition, particularly in parts of the country where N deposition is 
highest (as illustrated by comparison of Figures 6-13 and 6-18 of the 
PA). The PA finds that this situation--of an increasing, and spatially 
variable, portion of N deposition not being derived from N oxides or 
PM--complicates assessment of policy options for protection against 
ecological effects related to N deposition associated with N oxides and 
PM, and for secondary standards for those pollutants that may be 
considered to be associated with a desired level of welfare protection. 
That notwithstanding, recognizing that between the NO2 
primary and secondary NAAQS, the 1-hour primary standard (established 
in 2010) may be the more controlling on ambient air concentrations, the 
PA considered analyses of SLAMS air quality data with regard to trends 
in annual average NO2 concentrations (PA, Figure 7-9) and 
relationships between annual average NO2 concentrations (in 
a single year and averaged over three years) and design

[[Page 26674]]

values for the existing 1-hour primary standard (PA, Figure 7-10). In 
so doing, the PA noted that subsequent to 2011-2012, when ecoregion 
median N deposition levels in 95% of the eastern ecoregions of the 
continental U.S.\74\ have generally been at/below 11 kg N/ha-yr, annual 
average NO2 concentrations, averaged across three years, 
have been at/below 35 ppb (PA, Figures 7-6 and 7-9). Further, the SLAMS 
data indicate that single-year annual average NO2 
concentrations, averaged over three years, in areas that meet the 
current 1-hour primary standard have generally been below approximately 
35 to 40 ppb (PA, Figure 7-10). The PA recognizes, however, that this 
information as a whole also suggests the potential for future 
reductions in N oxide-related N deposition to be negated by increasing 
reduced N deposition.
---------------------------------------------------------------------------

    \74\ As noted in the PA, the eastern designation used throughout 
PA includes areas generally considered the Great Plains, while the 
West includes the states of ND, SD, CO, WY, MT, AZ, NM, UT, ID, CA, 
OR, WA (PA, p. 5-20).
---------------------------------------------------------------------------

    Further, the PA notes that the results also suggest that the 
PM2.5 annual average standard may provide some control of N 
deposition associated with PM and N oxides, but also notes that 
PM2.5 monitors, while capturing some compounds that 
contribute to S and N deposition across the U.S., also capture other 
non-S and non-N related pollutants (e.g., organic and elemental carbon) 
as part of the PM2.5 mass (PA, section 7.2.3.3). The amounts 
of each category of compounds varies regionally (and seasonally), and 
as noted above, N compounds generally comprise less than 30% of total 
PM2.5 mass (PA, section 6.3 and 6.4).
    In considering relationships between air quality metrics based on 
indicators other than those of the existing standards and N deposition 
(and associated uncertainties), the PA drew on the analyses of 
relationships for collocated measurements and modeled estimates of N 
compounds other than NO2 with N deposition in a subset of 27 
CASTNET sites located in 27 Class I areas, the majority of which (21 of 
27) are located in the western U.S. (PA, sections 6.2.2, 6.3 and 
6.4.2). The analyses indicate that total N deposition (TDep) in these 
rural areas has a moderate correlation with air concentrations of 
nitric acid and particulate nitrate for the 20-year dataset (2000-2020) 
(PA, Figure 6-32). The correlations are comparable to the correlation 
of NO2 with total N dep (TDep) at western SLAMS, a not 
unexpected observation given that more than 75% of the 27 CASTNET sites 
are in the West. A much lower correlation was observed at SLAMS in the 
East, and with the trajectory-based dataset. The PA notes that 
deposition at the western U.S sites is generally less affected by 
NH3 (PA, section 6.4.2). Further, the observed trend of 
increasing contribution to N deposition of NH3 emissions 
over the past decade suggests that such correlations of N deposition 
with oxidized N may be still further reduced in the future. Thus, the 
PA concludes that the evidence does not provide support for the 
oxidized N compounds (as analyzed at the 27 Class I sites) as 
indicators of total atmospheric N deposition, especially in areas where 
NH3 is prevalent (PA, section 7.2.3.3).
    The analyses involving N deposition and N-containing PM components 
at the 27 Class I area sites yield similar correlation coefficients as 
those for N deposition (TDep) and PM2.5 at SLAMS monitors 
(PA, section 7.2.3.3 and Figures 6-33, 6-39 [upper panel], and 6-32 
[left panel]). Further, the graphs of total N deposition estimates 
versus total N at the 27 Class I area sites indicate the calculated 
correlations (and slopes) likely to be appreciably influenced by the 
higher concentrations occurring in the first decade of the 20-year 
timeframe (PA, Figure 6-33). Thus, the PA concludes that the available 
analyses of N-containing PM2.5 components at the small 
dataset of sites remote from sources also do not indicate an overall 
benefit or advantage over consideration of PM2.5 (PA, 
section 7.4). As a whole, the PA finds that the limited dataset with 
varying analytical methods and monitor locations, generally distant 
from sources, does not clearly support a conclusion that such 
alternative indicators might provide better control of N deposition 
related to N oxides and PM over those used for the existing standards 
(PA, section 7.2.3.3). The PA also notes that use of the 
NO3- or particulate N measurements analyzed with 
deposition estimates at the 27 Class I area sites, alone or in 
combination with NO2, as an indicator for a new standard 
would entail development of sample collection and analysis FRM/FEMs 
\75\ and of a surveillance network.
---------------------------------------------------------------------------

    \75\ For example, sampling challenges have long been recognized 
for particulate NH4\+\ (e.g., ISA, Appendix 2, sections 
2.4.5; 2008 ISA, section 2.7.3).
---------------------------------------------------------------------------

2. CASAC Advice and Public Comments
    In evaluating the adequacy of the current secondary standards for 
SOX, oxides of N, and PM, in addition to evidence and air 
quality/exposure/risk-based information discussed above, we take note 
of the advice and recommendations of the CASAC, based on its review of 
the draft ISA and draft PA, as well as comments from the public. A 
limited number of public comments have been received in the docket for 
this review to date, including just a few comments on the draft PA, 
that primarily focused on technical analyses and information, which 
were considered in developing the final PA (PA, section 1.4). The few 
public commenters that addressed the adequacy of the current secondary 
standards or potential alternative options to achieve appropriate 
public welfare protection expressed the view that the available 
evidence does not indicate the need for revision of the existing 
standards. The remainder of this section focuses on advice and 
recommendations from the CASAC regarding the standards review based on 
the CASAC's review of the draft PA.
    In reviewing the draft PA, the CASAC first recognized that 
``translation of deposition-based effects to an ambient concentration 
in air is fraught with difficulties and complexities'' (Sheppard, 2023, 
pp. 1-2). Further, the CASAC expressed its view that, based on its 
interpretation of the Clean Air Act, NAAQS could be in terms of 
atmospheric deposition, which it concluded ``would be a cleaner, more 
scientifically defensible approach to standard setting.'' Accordingly, 
the CASAC recommended that direct atmospheric deposition standards be 
considered in future reviews (Sheppard, 2023, pp. 2 and 5). The CASAC 
then, as summarized below, provided recommendations regarding standards 
based on air concentrations.
    With regard to protection from effects other than those associated 
with ecosystem deposition of S and N compounds, the CASAC concluded 
that the existing SO2 and NO2 secondary standards 
provide adequate protection for direct effects of those pollutants on 
plants and lichens, providing consensus recommendations that these 
standards should be retained without revision for this purpose 
(Sheppard, 2023, p. 5 of letter and p. 23 of Response to Charge 
Questions). With regard to deposition-related effects of S and N 
compounds, the CASAC members did not reach consensus, with their advice 
divided between a majority opinion and a minority opinion. Advice 
conveyed from both the majority and minority groups of members 
concerning deposition-related effects is summarized here.
    With regard to deposition-related effects of S and standards for 
SOX, the majority of CASAC members recommended a new annual 
SO2

[[Page 26675]]

standard with a level in the range of 10 to 15 ppb,\76\ which these 
members concluded would generally maintain ecoregion median S 
deposition below 5 kg/ha-yr \77\ based on consideration of the 
trajectory-based SO2 analyses (and associated figures) in 
the draft PA (Sheppard, 2023, Response to Charge Questions, p. 25). 
They concluded that such a level of S deposition would afford 
protection for tree and lichen species \78\ and aquatic ecosystems. 
Regarding aquatic ecosystems, these members cited the ecoregion-scale 
estimates (from the aquatic acidification REA analyses) associated with 
median S deposition bins for the 90 ecoregion-time period combinations 
(PA, section 5.1.3.2) in conveying that for S deposition below 5 kg/ha-
yr, 80%, 80% and 70% of waterbodies per ecoregion are estimated to 
achieve an ANC at or above 20, 30 and 50 [micro]eq/L, respectively, in 
all ecoregion-time period combinations (Sheppard, 2023, Response to 
Charge Questions, p. 25).\79\ In recommending an annual SO2 
standard with a level in the range of 10 to 15 ppb, these members 
stated that such a standard would ``preclude the possibility of 
returning to deleterious deposition values as observed associated with 
the emergence of high annual average SO2 concentrations near 
industrial sources in 2019, 2020, and 2021,'' citing Figure 2-25 of the 
draft PA \80\ (Sheppard, 2023, Response to Charge Questions, p. 24).
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    \76\ Although the CASAC letter does not specify the form for 
such a new annual standard, the justification provided for this 
recommendation cites two figures in the draft PA (Figures 6-17 and 
6-18) which presented annual average SO2 concentrations 
averaged over three consecutive years (Sheppard, 2023, Response to 
Charge Questions, p. 25). Therefore, we are interpreting the CASAC 
majority recommendation to be for an annual standard, averaged over 
three years.
    \77\ Although the CASAC letter does not specify the statistic 
for the 5 kg/ha-yr value, the PA analyses referenced in citing that 
value, both the trajectory analyses and the ecoregion-scale summary 
of aquatic acidification results, focus on ecoregion medians. 
Therefore, we are interpreting the CASAC advice on this point to 
pertain to ecoregion means.
    \78\ In making this statement, these CASAC members cite two 
observational data studies with national-scale study areas published 
after the literature cut-off date for the ISA: one study is on 
lichen species richness and abundance and the second is on tree 
growth and mortality (Geiser et al., 2019; Pavlovic et al., 2023). 
The lichen study by Geiser et al. (2019) relies on lichen community 
surveys conducted at U.S. Forest Service sites from 1990 to 2012. 
The tree study by Pavlovic et al. (2023) uses machine learning 
models with the dataset from the observational study by Horn et al. 
(2018) to estimate confidence intervals for CLs for growth and 
survival for 108 species based on the dataset first analyzed by Horn 
et al. (2018).
    \79\ As seen in tables 3 and 4 in this preamble, these levels of 
protection are also achieved in ecoregion-time period combinations 
for which the ecoregion median S deposition estimate is at or below 
7 kg/ha-yr (PA, section 7.2.2.2, Table 7-1).
    \80\ The figure cited by the CASAC majority is the prior version 
of Figure 2-28 in section 2.4.2 of the final PA. The figure presents 
temporal trend in distribution (box and whiskers) of annual average 
SO2 concentrations at SLAMS.
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    One CASAC member dissented from this recommendation for an annual 
SO2 standard \81\ and instead recommended adoption of a new 
1-hour SO2 secondary standard identical in form, averaging 
time, and level to the existing primary standard based on the 
conclusion that the ecoregion 3-year average S deposition estimates for 
the most recent periods are generally below 5 kg/ha-yr and that those 
periods correspond to the timing of implementation of the existing 
primary SO2 standard (established in 2010), indicating the 
more recent lower deposition to be a product of current regulatory 
requirements (Sheppard, 2023, Appendix A, p. A-2).\82\
---------------------------------------------------------------------------

    \81\ Also dissenting from this advice was a member of the CASAC 
Oxides of Nitrogen, Oxides of Sulfur and Particulate Matter 
Secondary NAAQS Panel who was not also a member of the CASAC 
(Sheppard, 2023, Response to Charge Questions, p. 23). The former is 
a Panel formed for this review, while the latter is the standing 
Committee specified in the CAA.
    \82\ This member stated that the existing primary NAAQS for the 
three pollutants were significantly more restrictive than the 
existing secondary standards and provide adequate protection for 
deposition-related effects (Sheppard, 2023, Appendix A).
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    With regard to N oxides and protection against deposition-related 
welfare effects of N, the majority of CASAC members recommended 
revision of the existing annual NO2 standard to a level 
``<10-20 ppb'' (Sheppard, 2023, Response to Charge Questions, p. 24). 
The justification these members provided was related to their 
consideration of the relationship presented in the draft PA of median 
ecosystem N deposition with the weighted \83\ annual average 
NO2 metric concentrations, averaged over three years, at 
monitoring sites linked to the ecosystems by trajectory-based analyses 
and a focus on total N deposition estimates at or below 10 kg/ha-yr 
(Sheppard, 2023, Response to Charge Questions, p. 24). These members 
additionally recognized, however, that ``when considering all 
ecoregions, there is no correlation between annual average 
NO2 and N deposition'' (Sheppard, 2023, Response to Charge 
Questions, p. 24). Their focus on total N deposition estimates at or 
below 10 kg/ha-yr appears to relate to consideration of TMDL analyses 
in four East Coast estuaries: Chesapeake Bay, Tampa Bay, Neuse River 
Estuary and Waquoit Bay (Sheppard, 2023, Response to Charge Questions, 
pp. 12-14 and 29). Levels identified for allocation of atmospheric N 
loading in the first three of these estuaries were 6.1, 11.8 and 6.9 
\84\ kg/ha-yr, respectively, and atmospheric loading estimated in the 
fourth was below 5 kg/ha-yr (Sheppard, 2023, Response to Charge 
Questions, pp. 12-14). These members also conclude that 10 kg N/ha-yr 
is ``at the middle to upper end of the N critical load threshold for 
numerous species effects (e.g., richness) and ecosystem effects (e.g., 
tree growth) in U.S. forests grasslands, deserts, and shrublands (e.g., 
Pardo et al., 2011; Simkin et al., 2016) and thus 10 kg N/ha-yr 
provides a good benchmark for assessing the deposition-related effects 
of NO2 in ambient air'' (Sheppard, 2023, Response to Charge 
Questions, p. 23).
---------------------------------------------------------------------------

    \83\ The weighted metric is constructed by applying weighting to 
concentrations to the monitors identified as sites of influence, 
with the weighting equal to the relative contribution of air from 
the monitor location to the downwind ecoregion based on the 
trajectory analysis (PA, section 6.2.4). Values of this metric are 
not directly translatable to individual monitor concentrations or to 
potential standard levels.
    \84\ The CASAC letter states that the Neuse River Estuary TMDL 
specified a 30% reduction from the 1991-1995 loading estimate of 9.8 
kg/ha-yr, yielding a remaining atmospheric load target of 6.9 kg/ha-
yr (Sheppard, 2023, Response to Charge Questions, p. 13).
---------------------------------------------------------------------------

    One CASAC member disagreed with revision of the existing annual 
NO2 standard and instead recommended adoption of a new 1-
hour NO2 secondary standard identical in form, averaging 
time and level to the existing primary standard based on the conclusion 
that the N deposition estimates for the most recent periods generally 
reflect reduced deposition that is a product of current regulatory 
requirements, including implementation of the existing primary 
standards for NO2 and PM (Sheppard, 2023, Appendix A). This 
member additionally noted that bringing into attainment the areas still 
out of attainment with the 2013 primary annual PM2.5 
standard (12.0 [micro]g/m\3\) will provide further reductions in N 
deposition. This member also noted his analysis of NO2 
annual and 1-hour design values for the past 10 years (2013-2022) as 
indicating that the current primary NO2 standard provides 
protection for annual average NO2 concentrations below 31 
ppb (Sheppard, 2023, Appendix A).
    With regard to PM and effects related to deposition of N and S, the 
CASAC focused on the PM2.5 standards and made no 
recommendations regarding the PM10 standard. In considering 
the annual PM2.5 standard, the majority of CASAC members 
recommended revision of the annual secondary PM2.5 standard 
to a level of 6 to 10 [micro]g/m\3\. In their justification for this 
range, these

[[Page 26676]]

members focus on rates of total N deposition at or below 10 kg/ha-yr 
and total S deposition at or below 5 kg/ha-yr that they state would 
``afford an adequate level of protection to several species and 
ecosystems across the U.S.'' (Sheppard, 2023, Response to Charge 
Questions, p. 23). In reaching this conclusion for protection from N 
deposition, the CASAC majority cited studies of U.S. forests, 
grasslands, deserts and shrublands that are included in the ISA. For S 
deposition, the CASAC majority notes the Pavlovic et al. (2023) 
analysis of the dataset used by Horn et al. (2018). Conclusions of the 
latter study (Horn et al., 2018), which is characterized in the ISA and 
discussed in sections 5.3.2.3 and 7.2.2.2 of the PA (in noting median 
deposition of 5-12 kg S/ha-yr in ranges of species for which survival 
and/or growth was observed to be associated with S deposition), are 
consistent with the more recent analysis in the 2023 publication (ISA, 
Appendix 6, sections 6.2.3 and 6.3.3).
    As justification for their recommended range of annual 
PM2.5 levels (6-10 [micro]g/m\3\), this group of CASAC 
members made several statements regarding PM2.5 annual 
concentrations and estimates of S and N deposition for which they cited 
several figures in the draft PA. Citing figures in the draft PA with 
TDep deposition estimates and IMPROVE and CASTNET monitoring data, they 
stated that ``[i]n remote areas, IMPROVE PM2.5 
concentrations in the range of 2-8 [micro]g/m\3\ for the periods 2014-
2016 and 2017-2019 correspond with total S deposition levels <5 kg/ha-
yr (Figure 6-12), with levels generally below 3 kg/ha-yr, and with 
total N deposition levels <=10 kg/ha-yr (Figure 6-13)'' (Sheppard, 
2023, Response to Charge Questions, p. 23). With regard to S 
deposition, these members additionally cited a figure in the draft PA 
as indicating ecosystem median S deposition estimates at/below 5 kg/ha-
yr occurring with PM2.5 EAQM-max values in the range of 6 to 
12 [micro]g/m\3\ (Sheppard, 2023, Response to Charge Questions, pp. 23-
24). These members additionally cited figures in the draft PA as 
indicating that areas of 2019-2021 total N deposition estimates greater 
than 15 kg/ha-yr (in California, the Midwest, and the East) correspond 
with areas where the annual PM2.5 design values for 2019-
2021 range from 6 to 12 [micro]g/m\3\, and other figures (based on 
trajectory analyses) as indicating ecosystem median N deposition 
estimates below 10 kg N/ha-yr occurring only with PM2.5 
weighted EAQM values below 6 [micro]g/m\3\,\85\ and PM2.5 
EAQM-max values below 8 [micro]g/m\3\ (Sheppard, 2023, Response to 
Charge Questions, pp. 23-24). The CASAC also noted that the correlation 
coefficient for N deposition with the weighted EAQM is 0.52, while the 
correlation coefficient with the EAQM-max is near zero (0.03). The 
bases for the N and S deposition levels targeted in this CASAC majority 
recommendation are described in the paragraphs earlier in this section.
---------------------------------------------------------------------------

    \85\ As noted earlier in this section, weighted EAQM values are 
not directly translatable to concentrations at individual monitors 
or to potential standard levels.
---------------------------------------------------------------------------

    One CASAC member recommended revision of the annual secondary 
PM2.5 standard to a level of 12 [micro]g/m\3\ based on his 
interpretation of figures in the draft PA that present S and N 
deposition estimates for five different 3-year time periods from 2001 
to 2020. This member observed that these figures indicate ecoregion 
median S and N deposition estimates in the last 10 years below 5 and 10 
kg/ha-yr, respectively. This member concluded this to indicate that the 
2013 primary annual PM2.5 standard of 12.0 [micro]g/m\3\ 
provides adequate protection against long-term annual S and N 
deposition-related effects (Sheppard, 2023, Appendix A).
    Regarding the existing 24-hour PM2.5 secondary standard, 
the majority of CASAC members recommended revision of the level to 25 
ug/m\3\ or revision of the indicator and level to deciviews \86\ and 20 
to 25, respectively (Sheppard, 2023, Response to Charge Questions, p 
25). These members variously cited ``seasonal variabilities'' of 
``[e]cological sensitivities,'' describing sensitive lichen species to 
be influenced by fog or cloud water from which they state S and N 
contributions to be highly episodic, and visibility impairment 
(Sheppard, 2023, Response to Charge Questions, p 25). These members did 
not provide further specificity regarding their reference to lichen 
species and fog or cloud water. With regard to visibility impairment, 
these members described the EPA solicitation of comments that occurred 
with the separate EPA action to reconsider the 2020 decision to retain 
the existing PM2.5 standards as the basis for their 
recommendations on the secondary 24-hr PM2.5 standard 
(Sheppard, 2023, Response to Charge Questions, p 25; 88 FR 5562-5663, 
January 27, 2023).\87\ One CASAC member dissented from this view and 
supported retention of the existing secondary 24-hr PM2.5 
standard.
---------------------------------------------------------------------------

    \86\ Deciviews, units derived from light extinction, are 
frequently used in the scientific and regulatory literature to 
assess visibility (U.S. EPA 2019, section 13.2).
    \87\ Protection from impairment of visibility effects was one of 
the welfare effects within the scope of the PM NAAQS reconsideration 
rather than the scope of this review (U.S. EPA, 2016, 2017). In that 
action, the Administrator proposed not to change the 24-hour 
secondary PM NAAQS for visibility protection and also solicited 
comment on revising the level of the current secondary 24-hour 
PM2.5 standard to a level as low as 25 [micro]g/m\3\; in 
the final action, the Administrator concluded that the current 
secondary PM standards provide requisite protection against PM-
related visibility effects and retained the existing standards 
without revision (88 FR 5558, January 27, 2023; 89 FR 16202, March 
6, 2024).
---------------------------------------------------------------------------

    Among the CASAC comments on the draft PA \88\ was the comment that 
substantial new evidence has been published since development of the 
2020 ISA that supports changes to the draft PA conclusions on N 
deposition effects. Accordingly, in the final PA, a number of aspects 
of chapters 4 and 5 were revised from the draft PA; these changes took 
into account the information emphasized by the CASAC while also 
referring to the ISA and studies considered in it (PA, section 7.3). 
More recent studies cited by the CASAC generally concerned effects 
described in the ISA based on studies available at that time. While the 
newer studies include additional analyses and datasets, the ISA and 
studies in it also generally support the main points raised and 
observations made by the CASAC (PA, section 7.3).
---------------------------------------------------------------------------

    \88\ Consideration of CASAC comments and areas of the PA in 
which revisions have been made between the draft and this final 
document are described in section 1.4 of the PA.
---------------------------------------------------------------------------

3. Administrator's Proposed Conclusions
    In considering the adequacy of the existing secondary standards for 
SOX, N oxides, and PM, and what revisions or alternatives 
are appropriate, the Administrator has drawn on the ISA conclusions 
regarding the weight of the evidence for both the direct effects of 
SOX, N oxides, and PM in ambient air and for effects 
associated with ecosystem deposition of N and S compounds, and 
associated areas of uncertainty; quantitative analyses of aquatic 
acidification risk and of air quality and deposition estimates, and 
associated limitations and uncertainties; staff evaluations of the 
evidence, exposure/risk information, and air quality information in the 
PA; CASAC advice; and public comments received thus far in the review. 
In considering the available information in this review, the 
Administrator recognizes the evidence of direct biological effects 
associated with elevated short-term concentrations of SOX 
and N oxides that formed the basis for the existing secondary 
SO2 and NO2 standards, the evidence of ecological 
effects of PM in

