[Federal Register Volume 76, Number 147 (Monday, August 1, 2011)]
[Proposed Rules]
[Pages 46084-46147]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2011-18582]



[[Page 46083]]

Vol. 76

Monday,

No. 147

August 1, 2011

Part III





Environmental Protection Agency





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40 CFR Part 50





 Secondary National Ambient Air Quality Standards for Oxides of 
Nitrogen and Sulfur; Proposed Rule

  Federal Register / Vol. 76 , No. 147 / Monday, August 1, 2011 / 
Proposed Rules  

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 50

[EPA-HQ-OAR-2007-1145; FRL-9441-2]
RIN 2060-AO72


Secondary National Ambient Air Quality Standards for Oxides of 
Nitrogen and Sulfur

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: This proposed rule is being issued as required by a consent 
decree governing the schedule for completion of this review of the air 
quality criteria and the secondary national ambient air quality 
standards (NAAQS) for oxides of nitrogen and oxides of sulfur. Based on 
its review, EPA proposes to retain the current nitrogen dioxide 
(NO2) and sulfur dioxide (SO2) secondary 
standards to provide requisite protection for the direct effects on 
vegetation resulting from exposure to gaseous oxides of nitrogen and 
sulfur in the ambient air. Additionally, with regard to protection from 
the deposition of oxides of nitrogen and sulfur to sensitive aquatic 
and terrestrial ecosystems, including acidification and nutrient 
enrichment effects, EPA is proposing to add secondary standards 
identical to the NO2 and SO2 primary 1-hour 
standards and not set a new multi-pollutant secondary standard in this 
review. The proposed 1-hour secondary NO2 standard would be 
set at a level of 100 ppb and the proposed 1-hour secondary 
SO2 standard would be set at 75 ppb. In addition, EPA has 
decided to undertake a field pilot program to gather and analyze 
additional relevant data so as to enhance the Agency's understanding of 
the degree of protectiveness that a new multi-pollutant approach, 
defined in terms of an aquatic acidification index (AAI), would afford 
and to support development of an appropriate monitoring network for 
such a standard. The EPA solicits comment on the framework of such a 
standard and on the design of the field pilot program. The EPA will 
sign a notice of final rulemaking for this review no later than March 
20, 2012.

DATES: Written comments on this proposed rule must be received by 
September 30, 2011.
    Public Hearings: The EPA intends to hold a public hearing around 
the end of August to early September and will announce in a separate 
Federal Register notice the date, time, and address of the public 
hearing on this proposed rule.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2007-1145, by one of the following methods:
     http://www.regulations.gov: Follow the on-line 
instructions for submitting comments.
     E-mail: [email protected].
     Fax: 202-566-1741.
     Mail: Docket No. EPA-HQ-OAR-2007-1145, Environmental 
Protection Agency, Mail code 6102T, 1200 Pennsylvania Ave., NW., 
Washington, DC 20460. Please include a total of two copies.
     Hand Delivery: Docket No. EPA-HQ-OAR-2007-1145, 
Environmental Protection Agency, EPA West, Room 3334, 1301 Constitution 
Ave., NW., Washington, DC. Such deliveries are only accepted during the 
Docket's normal hours of operation, and special arrangements should be 
made for deliveries of boxed information.
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2007-1145. The EPA's policy is that all comments received will be 
included in the public docket without change and may be made available 
online at http://www.regulations.gov, including any personal 
information provided, unless the comment includes information claimed 
to be Confidential Business Information (CBI) or other information 
whose disclosure is restricted by statute. Do not submit information 
that you consider to be CBI or otherwise protected through http://www.regulations.gov or e-mail. The http://www.regulations.gov Web site 
is an ``anonymous access'' system, which means EPA will not know your 
identity or contact information unless you provide it in the body of 
your comment. If you send an e-mail comment directly to EPA without 
going through http://www.regulations.gov, your e-mail address will be 
automatically captured and included as part of the comment that is 
placed in the public docket and made available on the Internet. If you 
submit an electronic comment, EPA recommends that you include your name 
and other contact information in the body of your comment and with any 
disk or CD-ROM you submit. If EPA cannot read your comment due to 
technical difficulties and cannot contact you for clarification, EPA 
may not be able to consider your comment. Electronic files should avoid 
the use of special characters, any form of encryption, and be free of 
any defects or viruses. For additional information about EPA's public 
docket, visit the EPA Docket Center homepage at http://www.epa.gov/epahome/dockets.htm.
    Docket: All documents in the docket are listed in the http://www.regulations.gov index. 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, will be publicly available only in hard copy. 
Publicly available docket materials are available either electronically 
in http://www.regulations.gov or in hard copy at the Air and Radiation 
Docket and Information Center, EPA/DC, EPA West, Room 3334, 1301 
Constitution Ave., NW., Washington, DC. The Public Reading Room is open 
from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal 
holidays. The telephone number for the Public Reading Room is (202) 
566-1744 and the telephone number for the Air and Radiation Docket and 
Information Center is (202) 566-1742.

FOR FURTHER INFORMATION CONTACT: Dr. Richard Scheffe, Office of Air 
Quality Planning and Standards, U.S. Environmental Protection Agency, 
Mail code C304-02, Research Triangle Park, NC 27711; telephone: 919-
541-4650; fax: 919-541-2357; e-mail: [email protected].

SUPPLEMENTARY INFORMATION:

General Information

What should I consider as I prepare my comments for EPA?

    1. Submitting CBI. Do not submit this information to EPA through 
http://www.regulations.gov or e-mail. Clearly mark the part or all of 
the information that you claim to be CBI. For CBI information in a disk 
or CD ROM that you mail to EPA, mark the outside of the disk or CD ROM 
as CBI and then identify electronically within the disk or CD ROM the 
specific information that is claimed as CBI. In addition to one 
complete version of the comment that includes information claimed as 
CBI, a copy of the comment that does not contain the information 
claimed as CBI must be submitted for inclusion in the public docket. 
Information so marked will not be disclosed except in accordance with 
procedures set forth in 40 CFR part 2.
    2. Tips for Preparing Your Comments. When submitting comments, 
remember to:
     Identify the rulemaking by docket number and other 
identifying information (subject heading, Federal Register date and 
page number).

[[Page 46085]]

     Follow directions--The Agency may ask you to respond to 
specific questions or organize comments by referencing a Code of 
Federal Regulations (CFR) part or section 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.
     If you estimate potential costs or burdens, explain how 
you arrived at your estimate in sufficient detail to allow for it to be 
reproduced.
     Provide specific examples to illustrate your concerns, and 
suggest alternatives.
     Explain your views as clearly as possible.
     Make sure to submit your comments by the comment period 
deadline identified.

Availability of Related Information

    A number of documents relevant to this rulemaking are available on 
EPA web sites. The Integrated Science Assessment for Oxides of Nitrogen 
and Sulfur--Ecological Criteria: Final Report (ISA) is available on 
EPAs National Center for Environmental Assessment Web site. To obtain 
this document, go to http://www.epa.gov/ncea, and click on Air Quality 
then click on Oxides of Nitrogen and Sulfur. The Policy Assessment 
(PA), Risk and Exposure Assessment (REA), and other related technical 
documents are available on EPA's Office of Air Quality Planning and 
Standards (OAQPS) Technology Transfer Network (TTN) web site. The PA is 
available at http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_pa.html, and the exposure and risk assessments and other related 
technical documents are available at http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_rea.html. These and other related documents are 
also available for inspection and copying in the EPA docket identified 
above.

Table of Contents

    The following topics are discussed in this preamble:

I. Background
    A. Legislative Requirements
    B. History of Reviews of NAAQS for Nitrogen Oxides and Sulfur 
Oxides
    1. NAAQS for Oxides of Nitrogen
    2. NAAQS for Oxides of Sulfur
    C. History of Related Assessments and Agency Actions
    D. History of the Current Review
    E. Scope of the Current Review
II. Rationale for Proposed Decision on the Adequacy of the Current 
Secondary Standards
    A. Ecological Effects
    1. Effects Associated with Gas-Phase Oxides of Nitrogen and 
Sulfur
    a. Nature of ecosystem responses to gas-phase nitrogen and 
sulfur
    b. Magnitude of ecosystem response to gas-phase nitrogen and 
sulfur
    2. Acidification Effects Associated with Deposition of Oxides of 
Nitrogen and Sulfur
    a. Nature of Acidification-related Ecosystem Responses
    i. Aquatic Ecosystems
    ii. Terrestrial Ecosystems
    iii. Ecosystem Sensitivity
    b. Magnitude of Acidification-Related Ecosystem Responses
    i. Aquatic Acidification
    ii. Terrestrial Acidification
    c. Key Uncertainties Associated With Acidification
    i. Aquatic Acidification
    ii. Terrestrial Acidification
    3. Nutrient Enrichment Effects Associated With Deposition of 
Oxides of Nitrogen
    a. Nature of Nutrient Enrichment-Related Ecosystem Responses
    i. Aquatic Ecosystems
    ii. Terrestrial Ecosystems
    iii. Ecosystem Eensitivity to Nutrient Enrichment
    b. Magnitude of Nutrient Enrichment-Related Ecosystem Responses
    i. Aquatic Ecosystems
    ii. Terrestrial Ecosystems
    c. Key Uncertainties Associated With Nutrient Enrichment
    i. Aquatic Ecosystems
    ii. Terrestrial Ecosystems
    4. Other Ecological Effects
    B. Risk and Exposure Assessment
    1. Overview of Risk and Exposure Assessment
    2. Key Findings
    a. Air Quality Analyses
    b. Deposition-Related Aquatic Acidification
    c. Deposition-Related Terrestrial Acidification
    d. Deposition-Related Aquatic Nutrient Enrichment
    e. Deposition-Related Terrestrial Nutrient Enrichment
    f. Additional Effects
    3. Conclusions on Effects
    C. Adversity of Effects to Public Welfare
    1. Ecosystem Services
    2. Effects on Ecosystem Services
    a. Aquatic Acidification
    b. Terrestrial Acidification
    c. Nutrient Enrichment
    3. Summary
    D. Adequacy of the Current Standards
    1. Adequacy of the Current Standards for Direct Effects
    2. Appropriateness and Adequacy of the Current Standards for 
Deposition-Related Effects
    a. Appropriateness
    b. Adequacy of Protection
    i. Aquatic Acidification
    ii. Terrestrial Acidification
    iii. Terrestrial Nutrient Enrichment
    iv. Aquatic Nutrient Enrichment
    v. Other Effects
    3. CASAC Views
    4. Administrator's Proposed Conclusions Concerning Adequacy of 
Current Standard
III. Rationale for Proposed Decision on Alternative Multi-Pollutant 
Approach to Secondary Standards for Aquatic Acidification
    A. Ambient Air Indicators
    1. Oxides of Sulfur
    2. Oxides of Nitrogen
    B. Form
    1. Ecological Indicator
    2. Linking ANC to Deposition
    3. Linking Deposition to Ambient Air Indicators
    4. Aquatic Acidification Index
    5. Spatial Aggregation
    a. Ecoregion Sensitivity
    b. Representative Ecoregion-Specific Factors
    i. Factor F1
    (a) Acid-Sensitive Ecoregions
    (b) Non-Acid Sensitive Ecoregions
    ii. Factor F2
    iii. Factors F3 and F4
    c. Factors in Data-limited Ecoregions
    d. Application to Hawaii, Alaska, and the U.S. Territories
    6. Summary of the AAI Form
    C. Averaging Time
    D. Level
    1. Association Between pH Levels and Target ANC Levels
    2. ANC Levels Related to Effects on Aquatic Ecosystems
    3. Consideration of Episodic Acidity
    4. Consideration of Ecosystem Response Time
    5. Prior Examples of Target ANC Levels
    6. Consideration of Public Welfare Benefits
    7. Summary of Alternative Levels
    E. Combined Alternative Levels and Forms
    F. Characterization of Uncertainties
    1. Overview of Uncertainty
    2. Uncertainties Associated with Data Gaps
    3. Uncertainties in Modeled Processes
    4. Applying Knowledge of Uncertainties
    G. CASAC Advice
    H. Administrator's Proposed Conclusions
IV. Field Pilot Program and Ambient Monitoring
    A. Field Pilot Program
    1. Objectives
    2. Overview of Field Pilot Program
    3. Complementary Measurements
    4. Complementary Areas of Research Implementation Challenges
    5. Final Monitoring Plan Development and Stakeholder 
Participation
    B. Evaluation of Monitoring Methods
    1. Potential FRMs for SO2 and p-SO4
    2. Potential FRM for NOy
V. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    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 and Safety Risks

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    H. Executive Order 13211: Actions That Significantly Affect 
Energy Supply, Distribution, or Use
    I. National Technology Transfer and Advancement Act
    J. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations References

I. Background

A. Legislative Requirements

    Two sections of the Clean Air Act (CAA) govern the establishment 
and revision of the NAAQS. Section 108 (42 U.S.C. section 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 air pollutants that in her ``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 * * * [the Administrator] plans to issue air 
quality criteria * * *'' 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(b). Section 109 (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. Section 109(b)(1) defines a primary 
standard as one ``the attainment and maintenance of which in the 
judgment of the Administrator, based on such criteria and allowing an 
adequate margin of safety, are requisite to protect the public 
health.'' \1\ A secondary standard, as defined in section 109(b)(2), 
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.'' Welfare effects as defined in section 302(h) (42 U.S.C. 
7602(h)) include, but are not limited to, ``effects on soils, water, 
crops, vegetation, man-made 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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    \1\ The legislative history of section 109 indicates that a 
primary standard is to be set at ``the maximum permissible ambient 
air level * * * which will protect the health of any [sensitive] 
group of the population,'' and that for this purpose ``reference 
should be made to a representative sample of persons comprising the 
sensitive group rather than to a single person in such a group.'' S. 
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
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    In setting standards that are ``requisite'' to protect public 
health and welfare, as provided in section 109(b), EPA's task is to 
establish standards that are neither more nor less stringent than 
necessary for these purposes. In so doing, EPA may not consider the 
costs of implementing the standards. See generally, Whitman v. American 
Trucking Associations, 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 at 1185. 
Section 109(d)(1) requires that ``not later than December 31, 1980, and 
at 5-year intervals thereafter, the Administrator shall complete a 
thorough review of the criteria published under section 108 and the 
national ambient air quality standards * * * and shall make such 
revisions in such criteria and standards and promulgate such new 
standards as may be appropriate * * * .'' Section 109(d)(2) requires 
that an 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 
1980's, this independent review function has been performed by the 
Clean Air Scientific Advisory Committee (CASAC).

B. History of Reviews of NAAQS for Nitrogen Oxides and Sulfur Oxides

1. NAAQS for Oxides of Nitrogen
    After reviewing the relevant science on the public health and 
welfare effects associated with oxides of nitrogen, EPA promulgated 
identical primary and secondary NAAQS for NO2 in April 1971. 
These standards were set at a level of 0.053 parts per million (ppm) as 
an annual average (36 FR 8186). In 1982, EPA published Air Quality 
Criteria Document for Oxides of Nitrogen (US EPA, 1982), which updated 
the scientific criteria upon which the initial standards were based. In 
February 1984 EPA proposed to retain these standards (49 FR 6866). 
After taking into account public comments, EPA published the final 
decision to retain these standards in June 1985 (50 FR 25532).
    The EPA began the most recent previous review of the oxides of 
nitrogen secondary standards in 1987. In November 1991, EPA released an 
updated draft air quality criteria document (AQCD) for CASAC and public 
review and comment (56 FR 59285), which provided a comprehensive 
assessment of the available scientific and technical information on 
health and welfare effects associated with NO2 and other 
oxides of nitrogen. 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 ``provides a scientifically balanced 
and defensible summary of current knowledge of the effects of this 
pollutant and provides an adequate basis for EPA to make a decision as 
to the appropriate NAAQS for NO2'' (Wolff, 1993). The AQCD 
for Oxides of Nitrogen was then finalized (US EPA, 1995a). The EPA's 
OAQPS also prepared a Staff Paper that summarized and integrated the 
key studies and scientific evidence contained in the revised AQCD for 
oxides of nitrogen and identified the critical elements to be 
considered in the review of the NO2 NAAQS. The CASAC 
reviewed two drafts of the Staff Paper and concluded in a closure 
letter to the Administrator that the document provided a 
``scientifically adequate basis for regulatory decisions on nitrogen 
dioxide'' (Wolff, 1995).
    In October 1995, the Administrator announced her proposed decision 
not to revise either the primary or secondary NAAQS for NO2 
(60 FR 52874; October 11, 1995). A year later, the Administrator made a 
final determination not to revise the NAAQS for NO2 after 
careful evaluation of the comments received on the proposal (61 FR 
52852; October 8, 1996). While the primary NO2 standard was 
revised in January 2010 by supplementing the existing annual standard 
with the establishment of a new 1-hour standard, set at a level of 100 
ppb (75 FR 6474), the secondary NAAQS for NO2 remains 0.053 
ppm (100 micrograms per cubic meter [[mu]g/m3] of air), annual 
arithmetic average, calculated as the arithmetic mean of the 1-hour 
NO2 concentrations.
2. The NAAQS for Oxides of Sulfur
    The EPA promulgated primary and secondary NAAQS for SO2 
in April 1971 (36 FR 8186). The secondary standards included a standard 
set at 0.02 ppm, annual arithmetic mean, and a 3-hour average standard 
set at 0.5 ppm, not to be exceeded more than once per year. These 
secondary standards

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were established solely on the basis of evidence of adverse effects on 
vegetation. In 1973, revisions made to Chapter 5 (``Effects of Sulfur 
Oxide in the Atmosphere on Vegetation'') of the AQCD for Sulfur Oxides 
(US EPA, 1973) indicated that it could not properly be concluded that 
the vegetation injury reported resulted from the average SO2 
exposure over the growing season, rather than from short-term peak 
concentrations. Therefore, EPA proposed (38 FR 11355) and then 
finalized (38 FR 25678) a revocation of the annual mean secondary 
standard. At that time, EPA was aware that then-current concentrations 
of oxides of sulfur in the ambient air had other public welfare 
effects, including effects on materials, visibility, soils, and water. 
However, the available data were considered insufficient to establish a 
quantitative relationship between specific ambient concentrations of 
oxides of sulfur and such public welfare effects (38 FR 25679).
    In 1979, EPA announced that it was revising the AQCD for oxides of 
sulfur concurrently with that for particulate matter (PM) and would 
produce a combined PM and oxides of sulfur criteria document. Following 
its review of a draft revised criteria document in August 1980, CASAC 
concluded that acid deposition was a topic of extreme scientific 
complexity because of the difficulty in establishing firm quantitative 
relationships among (1) Emissions of relevant pollutants (e.g., 
SO2 and oxides of nitrogen), (2) formation of acidic wet and 
dry deposition products, and (3) effects on terrestrial and aquatic 
ecosystems. The CASAC also noted that acid deposition involves, at a 
minimum, several different criteria pollutants: Oxides of sulfur, 
oxides of nitrogen, and the fine particulate fraction of suspended 
particles. The CASAC 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.
    For these reasons, CASAC recommended that 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. The CASAC also suggested that a discussion of acid 
deposition be included in the AQCDs for oxides of nitrogen and PM and 
oxides of sulfur. Following CASAC closure on the AQCD for oxides of 
sulfur in December 1981, EPA's OAQPS published a Staff Paper in 
November 1982, although the paper did not directly assess the issue of 
acid deposition. Instead, EPA subsequently prepared the following 
documents to address acid deposition: The Acidic Deposition Phenomenon 
and Its Effects: Critical Assessment Review Papers, Volumes I and II 
(US EPA, 1984a, b) and The Acidic Deposition Phenomenon and Its 
Effects: Critical Assessment Document (US EPA, 1985) (53 FR 14935-
14936). These documents, though they were not considered criteria 
documents and did not undergo CASAC review, represented the most 
comprehensive summary of scientific information relevant to acid 
deposition completed by EPA at that point.
    In April 1988 (53 FR 14926), EPA proposed not to revise the 
existing primary and secondary standards for SO2. 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, EPA 
would draft and support an appropriate set of control measures. 
Although EPA revised the primary SO2 standard in June 2010 
by establishing a new 1-hour standard at a level of 75 ppb and revoking 
the existing 24-hour and annual standards (75 FR 35520), no further 
decisions on the secondary SO2 standard have been published.

C. History of Related Assessments and Agency Actions

    In 1980, the Congress created the National Acid Precipitation 
Assessment Program (NAPAP) in response to growing concern about acidic 
deposition. The NAPAP was given a broad 10-year mandate to examine the 
causes and effects of acidic deposition and to explore alternative 
control options to alleviate acidic deposition and its effects. During 
the course of the program, the NAPAP issued a series of publicly 
available interim reports prior to the completion of a final report in 
1990 (NAPAP, 1990).
    In spite of the complexities and significant remaining 
uncertainties associated with the acid deposition problem, it soon 
became clear that a program to address acid deposition was needed. The 
Clean Air Act Amendments of 1990 included numerous separate provisions 
related to the acid deposition problem. The primary and most important 
of the provisions, the amendments to Title IV of the Act, established 
the Acid Rain Program to reduce emissions of SO2 by 10 
million tons and emissions of nitrogen oxides by 2 million tons from 
1980 emission levels in order to achieve reductions over broad 
geographic regions. In this provision, Congress included a statement of 
findings that led them to take action, concluding that (1) The presence 
of acid 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; (2) 
the problem of acid deposition is of national and international 
significance; and (3) current and future generations of Americans will 
be adversely affected by delaying measures to remedy the problem.
    Second, Congress authorized the continuation of the NAPAP in order 
to assure that the research and monitoring efforts already undertaken 
would continue to be coordinated and would provide the basis for an 
impartial assessment of the effectiveness of the Title IV program.
    Third, Congress considered that further action might be necessary 
in the long term to address any problems remaining after implementation 
of the Title IV program and, reserving judgment on the form that action 
could take, included Section 404 of the 1990 Amendments (Clean Air Act 
Amendments of 1990, Pub. L. 101-549, Sec.  404) requiring 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.'' At the conclusion of the 
study, EPA was to submit a report to Congress. Five years later, EPA 
submitted its report, entitled Acid Deposition Standard Feasibility 
Study: Report to Congress (US EPA, 1995b) in fulfillment of this 
requirement. That report concluded that establishing acid deposition 
standards for sulfur and nitrogen deposition may 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.
    Fourth, the 1990 Amendments also added new language to sections of 
the CAA pertaining to the scope and application of the secondary NAAQS 
designed to protect the public welfare. Specifically, the definition of 
``effects on welfare'' in Section 302(h) was expanded to state that the 
welfare effects include effects ``* * * whether caused by 
transformation, conversion, or combination with other air pollutants.''

[[Page 46088]]

    In 1999, seven Northeastern states cited this amended language in 
Section 302(h) in a petition asking EPA to use its authority under the 
NAAQS program to promulgate secondary NAAQS for the criteria pollutants 
associated with the formation of acid rain. The petition stated that 
this language ``clearly references the transformation of pollutants 
resulting in the inevitable formation of sulfate and nitrate aerosols 
and/or their ultimate environmental impacts as wet and dry deposition, 
clearly signaling Congressional intent that the welfare damage 
occasioned by sulfur and nitrogen oxides be addressed through the 
secondary standard provisions of Section 109 of the Act.'' The petition 
further stated that ``recent federal studies, including the NAPAP 
Biennial Report to Congress: An Integrated Assessment, document the 
continued and increasing damage being inflicted by acid deposition to 
the lakes and forests of New York, New England and other parts of our 
nation, demonstrating that the Title IV program had proven 
insufficient.'' The petition also listed other adverse welfare effects 
associated with the transformation of these criteria pollutants, 
including impaired visibility, eutrophication of coastal estuaries, 
global warming, and tropospheric ozone and stratospheric ozone 
depletion.
    In a related matter, the Office of the Secretary of the U.S. 
Department of Interior (DOI) requested in 2000 that EPA initiate a 
rulemaking proceeding to enhance the air quality in national parks and 
wilderness areas in order to protect resources and values that are 
being adversely affected by air pollution. Included among the effects 
of concern identified in the request were the acidification of streams, 
surface waters, and/or soils; eutrophication of coastal waters; 
visibility impairment; and foliar injury from ozone.
    In a Federal Register notice in 2001 (65 FR 48699), EPA announced 
receipt of these requests and asked for comment on the issues raised in 
them. The EPA stated that it would consider any relevant comments and 
information submitted, along with the information provided by the 
petitioners and DOI, before making any decision concerning a response 
to these requests for rulemaking.
    The 2005 NAPAP report states that ``* * * scientific studies 
indicate that the emission reductions achieved by Title IV are not 
sufficient to allow recovery of acid-sensitive ecosystems. Estimates 
from the literature of the scope of additional emission reductions that 
are necessary in order to protect acid-sensitive ecosystems range from 
approximately 40-80% beyond full implementation of Title IV. * * *'' 
The results of the modeling presented in this Report to Congress 
indicate that broader recovery is not predicted without additional 
emission reductions (NAPAP, 2005).
    Given the state of the science as described in the ISA, REA, and in 
other recent reports, such as the NAPAP reports noted above, EPA has 
decided, in the context of evaluating the adequacy of the current 
NO2 and SO2 secondary standards in this review, 
to revisit the question of the appropriateness of setting secondary 
NAAQS to address remaining known or anticipated adverse public welfare 
effects resulting from the acidic and nutrient deposition of these 
criteria pollutants.

D. History of the Current Review

    The EPA initiated this current review in December 2005 with a call 
for information (70 FR 73236) for the development of a revised ISA. An 
Integrated Review Plan (IRP) was developed to provide the framework and 
schedule as well as the scope of the review and to identify policy-
relevant questions to be addressed in the components of the review. The 
IRP was released in 2007 (US EPA, 2007) for CASAC and public review. 
The EPA held a workshop in July 2007 on the ISA to obtain broad input 
from the relevant scientific communities. This workshop helped to 
inform the preparation of the first draft ISA, which was released for 
CASAC and public review in December 2007; a CASAC meeting was held on 
April 2-3, 2008 to review the first draft ISA. A second draft ISA was 
released for CASAC and public review in August 2008, and was discussed 
at a CASAC meeting held on October 1-2, 2008. The final ISA (US EPA, 
2008) was released in December 2008.
    Based on the science presented in the ISA, EPA developed the REA to 
further assess the national impact of the effects documented in the 
ISA. The Draft Scope and Methods Plan for Risk/Exposure Assessment: 
Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur 
outlining the scope and design of the future REA was prepared for CASAC 
consultation and public review in March 2008. A first draft REA was 
presented to CASAC and the public for review in August 2008 and a 
second draft was presented for review in June 2009. The final REA (US 
EPA, 2009) was released in September 2009. A first draft PA was 
released in March 2010 and reviewed by CASAC on April 1-2, 2010. In a 
June 22, 2010 letter to the Administrator, CASAC provided advice and 
recommendations to the Agency concerning the first draft PA (Russell 
and Samet, 2010a). A second draft PA was released to CASAC and the 
public in September 2010 and reviewed by CASAC on October 6-7, 2010. 
The CASAC provided advice and recommendations to the Agency regarding 
the second draft PA in a December 9, 2010 letter (Russell and Samet 
2010b). The CASAC and public comments on the second draft PA were 
considered by EPA staff in developing a final PA (US EPA, 2011). CASAC 
requested an additional meeting to provide additional advice to the 
Administrator based on the final PA on February 15-16, 2011. On January 
14, 2011, EPA released a version of the final PA prior to final 
document production, to provide sufficient time for CASAC review of the 
document in advance of this meeting. The final PA, incorporating final 
reference checks and document formatting, was released in February 
2011. In a May 17, 2011 letter (Russell and Samet, 2011a), CASAC 
offered additional advice and recommendations to the Administrator with 
regard to the review of the secondary NAAQS for oxides of nitrogen and 
oxides of sulfur.
    In 2005, the Center for Biological Diversity and four other 
plaintiffs filed a complaint alleging that EPA had failed to complete 
the current review within the period provided by statute.\2\ The 
schedule for completion of this review is governed by a consent decree 
resolving that lawsuit and the subsequent extension agreed to by the 
parties. The schedule presented in the original consent decree that 
governs this review, entered by the court on November 19, 2007, was 
revised on October 22, 2009 to allow for a 17-month extension of the 
schedule. The current decree provides that EPA sign for publication 
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.
---------------------------------------------------------------------------

    \2\ Center for Biological Diversity, et al. v. Johnson, No. 05-
1814 (D.D.C.)
---------------------------------------------------------------------------

    This action presents the Administrator's proposed decisions on the 
review of the current secondary oxides of nitrogen and oxides of sulfur 
standards. Throughout this preamble a number of conclusions, findings, 
and determinations proposed by the Administrator are noted. While they 
identify the reasoning that supports this proposal, they are only 
proposals and are not intended to be final or conclusive in nature. The 
EPA invites general, specific, and/or technical

[[Page 46089]]

comments on all issues involved with this proposal, including all such 
proposed judgments, conclusions, findings, and determinations.

E. Scope of the Current Review

    In conducting this periodic review of the secondary NAAQS for 
oxides of nitrogen and oxides of sulfur, as discussed in the IRP and 
REA, EPA decided to assess the scientific information, associated 
risks, and standards relevant to protecting the public welfare from 
adverse effects associated jointly with oxides of nitrogen and sulfur. 
Although EPA has historically adopted separate secondary standards for 
oxides of nitrogen and oxides of sulfur, EPA is conducting 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 National Research Council (NRC) has recommended that 
EPA consider multiple pollutants, as appropriate, in forming the 
scientific basis for the NAAQS (NRC, 2004). As discussed in the ISA and 
REA, there is a strong basis for considering these pollutants together, 
building upon EPA's past recognition of the interactions of these 
pollutants and on the growing body of scientific information that is 
now available related to these interactions and associated ecological 
effects.
    In defining the scope of this review, it must be considered that 
EPA has set secondary standards for two other criteria pollutants 
related to oxides of nitrogen and sulfur: Ozone and particulate matter 
(PM). Oxides of nitrogen are precursors to the formation of ozone in 
the atmosphere, and under certain conditions, can combine with 
atmospheric ammonia to form ammonium nitrate, a component of fine PM. 
Oxides of sulfur are precursors to the formation of particulate 
sulfate, which is a significant component of fine PM in many parts of 
the U.S. There are a number of welfare effects directly associated with 
ozone and fine PM, including ozone-related damage to vegetation and PM-
related visibility impairment. Protection against those effects is 
provided by the ozone and fine PM secondary standards. This review 
focuses on evaluation of the protection provided by secondary standards 
for oxides of nitrogen and sulfur for two general types of effects: (1) 
Direct effects on vegetation associated with exposure to gaseous oxides 
of nitrogen and sulfur in the ambient air, which are the effects that 
the current NO2 and SO2 secondary standards 
protect against; and (2) effects associated with the deposition of 
oxides of nitrogen and sulfur to sensitive aquatic and terrestrial 
ecosystems, including deposition in the form of particulate nitrate and 
particulate sulfate.
    The ISA focuses on the ecological effects associated with 
deposition of ambient oxides of nitrogen and sulfur to natural 
sensitive ecosystems, as distinguished from commercially managed 
forests and agricultural lands. This focus reflects the fact that the 
majority of the scientific evidence regarding acidification and 
nutrient enrichment is based on studies in unmanaged ecosystems. Non-
managed terrestrial ecosystems tend to have a higher fraction of 
nitrogen deposition resulting from atmospheric nitrogen (US EPA, 2008, 
section 3.3.2.5). In addition, the ISA notes that agricultural and 
commercial forest lands are routinely fertilized with amounts of 
nitrogen that exceed air pollutant inputs even in the most polluted 
areas (US EPA, 2008, section 3.3.9). This review recognizes that the 
effects of nitrogen deposition in managed areas are viewed differently 
from a public welfare perspective than are the effects of nitrogen 
deposition in natural, unmanaged ecosystems, largely due to the more 
homogeneous, controlled nature of species composition and development 
in managed ecosystems and the potential for benefits of increased 
productivity in those ecosystems.
    In focusing on natural sensitive ecosystems, the PA primarily 
considers the effects of ambient oxides of nitrogen and sulfur via 
deposition on multiple ecological receptors. The ISA highlights effects 
including those associated with acidification and nitrogen nutrient 
enrichment. With a focus on these deposition-related effects, EPA's 
objective is to develop a framework for oxides of nitrogen and sulfur 
standards that incorporates ecologically relevant factors and that 
recognizes the interactions between the two pollutants as they deposit 
to sensitive ecosystems. The overarching policy objective is to develop 
a secondary standard(s) based on the ecological criteria described in 
the ISA and the results of the assessments in the REA, and consistent 
with the requirement of the CAA to set secondary standards that are 
requisite to protect the public welfare from any known or anticipated 
adverse effects associated with the presence of these air pollutants in 
the ambient air. Consistent with the CAA, this policy objective 
includes consideration of ``variable factors * * * which of themselves 
or in combination with other factors may alter the effects on public 
welfare'' of the criteria air pollutants included in this review.
    In addition, we have chosen to focus on the effects of ambient 
oxides of nitrogen and sulfur on ecological impacts on sensitive 
aquatic ecosystems associated with acidifying deposition of nitrogen 
and sulfur, which is a transformation product of ambient oxides of 
nitrogen and sulfur. Based on the information in the ISA, the 
assessments presented in the REA, and advice from CASAC on earlier 
drafts of this PA (Russell and Samet, 2010a, 2010b), and as discussed 
in detail in the PA, we have the greatest confidence in the causal 
linkages between oxides of nitrogen and sulfur and aquatic 
acidification effects relative to other deposition-related effects, 
including terrestrial acidification and aquatic and terrestrial 
nutrient enrichment.

II. Rationale for Proposed Decision on the Adequacy of the Current 
Secondary Standards

    Decisions on retaining or revising the current secondary standards 
for oxides of nitrogen and sulfur are largely public welfare policy 
judgments based on the Administrator's informed assessment of what 
constitutes requisite protection against adverse effects to public 
welfare. A public welfare policy decision should draw 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.
    This section presents the rationale for the Administrator's 
proposed conclusions with regard to the adequacy of protection and 
ecological relevance of the current secondary standards for oxides of 
nitrogen and sulfur. As discussed more fully below, this rationale 
considered the latest scientific information on ecological effects 
associated with the presence of oxides of nitrogen and oxides of sulfur 
in the ambient air. This rationale also takes into account: (1) Staff 
assessments of the most policy-relevant information in the ISA and 
staff analyses of air quality,

[[Page 46090]]

exposure, and ecological risks, presented more fully in the REA and in 
the PA, upon which staff conclusions on revisions to the secondary 
oxides of nitrogen and oxides of sulfur standards are based; (2) CASAC 
advice and recommendations, as reflected in discussions of drafts of 
the ISA, REA, and PA at public meetings, in separate written comments, 
and in CASAC's letters to the Administrator; and (3) public comments 
received during the development of these documents, either in 
connection with CASAC meetings or separately.
    In developing this rationale, EPA has drawn upon an integrative 
synthesis of the entire body of evidence, published through early 2008, 
on ecological effects associated with the deposition of oxides of 
nitrogen and oxides of sulfur in the ambient air (US EPA, 2008). As 
discussed below in section II.A, this body of evidence addresses a 
broad range of ecological endpoints associated with ambient levels of 
oxides of nitrogen and oxides of sulfur. In considering this evidence, 
EPA focuses on those ecological endpoints, such as aquatic 
acidification, for which the ISA judges associations with oxides of 
nitrogen and oxides of sulfur to be causal, likely causal, or for which 
the evidence is suggestive that oxides of nitrogen and/or sulfur 
contribute to the reported effects. The categories of causality 
determinations have been developed in the ISA (US EPA, 2008) and are 
discussed in Section 1.6 of the ISA.
    Crucial to this review is the development of a form for an 
ecologically relevant standard that reflects both the geographically 
variable and deposition-dependent nature of the effects. The 
atmospheric levels of oxides of nitrogen and sulfur that afford a 
particular level of ecosystem protection are those levels that result 
in an amount of deposition that is less than the amount of deposition 
that a given ecosystem can accept without defined levels of 
degradation.
    Drawing from the framework developed in the REA, the framework we 
used to structure an ecologically meaningful secondary standard in the 
PA and to further develop the indicator, form, level, and averaging 
time of such a standard in section III of this proposal is depicted 
below and highlights the three key linkages that need to be considered 
in developing an ecologically relevant standard.
[GRAPHIC] [TIFF OMITTED] TP01AU11.023

    The following discussion relies heavily on chapters 2 and 3 of the 
PA. The PA includes staff's evaluation of the policy implications of 
the scientific assessment of the evidence presented and assessed in the 
ISA and the results of quantitative assessments based on that 
information presented and assessed in the REA. Taken together, this 
information informs staff conclusions and the development of policy 
options in the PA for consideration in addressing public and welfare 
effects associated with the presence of oxides of nitrogen and oxides 
of sulfur in the ambient air. Of particular note, chapter 2 of the PA 
presents information not repeated here that characterizes emissions, 
air quality, deposition and water quality. It includes discussions of 
the sources of nitrogen and sulfur in the atmosphere as well as current 
ambient air quality monitoring networks and models. Additional 
information in this section includes ecological modeling and water 
quality data sources.
    Section II.A presents a discussion of the effects associated with 
oxides of nitrogen and sulfur in the ambient air. The discussion is 
organized around the types of effects being considered, including 
direct effects of gaseous oxides of nitrogen and sulfur, deposition-
related effects related to acidification and nutrient enrichment, and 
other effects such as materials damage, climate-related effects and 
mercury methylation.
    Section II.B presents a summary and discussion of the risk and 
exposure assessment performed for each of the four major effects 
categories. The REA uses case studies representing the broad geographic 
variability of the impacts from oxides of nitrogen and sulfur to 
conclude that there are ongoing adverse effects in many ecosystems from 
deposition of oxides of nitrogen and sulfur and that under current 
emissions scenarios these effects are likely to continue.
    Section II.C presents a discussion of adversity linking ecological 
effects to measures that can be used to characterize the extent to 
which such effects are reasonably considered to be adverse to public 
welfare. This involves consideration of how to characterize adversity 
from a public welfare perspective. In so doing, consideration is given 
to the concept of ecosystem services, the evidence of effects on 
ecosystem services, and how ecosystem services can be linked to 
ecological indicators.
    Section II.D presents an assessment of the adequacy of the current 
oxides of nitrogen and oxides of sulfur secondary standards. 
Consideration is given to the adequacy of protection afforded by the 
current standards for both direct and deposition-related effects, as 
well as to the appropriateness of the fundamental structure and the 
basic elements of the current standards for providing protection from 
deposition-related effects. Considerations as to the extent to which 
deposition-related effects that could reasonably be judged to be 
adverse to public welfare are occurring under current conditions which 
are allowed by the current standards is also considered. Discussion of 
the structures and basic elements of the current NO2 and 
SO2 secondary standards and whether they are adequate to 
protect against such effects is presented.

[[Page 46091]]

A. Ecological Effects

    This section discusses the known or anticipated ecological effects 
associated with oxides of nitrogen and sulfur, including the direct 
effects of gas-phase exposure to oxides of nitrogen and sulfur (section 
II.A.1) and effects associated with deposition-related exposure 
(sections II.A.2 and 3). Section II.A. 2 addresses effects related to 
acidification of aquatic and terrestrial ecosystems and section II A.3 
addresses effects related to nutrient enrichment of aquatic and 
terrestrial ecosystems. These sections also address questions about the 
nature and magnitude of ecosystem responses to reactive nitrogen and 
sulfur deposition, including responses related to acidification, 
nutrient depletion, and, in Section II.A 4 the mobilization of toxic 
metals in sensitive aquatic and terrestrial ecosystems. The 
uncertainties and limitations associated with the evidence of such 
effects are also discussed throughout this section.
1. Effects Associated With Gas-Phase Oxides of Nitrogen and Sulfur
    Ecological effects on vegetation as discussed in earlier reviews as 
well as the ISA can be attributed to gas-phase oxides of nitrogen and 
sulfur. Acute and chronic exposures to gaseous pollutants such as 
SO2, NO2, nitric oxide (NO), nitric acid 
(HNO3) and peroxyacetyl nitrite (PAN) are associated with 
negative impacts to vegetation. The current secondary NAAQS were set to 
protect against direct damage to vegetation by exposure to gas-phase 
oxides of nitrogen and sulfur, such as foliar injury, decreased 
photosynthesis, and decreased growth. The following summary is a 
concise overview of the known or anticipated effects to vegetation 
caused by gas phase nitrogen and sulfur. Most phototoxic effects 
associated with gas phase oxides of nitrogen and sulfur occur at levels 
well above ambient concentrations observed in the U.S. (US EPA, 2008, 
section 3.4.2.4).
a. Nature of Ecosystem Responses to Gas-Phase Nitrogen And Sulfur
    The 2008 ISA found that gas phase nitrogen and sulfur are 
associated with direct phytotoxic effects (US EPA, 2008, section 4.4). 
The evidence is sufficient to infer a causal relationship between 
exposure to SO2 and injury to vegetation (US EPA, 2008, 
section 4.4.1 and 3.4.2.1). Acute foliar injury to vegetation from 
SO2 may occur at levels above the current secondary standard 
(3-h average of 0.50 ppm). Effects on growth, reduced photosynthesis 
and decreased yield of vegetation are also associated with increased 
SO2 exposure concentration and time of exposure.
    The evidence is sufficient to infer a causal relationship between 
exposure to NO, NO2 and PAN and injury to vegetation (US 
EPA, 2008, section 4.4.2 and 3.4.2.2). At sufficient concentrations, 
NO, NO2 and PAN can decrease photosynthesis and induce 
visible foliar injury to plants. Evidence is also sufficient to infer a 
causal relationship between exposure to HNO3 and changes to 
vegetation (US EPA, 2008, section 4.4.3 and 3.4.2.3). Phytotoxic 
effects of this pollutant include damage to the leaf cuticle in 
vascular plants and disappearance of some sensitive lichen species.
b. Magnitude of Ecosystem Response to Gas-Phase Nitrogen And Sulfur
    Vegetation in ecosystems near sources of gaseous oxides of nitrogen 
and sulfur or where SO2, NO, NO2, PAN and 
HNO3 are most concentrated are more likely to be impacted by 
these pollutants. Uptake of these pollutants in a plant canopy is a 
complex process involving adsorption to surfaces (leaves, stems and 
soil) and absorption into leaves (US EPA, 2008, section 3.4.2). The 
functional relationship between ambient concentrations of gas phase 
oxides of nitrogen and sulfur and specific plant response are impacted 
by internal factors such as rate of stomatal conductance and plant 
detoxification mechanisms, and external factors including plant water 
status, light, temperature, humidity, and pollutant exposure regime (US 
EPA, 2008, section 3.4.2).
    Entry of gases into a leaf is dependent upon physical and chemical 
processes of gas phase as well as to stomatal aperture. The aperture of 
the stomata is controlled largely by the prevailing environmental 
conditions, such as water availability, humidity, temperature, and 
light intensity. When the stomata are closed, resistance to gas uptake 
is high and the plant has a very low degree of susceptibility to 
injury. Mosses and lichens do not have a protective cuticle barrier to 
gaseous pollutants or stomata and are generally more sensitive to 
gaseous sulfur and nitrogen than vascular plants (US EPA, 2008, section 
3.4.2).
    The appearance of foliar injury can vary significantly across 
species and growth conditions affecting stomatal conductance in 
vascular plants (US EPA, 2009, section 6.4.1). For example, damage to 
lichens from SO2 exposure includes decreased photosynthesis 
and respiration, damage to the algal component of the lichen, leakage 
of electrolytes, inhibition of nitrogen fixation, decreased potassium 
(K+) absorption, and structural changes.
    The phytotoxic effects of gas phase oxides of nitrogen and sulfur 
are dependent on the exposure concentration and duration and species 
sensitivity to these pollutants. Effects to vegetation associated with 
oxides of nitrogen and sulfur are therefore variable across the U.S. 
and tend to be higher near sources of photochemical smog. For example, 
SO2 is considered to be the primary factor contributing to 
the death of lichens in many urban and industrial areas.
    The ISA states there is very limited new research on phytotoxic 
effects of NO, NO2, PAN and HNO3 at 
concentrations currently observed in the U.S. with the exception of 
some lichen species (US EPA, 2008, section 4.4). Past and current 
HNO3 concentrations may be contributing to the decline in 
lichen species in the Los Angeles basin. Most phytotoxic effects 
associated with gas phase oxides of nitrogen and sulfur occur at levels 
well above ambient concentrations observed in the U.S. (US EPA, 2008, 
section 3.4.2.4).
2. Acidification Effects Associated With Deposition of Oxides of 
Nitrogen and Sulfur
    Sulfur oxides and nitrogen oxides in the atmosphere undergo a 
complex mix of reactions in gaseous, liquid, and solid phases to form 
various acidic compounds. These acidic compounds are removed from the 
atmosphere through deposition: either wet (e.g., rain, snow), fog or 
cloud, or dry (e.g., gases, particles). Deposition of these acidic 
compounds to ecosystems can lead to effects on ecosystem structure and 
function. Following deposition, these compounds can, in some instances, 
unless retained by soil or biota, leach out of the soils in the form of 
sulfate (SO42-) and nitrate 
(NO3-), leading to the acidification of surface 
waters. The effects on ecosystems depend on the magnitude and rate of 
deposition, as well as a host of biogeochemical processes occurring in 
the soils and water bodies (US EPA, 2009, section 2.1). The chemical 
forms of nitrogen that may contribute to acidifying deposition include 
both oxidized and reduced chemical species, including reduced forms of 
nitrogen (NHx).
    When sulfur or nitrogen leaches from soils to surface waters in the 
form of SO42- or NO3-, an 
equivalent amount of positive cations, or countercharge, is also 
transported. This maintains electroneutrality. If the countercharge is 
provided by base cations, such as

