[Federal Register Volume 74, Number 234 (Tuesday, December 8, 2009)]
[Proposed Rules]
[Pages 64810-64881]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: E9-28058]



[[Page 64809]]

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Part II





Environmental Protection Agency





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40 CFR Parts 50, 53, and 58



 Primary National Ambient Air Quality Standard for Sulfur Dioxide; 
Proposed Rule

  Federal Register / Vol. 74, No. 234 / Tuesday, December 8, 2009 / 
Proposed Rules  

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53, and 58

 [EPA-HQ-OAR-2007-0352; FRL-8984-3]
RIN 2060-A048


Primary National Ambient Air Quality Standard for Sulfur Dioxide

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: Based on its review of the air quality criteria for oxides of 
sulfur and the primary national ambient air quality standard (NAAQS) 
for oxides of sulfur as measured by sulfur dioxide (SO2), 
EPA is proposing to revise the primary SO2 NAAQS to provide 
requisite protection of public health with an adequate margin of 
safety. Specifically, EPA proposes to establish a new 1-hour 
SO2 standard within the range of 50-100 parts per billion 
(ppb), based on the 3-year average of the annual 99th percentile (or 
4th highest) of 1-hour daily maximum concentrations. The EPA also 
proposes to revoke both the existing 24-hour and annual primary 
SO2 standards.

DATES: Comments must be received on or before February 8, 2010. Under 
the Paperwork Reduction Act, comments on the information collection 
provisions must be received by OMB on or before January 7, 2010.
    Public Hearings: A public hearing is scheduled for this proposed 
rule. The public hearing will be held on January 5, 2010 in Atlanta, 
Georgia.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2007-0352 by one of the following methods:
     http://www.regulations.gov: Follow the on-line 
instructions for submitting comments.
     E-mail: [email protected].
     Fax: 202-566-9744.
     Mail: Docket No. EPA-HQ-OAR-2007-0352, 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-2007-0352, 
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.
    Public Hearings: A public hearing is scheduled for this proposed 
rule. The public hearing will be held on January 5, 2010 in Atlanta, 
Georgia. The hearing will be held at the following location: Sam Nunn 
Atlanta Federal Center, Conference Rooms B and C, 61 Forsyth Street, 
SW., Atlanta, GA 30303, Telephone: (404) 562-9077.

    Note:  All persons entering the Atlanta Federal Center must have 
a valid picture ID such as a Driver's License and go through Federal 
security procedures. All persons must go through a magnetometer and 
all personal items must go through x-ray equipment, similar to 
airport security procedures. After passing through the equipment, 
all persons must sign in at the guard station and show their picture 
ID.

    See the SUPPLEMENTARY INFORMATION under ``Public Hearing'' for 
further information.
    Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2007-0352. EPA's policy is that all comments received will be included 
in the public docket without change and may be made available online at 
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 www.regulations.gov or e-mail. 
The 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 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 
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 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. Michael J. Stewart, Health and 
Environmental Impacts Division, Office of Air Quality Planning and 
Standards, U.S. Environmental Protection Agency, Mail Code C504-06, 
Research Triangle Park, NC 27711; telephone: 919-541-7524; fax: 919-
541-0237; 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 
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).
     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.

[[Page 64811]]

     Describe any assumptions and provide any technical 
information and/or data that you used.
     Provide specific examples to illustrate your concerns, and 
suggest alternatives.
     Explain your views as clearly as possible, avoiding the 
use of profanity or personal threats.
     Make sure to submit your comments by the comment period 
deadline identified.

Availability of Related Information

    A number of the documents that are relevant to this rulemaking are 
available through EPA's Office of Air Quality Planning and Standards 
(OAQPS) Technology Transfer Network (TTN) Web site at http://www.epa.gov/ttn/naaqs/standards/so2/s_so2_index.html. These documents 
include the Integrated Review Plan and the Health Assessment Plan, 
available at, the Integrated Science Assessment (ISA), available at 
http://www.epa.gov/ttn/naaqs/standards/so2/s_so2_cr_isa.html, and 
the Risk and Exposure Assessment (REA), available at http://www.epa.gov/ttn/naaqs/standards/so2/s_so2_cr_rea.html. These and 
other related documents are also available for inspection and copying 
in the EPA docket identified above.

Public Hearing

    The public hearing on January 5, 2010 will provide interested 
parties the opportunity to present data, views, or arguments concerning 
the proposed rule. The EPA may ask clarifying questions during the oral 
presentations, but will not respond to the presentations at that time. 
Written statements and supporting information submitted during the 
comment period will be considered with the same weight as any oral 
comments and supporting information presented at the public hearing. 
Written comments must be received by the last day of the comment 
period, as specified in this proposed rulemaking.
    The public hearing will begin at 10 a.m. and continue until 7 p.m. 
(local time) or later, if necessary, depending on the number of 
speakers wishing to participate. The EPA will make every effort to 
accommodate all speakers that arrive and register before 7 p.m. A lunch 
break is scheduled from 12:30 p.m. until 2 p.m.
    If you would like to present oral testimony at the hearing, please 
notify Ms. Tricia Crabtree (C504-02), U.S. EPA, Research Triangle Park, 
NC 27711. The preferred method for registering is by e-mail 
([email protected]). Ms. Crabtree may be reached by telephone at 
(919) 541-5688. She will arrange a general time slot for you to speak. 
The EPA will make every effort to follow the schedule as closely as 
possible on the day of the hearing.
    Oral testimony will be limited to five (5) minutes for each 
commenter to address the proposal. We will not be providing equipment 
for commenters to show overhead slides or make computerized slide 
presentations unless we receive special requests in advance. Commenters 
should notify Ms. Crabtree if they will need specific audiovisual (AV) 
equipment. Commenters should also notify Ms. Crabtree if they need 
specific translation services for non-English speaking commenters. The 
EPA encourages commenters to provide written versions of their oral 
testimonies either electronically on computer disk, CD-ROM, or in paper 
copy.
    The hearing schedule, including lists of speakers, will be posted 
on EPA's Web site for the proposal at http://www.epa.gov/ttn/naaqs/standards/so2/s_so2_index.html prior to the hearing. Verbatim 
transcripts of the hearing and written statements will be included in 
the rulemaking docket.

Table of Contents

    The following topics are discussed in this preamble:

I. Background
    A. Legislative requirements
    B. Related SO2 control programs
    C. History of reviews of the primary NAAQS for sulfur oxides
II. Rationale for proposed decisions on the primary standards
    A. Characterization of SO2 air quality
    1. Anthropogenic sources and current patterns of SO2 
air quality
    2. SO2 monitoring
    B. Health effects information
    1. Respiratory effects and 5-10 minute exposure to 
SO2
    a. Respiratory symptoms
    b. Lung function decrements
    c. Adversity of 5-10 minute respiratory effects
    2. Respiratory effects and 1 to 24-hour exposures to 
SO2
    a. Respiratory symptoms
    b. Emergency department visits and hospitalizations
    3. ISA conclusions regarding short-term (5-minutes to 24-hour) 
SO2 exposures
    4. Health effects and long-term exposures to SO2
    5. SO2-related impacts on public health
    a. Pre-existing respiratory disease
    b. Genetics
    c. Age
    d. Time spent outdoors
    e. Ventilation rate
    f. Socioeconomic status
    g. Size of at-risk population
    C. Human exposure and health risk characterization
    1. Evidence base for the risk characterization
    2. Overview of approaches
    3. Key limitations and uncertainties
    D. Considerations in review of the standards
    1. Background on the current standards
    2. Approach for reviewing the need to retain or revise the 
current standards
    E. Adequacy of the current standards
    1. Adequacy of the current 24-hour standard
    a. Evidence-based considerations
    b. Air quality, exposure, and risk-based considerations
    c. Summary of considerations from the REA regarding the 24-hour 
standard
    2. Adequacy of the current annual standard
    a. Evidence-based considerations
    b. Air quality, exposure, and risk-based considerations
    c. Summary of considerations from the REA regarding the annual 
standard
    3. CASAC views regarding adequacy of the current 24-hour and 
annual standards
    4. Administrator's conclusions regarding adequacy of the current 
24-hour and annual standards
    F. Conclusions on the elements of a proposed new short-term 
standard
    1. Indicator
    2. Averaging time
    a. Evidence and air quality, exposure, and risk-based 
considerations
    b. CASAC views
    c. Administrator's conclusions on averaging time
    3. Form
    a. Evidence, air quality, and risk-based considerations
    b. CASAC views
    c. Administrator's conclusions on form
    4. Level
    a. Evidence-based considerations
    b. Air quality, exposure and risk-based considerations
    c. Observations based on evidence and risk-based considerations
    d. CASAC views
    e. Administrator's conclusions on level for a 1-hour standard
    5. Implications for retaining or revoking current standards
    G. Summary of proposed decisions on primary standards
III. Proposed Amendments to Ambient Monitoring and Reporting 
Requirements
    A. Monitoring methods
    1. Background
    2. Proposed new FRM measurement technique
    3. Technical description of the proposed UVF FRM
    4. Implications to air monitoring networks
    5. Proposed revisions to 40 CFR Part 53
    B. Network design
    1. Background
    2. Proposed changes
    a. Population Weighted Emissions Index (PWEI) Triggered 
Monitoring
    b. State-level emissions triggered monitoring
    c. Monitor placement and siting
    d. Monitoring required by the Regional Administrator
    e. Alternative Network Design
    C. Data Reporting
IV. Proposed Appendix T--Interpretation of the Primary NAAQS for 
Oxides of Sulfur

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and Proposed Revisions to the Exceptional Events Rule
    A. Background
    B. Interpretation of the NAAQS for Oxides of Sulfur
    1. 1-hour standard based on the annual 4th highest daily value 
form
    2. 1-hour primary standard based on the 99th percentile value 
form
    C. Exceptional events information submission schedule
V. Designations for the SO2 NAAQS
VI. Clean Air Act Implementation Requirements
    A. How this rule applies to tribes
    B. Attainment dates
    1. Attaining the NAAQS
    2. Consequences of failing to attain by the Statutory Attainment 
Date
    C. Section 110(a)(2) NAAQS Infrastructure Requirements
    D. Attainment planning requirements
    1. SO2 Nonattainment area SIP requirements
    2. New source review and prevention of significant deterioration 
requirements
    3. General conformity
    E. Transition from the existing SO2 NAAQS to a 
revised SO2 NAAQS
VII. Communication of public health information
VIII. 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 & Safety Risks
    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 (Act or CAA) govern the 
establishment and revision of National Ambient Air Quality Standards 
NAAQS. Section 108 of the Act directs the Administrator to identify and 
list air pollutants that meet certain criteria, including that the air 
pollutant ``in his judgment, cause[s] or contribute[s] to air pollution 
which may reasonably be anticipated to endanger public health and 
welfare'' and ``the presence of which in the ambient air results from 
numerous or diverse mobile or stationary sources.'' CAA section 108 
(a)(1)(A) & (B). For those air pollutants listed, section 108 requires 
the Administrator to issue air quality criteria that ``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 ambient air 
* * *'' Section 108 (a) (2).
    Section 109(a) of the Act directs the Administrator to promulgate 
``primary'' and ``secondary'' NAAQS for pollutants for which air 
quality criteria have been 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 [the air quality] criteria and 
allowing an adequate margin of safety, are requisite to protect the 
public health.'' \1\ Section 109(b)(1). A secondary standard, in turn, 
must ``specify a level of air quality the attainment and maintenance of 
which, in the judgment of the Administrator, based on [the air quality] 
criteria, is requisite to protect the public welfare from any known or 
anticipated adverse effects associated with the presence of such 
pollutant in the ambient air.'' \2\ Section 109(b)(2) This proposal 
concerns exclusively the primary NAAQS for oxides of sulfur.
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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).
    \2\ EPA is currently conducting a separate review of the 
secondary SO2 NAAQS jointly with a review of the 
secondary NO2 NAAQS (see http://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.html for more information).
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    The requirement that primary standards include an adequate margin 
of safety is intended to address uncertainties associated with 
inconclusive scientific and technical information available at the time 
of standard setting. It is also intended to provide a reasonable degree 
of protection against hazards that research has not yet identified. 
Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (DC Cir 1980), 
cert. denied, 449 U.S. 1042 (1980); American Petroleum Institute v. 
Costle, 665 F.2d 1176, 1186 (DC Cir. 1981), cert. denied, 455 U.S. 1034 
(1982). Both kinds of uncertainties are components of the risk 
associated with pollution at levels below those at which human health 
effects can be said to occur with reasonable scientific certainty. 
Thus, in selecting primary standards that include an adequate margin of 
safety, the Administrator is seeking not only to prevent pollution 
levels that have been demonstrated to be harmful but also to prevent 
lower pollutant levels that may pose an unacceptable risk of harm, even 
if the risk is not precisely identified as to nature or degree.
    In addressing the requirement for a margin of safety, EPA considers 
such factors as the nature and severity of the health effects involved, 
the size of the at-risk population(s), and the kind and degree of the 
uncertainties that must be addressed. The selection of any particular 
approach to providing an adequate margin of safety is a policy choice 
left specifically to the Administrator's judgment. Lead Industries 
Association v. EPA, 647 F.2d at 1161-62.
    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. Whitman v. American Trucking 
Associations, 531 U.S. 457, 471, 475-76 (2001).
    Section 109(d)(1) of the Act requires the Administrator to 
periodically undertake a thorough review of the air quality criteria 
published under section 108 and the NAAQS and to revise the criteria 
and standards as may be appropriate. The Act also requires the 
Administrator to appoint an independent scientific review committee 
composed of seven members, including at least one member of the 
National Academy of Sciences, one physician, and one person 
representing State air pollution control agencies, to review the air 
quality criteria and NAAQS and to ``recommend to the Administrator any 
new * * * standards and revisions of existing criteria and standards as 
may be appropriate under section 108 and subsection (b) of this 
section.'' CAA section 109 (d)(2). This independent review function is 
performed by the Clean Air Scientific Advisory Committee (CASAC) of 
EPA's Science Advisory Board.

B. Related SO2 control programs

    States are primarily responsible for ensuring attainment and 
maintenance of ambient air quality standards once EPA has established 
them. Under section 110 of the Act, and related provisions, States are 
to submit, for EPA approval, State implementation plans (SIPs) that 
provide for the attainment and maintenance of such standards through 
control programs directed to sources of the pollutants involved. The 
States, in conjunction with EPA, also administer the prevention of 
significant deterioration program that covers these

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pollutants. See CAA sections 160-169. In addition, Federal programs 
provide for nationwide reductions in emissions of these and other air 
pollutants through the Federal motor vehicle and motor vehicle fuel 
control program under title II of the Act, (CAA sections 202-250) which 
involves controls for emissions from all moving sources and controls 
for the fuels used by these sources; new source performance standards 
under section 111; and title IV of the Act (CAA sections 402-416), 
which specifically provides for major reductions in SO2 
emissions. EPA has also promulgated the Clean Air Interstate Rule 
(CAIR) to define additional SO2 emission reductions needed 
in the Eastern United States to address the interstate impact 
provisions of CAA section 110(a)(2)(D), a rule which EPA is 
reevaluating pursuant to court remand.
    Currently, there are several areas designated as being in 
nonattainment of the primary SO2 NAAQS (see section VI). If 
the SO2 NAAQS is revised as a result of this review; 
however, some additional areas could be classified as non-attainment. 
Certain States would then be required to develop SIPs that identify and 
implement specific air pollution control measures to reduce ambient 
SO2 concentrations to attain and maintain the revised 
SO2 NAAQS, most likely by requiring air pollution controls 
on sources that emit oxides of sulfur (SOX).

C. History of reviews of the primary NAAQS for sulfur oxides

    On April 30, 1971, the EPA promulgated primary SO2 NAAQS 
(36 FR 8187). These primary standards, which were based on the findings 
outlined in the original 1969 Air Quality Criteria for Sulfur Oxides, 
were set at 0.14 parts per million averaged over a 24-hour period, not 
to be exceeded more than once per year, and 0.030 ppm annual arithmetic 
mean. In 1982, EPA published the Air Quality Criteria for Particulate 
Matter and Sulfur Oxides (EPA, 1982) along with an addendum of newly 
published controlled human exposure studies, which updated the 
scientific criteria upon which the initial standards were based (EPA, 
1982). In 1986, EPA published a second addendum presenting newly 
available evidence from epidemiologic and controlled human exposure 
studies (EPA, 1986). In 1988, EPA published a proposed decision not to 
revise the existing standards (53 FR 14926) (April 26, 1988). However, 
EPA specifically requested public comment on the alternative of 
revising the current standards and adding a new 1-hour primary standard 
of 0.4 ppm (400 ppb) to protect against 5-10 minute peak SO2 
concentrations.
    As a result of public comments on the 1988 proposal and other post-
proposal developments, EPA published a second proposal on November 15, 
1994 (59 FR 58958). The 1994 re-proposal was based in part on a 
supplement to the second addendum of the criteria document, which 
evaluated new findings on 5-10 minute SO2 exposures in 
asthmatics (EPA, 1994a). As in the 1988 proposal, EPA proposed to 
retain the existing 24-hour and annual standards. EPA also solicited 
comment on three regulatory alternatives to further reduce the health 
risk posed by exposure to high 5-minute peaks of SO2 if 
additional protection were judged to be necessary. The three 
alternatives were: (1) Revising the existing primary SO2 
NAAQS by adding a new 5-minute standard of 0.6 ppm (600 ppb) 
SO2; (2) establishing a new regulatory program under section 
303 of the Act to supplement protection provided by the existing NAAQS, 
with a trigger level of 0.6 ppm (600 ppb) SO2, one expected 
exceedance; and (3) augmenting implementation of existing standards by 
focusing on those sources or source types likely to produce high 5-
minute peak concentrations of SO2.
    On May 22, 1996, EPA announced its final decision not to revise the 
NAAQS for SOX (61 FR 25566). EPA found that asthmatics (a 
susceptible population group) could be exposed to such short-term 
SO2 bursts resulting in repeated `exposure events' such that 
tens or hundreds of thousands of asthmatics could be exposed annually 
to lung function effects ``distinctly exceeding * * * [the] typical 
daily variation in lung function'' that asthmatics routinely 
experience, and found further that repeated occurrences should be 
regarded as significant from a public health standpoint. 61 FR at 
25572, 25573. Nonetheless, the agency concluded that ``the likelihood 
that asthmatic individuals will be exposed * * * is very low when 
viewed from a national perspective'', that ``5-minute peak 
SO[2] levels do not pose a broad public health problem when 
viewed from a national perspective'', and that ``short-term peak 
concentrations of SO[2] do not constitute the type of 
ubiquitous public health problem for which establishing a NAAQS would 
be appropriate.'' Id. at 25575. EPA concluded, therefore, that it would 
not revise the existing standards or add a standard to specifically 
address 5-minute exposures. EPA also announced an intention to propose 
guidance, under section 303 of the Act, to assist states in responding 
to short-term peak of SO2 and later initiated a rulemaking 
to do so (62 FR 210 (Jan. 2, 1997).
    The American Lung Association and the Environmental Defense Fund 
challenged EPA's decision not to establish a 5-minute standard. On 
January 30, 1998, the Court of Appeals for the District of Columbia 
found that EPA had failed to adequately explain its determination that 
no revision to the SO2 NAAQS was appropriate and remanded 
the determination back to EPA for further explanation. American Lung 
Ass'n v. EPA, 134 F. 3d 388 (DC Cir. 1998). Specifically, the court 
held that EPA had failed to adequately explain the basis for its 
conclusion that short-term SO2 exposures to asthmatics do 
not constitute a public health problem, noting that the agency had 
failed to explain the link between its finding that repeated short-term 
exposures were significant, and that there would be tens to hundreds of 
thousands of such exposures annually to a susceptible subpopulation, 
but that a NAAQS was found not be appropriate. 134 F. 3d at 392. The 
court also rejected the explanation that short-term SO2 
bursts were ``localized, infrequent, and site-specific'' as a rational 
basis for the conclusion that no public health problem existed: 
``[N]othing in the Final Decision explains why `localized', `site-
specific', or even `infrequent' events might nevertheless create a 
public health problem, particularly since, in some sense, all pollution 
is local and site-specific * * *''. Id. The court accordingly remanded 
the case to EPA to adequately explain its determination or otherwise 
take action in accordance with the opinion. In response, EPA has 
collected and analyzed additional air quality data focused on 5-minute 
concentrations of SO2. These air quality analyses conducted 
since the last review will help inform the current review, which will 
address the issues raised in the court's remand of the Agency's last 
decision.
    EPA formally initiated the current review of the air quality 
criteria for oxides of sulfur and the SO2 primary NAAQS on 
May 15, 2006 (71 FR 28023) with a general call for information. EPA's 
draft Integrated Review Plan for the Primary National Ambient Air 
Quality Standards for Sulfur Dioxide (EPA, 2007a) was made available in 
April 2007 for public comment and was discussed by the CASAC via a 
publicly accessible teleconference on May 11, 2007. As noted in that 
plan, SOX includes multiple gaseous (e.g., SO3) 
and particulate (e.g., sulfate) species. Because the health effects 
associated with particulate species of SOx have been 
considered within the context of

[[Page 64814]]

the health effects of ambient particles in the Agency's review of the 
NAAQS for particulate matter (PM), the current review of the primary 
SO2 NAAQS is focused on the gaseous species of 
SOx and does not consider health effects directly associated 
with particulate species.
    The first draft of the Integrated Science Assessment for Oxides of 
Sulfur-Health Criteria (ISA) and the Sulfur Dioxide Health Assessment 
Plan: Scope and Methods for Exposure and Risk Assessment (EPA, 2007b) 
were reviewed by CASAC at a public meeting held on December 5-6, 2007. 
Based on comments received from CASAC and the public, EPA developed the 
second draft of the ISA and the first draft of the Risk and Exposure 
Assessment to Support the Review of the SO2 Primary National 
Ambient Air Quality Standard (Risk and Exposure Assessment (REA)). 
These documents were reviewed by CASAC at a public meeting held on July 
30-31, 2008. Based on comments received from CASAC and the public at 
this meeting, EPA released the final ISA in September of 2008 (EPA, 
2008a; henceforth referred to as ISA). In addition, comments received 
were considered in developing the second draft of the REA. Importantly, 
the second draft of the REA contained a draft staff policy assessment 
that considered the evidence presented in the final ISA and the air 
quality, exposure, and risk characterization results presented in the 
second draft REA, as they related to the adequacy of the current 
SO2 NAAQS and potential alternative primary SO2 
standards. This document was reviewed by CASAC at a public meeting held 
on April 16-17, 2009. In preparing the final REA report, which included 
the final staff policy assessment, EPA considered comments received 
from CASAC and the public at and subsequent to that meeting. The final 
REA containing the final staff policy assessment was completed in 
August 2009 (EPA 2009a; henceforth referred to as REA).
    The schedule for completion of this review is governed by a 
judicial order resolving a lawsuit filed in September 2005, concerning 
the timing of the current review. Center for Biologic Diversity v. 
Johnson (Civ. No. 05-1814) (D.D.C. 2007). The order that now governs 
this review, entered by the court in August 2007 and amended in 
December 2008, provides that the Administrator will sign, for 
publication, notices of proposed and final rulemaking concerning the 
review of the primary SO2 NAAQS no later than November 16, 
2009 and June 2, 2010, respectively.
    This action presents the Administrator's proposed decisions on the 
current primary SO2 standards. Throughout this preamble a 
number of conclusions, findings, and determinations proposed by the 
Administrator are noted. Although they identify the reasoning that 
supports this proposal, they are not intended to be final or 
conclusive. EPA invites general, specific, and/or technical comments on 
all issues involved with this proposal, including all such proposed 
judgments, conclusions, findings, and determinations. In addition to 
requesting comment on the overall approach, EPA invites specific 
comment on the level, or range of levels, appropriate for such a 
standard, as well as on the rationale that would support that level or 
range of levels.

II. Rationale for proposed decisions on the primary standards

    This section presents the rationale for the Administrator's 
proposed decision to revise the existing SO2 primary 
standards by replacing the current 24-hour and annual standards with a 
1-hour standard and to specify this 1-hour standard to the nearest 
parts per billion (ppb). As discussed more fully below, this rationale 
takes into account: (1) Judgments and conclusions presented in the ISA 
and the REA; (2) CASAC advice and recommendations, as reflected in the 
CASAC panel's discussions of drafts of the ISA and REA at public 
meetings, in separate written comments, and in CASAC letters to the 
Administrator (Henderson 2008; Samet, 2009); and (3) public comments 
received at CASAC meetings during the development of the ISA and the 
REA.
    In developing this rationale, EPA has drawn upon an integrative 
synthesis of the entire body of evidence on human health effects 
associated with the presence of SO2 in the ambient air, and 
upon the results of quantitative exposure and risk assessments 
reflecting this evidence. As discussed below, this body of evidence 
addresses a broad range of health endpoints associated with exposure to 
SO2 in the ambient air. In considering this entire body of 
evidence, EPA chose to focus in particular on those health endpoints 
for which the ISA finds associations with SO2 to be causal 
or likely causal (see section II.B below). Thus, the focus of this 
proposal will be on respiratory morbidity following short-term (5 
minutes to 24 hours) exposure to SO2, for which the ISA 
found a causal relationship.
    As discussed below, a substantial amount of new research has been 
conducted since EPA's last review of the SO2 NAAQS, with 
important new information coming from epidemiologic studies in 
particular. The newly available research studies evaluated in the ISA 
have undergone intensive scrutiny through multiple layers of peer 
review and opportunities for public review and comment. Although 
important uncertainties remain in the qualitative and quantitative 
characterizations of health effects attributable to exposure to ambient 
SO2, the review of this information has been extensive and 
deliberate.
    The remainder of this section discusses the Administrator's 
rationale for the proposed decisions on the primary standard. Section 
II.A presents a discussion of the principal emitting sources and 
current patterns of SO2 air quality, as well as the current 
SO2 monitoring network from which those air quality patterns 
are obtained. Section II.B includes an overview of the scientific 
evidence related to the respiratory effects associated with ambient 
SO2 exposure. This overview includes a discussion of the at-
risk populations considered in the ISA. Section II.C discusses the 
approaches taken by EPA to assess exposures and health risks associated 
with exposure to ambient SO2, including a discussion of key 
uncertainties associated with the analyses. Section II.D presents the 
approach that is being used in the current review of the SO2 
NAAQS with regard to consideration of the scientific evidence and the 
air quality, exposure, and risk-based results related to the adequacy 
of the current standards and potential alternative standards. Sections 
II.E and II.F discuss the scientific evidence and the air quality, 
exposure, and risk-based results specifically as they relate to the 
current and potential alternative standards, including discussion of 
the Administrator's proposed decisions on the standards. Section II.G 
summarizes the Administrator's proposed decisions with regard to the 
SO2 primary NAAQS.

A. Characterization of SO2 air quality

1. Anthropogenic sources and current patterns of SO2 Air 
Quality
    Anthropogenic SO2 emissions originate chiefly from point 
sources, with fossil fuel combustion at electric utilities (~66%) and 
other industrial facilities (~29%) accounting for the majority of total 
emissions (ISA, section 2.1). Other anthropogenic sources of 
SO2 include both the extraction of metal from ore as well as 
the burning of high sulfur-containing fuels by locomotives, large 
ships, and equipment utilizing diesel engines. SO2 emissions 
and

[[Page 64815]]

ambient concentrations follow a strong east to west gradient due to the 
large numbers of coal-fired electric generating units in the Ohio River 
Valley and upper Southeast regions. In the 12 Consolidated Metropolitan 
Statistical Areas (CMSAs) that had at least four SO2 
regulatory monitors from 2003-2005, 24-hour average concentrations in 
the continental U.S. ranged from a reported low of ~1 ppb in Riverside, 
CA and San Francisco, CA to a high of ~12 ppb in Pittsburgh, PA and 
Steubenville, OH (ISA, section 2.5.1). In addition, outside or inside 
all CMSAs from 2003-2005, the annual average SO2 
concentration was 4 ppb (ISA, Table 2-8). However, spikes in hourly 
concentrations occurred; the mean 1-hour maximum concentration outside 
or inside CMSAs was 13 ppb, with a maximum value of greater than 600 
ppb outside CMSAs and greater than 700 ppb inside CMSAs (ISA, Table 2-
8).
    Temporal and spatial patterns of 5-minute peaks of SO2 
are also important given that human clinical studies have demonstrated 
that exposure to these peaks can result in adverse respiratory effects 
in exercising asthmatics (see section II.B). For those monitors which 
voluntarily reported 5-minute block average data,\3\ when maximum 5-
minute concentrations were reported, the absolute highest concentration 
over the ten-year period exceeded 4000 ppb, but for all individual 
monitors, the 99th percentile was below 200 ppb (ISA, section 2.5.2 
Table 2-10). Median concentrations from these monitors reporting 5-
minute data ranged from 1 ppb to 8 ppb, and the average for each 
maximum 5-minute level ranged from 3 ppb to 17 ppb. Delaware, 
Pennsylvania, Louisiana, and West Virginia had mean values for maximum 
5-minute data exceeding 10 ppb. Among aggregated within-state data for 
the 16 monitors from which all 5-minute average intervals were 
reported, the median values ranged from 1 ppb to 5 ppb, and the means 
ranged from 3 ppb to 11 ppb (ISA, section 2.5.2). The highest reported 
concentration was 921 ppb, but the 99th percentile values for 
aggregated within-state data were all below 90 ppb (ISA, section 
2.5.2).
---------------------------------------------------------------------------

    \3\ A small number of sites, 98 total from 1997 to 2007 of the 
approximately 500 SO2 monitors, and not the same sites in 
all years, voluntarily reported 5-minute block average data to AQS 
(ISA, section 2.5.2). Of these, 16 reported all twelve 5-minute 
averages in each hour for at least part of the time between 1997 and 
2007. The remainder reported only the maximum 5-minute average in 
each hour.
---------------------------------------------------------------------------

2. SO2 monitoring
    Although the SO2 standard was established in 1971, 
uniform minimum monitoring requirements for SO2 monitoring 
did not appear until May 1979. From the time of the implementation of 
the 1979 monitoring rule through 2008, the SO2 network has 
steadily decreased in size from approximately 1496 sites in 1980 to the 
approximately 488 sites operating in 2008. At present, except for 
SO2 monitoring required at National Core Monitoring Stations 
(NCore stations), there are no minimum monitoring requirements for 
SO2 in 40 CFR part 58 Appendix D, other than a requirement 
for EPA Regional Administrator approval before removing any existing 
monitors and that any ongoing SO2 monitoring must have at 
least one monitor sited to measure the maximum concentration of 
SO2 in that area. EPA removed the specific minimum 
monitoring requirements for SO2 in the 2006 monitoring rule 
revisions, based on the fact that there were no SO2 
nonattainment areas at that time, coupled with trends evidence showing 
an increasing gap between national average SO2 
concentrations and the current 24-hour and annual standards. 
Additionally, the minimum requirements were removed to provide State, 
local, and tribal air monitoring agencies flexibility in meeting higher 
priority monitoring needs for pollutants such as ozone and 
PM2.5, or implementing the new multi-pollutant sites (NCore 
network) required by the 2006 rule revisions, by allowing them to 
discontinue lower priority monitoring. More information on 
SO2 monitoring can be found in section III.

B. Health effects information

    During the last review, EPA retained the current 24-hour and annual 
averaging times for the primary SO2 NAAQS. The 24-hour NAAQS 
was largely based on epidemiologic studies that observed associations 
between 24-hour average SO2 levels and adverse respiratory 
effects and daily mortality (EPA 1982, 1994a, 1994b). The annual 
standard was supported by a few epidemiologic studies that found an 
association between adverse respiratory effects and annual average 
SO2 concentrations (EPA 1982, 1994a, 1994b). However, it was 
noted that in the locations where these epidemiologic studies were 
conducted, high SO2 levels were usually accompanied by high 
levels of PM, thus making it difficult to disentangle the individual 
contribution each pollutant had on these health outcomes. Moreover, EPA 
noted that rather than 24-hour or annual average SO2 levels, 
the health effects observed in these studies may have been related, at 
least in part, to the occurrence of shorter-term peaks of 
SO2 within a 24-hour period (53 FR 14930; April 26, 1988).
    In the current review, the ISA along with its associated annexes, 
provided a comprehensive review and assessment of the scientific 
evidence related to the health effects associated with SO2 
exposures. For these health effects, the ISA characterized judgments 
about causality with a hierarchy that contains five levels (ISA, 
section 1-3): sufficient to infer a causal relationship, sufficient to 
infer a likely causal relationship (i.e., more likely than not), 
suggestive but not sufficient to infer a causal relationship, 
inadequate to infer the presence or absence of a causal relationship, 
and suggestive of no causal relationship. Judgments about causality 
were informed by a series of aspects that are based on those set forth 
by Sir Austin Bradford Hill in 1965 (ISA, Table 1-1). These aspects 
include strength of the observed association, availability of 
experimental evidence, consistency of the observed association, 
biological plausibility, coherence of the evidence, temporal 
relationship of the observed association, and the presence of an 
exposure-response relationship.
    Judgments made in the ISA about the extent to which relationships 
between various health endpoints and exposure to SO2 are 
likely causal have been informed by several factors. As discussed in 
the ISA in section 1.3, these factors include the nature of the 
evidence (i.e., controlled human exposure, epidemiologic, and/or 
toxicological studies) and the weight of evidence. The weight of 
evidence takes into account such considerations as biological 
plausibility, coherence of the evidence, strength of associations, and 
consistency of the evidence. Controlled human exposure studies provide 
directly applicable information for determining causality because these 
studies are not limited by differences in dosimetry and species 
sensitivity, which would need to be addressed in extrapolating animal 
toxicology data to human health effects, and because they provide data 
relating health effects specifically to SO2 exposures, in 
the absence of the co-occurring pollutants present in ambient air. 
Epidemiologic studies provide evidence of associations between 
SO2 concentrations and more serious health endpoints (e.g., 
hospital admissions and emergency department visits) that cannot be 
assessed in controlled human exposure studies. For these studies the 
degree of uncertainty introduced by confounding variables (e.g., other 
pollutants) affects the level of confidence that the health effects 
being investigated are attributable to

[[Page 64816]]

SO2 exposures alone and/or in combination with co-occurring 
pollutants.
    In using a weight of evidence approach to inform judgments about 
the degree of confidence that various health effects are likely to be 
caused by exposure to SO2, confidence increases with the 
number of studies consistently reporting a particular health endpoint, 
with increasing support for the biological plausibility of the health 
effects, and with the strength and coherence of the evidence. 
Conclusions regarding biological plausibility, consistency, and 
coherence of evidence of SO2-related health effects are 
drawn from the integration of epidemiologic studies with controlled 
human exposure studies and with mechanistic information from animal 
toxicological studies. As discussed below, the weight of evidence is 
strongest for respiratory morbidity endpoints (e.g., lung function 
decrements, respiratory symptoms, hospital admissions, and emergency 
department visits) associated with short-term (5-minutes to 24-hours) 
exposure to ambient SO2.
    For epidemiologic studies, strength of association refers to the 
magnitude of the association and its statistical strength, which 
includes assessment of both effect estimate size and precision. In 
general, when associations yield large relative risk estimates, it is 
less likely that the association could be completely accounted for by a 
potential confounder or some other bias. Consistency refers to the 
persistent finding of an association between exposure and outcome in 
multiple studies of adequate power in different persons, places, 
circumstances and times.
    Being mindful of the considerations discussed above, the ISA 
concluded that there was sufficient evidence to infer a causal 
relationship between respiratory morbidity and short-term (5-minutes to 
24-hours) exposure to SO2 (ISA, section 5.2). The ISA based 
this conclusion on the consistency, coherence, and plausibility of 
findings observed in controlled human exposure studies of 5-10 minutes, 
epidemiologic studies mostly using 1-hour daily maximum and 24-hour 
average SO2 concentrations, and animal toxicological studies 
using exposures of minutes to hours (ISA, section 5.2). The ISA judged 
evidence of an association between SO2 exposure and other 
health categories to be less convincing; other associations were judged 
to be suggestive but not sufficient to infer a causal relationship 
(i.e., short-term exposure to SO2 and mortality) or 
inadequate to infer the presence or absence of a causal relationship 
(i.e., short-term exposure to SO2 and cardiovascular 
morbidity, and long-term exposure to SO2 and respiratory 
morbidity, other morbidity, and mortality). Key conclusions from the 
ISA are described in greater detail in Table 5-3 of the ISA.
    As summarized above, the ISA found a ``causal'' association between 
short-term (5 minutes to 24 hour) exposure to SO2 and 
respiratory morbidity. The evidence leading to this conclusion will be 
discussed throughout this section as well as in the context of the 
adequacy of the current and proposed alternative standards (see section 
II.E and II.F) The ISA also found ``suggestive but not sufficient'' 
evidence to infer a causal relationship between short-term 
SO2 exposure and mortality. EPA considered this suggestive 
evidence within the context of proposing a new 1-hour averaging time 
(see section II.F.2). The association between short- and long-term 
SO2 exposure and other health categories was found to be 
inadequate to infer the presence or absence of a causal relationship 
and thus, will not be discussed in detail in this notice.
    Section II.B.1 discusses the results of controlled human exposure 
studies demonstrating respiratory effects in exercising asthmatics 
following 5-10 minute exposures to SO2, and conclusions in 
the REA regarding the adversity of such effects. Section II.B.2 
discusses the respiratory effects reported in U.S. epidemiologic 
studies of respiratory symptoms, as well as emergency department visits 
and hospital admissions for all respiratory causes and asthma. Section 
II.B.3 discusses ISA conclusions regarding short-term (5 minutes to 24-
hours) exposure to SO2 and respiratory effects, and section 
II.B.4 discusses long-term SO2 exposure and potentially 
adverse health effects. Finally, section II.B.5 discusses 
SO2-related impacts on public health.
1. Respiratory effects and 5-10 minute exposure to SO2
    As noted above, the ISA concluded that there was sufficient 
evidence to infer a causal relationship between respiratory morbidity 
and short-term (5-minutes to 24-hours) exposure to SO2 (ISA, 
section 5.2). This determination was primarily based on controlled 
human exposure studies demonstrating a relationship between 5-10 minute 
peak SO2 exposures and adverse effects on the respiratory 
system in exercising asthmatics. The ISA described the controlled human 
exposure results as being the ``definitive evidence'' for its causal 
finding (ISA, section 5.2; p. 5-2).
    Since the last review, several additional controlled human exposure 
studies have been published that provide supportive evidence of 
SO2-induced decrements in lung function and increases in 
respiratory symptoms among exercising asthmatics (see ISA, Annex Table 
D-2). However, based in part on recent guidance from the American 
Thoracic Society (ATS) regarding what constitutes an adverse health 
effect of air pollution (ATS, 2000), a much larger body of key older 
studies described in the prior review were analyzed in the ISA along 
with studies published since the last review. In their official 
statement, the ATS concluded that an air pollution-induced shift in a 
population distribution of a given health-related endpoint (e.g., lung 
function) should be considered adverse, even if this shift does not 
result in the immediate occurrence of illness in any one individual in 
the population (ATS 2000). The ATS also recommended that transient loss 
in lung function with accompanying respiratory symptoms attributable to 
air pollution should be considered adverse. However, it was noted in 
the ISA that symptom perception is highly variable among asthmatics 
even during severe episodes of asthmatic bronchoconstriction, and that 
an asymptomatic decrease in lung function may pose a significant health 
risk to asthmatic individuals as it is less likely that these 
individuals will seek treatment (ISA, section 3.1.3). Therefore, 
whereas the conclusions in the prior review of the SO2 NAAQS 
were based on SO2 exposure concentrations which resulted in 
large decrements in lung function and moderate to severe respiratory 
symptoms, the ISA's current review of data from controlled human 
exposure studies focused on moderate to large SO2-induced 
decrements in lung function and/or respiratory symptoms ranging from 
mild (perceptible wheeze or chest tightness) to severe (breathing 
distress requiring the use of a bronchodilator). See also section 
II.B.1.c below discussing adversity of effects. Key controlled human 
exposure studies of respiratory symptoms and lung function are 
described briefly below and in more detail in section 3.1.3 of the ISA.
a. Respiratory symptoms
    Numerous free-breathing controlled human exposure studies have 
evaluated respiratory symptoms (e.g. cough, wheeze, or chest tightness) 
in exercising asthmatic following 5-10 minute SO2 exposures. 
Linn et al. (1983) reported that 5-minute exposures to SO2 
levels as low as 400 ppb resulted in exercising asthmatics experiencing 
statistically significant increases in respiratory symptoms (e.g., 
wheeze, chest tightness,

[[Page 64817]]

cough, substernal irritation). In a separate study, exercising 
asthmatics exhibited respiratory symptoms following a 10-minute 
exposure to 400-600 ppb SO2 (Linn et al., (1987); Smith 
(1993)). Gong et al., (1995) exposed SO2-sensitive 
asthmatics to 0, 500 and 1000 ppb SO2 for 10 minutes while 
performing different levels of exercise (light, medium, or heavy) and 
reported that respiratory symptoms increased with increasing 
SO2 concentrations. The authors further reported that 
exposure to 500 ppb SO2 during light exercise evoked a more 
severe symptomatic response than heavy exercise in clean air.
    In addition to these free breathing chamber results described 
above, studies using mouthpiece exposure systems have reported 
respiratory symptoms within minutes of SO2 exposure.\4\ 
Balmes et al. (1987) reported that 7 out of 8 exercising asthmatics 
developed respiratory symptoms following a 500 ppb 3-minute exposure to 
SO2 via mouthpiece (ISA section 3.1.3.1). In an additional 
study, Trenga et al. (1999) reported increases in respiratory symptoms 
in exercising asthmatics following 10-minute exposures to 500 ppb 
SO2. Although not directly comparable to the free-breathing 
chamber results described above, these mouthpiece exposure results 
nonetheless support an association between SO2 exposure and 
respiratory symptoms.
---------------------------------------------------------------------------

    \4\ Studies utilizing a mouthpiece exposure system cannot be 
directly compared to studies involving freely breathing subjects, as 
nasal absorption of SO2 is bypassed during oral 
breathing, thus allowing a greater fraction of inhaled 
SO2 to reach the tracheobronchial airways. As a result, 
individuals exposed to SO2 through a mouthpiece are 
likely to experience greater respiratory effects from a given 
SO2 exposure.
---------------------------------------------------------------------------

b. Lung function decrements
    The ISA found that in free-breathing chamber studies, asthmatic 
individuals exposed to SO2 concentrations as low as 200-300 
ppb for 5-10 minutes during exercise have been shown to experience 
moderate or greater bronchoconstriction, measured as a decrease in 
Forced Expiratory Volume in the first second (FEV1) of >= 
15%, or an increase in specific airway resistance (sRaw) of >= 100% 
after correction for exercise-induced responses in clean air (Bethel et 
al., 1985; Linn et al., 1983, 1987; 1988; 1990; Roger et al., 1985).\5\ 
In addition, the ISA concluded that among asthmatics, both the 
percentage of individuals affected, and the severity of the response 
increases with increasing SO2 concentrations. That is, at 
concentrations ranging from 200-300 ppb, the lowest levels tested in 
free breathing chamber studies,\6\ approximately 5-30% of exercising 
asthmatics experience moderate or greater decrements in lung function 
(ISA, Table 3-1). At concentrations of 400-600 ppb, moderate or greater 
decrements in lung function occur in approximately 20-60% of exercising 
asthmatics, and compared to exposures at 200-300 ppb, a larger 
percentage of asthmatics experience severe decrements in lung function 
(i.e., >= 200% increase in sRaw, and/or a >= 20% decrease in 
FEV1) (ISA, Table 3-1). The ISA also noted that at 
SO2 concentrations >= 400 ppb, moderate or greater 
decrements in lung function are frequently accompanied by respiratory 
symptoms (e.g., cough, wheeze, chest tightness, shortness of breath) 
(ISA, Table 3-1). Further analysis and discussion of the individual 
studies presented above can be found in Sections 3.1.1 to 3.1.3.5 of 
the ISA.
---------------------------------------------------------------------------

    \5\ FEV1 and sRaw are measures of 
bronchoconstriction. Decreases in FEV1 or increases in 
sRaw can result in difficulty breathing.
    \6\ The ISA cites one chamber study with intermittent exercise 
where healthy and asthmatic children were exposed to 100 ppb 
SO2 in a mixture with ozone and sulfuric acid. The ISA 
notes that compared to exposure to filtered air, exposure to the 
pollutant mix did not result in statistically significant changes in 
lung function or respiratory symptoms (ISA section 3.1.3.4)
---------------------------------------------------------------------------

    In addition to the evidence from free-breathing chamber studies, 
the ISA notes very limited evidence of decrements in lung function in 
exercising asthmatics exposed to lower levels of SO2 via 
mouthpiece. That is, the ISA cites two studies where some exercising 
asthmatics had small changes in FEV1 or sRaw following 
exposure to 100 ppb SO2 via mouthpiece (Koenig et al., 1990 
and Sheppard et al., 1981).
c. Adversity of 5-10 minute respiratory effects
    The ATS has previously defined adverse respiratory health effects 
as ``medically significant physiologic changes generally evidenced by 
one or more of the following: (1) Interference with the normal activity 
of the affected person or persons, (2) episodic respiratory illness, 
(3) incapacitating illness, (4) permanent respiratory injury, and/or 
(5) progressive respiratory dysfunction'' (ATS 1985). The ATS has also 
recommended that transient loss in lung function with accompanying 
respiratory symptoms, or detectable effects of air pollution on 
clinical measures (e.g., medication use) be considered adverse (ATS 
1985). In addition, the REA noted that during the last O3 
NAAQS review, the Criteria Document (CD) and Staff Paper indicated that 
for many people with lung disease (e.g., asthma), even moderate 
decrements in lung function (e.g., FEV1 decrements > 10% but 
< 20% and/or >= 100% increases in sRaw) or respiratory symptoms would 
likely interfere with normal activities and result in additional and 
more frequent use of medication (EPA 2006, EPA 2007d). The REA also 
noted that CASAC has previously indicated that in the context of 
standard setting, a focus on the lower end of the range of moderate 
functional responses is most appropriate for estimating potentially 
adverse lung function decrements in people with lung disease (73 
FR16463). Finally, the REA noted that in the current SO2 
NAAQS review, clinicians on the CASAC Panel again advised that moderate 
or greater decrements in lung function can be clinically significant in 
some individuals with respiratory disease (hearing transcripts from 
USEPA Clean Air Scientific Advisory Committee (CASAC), July 30-31, 
2008, Sulfur Oxides-Health Criteria (part 3 of 4) pages 211-213).\7\
---------------------------------------------------------------------------

    \7\ These transcripts can be found in Docket ID No. EPA-HQ-ORD-
2006-0260. Available at www.regulations.gov.
---------------------------------------------------------------------------

    As previously mentioned, the ATS published updated guidelines on 
what constitutes an adverse health effect of air pollution in 2000 
(ATS, 2000). Among other considerations, the 2000 guidelines stated 
that measurable negative effects of air pollution on quality of life 
should be considered adverse (ATS 2000). These updated guidelines also 
indicated that exposure to air pollution that increases the risk of an 
adverse effect to the entire population is adverse, even though it may 
not increase the risk of any individual to an unacceptable level (ATS 
2000). For example, a population of asthmatics could have a 
distribution of lung function such that no individual has a level 
associated with significant impairment. Exposure to air pollution could 
shift the distribution to lower levels that still do not bring any 
individual to a level that is associated with clinically relevant 
effects. However, this would be considered adverse because individuals 
within the population would have diminished reserve function, and 
therefore would be at increased risk if affected by another agent (ATS 
2000).
    At SO2 concentrations >= 400 ppb, controlled human 
exposure studies have reported decrements in lung function that are 
often statistically significant at the group mean level, and that are 
frequently accompanied by respiratory symptoms. Being mindful that the 
ATS

[[Page 64818]]

guidelines described above specifically indicate decrements in lung 
function with accompanying respiratory symptoms as being adverse, 
exposure to 5-10 minute SO2 concentrations >= 400 ppb are 
clearly adverse.
    The ISA has also reported that exposure to SO2 
concentrations as low as 200-300 ppb for 5-10 minutes results in 
approximately 5-30% of exercising asthmatics experiencing moderate or 
greater decrements in lung function (defined in terms of a >= 15% 
decline in FEV1 or 100% increase in sRaw; ISA, Table 3-1). 
Considering the 2000 ATS guidelines mentioned above, the REA found that 
these results could reasonably indicate an SO2-induced shift 
in these lung function measurements for this population. As a result, a 
significant percentage of exercising asthmatics exposed to 
SO2 concentrations as low as 200 ppb would have diminished 
reserve lung function and would be at greater risk if affected by 
another respiratory agent (e.g., viral infection). Importantly, 
diminished reserve lung function in a population that is attributable 
to air pollution is an adverse effect under ATS guidance. In addition 
to the 2000 ATS guidelines, the REA was also mindful of: (1) Previous 
CASAC recommendations (Henderson 2006) and NAAQS review conclusions 
(EPA 2006, EPA 2007d) indicating that moderate decrements in lung 
function can be clinically significant in some asthmatics; and (2) 
subjects participating in these controlled human exposure studies not 
likely including the most severe asthmatics. Taken together, the REA 
concluded that exposure to SO2 concentrations at least as 
low as 200 ppb can result in adverse health effects in asthmatics.
    Importantly, the final REA noted that this conclusion was in 
agreement with CASAC comments following the first draft SO2 
REA (REA section 4.3). The first draft SO2 REA focused its 
analyses on exposures and risk associated with 5-minute SO2 
concentrations >= 400 ppb. However, CASAC strongly advised the 
Administrator that effects to exercising asthmatics at levels at least 
as low as 200 ppb can be adverse, and thus, should be considered in the 
second draft and final REAs (Henderson 2008).
2. Respiratory effects and 1- to 24-hour exposure to SO2
    In addition to the controlled human exposure evidence described 
above, the ISA based its causal finding of an association between 
short-term (5-minutes to 24-hours) exposure to SO2 and 
respiratory morbidity on results from epidemiologic studies of 
respiratory symptoms, as well as ED visits and hospital admissions for 
all respiratory causes and asthma. More specifically, the ISA describes 
the results from these epidemiologic studies as providing ``supporting 
evidence'' for its determination of causality (ISA section 5.2). Key 
epidemiologic studies of respiratory symptoms, as well as ED visits and 
hospital admissions are discussed below.
a. Respiratory symptoms
    The ISA found that the strongest epidemiologic evidence of an 
association between short-term SO2 concentrations and 
respiratory symptoms was in children. Studies conducted in North 
America and abroad generally reported positive associations between 
ambient SO2 concentrations and respiratory symptoms in 
children. U.S. studies of respiratory symptoms in children (identified 
from Table 5-4 of the ISA), including three large multi-city studies, 
are described briefly below and in more detail in section 3.1.4.1 of 
the ISA.
    The National Cooperative Inner-City Asthma Study (NCICAS, Mortimer 
et al. 2002) included asthmatic children (n = 846) from eight U.S. 
urban areas and examined the relationship between respiratory symptoms 
and summertime air pollution levels. The strongest associations were 
found between morning symptoms (e.g., morning cough) and the median 3-
hour average SO2 concentrations during morning hours (8 a.m. 
to 11 a.m.)--following a 1- to 2-day lag (ISA, Figure 3-2). Three-hour 
average concentrations in the morning hours ranged from 17 ppb in 
Detroit to 37 ppb in East Harlem, NY. This relationship remained robust 
and statistically significant in multi-pollutant models with ozone 
(O3), and nitrogen dioxide (NO2). When 
PM10 was also added to the model, the effect estimate 
remained relatively unchanged, although was no longer statistically 
significant (ISA, Figure 3-2). However, the ISA noted that the loss of 
statistical significance could have been the result of reduced 
statistical power since only three of the eight cities were included in 
the multi-pollutant analysis with PM (ISA, section 3.1.4.1).
    The Childhood Asthma Management Program (CAMP, Schildcrout et al. 
2006) examined the association between ambient air pollution and asthma 
exacerbations in children (n = 990) from eight North American cities. 
The median 24-hour average SO2 concentrations (collected in 
seven of the eight study locations) ranged from 2.2 ppb in San Diego to 
7.4 ppb in St. Louis. Positive associations with an increased risk of 
asthma symptoms were observed at all lags, but only the association at 
the 3-day moving average was statistically significant (ISA, Figure 3-
3). In joint-pollutant models with carbon monoxide (CO) and 
NO2, the 3-day moving average effect estimates remained 
robust and statistically significant. In a joint-pollutant model with 
PM10, the 3-day moving average effect estimate remained 
relatively unchanged, but was no longer statistically significant (ISA 
Figure 3-3).
    A longitudinal study of schoolchildren (n = 1,844) during the 
summer months from the Harvard Six Cities Study suggested that the 
association between SO2 and respiratory symptoms may 
potentially be confounded by PM10 (Schwartz et al., 1994). 
It should be noted that unlike the NCICAS and CAMP studies, this study 
was not limited to asthmatic children. The median 24-hour average 
SO2 concentration during this period was 4.1 ppb. 
SO2 concentrations were found to be statistically 
significantly associated with cough incidence and lower respiratory 
symptoms in single pollutant models. However, the effect of 
SO2 was substantially reduced and no longer statistically 
significant after adjustment for PM10 in a co-pollutant 
model. The ISA noted that because PM10 concentrations were 
correlated strongly to SO2-derived sulfate particles (r = 
0.80), the reduced SO2 effect estimate may indicate that for 
PM10 dominated by fine sulfate particles, PM10 
has a slightly stronger association than SO2 to cough 
incidence and lower respiratory symptoms (ISA, section 3.1.4.1.1).
    In addition to the three U.S. multi-city studies mentioned above, 
evidence of an association between ambient SO2 and 
respiratory symptoms in children was found in two additional U.S. 
respiratory symptom studies. Delfino et al., (2003) reported a 
statistically significant positive association between 1-hour daily 
maximum SO2 concentrations in Los Angeles and respiratory 
symptoms in Hispanic children with asthma (n = 22). Similarly, Neas et 
al., (1995) reported a positive association between 12-hour average 
SO2 concentrations in Uniontown, PA and incidence of evening 
cough in 4th and 5th graders (n = 83; ISA section 3.1.4.1). Neither of 
these single city studies employed multi-pollutant models, but given 
the consistency of results with other epidemiologic evidence, they 
nonetheless support the association between ambient SO2 
concentrations and respiratory symptoms in children.