[[Page 26677]]

ambient air, primarily associated with loading on vegetation surfaces, 
and also the extensive evidence of ecological effects associated with 
atmospheric deposition of N and S compounds into sensitive ecosystems. 
The Administrator also takes note of the quantitative analyses and 
policy evaluations documented in the PA that, with CASAC advice, inform 
his judgments in reaching his proposed decision on the secondary 
standards for SOX, N oxides, and PM that provide the 
requisite protection under the CAA.
    In reaching his proposed conclusions for the pollutants included in 
this review, the Administrator considers first the secondary standard 
for SOX and the adequacy of the existing standard for 
providing protection of the public welfare from direct effects on biota 
and from ecological effects related to ecosystem deposition of S 
compounds. In so doing, he considers the evidence regarding direct 
effects, as described in the ISA and evaluated in the PA, which is 
focused on SO2. He takes note of the PA finding that the 
evidence indicates SO2 concentrations associated with direct 
effects to be higher than those allowed by the existing SO2 
secondary standard (PA sections 5.4.1, 7.1.1 and 7.4). Additionally, he 
takes note of the CASAC unanimous conclusion that the existing standard 
provides protection from direct effects of SOX in ambient 
air, as summarized in section II.E.2 above. Based on all of these 
considerations, he judges the existing secondary SO2 
standard to provide the needed protection from direct effects of 
SOX. He next turns to consideration of ecological effects 
related to ecosystem deposition of S compounds.
    With regard to S deposition-related effects, as an initial matter, 
the Administrator recognizes the long-standing evidence of the role of 
SOX in ecosystem acidification and related ecological 
effects. While he additionally notes the ISA determinations of 
causality for S deposition with two other categories of effects related 
to mercury methylation and sulfide phytotoxicity (ISA, Table ES-1; PA, 
section 4.4), he recognizes that quantitative tools and approaches are 
not well developed for ecological effects associated with atmospheric 
deposition of S other than ecosystem acidification (PA, section 
7.2.2.1). Thus, he gives primary attention to effects related to 
acidifying deposition, given the robust evidence base and available 
quantitative tools, as well as the longstanding recognition of impacts 
in acid-sensitive ecosystems across the U.S. Accordingly, the 
Administrator takes note of the findings of the aquatic acidification 
REA and related policy evaluations in the PA. As summarized in the PA, 
the REA findings include that the range of ecoregion median deposition 
estimates across the contiguous U.S. analyzed during the 20-year period 
from 2001 through 2020 extended up to as high as 20 kg S/ha-yr, and 
that, except for one occasion (in 2011), the existing secondary 
SO2 standard was met in all states but Hawaii. Further, 
other than the design value in 2011, design values for the existing 
SO2 standard (second highest 3-hour average in a year) were 
well below its current level of 500 ppb (PA, section 6.2.1). For 
example, in the earliest 3-yr period analyzed (2001-03), when virtually 
all design values for the existing 3-hour secondary standard were below 
400 ppb and the 75th percentile of design values was below 100 ppb (PA, 
Figure 2-27), total S deposition was estimated to be greater than 14 
kg/ha-yr across the Ohio River valley and Mid-Atlantic states, ranging 
above 20 kg/ha-yr in portions of this area (PA, Figure 6-11). The PA 
also notes that the magnitude of S deposition estimates at the 90th 
percentile per ecoregion at sites assessed in the aquatic acidification 
REA was at or above 15 kg/ha-yr in half of the 18 eastern ecoregions 
and ranged up to nearly 25 kg/ha-yr during this time period (Figure 2; 
PA, Figure 7-2). The Administrator also takes note of the aquatic 
acidification risk estimates that indicate that this pattern of S 
deposition, estimated to have occurred during periods when the existing 
standard was met (e.g., 2000-2002), is associated with 20% to more than 
half of waterbody sites in affected eastern ecoregions \89\ being 
unable to achieve even the lowest of the three acid buffering capacity 
targets or benchmarks (ANC of 20 [micro]eq/L), and judges such risks to 
be of public welfare significance.
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    \89\ Aquatic acidification risk estimates for the 2001-2020 
deposition estimates in the eight western ecoregions indicated ANC 
levels achieving all three targets in at least 90% of all sites 
assessed in each ecoregion (PA, Table 5-4). Ecoregion median 
deposition estimates were at or below 2 kg/ha-yr in all eight 
western ecoregions (PA, Table 5-3).
---------------------------------------------------------------------------

    The Administrator also considers the advice from the CASAC in 
considering deposition-related effects of S. Although the CASAC 
provided two sets of advice regarding standards for protecting from 
such effects, both the majority and the minority of CASAC recommended 
adoption of a new SO2 standard for this purpose in light of 
conclusions that the existing standard did not provide such needed 
protection. Thus, based on the findings of the REA, associated policy 
evaluations in the PA with regard to S deposition and acidification-
related effects in sensitive ecosystems, and in consideration of advice 
from the CASAC, the Administrator proposes to judge that the current 
SO2 secondary standard is not requisite to protect the 
public welfare from adverse effects associated with acidic deposition 
of S compounds in sensitive ecosystems.
    Having reached this proposed conclusion that the existing secondary 
SO2 standard does not provide the requisite protection of 
the public welfare from adverse S deposition-related effects, most 
prominently those associated with aquatic acidification, the 
Administrator next considers options for a secondary standard that 
would provide the requisite protection from S deposition-related 
effects. In so doing, he turns first to the policy evaluations and 
staff conclusions in the PA, and the quantitative analyses and 
information described in Chapter 5 of the PA, for purposes of 
identifying S deposition rates that might be judged to provide an 
appropriate level of public welfare protection from acidification-
related effects. In this context, he takes note of the PA focus on the 
aquatic acidification risk estimates and the PA recognition of linkages 
between watershed soils and waterbody acidification, as well as 
terrestrial effects. He concurs with the PA view that such linkages 
indicate that protecting waterbodies from reduced acid buffering 
capacity (with ANC as the indicator) will also, necessarily, provide 
protection for watershed soils, and may reasonably be expected to also 
contribute protection for terrestrial effects. Accordingly, he 
considers the PA evaluation of the risk estimates in terms of 
waterbodies estimated to achieve the three acid buffering capacity 
benchmarks (20, 30 and 50 [micro]eq/L). Further, he concurs with the PA 
consideration of the ecosystem-scale estimates as appropriate for his 
purposes in identifying conditions that provide the requisite 
protection of the public welfare.
    In focusing on the ecoregion-scale findings of the aquatic 
acidification REA, with particular attention to the 18 well studied, 
acid-sensitive eastern ecoregions, the Administrator considers the PA 
evaluation of ecoregion median S deposition values at and below which 
the associated risk estimates indicated a high proportion of 
waterbodies in a high proportion of ecoregions would achieve ANC values 
at or above the three benchmarks (20, 30 and 50 [micro]eq/L), as 
summarized in Tables 7-1 and 5-5 of the PA. In so doing, he recognizes 
a number of factors, as described in the PA, which contribute 
variability and

[[Page 26678]]

uncertainty to waterbody estimates of ANC and to interpretation of 
acidification risk associated with different values of ANC (PA, section 
5.1.4 and Appendix 5A, section 5A.3).
    The Administrator additionally takes note of the approach taken by 
the CASAC majority in considering the ecoregion-scale risk estimates 
(summarized in section II.E.2 above). These members considered the 
summary of results for the ecoregion-scale analysis of ecoregion median 
deposition bins (in the draft PA \90\) and focused on a level of 
deposition (at or below 5 kg/ha-yr) estimated to achieve acid buffering 
capacity at or above the three ANC benchmarks in 80% (for ANC of 20 and 
30 [micro]eq/L) or 70% (for ANC of 50 [micro]eq/L) of waterbodies in 
all ecoregion-time period combinations \91\ (Sheppard, p. 25 of the 
Response to Charge Questions). As additionally recognized in the PA, 
the results for ecoregion-time period combinations for median S-
deposition in the 18 eastern ecoregions at or below 7 kg/ha-yr also 
indicate these percentages of waterbodies achieving the three ANC 
benchmarks (as seen in Tables 7-1 and 5-5 above).\92\ The results for 
median S deposition at or below 7 kg/ha-yr further indicate that 90% of 
waterbodies per ecoregion achieve ANC at or above 20, 30 and 50 
[micro]eq/L in 96%, 92% and 82%, respectively, of eastern ecoregion-
time period combinations. For median S deposition at or below 9 kg/ha-
yr, the percentages of eastern ecoregions meeting or exceeding the ANC 
benchmarks declines to 87%, 81% and 72% (as summarized in section 
II.E.1.c(2) above), and the percentages for all 25 analyzed ecoregions 
is higher.
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    \90\ While the final PA provides additional presentations of 
aquatic acidification risk estimates, including those at the 
ecoregion-scale, the estimates are unchanged from those in the draft 
PA (PA, section 5.1.3).
    \91\ The presentation of such percentages in the draft PA 
(reviewed by the CASAC) were specific to the 90 ecoregion-time 
period combinations for the 18 eastern ecoregions. Inclusion of the 
7 western ecoregions would yield higher percentages, as more than 
90% of waterbodies in those ecoregions were estimated to achieve all 
three ANC concentration in all time periods (PA, Table 5-4).
    \92\ Ecoregion median deposition was below 2 kg S/ha-yr in all 
35 ecoregion-time period combinations for the eight western 
ecoregions (PA, Table 5-4).
---------------------------------------------------------------------------

    The Administrator additionally considers the PA evaluation of the 
temporal trend or pattern of ecoregion-scale risk estimates across the 
five time periods in relation to the declining S deposition estimates 
for those periods. As summarized in section II.E.1.b above, based on 
the observation of appreciably improved acid buffering capacity (i.e., 
increased ANC) estimates by the third time period (2010-2012), the PA 
focused on the REA risk and deposition estimates for this and 
subsequent periods. The S deposition estimated to be occurring in the 
2010-2012 time period included ecoregion medians (across CL sites) 
ranging from 2.3 to 7.3 kg/ha-yr in the 18 eastern ecoregions and 
extending down below 1 kg/ha-year in the 7 western ecoregions; the 
highest ecoregion 90th percentile was approximately 8 kg/ha-yr (table 5 
and figure 2 above). For this pattern of deposition, more than 70% of 
waterbodies per ecoregion are estimated to be able to achieve an ANC of 
50 ueq/L in all 25 ecoregions (Figure 1, left panel), and more than 80% 
of waterbodies per ecoregion in all ecoregions are estimated to be able 
to achieve an ANC of 20 ueq/L (Figure 1, right panel). Further, by the 
2014-2016 period, when both median and 90th percentile S deposition in 
all 25 ecoregions was estimated to be at or below 5 kg/ha-yr, more than 
80% of waterbodies per ecoregion are estimated to be able to achieve an 
ANC of 50 ueq/L in all 25 ecoregions (more than 90% in 23 of the 25 
ecoregions) and more than 90% of waterbodies per ecoregion in all 
ecoregions are estimated to be able to achieve an ANC of 20 ueq/L 
(Figure 1, right panel).
    The Administrator observes that the estimates of acid buffering 
capacity achievement for the 2010-12 period deposition--achieving the 
ANC benchmarks in at least 70% to 80% (depending on the specific 
benchmark) of waterbodies per ecoregion--are consistent with the 
objectives identified by the CASAC majority (in considering estimates 
from the ecoregion-scale analysis). The advice from the CASAC majority 
emphasized ecoregion ANC achievement estimates of 70%, 80% and 80% for 
ANC benchmarks of 50, 30 and 20 [micro]eq/L, respectively. The 
estimates for the later time period are somewhat higher, with all 
ecoregions estimated to achieve the ANC benchmarks in at least 80% to 
90% (depending on the benchmarks) of waterbodies per ecoregion. In his 
consideration of these ANC achievement percentages identified by the 
CASAC, the Administrator notes the variation across the U.S. 
waterbodies with regard to site-specific factors that affect acid 
buffering (as summarized in sections II.C.1.b(1) and II.D.1 above and 
section 5.1.4 of the PA). Based on this and the CASAC majority advice, 
the Administrator concurs with the PA conclusion that both of these 
ecoregion-scale ANC achievement results (70% to 80% and 80% to 90%) may 
be reasonable to consider with regard to acid buffering capacity 
objectives for the purposes of protecting ecoregions from aquatic 
acidification risk of a magnitude with potential to be considered of 
public welfare significance.
    With regard to the variation in deposition across areas within 
ecoregions, the Administrator notes the PA recognition that the sites 
estimated to receive the higher levels of deposition are those most 
influencing the extent to which the potential objectives for aquatic 
acidification protection are or are not met. He further takes note of 
the PA observation of an appreciable reduction across the 20-year 
analysis period in the 90th percentile deposition estimates, as well as 
the median, for REA sites in each of the 25 ecoregions analyzed. 
Although the ecoregion 90th percentile and median estimates ranged up 
to 22 and 15 kg/ha-yr in the 2001-2003 time period, both types of 
estimates fall below approximately 5 to 8 kg/ha-yr by the 2010-2012 
period (PA, Figure 7-2). In light of this trend, as well as the 
temporal trend in the REA estimates, the Administrator takes note of 
the PA findings that the ecoregion-scale acid buffering objectives 
identified by the CASAC (more than 70% to 80% of waterbody sites in all 
ecoregions assessed achieving or exceeding the set of ANC benchmarks) 
might be expected to be met when ecoregion median and upper (90th) 
percentile deposition estimates at sensitive ecoregions are generally 
at and below about 5 to 8 kg/ha-yr. He additionally takes note of the 
PA recognition of uncertainties associated with the deposition 
estimates at individual waterbody sites and with the associated 
estimates of aquatic acidification risk (PA, section 5.1.4), and with 
the PA's consideration of estimates from the case study analyses, which 
together leads the PA to identify deposition rates at and below about 5 
to 8 or 10 \93\ kg/ha-yr as associated with a potential to achieve acid 
buffering capacity benchmarks in an appreciable portion of acid 
sensitive areas. Based on all of these considerations, the 
Administrator focuses on this range of deposition levels in turning his 
attention to identification of a secondary standard that might be 
associated with S deposition of such a magnitude.
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    \93\ Consideration of the case study analyses as well as the 
ecoregion-scale results for both the ecoregion-time period and 
temporal perspectives, indicates a range of S deposition below 
approximately 5 to 8 or 10 kg/ha-yr, on an areawide basis, to be 
associated with a potential to achieve acid buffering capacity 
levels of interest in an appreciable portion of acid sensitive areas 
(PA, section 7.4).

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[[Page 26679]]

    In considering options for a secondary standard based on 
consideration of S deposition-related effects, the Administrator takes 
note of the complexity of identifying a national ambient air quality 
standard focused on protection of the public welfare from adverse 
effects associated wth national patterns of atmospheric deposition 
(rather than on protection from direct exposure from patterns of 
ambient air concentrations of concern). As more specifically described 
in the PA, atmospheric deposition (ecosystem loading) of S, is, in a 
simple sense, the product of atmospheric concentrations of S compounds, 
factors affecting S transfer from air to surfaces, and time. Further, 
atmospheric concentrations in an ecosystem are, themselves, the result 
of emissions from multiple, distributed sources (near and far), 
atmospheric chemistry, and transport. Accordingly, the Administrator 
concurs with the PA judgment that consideration of the location of 
source emissions and expected pollutant transport (in addition to the 
influence of physical and chemical processes) is important to 
understanding relationships between SO2 concentrations at 
ambient air monitors and S deposition rates in sensitive ecosystems of 
interest. Based on these considerations in the PA, the Administrator 
concurs that to achieve a desired level of protection from aquatic 
acidification effects associated with S deposition in sensitive 
ecosystems, SO2 emissions must be controlled at their 
sources, and that associated NAAQS compliance monitoring is at 
regulatory SO2 monitors generally sited near large 
SO2 emissions sources.
    Further, the Administrator considers findings of the PA analyses of 
relationships between ambient air concentrations and S deposition 
estimates, conducted in recognition of the variation across the U.S. in 
the source locations and magnitude of SOX emissions, as well 
as the processes that govern transport and transformation of 
SOX to eventual deposition of S compounds. These analyses 
consider relationships between S deposition estimates and 
SO2 concentrations near SO2 monitors (both at 
NAAQS regulatory monitors, which are often near large sources of 
SO2 emissions, and in remote Class I areas) as well as 
relationships between ecoregion S deposition estimates and 
SO2 concentrations at upwind sites of influence, identified 
by trajectory analyses to account for the relationship between upwind 
concentrations near sources and deposition in downwind areas (section 
II.B above and PA, sections 6.2.2 through 6.2.4,). As evidence of the 
influence of SO2 in ambient air on S deposition, all of 
these analyses demonstrated there to be an association between 
SO2 concentrations and nearby or downwind S deposition (PA, 
section 7.4). The PA found the correlation coefficients to be strongest 
in the East and in the earliest two to three time periods, when 
deposition rates and air concentrations were much higher compared to 
the West and to more recent years, when deposition rates and 
concentrations are much lower (PA, Chapter 6).
    With regard to an indicator for a standard to address the effects 
of S deposition, the Administrator also takes note of the findings of 
the PA analyses and the parallel trends of SO2 emissions and 
S deposition in the U.S. over the past 20 years that indicate the 
strong influence of SO2 in ambient air on S deposition (PA, 
sections 6.4.1 and 7.4). The array of air quality analyses in the PA 
together illustrate the fact that atmospheric loading is a primary, but 
not the only, determinant of atmospheric deposition, as well as the 
complexity of how to consider concentrations at individual monitors, 
with variable spatial distribution, in relation to deposition rates. 
The distribution of monitor SO2 concentrations is 
appreciably flatter in the latter 10 years of the analysis period in 
comparison to the initial years, and S deposition rates during the 
latter 10 years are appreciably reduced from those in the earlier 
decade (PA, Figure 7-5). These parallel patterns indicate a broader 
distribution of concentrations across NAAQS monitors during these 
years. Additionally, the Administrator notes the PA finding of parallel 
temporal trends of ecoregion S deposition estimates and the REA aquatic 
acidification risk estimate across the five time periods analyzed.
    In light of all of the linkages connecting SOX emissions 
and S deposition-related effects, the Administrator considers the 
current information with regard to a new or revised standard for 
SOX that would be expected to provide protection from 
aquatic acidification-related risks of S deposition in sensitive 
ecoregions. With regard to the indicator for such a standard, he notes 
the PA findings of support for SO2 as a good indicator for a 
secondary standard to address S deposition (PA, sections 6.4.1 and 
7.4). This support includes the declining trend of S deposition that is 
consistent with and parallel to the sharp declines in annual average 
SO2 emissions across the 20-year period, as well as the 
general association of SLAMS with higher annual average SO2 
concentrations with higher local S deposition estimates in addition to 
the high correlations observed for ecoregion median S deposition with 
upwind SO2 monitoring sites of influence in the EAQM 
analyses. In light of all of these considerations, the Administrator 
judges SO2 to be the appropriate indicator for a standard 
addressing S deposition-related effects.
    With regard to the appropriate averaging time and form for such a 
standard, the Administrator takes note of the PA findings and advice 
from the CASAC. In the quantitative analyses of air quality and 
deposition, the PA generally focused on a year's averaging time based 
on the recognition that longer-term averages (such as over a year) most 
appropriately relate to deposition and associated ecosystem effects. 
The PA analyses also used a 3-year average form based on a recognition 
in the NAAQS program that such a form affords stability to the 
associated air quality management program that contributes to effective 
environmental protection. Similarly, in the advice of the CASAC 
majority on a standard addressing S deposition, these members 
recommended an annual average standard, and the information considered 
by the CASAC majority in drawing its conclusion also focused on an 
annual average SO2 metric with a form that involved 
averaging over three consecutive years, implying that to be the 
recommended form (section II.E.2 above). In consideration of these 
conclusions of the PA and the CASAC majority, the Administrator focuses 
on annual average SO2 concentrations, averaged over three 
years, as providing an averaging time and form \94\ that he judges 
appropriate for providing public welfare protection from adverse 
effects associated with long-term atmospheric deposition of S 
compounds.
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    \94\ A 3-year form is common to NAAQS adopted over the more 
recent past. This form provides a desired stability to the air 
quality management programs which is considered to contribute to 
improved public health and welfare protection (e.g., 78 FR 3198, 
January 15, 2013; 80 FR 65352, October 26, 2015; 85 FR 87267, 
December 31, 2020).
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    In turning to consideration of a level for such a standard, as an 
initial matter, the Administrator again notes the complexity 
(recognized above) associated with identifying a national ambient air 
quality standard focused on protection from national patterns of 
atmospheric deposition rather than on protection from patterns of 
direct exposure to SO2. As summarized in the PA, atmospheric 
deposition rates are a function of atmospheric loading, transformation, 
and transport, and are

[[Page 26680]]

not a one-to-one function of concentration at a specific monitoring 
location. Thus, the maximum concentration at a single upwind site is 
less important for total loading than the combined impact of all upwind 
emissions sources. This contributes uncertainty to the identification 
of the appropriate level for a national standard based on a single 
maximum concentration that, if occurring at any one or multiple 
locations, would be expected to constrain areawide deposition rates 
downwind to the desired level for protection. The atmospheric loading 
(and deposition) associated with the maximum concentration conceptually 
represented by a standard level depends on the number and spatial 
distribution of areas exhibiting that concentration. Reductions in 
deposition reflect geographically-broad emissions reductions and 
weighted concentration reductions (e.g., EAQM-weighted) more than 
reductions in the maximum concentration at individual locations. As 
shown by the 20-year trends in annual emissions and monitor annual 
average SO2 concentrations, the percentage reductions in 
deposition and emissions are greater than those in the highest monitor 
concentrations.\95\ Particularly in this case of identifying a standard 
to provide a pattern of ambient air concentrations that together 
contribute to deposition across the U.S., it is important to consider 
the distribution of air concentrations to which the standard will 
apply. The Administrator takes note of this complexity and associated 
uncertainty in his identification of a level for an annual average 
SO2 standard for S deposition.
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    \95\ As recognized in section II.E.1 above, in relating 
atmospheric loading to individual monitor concentrations, the higher 
correlations of the EAQM-weighted than the EAQM-max likely reflect 
the weighting of concentrations across multiple upwind monitors, 
with the trajectory analysis providing one approach that relates 
contributions from individual monitor locations to deposition in 
receiving ecosystems (without explicitly addressing the multiple 
factors at play).
---------------------------------------------------------------------------

    In considering an appropriate range of concentrations for a level 
for such a standard, the Administrator considers the evaluations and 
associated findings of the PA and advice from the CASAC. In considering 
the PA analyses and evaluation, the Administrator takes note of the 
uncertainties associated with potential limitations in the monitoring 
dataset across the 20-year period (e.g., with regard to the 
representation of source locations in the earlier years of the 
monitoring data), in addition to the complexities described above. In 
so doing, the Administrator considers the two options identified in the 
PA for a level of an annual average standard, with a 3-year average 
form. One option identified in the PA would establish a level in the 
range of somewhat below 15 ppb to a level of 10 ppb, and a second 
option would establish a standard with a level within the range of 10 
to 5 ppb. He additionally recognizes there to be uncertainties in 
aspects of the aquatic acidification risk modeling that contribute 
uncertainty to the resulting estimates, and in the significance, of 
aquatic acidification risk, which he finds to be greater with lower 
deposition levels (PA, section 5.1.4). Further, the Administrator takes 
note of the additional and appreciably greater uncertainty recognized 
in the PA to be associated with consideration of a standard level below 
10 ppb, including uncertainties in the relationships between S 
deposition and annual average SO2 concentrations below 10 
ppb (PA, Chapter 6, section 7.4). In general, there is uncertainty in 
identifying a specific level for a standard that may be expected to 
achieve a particular degree of S deposition-related protection for 
ecological effects. This uncertainty is coupled with the uncertainty 
associated with estimates of aquatic acidification risk in waterbodies 
across the U.S. associated with specific deposition levels, including 
with regard to interpretation of risk associated with different levels 
of acid buffering capacity. In this context and based on the PA 
findings, the Administrator recognizes there to be, on the whole across 
the various linkages, increased uncertainty for lower SO2 
concentrations and S deposition rates.
    The Administrator additionally considers the CASAC majority 
recommended range of levels for an annual average SO2 
standard to address S deposition-related ecological effects. As 
described in section II.E.2 above, the majority of the CASAC 
recommended adoption of an annual SO2 standard with a level 
within the range of 10 to 15 ppb. These members indicated that this 
range of levels was generally associated with S deposition below 5 kg/
ha-yr during the 2014-2016 and 2018-2020 trajectory analysis periods in 
the PA. The CASAC majority further conveyed that a standard level in 
this range (10-15 ppb) would afford protection to tree and lichen 
species as well as waterbodies, further stating that such a standard 
would ``preclude the possibility of returning to deleterious deposition 
values'' that these members indicated to be associated with relatively 
high annual average SO2 concentrations observed in 2019-2021 
near a location of industrial sources (Sheppard, Response to Charge 
Questions, pp. 24-25).
    Thus, based on analyses and evaluations in the PA, including 
judgments related to uncertainties in relating ambient air 
concentrations to deposition estimates for the purpose of identifying a 
standard level associated with a desired level of ecological 
protection, and based on advice from the CASAC majority, the 
Administrator judges that a level within the range from 10 to 15 ppb 
would be appropriate for an annual average SO2 standard 
requisite to protect the public welfare from adverse effects related to 
S deposition.
    The Administrator also considered the extent to which a new annual 
average standard might be expected to control short-term concentrations 
(e.g., of three hours duration) and accordingly provide protection from 
direct effects that is currently provided by the existing 3-hour 
secondary standard. In this context, he notes the analyses and 
conclusions of the PA with regard to the extent of control for short-
term concentrations (e.g., of three hours duration) that might be 
expected to be provided by an annual secondary SO2 standard. 
These analyses indicate that in areas and periods when the annual 
SO2 concentration (annual average, averaged over three 
years) is below 15 ppb, design values for the existing 3-hour standard 
are well below the existing secondary standard level of 0.5 ppm 
SO2 (PA, Figure 2-29). Based on these findings of the PA, 
the Administrator proposes that it is appropriate to consider revision 
of the existing secondary SO2 standard to an annual 
standard, with a 3-year average form and a level in the range from 10 
to 15 ppb.
    The Administrator also takes note of the recommendation from the 
CASAC minority to establish a 1-hour SO2 secondary standard, 
identical to the primary standard, based on its observation that most 
of the S deposition estimates for the last 10 years are less than 5 kg/
ha-yr and judgment that this indicates that the existing 1-hour primary 
SO2 standard adequately protects against long-term annual S 
deposition-related effects. Accordingly, the CASAC minority recommended 
setting the secondary SO2 standard equal to the current 
primary standard (section II.E.2 above; Sheppard, 2023, p. A-2). The 
Administrator preliminarily concludes, for the reasons discussed above, 
that an annual standard is a more appropriate form to address 
deposition-