[[Page 46092]]

calcium (Ca\2+\), magnesium (Mg\2+\), sodium (Na\+\), or K\+\, rather 
than hydrogen (H\+\) and dissolved inorganic aluminum, the acidity of 
the soil water is neutralized, but the base saturation of the soil 
decreases. Continued SO4\2-\ or NO3- 
leaching can deplete the available base cation pool in soil. As the 
base cations are removed, continued deposition and leaching of 
SO42- and/or NO3- (with 
H\+\ and Al\3+\) leads to acidification of soil water, and by 
connection, surface water. Introduction of strong acid anions such as 
sulfate and nitrate to an already acidic soil, whether naturally or due 
to anthropogenic activities, can lead to instantaneous acidification of 
waterbodies through direct runoff without any significant change in 
base cation saturation. 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 
hydrologic flowpaths (US EPA, 2009, section 2.1). Some of these factors 
such as vegetation and soil depth are highly variable over small 
spatial scales such as meters, but can be aggregated to evaluate 
patterns over larger spatial scales. Acidifying deposition of oxides of 
nitrogen and sulfur and the chemical and biological responses 
associated with these inputs vary temporally. Chronic or long-term 
deposition processes in the time scale of years to decades result in 
increases in inputs of nitrogen and sulfur to ecosystems and the 
associated ecological effects. Episodic or short term (i.e., hours or 
days) deposition refers to events in which the level of the acid 
neutralizing capacity (ANC) of a lake or stream is temporarily lowered. 
In aquatic ecosystems, short-term (i.e., hours or days) episodic 
changes in water chemistry can have significant biological effects. 
Episodic acidification refers to conditions during precipitation or 
snowmelt events when proportionately more drainage water is routed 
through upper soil horizons that tend to provide less acid neutralizing 
than is passing through deeper soil horizons (US EPA, 2009, section 
4.2). In addition, the accumulated sulfate and nitrate in snow packs 
can provide a surge of acidic inputs. Some streams and lakes may have 
chronic or base flow chemistry that is suitable for aquatic biota, but 
may be subject to occasional acidic episodes with deleterious 
consequences to sensitive biota.
    The following summary is a concise overview of the known or 
anticipated effects caused by acidification to ecosystems within the 
U.S. Acidification affects both terrestrial and freshwater aquatic 
ecosystems.
a. Nature of Acidification-Related Ecosystem Responses
    The ISA concluded that deposition of oxides of nitrogen and sulfur 
and NHx leads to the varying degrees of acidification of 
ecosystems (US EPA, 2008). In the process of acidification, 
biogeochemical components of terrestrial and freshwater aquatic 
ecosystems are altered in a way that leads to effects on biological 
organisms. Deposition to terrestrial ecosystems often moves through the 
soil and eventually leaches into adjacent water bodies.
i. Aquatic Ecosystems
    The scientific evidence is sufficient to infer a causal 
relationship between acidifying deposition and effects on 
biogeochemistry and biota in aquatic ecosystems (US EPA, 2008, section 
4.2.2). The strongest evidence comes from studies of surface water 
chemistry in which acidic deposition is observed to alter sulfate and 
nitrate concentrations in surface waters, the sum of base cations, ANC, 
dissolved inorganic aluminum and pH (US EPA, 2008, section 3.2.3.2). 
The ANC is a key indicator of acidification with relevance to both 
terrestrial and aquatic ecosystems. The ANC is useful because it 
integrates the overall acid-base status of a lake or stream and 
reflects how aquatic ecosystems respond to acidic deposition over time. 
There is also a relationship between ANC and the surface water 
constituents that directly contribute to or ameliorate acidity-related 
stress, in particular, concentrations of hydrogen ion (as pH), Ca\2+\ 
and aluminum (Al). Moreover, low pH surface waters leach aluminum from 
soils, which is quite lethal to fish and other aquatic organisms. In 
aquatic systems, there is a direct relationship between ANC and fish 
and phyto-zooplankton diversity and abundance.
    Low ANC coincides with effects on aquatic systems (e.g., individual 
species fitness loss or death, reduced species richness, altered 
community structure). At the community level, species richness is 
positively correlated with pH and ANC because energy cost in 
maintaining physiological homeostasis, growth, and reproduction is high 
at low ANC levels. For example, there is a logistic relationship 
between fish species richness and ANC class for Adirondack Case Study 
Area lakes that indicates the probability of occurrence of an organism 
for a given value of ANC. Biota are generally not harmed when ANC 
values are >100 microequivalents per liter ([mu]eq/L). The number of 
fish species also peaks at ANC values >100 [mu]eq/L. Below 100 [mu]eq/L 
ANC, fish fitness and community diversity begin to decline (US EPA, 
section 4.2). Specifically at ANC levels between 100 and 50 [mu]eq/L, 
the fitness of sensitive species (e.g., brook trout, zooplankton) 
begins to decline. When ANC concentrations are <50 [mu]eq/L, they are 
generally associated with death or loss of fitness of biota that are 
sensitive to acidification.
    Consistent and coherent documentation from multiple studies on 
various species from all major trophic levels of aquatic systems shows 
that geochemical alteration caused by acidification can result in the 
loss of acid-sensitive biological species (US EPA, 2008, section 
3.2.3.3). This is most often discussed with relation to pH. For 
example, in the Adirondacks, of the 53 fish species recorded in 
Adirondack lakes about half (26 species) were absent from lakes with pH 
below 6.0. Biological effects are linked to changes in water chemistry 
including decreases in ANC and pH and increases in inorganic Al 
concentration. The direct biological effects are caused by lowered pH 
which leads to increased inorganic Al concentrations (US EPA, 2011, 
Figures 3-1 and 3-2). While ANC level does not cause direct biological 
harm it is a good overall indicator of the risk of acidification (US 
EPA, 2011, section 3.1.3).
    There are clear associations between ANC, pH and aquatic species 
mortality and health which are summarized in section 3.1.1 of the PA. 
Significant harm to sensitive aquatic species has been observed at pH 
levels below 6. Normal stream pH levels with little to no toxicity 
range from 6 to 7 (MacAvoy et al, 1995). Baker et al (1990) observed 
that ``lakes with pH less than approximately 6.0 contain significantly 
fewer species than lakes with pH levels above 6.0.'' As noted in 
Chapter 3, typically at pH <4.5 and an ANC <0 [mu]eq/L, complete to 
near-complete loss of many taxa of organisms occur, including fish and 
aquatic insect populations, whereas other taxa are reduced to only 
acidophilic species. Acid Neutralizing Capacity is a measure of how 
much acid can be neutralized in a specific surface water system. An ANC 
value of 0 or below means that surface waters have no ability to 
neutralize any additional acid inputs.
    Additional evidence can help refine the understanding of effects 
occurring at pH levels between 4.5 and 6. When pH levels are below 5.6, 
relatively lower trout survival rates were observed in the

[[Page 46093]]

Shenandoah National Park. In field observations, when pH levels dropped 
to 5, mortality rates went to 100 percent (Bulger et al, 2000). At pH 
levels ranging from 5.4 to 5.8, cumulative mortality continues to 
increase. Several studies have shown that trout exposed to water with 
varying pH levels and fish larvae showed increasing mortality as pH 
levels decrease. In one study almost 100 percent mortality was observed 
at a pH of 4.5 compared to almost 100 percent survival at a pH of 6.5. 
Intermediate pH values (6.0, 5.5) in all cases showed reduced survival 
compared with the control (6.5), but not by statistically significant 
amounts (US EPA, 2008, section 3.2.3.3).
    One important indicator of acid stress is increased fish mortality. 
The response of fish to pH is not uniform across species. A number of 
synoptic surveys indicated loss of species diversity and absence of 
several fish species in the pH range of 5.0 to 5.5. If pH is lower, 
there is a greater likelihood that more fish species could be lost 
without replacement, resulting in decreased richness and diversity. In 
general, populations of salmonids are not found at pH levels less than 
5.0, and smallmouth bass (Micropterus dolomieu) populations are usually 
not found at pH values less than about 5.2 to 5.5. From Table 3-1, only 
one study showed significant mortality effects above a pH of 6, while a 
number of studies showed significant mortality when pH levels are at or 
below 5.5.
    The highest pH level for any of the studies reported in the ISA is 
6.0, suggesting that pH above 6.0 is protective against mortality 
effects for most species. Most thresholds are in the range of pH of 5.0 
to 6.0, which suggests that a target pH should be no lower than 5.0. 
Protection against mortality in some recreationally important species 
such as lake trout (pH threshold of 5.6) and crappie (pH threshold of 
5.5), combined with the evidence of effects on larval and embryo 
survival suggests that pH levels greater than 5.5 should be targeted to 
provide protection against mortality effects throughout the life stages 
of fish.
    Non-lethal effects have been observed at pH levels as high as 6. A 
study in the Shenandoah National Park found that the condition factor, 
a measure of fish health expressed as fish weight/length multiplied by 
a scaling constant, is positively correlated with stream pH levels, and 
that the condition factor is reduced in streams with a pH of 6.0 (US 
EPA, 2008, section 3.2.3.3).
    Biodiversity is another indicator of aquatic ecosystem health. A 
key study in the Adirondacks found that lakes with a pH of 6.0 had only 
half the potential species of fish (27 of 53 potential species). There 
is often a positive relationship between pH and number of fish species, 
at least for pH values between about 5.0 and 6.5, or ANC values between 
about 0 to 100 [mu]eq/L. Such observed relationships are complicated, 
however, by the tendency for smaller lakes and streams, having smaller 
watersheds, to also support fewer fish species, irrespective of acid-
base chemistry. This pattern may be due to a decrease in the number of 
available niches as stream or lake size decreases. Nevertheless, fish 
species richness is relatively easily determined and is one of the most 
useful indicators of biological effects of surface water acidification.
    Changes in stream water pH and ANC also contribute to declines in 
taxonomic richness of zooplankton, and macroinvertebrates which are 
often sources of food for fish, birds and other animal species in 
various ecosystems. These fish may also serve as a source of food and 
recreation for humans. Acidification of ecosystems has been shown to 
disrupt food web dynamics causing alteration to the diet, breeding 
distribution, and reproduction of certain species of birds (US EPA, 
2008, section 4.2.2.2. and Table 3-9). For example, breeding 
distribution of the common goldeneye (Bucephala clangula), an 
insectivorous duck, may be affected by changes in acidifying 
deposition. Similarly, decreases in prey diversity and quantity have 
been observed to create feeding problems for nesting pairs of loons on 
low-pH lakes in the Adirondacks.
ii. Terrestrial Ecosystems
    In terrestrial ecosystems, the evidence is sufficient to infer a 
causal relationship between acidifying deposition and changes in 
biogeochemistry (US EPA, 2008, section 4.2.1.1). The strongest evidence 
comes from studies of forested ecosystems, with supportive information 
on other plant taxa, including shrubs and lichens (US EPA, 2008, 
section 3.2.2.1.). Three useful indicators of chemical changes and 
acidification effects on terrestrial ecosystems, showing consistency 
and coherence among multiple studies are: soil base saturation, Al 
concentrations in soil water, and soil carbon to nitrogen (C:N) ratio 
(US EPA, 2008, section 3.2.2.2).
    As discussed in the ISA and REA, in soils with base saturation less 
than about 15 to 20 percent, exchange chemistry is dominated by Al. 
Under these conditions, responses to inputs of sulfuric acid and 
HNO3 largely involve the release and mobilization of 
dissolved inorganic Al. The effect can be neutralized by weathering 
from geologic parent material or base cation exchange. The Ca\2+\ and 
Al concentrations in soil water are strongly influenced by soil 
acidification and both have been shown to have quantitative links to 
tree health, including Al interference with Ca\2+\ uptake and Al 
toxicity to roots. Effects of nitrification and associated 
acidification and cation leaching have been consistently shown to occur 
only in soils with a C:N ratio below about 20 to 25.
    Soil acidification caused by acidic deposition has been shown to 
cause decreased growth and increased susceptibility to disease and 
injury in sensitive tree species. Red spruce (Picea rubens) dieback or 
decline has been observed across high elevation areas in the 
Adirondack, Green and White mountains. The frequency of freezing injury 
to red spruce needles has increased over the past 40 years, a period 
that coincided with increased emissions of sulfur and nitrogen oxides 
and increased acidifying deposition. Acidifying deposition can 
contribute to dieback in sugar maple (Acer saccharum) through depletion 
of cations from soil with low levels of available Ca. Grasslands are 
likely less sensitive to acidification than forests due to grassland 
soils being generally rich in base cations.
iii. Ecosystem Sensitivity
    The intersection between current deposition loading, historic 
loading and sensitivity defines the ecological vulnerability to the 
effects of acidification. Freshwater aquatic and some terrestrial 
ecosystems, notably forests, are the ecosystem types which are most 
sensitive to acidification. The ISA reports that the principal factor 
governing the sensitivity of terrestrial and aquatic ecosystems to 
acidification from sulfur and nitrogen deposition is geology 
(particularly surficial geology). Geologic formations having low base 
cation supply generally underlie the watersheds of acid-sensitive lakes 
and streams. Other factors that contribute to the sensitivity of soils 
and surface waters to acidifying deposition include topography, soil 
chemistry, land use, and hydrologic flowpaths. Episodic and chronic 
acidification tends to occur in areas that have base-poor bedrock, high 
relief, and shallow soils (US EPA, 2008, section 3.2.4.1).
b. Magnitude of Acidification-Related Ecosystem Responses
    Terrestrial and aquatic ecosystems differ in their response to 
acidifying

[[Page 46094]]

deposition. Therefore the magnitude of ecosystem response is described 
separately for aquatic and terrestrial ecosystems in the following 
sections. The magnitude of response refers to both the severity of 
effects and the spatial extent of the U.S. which is affected.
i. Aquatic Acidification
    Freshwater ecosystem surveys and monitoring in the eastern U.S. 
have been conducted by many programs since the mid-1980s, including 
EPA's Environmental Monitoring and Assessment Program (EMAP), National 
Surface Water Survey (NSWS), Temporally Integrated Monitoring of 
Ecosystems (TIME), and Long-term Monitoring (LTM) programs. Based on 
analyses of surface water data from these programs, New England, the 
Adirondack Mountains, the Appalachian Mountains (northern Appalachian 
Plateau and Ridge/Blue Ridge region) and the Upper Midwest contain the 
most sensitive lakes and streams (i.e., ANC less than about 50 [mu]eq/
L). Portions of northern Florida also contain many acidic and low-ANC 
lakes and streams, although the role of acidifying deposition in this 
region is less clear. The western U.S. contains many of the surface 
waters most sensitive to potential acidification effects, but with the 
exception of the Los Angeles Basin and surrounding areas, the levels of 
acidifying deposition are low in most areas. Therefore, acidification 
of surface waters by acidic deposition is not as prevalent in the 
western U.S., and the extent of chronic surface water acidification 
that has occurred in that region to date has likely been very limited 
relative to the Eastern U.S. (US EPA, 2008, section 3.2.4.2 and US EPA, 
2009, section 4.2.2).
    There are a number of species including fish, aquatic insects, 
other invertebrates and algae that are sensitive to acidification and 
cannot survive, compete or reproduce in acidic waters (US EPA, 2008, 
section 3.2.3.3). Decreases in ANC and pH have been shown to contribute 
to declines in species richness and declines in abundance of 
zooplankton, macroinvertebrates, and fish. Reduced growth rates have 
been attributed to acid stress in a number of fish species including 
Atlantic salmon (Salmo salar), Chinook salmon (Oncorhynchus 
tshawytscha), lake trout (Salvelinus namaycush), rainbow trout 
(Oncorhynchis mykiss), brook trout (Salvelinus Fontinalis), and brown 
trout (Salmo trutta). In response to small to moderate changes in 
acidity, acid-sensitive species are often replaced by other more acid-
tolerant species, resulting in changes in community composition and 
richness. The effects of acidification are continuous, with more 
species being affected at higher degrees of acidification. At a point, 
typically a pH <4.5 and an ANC <0 [mu]eq/L, complete to near-complete 
loss of many taxa of organisms occur, including fish and aquatic insect 
populations, whereas other taxa are reduced to only acidophilic 
species. These changes in taxa composition are associated with the high 
energy cost in maintaining physiological homeostasis, growth, and 
reproduction at low ANC levels (US EPA, 2008, section 3.2.3.3). 
Decreases in species richness related to acidification have been 
observed in the Adirondack Mountains and Catskill Mountains of New 
York, New England and Pennsylvania, and Virginia. From the sensitive 
areas identified by the ISA, further ``case study'' analyses on aquatic 
ecosystems in the Adirondack Mountains and Shenandoah National Park 
were conducted to better characterize ecological risk associated with 
acidification (US EPA, 2009, section 4).
    The ANC is the most widely used indicator of acid sensitivity and 
has been found in various studies to be the best single indicator of 
the biological response and health of aquatic communities in acid-
sensitive systems (Lien et al., 1992; Sullivan et al., 2006; US EPA, 
2008). In the REA, surface water trends in SO42- 
and NO3- concentrations and ANC levels were 
analyzed to affirm the understanding that reductions in deposition 
could influence the risk of acidification. The ANC values have been 
categorized according to their effects on biota, as shown in the table 
below. Monitoring data from TIME/LTM and EMAP programs were assessed 
for the years 1990 to 2006, and past, present and future water quality 
levels were estimated by both steady-state and dynamic biogeochemical 
models.

  Table II-1--Ecological Effects Associated With Alternative Levels of
                    Acid Neutralizing Capacity (ANC)
                   [Source: USEPA, Acid Rain Program]
------------------------------------------------------------------------
 
------------------------------------------------------------------------
        Category Label ANC Levels and Expected Ecological Effects
------------------------------------------------------------------------
Acute Concern.................  <0 [mu]eq/L......  Complete loss of fish
                                                    populations is
                                                    expected. Planktonic
                                                    communities have
                                                    extremely low
                                                    diversity and are
                                                    dominated by
                                                    acidophilic taxa.
                                                    The numbers of
                                                    individuals in
                                                    plankton species
                                                    that are present are
                                                    greatly reduced.
Severe Concern................  0-20 [mu]eq/L....  Highly sensitive to
                                                    episodic
                                                    acidification.
                                                    During episodes of
                                                    high acidifying
                                                    deposition, brook
                                                    trout populations
                                                    may experience
                                                    lethal effects. The
                                                    diversity and
                                                    distribution of
                                                    zooplankton
                                                    communities decline
                                                    sharply.
Elevated Concern..............  20-50 [mu]eq/L...  Fish species richness
                                                    is greatly reduced
                                                    (i.e., more than
                                                    half of expected
                                                    species can be
                                                    missing). On
                                                    average, brook trout
                                                    populations
                                                    experience sublethal
                                                    effects, including
                                                    loss of health,
                                                    ability to
                                                    reproduce, and
                                                    fitness. Diversity
                                                    and distribution of
                                                    zooplankton
                                                    communities decline.
Moderate Concern..............  50-100 [mu]eq/L..  Fish species richness
                                                    begins to decline
                                                    (i.e., sensitive
                                                    species are lost
                                                    from lakes). Brook
                                                    trout populations
                                                    are sensitive and
                                                    variable, with
                                                    possible sublethal
                                                    effects. Diversity
                                                    and distribution of
                                                    zooplankton
                                                    communities also
                                                    begin to decline as
                                                    species that are
                                                    sensitive to
                                                    acidifying
                                                    deposition are
                                                    affected.
Low Concern...................  >100 [mu]eq/L....  Fish species richness
                                                    may be unaffected.
                                                    Reproducing brook
                                                    trout populations
                                                    are expected where
                                                    habitat is suitable.
                                                    Zooplankton
                                                    communities are
                                                    unaffected and
                                                    exhibit expected
                                                    diversity and
                                                    distribution.
------------------------------------------------------------------------


[[Page 46095]]

    Studies on fish species richness in the Adirondacks Case Study Area 
demonstrated the effect of acidification. Of the 53 fish species 
recorded in Adirondack Case Study Area lakes, only 27 species were 
found in lakes with a pH <6.0. The 26 species missing from lakes with a 
pH <6.0 include important recreational species, such as Atlantic 
salmon, tiger trout (Salmo trutta X Salvelinus fontinalis), redbreast 
sunfish (Lepomis auritus), bluegill (Lepomis macrochirus), tiger musky 
(Esox masquinongy X lucius), walleye (Sander vitreus), alewife (Alosa 
pseudoharengus), and kokanee (Oncorhynchus nerka), as well as 
ecologically important minnows that are commonly consumed by sport 
fish. A survey of 1,469 lakes in the late 1980s found 346 lakes to be 
devoid of fish. Among lakes with fish, there was a relationship between 
the number of fish species and lake pH, ranging from about one species 
per lake for lakes having a pH <4.5 to about six species per lake for 
lakes having a pH >6.5. In the Adirondacks, a positive relationship 
exists between the pH and ANC in lakes and the number of fish species 
present in those lakes (US EPA, 2008, section 3.2.3.4).
    Since the mid-1990s, streams in the Shenandoah Case Study Area have 
shown slight declines in NO3- and 
SO42- concentrations in surface waters. The 2006 
concentrations are still above pre-acidification (1860) conditions. 
Model of Acidification of Groundwater in Catchments (MAGIC) modeling 
predicts surface water concentrations of NO3- and 
SO42- are 10- and 32-fold higher, respectively, 
in 2006 than in 1860. The estimated average ANC across 60 streams in 
the Shenandoah Case Study Area is 57.9 [mu]eq/L ( 4.5 
[mu]eq/L). Fifty-five percent of all monitored streams in the 
Shenandoah Case Study Area have a current risk of Elevated, Severe, or 
Acute. Of the 55 percent, 18 percent are chronically acidic today (US 
EPA, 2009, section 4.2.4.3).
    Based on a deposition scenario for this study area that maintains 
current emission levels from 2020 to 2050, the simulation forecast 
indicates that a large number of streams would still have Elevated to 
Acute problems with acidity in 2050.
    Biological effects of increased acidification documented in the 
Shenandoah Case Study Area include a decrease in the condition factor 
in blacknose dace and a decrease in fish biodiversity associated with 
decreasing stream ANC. On average, the fish species richness is lower 
by one fish species for every 21 [mu]eq/L decrease in ANC in Shenandoah 
National Park streams (US EPA, 2008, section 3.2.3.4).
ii. Terrestrial Acidification
    The ISA identified a variety of indicators that can be used to 
measure the effects of acidification in soils. Most effects of 
terrestrial acidification are observed in sensitive forest ecosystem in 
the U.S. Tree health has been linked to the availability of base 
cations (BC) in soil (such as Ca\2+\, Mg\2+\ and K\+\), as well as soil 
aluminum (Al) content. Tree species show a range of sensitivities to 
Ca/Al and BC/Al soil molar ratios, therefore these are good chemical 
indicators because they directly relate to the biological effects. 
Critical BC/Al molar ratios for a large variety of tree species ranged 
from 0.2 to 0.8. This range is similar to critical ratios of Ca/Al. 
Plant toxicity or nutrient antagonism was reported to occur at Ca/Al 
molar ratios ranging from 0.2 to 2.5 (US EPA, 2009).
    There has been no systematic national survey of terrestrial 
ecosystems to determine the extent and distribution of terrestrial 
ecosystem sensitivity to the effects of acidifying deposition. However, 
one preliminary national evaluation estimated that ~15 percent of 
forest ecosystems in the U.S. exceed the estimated critical load based 
on soil ANC leaching for sulfur and nitrogen deposition by >250 eq/ha/
yr (McNulty et al., 2007). Forests of the Adirondack Mountains of New 
York, Green Mountains of Vermont, White Mountains of New Hampshire, the 
Allegheny Plateau of Pennsylvania and high-elevation forest ecosystems 
in the southern Appalachians are the regions most sensitive to 
terrestrial acidification effects from acidifying deposition (US EPA, 
2008, section 3.2.4.2). While studies show some recovery of surface 
waters, there are widespread measurements of ongoing depletion of 
exchangeable base cations in forest soils in the northeastern U.S. 
despite recent decreases in acidifying deposition, indicating a slow 
recovery time.
    In the REA, a critical load analysis was performed for sugar maple 
and red spruce forests in the eastern U.S. by using BC/Al ratio in 
acidified forest soils as an indicator to assess the impact of nitrogen 
and sulfur deposition on tree health. These are the two most commonly 
studied tree species in North America for effects of acidification. At 
a BC/Al ratio of 1.2, red spruce growth can be decreased by 20 percent. 
Sugar maple growth can be decreased by 20 percent at a BC/Al ratio of 
0.6 (US EPA, 2009, section 4.4). The REA analysis determined the health 
of at least a portion of the sugar maple and red spruce growing in the 
U.S. may have been compromised with acidifying total nitrogen and 
sulfur deposition. Specifically, total nitrogen and sulfur deposition 
levels exceeded three selected critical loads for tree growth in 3 
percent to 75 percent of all sugar maple plots across 24 states--that 
is, it exceeded the highest (least stringent) of the three critical 
loads in 3 percent of plots, and the lowest (most stringent) in 75 
percent of plots. For red spruce, total nitrogen and sulfur deposition 
levels exceeded three selected critical loads in 3 percent to 36 
percent of all red spruce plots across eight states (US EPA, 2009, 
section 4.4).
c. Key Uncertainties Associated With Acidification
    There are different levels of uncertainty associated with 
relationships between deposition, ecological effects and ecological 
indicators. In Chapter 7 of the REA, the case study analyses associated 
with each targeted effect area were synthesized by identifying the 
strengths, limitations, and uncertainties associated with the available 
data, modeling approach, and relationship between the selected 
ecological indicator and atmospheric deposition as described by the 
ecological effect function (US EPA, 2009, Figure 1-1). A further 
discussion of uncertainty in aquatic and terrestrial ecosystems is 
presented below. The key uncertainties were characterized as follows to 
evaluate the strength of the scientific basis for setting a national 
standard to protect against a given effect (US EPA, 2009, section 7):
    (1) Data Availability: High, medium or low quality. This criterion 
is based on the availability and robustness of data sets, monitoring 
networks, availability of data that allows for extrapolation to larger 
assessment areas and input parameters for modeling and developing the 
ecological effect function. The scientific basis for the ecological 
indicator selected is also incorporated into this criterion.
    (2) Modeling Approach: High, fairly high, intermediate, or low 
confidence. This value is based on the strengths and limitations of the 
models used in the analysis and how accepted they are by the scientific 
community for their application in this analysis.
    (3) Ecological Effect Function: High, fairly high, intermediate or 
low confidence. This ranking is based on how well the ecological effect 
function describes the relationship between atmospheric deposition and 
the ecological indicator of an effect.

[[Page 46096]]

i. Aquatic Acidification
    The REA concludes that the available data are robust and considered 
high quality. There is high confidence about the use of these data and 
their value for extrapolating to a larger regional population of lakes. 
The EPA TIME/LTM network represents a source of long-term, 
representative sampling. Data on sulfate concentrations, nitrate 
concentrations and ANC from 1990 to 2006 used for this analysis as well 
as EPA EMAP and Regional Environmental Monitoring and Assessment 
Program (REMAP) surveys, provide considerable data on surface water 
trends.
    There is fairly high confidence associated with modeling and input 
parameters. Uncertainties in water quality estimates (i.e., ANC) from 
MAGIC were derived from multiple site calibrations. Pre-acidification 
refers to retrospective modeling to estimate water quality conditions 
before man-made contributions of acidifying inputs. The models are 
evaluated under current conditions to determine how well they replicate 
observed ANC values. The 95 percent confidence interval for pre-
acidification of lakes was an average of 15 [micro]eq/L difference in 
ANC concentrations, or 10 percent, and 8 [micro]eq/L, or 5 percent, for 
streams (US EPA, 2009, section 7.1.2). The use of the critical load 
model to estimate aquatic critical loads is limited by the 
uncertainties associated with runoff and surface water measurements and 
in estimating the catchment supply of base cations from the weathering 
of bedrock and soils (McNulty et al., 2007).
ii. Terrestrial Acidification
    The available data used to quantify the targeted effect of 
terrestrial acidification are robust and considered high quality. The 
U.S. Forest Service-Kane Experimental Forest and significant amounts of 
research work in the Allegheny Plateau have produced extensive, peer-
reviewed data sets. Sugar maple and red spruce were the focus of the 
REA since they are demonstrated to be negatively affected by soil 
available Ca\2+\ depletion and high concentrations of available Al, and 
occur in areas that receive high acidifying deposition. There is high 
confidence about the use of the REA terrestrial acidification data and 
their value for extrapolating to a larger regional population of 
forests.
    There is high confidence associated with the models, input 
parameters, and assessment of uncertainty used in the case study for 
terrestrial acidification. The Simple Mass Balance (SMB) model, a 
commonly used and widely applied approach for estimating critical 
loads, was used in the REA analysis (US EPA, 2008, section 7.2.2). 
There is fairly high confidence associated with the ecological effect 
function developed for terrestrial acidification (US EPA, 2009, section 
7.2.3).
3. Nutrient Enrichment Effects Associated With Deposition of Oxides of 
Nitrogen
    The following summary is a concise overview of the known or 
anticipated effects caused by nitrogen nutrient enrichment to 
ecosystems within the United States. Nutrient-enrichment affects 
terrestrial, freshwater and estuarine ecosystems. Nitrogen deposition 
is a major source of anthropogenic nitrogen. For many terrestrial and 
freshwater ecosystems other sources of nitrogen including fertilizer 
and waste treatment are greater than deposition. Nitrogen deposition 
often contributes to nitrogen-enrichment effects in estuaries, but does 
not drive the effects since other sources of nitrogen greatly exceed 
nitrogen deposition. Both oxides of nitrogen and NHX 
contribute to nitrogen deposition. For the most part, nitrogen effects 
on ecosystems do not depend on whether the nitrogen is in oxidized or 
reduced form. Thus, this summary focuses on the effects of nitrogen 
deposition in total.
a. Nature of Nutrient Enrichment-Related Ecosystem Responses
    The ISA found that deposition of nitrogen, including oxides of 
nitrogen and NHX, leads to the nitrogen enrichment of 
ecosystems (US EPA 2008). In the process of nitrogen enrichment, 
biogeochemical components of terrestrial and freshwater aquatic 
ecosystems are altered in a way that leads to effects on biological 
organisms.
i. Aquatic Ecosystems
    In freshwater ecosystems, the evidence is sufficient to infer a 
causal relationship between nitrogen deposition and the alteration of 
biogeochemical cycling in freshwater aquatic ecosystems (US EPA, 2008, 
section 3.3.2.3). Nitrogen deposition is the main source of nitrogen 
enrichment to headwater streams, lower order streams and high elevation 
lakes. The most common chemical indicators that were studied included 
NO32- and dissolved inorganic nitrogen (DIN) 
concentration in surface waters as well as the ratio of chlorophyll a 
to total phosphorus. Elevated surface water NO3- 
concentrations occur in both the eastern and western U.S. Studies 
report a significant correlation between nitrogen deposition and lake 
biogeochemistry by identifying a correlation between wet deposition and 
DIN and the ratio of chlorophyll a to total phosphate. Recent evidence 
provides examples of lakes and streams that are limited by nitrogen and 
show signs of eutrophication in response to nitrogen addition.
    The evidence is sufficient to infer a causal relationship between 
nitrogen deposition and the alteration of species richness, species 
composition and biodiversity in freshwater aquatic ecosystems (US EPA, 
2008, section 3.3.5.3). Increased nitrogen deposition can cause a shift 
in community composition and reduce algal biodiversity, especially in 
sensitive oligotrophic lakes.
    In the ISA, the evidence is sufficient to infer a causal 
relationship between nitrogen deposition and the biogeochemical cycling 
of nitrogen and carbon in estuaries (US EPA, 2008, section 4.3.4.1 and 
3.3.2.3). In general, estuaries tend to be nitrogen-limited, and many 
currently receive high levels of nitrogen input from human activities 
(US EPA, 2009, section 5.1.1). It is unknown if atmospheric deposition 
alone is sufficient to cause eutrophication; however, the contribution 
of atmospheric nitrogen deposition to total nitrogen load is calculated 
for some estuaries and can be >40 percent (US EPA, 2009, section 
5.1.1).
    The evidence is sufficient to infer a causal relationship between 
nitrogen deposition and the alteration of species richness, species 
composition and biodiversity in estuarine ecosystems (US EPA, 2008, 
section 4.3.4.2 and 3.3.5.4). Atmospheric and non-atmospheric sources 
of nitrogen contribute to increased phytoplankton and algal 
productivity, leading to eutrophication. Shifts in community 
composition, reduced hypolimnetic dissolved oxygen (DO), decreases in 
biodiversity, and mortality of submerged aquatic vegetation are 
associated with increased N deposition in estuarine systems.
ii. Terrestrial Ecosystems
    The evidence is sufficient to infer a causal relationship between 
nitrogen deposition and the alteration of biogeochemical cycling in 
terrestrial ecosystems (US EPA, 2008, section 4.3.1.1 and 3.3.2.1). 
This is supported by numerous observational, deposition gradient and 
field addition experiments in sensitive ecosystems. The leaching of 
NO3- in soil drainage waters and the export of 
NO3- in stream water were identified as two of 
the primary

[[Page 46097]]

indictors of nitrogen enrichment. Several nitrogen-addition studies 
indicate that NO3- leaching is induced by chronic 
additions of nitrogen. Studies identified in the ISA found that surface 
water NO3- concentrations exceeded 1 [mu]eq/L in 
watersheds receiving about 9 to 13 kg N/ha/yr of atmospheric nitrogen 
deposition. Nitrogen deposition disrupts the nutrient balance of 
ecosystems with numerous biogeochemical effects. The chemical 
indicators that are typically measured include 
NO3- leaching, soil C:N ratio, rates of nitrogen 
mineralization, nitrification, denitrification, foliar nitrogen 
concentration, and soil water NO3- and 
NH4+ concentrations. Note that nitrogen 
saturation (nitrogen leaching from ecosystems) does not need to occur 
to cause effects. Substantial leaching of NO3- 
from forest soils to stream water can acidify downstream waters, 
leading to effects described in the previous section on aquatic 
acidification. Due to the complexity of interactions between the 
nitrogen and carbon cycling, the effects of nitrogen on carbon budgets 
(quantified input and output of carbon to the ecosystem) are variable. 
Regional trends in net ecosystem productivity (NEP) of forests (not 
managed for silviculture) have been estimated through models based on 
gradient studies and meta-analysis. Atmospheric nitrogen deposition has 
been shown to cause increased litter accumulation and carbon storage in 
above-ground woody biomass. In the West, this has lead to increased 
susceptibility to more severe fires. Less is known regarding the 
effects of nitrogen deposition on carbon budgets of non-forest 
ecosystems.
    The evidence is sufficient to infer a causal relationship between 
nitrogen deposition on the alteration of species richness, species 
composition and biodiversity in terrestrial ecosystems (US EPA, 2008, 
section 4.3.1.2). Some organisms and ecosystems are more sensitive to 
nitrogen deposition and effects of nitrogen deposition are not observed 
in all habitats. The most sensitive terrestrial taxa to nitrogen 
deposition are lichens. Empirical evidence indicates that lichens in 
the U.S. are affected by deposition levels as low as 3 kg N/ha/yr. 
Alpine ecosystems are also sensitive to nitrogen deposition; changes in 
an individual species (Carex rupestris) were estimated to occur at 
deposition levels near 4 kg N/ha/yr and modeling indicates that 
deposition levels near 10 kg N/ha/yr alter plant community assemblages. 
In several grassland ecosystems, reduced species diversity and an 
increase in non-native, invasive species are associated with nitrogen 
deposition.
iii. Ecosystem Sensitivity to Nutrient Enrichment
    The numerous ecosystem types that occur across the U.S. have a 
broad range of sensitivity to nitrogen deposition (US EPA, 2008, Table 
4-4). Increased deposition to nitrogen-limited ecosystems can lead to 
production increases that may be either beneficial or adverse depending 
on the system and management goals.
    Organisms in their natural environment are commonly adapted to a 
specific regime of nutrient availability. Change in the availability of 
one important nutrient, such as nitrogen, may result in an imbalance in 
ecological stoichiometry, with effects on ecosystem processes, 
structure and function. In general, nitrogen deposition to terrestrial 
ecosystems causes accelerated growth rates in some species deemed 
desirable in commercial forests but may lead to altered competitive 
interactions among species and nutrient imbalances, ultimately 
affecting biodiversity. The onset of these effects occurs with nitrogen 
deposition levels as low as 3 kg N/ha/yr in sensitive terrestrial 
ecosystems to nitrogen deposition. In aquatic ecosystems, nitrogen that 
is both leached from the soil and directly deposited to the water 
surface can pollute the surface water. This causes alteration of the 
diatom community at levels as low as 1.5 kg N/ha/yr in sensitive 
freshwater ecosystems.
    The degree of ecosystem effects lies at the intersection of 
nitrogen loading and nitrogen-sensitivity. Nitrogen-sensitivity is 
predominately driven by the degree to which growth is limited by 
nitrogen availability. Grasslands in the western U.S. are typically 
nitrogen-limited ecosystems dominated by a diverse mix of perennial 
forbs and grass species. A meta-analysis discussed in the ISA (US EPA, 
2008, section 3.3.3), indicated that nitrogen fertilization increased 
aboveground growth in all non-forest ecosystems except for deserts. In 
other words, almost all terrestrial ecosystems are nitrogen-limited and 
will be altered by the addition of anthropogenic nitrogen. Likewise, a 
freshwater lake or stream must be nitrogen-limited to be sensitive to 
nitrogen-mediated eutrophication. There are many examples of fresh 
waters that are nitrogen-limited or nitrogen and phosphorous (P) co-
limited (US EPA, 2008, section 3.3.3.2). A large dataset meta-analysis 
discussed in the ISA (US EPA, 2008, section 3.3.3.2), found that 
nitrogen-limitation occurred as frequently as phosphorous-limitation in 
freshwater ecosystems. Additional factors that govern the sensitivity 
of ecosystems to nutrient enrichment from nitrogen deposition include 
rates and form of nitrogen deposition, elevation, climate, species 
composition, plant growth rate, length of growing season, and soil 
nitrogen retention capacity (US EPA, 2008, section 4.3). Less is known 
about the extent and distribution of the terrestrial ecosystems in the 
U.S. that are most sensitive to the effects of nutrient enrichment from 
atmospheric nirogen deposition compared to acidification.
    Because the productivity of estuarine and near shore marine 
ecosystems is generally limited by the availability of nitrogen, they 
are susceptible to the eutrophication effect of nitrogen deposition (US 
EPA, 2008, section 4.3.4.1). A recent national assessment of eutrophic 
conditions in estuaries found the most eutrophic estuaries were 
generally those that had large watershed-to-estuarine surface area, 
high human population density, high rainfall and runoff, low dilution 
and low flushing rates. In the REA, the National Oceanic and 
Atmospheric Administration's (NOAA) National Estuarine Eutrophication 
Assessment (NEEA) assessment tool, Assessment of Estuarine Tropic 
Status (ASSETS) categorical Eutrophication Index (EI) was used to 
evaluate eutrophication due to atmospheric loading of nitrogen. The 
ASSETS EI is an estimation of the likelihood that an estuary is 
experiencing eutrophication or will experience eutrophication based on 
five ecological indicators: Chlorophyll a, macroalgae, dissolved 
oxygen, nuisance/toxic algal blooms and submerged aquatic vegetation 
(SAV).
    In the REA, two regions were selected for case study analysis using 
ASSETS EI, the Chesapeake Bay and Pamlico Sound. Both regions received 
an ASSETS EI rating of Bad indicating that the estuary had moderate to 
high pressure due to overall human influence and a moderate high to 
high eutrophic condition (US EPA, 2009, sections 5.2.4.1 and 5.2.4.2). 
These results were then considered with SPAtially Referenced Regression 
on Watershed Attributes (SPARROW) modeling to develop a response curve 
to examine the role of atmospheric nitrogen deposition in achieving a 
desired decrease in load. To change the Neuse River Estuary's EI score 
from Bad to Poor not only must 100 percent of the total atmospheric 
nitrogen deposition be eliminated, but considerably more nitrogen from 
other sources as well must be controlled (US EPA, 2009, section 
5.2.7.2). In the Potomac River estuary, a 78 percent

[[Page 46098]]

decrease of total nitrogen could move the EI score from Bad to Poor (US 
EPA, 2009, section 5.2.7.1). The results of this analysis indicated 
decreases in atmospheric deposition alone could not eliminate coastal 
eutrophication problems due to multiple non-atmospheric nitrogen inputs 
(US EPA, 2009, section 7.3.3). However, the somewhat arbitrary 
discreteness of the EI scale can mask the benefits of decreases in 
nitrogen between categories.
    In general, estuaries tend to be nitrogen-limited, and many 
currently receive high levels of nitrogen input from human activities 
to cause eutrophication. As reported in the ISA (US EPA, 2008, section 
3.2.2.2), atmospheric nitrogen loads to estuaries in the U.S. are 
estimated to range from 2 to 8 percent for Guadalupe Bay, Texas on the 
lowest end to as high as 72 percent for St. Catherines-Sapelo estuary, 
Georgia. The Chesapeake Bay is an example of a large, well-studied and 
severely eutrophic estuary that is calculated to receive as much as 30 
percent of its total nitrogen load from the atmosphere.
b. Magnitude of Ecosystem Responses
i. Aquatic Ecosystems
    The magnitude of ecosystem response may be thought of on two time 
scales, current conditions and how ecosystems have been altered since 
the onset of anthropogenic nitrogen deposition. As noted previously, 
studies found that nitrogen-limitation occurs as frequently as 
phosphorous-limitation in freshwater ecosystems (US EPA, 2008, section 
3.3.3.2). Recently, a comprehensive study of available data from the 
northern hemisphere surveys of lakes along gradients of nitrogen 
deposition show increased inorganic nitrogen concentration and 
productivity to be correlated with atmospheric nitrogen deposition. The 
results are unequivocal evidence of nitrogen limitation in lakes with 
low ambient inputs of nitrogen, and increased nitrogen concentrations 
in lakes receiving nitrogen solely from atmospheric nitrogen 
deposition. It has been suggested that most lakes in the northern 
hemisphere may have originally been nitrogen-limited, and that 
atmospheric nitrogen deposition has changed the balance of nitrogen and 
phosphorous in lakes.
    Available data suggest that the increases in total nitrogen 
deposition do not have to be large to elicit an ecological effect. For 
example, a hindcasting exercise determined that the change in Rocky 
Mountain National Park lake algae that occurred between 1850 and 1964 
was associated with an increase in wet nitrogen deposition that was 
only about 1.5 kg N/ha. Similar changes inferred from lake sediment 
cores of the Beartooth Mountains of Wyoming also occurred at about 1.5 
kg N/ha deposition. Pre-industrial inorganic nitrogen deposition is 
estimated to have been only 0.1 to 0.7 kg N/ha based on measurements 
from remote parts of the world. In the western U.S., pre-industrial, or 
background, inorganic nitrogen deposition was estimated by to range 
from 0.4 to 0.7 kg N/ha/yr.
    Eutrophication effects from nitrogen deposition are most likely to 
be manifested in undisturbed, low nutrient surface waters such as those 
found in the higher elevation areas of the western U.S. The most severe 
eutrophication from nitrogen deposition effects is expected downwind of 
major urban and agricultural centers. High concentrations of lake or 
streamwater NO3-, indicative of ecosystem 
saturation, have been found at a variety of locations throughout the 
U.S., including the San Bernardino and San Gabriel Mountains within the 
Los Angeles Air Basin, the Front Range of Colorado, the Allegheny 
mountains of West Virginia, the Catskill Mountains of New York, the 
Adirondack Mountains of New York, and the Great Smoky Mountains in 
Tennessee (US EPA, 2008, section 3.3.8).
    In contrast to terrestrial and freshwater systems, atmospheric 
nitrogen load to estuaries contributes to the total load but does not 
necessarily drive the effects since other combined sources of nitrogen 
often greatly exceed nitrogen deposition. In estuaries, nitrogen-
loading from multiple anthropogenic and non-anthropogenic pathways 
leads to water quality deterioration, resulting in numerous effects 
including hypoxic zones, species mortality, changes in community 
composition and harmful algal blooms that are indicative of 
eutrophication. The following summary is a concise overview of the 
known or anticipated effects of nitrogen enrichment on estuaries within 
the U.S.
    There is a scientific consensus (US EPA, 2008, section 4.3.4) that 
nitrogen-driven eutrophication in shallow estuaries has increased over 
the past several decades and that the environmental degradation of 
coastal ecosystems due to nitrogen, phosphorus, and other inputs is now 
a widespread occurrence. For example, the frequency of phytoplankton 
blooms and the extent and severity of hypoxia have increased in the 
Chesapeake Bay and Pamlico estuaries in North Carolina and along the 
continental shelf adjacent to the Mississippi and Atchafalaya rivers' 
discharges to the Gulf of Mexico.
    A recent national assessment of eutrophic conditions in estuaries 
found that 65 percent of the assessed systems had moderate to high 
overall eutrophic conditions. Most eutrophic estuaries occurred in the 
mid-Atlantic region and the estuaries with the lowest degree of 
eutrophication were in the North Atlantic. Other regions had mixtures 
of low, moderate, and high degrees of eutrophication (US EPA, 2008, 
section 4.3.4.3).
    The mid-Atlantic region is the most heavily impacted area in terms 
of moderate or high loss of submerged aquatic vegetation due to 
eutrophication (US EPA, 2008, section 4.3.4.2). Submerged aquatic 
vegetation is important to the quality of estuarine ecosystem habitats 
because it provides habitat for a variety of aquatic organisms, absorbs 
excess nutrients, and traps sediments (US EPA, 2008, section 4.3.4.2). 
It is partly because many estuaries and near-coastal marine waters are 
degraded by nutrient enrichment that they are highly sensitive to 
potential negative impacts from nitrogen addition from atmospheric 
deposition.
ii. Terrestrial Ecosystems
    Little is known about the full extent and distribution of the 
terrestrial ecosystems in the U.S. that are most sensitive to impacts 
caused by nutrient enrichment from atmospheric nitrogen deposition. As 
previously stated, most terrestrial ecosystems are nitrogen-limited, 
therefore they are sensitive to perturbation caused by nitrogen 
additions (US EPA, 2008, section 4.3.1). Effects are most likely to 
occur where areas of relatively high atmospheric N deposition intersect 
with nitrogen-limited plant communities. The alpine ecosystems of the 
Colorado Front Range, chaparral watersheds of the Sierra Nevada, lichen 
and vascular plant communities in the San Bernardino Mountains and the 
Pacific Northwest, and the southern California coastal sage scrub (CSS) 
community are among the most sensitive terrestrial ecosystems. There is 
growing evidence (US EPA, 2008, section 4.3.1.2) that existing 
grassland ecosystems in the western U.S. are being altered by elevated 
levels of N inputs, including inputs from atmospheric deposition.
    In the eastern U.S., the degree of nitrogen saturation of the 
terrestrial ecosystem is often assessed in terms of the degree of 
NO3- leaching from watershed soils into ground 
water or surface water. Studies have estimated the number of surface 
waters at different

[[Page 46099]]

stages of saturation across several regions in the eastern U.S. Of the 
85 northeastern watersheds examined 60 percent were in Stage 1 or Stage 
2 of nitrogen saturation on a scale of 0 (background or pretreatment) 
to 3 (visible decline). Of the northeastern sites for which adequate 
data were available for assessment, those in Stage 1 or 2 were most 
prevalent in the Adirondack and Catskill Mountains. Effects on 
individual plant species have not been well studied in the U.S. More is 
known about the sensitivity of particular plant communities. Based 
largely on results obtained in more extensive studies conducted in 
Europe, it is expected that the more sensitive terrestrial ecosystems 
include hardwood forests, alpine meadows, arid and semi-arid lands, and 
grassland ecosystems (US EPA, 2008, section 3.3.5).
    The REA used published research results (US EPA, 2009, section 
5.3.1 and US EPA, 2008, Table 4.4) to identify meaningful ecological 
benchmarks associated with different levels of atmospheric nitrogen 
deposition. These are illustrated in Figure 3-4 of the PA. The 
sensitive areas and ecological indicators identified by the ISA were 
analyzed further in the REA to create a national map that illustrates 
effects observed from ambient and experimental atmospheric nitrogen 
deposition loads in relation to Community Multi-scale Air Quality 
(CMAQ) 2002 modeling results and National Atmospheric Deposition 
Program (NADP) monitoring data. This map, reproduced in Figure 3-5 of 
the PA, depicts the sites where empirical effects of terrestrial 
nutrient enrichment have been observed and site proximity to elevated 
atmospheric nitrogen deposition.
    Based on information in the ISA and initial analysis in the REA, 
further case study analyses on terrestrial nutrient enrichment of 
ecosystems were developed for the CS community and Mixed Conifer Forest 
(MCF) (US EPA, 2009). Geographic information systems (GIS) analysis 
supported a qualitative review of past field research to identify 
ecological benchmarks associated with CSS and mycorrhizal communities, 
as well as MCF nutrient-sensitive acidophyte lichen communities, fine-
root biomass in Ponderosa pine, and leached nitrate in receiving 
waters.
    The ecological benchmarks that were identified for the CSS and the 
MCF communities are included in the suite of benchmarks identified in 
the ISA (US EPA, 2008, section 3.3). There are sufficient data to 
confidently relate the ecological effect to a loading of atmospheric 
nitrogen. For the CSS community, the following ecological benchmarks 
were identified:

(1) 3.3 kg N/ha/yr--the amount of nitrogen uptake by a vigorous stand 
of CSS; above this level, nitrogen may no longer be limiting
(2) 10 kg N/ha/yr--mycorrhizal community changes

    For the MCF community, the following ecological benchmarks were 
identified:

(1) 3.1 kg N/ha/yr--shift from sensitive to tolerant lichen species
(2) 5.2 kg N/ha/yr--dominance of the tolerant lichen species
(3) 10.2 kg N/ha/yr--loss of sensitive lichen species
(4) 17 kg N/ha/yr--leaching of nitrate into streams.