[[Page 64819]]

b. Emergency department visits and hospitalizations
    Respiratory causes for ED and hospitalization visits typically 
include asthma, pneumonia, Chronic Obstructive Pulmonary Disorder 
(COPD), upper and lower respiratory infections, as well as other minor 
categories. Since the last review, there have been more than 50 peer 
reviewed epidemiologic studies published worldwide and overall, the ISA 
concluded that these studies provide evidence to support an association 
between ambient SO2 concentrations and ED visits and 
hospitalizations for all respiratory causes and asthma (ISA, section 
3.1.4.6). Notably, the ISA also found that when analyses of ED visit 
and hospitalizations for all respiratory causes were restricted by age, 
the results among children (0-14 years) and older adults (65+ years) 
were mainly positive, but not always statistically significant (ISA, 
section 3.1.4.6). In these same studies, when all age groups were 
combined, the ISA found that the results were mainly positive; however, 
the excess risk estimates were generally smaller compared to children 
and older adults (ISA, Figure 3-6). Results from key ED visit and 
hospital admission studies conducted in the U.S. are described in 
general below, and a more detailed discussion of both the U.S. and 
international literature can be found in the ISA (ISA, section 
3.1.4.6).
    Of the respiratory ED visit and hospital admission studies reviewed 
in the ISA, 10 key studies were conducted in the United States (ISA, 
Table 5-5). Of these 10 studies, three evaluated associations with 
SO2 using multi-pollutant models (Schwartz et al., (1995) in 
Tacoma, WA and New Haven CT; New York Department of Health (NYDOH), 
(2006) in Bronx and Manhattan, NY; and Ito et al., (2007) in New York 
City), while seven studies evaluated the SO2 effect using 
only single pollutant models (Wilson et al., (2005) in Manchester, NH 
and Portland, ME; Peel et al., (2005) in Atlanta, GA; Tolbert et al., 
(2007) in Atlanta GA; Jaffe et al., (2003) in Cleveland, Cincinnati and 
Columbus, OH; Schwartz et al., (1996) in Cleveland OH; Sheppard et al., 
(2003) in Seattle, WA; and Lin et al., (2004) in Bronx, NY). Taken 
together, these studies generally reported positive, but frequently not 
statistically significant associations between ambient SO2 
and ED visits and hospital admissions for all respiratory causes and 
for asthma. With regard to U.S. studies employing multi-pollutant 
models, results reported in Bronx, NY (NYDOH 2006) and New York City, 
NY (Ito et al., 2007) remained robust and statistically significant in 
the presence of PM2.5, [10% (4, 16) and 29.6% (14.3, 46.8), 
respectively] while in New Haven, CT (Schwartz et al., 1995) results 
remained robust and statistically significant in the presence of 
PM10 [2% (1, 3)]. However, in Manhattan, NY (NYDOH 2006) 
results reported from single, and multi-pollutant models were negative 
(although not statistically significantly negative), and in Tacoma, WA 
(Schwartz et al., 1995) the SO2 effect estimate [3% (1,6)] 
was reduced and no longer statistically significant in a multi-
pollutant model with PM10 [-1% (-4, 3)]. In models including 
gaseous co-pollutants, the SO2 effect estimate in the Bronx, 
NY (NYDOH 2006) remained statistically significant in the presence of 
NO2 [10% (4,15)], while in NYC (Ito et al., 2007) the 
SO2 effect estimate remained statistically significant in 
the presence of O3 [26.8% (13.7, 41.5)] and CO [31.1% (16.7, 
47.2)], but not in the presence of NO2 [-1.6% (-16.7, 
16.1)].
3. ISA conclusions regarding short-term (5-minutes to 24-hours) 
SO2 exposures
    As noted above, the ISA found that moderate or greater decrements 
in lung function occur in some exercising asthmatics exposed to 
SO2 concentrations as low as 200-300 ppb for 5-10 minutes. 
The ISA also found that among asthmatics, both the percentage of 
individuals affected, and the severity of the response increased with 
increasing SO2 concentrations. That is, at 5-10 minute 
concentrations ranging from 200-300 ppb, the lowest levels tested in 
free breathing chamber studies, approximately 5-30% percent of 
exercising asthmatics experienced moderate or greater decrements in 
lung function (ISA, Table 3-1). At concentrations of 400-600 ppb, 
moderate or greater decrements in lung function occurred in 
approximately 20-60% of exercising asthmatics, and compared to 
exposures at 200-300 ppb, a larger percentage of asthmatics experienced 
severe decrements in lung function (i.e., >=200% increase in sRaw, and/
or a >=20% decrease in FEV1) (ISA, Table 3-1). Moreover, at 
SO2 concentrations >=400 ppb (5-10 minute exposures), 
moderate or greater decrements in lung function were frequently 
accompanied by respiratory symptoms.
    In addition, the ISA concluded that epidemiologic studies of 
respiratory symptoms in children, as well as emergency department 
visits and hospitalizations for all respiratory causes and asthma were 
consistent and coherent. This evidence was consistent in that 
associations were reported in studies conducted in numerous locations 
and with a variety of methodological approaches (ISA, section 5.2). It 
was coherent in that respiratory symptom results from epidemiologic 
studies of short-term (predominantly 1-hour daily maximum or 24-hour 
average) SO2 concentrations were generally in agreement with 
respiratory symptom results from controlled human exposure studies of 
5-10 minutes. These results were also coherent in that the respiratory 
effects observed in controlled human exposure studies of 5-10 minutes 
provided a basis for a progression of respiratory morbidity that could 
lead to the ED visits and hospitalizations observed in epidemiologic 
studies (ISA, section 5.2). In addition, the ISA concluded that U.S. 
and international epidemiologic studies employing multi-pollutant 
models suggested that SO2 had a generally independent effect 
on respiratory morbidity outcomes (ISA, section 5.2).
    The ISA also found that the respiratory effects of SO2 
were consistent with the mode of action as it is currently understood 
from animal toxicological and human exposure studies (ISA, section 
5.2). The immediate effect of SO2 on the respiratory system 
is bronchoconstriction. This response is mediated by chemosensitive 
receptors in the tracheobronchial tree. Activation of these receptors 
triggers central nervous system reflexes that result in 
bronchoconstriction and respiratory symptoms that are often followed by 
rapid shallow breathing (ISA, section 5.2). The ISA noted that 
asthmatics are likely more sensitive to the respiratory effects of 
SO2 due to pre-existing inflammation associated with the 
disease. For example, pre-existing inflammation may lead to enhanced 
release of inflammatory mediators, and/or enhanced sensitization of the 
chemosensitive receptors (ISA, section 5.2).
    Taken together, the ISA concluded that the controlled human 
exposure, epidemiologic, and toxicological evidence supported its 
determination of a causal relationship between respiratory morbidity 
and short-term (5-minutes to 24-hours) exposure to SO2.
4. Health effects and long-term exposures to SO2
    There were numerous studies published since the last review 
examining possible associations between long-term SO2 
exposure and mortality and morbidity (respiratory morbidity, 
carcinogenesis, adverse prenatal and neonatal outcomes)

[[Page 64820]]

endpoints. However, the ISA concluded that the evidence relating long-
term (weeks to years) SO2 exposure to adverse health effects 
was ``inadequate to infer the presence or absence of a causal 
relationship'' (ISA, Table 5-3). That is, the ISA found the long-term 
health evidence to be of insufficient quantity, quality, consistency, 
or statistical power to make a determination as to whether 
SO2 was truly associated with these health outcomes (ISA, 
Table 1-2).
5. SO2-related impacts on public health
    Interindividual variation in human responses to air pollutants 
indicates that some subpopulations are at increased risk for the 
detrimental effects of ambient exposure to SO2. The NAAQS 
are intended to provide an adequate margin of safety for both general 
populations and sensitive subpopulations, or those subgroups 
potentially at increased risk for health effects in response to ambient 
air pollution. To facilitate the identification of subpopulations at 
the greatest risk for SO2-related health effects, studies 
have identified factors that contribute to the susceptibility and/or 
vulnerability of an individual to SO2. Susceptible 
individuals are broadly defined as those with a greater likelihood of 
an adverse outcome given a specific exposure in comparison with the 
general population (American Lung Association, 2001). The 
susceptibility of an individual to SO2 can encompass a 
multitude of factors which represent normal developmental phases (e.g., 
age) or biologic attributes (e.g., gender); however, other factors 
(e.g., socioeconomic status (SES)) may influence the manifestation of 
disease and also increase an individual's susceptibility (American Lung 
Association, 2001). In addition, subpopulations may be vulnerable to 
SO2 in response to an increase in their exposure during 
certain windows of life (e.g., childhood or old age) or as a result of 
external factors (e.g., SES) that contribute to an individual being 
disproportionately exposed to higher concentrations than the general 
population. It should be noted that in some cases specific factors may 
affect both the susceptibility and vulnerability of a subpopulation to 
SO2. For example, a subpopulation that is characterized as 
having low SES may have less access to healthcare resulting in the 
manifestation of a disease, which increases their susceptibility to 
SO2, but they may also reside in a location that results in 
exposure to higher concentrations of SO2, increasing their 
vulnerability to SO2.
    To examine whether SO2 differentially affects certain 
subpopulations, stratified analyses are often conducted in 
epidemiologic investigations to identify the presence or absence of 
effect modification. A thorough evaluation of potential effect 
modifiers may help identify subpopulations that are more susceptible 
and/or vulnerable to SO2. These analyses require the proper 
identification of confounders and their subsequent adjustment in 
statistical models, which helps separate a spurious from a true causal 
association. Although the design of toxicological and human clinical 
studies does not allow for an extensive examination of effect 
modifiers, the use of animal models of disease and the study of 
individuals with underlying disease or genetic polymorphisms do allow 
for comparisons between subgroups. Therefore, the results from these 
studies, combined with those results obtained through stratified 
analyses in epidemiologic studies, contribute to the overall weight of 
evidence for the increased susceptibility and vulnerability of specific 
subpopulations to SO2. Those groups identified in the ISA to 
be potentially at greater risk of experiencing an adverse health effect 
from SO2 exposure are described in more detail below.
a. Pre-existing respiratory disease
    In human clinical studies, asthmatics have been shown to be more 
responsive to the respiratory effects of SO2 exposure than 
healthy non-asthmatics. Although SO2-attributable decrements 
in lung function have generally not been demonstrated at concentrations 
<= 1000 ppb in non-asthmatics, statistically significant increases in 
respiratory symptoms and decreases in lung function have consistently 
been observed in exercising asthmatics following 5-10 minute 
SO2 exposures at concentrations ranging from 400-600 ppb 
(ISA, section 4.2.1.1). Moderate or greater SO2-induced 
decrements in lung function have also consistently been observed at 
SO2 concentrations ranging from 200-300 ppb in some 
asthmatics. The ISA also noted that a number of epidemiologic studies 
have reported respiratory morbidity in asthmatics associated with 
ambient SO2 concentrations (ISA 4.2.1.1). For example, 
numerous epidemiologic studies have observed positive associations 
between ambient SO2 concentrations and ED visits and 
hospitalizations for asthma (ISA section 4.2.1.1). Overall, the ISA 
concluded that epidemiologic and controlled human exposure studies 
indicated that individuals with pre-existing respiratory diseases, 
particularly asthma, are at greater risk than the general population of 
experiencing SO2-associated health effects (ISA, section 
4.2.1.1).
b. Genetics
    The ISA noted that a consensus now exists among scientists that the 
potential for genetic factors to increase the risk of experiencing 
adverse health effects due to ambient air pollution merits serious 
consideration. Several criteria must be satisfied in selecting and 
establishing useful links between polymorphisms in candidate genes and 
adverse respiratory effects. First, the product of the candidate gene 
must be significantly involved in the pathogenesis of the effect of 
interest, which is often a complex trait with many determinants. 
Second, polymorphisms in the gene must produce a functional change in 
either the protein product or in the level of expression of the 
protein. Third, in epidemiologic studies, the issue of effect 
modification by other genes or environmental exposures must be 
carefully considered (ISA section 4.2.2).
    Although many studies have examined the association between genetic 
polymorphisms and susceptibility to air pollution in general, only one 
study has specifically examined the effects of SO2 exposure 
on genetically distinct subpopulations. Winterton et al. (2001) found a 
significant association between SO2-induced decrements in 
FEV1 and the homozygous wild-type allele in the promoter 
region of Tumor Necrosis Factor-[alpha] (TNF- [alpha]; AA, position-
308). However, the ISA concluded that the overall body of evidence was 
too limited to reach a conclusion regarding the effects of 
SO2 exposure on genetically distinct subpopulations at this 
time.
c. Age
    The ISA identified children (i.e., < 18 years of age) and older 
adults (i.e., > 65 years of age) as groups that are potentially at 
greater risk of experiencing SO2-associated adverse health 
effects. In children, the developing lung is prone to damage from 
environmental toxicants as it continues to develop through adolescence. 
The biological basis for increased risk in the elderly is unknown, but 
one hypothesis is that it may be related to changes in antioxidant 
defenses in the fluid lining the respiratory tract. The ISA found a 
number of epidemiologic studies that observed increased respiratory 
symptoms in children associated with increasing SO2 
concentrations. In addition, several studies have reported

[[Page 64821]]

that the excess risk estimates for ED visits and hospitalizations for 
all respiratory causes, and to a lesser extent asthma, associated with 
a 10-ppb increase in 24-hour average SO2 concentrations were 
higher for children and older adults than for all ages together (ISA, 
section 4.2.3). However, the ISA also noted that the evidence from 
controlled human exposure studies does not suggest that adolescents are 
either more or less at risk than adults to the respiratory effects of 
SO2, but rather adolescents may experience similar 
respiratory effects at a given exposure concentration (ISA, sections 
3.1.3.5 and 4.2.3).\8\ Overall, the ISA found that compared to the 
general population, there was limited evidence to suggest that children 
and older adults are at greater risk of experiencing SO2-
associated health effects (ISA, section 4.2.3).
---------------------------------------------------------------------------

    \8\ Very young children are not included in controlled human 
exposure studies and this absence of data on what is likely to be a 
sensitive life stage is a source of uncertainty for children's 
susceptibility.
---------------------------------------------------------------------------

d. Time spent outdoors
    Outdoor SO2 concentrations are generally much higher 
than indoor concentrations. Thus, the ISA noted that individuals who 
spend a significant amount of time outdoors are likely at greater risk 
of experiencing SO2-associated health effects than those who 
spend most of their time indoors (ISA section 4.2.5).
e. Ventilation rate
    Controlled human exposure studies have demonstrated that decrements 
in lung function and respiratory symptoms occur at significantly lower 
SO2 exposure levels in exercising subjects compared to 
resting subjects. As ventilation rate increases, breathing shifts from 
nasal to oronasal, thus resulting in greater uptake of SO2 
in the tracheobronchial airways due to the diminished absorption of 
SO2 in the nasal passages. Therefore, individuals who spend 
a significant amount of time at elevated ventilation rates (e.g. while 
playing, exercising, or working) are expected to be at greater risk of 
experiencing SO2-associated health effects (ISA section 
4.2.5).
f. Socioeconomic status
    There is limited evidence that increased risk to SO2 
exposure is associated with lower SES (ISA section 4.2.5). Finkelstein 
et al. (2003) found that among people with below-median income, the 
relative risk for above-median exposure to SO2 was 1.18 (95% 
CI: 1.11, 1.26); the corresponding relative risk among subjects with 
above-median income was 1.03 (95% CI: 0.83, 1.28). However, the ISA 
concluded that there was insufficient evidence to reach a conclusion 
regarding SES and exposure to SO2 at this time (ISA section 
4.2.5).
g. Size of at-risk populations
    Considering the size of the groups mentioned above, large 
proportions of the U.S. population are likely to have a relatively high 
risk of experiencing SO2-related health effects. In the 
United States, approximately 7% of adults and 9% of children have been 
diagnosed with asthma. Notably, the prevalence and severity of asthma 
is higher among certain ethnic or racial groups such as Puerto Ricans, 
American Indians, Alaskan Natives, and African Americans (EPA 2008b). 
Furthermore, a higher prevalence of asthma among persons of lower SES 
and an excess burden of asthma hospitalizations and mortality in 
minority and inner-city communities have been observed. In addition, 
population groups based on age comprise substantial segments of 
individuals that may be potentially at risk for SO2-related 
health impacts. Based on U.S. census data from 2000, about 72.3 million 
(26%) of the U.S. population are under 18 years of age, 18.3 million 
(7.4%) are under 5 years of age, and 35 million (12%) are 65 years of 
age or older. There is also concern for the large segment of the 
population that is potentially at risk to SO2-related health 
effects because of increased time spent outdoors at elevated 
ventilation rates (those who work or play outdoors). Overall, the 
considerable size of the population groups at risk indicates that 
exposure to ambient SO2 could have a significant impact on 
public health in the United States.
C. Human exposure and health risk characterization
    To put judgments about SO2-associated health effects 
into a broader public health context, EPA has drawn upon the results of 
the quantitative exposure and risk assessments. Judgments reflecting 
the nature of the evidence and the overall weight of the evidence are 
taken into consideration in these quantitative exposure and risk 
assessments, discussed below. These assessments provide estimates of 
the likelihood that asthmatics at moderate or greater exertion (e.g. 
while exercising) would experience SO2 exposures of 
potential concern as well as an estimate of the number and percent of 
exposed asthmatic individuals likely to experience SO2-
induced lung function responses (i.e., moderate or greater decrements 
in lung function defined in terms of sRaw or FEV1) under 
varying air quality scenarios (e.g., just meeting the current or 
alternative standards). These assessments also characterize the kind 
and degree of uncertainties inherent in such estimates.
    This section describes the approach taken in the REA to 
characterize SO2-related exposures and health risks. Goals 
of the REA included estimating short-term exposures and potential human 
health risks associated with (1) recent levels of ambient 
SO2; (2) SO2 levels adjusted to simulate just 
meeting the current standards; and (3) SO2 levels adjusted 
to simulate just meeting potential alternative 1-hour standards. This 
section discusses the scientific evidence from the ISA that was used as 
the basis for the risk characterization (II.C.1), the approaches used 
in characterizing exposures and risks (II.C.2), and important 
uncertainties associated with these analyses (II.C.3). The results of 
the exposure and risk analyses, as they relate to the current and 
potential alternative standards, are discussed in subsequent sections 
of this proposal (sections II.E and II.F, respectively).
1. Evidence base for the risk characterization
    As previously mentioned, the ISA concluded that the evidence for an 
association between respiratory morbidity and SO2 exposure 
was ``sufficient to infer a causal relationship'' (ISA, section 5.2) 
and that the ``definitive evidence'' for this conclusion was from the 
results of 5-10 minute controlled human exposure studies demonstrating 
decrements in lung function and/or respiratory symptoms in exercising 
asthmatics (ISA, section 5.2). Accordingly, the REA concluded that 
quantitative exposure and risk analyses should focus on 5-minute levels 
of SO2 in excess of potential health effect benchmark values 
derived from the controlled human exposure literature (REA, section 
6.2). These benchmark levels are not potential standards, but rather 
are concentrations which represent ``exposures of potential concern'' 
which are used in the analyses to estimate potential exposures and 
risks associated with 5-minute concentrations of SO2. In 
addition, although the REA concluded that the epidemiologic evidence 
was not appropriate for use in quantitative risk analyses (REA, section 
6.3), these studies were considered in the selection of potential 
alternative standards for use in the air quality, exposure and risk 
analyses (REA, chapter 5), as well as in

[[Page 64822]]

the REA's assessment of the adequacy of the current and potential 
alternative primary standards (REA, sections 10.3; 10.4; and 10.5).
    As mentioned above, the health effect benchmark values used in the 
REA were derived primarily from the ISA's evaluation of the 5-10 minute 
controlled human exposure literature. The ISA concluded that moderate 
or greater decrements in lung function occurred in approximately 5-30% 
of exercising asthmatics following exposure to 200-300 ppb 
SO2 for 5-10 minutes. As explained in section II.B.1.b, the 
ISA concluded that moderate or greater decrements in lung function 
occurred in approximately 20-60% of exercising asthmatics following 
exposure to 400-600 ppb SO2 for 5-10 minutes. The ISA also 
concluded that at SO2 concentrations >= 400 ppb, 
statistically significant moderate or greater decrements in lung 
function at the group mean level have often been reported and are 
frequently accompanied by respiratory symptoms (ISA, section 3.1.3.5).
    In addition to the health evidence from the ISA presented above, 
when considering potential health effect benchmark levels, the REA 
noted: (1) Subjects participating in human exposure studies typically 
do not include individuals who may be most susceptible to the 
respiratory effects of SO2, (e.g., the most severe 
asthmatics given the obvious ethical issues of subjecting such persons 
to the clinical tests) and (2) given that approximately 5-30% of 
exercising asthmatics experienced moderate or greater decrements in 
lung function following exposure to 200-300 ppb SO2 (the 
lowest levels tested in free-breathing chamber studies), it is likely 
that a percentage of exercising asthmatics would also experience 
similar decrements in lung function following exposure to levels lower 
than 200 ppb (REA, section 6.2). That is, the REA concluded that there 
was no evidence to suggest that 200 ppb represented a threshold level 
below which no adverse respiratory effects would occur (REA, section 
6.2). Moreover, the REA considered that small SO2-induced 
lung function decrements have been observed in exercising asthmatics at 
concentrations as low as 100 ppb when SO2 is administered 
via mouthpiece (ISA, section 3.1.3).
    Taken together, the REA concluded it appropriate to examine 
potential 5-minute benchmark values in the range of 100-400 ppb (REA, 
section 6.2). The lower end of the range considered the factors 
mentioned above, while the upper end of the range recognized that 400 
ppb represents the lowest concentration at which moderate or greater 
decrements in lung function are frequently accompanied by respiratory 
symptoms (REA, section 6.2): a combination of effects which would 
clearly be considered adverse under ATS guidelines (ATS, 1985).
    Although the analysis of exposures of potential concern were 
conducted using discrete benchmark levels (i.e., 100, 200, 300, 400 
ppb), EPA recognizes that there is no sharp breakpoint within the 
continuum ranging from at and above 400 ppb down to 100 ppb. In 
considering the concept of exposures of potential concern, it is 
important to balance concerns about the potential for health effects 
and their severity with the increasing uncertainty associated with our 
understanding of the likelihood of such effects at lower SO2 
levels. Within the context of this continuum, estimates of exposures of 
potential concern at discrete benchmark levels provide some perspective 
on the potential public health impacts of SO2-related health 
effects that have been demonstrated in controlled human exposure 
studies. They also help in understanding the extent to which such 
impacts could change by just meeting the current and potential 
alternative standards. However, estimates of the number of asthmatics 
likely to experience exposures of potential concern cannot be 
translated directly into quantitative estimates of the number of people 
likely to experience specific health effects. Due to individual 
variability in responsiveness, only a subset of asthmatics exposed at 
and above a specific benchmark level can be expected to experience 
health effects. The amount of weight to place on the estimates of 
exposures of potential concern at any of these benchmark levels depends 
in part on the weight of the scientific evidence concerning health 
effects associated with SO2 exposures at and above that 
benchmark level. Such public health policy judgments are embodied in 
the NAAQS standard setting criteria (i.e., standards that, in the 
judgment of the Administrator, are requisite to protect public health 
with an adequate margin of safety).
    Since exposures of potential concern cannot be directly translated 
into quantitative estimates of the number of individuals likely to 
experience specific health effects, the REA not only characterizes 
exposure and risks utilizing exposures of potential concern, but also 
uses information from the controlled human exposure literature to 
conduct a quantitative risk assessment. The quantitative risk 
assessment estimated the number and percentage of exposed asthmatics at 
moderate or greater exertion expected to experience a moderate or 
greater lung function response (in terms of a >= 100% increase in sRaw 
and/or a >= 15% decline in FEV1; see section II.C.2).
2. Overview of approaches
    As noted above, the purpose of the assessments described in the REA 
was to characterize air quality, exposures, and health risks associated 
with recent ambient levels of SO2, with SO2 
levels that could be associated with just meeting the current 
SO2 NAAQS, and with SO2 levels that could be 
associated with just meeting potential alternative standards. The REA 
utilizes three approaches to characterize health risks In the first 
approach, for each air quality scenario, statistically estimated \9\ 
and measured ambient 5-minute SO2 concentrations were 
compared to the 5-minute potential health effect benchmark levels 
discussed above which (as noted) were derived from the controlled human 
exposure literature (REA, chapter 7). In the second approach, modeled 
estimates of 5-minute exposures in asthmatics at moderate or greater 
exertion (e.g. while exercising) were compared to these 5-minute 
potential health effect benchmark levels. In the third approach, 
exposure-response relationships from individual level data from 
controlled human exposure studies were used in conjunction with the 
outputs of the exposure analysis to estimate health impacts under the 
air quality scenarios mentioned above. A brief description of these 
approaches is provided below and each approach is described in detail 
in chapters 7 through 9 of the REA.
---------------------------------------------------------------------------

    \9\ Benchmark values derived from the controlled human exposure 
literature were associated with a 5-minute averaging time. However, 
only 98 ambient monitors located in 13 states from 1997-2007 
reported measured 5-minute SO2 concentrations since such 
monitoring is not required (see section III). In contrast, 809 
monitors in 48 states, DC, Puerto Rico, and the Virgin Islands 
reported 1-hour SO2 concentrations over a similar time 
period. Therefore, to broaden analyses to areas where measured 5-
minute SO2 concentrations were not available, the REA 
utilized a statistical relationship to estimate the highest 5-minute 
level in an hour, given a reported 1-hour average SO2 
concentration (REA, section 6.4). Then, similar to measured 5-minute 
SO2 levels, statistically estimated 5-minute 
SO2 concentrations were compared to 5-minute potential 
health effect benchmark values.
---------------------------------------------------------------------------

    In the first approach, statistically estimated and actual measured 
5-minute ambient SO2 concentrations were compared to 5-
minute potential health effect benchmark levels (REA, chapter 7). The 
results generated from the air quality analysis were considered a broad 
characterization of national air

[[Page 64823]]

quality and human exposures that might be associated with these 5-
minute SO2 concentrations. An advantage of the air quality 
analysis is its relative simplicity; however, there is uncertainty 
associated with the assumption that SO2 air quality can 
serve as an adequate surrogate for total exposure to ambient 
SO2. Actual exposures might be influenced by factors not 
considered by this approach, including small scale spatial variability 
in ambient SO2 concentrations (which might not be captured 
by the network of fixed-site ambient monitors) and spatial/temporal 
variability in human activity patterns.
    In the second approach, an inhalation exposure model was used to 
generate more realistic estimates of personal exposures in asthmatics 
(REA, chapter 8). This analysis estimated temporally and spatially 
variable ambient 5-minute SO2 concentrations and simulated 
asthmatics contact with these pollutant concentrations while at 
moderate or greater exertion (i.e., while at elevated ventilation 
rates). The approach was designed to estimate exposures that are not 
necessarily represented by the existing ambient monitoring data. 
AERMOD, an EPA dispersion model, was used to estimate 1-hour ambient 
SO2 concentrations using emissions estimates from 
stationary, non-point, and port sources. The Air Pollutants Exposure 
(APEX) model, an EPA human exposure model, was then used to estimate 
population exposures using the estimated hourly census block level 
SO2 concentrations. From these 1-hour census block 
concentrations, 5-minute maximum SO2 concentrations within 
each hour were estimated using the statistical relationship mentioned 
above. A probabilistic approach was then used to model asthmatics' 
exposures considering: (1) Time spent in different microenvironments; 
(2) time spent at moderate or greater exertion; and (3) the variable 
SO2 concentrations that occur within these microenvironments 
across time, space, and microenvironment type. Estimates of personal 
exposure to 5-minute SO2 levels were then compared to the 5-
minute potential health benchmark levels (i.e., 5-minute benchmark 
levels of 100, 200, 300, and 400 ppb). This approach to assessing 
exposures was more resource intensive than using ambient levels as an 
indicator of exposure; therefore, the final REA included the analysis 
of two locations: St Louis and Greene County, MO. Although the 
geographic scope of this analysis was limited, the approach provided 
estimates of SO2 exposures in asthmatics and asthmatic 
children in St. Louis and Greene Counties and thus, served to 
complement the broader air quality characterization.
    For the characterization of risks in both the air quality analysis 
and the exposure modeling analysis described above, the REA used a 
range of 5-minute potential health effect benchmarks: 100, 200, 300, 
and 400 ppb. These benchmark values were compared to both 
SO2 air quality levels and to estimates of SO2 
exposure in asthmatics. When SO2 air quality was used as an 
indicator of exposure, a key output of the analysis was an estimate of 
the number of days per year specific locations experienced 
statistically estimated 5-minute daily maximum levels of SO2 
that exceeded one of these 5-minute potential health effect benchmarks. 
When personal exposures were simulated, the output of the analysis was 
an estimate of the number and percent of asthmatics and asthmatic 
children at risk for experiencing, at least once per year, a 
statistically estimated 5-minute daily maximum level of SO2 
of ambient origin in excess of one of these benchmarks. An advantage of 
using the benchmark approach to characterize health risks is that the 
effects observed in the controlled human exposure studies clearly 
result from SO2 exposure, so the benchmarks are reliable 
levels at which effects to asthmatics from exposure to SO2 
can occur. A limitation of this approach is that the magnitude of the 
SO2 effect on decrements in lung function and respiratory 
symptoms can vary considerably from individual to individual and thus, 
not all asthmatics would be expected to respond to the same levels of 
SO2 exposure. Therefore, numbers of exposures can be 
quantified more readily than the number of individuals experiencing 
SO2-induced lung function decrements and/or respiratory 
symptoms.
    The third approach was a quantitative risk assessment. This 
approach combined results from the exposure analysis (i.e., the number 
of exposed total asthmatics or asthmatic children while at moderate or 
greater exertion) with exposure-response functions derived from 
individual level data from controlled human exposure studies (see ISA, 
Table 3-1 and Johns (2009) \10\) to estimate the percentage and number 
of exposed asthmatics and asthmatic children likely to experience a 
moderate or greater lung function response (i.e., decrements in lung 
function defined in terms of FEV1 and sRaw) under the air 
quality scenarios mentioned above (REA, chapter 9). The advantage of 
this approach is that it recognizes that not all exposed asthmatics at 
moderate or greater exertion will have a lung function response. 
Moreover, it is advantageous in that rather than considering discrete 
potential health effect benchmark levels, it quantitatively estimates 
the number and percent of asthmatics and asthmatic children likely to 
experience a moderate or greater lung function response considering the 
entire distribution of personal exposures.
---------------------------------------------------------------------------

    \10\ EPA recently conducted a complete quality assurance review 
of all individual subject data. The results of this review did not 
substantively change any of the entries in ISA, Table 3-1, and did 
not in anyway affect the conclusions of the ISA (see Johns and 
Simmons, 2009).
---------------------------------------------------------------------------

3. Key limitations and uncertainties
    The way in which air quality, exposure, and risk results will 
inform ultimate decisions regarding the current and potential 
alternative SO2 standards will depend upon the weight placed 
on each of the analyses when uncertainties associated with those 
analyses are taken into consideration. Sources of uncertainty 
associated with each of the analyses (air quality, exposure, and 
quantitative risk) are briefly presented below and are described in 
more detail in chapters 7-9 of the REA.
    In the air quality analysis, the REA used ambient SO2 
data from both the limited number of monitors reporting 5-minute 
concentrations and the broader network of monitors reporting 1-hour 
concentrations of SO2 to characterize U.S. air quality. 
There was general agreement in the monitor site attributes and 
emissions sources potentially influencing ambient monitoring 
concentrations for each set of data analyzed. However, the REA noted 
that the greatest relative uncertainty was in the spatial 
representativeness of both the overall monitoring network and the 
subsets of monitors chosen for detailed analyses (REA, section 
7.4.2.4).
    An additional source of uncertainty in the air quality analysis is 
associated with the statistical model used to estimate 5-minute maximum 
SO2 concentrations at monitors that reported only 1-hour 
SO2 concentrations (REA, section 7.4.2.6). Cross-validation 
of statistically estimated 5-minute concentrations with the limited 
number of reported 5-minute SO2 measurements indicated that 
the greatest difference in the predicted versus observed numbers of 
benchmark exceedances occurred at the lower and upper tails of the 
distribution. However, the REA noted that overall, the results of the 
cross-validation analysis indicated reasonable model performance (REA, 
sections 10.3.3.1 and 10.5.2).
    The air quality characterization assumes that the ambient 
monitoring

[[Page 64824]]

data and the estimated days per year with exceedances of the specified 
benchmark levels can serve as an indicator of exposure. Longer-term 
personal SO2 exposure (i.e., days to weeks) concentrations 
are correlated with and are a fraction of ambient SO2 
concentrations. However, uncertainty remains in this relationship when 
considering short-term (i.e., 5-minute) averaging times because of the 
lack of comparable measurement data (REA, section 7.4.2.7).
    The St. Louis and Greene county exposure assessments were also 
associated with a number of key uncertainties that should be considered 
when interpreting the results with regard to decisions on the standard. 
Such uncertainties are highlighted below, and these, as well as other 
sources of uncertainty, are also discussed in greater depth in section 
8.11 of the REA.
    In the exposure analyses, it was necessary to derive an area source 
emission profile rather than use a default profile to improve the 
agreement between ambient measurements and model predicted 1-hour 
SO2 concentrations. The improved model performance reduces 
uncertainty in the 1-hour SO2 concentrations predictions, 
but nonetheless remains as an important uncertainty in the absence of 
actual local source emission profiles (REA, section 8.11.2).
    The St. Louis and Greene county exposure assessments were performed 
to better reflect both the temporal and spatial representation of 
ambient concentrations and to estimate the rate of contact of asthmatic 
individuals with 5-minute SO2 concentrations while engaged 
in moderate or greater exertion. Estimated annual average 
SO2 exposures in the two exposure modeling domains are 
consistent with long-term personal exposures (i.e., days to weeks) 
measured in other U.S. locations (REA, chapter 8). However, uncertainty 
remains in the estimated number of persons with 5-minute SO2 
concentrations above benchmark levels because of the lack of comparable 
measurement data, particularly considering both the short-term 
averaging time and geographic location (REA, section 8.11.2).
    In addition, although all 5-minute ambient SO2 
concentrations in the exposure analyses were estimated by the exposure 
model, each hour was comprised of the maximum 5-minute SO2 
concentration and eleven other 5-minute SO2 concentrations 
normalized to the 1-hour mean concentration. The REA assumed that this 
approach would reasonably estimate the number of individuals exposed to 
peak concentrations. Sensitivity analyses revealed that both the number 
of persons exposed and where peak exposures occur can vary when 
considering an actual 5-minute temporal profile (REA, Section 8.11.2)
    A number of key uncertainties should also be considered when 
interpreting the results of the St. Louis and Greene County risk 
assessment with regard to decisions on the standard. Such uncertainties 
associated with the St Louis and Greene County risk assessment are 
discussed briefly below and in greater depth in section 9.4 of the REA.
    In the quantitative risk assessment, it was necessary to estimate 
responses at SO2 levels below the lowest exposure levels 
used in the free-breathing controlled human exposure studies (i.e., 
below 200 ppb). Probabilistic exposure-response relationships were 
derived in the REA using two different functional forms (i.e., probit 
and 2-parameter logistic), but nonetheless there remains greater 
uncertainty in responses below 200 ppb because of the lack of 
comparable experimental data. Moreover, because the controlled human 
exposure studies used in the risk assessment involved only 
SO2 exposures, it was assumed in the REA that estimates of 
SO2-induced health responses are not affected by the 
presence of other pollutants (e.g., PM2.5, O3, 
NO2; REA, section 9.4).
    The risk assessment assumes that the SO2-induced 
responses for individuals are reproducible. The REA noted that this 
assumption had some support in that one study (Linn et al., 1987) 
exposed the same subjects on two occasions to 600 ppb and the authors 
reported a high degree of correlation while observing a much lower 
correlation for the lung function response observed in the clean air 
with exercise exposure (REA, section 9.4).
    Because the vast majority of controlled human exposure studies 
investigating lung function responses were conducted with adult 
subjects, the risk assessment relies on data from adult asthmatic 
subjects to estimate exposure-response relationships that have been 
applied to all asthmatic individuals, including children. The ISA 
(section 3.1.3.5) indicates that there is a strong body of evidence 
that suggests adolescents may experience many of the same respiratory 
effects at similar SO2 levels, but recognizes that these 
studies administered SO2 via inhalation through a mouthpiece 
(which can result in an increase in lung SO2 uptake) rather 
than in an exposure chamber. Therefore, the uncertainty is greater in 
the risk estimates for asthmatic children (REA, section 9.4) \11\.
---------------------------------------------------------------------------

    \11\ Very young children were not included in the controlled 
human exposure data which served as the basis for the exposure-
response relationships used in the risk assessment. This absence of 
data on what is likely to be a sensitive life stage is an additional 
source of uncertainty in the risk assessment.
---------------------------------------------------------------------------

D. Considerations in review of the standards

    This section presents the integrative synthesis of the evidence and 
information contained in the ISA and the REA with regard to the current 
and potential alternative standards. EPA notes that the final decision 
on retaining or revising the current primary SO2 standards 
is a public health policy judgment to be made by the Administrator. The 
Administrator's final decision will draw upon scientific information 
and analyses related to health effects, population exposures, and 
risks; as well as judgments about the appropriate response to the range 
of uncertainties that are inherent in the scientific evidence and 
analyses; and comments received in response to this proposal.
1. Background on the current standards
    There are currently two SO2 primary standards. The 24-
hour average standard is 0.14 ppm not to be exceeded more than once per 
year and the annual average standard is 0.03 ppm. In the last review of 
the SO2 NAAQS, both the 24-hour and annual standards were 
retained. The rationale for the retention of these standards is 
discussed briefly below.
    In the last review, retention of the 24-hour standard was based 
largely on epidemiologic studies conducted in London in the 1950s and 
1960s. The results of those studies suggested an association between 
24-hour average levels of SO2 and increased daily mortality 
and aggravation of bronchitis when in the presence of elevated levels 
of PM (53 FR 14927). Additional epidemiologic evidence suggested that 
elevated SO2 levels were associated with the possibility of 
small, reversible declines in children's lung function (53 FR 14927). 
However, it was noted that in the locations where these epidemiologic 
studies were conducted, high SO2 levels were usually 
accompanied by high levels of PM, thus making it difficult to 
disentangle the individual contribution each pollutant had on these 
health outcomes. It was also noted that rather than 24-hour average 
SO2 levels, the health effects observed in these studies may 
have been related, at least in part, to the

[[Page 64825]]

occurrence of shorter-term peaks of SO2 within a 24-hour 
period (53 FR 14927).
    Retention of the annual standard in the last review was largely 
based on an assessment of qualitative evidence gathered from a limited 
number of epidemiologic studies. The strongest evidence for an 
association between annual SO2 concentrations and adverse 
health effects in the 1982 AQCD was from a study conducted by Lunn et 
al (1967). The authors found that among children, a likely association 
existed between chronic upper and lower respiratory tract illnesses and 
annual SO2 levels of 70-100 ppb in the presence of 230-301 
[micro]g/m\3\ black smoke. Three additional studies described in the 
1986 Second Addendum also suggested that long-term exposure to 
SO2 was associated with adverse respiratory effects. 
Notably, studies conducted by Chapman et al. (1985) and Dodge et al. 
(1985) found associations between long-term SO2 
concentrations (with or without high particle concentrations) and cough 
in children and young adults. However, it was noted that there was 
considerable uncertainty associated with these studies because they 
were conducted in locations subject to high, short-term peak 
SO2 concentrations (i.e., locations near point sources); 
therefore it was difficult to discern whether this increase in cough 
was the result of long-term, low level SO2 exposure, or 
repeated short-term peak SO2 exposures.
    It was concluded in the last review that there was no quantitative 
rationale to support a specific range for an annual standard (EPA, 
1994b). However, it was also found that although no single 
epidemiologic study provided clear quantitative conclusions, there 
appeared to be some consistency across studies indicating the 
possibility of respiratory effects associated with long-term exposure 
to SO2 just above the level of the existing annual standard 
(EPA, 1994b). In addition, air quality analyses conducted during the 
last review indicated that the short-term standards being considered 
(1-hour and/or 24-hour) could not by themselves prevent long-term 
concentrations of SO2 from exceeding the level of the 
existing annual standard in several large urban areas. Ultimately, both 
the scientific evidence and the air quality analyses were used by the 
Administrator to conclude that retaining the existing annual standard 
was requisite to protect human health.\12\
---------------------------------------------------------------------------

    \12\ Section I.C above discusses potential standards considered 
but not adopted in the last review, notably some type of standard to 
deal with effects of 5 to 10 minute exposures.
---------------------------------------------------------------------------

2. Approach for reviewing the need to retain or revise the current 
standards
    The decision in the present review on whether the current 24-hour 
and/or annual standards are requisite to protect public health with an 
adequate margin of safety will be informed by a number of scientific 
studies and analyses that were not available in the 1996 review. 
Specifically, as discussed above (section II.B), a large number of 
epidemiologic studies have been published since the 1996 review. Many 
of these studies evaluated associations between SO2 and 
adverse respiratory endpoints (e.g., respiratory symptoms, emergency 
department visits, hospital admissions) in locations where 24-hour and 
annual average SO2 concentrations were below the levels 
allowed by the current standards. In addition, with respect to adverse 
health effects associated with 5-minute SO2 concentrations, 
the REA described estimates of SO2-associated health risks 
that could be present in counties that just meet the current 24-hour or 
annual standards, whichever was controlling in a given county.\13\ The 
approach for considering this scientific evidence and exposure/risk 
information is discussed below.
---------------------------------------------------------------------------

    \13\ As noted in the REA, the controlling standard by definition 
would be the standard that allows air quality to just meet either 
the annual concentration level of 30.4 ppb (i.e., the annual 
standard is the controlling standard) or the 2nd highest 24-hour 
concentration level of 144 ppb (i.e., the 24-hour standard is the 
controlling standard). The factor selected is derived from a single 
monitor within each county (even if there is more than one monitor 
in the county) for a given year. A different (or the same) monitor 
in each county could be used to derive the factor for other years; 
the only requirement for selection is that it be the lowest factor, 
whether derived from the annual or 24-hour standard level.
---------------------------------------------------------------------------

    To evaluate whether the current primary SO2 standards 
are adequate or whether consideration of revisions is appropriate, EPA 
is using an approach in this review described in chapter 10 of the REA 
which builds upon the approaches used in reviews of other criteria 
pollutants, including the most recent reviews of the NO2, 
Pb, O3, and PM NAAQS (EPA, 2008c; EPA, 2007c; EPA, 2007d; 
EPA, 2005), and reflects the body of evidence and information that is 
currently available. As in other recent reviews, EPA's considerations 
will include the implications of placing more or less weight or 
emphasis on different aspects of the scientific evidence and the 
exposure/risk-based information, recognizing that the weight to be 
given to various elements of the evidence and exposure/risk information 
is part of the public health policy judgments that the Administrator 
will make in reaching decisions on the standard.
    A series of general questions frames this approach to considering 
the scientific evidence and exposure-/risk-based information. First, 
EPA's consideration of the scientific evidence and exposure/risk 
information with regard to the adequacy of the current standards is 
framed by the following questions:
     To what extent does evidence that has become available 
since the last review reinforce or call into question evidence for 
SO2-associated effects that were identified in the last 
review?
     To what extent has evidence for different health effects 
and/or sensitive populations become available since the last review?
     To what extent have uncertainties identified in the last 
review been reduced and/or have new uncertainties emerged?
     To what extent does evidence and exposure-/risk-based 
information that has become available since the last review reinforce 
or call into question any of the basic elements of the current 
standard?
    To the extent that the available evidence and exposure-/risk-based 
information suggests it may be appropriate to consider revision of the 
current standards, EPA considers that evidence and information with 
regard to its support for consideration of a standard that is either 
more or less stringent than the current standards. This evaluation is 
framed by the following questions:
     Is there evidence that associations, especially causal or 
likely causal associations, extend to ambient SO2 
concentrations as low as, or lower than, the concentrations that have 
previously been associated with health effects? If so, what are the 
important uncertainties associated with that evidence?
     Are exposures above benchmark levels and/or health risks 
estimated to occur in areas that meet the current standard? If so, are 
the estimated exposures and health risks important from a public health 
perspective? What are the important uncertainties associated with the 
estimated risks?
    To the extent that there is support for consideration of a revised 
standard, EPA then considers the specific elements of the standard 
(indicator, averaging time, form, and level) within the context of the 
currently available information. In so doing, the Agency addresses the 
following questions regarding the elements of the standard:
     Does the evidence provide support for considering a 
different indicator for gaseous SOX?