[[Page 26681]]

related effects, but he recognizes that greater weight could be given 
to the effectiveness of the existing 1-hour primary standard in 
controlling emissions and associated deposition. In light of these 
considerations, we solicit comment on this alternate option for 
revising the secondary SO2 standard to be identical to the 
current primary SO2 standard.
    In summary, based on all of the considerations identified above, 
including the currently available evidence in the ISA, the quantitative 
and policy evaluations in the PA, and the advice from the CASAC, the 
Administrator proposes to revise the existing secondary SO2 
standard, to an annual average standard, with a 3-year average form and 
a level within the range from 10 to 15 ppb as requisite to protect the 
public welfare. The EPA solicits comment on a lower level for a new 
annual standard down to 5 ppb, as well as whether the existing 3-hour 
secondary standard should be retained, in addition to establishing a 
new annual SO2 standard. Further, the EPA solicits comment 
on the option of revising the existing secondary SO2 
standard to be equal to the current primary standard in all respects.
    The Administrator additionally considers the available information 
and the PA evaluations and conclusions regarding the PM standard and S 
deposition-related effects. In so doing, he takes note of the 
information indicating varying composition of PM2.5 at sites 
across the U.S. (PA, section 2.4.3), with non-S containing compounds 
typically comprising more than 70% of the total annual PM2.5 
mass in the East and even more in the West. Further, he considers the 
PA findings of air quality analyses that indicate appreciable variation 
in associations, and generally low correlations, between S deposition 
and PM2.5, as summarized in section II.B above (PA, sections 
6.2.2.3 and 6.2.4.2). In addition, he takes note of the discussion 
above in support of his decision regarding a revised secondary 
SO2 standard, including the atmospheric chemistry 
information which indicates the dependency of S deposition on airborne 
SOX, as evidenced by the parallel trends of SO2 
emissions and S deposition. Based on all of these considerations, the 
Administrator judges that protection of sensitive ecosystems from S 
deposition is more effectively achieved through a revised 
SO2 standard than a standard for PM, and a revised PM 
standard is not warranted to provide protection against the effects of 
S deposition.
    Having reached his proposed decisions with regard to S deposition 
and SO2 and PM, the Administrator now turns to consideration 
of the secondary standards for N oxides and PM with regard to the 
protection afforded from direct ecological effects and from ecological 
effects related to ecosystem N deposition. As described below, the 
Administrator proposes to retain the existing NO2 and PM 
standards. In considering the secondary standard for oxides of N and 
protection from direct effects of N oxides in ambient air, the 
Administrator notes the evidence of welfare effects at the time this 
standard was established in 1971 indicated the direct effects of N 
oxides on vegetation, most particularly effects on foliar surfaces, and 
that the currently available information continues to document such 
effects, as summarized in section II.C.1.a above (ISA, Appendix 3, 
sections 3.3 and 3.4; PA, sections 4.1 and 5.4.2). With regard to 
NO2 and NO, the evidence does not indicate effects 
associated with ambient air concentrations allowed by the existing 
standard, as summarized in section II.C.3.c above (PA, section 7.4). 
Accordingly, the Administrator concurs with the PA conclusion that the 
evidence related to the direct effects of the N oxides, NO2 
and NO, does not call into question the adequacy of protection provided 
by the existing standard. With regard to the N oxide, HNO3, 
the PA provides additional evaluation in recognition of the evidence of 
effects associated with air concentrations and associated 
HNO3 dry deposition on plant and lichen surfaces, and 
uncertainty as to the extent to which exposures associated with such 
effects may be allowed by the existing secondary NO2 
standard (PA sections 7.1.2 and 5.4.2, and Appendix 5B, section 5B.4). 
Consistent with the conclusion in the PA, the Administrator judges the 
limited evidence to lack a clear basis for concluding that such effects 
might have been elicited by air quality that met the secondary 
NO2 standard. Thus, while the Administrator takes note of 
this discussion in the PA, he additionally recognizes the limitations 
of the evidence and associated uncertainties and judges them too great 
to provide support to a revised secondary NO2 standard. In 
so doing, he additionally takes note of the unanimous view of the CASAC 
that the existing secondary NO2 standard provides protection 
from direct effects of N oxides (section II.E.2 above).
    The Administrator next turns to consideration of the larger 
information base of effects related to N deposition in ecosystems. In 
so doing, he recognizes the complexities and challenges associated with 
quantitative characterization of N enrichment-related effects in 
terrestrial or aquatic ecosystems across the U.S. that might be 
expected to occur due to specific rates of atmospheric deposition of N 
over prolonged periods, and the associated uncertainties (PA, section 
7.2.3). These complexities and challenges are described further below. 
Some of the complexities associated with terrestrial deposition are 
similar to those for aquatic deposition, such as untangling the impacts 
of historic deposition from what might be expected from specific annual 
deposition rates absent that history, while other complexities related 
to available quantitative information and analyses differ. Further, 
with regard to many aquatic systems that receive N loading from sources 
other than atmospheric deposition, there is complexity to estimating 
the portion of N inputs, and associated contribution to effects, 
derived from atmospheric sources.
    It is important to note first that, as a general matter, the 
Administrator finds there are substantially more significant 
limitations and uncertainties associated with the evidence base for 
ecosystem effects related to N deposition associated with N oxides and 
PM, and with the available air quality information related to the 
limited potential for control of N deposition in areas across the U.S., 
in light of the impacts of other pollutants (i.e., NH3) on N 
deposition. The first set of limitations and uncertainties relates to 
quantitative relationships between N deposition and ecosystem effects, 
based on which differing judgments may be made in decisions regarding 
protection of the public welfare. In the case of protection of the 
public welfare from adverse effects associated with nutrient 
enrichment, we additionally recognize the complexity associated with 
identification of appropriate protection objectives in the context of 
changing conditions in aquatic and terrestrial systems as recent 
deposition has declined from the historical rates of loading. The 
second set of limitations and uncertainties relates to relatively lower 
correlations in more recent time periods of air quality metrics for N 
oxides with N deposition in ecosystems and the variation in PM 
composition across the U.S., particularly that between the eastern and 
western U.S. This latter set of limitations is considered to relate to 
the emergence of NH3, which is not a criteria pollutant, as 
a greater influence on N deposition than N oxides and PM over the more 
recent years. Further, this influence appears to

[[Page 26682]]

be exerted in areas with some of the highest N deposition estimates for 
those years.
    Additionally, the Administrator recognizes additional complexities 
in risk management and policy judgments, including with regard to 
identifying risk management objectives, such as judging the requisite 
public welfare protection for an ecosystem stressor like N enrichment, 
for which as the CASAC recognized, in terrestrial systems, there are 
both ``benefits and disbenefits'' (Sheppard, 2023, p. 8). As noted by 
the CASAC, ``[b]enefits include fertilization of crops and trees and 
the potential for improved sequestration of carbon in soils and plant 
biomass'' (Sheppard, 2023, p. 8), which, as noted in the PA, also 
complicates conclusions regarding the extent to which some ecological 
effects may be judged adverse to the public welfare (PA, section 7.4). 
In aquatic systems, identification of appropriate public welfare 
protection objectives is further complicated by N contributions to 
these systems from multiple sources other than atmospheric deposition, 
as well as by the effects of historical deposition that have influenced 
the current status of soils, surface waters, associated biota, and 
ecosystem structure and function. For example, changes to ecosystems 
that have resulted from past, appreciably higher levels of atmospheric 
deposition have the potential to affect how the ecosystem responds to 
current, lower levels of deposition or to still further reduced N 
inputs in the future.
    In turning to consideration of the evidence and air quality 
information related to N deposition, the Administrator takes note of 
the fact that ecosystem N deposition is influenced by air pollutants 
other than N oxides, particularly, NH3, which is not a CAA 
criteria pollutant (PA, sections 6.1, 6.2.1 and 7.2.3.3). As described 
in the PA, the extent of this contribution varies appreciably across 
the U.S. and has increased during the past 20 years.
    More specifically, while the PA historical trend analyses document 
the reductions in N deposition that correspond with reductions in 
emissions of N oxides, these analyses additionally document the 
increasing role of NH3 in N deposition since approximately 
2010 and the co-occurring tempering of total N deposition reductions, 
likely reflecting the countervailing pattern in contributions from 
NH3. Further, the areas of highest N deposition appear to 
correspond to the areas with the greatest deposition of NH3 
(PA, Figure 7-8).\96\ The Administrator concurs with the PA conclusion 
that this information complicates his consideration of the currently 
available information with regard to protection from N deposition-
related effects that might be afforded by the secondary standard for N 
oxides; particularly when considering the information since 2010 (and 
in more localized areas prior to that). That is, while the information 
regarding recent rates of ecoregion N deposition may in some individual 
areas (particularly those for which reduced N, specifically 
NH3, has a larger role) indicate rates greater than the 
range of values identified in the PA for consideration (e.g., 7-12 kg/
ha-yr based on the considerations in section 7.2.3 of the PA and the 
benchmark of 10 kg/ha-yr, as conveyed in the advice from the CASAC), 
the PA notes that the extent to which this occurrence relates to the 
existing NO2 secondary standard is unclear. The lack of 
clarity is both because of uncertainties in relating ambient air 
NO2 concentrations to rates of deposition, and because of 
the increasing contribution of NH3 to N deposition.
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    \96\ This associated lessening influence of N oxides on total N 
deposition is also evidenced by the poor correlations between N 
deposition and annual average NO2 concentrations (PA, 
sections 6.2.3 and 6.2.4), most particularly in more recent years 
and at eastern sites, which may be related to increasing emissions 
of NH3 in more recent years and at eastern sites (PA, 
section 2.2.3 and Figure 6-5).
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    The Administrator notes the PA finding that the temporal trend in 
ecoregion N deposition differs for ecoregions in which N deposition is 
driven by reduced N compared to those where reduced N comprises less of 
the total (e.g., PA, Figures 7-6 and 7-7). The N deposition trends in 
the latter ecoregions, which include reductions in the upper part of 
the distribution of ecoregion medians across the full 20-year period, 
as well as lower N deposition in the second as compared to the first 
decade (corresponding to the decline in NO2 emissions), 
appear to document the influence that NO2 emissions and 
concentrations had during this period. However, the influence of N 
oxides appears to be low in areas of the U.S. where N deposition is 
currently the highest, and where NH3 emissions have an 
influential role (PA, section 7.2.3.3). In light of the PA evaluations 
of N deposition and relative contribution from reduced and oxidized N 
compounds, the Administrator concurs with the PA conclusion that, based 
on the current air quality and deposition information and trends, a 
secondary standard for N oxides cannot be expected to effectively 
control total N deposition (PA, section 7.4).
    The Administrator additionally considers the two sets of advice 
from the CASAC regarding an NO2 annual standard in 
consideration of N deposition effects (section II.E.2 above). The CASAC 
majority recommended revision of the existing annual NO2 
standard level to a value ``<10 to 20 ppb'' (Sheppard, 2023, p. 24). As 
described in section II.E.2 above, however, the basis for this advice 
relates to a graph in the draft PA of the dataset of results from the 
trajectory-based analyses for the weighted annual NO2 metric 
(annual NO2 EAQM-weighted), which as noted above is not 
directly translatable to concentrations at individual monitors or to 
potential standard levels. These CASAC members additionally recognized 
that these results found no correlation between the ecoregion 
deposition and the EAQM-weighted values at upwind locations, and as 
described in section II.B above, the correlation coefficients are 
negative for N deposition with both annual NO2 EAQMs (PA, 
Table 6-10). Accordingly, based on the lack of a correlation for N 
deposition with the EAQMs, as well as the lack of translatability of 
the EAQM-weighted values to monitor concentrations or standard levels, 
the PA did not find the information highlighted by the CASAC majority 
for relating N deposition levels to ambient air concentrations to 
provide scientific support for their recommended levels. In light of 
this, the Administrator does not agree with the CASAC majority 
recommendations on revisions to the annual NO2 standard.
    The minority CASAC member recommended revision of the secondary 
NO2 standard to be identical to the primary standard based 
on their conclusion that the recent N deposition levels meet desired 
objectives and that the primary standard is currently the controlling 
standard (Sheppard, 2023, Appendix A). In consideration of this advice, 
the PA noted that among the NO2 primary and secondary NAAQS, 
the 1-hour primary standard (established in 2010) may currently be the 
controlling standard for ambient air concentrations, and observed that 
annual average NO2 concentrations, averaged over three 
years, in areas that meet the current 1-hour primary standard, have 
generally been below approximately 35 to 40 ppb.\97\
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    \97\ The air quality information regarding annual average 
NO2 concentrations at SLAMS monitors indicates more 
recent NO2 concentrations are well below the existing 
standard level of 53 ppb. As noted in the PA, the temporal trend 
figures indicate that, subsequent to 2011-2012, when median N 
deposition levels in 95% of the eastern ecoregions of the 
continental U.S. have generally been at or below 11 kg N/ha-yr, 
annual average NO2 concentrations, averaged across three 
years, have been at or below 35 ppb (PA, section 7.2.3.3).

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[[Page 26683]]

    In light of this finding, the PA identified a revision option for 
consideration based on the recent pattern in NO2 
concentrations (i.e., revision to a level below the current level of 53 
ppb to as low as 35 to 40 ppb [PA, section 7.4]).
    In considering this option as identified in the PA, the 
Administrator takes note of the PA characterization of the support for 
this option as ``not strong'' (PA, section 7.4). He further notes the 
PA conclusion that while the option may have potential to provide some 
level of protection from N deposition related to N oxides, there is 
significant uncertainty as to the level of protection that would be 
provided. The Administrator notes that this uncertainty relates 
prominently to the influence of NH3 on total N deposition 
separate from that of N oxides, and which in some areas of the U.S., 
particularly those areas where N deposition is highest, appears to be 
dominant (PA, section 7.2.3.3). Further, he gives weight to the PA 
statement that the extent to which the relative roles of these two 
pollutants (N oxides and NH3) may change in the future is 
not known. As evaluated in the PA, these factors together affect the 
extent of support for, and contribute significant uncertainty to, a 
judgment as to a level of N oxides in ambient air that might be 
expected to provide requisite protection from N deposition-related 
effects on the public welfare.
    In light of the considerations recognized above (summarized earlier 
in this section and in section II.E.1 above), the Administrator finds 
that the existing evidence does not clearly call into question the 
adequacy of the existing secondary NO2 standard. In so 
doing, he additionally notes, as recognized in the PA above, that 
recent median N deposition estimates are below the N deposition 
benchmark identified by the CASAC majority of 10 kg/ha-yr in ecoregions 
for which approximately half or more of recent total N deposition is 
estimated to be oxidized N, driven by N oxides (PA, section 7.2.3.3). 
In addition to the substantial uncertainty described above regarding 
the need for control of N deposition from N oxides that might be 
provided by a secondary standard for N oxides, the Administrator also 
takes note of the PA finding that there is substantial uncertainty 
about the effect of a secondary standard for N-oxides on the control of 
N deposition such that it is also not clear whether the available 
information provides a sufficient basis for a revised standard that 
might be judged to provide the requisite protection. In light of this 
PA finding, the current information on air quality and N deposition, 
and all of the above considerations, the Administrator proposes to also 
judge that the available evidence in this review is sufficient to 
conclude a revision to the secondary annual NO2 standard is 
not warranted. Based on all of these considerations, he proposes to 
retain the existing secondary NO2 standard, without 
revision. The EPA solicits comments on this proposed decision, and also 
solicits comment on the alternative of revising the level and form of 
the existing secondary NO2 standard to a level within the 
range from 35 to 40 ppb with a 3-year average form.
    Lastly, the Administrator turns to consideration of the existing 
standards for PM. As an initial matter, he takes note of the PA 
discussion and conclusion that the available information does not call 
into question the adequacy of protection afforded by the secondary 
PM2.5 standards from direct effects and deposition of 
pollutants other than S and N compounds (PA, sections 7.1.3 and 7.4). 
The evidence characterized in the ISA and summarized in the PA 
indicates such effects to be associated with conditions associated with 
concentrations much higher than the existing standards. Thus, the 
Administrator proposes to conclude that the current evidence does not 
call into question the adequacy of the existing PM standards with 
regard to direct effects and deposition of pollutants other than S and 
N compounds.
    With regard to N deposition and PM2.5, the Administrator 
considers the analyses and evaluations in the PA, as well as advice 
from the CASAC. As an initial matter, the Administrator takes note of 
the substantial and significant limitations and uncertainties 
associated with the evidence base for ecosystem effects related to N 
deposition associated with PM and with the available air quality 
information related to the limited potential for control of N 
deposition in areas across the U.S. in light of the impacts of 
NH3 on N deposition, as summarized earlier. For example, he 
notes limitations and uncertainties that relate to relatively lower 
correlations in more recent time periods of air quality metrics with N 
deposition and the variation in PM composition across the U.S. For 
example, the air quality analyses of relationships found only low to 
barely moderate correlations between N deposition estimates and annual 
average PM2.5 concentrations at nearby or upwind locations 
based on the full 20-year dataset, with somewhat higher correlations 
for the early years of the 20-year period, but with low or no 
correlation in the later years (PA, Chapter 6 and section 7.2.3.3). The 
PA also noted the variable composition of PM2.5 across the 
U.S., which contributes to geographic variability in the relationship 
between N deposition and PM2.5 concentrations, and that an 
appreciable percentage of PM2.5 mass does not contribute to 
N deposition. For example, the highest percentage of PM2.5 
represented by N compounds at CSN sites in 2020-2022 is 30% and it is 
less than 10% at an appreciable of sites (PA, section 6.4.2). The PA 
notes that this variability in percentage of PM2.5 
represented by N (or S) containing pollutants contributes a high level 
of uncertainty to our understanding of the potential effect of a 
PM2.5 standard on patterns of N deposition.
    In considering the advice from the CASAC for revision of the 
existing annual PM2.5 secondary standard, the Administrator 
notes the lack of consensus such that the Committee provided two 
different recommendations for revising the level of the standard, as 
summarized in section II.E.2 above: one for a level in the range from 6 
to 10 [mu]g/m\3\ and the second for a level of 12 [mu]g/m\3\. As 
summarized in the PA, the specific rationale for the range from 6 to 10 
ug/m\3\ is unclear, with levels within this range described as both 
relating to N deposition in a preferred range (at or below 10 kg N/ha-
yr) and relating to deposition above that range.\98\ The PA noted that 
this ``overlap'' illustrates the weakness and variability of 
relationships of PM2.5 with N deposition across the U.S. 
(PA, section 7.4). Further, the PA notes the low correlation for total 
N deposition estimates with annual average PM2.5 design 
values in the last 10 years at SLAMS (PA, Table 6-7). The second 
recommendation, from the CASAC minority, was based on their conclusion 
that the recent N (and S) deposition levels meet desired targets and 
that the primary annual PM2.5 standard, which has been 12 
[mu]g/m\3\ since 2013, has been

[[Page 26684]]

the controlling standard for annual PM2.5 concentrations 
(Sheppard, 2023, Appendix A).
---------------------------------------------------------------------------

    \98\ For example, the justification provided for the range of 
levels recommended by the CASAC majority for a revised 
PM2.5 annual standard (6 to 10 [mu]g/m\3\) refers both to 
annual average PM2.5 concentrations (3-yr averages) 
ranging from 2 to 8 [mu]g/m\3\ in 27 Class I areas (as corresponding 
to N deposition estimates at or below 10 kg/ha-yr) and to annual 
average PM2.5 concentrations (3-year averages) ranging 
from 6 to 12 [mu]g/m\3\ (at design value sites in areas of N 
deposition estimates greater than 15 kg/ha-yr), as summarized in 
section II.E.2 above.
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    Based on the currently available information, taking into account 
its limitations and associated uncertainties, and in consideration of 
all of the above, the Administrator proposes to conclude that 
PM2.5 is not an appropriate indicator for a secondary 
standard intended to provide protection of the public welfare from 
adverse effects related to N deposition. In reaching this proposed 
conclusion, the Administrator focuses in particular on the weak 
correlation between annual average PM2.5 design values and N 
deposition estimates in recent time periods, and additionally notes the 
PA conclusion that the available evidence, as evaluated in the PA, is 
reasonably judged insufficient to provide a basis for revising the 
PM2.5 annual standard with regard to effects of N deposition 
related to PM. Thus, based on consideration of the PA analyses and 
conclusions, as well as consideration of advice from the CASAC, the 
Administrator proposes to conclude that no change to the annual 
secondary PM2.5 standard is warranted and he proposes to 
retain the existing PM2.5 secondary standard, without 
revision. The EPA solicits comment on this proposed decision. 
Additionally, while recognizing the information and judgments regarding 
uncertainties that lead him to this proposed decision, the 
Administrator recognizes that there may be alternate views with regard 
to whether and to what extent a secondary standard with a 
PM2.5 indicator might be expected to provide control of N 
deposition. In this context, the Administrator additionally solicits 
comment on revising the existing standard level to a level of 12 [mu]g/
m\3\, in light of the recommendation and associated rationale provided 
by the CASAC minority.
    With regard to other PM standards, the Administrator concurs with 
the PA's finding of a lack of information that would call into question 
the adequacy of protection afforded by the existing PM10 
secondary standard for ecological effects, and thus concludes it is 
appropriate to propose retaining this standard without revision. With 
regard to the 24-hour PM2.5 standard, the Administrator 
takes note of the PA conclusion that the evidence available in this 
review, as documented in the ISA, or cited by the CASAC,\99\ does not 
call into question the adequacy of protection provided by the 24-hour 
PM2.5 standard from ecological effects (PA, section 7.4). 
Further, the Administrator considers the comments of the CASAC majority 
and recommendations for revision of this standard to a lower level or 
to an indicator of deciviews based on its consideration of short-term 
fog or cloud-related deposition events, without further specificity, as 
summarized in section II.E.2 above. In so doing, the Administrator 
notes the PA finding that, while the available evidence in the ISA 
recognizes there to be N deposition associated with cloud water or fog, 
it does not provide estimates of this deposition, describe associated 
temporal variability, or specify related effects on biota (ISA, 
Appendix 2; PA, section 7.3). Further, the specific revision options 
recommended by the CASAC majority were based on visibility 
considerations although the adequacy of protection provided by the 
secondary PM2.5 standard from visibility effects has been 
addressed in the reconsideration of the 2020 p.m. NAAQS decision (89 FR 
16202, March 6, 2024) and is not included in this review. The 
Administrator additionally notes the recommendation from the CASAC 
minority to retain the existing 24-hour secondary PM2.5 
standard without revision. Based on all of these considerations, the 
Administrator proposes to retain the existing 24-hour secondary 
PM2.5 standard, without revision. Further, based on the lack 
of evidence calling into question the adequacy of the secondary 
PM10 standards, he also proposes to retain the secondary 
PM10 standards without revision.
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    \99\ As summarized in section II.E.2 above, the CASAC majority, 
in its recommendation for revision of the existing standard, did not 
provide specificity regarding the basis for its references to lichen 
species and fog or cloud water, and the available evidence as 
characterized in the ISA does not provide estimates of this 
deposition or describe associated temporal variability, or 
specifically describe related effects on biota (ISA, Appendix 2).
---------------------------------------------------------------------------

    In reaching the conclusions described above regarding protection of 
the public welfare from ecological effects associated with ecosystem 
deposition of N and S compounds, the Administrator also takes note of 
consideration in the PA of the potential for indicators different from 
those for the current standards that may target specific chemicals that 
deposit N and S, e.g., NO3-, 
SO42-, NH4+ (PA, sections 
7.2.2.3, 7.2.3.3 and 7.4). In so doing, however, he recognizes a number 
of uncertainties and gaps in the available information important to 
such consideration. For example, one uncertainty relates to the depth 
of our understanding of the distribution of these chemicals in ambient 
air, including relationships between concentrations near sources and in 
areas of deposition, such as in protected areas. As described in the 
PA, depending on the indicator selected, the relationship exhibited 
between concentrations of the indicator and N or S deposition at the 
same location may not be expected to hold for concentrations of the 
indicator in more distant locations, including locations near emissions 
sources. Based on these considerations, the Administrator judges that 
the currently available information does not support standards based on 
such indicators at this time. Additionally, there are not currently 
approved regulatory monitoring methods for these potential indicators 
and there are practical considerations associated with establishing new 
standards with new indicators related to establishment of regulatory 
measurement methods and surveillance networks, that would impact 
effective implementation of the standards. Thus, he also recognizes 
that additional data collection and analysis is needed to develop the 
required evidence base to inform more comprehensive consideration of 
such alternatives.