    These benchmarks, ranging from 3.1 to 17 kg N/ha/yr, were compared 
to 2002 CMAQ/NADP data to discern any associations between atmospheric 
deposition and changing communities. Evidence supports the finding that 
nitrogen alters CSS and MCF communities. Key findings include the 
following: 2002 CMAQ/NADP nitrogen deposition data show that the 3.3 kg 
N/ha/yr benchmark has been exceeded in more than 93 percent of CSS 
areas (654,048 ha). These deposition levels are a driving force in the 
degradation of CSS communities. Although CSS decline has been observed 
in the absence of fire, the contributions of deposition and fire to the 
CSS decline require further research. The CSS is fragmented into many 
small parcels, and the 2002 CMAQ/NADP 12-km grid data are not fine 
enough to fully validate the relationship between CSS distribution, 
nitrogen deposition, and fire. The 2002 CMAQ/NADP nitrogen deposition 
data exceeds the 3.1 kg N/ha/yr benchmark in more than 38 percent 
(1,099,133 ha) of MCF areas, and nitrate leaching has been observed in 
surface waters. Ozone effects confound nitrogen effects on MCF 
acidophyte lichen, and the interrelationship between fire and nitrogen 
cycling requires additional research.
c. Key Uncertainties Associated With Nutrient Enrichment
    There are different levels of uncertainty associated with 
relationships between deposition, ecological effects and ecological 
indicators. The criteria used in the REA to evaluate the degree of 
confidence in the data, modeling and ecological effect function are 
detailed in chapter 7 of the REA. Below is a discussion of uncertainty 
relating aquatic and terrestrial ecosystems to nutrient enrichment 
effects.
i. Aquatic Ecosystems
    The approach for assessing atmospheric contributions to total 
nitrogen loading in the REA was to consider the main-stem river to an 
estuary (including the estuary) rather than an entire estuary system or 
bay. The biological indicators used in the NOAA ASSETS EI required the 
evaluation of many national databases including the US Geological 
Survey National Water Quality Assessment (NAWQA) files, EPA's STORage 
and RETrieval (STORET) database, NOAA's Estuarine Drainage Areas data 
and EPA's water quality standards nutrient criteria for rivers and 
lakes (US EPA, 2009, Appendix 6 and Table 1.2.-1). Both the SPARROW 
modeling for nitrogen loads and assessment of estuary conditions under 
NOAA ASSETS EI, have been applied on a national scale. The REA 
concludes that the available data are medium quality with intermediate 
confidence about the use of these data and their values for 
extrapolating to a larger regional area (US EPA, 2009, section 7.3.1). 
Intermediate confidence is associated with the modeling approach using 
ASSETS EI and SPARROW. The REA states there is low confidence with the 
ecological effect function due to the results of the analysis which 
indicated that reductions in atmospheric deposition alone could not 
solve coastal eutrophication problems due to multiple non-atmospheric 
nitrogen inputs (US EPA, 2009, section 7.3.3).
ii. Terrestrial Ecosystems
    Ecological thresholds are identified for CSS and MCF areas and 
these data are considered to be of high quality, however, the ability 
to extrapolate these data to larger regional areas is limited (US EPA, 
2009, section 7.4.1). No quantitative modeling was conducted or 
ecological effect function developed for terrestrial nutrient 
enrichment reflecting the uncertainties associated with these 
depositional effects.
4. Other Ecological Effects
    It is stated in the ISA (US EPA, 2008, section 3.4.1 and 4.5) that 
mercury is a highly neurotoxic contaminant that enters the food web as 
a methylated compound, methylmercury (MeHg). Mercury is principally 
methylated by sulfur-reducing bacteria and can be taken up by 
microorganisms, zooplankton and macroinvertebrates. The contaminant is 
concentrated in higher trophic levels, including fish eaten by humans. 
Experimental evidence has established that only inconsequential amounts 
of MeHg can

[[Page 46100]]

be produced in the absence of sulfate. Once MdHg is present, other 
variables influence how much accumulates in fish, but elevated mercury 
levels in fish can only occur where substantial amounts of MeHg are 
present. Current evidence indicates that in watersheds where mercury is 
present, increased oxides of sulfur deposition very likely results in 
additional production of MeHg which leads to greater accumulation of 
MeHg concentrations in fish. With respect to sulfur deposition and 
mercury methylation, the final ISA determined that ``[t]he evidence is 
sufficient to infer a causal relationship between sulfur deposition and 
increased mercury methylation in wetlands and aquatic environments.''
    The production of meaningful amounts of MeHg requires the presence 
of SO42- and mercury, and where mercury is 
present, increased availability of SO42- results 
in increased production of MeHg. There is increasing evidence on the 
relationship between sulfur deposition and increased methylation of 
mercury in aquatic environments; this effect occurs only where other 
factors are present at levels within a range to allow methylation. The 
production of MeHg requires the presence of SO42- 
and mercury, but the amount of MeHg produced varies with oxygen 
content, temperature, pH, and supply of labile organic carbon (US EPA, 
2008, section 3.4). In watersheds where changes in sulfate deposition 
did not produce an effect, one or several of those interacting factors 
were not in the range required for meaningful methylation to occur (US 
EPA, 2008, section 3.4). Watersheds with conditions known to be 
conducive to mercury methylation can be found in the northeastern U.S. 
and southeastern Canada.
    While the relationship between sulfur and MeHg production was 
concluded to be causal in the ISA, the REA concluded that there was 
insufficient evidence to quantify the relationship between sulfur and 
MeHg. Therefore only a qualitative assessment was included in chapter 6 
of the REA. The PA was then unable to make a determination as to the 
adequacy of the existing SO2 standards in protecting against 
welfare effects associated with increased mercury methylation.

B. Risk and Exposure Assessment

    The risk and exposure assessment conducted for the current review 
was developed to describe potential risk from current and future 
deposition of oxides of nitrogen and sulfur to sensitive ecosystems. 
The case study analyses in the REA show that there is confidence that 
known or anticipated adverse ecological effects are occurring under 
current ambient loadings of nitrogen and sulfur in sensitive ecosystems 
across the U.S. An overview of the material covered in the REA, a 
summary of the key findings from the air quality analyses, 
acidification and nutrient enrichment case studies, and general 
conclusions from evaluating additional welfare effects, are presented 
below.
1. Overview of the Risk and Exposure Assessment
    The REA evaluates the relationships between atmospheric 
concentrations, deposition, biologically relevant exposures, targeted 
ecosystem effects, and ecosystem services. To evaluate the nature and 
magnitude of adverse effects associated with deposition, the REA also 
examines various ways to quantify the relationships between air quality 
indicators, deposition of biologically available forms of nitrogen and 
sulfur, ecologically relevant indicators relating to deposition, 
exposure and effects on sensitive receptors, and related effects 
resulting in changes in ecosystem structure and services. The intent is 
to determine the exposure metrics that incorporate the temporal 
considerations (i.e., biologically relevant timescales), pathways, and 
ecologically relevant indicators necessary to determine the effects on 
these ecosystems. To the extent feasible, the REA evaluates the overall 
load to the system for nitrogen and sulfur, as well as the variability 
in ecosystem responses to these pollutants. It also evaluates the 
contributions of atmospherically deposited nitrogen and sulfur 
individually relative to the combined atmospheric loadings of both 
elements together.. Since oxidized nitrogen is the listed criteria 
pollutant (currently measured by the ambient air quality indicator 
NO2) for the atmospheric contribution to total nitrogen, the 
REA examines the contribution of nitrogen oxides to total reactive 
nitrogen in the atmosphere, relative to the contributions of reduced 
forms of nitrogen (e.g., ammonia, ammonium), to ultimately assess how a 
meaningful secondary NAAQS might be structured.
    The REA focuses on ecosystem welfare effects that result from the 
deposition of total reactive nitrogen and sulfur. Because ecosystems 
are diverse in biota, climate, geochemistry, and hydrology, response to 
pollutant exposures can vary greatly between ecosystems. In addition, 
these diverse ecosystems are not distributed evenly across the United 
States. To target nitrogen and sulfur acidification and nitrogen and 
sulfur enrichment, the REA addresses four main targeted ecosystem 
effects on terrestrial and aquatic systems identified by the ISA (US 
EPA, 2008): Aquatic acidification due to nitrogen and sulfur; 
terrestrial acidification due to nitrogen and sulfur; aquatic nutrient 
enrichment, including eutrophication; and terrestrial nutrient 
enrichment.
    In addition to these four targeted ecosystem effects, the REA also 
qualitatively addresses the influence of sulfur oxides deposition on 
MeHg production; nitrous oxide (N2O) effects on climate; 
nitrogen effects on primary productivity and biogenic greenhouse gas 
(GHG) fluxes; and phytotoxic effects on plants.
    Because the targeted ecosystem effects outlined above are not 
evenly distributed across the U.S., the REA identified case studies for 
each targeted effects based on ecosystems identified as sensitive to 
nitrogen and/or sulfur deposition effects. Eight case study areas and 
two supplemental study areas (Rocky Mountain National Park and Little 
Rock Lake, Wisconsin) are summarized in the REA based on ecosystem 
characteristics, indicators, and ecosystem service information. Case 
studies selected for aquatic acidification effects were the Adirondack 
Mountains and Shenandoah National Park. Kane Experimental Forest in 
Pennsylvania and Hubbard Brook Experimental Forest in New Hampshire 
were selected as case studies for terrestrial acidification. Aquatic 
nutrient enrichment case study locations were selected in the Potomac 
River Basin upstream of Chesapeake Bay and the Neuse River Basin 
upstream of the Pamlico Sound in North Carolina. The CSS communities in 
southern California and the MCF communities in the San Bernardino and 
Sierra Nevada Mountains of California were selected as case studies for 
terrestrial nutrient enrichment. Two supplemental areas were also 
chosen, one in Rocky Mountain National Park for terrestrial nutrient 
enrichment and one in Little Rock Lake, Wisconsin for aquatic nutrient 
enrichment.
2. Key Findings
    In summary, based on case study analyses, the REA concludes that 
known or anticipated adverse ecological effects are occurring under 
current conditions and further concludes that these adverse effects 
continue into the future. Key findings from the air quality analyses, 
acidification and nutrient enrichment case studies, as well as general 
conclusions from evaluating additional welfare effects, are summarized 
below.

[[Page 46101]]

a. Air Quality Analyses
    The air quality analyses in the REA encompass the current emissions 
sources of nitrogen and sulfur, as well as atmospheric concentrations, 
estimates of deposition of total nitrogen, policy-relevant background, 
and non-atmospheric loadings of nitrogen and sulfur to ecosystems, both 
nationwide and in the case study areas. Spatial fields of deposition 
were created using wet deposition measurements from the NADP National 
Trends Network and dry deposition predictions from the 2002 CMAQ model 
simulation. Some key conclusions from this analysis are:
    (1) Total reactive nitrogen deposition and sulfur deposition are 
much greater in the East compared to most areas of the West.
    (2) These regional differences in deposition correspond to the 
regional differences in oxides of nitrogen and SO2 
concentrations and emissions, which are also higher in the East. Oxides 
of nitrogen emissions are much greater and generally more widespread 
than NH3 emissions nationwide; high NH3 emissions 
tend to be more local (e.g., eastern North Carolina) or sub-regional 
(e.g., the upper Midwest and Plains states). The relative amounts of 
oxidized versus reduced nitrogen deposition are consistent with the 
relative amounts of oxides of nitrogen and NH3 emissions. 
Oxidized nitrogen deposition exceeds reduced nitrogen deposition in 
most of the case study areas; the major exception being the Neuse 
River/Neuse River Estuary Case Study Area.
    (3) Reduced nitrogen deposition exceeds oxidized nitrogen 
deposition in the vicinity of local sources of NH3.
    (4) There can be relatively large spatial variations in both total 
reactive nitrogen deposition and sulfur deposition within a case study 
area; this occurs particularly in those areas that contain or are near 
a high emissions source of oxides of nitrogen, NH3 and/or 
SO2.
    (5) The seasonal patterns in deposition differ between the case 
study areas. For the case study areas in the East, the season with the 
greatest amounts of total reactive nitrogen deposition correspond to 
the season with the greatest amounts of sulfur deposition. Deposition 
peaks in spring in the Adirondack, Hubbard Brook Experimental Forest, 
and Kane Experimental Forest case study areas, and it peaks in summer 
in the Potomac River/Potomac Estuary, Shenandoah, and Neuse River/Neuse 
River Estuary case study areas. For the case study areas in the West, 
there is less consistency in the seasons with greatest total reactive 
nitrogen and sulfur deposition in a given area. In general, both 
nitrogen and/or sulfur deposition peaks in spring or summer. The 
exception to this is the Sierra Nevada Range portion of the MCF Case 
Study Area, in which sulfur deposition is greatest in winter.
b. Deposition-Related Aquatic Acidification
    The role of aquatic acidification in two eastern United States 
areas--northeastern New York's Adirondack area and the Shenandoah area 
in Virginia--was analyzed in the REA to assess surface water trends in 
SO42- and NO3- 
concentrations and ANC levels and to affirm the understanding that 
reductions in deposition could influence the risk of acidification. 
Monitoring data from the EPA-administered TIME)/LTM programs and the 
EMAP were assessed for the years 1990 to 2006, and past, present and 
future water quality levels were estimated using both steady-state and 
dynamic biogeochemical models.
    Although wet deposition rates for SO2 and oxides of 
nitrogen in the Adirondack Case Study Area have reduced since the mid-
1990s, current concentrations are still well above pre-acidification 
(1860) conditions. The MAGIC modeling predicts 
NO3- and SO42- are 17- and 
5-fold higher today, respectively. The estimated average ANC for 44 
lakes in the Adirondack Case Study Area is 62.1 [mu]eq/L (15.7 [mu]eq/L); 78 percent of all monitored lakes in the 
Adirondack Case Study Area have a current risk of Elevated, Severe, or 
Acute. Of the 78 percent, 31 percent experience episodic acidification, 
and 18 percent are chronically acidic today.
    (1) Based on the steady-state critical load model for the year 
2002, 18 percent, 28 percent, 44 percent, and 58 percent of 169 modeled 
lakes received combined total sulfur and nitrogen deposition that 
exceeded critical loads corresponding to ANC limits of 0, 20, 50, and 
100 [mu]eq/L respectively.
    (2) Based on a deposition scenario that maintains current emission 
levels to 2020 and 2050, the simulation forecast indicates no 
improvement in water quality in the Adirondack Case Study Area. The 
percentage of lakes within the Elevated to Acute Concern classes 
remains the same in 2020 and 2050.
    (3) Since the mid-1990s, streams in the Shenandoah Case Study Area 
have shown slight declines in NO3 and 
SO42- concentrations in surface waters. The ANC 
levels increased from about 50 [mu]eq/L in the early 1990s to >75 
[mu]eq/L until 2002, when ANC levels declined back to 1991-1992 levels. 
Current concentrations are still above pre-acidification (1860) 
conditions. The MAGIC modeling predicts surface water concentrations of 
NO3 and SO42- are 10- and 32-fold 
higher today, respectively. The estimated average ANC for 60 streams in 
the Shenandoah Case Study Area is 57.9 [mu]eq/L (4.5 
[mu]eq/L). Fifty-five percent of all monitored streams in the 
Shenandoah Case Study Area have a current risk of Elevated, Severe, or 
Acute. Of the 55 percent, 18 percent experience episodic acidification, 
and 18 percent are chronically acidic today.
    (4) Based on the steady-state critical load model for the year 
2002, 52 percent, 72 percent, 85 percent and 93 percent of 60 modeled 
streams received combined total sulfur and nitrogen deposition that 
exceeded critical loads corresponding to ANC limits of 0, 20, 50, and 
100 [mu]eq/L respectively.
    (5) Based on a deposition scenario that maintains current emission 
levels to 2020 and 2050, the simulation forecast indicates that a large 
number of streams would still have Elevated to Acute problems with 
acidity.
c. Deposition-Related Terrestrial Acidification
    The role of terrestrial acidification was examined in the REA using 
a critical load analysis for sugar maple and red spruce forests in the 
eastern U.S. by using the BC/Al ratio in acidified forest soils as an 
indicator to assess the impact of nitrogen and sulfur deposition on 
tree health. These are the two most commonly studied species in North 
America for impacts of acidification. At a BC/Al ratio of 1.2, red 
spruce growth can be reduced by 20 percent. Sugar maple growth can be 
reduced by 20 percent at a BC/Al ratio of 0.6. Key findings of the case 
study are summarized below.
    (1) Case study results suggest that the health of at least a 
portion of the sugar maple and red spruce growing in the U.S. may have 
been compromised with acidifying total nitrogen and sulfur deposition 
in 2002. The 2002 CMAQ/NADP total nitrogen and sulfur deposition levels 
exceeded three selected critical loads in 3 percent to 75 percent of 
all sugar maple plots across 24 states. The three critical loads ranged 
from 6,008 to 107 eq/ha/yr for the BC/Al ratios of 0.6, 1.2, and 10.0 
(increasing levels of tree protection). The 2002 CMAQ/NADP total 
nitrogen and sulfur deposition levels exceeded three selected critical 
loads in 3 percent to 36 percent of all red spruce plots across eight 
states. The three critical loads

[[Page 46102]]

ranged from 4,278 to 180 eq/ha/yr for the Bc/Al ratios of 0.6, 1.2, and 
10.0 (increasing levels of tree protection).
    (2) The SMB model assumptions made for base cation weathering (Bcw) 
and forest soil ANC input parameters are the main sources of 
uncertainty since these parameters are rarely measured and require 
researchers to use default values.
    (3) The pattern of case study results suggests that nitrogen and 
sulfur acidifying deposition in the sugar maple and red spruce forest 
areas studied were similar in magnitude to the critical loads for those 
areas and both ecosystems are likely to be sensitive to any future 
changes in the levels of deposition.
d. Deposition-Related Aquatic Nutrient Enrichment
    The role of nitrogen deposition in two main stem rivers feeding 
their respective estuaries was analyzed in the REA to determine if 
decreases in deposition could influence the risk of eutrophication as 
predicted using the ASSETS EI scoring system in tandem with SPARROW 
modeling. This modeling approach provides a transferrable, 
intermediate-level analysis of the linkages between atmospheric 
deposition and receiving waters, while providing results on which 
conclusions could be drawn. A summary of findings follows:
    (1) The 2002 CMAQ/NADP results showed that an estimated 40,770,000 
kilograms (kg) of total nitrogen was deposited in the Potomac River 
watershed. The SPARROW modeling predicted that 7,380,000 kg N/yr of the 
deposited nitrogen reached the estuary (20 percent of the total load to 
the estuary). The overall ASSETS EI for the Potomac River and Potomac 
Estuary was Bad (based on all sources of N).
    (2) To improve the Potomac River and Potomac Estuary ASSETS EI 
score from Bad to Poor, a decrease of at least 78 percent in the 2002 
total nitrogen atmospheric deposition load to the watershed would be 
required.
    (3) The 2002 CMAQ/NADP results showed that an estimated 18,340,000 
kg of total nitrogen was deposited in the Neuse River watershed. The 
SPARROW modeling predicted that 1,150,000 kg N/yr of the deposited 
nitrogen reached the estuary (26 percent of the total load to the 
estuary). The overall ASSETS EI for the Neuse River/Neuse River Estuary 
was Bad.
    (4) It was found that the Neuse River/Neuse River Estuary ASSETS EI 
score could not be improved from Bad to Poor with decreases only in the 
2002 atmospheric deposition load to the watershed. Additional 
reductions would be required from other nitrogen sources within the 
watershed.
    The small effect of decreasing atmospheric deposition in the Neuse 
River watershed is because the other nitrogen sources within the 
watershed are more influential than atmospheric deposition in affecting 
the total nitrogen loadings to the Neuse River Estuary, as estimated 
with the SPARROW model. A water body's response to nutrient loading 
depends on the magnitude (e.g., agricultural sources have a higher 
influence in the Neuse than in the Potomac), spatial distribution, and 
other characteristics of the sources within the watershed; therefore a 
reduction in nitrogen deposition does not always produce a linear 
response in reduced load to the estuary, as demonstrated by these two 
case studies.
e. Deposition-Related Terrestrial Nutrient Enrichment
    California CSS and MCF communities were the focus of the 
Terrestrial Nutrient Enrichment Case Studies of the REA. Geographic 
information systems analysis supported a qualitative review of past 
field research to identify ecological benchmarks associated with CSS 
and mycorrhizal communities, as well as MCF's nutrient-sensitive 
acidophyte lichen communities, fine-root biomass in Ponderosa pine and 
leached nitrate in receiving waters. These benchmarks, ranging from 3.1 
to 17 kg N/ha/yr, were compared to 2002 CMAQ/NADP data to discern any 
associations between atmospheric deposition and changing communities. 
Evidence supports the finding that nitrogen alters CSS and MCF. Key 
findings include the following:
    (1) The 2002 CMAQ/NADP nitrogen deposition data show that the 3.3 
kg N/ha/yr benchmark has been exceeded in more than 93 percent of CSS 
areas (654,048 ha). This suggests that such deposition is a driving 
force in the degradation of CSS communities. One potentially 
confounding factor is the role of fire. Although CSS decline has been 
observed in the absence of fire, the contributions of deposition and 
fire to the CSS decline require further research. The CSS is fragmented 
into many small parcels, and the 2002 CMAQ/NADP 12-km grid data are not 
fine enough to fully validate the relationship between CSS 
distribution, nitrogen deposition, and fire.
    (2) The 2002 CMAQ/NADP nitrogen deposition data exceeds the 3.1 kg 
N/ha/yr benchmark in more than 38% (1,099,133 ha) of MCF areas, and 
nitrate leaching has been observed in surface waters. Ozone effects 
confound nitrogen effects on MCF acidophyte lichen, and the 
interrelationship between fire and nitrogen cycling requires additional 
research.
f. Additional Effects
    Ecological effects have also been documented across the U.S. where 
elevated nitrogen deposition has been observed, including the eastern 
slope of the Rocky Mountains where shifts in dominant algal species in 
alpine lakes have occurred where wet nitrogen deposition was only about 
1.5 kg N/ha/yr. High alpine terrestrial communities have a low capacity 
to sequester nitrogen deposition, and monitored deposition exceeding 3 
to 4 kg N/ha/yr could lead to community-level changes in plant species, 
lichens and mycorrhizae.
    Additional welfare effects are documented, but examined less 
extensively, in the REA. These effects include qualitative discussions 
related to visibility and materials damage, such as corrosion, erosion, 
and soiling of paint and buildings which are being addressed in the PM 
NAAQS review currently underway. A discussion of the causal 
relationship between sulfur deposition (as sulfate, 
SO42-) and increased mercury methylation in 
wetlands and aquatic environments is also included in the REA. On this 
subject the REA concludes that decreases in SO42- 
deposition will likely result in decreases in MeHg concentration; 
however, spatial and biogeochemical variations nationally hinder 
establishing large scale dose-response relationships.
    Several additional issues concerning oxides of nitrogen were 
addressed in the REA. Consideration was also given to N2O, a 
potent GHG. The REA concluded that it is most appropriate to analyze 
the role of N2O in the context of all of the GHGs rather 
than as part of the REA for this review. The REA considered nitrogen 
deposition and its correlation with the rate of photosynthesis and net 
primary productivity. Nitrogen addition ranging from 15.4 to 300 kg N/
ha/yr is documented as increasing wetland N2O production by 
an average of 207 percent across all ecosystems. Nitrogen addition 
ranging from 30 to 240 kg N/ha/yr increased CH4 emissions by 
115 percent, averaged across all ecosystems, and methane uptake was 
reduced by 38 percent averaged across all ecosystems when nitrogen 
addition ranged from 10 to 560 kg N/ha/yr, but reductions were only 
significant for coniferous and deciduous forests. The heterogeneity of 
ecosystems across the U.S., however, introduces variations into dose-
response relationships.

[[Page 46103]]

    The phytotoxic effects of oxides of nitrogen and sulfur on 
vegetation were also briefly discussed in the REA which concluded that 
since a unique secondary NAAQS exists for SO2, and 
concentrations of nitric oxide (NO), NO2 and PAN are rarely 
high enough to have phytotoxic effects on vegetation, further 
assessment was not warranted at this time.
3. Conclusions on Effects
    For aquatic and terrestrial acidification effects, a similar 
conceptual approach was used (critical loads) to evaluate the impacts 
of multiple pollutants on an ecological endpoint, whereas the 
approaches used for aquatic and terrestrial nutrient enrichment were 
fundamentally distinct. Although the ecological indicators for aquatic 
and terrestrial acidification (i.e., ANC and BC/Al) are very different, 
both ecological indicators are well-correlated with effects such as 
reduced biodiversity and growth. While aquatic acidification is clearly 
the targeted effect area with the highest level of confidence, the 
relationship between atmospheric deposition and an ecological indicator 
is also quite strong for terrestrial acidification. The main drawback 
with the understanding of terrestrial acidification is that the data 
are based on laboratory responses rather than field measurements. Other 
stressors that are present in the field but that are not present in the 
laboratory may confound this relationship.
    For nutrient enrichment effects, the REA utilized different types 
of indicators for aquatic and terrestrial effects to assess both the 
likelihood of adverse effects to ecosystems and the relationship 
between adverse effects and atmospheric sources of oxides of nitrogen. 
The ecological indicator chosen for aquatic nutrient enrichment, the 
ASSETS EI, seems to be inadequate to relate atmospheric deposition to 
the targeted ecological effect, likely due to the many other 
confounding factors. Further, there is far less confidence associated 
with the understanding of aquatic nutrient enrichment because of the 
large contributions from non-atmospheric sources of nitrogen and the 
influence of both oxidized and reduced forms of nitrogen, particularly 
in large watersheds and coastal areas. However, a strong relationship 
exists between atmospheric deposition of nitrogen and ecological 
effects in high alpine lakes in the Rocky Mountains because atmospheric 
deposition is the only source of nitrogen to these systems. There is 
also a strong weight-of-evidence regarding the relationships between 
ecological effects attributable to terrestrial nitrogen nutrient 
enrichment; however, ozone and climate change may be confounding 
factors. In addition, the response for other species or species in 
other regions of the U.S. has not been quantified.

C. Adversity of Effects to Public Welfare

    Characterizing a known or anticipated adverse effect to public 
welfare is an important component of developing any secondary NAAQS. 
According to the CAA, welfare effects include: ``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 effect on economic 
values and on personal comfort and well-being, whether caused by 
transformation, conversion, or combination with other air pollutants'' 
(CAA, Section 302(h)). While the text above lists a number of welfare 
effects, these effects do not define public welfare in and of 
themselves.
    Although there is no specific definition of adversity to public 
welfare, the paradigm of linking adversity to public welfare to 
disruptions in ecosystem structure and function has been used broadly 
by EPA to categorize effects of pollutants from the cellular to the 
ecosystem level. An evaluation of adversity to public welfare might 
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.
    Similar concepts were used in past reviews of secondary NAAQS for 
ozone and PM (relating to visibility), as well as in initial reviews of 
effects from lead deposition. Because oxides of nitrogen and sulfur are 
deposited from ambient sources into ecosystems where they affect 
changes to organisms, populations and ecosystems, the concept of 
adversity to public welfare as a result of alterations in structure and 
function of ecosystems is an appropriate consideration for this review.
    Based on information provided in the PA, the following section 
discusses how ecological effects from deposition of oxides of nitrogen 
and sulfur relate to adversity to public welfare. In the PA, public 
welfare was discussed in terms of loss of ecosystem services (defined 
below), which in some cases can be monetized. Each of the four main 
effect areas (aquatic and terrestrial acidification and aquatic and 
terrestrial nutrient over-enrichment) are discussed including current 
ecological effects and associated ecosystem services.
1. Ecosystem Services
    The PA defines ecosystem services as the benefits individuals and 
organizations obtain from ecosystems. Ecosystem services can be 
classified as provisioning (food and water), regulating (control of 
climate and disease), cultural (recreational, existence, spiritual, 
educational), and supporting (nutrient cycling). Conceptually, changes 
in ecosystem services may be used to aid in characterizing a known or 
anticipated adverse effect to public welfare. In the REA and PA 
ecosystem services are discussed as a method of assessing the magnitude 
and significance to the public of resources affected by ambient 
concentrations of oxides of nitrogen and sulfur and deposition in 
sensitive ecosystems.
    The EPA has in previous NAAQS reviews defined ecological goods and 
services for the purposes of a Regulatory Impact Analysis as the 
``outputs of ecological functions or processes that directly or 
indirectly contribute to social welfare or have the potential to do so 
in the future. Some outputs may be bought and sold, but most are not 
marketed.'' It is especially important to acknowledge that it is 
difficult to measure and/or monetize the goods and services supplied by 
ecosystems. It can be informative in characterizing adversity to public 
welfare to attempt to place an economic valuation on the set of goods 
and services that have been identified with respect to a change in 
policy; however it must be noted that this valuation will be incomplete 
and illustrative only.
    Knowledge about the relationships linking ambient concentrations 
and ecosystem services is considered in the PA as one method by which 
to inform a policy judgment on a known or anticipated adverse public 
welfare effect. For example, a change in an ecosystem structure and 
process, such as foliar injury, would be classified as an ecological 
effect, with the associated changes in ecosystem services, such as 
primary productivity, food availability, forest products, and 
aesthetics (e.g., scenic viewing), classified as public welfare 
effects. Additionally, changes in biodiversity would be classified as 
an ecological effect, and the associated changes in ecosystem 
services--productivity, existence (nonuse) value, recreational viewing 
and aesthetics--would also be classified as public welfare effects.
    As described in chapters 4 and 5 of the REA, case study analyses 
were performed that link deposition in sensitive ecosystems to changes 
in a

[[Page 46104]]

given ecological indicator (e.g., for aquatic acidification, to changes 
in ANC) and then to changes in ecosystems. Appendix 8 of the REA links 
the changes in ecosystems to the services they provide (e.g., fish 
species richness and its influence on recreational fishing). To the 
extent possible for each targeted effect area, the REA linked ambient 
concentrations of nitrogen and sulfur (i.e., ambient air quality 
indicators) to deposition in sensitive ecosystems (i.e., exposure 
pathways), and then to system response as measured by a given 
ecological indicator (e.g., lake and stream acidification as measured 
by ANC). The ecological effect (e.g., changes in fish species richness) 
was then, where possible, associated with changes in ecosystem services 
and the corresponding public welfare effects (e.g., recreational 
fishing).
2. Effects on Ecosystem Services
    The process used to link ecological indicators to ecosystem 
services is discussed extensively in appendix 8 of the REA. In brief, 
for each case study area assessed, the ecological indicators are linked 
to an ecological response that is subsequently linked to associated 
services to the extent possible. For example, in the case study for 
aquatic acidification the chosen ecological indicator is ANC which can 
be linked to the ecosystem service of recreational fishing. Although 
recreational fishing losses are the only service effects that can be 
independently quantified or monetized at this time, there are numerous 
other ecosystem services that may be related to the ecological effects 
of acidification.
    While aquatic acidification is the focus of this proposed standard, 
the other effect areas were also analyzed in the REA and these 
ecosystems are being harmed by nitrogen and sulfur deposition and will 
obtain some measure of protection with any decrease in that deposition 
regardless of the reason for the decrease. The following summarizes the 
current levels of specific ecosystem services for aquatic and 
terrestrial acidification and aquatic and terrestrial nutrient over-
enrichment and attempts to quantify and when possible monetize the harm 
to public welfare, as represented by ecosystem services, due to 
nitrogen and sulfur deposition.
a. Aquatic Acidification
    Acidification of aquatic ecosystems primarily affects the ecosystem 
services that are derived from the fish and other aquatic life found in 
surface waters. In the northeastern United States, the surface waters 
affected by acidification are not a major source of commercially raised 
or caught fish; however, they are a source of food for some 
recreational and subsistence fishers and for other consumers. Although 
data and models are available for examining the effects on recreational 
fishing, relatively little data are available for measuring the effects 
on subsistence and other consumers. Inland waters also provide 
aesthetic and educational services along with non-use services, such as 
existence value (protection and preservation with no expectation of 
direct use). In general, inland surface waters such as lakes, rivers, 
and streams also provide a number of regulating services, playing a 
role in hydrological regimes and climate regulation. There is little 
evidence that acidification of freshwaters in the northeastern U.S. has 
significantly degraded these specific services; however, freshwater 
ecosystems also provide biological control services by providing 
environments that sustain delicate aquatic food chains. The toxic 
effects of acidification on fish and other aquatic life impair these 
services by disrupting the trophic structure of surface waters. 
Although it is difficult to quantify these services and how they are 
affected by acidification, it is worth noting that some of these 
services may be captured through measures of provisioning and cultural 
services. For example, these biological control services may serve as 
``intermediate'' inputs that support the production of ``final'' 
recreational fishing and other cultural services.
    As summarized in Chapter 4 of the PA, recent studies indicate that 
acidification of lakes and streams can result in significant loss in 
economic value. For example, data indicate that more than 9 percent of 
adults in the northeastern part of the country participate annually in 
freshwater fishing yielding 140 million freshwater fishing days. Each 
fishing day has an estimated average value per day of $35. Therefore, 
the implied total annual value of freshwater fishing in the 
northeastern U.S. was $5 billion in 2006. Embedded in these numbers is 
a degree of harm to recreational fishing services due to acidification 
that has occurred over time. These harms have not been quantified on a 
regional scale; however, a case study was conducted in the Adirondacks 
area (US EPA, 2011, section 4.4.2).
    In the Adirondacks case study, estimates of changes in recreational 
fishing services were determined, as well as changes more broadly in 
``cultural'' ecosystem services (including recreational, aesthetic, and 
nonuse services). First, the MAGIC model (US EPA, 2009, Appendix 8 and 
section 2.2) was applied to 44 lakes to predict what ANC levels would 
be under both ``business as usual'' conditions (i.e., allowing for some 
decline in deposition due to existing regulations) and pre-emission 
(i.e., background) conditions. Second, to estimate the recreational 
fishing impacts of aquatic acidification in these lakes, an existing 
model of recreational fishing demand and site choice was applied. This 
model predicts how recreational fishing patterns in the Adirondacks 
would differ and how much higher the average annual value of 
recreational fishing services would be for New York residents if lake 
ANC levels corresponded to background (rather than business as usual) 
conditions. To estimate impacts on a broader category of cultural (and 
some provisioning) ecosystem services, results from the Banzhaf et al 
(2006) valuation survey of New York residents were adapted and applied 
to this context. The survey used a contingent valuation approach to 
estimate the average annual household willingness to pay (WTP) for 
future reductions in the percent of Adirondack lakes impaired by 
acidification. The focus of the survey was on impacts on aquatic 
resources. Pretesting of the survey indicated that respondents 
nonetheless tended to assume that benefits would occur in the condition 
of birds and forests as well as in recreational fishing.
    By extrapolating the 44 lake Adirondack case study to all 3,000 
Adirondack lakes and by applying the WTP survey results to all New York 
residents, the study estimated aggregated benefits between $300 and 
$800 million annually for the equivalent of improving lakes in the 
Adirondacks region to an ANC level of 50 [mu]eq/L. The REA estimated 44 
percent of the Adirondack lakes currently fall below an ANC of 50 
[mu]eq/L. Several states have set goals for improving the acid status 
of lakes and streams, generally targeting ANC in the range of 50 to 60 
[mu]eq/L, and have engaged in costly activities to decrease 
acidification.
    These results imply significant value to the public in addition to 
those derived from recreational fishing services. Note that the results 
are only applicable to improvements in the Adirondacks valued by 
residents of New York. If similar benefits exist in other acid-impacted 
areas, benefits for the nation as a whole could be substantial. The 
analysis provides results on only a subset of the impacts of 
acidification on ecosystem services and suggests that the

[[Page 46105]]

overall impact on these services is likely to be substantial.
b. Terrestrial Acidification
    Chapters 4.4.3 and 4.4.4 of the PA review several economic studies 
of areas sensitive to terrestrial acidification. Forests in the 
northeastern U.S. provide several important and valuable provisioning 
ecosystem services, which are reflected in the production and sales of 
tree products. Sugar maples are a particularly important commercial 
hardwood tree species in the United States, producing timber and maple 
syrup that provide hundreds of millions of dollars in economic value 
annually. Red spruce is also used in a variety of wood products and 
provides up to $100 million in economic value annually. Although the 
data do not exist to directly link acidification damages to economic 
values of lost recreational ecosystem services in forests, these 
resources are valuable to the public. A recent study, reviewed in the 
PA, suggests that the total annual value of recreational off-road 
driving was more than $9 billion and the value of hunting and wildlife 
viewing was more than $4 billion each in the northeastern States. The 
EPA is not able to quantify at this time the specific effects on these 
values of acid deposition, or of any specific reductions in deposition, 
relative to the effects of many other factors that may affect them.
c. Nutrient Enrichment
    Chapters 4.4.5 and 4.4.6 of the PA summarize economic studies of 
east coast estuaries affected by nutrient over-enrichment or 
eutrophication. Estuaries in the eastern United States are important 
for fish and shellfish production. The estuaries are capable of 
supporting large stocks of resident commercial species, and they serve 
as the breeding grounds and interim habitat for several migratory 
species. To provide an indication of the magnitude of provisioning 
services associated with coastal fisheries, from 2005 to 2007, the 
average value of total catch was $1.5 billion per year in 15 East Coast 
states. Estuaries also provide an important and substantial variety of 
cultural ecosystem services, including water-based recreational and 
aesthetic services. For example, data indicate that 4.8 percent of the 
population in coastal states from North Carolina to Massachusetts 
participated in saltwater fishing, with a total of 26 million saltwater 
fishing days in 2006. Based on estimates in the PA, total recreational 
value from these saltwater fishing days was approximately $1.3 billion. 
Recreational participation estimates for 1999-2000 showed almost 6 
million individuals participated in motorboating in coastal states from 
North Carolina to Massachusetts. The aggregate value of these coastal 
motorboating outings was $2 billion per year. EPA is not able to 
quantify at this time the specific effects on these values of nitrogen 
deposition, or of any specific reductions in deposition, relative to 
the effects of many other factors that may affect them.
    Terrestrial ecosystems can also suffer from nutrient over-
enrichment. Each ecosystem is different in its composition of species 
and nutrient requirements. Changes to individual ecosystems from 
changes in nitrogen deposition can be hard to assess economically. 
Relative recreational values are often determined by public use 
information. Chapter 4.4.7 of the PA reviewed studies related to park 
use in California. Data from California State Parks indicate that in 
2002, 68.7 percent of adult residents participated in trail hiking for 
an average of 24.1 days per year. The analyses in the PA indicate that 
the aggregate annual benefit for California residents from trail hiking 
in 2007 was $11.59 billion. EPA is not able to quantify at this time 
the specific effects on these values of nitrogen deposition, or of any 
specific reductions in deposition, relative to the effects of many 
other factors that may affect them.
    The PA also identified fire regulation as a service that could be 
affected by nutrient over-enrichment of the CSS and MCF ecosystems by 
encouraging growth of more flammable grasses, increasing fuel loads, 
and altering the fire cycle. Over the 5-year period from 2004 to 2008, 
Southern California experienced, on average, over 4,000 fires per year, 
burning, on average, over 400,000 acres per year. It is not possible at 
this time to quantify the contribution of nitrogen deposition, among 
many other factors, to increased fire risk.
3. Summary
    Adversity to public welfare can be understood by looking at how 
deposition of oxides of nitrogen and sulfur affect the ecological 
functions of an ecosystem (see II.A.), and then understanding the 
ecosystem services that are degraded. The monetized value of the 
ecosystem services provided by ecosystems that are sensitive to 
deposition of oxides of nitrogen and sulfur are in the billions of 
dollars each year, though it is not possible to quantify or monetize at 
this time the effects on these values of nitrogen and sulfur deposition 
or of any changes in deposition that may result from new secondary 
standards. Many lakes and streams are known to be degraded by acidic 
deposition which affects recreational fishing and tourism. Forest 
growth is likely suffering from acidic deposition in sensitive areas 
affecting red spruce and sugar maple timber production, sugar maple 
syrup production, hiking, aesthetic enjoyment and tourism. Nitrogen 
deposition contributes significantly to eutrophication in many 
estuaries affecting fish production, swimming, boating, aesthetic 
enjoyment and tourism. Ecosystem services are likely affected by 
nutrient enrichment in many natural and scenic terrestrial areas, 
affecting biodiversity, including habitat for rare and endangered 
species, fire control, hiking, aesthetic enjoyment and tourism.

D. Adequacy of the Current Standards

    An important issue to be addressed in the current review of the 
secondary standards for oxides of nitrogen and sulfur is whether, in 
view of the scientific evidence reflected in the ISA, additional 
information on exposure and risk discussed in the REA, and conclusions 
drawn from the PA, the existing standards provide adequate protection. 
The Administrator therefore, has considered the extent to which the 
current standards are adequate for the protection of public welfare. 
Having reached the general conclusion that aquatic and terrestrial 
ecosystems can be degraded by deposition of oxides of nitrogen and 
sulfur, it is then necessary to first evaluate the appropriateness (in 
terms of form and structure) of the current standards to address the 
ecological effects of oxides of nitrogen and sulfur as well as the 
adequacy of the current secondary standards for oxides of nitrogen and 
sulfur to provide requisite protection by considering to what degree 
risks to sensitive ecosystems would be expected to occur in areas that 
meet the current standards. Conclusions regarding the adequacy of the 
current standards are based on the available ecological effects, 
exposure and risk-based evidence. In evaluating the strength of this 
information, EPA has taken into account the uncertainties and 
limitations in the scientific evidence. This section addresses the 
adequacy of the current standards to protect against direct exposure 
effects on plants from oxides of nitrogen and sulfur, the 
appropriateness of the current structure of the standards to address 
deposition-related effects of oxides of nitrogen and sulfur on 
sensitive ecosystems and finally, the adequacy of such standards to 
protect against adverse effects related to the deposition of oxides of 
nitrogen and sulfur.