[[Page 64826]]

     Does the evidence provide support for considering 
different, or additional averaging times?
     What ranges of levels and forms of alternative standards 
are supported by the evidence, and what are the associated 
uncertainties and limitations?
     To what extent do specific averaging times, levels, and 
forms of alternative standards reduce the estimated exposures above 
benchmark levels and risks attributable to exposure to ambient 
SO2, and what are the uncertainties associated with the 
estimated exposure and risk reductions?
    The questions outlined above have been addressed in the REA. The 
following sections present considerations regarding the adequacy of the 
current standards and potential alternative standards, as discussed in 
chapter 10 of the REA, in terms of indicator, averaging time, form, and 
level.
E. Adequacy of the current standards
    In considering the adequacy of the current standards, the policy 
assessment chapter of the REA considered the scientific evidence 
assessed in the ISA, as well as the air quality, exposure, and risk-
based information presented in the REA. A summary of this evidence and 
information as well as CASAC recommendations and the Administrator's 
conclusions regarding the adequacy of the current standards are 
presented below. Section II.E.1 will discuss the adequacy of the 
current 24-hour standard and Section II.E.2 will then discuss adequacy 
of the current annual standard. Section II.E.3 will discuss CASAC views 
and finally, section II.E.4 discusses the Administrator's conclusions 
regarding the adequacy of the current 24-hour and annual standards.
1. Adequacy of the current 24-hour standard
a. Evidence-based considerations
    In considering the SO2 epidemiologic studies as they 
relate to the adequacy of the current 24-hour standard, the REA noted 
that 24-hour average SO2 concentrations were below the 
current 24-hour average SO2 NAAQS in many locations where 
positive and sometimes statistically significant associations were 
observed (REA, section 10.3). As discussed previously (see section 
II.B.3), the ISA characterized the epidemiologic evidence for 
respiratory effects as being consistent and coherent (ISA, section 
5.2). The evidence is consistent in that positive associations are 
reported in studies conducted in numerous locations and with a variety 
of methodological approaches (ISA, section 5.2). It is coherent in the 
sense that respiratory symptom results from epidemiologic studies 
predominantly using 1-hour daily maximum or 24-hour average 
SO2 concentrations are generally in agreement with the 
respiratory symptom results from controlled human exposure studies of 
5-10 minutes. These results are also coherent in that the respiratory 
effects observed in controlled human exposure studies of 5-10 minutes 
provide a basis for a progression of respiratory morbidity that could 
lead to the ED visits and hospitalizations observed in epidemiologic 
studies (ISA, section 5.2). The ISA also noted that when the 
epidemiologic literature is considered as a whole, there are generally 
positive associations between SO2 and respiratory symptoms 
in children, hospital admissions, and emergency department visits. 
Moreover, some of these associations were statistically significant, 
particularly the more precise effect estimates (ISA, section 5.2).
    The interpretation of these SO2 epidemiologic studies is 
complicated by the fact that SO2 is but one component of a 
complex mixture of pollutants present in the ambient air. In order to 
provide some perspective on this uncertainty, the ISA evaluates 
epidemiologic studies that employ multi-pollutant models. Specifically, 
the ISA noted that a number of SO2 epidemiologic studies 
have attempted to disentangle the effects of SO2 from those 
of co-occurring pollutants by utilizing multi-pollutant models. When 
evaluated as a whole, SO2 effect estimates in these models 
generally remained positive and relatively unchanged when co-pollutants 
were included. Therefore, although recognizing the uncertainties 
associated with separating the effects of SO2 from those of 
co-occurring pollutants, the ISA concluded that the limited available 
evidence indicates that the effect of SO2 on respiratory 
health outcomes appears to be generally robust and independent of the 
effects of gaseous co-pollutants, including NO2 and 
O3, as well as particulate co-pollutants, particularly 
PM2.5 (ISA, section 5.2; p. 5-9).
    In drawing broad conclusions regarding the evidence, the ISA 
considered the epidemiologic and experimental evidence as well as the 
uncertainties associated with that evidence. When this evidence and its 
associated uncertainties were taken together, the ISA concluded that 
the results of epidemiologic and experimental studies form a plausible 
and coherent data set that supports a relationship between 
SO2 exposures and respiratory endpoints, including 
respiratory symptoms and ED visits, at ambient concentrations that are 
present in areas that meet the current 24-hour SO2 NAAQS 
(ISA, section 5.5). Thus, taking into consideration the evidence 
discussed above, particularly the epidemiologic studies reporting 
SO2-associated health effects in locations that meet the 
current 24-hour standard, the REA concluded that the epidemiologic 
evidence calls into question the adequacy of the current 24-hour 
standard to protect public health (REA, section 10.3.4).
b. Air quality, exposure, and risk-based considerations
    As previously mentioned, the ISA found the evidence for an 
association between respiratory morbidity and SO2 exposure 
to be ``sufficient to infer a causal relationship'' (ISA, section 5.2) 
and that the ``definitive evidence'' for this conclusion comes from the 
results of controlled human exposure studies demonstrating decrements 
in lung function and/or respiratory symptoms in exercising asthmatics 
(ISA, section 5.2). Accordingly, the exposure and risk analyses 
presented in the REA focused on exposures and risks associated with 5-
minute peaks of SO2 in excess of the potential health effect 
benchmark values of 100, 200, 300, and 400 ppb SO2. In 
considering the results presented in these analyses, the REA 
particularly noted exceedances or exposures with respect to the 200 and 
400 ppb 5-minute benchmark levels. These benchmark levels were 
highlighted in the REA because (1) 400 ppb represents the lowest 
concentration in controlled human exposure studies where moderate or 
greater lung function decrements which were often statistically 
significant at the group mean level, were frequently accompanied by 
respiratory symptoms; and (2) 200 ppb is the lowest level at which 
moderate or greater decrements in lung function in free-breathing human 
exposure studies have been observed (notably, 200 ppb is also the 
lowest level that has been tested). The REA also recognized that there 
was very limited evidence demonstrating small decrements in lung 
function at 100 ppb from two mouthpiece exposure studies. However, as 
previously noted (see section II.B.1.b), the results of these studies 
are not directly comparable to free-breathing chamber studies, and 
thus, the REA primarily considered exceedences of the 200 ppb and 400 
ppb benchmark levels in its evaluation of the adequacy of the current 
24-hour (as well

[[Page 64827]]

as the annual; see section II.E.2) standard.
    A key output of the air quality analysis was the predicted number 
of statistically estimated 5-minute daily maximum SO2 
concentrations above benchmark levels given air quality simulated to 
just meet the level of the current 24-hour or annual SO2 
standard, whichever was controlling for a given county. Under this 
scenario, in 40 counties selected for detailed analysis, the REA found 
that the predicted yearly mean number of statistically estimated 5-
minute daily maximum concentrations > 400 ppb ranges from 1-102 days 
per year,\14\ with most counties in this analysis experiencing a mean 
of at least 20 days per year when statistically estimated 5-minute 
daily SO2 concentrations exceed 400 ppb (REA, Table 7-14). 
In addition, the predicted yearly mean number of statistically 
estimated 5-minute daily maximum concentrations > 200 ppb ranged from 
21-171 days per year, with about half of the counties in this analysis 
experiencing >= 70 days per year when 5-minute daily maximum 
SO2 concentrations exceed 200 ppb (REA, Table 7-12).
---------------------------------------------------------------------------

    \14\ Air quality estimates presented in this section represent 
the mean number of days per year when 5-minute daily maximum 
SO2 concentrations exceed a particular benchmark level 
given 2001-2006 air quality adjusted to just meet the current 
standards (see REA, Tables 7-11 to 7-14).
---------------------------------------------------------------------------

    The REA also generated exposure and risk estimates for two study 
areas in Missouri (i.e., Greene County and several counties 
representing the St. Louis urban area) which had significant emission 
sources of SO2. As noted in REA section 8.10, there were 
differences in the number of exposures above benchmark values when the 
results of the Greene County and St. Louis exposure assessments were 
compared. In addition, given that the results of the exposure 
assessment were used as inputs into the quantitative risk assessment, 
it was not surprising that there were also differences in the number of 
asthmatics at elevated ventilation rates estimated to have a moderate 
or greater lung function response in Greene County when compared to St. 
Louis. The REA noted that the differences in the St. Louis and Greene 
County exposure and quantitative risk results are likely indicative of 
the different types of locations they represent (see section 8.10). 
Greene County is a rural county with much lower population and emission 
densities, compared to the St. Louis study area which has population 
and emissions density similar to other urban areas in the U.S. It 
therefore follows that there would be greater exposures, and hence 
greater numbers and percentages of asthmatics at elevated ventilation 
rates experiencing moderate or greater lung function responses in the 
St. Louis study area. Thus, when considering the risk and exposure 
results as they relate to the adequacy of the current standards, the 
REA concluded that the St. Louis results were more informative in terms 
of ascertaining the extent to which the current standards protect 
against effects linked to the various benchmarks (linked in turn to 5-
minute exposures). The results in fact suggested that the current 
standards may not adequately protect public health (REA, section 
10.3.3). Moreover, the REA judged that the exposure and risk estimates 
for the St. Louis study area provided useful insights into exposures 
and risks for other urban areas in the U.S. with similar population and 
SO2 emissions densities (REA, section 10.3.3).
    When considering the St. Louis exposure results as they relate to 
the adequacy of the current standards, results discussed in the policy 
chapter of the REA included the percent of asthmatic children at 
moderate or greater exertion estimated to experience at least one 
exceedance of either the 200 or 400 ppb benchmark given air quality 
that was adjusted upward to simulate just meeting the current 24-hour 
standard (i.e., the controlling standard in St. Louis).\15\ Given this 
scenario, the REA found that approximately 24% of asthmatic children in 
that city would be estimated to experience at least one SO2 
exposure concentration greater than or equal to the 400 ppb benchmark 
level per year while at moderate or greater exertion (e.g., while 
exercising; REA, Figure 8-19). Similarly, the REA found that 
approximately 73% of asthmatic children would be expected to experience 
at least one SO2 exposure greater than or equal to a 200 ppb 
benchmark level while at moderate or greater exertion (REA, Figure 8-
19).
---------------------------------------------------------------------------

    \15\ Exposure and risk results presented in this notice are with 
respect to asthmatic children, results for all asthmatics are 
presented in REA chapters, 8, 9, and 10.
---------------------------------------------------------------------------

    When considering the St. Louis risk results as they relate to the 
adequacy of the current 24-hour standard, the policy assessment chapter 
of the REA included the percent of asthmatic children at elevated 
ventilation rates likely to experience at least one lung function 
response given air quality that is adjusted upward to simulate just 
meeting the current standards. Under this scenario, 19.1% to 19.2% of 
exposed asthmatic children at elevated ventilation rates were estimated 
to experience at least one moderate lung function response per year 
(defined as an increase in sRaw >= 100% (REA, Table 9-
8)).\16\ \17\ Furthermore, 7.9% to 8.1% of exposed asthmatic 
children at moderate or greater exertion were estimated to experience 
at least one large lung function response per year (defined as an 
increase in sRaw >= 200% (REA, Table 9-8)).
---------------------------------------------------------------------------

    \16\ The risk results presented represent the median estimate of 
exposed asthmatics expected to experience moderate or greater lung 
function decrements. Results are presented for both the probit and 
2-parameter logistic functional forms. The full range of estimates 
can be found in chapter 9 of the REA, and in all instances the 
smaller estimate is a result of using the probit function to 
estimate the exposure-response relationship.
    \17\ In this notice, risk results with respect to moderate or 
greater lung function responses are presented in terms of sRaw 
(i.e., >= 100% increases in sRaw). Risk results with respect to 
decrements in lung function defined in terms of FEV1 can 
be found in chapter 9 of the REA.
---------------------------------------------------------------------------

c. Summary of considerations from the REA regarding the 24-hour 
standard
    As noted above, the policy chapter of the REA considered several 
lines of scientific evidence when evaluating the adequacy of the 
current 24-hour standard to protect the public health. These included 
causality judgments made in the ISA, as well as the human exposure and 
epidemiologic evidence supporting those judgments. In particular, the 
REA concluded that numerous epidemiologic studies reporting positive 
associations between ambient SO2 and respiratory morbidity 
endpoints were conducted in locations that met, or were below the 
current 24-hour standard (REA, section 10.3.4). The REA concluded that 
to the extent that these considerations are emphasized, the adequacy of 
the current 24-hour standard to protect the public health would clearly 
be called into question (REA, section 10.3.4). The REA found this 
suggested consideration of a revised 24-hour standard and/or that an 
additional shorter-averaging time standard may be needed to provide 
additional health protection for sensitive groups, including asthmatics 
and individuals who spend time outdoors at elevated ventilation rates 
(REA, section 10.3.4). This also suggested that an alternative 
SO2 standard(s) should protect against health effects 
ranging from lung function responses and increased respiratory symptoms 
following 5-10 minute peak SO2 exposures, to increased 
respiratory symptoms and respiratory-related ED visits and hospital 
admissions associated with 1-hour daily maximum or 24-hour average

[[Page 64828]]

SO2 concentrations (REA, section 10.3.4).
    In examining the air quality, exposure, and risk-based information 
with regard to the adequacy of the current 24-hour SO2 
standard to protect the public health, the REA found that the results 
described above (and in more detail in chapters 7-9 of the REA) 
indicated that 5-minute exposures that could reasonably be judged 
important from a public health perspective (see section II.B.1.c) were 
associated with air quality adjusted upward to simulate just meeting 
the current 24-hour standard. These exposures were judged in the REA to 
be significant from a public health perspective due to their frequency: 
approximately 24% of child asthmatics at moderate or greater exertion 
in St. Louis are estimated to be exposed at least once per year to air 
quality exceeding the 5-minute 400 ppb benchmark, a level associated 
with lung function decrements in the presence of respiratory symptoms. 
Additionally, approximately 73% of child asthmatics in St. Louis would 
be expected to be exposed at least once per year to air quality 
exceeding the 5-minute 200 ppb benchmark. Moreover, slightly over 19% 
of exposed child asthmatics in St. Louis would be expected to 
experience at least one adverse lung function response (defined in 
terms of a >= 100% increase in sRaw) each year. Therefore, the REA 
concluded that the air quality, exposure, and risk-based considerations 
reinforced the epidemiologic evidence in supporting the conclusion that 
consideration should be given to revising the current 24-hour standard 
and/or setting a new shorter averaging time standard (e.g., 1-hour or 
less) to provide increased public health protection, especially for 
sensitive groups (e.g., asthmatics), from SO2-related 
adverse health effects (REA, section 10.3.4).
2. Adequacy of the current annual standard
    In considering the adequacy of the current annual standard, the 
policy assessment chapter of the REA considered the scientific evidence 
assessed in the ISA and the air quality, exposure, and risk-based 
information presented in the REA. A summary of this evidence and 
information is presented below.
a. Evidence-based considerations
    As an initial consideration with regard to the adequacy of the 
current annual standard, the REA noted that evidence relating long-term 
(weeks to years) SO2 exposure to adverse health effects 
(respiratory morbidity, carcinogenesis, adverse prenatal and neonatal 
outcomes, and mortality) was judged by the ISA to be ``inadequate to 
infer the presence or absence of a causal relationship'' (ISA, Table 5-
3). That is, the ISA found the health evidence to be of insufficient 
quantity, quality, consistency, or statistical power to make a 
determination as to whether SO2 is truly associated with 
these health endpoints (ISA, Table 1-2). With respect specifically to 
respiratory morbidity in children (in part, the basis for the current 
annual standard; see section II.D.1), the ISA presented recent 
epidemiologic evidence of an association with long-term exposure to 
SO2 (ISA, section 3.4.2). However, the ISA found the 
strength of these epidemiologic studies to be limited because of (1) 
variability in results across studies with respect to specific 
respiratory morbidity endpoints; (2) high correlations between long-
term average SO2 and co-pollutant concentrations, 
particularly PM; and (3) a lack of evaluation of potential confounding 
(ISA, section 3.4.2.1).
    The REA also noted that many epidemiologic studies demonstrating 
positive associations between 1-hour daily maximum or 24-hour average 
SO2 concentrations and respiratory symptoms, ED visits, and 
hospitalizations were conducted in areas where ambient SO2 
concentrations were well below the level of the current annual NAAQS 
(REA, section 10.4.2). The REA noted that this evidence suggested that 
the current annual standard was not providing adequate protection 
against health effects associated with shorter-term SO2 
concentrations found in epidemiologic studies (REA, section 10.4.2).
b. Air quality, exposure, and risk-based considerations
    Results of the risk characterization based on the air quality 
assessment provided additional insight into whether there is a need to 
revise the current annual standard, focusing again on the extent to 
which the annual standard may be providing protection against effects 
associated with short-term exposures. In general, analyses presented in 
the REA described the extent to which the current annual standard 
provided protection against 5-minute peaks of SO2 in excess 
of potential health effect benchmark levels (REA, chapter 7). The REA 
found that many of the monitors where frequent 5-minute exceedances 
were reported had annual average SO2 concentrations well 
below the level of the current annual standard. Moreover, the REA found 
that there was little to no correlation between the annual average 
SO2 concentration and the number of 5-minute daily maximum 
concentrations above potential health effect benchmark levels at these 
monitors (REA section 7.3.1). Thus, the REA concluded that the annual 
standard adds little in the way of protection against 5-minute peaks of 
SO2 (REA, section 10.4.4).
c. Summary of considerations from the REA regarding the annual standard
    As noted above, the ISA concluded that the evidence relating long-
term (weeks to years) SO2 exposure to adverse health effects 
(respiratory morbidity, carcinogenesis, adverse prenatal and neonatal 
outcomes, and mortality) was ``inadequate to infer the presence or 
absence of a causal relationship'' (ISA, Table 5-3). The ISA also 
reported that many epidemiologic studies demonstrating positive 
associations between short-term (e.g., 1-hour daily maximum, 24-hour 
average) SO2 concentrations and respiratory symptoms, as 
well as ED visits and hospitalizations, were conducted in areas where 
annual ambient SO2 concentrations were well below the level 
of the current annual NAAQS. In addition, analyses conducted in the REA 
suggested that the current annual standard is not providing protection 
against 5-10 minute peaks of SO2. Thus, the scientific 
evidence and the risk and exposure information suggest that the current 
annual SO2 standard: (1) Is likely not needed to protect 
against health risks associated with long term exposure to 
SO2; and 2) does not provide adequate protection from the 
health effects associated with shorter-term (i.e. <= 24-hours) 
SO2 exposures. Thus, the policy chapter of the REA 
accordingly concluded that consideration should be given to either 
revoking the annual standard or retaining it without revision, in 
conjunction with setting an appropriate short-term standard(s) (REA, 
section 10.4.4).
3. CASAC views regarding the adequacy of the current 24-hour and annual 
standards
    With regard to the adequacy of the current standards, CASAC 
conclusions were consistent with the views expressed in the policy 
assessment chapter of the REA.\18\ CASAC agreed

[[Page 64829]]

that the primary concern in this review is to protect against health 
effects that have been associated with short-term SO2 
exposures, particularly those of 5-10 minutes (Samet 2009). CASAC also 
agreed that the current 24-hour and annual standards are not sufficient 
to protect public health against the types of exposures that could lead 
to these health effects. Given these considerations, and as noted in 
their letter to the EPA Administrator, CASAC agreed ``that the current 
24-hour and annual standards are not adequate to protect public health, 
especially in relation to short term exposures to SO2 (5-10 
minutes) by exercising asthmatics'' (Samet, 2009, p. 15). CASAC also 
noted: ``assuming that EPA adopts a one hour standard in the range 
suggested, and if there is evidence showing that the short-term 
standard provides equivalent protection of public health in the long-
term as the annual standard, the panel is supportive of the REA 
discussion of discontinuing the annual standard'' (Samet 2009, p. 15). 
With regard to the current 24-hour standard, CASAC was generally 
supportive of using the air quality analyses in the REA as a means of 
determining whether the current 24-hour standard was needed in addition 
to a new 1-hour standard to protect public health. CASAC stated: ``the 
evidence presented [in REA Table 10-3] was convincing that some of the 
alternative one-hour standards could also adequately protect against 
exceedences of the current 24-hour standard'' (Samet 2009, p. 15) 
Discussion regarding CASAC's views on how the standard should be 
revised is provided below within the context of discussions on the 
elements (i.e., indicator, averaging time, form, level) of a new short-
term standard.
---------------------------------------------------------------------------

    \18\ CASAC views with respect to the current 24-hour and annual 
standards, as well as with respect to potential alternative 
standards are those following their review of the second draft 
SO2 REA, which contained a staff policy assessment 
chapter. EPA did not solicit, nor did it receive CASAC comments on 
the final policy assessment chapter contained in the final REA.
---------------------------------------------------------------------------

4. The Administrator's conclusions regarding adequacy of the current 
24-hour and annual standards
    Based on the epidemiologic evidence, the risk and exposure data set 
out in this section, as well as CASAC's advice and recommendations, the 
Administrator concludes (subject to consideration of public comment) 
that the current standards are not adequate to protect public health 
with an adequate margin of safety. The basis for this conclusion is as 
follows. First, the Administrator accepts and agrees with the ISA's 
conclusion that the results of controlled human exposure and 
epidemiologic studies form a plausible and coherent data set that 
supports a causal relationship between short-term (5-minutes to 24-
hours) SO2 exposures and adverse respiratory effects. The 
Administrator further agrees that the epidemiologic evidence 
(buttressed by the clinical evidence) indicates that the effects seen 
in the epidemiologic studies are attributable to exposure to 
SO2. She also accepts and agrees with the conclusion of the 
ISA that ``[i]n the epidemiologic studies, respiratory effects were 
observed in areas where the maximum ambient 24-h avg SO2 
concentration was below the current 24-h avg NAAQS level * * *'' (ISA, 
section 5.2, p. 5-2.) and so would occur at ambient SO2 
concentrations that are present in locations meeting the current 24-
hour NAAQS. The Administrator also notes that these effects occurred in 
areas with annual air quality levels considerably lower than those 
allowed by the current annual standard, indicating that the annual 
standard also is not providing protection against such effects. 
Existence of epidemiologic studies showing adverse effects occurring at 
levels allowed by the current standards is an accepted justification 
for finding that it is appropriate to revise the existing standards. 
See, e.g. American Trucking Ass'n v. EPA, 283 F. 3d 355, 370 (DC Cir. 
2002).
    With regard to the exposure and risk results, the Administrator 
notes and agrees with the analyses in the REA supporting that 5-minute 
exposures, reasonably judged important from a public health 
perspective, were associated with air quality adjusted upward to 
simulate just meeting the current standards. The Administrator 
especially notes the results of the St. Louis exposure analysis which, 
as summarized above, indicates that substantial percentages of 
asthmatic children at moderate or greater exertion would be exposed, at 
least once annually, to air quality exceeding the 400 and 200 ppb 
benchmarks. Moreover, in addition to the health evidence and risk-based 
information, the Administrator agrees with CASAC's conclusion that the 
current SO2 standards do not adequately protect the public's 
health.
    In considering approaches to revising the current standards, the 
Administrator is proposing that it is appropriate to consider setting a 
new short-term standard. The Administrator initially notes that a 1-
hour standard could provide increased public health protection, 
especially for members of at-risk groups, from health effects described 
in both controlled human exposure and epidemiologic studies, and hence, 
health effects associated with 5-minute to 24-hour exposures to 
SO2. As discussed in section II.F.5 below, depending on the 
degree of protection afforded by such a standard, it may be appropriate 
to replace, and not retain, the current 24-hour and annual standards in 
conjunction with setting a new short-term standard.

F. Conclusions on the elements of a proposed new short-term standard

    In considering alternative SO2 primary NAAQS, the 
Administrator notes the need to protect at-risk populations from: (1) 
1-hour daily maximum and 24-hour average exposures to SO2 
that could cause the types of respiratory morbidity effects reported in 
epidemiologic studies; and (2) 5-10 minute SO2 exposure 
concentrations reported in controlled human exposure studies to result 
in moderate or greater lung function responses and/or respiratory 
symptoms. Considerations with regard to potential alternative standards 
and the specific options being proposed are discussed in the following 
sections in terms of indicator, averaging time, form, and level 
(sections II.F.1 to II.F.4).
1. Indicator
    In the last review, EPA focused on SO2 as the most 
appropriate indicator for ambient SOX. In making a decision 
in the current review on the most appropriate indicator, the 
Administrator has considered the conclusions of the ISA and REA as well 
as the views expressed by CASAC. The REA noted that, although the 
presence of gaseous SOX species other than SO2 
has been recognized, no alternative to SO2 has been advanced 
as being a more appropriate surrogate for ambient gaseous 
SOX. Controlled human exposure studies and animal toxicology 
studies provide specific evidence for health effects following exposure 
to SO2. Epidemiologic studies also typically report levels 
of SO2, as opposed to other gaseous SOX. Because 
emissions that lead to the formation of SO2 generally also 
lead to the formation of other SOX oxidation products, 
measures leading to reductions in population exposures to 
SO2 can generally be expected to lead to reductions in 
population exposures to other gaseous SOX. Therefore, 
meeting an SO2 standard that protects the public health can 
also be expected to provide protection against potential health effects 
that may be independently associated with other gaseous SOX 
even though such effects are not discernable from currently available 
studies indexed by SO2 alone. See American Petroleum 
Institute v. EPA, 665 F, 2d 1176, 1186 (DC Cir. 1981) (reasonable for 
EPA to use ozone as the indicator for all photochemical oxidants even 
though

[[Page 64830]]

health information on the other photochemical oxidants is unknown; 
regulating ozone alone is reasonable since it presents a ``predictable 
danger'' and in doing so EPA did not abandon its responsibility to 
regulate other photochemical oxidants encompassed by the determination 
that photochemical oxidants as a class may be reasonably anticipated to 
endanger public health or welfare). Given these key points, the REA 
concluded that the available evidence supports the retention of 
SO2 as the indicator in the current review (REA, section 
10.5.1). Consistent with this conclusion, CASAC stated in a letter to 
the EPA Administrator that ``for indicator, SO2 is clearly 
the preferred choice'' (Samet 2009, p. 14). The Administrator agrees 
with this consensus, and therefore proposes to retain SO2 as 
the indicator for oxides of sulfur in the current review.
2. Averaging time
    In considering whether it is appropriate to revise the averaging 
times of the current standards, the first consideration is what health 
effects the standard is addressing, and specifically whether those 
effects are associated with short-term (i.e., 5-minutes to 24-hours), 
and/or long-term (i.e. weeks to years) exposure to SO2. 
There are distinct differences in the causality judgments in the ISA as 
to short-term versus long-term health effects of SO2. The 
ISA found evidence relating long-term (weeks to years) SO2 
exposures to adverse health effects to be ``inadequate to infer the 
presence or absence of a causal relationship'' (ISA, Table 5-3). In 
contrast, the ISA judged evidence relating short-term (5-minutes to 24-
hours) SO2 exposure to respiratory morbidity to be 
``sufficient to infer a causal relationship'' (the strongest possible 
conclusion as to causality) and short-term exposure to SO2 
and mortality to be ``suggestive of a causal relationship'' (ISA, Table 
5-3). Taken together, the REA concluded that these judgments most 
directly supported standard averaging time(s) that focus protection on 
SO2 exposures from 5-minutes to 24-hours (REA, section, 
10.5.2).
a. Evidence and air quality, exposure, and risk-based considerations
    In considering the level of support available for specific short-
term averaging times, the REA noted the strength of evidence from human 
exposure and epidemiologic studies evaluated in the ISA. As previously 
mentioned, controlled human exposure studies exposed exercising 
asthmatics to 5-10 minute peak concentrations of SO2 and 
consistently found decrements in lung function and/or respiratory 
symptoms. Importantly, the ISA described the controlled human exposure 
studies as being the ``definitive evidence'' for its conclusion that 
there exists a causal association between short-term (5-minutes to 24-
hours) SO2 exposure and respiratory morbidity (ISA, section 
5.2). In addition to the controlled human exposure evidence, there is a 
relatively small body of epidemiologic studies describing positive 
associations between 1-hour daily maximum SO2 levels and 
respiratory symptoms as well as hospital admissions and ED visits for 
all respiratory causes and asthma (ISA Tables 5.4 and 5.5). In addition 
to the evidence from these 1-hour daily maximum epidemiologic studies, 
there is a considerably larger body of epidemiologic studies reporting 
positive associations between 24-hour average SO2 levels and 
respiratory symptoms, as well as hospitalizations and ED visits for all 
respiratory causes and asthma. Moreover, with respect to these 
epidemiologic studies, there is support that adverse respiratory 
effects are more likely to occur at the upper end of the distribution 
of ambient SO2 concentrations (see section II.F.3 on Form). 
In addition, when describing epidemiologic studies observing positive 
associations between ambient SO2 and respiratory symptoms, 
the ISA stated ``that it is possible that these associations are 
determined in large part by peak exposures within a 24-hour period'' 
(ISA, section 5.2 at p. 5-5). Similarly, the ISA stated that: ``the 
effects of SO2 on respiratory symptoms, lung function, and 
airway inflammation observed in the human clinical studies using peak 
exposures further provides a basis for a progression of respiratory 
morbidity resulting in increased ED visits and hospital admissions'' 
and makes the associations observed in the epidemiologic studies 
``biologica[lly] plausib[le]'' (ISA, section 5.2 at p. 5-5).
    The controlled human exposure evidence described above provided 
support for an averaging time that protects against 5-10 minute peak 
SO2 exposures (REA, section 10.5.2). In addition, the REA 
found that results from epidemiologic studies provided support for both 
1-hour and 24-hour averaging times (REA, section 10.5.2). In addition, 
both the epidemiologic and controlled human exposure evidence suggests 
that a new short-term standard should be focused on limiting peak 
SO2 exposures. Thus, it can reasonably be concluded from the 
ISA and REA that it would be appropriate to consider the degree of 
protection potential alternative standards with averaging times under 
consideration provide against peak 5-minute to 24-hour SO2 
exposures. Moreover, as fully discussed in section II.F.3, this same 
information makes it reasonable that the form of a new short-term 
standard reflect a strategy to limit peak SO2 exposures. 
Thus, with respect to the analyses presented below regarding averaging 
time, a 99th percentile form will be considered. See American Petroleum 
Institute, 665 F. 2d at 1186 (selection of highest average ozone level 
in one hour to determine compliance with ozone NAAQS is reasonable 
``because it is calculated to measure the maximum exposure, which has 
been found to be a relevant factor in determining the likely 
consequences of ozone exposure'').
    In considering the level of support available for specific short-
term averaging times, the policy assessment chapter of the REA also 
took into account air quality considerations. More specifically, since 
the shortest averaging time for the current primary SO2 
standard is 24-hours, the REA evaluated the potential for a standard 
based on 24-hour average SO2 concentrations to limit 5-
minute peak SO2 exposures (REA, section 10.5.2). The REA 
evaluated ratios between 99th percentile 5-minute daily maximum and 
99th percentile 24-hour average SO2 concentrations for 42 
monitors reporting measured 5-minute data for any year between 2004-
2006 (REA, Table 10-1). Across this set of monitors, ratios of 99th 
percentile 5-minute daily maximum to 99th percentile 24-hour average 
SO2 concentrations spanned a range of 2.0 to 14.1 (REA, 
Table 10-1). These results suggested a standard based on 24-hour 
average SO2 concentrations would not likely be an effective 
or efficient approach for addressing 5-minute peak SO2 
concentrations. That is, the REA concluded using a 24-hour average 
standard to address 5-minute peaks would likely result in over-
controlling in some areas, while under-controlling in others (REA, 
section 10.5.2). This analysis also suggested that a 5-minute standard 
would not likely be an effective or efficient means for controlling 24-
hour average SO2 concentrations (REA, section 10.5.2).
    The REA also reported ratios between 99th percentile 5-minute daily 
maximum and 99th percentile 1-hour daily maximum SO2 levels 
from this set of monitors. Compared to the ratios discussed above (5-
minute daily maximum to 24-hour average), there was far less 
variability between 5-

[[Page 64831]]

minute daily maximum and 1-hour daily maximum ratios. More 
specifically, 39 of the 42 monitors had 99th percentile 5-minute daily 
maximum to 99th percentile 1-hour daily maximum ratios in the range of 
1.2 to 2.5 (REA, Table 10-1). The remaining three monitors had ratios 
of 3.6, 4.2 and 4.6 respectively. Overall, the REA found that this 
relatively narrow range of ratios (compared to the range of ratios 
presented above with respect to 5-minute daily maximum to 24-hour 
average) suggested that a standard with a 1-hour averaging time would 
be more efficient and effective at limiting 5-minute peaks of 
SO2 than a standard with a 24-hour averaging time (REA, 
section 10.5.2.2). This analysis also suggested that a 5-minute 
standard could be a relatively effective means of controlling 1-hour 
daily maximum SO2 concentrations.\19\
---------------------------------------------------------------------------

    \19\ The analysis of peak to mean ratios was used as an initial 
screen to evaluate which averaging times could be suited to control 
5-minute peaks of SO2. The more sophisticated analysis 
for ultimately determining that a one-hour averaging time set at an 
appropriate level could effectively limit these 5-minute peaks was 
the air quality, exposure, and risk analyses discussed in section 
II.F.4.
---------------------------------------------------------------------------

    The REA further evaluated the potential of the 1-hour daily maximum 
standards analyzed in the air quality, exposure, and risk analyses to 
limit peak 24-hour average SO2 exposures (REA, section 
10.5.2) since there is epidemiologic evidence to suggest that adverse 
respiratory effects are more likely to occur at the upper end of the 
distribution of ambient SO2 concentrations. The 99th 
percentile 24-hour average SO2 concentrations in cities 
where U.S. ED visit and hospitalization studies (for all respiratory 
causes and asthma; identified from Table 5-5 of the ISA) were conducted 
ranged from 16 ppb to 115 ppb (Thompson and Stewart, 2009). Moreover, 
effect estimates that remained statistically significant in multi-
pollutant models with PM were found in cities with 99th percentile 24-
hour average SO2 concentrations ranging from approximately 
36 ppb to 64 ppb. The REA found that a 99th percentile 1-hour daily 
maximum standard set at a level of 50-100 ppb would generally limit 
99th percentile 24-hour average SO2 concentrations in 
locations where epidemiologic studies reported statistically 
significant results in multi-pollutant models with PM (Table 1). That 
is, for 2004, given air quality adjusted to just meet a 50 ppb 99th 
percentile 1-hour daily maximum standard, the REA found that no county 
included in this analysis was estimated to have 24-hour average 
SO2 concentrations >= 36 ppb (Table 1). In addition, given 
air quality adjusted to just meet a 100 ppb 99th percentile 1-hour 
daily maximum standard, only 6 of the 39 counties (Linn, Union, Bronx, 
Fairfax, Hudson, and Wayne) included in this 2004 analysis were 
estimated to have 99th percentile 24-hour average SO2 
concentrations >= 36 ppb (Table 1). The REA repeated this analysis for 
the years 2005 and 2006 and found similar results (REA, Appendix Tables 
D1 and D2).\20\
---------------------------------------------------------------------------

    \20\ In 2005, given a 99th percentile 1-hour daily maximum 
standard at 50 ppb, Wayne County, West Virginia would have an 
estimated 99th percentile 24-hour average SO2 
concentration > 36 ppb (43 ppb; REA Appendix Table D-1).

 Table 1--99th Percentile 24-Hour Average SO2 Concentrations for 2004 Given Just Meeting the Alternative 1-Hour
        Daily Maximum 99th and 98th Percentile Potential Standards Analyzed in the Air Quality Assessment
                                         [Source: REA, Table 10-2].\21\
----------------------------------------------------------------------------------------------------------------
                                                            1-hour daily maximum standards
                                    ----------------------------------------------------------------------------
     State             County                           99th percentile                        98th percentile
                                    ----------------------------------------------------------------------------
                                         50        100        150        200        250        100        200
----------------------------------------------------------------------------------------------------------------
AZ............  Gila...............          6         12         18         25         31         16         32
DE............  New Castle.........         12         23         35         47         59         28         56
FL............  Hillsborough.......         10         20         30         40         50         28         55
IL............  Madison............         12         24         36         48         60         28         56
IL............  Wabash.............          7         13         20         27         33         19         38
IN............  Floyd..............          8         15         23         31         39         20         41
IN............  Gibson.............          9         18         27         36         45         20         41
IN............  Lake...............         12         24         36         48         60         31         62
IN............  Vigo...............         10         19         29         39         48         24         48
IA............  Linn...............         21         42         64         85        106         49         98
IA............  Muscatine..........         17         34         51         68         85         38         76
MI............  Wayne..............         17         33         50         66         83         37         74
MO............  Greene.............         12         24         36         48         60         31         62
MO............  Jefferson..........          9         18         27         36         45         25         51
NH............  Merrimack..........         17         33         50         66         83         39         79
NJ............  Hudson.............         19         38         57         76         95         48         96
NJ............  Union..............         18         36         54         72         90         44         89
NY............  Bronx..............         23         47         70         93        117         54        107
NY............  Chautauqua.........         13         27         40         54         67         32         65
NY............  Erie...............         14         27         41         54         68         30         61
OH............  Cuyahoga...........         17         34         51         67         84         40         80
OH............  Lake...............         10         19         29         39         48         23         47
OH............  Summit.............         12         24         36         48         61         27         55
OK............  Tulsa..............         16         32         47         63         79         36         72
PA............  Allegheny..........         12         23         35         47         59         30         60
PA............  Beaver.............         10         20         30         40         51         25         49
PA............  Northampton........         11         23         34         45         56         36         72
PA............  Warren.............         11         22         33         44         56         28         56
PA............  Washington.........         15         31         46         62         77         36         71
TN............  Blount.............         15         31         46         61         77         35         71

[[Page 64832]]

 
TN............  Shelby.............         17         34         51         68         85         41         81
TN............  Sullivan...........          8         16         24         32         39         23         46
TX............  Jefferson..........          9         17         26         35         44         21         41
VA............  Fairfax............         23         46         69         92        116         52        103
WV............  Brooke.............         12         24         37         49         61         31         62
WV............  Hancock............         15         29         44         58         73         35         69
WV............  Monongalia.........         10         20         30         40         50         25         51
WV............  Wayne..............         30         59         89        119        149         67        133
VI............  St Croix...........         14         27         41         54         68         51        101
----------------------------------------------------------------------------------------------------------------

    The air quality information presented above strongly support the 
likelihood that an alternative 99th percentile (see discussion of form 
below in II.F.3) 1-hour daily maximum standard set at an appropriate 
level (see discussion of level in II.F.4) can substantially reduce the 
upper end of the distribution of SO2 levels more likely to 
be associated with adverse respiratory effects; that is: (1) 99th 
percentile 1-hour daily maximum air quality concentrations in cities 
observing positive effect estimates in epidemiologic studies of 
hospital admissions and ED visits for all respiratory causes and 
asthma; and (2) 99th percentile 24-hour average air quality 
concentrations found in U.S. cities where ED visit and hospitalization 
studies (for all respiratory causes and asthma) observed statistically 
significant associations in multi-pollutant models with PM (i.e., 99th 
percentile 24-hour average SO2 concentration >= 36 ppb). In 
addition, based on the air quality and exposure analyses presented in 
chapters 7 and 8 of the REA, there is also a strong likelihood that a 
99th percentile 1-hour daily maximum standard will limit 5-10 minute 
peaks of SO2 shown in human exposure studies to result in 
decrements in lung function and/or respiratory symptoms in exercising 
asthmatics (see especially: REA Tables 7-11 to 7-14 and Figure 8-19). 
Such analyses are also summarized in section II.F.4 of this notice. 
Taken together, these results support that a 1-hour daily maximum 
standard, with an appropriate form and level, can provide adequate 
protection against the range of health outcomes associated with 
averaging times from 5-minutes to 24-hours (REA, section 10.5.2.3).
---------------------------------------------------------------------------

    \21\ 99th or 98th percentile 1-hour daily maximum concentrations 
were determined for each monitor in a given county for the years 
complete data were available from 2004-2006. These concentrations 
were averaged, and the monitor with the highest average in a given 
county was determined. Based on this highest average, all monitors 
in a given county were adjusted to just meet the potential 
alternative standards defined above, and for each of the years, the 
99th percentile 24-hour average SO2 concentration was 
identified. Results for the years 2005 and 2006 are presented in the 
REA, Appendix D.
---------------------------------------------------------------------------

    The REA also considered the possibility of a 5-minute averaging 
time based solely on the controlled human exposure evidence. However, 
the REA did not favor such an approach (REA 10.5.2.3). As in past NAAQS 
reviews, the stability of the design of pollution control programs in 
considering the elements of a NAAQS was considered, since more stable 
programs are more effective, and hence result in enhanced public 
safety. American Trucking Associations v. EPA, 283 F. 3d 355, 375 (DC 
Cir. 2002) (choice of 98th percentile form for 24-hour PM NAAQS, which 
allows a number of high exposure days per year to escape regulation 
under the NAAQS, justifiable as ``promot[ing] development of more 
`effective [pollution] control programs' '', since such programs would 
otherwise be ``less `stable'--and hence * * * less effective--than 
programs designed to address longer-term average conditions'', and 
there are other means (viz. emergency episode plans) to control those 
high exposure days). In this review, there were concerns about the 
stability of a standard using a 5-minute averaging time. Specifically, 
there was concern that compared to longer averaging times (e.g., 1-
hour, 24-hour), year-to-year variation in 5-minute SO2 
concentrations were likely to be substantially more temporally and 
spatially diverse. Thus, it is likely that locations would frequently 
shift in and out of attainment thereby reducing public health 
protection by disrupting an area's ongoing implementation plans and 
associated control programs. Consequently, the REA concluded that a 5-
minute averaging time would not provide a stable regulatory target and 
therefore would not be the preferred approach to provide adequate 
public health protection. However, as noted above, analyses in the REA 
support that a 1-hour averaging time, given an appropriate form and 
level (discussed below in sections II.F.3 and II.F.4, respectively) can 
adequately limit 5-minute SO2 exposures and provide a more 
stable regulatory target than setting a 5-minute standard.
b. CASAC views
    CASAC agreed with the conclusions of the policy assessment chapter 
of the REA that a primary consideration of the SO2 NAAQS 
should be the protection provided against health effects associated 
with short-term exposures. In their letter to the EPA Administrator, 
CASAC stated that they were ``in agreement with having a short-term 
standard and finds that the REA supports a one-hour standard as 
protective of public health'' (Samet 2009, p. 1). Furthermore, CASAC 
agreed with the REA that a ``one-hour standard is the preferred 
averaging time'' (Samet 2009, p.15).''
c. Administrator's conclusions on averaging time
    In considering the most appropriate averaging time(s) for the 
SO2 primary NAAQS, the Administrator notes the conclusions 
and judgments made in the ISA about the available scientific evidence, 
conclusions from the REA, and CASAC recommendations discussed above. 
Based on these considerations, the Administrator proposes to set a new 
standard based on 1-hour daily maximum SO2

[[Page 64833]]

concentrations to provide increased protection against effects 
associated with short-term (5-minutes to 24-hours) exposures. First, 
the Administrator agrees with the REA's conclusion that the standard 
should focus protection on short-term SO2 exposures from 5-
minutes to 24-hours. As noted above, CASAC's strong recommendation 
supports this approach as well. Second, the Administrator agrees that 
the standard must provide requisite protection from 5-10 minute 
exposure events (the critical issue in the previous review), but 
believes (subject to consideration of public comment) that this can be 
done without having a standard with a 5-minute averaging time. The 
Administrator agrees with the REA conclusion that it is likely a 1-hour 
standard--with the appropriate form and level--can substantially reduce 
5-10 minute peaks of SO2 shown in controlled human exposure 
studies to result in respiratory symptoms and/or decrements in lung 
function in exercising asthmatics. The Administrator further believes 
that a 5-minute averaging time would result in significant and 
unnecessary instability and is undesirable for that reason. The 
Administrator also notes the statements from CASAC addressing whether a 
one-hour averaging time can adequately control 5-10 minute peak 
exposures and whether there should be a 5-minute averaging time. CASAC 
stated that the REA had presented a ``convincing rationale'' for a one-
hour standard, and that ``a 1-hour standard is the preferred averaging 
time'' (Samet 2009, p. 16).
    Third, the Administrator agrees that a one-hour averaging time 
(again, with the appropriate form and level) would provide protection 
against the range of health outcomes associated with averaging times of 
one hour to 24 hours. Specifically, the Administrator finds that a 1-
hour standard can substantially reduce the upper end of the 
distribution of SO2 levels more likely to be associated with 
adverse respiratory effects; that is: (1) 99th percentile 1-hour daily 
maximum air quality concentrations in U.S. cities where positive effect 
estimates in epidemiologic studies of hospital admissions and ED visits 
for all respiratory causes and asthma were observed; and (2) 99th 
percentile 24-hour average air quality concentrations found in U.S. 
cities where ED visit and hospitalization studies (for all respiratory 
causes and asthma) observed statistically significant associations in 
multi-pollutant models with PM. Finally, the Administrator notes that 
the proposal to establish a new 1-hour averaging time is in agreement 
with CASAC recommendations. As noted above, CASAC stated that they were 
``in agreement with having a short-term standard and finds that the REA 
supports a one-hour standard as protective of public health'' (Samet, 
2009, p. 1).
3. Form
    When evaluating alternative forms in conjunction with specific 
levels, the REA considered the adequacy of the public health protection 
provided by the combination of level and form to be the foremost 
consideration. In addition, the REA recognized that it is important 
that the standard have a form that is reasonably stable. As just 
explained in the context of a five-minute averaging time, a standard 
set with a high degree of instability could have the effect of reducing 
public health protection because shifting in and out of attainment 
could disrupt an area's ongoing implementation plans and associated 
control programs.
a. Evidence, air quality, and risk-based considerations
    As previously mentioned, the policy chapter of the REA (chapter 10) 
recognized that the adequacy of the public health protection provided 
by a 1-hour daily maximum potential alternative standard will be 
dependent on the combination of form and level. It is therefore 
important that the particular form selected for a 1-hour daily maximum 
potential alternative standard reflect the nature of the health risks 
posed by increasing SO2 concentrations. That is, the REA 
noted that the form of the standard should reflect results from 
controlled human exposure studies demonstrating that the percentage of 
asthmatics affected, and the severity of the respiratory response (i.e. 
decrements in lung function, respiratory symptoms) increases as 
SO2 concentrations increase. Taking this into consideration, 
the REA concluded that a concentration-based form, averaged over three 
years, is more appropriate than an exceedance-based form (REA, section 
10.5.3). This is because a concentration-based form averaged over three 
years would give proportionally greater weight to years when 1-hour 
daily maximum SO2 concentrations are well above the level of 
the standard, than to years when 1-hour daily maximum SO2 
concentrations are just above the level of the standard. In contrast, 
an expected exceedance form would give the same weight to years when 1-
hour daily maximum SO2 concentrations are just above the 
level of the standard, as to years when 1-hour daily maximum 
SO2 concentrations are well above the level of the standard. 
Therefore, the REA concluded that a concentration-based form, averaged 
over three years (which also increases the stability of the standard) 
better reflects the continuum of health risks posed by increasing 
SO2 concentrations (i.e. the percentage of asthmatics 
affected and the severity of the response increases with increasing 
SO2 concentrations; REA, section 10.5.3).
    The form of the standard should also reflect health information in 
the ISA that suggests that adverse respiratory effects are more likely 
to occur at the upper end of the distribution of ambient SO2 
concentrations. Specifically, a few studies found that the increase in 
SO2-related respiratory health effects was observed at the 
upper end of the distribution of SO2 concentrations (ISA, 
section 5.3, p. 5-9). For example, an epidemiologic study conducted in 
Bronx, NY suggested an increased risk of asthma hospitalizations on the 
days with the highest SO2 concentrations (Lin et al., 2004). 
More specifically, the authors observed an increasing linear trend with 
respect to asthma hospitalizations across the range of SO2 
concentrations, with more marked effects observed at SO2 
concentrations somewhere between the 90th and 95th percentiles (ISA, 
section 4.1.2 and ISA, Figure 4-4).
    The epidemiologic evidence is consistent with the large body of 
controlled human exposure studies of exercising asthmatics exposed to 
short-term peak concentrations of SO2; these controlled 
human exposure studies provide the ``definitive evidence'' that short 
term peak SO2 exposure is associated with respiratory 
morbidity (SOx ISA, Section 5.3, page 5-2). These studies 
consistently found moderate or greater decrements in lung function 
(i.e. >= 100% increase in sRaw and/or >= 15% decline in 
FEV1)\22\ and/or respiratory symptoms in exercising 
asthmatics following 5-10 minute peak exposures to SO2. 
Moreover, as noted in the discussion on averaging time (section 
II.F.2), when discussing the possible relationship between effects 
observed in controlled human exposure studies and associations reported 
in epidemiologic analyses, the ISA stated with respect to epidemiologic 
studies of respiratory symptoms: ``it is possible that these 
associations are determined in large part by peak exposures within a 
24-hour period'' (ISA, section 5.2 at p. 5-5). Similarly, the ISA 
stated that: ``the effects of SO2 on respiratory symptoms,