F. Proposed Decision on the Secondary Standards

    The Administrator proposes to revise the existing secondary 
SO2 standard to be an annual average, averaged over three 
consecutive years with a level within the range from 10 to 15 ppb. The 
EPA solicits comments on this proposal, including the averaging time, 
form and range of levels for the revised standard. The EPA also 
solicits comments on the option of retaining the existing 3-hour 
standard, while also establishing such a new annual secondary standard. 
Additionally, the EPA solicits comment on the second option identified 
in the PA, for setting the level for a new annual standard in the range 
from 10 to 5 ppb, and also on the option of revising the secondary 
standard to be identical to the existing primary standard in all 
respects, along with the rationales on which such views are based.
    The Administrator proposes to retain the existing secondary 
standards for N oxides, and the existing suite of secondary standards 
for PM. The EPA solicits comments on these proposed decisions. The EPA 
also solicits comment on revising the level and form of the existing 
secondary NO2 standard to a level within the range from 40 
to 35 ppb and a 3-year average form. Regarding the PM secondary 
standards, the Administrator also solicits comment on revising the 
level of the existing annual secondary PM2.5 standard to 12 
[mu]g/m\3\.

[[Page 26685]]

III. Interpretation of the Secondary SO2 NAAQS

    The EPA is proposing to revise appendix T to 40 CFR part 50, 
Interpretation of the Primary National Ambient Air Quality Standards 
for Oxides of Sulfur, in order to provide data handling procedures for 
the proposed annual secondary SO2 standard. The proposed 
Sec.  50.21 which sets the averaging period, level, indicator, and form 
of the proposed annual standard refers to this appendix T. The proposed 
revised appendix T would detail the computations necessary for 
determining when the proposed annual secondary SO2 NAAQS is 
met. The proposed revised appendix T also would address data reporting, 
data completeness considerations, and rounding conventions.

A. Background

    The general purpose of a data interpretation appendix is to provide 
the practical details on how to make a comparison between multi-day and 
possibly multi-monitor ambient air concentration data and the level of 
the NAAQS, so that determinations of attainment and nonattainment are 
as objective as possible. Data interpretation guidelines also provide 
criteria for determining whether there are sufficient data to make a 
NAAQS level comparison at all. The regulatory language for the current 
secondary SO2 NAAQS, originally adopted in 1971, does not 
contain detailed data interpretation instructions. This situation 
contrasts with the primary NO2, ozone, PM2.5, 
PM10, lead, and primary SO2 NAAQS regulations, 
for which there are detailed data interpretation appendices in 40 CFR 
part 50 addressing issues that can arise in comparing monitoring data 
to the NAAQS. The existing appendix T includes these detailed data 
interpretation requirements for the primary 1-hour SO2 
NAAQS, thus the proposed revision would provide similar information for 
the proposed annual secondary SO2 NAAQS. The EPA has used 
its experience developing and applying this data interpretation 
appendix to develop the proposed revisions to the text in appendix T to 
address the proposed annual SO2 standard.

B. Interpretation of the Secondary SO2 Standard

    The purpose of a data interpretation rule for the secondary 
SO2 NAAQS is to give effect to the form, level, averaging 
time, and indicator specified in the proposed regulatory text at 40 CFR 
50.21, anticipating and resolving in advance various future situations 
that could occur. The proposed revised appendix T provides definitions 
and requirements that apply to the proposed annual secondary standard 
for SO2. The requirements clarify how ambient air data are 
to be reported, what ambient air data are to be used for comparisons 
with the SO2 NAAQS, and how to calculate design values for 
comparisons with the SO2 NAAQS. The data already required to 
be reported by ambient SO2 monitors for use in calculating 
design values for the current primary 1-hour SO2 NAAQS are 
also sufficient for use in calculating design values for the proposed 
secondary SO2 NAAQS.
    The proposed revised appendix T specifies that the annual secondary 
SO2 NAAQS would be met at an ambient air quality monitoring 
site when the valid annual secondary standard design value is less than 
or equal to [10-15] ppb, depending on the level finalized. The annual 
secondary standard design value for an ambient air quality monitoring 
site is described as the mean of the annual means for three consecutive 
years, with the annual mean derived as the annual average of daily 
means, with rounding and data completeness specified as described 
below. The use of a daily mean value in deriving the design value is 
consistent with the existing data handling requirements for the current 
1-hour primary SO2 NAAQS.
    Data completeness requirements for the proposed annual secondary 
standard in the proposed revised appendix T follows past EPA practice 
for other NAAQS pollutants by requiring that in general at least 75% of 
the monitoring data that should have resulted from following the 
planned monitoring schedule in a period must be available for the key 
air quality statistic from that period to be considered valid. These 
data completeness requirements are consistent with the current data 
completeness requirements for the 1-hour primary SO2 NAAQS 
in appendix T, and this proposal would not change those requirements. 
For the proposed annual secondary SO2 NAAQS, the key air 
quality statistics are the annual average of daily mean (24-hour 
average, midnight-to-midnight) concentrations in three successive 
years. It is important that daily means are representative of the 24-
hour period and that all seasons of the year are well represented. 
Hence, the 75% requirement is proposed to be applied at the daily and 
quarterly levels. These completeness requirements, including the 
calculation of the daily mean, are consistent with existing 
completeness requirements for the current 1-hour primary SO2 
NAAQS. The EPA invites comment on the proposed completeness 
requirements in appendix T.
    Recognizing that there may be years with incomplete data, the 
proposed text provides that a design value derived from incomplete data 
will nevertheless be considered valid if at least 75 percent of the 
days in each quarter of each of three consecutive years have at least 
one reported hourly value, and the 3-year annual average design value 
calculated according to the procedures specified in the proposed 
revised appendix T is above the level of the secondary annual standard. 
Additionally, following provisions in the proposed revised appendix T, 
a substitution test may be used to demonstrate validity of incomplete 
design values above the level of the standard by substituting a ``low'' 
daily mean value from the same calendar quarter in the 3-year design 
value period. Similarly, another substitution test may be used to 
demonstrate validity of incomplete design values below the level of the 
standard by substituting a ``high'' daily mean value from the same 
calendar quarter in the 3-year design value period. These substitution 
tests are consistent with existing substitution tests for the current 
1-hour primary SO2 NAAQS.
    It should be noted that one possible outcome of applying the 
proposed substitution test is that a year with incomplete data may 
nevertheless be determined to not have a valid design value and thus to 
be unusable in making annual secondary NAAQS compliance determinations 
for that 3-year period. However, the intention of the proposed 
substitution test is to reduce the frequency of such occurrences. The 
EPA invites comment on incorporating the proposed substitution test 
into the final rule.
    The EPA is proposing that the Administrator have general discretion 
to use incomplete data to calculate design values that would be treated 
as valid for comparison to the NAAQS despite the incompleteness, either 
at the request of a State or at the Administrator's own initiative. 
Similar provisions exist already for the PM2.5, 
NO2, lead, and 1-hour primary SO2 NAAQS. The 
Administrator would consider monitoring site closures/moves, monitoring 
diligence, and nearby concentrations in determining whether to use such 
data.
    Regarding rounding conventions for the annual secondary 
SO2 NAAQS, the EPA is proposing to be consistent with 
rounding conventions used for the current 1-hour primary SO2 
NAAQS.

[[Page 26686]]

Specifically, the EPA proposes that hourly SO2 measurement 
data shall be reported to EPA's regulatory database in units of ppb, to 
at most one place after the decimal, with additional digits to the 
right being truncated with no further rounding. Daily mean values and 
the annual mean of those daily values are not rounded. Further, the 
annual secondary standard design value is calculated pursuant to the 
proposed revised appendix T and then rounded to the nearest whole 
number or 1 ppb (decimals 0.5 and greater are rounded up to the nearest 
whole number, and any decimal lower than 0.5 is rounded down to the 
nearest whole number). The EPA invites comment on the proposed rounding 
conventions.

IV. Ambient Air Monitoring Network for SO2

    One of the primary purposes of collecting ambient air 
SO2 monitoring data is for use in determining whether 
ambient pollutant concentrations exceed the SO2 NAAQS. 
Ambient air SO2 monitoring data are collected by State, 
local, and Tribal monitoring agencies, as well as industry and the EPA 
in some instances, in accordance with the monitoring requirements 
contained in 40 CFR parts 50, 53, and 58. This section briefly 
discusses the current status of the SO2 monitoring network, 
its adequacy in supporting the existing secondary SO2 
standard, and its support for the proposed revision to the secondary 
SO2 NAAQS. Based on a thorough review of the SO2 
monitoring network, the EPA is not proposing any changes to the ambient 
air monitoring network design requirements.
    Ambient air SO2 monitoring data used for comparison to 
the NAAQS are typically collected by State, local, and Tribal 
monitoring agencies (``monitoring agencies''), as well as industry and 
Federal entities in some situations, in accordance with the monitoring 
requirements contained in 40 CFR parts 50, 53, and 58. A monitoring 
network is generally designed to measure and provide relevant air 
quality data as described and prescribed in 40 CFR part 58. To ensure 
that the data from the network are accurate and reliable to fulfill 
their intended purpose, the monitors in the network must comply with a 
set of requirements including the use of monitoring methods that EPA 
has designated as Federal Reference Methods (FRMs) or Federal 
Equivalent Methods (FEMs) per 40 CFR part 53, a network design to 
achieve monitoring objectives, and specific siting criteria, data 
reporting, quality assurance, and data handling rules or procedures.
    When promulgating the existing short-term 1-hour daily maximum 
primary SO2 NAAQS in 2010 (75 FR 35520, June 22, 2010), the 
EPA recognized that monitoring to support the health-based standard 
required a focus on measuring where SO2 emissions were 
elevated and to address population exposure. To that end, the EPA 
finalized minimum monitoring requirements for ambient air 
SO2 that prioritized monitoring resources in areas based on 
coincidence of elevated SO2 emissions and population, 
locking in a significant portion of the existing network at that time 
as well as introducing new monitors to the network. This approach was 
based on a hybrid analytical approach that was explained in the 
preamble to the 2010 primary SO2 NAAQS review that used both 
monitoring and modeling to assess compliance with the newly promulgated 
1-hour standard.
    In 2015, the EPA followed up on that hybrid monitoring and modeling 
concept to support the new 1-hour primary NAAQS by promulgating the 
Data Requirements Rule (DRR). The DRR (80 FR 51051, August 21, 2015) 
required air quality characterization in areas with large sources of 
SO2 emissions, specifically taking measures to assess and 
address the lack of information on SO2 concentrations around 
sources or source areas emitting 2,000 tons per year or more. Under the 
DRR, States had the option to employ monitoring, dispersion modeling, 
or to take a federally enforceable permit limit to comply. The 
implementation of the DRR resulted in approximately 78 monitoring sites 
across the country being focused on collecting data at locations of 
expected maximum SO2 concentrations around sources.
    There are approximately 434 ambient air SO2 monitors 
currently reporting data to EPA nationwide, with at least one 
SO2 monitor in every State, the District of Columbia, and 
Puerto Rico. The network reflects minimum monitoring requirements 
promulgated in the 2010 SO2 Primary NAAQS revision, the 
requirement to measure SO2 at all NCore multipollutant 
monitoring stations, actions taken by monitoring agencies to satisfy 
the DRR, plus additional monitoring conducted by State, local, and 
Tribal air agencies on their own prerogative to satisfy other data 
needs. In the memo in the docket for this action titled ``Ambient Air 
SO2 Monitoring Network Review and Background'' (Watkins et 
al., 2024), it is indicated that the current SO2 monitoring 
network includes a focus on characterizing air quality where 
SO2 concentrations are expected to be high in the ambient 
air. The network provides data needed for implementation of the current 
primary and secondary SO2 NAAQS and can also provide data 
that can be used to support the needs for an annual average standard 
like the one being proposed in this action.
    Based on the EPA's review of the SO2 network history, 
current design and objectives, and data, we believe that the current 
network is adequate to provide the data needed to implement the 
proposed secondary SO2 NAAQS; therefore, modification to the 
existing SO2 minimum monitoring requirements is not 
necessary. As noted in section II.B.1, spatial distribution of 
SO2 and sulfate deposition reflect the distribution for 
SOX emissions on which the network is largely focused upon. 
Additionally, as noted in section II.E.3, there is a general 
association of monitoring sites having higher SO2 
concentrations in areas with higher local sulfur deposition estimates, 
meaning that a network measuring SO2 in areas of expected 
higher concentrations would be expected to capture SO2 
concentrations contributing to areas experiencing higher deposition. We 
therefore believe that modifications to the existing SO2 
minimum monitoring requirements are not necessary to support 
implementation of the standard proposed in this action. In further 
support of this position, the EPA notes that the network is and will 
continue to be adaptable and can evolve in response to changing data 
needs, even without the Agency making changes to minimum monitoring 
requirements. The State, local, and Tribal air agencies that operate 
most of the network monitors, as well as industry stakeholders, can 
propose and make adjustments to their pieces of the network when a new 
need arises, or air quality conditions change. Finally, the EPA has 
authority through 40 CFR part 58, appendix D, section 4.4.3, for its 
Regional Administrators to work with State, local, and Tribal air 
agencies to require SO2 monitoring above the minimum 
monitoring requirements where the network is found to be insufficient 
to meet its objectives. This means that monitors can be added in an 
area that has the potential for concentrations that exceed or 
contribute to an exceedance of the level of the NAAQS.
    In summary, the EPA is not proposing any changes to the minimum 
monitoring requirements as part of this proposal to revise the 
secondary SO2 NAAQS because the network is currently 
adequate, and because the EPA, State, local, Tribal, and industry

[[Page 26687]]

stakeholders have the authority and ability to adjust monitoring 
efforts and redirect resources as needed to ensure that the monitoring 
objectives of the SO2 network continue to be met. The EPA 
solicits comment on this proposed determination.

V. Clean Air Act Implementation Considerations for the Proposed 
Secondary SO2 Standard

    The proposed SO2 secondary NAAQS, if finalized, would 
trigger a number of implementation processes which are discussed for 
informational purposes in this portion of the preamble. The Agency is 
proposing to retain the secondary NO2 and PM NAAQS; thus, 
discussion of implementation considerations related to those NAAQS is 
not included in this section.
    At the outset, promulgation of a new or revised NAAQS, including 
finalization of this proposed revision, would trigger a process through 
which States \100\ would make recommendations to the Administrator 
regarding initial area designations. States also would be required to 
make a new SIP submission to establish that they meet the necessary 
structural requirements for such new or revised NAAQS pursuant to CAA 
section 110(a)(1) and (2), also referred to as the ``infrastructure SIP 
submission'' (more on this submission below). Further, if finalized, 
the SO2 secondary NAAQS would need to be incorporated into 
the implementation of applicable air permitting requirements and SIP 
conformity processes. This section provides background information for 
understanding the possible implications of the proposed NAAQS changes 
and describes the EPA's intentions for providing States any guidance 
the EPA determines to be needed to assist their implementation efforts, 
if such proposed changes are finalized. This section also describes 
existing EPA interpretations of CAA requirements and other EPA guidance 
relevant to implementation of a new SO2 secondary NAAQS, if 
one is finalized.
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    \100\ This and all subsequent references to ``state'' are meant 
to include State, local and Tribal agencies responsible for the 
implementation of a SO2 control program.
---------------------------------------------------------------------------

    The EPA is not proposing any regulatory changes to SO2 
implementation as a part of this proposal. Therefore, EPA is not 
proposing action on such topics in this rulemaking. The public is 
encouraged to share information regarding implementation needs and 
considerations. Although this rulemaking is not requesting comment 
specifically on this topic, information on this topic may be submitted 
for informational purposes to the docket for this proposed rulemaking. 
The EPA welcomes the public to provide input to the Agency through 
comments. However, because these issues are not relevant to the 
establishment of the proposed secondary NAAQS, and because no specific 
revisions are proposed for the regulations implementing the proposed 
secondary NAAQS the EPA does not expect to respond to these comments in 
the final action on this proposal (nor is it required to do so).

A. Designation of Areas

    After the EPA establishes a new or revised NAAQS (primary or 
secondary), the CAA requires the EPA and States to take steps to ensure 
that the new or revised NAAQS is met. The first step, known as the 
initial area designations, involves identifying areas of the country 
that either meet or do not meet the new or revised NAAQS, along with 
the nearby areas contributing to NAAQS violations.
    Section 107(d)(1) of the CAA states that, ``By such date as the 
Administrator may reasonably require, but not later than 1 year after 
promulgation of a new or revised national ambient air quality standard 
for any pollutant under section 109, the Governor of each State shall . 
. . submit to the Administrator a list of all areas (or portions 
thereof) in the State'' and make recommendations for whether the EPA 
should designate those areas as ``nonattainment,'' ``attainment,'' or 
``unclassifiable.'' \101\ Section 107(d)(1)(A) of the CAA contains 
definitions of these terms. A nonattainment area is any area that does 
not meet (or that contributes to ambient air quality in a nearby area 
that does not meet) a NAAQS; an attainment area is any area (other than 
an area identified as a nonattainment area) that meets a NAAQS; and an 
unclassifiable area is any area that cannot be classified on the basis 
of available information as meeting or not meeting a NAAQS. The CAA 
provides the EPA discretion to require states to submit their 
designations recommendations within a reasonable amount of time not 
exceeding 1 year. The CAA also stipulates that ``the Administrator may 
not require the Governor to submit the required list sooner than 120 
days after promulgating a new or revised national ambient air quality 
standard.'' Section 107(d)(1)(B)(i) further provides, ``Upon 
promulgation or revision of a NAAQS, the Administrator shall promulgate 
the designations of all areas (or portions thereof) . . . as 
expeditiously as practicable, but in no case later than 2 years from 
the date of promulgation. Such period may be extended for up to one 
year in the event the Administrator has insufficient information to 
promulgate the designations.'' With respect to the NAAQS setting 
process, courts have interpreted the term ``promulgation'' to be 
signature and widespread dissemination of a final rule.\102\
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    \101\ While the CAA says ``designating'' with respect to the 
Governor's letter, in the full context of the CAA section it is 
clear that the Governor makes a recommendation to which the EPA must 
respond via a specified process if the EPA does not accept it.
    \102\ API v. Costle, 609 F.2d 20 (D.C. Cir. 1979).
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    If the EPA agrees that the state's designations recommendations are 
consistent with all relevant CAA requirements, then it may proceed to 
promulgate the designations for such areas. However, if the EPA 
disagrees that a state's recommendation is consistent with all relevant 
CAA requirements, then the EPA may make modifications to the 
recommended designations by following the process outlined in the CAA. 
By no later than 120 days prior to promulgating the final designations, 
the EPA is required to notify states of any intended modifications to 
the designations of any areas or portions thereof, including the 
boundaries of areas, as the EPA may deem necessary. States then have an 
opportunity to comment on the EPA's intended designations decisions. If 
a state elects not to provide designations recommendations, then the 
EPA must timely promulgate the designations that it deems appropriate. 
While section 107(d) of the CAA specifically addresses the designations 
process for states, the EPA intends to follow the same process for 
Tribes to the extent practicable, pursuant to section 301(d) of the CAA 
regarding Tribal authority, and the Tribal Authority Rule (63 FR 7254, 
February 12, 1998). To provide clarity and consistency in doing so, the 
EPA issued a guidance memorandum to our Regional Offices on working 
with Tribes during the designations process.\103\
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    \103\ ``Guidance to Regions for Working with Tribes during the 
National Ambient Air Quality Standards (NAAQS) Designations 
Process,'' December 20, 2011, Memorandum from Stephen D. Page to 
Regional Air Directors, Regions 1-X available at https://www.epa.gov/sites/default/files/2017-02/documents/12-20-11_guidance_to_regions_for_working_with_tribes_naaqs_designations.pdf
.
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    In this action, the EPA is proposing to add an annual average 
SO2 secondary standard with a level within the range of 10-
15 ppb, averaged over three consecutive years. Consistent with the 
process used in previous area designations efforts for both primary

[[Page 26688]]

and secondary standards, the EPA will employ a nationally consistent 
framework and approach to evaluate each state's designations 
recommendations, considering air quality and other area-specific facts 
and circumstances \104\ to support area designations and boundaries 
decisions for the NAAQS. Section 107(d) of the CAA explicitly requires 
that the EPA designate as nonattainment not only the area that is 
violating the pertinent standard, but also those nearby areas that 
contribute to ambient air quality in the violating area. Consistent 
with past practice, the EPA plans to address issues relevant to area 
designations more fully in a separate designations-specific memorandum 
around the time of promulgation of a new SO2 secondary 
NAAQS, if the proposal to establish a new standard is finalized. As 
this section is designed for informational purposes, the public may 
comment on the process and schedule for the initial area designations 
and nonattainment boundary setting effort associated with the proposed 
new SO2 secondary NAAQS. However, the EPA does not expect to 
respond to these comments in the final action containing the final 
decision on the proposed NAAQS (nor is it required to do so).
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    \104\ The EPA has historically used nationally consistent area-
specific analyses to support nonattainment area boundary 
recommendations and final boundary determinations by evaluating 
factors such as air quality data, emissions and emissions-related 
data (e.g., population density and degree of urbanization, traffic 
and commuting patterns), meteorology, geography/topography, and 
jurisdictional boundaries. We expect to follow a similar process 
when establishing area designations for any new or revised 
SO2 secondary NAAQS, if finalized.
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    As in past iterations of establishing a new or revised NAAQS, the 
EPA intends to make the designations for any new or revised NAAQS based 
on the most recent 3 years of complete, certified, and valid air 
quality data and other available information. The EPA intends to use 
such available air quality data from the current SO2 
monitoring network and other technical information. Monitoring data are 
currently available from existing FEM and FRM monitors sited and 
operated in accordance with 40 CFR parts 50 and 58 to determine 
compliance with the proposed new SO2 secondary NAAQS, if 
finalized.
    In some areas, State or Tribal air agencies may flag air quality 
data for certain days in the Air Quality System (AQS) database due to 
potential impacts from exceptional events (e.g., volcanic activity for 
SO2). Air quality monitoring data affected by exceptional 
events may be excluded from use in identifying a violation at a 
regulatory ambient air monitoring site if the data meet the criteria 
for exclusion under EPA's ``Treatment of Data Influenced by Exceptional 
Events'' Final Rule (81 FR 68216; October 3, 2016) (Exceptional Events 
Rule) and codified at 40 CFR 50.1, 50.14, and 51.930. For events 
affecting initial area designations, including designations under the 
proposed annual SO2 secondary NAAQS, if finalized, the air 
agency is required to follow the exceptional events demonstration 
submission deadlines that are identified in table 2 to 40 CFR 
50.14(c)(2)(vi)--``Schedule for Initial Notification and Demonstration 
Submission for Data Influenced by Exceptional Events for Use in Initial 
Area Designations.'' The EPA encourages air agencies to work 
collaboratively with the appropriate EPA Regional office after 
identifying any exceptional event influencing ambient air quality 
concentrations in a way that could affect area designations for the 
proposed annual SO2 secondary NAAQS, if finalized.\105\
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    \105\ Additional information, tools, and resources relating to 
exceptional events can be found at the EPA's exceptional events 
website located at https://www.epa.gov/air-quality-analysis/final-2016-exceptional-events-rule-supporting-guidance-documents-updated-faqs.
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B. Section 110(a)(1) and (2) Infrastructure SIP Requirements