[[Page 46106]]

1. Adequacy of the Current Standards for Direct Effects
    The current secondary oxides of nitrogen and sulfur standards are 
intended to protect against adverse effects to public welfare. For 
oxides of nitrogen, the current secondary standard was set identical to 
the primary standard,\3\ i.e., an annual standard set for 
NO2 to protect against adverse effects on vegetation from 
direct exposure to ambient oxides of nitrogen. For oxides of sulfur, 
the current secondary standard is a 3-hour standard intended to provide 
protection for plants from the direct foliar damage associated with 
atmospheric concentrations of SO2. It is appropriate to 
consider whether the current standards are adequate to protect against 
the direct effects on vegetation resulting from ambient NO2 
and SO2 which were the basis for the current secondary 
standards. The ISA concluded that there was sufficient evidence to 
infer a causal relationship between exposure to SO2, NO, 
NO2 and PAN and injury to vegetation. Additional research on 
acute foliar injury has been limited and there is no evidence to 
suggest foliar injury below the levels of the current secondary 
standards for oxides of nitrogen and sulfur. There is sufficient 
evidence to suggest that the levels of the current standards are likely 
adequate to protect against direct phytotoxic effects.
---------------------------------------------------------------------------

    \3\ The current primary NO2 standard has recently 
been changed to the 3-year average of the 98th percentile of the 
annual distribution of the 1 hour daily maximum of the concentration 
of NO2. The current secondary standard remains as it was 
set in 1971.
---------------------------------------------------------------------------

2. Appropriateness and Adequacy of the Current Standards for 
Deposition-Related Effects
    This section addresses two concepts necessary to evaluate the 
current standards in the context of deposition related effects. First, 
appropriateness of the current standards is considered with regard to 
indicator, form, level and averaging time. This discussion centers 
around the ability of the current standards to evaluate and provide 
protection against deposition related effects that vary spatially and 
temporally. It includes particular emphasis on the indicators and forms 
of the current standards and the degree to which they are ecologically 
relevant with regard to deposition related effects. Second, this 
section evaluates the current standards in terms of adequacy of 
protection.
a. Appropriateness
    The ISA has established that the major effects of concern for this 
review of the oxides of nitrogen and sulfur standards are associated 
with deposition of nitrogen and sulfur caused by atmospheric 
concentrations of oxides of nitrogen and sulfur. The current standards 
are not directed toward depositional effects, and none of the elements 
of the current NAAQS--indicator, form, averaging time, and level--are 
suited for addressing the effects of nitrogen and sulfur deposition.
    Five issues arise that call into question the ecological relevance 
of the structure of the current secondary standards for oxides of 
nitrogen and sulfur.
    (1) The current SO2 secondary standard (0.5 ppm 
SO2 over a 3-hour average) does not utilize an exposure 
period that is relevant for ecosystem impacts. The majority of 
deposition related impacts are associated with depositional loads that 
occur over periods of months to years. This differs significantly from 
exposures associated with hourly concentrations of SO2 as 
measured by the current secondary standard. By addressing short-term 
concentrations, the current SO2 secondary standard, while 
protective against direct foliar effects from gaseous oxides of sulfur, 
does not take into account the findings of effects in the ISA, which 
notes the relationship between annual deposition of sulfur and 
acidification effects which are likely to be more severe and widespread 
than phytotoxic effects under current ambient conditions, and include 
effects from long term deposition as well as short term. Acidification 
is a process that occurs over time because the ability of an aquatic 
system to counteract acidic inputs is reduced as natural buffers are 
used more rapidly than they can be replaced through geologic 
weathering. The relevant period of exposure for ecosystems is, 
therefore, not the exposures captured in the short averaging time of 
the current SO2 secondary standard. The current secondary 
standard for oxides of nitrogen is an annual standard (0.053 ppm 
averaged over 1 year) and as such is more ecologically relevant.
    (2) Current standards do not utilize appropriate atmospheric 
indicators. Nitrogen dioxide and SO2 are used as the 
component of oxides of nitrogen and sulfur that are measured, but they 
do not provide a complete link to the direct effects on ecosystems from 
deposition of oxides of nitrogen and sulfur as they do not capture all 
relevant chemical species of oxidized nitrogen and oxidized sulfur that 
contribute to deposition. The ISA provides evidence that deposition 
related effects are linked with total nitrogen and total sulfur 
deposition, and thus all forms of oxidized nitrogen and oxidized sulfur 
that are deposited will contribute to effects on ecosystems. Thus, by 
using atmospheric NO2 and SO2 concentrations as 
indicators, the current standards address only a fraction of total 
atmospheric oxides of nitrogen and sulfur, and do not take into account 
the effects from deposition of total atmospheric oxides of nitrogen and 
sulfur. This suggests that more comprehensive atmospheric indicators 
should be considered in designing ecologically relevant standards.
    (3) Current standards reflect separate assessments of the two 
individual pollutants, NO2 and SO2, rather than 
assessing the joint impacts of deposition to ecosystems. Recognizing 
the role that each pollutant plays in jointly affecting ecosystem 
indicators, functions, and services is vital to developing a meaningful 
standard. The clearest example of this interaction is in assessment of 
the impacts of acidifying deposition on aquatic ecosystems. 
Acidification in an aquatic ecosystem depends on the total acidifying 
potential of the deposition of both nitrogen and sulfur from both 
atmospheric deposition of oxides of nitrogen and sulfur as well as the 
inputs from other sources of nitrogen and sulfur such as reduced 
nitrogen and non-atmospheric sources. It is the joint impact of the two 
pollutants that determines the ultimate effect on organisms within the 
ecosystem, and critical ecosystem functions such as habitat provision 
and biodiversity. Standards that are set independently are less able to 
account for the contribution of the other pollutant. This suggests that 
interactions between oxides of nitrogen and oxides of sulfur should be 
a critical element of the conceptual framework for ecologically 
relevant standards. There are also important interactions between 
oxides of nitrogen and sulfur and reduced forms of nitrogen, which also 
contribute to acidification and nutrient enrichment. It is important 
that the structure of the standards address the role of reduced 
nitrogen in determining the ecological effects resulting from 
deposition of atmospheric oxides of nitrogen and sulfur. Consideration 
will also have to be given to total loadings as ecosystems respond to 
all sources of nitrogen and sulfur.
    (4) Current standards do not take into account variability in 
ecosystem sensitivity. Ecosystems are not uniformly distributed either 
spatially or temporally in their sensitivity to oxides of nitrogen and 
sulfur. Therefore, failure to account for the major determinants of 
variability, including geological and soil

[[Page 46107]]

characteristics related to the sensitivity to acidification or nutrient 
enrichment as well as atmospheric and landscape characteristics that 
govern rates of deposition, may lead to standards that do not provide 
requisite levels of protection across ecosystems. The current 
structures of the standards do not address the complexities in the 
responses of ecosystems to deposition of oxides of nitrogen and sulfur. 
Ecosystems contain complex groupings of organisms that respond in 
various ways to the alterations of soil and water that result from 
deposition of nitrogen and sulfur compounds. Different ecosystems 
therefore respond in different ways depending on a multitude of factors 
that control how deposition is integrated into the system. For example, 
the same levels of deposition falling on limestone dominated soils have 
a very different effect from those falling on shallow glaciated soils 
underlain with granite. One system may over time display no obvious 
detriment while the other may experience a catastrophic loss in fish 
communities. This degree of sensitivity is a function of many 
atmospheric factors that control rates of deposition as well as 
ecological factors that control how an ecosystem responds to that 
deposition. The current standards do not take into account spatial and 
seasonal variations, not only in depositional loadings, but also in 
sensitivity of ecosystems exposed to those loadings. Based on the 
discussion summarized above, the PA concludes that the current 
secondary standards for oxides of nitrogen and oxides of sulfur are not 
ecologically relevant in terms of averaging time, form, level or 
indicator.
b. Adequacy of Protection
    As described in the PA, ambient conditions in 2005 indicate that 
the current SO2 and NO2 secondary standards were 
not exceeded at that time (US EPA, 2011, Figures 6-1 and 6-2) in 
locations where negative ecological effects have been observed. In many 
locations, SO2 and NO2 concentrations are 
substantially below the levels of the secondary standards. This pattern 
suggests that levels of deposition and any negative effects on 
ecosystems due to deposition of oxides of nitrogen and sulfur under 
recent conditions are occurring even though areas meet or are below 
current standards. In addition, based on conclusions in the REA, these 
levels will not decline in the future to levels below which it is 
reasonable to anticipate effects.
    In determining the adequacy of the current secondary standards for 
oxides of nitrogen and sulfur the PA considered the extent to which 
ambient deposition contributes to loadings in ecosystems. Since the 
last review of the secondary standard for oxides of nitrogen, a great 
deal of information on the contribution of atmospheric deposition 
associated with ambient oxides of nitrogen has become available. The 
REA presents a thorough assessment of the contribution of oxidized 
nitrogen to nitrogen deposition throughout the U.S., and the relative 
contributions of ambient oxidized and reduced forms of nitrogen. The 
REA concludes that based on that analysis, ambient oxides of nitrogen 
are a significant component of atmospheric nitrogen deposition, even in 
areas with relatively high rates of deposition of reduced nitrogen. In 
addition, atmospheric deposition of oxidized nitrogen contributes 
significantly to total nitrogen loadings in nitrogen sensitive 
ecosystems.
    The ISA summarizes the available studies of relative nitrogen 
contribution and finds that in much of the U.S., oxides of nitrogen 
contribute from 50 to 75 percent of total atmospheric deposition 
relative to total reactive nitrogen, which includes oxidized and 
reduced nitrogen species (US EPA, 2008, section 2.8.4). Although the 
proportion of total nitrogen loadings associated with atmospheric 
deposition of nitrogen varies across locations, the ISA indicates that 
atmospheric nitrogen deposition is the main source of new anthropogenic 
nitrogen to most headwater streams, high elevation lakes, and low-order 
streams. Atmospheric nitrogen deposition contributes to the total 
nitrogen load in terrestrial, wetland, freshwater and estuarine 
ecosystems that receive nitrogen through multiple pathways. In several 
large estuarine systems, including the Chesapeake Bay, atmospheric 
deposition accounts for between 10 and 40 percent of total nitrogen 
loadings (US EPA, 2008).
    Atmospheric concentrations of oxides of sulfur account for nearly 
all sulfur deposition in the US. For the period 2004-2006, mean sulfur 
deposition in the U.S. was greatest east of the Mississippi River with 
the highest deposition amount, 21.3 kg S/ha-yr, in the Ohio River 
Valley where most recording stations reported 3-year averages >10 kg S/
ha-yr. Numerous other stations in the East reported S deposition >5 kg 
S/ha-yr. Total sulfur deposition in the U.S. west of the 100th meridian 
was relatively low, with all recording stations reporting <2 kg S/ha-yr 
and many reporting <1 kg S/ha-yr. Sulfur was primarily deposited in the 
form of wet SO42- followed in decreasing order by 
a smaller proportion of dry SO2 and a much smaller 
proportion of deposition as dry SO42-.
    As discussed throughout the REA (US EPA, 2009 and section II.B 
above), there are several key areas of risk that are associated with 
ambient concentrations of oxides of nitrogen and sulfur. As noted 
earlier, in previous reviews of the secondary standards for oxides of 
nitrogen and sulfur, the standards were designed to protect against 
direct exposure of plants to ambient concentrations of the pollutants. 
A significant shift in understanding of the effects of oxides of 
nitrogen and sulfur has occurred since the last reviews, reflecting the 
large amount of research that has been conducted on the effects of 
deposition of nitrogen and sulfur to ecosystems. The most significant 
current risks of adverse effects to public welfare are those related to 
deposition of oxides of nitrogen and sulfur to both terrestrial and 
aquatic ecosystems. These risks fall into two categories, acidification 
and nutrient enrichment, which were emphasized in the REA as most 
relevant to evaluating the adequacy of the existing standards in 
protecting public welfare from adverse ecological effects.
i. Aquatic Acidification
    The focus of the REA case studies was on determining whether 
deposition of sulfur and oxidized nitrogen in locations where ambient 
oxides of nitrogen and sulfur were at or below the current standards 
was resulting in acidification and related effects, including episodic 
acidification and mercury methylation. Based on the case studies 
conducted for lakes in the Adirondacks and streams in Shenandoah 
National Park (case studies are discussed more fully in section II.B 
and US EPA, 2009), there is significant risk to acid sensitive aquatic 
ecosystems at atmospheric concentrations of oxides of nitrogen and 
sulfur at or below the current standards. The REA also supports 
strongly a relationship between atmospheric deposition of oxides of 
nitrogen and sulfur and loss of ANC in sensitive ecosystems and 
indicates that ANC is an excellent indicator of aquatic acidification. 
The REA also concludes that at levels of deposition associated with 
oxides of nitrogen and sulfur concentrations at or below the current 
standards, ANC levels are expected to be below benchmark values that 
are associated with significant losses in fish species richness.
    Significant portions of the U.S. are acid sensitive, and current 
deposition levels exceed those that would allow

[[Page 46108]]

recovery of the most acid sensitive lakes in the Adirondacks (US EPA, 
2008, Executive Summary). In addition, because of past loadings, areas 
of the Shenandoah are sensitive to current deposition levels (US EPA, 
2008, Executive Summary). Parts of the West are naturally less 
sensitive to acidification and subjected to lower deposition 
(particularly SOX) levels relative to the eastern United 
States, and as such, less focus in the ISA is placed on the adequacy of 
the existing standards in these areas, with the exception of the 
mountainous areas of the West, which experience episodic acidification 
due to deposition.
    In describing the effects of acidification in the two case study 
areas the REA uses the approach of describing benchmarks in terms of 
ANC values. Many locations in sensitive areas of the U.S. have ANC 
levels below benchmark levels for ANC classified as severe, elevated, 
or moderate concern (US EPA, 2011, Figure 2-1). The average current ANC 
levels across 44 lakes in the Adirondack case study area is 62.1 
[mu]eq/L (moderate concern). However, 44 percent of lakes had 
deposition levels exceeding the critical load for an ANC of 50 [mu]eq/L 
(elevated), and 28 percent of lakes had deposition levels exceeding the 
(higher) critical load for an ANC of 20 [mu]eq/L (severe) (US EPA, 
2009, section 4.2.4.2). This information indicates that almost half of 
the 44 lakes in the Adirondacks case study area are at an elevated 
concern level, and almost a third are at a severe concern level. These 
levels are associated with greatly diminished fish species diversity, 
and losses in the health and reproductive capacity of remaining 
populations. Based on assessments of the relationship between number of 
fish species and ANC level in both the Adirondacks and Shenandoah 
areas, the number of fish species is decreased by over half at an ANC 
level of 20 [mu]eq/L relative to an ANC level at 100 [mu]eq/L (US EPA, 
2009, Figure 4.2-1). When extrapolated to the full population of lakes 
in the Adirondacks area using weights based on the EMAP probability 
survey (US EPA, 2009, section 4.2.6.1), 36 percent of lakes exceeded 
the critical load for an ANC of 50 [mu]eq/L and 13 percent of lakes 
exceeded the critical load for an ANC of 20 [mu]eq/L.
    Many streams in the Shenandoah case study area also have levels of 
deposition that are associated with ANC levels classified as severe, 
elevated, or moderate concern. The average ANC under recent conditions 
for the 60 streams evaluated in the Shenandoah case study area is 57.9 
[mu]eq/L, indicating moderate concern. However, 85 percent of these 
streams had recent deposition exceeding the critical load for an ANC of 
50 [mu]eq/L, and 72 percent exceeded the critical load for an ANC of 20 
[mu]eq/L. As with the Adirondacks area, this information suggests that 
ANC levels may decline in the future and significant numbers of 
sensitive streams in the Shenandoah area are at risk of adverse impacts 
on fish populations if recent conditions persist. Many other streams in 
the Shenandoah area are also likely to experience conditions of 
elevated to severe concern based on the prevalence in the area of 
bedrock geology associated with increased sensitivity to acidification 
suggesting that effects due to stream acidification could be widespread 
in the Shenandoah area (US EPA, 2009, section 4.2.6.2).
    In addition to these chronic acidification effects, the ISA notes 
that ``consideration of episodic acidification greatly increases the 
extent and degree of estimated effects for acidifying deposition on 
surface waters'' (US EPA, 2008, section 3.2.1.6). Some studies show 
that the number of lakes that could be classified as acid-impacted 
based on episodic acidification is 2 to 3 times the number of lakes 
classified as acid-impacted based on chronic ANC. These episodic 
acidification events can have long term effects on fish populations (US 
EPA, 2008, section 3.2.1.6). Under recent conditions, episodic 
acidification has been observed in locations in the eastern U.S. and in 
the mountainous western U.S. (US EPA, 2008, section 3.2.1.6).
    The ISA, REA and PA all conclude that the current standards are not 
adequate to protect against the adverse impacts of aquatic 
acidification on sensitive ecosystems. A recent survey, as reported in 
the ISA, found sensitive streams in many locations in the U.S., 
including the Appalachian Mountains, the Coastal Plain, and the 
Mountainous West (US EPA, 2008, section 4.2.2.3). In these sensitive 
areas, between 1 and 6 percent of stream kilometers are chronically 
acidified. The REA further concludes that both the Adirondack and 
Shenandoah case study areas are currently receiving deposition from 
ambient oxides of nitrogen and sulfur in excess of their ability to 
neutralize such inputs. In addition, based on the current emission 
scenarios, forecast modeling out to the year 2020 as well as 2050 
indicates a large number of streams in these areas will still be 
adversely impacted (section II.B). Based on these considerations, the 
PA concludes that the current secondary NAAQS for oxides of nitrogen 
and sulfur do not provide adequate protection of sensitive ecosystems 
with regard to aquatic acidification.
ii. Terrestrial Acidification
    Based on the terrestrial acidification case studies, Kane 
Experimental Forest in Pennsylvania and Hubbard Brook Experimental 
Forest described in section II.B) of sugar maple and red spruce 
habitat, the REA concludes that there is significant risk to sensitive 
terrestrial ecosystems from acidification at atmospheric concentrations 
of NO2 and SO2 at or below the current standards. 
The ecological indicator selected for terrestrial acidification is the 
BC/Al, which has been linked to tree health and growth. The results of 
the REA strongly support a relationship between atmospheric deposition 
of oxides of nitrogen and sulfur and BC/Al, and that BC/Al is a good 
indicator of terrestrial acidification. At levels of deposition 
associated with oxides of nitrogen and sulfur concentrations at or 
below the current standards, BC/Al levels are expected to be below 
benchmark values that are associated with significant effects on tree 
health and growth. Such degradation of terrestrial ecosystems could 
affect ecosystem services such as habitat provisioning, endangered 
species, goods production (timber, syrup, etc.) among others.
    Many locations in sensitive areas of the U.S. have BC/Al levels 
below benchmark levels classified as providing low to intermediate 
levels of protection to tree health. At a BC/Al ratio of 1.2 
(intermediate level of protection), red spruce growth can be reduced by 
20 percent. At a BC/Al ratio of 0.6 (low level of protection), sugar 
maple growth can be decreased by 20 percent. The REA did not evaluate 
broad sensitive regions. However, in the sugar maple case study area 
(Kane Experimental Forest), recent deposition levels are associated 
with a BC/Al ratio below 1.2, indicating between intermediate and low 
level of protection, which would indicate the potential for a greater 
than 20 percent reduction in growth. In the red spruce case study area 
(Hubbard Brook Experimental Forest), recent deposition levels are 
associated with a BC/Al ratio slightly above 1.2, indicating slightly 
better than an intermediate level of protection (US EPA, 2009, section 
4.3.5.1).
    Over the full range of sugar maple, 12 percent of evaluated forest 
plots exceeded the critical loads for a BC/Al ratio of 1.2, and 3 
percent exceeded the critical load for a BC/Al ratio of 0.6. However, 
there was large variability across states. In New Jersey, 67 percent of 
plots exceeded the critical load for a

[[Page 46109]]

BC/Al ratio of 1.2, while in several states on the outskirts of the 
range for sugar maple (e.g. Arkansas, Illinois) no plots exceeded the 
critical load for a BC/Al ratio of 1.2. For red spruce, overall 5 
percent of plots exceeded the critical load for a BC/Al ratio of 1.2, 
and 3 percent exceeded the critical load for a BC/Al ratio of 0.6. In 
the major red spruce producing states (Maine, New Hampshire, and 
Vermont), critical loads for a BC/Al ratio of 1.2 were exceeded in 0.5, 
38, and 6 percent of plots, respectively.
    The ISA, REA and PA all conclude that the current standards are not 
adequate to protect against the adverse impacts of terrestrial 
acidification on sensitive ecosystems. As stated in the REA and PA, the 
main drawback, with the understanding of terrestrial acidification lies 
in the sparseness of available data by which we can predict critical 
loads and that the data are based on laboratory responses rather than 
field measurements. Other stressors that are present in the field but 
that are not present in the laboratory may confound this relationship. 
The REA does however, conclude that the case study results, when 
extended to a 27 state region, show that nitrogen and sulfur acidifying 
deposition in the sugar maple and red spruce forest areas caused the 
calculated Bc/Al ratio to fall below 1.2 (the intermediate level of 
protection) in 12 percent of the sugar maple plots and 5 percent of the 
red spruce plots; however, results from individual states ranged from 0 
to 67 percent of the plots for sugar maple and 0 to 100 percent of the 
plots for red spruce.
iii. Terrestrial Nutrient Enrichment
    Nutrient enrichment effects are due to nitrogen loadings from both 
atmospheric and non-atmospheric sources. Evaluation of nutrient 
enrichment effects requires an understanding that nutrient inputs are 
essential to ecosystem health and that specific long term levels of 
nutrients in a system affect the types of species that occur over long 
periods of time. Short term additions of nutrients can affect species 
competition, and even small additions of nitrogen in areas that are 
traditionally nutrient poor can have significant impacts on 
productivity as well as species composition. Most ecosystems in the 
U.S. are nitrogen-limited, so regional decreases in emissions and 
deposition of airborne nitrogen compounds could lead to some decrease 
in growth of the vegetation that surrounds the targeted aquatic system 
but as discussed below evidence for this is mixed. Whether these 
changes in plant growth are seen as beneficial or adverse will depend 
on the nature of the ecosystem being assessed.
    Information on the effects of changes in nitrogen deposition on 
forestlands and other terrestrial ecosystems is very limited. The 
multiplicity of factors affecting forests, including other potential 
stressors such as ozone, and limiting factors such as moisture and 
other nutrients, confound assessments of marginal changes in any one 
stressor or nutrient in forest ecosystems. The ISA notes that only a 
fraction of the deposited nitrogen is taken up by the forests, most of 
the nitrogen is retained in the soils (US EPA, 2008, section 3.3.2.1). 
In addition, the ISA indicates that forest management practices can 
significantly affect the nitrogen cycling within a forest ecosystem, 
and as such, the response of managed forests to nitrogen deposition 
will be variable depending on the forest management practices employed 
in a given forest ecosystem (US EPA, 2008, Annex C C.6.3). Increases in 
the availability of nitrogen in nitrogen-limited forests via 
atmospheric deposition could increase forest production over large non-
managed areas, but the evidence is mixed, with some studies showing 
increased production and other showing little effect on wood production 
(US EPA, 2008, section 3.3.9). Because leaching of nitrate can promote 
cation losses, which in some cases create nutrient imbalances, slower 
growth and lessened disease and freezing tolerances for forest trees, 
the net effect of increased N on forests in the U.S. is uncertain (US 
EPA, 2008, section 3.3.9).
    The scientific literature has many examples of the deleterious 
effects caused by excessive nitrogen loadings to terrestrial systems. 
Several studies have set benchmark values for levels of N deposition at 
which scientifically adverse effects are known to occur. Large areas of 
the country appear to be experiencing deposition above these 
benchmarks. The ISA indicates studies that have found that at 3.1 kg N/
ha/yr, the community of lichens begins to change from acidophytic to 
tolerant species; at 5.2 kg N/ha/yr, the typical dominance by 
acidophytic species no longer occurs; and at 10.2 kg N/ha/yr, 
acidophytic lichens are totally lost from the community. Additional 
studies in the Colorado Front Range of the Rocky Mountain National Park 
support these findings. These three values (3.1, 5.2, and 10.2 kg/ha/
yr) are one set of ecologically meaningful benchmarks for the mixed 
conifer forest (MCF) of the pacific coast regions. Nearly all of the 
known sensitive communities receive total nitrogen deposition levels 
above the 3.1 N kg/ha/yr ecological benchmark according to the 12 km, 
2002 CMAQ/NADP data, with the exception of the easternmost Sierra 
Nevadas. The MCFs in the southern portion of the Sierra Nevada forests 
and nearly all MCF communities in the San Bernardino forests receive 
total nitrogen deposition levels above the 5.2 N kg/ha/yr ecological 
benchmark.
    Coastal Sage Scrub communities are also known to be sensitive to 
community shifts caused by excess nitrogen loadings. Studies have 
investigated the amount of nitrogen utilized by healthy and degraded 
CSS systems. In healthy stands, the authors estimated that 3.3 kg N/ha/
yr was used for CSS plant growth. It is assumed that 3.3 kg N/ha/yr is 
near the point where nitrogen is no longer limiting in the CSS 
community and above which level community changes occur, including 
dominance by invasive species and loss of coastal sage scrub. 
Therefore, this amount can be considered an ecological benchmark for 
the CSS community. The majority of the known CSS range is currently 
receiving deposition in excess of this benchmark. Thus, the REA 
concludes that recent conditions where oxides of nitrogen ambient 
concentrations are at or below the current oxides of nitrogen secondary 
standards are not adequate to protect against anticipated adverse 
impacts from N nutrient enrichment in sensitive ecosystems.
iv. Aquatic Nutrient Enrichment
    The REA aquatic nutrient enrichment case studies focused on coastal 
estuaries and revealed that while current ambient loadings of 
atmospheric oxides of nitrogen are contributing to the overall 
depositional loading of coastal estuaries, other non-atmospheric 
sources are contributing in far greater amounts in total, although 
atmospheric contributions are as large as some other individual source 
types. The ability of current data and models to characterize the 
incremental adverse impacts of nitrogen deposition is limited, both by 
the available ecological indicators, and by the inability to attribute 
specific effects to atmospheric sources of nitrogen. The REA case 
studies used ASSETS EI as the ecological indicator for aquatic nutrient 
enrichment. This index is a six level index characterizing overall 
eutrophication risk in a water body. This indicator is not sensitive to 
changes in nitrogen deposition within a single level of the index. In 
addition, this type of indicator does not reflect the impact of 
nitrogen deposition in conjunction with other sources of nitrogen.

[[Page 46110]]

    Based on the above considerations, the REA concludes that the 
ASSETS EI is not an appropriate ecological indicator for estuarine 
aquatic eutrophication and that additional analysis is required to 
develop an appropriate indicator for determining the appropriate levels 
of protection from N nutrient enrichment effects in estuaries related 
to deposition of oxides of nitrogen. As a result, EPA is unable to make 
a determination as to the adequacy of the existing secondary oxides of 
nitrogen standard in protecting public welfare from nitrogen nutrient 
enrichment effects in estuarine aquatic ecosystems.
    Additionally, nitrogen deposition can alter species composition and 
cause eutrophication in freshwater systems. In the Rocky Mountains, for 
example, deposition loads of 1.5 to 2 kg/ha/yr which are well within 
current ambient levels are known to cause changes in species 
composition in diatom communities indicating impaired water quality (US 
EPA, 2008, section 3.3.5.3). This suggests that the existing secondary 
standard for oxides of nitrogen does not protect such ecosystems and 
their resulting services from impairment.
v. Other Effects
    An important consideration in looking at the effects of deposition 
of oxides of sulfur in aquatic ecosystems is the potential for 
production of MeHg, a neurotoxic contaminant. The production of 
meaningful amounts of MeHg requires the presence of 
SO42- and mercury, and where mercury is present, 
increased availability of SO42- results in 
increased production of MeHg. There is increasing evidence on the 
relationship between sulfur deposition and increased methylation of 
mercury in aquatic environments; this effect occurs only where other 
factors are present at levels within a range to allow methylation. The 
production of MeHg requires the presence of SO42- 
and mercury, but the amount of MeHg produced varies with oxygen 
content, temperature, pH and supply of labile organic carbon (US EPA, 
2008, section 3.4). In watersheds where changes in sulfate deposition 
did not produce an effect, one or several of those interacting factors 
were not in the range required for meaningful methylation to occur (US 
EPA, 2008, section 3.4). Watersheds with conditions known to be 
conducive to mercury methylation can be found in the northeastern 
United States and southeastern Canada (US EPA, 2009, section 6).
    With respect to sulfur deposition and mercury methylation, the 
final ISA determined that ''[t]he evidence is sufficient to infer a 
causal relationship between sulfur deposition and increased mercury 
methylation in wetlands and aquatic environments.'' However, EPA did 
not conduct a quantitative assessment of the risks associated with 
increased mercury methylation under current conditions. As such, EPA is 
unable to make a determination as to the adequacy of the existing 
SO2 secondary standards in protecting against welfare 
effects associated with increased mercury methylation.
vi. Summary of Adequacy Considerations
    In summary, the PA concludes that currently available scientific 
evidence and assessments clearly call into question the adequacy of the 
current standards with regard to deposition-related effects on 
sensitive aquatic and terrestrial ecosystems, including acidification 
and nutrient enrichment. Further, the PA recognizes that the elements 
of the current standards--indicator, averaging time, level and form--
are not ecologically relevant, and are thus not appropriate for 
standards designed to provide such protection. Thus, the PA concludes 
that consideration should be given to establishing a new ecologically 
relevant multi-pollutant, multimedia standard to provide appropriate 
protection from deposition-related ecological effects of oxides of 
nitrogen and sulfur on sensitive ecosystems, with a focus on protecting 
against adverse effects associated with acidifying deposition in 
sensitive aquatic ecosystems.
3. CASAC Views
    In a letter to the Administrator (Russell and Samet 2011a), the 
CASAC Oxides of Nitrogen and Oxides of Sulfur Panel, with full 
endorsement of the chartered CASAC, unanimously concluded that:

    EPA staff has demonstrated through the Integrated Science 
Assessment (ISA), Risk and Exposure Characterization (REA) and the 
draft PA that ambient NOX and SOX can have, 
and are having, adverse environmental impacts. The Panel views that 
the current NOX and SOX secondary standards 
should be retained to protect against direct adverse impacts to 
vegetation from exposure to gas phase exposures of these two 
families of air pollutants. Further, the ISA, REA and draft PA 
demonstrate that adverse impacts to aquatic ecosystems are also 
occurring due to deposition of NOX and SOX. 
Those impacts include acidification and undesirable levels of 
nutrient enrichment in some aquatic ecosystems. The levels of the 
current NOX and SOX secondary NAAQS are not 
sufficient, nor the forms of those standards appropriate, to protect 
against adverse depositional effects; thus a revised NAAQS is 
warranted.

    In addition, with regard to the joint consideration of both oxides 
of nitrogen and oxides of sulfur as well as the consideration of 
deposition related effects, CASAC concluded that the PA had developed a 
credible methodology for considering such effects. The Panel stated 
that ``the Policy Assessment develops a framework for a multi-
pollutant, multimedia standard that is ecologically relevant and 
reflects the combined impacts of these two pollutants as they deposit 
to sensitive aquatic ecosystems.''
4. Administrator's Proposed Conclusions Concerning Adequacy of Current 
Standard
    Based on the above considerations and taking into account CASAC 
advice, the Administrator recognizes that the purpose of the secondary 
standard is to protect against ``adverse'' effects resulting from 
exposure to oxides of nitrogen and sulfur, discussed above in section 
II.A. The Administrator also recognizes the need for conclusions as to 
the adequacy of the current standards for both direct and deposition 
related effects as well as conclusions as to the appropriateness and 
ecological relevance of the current standards.
    In considering what constitutes an ecological effect that is also 
adverse to the public welfare, the Administrator took into account the 
ISA conclusions regarding the nature and strength of the effects 
evidence, the risk and exposure assessment results, the degree to which 
the associated uncertainties should be considered in interpreting the 
results, the conclusions presented in the PA, and the views of CASAC 
and members of the public. On these bases, the Administrator concludes 
that the current secondary standards are adequate to protect against 
direct phytotoxic effects on vegetation. Thus, the Administrator 
proposes to retain the current secondary standard for oxides of 
nitrogen at 53 ppb,\4\ annual average concentration, measured in the 
ambient air as NO2, and the current secondary standard for 
oxides of sulfur at 0.5 ppm,

[[Page 46111]]

3-hour average concentration, measured in the ambient air as 
SO2.
---------------------------------------------------------------------------

    \4\ The annual secondary standard for oxides of nitrogen is 
being specified in units of ppb to conform to the current version of 
the annual primary standard, as specified in the final rule for the 
most recent review of the NO2 primary NAAQS (75 FR 6531; 
February 9, 2010).
---------------------------------------------------------------------------

    With regard to deposition-related effects, the Administrator has 
first to consider the appropriateness of the structure of the current 
standards to address ecological effects of concern. Based on the 
evidence as well as considering the advice given by CASAC on this 
matter, the Administrator concludes that the elements of the current 
standards are not ecologically relevant and thus are not appropriate to 
provide protection of ecosystems. On the subject of adequacy of 
protection with regard to deposition-related effects, the Administrator 
considered the full nature of ecological effects related to the 
deposition of ambient oxides of nitrogen and sulfur into sensitive 
ecosystems across the country. Her conclusions are based on the 
evidence presented in the ISA with regard to acidification and nutrient 
enrichment effects, the findings of the REA with regard to scope and 
severity of the current and likely future effects of deposition, the 
synthesis of both the scientific evidence and risk and exposure results 
in the PA as to the adequacy of the current standards, and the advice 
of both CASAC and the public. After such consideration, the 
Administrator concludes that current levels of oxides of nitrogen and 
sulfur are sufficient to cause acidification of both aquatic and 
terrestrial ecosystems, nutrient enrichment of terrestrial ecosystems 
and contribute to nutrient enrichment effects in estuaries that could 
be considered adverse, and the current secondary standards do not 
provide adequate protection from such effects.
    Having reached these conclusions, the Administrator determines that 
it is appropriate to consider alternative standards that are 
ecologically relevant. These considerations support the conclusion that 
the current secondary standards is neither appropriate nor adequate to 
protect against deposition related effects. The Administrator's 
consideration of such alternative standards is discussed below in 
Section III.

III. Rationale for Proposed Decision on Alternative Multi-Pollutant 
Approach to Secondary Standards for Aquatic Acidification

    Having reached the conclusion that the current NO2 and 
SO2 secondary standards are not adequate to provide 
appropriate protection against deposition-related effects associated 
with oxides of nitrogen and sulfur, the Administrator then considered 
what new multi-pollutant standard might be appropriate, at this time, 
to address such effects on public welfare. The Administrator recognizes 
that the inherently complex and variable linkages between ambient 
concentrations of nitrogen and sulfur oxides, the related deposited 
forms of nitrogen and sulfur, and the ecological responses that are 
associated with public welfare effects call for consideration of an 
ecologically relevant design of a standard that reflects these 
linkages. The Administrator also recognizes that characterization of 
such complex and variable linkages will necessarily require 
consideration of information and analyses that have important 
limitations and uncertainties.
    Despite its complexity, an ecologically relevant multi-pollutant 
standard to address deposition-related effects could still 
appropriately be defined in terms of the same basic elements that are 
used to define any NAAQS--indicator, form, averaging time, and level. 
The form would incorporate additional structural elements that reflect 
relevant multi-pollutant and multimedia attributes. These structural 
elements include the use of an ecological indicator, tied to the 
ecological effect we are focused on, and other elements that account 
for ecologically relevant factors other than ambient air 
concentrations. All of these elements would be needed to enable a 
linkage from ambient air indicators to the ecological indicator to 
define an ecologically relevant standard. As a result, such a standard 
would necessarily be more complex than the NAAQS that have been set 
historically to address effects associated with ambient concentrations 
of a single pollutant.
    More specifically, the Administrator considered an ecologically 
relevant multi-pollutant standard to address effects associated with 
acidifying deposition related to ambient concentrations of oxides of 
nitrogen and sulfur in sensitive aquatic ecosystems. This focus is 
consistent with the information presented in the ISA, REA, and PA, 
which highlighted the sufficiency of the quantity and quality of the 
available evidence and assessments associated with aquatic 
acidification relative to the information and assessments available for 
other deposition-related effects, including terrestrial acidification 
and aquatic and terrestrial nutrient enrichment. Based on its review of 
these documents, CASAC agreed that aquatic acidification should be the 
focus for developing a new multi-pollutant standard in this review. In 
reaching conclusions about an air quality standard designed to address 
deposition-related aquatic acidification effects, the Administrator 
also recognizes that such a standard may also provide some degree of 
protection against other deposition-related effects.
    As discussed in chapter 7 of the PA, the development of a new 
multi-pollutant standard to address deposition-related aquatic 
acidification effects recognizes the need for consideration of a 
nationally applicable standard for protection against adverse effects 
of aquatic acidification on public welfare, while recognizing the 
complex and heterogeneous interactions between ambient air 
concentrations of nitrogen and sulfur oxides, the related deposition of 
nitrogen and sulfur, and associated ecological responses. The 
development of such a standard also needs to take into account the 
limitations and uncertainties in the available information and analyses 
upon which characterization of such interactions are based. The 
approach used in the PA also recognizes that while such a standard 
would be national in scope and coverage, the effects to public welfare 
from aquatic acidification will not occur to the same extent in all 
locations in the U.S., given the inherent variability of the responses 
of aquatic systems to the effects of acidifying deposition.
    As discussed above in section II, many locations in the U.S. are 
naturally protected against acid deposition due to underlying 
geological conditions. Likewise, some locations in the U.S., including 
lands managed for commercial agriculture and forestry, are not likely 
to be negatively impacted by current levels of nitrogen and sulfur 
deposition. As a result, while a new ecologically relevant secondary 
standard would apply everywhere, it would be structured to account for 
differences in the sensitivity of ecosystems across the country. This 
would allow for appropriate protection of sensitive aquatic ecosystems, 
which are relatively pristine and wild and generally in rural areas, 
and the services provided by such sensitive ecosystems, without 
requiring more protection than is needed elsewhere.
    As discussed below, the multi-pollutant standard developed in the 
PA would employ (1) total reactive oxidized nitrogen (NOy) 
and SOX as the atmospheric ambient air indicators; (2) a 
form that takes into account variable factors, such as atmospheric and 
ecosystem conditions that modify the amounts of deposited nitrogen and 
sulfur; the distinction between oxidized and reduced forms of nitrogen; 
effects of deposited nitrogen and sulfur on aquatic ecosystems in terms 
of the ecological indicator ANC; and the

[[Page 46112]]

representativeness of water bodies within a defined spatial area; (3) a 
multi-year averaging time, and (4) a standard level defined in terms of 
a single, national target ANC value that, in the context of the above 
form, identifies the levels of concentrations of NOy and 
SOX in the ambient air that would meet the standard. The 
form of such a standard has been defined by an index, AAI, which 
reflects the relationship between ambient concentrations of 
NOy and SOX and aquatic acidification effects 
that result from nitrogen and sulfur deposition related to these 
ambient concentrations.
    In presenting the considerations associated with such an air 
quality standard to address deposition-related aquatic acidification 
effects, the following sections focus on each element of the standard, 
including indicator (section III.A), form (section III.B), averaging 
time (section III.C), and level (section III.D). Alternative 
combinations of levels and forms are discussed in section III.E. 
Considerations related to important uncertainties inherent in such an 
approach are discussed in section III.F. Advice from CASAC on such a 
new standard is presented in section III.G. The Administrator's 
proposed decisions on such a new standard are presented in section 
III.H.

A. Ambient Air Indicators

    In considering alternative ambient air indicators, the PA primarily 
focuses on the important attribute of association. Association in a 
broad sense refers to how well an ambient air indicator relates to the 
ecological effects of interest by virtue of both the framework that 
links the ambient indicator and effects and the empirical evidence that 
quantifies the linkages. The PA also considers how measurable or 
quantifiable an indicator is to enable its use as an effective 
indicator of relevant ambient air concentrations.
    As discussed above in section II.C, the PA concludes that 
indicators other than NO2 and SO2 should be 
considered as the appropriate indicators of oxides of nitrogen and 
sulfur in the ambient air for protection against the acidification 
effects associated with deposition of the associated nitrogen and 
sulfur. This conclusion is based on the recognition that all forms of 
nitrogen and sulfur in the ambient air contribute to deposition and 
resulting acidification, and as such, NO2 and SO2 
are incomplete indicators. In principle, the ambient indicators should 
represent the species that are associated with oxides of nitrogen and 
sulfur in the ambient air and can contribute acidifying deposition. 
This includes both the species of oxides of nitrogen and sulfur that 
are directly emitted as well as species transformed in the atmosphere 
from oxides of nitrogen and sulfur that retain the nitrogen and sulfur 
atoms from directly emitted oxides of nitrogen and sulfur. All of these 
compounds are associated with oxides of nitrogen and sulfur in the 
ambient air and can contribute to acidifying deposition.
    The PA focuses in particular on the various compounds with nitrogen 
or sulfur atoms that are associated with oxides of nitrogen and sulfur, 
because the acidifying potential is specific to nitrogen and sulfur, 
and not other atoms (e.g., H, C, O) whether derived from the original 
source of oxides of nitrogen and sulfur emissions or from atmospheric 
transformations. For example, the acidifying potential of each molecule 
of NO2, NO, HNO3 or PAN is identical, as is the 
potential for each molecule of SO2 or ion of particulate 
sulfate, p-SO4. Each atom of sulfur affords twice the 
acidifying potential of each atom of nitrogen.
1. Oxides of Sulfur
    As discussed in the PA (US EPA, 2011, section 7.1.1), oxides of 
sulfur include the gases sulfur monoxide (SO), SO2, sulfur 
trioxide (SO3), disulfur monoxide (S2O), and 
particulate-phase sulfur compounds (referred to as SO4) that 
result from gas-phase sulfur oxides interacting with particles. 
However, the sum of SO2 and SO4 does represent 
virtually the entire ambient air mass of sulfur that contributes to 
acidification. In addition to accounting for virtually all the 
potential for acidification from oxidized sulfur in the ambient air, 
there are reliable methods to monitor the concentrations of 
SO2 and particulate SO4. In addition, much of the 
data used to develop the technical basis for the standard developed in 
the PA is based on monitoring or modeling of these species.\5\ The PA 
concludes that the sum of SO2 and SO4, referred 
to as SOX, are appropriate ambient air indicators of oxides 
of sulfur because they represent virtually all of the acidification 
potential of ambient air oxides of sulfur and there are reliable 
methods suitable for measuring SO2 and SO4.
---------------------------------------------------------------------------

    \5\ As discussed in chapter 2 of the PA, SO2 and 
particulate SO4 are routinely measured in ambient air 
monitoring networks, although only the Clean Air Status and Trends 
Network (CASTNET) filter packs do not intentionally exclude particle 
size fractions. The CMAQ treatment of SOX is the simple 
addition of both species, which are treated explicitly in the model 
formulation. All particle size fractions are included in the CMAQ 
SOX estimates.
---------------------------------------------------------------------------

2. Oxides of Nitrogen
    As discussed in the PA (US EPA, 2011, section 7.1.2), 
NOy, as defined in chapter 2 of the PA, incorporates 
basically all of the oxidized nitrogen species that have acidifying 
potential and as such, NOy should be considered as an 
appropriate indicator for oxides of nitrogen. Total reactive oxidized 
nitrogen is an aggregate measure of NO and NO2 and all of 
the reactive oxidized products of NO and NO2. That is, 
NOy is a group of nitrogen compounds in which all of the 
compounds are either an oxide of nitrogen or compounds in which the 
nitrogen atoms came from oxides of nitrogen. Total reactive oxidized 
nitrogen is especially relevant as an ambient indicator for 
acidification in that it both relates to the oxides of nitrogen in the 
ambient air and also represents the acidification potential of all 
oxidized nitrogen species in the ambient air, whether an oxide of 
nitrogen or derived from oxides of nitrogen.
    There are currently available reliable methods of measuring 
aggregate NOy. The term ``aggregate'' measure means that the 
NOy, as measured, is not based on measuring each individual 
species of NOy and calculating an NOy value by 
summing the individual species. Rather, as described in chapter 2 of 
the PA, current measurement techniques process all of the individual 
NOy species to produce a single aggregate measure of all of 
the nitrogen atoms associated with any NOy species. 
Consequently, the NOy measurement effectively provides the 
sum of all individual species, but the identity of the individual 
species is lost. As discussed above, the accounting for the individual 
nitrogen atoms is an accounting of the ambient air acidification 
potential of oxides of nitrogen and their transformation products and 
therefore the most relevant ambient indicator for aquatic acidification 
effects associated with oxides of nitrogen.
    This loss of the information on individual species motivated 
consideration of alternative or more narrowly defined indicators for 
oxides of nitrogen in the PA. Consideration of a subset of 
NOy species was based on the following reasoning. First, the 
actual dry deposition of nitrogen is determined on an individual 
species basis by multiplying the species concentration times a species-
specific deposition velocity and then summed to develop an estimate of 
total dry deposition. Consequently, the use of individual ambient 
species has the potential to be more consistent with the underlying

[[Page 46113]]

science of deposition and, therefore, has the potential to allow for a 
more rigorous evaluation of dry deposition with specialized field 
studies. In addition, there has been a suggestion of focusing only on 
the most quickly depositing NOy species, such as 
HNO3, as contributions from other NOy species 
such as NO2 may be negligible. These alternative indicators 
are discussed below.
    The PA considers the relative merits of using each individual 
NOy species as part of a group of indicators. In so doing, 
it was first noted that dry deposition of NOy is treated as 
the sum of the deposition of each individual species in advanced 
process-based air quality models like CMAQ, as described in chapter 2 
of the PA. Conceptually one could extend this process-based approach by 
using all NOy species individually as separate indicators 
for oxides of nitrogen and requiring, for example, measurements of each 
of the species, including the dominant species of HNO3, 
particulate nitrate (p-NO3), true NO2, NO and 
PAN. The potential attraction of using individual species would be the 
reliance on actual deposition velocities. This could have more physical 
meaning in comparison to a constructed model of aggregate deposition of 
NOy, which is difficult to evaluate with observations 
because of the assimilation of many species with disparate deposition 
behavior. The PA notes that the major drawback of using individual 
NOy species as the indicators is the lack of reliable 
measurement techniques, especially for PAN and NO2 in rural 
locations, which renders the use of virtually any individual 
NOy species, except for NO and perhaps p-NO3, as 
functionally inadequate from a measurement perspective.
    The PA next considered the relative merits of using a subset of 
NOy species as the indicators for oxides of nitrogen, as was 
discussed above for oxides of sulfur. To the extent that certain 
species provide relatively minor contributions to total NOy 
deposition, it may be appropriate to consider excluding them as part of 
the indicator. As discussed in chapter 2 of the PA, each nitrogen 
species within the array of NOy species has species-specific 
dry deposition velocities. For example, the deposition velocity of 
HNO3 is much greater than the velocity for NO2 
and, consequently, for a similar ambient air concentration, 
HNO3 contributes more deposition of acidifying nitrogen 
relative to NO2. In transitioning from source-oriented urban 
locations to rural environments, the ratio of the concentrations of 
HNO3 and PAN to NO2 increases.
    Based on the reasoning that a larger fraction of the deposited 
NOy is accounted for by total nitrate (the sum of 
HNO3 and p-NO3), a surrogate for the more rapidly 
depositing fraction of NOy, combined with the availability 
of reliable total nitrate measurements through the CASTNET, the PA 
considered using total nitrate as the indicator for oxides of nitrogen 
(US EPA, 2011, appendix E). Nitrate would be expected to correlate well 
with total reactive oxidized nitrogen deposition relative to 
NOy (US EPA, 2011, chapter 2) despite the inherent noise 
associated with variable contributions of low deposition velocity 
species (e.g., NO2) that may have relatively high ambient 
concentrations. However, modeling simulations suggest that 
NOy may be a more robust indicator, relative to 
HNO3, in terms of relating absolute changes in ambient air 
concentrations to changes in nitrogen deposition driven by changes in 
ambient concentrations of oxides of nitrogen (US EPA, 2011, Figure 2-
32).
    Based on the above considerations, the PA concludes that 
NOy should be considered as the appropriate ambient 
indicator for oxides of nitrogen based on its direct relationship to 
oxides of nitrogen in the ambient air and its direct relationship to 
deposition associated with aquatic acidification. Because 
NOy represents all of the potentially acidifying oxidized 
nitrogen species in the ambient air, it is appropriately associated 
with the deposition of potentially acidifying compounds associated with 
oxides of nitrogen in the ambient air. In addition, there are reliable 
methods available to measure NOy. Measurement of each 
individual species of NOy, or the measurement of only a 
subset of species of NOy, is less appropriate because there 
are not reliable measurements methods available to measure all of the 
individual species of NOy and a subset of species would fail 
to account for significant portions of the oxidized reactive nitrogen 
that relate to acidification.\6\
---------------------------------------------------------------------------

    \6\ The PA also notes that NOy is a useful 
measurement for model evaluation purposes, which is especially 
important, recognizing the unique role that CMAQ plays in the 
development of this standard, as described below in section III.B.
---------------------------------------------------------------------------

B. Form

    Based on the evidence of the aquatic acidification effects caused 
by the deposition of NOy and SOX, the PA (US EPA, 
2011, section 7.2) presents the development of a new form that is 
ecologically relevant for addressing such effects. The conceptual 
design for the form of such a standard includes three main components: 
an ecological indicator, deposition metrics that relate to the 
ecological indicator, and a function that relates ambient air 
indicators to deposition metrics. Collectively, these three components 
link the ecological indicator to ambient air indicators, as illustrated 
above in Fig II-1.
    The simplified flow diagram in Figure II-1 compresses the various 
atmospheric, biological, and geochemical processes associated with 
acidifying deposition to aquatic ecosystems into a simplified 
conceptual picture. The ecological indicator (left box) is related to 
atmospheric deposition through biogeochemical ecosystem models (middle 
box), which associate a target deposition load to a target ecological 
indicator. Once a target deposition is established, associated 
allowable air concentrations are determined (right box) through the 
relationships between concentration and deposition that are embodied in 
air quality models such as CMAQ. The following discussion describes the 
development and rationale for each of these components, as well as the 
integration of these components into the full expression of the form of 
the standard using the concept of a national AAI that represents a 
target ANC level as a function of ambient air concentrations. Spatial 
aggregation issues associated with defining each of the terms of this 
index are also addressed below.
    The AAI is designed to be an ecologically relevant form of the 
standard that determines the levels of NOy and 
SOX in the ambient air that would achieve a target ANC limit 
for the U.S. The intent of the AAI is to weight atmospheric 
concentrations of oxides of nitrogen and sulfur by their propensity to 
contribute to acidification through deposition, given the fundamental 
acidifying potential of each pollutant, and to take into account the 
ecological factors that govern acid sensitivity in different 
ecosystems. The index also accounts for the contribution of reduced 
nitrogen to acidification. Thus, the AAI encompasses those attributes 
of specific relevance to protecting ecosystems from the acidifying 
potential of ambient air concentrations of NOy and 
SOX.
1. Ecological Indicator
    In considering alternative ecological indicators, the PA again 
primarily focuses on the attribute of association. In the case of an 
ecological indicator for aquatic acidification, association refers to 
the relationship between the indicator and adverse effects as discussed 
in section II. Because of the conceptual structure of the form of an