[[Page 64834]]

lung function, and airway inflammation observed in the human clinical 
studies using peak exposures further provides a basis for a progression 
of respiratory morbidity resulting in increased ED visits and hospital 
admissions'' and makes the associations observed in the epidemiologic 
studies ``biologica[lly] plausib[le]'' (ISA, section 5.2 at p. 5-5). 
Thus, both the epidemiologic and controlled human exposure evidence 
suggests that the form of the standard should be focused on limiting 
peak SO2 exposures.
---------------------------------------------------------------------------

    \22\ See section II.B.1.b above explaining sRaw and FEV1.
---------------------------------------------------------------------------

    In considering specific concentration-based forms, the REA 
recognized the importance of: (1) Minimizing the number of days per 
year that an area could exceed the level of the standard and still 
attain the standard and thus, limiting the upper end of the 
distribution of SO2 levels most likely associated with 
adverse respiratory effects (2) limiting the prevalence of 5-minute 
peaks of SO2; and (3) providing a stable regulatory target 
to prevent areas from frequently shifting in and out of attainment. The 
REA focused on 98th and 99th percentile forms averaged over 3 years. 
The REA first noted that in most locations analyzed, the 99th 
percentile form of a 1-hour daily maximum standard would correspond to 
the 4th highest daily maximum concentration in a year, while a 98th 
percentile form would correspond approximately to the 7th to 8th 
highest daily maximum concentration in a year (REA, Table 10-5 and 
Thompson, 2009). In addition, results from the REA air quality analysis 
suggested that at a given SO2 standard level, a 99th 
percentile form is appreciably more effective at limiting 5-minute peak 
SO2 concentrations than a 98th percentile form (REA, section 
10.5.3 and REA, Figures 7-27 and 7-28). For example, the REA reported 
that compared to the same standard with a 99th percentile form, a 98th 
percentile 1-hour daily maximum standard set at a level of 100 ppb 
allows for on average, an estimated 90 and 74% more days per year when 
SO2 concentrations would likely exceed the 200 and 400 ppb 
benchmark values respectively (REA, section 10.5.3 and REA, Figure 7-
28). Moreover, in the counties selected for analysis in the REA air 
quality assessment, the estimated number of benchmark exceedances using 
a 98th percentile 1-hour daily maximum standard set at a level of 200 
ppb was similar to the corresponding 99th percentile standard set at a 
level of 250 ppb (REA, section 10.5.3 and REA, Tables 7-11 through 7-
14). Similarly, the estimated number of benchmark exceedances 
considering a 98th percentile standard set at a level of 100 ppb fell 
within the range of benchmark exceedances estimated for 99th percentile 
standards set at levels of 100 and 150 ppb (id.).
    As an additional matter, the REA compared trends in 98th and 99th 
percentile design values, as well as design values based on the 4th 
highest daily maximum from 54 sites located in the 40 counties selected 
for the detailed air quality analysis (REA section 10.5.3 and Thompson, 
2009). These results suggested that at the vast majority of sites, 
there would have been similar changes in 98th and 99th percentile 
design values over the last ten years (i.e. based evaluating 
overlapping three year intervals over the last ten years; see REA, 
Figure 10-1 and Thompson, 2009). These results also demonstrated that 
design values based on the 4th highest daily maximum are virtually 
indistinguishable from design values based on the 99th percentile (REA, 
Figure 10-1 and Thompson, 2009). As part of this analysis, all of the 
design values over this ten year period for all 54 sites were 
aggregated and the standard deviation calculated (REA, Figure 10-2 and 
Thompson, 2009). Results demonstrated similar standard deviations--i.e. 
similar stability--based on aggregated 98th or aggregated 99th 
percentile design values over the ten year period (see REA, Figure 10-2 
and Thompson 2009).
    Considering the evidence and air quality analyses presented above, 
the REA concluded that a concentration-based form provides the best 
protection against the health risks posed by increasing SO2 
concentrations (REA, section 10.5.3). Moreover, the REA found that at a 
given standard level, a 99th percentile or 4th highest daily maximum 
form provides appreciably more public health protection against 5-
minute peaks than a 98th percentile or 7th--8th highest daily maximum 
form (REA, section 10.5.3). In addition, over the last 10 years and for 
the vast majority of the sites examined, there appears to be little 
difference in 98th and 99th percentile design value stability (REA, 
section 10.5.3). Thus, the REA ultimately concluded that consideration 
should be given primarily to a 1-hour daily maximum standard with a 
99th percentile or 4th highest daily maximum form (REA, section 
10.5.3.3).
b. CASAC views
    CASAC agreed with the importance of considering the public health 
protection provided by the combination of form and level. Moreover, 
CASAC was in general agreement with the forms being considered. In a 
letter to the Administrator, CASAC stated: ``there is adequate 
information to justify the use of a concentration-based form averaged 
over 3 years'' (Samet 2009, p. 16). Moreover, when considering 98th vs. 
99th percentile forms, CASAC encouraged EPA to consider analyses in the 
REA (and perhaps additional analyses) with respect to the number of 
days per year 98th vs. 99th percentile forms would allow SO2 
concentrations to exceed the selected level. CASAC also encouraged EPA 
to consider analyses such as those presented above with respect to the 
number exceedences of 5-minute benchmarks given 98th vs. 99th 
percentile forms at a given standard level (Samet 2009).
c. Administrator's conclusions on form
    When considering alternative forms, the Administrator notes and 
agrees with the views expressed in the REA and the recommendations from 
CASAC, as described above. In particular, she agrees that the standard 
should use a concentration-based form averaged over three years in 
order to give due weight to years when 1-hour SO2 
concentrations are well above the level of the standard, than to years 
when 1-hour SO2 concentrations are just above the level of 
the standard. The Administrator agrees further, for the reasons given 
above, that a 99th percentile (or 4th highest) form could be 
appreciably more protective than a 98th (or 7th or 8th highest) form, 
and thus, should be utilized. Given these considerations, and in light 
of the specific range proposed for level below, the Administrator 
proposes to adopt either a 99th percentile or a 4th highest form, 
averaged over 3 years.
4. Level
    In assessing the level of a one-hour standard with either a 99th 
percentile or 4th highest average form (averaged over three years in 
either case) to propose, the Administrator has considered the broad 
range of scientific evidence assessed in the ISA, including the 
epidemiologic studies and controlled human exposure studies, as well as 
the results of air quality, exposure, and risk analyses presented in 
the REA. In light of this body of evidence and analyses, the 
Administrator reiterates that it is necessary to provide increased 
public health protection for at-risk populations against an array of 
adverse respiratory health effects related to short-term (i.e., 5 
minutes to 24 hours) exposures to ambient SO2. In 
considering the most appropriate way to provide this protection, the 
Administrator is mindful

[[Page 64835]]

of the extent to which the available evidence and analyses can inform a 
decision on the level of a standard. Specifically, the range of 
proposed standard levels discussed below is informed by epidemiologic 
and controlled human exposure studies.
a. Evidence-based considerations
    Evidence-based considerations take into account the full body of 
scientific evidence assessed in the ISA. When considering the extent to 
which this scientific evidence can inform a decision on the level of a 
1-hour standard, it is important to note that SO2 
concentrations represent different measures of exposure when drawn from 
experimental versus epidemiologic studies. Concentrations of 
SO2 tested in experimental studies, such as controlled human 
exposure studies, represent exposure concentrations in the breathing 
zone of the individual test subjects. In cases where controlled human 
exposure studies report effects, those effects are caused directly by 
exposure to a specified concentration of SO2. In contrast, 
concentrations of SO2 drawn from epidemiologic studies are 
often based on ambient monitoring data. SO2 concentrations 
recorded at these ambient monitors are used as surrogates for the 
distribution of SO2 exposures across the study area and over 
the time period of the study.
    Since the last review, there have been more than 50 peer reviewed 
epidemiologic studies published worldwide dealing with SO2 
exposure and effects (see ISA Tables 5-4 and 5-5). Overall, the ISA 
concluded that these studies provide evidence of an association between 
ambient SO2 concentrations and respiratory symptoms, as well 
as ED visits and hospitalizations for all respiratory causes and asthma 
(ISA, section 3.1.4). Moreover, the ISA indicates that many of these 
epidemiologic studies have reported that children and older adults may 
be at increased risk for SO2- associated adverse respiratory 
effects (ISA, section 5.2). In assessing the extent to which these 
studies and their associated air quality information can inform the 
level of a new 99th percentile (see sections II.F.2 and II.F.3) 1-hour 
daily maximum standard for the U.S., the REA considered U.S. and 
Canadian air quality information to be most relevant. EPA sent a 
request to the authors of U.S. and Canadian epidemiologic studies 
(studies were identified from Tables 5-4 and 5-5 of the ISA) for 99th 
(and 98th) percentile 1-hour daily maximum SO2 
concentrations from the monitor recording the highest SO2 
level in the location and time period corresponding to their studies 
(see Thompson and Stewart (2009)). Air quality information was received 
from authors of both U.S. and Canadian studies; however, as noted in 
the REA (REA, section 5.5), SO2 concentrations reported for 
Canadian studies are not directly comparable to those reported for 
studies in the U.S. because SO2 levels reported for Canadian 
analyses represent the average 1-hour daily maximum level across 
multiple monitors in a given city (see REA Figure 5-5), rather than the 
concentration from the single monitor that recorded the highest 
SO2 concentration (see Thompson and Stewart, 2009). Thus, 
the REA noted that SO2 concentrations associated with 
Canadian studies would be relatively lower (potentially significantly 
lower) than those levels presented for U.S. epidemiologic studies, and 
therefore the REA focused on 99th percentile air quality information 
from U.S. studies for informing potential 1-hour standard levels.
    Figures 1 to 4 present 99th (and 98th) percentile 1-hour daily 
maximum SO2 concentrations from ten U.S. epidemiologic 
studies (some of which were conducted in multiple cities) of ED visits 
and hospital admissions \23\ (Figures 5-1 to 5-4 in the REA). The REA 
noted that this information provides evidence for effects in cities 
with particular 99th percentile 1-hour SO2 levels, and 
hence, was of particular relevance for identifying standard levels that 
could protect against the SO2 concentrations observed in 
these studies. The air quality information presented in these figures 
generally shows that positive associations between ambient 
SO2 concentrations and ED visit and hospitalizations have 
been reported in cities where 99th percentile 1-hour daily maximum 
SO2 concentrations ranged from approximately 50-460 ppb. 
More specifically, seven of these studies were in cities where 99th 
percentile 1-hour daily maximum SO2 concentrations ranged 
from approximately 75-150 ppb. Among these epidemiologic studies in the 
range of 75-150 ppb, there is a cluster of three studies reporting 
statistically significant results in multi-pollutant models with PM. 
Specifically, in epidemiologic studies conducted in the Bronx, NY 
(NYDOH 2006), and in NYC, NY (Ito et al., 2007), the SO2 
effect estimate remained positive and statistically significant in 
multi-pollutant models with PM2.5 in these locations when 
99th percentile 1-hour daily maximum SO2 levels were 78 and 
82 ppb respectively. (ISA, Table 5-5). Moreover, in an epidemiologic 
study conducted in New Haven, CT (Schwartz et al., 1995), the 
SO2 effect estimate remained positive and statistically 
significant in a multi-pollutant model with PM10 in this 
location when the 99th percentile 1-hour daily maximum SO2 
concentration was 150 ppb. The REA noted that although 
statistical significance in co-pollutant models is an important 
consideration, it is not necessary for appropriate consideration of and 
reliance on such epidemiologic evidence.\24\ However, as noted earlier, 
there is special sensitivity in this review in disentangling PM-related 
effects (especially sulfate PM) from SO2-related effects in 
interpreting the epidemiologic studies; thus, these studies are of 
particular relevance here, lending strong support both to the 
conclusion that SO2 effects are generally independent of PM 
(ISA, section 5.2) and that these independent adverse effects of 
SO2 have occurred in cities with 1-hour daily maximum, 99th 
percentile concentrations in the range of 78-150 ppb.
---------------------------------------------------------------------------

    \23\ In some cases, U.S. authors provided the AQS monitor IDs 
used in their studies and the statistics from the highest reporting 
monitor were calculated by EPA. In cases where U.S. authors were 
unable to provide the requested data (Schwartz 1995, Schwartz 1996, 
and Jaffe 2003), EPA identified the maximum reporting monitor from 
all monitors located in the study area and calculated the 98th and 
99th percentile statistics (see Thompson and Stewart 2009). Results 
presented from study locations for which effect estimates were 
reported.
    \24\ For example, evidence of a pattern of results from a group 
of studies that find effect estimates similar in direction and 
magnitude would warrant consideration of and reliance on such 
studies even if the studies did not all report statistically 
significant associations in single- or multi-pollutant models. The 
SO2 epidemiologic studies fit this pattern, and are 
buttressed further by the results of the clinical studies. ISA, 
section 5.2.
---------------------------------------------------------------------------

    In addition to the study locations where SO2 
concentrations ranged from 75-150 ppb, the REA noted that two 
epidemiologic studies included cities reporting positive associations 
between ambient SO2 levels and ED visits when 99th 
percentile 1-hour daily maximum SO2 concentrations were 
approximately 50 ppb (Wilson et al., (2005) in Portland, ME and Jaffe 
et al., (2003) in Columbus, OH). These studies reported generally 
positive and sometimes statistically significant results using single 
pollutant models (Figures 1 and 2), and did not evaluate potential 
confounding through the use of multi-pollutant models. Nonetheless, 
these studies provide limited evidence of an association between ED 
visits and 99th percentile 1-hour daily maximum SO2 
concentrations in locations where SO2 levels were 
approximately 50 ppb. Finally, the REA noted that studies

[[Page 64836]]

conducted in Cleveland and Cincinnati, OH (Schwartz et al. 1996 and 
Jaffe et al. 2003) reported positive associations between ambient 
SO2 levels and ED visits and hospital admissions when 99th 
percentile 1-hour daily maximum SO2 concentrations in these 
cities ranged from 170-457 ppb (REA, section 5.5). The REA found the 
SO2 level in Cincinnati (Jaffe et al., 2003; REA section 
5.5) to be of particular concern. The 99th percentile 1-hour daily 
maximum SO2 level in Cincinnati was > 400 ppb (Figure 2), 
which in 5-10 minute controlled human exposure studies, was an 
SO2 concentration range consistently shown to result in 
clearly adverse health effects in exercising asthmatics (i.e., 
decrements in lung function accompanied by respiratory symptoms).
    Taken together, the epidemiologic evidence described above suggests 
that standard levels at and below 75 ppb should be considered to limit 
SO2 concentrations such that the upper end of the 
distribution of daily maximum hourly concentrations would likely be 
below that observed in most of these U.S. studies. Notably, a standard 
at or below 75 ppb would be lower than the SO2 air quality 
levels found in the cluster of three epidemiologic studies finding 
statistically significant effects in multi-pollutant models with PM 
(i.e., 99th percentile 1-hour daily maximum SO2 
concentrations >= 78 ppb). Moreover, standard levels at or below 75 ppb 
recognize the limited evidence from two epidemiologic studies reporting 
mostly positive and sometimes statistically significant associations in 
single pollutant models when 99th percentile 1-hour daily maximum 
SO2 concentrations were approximately 50 ppb (Wilson et al., 
(2005) in Portland, ME and Jaffe et al., (2003) in Columbus, OH; see 
Figures 1 and 2). Judgments about the weight to place on uncertainties 
inherent in such studies should also inform selection of a specific 
standard level.
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BILLING CODE 6560-50-C
    The REA also considered findings from controlled human exposure 
studies when evaluating potential alternative standard levels. The ISA 
found that the most consistent evidence of decrements in lung function 
and/or respiratory symptoms was from controlled human exposure studies 
exposing exercising asthmatics to SO2 concentrations >= 400 
ppb for 5-10 minute durations (ISA, section 3.1.3.5). As previously 
mentioned, at SO2 concentrations ranging from 400-600 ppb, 
moderate or greater decrements in lung function occur in approximately 
20-60% of exercising asthmatics, and compared to exposures at 200-300 
ppb, a larger percentage of subjects experience severe decrements in 
lung function. Moreover, at concentrations >= 400 ppb, decrements in 
lung function are often statistically significant at the group mean 
level, and are frequently accompanied by respiratory symptoms (ISA, 
Table 5-1).
---------------------------------------------------------------------------

    \25\ There were no U.S. hospitalization studies with 1-hour 
effect estimates identified in Table 5-5 of the ISA.
---------------------------------------------------------------------------

    Controlled human exposure studies have also demonstrated decrements 
in lung function in exercising asthmatics following 5-10 minute 
SO2 exposures starting as low as 200-300 ppb in free-
breathing chamber studies. At concentrations ranging from 200-300 ppb, 
the lowest levels tested in free breathing chamber studies, 
approximately 5-30% percent of exercising asthmatics are likely to 
experience moderate or greater decrements in lung function in these 
studies. Moreover, although these individuals experienced lung function 
decrements, they were not frequently accompanied by respiratory 
symptoms and at these SO2 concentrations, group mean changes 
in lung function have not been shown to be statistically significant. 
However, the ISA and REA noted that for evident ethical reasons, the 
subjects participating in the controlled human exposure studies 
described above do not include the most severe asthmatics. Thus, the 
REA found it is reasonable to anticipate that individuals who are more 
sensitive to SO2 would have a greater response at 200-300 
ppb SO2, and/or would respond to SO2 
concentrations even lower than 200 ppb (REA, section 10.5.4). 
Similarly, the REA noted that there is no evidence to suggest that 200 
ppb represents a threshold below which no adverse respiratory effects 
occur (REA, section 10.5.4). In fact, limited evidence from two 
mouthpiece exposure studies suggests that exposure to 100 ppb 
SO2 can result in small decrements in lung function.\26\
---------------------------------------------------------------------------

    \26\ Although not directly comparable to free-breathing chamber 
studies, findings from these mouthpiece studies may be particularly 
relevant to those asthmatics who breathe oronasally even at rest 
(EPA, 1994b).
---------------------------------------------------------------------------

    Considering the controlled human exposure evidence presented above, 
the ISA concluded that as SO2 concentrations increase the 
percentage of asthmatics affected increases as does the severity of the 
response. Moreover, as previously noted, effects associated with 
SO2 concentrations >= 400 ppb are clearly considered adverse 
effects of air pollution under ATS guidelines, while effects at 200-300 
ppb were considered adverse in the REA based on interpretation of ATS 
guidelines, CASAC recommendations, and previous conclusions from 
comparable situations in other NAAQS reviews (see section II.B.1.c). 
Taken together, the REA concluded that the level of a new 99th 
percentile 1-hour daily maximum

[[Page 64840]]

standard should provide substantial protection against SO2 
concentrations >= 400 ppb, and appreciable protection against 5-minute 
SO2 concentrations >= 200 ppb (REA, section 10.5.4).
b. Air quality, exposure and risk-based considerations
    In evaluating the extent to which 99th percentile 1-hour daily 
maximum alternative standard levels limit 5-minute SO2 
concentrations >= 400 and >= 200 ppb, the REA first considered key 
results of the air quality analysis. As previously noted, the results 
generated from the air quality analysis were from 40 counties and 
considered a broad characterization of national air quality and human 
exposures that might be associated with these 5-minute SO2 
concentrations (see section II.C). However, there is uncertainty 
associated with the assumption that SO2 air quality measured 
at fixed site monitors can serve as an adequate surrogate for total 
exposure to ambient SO2. Actual exposures might be 
influenced by factors not considered in this analysis including small 
scale spatial variability in ambient SO2 concentrations 
(which might not be captured by the network of fixed-site ambient 
monitors) and spatial/temporal variability in human activity patterns.
    Table 2 reports the maximum mean number of days per year 5-minute 
daily maximum SO2 levels would be expected to exceed a given 
5-minute potential health effect benchmark level in any of the 40 
counties included in the air quality analysis, given air quality 
simulated to just meet the current, and potential alternative 99th 
percentile 1-hour daily maximum standards analyzed in the REA. In 
addition, although not directly analyzed in the REA, these tables 
include air quality results given a 99th percentile 1-hour daily 
maximum standard at 75 ppb; this concentration was included in these 
tables because as mentioned above, the epidemiologic evidence suggested 
consideration of a standard level at or below 75 ppb.\27\ Table 2 shows 
that at standard levels ranging from 50-100 ppb, there would be at most 
two days per year when statistically estimated 5-minute SO2 
concentrations in these counties exceed the 400 ppb benchmark, while at 
standard levels of 150 ppb and above there is a marked increase in the 
maximum number of days per year the 400 ppb benchmark is exceeded. 
Similar trends are seen with respect to the 300 ppb benchmark level. 
With respect to the 200 and 100 ppb benchmarks, the 50 ppb standard is 
clearly the most effective at limiting these 5-minute SO2 
concentrations. However, compared to standards at 150 ppb and above, 
standards in the range of 75-100 ppb would allow considerably less 
exceedence of the 200 and 100 ppb benchmarks. Additional and more 
detailed results from the air quality analysis can be found in chapter 
7 of the REA.
---------------------------------------------------------------------------

    \27\ Air quality, exposure, and risk numbers reported in Chapter 
10 of the REA for a 75 ppb standard were bound by the estimates from 
air quality adjusted to just meet 99th percentile 1-hour daily 
maximum standards at 50 and 100 ppb.

Table 2--Maximum Mean Number of Days Per Year in Any of the Counties Included in the Air Quality Analysis When 5-Minute Daily Maximum SO2 Concentrations
    Exceed the 100, 200, 300, and 400 ppb Potential Health Effect Benchmark Values Given Air Quality Adjusted To Just Meet the Current Standards, or
                                               Alternative 99th Percentile 1-Hour Daily Maximum Standards
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                                 Air quality scenarios
                                                              ------------------------------------------------------------------------------------------
                                                                   Just                    99th percentile 1-hour daily maximum standards
           Exposure benchmarks (5-minute exposures)              meeting   -----------------------------------------------------------------------------
                                                                 current
                                                                standards      50 ppb       75 ppb      100 ppb      150 ppb      200 ppb      250 ppb
--------------------------------------------------------------------------------------------------------------------------------------------------------
400 ppb......................................................          102            0        (0-2)            2            7           13           18
300 ppb......................................................          130            0        (0-5)            5           13           20           27
200 ppb......................................................          171            2       (2-13)           13           24           42           69
100 ppb......................................................          234           13      (13-43)           43           93          133          180
--------------------------------------------------------------------------------------------------------------------------------------------------------

    While the air quality analysis results presented in Table 2 used 
estimated 5-minute SO2 concentrations as a surrogate for 
exposure, the results from the exposure analysis considered the 
likelihood that an asthmatic at elevated ventilation rate would come 
into contact with a 5-minute SO2 concentration at or above a 
given benchmark level one or more times per year. As previously noted, 
this resource intensive analysis was performed for St. Louis and Greene 
County, MO, but results from the St. Louis analysis were found to be 
more informative with respect to informing standard levels given that 
the St. Louis results: (1) Suggested that the current standards were 
not adequate to protect public health; and (2) likely provide useful 
insights into exposures and risk for other urban areas in the U.S. with 
similar population and SO2 emissions density (i.e., areas 
where SO2 exposures are more likely).
    Table 3 reports the estimated percent of asthmatic children at 
moderate or greater exertion in St. Louis, that would be expected to 
experience at least one SO2 exposure per year, at or above a 
health effect benchmark level in scenarios in which air quality was 
adjusted to meet the current, and alternative 99th percentile 1-hour 
daily maximum standards. This analysis estimates that standard levels 
ranging from 50-100 ppb would protect > 99% of asthmatic children, at 
moderate or greater exertion, from experiencing at least one 
SO2 exposure >= 400 ppb per year.\28\ Similarly, a standard 
at 150 ppb is estimated to protect ~ 99% of asthmatic children at 
moderate or greater exertion from experiencing at least one 
SO2 exposure >= 400 ppb. Compared to standards ranging from 
50-150 ppb, standards at 200 and 250 ppb are estimated to allow 
appreciably more exposures >= 400 ppb (Table 3). With respect to the 
300 ppb benchmark, standards at 50, 75, and 100 ppb provide similar 
protection, while there is a marked increase in exposures of asthmatic 
children at moderate or greater exertion at standard levels >= 150 ppb 
(Table 3). Considering the 200 ppb benchmark level, it is estimated 
that 1-hour standard levels ranging from 50-100 ppb limit 5-minute 
SO2 exposures >= 200 ppb considerably more than 1-hour 
standard levels >= 150 ppb. More

[[Page 64841]]

specifically, standards in the range of 50-100 ppb are estimated to 
protect approximately 97 to > 99% of asthmatic children at moderate or 
greater exertion from experiencing at least one 5-minute exposure >= 
200 ppb per year, while standards ranging from 150-250 ppb are 
estimated to protect approximately 60 to 88% of these children from 
experiencing at least one 5-minute SO2 exposure >= 200 ppb 
per year. Finally, similar to the air quality analysis, a standard at 
50 ppb is clearly most effective at limiting 5-minute SO2 
exposures >= 100 ppb. Additional and more detailed results from the 
exposure assessment can be found in chapter 8 of the REA.
---------------------------------------------------------------------------

    \28\ Table 3 reports that given a 99th percentile 1-hour daily 
maximum standard in the range of 50-100 ppb, < 1% of asthmatic 
children at moderate or greater exertion would be estimated to 
experience an SO2 exposure >= 400 ppb, hence it can be 
stated that this range of levels would protect > 99% of asthmatic 
children at moderate or greater exertion from experiencing at least 
one SO2 exposure >= 400 ppb per year.

   Table 3--Estimated Percent of Asthmatic Children in St. Louis at Moderate or Greater Exertion Expected to Experience at Least One 5-Minute Exposure
     Above the 100, 200, 300, and 400 ppb Potential Health Effect Benchmark Levels Given Air Quality Adjusted To Just Meet the Current Standards, or
                                               Alternative 99th Percentile 1-Hour Daily Maximum Standards
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                        Air quality scenarios
                                            ------------------------------------------------------------------------------------------------------------
                                                 Just                             99th Percentile 1-hour daily maximum standards
  Exposure benchmarks (5-minute exposures)     meeting   -----------------------------------------------------------------------------------------------
                                               current
                                              standards      50 ppb                75 ppb               100 ppb      150 ppb      200 ppb      250 ppb
--------------------------------------------------------------------------------------------------------------------------------------------------------
400 ppb....................................          24%         < 1%  < 1%.........................         < 1%          ~1%         2.7%         6.3%
300 ppb....................................        43.8%         < 1%  < 1%.........................         < 1%         2.7%           8%          16%
200 ppb....................................        73.1%         < 1%  (~1 to 2.7%).................         2.7%        11.6%        24.5%          40%
100 ppb....................................        96.7%         2.7%  (2.7 to 24.5%)...............        24.5%        54.5%        73.6%        84.8%
--------------------------------------------------------------------------------------------------------------------------------------------------------

    In evaluating the extent to which alternative standard levels 
provide protection against the health effects associated with 5-minute 
SO2 exposures, the REA also considered key results from the 
quantitative risk assessment (REA, chapter 9). Table 4 presents the 
percent of exposed asthmatic children at moderate or greater exertion 
in St. Louis expected to experience at least one moderate or greater 
lung function response per year, in terms of sRaw, given the 99th 
percentile 1-hour daily maximum standards analyzed in the REA. Results 
presented in Table 4 show that standard levels in the range of 100 to 
150 ppb would generally be expected to protect approximately 95 to 98% 
of exposed asthmatic children at moderate or greater exertion from 
experiencing at least one >= 100% increase in sRaw per year, while 
standards around and below 75 ppb would be estimated to provide exposed 
asthmatic children with protection approaching 99% or greater. 
Additional and more detailed risk analyses can be found in chapter 9 of 
the REA.

    Table 4--Estimated Percent of Asthmatic Children in St. Louis at Moderate or Greater Exertion Expected To
 Experience a >= 100% Increase in sRaw Given Air Quality Adjusted To Just Meet Either the Current Standards, or
                           Alternative 99th Percentile 1-Hour Daily Maximum Standards
----------------------------------------------------------------------------------------------------------------
                                              Air quality scenarios
-----------------------------------------------------------------------------------------------------------------
                                                   99th Percentile 1-hour daily maximum standards
  Just meeting current standards   -----------------------------------------------------------------------------
                                       50 ppb       75 ppb      100 ppb      150 ppb      200 ppb      250 ppb
----------------------------------------------------------------------------------------------------------------
19.1-19.2%........................     0.4-0.9%   (0.4-2.9%)     2.1-2.9%     4.6-5.4%     7.4-8.1%   10.4-10.9%
----------------------------------------------------------------------------------------------------------------

c. Observations based on evidence and risk-based considerations
    The policy assessment chapter of the REA considered the scientific 
evidence and the air quality, exposure, and risk information as they 
relate to considering alternative 1-hour SO2 standards that 
could be judged to be requisite to protect public health with an 
adequate margin of safety. This evidence and information supports the 
following conclusions:
     Given the U.S. epidemiologic evidence and their associated 
air quality levels (see Figures 1-4), 99th percentile 1-hour standard 
levels at and below 75 ppb should be considered to limit SO2 
concentrations such that the upper end of the distribution of daily 
maximum hourly concentrations would likely be below that observed in 
most of the U.S. studies. Judgments about the weight to place on 
uncertainties inherent in such studies should also inform selection of 
a specific standard level.
     Based on the air quality and exposure results, 1-hour 
standard levels in the range of 50-100 ppb should be considered to 
substantially limit 5-minute SO2 concentrations >= 400 ppb 
and appreciably limit 5-minute SO2 concentrations >= 200 
ppb.
     Based on the air quality and exposure results, compared to 
a 1-hour standard in the range of 50-100 ppb, a 1-hour standard level 
at 150 ppb would be expected similarly limit 5-minute SO2 
concentrations >= 400 ppb, but would limit 5-minute SO2 
concentrations >= 200 ppb considerably less.
     If relatively more weight is placed on certain types of 
uncertainties in the epidemiologic and controlled human exposure 
evidence, levels up to 150 ppb could be considered, recognizing the 
questions as to the adequacy of protection that would be raised by 
levels at the higher end of this range.
     Placing relatively more weight on the consideration that 
participants in controlled human exposure studies do not include the 
most severe asthmatics would add support to considering standard levels 
down to 50 ppb.
d. CASAC views
    CASAC expressed their views on potential levels for a standard in a 
letter to the EPA Administrator (Samet, 2009) within the context of 
their review of the 2nd draft REA, which also contained the draft 
policy assessment chapter. In drawing conclusions regarding the level 
of a short-term standard, CASAC considered the scientific evidence

[[Page 64842]]

evaluated in the ISA, the air quality, exposure, and risk results 
presented in the 2nd draft REA, and the evidence- and risk-based 
considerations presented in the policy assessment chapter of the 2nd 
draft REA. CASAC concurred with the conclusion from the policy 
assessment chapter for a range of standard levels beginning at 50 ppb: 
``[that chapter 10] clearly provides sufficient rationale for the range 
of levels beginning at a lower limit of 50 ppb'' (Samet 2009, p. 16). 
For instance, CASAC has previously indicated that EPA should consider 
in its analyses the uncertainty that asthmatics participating in 
controlled human exposure studies do not represent the most 
SO2 sensitive asthmatics (Henderson 2008 p. 6). With respect 
to the upper end of the range, CASAC stated, ``an upper limit of 150 
ppb posited in Chapter 10 could be justified under some interpretations 
of weight of evidence, uncertainties, and policy choices regarding 
margin of safety,'' (Samet 2009, p. 16) although the letter did not 
provide any indication of what interpretations, uncertainties, or 
policy choices might support selection of a level as high as 150 ppb. 
Further, CASAC stated that ``the draft REA appropriately implies that 
levels greater than 150 ppb are not adequately supported'' (id). 
Moreover, CASAC stated that: ``the panel agrees that the posited range 
of 50 to 150 ppb and the exposition of factors to consider when 
comparing values within the range are appropriately conveyed (Samet 
2009, p. 16).''
e. Administrator's conclusions on level for a 1-hour standard
    As discussed above, in sections II.F.2 and II.F.3, the 
Administrator has proposed setting a 1-hour standard with a 99th 
percentile form. For the reasons discussed below, the Administrator 
proposes to set a level for a new 99th percentile 1-hour daily maximum 
primary SO2 standard within the range from 50 to 100 ppb. In 
reaching this proposed decision, the Administrator has considered: (1) 
The evidence-based considerations from the final ISA and the final REA; 
(2) the results of the air quality, exposure, and risk assessments 
discussed above and in the final REA; (3) CASAC advice and 
recommendations on both the ISA and REA discussed above and provided in 
CASAC's letters to the Administrator; and (4) public comments received 
on the first and second drafts of the ISA and REA. In considering what 
level of a 1-hour SO2 standard is requisite to protect 
public health with an adequate margin of safety, the Administrator is 
mindful that this choice requires judgments based on an interpretation 
of the evidence and other information that neither overstates nor 
understates the strength and limitations of that evidence and 
information.
    The Administrator notes that the most direct evidence of 
respiratory effects from exposure to SO2 comes from the 
controlled human exposure studies. These studies exposed groups of 
exercising asthmatics to defined concentrations of SO2 for 
5-10 minutes and found adverse respiratory effects. As discussed above, 
SO2 exposure levels which resulted in respiratory effects in 
controlled human exposure studies were used in the REA as 5-minute 
benchmark exposures of potential concern. With respect to these 5-
minute benchmarks, the Administrator focused on exceedences of the 400 
and 200 ppb benchmarks. She notes that under ATS guidelines (ATS 1985, 
2000) exposure to 5-10 minute SO2 concentrations >= 400 ppb 
results in health effects which are clearly adverse: moderate or 
greater decrements in lung function (in terms of FEV1 or 
sRaw \29\) that are frequently accompanied by respiratory symptoms.\30\
---------------------------------------------------------------------------

    \29\ Decreases of 10-20% in FEV1 (forced expiratory 
volume) and/or 100-200% increases in sRaw (specific airway 
resistance) are defined as moderate decrements in lung function.
    \30\ The ISA concluded that collective evidence from controlled 
human exposure studies considered in the previous review, along with 
a limited number of new controlled human exposure studies, 
consistently indicates that with elevated ventilation rates a large 
percentage of asthmatic individuals tested in a given chamber study 
(up to 60%, depending on the study) experience moderate or greater 
decrements in lung function, frequently accompanied by respiratory 
symptoms, following peak exposures to SO2 at 
concentrations of 0.4-0.6 ppm. (ISA, p 3-9).
---------------------------------------------------------------------------

    The Administrator also focused on exceedences of the 200 ppb 
benchmark, the lowest SO2 concentration tested in free-
breathing chamber studies. In these studies, moderate or greater 
decrements in lung function occurred in approximately 5 to 30% of 
exercising asthmatics, depending on the study. The Administrator 
further notes that while concentrations as low as 200 ppb have not been 
frequently accompanied by respiratory symptoms, she considers these 
effects to be adverse in light of CASAC advice and ATS guidelines. The 
REA concluded that these controlled human exposure studies could 
reasonably be interpreted to indicate an SO2-induced shift 
in lung function for this population of asthmatics (REA, section 4.3), 
such that asthmatics would have diminished reserve lung function and 
would be at greater risk if affected by another respiratory agent 
(e.g., viral infection). Importantly, diminished reserve lung function 
in a population that is attributable to air pollution is an adverse 
effect under ATS guidelines as discussed in section II.B.1.c.
    As discussed below, the Administrator also considered the results 
of the air quality, exposure, and risk analyses, as they serve to 
estimate the extent to which a given 1-hour standard limits peaks of 
SO2 above the 5-minute benchmark concentrations derived from 
controlled human exposure studies. In considering these results as they 
relate to limiting 5-minute SO2 concentrations >= 400 ppb 
and >= 200 ppb, and being mindful that more severe effects occur 
following 5-minute SO2 exposures >= 400 ppb, the 
Administrator finds the most support for 99th percentile 1-hour daily 
maximum standard levels up to 100 ppb to protect against 5-minute 
SO2 exposures >= 200 ppb. She notes that the 40-county air 
quality analysis estimates that a 100 ppb 1-hour standard would allow 
at most 2 days per year on average when estimated 5-minute daily 
maximum SO2 concentrations exceed the 400 ppb benchmark, and 
at most 13 days per year on average when 5-minute SO2 
concentrations exceed the 200 ppb benchmark (Table 2). Furthermore, 
given a simulated 1-hour 100 ppb standard level, most counties in the 
air quality analysis were estimated to experience 0 days per year on 
average when 5-minute SO2 concentrations exceed the 400 ppb 
benchmark and <= 3 days per year on average when 5-minute 
SO2 concentrations were estimated to exceed the 200 ppb 
benchmark (see REA, Tables 7-14 and 7-12).
    In addition, the St. Louis exposure analysis estimates that a 99th 
percentile 1-hour standard at a level of 100 ppb would likely protect > 
99% of asthmatic children at moderate or greater exertion from 
experiencing at least one 5-minute exposure >= 400 ppb per year, and 
approximately 97% of asthmatic children at moderate or greater exertion 
from experiencing at least one exposure >= 200 ppb per year. In 
contrast, the Administrator notes that the St. Louis exposure analysis 
estimates a 99th percentile 1-hour daily maximum standard at a level of 
150 ppb would likely protect only about 88% of asthmatic children at 
moderate or greater exertion from experiencing at least one 5-minute 
exposure >= 200 ppb per year. Finally, the Administrator notes that the 
St. Louis risk assessment estimates that a 99th percentile 1-hour 
standard level at 100 ppb would likely protect about 97-98% of exposed 
asthmatic children from experiencing at least one moderate or greater 
lung function response (defined as a >= 100% increase in sRaw). Based 
on these

[[Page 64843]]

considerations, she concludes that there is support for a 99th 
percentile 1-hour daily maximum standard level at or below 100 ppb to 
appreciably limit 5-minute exposures to SO2 above the 200 
ppb benchmark level.
    Turning to the epidemiologic evidence, the Administrator notes that 
epidemiologic studies have reported associations between more serious 
health outcomes (i.e. respiratory-related ED visits and 
hospitalizations) and ambient SO2 concentrations. Unlike the 
controlled human exposure studies however, results from epidemiologic 
studies can be complicated by the fact that SO2 is but one 
component of a complex mixture of pollutants in the ambient air. This 
uncertainty is addressed by the ISA which concluded that the limited 
available evidence indicates that the effect of SO2 on 
respiratory health outcomes appears to be generally robust and 
independent of the effects of gaseous co-pollutants, including 
NO2 and O3, as well as particulate co-pollutants, 
particularly PM2.5 (ISA, section 5.2; p. 5-9).
    The Administrator also notes that in general, associations reported 
in epidemiologic analyses are not associated with a defined exposure 
level of a pollutant (unlike the controlled human exposure studies), 
but represent concentrations of a pollutant taken from ambient 
monitoring data during the study period. These concentrations are used 
as surrogates for the distribution of pollutant exposures across the 
study area over the time period of the study. This introduces a degree 
of uncertainty in the interpretation of epidemiologic results in that 
it can be difficult to discern what part of the distribution of 
pollutant levels are likely most linked to the associations reported in 
epidemiologic analyses.
    With respect to SO2 specifically, the Administrator 
notes that adverse respiratory effects in epidemiologic studies are 
especially likely to occur at the upper end of the distribution of 
ambient SO2 concentrations. Although some epidemiologic 
studies reported a linear relationship across the entire range of 
SO2 concentrations, a few other studies found that the 
increase in SO2-related respiratory health effects was 
observed at the upper end of the distribution of SO2 
concentrations (ISA, section 5.3, p. 5-9). For example, an 
epidemiologic study conducted in Bronx, NY suggested an increased risk 
of asthma hospitalizations on the days with the highest SO2 
concentrations (Lin et al., 2004). More specifically, these authors 
observed increased risk of asthma hospitalizations at SO2 
concentrations somewhere between the 90th and 95th percentiles (ISA, 
section 4.1.2 and ISA, Figure 4-4).
    This epidemiologic evidence, though not independently sufficient to 
draw conclusions regarding causation, is consistent with, and informed 
by, the large body of controlled human exposure studies of exercising 
asthmatics exposed to short-term peak concentrations of SO2; 
these controlled human exposure studies provide the ``definitive 
evidence'' that short-term peak SO2 exposure is associated 
with respiratory morbidity (ISA, Section 5.3, page 5-8). These studies 
consistently found moderate or greater decrements in lung function 
(i.e. >= 100% increase in sRaw and/or >= 15% decline in 
FEV1) and/or respiratory symptoms in exercising asthmatics 
following 5-10 minute peak exposures to SO2. Discussing the 
possible relationship between effects observed in these controlled 
human exposure studies and the associations reported in the 
epidemiologic studies, the ISA stated: ``it is possible that these 
associations [in the epidemiologic studies] are determined in large 
part by peak exposures within a 24-hour period'' (ISA, section 5.2 at 
p. 5-5). Similarly, the ISA stated that: ``the effects of 
SO2 on respiratory symptoms, lung function, and airway 
inflammation observed in the human clinical studies using peak 
exposures further provides a basis for a progression of respiratory 
morbidity resulting in increased ED visits and hospital admissions'' 
and makes the associations observed in the epidemiologic studies 
``biologica[lly] plausib[le]'' (ISA, section 5.2 at p. 5-5). Thus, 
considered together, the epidemiologic and controlled human exposure 
evidence suggest that it is a reasonable approach to move the air 
quality distribution lower in a manner that targets control of both 
hourly and 5-10 minute peak SO2 exposures.
    For the reasons discussed above in section II.F.3, the 
Administrator has proposed a 99th percentile of the 1-hour daily 
maximum concentration as an appropriate form.\31\ Moreover, as just 
discussed, there is support for the Agency's view that adverse 
respiratory effects in epidemiologic studies are especially likely to 
occur at the upper end of the distribution of ambient SO2 
concentrations. Therefore, the Administrator finds it reasonable to 
focus on limiting the 99th percentile SO2 levels reported in 
locations where positive associations were found in key epidemiologic 
studies. Adjusting the distribution of SO2 levels in this 
manner will target control of those hourly and 5-10 minute peak 
SO2 concentrations that are of most concern.
---------------------------------------------------------------------------

    \31\ As previously discussed in section II.F.3, a 99th 
percentile form was proposed to: (1) Minimize the number of days per 
year that an area could exceed the level of the standard and still 
attain the standard; (2) limit the prevalence of 5-minute peaks of 
SO2; and (3) provide a stable regulatory target to 
prevent areas from frequently shifting in and out of attainment.
---------------------------------------------------------------------------

    In considering the epidemiologic evidence with regard to level, the 
Administrator notes that there have been more than 50 peer reviewed 
epidemiologic studies evaluating SO2 published worldwide 
(ISA, Tables 5-4 and 5-5). The Administrator finds that in assessing 
the extent to which these studies and their associated air quality 
information can inform the level of a new 99th percentile 1-hour daily 
maximum standard, U.S. and Canadian air quality information is most 
relevant. As described in section II.F.4.a, SO2 
concentrations reported for Canadian studies are not directly 
comparable to those reported for U.S. studies. That is, concentrations 
reported for Canadian analyses represent the average 99th percentile 1-
hour daily maximum level across multiple monitors in a given city (REA 
Figure 5-5), rather than the concentration from the single monitor that 
recorded the highest SO2 level (see Thompson and Stewart, 
2009). Thus, the Administrator focused on 99th percentile air quality 
information from U.S. studies for informing potential 1-hour standard 
levels.
    The Administrator notes that Figures 1 to 4 include 99th percentile 
1-hour daily maximum SO2 concentrations from ten U.S. 
epidemiologic studies of ED visits and hospital admissions (Figures 5-1 
to 5-4 in the REA). The Administrator agrees with the REA finding that 
this information provides evidence of associations between ambient 
SO2 and ED visits and hospital admissions in cities with 
particular 99th percentile 1-hour SO2 levels. This 
information is relevant for identifying standard levels that could 
significantly limit SO2 concentrations so that the upper end 
of the distribution of daily maximum hourly concentrations would likely 
be below that observed in most of these studies. These figures report 
mostly positive, and sometimes statistically significant, associations 
between ambient SO2 concentrations and ED visit and hospital 
admissions in locations where 99th percentile 1-hour daily maximum 
SO2 levels ranged from 50-460 ppb. Moreover, within this 
broader range of SO2 concentrations, seven of these studies 
were in locations where the 99th percentile of the 1-hour daily maximum 
SO2 concentrations were in the range of 75-150 ppb. The 
Administrator particularly notes the