    The CAA directs states to address basic SIP requirements to 
implement, maintain, and enforce the NAAQS. Under CAA sections 
110(a)(1) and (2), states are required to have state implementation 
plans that provide the necessary air quality management infrastructure 
including, among other things, enforceable emissions limitations, an 
ambient air monitoring program, an enforcement program, air quality 
modeling capabilities, and adequate personnel, resources, and legal 
authority. After the EPA promulgates a new or revised NAAQS, states are 
required to make a new SIP submission to establish that they meet the 
necessary structural requirements for such new or revised NAAQS or make 
changes to do so. The EPA refers to this type of SIP submission as an 
``infrastructure SIP submission.'' Under CAA sections 110(a)(1), all 
states are required to make these infrastructure SIP submissions within 
3 years after promulgation of a new or revised standard, or such 
shorter deadline as the EPA may impose. Although the CAA authorizes the 
EPA to set a shorter time for states to make these SIP submissions, the 
EPA is not proposing to do so. Section 110(b) of the CAA also provides 
that the EPA may extend the deadline for the ``infrastructure'' SIP 
submission for a new secondary NAAQS by up to 18 months beyond the 
initial 3 years. If the proposed new annual SO2 secondary 
NAAQS is finalized, the EPA believes it would be more efficient for 
states and the EPA if each affected state submits the section 110 
infrastructure SIP that addresses the secondary standard within 3 years 
of promulgation of a new or revised NAAQS, and so is not proposing to 
apply a shorter deadline. However, the EPA also recognizes that 
individual states may prefer the flexibility to submit the secondary 
NAAQS infrastructure SIP at a later date, and if requested, the EPA 
would review such requests on a case-by-case basis as is provided by 
the EPA's existing regulations implementing CAA section 110(b) at 40 
CFR 51.341.
    Under CAA section 110(a)(1) and (2), states are required to make 
SIP submissions that address a number of requirements pertaining to 
implementation, maintenance, and enforcement of a new or revised NAAQS. 
The specific subsections in CAA section 110(a)(2) require states to 
address a number of requirements, as applicable: (A) emissions limits 
and other control measures; (B) ambient air quality monitoring/data 
system; (C) programs for enforcement of control measures and for 
construction or modification of stationary sources; (D)(i) interstate 
pollution transport and (ii) interstate and international pollution 
abatement; (E) adequate resources and authority, conflict of interest, 
and oversight of local governments and regional agencies; (F) 
stationary source monitoring and reporting; (G) emergency episodes; (H) 
SIP revisions; (I) plan revisions for nonattainment areas; (J) 
consultation with government officials, public notification, Prevention 
of Significant Deterioration (PSD) and visibility protection; (K) air 
quality modeling and submission of modeling data; (L) permitting fees; 
and (M) consultation and participation by affected local entities. 
These requirements apply to all SIP submissions in general, but the EPA 
has provided specific guidance to states concerning its interpretation 
of these requirements in the specific context of infrastructure SIP 
submissions for a new or revised NAAQS.\106\
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    \106\ See ``Guidance on Infrastructure State Implementation Plan 
(SIP) Elements under Clean Air Act sections 110(a)(1) and 
110(a)(2)'' September 2013, Memorandum from Stephen D. Page to 
Regional Air Directors, Regions 1-10.
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    The EPA interprets the CAA such that two elements identified in 
section 110(a)(2) are not subject to the 3-year

[[Page 26689]]

submission deadline of section 110(a)(1), and thus states are not 
required to address them in the context of an infrastructure SIP 
submission. The elements pertain to part D, in title I of the CAA, 
which addresses plan requirements for nonattainment areas. Therefore, 
for the reasons explained below, the following section 110(a)(2) 
elements are considered by the EPA to be outside the scope of 
infrastructure SIP actions: (1) the portion of section 110(a)(2)(C), 
programs for enforcement of control measures and for construction or 
modification of stationary sources that applies to permit programs 
applicable in designated nonattainment areas, (known as ``nonattainment 
new source review'') under part D; and (2) section 110(a)(2)(I) in its 
entirety. The EPA does not expect states to address these two elements 
pertaining to part D for a new or revised NAAQS in the infrastructure 
SIP submissions to include regulations or emissions limits developed 
specifically for attaining the relevant standard as it pertains to 
areas designated nonattainment for the proposed SO2 
secondary NAAQS, if finalized. States would be required to submit 
infrastructure SIP submissions for the proposed new SO2 
secondary NAAQS, if finalized, before they are required to submit 
nonattainment plan SIP submissions to demonstrate attainment with the 
same NAAQS. As a general matter, states would be required to submit 
nonattainment plans to provide for attainment and maintenance of the 
proposed new SO2 secondary NAAQS, if finalized, within 3 
years from the effective date of nonattainment area designations as 
required under CAA section 172(b). In addition, because this NAAQS, if 
finalized, would be a secondary standard, section 110(b) of the CAA 
also provides that the EPA may extend the deadline for the 
nonattainment plan for up to 18 months beyond the initial 3 years. The 
EPA reviews and acts upon these later SIP submissions through a 
separate process. For this reason, the EPA does not expect states to 
address new nonattainment area emissions controls per section 
110(a)(2)(I) in their infrastructure SIP submissions.
    Another required infrastructure SIP element is that each state's 
SIP must contain adequate provisions to prohibit, consistent with the 
provisions of title I of the CAA, emissions from within the state that 
will significantly contribute to nonattainment in, or interfere with 
maintenance by, any other state of the primary or secondary NAAQS.\107\ 
This element is often referred to as the ``good neighbor'' or 
``interstate transport'' provision.\108\ The provision has two prongs: 
significant contribution to nonattainment (prong 1) and interference 
with maintenance (prong 2). The EPA and states must give independent 
significance to prong 1 and prong 2 when evaluating downwind air 
quality problems under CAA section 110(a)(2)(D)(i)(I).\109\ Further, 
case law has established that the EPA and states must implement 
requirements to meet interstate transport obligations in alignment with 
the applicable statutory attainment schedule of the downwind areas 
impacted by upwind-state emissions.\110\
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    \107\ CAA section 110(a)(2)(D)(i)(I)
    \108\ CAA section 110(a)(2)(D)(i)(II) also addresses certain 
interstate effects that states must address and thus is also 
sometimes referred to as relating to ``interstate transport.''
    \109\ See North Carolina v. EPA, 531 F.3d 896, 909-11 (D.C. Cir. 
2008).
    \110\ See id. at 911-13. See also Wisconsin v. EPA, 938 F.3d 
303, 313-20 (D.C. Cir. 2019); Maryland v. EPA, 958 F.3d 1185, 1203-
04 (D.C. Cir. 2020).
---------------------------------------------------------------------------

    The EPA anticipates coordinating with states with respect to the 
requirements of CAA section 110(a)(2)(D)(i)(I) for implementation of 
the proposed SO2 secondary NAAQS, if finalized. We note that 
states may elect to make SIP submissions that address certain 
infrastructure SIP elements separately from the others. In recent 
years, due in part to the complexity of addressing interstate transport 
obligations, some states have found it efficient to make SIP 
submissions to address the interstate transport provisions separately 
from other infrastructure SIP elements.
    It is the responsibility of each state to review its air quality 
management program's existing SIP provisions in light of each new or 
revised NAAQS to determine whether any revisions are necessary to 
implement a new or revised NAAQS. Most states have revised and updated 
their SIPs in recent years to address requirements associated with 
other revised NAAQS. For some states, it may be the case that, for a 
number of infrastructure elements, the state may believe it already has 
adequate state regulations adopted and approved into the SIP to address 
a particular requirement with respect to any new or revised NAAQS. For 
such portions of the state's infrastructure SIP submission, the state 
could provide an explanation of how its existing SIP provisions are 
adequate.
    If a state determines that existing SIP-approved provisions are 
adequate in light of the proposed SO2 secondary NAAQS, if 
finalized, with respect to a given infrastructure SIP element (or sub-
element), then the state could make a SIP submission ``certifying'' 
that the existing SIP contains provisions that address those 
requirements of the specific section 110(a)(2) infrastructure 
elements.\111\ In the case of such a certification submission, the 
state would not have to include a copy of the relevant provision (e.g., 
rule or statute) itself. Rather, the state in its infrastructure SIP 
submission could provide citations to the SIP-approved state statutes, 
regulations, or non-regulatory measures, as appropriate, which meet the 
relevant CAA requirement. Like any other SIP submission, that state 
could make such a certification only after it has provided reasonable 
notice and opportunity for public hearing. This ``reasonable notice and 
opportunity for public hearing'' requirement for infrastructure SIP 
submissions is to meet the requirements of CAA sections 110(a) and 
110(l). Under the EPA's regulations at 40 CFR part 51, if a public 
hearing is held, an infrastructure SIP submittal must include a 
certification by the state that the public hearing was held in 
accordance with the EPA's procedural requirements for public hearings. 
See 40 CFR part 51, appendix V, 2.1(g), and see 40 CFR 51.102.
---------------------------------------------------------------------------

    \111\ A ``certification'' approach would not be appropriate for 
the interstate pollution control requirements of CAA section 
110(a)(2)(D)(i).
---------------------------------------------------------------------------

    In consultation with its EPA Regional Office, a state should follow 
all applicable EPA regulations governing infrastructure SIP submissions 
in 40 CFR part 51--e.g., subpart I (Review of New Sources and 
Modifications), subpart J (Ambient Air Quality Surveillance), subpart K 
(Source Surveillance), subpart L (Legal Authority), subpart M 
(Intergovernmental Consultation), subpart O (Miscellaneous Plan Content 
Requirements), subpart P (Protection of Visibility), and subpart Q 
(Reports). For the EPA's general criteria for infrastructure SIP 
submissions, refer to 40 CFR part 51, appendix V, Criteria for 
Determining the Completeness of Plan Submissions. The EPA recommends 
that states electronically submit their infrastructure SIPs to the EPA 
through the State Plan Electronic Collaboration System (SPeCS),\112\ an 
online system available through the EPA's Central Data Exchange.
---------------------------------------------------------------------------

    \112\ https://cdx.epa.gov/.
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C. Prevention of Significant Deterioration and Nonattainment New Source 
Review Programs for the Proposed Secondary SO2 Standard

    The CAA, at parts C and D of title I, contains preconstruction 
review and

[[Page 26690]]

permitting programs applicable to new major stationary sources and 
major modifications of existing major sources. The preconstruction 
review of each new major stationary source and major modification 
applies on a pollutant-specific basis, and the requirements that apply 
for each pollutant depend on whether the area in which the source is 
situated is designated as attainment (or unclassifiable) or 
nonattainment for that pollutant. In areas designated attainment or 
unclassifiable for a pollutant, the PSD requirements under part C apply 
to construction at major sources. In areas designated nonattainment for 
a pollutant, the Nonattainment New Source Review (NNSR) requirements 
under part D apply to major source construction. Collectively, those 
two sets of permit requirements are commonly referred to as the ``major 
New Source Review'' or ``major NSR'' programs.
    The EPA is not proposing any changes to the NSR program regulations 
as part of this proposal to implement the proposed SO2 
secondary NAAQS, if finalized. Under the PSD program, at the effective 
date of a final new or revised NAAQS, the applicant must demonstrate 
that any new construction or major modification and associated source 
emissions increase triggering PSD requirements does not cause or 
contribute to violation of that new or revised NAAQS. The EPA has 
models, guidance, and other tools for making this showing. The EPA 
anticipates that sources and reviewing authorities will be able to use 
most of these existing tools to demonstrate compliance with the 
secondary SO2 standard, if finalized as proposed. However, 
some adjustment and updates to these tools may be appropriate. The EPA 
is also considering an alternative compliance demonstration approach 
(described in section V.D. of this action) that the Agency may support 
using to make this PSD permitting demonstration. Considering these 
topics, the EPA has developed a separate technical document (Tillerson 
et al., 2024),\113\ which provides a technical justification for how a 
demonstration of compliance with the 1-hour primary SO2 
standard can suffice to demonstrate compliance with the proposed new 
SO2 secondary standard, if such a standard is finalized.
---------------------------------------------------------------------------

    \113\ This technical memo (Tillerson et al., 2024) is available 
in the docket for this NAAQS review (EPA-HQ-OAR-2014-0128).
---------------------------------------------------------------------------

    The statutory requirements for a PSD permit program set forth under 
part C of title I of the CAA (sections 160 through 169) are implemented 
through the EPA's PSD regulations found at 40 CFR 51.166 (minimum 
requirements for an approvable PSD SIP) and 40 CFR 52.21 (PSD 
permitting program for permits issued under the EPA's Federal 
permitting authority). Whenever a proposed new major source or major 
modification triggers PSD requirements for SO2, either 40 
CFR 52.21 or state regulations based on 40 CFR 51.166 will apply for 
undesignated areas and for areas that are designated as attainment or 
as unclassifiable for the proposed SO2 secondary NAAQS, if 
such standard is finalized.
    For PSD, a ``major stationary source'' is one with the potential to 
emit 250 tons per year (tpy) or more of any regulated NSR pollutant, 
unless the new or modified source is classified under a list of 28 
source categories contained in the statutory definition of ``major 
emitting facility'' in section 169(1) of the CAA. For those 28 source 
categories, a ``major stationary source'' is one with the potential to 
emit 100 tpy or more of any regulated NSR pollutant. A ``major 
modification'' is a physical change or a change in the method of 
operation of an existing major stationary source that results, first, 
in a significant emissions increase of a regulated NSR pollutant and, 
second, in a significant net emissions increase of that pollutant. See 
40 CFR 51.166(b)(2)(i), 52.21(b)(2)(i). The EPA PSD regulations define 
the term ``regulated NSR pollutant'' to include any pollutant for which 
a NAAQS has been promulgated and any pollutant identified in the EPA 
regulations as a constituent or precursor to such pollutant. See 40 CFR 
51.166(b)(49), 52.21(b)(50). Thus, the PSD program currently requires 
the review and control of emissions of SO2, as applicable. 
Among other things, for each regulated NSR pollutant emitted or 
increased in a significant amount, the PSD program requires a new major 
stationary source or a major modification to apply the ``best available 
control technology'' (BACT) and to conduct an air quality impact 
analysis to demonstrate that the proposed major stationary source or 
major modification will not cause or contribute to a violation of any 
NAAQS or PSD increment.\114\ See CAA section 165(a)(3)-(4), 40 CFR 
51.166(j) and (k), 52.21(j) and (k). The PSD requirements may also 
include, in appropriate cases, an analysis of potential adverse impacts 
on Class I areas. See CAA section 162(a) and 165, 40 CFR 51.166(p), 
52.21(p)).\115\
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    \114\ By establishing the maximum allowable level of ambient air 
pollutant concentration increase in a particular area, an increment 
defines ``significant deterioration'' of air quality in that area. 
Increments are defined by the CAA as maximum allowable increases in 
ambient air concentrations above a baseline concentration and are 
specified in the PSD regulations by pollutant and area 
classification (Class I, II and III). 40 CFR 51.166(c), 52.21(c); 75 
FR 64864; October 20, 2010; The EPA has developed the Guideline on 
Air Quality Models and other documents to, among other things, 
provide methods and guidance for demonstrating compliance the NAAQS 
and PSD increments including the annual SO2 standard. See 
40 CFR part 51, appendix W; 82 FR 5182, January 17, 2017.
    \115\ Congress established certain Class I areas in section 
162(a) of the CAA, including international parks, national 
wilderness areas, and national parks that meet certain criteria. 
Such Class I areas, known as mandatory Federal Class I areas, are 
afforded special protection under the CAA. In addition, states and 
Tribal governments may establish Class I areas within their own 
political jurisdictions to provide similar special air quality 
protection.
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    With regard to nonattainment NSR, the EPA's regulations for the 
NNSR programs are contained in 40 CFR 51.165, 52.24 and 40 CFR part 51, 
appendix S. Specifically, the EPA developed minimum program 
requirements for a NNSR program that is approvable in a SIP, and those 
requirements, which include requirements for SO2, are 
contained in 40 CFR 51.165. In addition, 40 CFR part 51, appendix S 
contains requirements constituting an interim NNSR program. This 
program enables NNSR permitting in nonattainment areas by states that 
lack a SIP-approved NNSR permitting program (or a program that does not 
apply to the relevant pollutant) during the time between the date of 
the relevant designation and the date that the EPA approves into the 
SIP a NNSR program. See 40 CFR part 51, appendix S, part I; 40 CFR 
52.24(k). Any new NNSR requirements for SO2 associated with 
the proposed secondary standard, should it be finalized, would not 
become applicable until the effective date of any nonattainment 
designation for the final standard.
    As noted above, the EPA intends to review and consider the 
appropriateness of existing PSD compliance demonstration tools for 
implementation of any new secondary SO2 NAAQS. In addition, 
as described below and in section V.D. of this document, the EPA 
acknowledges that there may be added burdens associated with making the 
required PSD air quality impact demonstration for the annual secondary 
standard if finalized, and the EPA may develop an alternative 
compliance demonstration based upon Tillerson et al. (2024) if the 
secondary SO2 NAAQS is finalized as proposed. Under such an 
alternative compliance demonstration, sources and reviewing authorities 
would be able to sufficiently demonstrate compliance with the proposed 
new SO2 secondary standard by demonstrating compliance with 
the primary 1-hour standard. Section V.D.

[[Page 26691]]

of this document includes further discussion of an alternative 
compliance demonstration approach and the technical justification that 
sources and permitting authorities may apply in permitting actions.

D. Alternative PSD Compliance Demonstration Approach for the Proposed 
Secondary SO2 Standard

    If the new secondary SO2 NAAQS is finalized as proposed, 
the EPA would plan to calculate design values for the new secondary 
NAAQS using the procedures described in section III of this preamble, 
relying upon ambient air SO2 measurement data. The PSD 
program requires that new or modified stationary sources complete a 
compliance demonstration using air quality modeling or other methods to 
demonstrate that their proposed emissions increases will not cause or 
contribute to a violation of any NAAQS, including this secondary 
SO2 NAAQS, if finalized. See 40 CFR 51.166(k), (m), 
52.21(k), (m). Under 40 CFR 51.166(l), 52.21(l), all PSD demonstrations 
for purposes of determining whether a new or modified source will cause 
or contribute to a NAAQS violation, including a violation of the 
secondary NAAQS for SO2, if finalized, must be based upon 
air quality models, databases, and other requirements specified 40 CFR 
part 51, appendix W.
    Under section 9.2.3 of appendix W, the EPA recommends a multi-stage 
approach to making the required demonstration of compliance with the 
NAAQS, which enables a streamlined demonstration in many cases using 
PSD screening tools. If a cumulative impact assessment is necessary, 
due to the source-oriented nature of the current monitoring network for 
SO2, there is some uncertainty as to whether sources may be 
able to rely on existing monitoring data to adequately represent 
background for their PSD compliance demonstrations. Although the 
current SO2 monitoring network is primarily geared to 
measure in areas of high SO2 emissions proximate to 
populations and to sources, it has a limited number of monitors away 
from emissions areas that are needed to provide the information 
necessary for area specific estimates of background concentrations. 
Therefore, there may be situations where prospective PSD sources could 
be required to collect new data in order to determine the 
representative background concentrations of annual SO2.
    Because of the added burdens that may result for applicants and 
permitting authorities from these considerations, the EPA is 
considering alternative approaches to enable prospective PSD sources to 
more readily demonstrate that they will not cause or contribute to a 
violation of the proposed secondary SO2 NAAQS, if finalized 
as proposed. The EPA believes that it is reasonable to allow the use of 
an alternative demonstration approach where such an approach is 
technically justified. The EPA is providing notice of the potential for 
an alternative PSD compliance demonstration approach discussed in this 
section and based upon the technical analysis detailed in Tillerson et 
al. (2024) included in the docket. The public is encouraged to share 
information on this alternative compliance demonstration approach. The 
EPA may consider information provided by the public in developing any 
future guidance on this approach for the new secondary SO2 
NAAQS. The EPA is not proposing this alternative compliance 
demonstration approach for the proposed secondary standard nor is the 
EPA taking any action to implement this alternative compliance 
demonstration approach in this rulemaking. Consequently, the EPA would 
not be obligated to respond to any comments received on this topic as 
part of the final rulemaking.
    The Agency believes that following an alternative compliance 
demonstration approach could aid implementation of the PSD permitting 
program after enactment of the proposed secondary SO2 NAAQS, 
if finalized. To support consideration of alternative approaches that 
could be used by prospective PSD sources, the EPA conducted a two-
pronged technical analysis of the relationships between the proposed 
secondary standard and the existing 1-hour SO2 primary NAAQS 
(See Tillerson et al., 2024). The first prong of the analysis addressed 
aspects of a PSD source impact analysis by evaluating whether an 
individual source's impact resulting in a small increase in 1-hour 
SO2 concentration, at the level of the significant impact 
level (SIL) for the primary SO2 NAAQS, would produce a 
comparably small increase in the annual SO2 concentration. 
This analysis included modeled estimates of SO2 for a range 
of source categories and scenarios. It indicated that small increases 
in 1-hour SO2 concentrations caused by individual sources 
produce similarly small changes in the annual SO2 
concentrations. The second prong of the analysis addressed aspects of a 
PSD cumulative impact analysis indicating that a demonstration showing 
attainment of the 1-hour SO2 standard is expected to also 
show attainment of the proposed secondary SO2 standard. This 
analysis was based on 2017 to 2022 air quality data and compared the 
air quality that would meet the current 1-hour SO2 standard 
(with its level of 75 ppb in conjunction with a 99th percentile 
averaged over 3 years) with air quality that would meet the proposed 
secondary SO2 standard (with a level of 10-15 ppb in 
conjunction with an annual mean averaged over three years). As shown in 
Tillerson et al. (2024), this analysis indicated that all areas for 
which existing monitoring data showed attainment of the 1-hour 
SO2 standards would also likely be in attainment of the 
proposed secondary SO2 standard. The EPA believes that this 
technical analysis is robust and that its conclusions can be applied 
across the United States.
    Based on this technical analysis, the EPA currently believes that 
there is sufficient evidence that, for the purposes of making a 
demonstration under the PSD program that a new or modified source will 
not cause or contribute to a violation of the proposed secondary 
SO2 NAAQS, if finalized, a persuasive demonstration that the 
source will not cause or contribute to a violation of the 1-hour 
SO2 NAAQS could serve as a suitable alternate compliance 
demonstration. As such, many or all sources undergoing PSD review for 
SO2 would be able to rely upon their analysis demonstrating 
that they will not cause or contribute to a violation of the 1-hour 
SO2 NAAQS to also demonstrate that they will not cause or 
contribute to a violation of the proposed secondary SO2 
NAAQS, if finalized. This alternative compliance demonstration approach 
would thus serve to streamline air quality analyses in a manner 
consistent with the CAA and NSR regulations. Using this approach would 
result in a source not needing to provide a separate and distinct 
analysis to demonstrate compliance with the proposed secondary 
SO2 standard, if finalized. The EPA believes this 
alternative compliance demonstration approach could fulfill PSD 
requirements for individual sources in PSD areas for the proposed 
secondary SO2 NAAQS, if finalized. This approach would apply 
in both areas that would not yet have been designated as nonattainment 
for the new secondary SO2 NAAQS, if finalized, and those 
that would be ultimately designated as attainment or unclassifiable 
areas. The EPA will continue to evaluate this potential approach and 
may consider it in guidance addressing implementation of the proposed 
secondary SO2 NAAQS, if

[[Page 26692]]

finalized, separate from this rulemaking setting the standard itself.

E. Transportation Conformity Program

    Transportation conformity is required under CAA section 176(c) (42 
U.S.C. 7506(c)) to ensure that federally supported highway and transit 
activities are consistent with (``conform to'') the purpose of the SIP. 
Transportation conformity applies to areas that are designated as 
nonattainment areas and to nonattainment areas that have been 
redesignated to attainment with an approved CAA section 175A 
maintenance plan (i.e., maintenance areas) for transportation-related 
criteria pollutants: carbon monoxide, ozone, NO2, 
PM2.5, and PM10. Motor vehicles are not 
significant sources of SO2, and thus transportation 
conformity does not apply to any SO2 NAAQS (40 CFR 
93.102(b)(1)), existing or proposed.\116\ Therefore, the EPA is not 
proposing any changes to the transportation conformity rule (40 CFR 
51.390 and 40 CFR part 93, subpart A) for the proposed SO2 
secondary NAAQS.
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    \116\ See ``VII. Description of the Proposal'' in ``Criteria and 
Procedures for Determining Conformity to State or Federal 
Implementation Plans of Transportation Plans, Programs, and Projects 
funded or Approved Under Title 23 U.S.C. or the Federal Transit 
Act.'' (58 FR 3768, January 11, 1993). EPA finalized the original 
transportation conformity regulations on November 24, 1993 (58 FR 
62188). The rule has subsequently been revised and the current 
provisions of the transportation conformity rule are found at 40 CFR 
part 93, subpart A.
---------------------------------------------------------------------------

F. General Conformity Program

    The General Conformity program requirements at 40 CFR part 93, 
subpart B establish criteria and procedures for determining conformity 
as required under CAA section 176(c),\117\ which prohibits a Federal 
agency from taking an action that would interfere with the ability of a 
state or Tribe to attain or maintain the NAAQS. General Conformity 
applies only to Federal activities not defined as transportation plans, 
programs, or projects under 40 CFR 93.102. The program requirements 
apply to emissions of all six criteria pollutants and their precursors 
per 40 CFR 93.153(b)(1) and (2), but only to the extent the emissions 
can be characterized as ``direct emissions'' or ``indirect emissions'' 
as defined under 40 CFR 93.152.
---------------------------------------------------------------------------

    \117\ Under CAA section 176(c)(1), Federal agencies have the 
affirmative responsibility to assure their actions achieve 
conformity to the purpose of an implementation plan, where the term 
``conformity to an implementation plan'' is defined at CAA sections 
176(c)(1)(A) and 176(c)(1)(B). Under CAA section 176(c)(4) the EPA 
is required to establish criteria and procedures for determining 
conformity.
---------------------------------------------------------------------------

    The General Conformity program applies to activities that cause 
emissions of the criteria or precursor pollutants to originate within 
designated nonattainment areas \118\ or redesignated attainment areas 
that operate under approved CAA section 175A maintenance plans (i.e., 
maintenance areas).
---------------------------------------------------------------------------

    \118\ Applicability of the General Conformity program to any 
newly designated nonattainment area for a specific NAAQS begins one 
year following the effective date of the final nonattainment 
designation, as allowed under CAA section 176(c)(6) and 40 CFR 
93.153(k).
---------------------------------------------------------------------------

    The EPA is not proposing changes to the General Conformity 
regulations in this proposed rulemaking. However, in the future, the 
EPA intends to review the need to issue or revise guidance describing 
how the current conformity regulations apply in nonattainment and 
maintenance areas for any new or revised NAAQS, as needed.