[[Page 46114]]

AAI-based standard (Figure III-1), this particular ecological indicator 
must also link up in a meaningful and quantifiable manner with 
acidifying atmospheric deposition. In effect, the ecological indicator 
for aquatic acidification is the bridge between biological impairment 
and deposition of NOy and SOX.
    This section presents the rationale in the PA for selecting ANC as 
the appropriate ecological indicator for consideration. Recognizing 
that ANC is not itself the causative or toxic agent for adverse aquatic 
acidification effects, the rationale for using ANC as the relevant 
ecological indicator is based on the following:
    (1) The ANC is directly associated with the causative agents, pH 
and dissolved Al, both through empirical evidence and mechanistic 
relationships;
    (2) Empirical evidence shows very clear and strong relationships 
between adverse effects and ANC;
    (3) The ANC is a more reliable indicator from a modeling 
perspective, allowing use of a body of studies and technical analyses 
related to ANC and acidification to inform the development of the 
standard; and
    (4) The ANC literally embodies the concept of acidification as 
posed by the basic principles of acid base chemistry and the 
measurement method used to estimate ANC and, therefore, serves as a 
direct index to protect against acidification.
    Ecological indicators of acidification in aquatic ecosystems can be 
chemical or biological components of the ecosystem that are altered by 
the acidifying effects of nitrogen and sulfur deposition. A desirable 
ecological indicator for aquatic acidification is one that is 
measurable or estimable, linked causally to deposition of nitrogen and 
sulfur, and linked causally, either directly or indirectly to 
ecological effects known or anticipated to adversely affect public 
welfare.
    As summarized in chapter 2 of the PA, atmospheric deposition of 
NOy and SOX causes aquatic acidification through 
the input of strong acid anions (e.g., NO3- and 
SO42-) that ultimately shifts the water chemistry 
equilibrium toward increased hydrogen ion levels (or decreased pH). The 
anions are deposited either directly to the aquatic ecosystem or 
indirectly via transformation through soil nitrification processes and 
subsequent drainage from terrestrial ecosystems. In other words, when 
these anions are mobilized in the terrestrial soil, they can leach into 
adjacent water bodies. Aquatic acidification is indicated by changes in 
the surface water chemistry of ecosystems. In turn, the alteration of 
surface water chemistry has been linked to negative effects on the 
biotic integrity of freshwater ecosystems. There is a suite of chemical 
indicators that could be used to assess the effects of acidifying 
deposition on lake or stream acid-base chemistry. These indicators 
include ANC; alkalinity (ALK); base neutralizing capacity, commonly 
referred to as acidity (ACY); surface water pH; concentrations of 
trivalent aluminum, Al\+3\; and concentrations of major anions 
(SO42-, NO3-), cations 
(Ca\2+\, Mg\+2\, K\+\), or sums of cations or anions.
    The ANC and ALK are very similar quantities and are used 
interchangeably in the literature and for some of the analyses 
presented in this document. Both ANC and ALK are defined as the amount 
of strong acid required to reach a specified equivalence point. For 
acid-base solutions, an equivalence point can be thought of as the 
point at which the addition of strong acids (i.e., titration) is no 
longer neutralized by the solution. This explains the term acid 
neutralizing capacity, or ANC, as ANC relates directly to the capacity 
of a system to neutralize acids. The differences between ANC and ALK 
are based on operational definitions and subject to various 
interpretations. The ANC is preferred over ALK as the body of 
scientific evidence has focused on ANC and effects relationships. The 
ALK is more widely associated with more general characterizations of 
water quality such as the relative hardness of water associated with 
carbonates.
    Indictors such as the concentrations of specific anions, cations, 
or their groupings, while relevant to acidification processes, are not 
robust acidification indicators as it is the relative balance of 
cations and anions that is more directly associated with acidification. 
That balance is captured by ANC and ALK. Acidity, ACY, is the converse 
of ANC and indicates how much strong base it takes to reach an 
equivalence point. Because ACY is not used in most ecosystem 
assessments, the body of information relating ACY to effects is too 
limited to serve as a basis for an appropriate ecological indicator. 
Aluminum and other metals are causative toxic agents that directly 
impair biological functions. However, Al, or metals in general, have 
high variability in concentrations that can be linked to effects, often 
at extremely low levels which in some cases approach detectability 
limits, exhibit rapid transient responses, and are often confounded by 
the presence of other toxic metals. These concerns limit the use of 
metals as reliable and measurable ecological indicators. Hydrogen ion 
(H\+\) concentrations, using their negative logarithmic values, or pH, 
are well correlated with adverse effects, as discussed above in section 
II.A, and determine the solubility of metals such as aluminum. However, 
pH is not a preferred acidification indicator due to its highly 
transient nature and other concerns, as discussed below.
    Having reasoned that ANC is a preferred indicator to ALK, ACY, 
individual metals or groupings of ions, the PA considers the relative 
merits of ANC compared to pH, which is a well recognized indicator of 
acidity and a more direct causative agent with regard to adverse 
effects. First, the linkage between ANC and pH is considered in 
recognition of the causative association between pH and effects.
    The ANC is not the direct causative toxic agent impacting aquatic 
species diversity. The scientific literature generally emphasizes the 
links between pH and adverse effects as described above in section 
II.A. It is important, therefore, to consider the extent to which ANC 
and pH are well related from a mechanistic perspective as well as 
through empirical evidence. The ANC and pH are co-dependent on each 
other based on the requirement that all solutions are electrically 
neutral, meaning that any solution must satisfy the condition that all 
negatively charged species must be balanced by all positively charged 
species. The ANC is defined as the difference between strong anions and 
cations (US EPA, 2011, equation 7-13).
    While the chemistry can be complex, the co-dependency between ANC 
and pH is explained by recognizing that positively charged hydrogen, 
H\+\, is incorporated in the charge balance relationships related to 
the overall solution chemistry which also defines ANC. The positive, 
directional co-dependency (i.e., ANC and pH increase together) is 
further explained in concept as ANC reflects how much strong acid 
(i.e., how much hydrogen ion) it takes to titrate to an equivalence 
point. Strong observed correlations between pH and ANC as described in 
the PA support these mechanistic relationships.
    As discussed above in section II.A, there are well established 
examples of ANC correlating strongly with a variety of ecological 
effects which are summarized in the PA (US EPA, 2011, Table 3-1). 
Because pH and ANC are well correlated and linearly dependent over the 
pH ranges (4.5-6) where adverse ecological effects are observed, 
evidence of clear associations exist between ANC and adverse ecological 
effects as described in the PA. In large measure, this dependence 
between pH

[[Page 46115]]

and ANC and the relationship of both pH and ANC to effects, speak 
directly to the appropriateness of ANC with respect to its use as an 
ecological indicator.
    Thus, there is a clear association between ANC and ecological 
effects, although there is a more direct causal relationship between pH 
and ecological effects. Nonetheless, ANC is preferred as an ecological 
indicator based on its superior ability to provide a linkage with 
deposition in a meaningful and quantifiable manner, a role that is 
served far more effectively by ANC than by pH. While both ANC and pH 
are clearly associated with the effects of concern, ANC is superior in 
linking these effects to deposition.
    The PA notes that the basis for this conclusion is that acidifying 
atmospheric deposition of nitrogen and sulfur is a direct input of 
potential acidity (ACY), or, in terms of ANC, such deposition is 
relevant to the major anions that reduce the capacity of a water body 
to neutralize acidity. Consequently, there is a well defined linear 
relationship between potential acidifying deposition and ANC. This ANC-
deposition relationship facilitates the linkage between ecosystem 
models that calculate an ecological indicator and the atmospheric 
deposition of NOy and SOX. On the other hand, 
there is no direct linear relationship between deposition and pH. While 
acid inputs from deposition lower pH, the relationship can be extremely 
nonlinear and there is no direct connection from a modeling or mass 
balance perspective between the amount of deposition entering a system 
and pH. The term ``mass balance'' underlies the basic formulation of 
any physical modeling construct, for atmospheric or aquatic systems, 
and refers to the accounting of the flow of mass into a system, the 
transformation to other forms, and the loss due to flow out of a system 
and other removal processes. The ANC is a conserved property. This 
means that ANC in a water body can be accounted for by knowledge of how 
much ANC initially exists, how much flows in and is deposited, and how 
much flows out. In contrast, hydrogen ion concentration in the water, 
the basis for pH, is not a conserved property as its concentration is 
affected by several factors such as temperature, atmospheric pressure, 
mixing conditions of a water body, and the levels of several other 
chemical species in the system. The disadvantage of pH lacking 
conservative properties is that there is a very complex connection 
between changes in ambient air concentrations of NOy and 
SOX and pH.
    The discussion of basic water chemistry of natural systems in 
chapter 2 of the PA provides further details on why pH is not a 
conserved quantity and is subject to rapid transient response behavior 
that makes it difficult to use as a reliable and functional ecological 
indicator. The observed pH-to-ANC relationship (US EPA, 2011, figure 7-
2) partially explains the concern with pH responding too abruptly. In 
the region where pH ranges roughly from 4.5 to 6 and is of greatest 
relevance to effects (US EPA, 2011, figure 7-4), there clearly is more 
sensitivity of pH to changes in ANC in the ANC range from approximately 
0 to 50 [micro]eq/L. A focus on this part of the ANC-to-pH relationship 
shows that ANC associates well with pH in a fairly linear manner. 
However, the pH range from 4.5 to 6 also includes one of the very 
steepest parts of the slope relating pH as a function of ANC, where ANC 
ranges down below 0 [micro]eq/L, which is subject to very rapid change 
in ANC, or deposition inputs. This part of the relationship coincides 
with reduced levels of ANC and hence with reduced ability to neutralize 
acids and moderate pH fluctuations. This response behavior can be 
extended to considering how pH would change in response to deposition, 
or ambient concentrations, of NOy and SOX, which 
can be viewed as ``ANC-like'' inputs.
    In summary, because ANC clearly links both to biological effects of 
aquatic acidification as well as to acidifying inputs of NOy 
and SOX deposition, the PA concludes that ANC is an 
appropriate ecological indicator for relating adverse aquatic ecosystem 
effects to acidifying atmospheric deposition of SOx and 
NOy, and is preferred to other potential indicators. In 
reaching this conclusion, the PA notes that in its review of the first 
draft PA, CASAC concluded that ``information on levels of ANC 
protective to fish and other aquatic biota has been well developed and 
presents probably the lowest level of uncertainty in the entire 
methodology'' (Russell and Samet, 2010a). In its more recent review of 
the second draft PA, CASAC agreed ``that acid neutralizing capacity is 
an appropriate ecological measure for reflecting the effects of aquatic 
acidification'' (Russell and Samet, 2010b; p. 4).
2. Linking ANC to Deposition
    There is evidence to support a quantified relationship between 
deposition of nitrogen and sulfur and ANC. This relationship was 
analyzed in the REA for two case study areas, the Adirondack and 
Shenandoah Mountains, based on time-series modeling and observed 
trends. In the REA analysis, long-term trends in surface water nitrate, 
sulfate and ANC were modeled using MAGIC for the two case study areas. 
These data were used to compare recent surface water conditions in 2006 
with preindustrial conditions (i.e. preacidification 1860). The results 
showed a marked increase in the number of acid impacted lakes, 
characterized as a decrease in ANC levels, since the onset of 
anthropogenic nitrogen and sulfur deposition, as discussed in chapter 2 
of the PA.
    In the REA, more recent trends in ANC, over the period from 1990 to 
2006, were assessed using monitoring data collected at the two case 
study areas. In both case study areas, nitrate and sulfate deposition 
decreased over this time period. In the Adirondack Mountains, this 
corresponded to a decreased concentration of nitrate and sulfate in the 
surface waters and an increase in ANC (U.S. EPA, 2009, section 
4.2.4.2). In the Shenandoah Mountains, there was a slight decrease in 
nitrate and sulfate concentration in surface waters corresponding to 
modest increase in ANC from 50 [mu]eq/L in 1990 to 67 [mu]eq/L in 2006 
(U.S. EPA, 2009, section 4.2.4.3, Appendix 4, and section 3.4).
    In the REA, the quantified relationship between deposition and ANC 
was investigated using ecosystem acidification models, also referred to 
as acid balance models or critical loads models (U.S. EPA, 2011, 
section 2 and U.S. EPA, 2009, section 4 and Appendix 4). These models 
quantify the relationship between deposition of nitrogen and sulfur and 
the resulting ANC in surface waters based on an ecosystem's inherent 
generation of ANC and ability to neutralize nitrogen deposition through 
biological and physical processes. A critical load is defined as the 
amount of acidifying atmospheric deposition of nitrogen and sulfur 
beyond which a target ANC is not reached. Relatively high critical load 
values imply that an ecosystem can accommodate greater deposition 
levels than lower critical loads for a specific target ANC level. 
Ecosystem models that calculate critical loads form the basis for 
linking deposition to ANC.
    As discussed in chapter 2 of the PA, both dynamic and steady state 
models calculate ANC as a function of ecosystem attributes and 
atmospheric nitrogen and sulfur deposition, and can be used to 
calculate critical loads. Steady state models are time invariant and 
reflect the long term consequences associated with an ecosystem 
reaching equilibrium under a constant level of atmospheric deposition. 
Dynamic models are time variant and take into account the time 
dependencies inherent in ecosystem hydrology, soil and

[[Page 46116]]

biological processes. Dynamic models like MAGIC can provide the time 
series response of ANC to deposition whereas steady state models 
provide a single ANC relationship to any fixed deposition level. 
Dynamic models naturally are more complex than steady state models as 
they attempt to capture as much of the fundamental biogeochemical 
processes as practicable, whereas steady state models depend on far 
greater parameterization and generalization of processes that is 
afforded, somewhat, by not having to accounting for temporal 
variability.
    The PA notes that steady state models are capable of addressing the 
question of what does it take to reach and sustain a specific level of 
ANC. Dynamic models are also capable of addressing that question, but 
can also address the question of how long it takes to achieve that 
result. Dynamic models afford the ability for more comprehensive 
treatment of a variety of processes throughout the surface, soil and 
bedrock layers within an ecosystem. For example, steady state models 
treat sulfate as a mobile anion throughout the system, meaning that the 
sulfate that is deposited to a watershed enters the water column and is 
not influenced by soil adsorption or cation exchange. Dynamic models 
can incorporate these time variant processes. The use of a steady state 
model treating sulfate as totally mobile does not necessarily conflict 
with the possibility of sulfate acting as a less than mobile ion at 
certain times. The steady state assumption is premised on the long term 
behavior of sulfate which can undergo periods of net adsorption 
followed by periods of net desorption which can balance out over time. 
The PA recognizes that as the richness of the available data increases, 
in terms of parameters and spatial resolution, the incorporation of 
dynamic modeling approaches in the standard setting process should 
become more feasible. In determining an appropriate modeling approach 
for the development of a NAAQS in this review, the PA considers both 
the relevance of the question addressed as well as the ability to 
perform modeling that provides relevant information for geographic 
areas across the country.
    Dynamic models require a large amount of catchment level-specific 
data relative to steady state models. Because of the time invariant 
nature of steady state models, the data requirements that integrate 
across a broad spectrum of ecosystem processes is achievable and 
available now at the national level. Water quality data to support 
steady state models currently exist for developing a national data base 
for modeling nearly 10,000 catchments in the contiguous U.S. In 
contrast, the data needs to support dynamic models for national-scale 
analyses simply are not available at this time. Further, the 
information provided by steady state modeling would be sufficient to 
develop and analyze alternative NAAQS and the kind of protection they 
would afford. While it would be of interest to also obtain information 
about how much time it would take for a target ANC level to be 
achieved, the absence of such information does not preclude developing 
and evaluating alternative NAAQS using the AAI structure. Based on the 
above considerations, the PA concludes that at this time steady state 
critical load modeling is an appropriate tool for linking long-term ANC 
levels to atmospheric deposition of nitrogen and sulfur for development 
of an AAI that has national applicability.
    A steady state model is used to define the critical load, which is 
the amount of atmospheric deposition of nitrogen (N) and sulfur (S) 
beyond which a target ANC is not achieved and sustained.\7\ It is 
expressed as:
---------------------------------------------------------------------------

    \7\ This section discusses the linkages between deposition of 
nitrogen and sulfur and ANC. Section III.B.3 then discusses the 
linkages between atmospheric concentrations of NOY and 
SOX and deposition of nitrogen and sulfur.

[GRAPHIC] [TIFF OMITTED] TP01AU11.024

---------------------------------------------------------------------------
Where:

CLANClim(N + S) is the critical load of deposition, with 
units of equivalent charge/(area-time);
    [BC]0-* is the natural contribution of 
base cations from weathering, soil processes and preindustrial 
deposition, with units of equivalent charge/volume;
[ANClim] is the target ANC value, with units of 
equivalent charge/volume; Q is the catchment level runoff rate 
governed by water mass balance and dominated by precipitation, with 
units of distance/time; and
Neco is the amount of nitrogen deposition that is effectively 
neutralized by a variety of biological (e.g., nutrient uptake) and 
physical processes, with units of equivalent charge/(area-time).

    Equation III-1 is a modified expression that adopts the basic 
formulation of the steady state models that are described in chapter 2 
of the PA. More detailed discussion of the rationale, assumptions and 
derivation of equation III-1, as well as all of the equations in this 
section, are included in Appendix B of the PA. The equation simply 
reflects the amount of deposition of nitrogen and sulfur from the 
atmosphere, CLANClim(N + S), that is associated with a 
sustainable long-term ANC target, [ANClim], given the 
capacity of the natural system to generate ANC, 
[BC]0-*, and the capacity of the natural system 
to neutralize nitrogen deposition, Neco. This expression of critical 
load is valid when nitrogen deposition is greater than Neco.\8\ The 
runoff rate, Q, allows for balancing mass in the two environmental 
mediums--atmosphere and catchment. This critical load expression can be 
focused on a single water system or more broadly. To extend 
applicability of the critical load expression (equation III-1) from the 
catchment level to broader spatial areas, the terms Qr and 
CLr, are used, which are the runoff rate and critical load, 
respectively, of the region over which all the atmospheric terms in the 
equation are defined.
---------------------------------------------------------------------------

    \8\ Because Neco is only relevant to nitrogen deposition, in 
rare cases where Neco is greater than the total nitrogen deposition, 
the critical load would be defined only in terms of acidifying 
deposition of sulfur and the Neco term in equation III-1 would be 
set to zero.
---------------------------------------------------------------------------

    In considering the contributions of SOx or 
NOy species to acidification, it is useful to think of every 
depositing nitrogen atom as supplying one equivalent charge unit and 
every sulfur atom as depositing two charge units. The PA uses 
equivalent charge per volume as a normalizing tool in place of the more 
familiar metrics such as mass or moles per volume. This allows for a 
clearer explanation of many of the relationships between atmospheric 
and ecosystem processes that incorporate mass and volume unit 
conventions somewhat specific to the environmental media of concern 
(e.g., m\3\ for air and liter for liquid water). Equivalent charge 
reflects the chemistry equilibrium fundamentals that assume 
electroneutrality, or balancing charge where the sum of cations always 
equals the sum of anions.
    As presented above, the terms S and N in the CLANClim (N 
+ S) term broadly represent all species of sulfur or nitrogen that can 
contribute to

[[Page 46117]]

acidifying deposition. This follows conventions used in the scientific 
literature that addresses critical loads, and it reflects all possible 
acidifying contributions from any sulfur or nitrogen species. For all 
practical purposes, S reflects SOx as described above, the 
sum of sulfur dioxide gas and particulate sulfate. However, N in 
equation III-1 includes both oxidized forms, consistent with the 
ambient indicator, NOy, in addition to the reduced nitrogen 
species, ammonia and ammonium ion, referred to as NHx. The 
NHX is included in the critical load formulation because it 
contributes to potentially acidifying nitrogen deposition. 
Consequently, from a mass balance or modeling perspective, the form of 
the standard needs to account for NHX, as described below.
3. Linking Deposition to Ambient Air Indicators
    The last major component of the form illustrated in Figure III-1 
addresses the linkage between deposition of nitrogen and sulfur and 
concentrations of the ambient air indicators, NOY and 
SOX. To link ambient air concentrations with deposition, the 
PA defines a transference ratio, T, as the ratio of total wet and dry 
deposition to ambient concentration, consistent with the area and time 
period over which the standard is defined. To express deposition of 
NOY and SOX in terms of NOY and 
SOX ambient concentrations, two transference ratios were 
defined, where TSOx equals the ratio of the combined dry and 
wet deposition of SOx to the ambient air concentration of 
SOx, and TNOY equals the ratio of the combined 
dry and wet deposition of NOY to the ambient air 
concentration of NOY.
    As described in chapter 7 of the PA, reduced forms of nitrogen 
(NHx) are included in total nitrogen in the critical load 
equation, III-1. Reduced forms of nitrogen are treated separately, as 
are NOy and SOx, and the transference ratios are 
applied. This results in the following critical load expression that is 
defined explicitly in terms of the indicators NOY and 
SOx:
[GRAPHIC] [TIFF OMITTED] TP01AU11.025

This is the same equation as III-1, with the deposition associated with 
the critical load translated to deposition from ambient air 
concentrations via transference ratios. In addition, deposition of 
reduced nitrogen, oxidized nitrogen and oxidized sulfur are treated 
separately.
    Transference ratios are a modeled construct, and therefore cannot 
be compared directly to measurable quantities. There is an analogy to 
deposition velocity, as a transference ratio is basically an aggregated 
weighted average of the deposition velocities of all contributing 
species across dry and wet deposition, and transference ratio units are 
expressed as distance/time. However, wet deposition commonly is not 
interpreted as the product of a concentration times a velocity. Direct 
wet deposition observations are available which integrate all of the 
processes, regardless of how well they may be understood, related to 
wet deposition into a measurable quantity. There are reasonable 
analogies between the processes governing dry and wet deposition, from 
a fundamental mass transfer perspective. In both cases there is a 
transfer of mass between the dry ambient phase and another medium, 
either a surface or vegetation in the case of dry deposition, or a rain 
droplet or cloud in the case of wet precipitation. The specific 
thermodynamic properties and chemical/biological reactions that govern 
the transfer of dry mass to plants or aqueous droplets differ, but 
either process can be based on conceptualizing the product of a 
concentration, or concentration difference, times a mass transfer 
coefficient which is analogous to the basic dry deposition model: dry 
deposition = concentration x velocity (U.S. EPA, 2011, Appendix F).
    Transference ratios require estimates of wet deposition of 
NOy and SOX, dry deposition of NOY and 
SOX, and ambient air concentrations of NOY and 
SOx. Possible sources of information include model estimates 
or a combination of model estimates and observations, recognizing that 
dry deposition is a modeled quantity that can use observed or modeled 
estimates of concentration. The limited amount of NOY 
measurements in acid-sensitive areas as well as the combination of 
representative NOY, SO2 and SO4 
observations generally preclude the use of observations for development 
of a standard that is applicable nationally.
    The PA considers a blending of observations and models to take 
advantage of their relative strengths; e.g., combining the NADP wet 
deposition observations, modeled dry deposition, and a mix of modeled 
and observed concentrations, using the model for those species not 
measured or measured with very sparse spatial coverage. A potential 
disadvantage of mixing and matching observations and model estimates is 
to lose consistency afforded by using just modeling alone. A modeling 
platform like CMAQ is based on adhering to consistent treatment of mass 
conservation, by linking emission inputs with air concentrations and 
concentrations to deposition. Inconsistencies from combining processes 
from different analytical platforms increase the chance that mass (of 
nitrogen or sulfur) would unintentionally be increased or decreased as 
the internal checking that assures mass conservation is lost. 
Transference ratios incorporate a broad suite of atmospheric processes 
and consequently an analytical approach that instills consistency in 
the linkage of these processes is preferable to an approach lacking 
such inherent consistency. This contention does not mean that 
observations alone, if available, could not be used, but suggests that 
the inconsistencies in combining models and observations for the 
purposes of developing transference ratios has the potential for 
creating unintended artifacts.
    While there is a reasonable conceptual basis for the concept of an 
aggregated deposition velocity referred to in the PA as a transference 
ratio, there is very limited ability to compare observed and calculated 
ratios. This is because the deposition velocity is dependent on 
individual species, and the mass transfer processes of wet and dry 
removal, while conceptually similar, are different. Consequently, there 
does not exist a meaningful approach to measure such an aggregated or 
lumped parameter. Therefore, at this time, the evaluation of 
transference ratios is based on sensitivity studies, analysis of 
variability, and comparisons with other models, as described in 
Appendix F of the PA.
    As discussed in Appendix F, the interannual variability, as well as 
the sensitivity to emission changes of roughly 50 percent, results in 
changes of transference ratios of approximately 5 to 10 percent. Part 
of the reason for this inherent stability is due to the co-dependence 
of concentration and deposition. For example, as concentrations are 
reduced as a result of emissions reductions, deposition in turn

[[Page 46118]]

is reduced since deposition is a direct linear function of 
concentration leading to negligible impact on the deposition-to-
concentration ratio. Likewise, an overestimate of concentration likely 
does not induce a bias in the transference ratio. While it is important 
to continue to improve the model's ability to match ambient 
concentrations in time and space, the bias of a modeled estimate of 
concentration relative to observations does not necessarily result in a 
bias in a calculated transference ratio. In effect, this consideration 
of bias cancellation reduces the sensitivity of transference ratios to 
model uncertainties and affords increased confidence in the stability 
of these ratios. Based on the series of sensitivity and variability 
analyses, the PA concludes that the transference ratios are relatively 
stable and provide a sound metric for linking deposition and 
concentration.
    As discussed in the PA, transference ratios are dependent on the 
platform upon which they are constructed. Comparisons of transference 
ratios constructed from different modeling platforms do exhibit 
significant differences. While this divergence of results may be 
explained by a variety of differences in process treatments, input 
fields and incommensurabilities in species definitions and spatial 
configurations, it does suggest two very important conclusions. First, 
the idea of using multiple platforms for different parts of the country 
may be problematic as there does not exist a reliable approach to judge 
acceptance which is almost always based on comparisons to observations. 
Second, since transference ratios are based on concentrations and 
deposition, as the uncertainties in each of those components are 
reduced, the relative uncertainty in the ratios also is reduced. This 
means that basic improvements in the model's ability to reproduce 
observed wet deposition and ambient concentration fields enhance the 
relative confidence in the constructed transference ratios. Similarly, 
as in-situ dry deposition flux measurements become available that 
enable a more rigorous evaluation and diagnosis of modeled dry 
deposition processes, the expected improved treatment of dry deposition 
also would increase confidence in transference ratios. Finally, 
deposition is directly related to ambient air concentrations. Models 
like CMAQ rely on the concentration-to-deposition linkage to calculate 
deposition, which is the foundation for broadly based and robust 
assessments addressing atmospheric deposition. In principle, the use of 
a modeled constructed transference ratio is based on the same premise 
by which we use models to estimate deposition in the first place.
    The shortage of widely available ambient air observations and the 
fact that estimates of dry deposition requires modeling, collectively 
suggests that a unified modeling platform is the best approach for 
constructing transference ratios. The PA (U.S. EPA, 2011, section 2) 
considers CMAQ and other models, such as CAMx and Canada's AURAMS--A 
Unified Regional Air-quality Modeling System (Smythe et al., 2008), and 
concludes that CMAQ is the preferred modeling platform for constructing 
transference ratios. This conclusion reflects the view that for the 
purposes of defining transference ratios, a modeling platform should: 
(1) Be a multiple pollutant model recognizing the myriad of connections 
across pollutant categories that directly and indirectly impact 
nitrogen and sulfur characterization, (2) include the most 
comprehensive scientific treatments of atmospheric processes that 
relate directly and indirectly to characterizing concentrations and 
deposition, (3) have an infrastructure capability that accommodates the 
inclusion of improved scientific treatments of relevant processes and 
important input fields, and (4) undergo frequent reviews regarding the 
adequacy of the underlying science as well as the appropriateness in 
applications. The CMAQ platform exhibits all these characteristics. It 
has been (and continues to be) extensively evaluated for several 
pollutant categories, and is supported by a central infrastructure of 
EPA scientists, whose mission is to improve and evaluate the CMAQ 
platform. More directly, CMAQ, and its predecessor versions, has a long 
track record going back to the NAPAP in the 1980s of specific 
improvements in deposition processes, which are described in Appendix F 
of the PA.
4. Aquatic Acidification Index
    Having established the various expressions that link atmospheric 
deposition of nitrogen and sulfur to ANC and the transference ratios 
that translate atmospheric concentrations to deposition of nitrogen and 
sulfur, the PA derived the following expression of these linkages, 
which separates reduced forms of nitrogen, NHX, from 
oxidized forms:
[GRAPHIC] [TIFF OMITTED] TP01AU11.026

    Equation III-3 is the basic expression of the form of a standard 
that translates the conceptual framework into an explicit expression 
that defines ANC as a function of the ambient air indicators, 
NOY and SOX reduced nitrogen deposition,\9\ and 
the critical load necessary to achieve a target ANC level. This 
equation calculates an expected ANC value based on ambient 
concentrations of NOY and SOX. The calculated ANC 
will differ from the target ANC (ANClim) depending on how much the 
nitrogen and sulfur deposition associated with NOY, 
SOX, and NHX differs from the critical load 
associated with just achieving the target ANC.
---------------------------------------------------------------------------

    \9\ Because NHx is characterized directly as deposition, not as 
an ambient concentration in this equation, no transference ratio is 
needed for this term.
---------------------------------------------------------------------------

    Based on equation III-3, the PA defines an AAI that is more simply 
stated using terms that highlight the ambient air indicators:
[GRAPHIC] [TIFF OMITTED] TP01AU11.027

where the AAI represents the long term (or steady state) ANC level 
associated with ambient air concentrations of NOY and 
SOX. The factors F1 through F4 convey three attributes: a 
relative measure of the ecosystem's ability to neutralize acids (F1), 
the acidifying potential of reduced nitrogen deposition (F2), and the 
deposition-to-concentration translators for NOY (F3) and 
SOX (F4).

    Specifically:

F1 = ANClim + CLr/Qr;

[[Page 46119]]

F2 = NHx/Qr = NHx deposition divided by Qr;
F3 = TNOy/ Qr; TNOy is the transference ratio 
that converts ambient air concentrations of NOy to deposition of 
NOy; and
F4 = TSOx/ Qr; TSOx is the transference ratio 
that converts ambient air concentrations of SOX to 
deposition of SOX.

All of these factors include representative Qr to maintain unit (and 
mass) consistency between the AAI and the terms on the right side of 
equation III-4.
    The F1 factor is the target ANC level plus the amount of deposition 
(critical load) the ecosystem can receive and still achieve the target 
level. It incorporates an ecosystem's ability to generate acid 
neutralizing capacity through base cation supply ([BC]*0) and to 
neutralize acidifying nitrogen deposition through Neco, both of which 
are incorporated in the CL term. As noted above, because Neco can only 
neutralize nitrogen deposition (oxidized or reduced) there may be rare 
cases where Neco exceeds the combination of reduced and oxidized 
nitrogen deposition. Consequently, to ensure that the AAI equation is 
applicable in all cases that may occur, equation III-4 is conditional 
on total nitrogen deposition, {NHX + F3[NOy]{time} , being 
greater than Neco. In rare cases where Neco is greater than 
{NHX + F3[NOy]{time} , F2, F3, and Neco would be set equal 
to 0 in the AAI equation. The consequence of setting F2 and F3 to zero 
is simply to constrain the AAI calculation just to SOx, as 
nitrogen would have no bearing on acidifying contributions in this 
case.
    The PA concludes that equation III-4 (U.S. EPA, 2011,equation 7-
12), which defines the AAI, is ecologically relevant and appropriate 
for use as the form of a national standard designed to provide 
protection for aquatic ecosystems from the effects of acidifying 
deposition associated with concentrations of oxides of nitrogen and 
sulfur in the ambient air. This AAI equation does not, however, in 
itself, define the spatial areas over which the terms of the equation 
would apply. To specify values for factors F1 through F4, it is 
necessary to define spatial areas over which these factors are 
determined. Thus, it is necessary to identify an approach for spatially 
aggregating water bodies into ecologically meaningful regions across 
the U.S., as discussed below.
5. Spatial Aggregation
    As discussed in the PA, one of the unique aspects of this form is 
the need to consider the spatial areas over which values for the F 
factors in the AAI equation are quantified. Ecosystems across the U.S. 
exhibit a wide range of geological, hydrological and vegetation 
characteristics that influence greatly the ecosystem parameters, Q, 
BC0-* and Neco that are incorporated in the AAI. Variations 
in ecosystem attributes naturally lead to wide variability in the 
sensitivities of water bodies in the U.S. to acidification, as well as 
in the responsiveness of water bodies to changes in acidifying 
deposition. Consequently, variations in ecosystem sensitivity, and the 
uncertainties inherent in characterizing these variations, must be 
taken into account in developing a national standard. In developing a 
secondary NAAQS to protect public welfare, the focus of the PA is on 
protecting sensitive populations of water bodies, not on each 
individual water body, which is consistent with the Agency's approach 
to protecting public health through primary NAAQS that focus on 
susceptible populations, not on each individual.
    The approach used for defining ecologically relevant regions across 
the U.S. in the PA (U.S. EPA, 2011, section 7.2.5) is described below, 
along with approaches to characterizing each region as acid sensitive 
or relatively non-acid sensitive. This characterization facilitates a 
more detailed analysis and focus on those regions that are relatively 
more acid sensitive. This characterization is also used to avoid over-
protection in relatively non-acid sensitive regions, regions that would 
receive limited benefit from reductions in the deposition of oxides of 
nitrogen and sulfur with respect to aquatic acidification effects. 
Approaches to developing representative values for each of the terms in 
the AAI equation (factors F1 through F4) for each ecologically relevant 
region for which sufficient data are available are then discussed. 
These spatial aggregation approaches are generally applicable to the 
contiguous U.S. The following discussion also addresses the development 
of factors for data-limited regions and specifically for Hawaii, Alaska 
and the U.S. territories.
    Stated more simply, this section discusses appropriate ways to 
divide the country into ecologically relevant regions; to characterize 
each region as either acid sensitive or relatively non-acid sensitive; 
and to determine values of factors F1 through F4 for each region, 
taking into consideration the acid sensitivity of each region and the 
availability of relevant data. For each such region, the AAI would be 
calculated based on the values of factors F1 through F4 specified for 
that region.
    In considering approaches to spatial aggregation, the PA focuses on 
methods that have been developed to define ecologically relevant 
regions, referred to as ecoregions, which are meaningfully related to 
the factors that are relevant to aquatic acidification. As noted above, 
the PA did not focus on looking at each individual water body, nor did 
it focus on aggregating over the entire nation, which would preclude 
taking into account the inherent variability in atmospheric and 
ecological factors that fundamentally modify the relationships that are 
central to the development of an ecologically relevant AAI.
    Based on considering available classification schemes, the PA 
concludes that Omernik's ecoregion classification (as described at 
http://www.epa.gov/wed/pages/ecoregions) is the most appropriate method 
to consider for the purposes of this review. This classification offers 
several levels of spatial delineation, has undergone an extensive 
scientific peer review process, and has explicitly been applied to 
delineating acid sensitive areas within the U.S. Further, the PA 
concludes that ecoregion level III (Figure III-1) resolution, with 84 
defined ecoregions in the contiguous U.S.,\10\ is the most appropriate 
level to consider for this purpose. The spatial resolution afforded by 
level III strikes an appropriate balance relative to the reasoning that 
supports conclusions on indicators, as discussed above. The PA 
concludes that the most detailed level of resolution (level IV) is not 
appropriate given the limited data availability to address nearly 1,000 
subdivisons within that level and the currently evolving nature of 
level IV regions. Further, level III ecoregions are preferred to level 
II in that level III ecoregions, but not level II ecoregions, are 
largely contiguous in space which allows for a more coherent 
development of information to quantify the AAI factors and to 
characterize the concentrations of NOy and SOx in the ambient air 
within each ecoregion.
---------------------------------------------------------------------------

    \10\ We note that an 85th area within Omernik's Ecoregion Level 
III is currently being developed for California.
---------------------------------------------------------------------------

    Appendix C of the PA includes a description of each level III 
ecoregion. The PA notes that the use of ecoregions is an appropriate 
spatial aggregation scheme for an AAI-based standard focused on 
deposition-related aquatic acidification effects, while many of the 
same ecoregion attributes may be applicable in subsequent NAAQS reviews 
that may address other deposition-related aquatic and terrestrial 
ecological effects. Because atmospheric deposition is modified by 
ecosystem attributes, the types of vegetation, soils, bedrock geology, 
and

[[Page 46120]]

topographic features that are the basis of this ecoregion 
classification approach also will likely be key attributes for other 
deposition-related effects (e.g., terrestrial acidification, nutrient 
enrichment) that link atmospheric concentrations to an aquatic or 
terrestrial ecological indicator.
[GRAPHIC] [TIFF OMITTED] TP01AU11.028

a. Ecoregion Sensitivity
    The PA used Omernik's original alkalinity data (U.S. EPA, 2011, 
section 2) and more recent ANC data to delineate two broad groupings of 
ecoregions: Acid-sensitive and relatively non-acid sensitive 
ecoregions. This delineation was made to facilitate greater focus on 
those ecoregions with water bodies that generally have greater acid 
sensitivity and to avoid over-protection in regions with generally less 
sensitive water bodies. The approach used to delineate acid-sensitive 
and relatively non-acid sensitive regions included an initial 
numerical-based sorting scheme using ANC data, which categorized 
ecoregions with relatively high ANC values as being relatively non-acid 
sensitive. This initial delineation resulted in 29 of the 84 Omernik 
ecoregions being categorized as acid sensitive. Subsequently, land use 
data were also considered to determine to what extent an ecoregion is 
of a relatively pristine and rural nature by quantifying the degree to 
which active management practices related to development and 
agriculture occur in each ecoregion.
    The overall objective is to produce a logical and practical 
grouping of ecoregions that experience adverse conditions with respect 
to aquatic acidification and are likely to respond to changes in 
concentrations of NOy and SOx in the ambient air 
and to the related deposition levels. To achieve this goal, a two-step 
process has been applied, first identifying acid sensitive ecoregions 
based on water quality data alone, and second identifying among those 
acid-sensitive ecoregions those with highly developed and managed 
areas. These highly developed and managed ecoregions are placed in a 
non-acid sensitive category to avoid over protection beyond what is 
requisite to protect public welfare. More specifically, in determining 
an ecoregion's acid sensitivity status in step 1, ANC data across the 
84 ecoregions are sorted (U.S. EPA, 2011, section 7) to determine the 
number of water bodies within a region with ANC values suggestive of 
acid sensitivity, so as to screen out regions with an overabundance of 
high ANC values. In reviewing the ANC data, the PA identified 29 
ecoregions that meet two criteria: (1) Greater than 5 percent of water 
bodies with data with ANC values less than 200 [micro]eq/L and (2) 
greater than 1 percent of water bodies with ANC values less than 100 
[micro]eq/L. In step 2, land use data were used to identify those acid 
sensitive ecoregions with significant managed areas that would not be 
considered as having a relatively pristine and rural character. The 
percentage of the combination of developed (residential, 
transportation, industrial and commercial) and agricultural (croplands, 
pastures, orchards, vineyards) land use was used as an indicator of 
managed land use area. Forest cover was used as an indicator of non-
managed land use more directly reflecting the pristine quality of a 
region. Based on the 2006 National Land Cover Data base (NLCD, http://www.epa.gov/mrlc/nlcd-2006.html), acid sensitive ecoregions would meet 
both of the following land use data

[[Page 46121]]

criteria: Percent of developed and agricultural area less than 20 
percent combined with forested area greater than 50 percent. The 
combination of steps 1 and 2 identify 22 relatively acid sensitive 
areas (Table III-1 and Figure III-2).
    Consideration was also given to the use of naturally acidic 
conditions in defining relatively non-acid sensitive areas. For 
example, several of the ecoregions located in plains near the coast 
exhibit elevated dissolved organic carbon (DOC) levels, which is 
associated with naturally acidic conditions. The DOC in surface waters 
is derived from a variety of weak organic acid compounds generated from 
the natural availability and decomposition of organic matter from 
biota. Consequently, high DOC is associated with ``natural'' acidity, 
with the implication that a standard intended to protect against 
atmospheric contributions to acidity is not an area of focus. The 
evidence suggests that several of the more highly managed ecoregions in 
coastal or near coastal transition zones are associated with relatively 
high DOC values, typically exceeding on average 5 mg/l, compared to 
other acid sensitive areas. Although there is sound logic to interpret 
naturally acidic areas as relatively non-acid sensitive, natural 
acidity indicators were not explicitly included in defining relatively 
non-acid sensitive areas as there does not exist a consensus-based 
quantifiable scientific definition of natural acidity. Approaches to 
explicitly define natural acidity likely will be pursued in future 
reviews of the standard.