[[Page 64844]]

cluster of three epidemiologic studies between 78-150 ppb (for the 99th 
percentile of the 1-hour SO2 concentrations) where the 
SO2 effect estimate remained positive and statistically 
significant in multi-pollutant models with PM (NYDOH (2006), Ito et 
al., (2007), and Schwartz et al., (1995)). The Administrator also notes 
the limited evidence from two epidemiologic studies employing single 
pollutant models that found mostly positive, and sometimes 
statistically significant, associations between ambient SO2 
and ED visits in locations where 1-hour SO2 concentrations 
were approximately 50 ppb (Figures 1 and 2). Based on the 
interpretation of the epidemiologic evidence discussed above, the 
Administrator concludes that this evidence provides support for 
consideration of a 99th percentile 1-hour daily maximum standard level 
at or below 75 ppb to limit SO2 concentrations such that the 
upper end of the distribution of daily maximum hourly concentrations 
would likely be below that observed in most of the U.S. studies. The 
Administrator also recognizes that judgments about the weight to place 
on uncertainties inherent in such studies should inform selection of a 
specific standard level.
    Based on the epidemiologic and controlled human exposure 
information presented above, the Administrator considered what range of 
standard levels would be requisite to protect public health, including 
the health of at-risk groups, with an adequate margin of safety that is 
sufficient but not more than necessary to achieve that result. The 
assessment of a standard level calls for consideration of both the 
degree of risk to public health at alternative levels of the standard 
as well as the certainty that such risk will occur at any specific 
level. Based on the information available in the ISA, there is no 
evidence-based bright line that indicates a single appropriate level. 
Moreover, given that a 1-hour averaging time is being used to control 
5-minute peaks of SO2, the Administrator also recognizes 
that the results of the air quality, exposure, and risk analyses will 
have to be considered given that these analyses indicate the extent to 
which a particular 99th percentile 1-hour daily maximum standard will 
likely limit 5-minute SO2 peaks of a given concentration. 
Thus, the combination of scientific evidence and air quality, exposure, 
and risk-based information needs to be considered as a whole in making 
this public health policy judgment.
    In selecting a level that would serve as an appropriate upper end 
for a range of levels to propose, the Administrator has considered a 
cautious approach to interpreting the available evidence and exposure/
risk-based information--that is, an approach that places relatively 
more weight on those types of uncertainties and limitations in the 
information that would lead to placing less reliance on the results of 
the epidemiologic studies. This approach would tend to avoid 
potentially overestimating public health risks and the degree of 
protection likely to be associated with just meeting a particular 
standard level. This approach would place more weight in particular on 
uncertainties in epidemiologic evidence such as concerns related to 
exposure measurement error, the possible role of co-pollutants and 
effects modifiers, and interindividual differences in susceptibility to 
SO2-related effects.
    In applying this approach, the Administrator has selected an upper 
end of a range of levels to propose at 100 ppb. The selection of this 
level focuses on the results of the controlled human exposure studies 
and is primarily based on the results of the air quality and exposure 
analyses which suggest that a 1-hour standard should be at or below 100 
ppb to appreciably limit 5-minute SO2 benchmark 
concentrations >= 200 ppb. That is, as mentioned above, the St. Louis 
exposure analysis indicates that a 1-hour standard at 100 ppb would 
still be estimated to protect about 97% of asthmatic children at 
moderate or greater exertion from experiencing at least one 5-minute 
SO2 exposure >= 200 ppb. In contrast, the St. Louis exposure 
analysis estimates that a 1-hour standard at 150 ppb would likely only 
protect about 88% of asthmatic children at moderate or greater exertion 
from experiencing at least one 5-minute SO2 exposure >= 200 
ppb.
    In selecting a level that would serve as an appropriate lower end 
for a range of levels to propose, the Administrator has considered a 
precautionary approach to interpreting the available evidence and 
exposure/risk-based information--that is, an approach that places 
relatively more weight on the results of the epidemiological studies, 
as well as more weight on those types of uncertainties that may be 
associated with potentially underestimating health effects in the most 
sensitive populations. This approach would tend to avoid potentially 
underestimating public health risks and the degree of protection likely 
to be associated with just meeting a particular standard level. This 
approach would place more weight on the consideration that the 
participants in controlled human exposure studies did not include 
individuals with severe asthma.
    In applying this approach, she has selected 50 ppb as the lower end 
of a range of levels to propose, which is consistent with CASAC's 
advice. The selection of this level focuses in part on the 
epidemiologic evidence. With respect to the epidemiologic studies, 
seven of ten U.S. ED visit and hospital admission studies reporting 
generally positive associations with ambient SO2 were 
conducted in locations where 99th percentile 1-hour daily maximum 
SO2 levels were about 75-150 ppb, and three of these studies 
observed statistically significant positive associations between 
ambient SO2 and respiratory-related ED visits and 
hospitalizations in multi-pollutant models with PM (NYDOH (2006), Ito 
et al., (2007), and Schwartz et al., (1995)). Further, the 
Administrator notes that a 99th percentile 1-hour daily maximum 
standard set at a level of 50 ppb is well below the 99th percentile 1-
hour daily maximum SO2 concentrations reported in locations 
where these studies were conducted (i.e. well below 99th percentile 1-
hour daily maximum SO2 levels of 78-150 ppb). Finally, the 
Administrator notes that two epidemiologic studies reported generally 
positive associations between ambient SO2 and ED visits in 
cities when 99th percentile 1-hour daily maximum SO2 
concentrations were approximately 50 ppb, but does not consider that 
evidence strong enough to set a lower standard level.
    In considering the results of the air quality and exposure 
analyses, the Administrator also notes that the 40-county air quality 
analysis estimates that a 99th percentile 1-hour daily maximum standard 
set at a level of 50 ppb would result in zero days per year when 
estimated 5-minute SO2 concentrations exceed the 400 ppb 5-
minute benchmark level and at most 2 days per year when modeled 5-
minute SO2 concentrations exceed the 200 ppb 5-minute 
benchmark level. In addition, the St. Louis exposure analysis estimates 
that a 99th percentile 1-hour daily maximum standard set at a level of 
50 ppb would likely protect > 99% of asthmatic children at moderate or 
greater exertion from experiencing at least one 5-minute exposure both 
>= 400 and >= 200 ppb per year.
    The Administrator thus proposes to set the level of a new 1-hour 
standard that would protect public health with an adequate margin of 
safety between 50 ppb and 100 ppb. In so doing, the Administrator is 
relying on reported findings from both epidemiologic and controlled 
human exposure studies, as well as the results of air quality and 
exposure analyses. The Administrator

[[Page 64845]]

solicits comment on this proposed range of standard levels as well as 
on the approach she has used to identify the range. Specifically, the 
Administrator solicits comment on the following:
     The weight she has placed on the epidemiologic evidence, 
the controlled human exposure evidence, and the air quality, exposure, 
and risk information, the benchmark used to select the proposed range, 
and the uncertainties associated with each of these.
     The most appropriate level within this proposed range 
given the available scientific evidence, and air quality, exposure, and 
risk information, and the uncertainties associated with each.
    With regard to the proposed range of standard levels, the 
Administrator notes that the lower end of the proposed range is 
consistent with CASAC advice that there is clearly sufficient evidence 
for consideration of standard levels starting at 50 ppb (Samet 2009). 
With respect to the upper end of the proposed range, the Administrator 
notes that CASAC concluded that standards up to 150 ppb ``could be 
justified under some interpretations of weight of evidence, 
uncertainties, and policy choices regarding margin of safety'' (Samet 
2009, p. 16), although the letter did not provide any indication of 
what interpretations, uncertainties, or policy choices might support 
selection of a level as high as 150 ppb.
    In light of the range of levels included in CASAC's advice, the 
Administrator solicits comment on setting a standard level above 100 
ppb and up to 150 ppb. In so doing, the Administrator again recognizes 
that there are uncertainties with the scientific evidence, such as 
attributing effects reported in epidemiologic studies specifically to 
SO2 given the presence of co-occurring pollutants, 
especially PM, and the uncertainties associated with using ambient 
SO2 concentrations as a surrogate for exposure. Any comments 
should specifically address the cluster of epidemiologic studies that 
remained statistically significant in co-pollutant models with PM, two 
of which had 99th percentile levels appreciably lower than 150 ppb. 
Commenters should also address the conclusion in the ISA that the 
respiratory effects seen in the epidemiologic studies are generally 
robust and independent of co-pollutants. In addition, the Administrator 
notes that compared to the proposed range of 50-100 ppb, a standard 
level as high as 150 ppb would not comparably limit 5-minute 
SO2 exposures >= 200 ppb. She notes that the St. Louis 
exposure analysis estimates that a 150 ppb standard would protect 
approximately 88% of asthmatic children at moderate or greater exertion 
from experiencing at least one SO2 exposure >= 200 ppb per 
year (compared to > 99% and approximately 97% given standards at 50 and 
100 ppb respectively; see Table 3). There are also questions as to 
whether a standard set at this level would provide an adequate margin 
of safety. Thus, with respect to considering whether it would be 
appropriate to set a standard level as high as 150 ppb, the 
Administrator invites comment on the extent to which it is appropriate 
to emphasize uncertainties with respect to the epidemiologic evidence. 
She also invites comment on the implications such considerations would 
have on limiting 5-minute SO2 exposures >= 200 ppb.
5. Implications for retaining or revoking current standards
    The REA recognized that the particular level selected for a new 1-
hour daily maximum standard would have implications for reaching 
decisions on whether to retain or revoke the current 24-hour and annual 
standards. That is, with respect to SO2-induced respiratory 
morbidity, the lower the level selected for a 99th percentile 1-hour 
daily maximum standard, the less additional public health protection 
the current standards would be expected to provide. As previously 
mentioned (see section II.E.3), CASAC expressed a similar view 
following their review of the 2nd draft REA: ``assuming that EPA adopts 
a one hour standard in the range suggested, and if there is evidence 
showing that the short-term standard provides equivalent protection of 
public health in the long-term as the annual standard, the panel is 
supportive of the REA discussion of discontinuing the annual standard'' 
(Samet 2009, p. 15). With regard to the current 24-hour standard, CASAC 
was generally supportive of using the air quality analyses in the REA 
as a means of determining whether the current 24-hour standard was 
needed in addition to a new 1-hour standard to protect public health. 
CASAC stated: ``the evidence presented [in REA Table 10-3] was 
convincing that some of the alternative one-hour standards could also 
adequately protect against exceedances of the current 24-hour 
standard'' (Samet 2009, p. 15).
    In accordance with the REA findings and CASAC recommendations 
mentioned above, the Administrator notes that the 1-hour standards 
being proposed (i.e., 99th percentile 1-hour daily maximum 
SO2 standards at 50-100 ppb) would have the effect of 
maintaining 24-hour and annual SO2 concentrations generally 
well below the levels of the current 24-hour and annual NAAQS (see REA 
Tables 10-3 and 10-4 and REA Appendix Tables D-3 to D-6). Thus, if a 
new 99th percentile 1-hour daily maximum standard is set in the 
proposed range of 50-100 ppb, than the Administrator proposes to revoke 
the current 24-hour and annual standards. However, if a standard is set 
at a level >100 ppb and up to 150 ppb, then the Administrator proposes 
to retain the existing 24-hour standard, recognizing that a 99th 
percentile 1-hour daily maximum standard at 150 ppb would not have the 
effect of maintaining 24-hour average SO2 concentrations 
below the level of the current 24-hour standard in all locations 
analyzed (see REA Appendix Table D-4). However, the Administrator would 
revoke the current annual standard recognizing: (1) 99th percentile 1-
hour daily maximum standards in the range of 50-150 ppb would maintain 
annual average SO2 concentrations below the level of the 
current annual standard (see REA Table 10-4 and REA Appendix tables D-5 
and D-6); and (2) the lack of sufficient evidence linking long-term 
SO2 exposure to adverse health effects.

G. Summary of proposed decisions on the primary standard

    For the reasons discussed above, and taking into account 
information and assessments presented in the ISA and REA as well as the 
advice and recommendations of CASAC, the Administrator proposes that 
the current 24-hour and annual standards are not requisite to protect 
public health with an adequate margin of safety. The Administrator 
proposes to establish a new 1-hour standard that will afford increased 
protection for asthmatics and other at-risk populations against an 
array of adverse respiratory health effects related to short-term (5-
minutes to 24-hours) SO2 exposure. These effects include 
increased decrements in lung function (defined in terms of sRaw and 
FEV1), increases in respiratory symptoms, and related 
serious indicators of respiratory morbidity including emergency 
department visits and hospital admissions for respiratory causes.
    Specifically, the Administrator proposes to set a new short-term 
primary SO2 standard with a 1-hour (daily maximum) averaging 
time and a form defined as the 3-year average of the 99th percentile or 
the 4th highest daily maximum concentration. The level for the new 
standard is proposed to be within the range of 50-100 ppb. The 
Administrator also solicits comment on levels as high as 150 ppb. In 
addition to

[[Page 64846]]

setting a new 1-hour standard in the proposed rage of 50-100 ppb, the 
Administrator proposes to revoke the current 24-hour and annual 
standards recognizing that a 1-hour standard set in the proposed range 
of 50-100 ppb will have the effect of generally maintaining 24-hour and 
annual SO2 concentrations well below the levels of the 
current 24-hour and annual standards. Moreover, the Administrator notes 
that there is little health evidence to support an annual standard for 
the purpose of protecting against health effects associated with long-
term SO2 exposures.

III. Proposed Amendments to Ambient Monitoring and Reporting 
Requirements

    EPA is proposing changes to the ambient air monitoring, reporting, 
and network design requirements for the SO2 NAAQS. This 
section discusses the changes we are proposing that are intended to 
support the proposed 1-hour NAAQS, and the possible retention of the 
existing 24-hour NAAQS depending on the selected level of the 1-hour 
NAAQS, as described in Section II above. Ambient SO2 
monitoring data are used to determine whether an area is in violation 
of the SO2 NAAQS. Ambient SO2 monitoring data are 
collected by state, local, and tribal monitoring agencies (``monitoring 
agencies'') in accordance with the monitoring requirements contained in 
40 CFR parts 50, 53, and 58.

A. Monitoring methods

    To be used in a determination of compliance with the SO2 
NAAQS, SO2 data must be collected using either a Federal 
Reference Method (FRM) or a Federal Equivalent Method (FEM) as defined 
in 40 CFR Parts 50 and 53. The current monitoring methods in use by 
most State and local monitoring agencies are FEM analyzers based on the 
ultraviolet fluorescence (UVF) measurement principle. These continuous 
analyzers were implemented into the SO2 monitoring networks 
in the early 1980s, and the current manual FRM for SO2 is no 
longer used for field monitoring. The current list of all approved FRMs 
and FEMs capable of providing ambient SO2 data for use in 
attainment designations may be found on the EPA Web site http://www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent-methods-list.pdf.
    For reasons explained subsequently, EPA proposes to establish a new 
FRM for measuring SO2 in the ambient air. This proposed new 
FRM for SO2 would be an automated method based on UVF (the 
same type of analyzers now in widespread use), and it would be 
specified in the form of a reference measurement principle and a 
calibration procedure. It would be in a new Appendix A-1 to 40 CFR Part 
50. Analyzers approved as FRMs for SO2 after the effective 
date of the final rule would be subject to performance specifications 
and other requirements set forth in 40 CFR Part 53, under associated 
amendments proposed for Part 53. The existing FRM for SO2 (a 
wet-chemical, manual method) would be retained for some period of time, 
thereby permitting continued use of currently designated FEMs to avoid 
any disruption to existing SO2 monitoring networks.
1. Background
    FRMs, as set forth in several appendices to 40 CFR Part 50, serve 
either or both of two primary purposes. The first is to provide a 
specified, definitive methodology for routinely measuring 
concentrations of various ambient air pollutants for comparison to the 
NAAQS in Part 50 and for other air monitoring objectives. The second is 
to provide a standard of comparison for determining equivalence to the 
specified reference method of alternative and perhaps more practical 
pollutant measurement methods (FEMs) that can be used in lieu of the 
FRM for routine monitoring.
    Some of the FRMs contained in appendices to Part 50 (such as the 
current SO2 FRM) are manual methods that are completely 
specified within their respective appendices. Others (such as the ozone 
FRM) are in the form of a measurement principle and associated 
calibration procedure that must be implemented in a commercial FRM 
analyzer model. Such FRM analyzers must be tested and shown to meet 
explicit performance and other requirements that are set forth in 40 
CFR Part 53 (Ambient Air Monitoring Reference and Equivalent Methods). 
Each of these analyzer models is considered to be an FRM only upon 
specific designation as such by EPA under the provisions of Part 53.
    From time to time, as pollutant measurement technology advances, 
the reference methods in these Part 50 appendices need to be assessed 
to determine if improved or more suitable measurement technology has 
become available to better meet current FRM needs as well as potential 
future FRM requirements. Such new technology can either be presented to 
EPA for evaluation by an FEM applicant under Sec.  53.16, or (as in 
this case) EPA can originate the process itself as provided in Sec.  
53.7. If, after reviewing a new methodology, the Administrator 
determines that the new methodology is substantially superior, Sec.  
53.16 of Part 53 provides for supersession of FRMs under these 
circumstances.
    The FRM for measuring SO2 in the ambient air was 
promulgated on April 30, 1971 (36 FR 8186), in conjunction with EPA's 
establishment (originally as 42 CFR Part 410) of the first national 
ambient air quality standards (NAAQS) for six pollutants (including 
sulfur dioxide) as now set forth in 40 CFR Part 50. This SO2 
FRM is specified in Appendix A of Part 50 and identified as the 
pararosaniline method. It is a manual, wet-chemical method requiring 
sample air to be bubbled through an absorbing reagent 
(tetrachloromecurate), which is then returned to a laboratory for 
chemical analysis. At the time of its promulgation, the method was 
considered the best available method and was in considerable use for 
monitoring SO2 in the air. However, newly developed 
automated continuous analyzers approved as FEMs rapidly supplanted use 
of this manual method for air monitoring in the U.S. By the 1990's, the 
FRM was no longer used at all in domestic air monitoring (EPA, 2009b), 
and since then the method has been used mainly as a comparison 
reference method for the testing and designation of candidate FEMs for 
SO2 in accordance with 40 CFR Part 53.
    The pararosaniline manual FRM has served its role for many years, 
but now a better method is needed that more fully meets the needs of 
contemporary SO2 monitoring. The existing FRM is primarily a 
24-hour integrated method, whereas a 1-hour SO2 FRM 
measurement capability would be needed to implement the proposed 1-hour 
SO2 NAAQS. Existing FEM analyzers can and do provide 1-hour 
measurement capability, but EPA wishes to facilitate the approval of 
new monitoring technologies as well. While the existing manual 
reference method can produce 1-hour averages, it is clearly impractical 
for routine use in making 1-hour SO2 measurements. Also, the 
1-hour mode of the manual method is not a good standard for approving 
new FEMs with 1-hour measurement capability, because scores of 1-hour 
measurements would be needed during equivalency testing. Further, the 
existing FRM is cumbersome to use and requires a mercury-containing 
reagent that is potentially hazardous to operators or to the 
environment if it is mishandled.
    These operational shortcomings suggest that the existing FRM should 
be replaced with a more suitable

[[Page 64847]]

methodology. Fortunately, the existing SO2 instrumental 
measurement technique based on the UVF measurement principle offers 
superior performance and substantial operational advantages, as 
reported in an FRM evaluation for EPA produced by Research Triangle 
Institute (Rickman, 1987). Analyzers using this technique can well 
provide the needed detection limits, precision, and accuracy and 
fulfill other purposes of an FRM, including use as an appropriate 
standard of reference for testing and designation of new FEM analyzers. 
After reviewing these factors, EPA has determined that a new, automated 
FRM for SO2 based on the UVF measurement principle should be 
adopted. EPA is proposing to add the new FRM in a new Appendix A-1 to 
Part 50.
    In association with the proposed new FRM, EPA is also proposing to 
update the performance-based requirements for FEM SO2 
analyzers currently in 40 CFR Part 53. These requirements were 
established in the 1970's, based primarily on the wet-chemical 
measurement technology available at that time. Those initial 
requirements have become significantly outdated and should be modified 
to match current technology, particularly because they would apply to 
new FRM analyzers under the proposed new FRM. The better instrumental 
performance available with the proposed new UVF reference method 
technique allows the performance requirements for SO2 in 40 
CFR Part 53 to be made more stringent for both FRM and FEM analyzers 
(EPA, 2009c).
2. Proposed new FRM measurement technique
    Since the 1970's, a variety of measurement principles have been 
successfully used to produce continuous analyzers for SO2, 
some of which have qualified for EPA designation as equivalent methods 
(found at http://www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent-methods-list.pdf). These include methods based on 
ultraviolet fluorescence, flame photometry, differential optical 
absorption spectroscopy, coulometric and conductometric techniques, and 
second derivative ultraviolet absorption spectrometry. Although some of 
these techniques saw considerable utilization in the 1970's, their use 
dwindled after the introduction of UVF analyzers because of various 
shortcomings such as non-specificity for SO2, susceptibility 
to interferences, marginal performance, or operational disadvantages 
(e.g. requiring hydrogen gas or wet-chemical reagents). Consequently, 
the UVF technique has emerged as the clearly dominant measurement 
technique for SO2, providing a majority of the domestic air 
monitoring data obtained over the last three decades, and virtually 
100% of the current monitoring data (EPA, 2009b). As the proposed new 
reference method, the UVF technique would be specified in performance-
based form, with a generic reference measurement principle and 
associated calibration procedure in a new Appendix A-1 to 40 CFR Part 
50. Associated performance requirements applicable to candidate UVF FRM 
analyzers would be specified in 40 CFR Part 53. This form of the FRM is 
consistent with that specified for FRMs for CO, O3, and 
NO2 in Appendices C, D, and F (respectively) to 40 CFR Part 
50.
    Reasonable commercial availability of high quality analyzers 
utilizing the reference measurement principle that can be offered by 
multiple manufacturers, ideally over many years, is an important aspect 
of any new reference measurement principle. EPA has designated more 
than a dozen UVF analyzers as equivalent to the current reference 
method over the last 30 years. Although most of the early model UVF 
analyzers are no longer in production, many have been replaced by 
redesigned and improved models, and entirely new models continue to 
become designated as FEMs. Currently, more than a half-dozen designated 
FEM models offered by multiple manufacturers are commercially 
available. The widespread use of the method has three important 
technical advantages for an FRM: (1) A variety of analyzer models are 
available and will likely continue to be available from multiple 
manufacturers for many years to come, (2) analyzer manufacturers have 
had (and continue to have) a strong marketing incentive to improve, 
refine, perfect, and continue to market such analyzers, and (3) the 
number of accumulated UVF field monitoring datasets (including related 
QC data) provide an extensive, available performance track record that 
can be evaluated to assess the performance of the analyzers in actual 
monitoring use.
    The only other equivalent method measurement technique that has 
even a small representation among currently available FEM analyzers is 
the differential optical absorption spectrometric method. The open-path 
nature of this method (measurement of pollutants in the open air 
without a closed measurement cell) is not suitable for many of the 
purposes of a reference method. Further, this method is only available 
as two product models from two manufacturers, and very few State and 
local monitoring agencies are using such analyzers.
    The UVF technique is not without some imperfections as a reference 
method. Analyzers utilizing the technique are, to a limited degree, 
susceptible to interference from aromatic hydrocarbon species and 
potentially other compounds at existing levels or levels that may occur 
at many monitoring sites. However, analyzer manufacturers have 
developed very effective ways to reduce these potential limitations, 
including careful selection of wavelengths, optimum optical design, and 
sample air scrubbers, such that typical interferences are minimal.
    All UVF analyzers that have been designated as SO2 FEMs 
have been tested and shown to meet the existing performance 
requirements of 40 CFR Part 53. These include required testing for both 
positive and negative potential interferents, minimum level of 
measurement, zero and span drift, and precision. The results of these 
tests have been submitted to EPA and are in the archived FEM 
applications for these analyzers. Many newer models substantially 
exceed those requirements, with sensitivities down to less than 1 ppb, 
and typically commensurate levels of signal noise, precision, and zero 
drift (EPA, 2009c). In addition, UVF analyzers can accommodate a wide 
range of concentration measurement ranges. They are quite well suited 
to measure high, short-term SO2 concentrations near sources, 
and they can also be used to measure trace-level concentrations in 
clean areas.
    For these reasons, EPA has decided to propose a new automated 
SO2 FRM based on the UVF measurement technology. EPA is 
confident that commercially available UVF instrument models would 
provide capability to serve not only current monitoring and FRM 
applications but anticipated monitoring and FRM needs well into future 
years. EPA solicits comment on the proposal to promulgate an FRM for 
SO2 that would be an automated method based on ultraviolet 
fluorescence, which would be specified in the form of a reference 
measurement principle and calibration procedure, as stated here, and 
contained in a new Appendix A-1 to 40 CFR Part 50.
3. Technical description of the proposed UVF FRM
    The proposed new reference method is based on automated measurement 
of the intensity of the characteristic fluorescence released by 
SO2 in an ambient air sample when irradiated by ultraviolet 
light. The SO2 fluorescence produced is also in the 
ultraviolet range,

[[Page 64848]]

but is measured at a longer wavelength. An analyzer implementing this 
measurement principle would include a measurement cell, an ultraviolet 
light source of appropriate wavelength, an ultraviolet detector system 
with appropriate wavelength sensitivity, and a pump and flow control 
system for sampling the ambient air. Generally, the analyzer also 
requires a means to reduce concentrations of aromatic hydrocarbons and 
possibly other compounds (depending on target wavelengths and other 
parameters used) in the air sample to control for potential measurement 
interferences. The analyzer is calibrated by referencing the 
instrumental fluorescence measurements to SO2 standard 
concentrations traceable to a NIST (National Institute of Standards and 
Technology) primary standard for SO2. This generic 
description of the FRM would be contained in Appendix A-1 to 40 CFR 
Part 50 and would be coupled with explicit analyzer performance 
requirements specified in Subpart B of 40 CFR Part 53. To qualify as an 
FRM, an analyzer model based on this principle would have to be tested 
in accordance with test procedures in Subpart B Part 53 and shown to 
meet the performance requirements specified in that Subpart. EPA could 
then designate the analyzer model as an FRM analyzer, as provided in 
Part 53.
4. Implications to air monitoring networks
    Under Sec.  53.16, EPA must consider the benefits of a proposed 
supersession of an existing reference method, the potential economic 
consequences of such action for State and local monitoring agencies, 
and any disruption of State and local air quality monitoring programs 
that might result from such action. Supersession of an existing 
reference method, as described in Sec.  53.16, presumes that the 
existing FRM would be deleted from Part 50 and replaced with a new FRM, 
and that all equivalent methods based on the old FRM would be 
cancelled. In the case of SO2, essentially all current 
domestic air monitoring activity is carried out using FEM UVF 
analyzers. Cancellation of the FEM designations of all these analyzers 
now would be potentially very disruptive to State, local, and other 
monitoring networks, even though Sec.  53.16 alludes to a possible 
transition period to allow monitoring agencies some period of time to 
replace cancelled FEM analyzers.
    EPA recognizes that these existing SO2 FEMs are 
providing monitoring data that are adequate for the current and the 
proposed SO2 NAAQS and for many other purposes, and there 
appears to be no need or purpose served by their withdrawal. Therefore, 
in this case, EPA proposes instead to retain the existing manual FRM 
for SO2 and to promulgate an entirely new automated FRM for 
SO2. The new FRM description would be contained in a new 
Appendix A-1 to 40 CFR Part 50, and the existing FRM would be re-
codified as Appendix A-2 to 40 CFR Part 50, with both reference methods 
coexisting. Following adoption of the new Appendix A-1, new language 
proposed for Sec.  53.2(a) and (b) would provide that new FRM and FEM 
analyzers for SO2 be designated only with reference to the 
proposed new Appendix A-1. At the same time, retention of the existing 
SO2 reference method will preclude the need to cancel the 
designations of all existing FEMs for SO2.
    Under this proposal, no monitoring agencies would be required to 
change their SO2 monitoring procedures as a result of the 
proposed changes, so it would have no economic costs for implementation 
and no disruptive effects on state, local, or tribal air quality 
monitoring programs. Further, since UVF FEM analyzers have been in 
dominant use for many years, no bias or discontinuity in any aspect of 
the monitoring data obtained subsequently would result from the 
proposed change in the SO2 reference methodology.
    In conjunction with the proposed new FRM, EPA is also proposing to 
adopt updated performance requirements in 40 CFR Part 53, applicable to 
both FRM and FEM analyzers, consistent with the automated methods and 
in anticipation of future NAAQS needs. This would ensure that, going 
forward, all new SO2 monitors would have improved 
performance. EPA believes that the proposal to retain the existing FRM 
while adding the new FRM would provide for a smooth, evolutionary 
transition from the older, manual FRM to the new, modern, automated FRM 
and FEM technology and the associated better performance requirements, 
with no immediate impact to current monitoring activities. For purposes 
of comparing SO2 monitoring data to the SO2 
NAAQS, the EPA believes that the UVF FEMs are appropriate for continued 
use under the current standards and under the option being considered 
for a new 1-hour averaged primary SO2 NAAQS. After several 
years, at a time when either a new SO2 NAAQS would require 
higher monitoring data quality or there would be no further potential 
for disruption to monitoring agencies, EPA would plan to withdraw the 
older reference method and it's associated FEMs.
5. Proposed revisions to 40 CFR Part 53
    Several amendments associated with the proposed new SO2 
reference measurement principle are proposed to 40 CFR Part 53. The 
most significant of these would update the performance requirements for 
both new FRM and new FEM analyzers for SO2, as set forth in 
proposed revised Table B-1. Based on typical performance capabilities 
available for UVF analyzers, EPA is proposing to reduce the allowable 
noise from 5 ppb to 1 ppb, the lower detectable limit from 10 ppb to 2 
ppb, and the allowable interference equivalent limits from 20 ppb to 5 ppb for each interferent and from 60 ppb 
to 20 ppb for the total of all interferents. Also, EPA proposes to 
change the allowable zero drift limits from 20 ppb to 
4 ppb, and to delete the specified limits for span drift at 
20% of the upper range limit (URL) for SO2 analyzers. Review 
of FEM analyzer performance test results has shown that the 20% URL 
span limit requirements are unnecessary because drift performance 
requirements are adequately covered by the zero drift and 80% URL span 
drift limits. EPA proposes to change the lag time allowed from 20 to 2 
minutes and change the rise and fall time limits from 15 to 2 minutes. 
For precision, EPA proposes to change the form of the precision limit 
specifications from ppm to percent (of the URL) for SO2 
analyzers and to set the limit at 2 percent for both 20% and 80% of the 
URL. Two percent is equivalent to 10 ppb for the standard (500 ppb) 
range, which is equivalent to the existing limit value for precision at 
20% of the URL, but would be a reduction from 15 ppb to 10 ppb for the 
limit value at 80% of the URL. This change in units from ppm (or ppb as 
given here) to percent makes the requirement responsive to higher and 
lower measurement ranges. Also, a new footnote is proposed to be added 
to Table B-1 to clarify how noise tests are to be carried out for 
candidate analyzers having an adjustable or automatic time constant 
capability.
    EPA recognizes that SO2 monitoring needs can vary 
widely, from monitoring background levels in pristine areas to 
measuring short-term (1-hour) or even very short-term (less than 1-
hour) high-level averages in the vicinity of substantial sources of 
SO2. To address the need for more sensitive, lower 
measurement ranges for SO2 analyzers, EPA is proposing a 
separate set of performance requirements that would apply specifically 
to narrower measurement ranges, i.e. ranges extending from zero to 
concentrations

[[Page 64849]]

less than 0.5 ppm. These additional requirements are listed in the 
proposed revised Table B-1. A candidate analyzer that meets the Table 
B-1 requirements for the standard measurement range (0 to 0.5 ppm) 
could optionally have one or more narrower ranges included in its FRM 
or FEM designation by further testing to show that it meets these 
supplemental, narrower-range requirements.
    At the other (high) end of the concentration measurement spectrum, 
another related change proposed for Sec.  53.20 would allow optional 
designation of measurement ranges for SO2 up to 2 ppm rather 
than 1 ppm as is now permitted, and designation of these higher ranges 
would be applicable to both FRM and FEM analyzers. Such higher ranges 
are often needed for measurement of short-interval SO2 
averages. Finally, EPA is proposing to clarify in Sec.  53.20 that 
optional testing for auxiliary higher or lower measurement ranges (for 
all gaseous pollutants) may include tests for only some of the 
performance parameters, since the test results for the other 
performance parameters carried out for the standard measurement range 
would be technically applicable and adequate for the higher and/or 
lower ranges as well.
    EPA believes that these changes in performance requirements are 
appropriate, based on analyzer performance data available from analyzer 
manuals and recent FEM applications. EPA solicits comments especially 
from UVF instrument users and manufacturers on these proposed changes, 
particularly in regard to whether they are reasonable, appropriate, of 
significant benefit, and achievable without undue cost. Comments are 
also requested on such issues as the trade off between a high 
measurement range and the need for adequate resolution at 
concentrations near the annual NAAQS, a similar trade off between noise 
level and response time (some analyzers allow these parameters to be 
adjusted by the operator or may adjust them automatically based on the 
rate of change of the concentration level), and whether such 
performance parameters should be addressed in more detail in 40 CFR 
Part 53. In particular, should SO2 analyzer requirements 
address the potential need for faster measurement response time to 
permit more accurate monitoring of short-term intervals such as 5-
minute or 10-minute averages, and are the special performance 
requirements EPA is proposing for measuring very low levels (trace 
levels) of SO2 appropriate and effective?
    Another significant change proposed to 40 CFR Part 53 would add 
some low and medium level 1-hour comparability tests to the Subpart C 
comparability test requirements, as specified in Table C-1. These would 
help to ensure that the 1-hour measurement performance of candidate 
FEMs are adequate, relative to the FRM. Also, EPA proposes to amend 
Table A-1 in Subpart A to reflect the new FRM description in proposed 
new Appendix A-1 of 40 CFR Part 50. This table would also be amended to 
correct some printing errors in the current table as well as to add new 
entries related to the new FRM for lead in PM10 that was 
recently promulgated. Other minor changes would be necessary in the 
wording of a few sections of Subparts A and B due to the proposed 
change in the nature of the SO2 FRM from a manual to an 
automated method or to update the language. These changes are reflected 
in the proposed regulatory text section of this notice.
    EPA proposes additional minor revisions to Tables B-2 and B-3 of 
Subpart B. The changes proposed to Table B-2 would update some of the 
analytical methods for generation or verification of SO2 and 
interferent test concentrations and their associated references. 
Similarly, Table B-3 would be updated to add a specific listing for 
ultraviolet fluorescent methods and to add a few additional interferent 
test species for some other measurement techniques that have been found 
from experience to be needed.

B. Network design

1. Background
    The basic objectives of an ambient monitoring network, as noted in 
40 CFR Part 58 Appendix D, include (1) providing air pollution data to 
the general public in a timely manner, (2) supporting compliance with 
ambient air quality standards and emissions strategy development, and 
(3) providing support for air pollution research. The SO2 
network was originally deployed to support implementation of the 
SO2 NAAQS established in 1971. Although the SO2 
standard was established in 1971, EPA did not establish uniform minimum 
monitoring requirements for SO2 monitoring until May 1979. 
From the time of the implementation of the 1979 monitoring rule, 
through 2008, the SO2 network has steadily decreased in size 
from approximately 1496 sites in 1980 to the approximately 488 sites 
operating in 2008 (Watkins and Thompson, 2009). The reduction in 
network size is due in part to the change in the source sector 
contributions to the overall SO2 inventory and the general 
decline of ambient SO2 levels over time. In the early 
decades of the SO2 network, particularly the 1970s, there 
was a wider variety of more ubiquitous SO2 sources in urban 
areas, including residential coal and oil furnaces, when compared to 
the stationary source, electric generating unit (EGU)-dominated 
inventories of today (see below). The situation in the 1970s led to a 
network design keyed on population, an appropriate approach at the time 
considering the close proximity of sources and people, particularly in 
urban, residential settings (Watkins and Thompson, 2009).
    An analysis of the approximately 488 monitoring sites comprising 
the current (2008) SO2 monitoring network indicates that 
just under half (46%) of the sites in the current SO2 
network are reported to be for the assessment of concentrations for 
general population exposure. As for the present day inventory, the 2005 
NEI (http://www.epa.gov/ttn/chief/net/2005inventory.html) indicates 
that SO2 emissions from EGUs contribute approximately 70% of 
the anthropogenic SO2 emissions in the U.S. However, only 
approximately one third (35%) of the network is reported to be 
addressing locations of maximum (highest) concentrations, likely linked 
to a specific source or group of sources such as EGUs.
    The current network supports the reporting of 1-hour data to EPA's 
Air Quality System (AQS) database, as required in Sec.  58.12 of 40 CFR 
Part 58, since the network utilizes the continuous UVF FEM, which can 
provide time-resolved data averaged over periods as short as several 
minutes. The routine submittal of hourly data by state, local, and 
tribal air monitoring agencies to AQS is suitable for use in comparison 
to both of the current primary 24-hour and annual NAAQS. There are a 
few monitoring agencies who also report 5-minute data voluntarily to 
AQS.
    The current network is sited at a variety of spatial scales; 
however a majority of the network, just over sixty percent, is sited at 
the neighborhood spatial scale\32\ (Watkins and Thompson,

[[Page 64850]]

2009). Although there are 488 SO2 monitors operating in the 
network, there are currently no minimum monitoring requirements for 
SO2 in 40 CFR part 58 Appendix D, other than the following 
three: (1) SO2 must be monitored at National Core (NCore) 
monitoring sites (discussed below), (2) the EPA Regional Administrator 
must approve the removal of any existing monitors, and (3) any ongoing 
SO2 monitoring must have at least one monitor sited to 
measure the maximum concentration of SO2 in that area.
---------------------------------------------------------------------------

    \32\ Spatial scales are defined in 40 CFR Part 58 Appendix D, 
Section 1.2, where the scales of representativeness include:
    1. Microscale--Defines the concentration in air volumes 
associated with area dimensions ranging from several meters up to 
about 100 meters.
    2. Middle scale--Defines the concentration typical of areas up 
to several city blocks in size, with dimensions ranging from about 
100 meters to 0.5 kilometers.
    3. Neighborhood scale--Defines concentrations within some 
extended area of the city that has relatively uniform land use with 
dimensions in the 0.5 to 4.0 kilometers range.
    4. Urban scale--Defines concentrations within an area of city-
like dimensions, on the order of 4 to 50 kilometers. Within a city, 
the geographic placement of sources may result in there being no 
single site that can be said to represent air quality on an urban 
scale. The neighborhood and urban scales have the potential to 
overlap in applications that concern secondarily formed or 
homogeneously distributed air pollutants.
    5. Regional scale--Defines usually a rural area of reasonably 
homogeneous geography without large sources, and extends from tens 
to hundreds of kilometers.
---------------------------------------------------------------------------

    The SO2 monitors that are required as part of the 
National Core monitoring network (NCore) were not required solely for 
providing direct support of the SO2 NAAQS. The monitoring 
rule promulgated in 2006 (71 FR 61236) and codified at 40 CFR Part 58 
and its Appendices established the NCore multi-pollutant network 
requirement to support integrated air quality management data needs. 
Further, NCore is intended to establish long-term sites providing data 
for air quality trends analysis, model evaluation, and, for urban 
sites, tracking metropolitan air quality statistics. To do this, NCore 
sites are required to measure various pollutants, including 
SO2, but are not sited to monitor maximum concentrations of 
SO2. NCore sites provide data representing concentrations at 
the broader neighborhood and urban spatial scales. The data from the 
NCore sites will be compared to the NAAQS although, as noted earlier, 
NAAQS comparisons are not the primary objective of NCore. The NCore 
network, which will be fully implemented by January 1, 2011, will 
result in approximately 83 sites, each with an SO2 monitor, 
with approximately 60 sites being located in urban areas.
    As set out in detail in section II.B of this notice, there is a 
causal relationship between short-term SO2 exposure and 
respiratory morbidity, with ''short-term'' meaning exposures from 5-10 
minutes up to and including 24 hours. This finding is based primarily 
on results from controlled human exposure studies of 5-10 minutes as 
well as epidemiologic studies using mostly 1-hour daily maximum and 24-
hour average SO2 concentrations. Importantly, the ISA 
described the controlled human exposure studies of 5-10 minutes as 
being the ``definitive evidence'' for this conclusion (ISA, section 
5.2). In addition, when describing epidemiologic studies observing 
positive associations between ambient SO2 and respiratory 
symptoms, the ISA stated ``that it is possible that these associations 
are determined in large part by peak exposures within a 24-hour 
period'' (ISA, section 5.2 at p. 5-5). The ISA also stated that the 
respiratory effects following 5- to 10-minute SO2 exposures 
in controlled human exposure studies provide a basis for a progression 
of respiratory morbidity that could result in increased ED visits and 
hospital admissions (ISA, section 5.2). Thus, the monitoring network to 
support the proposed NAAQS should be focused on identifying the 
expected maximum short-term concentrations in any particular area.
    The ISA (Section 2.1) indicates that point (i.e., stationary) 
sources account for approximately 95% of the total anthropogenic 
SO2 emissions in the U.S. According to the 2005 National 
Emissions Inventory (http://www.epa.gov/ttn/chief/net/2005inventory.html), electrical generating units (EGUs) emit 
approximately 70% of the anthropogenic SO2 emissions in the 
U.S. The 2005 NEI indicates that the total anthropogenic emission 
inventory of SO2 is approximately 14,742 thousand tons per 
year. Of those 14,742 thousand tons per year of emitted SO2, 
approximately 85% were emitted by stationary sources that emit 100 or 
more tons per year (comprising approximately 1,928 of the 32,988 
facilities listed in the 2005 NEI). This information indicates that a 
relatively small number (6%) of all SO2 emitting stationary 
sources are responsible for a large majority of the total anthropogenic 
emissions inventory (85%) in the U.S. Therefore, monitors sited to 
reflect locations of expected maximum concentrations should be 
primarily oriented towards locations influenced by one or a cluster of 
high SO2 emitting sources.
    As noted in the key observations of the exposure analysis of the 
REA (REA, Section 8.12), there are a variety of factors that influence 
overall population exposure to ground-level concentrations in a given 
area, including population density and proximity to sources, emissions 
density in an area, and source specific emission parameters such as 
stack height, among other factors. In general, however, it is expected 
that any short-term peaks that may occur in an area are more likely to 
occur nearer to a source or sources, or in an area where multiple 
sources are significantly contributing to increased ground level 
concentrations (an area with high emissions density).\33\ Given that 
maximum ground-level concentrations of SO2 are usually 
directly traceable to specific sources, or a cluster of sources, the 
network design should support implementation of the proposed 1-hour 
SO2 NAAQS by targeting maximum ground-level concentrations 
in areas of both higher population and higher emissions.
---------------------------------------------------------------------------

    \33\ There is inherent variability in where peak ground level 
concentrations may occur in space and time from an individual source 
or group of sources, due to multiple factors including tons emitted, 
stack height, meteorology, among others. These factors are discussed 
further in the Monitor Placement and Siting section of this chapter.
---------------------------------------------------------------------------

2. Proposed changes
    In conjunction with the proposed 1-hour primary NAAQS and (if EPA 
should adopt a standard at the upper end of the range of levels for 
which the Agency is soliciting comment) the potential retention of the 
current 24-hour NAAQS, we are proposing a number of changes to the 
SO2 monitoring network. As just noted, there are currently 
minimum monitoring requirements for SO2 only at NCore sites. 
The proposal for a new 1-hour NAAQS necessitates the re-introduction of 
minimum monitoring requirements. An analysis of the approximately 488 
monitoring sites comprising the current (2008) SO2 
monitoring network indicates that just under half (~46%) of the sites 
in the current SO2 network are reported to be for the 
assessment of concentrations for general population exposure. The 
current network was not originally deployed to address current short-
term, peak concentrations, such as those locations nearer to stationary 
sources or in areas of higher emissions densities, where maximum hourly 
and 5- to 10-minute concentrations are likely to occur. The Agency has 
data indicating that only about one third of the existing 
SO2 network may be source-oriented monitors and/or sited in 
locations of expected maximum concentrations (Watkins and Thompson, 
2009).
    To fully support the proposed SO2 NAAQS, the monitoring 
network needs to identify where short-term, peak ground-level 
concentrations--i.e. concentrations from 5 minutes to one hour (or 
potentially up to 24 hours)--

[[Page 64851]]

may occur. Due to the multiple variables that affect ground level 
SO2 concentrations caused by one or more stationary sources, 
it is difficult to specify a source specific threshold, algorithm, or 
metric by which to require monitoring in a rule such as this. To 
achieve this goal, therefore, EPA is proposing a two-pronged network 
design to ensure that States perform a sufficient amount of monitoring 
of ambient concentrations of SO2 to determine attainment of 
the proposed SO2 NAAQS that intends to prevent exposure to 
peak concentrations. EPA anticipates this two-pronged network would 
require approximately 345 monitors nationwide, providing data for 
comparison with both the proposed 1-hour and the 24-hour standard if 
retained. The network would be wholly comprised of monitors sited at 
locations of expected maximum hourly concentrations. EPA is proposing 
that the two prongs of this SO2 network design would be 
distributed based on: (1) A Population Weighted Emissions Index (PWEI) 
and (2) the state-level contribution to the national, SO2 
emissions inventory. EPA notes that although we propose that the 
network include a minimum number of required monitors, State, local, 
and tribal agencies may conduct additional monitoring above the minimum 
requirements. If those additional monitors satisfy all applicable 
requirements in 40 CFR Part 58, the data from those monitors would be 
comparable to the NAAQS. EPA estimates that one-half to two-thirds of 
the monitors in the existing network (excluding any currently operating 
NCore sites) may have to be moved in order to be counted towards the 
requirement for monitors sited at locations of expected maximum short-
term concentrations of SO2.
    We solicit comment on whether the estimated 348 monitors required 
by this proposal, distributed based on the two network design 
components presented below, are too few, too many, or suitable to 
establish a minimum network sufficient to meet the monitoring 
objectives noted above, including supporting compliance with the 
proposed 1-hour SO2 NAAQS.
    We propose that state and, where appropriate, local air monitoring 
agencies submit a plan for deploying SO2 monitors in 
accordance with the proposed requirements presented below by July 1, 
2011. We also propose that the SO2 network being proposed be 
physically established no later than January 1, 2013. Considering the 
proposed timeline and criteria presented in the network design, we 
solicit comment on whether alternative dates would be more appropriate 
as deadlines for state and local monitoring agencies to submit a 
monitoring plan. We also solicit comments on whether alternative dates 
would be more appropriate as deadlines for state and local monitoring 
agencies to physically deploy monitors.
a. Population weighted emissions index (PWEI) triggered monitoring
    The EPA proposes that the first prong of the ambient SO2 
monitoring network account for SO2 exposure by requiring 
monitors in locations where population and emissions may lead to higher 
potential for population exposure to peak hourly SO2 
concentrations. In order to do this, EPA has developed a Population 
Weighted Emissions Index (PWEI) that uses population and emissions 
inventory data at the CBSA \34\ level to assign required monitoring for 
a given CBSA (population and emissions being obvious relevant factors 
in prioritizing numbers of required monitors). The PWEI for a 
particular CBSA is calculated by multiplying the population (using the 
latest Census Bureau estimates) of a CBSA by the total amount of 
SO2 emissions in that CBSA. The CBSA emission value is in 
tons per year, and is calculated by aggregating the county level 
emissions for each county in a CBSA. We then normalize by dividing the 
resulting product of CBSA population and CBSA SO2 emissions 
by 1,000,000 to provide a PWEI value, the units of which are millions 
of people-tons per year. This calculation has been performed for each 
CBSA and has been posted in the docket as ``CBSA PWEI Calculation, 
2009''. EPA believes that using this PWEI metric to inform where 
monitoring is required is more appropriate for the SO2 
network design than utilizing a population-only type of approach, so 
that we may focus monitoring resources in areas of the country where 
people and emission sources are in greater proximity. In addition, 
EPA's initial view is that this PWEI concept is appropriate for 
SO2 but is not necessarily transferrable to the other 
criteria pollutants. From a very broad vantage point, SO2 is 
exclusively a primarily emitted pollutant (i.e. unlike PM2.5 
and ozone there is no secondary formation of SO2), is almost 
exclusively emitted by stationary sources (unlike NO2, CO, 
PM2.5, thoracic coarse PM, and ozone), and is a gaseous 
pollutant which is somewhat more subject to transport (unlike Pb in the 
Total Suspended Particulate (TSP) and PM10 size fractions).
---------------------------------------------------------------------------

    \34\ CBSAs are defined by the U.S. Census Bureau, and are 
comprised of both Metropolitan Statistical Areas and Micropolitan 
Statistical Areas (http://www.census.gov).
---------------------------------------------------------------------------

    We propose that the first prong of the SO2 network 
design require monitors in CBSAs, according to the following criteria. 
For any CBSA with a calculated PWEI value equal to or greater than 
1,000,000, a minimum of three SO2 monitors are required 
within that CBSA. For any CBSA with a calculated PWEI value equal to or 
greater than 10,000, but less than 1,000,000, a minimum of two 
SO2 monitors are required within that CBSA. For any CBSA 
with a calculated PWEI value equal to or greater than 5,000, but less 
than 10,000, a minimum of one SO2 monitor is required within 
that CBSA. EPA believes that the monitors required within these 
breakpoints provide a reasonable minimum number of monitors in a CBSA 
that considers the combination of population and emissions that exist 
in a given CBSA. This proposed requirement is based on factors that 
will ensure highly populated areas will receive monitoring even if the 
emissions in that area are moderate, which is appropriate given the 
fact that the greater population creates increased potential for 
exposure to those moderate sources. Additionally, this proposed 
requirement also ensures that those areas with higher emissions or 
emission densities, with moderate or modest populations will receive 
monitoring since those increased emissions are likely to have a 
significant impact on whatever population may exist nearby.
    EPA estimates that these criteria will result in 231 required sites 
in 132 CBSAs. We propose that monitors triggered in this first prong of 
the network design must be sited in locations of expected maximum 1-
hour concentrations, at the appropriate spatial scale\35\, within the 
boundaries of a given CBSA. EPA also proposes that when state or local 
agencies make selections for monitoring sites from a pool of similar 
candidate site locations, they shall prioritize monitoring where the 
maximum expected hourly concentrations occur in relative greater 
proximity to populations. EPA believes that states will likely need to 
use some form of quantitative analysis, such as

[[Page 64852]]

modeling, data analysis, or saturation studies to aid in determining 
where ground-level SO2 maxima may occur in a given CBSA. The 
selection of these sites shall be documented in the Annual Monitoring 
Network Plan per Sec.  58.10, which includes a requirement for public 
inspection or comment, and approval by the EPA Regional Administrator.
---------------------------------------------------------------------------

    \35\ Due to the variability in where maximum ground-level 
concentrations may occur (discussed in the Monitor Siting and 
Placement section of this chapter), the appropriate spatial scales 
within which an SO2 monitor might be placed include the 
microscale, middle, and neighborhood scales, which are defined in 40 
CFR Part 58 Appendix D. [could also refer to the fn above where 
these are described]
---------------------------------------------------------------------------