VI. Statutory and Executive Order Reviews

    Additional information about these statutes and Executive orders 
can be found at https://www.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive 
Order 14094: Modernizing Regulatory Review

    This action is a ``significant regulatory action'' as defined in 
Executive Order 12866, as amended by Executive Order 14094. 
Accordingly, EPA submitted this action to the Office of Management and 
Budget (OMB) for Executive Order 12866 review. Documentation of any 
changes made in response to the Executive Order 12866 review is 
available in the docket. The EPA prepared an analysis to determine if 
additional emission reductions would be needed to meet the proposed 
revision of the secondary SO2 NAAQS. This analysis is 
contained in the document ``Air Quality Analyses Using Sulfur Dioxide 
(SO2) Air Quality Data,'' which is available in the docket 
for this NAAQS review (ID No. EPA-HQ-OAR-2014-0128). The analysis 
concluded that no additional emissions reductions beyond any needed to 
meet the current 1-hour primary SO2 NAAQS would be expected 
to be necessary to meet the proposed annual secondary SO2 
NAAQS, resulting in no costs or benefits associated with pollution 
controls for this proposed NAAQS revision, if finalized. Accordingly, 
no regulatory impact analysis has been prepared for this proposal.

B. Paperwork Reduction Act (PRA)

    This action does not impose an information collection burden under 
the PRA. There are no information collection requirements directly 
associated with a proposed decision to revise or retain a NAAQS under 
section 109 of the CAA.

C. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic 
impact on a substantial number of small entities under the RFA. This 
action will not impose any requirements on small entities. Rather, this 
proposed rule establishes national standards for allowable annual 
average concentrations of SO2 in ambient air as required by 
section 109 of the CAA. See also American Trucking Associations v. EPA, 
175 F.3d 1027, 1044-45 (D.C. Cir. 1999) (NAAQS do not have significant 
impacts upon small entities because NAAQS themselves impose no 
regulations upon small entities), rev'd in part on other grounds, 
Whitman v. American Trucking Associations, 531 U.S. 457 (2001).

D. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in 
the Unfunded Mandates Reform Act (UMRA), 2 U.S.C. 1531-1538, and does 
not significantly or uniquely affect small governments. Furthermore, as 
indicated previously, in setting a NAAQS the EPA cannot consider the 
economic or technological feasibility of attaining ambient air quality 
standards, although such factors may be considered to a degree in the 
development of state plans to implement the standards. See also 
American Trucking Associations v. EPA, 175 F. 3d at 1043 (noting that 
because the EPA is precluded from considering costs of implementation 
in establishing NAAQS, preparation of the RIA pursuant to the Unfunded 
Mandates Reform Act would not furnish any information that the court 
could consider in reviewing the NAAQS).
    The EPA acknowledges, however, that if corresponding revisions to 
associated SIP requirements and air quality surveillance requirements 
are proposed at a later time, those revisions might result in such 
effects. Any such effects would be addressed as appropriate if and when 
such revisions are proposed.

E. Executive Order 13132: Federalism

    This action does not have federalism implications. It will not have 
substantial direct effects on the states, on the relationship between 
the national government and the states, or on the distribution of power 
and responsibilities among the various levels of government. However, 
the EPA

[[Page 26693]]

recognizes that states will have a substantial interest in this action 
and any future revisions to associated requirements.

F. Executive Order 13175: Consultation and Coordination With Indian 
Tribal Governments

    This action does not have Tribal implications, as specified in 
Executive Order 13175. It does not have a substantial direct effect on 
one or more Indian Tribes as Tribes are not obligated to adopt or 
implement any NAAQS. Thus, Executive Order 13175 does not apply to this 
action. However, consistent with the EPA Policy on Consultation and 
Coordination with Indian Tribes, the EPA will offer government-to-
government consultation with Tribes as requested.

G. Executive Order 13045: Protection of Children From Environmental 
Health Risks and Safety Risks

    EPA interprets Executive Order 13045 as applying only to those 
regulatory actions that concern environmental health or safety risks 
that EPA has reason to believe may disproportionately affect children, 
per the definition of ``covered regulatory action'' in section 2-202 of 
the Executive order.
    Therefore, this action is not subject to Executive Order 13045 
because it does not concern an environmental health risk or safety 
risk. Since this action does not concern human health, EPA's Policy on 
Children's Health also does not apply.

H. Executive Order 13211: Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution or Use

    This action is not a ``significant energy action'' because it is 
not likely to have a significant adverse effect on the supply, 
distribution, or use of energy. The purpose of this action is to 
propose to revise the existing secondary SO2 standard, and 
also to propose to retain the current secondary standards for 
NO2, PM2.5 and PM10. The action does 
not prescribe specific pollution control strategies by which these 
ambient air standards and monitoring revisions will be met. Such 
strategies will be developed by states on a case-by-case basis, and the 
EPA cannot predict whether the control options selected by states will 
include regulations on energy suppliers, distributors, or users. Thus, 
the EPA concludes that this proposal does not constitute a significant 
energy action as defined in Executive Order 13211.

I. National Technology Transfer and Advancement Act (NTTAA)

    This action involves technical standards. The EPA is proposing to 
use the existing indicator, SO2, for measurements in support 
of this action. To the extent feasible, the EPA employs a Performance-
Based Measurement System (PBMS), which does not require the use of 
specific, prescribed analytic methods. The PBMS is defined as a set of 
processes wherein the data quality needs, mandates or limitations of a 
program or project are specified and serve as criteria for selecting 
appropriate methods to meet those needs in a cost-effective manner. It 
is intended to be more flexible and cost effective for the regulated 
community; it is also intended to encourage innovation in analytical 
technology and improved data quality. Though the FRM for the NAAQS 
indicators defines the particular specifications for ambient air 
monitors, there is some variability with regard to how monitors can 
measure the pollutants, including SO2. Therefore, it is not 
practically possible to fully define the FRM in performance terms to 
account for this possible or realized variability in measurement 
principles of operation. Nevertheless, our approach in the past has 
resulted in multiple brands of monitors being approved as FRM for 
SO2, and we expect this to continue. Also, the FRMs 
described in 40 CFR part 50 and the equivalency criteria described in 
40 CFR part 53, constitute a performance-based measurement system for 
SO2, since methods that meet the field testing and 
performance criteria can be approved as FEMs. The EPA is not precluding 
the use of any other method, whether it constitutes a voluntary 
consensus standard or not, as long as it meets the specified 
performance criteria and is approved as an FRM or FEM.

J. Executive Order 12898: Federal Actions To Address Environmental 
Justice in Minority Populations and Low-Income Populations and 
Executive Order 14096: Revitalizing Our Nation's Commitment to 
Environmental Justice for All

    The EPA believes that the human health and environmental conditions 
that exist prior to this action do not result in disproportionate and 
adverse effects on communities with Environmental Justice (EJ) 
concerns. As discussed in sections II.D and II.E above, and Chapters 5 
and 7 of the PA, the acid buffering capacity of waterbodies in key 
acid-sensitive ecoregions in recent years is estimated to meet 
protection targets in high percentages. As discussed in section II.C.2 
above, impacts on acid-sensitive waterbodies, if sufficiently severe, 
would have the potential to impact the public welfare through impacts 
to fisheries. Although recent conditions do not indicate such a level 
of severity, to the extent local communities relied on such fisheries 
disproportionately to their representation in the population, such 
effects of the past (e.g., effects associated with acidification risks 
of 20 or more years ago) would have had the potential for 
disproportionate impacts. Recent conditions do not indicate risk of 
aquatic acidification to such a level of severity and the available 
information for recent acid buffering capacity levels does not include 
evidence of disproportionate and adverse impacts on communities with EJ 
concerns. As the proposed action is to establish a new, more stringent 
standard to protect acid-sensitive waterbodies to recent levels and 
protect against recurrence of acidification effects from the past, for 
which the potential for disproportionate and adverse effects on local 
communities is unknown, the EPA believes that this action is not likely 
to result in new disproportionate and adverse effects on communities 
with EJ concerns. The information supporting this Executive order 
review is contained in the PA for this review and sections II.C, II.D 
and II.E of this document.

References

Baker, LA, Herlihy, AT, Kaufmann, PR, and Eilers, JM (1991). Acidic 
lakes and streams in the United States: the role of acidic 
deposition. Science 252: 1151-1154.
Banzhaf, S, Burtraw, D, Evans, D and Krupnick, A (2006). Valuation 
of Natural Resource Improvements in the Adirondacks. Land Econ 82: 
445-464.
Belnap, J, Sigal, L, Moir, W and Eversman, S (1993). Lichens as 
Bioindicators of Air Quality: Identification of sensitive species. 
General Technical Report RM-224. United States Department of 
Agriculture, US Forest Service, Rocky Mountain Forest and Range 
Experimental Station.
Bethers, S, Day, ME, Wiersma, GB, Fernandez, IJ and Elvir, JA 
(2009). Effects of chronically elevated nitrogen and sulfur 
deposition on sugar maple saplings: Nutrition, growth and 
physiology. For Ecol Manage 258: 895-902.
Boonpragob, K, Nash, T, III and Fox, CA (1989). Seasonal deposition 
patterns of acidic ions and ammonium to the lichen Ramalina 
menziesii tayl. in Southern California. Environ Exp Bot 29: 187-197.
Boyer, EW, Goodale, CL, Jaworski, NA and Howarth, RW (2002). 
Anthropogenic nitrogen sources and relationships to riverine 
nitrogen export in the

[[Page 26694]]

northeastern USA. Biogeochemistry 57: 137-169.
Brown, CA and Ozretich, RJ (2009). Coupling between the coastal 
ocean and Yaquina Bay, Oregon: Importance of oceanic inputs relative 
to other nitrogen sources. Estuaries Coasts 32: 219-237.
Bulger, AJ, Cosby, BJ, Dolloff, CA, Eshleman, KN, Webb, JR and 
Galloway, JN (1999). SNP:FISH. Shenandoah National Park: Fish in 
sensitive habitats. Project Final Report-Volume 1-4. 
Charlottesville, VA, University of Virginia.
Bytnerowicz, A and Fenn, ME (1996). Nitrogen deposition in 
California forests: A review. Environ Pollut 92: 127-146.
Clark, CM and Tilman, D (2008). Loss of plant species after chronic 
low-level nitrogen deposition to prairie grasslands. Nature 451: 
712-715.
Cosby, BJ, Hornberger, GM, Galloway, JN and Wright, RF (1985). 
Modeling the effects of acid deposition: Assessment of a lumped 
parameter model of soil water and streamwater chemistry. Water 
Resour Res 21(1): 51-63.
Cosby, BJ, Webb, JR, Galloway, JN and Deviney, FA (2006). Acidic 
deposition impacts on natural resources in Shenandoah National Park. 
Technical Report NPS/NER/NRTR--2006-066. United States Department of 
the Interior, National Park Service, Northeast Region.
Costanza, R, De Groot, R, Braat, L, Kubiszewski, I, Fioramonti, L, 
Sutton, P, Farber, S and Grasso, M (2017). Twenty years of ecosystem 
services: How far have we come and how far do we still need to go? 
Ecosyst Serv 28: 1-16.
Cox, L, Kendall, R and Fernandez, I (2020a). Letter from Louis Cox, 
Chair, Clean Air Scientific Advisory Committee, Ronald Kendall, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur and Ivan Fernandez, Immediate Past Chair, Secondary NAAQS 
Review Panel for Oxides of Nitrogen and Sulfur to the Administrator 
Andrew R. Wheeler, Re: CASAC Review of the EPA's Integrated Science 
Assessment for Oxides of Nitrogen, Oxides of Sulfur, and Particulate 
Matter--Ecological Criteria (Second External Review Draft--June 
2018). May 5, 2020. EPA-CASAC-20-004. Office of the Administrator, 
Science Advisory Board Washing, DC Available at: https://casac.epa.gov/ords/sab/f?p=113:12:1342972375271:::12.
Cox, L, Kendall, R and Fernandez, I (2020b). Letter from Louis Cox, 
Chair, Clean Air Scientific Advisory Committee, Ronald Kendall, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur and Ivan Fernandez, Immediate Past Chair, Secondary NAAQS 
Review Panel for Oxides of Nitrogen and Sulfur to Administrator 
Andrew R. Wheeler, Re: Consultation on the EPA's Review of the 
Secondary Standards for Ecological Effects of Oxides of Nitrogen, 
Oxides of Sulfur, and Particulate Matter: Risk and Exposure 
Assessment Planning Document (August-2018). May 5, 2020. EPA-CASAC-
20-005. Office of the Administrator, Science Advisory Board Washing, 
DC Available at: https://casac.epa.gov/ords/sab/f?p=113:12:1342972375271:::12.
Cox, RD, Preston, KL, Johnson, RF, Minnich, RA and Allen, EB (2014). 
Influence of landscape scale variables on vegetation conversion to 
exotic annual grassland in southern California, USA. Glob Ecol 
Conserv 2: 190-203.
Davis, TW, Bullerjahn, GS, Tuttle, T, Mckay, RM and Watson, SB 
(2015). Effects of increasing nitrogen and phosphorus concentrations 
on phytoplankton community growth and toxicity during planktothrix 
blooms in Sandusky Bay, Lake Erie. Environ Sci Technol 49: 7197-
7207.
Dietze, MC and Moorcroft, PR (2011). Tree mortality in the eastern 
and central United States:Patterns and drivers. Glob Change Biol 
17(11): 3312-3326.
Diez Roux, A and Fernandez, I (2016). Letter from Anna Diez Roux, 
Chair, Clean Air Scientific Advisory Committee and Ivan Fernandez, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur, to Administrator Gina McCarthy, Re: CASAC Review of the 
EPA's Draft Integrated Review Plan for the National Ambient Air 
Quality Standards for Oxides of Nitrogen and Oxides of Sulfur. April 
1, 2016. EPA-CASAC-16-001. Office of the Administrator, Science 
Advisory Board Washing, DC Available at: https://casac.epa.gov/ords/sab/f?p=113:12:1342972375271:::12.
Diez Roux, A and Fernandez, I (2017). Letter from Anna Diez Roux, 
Chair, Clean Air Scientific Advisory Committee and Ivan Fernandez, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur, to the Honorable Gina McCarthy, Administrator, Re: CASAC 
Review of the EPA's Integrated Science Assessment for Oxides of 
Nitrogen, Oxides of Sulfur, and Particulate Matter--Ecological 
Criteria (First External Review Draft--February 2017). September 28, 
2017. EPA-CASAC-17-004. Office of the Administrator, Science 
Advisory Board Washing, DC Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100X9FA.PDF.
Driscoll, CT, Driscoll, KM, Fakhraei, H, Civerolo, K (2016). Long-
term temporal trends and spatial patterns in the acid-base chemistry 
of lakes in the Adirondack region of New York in response to 
decreases in acidic deposition. Atm Chem 146: 5-14.
Driscoll, CT, Lawrence, GB, Bulger, AJ, Butler, TJ, Cronan, CS, 
Eagar, C, Lambert, KF, Likens, GE, Stoddard, JL and Weathers, KC 
(2001). Acidic deposition in the northeastern United States: Sources 
and inputs, ecosystem effects, and management strategies. Bioscience 
51: 180-198.
Driscoll, CT, Lehtinen, MD and Sullivan, TJ (1994). Modeling the 
acid-base chemistry of organic solutes in Adirondack, New York, 
lakes. Water Resour Res 30: 297-306.
Duchesne, L and Ouimet, R (2009). Present-day expansion of American 
beech in northeastern hardwood forests: Does soil base status 
matter? Can J For Res 39: 2273-2282.
Dupont, J, Clair, TA, Gagnon, C, Jeffries, DS, Kahl, JS, Nelson, SJ 
and Peckenham, JM (2005). Estimation of critical loads of acidity 
for lakes in northeastern United States and eastern Canada. Environ 
Monit Assess 109(1): 275-291.
Emmett, BA, Boxman, D, Bredemeier, M, Gunderson, P, Kjonaas, OJ, 
Moldan, F, Schleppi, P, Tietema, A and Wright, RF (1998). Predicting 
the effects of atmospheric nitrogen deposition in conifer stands: 
evidence from the NITREX ecosystem-scale experiments. Ecosystems 1: 
352-360.
Farmer, AM, Bates, JW and Bell, JNB. (1992). Bryophytes and Lichens 
in a Changing Environment: Ecophysiological effects of acid rain on 
bryophytes and lichens. Claredon Press. Oxford, UK.
Fenn, ME, Allen, EB, Weiss, SB, Jovan, S, Geiser, LH, Tonnesen, GS, 
Johnson, RF, Rao, LE, Gimeno, BS, Yuan, F, Meixner, T and 
Bytnerowicz, A (2010). Nitrogen critical loads and management 
alternatives for N-impacted ecosystems in California. J Environ 
Manage 91: 2404-2423.
Friedlander, S (1982). Letter from Sheldon Friedlander, Chair, Clean 
Air Scientific Advisory Committee to Anne Gorsuch, Administrator, 
Re: CASAC Review and Closure of the Criteria Document for Sulfur 
Oxides/Particulate Matter. January 29, 1982. EPA-SAB-CASAC-82-003. 
Office of the Administrator, Science Advisory Board Washing, DC 
Available at: https://casac.epa.gov/ords/sab/f?p=113:12:1342972375271:::12.
Fuss, CB, Driscoll, CT and Campbell, JL (2015). Recovery from 
chronic and snowmelt acidification: Long-term trends in stream and 
soil water chemistry at the Hubbard Brook Experimental Forest, New 
Hampshire, USA. Jour Geo Res: Biog 120: 2360-2374.
Geiser, LH, Nelson, PR, Jovan, SE, Root, HT and Clark, CM (2019). 
Assessing ecological risks from atmospheric deposition of nitrogen 
and sulfur to us forests using epiphytic macrolichens. Diversity 
11(6): 87.
Gobler, CJ, Burkholder, JM, Davis, TW, Harke, MJ, Johengen, T, Stow, 
CA and Van de Waal, DB (2016). The dual role of nitrogen supply in 
controlling the growth and toxicity of cyanobacterial blooms. 
Harmful Algae 54: 87-97.
Herlihy AT, Kaufman, PR and Mitch, ME (1991). Stream chemistry in 
the Eastern United States 2. Current sources of acidity in acidic 
and low acid-neutralizing capacity streams. Water Resources Res 
27(4): 629-642.
Horn, KJ, Thomas, RQ, Clark, CM, Pardo, LH, Fenn, ME, Lawrence, GB, 
Perakis, SS, Smithwick, EA, Baldwin, D, Braun, S and Nordin, A 
(2018). Growth and survival relationships of 71 tree species with 
nitrogen and sulfur deposition across the conterminous U.S. PLoS ONE 
13(10): e0205296.
Howarth, RW. (2008). Science for ecosystem-based management: 
Narragansett Bay in the 21st century: Estimating atmospheric 
nitrogen deposition in the Northeastern United States: Relevance to 
Narragansett Bay. Springer. New York, NY.

[[Page 26695]]

Hutchinson, J, Maynard, D and Geiser, L (1996). Air quality and 
lichens--a literature review emphasizing the Pacific Northwest, USA. 
United States Department of Agriculture.
Isbell, F, Tilman, D, Polasky, S, Binder, S and Hawthorne, P (2013). 
Low biodiversity state persists two decades after cessation of 
nutrient enrichment. Ecol Lett 16: 454-460.
Janicki Environmental, Inc. (2013). Estimates of total nitrogen, 
total phosphorus, total suspended solids, and biochemical oxygen 
demand loadings to Tampa Bay, Florida: 2007-2011. St. Petersburg, 
FL: Tampa Bay Estuary Program. Available at: http://www.tbeptech.org/TBEP_TECH_PUBS/2013/TBEP_03_13_FINAL_TBEP_Loads_2007-2011%2019Mar2013.pdf.
Jensen, NK, Holzmueller, EJ, Edwards, PJ, Thomas-Van Gundy, M, 
DeWalle, DR and Williard, KWJ (2014). Tree response to experimental 
watershed acidification. Water Air Soil Pollut 225:1-12.
Latimer, JS and Charpentier, MA (2010). Nitrogen inputs to seventy-
four southern New England estuaries: Application of a watershed 
nitrogen loading model. Estuar Coast Shelf Sci 89: 125-136.
Latimer, JS and Rego, SA (2010). Empirical relationship between 
eelgrass extent and predicted watershed-derived nitrogen loading for 
shallow New England estuaries. Estuar Coast Shelf Sci 90: 231-240.
Lawrence, GB, Hazlett, PW, Fernandez, IJ, Ouimet, R, Bailey, SW, 
Shortle, WC, Smith, KT and Antidormi, MR (2015). Declining acidic 
deposition begins reversal of forest-soil acidification in the 
northeastern US and eastern Canada. Environ Sci Technol 49: 13103-
13111.
Lebo, ME; Paerl, HW; Peierls, BL. (2012). Evaluation of progress in 
achieving TMDL mandated nitrogen reductions in the Neuse River 
Basin, North Carolina. Environ Manage 49: 253-266. http://dx.doi.org/10.1007/s00267-011-9774-5
Li, H and McNulty, SG (2007). Uncertainty analysis on simple mass 
balance model to calculate critical loads for soil acidity. Environ 
Pollut 149: 315-326.
Lippman, M (1986). Letter from Morton Lippman, Chair, Clean Air 
Scientific Advisory Committee to the Honorable Lee Thomas, 
Administrator, Re: Review of the 1986 Addendum to the 1982 Staff 
Paper on Particulate Matter (National Ambient Air Quality Standards 
for Particulate Matter: Assessment of Scientific and Technical 
Information). December 16, 1986. SAB-CASAC-87-010. Office of the 
Administrator, Science Advisory Board Washing, DC Available at: 
https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100RZZ1.PDF
Lippman, M (1987). Letter from Morton Lippman, Chair, Clean Air 
Scientific Advisory Committee to the Honorable Lee Thomas, 
Administrator, Re: Review of the 1986 Addendum to the 1982 Staff 
Paper on Sulfur Oxides (Review of the National Ambient Air Quality 
Standards for Sulfur Oxides: Updated assessment of Scientific and 
Technical Information). February 19, 1987. SAB-CASAC-87-022. Office 
of the Administrator, Science Advisory Board Washing, DC Available 
at: https://casac.epa.gov/ords/sab/f?p=113:12:1342972375271:::12.
Lynch, JA, Phelan, J, Pardo, LH, McDonnell, TC, Clark, CM, Bell, MD, 
Geiser, LH and Smith, RJ (2022). Detailed Documentation of the 
National Critical Load Database (NCLD) for U.S. Critical Loads of 
Sulfur and Nitrogen, version 3.2.1 National Atmospheric Deposition 
Program, Wisconsin State Laboratory of Hygiene, Madison, WI.
Magill, AH, Aber, JD, Currie, WS, Nadelhoffer, KJ, Martin, ME, 
McDowell, WH, Melillo, JM and Steudler, P (2004). Ecosystem response 
to 15 years of chronic nitrogen additions at the Harvard Forest 
LTER, Massachusetts, USA. For Ecol Manage 196: 7-28.
McCrackin, ML, Harrison, JA and Compton, JE (2013). A comparison of 
NEWS and SPARROW models to understand sources of nitrogen delivered 
to US coastal areas. Biogeochemistry 114: 281-297.
McDonnell, TC, Cosby, BJ and Sullivan, TJ (2012). Regionalization of 
soil base cation weathering for evaluating stream water 
acidification in the Appalachian Mountains, USA. Environ Pollut 
Control 162: 338-344.
McDonnell, TC, Sullivan, TJ, Hessburg, PF, Reynolds, KM, Povak, NA, 
Cosby, BJ, Jackson, W and Salter, RB (2014). Steady-state sulfur 
critical loads and exceedances for protection of aquatic ecosystems 
in the U.S. Southern Appalachian Mountains. J Environ Manage 146: 
407-419.
McNulty, SG, Boggs, J, Aber, JD, Rustad, L and Magill, A (2005). Red 
spruce ecosystem level changes following 14 years of chronic N 
fertilization. For Ecol Manage 219: 279-291.
Moore, RB, Johnston, CM, Smith, RA and Milstead, B (2011). Source 
and delivery of nutrients to receiving waters in the northeastern 
and mid-atlantic regions of the United States. J Am Water Resour 
Assoc 47: 965-990.
NADP (National Atmospheric Deposition Program) (2021). National 
Atmospheric Deposition Program 2021 Annual Summary. Wisconsin State 
Laboratory of Hygiene, University of Wisconsin-Madison, WI. 
Available at: https://nadp.slh.wisc.edu/wp-content/uploads/2022/11/2021as.pdf.
NAPAP (National Acid Precipitation Assessment Program) (1991). 1990 
Integrated Assessment Report. NAPAP Office of the Director, 
Washington, DC
NRC (National Research Council) (2004). Air quality management in 
the United States. National Academies Press. Washington, DC.
Officer, CB, Biggs, RB, Taft, JL, Cronin, LE, Tyler, MA and Boynton, 
WR (1984). Chesapeake Bay anoxia: Origin, development, and 
significance. Science 223: 22-27.
Omernik, JM (1987). Ecoregions of the Conterminous United States. 
Ann Ass Am Geogr 77(1): 118-125.
Omernik, JM and Griffith, GE (2014). Ecoregions of the Conterminous 
United States: Evolution of a Hierarchical Spatial Framework. 
Environ Manag 54: 1249-1266.
Padgett, PE, Parry, SD, Bytnerowicz, A and Heath, RL (2009). Image 
analysis of epicuticular damage to foliage caused by dry deposition 
of the air pollutant nitric acid. J Environ Monit 11: 63-74.
Pardo, LH, Fenn, ME, Goodale, CL, Geiser, LH, Driscoll, CT, Allen, 
EB, Baron, JS, Bobbink, R, Bowman, WD, Clark, CM, Emmett, B, 
Gilliam, FS, Greaver, TL, Hall, SJ, Lilleskov, EA, Liu, L, Lynch, 
JA, Nadelhoffer, KJ, Perakis, SS, Robin-Abbott, MJ, Stoddard, JL, 
Weathers, KC and Dennis, RL (2011). Effects of nitrogen deposition 
and empirical nitrogen critical loads for ecoregions of the United 
States. Ecol Appl 21: 3049-3082.
Pavlovic, NR, Chang, SY, Huang, J, Craig, K, Clark, C, Horn, K and 
Driscoll, CT (2023). Empirical nitrogen and sulfur critical loads of 
U.S. tree species and their uncertainties with machine learning. Sci 
Total Environ 857: 159252.
Pregitzer, KS, Burton, AJ, Zak, DR and Talhelm, AF (2008). Simulated 
chronic nitrogen deposition increases carbon storage in Northern 
Temperate forests. Global Change Biol 14: 142-153.
Riddell, J, Nash, TH, III and Padgett, P (2008). The effect of HNO3 
gas on the lichen Ramalina menziesii. Flora 203: 47-54.
Riddell, J, Padgett, PE and Nash, TH III (2012). Physiological 
responses of lichens to factorial fumigations with nitric acid and 
ozone. Environ Pollut 170: 202-210.
Robertson, DM amd Saad, DA (2013). SPARROW models used to understand 
nutrient sources in the Mississippi/Atchafalaya river basin. J 
Environ Qual 42: 1422-1440.
Robinson, RB, Barnett, TW, Harwell, GR, Moore, SE, Kulp, M and 
Schwartz, JS (2008). pH and acid anion time trends in different 
elevation ranges in the Great Smoky Mountains National Park. J 
Environ Eng 134(9): 800-808.
Russell, A (2007). Letter from Armistead Russel, Chair, Secondary 
NAAQS Review Panel for Oxides of Nitrogen and Sulfur, to the 
Honorable Stephen L. Johnson, Administrator, Re: Clean Air 
Scientific Advisory Committee's (CASAC) NOX & 
SOX Secondary NAAQS Review Panel's Consultation on EPA's 
Draft Plan for Review of the Secondary NAAQS for Nitrogen Dioxide 
and Sulfur Dioxide (September 2007 Draft). November 29, 2007. EPA-
CASAC-08-003. Office of the Administrator, Science Advisory Board 
Washing, DC Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1000QHW.PDF.
Russell, A and Henderson, R (2008). Letter from Armistead Russel, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur and Rogene Henderson, Chair, Clean Air Scientific Advisory 
Committee to the Honorable Stephen L. Johnson, Administrator, Re: 
Clean Air Scientific Advisory