              Table III-1--List of 22 Acid-Sensitive Areas
------------------------------------------------------------------------
                                                               Ecoregion
                       Ecoregion name                             No.
------------------------------------------------------------------------
Ridge and Valley............................................       8.4.1
Northern Appalachian Plateau and Uplands....................       8.1.3
Piedmont....................................................       8.3.4
Western Allegheny Plateau...................................       8.4.3
Southwestern Appalachians...................................       8.4.9
Boston Mountains............................................       8.4.6
Blue Ridge..................................................       8.4.4
Ouachita Mountains..........................................       8.4.8
Central Appalachians........................................       8.4.2
Northern Lakes and Forests..................................       5.2.1
Maine/New Brunswick Plains and Hills........................       8.1.8
North Central Appalachians..................................       5.3.3
Northern Appalachian and Atlantic Maritime Highlands........       5.3.1
Columbia Mountains/Northern Rockies.........................       6.2.3
Middle Rockies..............................................      6.2.10
Wasatch and Uinta Mountains.................................      6.2.13
North Cascades..............................................       6.2.5
Cascades....................................................       6.2.7
Southern Rockies............................................      6.2.14
Sierra Nevada...............................................      6.2.12
Idaho Batholith.............................................      6.2.15
Canadian Rockies............................................       6.2.4
------------------------------------------------------------------------

                                                              [GRAPHIC] [TIFF OMITTED] TP01AU11.029
                                                              

[[Page 46122]]

b. Representative Ecoregion-Specific Factors
    Having concluded that the Omernik level III ecoregions are an 
appropriate approach to spatial aggregation for the purpose of a 
standard to address deposition-related aquatic acidification effects, 
the PA uses those ecoregions to define each of the factors in the AAI 
equation. As discussed below, factors F1 through F4 in equation III-4 
are defined for each ecoregion by specifying ecoregion-specific values 
for each factor based on monitored or modeled data that are 
representative of each ecoregion.
i. Factor F1
    As discussed above, factor F1 reflects a relative measure of an 
ecosystem's ability to neutralize acidifying deposition, and is defined 
as: F1 = ANClim + CLr/Qr. The value of F1 for each ecoregion would be 
based on a representative critical load for the ecoregion 
(CLr) associated with a single national target ANC level 
(ANClim, discussed below in section III.D), as well as on a 
representative runoff rate (Qr). To specify ecoregion-
specific values for the term Qr, the PA used the median 
value of the distribution of Q values that are available for water 
bodies within each ecoregion. To specify ecoregion-specific 
representative values for the term CLr in factor F1, a 
distribution \11\ of calculated critical loads was created for the 
water bodies in each ecoregion for which sufficient water quality and 
hydrology data are available.\12\ The representative critical load was 
then defined to be a specific percentile of the distribution of 
critical loads in the ecoregion. Thus, for example, using the 90th 
percentile means that within an ecoregion, 90 percent of the water 
bodies would be expected to have higher calculated critical loads than 
the representative critical load. That is, if the representative 
critical load were to occur across the ecoregion, 90 percent of the 
water bodies would be expected to achieve the national ANC target or 
better.
---------------------------------------------------------------------------

    \11\ The distribution of critical loads was based on CL values 
calculated with Neco at the lake level. Consideration could also be 
given to using a distribution of CLs without Neco and adding the 
ecoregion average Neco value to the nth percentile critical load. 
This would avoid cases where the lake-level Neco value potentially 
could be greater than total nitrogen deposition. The CL at the lake 
level represents the CL for the lake to achieve the specified 
national target ANC value.
    \12\ The PA judged the data to be sufficient for this purpose if 
data are available from more than 10 water bodies in an ecoregion.
---------------------------------------------------------------------------

    The specific percentile selected as part of the definition of F1 is 
an important parameter that directly impacts the representative 
critical load specified for each ecoregion, and therefore the degree of 
protectiveness of the standard. A higher percentile corresponds to a 
lower critical load and, therefore, to lower allowable ambient air 
concentrations of NOy and SOx and related 
deposition to achieve a target AAI level. In conjunction with the other 
terms in the AAI equation, alternative forms can be appropriately 
characterized in part by identifying a range of alternative 
percentiles. The choice of an appropriate range of percentiles to 
consider for acid-sensitive and relatively non-acid sensitive 
ecoregions, respectively, is discussed below.
a. Acid-Sensitive Ecoregions
    In identifying percentiles that are appropriate to consider for the 
purpose of calculating factor F1 for ecoregions characterized as acid 
sensitive, the PA concludes that it is appropriate to focus on the 
lower (more sensitive) part of the distribution of critical loads, so 
as to ensure that the ecoregion would be represented by relatively more 
acid sensitive water bodies within the ecoregion. Specifying factor F1 
in this way would help to define a standard that would be protective of 
the population of acid sensitive water bodies within an ecoregion, 
recognizing that even ecoregions characterized as acid sensitive may 
contain a number of individual water bodies that are not acid 
sensitive. The PA recognizes that there is no basis for independently 
evaluating the degree of protectiveness afforded by any specific 
percentile value, since it is the combination of form and level, in 
conjunction with the indicator and averaging time, which determine the 
degree of protectiveness of a standard. In light of this, the PA 
concludes that it is appropriate to consider a range of percentiles, 
from well above the 50th percentile, or median, of the distribution to 
somewhat below the highest value (in terms of sensitivity; a high 
degree of sensitivity corresponds to a low value for critical load). 
More specifically, the PA concludes it is appropriate to consider 
percentiles in the range of the 70th to the 90th percentile (of 
sensitivity). This conclusion is based on the judgment that it would 
not be appropriate to represent an ecoregion with the lowest or near 
lowest critical load, so as to avoid potential extreme outliers that 
can be seen to exist at the extreme end of the data distributions, 
which would not be representative of the population of acid sensitive 
water bodies within the ecoregion and could lead to an overly 
protective standard. At the same time, in considering ecoregions that 
are inherently acid sensitive, it is judged to be appropriate to limit 
the lower end of the range for consideration to the 70th percentile, a 
value well above the median of the distribution, so that a substantial 
majority of acid-sensitive water bodies are protected.
    In considering this conclusion, the CASAC Panel noted that the data 
bases for calculating critical loads within an ecoregion are not 
necessarily representative of all water bodies within an ecoregion. 
That is, in many ecoregions the lake sampling design used in studies 
that generated the relevant data may have focused on the relatively 
more sensitive water bodies within an ecoregion (Russell and Samet, 
2011a). Consequently, a given percentile of the distribution of 
calculated critical loads, based on sampled water bodies, may not be 
representative of that percentile of all water bodies across an entire 
ecoregion. To the extent that the sampling of water bodies within an 
ecoregion was skewed toward the relatively more sensitive water bodies, 
selecting a given percentile from the distribution of available 
critical loads would result in a somewhat higher percentile of all 
water bodies within that ecoregion having a higher calculated critical 
load than the representative critical load value. Thus, the extent to 
which study sampling designs have resulted in skewed distributions of 
calculated critical loads is an uncertainty that is appropriate to 
consider in selecting a percentile for the purpose of defining the 
factor F1 in the AAI equation.
b. Non-Acid Sensitive Ecoregions
    With regard to identifying percentiles that are appropriate to 
consider for the purpose of calculating factor F1 for ecoregions 
characterized as relatively non-acid sensitive, the PA recognizes that 
while such ecoregions are generally less sensitive to acidifying 
deposition from oxides of nitrogen and sulfur, they may contain a 
number of water bodies that are acid sensitive. This category includes 
ecoregions that are well protected from acidification effects due to 
natural production of base cations and high ANC levels, as well as 
naturally acidic systems with limited base cation production and 
consequently very low critical loads. Therefore, the use of a critical 
load that would be associated with a highly sensitive water body in a 
naturally acidic system would impose a high degree of relative 
protection in terms of allowable ambient air concentrations of oxides 
of nitrogen and sulfur and

[[Page 46123]]

related deposition, while potentially affording little or no public 
welfare benefit from attempting to improve a naturally acidic system.
    Based on these considerations, the PA concludes it is appropriate 
to consider the use of a range of percentiles that extends lower than 
the range identified above for acid-sensitive ecoregions. Consideration 
of a lower percentile would avoid representing a relatively non-acid 
sensitive ecoregion by a critical load associated with relatively more 
acid-sensitive water bodies. In particular, the PA concludes it is 
appropriate to focus on the median or 50th percentile of the 
distribution of critical loads so as to avoid over-protection in such 
ecoregions. Recognizing that relatively non-acid sensitive ecoregions 
generally are not sampled to the extent that acid-sensitive ecoregions 
are, it also is appropriate to consider using the median critical load 
of all relatively non-acid sensitive ecoregions for each such 
ecoregion.
ii. Factor F2
    As discussed above, factor F2 is the amount of reduced nitrogen 
deposition within an ecoregion, including the deposition of both 
ammonia gas and ammonium ion, and is defined as: F2 = NHX/
Qr. The PA calculated the representative runoff rate, Qr, 
using a similar approach as noted above for factor F1; i.e., the median 
value of the distribution of Q values that are available for water 
bodies within each ecoregion. In the PA, 2005 CMAQ model simulations 
over 12-km grids are used to calculate an average value of 
NHX for each ecoregion. The NHX term is based on 
annual average model outputs for each grid cell, which are spatially 
averaged across all the grid cells contained in each ecoregion to 
calculate a representative annual average value for each ecoregion. The 
PA concludes that this approach of using spatially averaged values is 
appropriate for modeling, largely due to the relatively rapid mixing of 
air masses that typically results in relatively homogeneous air quality 
patterns for regionally dispersed pollutants. In addition, there is 
greater confidence in using spatially averaged modeled atmospheric 
fields than in using modeled point-specific fields.
    This averaging approach is also used for the air concentration and 
deposition terms in factors F3 and F4, as discussed below. The PA notes 
that modeled NHX deposition exhibits greater spatial 
variability than the other modeled terms in factors F3 and F4. 
Recognizing this greater variability, the PA concludes that it would be 
appropriate to consider alternative approaches to specifying the value 
of NHX. One such approach might involve the use of more 
localized and/or contemporaneous modeling in areas where this term is 
likely to be particularly variable and important.
iii. Factors F3 and F4
    As discussed above, factors F3 and F4 are the ratios that relate 
ambient air concentrations of NOy and SOX to the 
associated deposition, and are defined as follow: F3 = TNOy/ 
Qr and F4 = TSOx/ Qr. TNOy is the transference 
ratio that converts ambient air concentrations of NOy to 
deposition of NOy and TSOx is the transference 
ratio that converts ambient air concentrations of SOX to 
deposition of SOX. The representative runoff rate, 
Qr, is calculated as for factors F1 and F2. The transference 
ratios are based on the 2005 CMAQ simulations, using average values for 
each ecoregion, as noted above for factor F2. More specifically, the 
transference ratios are calculated as the annual deposition of 
NOy or SOX spatially averaged across the 
ecoregion and divided by the annual ambient air concentration of 
NOy or SOX, respectively, spatially averaged 
across the ecoregion.
c. Factors in Data-Limited Ecoregions
    As discussed above in section III.B.5.a, in the PA the initial 
delineation of acid-sensitive and relatively non-acid sensitive 
ecoregions was based on available ANC and alkalinity data. Areas not 
meeting the ANC criteria described above are categorized as relatively 
non-acid sensitive. The development of a reasonable distribution of 
critical loads for water bodies within an ecoregion for the purpose of 
identifying the representative critical load requires additional data, 
including more specific water quality data for major cations and 
anions. This means that the water bodies that can be used to develop a 
distribution of critical loads is generally a subset of those water 
bodies for which ANC data are available Consequently, there are certain 
ecoregions with sparse data that are not suitable for developing a 
distribution of critical loads.
    As noted above, the PA judges that it is not appropriate to develop 
such distributions based on data from less than ten water bodies within 
an ecoregion. Twelve such ecoregions, which included only relatively 
non-acid sensitive ecoregions, were characterized as being data-
limited. For these ecoregions, the PA considered alternative approaches 
to specifying values for the terms CLr and Qr for 
the purpose of determining values for each of the factors in the AAI 
equation. For these data-limited ecoregions, the PA judges that it is 
appropriate to use the median values of CLr and 
Qr from the distributions of these terms for all other 
relatively non-acid sensitive ecoregions, rather than attempting to use 
severely limited data to develop a value for these terms based solely 
on data from such an ecoregion. Further, consideration could be given 
to using a single national default value for all relatively non-acid 
sensitive ecoregions. The PA notes that this data limitation is not a 
concern in specifying values for the other terms in the AAI equation 
for such ecoregions, since those terms are based on data from the 2005 
CMAQ model simulation, which covers all ecoregions across the 
contiguous U.S.
d. Application to Hawaii, Alaska, and the U.S. Territories
    The above methods for specifying ecoregion-specific values for the 
factors in the AAI equation apply to those ecoregions within the 
contiguous U.S. For areas outside the continental U.S., including 
Hawaii, Alaska, and the U.S. Territories, there is currently a lack of 
available data to characterize the sensitivity of such areas, as well 
as a lack of water body-specific data and CMAQ-type modeling to specify 
values for the F1 through F4 factors. Thus, the PA has considered 
possible alternative approaches to specifying values for factors F1 
through F4 in the AAI equation for these areas.
    One such approach could be to specify area-specific values for the 
factors based on values derived for ecoregions with similar acid 
sensitivities, to the extent that relevant information can be obtained 
to determine such similarities. Such an approach would involve 
conducting an analysis to characterize similarities in relevant 
ecological attributes between ecoregions in the contiguous U.S. and 
these areas outside the contiguous U.S. so as to determine the 
appropriateness of utilizing ecoregion-specific values for the 
CLr and Qr terms from one or more ecoregions 
within the contiguous U.S. This approach would also involve conducting 
additional air quality modeling for the areas that are outside the 
geographical scope of the currently available CMAQ model simulations, 
so as to develop the other information necessary to specify values for 
factors F2 through F4 for these areas.
    A second approach could rely on future data collection efforts to 
establish relevant ecological data within these areas that, together 
with additional air quality modeling, could be used to specify area-
specific values for factors

[[Page 46124]]

F1 through F4. Until such time as relevant data become available, these 
areas could be treated the same as data-limited ecoregions in the 
contiguous U.S. that are relatively non-acid sensitive.
    The PA concludes that either approach would introduce substantial 
uncertainties that arise from attempting to extrapolate values based on 
similarity assumptions or arbitrarily assigning values for factors in 
the AAI equation that would be applicable to these areas outside the 
contiguous U.S. In light of such uncertainties, the PA concludes that 
it would also be appropriate to consider relying on the existing 
NO2 and SO2 secondary standards in these areas 
for protection of any potential direct or deposition-related ecological 
effects that may be associated with the presence of oxides of nitrogen 
and sulfur in the ambient air. The PA concludes that relying on 
existing secondary standards in these areas is preferable to using a 
highly uncertain approach to allow for the application of a new 
standard based on the AAI in the absence of relevant area-specific 
data.
6. Summary of the AAI Form
    With regard to the form of a multi-pollutant air quality standard 
to address deposition-related aquatic acidification effects, the PA 
concludes that consideration should be given to an ecologically 
relevant form that characterizes the relationships between the ambient 
air indicators for oxides of nitrogen and sulfur, the related 
deposition of nitrogen and sulfur, and the associated aquatic 
acidification effects in terms of a relevant ecological indicator. 
Based on the available information and assessments, consideration 
should be given to using ANC as the most appropriate ecological 
indicator for this purpose, in that it provides the most stable metric 
that is highly associated with the water quality properties that are 
directly responsible for the principal adverse effects associated with 
aquatic acidification: Fish mortality and reduced aquatic species 
diversity.
    The PA developed such a form, using a simple equation to calculate 
an AAI value in terms of the ambient air indicators of oxides and 
nitrogen and sulfur and the relevant ecological and atmospheric factors 
that modify the relationships between the ambient air indicators and 
ANC. Recognizing the spatial variability of such factors across the 
U.S., the PA concludes it is appropriate to divide the country into 
ecologically relevant regions, characterized as acid-sensitive or 
relatively non-acid-sensitive, and specify the value of each of the 
factors in the AAI equation for each such region. Omernik ecoregions, 
level III, are identified as the appropriate set of regions over which 
to define the AAI. There are 84 such ecoregions that cover the 
continental U.S. This set of ecoregions is based on grouping a variety 
of vegetation, geological, and hydrological attributes that are 
directly relevant to aquatic acidification assessments and that allow 
for a practical application of an aquatic acidification standard on a 
national scale.
    The PA defines AAI by the following equation: AAI = F1 - F2 - 
F3[NOy] - F4[SOX]. Factors F1 through F4 would be 
defined for each ecoregion by specifying ecoregion-specific values for 
each factor based on monitored or modeled data that are representative 
of each ecoregion. The F1 factor is also defined by a target ANC value. 
More specifically:
    (1) F1 reflects a relative measure of an ecosystem's ability to 
neutralize acidifying deposition. The value of F1 for each ecoregion 
would be based on a representative critical load for the ecoregion 
associated with a single national target ANC level, as well as on a 
representative runoff rate. The representative runoff rate, which is 
also used in specifying values for the other factors, would be the 
median value of the distributions of runoff rates within the ecoregion. 
The representative critical load would be derived from a distribution 
of critical loads calculated for each water body in the ecoregion for 
which sufficient water quality and hydrology data are available. The 
representative critical load would be defined by selecting a specific 
percentile of the distribution.
    In identifying a range of percentiles that are appropriate to 
consider for this purpose, regions categorized as acid sensitive were 
considered separately from regions categorized as relatively non-acid 
sensitive. For acid sensitive regions, the PA concludes that 
consideration should be given to selecting a percentile from within the 
range of the 70th to the 90th percentile. The lower end of this range 
was selected to be appreciably above the median value so as to ensure 
that the critical load would be representative of the population of 
relatively more acid sensitive water bodies within the region, while 
the upper end was selected to avoid the use of a critical load from the 
extreme tail of the distribution which is subject to a high degree of 
variability and potential outliers. For relatively non-acid sensitive 
regions, the PA concludes that consideration should be given to 
selecting the 50th percentile to best represent the distribution of 
water bodies within such a region, or alternatively to using the median 
critical load of all relatively non-acid sensitive areas, recognizing 
that such areas are far less frequently evaluated than acid sensitive 
areas. Using either of these approaches would avoid characterizing a 
generally non-acid-sensitive region with a critical load that is 
representative of relatively acid sensitive water bodies that may exist 
within a generally non-acid sensitive region.
    (2) F2 reflects the deposition of reduced nitrogen. Consideration 
should be given to specifying the value of F2 for each region based on 
the averaged modeled value across the region, using national CMAQ 
modeling that has been conducted by EPA. Consideration could also be 
given to alternative approaches to specifying this value, such as the 
use of more localized and/or contemporaneous modeling in areas where 
this term is likely to be particularly variable and important.
    (3) F3 and F4 reflect transference ratios that convert ambient air 
concentrations of NOy and SOX, respectively, into 
related deposition of nitrogen and sulfur. Consideration should be 
given to specifying the values for F3 and F4 for each region based on 
CMAQ modeling results averaged across the region. We conclude that 
specifying the values or the transference ratios based on CMAQ modeling 
results alone is preferred to an alternative approach that combines 
CMAQ model estimates with observational data.
    (4) The terms [NOy] and [SOX] reflect ambient 
air concentrations measured at monitoring sites within each region.
    Using the equation, a value of AAI can be calculated for any 
measured values of ambient NOy and SOX. For such 
a NAAQS, the Administrator would set a single, national value for the 
level of the AAI used to determine achievement of the NAAQS, as 
discussed below in section III.D. The ecoregion-specific values for 
factors F1 through F4 would be specified by EPA based on the most 
recent data and CMAQ model simulations, and codified as part of such a 
standard. These factors would be reviewed and updated as appropriate in 
the context of each periodic review of the NAAQS.
    The PA developed specific F factors for each ecoregion based on the 
approach discussed above, using alternative percentiles and alternative 
national target ANC levels. The results of this analysis for ecoregions 
characterized as acid sensitive are presented in Table 7-1a-d in the 
PA.

[[Page 46125]]

C. Averaging Time

    As discussed in section 7.3 of the PA, aquatic acidification can 
occur over both long- and short-term timescales. Long-term cumulative 
deposition of nitrogen and sulfur is reflected in the chronic acid-base 
balance of surface waters as indicated by measured annual ANC levels. 
Similarly, the use of steady state critical load modeling, which 
generates critical loads in terms of annual cumulative deposition of 
nitrogen and sulfur, means that the focus of ecological effects studies 
based on critical loads is on the long-term equilibrium status of water 
quality in aquatic ecosystems. Much of the evidence of adverse 
ecological effects associated with aquatic acidification, as discussed 
above in section II.A, is associated with chronically low ANC levels. 
Protection against a chronic ANC level that is too low is provided by 
reducing overall annual average deposition levels for nitrogen and 
sulfur.
    Reflecting this focus on long-term acidifying deposition, the PA 
developed the AAI that links ambient air indicators to deposition-
related ecological effects, in terms of several factors, F1 through F4. 
As discussed above, these factors are all calculated as annual average 
values, whether based on water quality and hydrology data or on CMAQ 
model simulations. In the context of a standard defined in terms of the 
AAI, the PA concludes that it is appropriate to consider the same 
annual averaging time for the ambient air indicators as is used for the 
factors in the AAI equation.
    We also recognize that short-term (i.e., hours or days) episodic 
changes in water chemistry, often due to changes in the hydrologic flow 
paths, can have important biological effects in aquatic ecosystems. 
Such short-term changes in water chemistry are termed ``episodic 
acidification.'' Some streams may have chronic or base flow chemistry 
that is generally healthy for aquatic biota, but may be subject to 
occasional acidic episodes with potentially lethal consequences. Thus, 
short-term episodic ecological effects can occur even in the absence of 
long-term chronic acidification effects.
    Episodic declines in pH and ANC are nearly ubiquitous in drainage 
waters throughout the eastern U.S. Episodic acidification can result 
from several mechanisms related to changes in hydrologic flow paths. 
For example, snow can store nitrogen deposited throughout the winter 
and snowmelt can then release this stored nitrogen, together with 
nitrogen derived from nitrification in the soil itself, in a pulse that 
leads to episodic acidification in the absence of increased deposition 
during the actual episodic acidification event. The PA notes that 
inputs of nitrogen and sulfur from snowpack and atmospheric deposition 
largely cycle through soil. As a result, short-term direct deposition 
inputs are not necessarily important in episodic acidification. Thus, 
as noted in chapter 3 of the ISA, protection against episodic acidity 
events can be achieved by establishing a higher chronic ANC level.
    Taken together, the above considerations support the conclusion 
that it is appropriate to consider the use of a long-term average for 
the ambient air indicators NOy and SOX for an 
aquatic acidification standard defined in terms of the AAI. The use of 
an annual averaging time for NOy and SOX 
concentrations would be appropriate to provide protection against low 
chronic ANC levels, which in turn would protect against both long-term 
acidification and acute acidic episodes.
    The PA has also considered interannual variability in both ambient 
air quality and in precipitation, which is directly related to the 
deposition of oxides of nitrogen and sulfur from the ambient air. While 
ambient air concentrations show year-to-year variability, often the 
year-to-year variability in precipitation is considerably greater, 
given the highly stochastic nature of precipitation. The use of 
multiple years over which annual averages are determined would dampen 
the effects of interannual variability in both air quality and 
precipitation. For the ambient air indicators, the use of multiple-year 
averages would also add stability to calculations used to judge whether 
an area meets a standard defined in terms of the AAI. Consequently, the 
PA concludes that an annual averaging time based on the average of each 
year over a consecutive 3- to 5-year period is appropriate to consider 
for the ambient air indicators NOy and SOX. In 
reaching this conclusion, the PA notes that in its comments on the 
second draft PA, CASAC agreed that a 3- to 5-year averaging time was 
appropriate to consider (Russell and Samet, 2010b).

D. Level

    As discussed above, the PA concludes that ANC is the ecological 
indicator best suited to reflect the sensitivity of aquatic ecosystems 
to acidifying deposition from oxides of nitrogen and sulfur in the 
ambient air. The ANC is an indicator of the aquatic acidification 
expected to occur given the natural buffering capacity of an ecosystem 
and the loadings of nitrogen and sulfur resulting from atmospheric 
deposition. Thus, the PA developed a new standard for aquatic 
acidification that is based on the use of chronic ANC as the ecological 
indicator as a component in the AAI.
    The level of the standard would be defined in terms of a single, 
national value of the AAI. The standard would be met at a monitoring 
site when the multi-year average of the calculated annual values of the 
AAI was equal to or above the specified level of the standard.\13\ The 
annual values of the AAI would be calculated based on the AAI equation 
using the assigned ecoregion-specific values for factors F1 through F4 
and monitored annual average NOy and SOX 
concentrations. Since the AAI equation is based on chronic ANC as the 
ecological indicator, the level chosen for the standard would reflect a 
target chronic ANC value. As noted above, the assigned F factors for 
each ecoregion would be determined by EPA in the rulemaking to set the 
NAAQS, based on water quality and hydrology data, CMAQ modeling, the 
selected percentile that is used to identify a representative critical 
load within the ecoregion, and the selected level of the standard. The 
combination of the form of the standard, discussed above in section 
III.B, defined by the AAI equation and the assigned values of the F 
factors in the equation, other elements of the standard including the 
ambient air indicators (section III.A) and their averaging time 
(section III.C), and the level of the standard determines the allowable 
levels of NOy and SOX in the ambient air within 
each ecoregion. All of the elements of the standard together determine 
the degree of protection from adverse aquatic acidification effects 
associated with oxides of nitrogen and sulfur in the ambient air. The 
level of the standard plays a central role in determining the degree of 
protection provided and is discussed below.
---------------------------------------------------------------------------

    \13\ Unlike other NAAQS, where the standard is met when the 
relevant value is at or below the level of the standard since a 
lower standard level is more protective, in this case a higher 
standard level is more protective.
---------------------------------------------------------------------------

    The PA focuses primarily on information that relates degrees of 
biological impairment associated with adverse ecological effects to 
aquatic ecosystems to alternative levels of ANC in reaching conclusions 
regarding the range of target ANC levels that is appropriate to 
consider for the level of the standard. The PA develops the rationale 
for identifying a range of target ANC levels that is appropriate to 
consider by addressing questions related to the following areas: (1) 
Associations between ANC and pH levels to provide an initial bounding 
for the range of ANC

[[Page 46126]]

values to be considered; (2) evidence that allows for the delineation 
of specific ANC ranges associated with varying degrees of severity of 
biological impairment ecological effects; (3) the role of ANC in 
affording protection against episodic acidity; (4) implications of the 
time lag response of ANC to changes in deposition; (5) past and current 
examples of target ANC values applied in environmental management 
practices; and (6) data linking public welfare benefits and ANC.
1. Association Between pH Levels and Target ANC Levels
    As discussed above in section II.A and more fully in chapter 3 of 
the PA, specific levels of ANC are associated with differing levels of 
risk of biological impairment in aquatic ecosystems, with higher levels 
of ANC resulting in lower risk of ecosystem impacts, and lower ANC 
levels resulting in risk of both higher intensity of impacts and a 
broader set of impacts. While ANC is not the causal agent determining 
biological effects in aquatic ecosystem, it is a useful metric for 
determining the level at which a water body is protected against risks 
of acidification. There is a direct correlation between ANC and pH 
levels which, along with dissolved aluminum, are more closely linked to 
the biological causes of ecosystem response to acidification.
    Because there is a direct correlation between ANC and pH levels, 
the selection of target ANC levels is informed in part through 
information on effects of pH as well as direct studies of effects 
related to ANC. Levels of pH are closely associated with ANC in the pH 
range of approximately 4.5 to 7. Within this range, higher ANC levels 
are associated with higher pH levels. At a pH level of approximately 
4.5, further reductions in ANC generally do not correlate with pH, as 
pH levels remain at approximately 4.5 while ANC values fall 
substantially. Similarly, at a pH value of approximately 7, ANC values 
can continue to increase with no corresponding increase in pH. As pH is 
the primary causal indicator of effects related to aquatic 
acidification, this suggests that ANC values below approximately -50 
[mu]eq/L (the apparent point in the relationship between pH and ANC 
where pH reaches a minimum) are not likely to result in further damage. 
In addition, ANC values around and above approximately 100 [mu]eq/L 
(the apparent region in the relationship where pH reaches a maximum) 
are not likely to confer additional protection. As a result, the 
initial focus in the PA was on target ANC values in the range of -50 to 
100 [mu]eq/L.
2. ANC Levels Related to Effects on Aquatic Ecosystems
    As discussed above in section II.A, the number of fish species 
present in a water body has been shown to be positively correlated with 
the ANC level in the water, with higher values supporting a greater 
richness and diversity of fish species. The diversity and distribution 
of phyto-zooplankton communities also are positively correlated with 
ANC.
    A summary of effects related to ANC ranges is shown above in Table 
II-1. Within the ANC range from approximately -50 to 100 [mu]eq/L, 
linear and sigmoidal relationships are observed between ANC and 
ecosystem effects. On average, fish species richness is lower by one 
fish species for every 21 [mu]eq/L decrease in ANC in Shenandoah 
National Park streams (ISA, section 3.2.3.4). As shown in Table II-1, 
ANC levels have been grouped into five categories related to expected 
ecological effects, including categories of acute concern (<0 [mu]eq/
L), severe concern (0-20 [mu]eq/L), elevated concern (20-50 [mu]eq/L), 
moderate concern (50-100 [mu]eq/L), and low concern (>100 [mu]eq/L). 
This categorization is supported by a large body of research completed 
throughout the eastern U.S. (Sullivan et al., 2006).
    Water bodies with ANC values less than or equal to 0 [mu]eq/L at 
based flow are chronically acidic. Such ANC levels can lead to complete 
loss of species and major changes in the ability of water bodies to 
support diverse biota, especially in water bodies that are highly 
sensitive to episodic acidification. Based on the above considerations, 
the PA has focused on target ANC levels no lower than 0 [mu]eq/L.
    As discussed in the PA, biota generally are not harmed when ANC 
values are >100 [mu]eq/L, due to the low probability that pH levels 
will be below 7. In the Adirondacks, the number of fish species also 
peaks at ANC values >100 [mu]eq/L. This suggests that at ANC levels 
greater than 100 [mu]eq/L, little risk from acidification exists in 
many aquatic ecosystems. At ANC levels below 100 [mu]eq/L, overall 
health of aquatic communities can be maintained, although fish fitness 
and community diversity begin to decline. At ANC levels ranging from 
100 down to 50 [mu]eq/L, there is increasing likelihood that the 
fitness of sensitive species (e.g., brook trout, zooplankton) will 
begin to decline. When ANC concentrations are below 50 [mu]eq/L, the 
probability of acidification increases substantially, and negative 
effects on aquatic biota are observed, including large reductions in 
diversity of fish species and changes in the health of fish 
populations, affecting reproductive ability and fitness, especially in 
water bodies that are affected by episodic acidification. While there 
is evidence that ANC levels above 50 can confer additional protection 
from adverse ecological effects associated with aquatic acidification 
in some sensitive ecosystems, the expectation that such incremental 
protection from adverse effects will continue up to an ANC level of 100 
is substantially reduced. The PA concludes that the above 
considerations support a focus on target ANC levels up to a level 
greater than 50 [mu]eq/L but below 100 [mu]eq/L, such as up to a level 
of 75 [mu]eq/L.
    In considering the available scientific evidence, as summarized 
here and discussed in more detail in the ISA and REA, in its review of 
the second draft PA, CASAC expressed the following views about the 
range of biological responses that corresponds to this range of ANC 
levels (i.e., 0-100 [mu]eq/L):

    There will likely be biological effects of acidification at 
higher ANC values within this range, and there are relatively 
insensitive organisms that are not impacted at ANC values at the low 
end of this range. Adverse effects of acidification on aquatic biota 
are fairly certain at the low end of this range of ANC and 
incremental benefits of shifting waters to higher ANC become more 
uncertain at higher ANC levels. There is substantial confidence that 
there are adverse effects at ANC levels below 20 [mu]eq/L, and 
reasonable confidence that there are adverse effects below 50 
[mu]eq/L. Levels 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, 2010b)

    The PA concludes that the above considerations, including the views 
of CASAC, provide support for focusing on target ANC levels in the 
range of 20 to 75 [mu]eq/L.
3. Consideration of Episodic Acidity
    As discussed in the PA, across the broad range of ANC values from 0 
to 100 [mu]eq/L, ANC affords protection against the likelihood of 
decreased pH (and associated increases in Al) during long or short 
periods. In general, the higher the ANC within this range, the lower 
the probability of reaching low pH levels where direct effects such as 
increased fish mortality occur, as shown in Table 3-1 of the PA. 
Accordingly, greater protection would be achieved by target chronic ANC 
values set high enough to avoid pH depression to levels associated with 
elevated risk.

[[Page 46127]]

    The specific relationship between ANC and the probability of 
reaching pH levels of elevated risk varies by water body and fish 
species. The ANC levels below 20 [mu]eq/L are generally associated with 
high probability of low pH, leading to death or loss of fitness of 
biota that are sensitive to acidification (US EPA, 2008, section 
5.2.2.1; US EPA, 2009, section 5.2.1.2). At these levels, during 
episodes of high acidifying deposition, brook trout populations may 
experience lethal effects. In addition, the diversity and distribution 
of zooplankton communities decline sharply at ANC levels below 20 
[mu]eq/L. Overall, there is little uncertainty that significant effects 
on aquatic biota are occurring at ANC levels below 20 [mu]eq/L.
    It is clear that at ANC levels approaching 0 [mu]eq/L (Table II-1), 
there is significant impairment of sensitive aquatic ecosystems with 
almost complete loss of fish species. Avoiding ANC levels approaching 0 
[mu]eq/L is particularly relevant to episodic spikes in acidity that 
occur during periods of rapid snow melt and during and after major 
precipitation events. Since the ANC range considered in the PA reflects 
average, long-term base flow values, it is appropriate to consider 
protecting against episodic drops in ANC values to a level as low as 0 
[mu]eq/L. Staddard et al. (2003) noted on average a 30 [mu]eq/L 
depression of ANC between spring and summer time values, indicating the 
need to maintain higher base flow ANC levels to protect against ANC 
levels below 0 [mu]eq/L. The above considerations do not provide 
support for a target chronic ANC level as low as 0 [mu]eq/L for a 
standard that would protect against significant harm to aquatic 
ecosystems, including harm from episodic acidification. The PA 
concludes that these considerations also support a lower end of the 
range for consideration no lower than 20 [mu]eq/L.
    The CASAC agreed with this conclusion in its comments on the second 
draft PA (Russell and Samet, 2010b). The CASAC noted that ``there are 
clear and marked biological effects at ANC values near 0 [mu]eq/L, so 
this is probably not an appropriate target value'' for the AAI. With 
regard to the likelihood of impairment of aquatic ecosystems due to 
episodic acidification, in terms of specific target levels for chronic 
ANC, CASAC expressed the following view:

    Based on surface waters studied in the Northeast, decreases in 
ANC associated with snowmelt [are] approximately 50 [mu]eq/L. Thus, 
based on these studies, a long term ANC target level of 75 [mu]eq/L 
would generally guard against effects from episodic acidification 
down to a level of about 25 [mu]eq/L. (Russell and Samet, 2010b)

4. Consideration of Ecosystem Response Time
    The PA notes that when considering a standard level to protect 
against aquatic acidification, it is appropriate to take into account 
both the time period to recovery as well as the potential for recovery 
in acid-sensitive ecoregions. Ecosystems become adversely impacted by 
acidifying deposition over long periods of time and have variable time 
frames and abilities to recover from such perturbations. Modeling 
presented in the REA (U.S. EPA, 2009, section 4.2.4) shows the 
estimated ANC values for Adirondack lakes and Shenandoah streams under 
pre-acidification conditions and indicates that for a small percentage 
of lakes and streams, natural ANC levels would have been below 50 
[mu]eq/L. Therefore, for these water bodies, reductions in acidifying 
deposition are not likely to achieve an ANC of 50 [mu]eq/L or greater. 
Conversely, for some lakes and streams the level of perturbation from 
long periods of acidifying deposition has resulted in very low ANC 
values compared to estimated natural conditions. For such water bodies, 
the time to recovery would be largely dependent on future inputs of 
acidifying deposition.
    Setting a standard level in terms of a target chronic ANC level is 
based on the long-term response of aquatic ecosystems. The time 
required for a water body to achieve the target ANC level--given a 
decrease in ambient air concentrations of NOy and 
SOx and related acidifying deposition such that the critical 
load for a target ANC is not exceeded--is often decades if not 
centuries. In recognition of the potential public welfare benefits of 
achieving the target ANC in a shorter time frame, the concept of target 
loads had been developed. Target loads represent the depositional 
loading that is expected to achieve a particular level of the 
ecological indicator by a given time. For example, to achieve an ANC 
level of 20 [mu]eq/L by 2030, it might be necessary to specify a higher 
target ANC level of, for example, 50 [mu]eq/L, such that the 
depositional loading would be reduced more quickly than would occur if 
the depositional loading was based on achieving a target ANC level of 
20 [mu]eq/L as a long-term equilibrium level. In this example, the 
target ANC of 50 [mu]eq/L would ultimately be realized many years 
later.
    The above considerations have implications for selecting an 
appropriate standard level, in that the standard level affects not only 
the ultimate degree of protection that would be afforded by the 
standard, but also the time frame in which such protection would be 
realized. However, the PA recognizes that there is a great deal of 
heterogeneity in response times among water bodies and that there is 
only very limited information from dynamic modeling that would help to 
quantify recovery time frames in areas across the country. As a 
consequence, quantification of a general relationship between critical 
loads associated with a specific long-term target ANC level and target 
loads associated with achieving the target ANC level within a specific 
time frame is not currently possible. Thus, while the time frame for 
recovery is an important consideration in selecting an appropriate 
range of levels to consider, the PA concludes that it can only be 
considered in a qualitative sense at this time.
5. Prior Examples of Target ANC Levels
    A number of regional organizations, states, and international 
organizations have developed critical load frameworks to protect 
against acidification of sensitive aquatic ecosystems. In considering 
the appropriate range of target ANC levels for consideration in this 
review, it is informative to evaluate the target ANC levels selected by 
these different organizations, as well as the rationale provided in 
support of the selected levels. Chapter 4 of the PA provides a detailed 
discussion of how critical loads have been developed and used in other 
contexts. Specific target values and their rationales are summarized 
below.
    The UNECE has developed critical loads in support of international 
emissions reduction agreements. As noted in chapter 4 of the PA, 
critical loads were established to protect 95 percent of surface waters 
in Europe from an ANC less than 20 [micro]eq/L based on protection of 
brown trout. Individual countries have set alternative ANC targets; for 
example, Norway targets an ANC of 30 [micro]eq/L based on protection of 
Atlantic salmon. Several states have established target ANC or pH 
values related to protection of lakes and streams from acidification. 
While recognizing that some lakes in the Adirondacks will have a 
naturally low pH, the state of New York has established a target pH 
value of 6.5 for lakes that are not naturally below 6.5. As noted 
above, this level is associated with an ANC value that is likely to be 
between 20 and 50 [micro]eq/L or possibly higher. New Hampshire and 
Vermont have set ANC targets of 60 [micro]eq/L and 50 [micro]eq/L, 
respectively. Tennessee has established site-specific target ANC

[[Page 46128]]

values based on assessments of natural acidity, with a default value of 
50 [micro]eq/L when specific data are not available.
    Taken together, these policy responses to concerns about ecological 
effects associated with aquatic acidification indicate that target ANC 
values between 20 and 60 [mu]eq/L have been selected by states and 
other nations to provide protection of lakes and streams in some of the 
more sensitive aquatic ecosystems.
6. Consideration of Public Welfare Benefits
    The point at which effects on public welfare become adverse is not 
defined in the CAA. Characterizing a known or anticipated adverse 
effect to public welfare is an important component of developing any 
secondary NAAQS. According to the CAA, welfare effects include:

* * * 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 
effect on economic values and on personal comfort and well-being, 
whether caused by transformation, conversion, or combination with 
other air pollutants. (CAA, section 302(h)).

    Consideration of adversity to public welfare in the context of the 
secondary NAAQS for oxides of nitrogen and sulfur can be informed by 
information about losses in ecosystem services associated with 
acidifying deposition and the potential economic value of those losses, 
as summarized above in section II.C and discussed more fully in chapter 
4 of the PA.
    Ecosystem service losses at alternative ANC levels are difficult to 
enumerate. However, in general there are categories of ecosystem 
services, discussed in chapter 4 of the PA, that are related to the 
specific ecosystem damages expected to occur at alternative ANC levels. 
Losses in fish populations due to very low ANC (below 20 [mu]eq/L) are 
likely associated with significant losses in value for recreational and 
subsistence fishers. Many acid sensitive lakes are located in areas 
with high levels of recreational fishing activity. For example, in the 
northeastern U.S., where nearly 8 percent of lakes are considered 
acidic, more than 9 percent of adults participate in freshwater 
fishing, with an estimated value of approximately $5 billion in 2006. 
This suggests that improvements in lake fish populations may be 
associated with significant recreational fishing value.
    As discussed in the PA, inland surface waters also provide cultural 
services such as aesthetic and existence value and educational 
services. To the extent that piscivorous birds and other wildlife are 
harmed by the absence of fish in these waters, hunting and birdwatching 
activities are likely to be adversely affected. A case study of the 
value to New York residents of improving the health of lakes in the 
Adirondacks found significant willingness to pay for those 
improvements. When scaled to evaluate the improvement in lake health 
from achieving ANC values of either 20 or 50 [mu]eq/L, the study 
implies benefits to the New York population roughly on the order of 
$300-900 million per year (in constant 2007$). The survey administered 
in this study recognized that participants were thinking about the full 
range of services provided by the lakes in question--not just the 
recreational fishing services. Therefore the estimates of willingness 
to pay include resident's benefits for potential hunting and 
birdwatching activities and other ancillary services. These results are 
just for New York populations. The PA concludes that if similar 
benefits exist for improvements in other acid sensitive lakes, the 
economic value to U.S. populations could be very substantial, 
suggesting that, at least by one measure of impact on public welfare, 
impacts associated with ANC less than 50 [mu]eq/L may be adverse to 
public welfare.
7. Summary of Alternative Levels
    Based on all the above considerations, the PA concludes that 
consideration should be given to a range of standard levels from 20 to 
75 [mu]eq/L. The available evidence indicates that target ANC levels 
below 20 [mu]eq/L would be inadequate to protect against substantial 
ecological effects and potential catastrophic loss of ecosystem 
function in some sensitive aquatic ecosystems. While ecological effects 
occur at ANC levels below 50 [mu]eq/L in some sensitive ecosystems, the 
degree and nature of those effects are less significant than at levels 
below 20 [mu]eq/L. Levels at and above 50 [mu]eq/L would be expected to 
provide additional protection, although uncertainties regarding the 
potential for additional protection from adverse ecological effects are 
much larger for target ANC levels above about 75 [mu]eq/L, as effects 
are generally appreciably less sensitive to changes in ANC at such 
higher levels.
    In reaching this conclusion in the PA, consideration was given to 
the extent to which a target ANC level within this range would protect 
against episodic as well as long-term ecological effects. Levels in the 
mid- to upper-part of this range would be expected to provide greater 
protection against short-term, episodic peaks in aquatic acidification, 
while lower levels within this range would give more weight to 
protection from long-term rather than episodic acidification. 
Similarly, levels in the mid- to upper-part of this range would be 
expected to result in shorter time periods for recovery given the lag 
in ecosystem response in some sensitive ecosystems relative to levels 
in the lower part of this range. The PA also notes that this range 
encompasses target ANC values that have been established by various 
States and regional and international organizations to protect against 
acidification of aquatic ecosystems.
    The PA recognizes that the level of the standard together with the 
other elements of the standard, including the ambient air indicators, 
averaging time, and form, determine the overall protectiveness of the 
standard. Thus, consideration of a standard level should reflect the 
strengths and limitations of the evidence and assessments as well as 
the inherent uncertainties in the development of each of the elements 
of the standard. The implications of considering alternative standards, 
defined in terms of alternative combinations of levels and percentile 
values that are a critical component of factor F1 in the form of the 
standard, are discussed below in section III.E. Key uncertainties in 
the various components of the standard are summarized and considered 
below in section III.F.

E. Combined Alternative Levels and Forms

    To provide some perspective on the implications of various 
alternative multi-pollutant, AAI-based standards, the PA presented the 
number of acid-sensitive ecoregions that would likely not meet various 
sets of alternative standards. The alternative standards considered 
were based on combinations of alternative target ANC levels, within the 
range of 20 to 75 [mu]eq/L, and alternative forms, characterized by 
alternative representative percentiles within the range of the 70th to 
90th percentile. These alternative standards are also defined in terms 
of the other elements of the standard: ambient air indicators NOy and 
SOx, discussed above in section III.A; other elements of the form of 
the standard, including ecoregion-specific values for factors F1 
through F4 in the AAI equation, discussed above in section III.B.5; and 
an annual averaging time for NOy and SOx, 
discussed above in section III.C. With regard to the averaging time, 
the assessment did not consider multi-year averaging of the calculated 
annual AAI

[[Page 46129]]

values due to data limitations, including, for example, the lack of 
CMAQ modeling for multiple consecutive years. In this assessment, we 
characterize an ecoregion as likely not meeting a given alternative 
standard if the calculated AAI value is less than the target ANC level 
of the standard, recognizing that higher AAI values are more protective 
than lower values.
    The results of this assessment are presented in Table 7-1a-d in the 
PA for a subset of ecoregions including those characterized as acid 
sensitive. Calculated annual AAI values at the ecoregion level are 
shown for each alternative standard considered. Based on these AAI 
values, Table 7-2 in the PA summarizes the number of acid-sensitive 
ecoregions that would likely not meet each of the alternative standards 
considered.\14\ Calculated AAI values for all ecoregions categorized as 
relatively non-acid sensitive are shown in Table D-5 in Appendix D of 
the PA. In all cases, these relatively non-acid sensitive ecoregions 
were estimated to meet all of the alternative standards considered in 
this assessment.
---------------------------------------------------------------------------

    \14\ Tables 7-1a-d and 7-2 in the PA present assessment results 
for 29 ecoregions that had been initially characterized as acid 
sensitive. Subsequently, based on a broader set of criteria used to 
characterize ecoregions as acid sensitive, as discussed above in 
section III.B.5.a, the set of ecoregions characterized as acid 
sensitive was narrowed to include 22 ecoregions.
---------------------------------------------------------------------------

    As described above, the AAI values presented in Table 7-1a-d of the 
PA are based in part on data from 2005 CMAQ model simulations, which 
was used to generate values for F2 through F4 in the AAI equation, as 
well as to estimate annual average ambient air concentrations of 
NOy and SOx that reflect recent air quality in 
the absence of currently available monitored concentrations in 
sensitive ecoregions across the country. Water quality and hydrology 
data from water bodies within each ecoregion were also used in 
calculating the AAI values. Such data were initially used to calculate 
critical loads for each water body with sufficient data within an 
ecoregion so as to identify the nth percentile critical load 
representative of the ecoregion used in calculating the F1 factor for 
the ecoregion. As expected, the number of ecoregions that likely would 
not meet alternative standards increases with increasing percentile 
values and target ANC levels (U.S. EPA, 2011, Table 7-2). Out of 22 
acid-sensitive ecoregions, the number of ecoregions that would likely 
not meet the alternative standards ranges from 22 for the most 
protective alternative standard considered (75 [mu]eq/L, 90th 
percentile) to 4 for the least protective alternative standard (20 
[mu]eq/L, 70th percentile). It is apparent that both the percentile and 
the level chosen have a strong influence, over the ranges considered, 
in determining the number of areas that would likely not meet this set 
of alternative standards.
    The PA observes that there is one grouping of these acid-sensitive 
ecoregions that would likely not meet almost all combinations of level 
and form under consideration (U.S. EPA, 2011, Table 7-2 and Appendix 
D). This group is made up of southern Appalachian mountain areas, 
including North Central Appalachians, 5.3.3; Ridge and Valley, 8.4.1; 
Central Appalachians, 8.4.2; Blue Ridge, 8.4.4; and Southwestern 
Appalachians, 8.4.9. In addition, these ecoregions exhibit the highest 
amounts of exceedance relative to alternative standards.
    The Northern Appalachian and Atlantic Maritime Highlands (5.3.1), 
which includes the Adirondacks, and the Northern Lakes and Forests 
(5.2.1) of the upper midwest exhibit similar patterns with respect to 
in the role of level and percentile in identifying regions not likely 
to meet alternative standards, although there are considerably fewer 
cases compared to the regions in the Appalachians.
    In the mountainous west, the Sierra Nevada (6.2.12), Idaho 
Batholith (6.2.15) and the Cascades (6.2.7) ecoregions likely would not 
meet alternative standards in fewer cases relative to eastern regions, 
with the Sierra Nevada ecoregion exhibiting relatively greater 
sensitivity compared to all western regions. Only in the upper part of 
the ranges of level and percentile do regions in the northern and 
central Rockies likely not meet alternative standards.
    In considering these findings, the PA observes that the standard as 
defined by the AAI behaves in an intuitively logical manner. That is, 
an increase in ecoregions likely not to meet the standard is associated 
with higher alternative levels and percentiles, both of which 
contribute to a lower regionally representative critical load. 
Moreover, the areas of known adverse aquatic acidification effects are 
identified, mostly in high elevation regions or in the northern 
latitudes--the Adirondacks, Shenandoahs, northern midwest lakes and the 
mountainous west. These results reflect the first application of a 
nationwide model that integrates water quality and atmospheric 
processes at a national scale and provides findings that are consistent 
with our basic understanding of the extent of aquatic acidification 
across the U.S. What is particularly noteworthy is that this model is 
not initialized with a starting ANC based on water quality data, which 
likely would result in a reproduction of water quality observations. 
Rather, this standard reflects the potential of the changes in 
atmospheric concentrations of NOy and SOx to 
induce long-term sustained changes in surface water systems. The PA 
notes that the fact that the patterns of adversity based on applying 
this standard are commensurate with what is observed in surface water 
systems provides confidence in the basic underlying formulation of the 
standard.
    The PA notes that the Appalachian mountain regions merit further 
inspection as they stand out as areas with the largest relative 
exceedances from a national perspective. Water quality data from these 
regions as well as an emissions sensitivity CMAQ simulation were 
considered to better understand the simulated behavior of these 
regions. The maps and tables in appendix D of the PA include paired 
comparisons of the CMAQ 2005 and emissions sensitivity simulations. The 
emissions sensitivity simulation reflects domain-wide reductions in 
NOy and SOx emissions of 48 percent and 42 
percent, respectively, relative to 2005 base year emissions. The PA 
assumes that this emissions sensitivity simulation is indicative of 
future conditions.
    The emissions sensitivity results project that many of the regions 
that likely would not meet the alternative standards based on recent 
air quality, especially at alternative levels of 20 and 35 [micro]eq/L, 
would likely meet such standards in the future year scenario for the 
Appalachian mountain regions. It is apparent that the AAI calculations 
are especially sensitive to changes in SOx emissions as the 
Appalachian regions have the highest SOx concentrations and 
deposition rates (U.S. EPA, 2011,section 2), and the AAI equation 
responds as expected to modeled reductions in SOx. The 
emissions sensitivity scenario is a prospective application of the 
standard, in the sense that rules derived from the air quality 
management process result in reductions of NOy and 
SOx emissions. Expected emission changes over the next two 
decades should be far greater than the 42 percent and 48 percent, 
respectively, SOx and NOy reductions used in this 
analysis, with a consequent further reduction in areas that would 
likely not meet alternative standards.
    The Appalachian mountain regions generally have low DOC levels, 
average runoff rates, moderately low base cation supply and highly 
elevated sulfate concentrations. Collectively, those attributes do not 
suggest naturally acidic conditions as the availability of

[[Page 46130]]

anthropogenic contributions of mineral acids is likely responsible for 
observed low ANC values in those regions.
    The PA notes the Sierra Nevada region as an interesting case study, 
as it has some of the lowest critical load values nationally (U.S. EPA, 
2011, Table D-3). Water quality data indicate extremely low sulfate, as 
expected given the relatively low SO2 emissions in the 
western U.S. Extremely low base cation supply and low Neco, which 
mitigate the effect of nitrogen deposition, explain the low critical 
load values. Low Neco values appear to associate well with high 
elevation western U.S. regions, perhaps reflecting the more arid and 
reduced vegetation density relative to eastern U.S. regions. The 
proximity to high level nitrogen emissions combined with very low base 
cation supply explains the cases where the Sierra region likely does 
not meet alternative standards. Because Neco values are low in the 
Sierras, the system responds effectively to reductions of 
NOx emissions, as illustrated in the maps and tables of 
Appendix D of the PA. Although Neco affords protection from the 
acidifying effects of nitrogen deposition, the availability of 
excessive nitrogen neutralization capacity also means that reductions 
in nitrogen are not as effective as reductions in SOx in 
reducing the calculated AAI.
    In reviewing these results, the PA observes that the analysis of 
the alternative combinations of level and form presented provide 
context for considering the impact of different standards. Since the 
AAI equation has been newly developed in the PA, these examples of 
estimated exceedances help to address the question of whether the AAI 
equation responds in a reasonable manner with regard to identifying 
areas of concern and to prospective changes in atmospheric conditions 
likely to result from future emissions reduction strategies. The PA 
concludes that the behavior of the AAI calculations is both reasonable 
and explainable, which the PA concludes serves to increase confidence 
in considering a standard defined in terms of the AAI.