    EPA solicits comment on (1) the use of the Population Weighted 
Emissions Index (PWEI), (2) the PWEI calculation method, (3) the PWEI 
breakpoints that correlate to a number of required monitors, (4) the 
requirement that the monitors shall be sited in locations of expected 
maximum 1-hour concentration, and (5) that state or local agencies 
making selections for monitoring sites from a pool of similar candidate 
site locations shall prioritize monitoring where the maximum expected 
hourly concentrations occur in relative greater proximity to 
populations.
    EPA recognizes that CBSA populations and emissions inventories 
change over time, suggesting a need for periodic review of the 
monitoring network. At the same time, EPA recognizes the advantages of 
a stable monitoring network. Therefore, while EPA currently provides 
for updates of the NEI every 3 years, EPA believes that the current 
network review requirements per Sec.  58.10 which requires an annual 
network plan and recurring 5-year assessments provide a suitable 
schedule for planning and assessing the monitoring network. Through the 
5-year assessments, states will be in a position to review emissions 
distributions from updated NEIs to calculate PWEI values for each CBSA 
and subsequently assess whether the operational monitoring network 
remains appropriate. EPA proposes that the number of sites required to 
operate as a result of the PWEI values calculated for each CBSA be 
reviewed and revised for each CBSA through the 5-year network 
assessment cycle required in Sec.  58.10. EPA solicits comment on 
whether such adjustments to the network should be required on a 5-year 
cycle that matches the general frequency of network assessments or some 
other frequency.
b. State-level emissions triggered monitoring
    As the second prong of the SO2 network, we are proposing 
to require a monitor or monitors in each state, allocated by state-
level SO2 emissions. In this prong, EPA proposes to 
distribute approximately 117 sites, based on the corresponding percent 
contribution of each individual state to the national anthropogenic 
SO2 emission inventory. This prong of the network design is 
intended to allow a portion of the overall required monitors to be 
placed where needed, independent of the PWEI, inside or outside of 
CBSAs. EPA proposes to require monitors, using state boundaries as the 
geographic unit for allocation purposes, in proportion to a state's SO2 
emissions, i.e., a state with higher emissions will be required to have 
a proportionally higher number of monitors. The proposed percent 
contribution of individual states is based on the most recent NEI, with 
SO2 emissions being aggregated by state. Each one percent 
(after rounding) would correspond to one required monitor. For example, 
according to the 2005 NEI, the State of Ohio contributes 8.66% of the 
total anthropogenic SO2 inventory, which would correspond to 
requiring nine monitors to be distributed within Ohio. Further, EPA 
proposes that each state have at least one monitor required as part of 
this second prong, even if a particular state contributes less than 
0.5% of the total anthropogenic national emissions inventory. As a 
result, approximately 117 monitoring sites would be required and 
distributed based on state-level SO2 emissions in the most 
recent NEI, which in this case, is the 2005 NEI. EPA solicits comment 
on the use of state-level emission inventories based on the most recent 
NEI to proportionally distribute approximately one third (117 sites) of 
the required monitoring network.
    According to the most recent NEI, for this proposed second prong, 
we estimate the state/percent contribution to the national inventory/
required monitor distribution to be:

Table 5--State-level Emission Triggered Monitors--This Table Shows State
 and Territory Level Contributions to the National SO2 Inventory and the
 Corresponding Number of Monitors Required for Each State as Proposed in
                 Prong 2 of the Proposed Network Design
------------------------------------------------------------------------
                                         Percent
                                     contribution to    Proposed number
        State or Territory           the national SO2     of required
                                        inventory           monitors
                                        (percent)
------------------------------------------------------------------------
Alabama...........................               4.02                  4
Alaska............................               0.46                  1
American Samoa....................                N/A                  1
Arizona...........................               0.60                  1
Arkansas..........................               0.77                  1
California........................               1.48                  1
Colorado..........................               0.54                  1
Connecticut.......................               0.23                  1
Delaware..........................               0.58                  1
District of Columbia..............               0.03                  1
Florida...........................               4.40                  4
Georgia...........................               5.07                  5
Guam..............................                N/A                  1
Hawaii............................               0.08                  1
Idaho.............................               0.16                  1
Illinois..........................               3.51                  4
Indiana...........................               7.10                  7
Iowa..............................               1.50                  2
Kansas............................               1.33                  1
Kentucky..........................               3.88                  4
Louisiana.........................               2.40                  2
Maine.............................               0.25                  1
Maryland..........................               2.58                  3
Massachusetts.....................               1.07                  1

[[Page 64853]]

 
Michigan..........................               3.32                  3
Minnesota.........................               1.05                  1
Mississippi.......................               0.81                  1
Missouri..........................                2.8                  3
Montana...........................               0.26                  1
Nebraska..........................               0.82                  1
Nevada............................               0.49                  1
New Hampshire.....................               0.43                  1
New Jersey........................               0.69                  1
New Mexico........................               0.32                  1
New York..........................               2.65                  3
North Carolina....................               4.40                  4
North Dakota......................               1.08                  1
Northern Mariana Islands..........                N/A                  1
Ohio..............................               8.66                  9
Oklahoma..........................               1.12                  1
Oregon............................               0.32                  1
Pennsylvania......................               7.96                  8
Puerto Rico.......................                N/A                  1
Rhode Island......................               0.06                  1
South Carolina....................               2.06                  2
South Dakota......................               0.19                  1
Tennessee.........................               2.63                  3
Texas.............................               6.34                  6
Utah..............................               0.35                  1
Vermont...........................               0.05                  1
Virgin Islands....................                N/A                  1
Virginia..........................               2.34                  2
Washington........................               0.45                  1
West Virginia.....................               3.63                  4
Wisconsin.........................               1.79                  2
Wyoming...........................               0.83                  1
------------------------------------------------------------------------

    EPA proposes siting requirements for this second prong of required 
monitors to be the same as those in the first prong: siting in 
locations of expected maximum 1-hour concentrations, at the appropriate 
spatial scale, within the boundaries of a given state, and prioritizing 
the selection of candidate sites where the maximum expected hourly 
concentrations occur in greater proximity to populations. This again 
would need to be determined case-by-case using quantitative analysis, 
such as modeling, data analysis, or saturation studies to aid in 
determining where ground-level SO2 maxima may occur in a 
given state. We propose that these monitors can be located inside or 
outside of CBSA boundaries. However, if a monitor required by the 
second prong is placed inside a CBSA that already has a requirement for 
monitoring due to the first prong of this network design, that monitor 
would not be allowed to count towards satisfying the first prong 
requirements. As noted for the first prong of required monitors, the 
selection of these sites shall be documented in the Annual Monitoring 
Network Plan per Sec.  58.10, which includes a requirement for public 
inspection or comment, and approval by the EPA Regional Administrator.
    The EPA solicits comment on (1) the use of state-level emission 
inventories to proportionally distribute required monitors, (2) 
requiring each state to have at least one monitor under this prong of 
the network design, and (3) requiring all monitors to be sited in 
locations of expected maximum 1-hour concentration inside or outside of 
CBSAs.
    EPA recognizes that emissions inventories change over time, 
suggesting a need for periodic review of the monitoring network. At the 
same time, EPA recognizes the advantages of a stable monitoring 
network. Therefore, while EPA currently provides for updates of the NEI 
every 3 years, EPA believes that the current network review 
requirements per Sec.  58.10 which requires an annual network plan and 
recurring 5-year assessments provide a suitable schedule for planning 
and assessing the monitoring network. Through the 5-year assessments, 
states will be in a position to review emissions distributions from 
updated NEIs to assess whether the monitoring requirements remain 
appropriate. EPA proposes that the number of sites required to operate 
as a result of state-level emissions be reviewed and revised for each 
state through the 5-year network assessment cycle required Sec.  58.10. 
EPA solicits comment on whether such adjustments to the network should 
be required on a 5-year cycle that matches the general frequency of 
network assessments or some other frequency.
c. Monitor placement and siting
    Sites that are to be placed in locations of expected maximum 1-hour 
concentrations, will also likely discern 5-minute peaks as well. EPA 
expects that in general, these locations will be in proximity to larger 
emitting sources (in tons per year) and/or areas of relatively high 
emissions densities where multiple sources may be contributing to peak 
ground-level concentrations. The variability in where such locations 
exist relative to the

[[Page 64854]]

responsible emission source(s) depends on multiple factors including 
the tonnage emitted by a source (or group of sources), stack height, 
stack diameter, emission exit velocity, emission temperature, terrain, 
and meteorology. Depending on these variables, plumes may heavily 
fumigate areas immediately downwind of a source, or may never truly 
touch down at all, dispersing into ambient air where SO2 
concentrations continually decrease with increasing distance away from 
the source. This is illustrated in an example where a relatively large 
source with a tall stack height may not produce exceedingly high ground 
level concentrations anywhere along its plume trajectory while a 
smaller source with a relatively short stack may cause relatively 
higher ground level concentrations under the same meteorological 
conditions at the same location. The primary reason for this 
variability is because the peak impacts of sources with higher stacks 
will generally be farther downwind and may be more variably located 
than is the case for sources with shorter stacks. Further, depending on 
meteorology, an emission plume from an individual source may cause 
increased ground-level concentrations at any heading, relative to the 
parent source, corresponding to the prevailing winds.
    When analyzing a particular source, a state may find multiple 
locations where peak ground-level concentrations may occur around an 
individual source. EPA does not intend for multiple monitors to be 
sited around or in proximity to one source. Not siting multiple 
monitors around, or in proximity, to one source ensures that more 
individual sources or groups of sources will receive attention by the 
monitoring network. States always have the discretion to perform 
additional monitoring above the minimum requirements to increase 
monitoring around a particular source or group of sources.
    Due to the variability of how, when, where, and to what degree a 
source or group of sources can contribute to peak, ground-level 
SO2 concentrations, EPA expects that State and local 
monitoring agencies will need to analyze all relevant information, 
including available ambient and emissions data, and potentially use air 
quality modeling or saturation studies to select appropriate monitoring 
site locations. Further, due to the variability in where maximum 
ground-level concentrations may occur, the appropriate spatial scales 
within which a monitor might be placed include the microscale, middle, 
and neighborhood scales, which are defined in 40 CFR Part 58 Appendix 
D. EPA believes that states, in evaluating a source (or group of 
sources) that contribute to a peak ground-level SO2 
concentration that varies with space and time, should identify where 
the highest concentrations are expected to occur in developing 
candidate site locations. EPA proposes that when state and local 
agencies make selections for monitoring sites from candidate site 
locations, they shall prioritize monitoring where the maximum expected 
hourly concentrations occur in greater proximity to populations. EPA 
solicits comment on the role of population exposure in the site 
selection process.
d. Monitoring required by the regional administrator
    In addition to the two prongs of the proposed SO2 
network design, we propose that the Regional Administrator will have 
discretion to require monitoring above these minimum requirements under 
prongs 1 and 2, as necessary to address situations where the minimum 
monitoring requirements are not sufficient to meet monitoring 
objectives noted above. EPA recognizes that the minimum required 
monitors in the proposed network design under the two prongs described 
above are based on indicators that may not provide for all the 
monitoring that may be necessary in an area. An example where EPA 
envisions requiring an additional monitor might be a case where a 
source having modest emissions still has high potential to cause a 
violation of the NAAQS in a community or neighborhood. This situation 
might occur where a modest SO2 source has, for example, a 
low emission stack and/or is in an area where meteorological conditions 
cause situations, such as inversions or stagnation, that might lead to 
high ground-level concentrations of SO2. In this example, 
such a monitor might be needed even though a state is fulfilling its 
monitoring requirements under the first and second prongs of the 
proposed network design. The purpose of this provision is to monitor in 
and provide data for otherwise non-monitored locations that have the 
potential to exceed the level of the NAAQS or that are perceived to 
have higher exposure risks due to proximity to a source or sources. In 
such an example, the Regional Administrators may make use of any 
available data including existing model data, existing data analyses, 
or screening tools such as AERSCREEN or SCREEN3, to inform a decision 
of whether or not a monitor should be required for a given area or 
location. Any monitor required through the Regional Administrator and 
selected by the state or local agency would be included in the Annual 
Monitoring Network Plan per Sec.  58.10, which includes a requirement 
for public inspection or comment, and approval by the EPA Regional 
Administrator. In any case, EPA encourages state, local, and tribal 
monitoring agencies to provide input and information to the appropriate 
Regional Administrators in determining whether additional monitors are 
needed and the locations of such monitors. We solicit comment on the 
proposal to allow Regional Administrators the discretion to require 
monitoring above the requirements under prongs 1 and 2 for any area or 
location where those monitoring requirements are not sufficient to meet 
monitoring objectives.
    EPA notes that existing requirements detailed in Sec.  58.14(c) 
address certain conditions where existing monitors can be shut down, 
with EPA Regional Administrator approval. EPA is not reopening or 
otherwise reconsidering this provision. However, this requirement is 
noted here so that state or local agency requests to potentially 
relocate SO2 monitors to meet the proposed requirements of 
prongs 1 or 2 will be considered with the specific provisions of Sec.  
58.14(c) in mind.
e. Alternative network design
    EPA solicits comments on alternative network designs, including 
alternative methods to determine the minimum number of monitors per 
state. We are particularly interested in whether a screening approach 
for assessing the likelihood of a NAAQS exceedance could be developed 
and serve as a basis for determining the number and location of 
required monitors.
    More specifically, EPA requests comment on whether it should 
utilize existing screening tools such as AERSCREEN or SCREEN3, which 
use parameters such as effective stack height and emissions levels to 
identify facilities with the potential to cause an exceedance of the 
proposed standard. For that set of sources, EPA could then require 
states to conduct more refined modeling (likely using the American 
Meteorological Society (AMS)/EPA Regulatory Model (AERMOD)) to 
determine locations where monitoring should be conducted. Any screening 
or modeling would likely be carried out by states by using EPA 
recommended models and techniques referenced by 40 CFR Part 51, 
Appendix W, which provides guidance on air quality modeling. Such 
screening or modeling uses facility emission tonnage, stack heights, 
stack diameters, emission temperatures, emission velocities, and 
accounts for local terrain and meteorology in determining where

[[Page 64855]]

expected maximum hourly concentrations may occur. In using this 
approach, EPA would then require states to locate monitors at the point 
of maximum concentration around sources identified as likely causing 
NAAQS exceedances.
    This approach could lead to monitors being required at a 
significantly larger number of locations than under the proposed 
approach. For example, the NEI shows that 2,407 sources emit 50 tons 
per year or more of SO2, while 1,928 sources emit 100 tons 
per year or more of SO2. If, for example, the state 
screening approach found that a substantial fraction of those 50 or 100 
ton per year sources had a significant probability of violating the 
NAAQS, states could be required to model, evaluate, and potentially 
monitor a corresponding number of sources. EPA also notes that this 
alternative approach would not distinctly use population as a factor 
for where monitors should be placed. EPA solicits comment on the 
resource implications for state and local agencies associated with this 
approach.
    If EPA selects a standard level near the lower end of the proposed 
range, it is likely that a greater number of areas would exceed the 
NAAQS, leading to the need for additional monitors. A facility 
screening approach, as described above would explicitly account for the 
specific parameters of a facility, air quality information, and the 
stringency of the standard for determining the number of monitors, in 
contrast to the proposed approach. EPA solicits comment on how, in the 
absence of a facility screening approach, the number of monitors 
required nationwide could be adjusted if EPA finalizes a standard near 
the lower end of the proposed range.

C. Data reporting

    SO2 UV fluorescence FEMs are continuous gas analyzers, 
producing updated data values on the order of every 20 seconds. Data 
values are typically aggregated into minute averages and then compiled 
into hourly averages for reporting purposes. EPA proposes to retain the 
existing requirement that State and local monitoring agencies report 
hourly SO2 data to AQS within 90 days of the end of each 
calendar quarter. EPA encourages monitoring agencies to voluntarily 
report their pre-validated data on an hourly basis to EPA's real time 
AIRNow data system.
    The definitive evidence for the ISA's conclusion of causal 
association between short-term SO2 exposure and respiratory 
morbidity is from controlled human exposure studies of 5-10 minutes in 
exercising asthmatics (ISA, section 5.2). The REA therefore assessed 
exposure and risks associated with 5-minute SO2 
concentrations above 5-minute health effect benchmark levels derived 
from these controlled human exposure studies. In performing these 
analyses, the REA noted that: (1) The majority of the current 
SO2 monitoring network reported 1-hour SO2 
concentrations (REA section 7.2.3); (2) very few state and local 
agencies in the U.S. voluntary reported ambient 5-minute SO2 
concentrations, as such reporting is not required (REA, section 
10.3.3.2); and (3) the lack of 5-minute monitoring data necessitated 
the use of statistically estimated 5-minute SO2 
concentrations derived from reported 1-hour SO2 levels (see 
REA section 7.2.3) in order to expand the geographic scope of the 
exposure and risk analyses. Thus given the demonstrated importance of 
5-minute SO2 concentrations, EPA proposes that State and 
local agencies shall report to AQS the maximum 5-minute block average 
of the twelve 5-minute block averages of SO2 for each hour, 
in addition to the existing requirement to report the 1-hour average.
    EPA solicits comment on the proposed requirement for state and 
local monitoring agencies to report both hourly average and the maximum 
5-minute block average out of the twelve 5-minute block averages of 
SO2 for each hour. EPA also solicits comment on the 
advantages and disadvantages of alternatively requiring state and local 
agencies to report all twelve 5-minute SO2 values for each 
hour. Having all twelve 5-minute SO2 values for each hour 
would provide more detailed information for health research purposes 
and provide additional information to help inform the next review of 
the SO2 standard. We also solicit comment on alternatively 
requiring state and local agencies to report the maximum 5-minute 
concentration in an hour based on a moving 5-minute averaging period 
rather than time block averaging.
    EPA notes the potential resource burden with the proposed 
requirement to report 5-minute average values in addition to 1-hour 
average values, as is currently required. Accordingly, we solicit 
comment on the magnitude and importance of this resource burden, 
recognizing that monitoring agencies utilize a variety of automated 
data acquisition and management programs, and that the resulting burden 
of validating and reporting 5-minute data may vary from a relatively 
trivial matter to an issue of greater importance, depending on the 
procedures utilized within each agency's data reporting process.
    As a part of the larger data quality performance requirements of 
the ambient monitoring program, we are proposing data quality 
objectives (DQOs) for the proposed SO2 network. The DQOs are 
meant to identify measurement uncertainty for a given pollutant method. 
We propose a goal for acceptable measurement uncertainty for 
SO2 methods to be defined for precision as an upper 90 
percent confidence limit for the coefficient of variation (CV) of 15 
percent and for bias as an upper 95 percent confidence limit for the 
absolute bias of 15 percent. We solicit comment on the proposed DQOs 
and on what the acceptable measurement uncertainty should be.

IV. Proposed Appendix T--Interpretation of the Primary NAAQS for Oxides 
of Sulfur and Proposed Revisions to the Exceptional Events Rule

    The EPA is proposing to add Appendix T, Interpretation of the 
Primary National Ambient Air Quality Standards for Oxides of Sulfur, to 
40 CFR Part 50 in order to provide data handling procedures for the 
proposed SO2 1-hour primary standard. The proposed Sec.  
50.11 which sets the averaging period, level, indicator and form of the 
NAAQS refers to this Appendix T. The proposed Appendix T would detail 
the computations necessary for determining when the proposed 1-hour 
primary SO2 NAAQS is met. The proposed Appendix T also would 
address data reporting, data completeness considerations, and rounding 
conventions.
    Two versions of the proposed Appendix T are printed at the end of 
this notice. The first applies to a 1-hour primary standard based on 
the annual 4th high value form, while the second applies to a 1-hour 
primary standard based on the 99th percentile daily value form. (As 
explained in section II.F. 3 above, EPA is proposing alternative forms 
here based on technical analysis that they are equally effective.) The 
discussion here addresses the first of these versions, followed by a 
brief description of the differences found in the second version.
    For the proposed 1-hour primary standard, EPA is proposing data 
handling procedures, a proposed addition of a cross-reference to the 
Exceptional Events Rule, a proposed addition to allow the Administrator 
discretion to consider otherwise incomplete data to be complete, and a 
proposed provision addressing the

[[Page 64856]]

possibility of there being multiple SO2 monitors at one 
site.
    The EPA is also proposing SO2-specific changes to the 
deadlines in 40 CFR 50.14, by which states must flag ambient air data 
that they believe have been affected by exceptional events and submit 
initial descriptions of those events, and to the deadlines by which 
states must submit detailed justifications to support the exclusion of 
that data from EPA determinations of attainment or nonattainment with 
the NAAQS. The deadlines now contained in 40 CFR 50.14 are generic, and 
are not always appropriate for SO2 given the anticipated 
schedule for the designations of areas under the proposed 
SO2 NAAQS.

A. Background

    The general purpose of a data interpretation appendix is to provide 
the practical details on how to make a comparison between multi-day and 
possibly multi-monitor ambient air concentration data and the level of 
the NAAQS, so that determinations of attainment and nonattainment are 
as objective as possible. Data interpretation guidelines also provide 
criteria for determining whether there are sufficient data to make a 
NAAQS level comparison at all.
     The regulatory language for the current SO2 NAAQS, 
originally adopted in 1977, contains data interpretation instructions 
only for the issue of data completeness. This situation contrasts with 
the situations for ozone, PM2.5, PM10, and most 
recently Pb for which there are detailed data interpretation appendices 
in 40 CFR Part 50 addressing issues that can arise in comparing 
monitoring data to the NAAQS. EPA has used its experience developing 
and applying these other data interpretation appendices to develop the 
proposed text for Appendix T.
    An exceptional event is defined in 40 CFR 50.1 as an event that 
affects air quality, is not reasonably controllable or preventable, is 
an event caused by human activity that is unlikely to recur at a 
particular location or is a natural event, and is determined by the 
Administrator in accordance with 40 CFR 50.14 to be an exceptional 
event. Air quality data that is determined, under the procedural steps 
and substantive criteria specified in section 50.14, to have been 
affected by an exceptional event may be excluded from consideration 
when EPA makes a determination that an area is meeting or not meeting 
the associated NAAQS. The key procedural deadlines in section 50.14 are 
that a State must notify EPA that data have been affected by an event, 
i.e., ``flag'' the data in the Air Quality Systems (AQS) database, and 
provide an initial description of the event by July 1 of the year after 
the data are collected, and that the State must submit the full 
justification for exclusion within 3 years after the quarter in which 
the data were collected. However, if a regulatory decision based on the 
data, for example a designation action, is anticipated, the schedule is 
shortened and all information must be submitted to EPA no later than a 
year before the decision is to be made. This generic schedule presents 
problems when a NAAQS has been recently revised, as discussed below.

B. Interpretation of the primary NAAQS for oxides of sulfur

    The purpose of a data interpretation rule for the SO2 
NAAQS is to give effect to the form, level, averaging time, and 
indicator specified in the proposed regulatory text at 40 CFR 50.11, 
anticipating and resolving in advance various future situations that 
could occur. The proposed Appendix T provides definitions and 
requirements that apply to the proposed 1-hour primary standard for 
SO2. The requirements concern how ambient data are to be 
reported, what ambient data are to be considered (including the issue 
of which of multiple monitors' data sets will be used when more than 
one monitor has operated at a site), and the applicability of the 
Exceptional Events Rule to the primary SO2 NAAQS.
1. 1-hour primary standard based on the annual 4th high value form
    With regard to data completeness for the proposed 1-hour primary 
standard, the proposed Appendix follows past EPA practice for other 
NAAQS pollutants by requiring that in general at least 75% of the 
monitoring data that should have resulted from following the planned 
monitoring schedule in a period must be available for the key air 
quality statistic from that period to be considered valid. For the 
proposed 1-hour primary SO2 NAAQS, the key air quality 
statistics are the daily maximum 1-hour concentrations in three 
successive years. It is important that sampling within a day encompass 
the period when concentrations are likely to be highest and that all 
seasons of the year are well represented. Hence, the 75% requirement is 
proposed to be applied at the daily and quarterly levels. EPA invites 
comment on the proposed completeness requirements.
    Recognizing that there may be years with incomplete data, the 
proposed text provides that a design value derived from incomplete data 
will nevertheless be considered valid in either of two situations.
    First, if the design value calculated from at least four days of 
monitoring observations in each of these years exceeds the level of the 
1-hour primary standard, it would be valid. This situation could arise 
if monitoring was intermittent but high SO2 levels were 
measured on enough hours and days for the mean of the three annual 4th 
highest values to exceed the standard. In this situation, more complete 
monitoring could not possibly have indicated that the standard was 
actually met.
    Second, we are proposing a diagnostic data substitution test which 
is intended to identify those cases with incomplete data in which it 
nevertheless is very likely, if not virtually certain, that the daily 
1-hour design value would have been observed to be below the level of 
the NAAQS if monitoring data had been minimally complete.
    The diagnostic test would be applied only if there is at least 50% 
data capture in each quarter of each year and if the 3-year mean of the 
observed annual 4th highest maximum hourly values in the incomplete 
data is below the NAAQS level. The test would substitute a high 
hypothetical concentration for as much of the missing data as needed to 
meet the 100% requirement in each quarter. The value that is 
substituted for the missing values is the highest daily maximum 1-hour 
observed in the same quarter, looking across all three years under 
evaluation. If the resulting 3-year design value is below the NAAQS, it 
is highly likely that the design value calculated from complete data 
would also have been below the NAAQS, so the original design value 
indicating compliance would be considered valid.
    It should be noted that one possible outcome of applying the 
proposed substitution test is that a year with incomplete data may 
nevertheless be determined to not have a valid design value and thus to 
be unusable in making 1-hour primary NAAQS compliance determinations 
for that 3-year period. EPA invites comment on incorporating the 
proposed substitution test into the final rule.
    EPA is proposing that the Administrator have general discretion to 
use incomplete data to calculate design values that would be treated as 
valid for comparison to the NAAQS despite the incompleteness, either at 
the request of a state or at her own initiative. Similar provisions 
exist already for the PM2.5 and lead NAAQS, and EPA has 
recently proposed such provisions to accompany the proposed 1-hour 
NO2 and SO2 NAAQS. The Administrator would

[[Page 64857]]

consider monitoring site closures/moves, monitoring diligence, and 
nearby concentrations in determining whether to use such data.
2. 1-hour primary standard based on the annual 99th percentile daily 
value form
    The second version of the proposed Appendix T appearing at the end 
of this notice contains proposed interpretation procedures for a 1-hour 
primary standard based on the 99th percentile daily value form. The 4th 
high daily value form and the 99th percentile daily value form would 
yield the same design value in a situation in which every hour and day 
of the year has reported monitoring data, since the 99th percentile of 
365 daily values is the 4th highest value. However, the two forms 
diverge if data completeness is 82% or less, because in that case the 
99th percentile value is the 3rd highest (or higher) value, to 
compensate for the lack of monitoring data on days when concentrations 
could also have been high.
    Logically, provisions to address possible data incompleteness under 
the 99th percentile daily value form should be somewhat different from 
those for the 4th highest form. With a 4th highest form, incompleteness 
should not invalidate a design value that exceeds the standard, for 
reasons explained above. With the 99th percentile form, however, a 
design value exceeding the standard stemming from incomplete data 
should not automatically be considered valid, because concentrations on 
the unmonitored days could have been relatively low, such that the 
actual 99th percentile value for the year could have been lower, and 
the design value could have been below the standard. The second 
proposed version of Appendix T accordingly has somewhat different 
provisions for dealing with data incompleteness. One difference is the 
addition of another diagnostic test based on data substitution, which 
in some cases can validate a design value based on incomplete data that 
exceeds the standard.
    The second version of the proposed Appendix T provides a table for 
determining which day's maximum 1-hour concentration will be used as 
the 99th percentile concentration for the year. The proposed table is 
similar to one used now for the 24-hour PM2.5 NAAQS, which 
is based on a 98th percentile form, but adjusted to reflect a 99th 
percentile form for the 1-hour primary SO2 standard. The 
proposed Appendix T also provides instructions for rounding (not 
truncating) the average of three annual 99th percentile hourly 
concentrations before comparison to the level of the primary NAAQS.

C. Exceptional events information submission schedule

    The Exceptional Events Rule at 40 CFR 50.14 contains generic 
deadlines for a state to submit to EPA specified information about 
exceptional events and associated air pollutant concentration data. A 
state must initially notify EPA that data have been affected by an 
event by July 1 of the calendar year following the year in which the 
event occurred; this is done by flagging the data in AQS and providing 
an initial event description. The state must also, after notice and 
opportunity for public comment, submit a demonstration to justify any 
claim within 3 years after the quarter in which the data were 
collected. However, if a regulatory decision based on the data (for 
example, a designation action) is anticipated, the schedule to flag 
data in AQS and submit complete documentation to EPA for review is 
shortened, and all information must be submitted to EPA no later than 
one year before the decision is to be made.
    These generic deadlines are suitable for the period after initial 
designations have been made under a NAAQS, when the decision that may 
depend on data exclusion is a redesignation from attainment to 
nonattainment or from nonattainment to attainment. However, these 
deadlines present problems with respect to initial designations under a 
newly revised NAAQS. One problem is that some of the deadlines, 
especially the deadlines for flagging some relevant data, may have 
already passed by the time the revised NAAQS is promulgated. Until the 
level and form of the NAAQS have been promulgated a state does not know 
whether the criteria for excluding data (which are tied to the level 
and form of the NAAQS) were met on a given day. Another problem is that 
it may not be feasible for information on some exceptional events that 
may affect final designations to be collected and submitted to EPA at 
least one year in advance of the final designation decision. This could 
have the unintended consequence of EPA designating an area 
nonattainment because of uncontrollable natural or other qualified 
exceptional events.
    The Exceptional Events Rule at Sec.  50.14(c)(2)(v) indicates 
``when EPA sets a NAAQS for a new pollutant, or revises the NAAQS for 
an existing pollutant, it may revise or set a new schedule for flagging 
data for initial designation of areas for those NAAQS.''
    For the specific case of SO2, EPA anticipates that the 
signature date for the revised SO2 NAAQS will be June 2, 
2010 (a date specified by Consent Decree), that state/tribal 
designations recommendations will be due by June 2, 2011, and that 
initial designations under the revised NAAQS will be made by June 1, 
2012 (since June 2, 2012 would be on a Saturday) and will be based on 
air quality data from the years 2008-2010 or 2009-2011 if there is 
sufficient data for these data years. (See Section VI below for more 
detailed discussion of the designation schedule and what data EPA 
intends to use.) Under the current rule, because final designations 
would be made by June 1, 2012, all events to be considered during the 
designations process would have to be flagged and fully documented by 
states one year prior to designations, by June 1, 2011. A state would 
not be able to flag and submit documentation regarding events that 
occurred between June to December 2011 by one year before designations 
are made in June 2012.
    EPA is proposing revisions to 40 CFR 50.14 only to change 
submission dates for information supporting claimed exceptional events 
affecting SO2 data. The proposed rule text at the end of 
this notice shows the changes that would apply if a revised 
SO2 NAAQS is promulgated by June 2, 2010, and designations 
are made two years after such promulgation. For air quality data 
collected in 2008, we propose to extend the generic July 1, 2009 
deadline for flagging data (and providing a brief initial description 
of the event) to October 1, 2010. EPA believes this extension would 
provide adequate time for states to review the impact of exceptional 
events from 2008 on the revised standard and notify EPA by flagging the 
relevant data in AQS. EPA is not proposing to change the foreshortened 
deadline of June 1, 2011 for submitting documentation to justify an 
SO2-related exceptional event from 2008. We believe the 
generic deadline provides adequate time for states to develop and 
submit proper documentation.
    For data collected in 2009, EPA proposes to extend generic deadline 
of July 1, 2010 for flagging data and providing initial event 
descriptions to October 1, 2010. EPA is retaining the deadline of June 
1, 2011 for states to submit documentation to justify an 
SO2-related exceptional event from 2009. EPA plans to assist 
the states by providing at the time of signature our assessment of 
which monitoring sites and days have exceeded the NAAQS in 2008 and 
2009. For data collected in 2010, EPA is proposing a deadline of June 
1, 2011 for flagging data and providing initial event descriptions and

[[Page 64858]]

for submitting documentation to justify exclusion of the flagged data. 
EPA believes that this deadline provides states with adequate time to 
review and identify potential exceptional events that occur in calendar 
year 2010, even for those events that might occur late in the year. EPA 
believes these deadlines will be feasible because experience suggest 
that exceptional events affecting SO2 data are few in number 
and easily assessed, so no state is likely to have a large workload.
    If a state intends 2011 data to be considered in SO2 
designations, 2011 data must be flagged and detailed event 
documentation submitted 60 days after the end of the calendar quarter 
in which the event occurred or by March 31, 2011, whichever date occurs 
first. Again, EPA believes these deadlines will be feasible because 
experience suggest that exceptional events affecting SO2 
data are few in number and easily assessed, so no state is likely to 
have a large workload.
    Table 6 summarizes the proposed designation deadlines discussed in 
this section and provides designation schedule information from recent, 
pending or prior NAAQS revisions for other pollutants. If the 
promulgation date for a revised SO2 NAAQS occurs on a 
different date than June 1, 2010 (i.e. if the consent decree should be 
amended--which EPA does not presently anticipate), EPA will revise the 
final SO2 exceptional event flagging and documentation 
submission deadlines accordingly, consistent with this proposal, to 
provide states with reasonably adequate opportunity to review, 
identify, and document exceptional events that may affect an area 
designation under a revised NAAQS. EPA invites comment on these 
proposed changes in the exceptional event flagging and documentation 
submission deadlines for the revised SO2 NAAQS shown in 
Table 6.

      Table 6--Schedule for Exceptional Event Flagging and Documentation Submission for Data To Be Used in
                                 Designations Decisions for New or Revised NAAQS
----------------------------------------------------------------------------------------------------------------
                                          Air quality data
   NAAQS pollutant/ standard/(level)/      collected for     Event flagging & initial    Detailed documentation
           promulgation date               calendar year       description deadline       submission deadline
----------------------------------------------------------------------------------------------------------------
PM2.5/24-Hr Standard (35 [mu]g/m\3\)             2004-2006  October 1, 2007 \a\......  April 15, 2008 \a\.
 Promulgated October 17, 2006.
Ozone/8-Hr Standard (0.075 ppm)                  2005-2007  June 18, 2009 \a\........  June 18, 2009 \a\.
 Promulgated March 12, 2008.
                                                      2008  June 18, 2009 \a\........  June 18, 2009 \a\.
                                                      2009  60 Days after the end of   60 Days after the end of
                                                             the calendar quarter in    the calendar quarter in
                                                             which the event occurred   which the event occurred
                                                             or February 5, 2010,       or February 5, 2010,
                                                             whichever date occurs      whichever date occurs
                                                             first \b\.                 first \b\.
NO2/1-Hour Standard (80-100 Ppb, Final                2008  July 1, 2010 \a\.........  January 22, 2011 \a\.
 Level Tbd).
                                                      2009  July 1, 2010 \a\.........  January 22, 2011 \a\.
                                                      2010  April 1, 2011 \a\........  July 1, 2011 \a\.
SO2/1-Hour Standard (50-100 PPB, Final                2008  October 1, 2010 \b\......  June 1, 2011 \b\.
 Level Tbd).
                                                      2009  October 1, 2010 \b\......  June 1, 2011 \b\.
                                                      2010  June 1, 2011 \b\.........  June 1, 2011 \b\.
                                                      2011  60 Days after the end of   60 Days after the end of
                                                             the calendar quarter in    the calendar quarter in
                                                             which the event occurred   which the event occurred
                                                             or March 31, 2011,         or March 31, 2011,
                                                             whichever date occurs      whichever date occurs
                                                             first \b\.                 first \b\.
----------------------------------------------------------------------------------------------------------------
\a\ These dates are unchanged from those published In the original rulemaking, or are being proposed elsewhere
  and are shown in this table for informational purposes--the agency is not opening these dates for comment
  under this rulemaking.
\b\ Indicates change from general schedule In 40 CFR 50.14.
Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final
  initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most
  notably, for data used by EPA for redesignations to attainment.

V. Designations for the SO2 NAAQS

    After EPA establishes or revises a NAAQS, the CAA directs EPA and 
the states to begin taking steps to ensure that the new or revised 
NAAQS is met. The first step is to identify areas of the country that 
do not meet the new or revised NAAQS. This step is known as the initial 
area designations.
    Section 107(d)(1)(A) of the CAA provides that, ``By such date as 
the Administrator may reasonably require, but not later than 1 year 
after promulgation of a new or revised NAAQS for any pollutant under 
section 109, the Governor of each state shall * * * submit to the 
Administrator a list of all areas (or portions thereof) in the state'' 
that designates those areas as nonattainment, attainment, or 
unclassifiable. The CAA section 107(d)(1)(A)(i) defines an area as 
nonattainment if it is violating the NAAQS or if it is contributing to 
a violation in a nearby area.
    Section 107(d)(1)(B)(i) further provides, ``Upon promulgation or 
revision of a NAAQS, the Administrator shall promulgate the 
designations of all areas (or portions thereof) * * * as expeditiously 
as practicable, but in no case later than 2 years from the date of 
promulgation. Such period may be extended for up to one year in the 
event the Administrator has insufficient information to promulgate the 
designations within 2 years. By no later than 120 days prior to 
promulgating designations, EPA is required to notify states of any 
intended modifications to their boundaries as EPA may deem necessary. 
States then have an opportunity to comment on EPA's intended decisions. 
(See section 107(d)(1)(B)(ii).) Whether or not a state provides a 
recommendation, EPA must promulgate the designation that the Agency 
deems appropriate.
    Therefore, following promulgation of any revised SO2 
NAAQS in June 2010, EPA must promulgate initial designations by June 
2012, or, by June 2013 in the event that the Administrator has 
insufficient information to promulgate initial designations within 2 
years. Along with the proposal to set a

[[Page 64859]]

new 1-hour primary SO2 NAAQS, elsewhere in this action, EPA 
is proposing new SO2 ambient air monitoring network 
requirements. As proposed, any new monitors would be deployed no later 
than January 1, 2013. Compliance with the proposed 1-hour 
SO2 NAAQS would be determined based on 3 years of complete, 
quality assured, certified monitoring data. We do not expect newly 
sited monitors for the proposed new network to generate sufficient 
monitoring data for EPA to use in determining whether areas are in 
compliance with the revised SO2 NAAQS by the statutory 
deadline for EPA to complete initial designations, even if EPA were to 
take an additional third year. Therefore, EPA intends to complete the 
designations on a 2-year schedule, by June 2012, based on 3 years of 
complete, quality assured, certified air quality monitoring data from 
the current monitoring network.
    EPA expects to base designations on air quality data from the years 
2008-2010 or 2009-2011. Because the new monitoring network requirements 
would not apply until January 1, 2013, EPA expects that many 
SO2 monitors now operating will continue in operation at 
their current locations at least through the end of 2011.\36\ The 
SO2 monitors in the current network were generally sited to 
measure the highest 24-hour and annual average SO2 
concentrations. However, all of the monitors report hourly data. EPA 
estimates that around 488 monitors operated in 2008. EPA believes at 
least one third of the monitors meet the proposed network design 
requirements and therefore would not need to be moved. Additional 
monitors may be retained in their current locations if they are 
measuring high levels of SO2. If a monitor in the existing 
network indicates a violation of the 1-hour SO2 NAAQS, EPA 
intends to designate the area nonattainment, regardless of whether or 
not the monitor is located such that it could be counted towards 
meeting the proposed new network requirements. However, if the monitor 
indicates that the monitoring site meets the 1-hour SO2 
NAAQS, EPA's decision on the designation of the area would be made on a 
case-by-case basis. One possible outcome is that the area may be 
designated as unclassifiable because EPA would be unable to determine 
whether the area is violating the 1-hour SO2 NAAQS, or 
contributing to a violation in a nearby area, because of a lack of a 
complete monitoring network meeting the new network requirements.
---------------------------------------------------------------------------

    \36\ EPA Regional Administrator approval will be required for 
any state to discontinue an existing monitoring site, and EPA does 
not expect that it will before 2011 approve discontinuation of 
monitoring at any site which appears to have a substantial 
likelihood of violating the 1-hour NAAQS.
---------------------------------------------------------------------------

    Accordingly, state Governors would need to submit their initial 
designation recommendations to EPA no later than June 2011. If the 
Administrator intends to modify any state recommendation, EPA would 
notify the state's Governor no later than February 2012, 120 days prior 
to promulgating the final designations. States would then have an 
opportunity to comment on EPA's tentative decisions before EPA 
promulgates the final designations in June 2012.
    While CAA section 107 specifically addresses states, EPA intends to 
follow the same process for tribes to the extent practicable, pursuant 
to section 301(d) of the CAA regarding tribal authority, and the Tribal 
Authority Rule (63 FR 7254; February 12, 1998). Pursuant to the Tribal 
Authority Rule, Tribes are not subject to the schedule requirements 
that apply to states. However, EPA intends to promulgate designations 
for Tribal land as well as state land according to the schedule 
mandated for state land, so EPA encourages Tribes that wish to provide 
input on EPA's designations to provide this input on the schedule 
mandated for states.

VI. Clean Air Act Implementation Requirements

    This section of the preamble discusses the Clean Air Act (CAA) 
requirements that states and emissions sources would need to address 
when implementing new or revised SO2 NAAQS based on the 
structure outlined in the CAA and existing rules.\37\ The EPA believes 
that there are sufficient guidance documents and regulations currently 
in place to fully implement the proposed revision to the SO2 
NAAQS.\38\ However, EPA may provide additional guidance in the future, 
as necessary, to assist states and emissions sources to comply with the 
CAA provisions for implementing a new or revised SO2 NAAQS.
---------------------------------------------------------------------------

    \37\ Since EPA is proposing to take comments on retaining the 
current 24-hr standards without revision if the 1-hr standard is set 
at 100-150 ppb, the discussion in this section relates to 
implementation of the proposed 1-hour standard and the possible 
retention or revocation of the current 24-hr standard.
    \38\ See SO2 Guideline Document, Office of Air 
Quality Planning and Standards, Research Triangle Park, NC 27711, 
EPA-452/R-94-008, February 1994.
---------------------------------------------------------------------------

    The CAA assigns important roles to EPA, states and tribal 
governments to achieve the NAAQS. States have the primary 
responsibility for developing and implementing state implementation 
plans (SIPs) that contain state measures necessary to achieve the air 
quality standards in each area once EPA has established the NAAQS. EPA 
provides assistance to states and tribes by providing technical tools, 
assistance, and guidance, including information on the potential 
control measures that may assist in helping areas attain the standards.
    Under section 110 of the CAA, 42 U.S.C. 7410, and related 
provisions, states are directed to submit, for EPA approval, SIPs that 
provide for the attainment and maintenance of such standards through 
control programs directed at sources of SO2 emissions. If a 
state fails to adopt and implement the required SIPs by the time 
periods provided in the CAA, EPA has the responsibility under the CAA 
to adopt a federal implementation plan (FIP) to assure that areas 
attain the NAAQS in an expeditious manner. The states, in conjunction 
with EPA, also administer the prevention of significant deterioration 
(PSD) program for SO2. See sections 160-169 of the CAA, 42 
U.S.C. 7470-7479. In addition, federal programs provide for nationwide 
reductions in emissions of SO2 and other air pollutants 
under Title II of the Act, 42 U.S.C. 7521-7574. These programs involve 
limits on the sulfur content of the fuel used by automobiles, trucks, 
buses, motorcycles, non-road engines and equipment, marine vessels and 
locomotives. EPA is also in the process of establishing limits on the 
sulfur content of the fuel used by ocean going vessels. Emissions 
reductions for SO2 are also obtained from implementation of 
the new source performance standards (NSPS) for stationary sources 
under sections 111 and 129 of the CAA, 42 U.S.C. 7411 and 7429; and the 
national emission standards for hazardous air pollutants (NESHAP) for 
stationary sources under section 112 of the CAA, 42 U.S.C. 7412.

A. How this rule applies to tribes

    CAA section 301(d) authorizes EPA to treat eligible Indian tribes 
in the same manner as states (TAS) under the CAA and requires EPA to 
promulgate regulations specifying the provisions of the statute for 
which such treatment is appropriate. EPA has promulgated these 
regulations--known as the Tribal Authority Rule or TAR--at 40 CFR Part 
49. See 63 FR 7254 (February 12, 1998). The TAR establishes the process 
for Indian tribes to seek TAS eligibility and sets forth the CAA 
functions for which TAS will be available. Under the TAR, eligible 
tribes may seek approval for all CAA and regulatory purposes other than 
a small number of functions enumerated at section 49.4. Implementation 
plans

[[Page 64860]]

under section 110 are included within the scope of CAA functions for 
which eligible tribes may obtain approval. Section 110(o) also 
specifically describes tribal roles in submitting implementation plans. 
Eligible Indian tribes may thus submit implementation plans covering 
their reservations and other areas under their jurisdiction.
    The CAA and TAR do not, however, direct tribes to apply for TAS or 
implement any CAA program. In promulgating the TAR EPA explicitly 
determined that it was not appropriate to treat tribes similarly to 
states for purposes of, among other things, specific plan submittal and 
implementation deadlines for NAAQS-related requirements. 40 CFR 
49.4(a). In addition, where tribes do seek approval of CAA programs, 
including section 110 implementation plans, the TAR provides 
flexibility and allows them to submit partial program elements, so long 
as such elements are reasonably severable--i.e., ``not integrally 
related to program elements that are not included in the plan 
submittal, and are consistent with applicable statutory and regulatory 
requirements''. 40 CFR 49.7.
    To date, very few tribes have sought TAS for purposes of section 
110 implementation plans. However, some tribes may be interested in 
pursuing such plans to implement today's proposed standard, once it is 
promulgated. In several sections of this preamble, EPA describes the 
various roles and requirements states will address in implementing 
today's proposed standard. Such references to states generally include 
eligible Indian tribes to the extent consistent with the flexibility 
provided to tribes under the TAR. Where tribes do not seek TAS for 
section 110 implementation plans, EPA under its discretionary authority 
will promulgate FIPs as ``necessary or appropriate to protect air 
quality.'' 40 CFR 49.11(a). EPA also notes that some tribes operate air 
quality monitoring networks in their areas. For such monitors to be 
used to measure attainment with the proposed revised primary NAAQS for 
SO2, the criteria and procedures identified in this proposed 
rule would apply.

B. Attainment dates

    The latest date by which an area is required to attain the 
SO2 NAAQS is determined from the effective date of the 
nonattainment designation for the affected area. For areas designated 
nonattainment for the revised SO2 NAAQS, SIPs must provide 
for attainment of the NAAQS as expeditiously as practicable, but no 
later than 5 years from the effective date of the nonattainment 
designation for the area. See section 192(a) of the CAA. The EPA will 
determine whether an area has demonstrated attainment of the 
SO2 NAAQS by evaluating air quality monitoring data 
consistent with the form of the NAAQS for SO2, if revised, 
which will be codified at 40 CFR part 50, Appendix T.
1. Attaining the NAAQS
    In order for an area to be redesignated as attainment, it must meet 
five conditions provided under section 107(d)(3)(E) of the CAA. This 
section requires that:
     EPA must have determined that the area has met the 
SO2 NAAQS;
     EPA has fully approved the state's implementation plan;
     The improvement in air quality in the affected area is due 
to permanent and enforceable reductions in emissions;
     EPA has fully approved a maintenance plan for the area; 
and
     The state(s) containing the area have met all applicable 
requirements under section 110 and part D.
2. Consequences of failing to attain by the statutory attainment date
    Any SO2 nonattainment area that fails to attain by its 
statutory attainment date would be subject to the requirements of 
sections 179(c) and (d) of the CAA. EPA is required to make a finding 
of failure to attain no later than 6 months after the specified 
attainment date and publish a notice in the Federal Register. The state 
would then need to submit an implementation plan revision no later than 
one year following the effective date of the Federal Register notice 
making the determination of the area's failure to attain. This 
submission must demonstrate that the standard will be attained as 
expeditiously as practicable, but no later than 5 years from the 
effective date of EPA's finding that the area failed to attain. In 
addition, section 179(d)(2) provides that the SIP revision must include 
any specific additional measures as may be reasonably prescribed by 
EPA, including ``all measures that can be feasibly implemented in the 
area in light of technological achievability, costs, and any nonair 
quality and other air quality-related health and environmental 
impacts.''