[[Page 26696]]

Committee's (CASAC) Peer Review of EPA's Integrated Science 
Assessment (ISA) for Oxides of Nitrogen and Sulfur--Environmental 
Criteria (First External Review Draft, December 2007). May 19, 2008. 
EPA-CASAC-08-012. Office of the Administrator, Science Advisory 
Board Washing, DC Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1000K05.PDF.
Russell, A and Samet, JM (2008a). Letter from Armistead Russel, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur and Jonathan M. Samet, Chair, Clean Air Scientific Advisory 
Committee to the Honorable Stephen L. Johnson, Administrator, Re: 
Peer Review of EPA's Integrated Science Assessment (ISA) for Oxides 
of Nitrogen and Sulfur--Environmental Criteria (Second External 
Review Draft). November 18, 2008. EPA-CASAC-09-002. Office of the 
Administrator, Science Advisory Board Washing, DC Available at: 
https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1002E7G.PDF.
Russell, A and Samet, JM (2008b). Letter from Armistead Russel, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur and Jonathan M. Samet, Chair, Clean Air Scientific Advisory 
Committee to the Honorable Stephen L. Johnson, Administrator, Re: 
Peer Review of EPA's Risk and Exposure Assessment to Support the 
Review of the Secondary National Ambient Air Quality Standard for 
Oxides of Nitrogen and Sulfur: First Draft. December 23, 2008. EPA-
CASAC-09-004. Office of the Administrator, Science Advisory Board 
Washing, DC Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1002ZCZ.PDF.
Russell, A and Samet, JM (2009). Letter from Armistead Russel, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur and Jonathan M. Samet, Chair, Clean Air Scientific Advisory 
Committee to the Honorable Lisa P. Jackson, Administrator, Re: Peer 
Review of EPA's Risk and Exposure Assessment to Support the Review 
of the Secondary National Ambient Air Quality Standard (NAAQS) for 
Oxides of Nitrogen and Sulfur: Second Draft. August 28, 2009. EPA-
CASAC-09-013. Office of the Administrator, Science Advisory Board 
Washing, DC Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1005A11.PDF.
Russell, A and Samet, JM (2010a). Letter from Armistead Russel, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur and Jonathan M. Samet, Chair, Clean Air Scientific Advisory 
Committee to the Honorable Lisa P. Jackson, Administrator, Re: 
Review of the Policy Assessment for the Review of the Secondary 
National Ambient Air Quality Standard for NOX and 
SOX: Second Draft. December 9, 2010. EPA-CASAC-11-003. 
Office of the Administrator, Science Advisory Board Washing, DC 
Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=9101XP6G.PDF.
Russell, A and Samet, JM (2010b). Letter from Armistead Russel, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur and Jonathan M. Samet, Chair, Clean Air Scientific Advisory 
Committee to the Honorable Lisa P. Jackson, Administrator, Re: 
Review of the Policy Assessment for the Review of the Secondary 
National Ambient Air Quality Standard for NOX and 
SOX: First Draft (March 2010). June 22, 2010. EPA-CASAC-
10-014. Office of the Administrator, Science Advisory Board Washing, 
DC Available at: https://casac.epa.gov/ords/sab/f?p=113:12:1342972375271:::12.
Russell, A and Samet, JM (2011). Letter from Armistead Russel, 
Chair, Secondary NAAQS Review Panel for Oxides of Nitrogen and 
Sulfur and Jonathan M. Samet, Chair, Clean Air Scientific Advisory 
Committee to the Honorable Lisa P. Jackson, Administrator, Re: CASAC 
Comments on the Policy Assessment for the Review of the Secondary 
National Ambient Air Quality Standard for Oxides of Nitrogen and 
Oxides of Sulfur (February 2011). May 17, 2011. EPA-CASAC-11-005. 
Office of the Administrator, Science Advisory Board Washing, DC 
Available at: https://casac.epa.gov/ords/sab/f?p=113:12:1342972375271:::12.
Schaberg, PG, Hawley, GJ, Rayback, SA, Halman, JM and Kosiba, AM 
(2014). Inconclusive evidence of Juniperus virginiana recovery 
following sulfur pollution reductions. Proc Natl Acad Sci 111: E1.
Scheffe, RD, Lynch, JA, Reff, A, Kelly, JT, Hubbell, B, Greaver, TL 
and Smith, JT (2014). The aquatic acidification index: A new 
regulatory metric linking atmospheric and biogeochemical models to 
assess potential aquatic ecosystem recovery. Water Air Soil Pollut 
225: 1-15.
Schwede, DB and Lear, GG (2014). A novel hybrid approach for 
estimating total deposition in the United States. Atmos Environ 92: 
207-220.
Shaw, GD, Cisneros, R, Schweizer, D, Sickman, JO and Fenn, ME 
(2014). Critical Loads of Acid Deposition for Wilderness Lakes in 
the Sierra Nevada (California) Estimated by the Steady-State Water 
Chemistry Model. Water Air Soil Pollut 225:1-15.
Sheppard, EA (2023). Letter from Elizabeth A. Sheppard, Chair, Clean 
Air Scientific Advisory Committee, to the Honorable Michael S. 
Regan, Administrator, Re: CASAC Review of the EPA's Policy 
Assessment for the Review of the Secondary National Ambient Air 
Quality Standards for Oxides of Nitrogen, Oxides of Sulfur and 
Particulate Matter (External Review Draft--May 2023). September 27, 
2023. EPA-CASAC-23-005. Office of the Administrator, Science 
Advisory Board Washing, DC Available at: https://casac.epa.gov/ords/sab/f?p=113:12:1342972375271:::12.
Simkin, SM, Allen, EB, Bowman, WD, Clark, CM, Belnap, J, Brooks, ML, 
Cade, BS, Collins, SL, Geiser, LH, Gilliam, FS and Jovan, SE (2016). 
Conditional vulnerability of plant diversity to atmospheric nitrogen 
deposition across the United States. Proc Natl Acad Sci 113(15): 
4086-4091.
Stevens, CJ (2016). How long do ecosystems take to recover from 
atmospheric nitrogen deposition? Biol Conserv 200: 160-167.
Strengbom, J, Nordin, A, N[auml]sholm, T and Ericson, L (2001). Slow 
recovery of boreal forest ecosystem following decreased nitrogen 
input. Funct Ecol 15: 451-457.
Sullivan, TJ, Cosby, BJ, Driscoll, CT, McDonnell, TC, Herlihy, AT 
and Burns, DA (2012a). Target loads of atmospheric sulfur and 
nitrogen deposition for protection of acid sensitive aquatic 
resources in the Adirondack Mountains, New York. Water Resour Res 
48(1): W01547.
Sullivan, TJ, Cosby, BJ, Jackson, WA, Snyder, K and Herlihy, AT 
(2011). Acidification and prognosis for future recovery of acid-
sensitive streams in the Southern Blue Ridge province. Water Air 
Soil Pollut 219: 11-16.
Sullivan, TJ, Cosby, BJ, McDonnell, TC, Porter, EM, Blett, T, 
Haeuber, R, Huber, CM and Lynch, J (2012b). Critical loads of 
acidity to protect and restore acid-sensitive streams in Virginia 
and West Virginia. Water Air Soil Pollut 223: 5759-5771.
Sullivan, TJ, Driscoll, CT, Cosby, BJ, Fernandez, IJ, Herlihy, AT, 
Zhai, J, Stemberger, R, Snyder, KU, Sutherland, JW, Nierzwicki-
Bauer, SA, Boylen, CW, McDonnell, TC and Nowicki, NA (2006). 
Assessment of the extent to which intensively-studied lakes are 
representative of the Adirondack Mountain region. Final Report 06-
17. Corvallis, OR, E&S Environmental Chemistry, Inc.
Thomas, RB, Spal, SE, Smith, KR and Nippert, JB (2013). Evidence of 
recovery of Juniperus virginiana trees from sulfur pollution after 
the Clean Air Act. Proc Natl Acad Sci 110: 15319-15324.
Thomas, RQ, Canham, CD, Weathers, KC and Goodale, CL (2010). 
Increased tree carbon storage in response to nitrogen deposition in 
the US. Nat Geosci 3(1): 13-17.
Tillerson, C, Mintz, D and Hawes, T (2024). Memorandum to Secondary 
NOX/SOX/PM NAAQS Review Docket (EPA-HQ-OAR-
2014-0128). Technical Analyses to Support Alternative Demonstration 
Approach for Proposed Secondary SO2 NAAQS under NSR/PSD 
Program. January XX, 2024. Office of Air Quality Planning and 
Standards, Research Triangle Park, NC.
U.S. DHEW (U.S. Department of Health, Education and Welfare) 
(1969a). Air quality criteria for sulfur oxides. National Air 
Pollution Control Administration. Washing, DC Pub. No. AP-50. 
January 1969. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=20013JXZ.PDF.
U.S. DHEW (U.S. Department of Health, Education and Welfare). 
(1969b). Air

[[Page 26697]]

quality criteria for particulate matter. National Air Pollution 
Control Administration. Washing, DC Pub. No. AP-49. January 1969. 
Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=20013C3W.PDF.
U.S. EPA (1971). Air Quality Criteria for Nitrogen Oxides. Air 
Pollution Control Office. Washington DC. EPA 450-R-71-001. January 
1971. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=20013K3B.PDF.
U.S. EPA (1973). ``Effects of Sulfur Oxide in the Atmosphere on 
Vegetation''. Revised Chapter 5 of Air Quality Criteria for Sulfur 
Oxides. Office of Research and Development. Research Triangle Park, 
N.C. EPA-R3-73-030. September 1973. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=2000X8F8.PDF.
U.S. EPA (1982a). Air Quality Criteria for Oxides of Nitrogen. 
Office of Research and Development. Research Triangle Park, N.C. 
EPA/600/8-82/026F. December 1982. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=500021LI.PDF.
U.S. EPA (1982b). Air Quality Criteria for Particulate Matter and 
Sulfur Oxides. Volume I-III. Office of Research and Development. 
Research Triangle Park, N.C. EPA/600/8-82/029. December 1982. 
Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=3000188Z.PDF https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=300018EV.PDF https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=300053KV.PDF.
U.S. EPA (1982c). Review of the National Ambient Air Quality 
Standards for Sulfur Oxides: Assessment of Scientific and Technical 
Information. OAQPS Staff Paper. Office of Air Quality Planning and 
Standards. Research Triangle Park, NC. EPA-450/5-82-007. November 
1982. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=300068A0.PDF.
U.S. EPA (1982d). Review of the National Ambient Air Quality 
Standards for Particulate Matter: Assessment of Scientific and 
Technical Information. OAQPS Staff Paper. Office of Air Quality 
Planning and Standards. Research Triangle Park, NC. EPA-450/5-82-
001. January 1982. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=2000NH6N.PDF.
U.S. EPA (1984a). The Acidic Deposition Phenomenon and Its Effects: 
Critical Assessment Review Papers. Volume I Atmospheric Sciences. 
Office of Research and Development, Washington DC. EPA600/8-83-
016AF. July 1984. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=2000G4AJ.PDF.
U.S. EPA (1984b). The Acidic Deposition Phenomenon and Its Effects: 
Critical Assessment Review Papers. Volume II Effects Sciences. 
Office of Research and Development, Washington DC. EPA-600/8- 83-
016BF. July 1984. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=2000G5FI.PDF.
U.S. EPA (1985). The Acidic Deposition Phenomenon and Its Effects: 
Critical Assessment Document. Office of Research and Development, 
Washington, DC. EPA-600/8-85/001. August 1985. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=2000AD53.PDF.
U.S. EPA (1986). Review of the National Ambient Air Quality 
Standards for Particulate Matter: Updated Assessment of Scientific 
and Technical Information. Addendum to the 1982 OAQPS Staff Paper. 
Office of Air Quality Planning and Standards, Research Triangle 
Park, NC. EPA-450/05-86-012. December 1986. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=910113UH.PDF.
U.S. EPA (1987). National Air Quality and Emissions Trends Report, 
1985. Office of Air Quality Planning and Standards, Research 
Triangle Park, NC. EPA 450/4-87-001. February 1987. Available at: 
https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=2000J2BU.PDF.
U.S. EPA (1993). Air Quality Criteria for Oxides of Nitrogen. Volume 
I-III. U.S. Office of Research and Development, Research Triangle 
Park, NC. EPA/600/8-91/049aF-cF. August 1993. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=30001LZT.PDF https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=300056QV.PDF https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=30001NI2.PDF.
U.S. EPA (1995a). Review of the National Ambient Air Quality 
Standards for Nitrogen Dioxide: Assessment of Scientific and 
Technical Information, OAQPS Staff Paper. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452/R-95-
005. September 1995. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=00002UBE.PDF.
U.S. EPA (1995b). Acid Deposition Standard Feasibility Study: Report 
to Congress. Office of Air and Radiation, Acid Rain Division, 
Washington, DC. EPA-430-R-95-001a. October 1995. Available at: 
https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=2000WTGY.PDF.
U.S. EPA (1996). Review of the National Ambient Air Quality 
Standards for Particulate Matter: Policy Assessment of Scientific 
and Technical Information (OAQPS Staff Paper). Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452\R-96-
013. July 1996. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=2000DLIE.PDF.
U.S. EPA (2004a). Air Quality Criteria for Particulate Matter. (Vol 
I of II). Office of Research and Development, Research Triangle 
Park, NC. EPA-600/P-99-002aF. October 2004. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100LFIQ.PDF.
U.S. EPA (2004b). Air Quality Criteria for Particulate Matter. (Vol 
II of II). Office of Research and Development, Research Triangle 
Park, NC. EPA-600/P-99-002bF. October 2004. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100LG7Q.PDF.
U.S. EPA (2005). Review of the National Ambient Air Quality 
Standards for Particulate Matter: Policy Assessment of Scientific 
and Technical Information, OAQPS Staff Paper. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452/R-05-
005a. December 2005. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1009MZM.PDF.
U.S. EPA (2007). Integrated Review Plan for the Secondary National 
Ambient Air Quality Standards for Nitrogen Dioxide and Sulfur 
Dioxide. Office of Research and Development, Research Triangle Park, 
NC, EPA-452/R-08-006. December 2007. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1001FDM.PDF.
U.S. EPA (2008a). Integrated Science Assessment (ISA) for Oxides of 
Nitrogen and Sulfur Ecological Criteria. Office of Research and 
Development, Research Triangle Park, NC. EPA/600/R-08/082F. December 
2008. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100R7MG.PDF.
U.S. EPA (2008b). Integrated Review Plan for the National Ambient 
Air Quality Standards for Particulate Matter. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA 452/R-08-
004. March 2008. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1001FB9.PDF.
U.S. EPA (2009a). Risk and Exposure Assessment for Review of the 
Secondary National Ambient Air Quality Standards for Oxides of 
Nitrogen and Oxides of Sulfur (Main Content). Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452/R-09-
008a. September 2009. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100FNQV.PDF.
U.S. EPA (2009b). Integrated Science Assessment for Particulate 
Matter. Office of Research and Development, Research Triangle Park, 
NC. EPA/600/R-08/139F. December 2009. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P10060Z4.PDF.
U.S. EPA (2009c). Particulate Matter National Ambient Air Quality 
Standards (NAAQS): Scope and Methods Plan for Urban Visibility 
Impact Assessment. Office of Air Quality Planning and Standards, 
Research Triangle Park, NC. EPA-452/P-09-001. February 2009. 
Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100FLUX.PDF.
U.S. EPA (2010). Particulate Matter Urban-Focused Visibility 
Assessment--Final Document. Office of Air Quality Planning and 
Standards, Research Triangle Park, NC. EPA-452/R- 10-004. July 2010. 
Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100FO5D.PDF.
U.S. EPA (2011). Policy Assessment for the Review of the Secondary 
National Ambient Air Quality Standards for Oxides of Nitrogen and 
Oxides of Sulfur. Office of Air Quality Planning and Standards, 
Research Triangle Park, NC. EPA-452/R-11-005a, b. February 2011. 
Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1009R7U.PDF https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1009RHY.PDF.

[[Page 26698]]

U.S. EPA (2016). Integrated Review Plan for the National Ambient Air 
Quality Standards for Particulate Matter. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452/R-16-
005. December 2016. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100R5VE.PDF.
U.S. EPA (2017). Integrated Review Plan for the Secondary NAAQS for 
Oxides of Nitrogen and Oxides of Sulfur and Particulate Matter--
Final. Office of Air Quality Planning and Standards, Research 
Triangle Park, NC. EPA-452/R-17-002. January 2017. Available at: 
https://nepis.epa.gov/Exe/ZyPDf.cgi?Dockey=P100R607.PDF.
U.S. EPA (2018). Review of the Secondary Standards for Ecological 
Effects of Oxides of Nitrogen, Oxides of Sulfur, and Particulate 
Matter: Risk and Exposure Assessment Planning Document. Office of 
Air Quality Planning and Standards, Research Triangle Park, NC. EPA-
452/D-18-001. August 2018. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100V7JA.PDF.
U.S. EPA (2019). Integrated Science Assessment (ISA) for for 
Particulate Matter (Final Report, Dec 2019). U.S. Environmental 
Protection Agency, Washington, DC, EPA/600/R-19/188, 2019.
U.S. EPA (2020a). Integrated Science Assessment (ISA) for Oxides of 
Nitrogen, Oxides of Sulfur and Particulate Matter Ecological 
Criteria (Final Report, 2020). Office of Air Quality Planning and 
Standards, Research Triangle Park, NC. EPA/600/R-20/278. September 
2020. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P1010WR3.PDF.
U.S. EPA (2020b). Policy Assessment for the Review of the National 
Ambient Air Quality Standards for Particulate Matter. Office of Air 
Quality Planning and Standards, Research Triangle Park, NC. EPA-452/
R-20-002. January 2020. Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100YGMN.pdf.
U.S. EPA (2020c). Policy Assessment for the Review of the Ozone 
National Ambient Air Quality Standards. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452/R-20-
001. May 2020. Available at: https://nepis.epa.gov/Exe/ZyPdf.cgi?Dockey=P100ZES4.pdf.
U.S. EPA (2023). Overview of Particulate Matter (PM) Air Quality in 
the United States. Available at: https://www.epa.gov/air-quality-analysis/particulate-matter-naaqs-review-analyses-and-data-sets
U.S. EPA (2024). Policy Assessment for the Review of the Secondary 
National Ambient Air Quality Standards for Oxides of Nitrogen, 
Oxides of Sulfur and Particulate Matter. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC. EPA-452/R-24-
003. January 2024. Available at: https://www.epa.gov/system/files/documents/2024-01/noxsoxpm-final.pdf.
Wallace, ZP, Lovett, GM, Hart, JE and Machona, B (2007). Effects of 
nitrogen saturation on tree growth and death in a mixed-oak forest. 
For Ecol Manage 243: 210-218.
Waller, K, Driscoll, C, Lynch, J, Newcomb, D and Roy, K (2012). 
Long-term recovery of lakes in the Adirondack region of New York to 
decreases in acidic deposition. Atmos Environ 46: 56-64.
Watkins, N, Boyette, L and Jager, D (2024). Memorandum to Secondary 
NOX/SOX/PM NAAQS Review Docket (EPA-HQ-OAR-
2014-0128). Ambient Air SO2 Monitoring Network Review and 
Background (January 2024). January 18, 2024. Office of Air Quality 
Planning and Standards, Research Triangle Park, NC.
WHO (2008). WHO/IPCS Harmonization Project Document No. 6. Part 1: 
Guidance Document on Characterizing and Communicating Uncertainty in 
Exposure Assessment. International Programme on Chemical Safety. 
World Health Organization. Geneva, Switzerland. Available at: http://www.who.int/ipcs/methods/harmonization/areas/exposure/en/.
Williams, J and Labou, S (2017). A database of georeferenced 
nutrient chemistry data for mountain lakes of the Western United 
States. Sci Data 4: 170069.
Wolff, GT (1993). Letter from George T. Wolff, Chair, Clean Air 
Scientific Advisory Committee to the Honorable Carol M. Browner, 
Administrator, U.S. EPA. Re: Clean Air Scientific Advisory Committee 
Closure on the Air Quality Criteria Document for Oxides of Nitrogen. 
September 30, 1993. EPA-SAB-CASAC-LTR-93-015. Office of the 
Administrator, Science Advisory Board Washing, DC Available at: 
https://casac.epa.gov/ords/sab/f?p=113:12:1342972375271:::12.
Wolff, GT (1995). Letter from George T. Wolff, Chair, Clean Air 
Scientific Advisory Committee to the Honorable Carol M. Browner, 
Administrator, Re: CASAC Review of the Staff Paper for the Review of 
the National Ambient Air Quality Standards for Nitrogen Dioxide: 
Assessment of Scientific and Technical Information. August 22, 1995. 
EPA-SAB-CASAC-LTR-95-004. Office of the Administrator, Science 
Advisory Board Washing, DC Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100FL6Q.PDF.
Wolff, GT (1996). Letter from George T. Wolff, Chair, Clean Air 
Scientific Advisory Committee to the Honorable Carol M. Browner, 
Administrator, Re: Closure by the Clean Air Scientific Advisory 
Committee (CASAC) on the Staff Paper for Particulate Matter. June 
13, 1996. EPA-SAB-CASAC-LTR-96-008. Office of the Administrator, 
Science Advisory Board Washing, DC Available at: https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=9100TTBM.PDF.
Zhou, Q, Driscoll, CT and Sullivan, TJ (2015). Responses of 20 lake-
watersheds in the Adirondack region of New York to historical and 
potential future acidic deposition. Sci Total Environ 511: 186-194.