F. Characterization of Uncertainties

    This section summarizes discussions of the results of analyses and 
assessments, presented more fully in the PA (U.S. EPA, 2011, section 
7.6 and Appendices F and G), intended to address the relative 
confidence associated with the linked atmospheric-ecological effects 
system described above. An overview of uncertainties is presented in 
the context of the major structural components underlying the standard, 
as well as with regard to areas of relatively high uncertainty. The 
section closes with a discussion of data gaps and uncertainties 
associated with the use of ecological and atmospheric modeling to 
specify the factors in the AAI equation, which can be used to guide 
future field programs and longer-term research efforts.
1. Overview of Uncertainty
    As discussed in the PA (U.S. EPA, 2011, Table 7-3), there is 
relatively low uncertainty with regard to the conceptual formulation of 
the overall structure of the AAI-based standard that incorporates the 
major associations linking biological effects to air concentrations. 
Based on the strength of the evidence that links species richness and 
mortality to water quality, the associations are strongly causal and 
without any obvious confounding influence. The strong association 
between the ecosystem indicator (ANC) and the causative water chemistry 
species (dissolved aluminum and hydrogen ion) reinforces the confidence 
in the linkage between deposition of nitrogen and sulfur and effects. 
This strong association between ANC and effects is supported by a sound 
mechanistic foundation between deposition and ANC. The same mechanistic 
strength holds true for the relationship between ambient air levels of 
nitrogen and sulfur and deposition, which completes the linkage from 
ambient air indicators through deposition to ecological effects.
    There are relatively higher uncertainties, however, in considering 
specific elements within the structure of an AAI-based standard, 
including the deposition of SOx, NOy, and 
NHx as well as the critical load-related component, each of 
which can vary within and across ecoregions. Overall system uncertainty 
relates not just to the uncertainty in each such element, but also to 
the combined uncertainties that result from linking these elements 
together within the AAI-based structure. Some of these elements--
including, for example, dry deposition, pre-industrial base cation 
production, and reduced nitrogen deposition--are estimated with less 
confidence than other elements (U.S. EPA, 2011, Table 7.3). The 
uncertainties associated with all of these elements, and the 
combination of these elements through the AAI equation, are discussed 
below and in the following sections related to measured data gaps and 
modeled processes for both air quality and water quality.
    The lack of observed dry deposition data is constrained by 
resources and the lack of efficient measurement technologies. Progress 
in reducing uncertainties in dry deposition will depend on improved 
atmospheric concentration data and direct deposition flux measurements 
of the relevant suite of NOy and SOx species.
    Pre-industrial base cation productivity by definition is not 
observable. Contemporary observations and inter-model comparisons are 
useful tools that would help reduce the uncertainty in estimates of 
preindustrial base cation productivity used in the AAI equation. In 
characterizing contemporary base cation flux using basic water quality 
measurements (i.e., major anion and cation species as defined in 
equation 2.11 in the PA), it is reasonable to assume that a major 
component of contemporary base cation flux is associated with pre-
industrial weathering rates. To the extent that multiple models 
converge on similar solutions, greater confidence in estimating pre-
industrial base cation production would be achieved.
    Characterization of NHx deposition has been evolving 
over the last decade. The relatively high uncertainty in characterizing 
NHx deposition is due to both the lack of field measurements 
and the inherent complexity of characterizing NHx with 
respect to source emissions and dry deposition. Because ammonia 
emissions are generated through a combination of man-made and 
biological activities, and ammonia is semi-volatile, the ability to 
characterize spatial and temporal distributions of NHx 
concentrations and deposition patterns is challenging. While direct 
measurement of NHx deposition is resource intensive because 
of the diffuse nature of sources (i.e., area-wide and non-point 
sources), there have been more frequent deposition flux studies, 
relative to other nitrogen species, that enable the estimation of both 
emissions and dry deposition. Also, while ammonia has a relatively high 
deposition velocity and traditionally was thought to deposit close to 
the emissions release areas, the semi-volatile nature of ammonia 
results in re-entrainment back into the lower boundary layer resulting 
in a more dispersed concentration pattern exhibiting transport type 
characteristics similar to longer lived atmospheric species. These 
inherent complexities in source characterization and ambient 
concentration patterns raise the uncertainty level of NHx in 
general. However, the PA notes that progress is being made in measuring 
ammonia with cost efficient samplers and anticipates the gradual 
evolution of a spatially robust ammonia sampling network that would 
help support analyses to reduce underlying uncertainties in 
NHx

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deposition. Also, from an aquatic acidification perspective, 
NHx is not as important a driver as NOy and 
SOx in the mountainous areas in the eastern U.S. However, 
the relative importance of NHx is likely to increase over 
time, in light of air quality rules in place designed to reduce 
emissions of NOy and SOx.
2. Uncertainties Associated With Data Gaps
    In summarizing uncertainties with respect to available measurement 
data and the use of ecological and atmospheric models, the PA 
indentified data gaps and model uncertainties in relative terms by 
comparing, for example, the relative richness of data between 
geographic areas or environmental media. With regard to relevant air 
quality measurements, the PA notes that such measurements are 
relatively sparse in the western U.S. While the spatial extent of 
CASTNET coverage has gradually incorporated western U.S. locations with 
support from the NPS, the relative density of monitoring sites is much 
less than that in the eastern U.S. This relative disparity in spatial 
density of monitors is exacerbated as air quality patterns in the 
mountainous west generally exhibit greater spatial heterogeneity due to 
dramatic elevation gradients that impact meteorology and air mass flow 
patterns. Similarly, water quality data coverage is far more 
comprehensive in the eastern U.S. relative to the west
    Measurements of NOy notably are lacking in both eastern 
and western acid-sensitive ecoregions. This adds uncertainty to the use 
of the AAI equation as the lack of NOy data limits efforts 
to evaluate air quality modeling of NOy that is the basis 
for quantifying factor F3 in the AAI equation. The lack of 
NOy measurements also limits efforts to characterize the 
variability and representativeness of modeled NOy 
concentrations within and across ecoregions. Currently, the Agency's 
ability to define the protection likely to be afforded by alternative 
standards (in terms of alternative levels and percentiles) is 
compromised by the lack of a full set of ambient air quality indicator 
measurements, notably including NOy, throughout sensitive 
ecoregions across the U.S.
    Further, obtaining measurement of the dominant species that 
comprise NOy (HNO3, true NO2, NO, p-
NO3, and PAN) would be useful to evaluate performance of 
NOy samplers. Beyond the more well known dominant components 
of NOy, research efforts would be needed to characterize 
total reactive nitrogen that may include significant amounts of 
organically-bound nitrogen (beyond PAN) which is poorly understood with 
regard to emission sources and concentration levels.
    Field measurements of NHx have been extremely limited, 
but have begun to be enhanced through the NADP's passive ammonia 
network (AMoN). The AMoN measures ammonia at over 50 sites, with more 
than 35 at CASTNET locations. Enhanced spatial coverage of reduced 
nitrogen measurements, particularly to understand within and across 
ecoregion variability, and the inclusion of some continuous 
observations would provide a better understanding of the uncertainty in 
the F2 factor in the AAI equation and of the representativeness of 
modeled NHx deposition within and across ecoregions.
    With regard to water quality data, the PA notes that such data are 
typically limited relative to air quality data sets, and are also 
relatively sparse in the western U.S. The TIME/LTM water quality 
sampling program in the eastern U.S. (as described in chapter 2 of the 
PA) is an appropriate complement to national air monitoring programs as 
it affords consistency across water bodies in terms of sampling 
frequency and analysis protocols. Consideration should be given to 
extending the TIME/LTM design to all acid sensitive ecoregions, with 
priority for areas in the western mountains that are data limited and 
showing initial signs of adversity particularly with respect to aquatic 
acidification. The lack of a regulatory requirement for TIME/LTM often 
jeopardizes funding support of this resource that is especially 
valuable and cost effective. While there are several state and local 
agency water quality data bases, it is unclear the extent to which 
differences in sampling, chemical analysis and reporting protocols 
would impact the use of such data for the purpose of better 
understanding the degree of protectiveness that would be afforded by an 
AAI-based standard within sensitive ecoregions across the country. In 
addition, our understanding of water quality in Alaska and Hawaii and 
the acid sensitivity of their ecoregions is particularly limited.
    Water quality data and modeling support the standard setting 
process. As more water bodies are sampled, the critical load data bases 
would expand, enabling clearer delineation of ecoregion representative 
critical loads in terms of the nth percentile. This would provide more 
refined characterization of the degree of protection afforded by a 
given standard. Longer term, the availability of water quality trend 
data (annual to monthly sampled) would support accountability 
assessments that examine if an ecoregion's response to air management 
efforts is as predicted by earlier model forecasting. The most obvious 
example is the long-term response of water quality ANC change to 
changes in calculated AAI, deposition, ambient NOy and 
SOx concentrations, and emissions. In addition, water 
quality trends data provide a basis for evaluating and improving the 
parameterizations of processes in critical load models applied at the 
ecoregion scale related to nitrogen retention and base cation supply. A 
better understanding of soil processes, especially in the southern 
Appalachians, would enhance efforts to examine the variability within 
ecoregions of the soil-based adsorption and exchange processes which 
moderate the supply of major cations and anions to surface waters and 
strongly influence the response of surface water ANC to changes in 
deposition of nitrogen and sulfur.
3. Uncertainties in Modeled Processes
    As discussed in the PA, from an uncertainty perspective, gaps in 
field measurement data are related to uncertainties in modeled 
processes and in the specific application of such models. As noted 
above, processes that are embodied in an AAI-based standard are modeled 
using the CMAQ atmospheric model and steady state ecological models. 
These models are characterized in the ISA as being well established and 
they have undergone extensive peer review. Nonetheless, the application 
of these models for purposes of specifying the factors in the AAI 
equation, on an ecoregion scale, is a new application that introduces 
uncertainties, as noted below, especially in areas with limited 
observational data that can be used to evaluate this specific 
application. Understanding uncertainties in relevant modeled process 
thus involves consideration of the uncertainties associated with 
applying each model as well as the combination of these uncertainties 
as the models are applied in combination within the AAI framework.
    With regard to the application of CMAQ for purposes of use in an 
AAI-based standard, the modeling of dry deposition has been identified 
as having a relatively high degree of uncertainty. Due to a combination 
of system complexity and resource constraints, there is no routine 
observational basis for directly comparing modeled dry deposition and 
measurements. Periodic dry deposition flux experiments covering a 
variety of vegetation, surfaces and meteorology across seasons would

[[Page 46132]]

enable a more robust evaluation of modeled deposition of nitrogen and 
sulfur. Given the difficulty in acquiring dry deposition observations, 
it becomes especially important to evaluate the model's ability to 
capture temporal and spatial ambient air patterns of individual 
nitrogen and sulfur species which are used to drive dry deposition 
calculations in models. For example, reducing a generally acknowledged 
positive bias in model-predicted SO2 relative to 
observations is especially relevant to the AAI-based standard, as 
SO2 deposition is a dominant contributor to total acidifying 
deposition in the eastern U.S. With respect to oxidized nitrogen, 
observations of individual NOy species are important as air 
quality models calculate the individual deposition of each species. The 
modeled transference ratios, TNOy and TSOx used 
in factors F3 and F4 rely on CMAQ's ability to characterize both 
deposition and concentration. Consequently, a better understanding of 
the variability of these factors within and across ecoregions could be 
achieved by improved availability of measured ambient concentrations 
and deposition observations.
    Steady state biogeochemical ecosystem modeling is used to develop 
critical load estimates that are incorporated in the AAI equation 
through factor F1. Consequently, the PA notes that an estimate of the 
temporal response of surface water ANC to deposition and air 
concentration changes is not directly available. Lacking a predicted 
temporal response impairs the ability to conduct accountability 
assessments down to the effects level. Accountability assessments would 
examine the response of each step in the emissions source through air 
concentration--deposition--surface water quality--biota continuum. The 
steady state assumption at the ecosystem level does not impair 
accountability assessments through the air concentration/deposition 
range of that continuum. However, in using steady state ecosystem 
modeling, several assumptions are made relative to the long-term 
importance of processes related to soil adsorption of major ions and 
ecosystem nitrogen dynamics. Because these models often were developed 
and applied in glaciated areas with relatively thin and organically 
rich soils, their applicability is relatively more uncertain in areas 
such as those in the non-glaciated clay-based soil regions of the 
central Appalachians. Consequently, it is desirable to develop the 
information bases to drive simple dynamic ecosystem models that 
incorporate more detailed treatment of subsurface processes, such as 
adsorption and exchange processes and sulfate absorption.
4. Applying Knowledge of Uncertainties
    An understanding of the relative uncertainties in a system assists 
in setting priorities for data collection efforts and research, with 
the expectation that such efforts would reduce uncertainties over time 
and afford greater confidence in applications of an AAI-based standard. 
Because of the uniquely wide breadth of pollutants and environmental 
media addressed by an AAI-based multi-pollutant standard, there are a 
wide range of uncertainties that are important to consider relative to 
single pollutant standards that typically address only direct effects 
of ambient air exposures. For an AAI-based standard, a reduction of the 
uncertainties across the various modeled processes at the ecoregion 
scale would lead to greater confidence in the degree of protection 
afforded by the standard.
    The PA notes that there is generally low uncertainty with regard to 
the conceptual development and related major components of this 
standard. In recognizing the scientific soundness of the basic 
structure of this standard, the PA notes that future efforts would be 
appropriately directed at expanding the availability of relevant data 
for ecoregion-specific evaluation and application of the relevant 
modeling of ecological and atmospheric processes, as identified above. 
Such efforts would further support consideration of an AAI-based 
standard and would guide field studies and analyses designed to improve 
the longer-term confidence in such a standard.

G. CASAC Advice

    The CASAC has advised EPA concerning the ISA, the REA, and the PA. 
The CASAC has endorsed EPA's interpretation of the science embodied in 
the ISA and the assessment approaches and conclusions incorporated in 
the REA.
    Most recently, CASAC has considered the information in the final PA 
in providing its recommendations on the review of the new multi-
pollutant standard developed in that document and discussed above 
(Russell and Samet, 2011a). In so doing, CASAC has expressed general 
support for the conceptual framework of the standard based on the 
underlying scientific information, as well as for the conclusions in 
the PA with regard to indicators, form, averaging time, and level of 
the standard that are appropriate for consideration by the Agency in 
reaching decisions on the review of the secondary NAAQS for oxides of 
nitrogen and sulfur:

    The final Policy Assessment clearly sets out the basis for the 
recommended ranges for each of the four elements (indicator, 
averaging time, level and form) of a potential NAAQS that uses 
ambient air indicators to address the combined effects of oxides of 
nitrogen and oxides of sulfur on aquatic ecosystems, primarily 
streams and lakes. As requested in our previous letters, the Policy 
Assessment also describes the implications of choosing specific 
combinations of elements and provides numerous maps and tabular 
estimates of the spatial extent and degree of severity of NAAQS 
exceedances expected to result from possible combinations of the 
elements of the standard.
    We believe this final PA is appropriate for use in determining a 
secondary standard to help protect aquatic ecosystems from 
acidifying deposition of oxides of sulfur and nitrogen. EPA staff 
has done a commendable job developing the innovative Aquatic 
Acidification Index (AAI), which provides a framework for a national 
standard based on ambient concentrations that also takes into 
account regional differences in sensitivities of ecosystems across 
the country to effects of acidifying deposition. (Russell and Samet, 
2011a)

    The CASAC also recommended that as EPA moves forward in the 
regulatory process ``some attention should be given to our residual 
concern that the available data may reflect the more sensitive water 
bodies and thus, the selection of percentiles of waterbodies to be 
protected could be conservatively biased'' (Russell and Samet, 2011a). 
In addition, CASAC found some improvements could be made to the 
uncertainty analysis, as noted below. With respect to indicators, CASAC 
supports the use of SOx and NOy as ambient air 
indicators (discussed above in section III.A) and ANC as the ecological 
indicator (discussed above in section III.B.1):
    The use of NOy and SOx as atmospheric 
indicators of oxides of nitrogen and sulfur atmospheric concentrations 
is well justified. The use in the AAI of NOy and 
SOx as atmospheric indicators of oxides of nitrogen and 
sulfur concentrations is useful and corresponds with other efforts by 
EPA. As we have stated previously, CASAC also agrees that ANC is the 
most appropriate ecological indicator of aquatic ecosystem response and 
resiliency to acidification (Russell and Samet, 2011a).
    With respect to the form of the standard (discussed above in 
section III.B), CASAC stated the following:

    EPA has developed the AAI, an innovative ``form'' of the NAAQS 
itself that incorporates

[[Page 46133]]

the multi-pollutant, multi-media, environmentally modified, 
geographically variable nature of SOx/NOy 
deposition-related aquatic acidification effects. With the caveats 
noted below, CASAC believes that this form of the NAAQS as described 
in the final Policy Assessment is consistent with and directly 
reflective of current scientific understanding of effects of 
acidifying deposition on aquatic ecosystems. (Russell and Samet, 
2011a)
    CASAC agrees that the spatial components of the form in the 
Policy Assessment are reasonable and that use of Omernick's 
ecoregions (Level III) is appropriate for a secondary NAAQs intended 
to protect the aquatic environment from acidification * * * (Russell 
and Samet, 2011a)

    The ``caveats'' noted by CASAC include a recognition of the 
importance of continuing to evaluate the performance of the CMAQ and 
ecological models to account for model uncertainties and to make the 
model-dependent factors in the AAI more transparent. In addition, CASAC 
noted that the role of DOC and its effects on ANC would benefit from 
further refinement and clarification (Russell and Samet, 2011a). While 
CASAC expressed the view that the ``division of ecoregions into 
`sensitive' and `non-sensitive' subsets, with a more protective 
percentile applied to the sensitive areas, also seems reasonable'' 
(Russell and Samet, 2011a), CASAC also noted that there was the need 
for greater clarity in specifying how appropriate screening criteria 
would be applied in assigning ecoregions to these categories. Further, 
CASAC identified potential biases in critical load calculations and in 
the regional representativeness of available water chemistry data, 
leading to the observation that a given percentile of the distribution 
of estimated critical loads may be protective of a higher percentage of 
surface waters in some regions (Russell and Samet, 2011a).
    With respect to averaging time (discussed above in section III.C), 
CASAC stated the following:

    Considering the cumulative nature of the long-term adverse 
ecological effects and the year-to-year variability of atmospheric 
conditions (mainly in the amount of precipitation), CASAC concurs 
with EPA that an averaging time of three to five years for the AAI 
parameters is appropriate. A longer averaging time would mask 
possible trends of AAI, while a shorter averaging time would make 
the AAI being more influenced by the conditions of the particular 
years selected. (Russell and Samet, 2011a)

    With respect to level as well as the combination of level and form 
as they are presented as alternative standards (discussed above in 
sections III.D-E), CASAC stated the following:

    CASAC agrees with EPA staff's recommendation that the ``level'' 
of the alternative AAI standards should be within the range of 20 
and 75 [mu]eq/L. We also recognize that both the ``level'' and the 
form of any AAI standard are so closely linked in their 
effectiveness that these two elements should be considered together. 
(Russell and Samet, 2011a)
    When considered in isolation, it is difficult to evaluate the 
logic or implications of selecting from percentiles (70th to 90th) 
of the distribution of estimated critical loads for lakes in 
sensitive ecoregions to determine an acceptable amount of deposition 
for a given ecoregion. However, when these percentile ranges are 
combined with alternative levels within the staff-recommended ANC 
range of 20 to 75 microequivalents per liter ([mu]eq/L), the results 
using the AAI point to the ecoregions across the country that would 
be expected to require additional protection from acidifying 
deposition. Reasonable choices were made in developing the form. The 
number of acid sensitive regions not likely to meet the standard 
will be affected both by choice of ANC level and the percentile of 
the distribution of critical loads for lakes to meet alternative ANC 
levels in each region. These combined recommendations provide the 
Administrator with a broad but reasonable range of minimally to 
substantially protective options for the standard. (Russell and 
Samet, 2011a)

    CASAC also commented on EPA's uncertainty analysis, and provided 
advice on areas requiring further clarification in the proposed rule 
and future research. The CASAC found it ``difficult to judge the 
adequacy of the uncertainty analysis performed by EPA because of lack 
of details on data inputs and the methodology used, and lack of clarity 
in presentation'' (Russell and Samet, 2011a). In particular, CASAC 
identified the need for more thorough model evaluations of critical 
load and atmospheric modeling, recognizing the important role of models 
as they are incorporated in the form of the standard. In light of the 
innovative nature of the standard developed in the PA, CASAC identified 
``a number of areas that should be the focus of further research'' 
(Russell and Samet, 2011a). While CASAC recognized that EPA staff was 
able to address some of the issues in the PA, they also noted areas 
``that would benefit from further study or consideration in potential 
revisions or modifications to the form of the standard.'' Such research 
areas include ``sulfur retention and mobilization in the soils, 
aluminum availability, soil versus water acidification and ecosystem 
recovery times.'' Further, CASAC encouraged future efforts to monitor 
individual ambient nitrogen species, which would help inform further 
CMAQ evaluations and the specification of model-derived elements in the 
AAI equation (Russell and Samet, 2011a).

H. Administrator's Proposed Conclusions

    Having concluded that the existing NO2 and 
SO2 secondary standards are neither sufficiently protective 
nor appropriate to address deposition-related effects associated with 
oxides of nitrogen and sulfur (section II.D above), the Administrator 
has considered whether it is appropriate at this time to set a new 
multi-pollutant standard for that purpose, with a structure that would 
better reflect the available science regarding acid deposition. In 
considering this, she recognizes that such an appropriate standard, for 
purposes of section 109(b) and (d) of the CAA,\15\ must in her judgment 
be requisite to protect public welfare, such that it would be neither 
more nor less stringent that necessary for that purpose. In particular, 
she has focused on the new standard developed in the PA and reviewed by 
CASAC, as discussed above. In so doing, the Administrator first 
considered the extent to which there is a scientific basis for 
development of such a standard, specifically with regard to a standard 
that would provide protection from deposition-related aquatic 
acidification in sensitive aquatic ecosystems in areas across the 
country. As discussed above, the Administrator notes that the ISA 
concludes that the available scientific evidence is sufficient to infer 
a causal relationship between acidifying deposition of nitrogen and 
sulfur in aquatic ecosystems, and that the deposition of oxides of 
nitrogen and sulfur both cause such acidification under current 
conditions in the U.S. Further, the ISA concludes that there are well-
established water quality and biological indicators of aquatic 
acidification as well as well-established models that address 
deposition, water quality, and effects on ecosystem biota, and that 
ecosystem sensitivity to acidification varies across the country 
according to present and historic nitrogen and sulfur deposition as 
well as geologic, soil, vegetative, and hydrologic factors. Based on 
these considerations, the Administrator agrees with the conclusion in 
the PA, and supported by CASAC, that there is a strong scientific basis 
for development

[[Page 46134]]

of a standard with the general structure presented in the PA.
---------------------------------------------------------------------------

    \15\ Section 109(d)(1) requires that ``* * * the Administrator 
shall complete a thorough review * * * and shall make such revisions 
in such criteria and standards and promulgate such new standards as 
may be appropriate under * * * subsection 109(b) of this section.'' 
[emphasis added]
---------------------------------------------------------------------------

    The Administrator also recognizes that the conceptual framework for 
an ecologically relevant, multi-pollutant standard, which was initially 
explored in the REA and further developed in the PA, builds on the 
information in the ISA. She notes that the structure of the standard 
addresses the combined effects of deposition from oxides of nitrogen 
and sulfur by characterizing the linkages between ambient 
concentrations, deposition, and aquatic acidification, and that the 
structure of the standard takes into account relevant variations in 
these linkages across the country. She recognizes that while the 
standard is innovative and unique, the structure of the standard is 
well grounded in the science underlying the relationships between 
ambient concentrations of oxides of nitrogen and sulfur and the aquatic 
acidification related to deposition of nitrogen and sulfur associated 
with such ambient concentrations.
    While the Administrator recognizes the strong scientific foundation 
for the structure of an AAI-based standard, she also recognizes that 
the standard depends on atmospheric and ecological modeling, based on 
appropriate data, to specify the terms of an equation that incorporates 
the linkages between ambient concentrations, deposition, and aquatic 
acidification. This equation, which defines an aquatic acidification 
index (AAI), has the effect of translating spatially variable 
ecological effects into a potential national standard. With respect to 
establishing the specific terms of this equation, there are a number of 
inherent uncertainties and complexities that are relevant to the 
question of whether it is appropriate under section 109 to set a 
specific AAI-based standard at this time, recognizing that such a 
standard must be requisite to protect public welfare without being 
either more or less stringent than necessary for this purpose. As 
discussed above, these uncertainties and complexities generally relate 
not to the structure of the standard, but to the quantification of the 
various elements of the standard, such as the F factors discussed 
earlier in this section and their representativeness at an ecoregion 
scale. These uncertainties and complexities currently limit efforts to 
characterize the degree of protectiveness that would be afforded by 
such a standard, within the ranges of levels and forms identified in 
the PA, and the representativeness of F factors in the AAI equation 
described above and in the PA. These important uncertainties have been 
generally categorized as limitations in available field data as well as 
uncertainties that are related to reliance on the application of 
ecological and atmospheric modeling at the ecoregion scale to specify 
the various elements of the AAI.
    With regard to data limitations, the Administrator observes that 
there are several important limitations in the available data upon 
which elements of the AAI are based. For example, while ambient 
measurements of NOy are made as part of a national 
monitoring network, the monitors are not located in locations that are 
representative of sensitive aquatic ecosystems. While air and water 
quality data are generally available in areas in the eastern U.S., 
there is relatively sparse coverage in mountainous western areas where 
a number of sensitive aquatic ecosystems are located. Further, even in 
areas where relevant data are available, small sample sizes impede 
efforts to characterize the representativeness of the available data, 
which was noted by CASAC as being of particular concern. Also, 
measurements of reduced forms of nitrogen are available from only a 
small number of monitoring sites, and emission inventories for reduced 
forms of nitrogen used in atmospheric modeling are subject to 
considerable uncertainty.
    With regard to uncertainties related to the use of ecological and 
atmospheric modeling, the Administrator notes in particular that model 
results are difficult to evaluate due to a lack of relevant 
observational data. For example, relatively large uncertainties are 
introduced by a lack of data with regard to pre-industrial 
environmental conditions and other parameters that are necessary inputs 
to critical load models that are the basis for factor F1 in the AAI 
equation. Also, observational data are not generally available to 
evaluate the modeled relationships between nitrogen and sulfur in the 
ambient air and associated deposition, which are the basis for the 
other factors (i.e., F2, F3, and F4) in the AAI equation.
    In combination, these limitations and uncertainties result in a 
considerable degree of uncertainty as to how well the quantified 
elements of the AAI standard would predict the actual relationship 
between varying ambient concentrations of oxides of nitrogen and sulfur 
and steady state ANC levels across the distribution of water bodies 
within the various ecoregions in the U.S. Because of this, there is 
considerable uncertainty as to the actual degree of protectiveness that 
such a standard would provide, especially for acid-sensitive 
ecoregions. The Administrator recognizes that the AAI equation, with 
factors quantified in the ranges discussed above and described more 
fully in the PA, generally performs well in identifying areas of the 
country that are sensitive to such acidifying deposition and indicates, 
as expected, that lower ambient levels of oxides of nitrogen and sulfur 
would lead to higher calculated AAI values. However, the uncertainties 
discussed here are critical for determining the actual degree of 
protection that would be afforded such areas by any specific target ANC 
level and percentile of water bodies that would be chosen in setting a 
new AAI-based standard, and thus for determining an appropriate AAI-
based standard that meets the requirements of section 109.
    In considering these uncertainties, the Administrator notes that 
CASAC acknowledged that important uncertainties remain that would 
benefit from further study and data collection efforts, which might 
lead to potential revisions or modifications to the form of the 
standard developed in the PA. She also notes that CASAC encouraged the 
Agency to engage in future monitoring and model evaluation efforts to 
help inform the specification of model-derived elements in the AAI 
equation.
    Based on the above considerations, the Administrator has determined 
that it is not appropriate under section 109 to set a new multi-
pollutant standard to address deposition-related effects of oxides of 
nitrogen and sulfur on aquatic acidification at this time. Setting a 
NAAQS generally involves consideration of the degree of uncertainties 
in the science and other information, such as gaps in the relevant data 
and, in this case, limitations in the evaluation of the application of 
relevant ecological and atmospheric models at an ecoregion scale. As 
noted above, the issue here is not a question of uncertainties about 
the scientific soundness of the structure of the AAI, but instead 
uncertainties in the quantification and representativeness of the 
elements of the AAI as they vary in ecoregions across the country. At 
present, these uncertainties prevent an understanding of the degree of 
protectiveness that would be afforded to various ecoregions across the 
country by a new standard defined in terms of a specific nationwide 
target ANC level and a specific percentile of water bodies for acid-
sensitive ecoregions and thus prevent identification of an appropriate 
standard.. The Administrator has considered whether these uncertainties 
could be appropriately accounted for by choosing either a more or less 
protective target ANC level and percentile of water bodies than would 
otherwise be chosen if the uncertainties did not substantially limit 
the confidence that can

[[Page 46135]]

appropriately be ascribed to the quantification of the AAI elements. 
However, in the Administrator's judgment, the uncertainties are of such 
nature and magnitude that there is no reasoned way to choose such a 
specific nationwide target ANC level or percentile of water bodies that 
would appropriately account for the uncertainties, since neither the 
direction nor the magnitude of change from the target level and 
percentile that would otherwise be chosen can reasonably be ascertained 
at this time.
    Based on the above considerations, the Administrator judges that 
the current limitations in relevant data and the uncertainties 
associated with specifying the elements of the AAI based on modeled 
factors are of such nature and degree as to prevent her from reaching a 
reasoned decision such that she is adequately confident as to what 
level and form (in terms of a selected percentile) of such a standard 
would provide any particular intended degree of protection of public 
welfare that the Administrator determined satisfied the requirements to 
set an appropriate standard under section 109. While acknowledging that 
CASAC supported moving forward to establish the standard developed in 
the PA, the Administrator also observes that CASAC supported conducting 
further field studies that would better inform the continued 
development or modification of such a standard. Given the large 
uncertainties and complexities inherent in quantifying the elements of 
such a standard, largely deriving from the unprecedented nature of the 
standard under consideration in this review, and having fully 
considered CASAC's advice, the Administrator provisionally concludes 
that it is premature to set a new, multi-pollutant secondary standard 
for oxides of nitrogen and sulfur at this time, and as such she is 
proposing not to set such a new secondary standard.
    While it is premature to set such a multi-pollutant standard at 
this time, the Administrator determines that the Agency should 
undertake a field pilot program to gather additional data, and that it 
is appropriate that such a program be undertaken before, rather than 
after, reaching a decision to set such a standard. As described below 
in section IV, the purpose of the program is to collect and analyze 
data so as to enhance our understanding of the degree of protectiveness 
that would likely be afforded by a standard based on the AAI as 
developed in the PA. This will provide additional information to aid 
the Agency in considering an appropriate multi-pollutant standard, 
specifically with respect to the acidifying effects of deposition of 
oxides of nitrogen and sulfur. PA. Data generated by this field program 
will also support development of an appropriate monitoring network that 
would work in concert with such a standard to result in the intended 
degree of protection. The data and analyses generated as a result of 
this program will serve to inform the next review of the NAAQS for 
oxides of nitrogen and sulfur. The information generated during the 
field program can also be used to help state agencies and EPA better 
understand how an AAI-based standard would work in terms of the 
implementation of such a standard.
    Based on the above considerations, the Administrator is proposing 
not to set a new multi-pollutant AAI-based secondary standard for 
oxides of nitrogen and sulfur in this review. In reaching this 
decision, the Administrator recognizes that the new NO2 and 
SO2 primary 1-hour standards set in 2010, while not 
ecologically relevant for a secondary standard, will nonetheless result 
in reductions in oxides of nitrogen and sulfur that will directionally 
benefit the environment by reducing NOy and SOx deposition to sensitive 
ecosystems. EPA is proposing to revise the secondary standards by 
adding secondary standards identical to the NO2 and 
SO2 primary 1-hour standards set in 2010. More specifically, 
EPA is proposing a 1-hour secondary NO2 standard set at a 
level of 100 ppb and a 1-hour secondary SO2 standard set at 
a level of 75 ppb. While this will not add secondary standards of an 
ecologically relevant form to address deposition-related effects, it 
will directionally provide some degree of additional protection. This 
is consistent with the view that the current secondary standards are 
neither sufficiently protective nor appropriate in form, but that it is 
not appropriate to propose to set a new, ecologically relevant multi-
pollutant secondary standard at this time, for all of the reasons 
discussed above.
    While not a basis for this decision, the Administrator also 
recognizes that a new, innovative AAI-based standard would raise 
significant implementation issues that would need to be addressed 
consistent with the CAA requirements for implementation-related actions 
following the setting of a new NAAQS. It will take time to address 
these issues, during which the Agency will be conducting a field pilot 
program to gather relevant data and the environment will benefit from 
reductions in oxides of nitrogen and sulfur resulting from the new 
NO2 and SO2 primary standards, as noted above, as 
well as reductions expected to be achieved from EPA's Cross-State Air 
Pollution Rule and Mercury and Air Toxics standards. These 
implementation-related issues are discussed in more detail below in 
section IV.A.5.
    The Administrator solicits comment on all aspects of this proposed 
decision, including the framework and elements of a multi-pollutant 
standard for oxides of nitrogen and sulfur to address deposition-
related effects on sensitive ecosystems, with a focus on aquatic 
acidification, and the uncertainties and complexities associated with 
the development of such a standard at this time. The Administrator also 
solicits comment on the field pilot program and related monitoring 
methods as discussed below in section IV.

IV. Field Pilot Program and Ambient Monitoring

    This section describes EPA's plans for a field pilot program and 
the evaluation of monitoring methods for ambient air indicators of 
NOy and SOx to implement the Administrator's 
decision to undertake such a field monitoring program in conjunction 
with her decision to propose not to set a new multi-pollutant secondary 
standard in this review, as discussed above in section III.H. As noted 
above and discussed below in section IV.A, the field pilot program is 
intended to collect and analyze data so as to enhance our understanding 
of the degree of protectiveness that would likely be afforded by a 
standard based on the AAI as developed in the PA. Data generated by 
this field program would also support development of an appropriate 
monitoring network that would work in concert with such a standard to 
result in the intended degree of protection. As discussed below in 
section IV.B, the evaluation of monitoring methods focuses on the 
development of Federal Reference Methods/Federal Equivalent Methods 
(FRM/FEM) for NOy and SOx. The EPA notes that the 
monitoring program described here is intended to be coordinated with 
EPA's CASTNET as a supplement to existing monitoring programs and is 
beyond the scope of the current CASTNET program.

A. Field Pilot Program

    This section presents the objectives of a field pilot program 
(section IV.A.1) that would gather relevant field data over a 5-year 
period in a sample of three to five sensitive ecoregions across the 
country. An overview of the scope and structure of the field program, 
with a focus on measurements of ambient air indicators of oxides of 
nitrogen and

[[Page 46136]]

sulfur, is presented in section IV.A.2. Section IV.A.3 explains the 
role of additional complementary measurements beyond the ambient air 
indicators that would be included in the program, and section IV.A.4 
discusses a parallel longer-term research agenda, both of which are 
guided by the uncertainties discussed above in section III. Section 
IV.A.5 identifies implementation challenges presented by an AAI-based 
standard that could be addressed in parallel with a field pilot 
program. Section IV.A.6 discusses engagement with stakeholder groups as 
part of the planned pilot program.
1. Objectives
    Consideration of a new multi-pollutant standard to address 
deposition-related effects on sensitive aquatic ecoregions raises 
unique challenges relative to those typically raised in reviews of 
existing NAAQS for which an established network of FRM/FEM monitors, 
designed to measure the indicator pollutant, is generally available. 
The primary goal of this field pilot program, and the related 
monitoring program discussed in section IV.B, is to enhance our 
understanding of the degree of protectiveness that would likely be 
afforded by a standard based on the AAI, as described above in section 
III, so as to aid the Agency in considering an appropriate multi-
pollutant standard that would be requisite to protect public welfare 
consistent with section 109 of the CAA, through the following 
objectives:
    (1) Evaluate measurement methods for the ambient air indicators of 
NOy and SOx and consider designation of such 
methods as FRMs;
    (2) Examine the variability and improve characterization of 
concentration and deposition patterns of NOy and 
SOx, as well as reduced forms of nitrogen, within and across 
a number of sensitive ecoregions across the country;
    (3) Develop updated ecoregion-specific factors (i.e., F1 through 
F4) for the AAI equation based in part on new observed air quality data 
within the sample ecoregions as well as on updated nationwide air 
quality model results and expanded critical load data bases, and 
explore alternative approaches for developing such representative 
factors;
    (4) Calculate ecoregion-specific AAI values using observed 
NOy and SOx data and updated ecoregion-specific 
factors to examine the extent to which the sample ecoregions would meet 
a set of alternative AAI-based standards;
    (5) Develop air monitoring network design criteria for an AAI-based 
standard;
    (6) assess the use of total nitrate measurements as a potential 
alternative indicator for NOy;
    (7) Support related longer-term research efforts, including 
enhancements to and evaluation of modeled dry deposition algorithms; 
and
    (8) Facilitate stakeholder engagement in addressing implementation 
issues associated with possible future adoption of an AAI-based 
standard.
2. Overview of Field Pilot Program
    The CASTNET program (Figure IV-1) affords an available 
infrastructure relevant to an AAI-based standard, given the location of 
sites in some acid-sensitive ecoregions and various measurements of 
sulfur and nitrogen species. The EPA plans to use CASTNET sites in 
selected acid-sensitive ecoregions to serve as the platform for this 
pilot program, potentially starting in late 2012 and extending through 
2018. The CASTNET sites in three to five ecoregions in acid-sensitive 
areas would collect NOy and SOx (i.e., 
SO2 and p-SO4) measurements over a 5-year period. 
The initial step in developing a data base of observed ambient air 
indicators for oxides of nitrogen and sulfur requires the addition of 
NOy samplers at the pilot study sites so that a full 
complement of indicator measurements are available to calculate AAI 
values. These CASTNET sites would also be used to make supplemental 
observations useful for evaluation of CMAQ's characterization of 
factors F2 -F4 in the AAI equation.
    The selected ecoregions would account for geographic variability by 
including regions from across the U.S., including the east, upper 
midwest and west. Each selected region would have at least two existing 
CASTNET sites. Each of the pilot CASTNET sites would be used to 
evaluate the performance of the established methods, data retrieval and 
reporting procedures used in the AAI equation.

[[Page 46137]]

[GRAPHIC] [TIFF OMITTED] TP01AU11.030

    Over the course of this 5-year pilot program, the most current 
national air quality modeling, based on the most current national 
emissions inventory, would be used to develop an updated set of F2--F4 
factors. A parallel multi-agency national critical load data base 
development effort would be used as the basis for calculating updated 
F1 factors. As discussed above in section III.B, these factors would be 
based on average parameter values across an ecoregion. Using this new 
set of F factors, observations of NOy and SOx 
derived from the pilot program, averaged across each ecoregion, would 
be used to calculate AAI values in the sample ecoregions. The data from 
the pilot program would also be used to examine alternative approaches 
to generating representative air quality values, such as examining the 
appropriateness of spatial averaging in areas of high spatial 
variability.
3. Complementary Measurements
    Complementary measurements may be performed at some sites in the 
pilot network to reduce uncertainties in the recommended methods and 
better characterize model performance and application to the AAI. The 
CASAC Air Monitoring and Methods Subcommittee (AMMS) advised EPA that 
such supplemental measurements were of critical importance in a field 
measurement program related to an AAI-based standard (Russell and 
Samet, 2011b).
    Candidate complementary measurements to address sulfur, in addition 
to those provided by the CASTNET filter pack (CFP), include trace gas 
continuous SO2 and speciated PM2.5 measurements. 
The co-located deployment of a continuous SO2 analyzer with 
the CFP for SO2 will provide test data for determining 
suitability of continuous SO2 measurements as a Federal 
Equivalent Method (FEM), as well as producing valuable time series data 
for model evaluation purposes. The weekly averaging time provided by 
the CFP adequately addresses the annual-average basis of an AAI-based 
secondary standard, but would not be applicable to short-term (i.e., 1-
hour) averages associated with the primary SO2 standard. 
Conversely, because of the low concentrations associated with many 
acid-sensitive ecoregions, existing SO2 Federal Reference 
Methods (FRMs) designated for use in determining compliance with the 
primary standard would not necessarily be appropriate for use in 
conjunction with an AAI-based secondary standard.
    Co-locating the PM2.5 sampler used in the EPA Chemical 
Speciation Network and the Interagency Monitoring of Protected Visual 
Environments (IMPROVE) network at pilot network sites would allow for 
characterizing the relationship between the CFP-derived p-
SO4 and the speciation samplers used throughout the state 
and local air quality networks. Note that CASTNET already has several 
co-located IMPROVE chemical speciation samplers. Because the AAI 
equation is based on concentration of p-SO4, the original 
motivation for capturing all particle size fractions is not as 
important relative to simply capturing the concentration of total p-
SO4.
    Candidate measurements to complement oxidized nitrogen 
measurements, in addition to the CFP, include a mix of continuous and 
periodic sampling for the dominant NOy species, namely NO, 
true NO2, PAN,

[[Page 46138]]

HNO3, and p-NO3. While there are several 
approaches to acquiring these measurements, perhaps the most efficient 
strategy would take advantage of the available CFP for total nitrate, 
and add a three-channel chemiluminescence instrument that will cycle 
between NOy, true NO2 and NO by adding photolytic 
detection for true NO2. Other options for measuring true 
NO2 would include adding either a stand-alone photolytic or 
cavity ring-down spectroscopy instrument. Measurements of PAN may be 
acquired either on a periodic basis through canister sampling and 
subsequent laboratory analysis or through emerging in-situ sampling and 
analysis methods. Although the CFP yields a reliable measurement of 
total nitrate, the t-NO3 (i.e., the sum of HNO3 
and p-NH4) value, strong consideration may be given to 
direct measurement of HNO3, which has the highest deposition 
velocity of all the dominant NOy species. Similar to the use 
of continuous SO2 data, these speciated NOy data 
serve two purposes: evaluating total NOy instrument behavior 
and evaluating air quality models. The measurement of individual 
NOy species can be used to generate site-specific 
NOy values for comparison to modeled NOy, and 
will likely provide insight into and improvement of modeled dry 
deposition.
    The CASAC AMMS (Russell and Samet, 2011b) recommended that EPA 
consider the use of t-NO3 obtained from CASTNET sampling as 
an indicator for NOy, reasoning that t-NO3 is 
typically a significant fraction of deposited oxidized nitrogen in 
rural environments and CASTNET measurements are widely available. 
Collection of this data would support further consideration of using 
the CFP for t-NO3 as the indicator of oxides of nitrogen for 
use in an AAI-based secondary standard.
    The CASAC AMMS also recommended that total NHx 
(NH3 and p-NH4) be considered as a proxy for 
reduced nitrogen species, reasoning that the subsequent partitioning to 
NH3 and p-NH4 may be estimated using equilibrium 
chemistry calculations. Reduced nitrogen measurements are used to 
evaluate air quality modeling which is used in generating factor F2. 
Additional studies are needed to determine the applicability of 
NHx measurements and calculated values of NH3 and 
NH4 to the AAI.
    The additional supplemental measurements of speciated 
NOy, continuous SO2 and NHx will be 
used in future air quality modeling evaluation efforts. Because there 
often is significant lag in the availability of contemporary emissions 
data to drive air quality modeling, the complete use of these data sets 
will extend beyond the 5-year collection period of the pilot program. 
Consequently, the immediate application of those data will address 
instrument performance comparisons that explore the feasibility of 
using continuous SO2 instruments in rural environments, and 
using the speciated NOy data to assess NOy 
instrument performance. Although contemporary air quality modeling will 
lag behind measurement data availability, the observations can be used 
in deposition models to compare observed transference ratios with the 
previously calculated transference ratios to test temporal stability of 
the ratios.
    An extended water quality sampling effort should parallel the air 
quality measurement program to address some of the uncertainties 
related to factor F1 and the representativeness of the nth percentile 
critical load as discussed in section III.B.5.b.i. The objective of the 
water quality sampling would be to develop a larger data base of 
critical loads in each of the pilot ecoregions such that the nth 
percentile can adequately be characterized in terms of representing all 
water bodies. Opportunities to leverage and perhaps enhance existing 
ecosystem modeling efforts enabling more advanced critical load 
modeling and improved methods to estimate base cation production would 
be pursued. For example, areas with ongoing research studies producing 
data for dynamic critical load modeling would be considered when 
selecting the pilot ecoregions.
4. Complementary areas of research
    The EPA recognizes that a source of uncertainty in an AAI-based 
secondary standard that would not be directly addressed in the pilot 
program stems from the uncertainty in the model used to link 
atmospheric concentrations to dry deposition fluxes. Currently, there 
are no ongoing direct dry deposition measurement studies at CASTNET 
sites that can be used to evaluate modeled results. It was strongly 
recommended by CASAC AMMS that a comprehensive sampling-intensive study 
be conducted in at least one, preferably two sites in different 
ecoregions to assess characterization of dry deposition of sulfur and 
nitrogen. These sites would be the same as those for the complementary 
measurements described above, but they would afford an opportunity to 
also complement dry deposition process research that benefits from the 
ambient air measurements collected in the pilot program. The concerns 
regarding uncertainties underlying an AAI-based secondary standard 
suggest that research that includes dry deposition measurements and 
evaluation of dry deposition models should be a high priority.
    Similar leveraging should be pursued with respect to ecosystem 
research activities. For example, studies that capture a suite of soil, 
vegetation, hydrological, and water quality properties that can help 
evaluate more advanced critical load models would complement the 
atmospheric-based pilot program. In concept, such studies could provide 
the infrastructure for true multi-pollutant, multi-media ``super'' 
sites assuming the planning, coordination, and resource facets can be 
aligned. While this discussion emphasizes the opportunity of leveraging 
ongoing research efforts, consideration could be given to explicitly 
including related research components directly in the pilot program.
5. Implementation challenges
    The CAA requires that once a NAAQS is established, designation and 
implementation must move forward. With a standard as innovative as the 
AAI-based standard considered in this review, the Administrator 
believes that its success will be greatly improved if, while additional 
data are being collected to reduce the uncertainties discussed above, 
the implementing agencies and other stakeholders have an opportunity to 
discuss and thoroughly understand how such a standard would work. And 
since, as noted above, emissions reductions that are directionally 
correct to reduce aquatic acidification will be occurring as a result 
of other CAA programs, the Administrator believes that this period of 
further discussion will not delay progress but will ensure that once 
implementation is triggered, agencies will be prepared to implement it 
successfully.
    Consideration of an AAI-based secondary standard for oxides of 
nitrogen and sulfur would present significant implementation challenges 
because it involves multiple, regionally-dispersed pollutants and 
relatively complex compliance determinations based on regionally 
variable levels of NOy and SOx concentrations 
that would be necessary to achieve a national ANC target. The 
anticipated implementation challenges fall into three main categories: 
monitoring and compliance determinations for area designations, pre-
construction permit application analyses of individual source impacts, 
and State Implementation Plan (SIP) development. Several overarching 
implementation questions that we

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anticipate will be addressed in parallel with the field pilot program's 
five-year data collection period include:
    (1) What are the appropriate monitoring network density and siting 
requirements to support a compliance system based on ecoregions?
    (2) Given the unique spatial nature of the secondary standard 
(e.g., ecoregions), what are the appropriate parameters for 
establishing nonattainment areas?
    (3) How can new or modified major sources of oxides of nitrogen and 
oxides of sulfur emissions assess their ambient impacts on the standard 
and demonstrate that they are not causing or contributing to a 
violation of the NAAQS for preconstruction permitting? To what extent 
does the fact that a single source may be impacting multiple areas, 
with different acid sensitivities and variable levels of NOy 
and SOx concentrations that would be necessary to achieve a 
national ANC target, complicate this assessment and how can these 
additional complexities best be addressed?
    (4) What additional tools, information, and planning structures are 
needed to assist states with SIP development, including the assessment 
of interstate pollutant transport and deposition?
    (5) Would transportation conformity apply in nonattainment and 
maintenance areas for this secondary standard, and, if it does, would 
satisfying requirements that apply for related primary standards (e.g., 
ozone, PM2.5, and NO2) be demonstrated to satisfy 
requirements for this secondary standard?
6. Final Monitoring Plan Development and Stakeholder Participation
    The existing CASTNET sampling site infrastructure provides an 
effective means of quickly and efficiently deploying a monitoring 
program to support potential implementation of an AAI-based secondary 
standard, and also provides an additional opportunity for federally 
managed networks to collaborate and support the states, local agencies 
and tribes (SLT) in determining compliance with a secondary standard. A 
collaborative effort would help to optimize limited federal and SLT 
monitoring funds and would be beneficial to all involved. The CASTNET 
is already a stakeholder-based program with over 20 participants and 
contributors, including federal, state and tribal partners.
    The CASAC AMMS generally endorsed the technical approaches used in 
CASTNET, but concerns were raised by individual representatives of 
state agencies concerning the perception of EPA-controlled management 
aspects of CASTNET and data ownership. Potential approaches to resolve 
these issues will be developed and evaluated in existing National 
Association of Clean Air Agencies (NACAA)/EPA ambient air monitoring 
workgroups. The EPA Office of Air and Radiation (which includes the 
Office of Air Quality Planning Standards, OAQPS; and the Office of 
Atmospheric Program's Clean Air Markets Division, OAP-CAMD), and their 
partners on the NACAA monitor steering committee will develop a 
prioritized specific plan that identifies the three to five ecoregions 
and the instrumentation to be deployed. The EPA anticipates that a cost 
estimate of the plan with priorities and options will be developed by 
January, 2012. Although this pilot program is focused on data 
collection, the plan will include details of the data analysis 
approaches as well as a vehicle that incorporates engagement from those 
within EPA and SLTs to foster progress on the implementation questions 
noted above in section IV.A.5.
    If an AAI-based secondary standard were to be set in the future, 
deployment of a full national network would follow the pilot monitoring 
program. The number of sites deployed in the network will lead to 
increased confidence in capturing spatial patterns of air quality. 
Recognizing that this section presents the general elements of the 
field pilot programs, EPA intends to develop a more detailed field 
pilot program plan through a process that will engage the air quality 
management and research (atmospheric and ecosystem) communities, as 
well as other federal agencies, state and local agencies, and non-
government based centers of expertise. The EPA is seeking comment and 
input on all aspects of this field pilot program.