C. Section 110(a)(1) and (2) NAAQS infrastructure requirements

    Section 110(a)(2) of the CAA directs all states to develop and 
maintain a solid air quality management infrastructure, including 
enforceable emission limitations, an ambient monitoring program, an 
enforcement program, air quality modeling capabilities, and adequate 
personnel, resources, and legal authority. Section 110(a)(2)(D) also 
requires state plans to prohibit emissions from within the state which 
contribute significantly to nonattainment or maintenance areas in any 
other state, or which interfere with programs under part C of the CAA 
to prevent significant deterioration of air quality or to achieve 
reasonable progress toward the national visibility goal for Federal 
class I areas (national parks and wilderness areas).
    Under sections 110(a)(1) and (2) of the CAA, all states are 
directed to submit SIPs to EPA which demonstrate that basic program 
elements have been addressed within 3 years of the promulgation of any 
new or revised NAAQS. Subsections (A) through (M) of section 110(a)(2) 
set forth the elements that a state's program must contain in the 
SIP.\39\ The list of section 110(a)(2) NAAQS implementation 
requirements are the following:
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    \39\ Two elements identified in section 110(a)(2) are not listed 
below because, as EPA interprets the CAA, SIPs incorporating any 
necessary local nonattainment area controls would not be due within 
3 years, but rather are due at the time the nonattainment area 
planning requirements are due. These elements are: (1) Emission 
limits and other control measures, section 110(a)(2)(A), and (2) 
Provisions for meeting part D, section 110(a)(2)(I), which requires 
areas designated as nonattainment to meet the applicable 
nonattainment planning requirements of part D, title I of the CAA.
---------------------------------------------------------------------------

     Ambient air quality monitoring/data system: Section 
110(a)(2)(B) requires SIPs to provide for setting up and operating 
ambient air quality monitors, collecting and analyzing data and making 
these data available to EPA upon request.
     Program for enforcement of control measures: Section 
110(a)(2)(C) requires SIPs to include a program providing for 
enforcement of SIP measures and the regulation and permitting of new/
modified sources.
     Interstate transport: Section 110(a)(2)(D) requires SIPs 
to include provisions prohibiting any source or other type of emissions 
activity in the state from contributing significantly to nonattainment 
or interfering with maintenance of the NAAQS in another state, or from 
interfering with measures required to prevent significant deterioration 
of air quality or to protect visibility.
     Adequate resources: Section 110(a)(2)(E) directs states to 
provide assurances of adequate funding, personnel and legal authority 
to implement their SIPs.
     Stationary source monitoring system: Section 110(a)(2)(F) 
directs

[[Page 64861]]

states to establish a system to monitor emissions from stationary 
sources and to submit periodic emissions reports to EPA.
     Emergency power: Section 110(a)(2)(G) directs states to 
include contingency plans, and adequate authority to implement them, 
for emergency episodes in their SIPs.
     Provisions for SIP revision due to NAAQS changes or 
findings of inadequacies: Section 110(a)(2)(H) directs states to 
provide for revisions of their SIPs in response to changes in the 
NAAQS, availability of improved methods for attaining the NAAQS, or in 
response to an EPA finding that the SIP is inadequate.
     Consultation with local and Federal government officials: 
Section 110(a)(2)(J) directs states to meet applicable local and 
Federal government consultation requirements when developing SIPs and 
reviewing preconstruction permits.
     Public notification of NAAQS exceedances: Section 
110(a)(2)(J) directs states to adopt measures to notify the public of 
instances or areas in which a NAAQS is exceeded.
     PSD and visibility protection: Section 110(a)(2)(J) also 
directs states to adopt emissions limitations, and such other measures, 
as may be necessary to prevent significant deterioration of air quality 
in attainment areas and protect visibility in Federal Class I areas in 
accordance with the requirements of CAA Title I, part C.
     Air quality modeling/data: Section 110(a)(2)(K) requires 
that SIPs provide for performing air quality modeling for predicting 
effects on air quality of emissions of any NAAQS pollutant and 
submission of data to EPA upon request.
     Permitting fees: Section 110(a)(2)(L) requires the SIP to 
include requirements for each major stationary source to pay permitting 
fees to cover the cost of reviewing, approving, implementing and 
enforcing a permit.
     Consultation/participation by affected local government: 
Section 110(a)(2)(M) directs states to provide for consultation and 
participation by local political subdivisions affected by the SIP.

D. Attainment planning requirements

1. SO2 nonattainment area SIP requirements
    Any state containing an area designated as nonattainment with 
respect to the SO2 NAAQS would need to develop for 
submission to EPA a SIP meeting the requirements of part D, Title I, of 
the CAA, providing for attainment by the applicable statutory 
attainment date. See sections 191(a) and 192(a) of the CAA. As 
indicated in section 191(a), all components of the SO2 part 
D SIP must be submitted within 18 months of the effective date of an 
area's designation as nonattainment.
    Section 172 of the CAA addresses the general requirements for areas 
designated as nonattainment. Section 172(c) directs states with 
nonattainment areas to submit a SIP which contains an attainment 
demonstration showing that the affected area will attain the standard 
by the applicable statutory attainment date. The SIP must show that the 
area will attain the standard as expeditiously as practicable, and must 
``provide for the implementation of all Reasonably Available Control 
Measures (RACM) as expeditiously as practicable (including such 
reductions in emissions from existing sources in the area as may be 
obtained through the adoption, at a minimum, of Reasonably Available 
Control Technology (RACT)).''
    SIPs required under Part D of the CAA must also provide for 
reasonable further progress (RFP). See section 172(c)(2) of the CAA. 
The CAA defines RFP as ``such annual incremental reductions in 
emissions of the relevant air pollution as are required by part D, or 
may reasonably be required by the Administrator for the purpose of 
ensuring attainment of the applicable NAAQS by the applicable 
attainment date.'' See section 171 of the CAA. Historically, for some 
pollutants, RFP has been met by showing annual incremental emission 
reductions sufficient to maintain generally linear progress toward 
attainment by the applicable attainment date.
    All SO2 nonattainment area SIPs must include contingency 
measures which must be implemented in the event that an area fails to 
meet RFP or fails to attain the standards by its attainment date. See 
section 172(c)(9) of the CAA. These contingency measures must be fully 
adopted rules or control measures that take effect without further 
action by the state or the Administrator. The EPA interprets this 
requirement to mean that the contingency measures must be implemented 
with only minimal further action by the state or the affected sources 
with no additional rulemaking actions such as public hearings or 
legislative review.
    Emission inventories are also critical for the efforts of state, 
local, and federal agencies to attain and maintain the NAAQS that EPA 
has established for criteria pollutants including SO2. 
Section 191(a) in conjunction with section 172(c) requires that areas 
designated as nonattainment for SO2 submit an emission 
inventory to EPA no later than 18 months after designation as 
nonattainment. In the case of SO2, sections 191(a) and 
172(c) also direct states to submit periodic emission inventories for 
nonattainment areas. The periodic inventory must include emissions of 
SO2 for point, nonpoint, mobile, and area sources.
2. New source review and prevention of significant deterioration 
requirements
    The Prevention of Significant Deterioration (PSD) and nonattainment 
New Source Review (NSR) programs contained in parts C and D of Title I 
of the CAA govern preconstruction review of any new or modified major 
stationary sources of air pollutants regulated under the CAA as well as 
any precursors to the formation of that pollutant when identified for 
regulation by the Administrator.\40\ The EPA rules addressing these 
programs can be found at 40 CFR 51.165, 51.166, 52.21, 52.24, and Part 
51, appendix S.
---------------------------------------------------------------------------

    \40\ The terms ``major'' and ``minor'' define the size of a 
stationary source, for applicability purposes, in terms of an annual 
emissions rate (tons per year, tpy) for a pollutant. Generally, a 
minor source is any source that is not ``major.'' ``Major'' is 
defined by the applicable regulations--PSD or nonattainment NSR.
---------------------------------------------------------------------------

    The PSD program applies when a major source located in an area that 
is designated as attainment or unclassifiable for any criteria 
pollutant is constructed or undergoes a major modification.\41\ The 
nonattainment NSR program applies on a pollutant-specific basis when a 
major source constructs or modifies in an area that is designated as 
nonattainment for that pollutant. The minor NSR program addresses major 
and minor sources that undergo construction or modification activities 
that do not qualify as major, and it applies, as necessary to assure 
attainment, regardless of the designation of the area in which a source 
is located.
---------------------------------------------------------------------------

    \41\ In addition, the PSD program applies to non-criteria 
pollutants subject to regulation under the Act, except those 
pollutants regulated under section 112 and pollutants subject to 
regulation only under section 211(o).
---------------------------------------------------------------------------

    PSD permit requirements are effective on the promulgation date of a 
new or revised standard. SIPs that address the PSD requirements related 
to attainment areas are due no later than 3 years after the 
promulgation of a revised NAAQS for SO2. The PSD 
requirements include but are not limited to the following:
     Installation of Best Available Control Technology (BACT);
     Air quality monitoring and modeling analyses to ensure 
that a project's emissions will not cause or contribute to a violation 
of any NAAQS

[[Page 64862]]

or maximum allowable pollutant increase (PSD increment);
     Notification of Federal Land Manager of nearby Class I 
areas; and public comment on the permit.
    If EPA establishes a 1-hour NAAQS for SO2, the owner or 
operator of any major stationary source or major modification locating 
in an attainment or unclassifiable area for SO2 will be 
required, as a prerequisite for a PSD permit, to demonstrate that the 
emissions increases from the new or modified source will not cause or 
contribute to a violation of the that new NAAQS. The EPA does not 
anticipate that this will pose a technical problem, since the modeling 
capability and SO2 emissions input data already exist. 
Depending on the final form of the 1-hour NAAQS, it may be necessary to 
make adjustments to the AERMOD modeling system to accommodate the form 
of the standard; however, EPA anticipates that any such adjustments can 
be readily accomplished in coordination with the promulgation of any 
new NAAQS for SO2 in time to enable states to implement such 
standard via the PSD program. The analyses for the 1-hour NAAQS will be 
in addition to the existing demonstration of compliance for the annual 
and 24-hour SO2 NAAQS, which will continue to be required 
unless EPA revokes these standards in conjunction with its promulgation 
of a new 1-hour NAAQS for SO2.
    The owner or operator of a new or modified source will still be 
required to demonstrate compliance with the annual and 24-hour 
SO2 increments, even if their counterpart NAAQS are revoked. 
The annual and 24-hour increments are established in the CAA and will 
need to remain in the PSD regulations because EPA does not interpret 
the Clean Air Act to authorize EPA to remove them. It appears necessary 
for Congress to amend the Act to make appropriate changes to the 
statutory SO2 increments, perhaps similar to the way the Act 
was amended to accommodate PM10 increments in lieu of the 
statutory TSP increments. If we establish a new 1-hour SO2 
NAAQS, EPA will consider the need to adopt new 1-hour SO2 
increments.
    In association with the requirement to demonstrate compliance with 
the NAAQS and increments, the owner or operator of a new or modified 
source must submit for review and approval a source impact analysis and 
an air quality analysis. The source impact analysis, primarily a 
modeling analysis, must demonstrate that allowable emissions increases 
from the proposed source or modification, in conjunction with emissions 
from other existing sources will not cause or contribute to either a 
NAAQS or increment violation. The air quality analysis must assess the 
ambient air quality in the area that the proposed source or 
modification would affect.
    For the air quality analysis, the owner or operator must submit in 
its permit application air quality monitoring data that shall have been 
gathered over a period of one year and is representative of air quality 
in the area of the proposed project. If existing data representative of 
the area of the proposed project is not available, new data may need to 
be collected by the owner or operator of the source or modification. 
Where data is already available, it might be necessary to evaluate the 
location of the monitoring sites from which the SO2 data 
were collected in comparison to any new siting requirements associated 
with the 1-hour NAAQS. If existing sites are inappropriate for 
providing the necessary representative data, then new monitoring data 
will need to be collected by the owner or operator of the proposed 
project.
    Historically, EPA has allowed the use of several screening tools to 
help facilitate the implementation of the new source review program by 
reducing the permit applicant's burden, and streamlining the permitting 
process for de minimis circumstances. These screening tools include a 
significant emissions rate (SER), significant impact levels (SILs), and 
a significant monitoring concentration (SMC). The SER, as defined in 
tons per year for each regulated pollutant, is used to determine 
whether any proposed source or modification will emit sufficient 
amounts of a particular pollutant to require the review of that 
pollutant under the NSR permit program. EPA will consider whether to 
evaluate the existing significant emissions rate (SER) for 
SO2 to see if it would change substantially based on the 
NAAQS levels for the 1-hour averaging period. Historically, we have 
defined a de minimis pollutant impact as one that results in a modeled 
ambient impact of less than approximately 4% of the short-term NAAQS. 
The current SER for SO2 (40 tpy) is based on the impact on 
the 24-hour SO2 NAAQS. See, 45 FR 52676, 52707 (August 7, 
1980). We have typically used the most sensitive averaging period to 
calculate the SER, and we may want to evaluate the new 1-hour period 
for SO2 because it is likely to represent most sensitive 
averaging period for SO2.
    The SIL, expressed as an ambient pollutant concentration ([mu]g/
m\3\), is used to determine whether the impact of a particular 
pollutant is significant enough to warrant a complete air quality 
impact analysis for any applicable NAAQS and increments. EPA has 
promulgated regulations under 40 CFR 51.165(b) which include SILs for 
SO2 to determine whether a source's impact would be 
considered to cause or contribute to a NAAQS violation for either the 
3-hour, 24-hour or annual averaging periods. These SILs were originally 
developed in 1978 to limit the application of air quality dispersion 
models to a downwind distance of no more than 50 kilometers or to 
``insignificant levels.'' See, 43 FR 26398, June 19, 1978. Through 
guidance, EPA has also allowed the use of SILs to determine whether or 
not it is necessary for a source to carry out a comprehensive source 
impact analysis and to determine the extent of the impact area in which 
the analysis will be carried out. The existing SILs for SO2 
were not developed on the basis of specific SO2 NAAQS 
levels, so if the existing NAAQS are not being revised, there is 
probably no need to revise the existing SILs. Even if we decide to 
revoke any of the existing NAAQS, the corresponding SIL should still be 
useful for increment assessment. A SIL for the 1-hour averaging period 
does not exist, and would need to be developed for use with modeling 
for 1-hour SO2 NAAQS and increments (if and when developed).
    Finally, the SMC, also measured as an ambient pollutant 
concentration ([mu]g/m\3\), is used to determine whether it may be 
appropriate to exempt a proposed project from the requirement to 
collect ambient monitoring data for a particular pollutant as part of a 
complete permit application. EPA first defined SMCs for regulated 
pollutants under the PSD program in 1980. See, 45 FR 52676, 52709-10 
(August 7, 1980). The existing SMC for SO2, based on a 24-
hour averaging period, may need to be re-evaluated to consider the 
effect of basing the SMC on the 1-hour averaging period, especially in 
light of the fact that we may revoke the NAAQS for the 24-hour 
averaging period. Third, even if the 1-hour averaging period does not 
indicate the need for a revised SMC for SO2, the fact that 
the original SMC for SO2 is based on 1980 monitoring data 
(Lowest Detectable Level, correction factor of ``5''), could be a basis 
for revising the existing value. More up-to-date monitoring data and 
statistical analyses of monitoring accuracy may yield a different--
possibly lower--correction factor today. A new 1-hour NAAQS would not 
necessarily cause this result, but may provide a ``window

[[Page 64863]]

of opportunity'' to re-evaluate the SMC for SO2. See 
sections II.E.2 and II.F.2 above.
    As a means of reducing the permit applicant's burden, and to 
streamline permitting, permit authorities use screening tools referred 
to as significant impact levels (SILs) and a significant monitoring 
concentration (SMC). EPA issued unofficial SO2 SILs for the 
3-hour (secondary standard), 24-hour and annual averaging periods. 
These SILs were developed in 1978 to limit the application of air 
quality dispersion models to a downwind distance of no more than 50 
kilometers or to ``insignificant levels.'' See, 43 FR 263--, 26398, 
(June 19, 1978). These values were not developed on the basis of 
specific SO2 NAAQS levels, so if the existing NAAQS are not 
being revised, there is probably no need to revise the existing SILs. 
Even if we decide to revoke any of the existing NAAQS, the 
corresponding SIL should still be useful for increment assessment. A 
SIL for the 1-hour averaging period does not exist, and would need to 
be developed for use with modeling for the 1-hour SO2 NAAQS 
and increments (if and when developed).
    States which have areas designated as nonattainment for the 
SO2 NAAQS are directed to submit, as a part of the SIP due 
18 months after an area is designated as nonattainment, provisions 
requiring permits for the construction and operation of new or modified 
stationary sources anywhere in the nonattainment area. Prior to 
adoption of the SIP revision addressing major source nonattainment NSR 
for SO2 nonattainment areas, the requirements of 40 CFR part 
51, appendix S will apply. Nonattainment NSR requirements include but 
are not limited to:
     Installation of Lowest Achievable Emissions Rate (LAER) 
control technology;
     Offsetting new emissions with creditable emissions 
reductions;
     A certification that all major sources owned and operated 
in the state by the same owner are in compliance with all applicable 
requirements under the CAA;
     An alternative siting analysis demonstrating that the 
benefits of a proposed source significantly outweigh the environmental 
and social costs imposed as a result of its location, construction, or 
modification; and
     Public comment on the permit.
    Minor NSR programs must meet the statutory requirements in section 
110(a)(2)(C) of the CAA which requires ``* * * regulation of the 
modification and construction of any stationary source * * * as 
necessary to assure that the [NAAQS] are achieved.'' These programs 
must be established in each state within 3 years of the promulgation of 
a new or revised NAAQS.
3. General conformity
    Section 176(c) of the CAA requires that all federal actions conform 
to an applicable implementation plan developed pursuant to section 110 
and part D of the CAA. The EPA rules developed under section 176(c) 
prescribe the criteria and procedures for demonstrating and assuring 
conformity of federal actions to a SIP. Each federal agency must 
determine that any actions covered by the general conformity rule 
conform to the applicable SIP before the action is taken. The criteria 
and procedures for conformity apply only in nonattainment areas and 
those areas redesignated attainment since 1990 (``maintenance areas'') 
with respect to the criteria pollutants under the CAA \42\: carbon 
monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone 
(O3), particulate matter (PM2.5 and PM10), and 
sulfur dioxide (SO2). The general conformity rules apply one 
year following the effective date of designations for any new or 
revised NAAQS.\43\
---------------------------------------------------------------------------

    \42\ Criteria pollutants are those pollutants for which EPA has 
established a NAAQS under section 109 of the CAA.
    \43\ Transportation conformity is required under CAA section 
176(c) (42 U.S.C. 7506(c) to ensure that federally supported highway 
and transit project activities are consistent with (``conform to'') 
the purpose of the SIP. Transportation conformity applies to areas 
that are designated nonattainment, and those areas redesignated to 
attainment after 1990 (``maintenance areas'' with plans developed 
under CAA section 175A) for transportation-related criteria 
pollutants. Due to the relatively small amounts of sulfur in 
gasoline and on-road diesel fuel, transportation conformity does not 
apply to the SO2 NAAQS. 40 CFR 93.102(b)(1).
---------------------------------------------------------------------------

    The general conformity determination examines the impacts of direct 
and indirect emissions related to federal actions. The general 
conformity rule provides several options to satisfy air quality 
criteria, such as modeling or offsets, and requires the federal action 
to also meet any applicable SIP requirements and emissions milestones. 
The general conformity rule also requires that notices of draft and 
final general conformity determinations be provided directly to air 
quality regulatory agencies and to the public by publication in a local 
newspaper.

E. Transition from the existing SO2 NAAQS to a revised SO2 NAAQS

    As stated in section II.F.5 of this notice, in addition to 
proposing a short-term 1-hour SO2 NAAQS, EPA is proposing to 
revoke the current annual and 24-hour standards, (annual 0.03 ppm and 
24-hour 0.14 ppm). Specifically, EPA is proposing that the level for 
the 1-hour standard for SO2 be a range between 50-100 ppb, 
and is taking comment on setting the level of the standard up to 150 
ppb. If the Administrator sets the 1-hour standard at 100 ppb or lower, 
EPA is proposing to revoke the current 24-hour standard. If the 
Administrator sets the level of the 1-hour standard between a range of 
100-150 ppb, then EPA would retain the current 24-hour standard.
    If EPA revises the SO2 NAAQS and revokes either the 
current annual or 24-hour standard, EPA would need to promulgate 
adequate anti-backsliding provisions. The CAA establishes anti-
backsliding requirements where EPA relaxes a NAAQS. Here, if EPA were 
to replace the annual and/or 24-hour standard with a short term 1-hour 
standard, EPA would need to address the section 172(e) anti-backsliding 
provision of the CAA and determine whether it applies on its face or by 
analogy, and what provisions would be appropriate to provide for 
transition to the new standard. States would need to insure that the 
health protection provided under the existing SO2 NAAQS 
continues to be achieved as well as maintained as states begin to 
implement a revised NAAQS. This means that states would be directed to 
continue implementing attainment and maintenance SIPs associated with 
the existing SO2 NAAQS until such time as they are subsumed 
by any new planning and control requirements associated with a revised 
NAAQS.
    Whether or not section 172(e) directly applies to EPA's final 
action on the SO2 NAAQS, EPA has previously looked to other 
provisions of the CAA to determine how to address anti-backsliding. The 
CAA contains a number of provisions that indicate Congress's intent to 
not allow provisions from implementation plans to be altered or removed 
if the plan revision would jeopardize the air quality protection being 
provided by the existing plan when EPA revises a NAAQS to make it more 
stringent. For example, section 110(l) provides that EPA may not 
approve a SIP revision if it interferes with any applicable requirement 
concerning attainment and RFP, or any other applicable requirement 
under the CAA. In addition, section 193 of the CAA prohibits the 
modification of a control, or a control requirement, in effect or 
required to be adopted as of November 15, 1990 (i.e., prior to the 
promulgation of the Clean Air Act Amendments of 1990), unless such a 
modification would

[[Page 64864]]

ensure equivalent or greater emissions reductions. Further, section 
172(e) of the CAA specifies that if EPA revises a NAAQS to make it less 
stringent than a previous NAAQS, control obligations that apply in 
nonattainment area SIPs may not be relaxed, and adopting those controls 
that have not yet been adopted as needed may not be avoided. The intent 
of Congress, concerning the aforementioned sections of the CAA, was 
confirmed in a recent DC Circuit Court opinion on the Phase I ozone 
implementation rule. See South Coast Air Quality Management Dist. v. 
EPA, 472 F.3d 882 (DC Cir. 2006).
    To ensure that the antibacksliding provisions and principles of 
section 172(e) are met and applied if EPA revokes the current 
standards, EPA is proposing that the current SO2 NAAQS would 
remain in effect for one year following the effective date of the 
initial designations under section 107(d)(1) for the revised 
SO2 NAAQS before the current NAAQS are revoked in most 
attainment areas. However, any existing SIP provisions under CAA 
sections 110, 191 and 192 associated with the existing annual and 24-
hour SO2 NAAQS would remain in effect, including all 
currently implemented planning and emissions control obligations, 
including both those in the state's SIP and that have been promulgated 
by EPA in FIPs. This would ensure that both the new nonattainment NSR 
requirements and the general conformity requirements for a revised 
standard are in place so that there will be no gap in the public health 
protections provided by these two programs. It will also insure that 
all nonattainment areas under the current NAAQS and all areas for which 
SIP calls have been issued would continue to be protected by currently 
required control measures.
    EPA is also proposing that the existing NAAQS remain in place for 
any current nonattainment area, or any area for which a state has not 
fulfilled the requirements of a SIP call, until the affected area 
submits, and EPA approves, a SIP with an attainment demonstration which 
fully addresses the attainment requirements of the revised 
SO2 NAAQS. This, in combination with the CAA mechanisms 
provided in sections 110(l), 193, and 172(e) will help to ensure that 
continued progress is made toward timely attainment of the 
SO2 NAAQS. Also, in light of the nature of the proposed 
revision of the SO2 NAAQS, the lack of classifications (and 
mandatory controls associated with such classifications pursuant to the 
CAA), and the small number of current nonattainment areas, and areas 
subject to SIP calls, EPA believes (subject to consideration of public 
comment) that retaining the current standard for a limited period of 
time until attainment SIPs are approved for the new standard in current 
nonattainment areas and SIP call areas, and one year after designations 
in other areas, will adequately serve the anti-backsliding requirements 
and goals of the CAA.\44\
---------------------------------------------------------------------------

    \44\ The areas that are currently designated as nonattainment 
for the pre-existing SO2 primary NAAQS are Hayden, AZ; 
Armstrong, PA; Laurel, MT; Piti, GU; and Tanguisson, GU. The areas 
that are designated nonattainment for both the primary and the 
secondary standards are East Helena, MT, Salt Lake Co, MT, Toole Co, 
UT, and Warren Co, NJ. (See http://www.epa.gov/oar/oaqps/greenbk/lnc.html). The Billings/Laurel, MT, area is the only area currently 
subject to a SIP call.
---------------------------------------------------------------------------

VII. Communication of Public Health Information

    Information on the public health implications of ambient 
concentrations of criteria pollutants is currently made available 
primarily through EPA's Air Quality Index (AQI) program. The current 
Air Quality Index has been in use since its inception in 1999 (64 FR 
42530). It provides accurate, timely, and easily understandable 
information about daily levels of pollution (40 CFR 58.50). The AQI 
establishes a nationally uniform system of indexing pollution levels 
for NO2, carbon monoxide, ozone, particulate matter and 
sulfur dioxide. The AQI converts pollutant concentrations in a 
community's air to a number on a scale from 0 to 500. Reported AQI 
values enable the public to know whether air pollution levels in a 
particular location are characterized as good (0-50), moderate (51-
100), unhealthy for sensitive groups (101-150), unhealthy (151-200), 
very unhealthy (201-300), or hazardous (300-500). The AQI index value 
of 100 typically corresponds to the level of the short-term primary 
NAAQS for each pollutant. An AQI value greater than 100 means that a 
pollutant is in one of the unhealthy categories (i.e., unhealthy for 
sensitive groups, unhealthy, very unhealthy, or hazardous) on a given 
day; an AQI value at or below 100 means that a pollutant concentration 
is in one of the satisfactory categories (i.e., moderate or good). 
Decisions about the pollutant concentrations at which to set the 
various AQI breakpoints, that delineate the various AQI categories, 
draw directly from the underlying health information that supports the 
review of the primary NAAQS.
    The Agency recognizes the importance of revising the AQI in a 
timely manner to be consistent with any revisions to the primary NAAQS. 
Therefore EPA proposes to finalize conforming changes to the AQI, in 
connection with the Agency's final decision on the SO2 NAAQS 
if revisions to the primary standard are promulgated. If EPA 
promulgates a short-term primary SO2 NAAQS, conforming 
changes would include setting the 100 level of the AQI at the same 
level as the revised primary SO2 NAAQS. Conforming changes 
also would include setting the other AQI breakpoints at the lower end 
of the AQI scale (i.e., AQI values of 50 and 150). EPA does not propose 
to change breakpoints at the higher end of the AQI scale (from 200 to 
500), which would apply to state contingency plans or the Significant 
Harm Level (40 CFR 51.16), because the information from this review 
does not inform decisions about breakpoints at those higher levels.
    With regard to an AQI value of 50, the breakpoint between the good 
and moderate categories, historically this value is set at the level of 
the annual NAAQS, if there is one, or one-half the level of the short-
term NAAQS in the absence of an annual NAAQS (63 FR 67823, Dec. 12, 
1998). Taking into consideration this practice, EPA is proposing to set 
the AQI value of 50 to be between 25 and 50 ppb SO2, 1-hour 
average. EPA anticipates that figures towards the lower end of this 
range would be appropriate if the standard is set towards the lower end 
of the range for the proposed standard (e.g. 50 ppb), while figures 
towards the higher end of the range would be more appropriate for 
standards set at the higher end of the range (e.g., 100 ppb). If the 
short-term standard is set at a level above 100 ppb, and (contrary to 
the proposal) the annual standard is not revoked, then consideration 
could be given to setting an AQI value of 50 at the level of the annual 
standard, or 30 ppb. EPA solicits comments on this range for an AQI of 
50, and the appropriate basis for selecting an AQI of 50 both within 
this range and, in light of EPA's solicitation of comment on 1-hour 
standard levels above 100 ppb, above this range.
    With regard to an AQI value of 150, the breakpoint between the 
unhealthy for sensitive groups and unhealthy categories, historically 
values between the short-term standard and an AQI value of 500 are set 
at levels that are approximately equidistant between the AQI values of 
100 and 500 unless there is health evidence that suggests a specific 
level would be appropriate (63 FR 67829, Dec. 12, 1998). For an AQI 
value of 150, the range of 175 to 200 ppb SO2, 1-hour 
average, represents the midpoint between the proposed range for the 
short-term standard and the level

[[Page 64865]]

of an AQI value of 200 (300 ppb SO2, 1-hour average).

VIII Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

    Under section 3(f)(1) of Executive Order 12866 (58 FR 51735, 
October 4, 1993), this action is an ``economically significant 
regulatory action'' because it is likely to have an annual effect on 
the economy of $100 million or more. Accordingly, EPA submitted this 
action to the Office of Management and Budget (OMB) for review under EO 
12866 and any changes made in response to OMB recommendations have been 
documented in the docket for this action. In addition, EPA prepared a 
Regulatory Impact Analysis (RIA) of the potential costs and benefits 
associated with this action. However, the CAA and judicial decisions 
make clear that the economic and technical feasibility of attaining the 
national ambient standards cannot be considered in setting or revising 
NAAQS, although such factors may be considered in the development of 
State implementation plans to implement the standards. Accordingly, 
although an RIA has been prepared, the results of the RIA have not been 
considered by EPA in developing this proposed rule.

B. Paperwork Reduction Act

    The information collection requirements in this proposed rule have 
been submitted for approval to the Office of Management and Budget 
(OMB) under the Paperwork Reduction Act, 44 U.S.C. 3501 et seq. The 
Information Collection Request (ICR) document prepared by EPA for these 
proposed revisions to part 58 has been assigned EPA ICR number 2370.01
    The information collected under 40 CFR part 53 (e.g., test results, 
monitoring records, instruction manual, and other associated 
information) is needed to determine whether a candidate method intended 
for use in determining attainment of the NAAQS in 40 CFR part 50 will 
meet the design, performance, and/or comparability requirements for 
designation as a Federal reference method (FRM) or Federal equivalent 
method (FEM). We do not expect the number of FRM or FEM determinations 
to increase over the number that is currently used to estimate burden 
associated with SO2 FRM/FEM determinations provided in the 
current ICR for 40 CFR part 53 (EPA ICR numbers 2370.01). As such, no 
change in the burden estimate for 40 CFR part 53 has been made as part 
of this rulemaking.
    The information collected and reported under 40 CFR part 58 is 
needed to determine compliance with the NAAQS, to characterize air 
quality and associated health impacts, to develop emissions control 
strategies, and to measure progress for the air pollution program. The 
proposed amendments would revise the technical requirements for 
SO2 monitoring sites, require the siting and operation of 
additional SO2 ambient air monitors, and the reporting of 
the collected ambient SO2 monitoring data to EPA's Air 
Quality System (AQS). The annual average reporting burden for the 
collection under 40 CFR part 58 (averaged over the first 3 years of 
this ICR) is $13,863,950. Burden is defined at 5 CFR 1320.3(b). State, 
local, and tribal entities are eligible for State assistance grants 
provided by the Federal government under the CAA which can be used for 
monitors and related activities.
    An agency may not conduct or sponsor, and a person is not required 
to respond to, a collection of information unless it displays a 
currently valid OMB control number. The OMB control numbers for EPA's 
regulations in 40 CFR are listed in 40 CFR part 9.
    To comment on the Agency's need for this information, the accuracy 
of the provided burden estimates, and any suggested methods for 
minimizing respondent burden, EPA has established a public docket for 
this rule, which includes this ICR, under Docket ID number EPA-HQ-OAR-
2007-0352. Submit any comments related to the ICR to EPA and OMB. See 
ADDRESSES section at the beginning of this notice for where to submit 
comments to EPA. Send comments to OMB at the Office of Information and 
Regulatory Affairs, Office of Management and Budget, 725 17th Street, 
NW, Washington, DC 20503, Attention: Desk Office for EPA. Since OMB is 
required to make a decision concerning the ICR between 30 and 60 days 
after December 8, 2009, a comment to OMB is best assured of having its 
full effect if OMB receives it by January 7, 2010. The final rule will 
respond to any OMB or public comments on the information collection 
requirements contained in this proposal.

C. Regulatory Flexibility Act

    The Regulatory Flexibility Act (RFA) generally requires an agency 
to prepare a regulatory flexibility analysis of any rule subject to 
notice and comment rulemaking requirements under the Administrative 
Procedure Act or any other statute unless the agency certifies that the 
rule will not have a significant economic impact on a substantial 
number of small entities. Small entities include small businesses, 
small organizations, and small governmental jurisdictions.
    For purposes of assessing the impacts of this 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 this 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 SO2 in ambient air as required by section 
109 of the CAA. American Trucking Ass'ns v. EPA, 175 F. 3d 1027, 1044-
45 (DC Cir. 1999) (NAAQS do not have significant impacts upon small 
entities because NAAQS themselves impose no regulations upon small 
entities). Similarly, the proposed amendments to 40 CFR Part 58 address 
the requirements for States to collect information and report 
compliance with the NAAQS and will not impose any requirements on 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. Unless otherwise prohibited by law, 
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 one year. Before 
promulgating an EPA rule for which a written statement is required 
under section 202, section 205 of the UMRA generally requires EPA to 
identify and consider a reasonable number of

[[Page 64866]]

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 is not subject to the requirements of sections 202 and 
205 of the UMRA. EPA has determined that this proposed rule does not 
contain a Federal mandate that may result in expenditures of $100 
million or more for State, local, and tribal governments, in the 
aggregate, or the private sector in any one year. The revisions to the 
SO2 NAAQS impose no enforceable duty on any State, local or 
Tribal governments or the private sector. The expected costs associated 
with the monitoring requirements are described in EPA's ICR document, 
but those costs are not expected to exceed $100 million in the 
aggregate for any year. Furthermore, as indicated previously, in 
setting a NAAQS, EPA cannot consider the economic or technological 
feasibility of attaining ambient air quality standards. Because the CAA 
prohibits EPA from considering the types of estimates and assessments 
described in section 202 when setting the NAAQS, the UMRA does not 
require EPA to prepare a written statement under section 202 for the 
revisions to the SO2 NAAQS.
    With regard to implementation guidance, the CAA imposes the 
obligation for States to submit SIPs to implement the SO2 
NAAQS. In this proposed rule, EPA is merely providing an interpretation 
of those requirements. However, even if this rule did establish an 
independent obligation for States to submit SIPs, it is questionable 
whether an obligation to submit a SIP revision would constitute a 
Federal mandate in any case. The obligation for a State to submit a SIP 
that arises out of section 110 and section 191 of the CAA is not 
legally enforceable by a court of law, and at most is a condition for 
continued receipt of highway funds. Therefore, it is possible to view 
an action requiring such a submittal as not creating any enforceable 
duty within the meaning of U.S.C. 658 for purposes of the UMRA. Even if 
it did, the duty could be viewed as falling within the exception for a 
condition of Federal assistance under U.S.C. 658.
    EPA has determined that this proposed rule contains no regulatory 
requirements that might significantly or uniquely affect small 
governments because it imposes no enforceable duty on any small 
governments. Therefore, this rule is not subject to the requirements of 
section 203 of the UMRA.

E. Executive Order 13132: Federalism

    Executive Order 13132, entitled ``Federalism'' (64 FR 43255; August 
10, 1999), requires EPA to develop an accountable process to ensure 
``meaningful and timely input by State and local officials in the 
development of regulatory policies that have federalism implications.'' 
``Policies that have federalism implications'' is defined in the 
Executive Order to include regulations that 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.''
    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. The rule does not alter the 
relationship between the Federal government and the States regarding 
the establishment and implementation of air quality improvement 
programs as codified in the CAA. Under section 109 of the CAA, EPA is 
mandated to establish NAAQS; however, CAA section 116 preserves the 
rights of States to establish more stringent requirements if deemed 
necessary by a State. Furthermore, this rule does not impact CAA 
section 107 which establishes that the States have primary 
responsibility for implementation of the NAAQS. Finally, as noted in 
section E (above) on UMRA, this rule does not impose significant costs 
on State, local, or tribal governments or the private sector. Thus, 
Executive Order 13132 does not apply to this rule.
    However, EPA recognizes that States will have a substantial 
interest in this rule and any corresponding revisions to associated air 
quality surveillance requirements, 40 CFR part 58. 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 proposed rule does not 
have tribal implications, as specified in Executive Order 13175. It 
does not have a substantial direct effect on one or more Indian tribes, 
on the relationship between the Federal government and Indian tribes, 
or on the distribution of power and responsibilities between the 
Federal government and tribes. The rule does not alter the relationship 
between the Federal government and tribes as established in the CAA and 
the TAR. Under section 109 of the CAA, EPA is mandated to establish 
NAAQS; however, this rule does not infringe existing tribal authorities 
to regulate air quality under their own programs or under programs 
submitted to EPA for approval. Furthermore, this rule does not affect 
the flexibility afforded to tribes in seeking to implement CAA programs 
consistent with the TAR, nor does it impose any new obligation on 
tribes to adopt or implement any NAAQS. Finally, as noted in section E 
(above) on UMRA, this rule does not impose significant costs on tribal 
governments. Thus, Executive Order 13175 does not apply to this rule. 
However, EPA recognizes that tribes may be interested in this rule and 
any corresponding revisions to associated air quality surveillance 
requirements. Therefore, in the spirit of Executive Order 13175, and 
consistent with EPA policy to promote communications between EPA and 
tribes, EPA specifically solicits additional comment on this proposed 
rule from tribal officials.

[[Page 64867]]

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

    This action is subject to Executive Order (62 FR 19885, April 23, 
1997) because it is an economically significant regulatory action as 
defined by Executive Order 12866, and we believe that the environmental 
health risk addressed by this action has a disproportionate effect on 
children. The proposed rule will establish uniform national ambient air 
quality standards for SO2; these standards are designed to 
protect public health with an adequate margin of safety, as required by 
CAA section 109. The protection offered by these standards may be 
especially important for asthmatics, including asthmatic children, 
because respiratory effects in asthmatics are among the most sensitive 
health endpoints for SO2 exposure. Because asthmatic 
children are considered a sensitive population, we have evaluated the 
potential health effects of exposure to SO2 pollution among 
asthmatic children. These effects and the size of the population 
affected are discussed in chapters 3 and 4 of the ISA; chapters 3, 4, 
7, 8, 9 of the REA, and sections II.A through II.E of this preamble.

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

    This rule is not a ``significant energy action'' as defined in 
Executive Order 13211, ``Actions Concerning Regulations That 
Significantly Affect Energy Supply, Distribution, or Use'' (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. The 
purpose of this rule is to establish revised NAAQS for SO2. 
The rule does not prescribe specific 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. Thus, EPA concludes that this rule 
is not likely to have any adverse energy effects.

 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, section 12(d) (15 U.S.C. 27) 
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.
    This proposed rulemaking involves technical standards with regard 
to ambient monitoring of SO2. The use of this voluntary 
consensus standard would be impractical because the analysis method 
does not provide for the method detection limits necessary to 
adequately characterize ambient SO2 concentrations for the 
purpose of determining compliance with the proposed revisions to the 
SO2 NAAQS.
    EPA welcomes comments on this aspect of the proposed rule, and 
specifically invites the public to identify potentially applicable 
voluntary consensus standards and to explain why such standards should 
be used in the regulation.

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 increases the 
level of environmental protection for all affected populations without 
having any disproportionately high and adverse human health effects on 
any population, including any minority or low-income population. The 
proposed rule will establish uniform national standards for 
SO2 in ambient air. EPA solicits comment on environmental 
justice issues related to the proposed revision of the SO2 
NAAQS.

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0352-0044. Available at www.regulations.gov.
Henderson. (2008). Letter to EPA Administrator Stephen Johnson: 
Clean Air Scientific Advisory Committee's (CASAC) Peer Review of 
EPA's Risk and Exposure Assessment to Support the Review of the 
SO2 Primary National Ambient Air Quality Standards (First 
Draft, July 2008). EPA-CASAC-08-019. August 22, 2008. Sulfur Dioxide 
Review Docket. Docket ID No. EPA-HQ-OAR-2007-0352-0034. Available at 
www.regulations.gov.
Ito K. (2007). Characterization of PM2.5, gaseous 
pollutants, and meteorological interactions in the context of time-
series health effects models. J Expos Sci Environ Epidemiol. 17:S45-
S60.
Jaffe DH, Singer ME, Rimm AA. (2003). Air pollution and emergency 
department visits for asthma among Ohio medicaid recipients, 1991-
1996. Environ Res. 91:21-28.
Johns. (2009). Presentation and analysis of controlled human 
exposure data described in Table 3-1 of the 2008 Integrated Science 
Assessment (ISA) for Sulfur Oxides; April 29, 2009. Available at: 
http://www.epa.gov/ttn/naaqs/standards/so2/s_so2_cr_rea.html.
Johns and Simmons (2009). Memorandum to the Sulfur Oxides NAAQS 
Review Docket. Quality Assurance Review of Individual Subject Data 
Presented in Table 3-1 of the 2008 Integrated Science Assessment 
(ISA) for Sulfur Oxides. Air Quality Criteria for Sulfur Oxides 
Docket. Docket ID No. EPA-HQ-ORD-2006-0260-0036. Available at 
www.regulations.gov.
Koenig JQ, Covert DS, Hanley QS, van Belle G, Pierson WE. (1990). 
Prior exposure to ozone potentiates subsequent response to sulfur 
dioxide in adolescent asthmatic subjects. Am Rev Respir Dis. 
141:377-380.
Lin S, Hwang S-A, Pantea C, Kielb C, Fitzgerald E. (2004). Childhood 
asthma hospitalizations and ambient air sulfur dioxide 
concentrations in Bronx County, New York. Arch Environ Health. 
59:266-275.
Linn WS, Venet TG, Shamoo DA, Valencia LM, Anzar UT, Spier CE, 
Hackney JD. (1983). Respiratory effects of sulfur dioxide in heavily 
exercising asthmatics. A dose-response study. Am Rev Respir Dis. 
127:278-83.
Linn WS, Avol EL, Peng RC, Shamoo DA, Hackney JD. (1987). Replicated 
dose-response study of sulfur dioxide effects in normal, atopic, and 
asthmatic volunteers. Am Rev Respir Dis. 136:1127-1134.
Linn WS, Avol EL, Shamoo DA, Peng RC, Spier CE, Smith MN, Hackney 
JD. (1988). Effect of metaproterenol sulfate on mild asthmatics' 
response to sulfur dioxide exposure and exercise. Arch Environ 
Health. 43:399-406.
Linn WS, Shamoo DA, Peng RC, Clark KW, Avol EL, Hackney JD. (1990). 
Responses to sulfur dioxide and exercise by medication-dependent 
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Mortimer KM, Neas LM, Dockery DW, Redline S, Tager IB. (2002). The 
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Authority; New York State Department of Health, for Atlanta, GA; 
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Health and Human Services.
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Bronchoconstriction in asthmatics exposed to sulfur dioxide during 
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[[Page 64869]]

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Multipollutant modeling issues in a study of ambient air quality and 
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Allergy Asthma Immunol. 86:232-238.

List of Subjects

40 CFR Part 50

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

40 CFR Part 53

    Environmental protection, Administrative practice and procedure, 
Air pollution control, Intergovernmental relations, Reporting and 
recordkeeping requirements.

40 CFR Part 58

    Environmental protection, Administrative practice and procedure, 
Air pollution control, Intergovernmental relations, Reporting and 
recordkeeping requirements.

    Dated: November 16, 2009.
Lisa P. Jackson,
Administrator.
    For the reasons stated in the preamble, title 40, chapter I 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 50.4 is amended by adding paragraph (e) to read as 
follows:


Sec.  50.4  National primary ambient air quality standards for sulfur 
oxides (sulfur dioxide).

* * * * *
    (e) The standards set forth in this section will remain applicable 
to all areas notwithstanding the promulgation of SO2 
national ambient air quality standards (NAAQS) in Sec.  50.17. The 
SO2 NAAQS set forth in this section will no longer apply to 
an area one year after the effective date of the designation of that 
area, pursuant to section 107 of the Clean Air Act, for the 
SO2 NAAQS set forth in Sec.  50.17; except that for areas 
designated nonattainment for the SO2 NAAQS set forth in this 
section as of the effective date of Sec.  50.17, and areas not meeting 
the requirements of a SIP call with respect to requirements for the 
SO2 NAAQS set forth in this section, the SO2 
NAAQS set forth in this section will apply until that area submits, 
pursuant to section 191 of the Clean Air Act, and EPA approves, an 
implementation plan providing for attainment of the SO2 
NAAQS set forth in Sec.  50.17.
    3. Section 50.14 is amended by revising paragraph (c)(2)(vi) to 
read as follows:


Sec.  50.14  Treatment of air quality monitoring data influenced by 
exceptional events.

* * * * *
    (c) * * *
    (2) * * *
    (vi) When EPA sets a NAAQS for a new pollutant or revises the NAAQS 
for an existing pollutant, it may revise or set a new schedule for 
flagging exceptional event data, providing initial data descriptions 
and providing detailed data documentation in AQS for the initial 
designations of areas for those NAAQS. Table 1 provides the schedule 
for submission of flags with initial descriptions in AQS and detailed 
documentation. These schedules shall apply for those data which will or 
may influence the initial designation of areas for those NAAQS. EPA 
anticipates revising Table 1 as necessary to accommodate revised data 
submission schedules for new or revised NAAQS.

Table 1--Schedule or Exceptional Event Flagging and Documentation Submission for Data To Be Used in Designations
                                       Decisions for New or Revised NAAQS
----------------------------------------------------------------------------------------------------------------
                                         Air quality data
   NAAQS pollutant/ standard/(level)/      collected for   Event flagging and initial    Detailed documentation
           promulgation date               calendar year      description deadline        submission deadline
----------------------------------------------------------------------------------------------------------------
PM2.5/24-Hr Standard (35 [mu]g/m\3\)            2004-2006  October 1, 2007 \a\.......  April 15, 2008 \a\.
 Promulgated October 17, 2006.
Ozone/8-Hr Standard (0.075 ppm)                 2005-2007  June 18, 2009 \a\.........  June 18, 2009 \a\.
 Promulgated March 12, 2008.
                                                     2008  June 18, 2009 \a\.........  June 18, 2009 \a\.

[[Page 64870]]

 
                                                     2009  60 Days after the end of    60 Days after the end of
                                                            the calendar quarter in     the calendar quarter in
                                                            which the event occurred    which the event occurred
                                                            or February 5, 2010,        or February 5, 2010,
                                                            whichever date occurs       whichever date occurs
                                                            first \b\.                  first \b\.
NO2/1-Hour Standard (80-100 PPB, final               2008  July 1, 2010 \a\..........  January 22, 2011 \a\.
 level TBD).
                                                     2009  July 1, 2010 \a\..........  January 22, 2011 \a\.
                                                     2010  April 1, 2011 \a\.........  July 1, 2011 \ a\.
SO2/1-Hour Standard (50-100 PPB, final               2008  October 1, 2010 \b\.......  June 1, 2011 \b\.
 level TBD).
                                                     2009  October 1, 2010 \b\.......  June 1, 2011 \b\.
                                                     2010  June 1, 2011 \b\..........  June 1, 2011 \b\.
                                                     2011  60 Days after the end of    60 Days after the end of
                                                            the calendar quarter in     the calendar quarter in
                                                            which the event occurred    which the event occurred
                                                            or March 31, 2011,          or March 31, 2011,
                                                            whichever date occurs       whichever date occurs
                                                            first \b\.                  first \b\.
----------------------------------------------------------------------------------------------------------------
\a\ These dates are unchanged from those published in the original rulemaking, or are being proposed elsewhere
  and are shown in this table for informational purposes--the Agency is not opening these dates for comment
  under this rulemaking.
\b\ Indicates change from general schedule in 40 CFR 50.14.
Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final
  initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most
  notably, for data used by EPA for redesignations to attainment.

* * * * *
    4. A new 50.17 is added to read as follows:


Sec.  50.17  National primary ambient air quality standards for sulfur 
oxides (sulfur dioxide).