List of Subjects in 40 CFR Part 50

    Environmental protection, Air pollution control, Carbon monoxide, 
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.

Michael S. Regan,
Administrator.

    For the reasons set forth in the preamble, the Environmental 
Protection Agency proposes to amend chapter I of title 40 of the Code 
of Federal Regulations as follows:

PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY 
STANDARDS

0
1. The authority citation for part 50 continues to read as follows:

    Authority:  42 U.S.C. 7401, et seq.

0
2. Add Sec.  50.21 to read as follows:


Sec.  50.21  National secondary ambient air quality standards for 
sulfur oxides (sulfur dioxide).

    (a) The national secondary ambient annual air quality standard for 
oxides of sulfur is [10-15] parts per billion (ppb), measured in the 
ambient air as sulfur dioxide (SO2) by a reference method 
based on appendix A-1 and appendix A-2 of this part, or by a Federal 
Equivalent Method (FEM) designated in accordance with part 53 of this 
chapter.
    (b) The secondary annual standard is met when the 3-year average of 
the annual SO2 concentration is less than or equal to [10-
15] ppb, as determined in accordance with appendix T of this part.
0
3. Revise appendix T to part 50 to read as follows:

Appendix T to Part 50--Interpretation of the Primary and Secondary 
National Ambient Air Quality Standards for Oxides of Sulfur (Sulfur 
Dioxide)

1. General

    (a) This appendix explains the data handling conventions and 
computations necessary for determining when the primary and 
secondary national ambient air quality standards for Oxides of 
Sulfur as measured by Sulfur Dioxide (``SO2 NAAQS'') 
specified in Sec.  50.17 are met at an ambient air quality 
monitoring site. Sulfur dioxide (SO2) is measured in the 
ambient air by a Federal reference method (FRM) based on appendix A 
or A-1 to this part or by a Federal equivalent method (FEM) 
designated in accordance with part 53 of this chapter. Data handling 
and computation procedures to be used in making comparisons between 
reported SO2 concentrations and the levels of the 
SO2 NAAQS are specified in the following sections.
    (b) Decisions to exclude, retain, or make adjustments to the 
data affected by exceptional events, including natural events, are 
made according to the requirements and

[[Page 26699]]

process deadlines specified in Sec. Sec.  50.1, 50.14 and 51.930 of 
this chapter.
    (c) The terms used in this appendix are defined as follows:
    Annual mean refers to the annual average of all the daily mean 
values as defined in section 5.2 of this appendix.
    Daily maximum 1-hour values for SO2 refers to the 
maximum 1-hour SO2 concentration values measured from 
midnight to midnight (local standard time) that are used in NAAQS 
computations.
    Daily mean values for SO2 refers to the 24-hour 
average of 1-hour SO2 concentration values measured from 
midnight to midnight (local standard time) that are used in NAAQS 
computations.
    Design values are the metrics (i.e., statistics) that are 
compared to the NAAQS levels to determine compliance, calculated as 
specified in section 5 of this appendix. The design value for the 
primary 1-hour NAAQS is the 3-year average of annual 99th percentile 
daily maximum 1-hour values for a monitoring site (referred to as 
the ``1-hour primary standard design value''). The design value for 
the secondary annual NAAQS is the 3-year average of the annual mean 
of daily mean values for a monitoring site (referred to as the 
``annual secondary standard'').
    99th percentile daily maximum 1-hour value is the value below 
which nominally 99 percent of all daily maximum 1-hour concentration 
values fall, using the ranking and selection method specified in 
section 5.1 of this appendix.
    Pollutant Occurrence Code (POC) refers to a numerical code (1, 
2, 3, etc.) used to distinguish the data from two or more monitors 
for the same parameter at a single monitoring site.
    Quarter refers to a calendar quarter.
    Year refers to a calendar year.

2. Requirements for Data Used for Comparisons With the SO2 
NAAQS and Data Reporting Considerations

    (a) All valid FRM/FEM SO2 hourly data required to be 
submitted to EPA's Air Quality System (AQS), or otherwise available 
to EPA, meeting the requirements of part 58 of this chapter 
including appendices A, C, and E shall be used in design value 
calculations. Multi-hour average concentration values collected by 
wet chemistry methods shall not be used.
    (b) Data from two or more monitors from the same year at the 
same site reported to EPA under distinct Pollutant Occurrence Codes 
shall not be combined in an attempt to meet data completeness 
requirements. The Administrator will combine annual 99th percentile 
daily maximum concentration values from different monitors in 
different years, selected as described here, for the purpose of 
developing a valid 1-hour primary standard design value. If more 
than one of the monitors meets the completeness requirement for all 
four quarters of a year, the steps specified in section 5.1(a) of 
this appendix shall be applied to the data from the monitor with the 
highest average of the four quarterly completeness values to derive 
a valid annual 99th percentile daily maximum concentration. If no 
monitor is complete for all four quarters in a year, the steps 
specified in sections 3.1(c) and 5.1(a) of this appendix shall be 
applied to the data from the monitor with the highest average of the 
four quarterly completeness values in an attempt to derive a valid 
annual 99th percentile daily maximum concentration. Similarly, the 
Administrator will combine annual means from different monitors in 
different years, selected as described here, for the purpose of 
developing a valid annual secondary standard design value. If more 
than one of the monitors meets the completeness requirement for all 
four quarters of a year, the steps specified in section 5.2(a) of 
this appendix shall be applied to the data from the monitor with the 
highest average of the four quarterly completeness values to derive 
a valid annual mean. If no monitor is complete for all four quarters 
in a year, the steps specified in sections 3.2(c) and 5.2(a) of this 
appendix shall be applied to the data from the monitor with the 
highest average of the four quarterly completeness values in an 
attempt to derive a valid annual mean. This paragraph does not 
prohibit a monitoring agency from making a local designation of one 
physical monitor as the primary monitor for a Pollutant Occurrence 
Code and substituting the 1-hour data from a second physical monitor 
whenever a valid concentration value is not obtained from the 
primary monitor; if a monitoring agency substitutes data in this 
manner, each substituted value must be accompanied by an AQS 
qualifier code indicating that substitution with a value from a 
second physical monitor has taken place.
    (c) Hourly SO2 measurement data shall be reported to 
AQS in units of parts per billion (ppb), to at most one place after 
the decimal, with additional digits to the right being truncated 
with no further rounding.

3. Comparisons With the NAAQS

3.1 Comparisons With the 1-Hour Primary SO2 NAAQS

    (a) The 1-hour primary SO2 NAAQS is met at an ambient 
air quality monitoring site when the valid 1-hour primary standard 
design value is less than or equal to 75 parts per billion (ppb).
    (b) An SO2 1-hour primary standard design value is 
valid if it encompasses three consecutive calendar years of complete 
data. A year meets data completeness requirements when all four 
quarters are complete. A quarter is complete when at least 75 
percent of the sampling days for each quarter have complete data. A 
sampling day has complete data if 75 percent of the hourly 
concentration values, including State-flagged data affected by 
exceptional events which have been approved for exclusion by the 
Administrator, are reported.
    (c) In the case of one, two, or three years that do not meet the 
completeness requirements of section 3.1(b) of this appendix and 
thus would normally not be useable for the calculation of a valid 3-
year 1-hour primary standard design value, the 3-year 1-hour primary 
standard design value shall nevertheless be considered valid if one 
of the following conditions is true.
    (i) At least 75 percent of the days in each quarter of each of 
three consecutive years have at least one reported hourly value, and 
the design value calculated according to the procedures specified in 
section 5.1 is above the level of the primary 1-hour standard.
    (ii)(A) A 1-hour primary standard design value that is equal to 
or below the level of the NAAQS can be validated if the substitution 
test in section 3.1(c)(ii)(B) of this appendix results in a ``test 
design value'' that is below the level of the NAAQS. The test 
substitutes actual ``high'' reported daily maximum 1-hour values 
from the same site at about the same time of the year (specifically, 
in the same calendar quarter) for unknown values that were not 
successfully measured. Note that the test is merely diagnostic in 
nature, intended to confirm that there is a very high likelihood 
that the original design value (the one with less than 75 percent 
data capture of hours by day and of days by quarter) reflects the 
true under-NAAQS-level status for that 3-year period; the result of 
this data substitution test (the ``test design value,'' as defined 
in section 3.1(c)(ii)(B) of this appendix) is not considered the 
actual design value. For this test, substitution is permitted only 
if there are at least 200 days across the three matching quarters of 
the three years under consideration (which is about 75 percent of 
all possible daily values in those three quarters) for which 75 
percent of the hours in the day, including State-flagged data 
affected by exceptional events which have been approved for 
exclusion by the Administrator, have reported concentrations. 
However, maximum 1-hour values from days with less than 75 percent 
of the hours reported shall also be considered in identifying the 
high value to be used for substitution.
    (B) The substitution test is as follows: Data substitution will 
be performed in all quarter periods that have less than 75 percent 
data capture but at least 50 percent data capture, including State-
flagged data affected by exceptional events which have been approved 
for exclusion by the Administrator; if any quarter has less than 50 
percent data capture then this substitution test cannot be used. 
Identify for each quarter (e.g., January-March) the highest reported 
daily maximum 1-hour value for that quarter, excluding State-flagged 
data affected by exceptional events which have been approved for 
exclusion by the Administrator, looking across those three months of 
all three years under consideration. All daily maximum 1-hour values 
from all days in the quarter period shall be considered when 
identifying this highest value, including days with less than 75 
percent data capture. If after substituting the highest reported 
daily maximum 1-hour value for a quarter for as much of the missing 
daily data in the matching deficient quarter(s) as is needed to make 
them 100 percent complete, the procedure in section 5 yields a 
recalculated 3-year 1-hour standard ``test design value'' less than 
or equal to the level of the standard, then the 1-hour primary 
standard design value is deemed to have passed the diagnostic test 
and is valid, and the level of the standard is deemed to have been 
met in that 3-year period. As noted in section 3.1(c)(i) of this 
appendix, in such a case, the 3-year design value based on the data 
actually reported, not the ``test design

[[Page 26700]]

value,'' shall be used as the valid design value.
    (iii)(A) A 1-hour primary standard design value that is above 
the level of the NAAQS can be validated if the substitution test in 
section 3.1(c)(iii)(B) of this appendix results in a ``test design 
value'' that is above the level of the NAAQS. The test substitutes 
actual ``low'' reported daily maximum 1-hour values from the same 
site at about the same time of the year (specifically, in the same 
three months of the calendar) for unknown hourly values that were 
not successfully measured. Note that the test is merely diagnostic 
in nature, intended to confirm that there is a very high likelihood 
that the original design value (the one with less than 75 percent 
data capture of hours by day and of days by quarter) reflects the 
true above-NAAQS-level status for that 3-year period; the result of 
this data substitution test (the ``test design value,'' as defined 
in section 3.1(c)(iii)(B) of this appendix) is not considered the 
actual design value. For this test, substitution is permitted only 
if there are a minimum number of available daily data points from 
which to identify the low quarter-specific daily maximum 1-hour 
values, specifically if there are at least 200 days across the three 
matching quarters of the three years under consideration (which is 
about 75 percent of all possible daily values in those three 
quarters) for which 75 percent of the hours in the day have reported 
concentrations. Only days with at least 75 percent of the hours 
reported shall be considered in identifying the low value to be used 
for substitution.
    (B) The substitution test is as follows: Data substitution will 
be performed in all quarter periods that have less than 75 percent 
data capture. Identify for each quarter (e.g., January-March) the 
lowest reported daily maximum 1-hour value for that quarter, looking 
across those three months of all three years under consideration. 
All daily maximum 1-hour values from all days with at least 75 
percent capture in the quarter period shall be considered when 
identifying this lowest value. If after substituting the lowest 
reported daily maximum 1-hour value for a quarter for as much of the 
missing daily data in the matching deficient quarter(s) as is needed 
to make them 75 percent complete, the procedure in section 5.1 of 
this appendix yields a recalculated 3-year 1-hour standard ``test 
design value'' above the level of the standard, then the 1-hour 
primary standard design value is deemed to have passed the 
diagnostic test and is valid, and the level of the standard is 
deemed to have been exceeded in that 3-year period. As noted in 
section 3.1(c)(i) of this appendix, in such a case, the 3-year 
design value based on the data actually reported, not the ``test 
design value'', shall be used as the valid design value.
    (d) A 1-hour primary standard design value based on data that do 
not meet the completeness criteria stated in section 3.1(b) of this 
appendix and also do not satisfy section 3.1(c) of this appendix, 
may also be considered valid with the approval of, or at the 
initiative of, the Administrator, who may consider factors such as 
monitoring site closures/moves, monitoring diligence, the 
consistency and levels of the valid concentration measurements that 
are available, and nearby concentrations in determining whether to 
use such data.
    (e) The procedures for calculating the 1-hour primary standard 
design values are given in section 5.1 of this appendix.

3.2 Comparisons With the Annual Secondary SO2 NAAQS

    (a) The annual secondary SO2 NAAQS is met at an 
ambient air quality monitoring site when the valid annual secondary 
standard design value is less than or equal to [10-15] parts per 
billion (ppb).
    (b) An SO2 annual secondary standard design value is 
valid if it encompasses three consecutive calendar years of complete 
data. A year meets data completeness requirements when all four 
quarters are complete. A quarter is complete when at least 75 
percent of the sampling days for each quarter have complete data. A 
sampling day has complete data if 75 percent of the hourly 
concentration values, including State-flagged data affected by 
exceptional events which have been approved for exclusion by the 
Administrator, are reported.
    (c) In the case of one, two, or three years that do not meet the 
completeness requirements of section 3.2(b) of this appendix and 
thus would normally not be useable for the calculation of a valid 3-
year annual secondary standard design value, the 3-year annual 
secondary standard design value shall nevertheless be considered 
valid if one of the following conditions is true.
    (i) At least 75 percent of the days in each quarter of each of 
three consecutive years have at least one reported hourly value, and 
the design value calculated according to the procedures specified in 
section 5.2 of this appendix is above the level of the secondary 
annual standard.
    (ii)(A) An annual secondary standard design value that is equal 
to or below the level of the NAAQS can be validated if the 
substitution test in section 3.2(c)(ii)(B) of this appendix results 
in a ``test design value'' that is below the level of the NAAQS. The 
test substitutes actual ``high'' reported daily mean values from the 
same site at about the same time of the year (specifically, in the 
same calendar quarter) for unknown or incomplete (less than 75 
percent of hours reported) daily mean values. Note that the test is 
merely diagnostic in nature, intended to confirm that there is a 
very high likelihood that the original design value (the one with 
less than 75 percent data capture of hours by day and of days by 
quarter) reflects the true under-NAAQS-level status for that 3-year 
period; the result of this data substitution test (the ``test design 
value,'' as defined in section 3.2(c)(ii)(B)) of this appendix is 
not considered the actual design value. For this test, substitution 
is permitted only if there are at least 200 days across the three 
matching quarters of the three years under consideration (which is 
about 75 percent of all possible daily values in those three 
quarters) for which 75 percent of the hours in the day, including 
State-flagged data affected by exceptional events which have been 
approved for exclusion by the Administrator, have reported 
concentrations. However, daily mean values from days with less than 
75 percent of the hours reported shall also be considered in 
identifying the high daily mean value to be used for substitution.
    (B) The substitution test is as follows: Data substitution will 
be performed in all quarter periods that have less than 75 percent 
data capture but at least 50 percent data capture, including State-
flagged data affected by exceptional events which have been approved 
for exclusion by the Administrator; if any quarter has less than 50 
percent data capture then this substitution test cannot be used. 
Identify for each quarter (e.g., January-March) the highest reported 
daily mean value for that quarter, excluding State-flagged data 
affected by exceptional events which have been approved for 
exclusion by the Administrator, looking across those three months of 
all three years under consideration. All daily mean values from all 
days in the quarter period shall be considered when identifying this 
highest value, including days with less than 75 percent data 
capture. If after substituting the highest daily mean value for a 
quarter for as much of the missing daily data in the matching 
deficient quarter(s) as is needed to make them 100 percent complete, 
the procedure in section 5 of this appendix yields a recalculated 3-
year annual standard ``test design value'' less than or equal to the 
level of the standard, then the annual secondary standard design 
value is deemed to have passed the diagnostic test and is valid, and 
the level of the standard is deemed to have been met in that 3-year 
period. As noted in section 3.2(c)(i) of this appendix, in such a 
case, the 3-year design value based on the data actually reported, 
not the ``test design value,'' shall be used as the valid design 
value.
    (iii)(A) An annual secondary standard design value that is above 
the level of the NAAQS can be validated if the substitution test in 
section 3.2(c)(iii)(B) of this appendix results in a ``test design 
value'' that is above the level of the NAAQS. The test substitutes 
actual ``low'' reported daily mean values from the same site at 
about the same time of the year (specifically, in the same three 
months of the calendar) for unknown or incomplete (less than 75 
percent of hours reported) daily mean values. Note that the test is 
merely diagnostic in nature, intended to confirm that there is a 
very high likelihood that the original design value (the one with 
less than 75 percent data capture of hours by day and of days by 
quarter) reflects the true above-NAAQS-level status for that 3-year 
period; the result of this data substitution test (the ``test design 
value,'' as defined in section 3.2(c)(iii)(B) of this appendix) is 
not considered the actual design value. For this test, substitution 
is permitted only if there are a minimum number of valid daily mean 
values from which to identify the low quarter-specific daily mean 
values, specifically if there are at least 200 days across the three 
matching quarters of the three years under consideration (which is 
about 75 percent of all possible daily values in those three 
quarters) for which 75 percent of the hours in the day have reported 
concentrations. Only days with at least 75

[[Page 26701]]

percent of the hours reported shall be considered in identifying the 
low daily mean value to be used for substitution.
    (B) The substitution test is as follows: Data substitution will 
be performed in all quarter periods that have less than 75 percent 
data capture. Identify for each quarter (e.g., January-March) the 
lowest reported daily mean value for that quarter, looking across 
those three months of all three years under consideration. All daily 
mean values from all days with at least 75 percent capture in the 
quarter period shall be considered when identifying this lowest 
value. If after substituting the lowest reported daily mean value 
for a quarter for as much of the missing daily data in the matching 
deficient quarter(s) as is needed to make them 75 percent complete, 
the procedure in section 5.2 of this appendix yields a recalculated 
3-year annual standard ``test design value'' above the level of the 
standard, then the annual secondary standard design value is deemed 
to have passed the diagnostic test and is valid, and the level of 
the standard is deemed to have been exceeded in that 3-year period. 
As noted in section 3.2(c)(i) of this appendix, in such a case, the 
3-year design value based on the data actually reported, not the 
``test design value,'' shall be used as the valid design value.
    (d) An annual secondary standard design value based on data that 
do not meet the completeness criteria stated in section 3.2(b) of 
this appendix and also do not satisfy section 3.2(c) of this 
appendix, may also be considered valid with the approval of, or at 
the initiative of, the Administrator, who may consider factors such 
as monitoring site closures/moves, monitoring diligence, the 
consistency and levels of the valid concentration measurements that 
are available, and nearby concentrations in determining whether to 
use such data.
    (e) The procedures for calculating the annual secondary standard 
design values are given in section 5.2 of this appendix.

4. Rounding Conventions

4.1 Rounding Conventions for the 1-Hour Primary SO2 NAAQS

    (a) Hourly SO2 measurement data shall be reported to 
AQS in units of parts per billion (ppb), to at most one place after 
the decimal, with additional digits to the right being truncated 
with no further rounding.
    (b) Daily maximum 1-hour values and, therefore, the annual 99th 
percentile of those daily values are not rounded.
    (c) The 1-hour primary standard design value is calculated 
pursuant to section 5.1 of this appendix and then rounded to the 
nearest whole number or 1 ppb (decimals 0.5 and greater are rounded 
up to the nearest whole number, and any decimal lower than 0.5 is 
rounded down to the nearest whole number).

4.2 Rounding Conventions for the Annual Secondary SO2 NAAQS

    (a) Hourly SO2 measurement data shall be reported to 
AQS in units of parts per billion (ppb), to at most one place after 
the decimal, with additional digits to the right being truncated 
with no further rounding.
    (b) Daily mean values and the annual mean of those daily values 
are not rounded.
    (c) The annual secondary standard design value is calculated 
pursuant to section 5.2 of this appendix and then rounded to the 
nearest whole number or 1 ppb (decimals 0.5 and greater are rounded 
up to the nearest whole number, and any decimal lower than 0.5 is 
rounded down to the nearest whole number).

5. Calculation Procedures

5.1 Calculation Procedures for the 1-Hour Primary SO2 NAAQS

    (a) Procedure for identifying annual 99th percentile values. 
When the data for a particular ambient air quality monitoring site 
and year meet the data completeness requirements in section 3.1(b) 
of this appendix, or if one of the conditions of section 3.1(c) of 
this appendix is met, or if the Administrator exercises the 
discretionary authority in section 3.1(d) of this appendix, 
identification of annual 99th percentile value is accomplished as 
follows.
    (i) The annual 99th percentile value for a year is the higher of 
the two values resulting from the following two procedures.
    (A) Procedure 1. For the year, determine the number of days with 
at least 75 percent of the hourly values reported.
    (1) For the year, determine the number of days with at least 75 
percent of the hourly values reported including State-flagged data 
affected by exceptional events which have been approved for 
exclusion by the Administrator.
    (2) For the year, from only the days with at least 75 percent of 
the hourly values reported, select from each day the maximum hourly 
value excluding State-flagged data affected by exceptional events 
which have been approved for exclusion by the Administrator.
    (3) Sort all these daily maximum hourly values from a particular 
site and year by descending value. (For example: (x[1], x[2], x[3], 
* * *, x[n]). In this case, x[1] is the largest number and x[n] is 
the smallest value.) The 99th percentile is determined from this 
sorted series of daily values which is ordered from the highest to 
the lowest number. Using the left column of table 1, determine the 
appropriate range (i.e., row) for the annual number of days with 
valid data for year y (cny). The corresponding ``n'' 
value in the right column identifies the rank of the annual 99th 
percentile value in the descending sorted list of daily site values 
for year y. Thus, P0.99, y = the nth largest value.
    (B) Procedure 2. For the year, determine the number of days with 
at least one hourly value reported.
    (1) For the year, determine the number of days with at least one 
hourly value reported including State-flagged data affected by 
exceptional events which have been approved for exclusion by the 
Administrator.
    (2) For the year, from all the days with at least one hourly 
value reported, select from each day the maximum hourly value 
excluding State-flagged data affected by exceptional events which 
have been approved for exclusion by the Administrator.
    (3) Sort all these daily maximum values from a particular site 
and year by descending value. (For example: (x[1], x[2], x[3], * * 
*, x[n]). In this case, x[1] is the largest number and x[n] is the 
smallest value.) The 99th percentile is determined from this sorted 
series of daily values which is ordered from the highest to the 
lowest number. Using the left column of table 1, determine the 
appropriate range (i.e., row) for the annual number of days with 
valid data for year y (cny). The corresponding ``n'' 
value in the right column identifies the rank of the annual 99th 
percentile value in the descending sorted list of daily site values 
for year y. Thus, P0.99,y = the nth largest value.
    (b) The 1-hour primary standard design value for an ambient air 
quality monitoring site is mean of the three annual 99th percentile 
values, rounded according to the conventions in section 4.1 of this 
appendix.

                    Table 1 of Appendix T of Part 50
------------------------------------------------------------------------
                                                    P0.99,y is the nth
 Annual number of days with valid data for year   maximum value of  the
                  ``y''  (cny)                    year, where n  is the
                                                      listed number
------------------------------------------------------------------------
1-100..........................................                        1
101-200........................................                        2
201-300........................................                        3
301-366........................................                        4
------------------------------------------------------------------------

5.2 Calculation Procedures for the Annual Secondary SO2 NAAQS

    (a) When the data for a site and year meet the data completeness 
requirements in section 3.2(b) of this appendix, or if the 
Administrator exercises the discretionary authority in section 
3.2(c), the annual mean is simply the arithmetic average of all the 
daily mean values.
    (b) The annual secondary standard design value for an ambient 
air quality monitoring site is the mean of the annual means for 
three consecutive years, rounded according to the conventions in 
section 4.2 of this appendix.

[FR Doc. 2024-07397 Filed 4-12-24; 8:45 am]
BILLING CODE 6560-50-P