B. Evaluation of Monitoring Methods

    The EPA generally relies on monitoring methods that have been 
designated as FRMs or FEMs for the purpose of determining the 
attainment status of areas with regard to existing NAAQS. Such FRMs or 
FEMs are generally required to measure the air quality indicators that 
are compared to the level of a standard to assess compliance with a 
NAAQS. Prior to their designation by EPA as FRM/FEMs through a 
rulemaking process, these methods must be determined to be applicable 
for routine field use and need to have been experimentally validated by 
meeting or exceeding specific accuracy, reproducibility, and 
reliability criteria established by EPA for this purpose. As discussed 
above in section III.A, the ambient air indicators being considered for 
use in an AAI-based standard include SO2, particulate 
sulfate (p-SO4), and total reactive oxides of nitrogen 
(NOy).
    The CASTNET provides a well established infrastructure that would 
meet the basic location and measurement requirements of an AAI-based 
secondary standard given the rural placement of sites in acid sensitive 
areas. In addition, CFPs currently provide very economical weekly, 
integrated average concentration measurements of SO2, p-
SO4, ammonium ion (NH4) and t-NO3, the 
sum of HNO3 and p-NO3.
    While routinely operated instruments that measure SO2, 
p-SO4, NOy and/or t-NO3 exist, 
instruments that measure p-SO4, NOy, t-
NO3, or the CFP for SO2 have not been designated 
by EPA as FRMs or FEMs. The EPA's Office of Research and Development 
has initiated work that will support future FRM designations by EPA for 
SO2 and p-SO4 measurements based on the CFP. Such 
a designation by EPA could be done for the purpose of facilitating 
consistent research related to an AAI-based standard and/or in 
conjunction with setting and supporting an AAI-based secondary 
standard.
    Based on extensive review of literature and available data, the EPA 
has identified potential methods that appear suitable for measuring 
each of the three components of the indicators. These three methods are 
being considered as new FRMs to be used for measuring the ambient 
concentrations of the three components that would be needed to 
determine compliance with an AAI-based secondary standard.
    For the SO2 and p-SO4 measurements, EPA is 
considering the CFP method, which provides weekly average concentration 
measurements for SO2 and p-SO4. This method has 
been used in the EPA's CASTNET monitoring network for 15 years, and 
strongly indicates that it will meet the requirements for use as an FRM 
for the SO2 and p-SO4 concentrations for an AAI-
based secondary standard.
    Although the CFP method would provide measurements of both the 
SO2 and p-SO4 components in a unified sampling 
and analysis procedure, individual FRMs will be considered for each. 
The EPA recognizes that an existing FRM to measure SO2 
concentrations using ultra-violet fluorescence (UVF) exists (40 CFR 
Part 50, Appendix A-1) for the purpose of monitoring compliance for the 
primary SO2 NAAQS. However, several factors suggest that the 
CFP method would be

[[Page 46140]]

superior to that UVF FRM for monitoring compliance with an AAI-based 
secondary standard and will be discussed in more detail below.
    For monitoring the NOy component, a continuous analyzer 
for measuring NOy is commercially available and is 
considered to be suitable for use as an FRM. This method is similar in 
design to the existing NO2 FRM (described in 40 CFR Part 50, 
Appendix F), which is based on the ozone chemiluminescence measurement 
technique. The method is adapted to and further optimized to measure 
all NOy. However, this NOy method requires 
further evaluation before it can be fully confirmed as a suitable FRM. 
The EPA is currently completing a full scientific assessment of the 
NOy method to determine whether it would be appropriate to 
consider for designation by EPA as an FRM. Specific details on these 
three methods are given below.
    On February 16, 2011, EPA presented this set of potential FRMs to 
the CASAC AMMS for their consideration and comment. In response, the 
CASAC AMMS stated that, overall, it believes that EPA's planned 
evaluation of methods for measuring NOy, SO2 and 
p-SO4 as ambient air indicators is a suitable approach in 
concept. On supporting the CFP method as a potential FRM for 
SO2, CASAC stated that they felt that the CFP is adequate 
for measuring long-term average SO2 gas concentrations in 
rural areas with low levels (less than 5 parts per billion by volume 
(ppbv)) and is therefore suitable for consideration as an FRM. For p-
SO4, CASAC generally supports the use of the CFP as a 
potential FRM for measuring p-SO4 for an AAI-based secondary 
standard. The method has been relatively well-characterized and 
evaluated, and it has a documented, long-term track record of 
successful use in a field network designed to assess spatial patterns 
and long-term trends.
    On supporting the photometric NOy method as a potential 
FRM, CASAC concluded that the existing NOy method is 
generally an appropriate approach for the indicator. However, CASAC 
agrees that additional characterization and research is needed to fully 
understand the method in order to designate it as a FRM. The EPA is now 
soliciting public comment on these methods as to their adequacy, 
suitability, and relative merits as FRMs for purposes of monitoring to 
determine compliance with an AAI-based secondary standard.
1. Potential FRMs for SO2 and p-SO4
    The CFP is a combined, integrated sampling and analysis method 
based on the well-established measurement technology that has been used 
extensively in EPA's CASTNET monitoring network (see http://www.epa.gov/castnet). This method is in current use at over 80 
monitoring sites and has been in use at not less than 40 sites for over 
15 years. This method employs a relatively simple and inexpensive 
sampler and uses four 47-mm filters placed in an open-faced filter pack 
to simultaneously collect integrated filter samples for the 
SO2 and p-SO4 components. In addition, the CFP is 
also capable of the collection of t-NO3, the sum of 
HNO3 and p-NO3.
    The first stage of the filter pack assembly contains a Teflon[reg] 
filter that collects p-SO42- and p-
NO3, the second stage contains a nylon filter that collects 
SO2 (as SO42-) and HNO3, 
and the third stage contains two cellulose fiber filters impregnated 
with potassium carbonate (K2CO3) that collect any 
remaining SO2 (as SO42-). The sampler 
collects 1-week integrated samples at a very low, controlled flow rate 
(1.5 or 3 L/min) in an attempt simulate actual deposition. Weekly 
averaged SO2 and p-SO4 concentrations could then 
be averaged over a 1-year period to calculate annual average values.
    Upon sample completion, the species-specific filters are extracted, 
with subsequent analysis by the well-established and documented ion 
chromatographic (IC) analytical technique. During the IC analysis, an 
aliquot of a filter extract is injected into a stream of eluent (ion 
chromatography mobile phase, generally a millimolar-strength solution 
of carbonate-bicarbonate) and passed through a series of ion 
exchangers. The anions of interest are separated on the basis of their 
relative affinities for a low capacity and the strongly basic anion 
exchanger (guard and separator column). The separated anions are 
directed onto a cation exchanger (suppressor column) where they are 
converted to their highly conductive acid form, and the eluent is 
converted to a weakly conductive form. The now-separated anions, each 
in their acid form, are measured by conductivity. They are identified 
on the basis of retention time compared to that of standards and 
quantified by measurement of peak area compared to the peak areas of 
calibration standards.
    Calibration and quality assurance for the method are applied to the 
sample filters, the analytical processes, and the flow rate measurement 
and control aspects of the sampler. Overall method performance is 
typically assessed with collocated samplers. These quality assurance 
techniques are routinely used and have proved adequate for other types 
of FRMs and equivalent methods in air monitoring network service.
    The measurement and analytical procedures and past performance data 
associated with the CFP method are well documented and available 
through Quality Assurance Performance Plans (QAPPs), Standard Operating 
Procedures (SOPs) and annual reports (US EPA, 2010a and 2010b). The 
accumulated database on the CFP method is substantial and indicates 
that the method is sound, stable and has good reliability in routine, 
field operation. Data quality assessment results show the method to 
have good reproducibility, with collocated and analytical precision 
values in the range of 2 percent to 10 percent (excluding very low 
concentration measurements near the method detection limits; US EPA 
2010b).
    Data quality objectives (DQOs) for a new FRM would be based upon 
current DQOs being used for this method by EPA's OAP/CAMD and the NPS, 
the federal managers of CASTNET (US EPA, 2010a). In its current state, 
the CFP method is expected to meet or exceed (as past CASTNET data have 
indicated; US EPA, 2010b) the expected FRM DQOs, even when deployed in 
new monitoring networks outside of CASTNET. In addition, CASTNET 
samples have agreed favorably with other measures of SO2 and 
p-SO4 in comparison studies. For example, in direct 
comparison with an annular denuder sampler (ADS) method, CASTNET/ADS 
ratios for SO2 and p-SO4 were generally on the 
order of 0.9-1.1 (Lavery et al, 2009; Sickles et al, 1999; Sickles et 
al, 2008), thus illustrating the accuracy of the CFP method in the 
determination of long-term average SO2 and p-SO4 
concentrations. The EPA believes that the CFP method would be fully 
adequate as an FRM in determining yearly average SO2 and p-
SO4 concentrations for compliance determination purposes.
    The EPA recognizes that an existing FRM for SO2 has 
proven adequate for the purposes of monitoring compliance for the 
primary SO2 NAAQS, specifically the newly-promulgated 1-hour 
standard. However, this FRM is better suited to the shorter-term, 
higher concentration primary and secondary SO2 NAAQS, and 
there is substantial uncertainty as to the adequacy of this 
SO2 FRM for monitoring the lower concentrations relevant to 
determining compliance with an AAI-based secondary standard. The 
performance specifications for SO2 FRM analyzers (40 CFR 
Part 53, Table B-1) require a lower detectable limit (LDL) of 0.002

[[Page 46141]]

ppm for the standard measurement range and 0.001 ppm for the lower 
measurement range. These requirements correspond to mass per unit 
volume concentrations of 5.24 and 2.62 [micro]g/m\3\, respectively. 
Analysis of 2009 CASTNET data shows that of the 84 CASTNET sampling 
sites, 63 measured annual average SO2 concentrations below 
even the lower of these LDL requirements of 2.62 [micro]g/m\3\ for the 
lower range SO2 FRM (US EPA, 2010a). In addition, 11 of the 
84 sites measured annual (2009) average SO2 concentrations 
very near or below the manufacturers' reported detection limits for 
trace level UVF SO2 monitors. Further, it is likely that the 
number of sites with annual average SO2 concentration below 
both the SO2 FRM LDL and the manufacturers reported 
detection limits will increase due to expected declines in mean 
SO2 concentrations (US EPA, 2010b). For these reasons, EPA 
is considering the CFP method for use as the FRM for monitoring the 
SO2 component of an ambient air indicator for oxides of 
sulfur, with a recommendation for additional study and data collection 
to evaluate further the possible applicability of the continuous UVF 
SO2 FRM for this purpose.
2. Potential FRM for NOy
    Atmospheric concentrations of NOy are measured 
continuously by an analyzer that photometrically measures the light 
intensity, at wavelengths greater than 600 nanometers (nm), resulting 
from the chemiluminescent reaction of ozone (O3) with NO in 
sampled air. This method is very similar to the chemiluminescence NO/
NO2 analyzers widely used to collect NO2 
monitoring data for determining compliance with the NO2 
NAAQS. The various oxides of nitrogen species, excluding NO, are first 
quantitatively reduced to NO by means of a catalytic converter. These 
species include NO2, HNO2, PANs, HNO3 
and p-NO3. The NO, which commonly exists in ambient air, 
passes through the converter unchanged, and, when combined with the NO 
resulting from the catalytic conversion of the other oxides of 
nitrogen, a measurement of the total NOy concentration 
results. To maximize the conversion of the more chemically active 
oxides of nitrogen species, the converter is located externally, at or 
near the air sample inlet probe. This location minimizes losses of 
these active species that could otherwise occur from chemical reactions 
and wall losses in the sample inlet line.
    The NOy analyzer is a suitable, commercially produced 
continuous chemiluminescence analyzer that includes an ozone generator, 
a reaction cell, a photometric detector, wavelength filters as 
necessary to reduce sensitivity to wavelengths below 600 nanometer 
(nm), a pump and flow control system to draw atmospheric air through 
the converter and into the reaction cell, a suitable converter, a 
system to control the operation of the analyzer, and appropriate 
electronics to process and quantitatively scale the photometric 
signals. The converter contains a catalyst such as molybdenum and is 
heated to an optimum temperature designed to optimize the conversion of 
the various oxides of nitrogen to NO. It is connected to the analyzer 
via suitable lengths of Teflon[reg] tubing. Hourly NOy 
measurements obtained by the analyzer would be averaged over the same 
7-day period used by the CFP method to measure the SO2 and 
p-SO4 components, with further averaging over a 1-year 
period.
    Commercial NOy analyzers are currently available, and 
the analyzers have been used for a variety of monitoring applications. 
During the 2006 TexAQS Radical and Aerosol Measurement Project (TRAMP), 
Luke et al., 2010, compared measured NOy concentrations 
obtained with an NOy instrument based upon the above 
mentioned methodology with the sum of measured individual 
NOy species (i.e., NOyi = 
NO+NO2+HNO3+PANs+HNO2+p-
NO3). This comparison yielded excellent overall agreement 
during both day ([NOy](ppb) = [NOyi](ppb) x 1.03-0.42; r\2\ = 0.9933) 
and night time ([NOy](ppb) = [NOyi](ppb) x 1.01-0.18; r\2\ = 0.9975) 
periods (Luke et al, 2010). The results of this study show that this 
NOy method is capable of the accurate determination of all 
the atmospherically relevant NOy components, resulting in an 
accurate determination of total NOy concentrations. The 
NOy instruments have been routinely operated in networks 
such as SouthEastern Aerosol Research and Characterization (SEARCH), 
dating back several years. In addition, state monitoring agencies 
across the U.S. have begun, starting in 2009, the routine operation of 
commercially available NOy instrumentation in anticipation 
of EPA's NCore network transitioning to full operation in 2011.
    These initial assessments described above are promising and 
indicate that the photometric NOy method appears to be 
accurate, reliable, and capable of routine network operation. As a 
result, the method is most likely capable for use as an FRM for 
determining atmospheric NOy concentrations as a component in 
determining compliance with an AAI-based secondary standard. However, 
as described below, this continuous method for NOy requires 
additional time for further evaluation before it can be fully confirmed 
for adoption as a FRM. The EPA has identified measurement uncertainties 
and some remaining science questions associated with this method. Among 
these are: (a) The ability of the method to capture all components of 
NOy relevant to nitrogen deposition, (b) the efficiency of 
the molybdenum converter in converting all oxides of nitrogen to NO for 
detection (excluding NO2, as this conversion is already well 
documented), (c) appropriate inlet height specifications to minimize 
any bias associated with vertical concentration gradients of key 
NOy components, (d) identification and quantification of 
potential measurement interferences in the NOy 
determination, and (e) development and demonstration of effective 
calibration/challenge procedures to best represent the various mixtures 
of NOy components that are expected to be present in the 
different air sheds across the U.S.
    To address these NOy method uncertainties and to fully 
assess this method for use as the NOy FRM, EPA has developed 
a detailed research plan (Russell and Samet, 2011b) which was presented 
to the CASAC AMMS on February 16, 2011. In response, CASAC recognized 
the need for, and supported the general outline of EPA's research plan 
to evaluate the NOy method for potential designation as an 
FRM (US EPA, 2011). In addition, the CASAC AMMS suggested additional 
areas of research associated with the photometric NOy method 
that warrant further assessment prior to final designation of the 
method as the NOy FRM. These include operation of the method 
during extremely low temperature conditions to investigate possible 
condensation in sample lines, method detection limits relative to low 
levels expected in remote areas, and ambient-based method evaluations 
in various air sheds across the U.S. In response to these CASAC AMMS 
suggestions, EPA is carrying out studies, in addition to the tasks 
outlined in the research plan, for the NOy method. The 
results of these studies will likely take a year or more to become 
available. As noted previously, EPA anticipates that these results will 
be favorable and will confirm the adequacy of the NOy method 
as a suitable FRM for determining compliance with an AAI-based 
secondary standard.

[[Page 46142]]

V. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Review and 
Executive Order 13563: Improving Regulation and Regulatory Review

    Under Executive Order 12866 (58 FR 51735, October 4, 1993), this 
action is a ``significant regulatory action.'' Accordingly, EPA 
submitted this action to the Office of Management and Budget (OMB) for 
review under Executive Orders 12866 and 13563 (76 FR 3821, January 21, 
2011), and any changes made in response to OMB recommendations have 
been documented in the docket for this action.

B. Paperwork Reduction Act

    This action does not impose an information collection burden under 
the provisions of the Paperwork Reduction Act, 44 U.S.C. 3501 et seq. 
Burden is defined at 5 CFR 1320.3(b). There are no information 
collection requirements directly associated with the establishment of a 
NAAQS under section 109 of the CAA.

C. Regulatory Flexibility Act

    For purposes of assessing the impacts of today's rule on small 
entities, small entity is defined as: (1) A small business that is a 
small industrial entity as defined by the Small Business 
Administration's (SBA) regulations at 13 CFR 121.201; (2) a small 
governmental jurisdiction that is a government of a city, county, town, 
school district or special district with a population of less than 
50,000; and (3) a small organization that is any not-for-profit 
enterprise which is independently owned and operated and is not 
dominant in its field.
    After considering the economic impacts of today's proposed rule on 
small entities, I certify that this action will not have a significant 
economic impact on a substantial number of small entities. This 
proposed rule will not impose any requirements on small entities. 
Rather, this rule establishes national standards for allowable 
concentrations of oxides of nitrogen and sulfur in ambient air as 
required by section 109 of the CAA. See also American Trucking 
Associations v. EPA. 175 F. 3d at 1044-45 (NAAQS do not have 
significant impacts upon small entities because NAAQS themselves impose 
no regulations upon small entities). We continue to be interested in 
the potential impacts of the proposed rule on small entities and 
welcome comments on issues related to such impacts.

D. Unfunded Mandates Reform Act

    Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public 
Law 104-4, establishes requirements for Federal agencies to assess the 
effects of their regulatory actions on State, local, and Tribal 
governments and the private sector. Under section 202 of the UMRA, EPA 
generally must prepare a written statement, including a cost-benefit 
analysis, for proposed and final rules with ``Federal mandates'' that 
may result in expenditures to state, local, and tribal governments, in 
the aggregate, or to the private sector, of $100 million or more in any 
1 year. Before promulgating an EPA rule for which a written statement 
is needed, section 205 of the UMRA generally requires EPA to identify 
and consider a reasonable number of regulatory alternatives and to 
adopt the least costly, most cost-effective or least burdensome 
alternative that achieves the objectives of the rule. The provisions of 
section 205 do not apply when they are inconsistent with applicable 
law. Moreover, section 205 allows EPA to adopt an alternative other 
than the least costly, most cost-effective or least burdensome 
alternative if the Administrator publishes with the final rule an 
explanation why that alternative was not adopted. Before EPA 
establishes any regulatory requirements that may significantly or 
uniquely affect small governments, including tribal governments, it 
must have developed under section 203 of the UMRA a small government 
agency plan. The plan must provide for notifying potentially affected 
small governments, enabling officials of affected small governments to 
have meaningful and timely input in the development of EPA regulatory 
proposals with significant Federal intergovernmental mandates, and 
informing, educating, and advising small governments on compliance with 
the regulatory requirements.
    This action contains no Federal mandates under the provisions of 
Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), 2 U.S.C. 
1531-1538 for state, local, or tribal governments or the private 
sector. Therefore, this action is not subject to the requirements of 
sections 202 or 205. Furthermore, as indicated previously, in setting a 
NAAQS 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 EPA is precluded from considering costs 
of implementation in establishing NAAQS, preparation of a Regulatory 
Impact Analysis pursuant to the Unfunded Mandates Reform Act would not 
furnish any information which the court could consider in reviewing the 
NAAQS). Accordingly, EPA has determined that the provisions of sections 
202, 203, and 205 of the UMRA do not apply to this proposed decision. 
The EPA acknowledges, however, that any corresponding revisions to 
associated state implementation plan (SIP) requirements and air quality 
surveillance requirements, 40 CFR part 51 and 40 CFR part 58, 
respectively, might result in such effects. Accordingly, EPA will 
address, as appropriate, unfunded mandates if and when it proposes any 
revisions to 40 CFR parts 51 or 58.

E. Executive Order 13132: Federalism

    This proposed rule 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, 
as specified in Executive Order 13132 because it does not contain 
legally binding requirements. Thus, the requirements of Executive Order 
13132 do not apply to this rule.
    EPA believes, however, that this proposed rule may be of 
significant interest to state governments. As also noted in section E 
(above) on UMRA, EPA recognizes that states will have a substantial 
interest in this rule and any corresponding revisions to associated SIP 
requirements and air quality surveillance requirements, 40 CFR part 51 
and 40 CFR part 58, respectively. Therefore, in the spirit of Executive 
Order 13132 and consistent with EPA policy to promote communications 
between EPA and state and local governments, EPA specifically solicits 
comment on this proposed rule from state and local officials.

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

    Executive Order 13175, entitled ``Consultation and Coordination 
with Indian Tribal Governments'' (65 FR 67249, November 9, 2000), 
requires EPA to develop an accountable process to ensure ``meaningful 
and timely input by tribal officials in the development of regulatory 
policies that have tribal implications.'' This rule concerns the 
establishment of national standards to address the public welfare 
effects of oxides of nitrogen and sulfur.

[[Page 46143]]

    This action does not have Tribal implications, as specified in 
Executive Order 13175 (65 FR 67249, November 9, 2000). It does not have 
a substantial direct effect on one or more Indian tribes, since tribes 
are not obligated to adopt or implement any NAAQS. Thus, Executive 
Order 13175 does not apply to this rule.

G. Executive Order 13045: Protection of Children from Environmental 
Health & Safety Risks

    This action is not subject to EO 13045 because it is not an 
economically significant rule as defined in EO 12866.

H. Executive Order 13211: Actions that Significantly Affect Energy 
Supply, Distribution or Use

    This action is not a ``significant energy action'' as defined in 
Executive Order 13211 (66 FR 28355, May 22, 2001), because it is not 
likely to have a significant adverse effect on the supply, 
distribution, or use of energy. This action concerns the establishment 
of national standards to address the public welfare effects of oxides 
of nitrogen and sulfur. This action does not prescribe specific 
pollution control strategies by which these ambient standards will be 
met. Such strategies will be developed by states on a case-by-case 
basis, and EPA cannot predict whether the control options selected by 
states will include regulations on energy suppliers, distributors, or 
users.

I. National Technology Transfer and Advancement Act

    Section 12(d) of the National Technology Transfer and Advancement 
Act of 1995 (NTTAA), Public Law 104-113, 12(d) (15 U.S.C. 272 note) 
directs EPA to use voluntary consensus standards in its regulatory 
activities unless to do so would be inconsistent with applicable law or 
otherwise impractical. Voluntary consensus standards are technical 
standards (e.g., materials specifications, test methods, sampling 
procedures, and business practices) that are developed or adopted by 
voluntary consensus standards bodies. The NTTAA directs EPA to provide 
Congress, through OMB, explanations when the Agency decides not to use 
available and applicable voluntary consensus standards.
    The EPA is not aware of any voluntary consensus standards that are 
relevant to the provisions of this proposed rule. The EPA welcomes any 
feedback on such standards that may be applicable.

J. Executive Order 12898: Federal Actions To Address Environmental 
Justice in Minority Populations and Low-Income Populations

    Executive Order 12898 (59 FR 7629 (Feb. 16, 1994)) establishes 
federal executive policy on environmental justice. Its main provision 
directs federal agencies, to the greatest extent practicable and 
permitted by law, to make environmental justice part of their mission 
by identifying and addressing, as appropriate, disproportionately high 
and adverse human health or environmental effects of their programs, 
policies, and activities on minority populations and low-income 
populations in the United States.
    EPA has determined that this proposed rule will not have 
disproportionately high and adverse human health or environmental 
effects on minority or low-income populations because it retains the 
level of environmental protection for all affected populations without 
having any disproportionately high and adverse human health or 
environmental effects on any population, including any minority or low-
income population.

References

    Baker JP; Bernard DP; Christensen SW; Sale MJ. 1990. Biological 
effects of changes in surface water acid-base chemistry. (State of 
science/technology report 13). Washington DC: National Acid 
Precipitation Assessment Program (NAPAP).
    Bulger AJ; Cosby BJ; Webb JR. 2000. Current, reconstructed past, 
and projected future status of brook trout (Salvelinus fontinalis) 
streams in Virginia. Can J Fish Aquat Sci, 57, 1515-1523.
    Banzhaf, S., D. Burtraw, D. Evans, and A. Krupnick. 2006. 
``Valuation of Natural Resource Improvements in the Adirondacks.'' 
Land Economics 82:445-464.
    Lavery, T.F. C.M. Rogers, R. Baumgardner, and K.P. Mishoe. 2009. 
Intercomparison of Clean Air Status and Trends Network Nitrate and 
Nitric Acid Measurements with Data from Other Monitoring Programs. 
J. Air & Waste Manage. Assoc., 59, 214-226.
    Lien L; Raddum GG; Fjellheim A. 1992. Critical loads for surface 
waters: invertebrates and fish. (Acid rain research report no 21). 
Oslo, Norway: Norwegian Institute for Water Research.
    Luke, W.T., P.K. Barry, L. Lefer, J. Flynn, B. Rappengl[uuml]ck, 
M. Leuchner, J.E. Dibb, L.D. Ziemba, C.H. Anderson, and M. Buhr. 
2010. Measurements of primary trace gases and NOY 
composition in Houston, Texas. Atmospheric Environment, 44, 4068-
4080.
    MacAvoy SW; Bulger AJ. 1995. Survival of brook trout (Salvelinus 
fontinalis) embryos and fry in streams of different acid sensitivity 
in Shenandoah National Park, USA. Water Air Soil Pollut, 85, 445-
450.
    McNulty SG; Cohen EC; Myers JAM; Sullivan TJ; Li H. 2007. 
Estimates of critical acid loads and exceedances for forest soils 
across the conterminous United States. Environ Pollut, 149, 281-292.
    NAPAP. 1990. Acid Deposition: State of Science and Technology. 
National Acid Precipitation Assessment Program. Office of the 
Director, Washington, DC.
    NAPAP. (2005). National acid precipitation assessment program 
report to Congress: An integrated assessment. http://www.esrl.noaa.gov/csd/aqrs/reports/napapreport05.pdf. Silver Spring, 
MD: National Acid Precipitation Assessment Program (NAPAP); 
Committee on Environment and Natural Resources (CENR) of the 
National Science and Technology Council (NSTC).
    NRC (National Research Council). 2004. Air quality management in 
the United States. Washington, DC: National Research Council (NRC); 
The National Academies Press.
    Russell, A and J. M. Samet, 2010a. Review of the Policy 
Assessment for the Review of the Secondary National Ambient Air 
Quality Standard for NOX and SOX: First Draft. 
EPA-CASAC-10-014.
    Russell, A and J. M. Samet, 2010b. Review of the Policy 
Assessment for the Review of the Secondary National Ambient Air 
Quality Standard for NOX and SOX: Second 
Draft. EPA-CASAC-11-003.
    Russell, A and J. M. Samet, 2011. Review of the Policy 
Assessment for the Review of the Secondary National Ambient Air 
Quality Standard for NOX and SOX: FINAL. EPA-
CASAC-11-005.
    Russell and Samet, 2011b Review of EPA Draft Documents on 
Monitoring and Methods for Oxides of Nitrogen (NOX) and 
Sulfur (SOX) http://yosemite.epa.gov/sab/sabpeople.nsf/WebCommittees/CASAC.
    Sickles II, J.E., L. L. Hodson, and L. M. Vorburger. 1999. 
Evaluation of the filter pack for long-duration sampling of ambient 
air. Atmospheric Environment, 33, 2187-2202.
    Sickles II, J.E. and D.S. Shadwick. 2008. Comparison of 
particulate sulfate and nitrate at collocated CASTNET and IMPROVE 
sites in the eastern US. Atmospheric Environment, 42, 2062-2073.
    Smyth SC, W. Jiang, and H. Roth. 2008. A comparative performance 
evaluation of the AURAMS and CMAQ air quality modeling systems. 
Atmos Envir 43:1059-1070.
    Stoddard J; Kahl JS; Deviney FA; DeWalle DR; Driscoll CT; 
Herlihy AT; Kellogg JH; Murdoch PS; Webb JR; Webster KE. (2003). 
Response of surface water chemistry to the Clean Air Act Amendments 
of 1990 (No. EPA 620/R-03/001). Research Triangle Park, NC; National 
Health and Environmental Effects Research Laboratory; Office of 
Research and Development; U.S. Environmental Protection Agency.
    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; Nowicki NA. 2006. Assessment of the extent 
to which intensively studied lakes are representative of the 
Adirondack Mountain region. (Final Report no 06-17).Corvallis, OR; 
prepared by Environmental Chemistry, Inc. for: Albany, NY; 
Environmental Monitoring Evaluation and Protection Program of the 
New York State Energy Research and Development Authority (NYSERDA).

[[Page 46144]]

    US EPA, 1973. ``Effects of Sulfur Oxide in the Atmosphere on 
Vegetation''. Revised Chapter 5 of Air Quality Criteria For Sulfur 
Oxides. U.S. Environmental Protection Agency. Research Triangle 
Park, N.C. EPA-R3-73-030.
    US EPA. 1982. Review of the National Ambient Air Quality 
Standards for Sulfur Oxides: Assessment of Scientific and Technical 
Information. OAQPS Staff Paper. EPA-450/5-82-007. U.S. Environmental 
Protection Agency, Office of Air Quality Planning and Standards, 
Research Triangle Park, NC.
    US EPA, 1984a. The Acidic Deposition Phenomenon and Its Effects: 
Critical Assessment Review Papers. Volume I Atmospheric Sciences. 
EPA-600/8-83-016AF. Office of Research and Development, Washington, 
DC.
    US EPA, 1984b. The Acidic Deposition Phenomenon and Its Effects: 
Critical Assessment Review Papers. Volume II Effects Sciences. EPA-
600/8-83-016BF. Office of Research and Development, Washington, DC.
    US EPA, 1985. The Acidic Deposition Phenomenon and Its Effects: 
Critical Assessment Document. EPA-600/8-85/001. Office of Research 
and Development, Washington, DC.
    US EPA. 1995a. Review of the National Ambient Air Quality 
Standards for Nitrogen Dioxide: Assessment of Scientific and 
Technical Information. OAQPS Staff Paper. EPA-452/R-95-005. U.S. 
Environmental Protection Agency, Office of Air Quality Planning and 
Standards, Research Triangle Park, NC. September.
    US EPA. 1995b. Acid Deposition Standard Feasibility Study Report 
to Congress. U.S. Environmental Protection Agency, Washington, DC. 
EPA-430/R-95-001a.
    US EPA 2007. Integrated Review Plan for the Secondary National 
Ambient Air Quality Standards for Nitrogen Dioxide and Sulfur 
Dioxide. U.S. Environmental Protection Agency, Research Triangle 
Park, NC, EPA-452/R-08-006.
    US EPA 2008. Integrated Science Assessment (ISA) for Oxides of 
Nitrogen and Sulfur Ecological Criteria (Final Report). U.S. 
Environmental Protection Agency, Washington, D.C., EPA/600/R-08/
082F, 2008.
    US EPA 2009. Risk and Exposure Assessment for Review of the 
Secondary National Ambient Air Quality Standards for Oxides of 
Nitrogen and Oxides of Sulfur-Main Content--Final Report. U.S. 
Environmental Protection Agency, Washington, D.C., EPA-452/R-09-
008a.
    US EPA, 2010a. CASTNET Quality Assurance Project Plan, Revision 
7.0, October 2010, http://java.epa.gov/castnet/.
    US EPA, 2010b. CASTNET Annual Reports, 2004-2009, http://java.epa.gov/castnet/.
    US EPA 2011. Policy Assessment for the Review of the Secondary 
National Ambient Air Quality Standards for Oxides of Nitrogen and 
Oxides of Sulfur. U.S. Environmental Protection Agency, Washington, 
DC, EPA-452/R-11-005a.
    US EPA, 2011b. Federal Reference Methods for NOy and 
p-SO4 for the New Combined NOX and SOx 
Secondary NAAQS Research Plan, EPA/600/1-11/002 January 20, 2011.
    Wolff, G. T. 1993. CASAC closure letter for the 1993 Criteria 
Document for Oxides of Nitrogen addressed to U.S. EPA Administrator 
Carol M. Browner dated September 30, 1993.
    Wolff, G. T. 1995. CASAC closure letter for the 1995 OAQPS Staff 
Paper addressed to U.S. EPA Administrator Carol M. Browner dated 
August 22, 1995.

List of Subjects in 40 CFR Part 50

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

    Dated: July 12, 2011.
Lisa P. Jackson,
Administrator.

    For the reasons set forth in the preamble, part 50 of chapter 1 of 
title 40 of the code of Federal regulations is proposed to be amended 
as follows:

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

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

    Authority: 42 U.S.C. 7401, et seq.
    2. Section 50.5 is amended by revising paragraphs (b) and (c) and 
by adding paragraphs (d) and (e) to read as follows:


Sec.  50.5  National secondary ambient air quality standards for sulfur 
oxides (sulfur dioxide).

* * * * *
    (b) The level of the national secondary 1-hour ambient air quality 
standard for oxides of sulfur is 75 parts per billion (ppb, which is 1 
part in 1,000,000,000), measured in the ambient air as sulfur dioxide 
(SO2).
    (c) The levels of the standards shall be measured by a reference 
method based on Appendix A-1 or A-2 of this part, or by a Federal 
Equivalent Method (FEM) designated in accordance with part 53 of this 
chapter.
    (d) To demonstrate attainment with the 3-hour secondary standard, 
the second-highest 3-hour average must be based upon hourly data that 
are at least 75 percent complete in each calendar quarter. A 3-hour 
block average shall be considered valid only if all three hourly 
averages for the 3-hour period are available. If only one or two hourly 
averages are available, but the 3-hour average would exceed the level 
of the standard when zeros are substituted for the missing values, 
subject to the rounding rule of paragraph (a) of this section, then 
this shall be considered a valid 3-hour average. In all cases, the 3-
hour block average shall be computed as the sum of the hourly averages 
divided by 3.
    (e) The 1-hour secondary standard is met at an ambient air quality 
monitoring site when the three-year average of the annual 99th 
percentile of the daily maximum 1-hour average concentrations is less 
than or equal to 75 ppb, as determined in accordance with Appendix T of 
this part.
    3. Section 50.11 is revised to read as follows:


Sec.  50.11  National primary and secondary ambient air quality 
standards for oxides of nitrogen (with nitrogen dioxide as the 
indicator).

    (a) The level of the national primary and secondary annual ambient 
air quality standards for oxides of nitrogen is 53 parts per billion 
(ppb, which is 1 part in 1,000,000,000), annual average concentration, 
measured in the ambient air as nitrogen dioxide.
    (b) The level of the national primary and secondary 1-hour ambient 
air quality standards for oxides of nitrogen is 100 ppb, 1-hour average 
concentration, measured in the ambient air as nitrogen dioxide.
    (c) The levels of the standards shall be measured by:
    (1) A reference method based on appendix F to this part; or
    (2) A Federal equivalent method (FEM) designated in accordance with 
part 53 of this chapter.
    (d) The annual primary and secondary standards are met when the 
annual average concentration in a calendar year is less than or equal 
to 53 ppb, as determined in accordance with Appendix S of this part for 
the annual standard.
    (e) The 1-hour primary and secondary standards are met when the 
three-year average of the annual 98th percentile of the daily maximum 
1-hour average concentration is less than or equal to 100 ppb, as 
determined in accordance with Appendix S of this part for the 1-hour 
standard.
    4. Appendix S is amended as follows:
    a. by revising paragraph 1.(a),
    b. by revising the definition of ``Design values'' under paragraph 
1.(c),
    c. by revising paragraph 2.(b),
    d. by revising paragraphs 3.1(a) through (d),
    e. by revising paragraphs 3.2(a) through (e),
    f. by revising paragraph 4.1(b),
    g. by revising paragraph 4.2(c),
    h. by revising paragraph 5.1(b), and
    i. by revising paragraph 5.2(b) to read as follows:

[[Page 46145]]

Appendix S to Part 50--Interpretation of the Primary and Secondary 
National Ambient Air Quality Standards for Oxides of Nitrogen (Nitrogen 
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 
nitrogen as measured by nitrogen dioxide (``NO2 NAAQS'') 
specified in Sec.  50.11 are met. Nitrogen dioxide (NO2) 
is measured in the ambient air by a Federal reference method (FRM) 
based on appendix F 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 NO2 concentrations and the levels of the 
NO2 NAAQS are specified in the following sections.
* * * * *
    (c) * * *
    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 values for the 
primary and secondary NAAQS are:
    (1) The annual mean value for a monitoring site for one year 
(referred to as the ``annual primary or secondary standard design 
value'').
    (2) The 3-year average of annual 98th percentile daily maximum 
1-hour values for a monitoring site (referred to as the ``1-hour 
primary or secondary standard design value'').
* * * * *
    2. Requirements for Data Used for Comparisons With the 
NO2 NAAQS and Data
Reporting Considerations.
* * * * *
    (b) When two or more NO2 monitors are operated at a 
site, the state may in advance designate one of them as the primary 
monitor. If the state has not made this designation, the 
Administrator will make the designation, either in advance or 
retrospectively. Design values will be developed using only the data 
from the primary monitor, if this results in a valid design value. 
If data from the primary monitor do not allow the development of a 
valid design value, data solely from the other monitor(s) will be 
used in turn to develop a valid design value, if this results in a 
valid design value. If there are three or more monitors, the order 
for such comparison of the other monitors will be determined by the 
Administrator. The Administrator may combine data from different 
monitors in different years for the purpose of developing a valid 1-
hour primary or secondary standard design value, if a valid design 
value cannot be developed solely with the data from a single 
monitor. However, data from two or more monitors in the same year at 
the same site will not be combined in an attempt to meet data 
completeness requirements, except if one monitor has physically 
replaced another instrument permanently, in which case the two 
instruments will be considered to be the same monitor, or if the 
state has switched the designation of the primary monitor from one 
instrument to another during the year.
* * * * *
    3. Comparisons with the NO2 NAAQS.
    3.1 The Annual Primary and Secondary NO2 NAAQS.
    (a) The annual primary and secondary NO2 NAAQS are 
met at a site when the valid annual primary standard design value is 
less than or equal to 53 parts per billion (ppb).
    (b) An annual primary or secondary standard design value is 
valid when at least 75 percent of the hours in the year are 
reported.
    (c) An annual primary or secondary standard design value based 
on data that do not meet the completeness criteria stated in section 
3.1(b) 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.
    (d) The procedures for calculating the annual primary and 
secondary standard design values are given in section 5.1 of this 
appendix.
    3.2 The 1-Hour Primary and Secondary NO2 NAAQS.
    (a) The 1-hour primary or secondary NO2 NAAQS is met 
at a site when the valid 1-hour primary or secondary standard design 
value is less than or equal to 100 parts per billion (ppb).
    (b) An NO2 1-hour primary or secondary standard 
design value is valid if it encompasses three consecutive calendar 
years of complete data. A year meets data completeness requirements 
when all 4 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 1-hour primary or secondary standard design value, the 3-year 
1-hour primary or 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 is above the level of the primary or secondary 1-hour 
standard.
    (ii) (A) A 1-hour primary or secondary standard design value 
that is below the level of the NAAQS can be validated if the 
substitution test in section 3.2(c)(ii)(B) 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 3.2(c)(ii)(B)) 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 non-excluded 
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 5.2 
yields a recalculated 3-year 1-hour standard ``test design value'' 
below the level of the standard, then the 1-hour primary or 
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), 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) A 1-hour primary or 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) 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

[[Page 46146]]

calendar) 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 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)) 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 5.2 
yields a recalculated 3-year 1-hour standard ``test design value'' 
above the level of the standard, then the 1-hour primary or 
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), 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 or secondary standard design value based on 
data that do not meet the completeness criteria stated in 3.2(b) and 
also do not satisfy section 3.2(c), 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 and 
secondary standard design values are given in section 5.2 of this 
appendix.
    4. Rounding Conventions.
    4.1 Rounding Conventions for the Annual Primary and Secondary 
NO2 NAAQS.
* * * * *
    (b) The annual primary or secondary standard design value is 
calculated pursuant to section 5.1 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 1-hour Primary and Secondary 
NO2 NAAQS.
* * * * *
    (c) The 1-hour primary or secondary standard design value is 
calculated pursuant to section 5.2 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 for the Primary and Secondary 
NO2 NAAQS.
    5.1 Procedures for the Annual Primary and Secondary 
NO2 NAAQS.
* * * * *
    (b) The annual primary or secondary standard design value for a 
site is the valid annual mean rounded according to the conventions 
in section 4.1.
    5.2 Calculation Procedures for the 1-hour Primary and Secondary 
NO2 NAAQS.
* * * * *
    (b) The 1-hour primary or secondary standard design value for a 
site is the mean of the three annual 98th percentile values, rounded 
according to the conventions in section 4.
* * * * *

    5. Appendix T is amended as follows:
    a. by revising paragraph 1.(a),
    b. by revising the definition of ``Design values'' under paragraph 
1.(c),
    c. by revising paragraph 2.(b),
    d. by revising paragraphs 3.(a) through (e),
    e. by revising paragraph 4.(c), and
    f. by revising paragraph 5.(b) 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 and Sec.  50.5 (b), respectively, 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-1 or A-2 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.
* * * * *
    (c) * * *
    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 and secondary 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'').
* * * * *
    2. Requirements for Data Used for Comparisons With the 
SO2 NAAQS and Data Reporting Considerations.
* * * * *
    (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 or 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(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 section 3(c) and 5(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. 
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.
* * * * *
    3. Comparisons with the 1-hour Primary and Secondary 
SO2 NAAQS.
    (a) The 1-hour primary or secondary SO2 NAAQS is met 
at an ambient air quality monitoring site when the valid 1-hour 
primary or secondary standard design value is less than or equal to 
75 parts per billion (ppb).
    (b) An SO2 1-hour primary or secondary standard 
design value is valid if it encompasses three consecutive calendar 
years of complete data. A year meets data completeness requirements 
when all 4 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.

[[Page 46147]]

    (c) In the case of one, two, or three years that do not meet the 
completeness requirements of section 3(b) of this appendix and thus 
would normally not be useable for the calculation of a valid 3-year 
1-hour primary or secondary standard design value, the 3-year 1-hour 
primary or 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 is above the level of the primary or secondary 1-hour 
standard.
    (ii) (A) A 1-hour primary or 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(c)(ii)(B) 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(c)(ii)(B)) 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 or 
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(c)(i), 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) A 1-hour primary or secondary standard design value 
that is above the level of the NAAQS can be validated if the 
substitution test in section 3(c)(iii)(B) 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(c)(iii)(B)) 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 yields 
a recalculated 3-year 1-hour standard ``test design value'' above 
the level of the standard, then the 1-hour primary or 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(c)(i), 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 or secondary standard design value based on 
data that do not meet the completeness criteria stated in 3(b) and 
also do not satisfy section 3(c), 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 or 
secondary standard design values are given in section 5 of this 
appendix.
    4. Rounding Conventions for the 1-hour Primary and Secondary 
SO2 NAAQS.
* * * * *
    (c) The 1-hour primary or secondary standard design value is 
calculated pursuant to section 5 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 for the 1-hour Primary and Secondary 
SO2 NAAQS.
* * * * *
    (b) The 1-hour primary or secondary standard design value for an 
ambient air quality monitoring site is the mean of the three annual 
99th percentile values, rounded according to the conventions in 
section 4.

[FR Doc. 2011-18582 Filed 7-29-11; 8:45 am]
BILLING CODE 6560-50-P