    (a) The level of the national primary 1-hour annual ambient air 
quality standard for oxides of sulfur is (50-100) parts per billion 
(ppb, which is 1 part in 1,000,000,000), measured in the ambient air as 
sulfur dioxide (SO2).
    (b) The 1-hour primary standard is met when the three-year average 
of the annual (99th percentile)(fourth highest) of the daily maximum 1-
hour average concentrations is less than or equal to (50-100) ppb, as 
determined in accordance with Appendix T of this part.
    5. Add Appendix A-1 to Part 50 to read as follows:

Appendix A-1 to Part 50--Reference Measurement Principle and 
Calibration Procedure for the Measurement of Sulfur Dioxide in the 
Atmosphere (Ultraviolet Fluorescence Method)

    1.0 Applicability.
    1.1 This ultraviolet fluorescence (UVF) method provides a 
measurement of the concentration of sulfur dioxide (SO2) 
in ambient air for determining compliance with the national primary 
and secondary ambient air quality standards for sulfur oxides 
(sulfur dioxide) as specified in Sec.  50.4 and Sec.  50.5 of this 
chapter. The method is applicable to the measurement of ambient 
SO2 concentrations using continuous (real-time) sampling. 
Additional quality assurance procedures and guidance are provided in 
part 58, appendix A, of this chapter and in Reference 3.
    2.0 Principle.
    2.1 This reference method is based on automated measurement of 
the intensity of the characteristic fluorescence released by 
SO2 in an ambient air sample contained in a measurement 
cell of an analyzer when the air sample is irradiated by ultraviolet 
(UV) light passed through the cell. The fluorescent light released 
by the SO2 is also in the ultraviolet region, but at 
longer wavelengths than the excitation light. Typically, optimum 
instrumental measurement of SO2 concentrations is 
obtained with an excitation wavelength in a band between 
approximately 190 to 230 nm, and measurement of the SO2 
fluorescence in a broad band around 320 nm, but these wavelengths 
are not necessarily constraints of this reference method. Generally, 
the measurement system (analyzer) also requires means to reduce the 
effects of aromatic hydrocarbon species, and possibly other 
compounds, in the air sample to control measurement interferences 
from these compounds, which may be present in the ambient air. 
References 1 and 2 describe UVF method.
    2.2. The measurement system is calibrated by referencing the 
instrumental fluorescence measurements to SO2 standard 
concentrations traceable to a National Institute of Science and 
Technology (NIST) primary standard for SO2 (see 
Calibration Procedure below).
    2.3. An analyzer implementing this measurement principle is 
shown schematically in Figure 1. Designs should include a 
measurement cell, a UV light source of appropriate wavelength, a UV 
detector system with appropriate wave length sensitivity, a pump and 
flow control system for sampling the ambient air and moving it into 
the measurement cell, sample air conditioning components as 
necessary to minimize measurement interferences, suitable control 
and measurement processing capability, and other apparatus as may be 
necessary. The analyzer must be designed to provide accurate, 
repeatable, and continuous measurements of SO2 
concentrations in ambient air, with measurement performance as 
specified in subpart B of part 53 of this chapter.
    2.4. Sampling considerations: The use of a particle filter on 
the sample inlet line of a UVF SO2 analyzer is required 
to prevent interference, malfunction, or damage due to particles in 
the sampled air.
    3.0 Interferences.
    3.1 The effects of the principal potential interferences may 
need to be mitigated to meet the interference equivalent 
requirements of part 53 of this chapter. Poly-nuclear aromatic (PNA) 
hydrocarbons such as xylene and naphthalene can fluoresce and act as 
strong positive interferences. These gases can be removed by using a 
permeation type scrubber (hydrocarbon ``kicker''). Nitrogen oxide 
(NO) in high concentrations can also fluoresce and cause positive 
interference. Optical filtering can be employed to improve the 
rejection of interference from high NO. Ozone can absorb UV light 
given off by the SO2 molecule and cause a measurement 
offset. This effect can be reduced by minimizing the measurement 
path length between the area where SO2 fluorescence 
occurs and the photomultiplier tube detector (e.g. <5 cm). A 
hydrocarbon scrubber, optical filter and appropriate distancing of 
the measurement path length may be required method components to 
reduce interference.
    4.0 Calibration Procedure. Atmospheres containing accurately 
known concentrations of sulfur dioxide are prepared using a 
compressed gas transfer standard diluted with accurately metered 
clean air flow rates.
    4.1 Apparatus: Figure 2 shows a typical generic system suitable 
for diluting a SO2 gas cylinder concentration standard 
with clean air through a mixing chamber to produce the desired 
calibration concentration standards.

[[Page 64871]]

A valve may be used to conveniently divert the SO2 from 
the sampling manifold to provide clean zero air at the output 
manifold for zero adjustment. The system may be made up using common 
laboratory components, or it may be a commercially manufactured 
system. In either case, the principle components are as follows:
    4.1.1 Air and standard gas flow controllers, capable of 
maintaining constant gas flow rates to within  2 
percent.
    4.1.2 Air and standard gas flow meters, capable of measuring and 
monitoring air or N2 (standard gas) flow rates to within 
 2 percent and properly calibrated to a NIST-traceable 
standard.
    4.1.3 Mixing chamber, of an inert material such as glass and of 
proper design to provide thorough mixing of pollutant gas and 
diluent air streams.
    4.1.4 Sampling manifold, constructed of glass, 
polytetrafluoroethylene (PTFE TeflonTM), or other 
suitably inert material and of sufficient diameter to insure a 
minimum pressure drop at the analyzer connection, with a vent 
designed to insure a minimum over-pressure (relative to ambient air 
pressure) at the analyzer connection and to prevent ambient air from 
entering the manifold.
    4.1.5 Standard gas pressure regulator, of clean stainless steel 
with a stainless steel diaphragm, suitable for use with a high 
pressure SO2 gas cylinder.
    4.1.6 Reagents.
    4.1.6.1 SO2 gas transfer standard, in N2, 
with the concentration traceable to a NIST Standard Reference 
Material (SRM) such as SRM 1693a (50 [mu]mole/mole) or SRM 1694a 
(100 [mu]mole/mole) Since UVF analyzers may be sensitive to 
O2-to-N2 ratios, it is important that the 
SO2 standard concentration be sufficiently high (50 to 
100 ppm) such that the O2 content in the diluent air is 
not significantly changed by the added standard gas.
    4.1.6.2 Clean zero air, free of contaminants that could cause a 
detectable response or a change in sensitivity of the analyzer. 
Since ultraviolet fluorescence analyzers may be sensitive to 
aromatic hydrocarbons and O2-to-N2 ratios, it 
is important that the clean zero air contains less than 0.1 ppm 
aromatic hydrocarbons and O2 and N2 
percentages approximately the same as in ambient air. A procedure 
for generating zero air is given in reference 1.
    4.2 Procedure
    4.2.1 Obtain a suitable calibration apparatus, such as the one 
shown schematically in Figure 1, and verify that all materials in 
contact with the pollutant are of glass, Teflon\TM\, or other 
suitably inert material and completely clean.
    4.2.2 Purge the SO2 standard gas lines and pressure 
regulator to remove any residual air.
    4.2.3 Ensure that there are no leaks in the system and that the 
flow measuring devices are properly and accurately calibrated under 
the conditions of use against a reliable volume or flow rate 
standard such as a soap-bubble meter or a wet-test meter traceable 
to a NIST standard. All volumetric flow rates should be corrected to 
the same reference temperature and pressure by using the formula 
below:
[GRAPHIC] [TIFF OMITTED] TP08DE09.004

Where:
Fc = corrected flow rate (L/min at 25[deg] C and 760 mm Hg),
Fm = measured flow rate, (at temperature, Tm and 
pressure, Pm),
Pm = measured pressure in mm Hg, (absolute), and
Tm = measured temperature in degrees Celsius.

    4.2.4 Allow the SO2 analyzer under calibration to 
sample zero air until a stable response is obtained, then make the 
proper zero adjustment.
    4.2.5 Adjust the airflow to provide an SO2 
concentration of approximately 80 percent of the upper measurement 
range limit of the SO2 instrument and verify that the 
total air flow of the calibration system exceeds the demand of all 
analyzers sampling from the output manifold (with the excess 
vented).
    4.2.6 Calculate the actual SO2 calibration 
concentration standard as:
[GRAPHIC] [TIFF OMITTED] TP08DE09.005

Where:
C = the concentration of the SO2 gas standard
Fp = the flow rate of SO2 gas standard
Ft = the total air flow rate of pollutant and diluent 
gases

    4.2.7 When the analyzer response has stabilized, adjust the 
SO2 span control to obtain the desired response 
equivalent to the calculated standard concentration. If substantial 
adjustment of the span control is needed, it may be necessary to re-
check the zero and span adjustments by repeating steps 4.2.4 through 
4.2.7 until no further adjustments are needed.
    4.2.8 Adjust the flow rate(s) to provide several other 
SO2 calibration concentrations over the analyzer's 
measurement range. At least five different concentrations evenly 
spaced throughout the analyzer's range are suggested.
    4.2.9 Plot the analyzer response (vertical or Y-axis) versus 
SO2 concentration (horizontal or X-axis). Compute the 
linear regression slope and intercept and plot the regression line 
to verify that no point deviates from this line by more than 2 
percent of the maximum concentration tested.

    Note: Additional information on calibration and pollutant 
standards is provided in Section 12 of Reference 3.

    5.0 Frequency of calibration.
    The frequency of calibration, as well as the number of points 
necessary to establish the calibration curve and the frequency of 
other performance checking will vary by analyzer; however, the 
minimum frequency, acceptance criteria, and subsequent actions are 
specified in Reference 3, Appendix D: Measurement Quality Objectives 
and Validation Template for SO2 (page 9 of 30). The 
user's quality control program should provide guidelines for initial 
establishment of these variables and for subsequent alteration as 
operational experience is accumulated. Manufacturers of analyzers 
should include in their instruction/operation manuals information 
and guidance as to these variables and on other matters of 
operation, calibration, routine maintenance, and quality control.
    6.0 References for SO2 Method.
1. H. Okabe, P.L. Splitstone, and J.J. Ball, ``Ambient and Source 
SO2 Detector Based on a Fluorescence Method'', Journal of 
the Air Control Pollution Association, vol. 23, p. 514-516 (1973).
2. F.P. Schwarz, H. Okabe, and J.K. Whittaker, ``Fluorescence 
Detection of Sulfur Dioxide in Air at the Parts per Billion Level,'' 
Analytical Chemistry, vol. 46, pp. 1024-1028 (1974).
3. QA Handbook for Air Pollution Measurement Systems--Volume II. 
Ambient Air Quality Monitoring Programs. U. S. EPA. EPA-454/B-08-003 
(2008). (Available at http://www.epa.gov/ttn/amtic/qabook.html.)

[[Page 64872]]

[GRAPHIC] [TIFF OMITTED] TP08DE09.006

[GRAPHIC] [TIFF OMITTED] TP08DE09.007


[[Page 64873]]


    6. Appendix A to Part 50 is redesignated as Appendix A-2 to Part 
50.
    7. Appendix T to Part 50 is added to read as follows:

Option 1 for Appendix T to Part 50

Appendix T to Part 50--Interpretation of the Primary National Ambient 
Air Quality Standards for Oxides of Sulfur (Sulfur Dioxide) [1-hour 
primary standard based on the 4th highest daily maximum value form]

    1. General.
    (a) This appendix explains the data handling conventions and 
computations necessary for determining when the primary national 
ambient air quality standards for Oxides of Sulfur as measured by 
Sulfur Dioxide (``SO2 NAAQS'') specified in Sec.  50.4 
are met. Sulfur Dioxide (SO2) is measured in the ambient 
air by a Federal reference method (FRM) based on appendix A to this 
part or by a Federal equivalent method (FEM) designated in 
accordance with part 53 of this chapter. Data handling and 
computation procedures to be used in making comparisons between 
reported SO2 concentrations and the levels of the 
SO2 NAAQS are specified in the following sections.
    (b) Decisions to exclude, retain, or make adjustments to the 
data affected by exceptional events, including natural events, are 
made according to the requirements and process deadlines specified 
in Sec. Sec.  50.1, 50.14 and 51.930 of this chapter.
    (c) The terms used in this appendix are defined as follows:
    Annual 4th highest daily maximum 1-hour value refers to the 4th 
highest daily 1-hour maximum value at a site in a particular year.
    Daily maximum 1-hour values for SO2 refers to the 
maximum 1-hour SO2 concentration values measured from 
midnight to midnight (local standard time) that are used in NAAQS 
computations.
    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 NAAQS is the 3-year average of annual 4th highest daily 
maximum 1-hour values for a monitoring site (referred to as the ``1-
hour primary standard design value'').
    Quarter refers to a calendar quarter.
    Year refers to a calendar year.
    2. Requirements for Data Used for Comparisons With the 
SO2 NAAQS and Data Reporting Considerations.
    (a) All valid FRM/FEM SO2 hourly data required to be 
submitted to EPA's Air Quality System (AQS), or otherwise available 
to EPA, meeting the requirements of part 58 of this chapter 
including appendices A, C, and E shall be used in design value 
calculations. Multi-hour average concentration values collected by 
wet chemistry methods shall not be used.
    (b) When two or more SO2 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 in advance, 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 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.
    (c) Hourly SO2 measurement data shall be reported to 
AQS in units of parts per billion (ppb), to at most one place after 
the decimal, with additional digits to the right being truncated 
with no further rounding.
    3. Comparisons with the 1-hour Primary SO2 NAAQS.
    (a) The 1-hour primary SO2 NAAQS is met at a site 
when the valid 1-hour primary standard design value is less than or 
equal to [50-150] parts per billion (ppb).
    (b) An SO2 1-hour primary standard design value is 
valid if it encompasses three consecutive calendar years of complete 
data. A year meets data completeness requirements when all 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 are reported.
    (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 usable for the calculation of a valid 3-year 
1-hour primary standard design value, the 3-year 1-hour primary 
standard design value shall nevertheless be considered valid if 
either of the following conditions is true:
    (i) If there are at least four days in each of the 3 years that 
have at least one reported hourly value, and the resulting 3-year 1-
hour primary standard design value exceeds the 1-hour primary NAAQS. 
In this situation, more complete data capture could not possibly 
have resulted in a design value below the 1-hour primary NAAQS:
    (ii)(A) A 1-hour primary standard design value that is 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 calendar quarter) 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 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 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; 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, 
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'' below the level of the standard, then 
the 1-hour primary standard design value is deemed to have passed 
the diagnostic test and is valid, and the level of the standard is 
deemed to have been met in that 3-year period. As noted in section 
3(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 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 standard 
design values are given in section 5 of this appendix.
    4. Rounding Conventions for the 1-hour Primary SO2 
NAAQS.
    (a) Hourly SO2 measurement data shall be reported to 
AQS in units of parts per billion (ppb), to at most one place after 
the decimal, with additional digits to the right being truncated 
with no further rounding.

[[Page 64874]]

    (b) Daily maximum 1-hour values, including the annual 4th 
highest of those daily values, are not rounded.
    (c) The 1-hour primary 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 SO2 
NAAQS.
    (a) When the data for a particular site and year meet the data 
completeness requirements in section 3(b), or if one of the 
conditions of section 3(c) is met, or if the Administrator exercises 
the discretionary authority in section 3(d), calculation of the 4th 
highest daily 1-hour maximum is accomplished as follows.
    (i) For each year, select from each day the highest hourly 
value. All daily maximum 1-hour values from all days in the quarter 
period shall be considered at this step, including days with less 
than 75 percent data capture.
    (ii) For each year, order these daily values and take the 4th 
highest.
    (iii) The 1-hour primary standard design value for a site is 
mean of the three annual 4th highest values, rounded according to 
the conventions in section 4.

Option 2 for Appendix T to Part 50

Appendix T to Part 50--Interpretation of the Primary National Ambient 
Air Quality Standards for Oxides of Sulfur (Sulfur Dioxide) [1-hour 
primary standard based on the 99th percentile form]

    1. General.
    (a) This appendix explains the data handling conventions and 
computations necessary for determining when the primary national 
ambient air quality standards for Oxides of Sulfur as measured by 
Sulfur Dioxide (``SO2 NAAQS'') specified in Sec.  50.4 
are met. Sulfur Dioxide (SO2) is measured in the ambient 
air by a Federal reference method (FRM) based on appendix A to this 
part or by a Federal equivalent method (FEM) designated in 
accordance with part 53 of this chapter. Data handling and 
computation procedures to be used in making comparisons between 
reported SO2 concentrations and the levels of the 
SO2 NAAQS are specified in the following sections.
    (b) Decisions to exclude, retain, or make adjustments to the 
data affected by exceptional events, including natural events, are 
made according to the requirements and process deadlines specified 
in Sec. Sec.  50.1, 50.14 and 51.930 of this chapter.
    (c) The terms used in this appendix are defined as follows:
    Daily maximum 1-hour values for SO2 refers to the 
maximum 1-hour SO2 concentration values measured from 
midnight to midnight (local standard time) that are used in NAAQS 
computations.
    Design values are the metrics (i.e., statistics) that are 
compared to the NAAQS levels to determine compliance, calculated as 
specified in section 5 of this appendix. The design value for the 
primary 1-hour NAAQS is the 3-year average of annual 99th percentile 
daily maximum 1-hour values for a monitoring site (referred to as 
the ``1-hour primary standard design value'').
    99th percentile daily maximum 1-hour value is the value below 
which nominally 99 percent of all daily maximum 1-hour concentration 
values fall, using the ranking and selection method specified in 
section 5 of this appendix.
    Quarter refers to a calendar quarter.
    Year refers to a calendar year.
    2. Requirements for Data Used for Comparisons With the 
SO2 NAAQS and Data Reporting Considerations.
    (a) All valid FRM/FEM SO2 hourly data required to be 
submitted to EPA's Air Quality System (AQS), or otherwise available 
to EPA, meeting the requirements of part 58 of this chapter 
including appendices A, C, and E shall be used in design value 
calculations. Multi-hour average concentration values collected by 
wet chemistry methods shall not be used.
    (b) When two or more SO2 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 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.
    (c) Hourly SO2 measurement data shall be reported to 
AQS in units of parts per billion (ppb), to at most one place after 
the decimal, with additional digits to the right being truncated 
with no further rounding.
    3. Comparisons with the 1-hour Primary SO2 NAAQS.
    (a) The 1-hour primary SO2 NAAQS is met at a site 
when the valid 1-hour primary standard design value is less than or 
equal to [50-150] parts per billion (ppb).
    (b) An SO2 1-hour primary standard design value is 
valid if it encompasses three consecutive calendar years of complete 
data. A year meets data completeness requirements when all 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 are reported.
    (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 standard design value, the 3-year 1-hour primary 
standard design value shall nevertheless be considered valid if one 
of the following conditions is true.
    (i) At least 75 percent of the days in each quarter of each of 
three consecutive years have at least one reported hourly value, and 
the design value calculated according to the procedures specified in 
section 5 is above the level of the primary 1-hour standard.
    (ii) (A) A 1-hour primary standard design value that is 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 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; 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, 
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'' below the level of the standard, then 
the 1-hour primary standard design value is deemed to have passed 
the diagnostic test and is valid, and the level of

[[Page 64875]]

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 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 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 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 standard 
design values are given in section 5 of this appendix.
    4. Rounding Conventions for the 1-hour Primary SO2 
NAAQS.
    (a) Hourly SO2 measurement data shall be reported to 
AQS in units of parts per billion (ppb), to at most one place after 
the decimal, with additional digits to the right being truncated 
with no further rounding.
    (b) Daily maximum 1-hour values and therefore the annual 4th 
highest of those daily values are not rounded.
    (c) The 1-hour primary 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 SO2 
NAAQS.
    (a) Procedure for identifying annual 99th percentile values. 
When the data for a particular site and year meet the data 
completeness requirements in section 3(b), or if one of the 
conditions of section 3(c) is met, or if the Administrator exercises 
the discretionary authority in section 3(d), identification of 
annual 99th percentile value is accomplished as follows.
    (i) The annual 99th percentile value for a year is the higher of 
the two values resulting from the following two procedures.
    (1) Procedure 1. For the year, determine the number of days with 
at least 75 percent of the hourly values reported.
    (A) For the year, from only the days with at least 75 percent of 
the hourly values reported, select from each day the maximum hourly 
value.
    (B) Sort all these daily maximum hourly values from a particular 
site and year by descending value. (For example: (x[1], x[2], x[3], 
* * *, x[n]). In this case, x[1] is the largest number and x[n] is 
the smallest value.) The 99th percentile is determined from this 
sorted series of daily values which is ordered from the highest to 
the lowest number. Using the left column of Table 1, determine the 
appropriate range (i.e., row) for the annual number of days with 
valid data for year y (cny). The corresponding ``n'' 
value in the right column identifies the rank of the annual 99th 
percentile value in the descending sorted list of daily site values 
for year y. Thus, P0.99, y = the nth largest value.
    (2) Procedure 2. For the year, determine the number of days with 
at least one hourly value reported.
    (A) For the year, from all the days with at least one hourly 
value reported, select from each day the maximum hourly value.
    (B) Sort all these daily maximum values from a particular site 
and year by descending value. (For example: (x[1], x[2], x[3], * * 
*, x[n]). In this case, x[1] is the largest number and x[n] is the 
smallest value.) The 99th percentile is determined from this sorted 
series of daily values which is ordered from the highest to the 
lowest number. Using the left column of Table 1, determine the 
appropriate range (i.e., row) for the annual number of days with 
valid data for year y (cny). The corresponding ``n'' 
value in the right column identifies the rank of the annual 99th 
percentile value in the descending sorted list of daily site values 
for year y. Thus, P0.99, y = the nth largest value.
    (b) The 1-hour primary standard design value for a site is mean 
of the three annual 99th percentile values, rounded according to the 
conventions in section 4.

                                 Table 1
------------------------------------------------------------------------
                                       P0.99, y is the nth maximum value
Annual number of days with valid data     of the year, where n is the
         for year ``y'' (cny)                    listed number
------------------------------------------------------------------------
1-100................................  1
101-200..............................  2
201-300..............................  3
301-366..............................  4
------------------------------------------------------------------------

PART 53--AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT METHODS

    8. The authority citation for part 53 continues to read as follows:

    Authority:  Sec. 301(a) of the Clean Air Act (42 U.S.C. sec. 
1857g(a)), as amended by sec. 15(c)(2) of Public Law 91-604, 84 
Stat. 1713, unless otherwise noted.

Subpart A--[Amended]

    9. Section 53.2 is amended by revising paragraphs (a)(1) and (b) to 
read as follows:


Sec.  53.2.  General requirements for a reference method determination.

* * * * *
    (a) Manual methods--(1) Sulfur dioxide (SO2) and Lead. For 
measuring SO2 and lead, Appendixes A-2 and G of part 50 of 
this chapter specify unique manual FRM for measuring those pollutants. 
After [effective date of Appendix A-1], a new FRM for SO2 
must be an automated method that utilizes the measurement principle and 
calibration procedure specified in Appendix A-1 to part 50 of this 
chapter and must meet applicable requirements of this part, as 
specified in paragraph (b) of this section. Except as provided in Sec.  
53.16, other manual methods for lead will not be considered for a 
reference method determination under this part.
* * * * *
    (b) Automated methods. An automated FRM for measuring 
SO2, CO, O3, or NO2 must utilize the 
measurement principle and calibration procedure specified in the 
appropriate appendix to part 50 of this chapter (appendix A-1 only for 
SO2 methods) and must have been shown in accordance with 
this part to meet the

[[Page 64876]]

requirements specified in this subpart A and subpart B of this part.
    10. Section 53.8 is amended by revising paragraph (c) to read as 
follows:


Sec.  53.8  Designation of reference and equivalent methods.

* * * * *
    (c) The Administrator will maintain a current list of methods 
designated as FRM or FEM in accordance with this part and will send a 
copy of the list to any person or group upon request. A copy of the 
list will be available via the Internet and may be available from other 
sources.
    11. Table A-1 to Subpart A is revised to read as follows:

  Table A-1 to Subpart A of Part 53--Summary of Applicable Requirements for Reference and Equivalent Methods for Air Monitoring of Criteria Pollutants
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                                            Applicable subparts of part 53
     Pollutant          Reference or       Manual or automated   Applicable part 50 appendix -----------------------------------------------------------
                         equivalent                                                               A         B         C         D         E         F
--------------------------------------------------------------------------------------------------------------------------------------------------------
SO2...............  Reference...........  Manual..............  A-2
                                          Automated...........  A-1                           [check]   [check]
                    Equivalent..........  Manual..............  A-1                           [check]   ........  [check]
                                          Automated...........  A-1                           [check]   [check]   [check]
CO................  Reference...........  Automated...........  C                             [check]   [check]
                    Equivalent..........  Manual..............  C                             [check]   ........  [check]
                                          Automated...........  C                             [check]   [check]   [check]
O3................  Reference...........  Automated...........  D                             [check]   [check]
                    Equivalent..........  Manual..............  D                             [check]   ........  [check]
                                          Automated...........  D                             [check]   [check]   [check]
NO2...............  Reference...........  Automated...........  F                             [check]   [check]
                    Equivalent..........  Manual..............  F                             [check]   ........  [check]
                                          Automated...........  F                             [check]   [check]   [check]
Pb................  Reference...........  Manual..............  G
                    Equivalent..........  Manual..............  G                             [check]   ........  [check]
                                          Automated...........  G                             [check]   ........  [check]
PM10-Pb...........  Reference...........  Manual..............  Q
                    Equivalent..........  Manual..............  Q                             [check]   ........  [check]
                                          Automated...........  Q                             [check]   ........  [check]
PM10..............  Reference...........  Manual..............  J                             [check]   ........  ........  [check]   ........  ........
                    Equivalent..........  Manual..............  J                             [check]   ........  [check]   [check]   ........  ........
                                          Automated...........  J                             [check]   ........  [check]   [check]   ........  ........
PM2.5.............  Reference...........  Manual..............  L                             [check]   ........  ........  ........  [check]   ........
                    Equivalent Class I..  Manual..............  L                             [check]   ........  [check]   ........  [check]   ........
                    Equivalent Class II.  Manual..............  L \1\                         [check]   ........  [check]   ........  [check]   [check]
                                                                                                                      \2\                           1 2
                    Equivalent Class III  Automated...........  L \1\                         [check]   ........  [check]   ........  [check]   [check]
                                                                                                                                                    \1\
PM10	2.5..........  Reference...........  Manual..............  L, O                          [check]   ........  ........  ........  [check]   ........
                    Equivalent Class I..  Manual..............  L, O                          [check]   ........  [check]   ........  [check]   ........
                    Equivalent Class II.  Manual..............  L, O                          [check]   ........  [check]   ........  [check]   [check]
                                                                                                                      \2\                           1 2
                    Equivalent Class III  Automated...........  L \1\, O \1\                  [check]   ........  [check]   ........  [check]   [check]
                                                                                                                                                    \1\
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Some requirements may apply, based on the nature of each particular candidate method, as determined by the Administrator.
\2\ Alternative Class III requirements may be substituted.

Subpart B--[Amended]

    12. Section 53.20 is amended as follows:
    A. By revising paragraph (b).
    B. In paragraph (c), by revising Table B-1.
    The revisions read as follows:


Sec.  53.20  General provisions.

* * * * *
    (b) For a candidate method having more than one selectable 
measurement range, one range must be that specified in table B-1 
(standard range for SO2), and a test analyzer representative 
of the method must pass the tests required by this subpart while 
operated in that range. The tests may be repeated for one or more 
broader ranges (i.e., ones extending to higher concentrations) than the 
range specified in table B-1, provided that the range does not extend 
to concentrations more than four times the upper range limit specified 
in table B-1. For broader ranges, only the tests for range 
(calibration), noise at 80% of the upper range limit, and lag, rise and 
fall time are required to be repeated. The tests may be repeated for 
one or more narrower ranges (ones extending to lower concentrations) 
than that specified in table B-1. For SO2 methods, table B-1 
specifies special performance requirements for narrower (lower) ranges. 
For methods other than SO2, only the tests for range 
(calibration), noise at 0% of the measurement range, and lower 
detectable limit are required to be repeated. If the tests are 
conducted or passed only for the specified range (standard range for 
SO2), any FRM or FEM method determination with respect to 
the method will be limited to that range. If the tests are passed for 
both the specified range and one or more broader ranges, any such 
determination will include the additional range(s) as well as the 
specified range, provided that the tests required by subpart C of this 
part (if applicable) are met for the broader range(s). If the tests are 
passed for both the specified range and one or more narrower ranges, 
any FRM or FEM method determination for the method will include the 
narrower range(s) as well as the specified range. Appropriate test data 
shall be submitted for each range sought to be included in a FRM or FEM 
method determination under this paragraph (b).
    (c) * * *

[[Page 64877]]



                                               Table B-1--Performance Specifications for Automated Methods
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                         SO2
                                                            ----------------------------
       Performance parameter                Units \1\                           Lower        O3           CO          NO2        Definitions and test
                                                             Std. range \3\   range \2\                                               procedures
                                                                                 \3\
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. Range...........................  ppm...................         0-0.5       <0.5        0-0.5           0-50     0-0.5    Sec. 53.23(a).
2. Noise...........................  ppm...................           0.001      0.0005       0.005           50       0.005  Sec. 53.23(b).
3. Lower detectable limit..........  ppm...................           0.002      0.001        0.010          1.0       0.010  Sec. 53.23(c).
4. Interference equivalent
    Each interferent...............  ppm...................      0.005  minus>0.00  minus>0.02    minus>1.0  minus>0.02
                                                                                 5
    Total, all interferents........  ppm...................           0.020      0.020        0.06           1.5       0.04   Sec. 53.23(d).
5. Zero drift, 12 and 24 hour......  ppm...................      0.004  minus>0.00  minus>0.02    minus>1.0  minus>0.02
                                                                                 2
7. Span drift, 24 hour:
    20% of upper range limit.......  Percent...............  ..............  ..........  20.0   minus>10.0  minus>20.0
    80% of upper range limit.......  Percent...............      5.0    minus>5.0   minus>5.0     minus>2.5  minus>5.0
8. Lag time........................  Minutes...............           2          2           20               10      20      Sec. 53.23(e).
9. Rise time.......................  Minutes...............           2          2           15                5      15      Sec. 53.23(e).
10. Fall time......................  Minutes...............           2          2           15                5      15      Sec. 53.23(e).
11. Precision:
    20% of upper range limit.......  ppm...................  ..............  ..........       0.010          0.5       0.020  Sec. 53.23(e).
                                     Percent...............           2          2       ..........  ...........  ..........  Sec. 53.23(e).
    80% of upper range limit.......  ppm...................  ..............  ..........       0.010          0.5       0.030  Sec. 53.23(e).
                                     Percent...............           2          2       ..........  ...........  ..........  Sec. 53.23(e).
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ To convert from parts per million (ppm) to [mu]g/m\3\ at 25 [deg]C and 760 mm Hg, multiply by M/0.02447, where M is the molecular weight of the gas.
  Percent means percent of the upper range limit.
\2\ Tests for interference equivalent and lag time do not need to be repeated for any lower SO2 range provided the test for the standard range shows
  that the lower range specification is met for each of these test parameters.
\3\ For candidate analyzers having automatic or adaptive time constants or smoothing filters, describe their functional nature, and describe and conduct
  suitable tests to demonstrate their function aspects and verify that performances for calibration, noise, lag, rise, fall times, and precision are
  within specifications under all applicable conditions. For candidate analyzers with operator-selectable time constants or smoothing filters, conduct
  calibration, noise, lag, rise, fall times, and precision tests at the highest and lowest settings that are to be included in the FRM or FEM
  designation.

* * * * *
    13. Section 53.21 is amended by revising paragraph (a) to read as 
follows:


Sec.  53.21  Test conditions.

    (a) Set-up and start-up of the test analyzer shall be in strict 
accordance with the operating instructions specified in the manual 
referred to in Sec.  53.4(b)(3). Allow adequate warm-up or 
stabilization time as indicated in the operating instructions before 
beginning the tests. The test procedures assume that the test analyzer 
has an analog measurement signal output that is connected to a suitable 
strip chart recorder of the servo, null-balance type. This recorder 
shall have a chart width of a least 25 centimeters, chart speeds up to 
10 cm per hour, a response time of 1 second or less, a deadband of not 
more than 0.25 percent of full scale, and capability either of reading 
measurements at least 5 percent below zero or of offsetting the zero by 
at least 5 percent. If the test analyzer does not have an analog signal 
output, or if other types of measurement data output are used, an 
alternative measurement data recording device (or devices) may be used 
for the tests, provided it is reasonably suited to the nature and 
purposes of the tests and an analog representation of the analyzer 
measurements for each test can be plotted or otherwise generated that 
is reasonably similar to the analog measurement recordings that would 
be produced by a conventional chart recorder.
* * * * *
    14. Section 53.22(d) is amended by revising Table B-2 to read as 
follows:


Sec.  53.22  Generation of test atmospheres.

* * * * *
    (d) * * *

                       Table B-2--Test Atmospheres
------------------------------------------------------------------------
           Test gas                  Generation          Verification
------------------------------------------------------------------------
Ammonia.......................  Permeation device.   Indophenol method,
                                 Similar to system    reference 3.
                                 described in
                                 references 1 and 2.
Carbon dioxide................  Cylinder of zero     Use NIST-certified
                                 air or nitrogen      standards whenever
                                 containing CO2 as    possible. If NIST
                                 required to obtain   standards are not
                                 the concentration    available, obtain
                                 specified in table   2 standards from
                                 B-3.                 independent
                                                      sources which
                                                      agree within 2
                                                      percent, or obtain
                                                      one standard and
                                                      submit it to an
                                                      independent
                                                      laboratory for
                                                      analysis, which
                                                      must agree within
                                                      2 percent of the
                                                      supplier's nominal
                                                      analysis.
Carbon monoxide...............  Cylinder of zero     Use a FRM CO
                                 air or nitrogen      analyzer as
                                 containing CO as     described in
                                 required to obtain   reference 8.
                                 the concentration
                                 specified in table
                                 B-3.
Ethane........................  Cylinder of zero     Gas chromatography,
                                 air or nitrogen      ASTM D2820,
                                 containing ethane    reference 10. Use
                                 as required to       NIST-traceable
                                 obtain the           gaseous methane or
                                 concentration        propane standards
                                 specified in table   for calibration.
                                 B-3.
Ethylene......................  Cylinder of pre-     Do.
                                 purified nitrogen
                                 containing
                                 ethylene as
                                 required to obtain
                                 the concentration
                                 specified in table
                                 B-3.

[[Page 64878]]

 
Hydrogen chloride.............  Cylinder \1\ of pre- Collect samples in
                                 purified nitrogen    bubbler containing
                                 containing           distilled water
                                 approximately 100    and analyze by the
                                 ppm of gaseous       mercuric
                                 HCL. Dilute with     thiocyante method,
                                 zero air to          ASTM (D612), p.
                                 concentration        29, reference 4.
                                 specified in table
                                 B-3.
Hydrogen sulfide..............  Permeation device    Tentative method of
                                 system described     analysis for H2S
                                 in references 1      content of the
                                 and 2.               atmosphere, p.
                                                      426, reference 5.
Methane.......................  Cylinder of zero     Gas chromatography
                                 air containing       ASTM D2820,
                                 methane as           reference 10. Use
                                 required to obtain   NIST-traceable
                                 the concentration    methane standards
                                 specified in table   for calibration.
                                 B-3.
Nitric oxide..................  Cylinder\1\ of pre-  Gas phase titration
                                 purified nitrogen    as described in
                                 containing           reference 6,
                                 approximately 100    section 7.1.
                                 ppm NO. Dilute
                                 with zero air to
                                 required
                                 concentration.
Nitrogen dioxide..............  1. Gas phase         1. Use an FRM NO2
                                 titration as         analyzer
                                 described in         calibrated with a
                                 reference 6.         gravimetrically
                                2. Permeation         calibrated
                                 device, similar to   permeation device.
                                 system described    2. Use an FRM NO2
                                 in reference 6.      analyzer
                                                      calibrated by gas-
                                                      phase titration as
                                                      described in
                                                      reference 6.
Ozone.........................  Calibrated ozone     Use an FEM ozone
                                 generator as         analyzer
                                 described in         calibrated as
                                 reference 9.         described in
                                                      reference 9.
Sulfur dioxide................  1. Permeation        Use an SO2 FRM or
                                 device as            FEM analyzer as
                                 described in         described in
                                 references 1 and 2.  reference 7.
                                2. Dynamic dilution
                                 of a cylinder
                                 containing
                                 approximately 100
                                 ppm SO2 as
                                 described in
                                 reference 7.
Water.........................  Pass zero air        Measure relative
                                 through distilled    humidity by means
                                 water at a fixed     of a dew-point
                                 known temperature    indicator,
                                 between 20 [deg]     calibrated
                                 and 30 [deg]C such   electrolytic or
                                 that the air         piezo electric
                                 stream becomes       hygrometer, or wet/
                                 saturated. Dilute    dry bulb
                                 with zero air to     thermometer.
                                 concentration
                                 specified in table
                                 B-3.
Xylene........................  Cylinder of pre-     Use NIST-certified
                                 purified nitrogen    standards whenever
                                 containing 100 ppm   possible. If NIST
                                 xylene. Dilute       standards are not
                                 with zero air to     available, obtain
                                 concentration        2 standards from
                                 specified in table   independent
                                 B-3.                 sources which
                                                      agree within 2
                                                      percent, or obtain
                                                      one standard and
                                                      submit it to an
                                                      independent
                                                      laboratory for
                                                      analysis, which
                                                      must agree within
                                                      2 percent of the
                                                      supplier's nominal
                                                      analysis.
Zero air......................  1. Ambient air
                                 purified by
                                 appropriate
                                 scrubbers or other
                                 devices such that
                                 it is free of
                                 contaminants
                                 likely to cause a
                                 detectable
                                 response on the
                                 analyzer.
                                2. Cylinder of
                                 compressed zero
                                 air certified by
                                 the supplier or an
                                 independent
                                 laboratory to be
                                 free of
                                 contaminants
                                 likely to cause a
                                 detectable
                                 response on the
                                 analyzer.
------------------------------------------------------------------------
\1\ Use stainless steel pressure regulator dedicated to the pollutant
  measured.
Reference 1. O'Keefe, A. E., and Ortaman, G. C. ``Primary Standards for
  Trace Gas Analysis,'' Anal. Chem. 38, 760 (1966).
Reference 2. Scaringelli, F. P., A. E. Rosenberg, E*, and Bell, J. P.,
  ``Primary Standards for Trace Gas Analysis.'' Anal. Chem. 42, 871
  (1970).
Reference 3. ``Tentative Method of Analysis for Ammonia in the
  Atmosphere (Indophenol Method)'', Health Lab Sciences, vol. 10, No. 2,
  115-118, April 1973.
Reference 4. 1973 Annual Book of ASTM Standards, American Society for
  Testing and Materials, 1916 Race St., Philadelphia, PA.
Reference 5. Methods for Air Sampling and Analysis, Intersociety
  Committee, 1972, American Public Health Association, 1015.
Reference 6. 40 CFR 50 Appendix F, ``Measurement Principle and
  Calibration Principle for the Measurement of Nitrogen Dioxide in the
  Atmosphere (Gas Phase Chemiluminescence).''
Reference 7. 40 CFR 50 Appendix A-1, ``Measurement Principle and
  Calibration Procedure for the Measurement of Sulfur Dioxide in the
  Atmosphere (Ultraviolet Fluorescence).''
Reference 8. 40 CFR 50 Appendix C, ``Measurement Principle and
  Calibration Procedure for the Measurement of Carbon Monoxide in the
  Atmosphere'' (Non-Dispersive Infrared Photometry)''.
Reference 9. 40 CFR 50 Appendix D, ``Measurement Principle and
  Calibration Procedure for the Measurement of Ozone in the
  Atmosphere''.
Reference 10. ``Standard Test Method for C, through C5 Hydrocarbons in
  the Atmosphere by Gas Chromatography'', D 2820, 1987 Annual Book of
  Aston Standards, vol 11.03, American Society for Testing and
  Materials, 1916 Race St., Philadelphia, PA 19103.

* * * * *
    15. Section 53.23(d) is amended by revising Table B-3 to read as 
follows:


Sec.  53.23  Test procedures.

* * * * *
    (d) * * *

                                                                 Table B-3--Interferent Test Concentration,\1\ Parts Per Million
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                       Hydro-            Hydro-                                                                                Carbon
        Pollutant                Analyzer type        chloric   Ammo-     gen     Sulfur  Nitrogen   Nitric   Carbon   Ethy-    Ozone      M-    Water vapor    mon-    Meth-    Ethane   Naph-
                                                        acid     nia    sulfide  dioxide   dioxide   oxide   dioxide    lene             xylene                oxide     ane             thalene
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SO2.....................  Ultraviolet fluorescence..  .......  .......  \5\ 0.1      \4\       0.5      0.5  .......  .......      0.5      0.2       20,000  .......  .......  .......      \6\
                                                                                    0.14                                                                                                    0.05
SO2.....................  Flame photometric.........  .......  .......     0.01      \4\  ........  .......      750  .......  .......  .......   \3\ 20,000       50  .......  .......  .......
                                                                                    0.14
SO2.....................  Gas chromatography........  .......  .......      0.1      \4\  ........  .......      750  .......  .......  .......   \3\ 20,000       50  .......  .......  .......
                                                                                    0.14
SO2.....................  Spectrophotometric-wet          0.2      0.1      0.1      \4\       0.5  .......      750  .......      0.5  .......  ...........  .......  .......  .......  .......
                           chemical (pararosanaline).                               0.14
SO2.....................  Electrochemical...........      0.2      0.1      0.1      \4\       0.5      0.5  .......      0.2      0.5  .......   \3\ 20,000  .......  .......  .......  .......
                                                                                    0.14
SO2.....................  Conductivity..............      0.2      0.1  .......      \4\       0.5  .......      750  .......  .......  .......  ...........  .......  .......  .......  .......
                                                                                    0.14
SO2.....................  Spectrophotometric-gas      .......  .......  .......      \4\       0.5  .......  .......  .......      0.5      0.2  ...........  .......  .......  .......  .......
                           phase, including DOAS.                                   0.14
O3......................  Chemiluminescent..........  .......  .......  \3\ 0.1  .......  ........  .......      750  .......      \4\  .......   \3\ 20,000  .......  .......  .......  .......
                                                                                                                                  0.08
O3......................  Electrochemical...........  .......  \3\ 0.1  .......      0.5       0.5  .......  .......  .......      \4\  .......  ...........  .......  .......  .......  .......
                                                                                                                                  0.08

[[Page 64879]]

 
O3......................  Spectrophotometric-wet      .......  \3\ 0.1  .......      0.5       0.5  \3\ 0.5  .......  .......      \4\  .......  ...........  .......  .......  .......  .......
                           chemical (potassium                                                                                    0.08
                           iodide).
O3......................  Spectrophotometric-gas      .......  .......  .......      0.5       0.5      0.5  .......  .......      \4\     0.02       20,000  .......  .......  .......  .......
                           phase, including                                                                                       0.08
                           ultraviolet absorption
                           and DOAS).
CO......................  Infrared..................  .......  .......  .......  .......  ........  .......      750  .......  .......  .......       20,000   \4\ 10  .......  .......  .......
CO......................  Gas chromatography with     .......  .......  .......  .......  ........  .......  .......  .......  .......  .......       20,000   \4\ 10  .......      0.5  .......
                           flame ionization detector.
CO......................  Electrochemical...........  .......  .......  .......  .......  ........      0.5  .......      0.2  .......  .......       20,000   \4\ 10  .......  .......  .......
CO......................  Catalytic combustion-       .......      0.1  .......  .......  ........  .......      750      0.2  .......  .......       20,000   \4\ 10      5.0      0.5  .......
                           thermal detection.
CO......................  IR fluorescence...........  .......  .......  .......  .......  ........  .......      750  .......  .......  .......       20,000   \4\ 10  .......      0.5  .......
CO......................  Mercury replacement-UV      .......  .......  .......  .......  ........  .......  .......      0.2  .......  .......  ...........   \4\ 10  .......      0.5  .......
                           photometric.
NO2.....................  Chemiluminescent..........  .......  \3\ 0.1  .......      0.5   \4\ 0.1      0.5  .......  .......  .......  .......       20,000  .......  .......  .......  .......
NO2.....................  Spectrophotometric-wet      .......  .......  .......      0.5   \4\ 0.1      0.5      750  .......      0.5  .......  ...........  .......  .......  .......  .......
                           chemical (azo-dye
                           reaction).
NO2.....................  Electrochemical...........      0.2  \3\ 0.1  .......      0.5   \4\ 0.1      0.5      750  .......      0.5  .......       20,000       50  .......  .......  .......
NO2.....................  Spectrophotometric-gas      .......  \3\ 0.1  .......      0.5   \4\ 0.1      0.5  .......  .......      0.5  .......       20,000       50  .......  .......  .......
                           phase.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Concentrations of interferent listed must be prepared and controlled to 10 percent of the stated value.
\2\ Analyzer types not listed will be considered by the Administrator as special cases.
\3\ Do not mix with the pollutant.
\4\ Concentration of pollutant used for test. These pollutant concentrations must be prepared to 10 percent of the stated value.
\5\ If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test.
\6\ If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full scale response, reattach the scrubber, and evaluate
  response for interference.

* * * * *

Subpart C--[Amended]

    16. Section 53.32 is amended by revising paragraph (e)(2) to read 
as follows:


Sec.  53.32  Test procedures for methods for SO2, CO, O3, and NO2.

* * * * *
    (e) * * *
    (2) For a candidate method having more than one selectable range, 
one range must be that specified in table B-1 of subpart B of this 
part, and a test analyzer representative of the method must pass the 
tests required by this subpart while operated on that range. The tests 
may be repeated for one or more broader ranges (i.e., ones extending to 
higher concentrations) than the one specified in table B-1 of subpart B 
of this part, provided that such a range does not extend to 
concentrations more than four times the upper range limit specified in 
table B-1 of subpart B of this part and that the test analyzer has 
passed the tests required by subpart B of this part (if applicable) for 
the broader range. If the tests required by this subpart are conducted 
or passed only for the range specified in table B-1 of subpart B of 
this part, any equivalent method determination with respect to the 
method will be limited to that range. If the tests are passed for both 
the specified range and a broader range (or ranges), any such 
determination will include the broader range(s) as well as the 
specified range. Appropriate test data shall be submitted for each 
range sought to be included in such a determination.
* * * * *
    17. Table C-1 to Subpart C is revised to read as follows:

          Table C-1 to Subpart C of Part 53--Test Concentration Ranges, Number of Measurements Required, and Maximum Discrepancy Specifications
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                            Simultaneous measurements required               Maximum
                                                                                   ----------------------------------------------------    discrepancy
                   Pollutant                       Concentration range, parts per            1-hour                    24-hour           specification,
                                                           million (ppm)           ----------------------------------------------------     parts per
                                                                                     First set    Second set   First set    Second set       million
--------------------------------------------------------------------------------------------------------------------------------------------------------
Ozone..........................................  Low 0.06 to 0.10.................            5            6  ...........  ...........              0.02
                                                 Med. 0.15 to 0.25................            5            6  ...........  ...........              0.03
                                                 High 0.35 to 0.46................            4            6  ...........  ...........              0.04
                                                                                   ---------------------------------------------------------------------
                                                    Total.........................           14           18  ...........  ...........  ................
--------------------------------------------------------------------------------------------------------------------------------------------------------
Carbon monoxide................................  Low 7 to 11......................            5            6  ...........  ...........               1.5
                                                 Med. 20 to 30....................            5            6  ...........  ...........               2.0
                                                 High 25 to 45....................            4            6  ...........  ...........               3.0
                                                                                   ---------------------------------------------------------------------
                                                    Total.........................           14           18  ...........  ...........  ................
--------------------------------------------------------------------------------------------------------------------------------------------------------
Sulfur dioxide.................................  Low 0.02 to 0.05.................            5            6            3            3              0.02
                                                 Med. 0.10 to 0.15................            5            6            2            3              0.03
                                                 High 0.30 to 0.50................            4            6            2            2              0.04
                                                                                   ---------------------------------------------------------------------
                                                    Total.........................           14           18            7            8  ................
--------------------------------------------------------------------------------------------------------------------------------------------------------

[[Page 64880]]

 
Nitrogen dioxide...............................  Low 0.02 to 0.08.................  ...........  ...........            3            3              0.02
                                                 Med. 0.10 to 0.20................  ...........  ...........            2            2              0.02
                                                 High 0.25 to 0.35................  ...........  ...........            2            2              0.03
                                                                                   ---------------------------------------------------------------------
                                                    Total.........................  ...........  ...........            7            8  ................
--------------------------------------------------------------------------------------------------------------------------------------------------------

PART 58--AMBIENT AIR QUALITY SURVEILLANCE

    18. The authority citation for part 58 continues to read as 
follows:

    Authority: 42 U.S.C. 7403, 7410, 7601(a), 7611, and 7619.

Subpart B--[Amended]

    19. Section 58.10 is amended by adding paragraph (a)(6) to read as 
follows:


Sec.  58.10  Annual monitoring network plan and periodic network 
assessment.

    (a) * * *
    (6) A plan for establishing SO2 monitoring sites in 
accordance with the requirements of appendix D to this part shall be 
submitted to the EPA Regional Administrator by July 1, 2011 as part of 
the annual network plan required in paragraph (a)(1) of this section. 
The plan shall provide for all required SO2 monitoring sites 
to be operational by January 1, 2013.
* * * * *
    20. Section 58.12 is amended by adding paragraph (g) to read as 
follows:


Sec.  58.12  Operating schedules.

* * * * *
    (g) For continuous SO2 analyzers, the maximum 5-minute 
block average concentration of the twelve 5-minute blocks in the hour 
must be collected except as noted in Sec.  58.12(a).
    21. Section 58.13 is amended by adding paragraph (d) to read as 
follows:


Sec.  58.13  Monitoring network completion.

* * * * *
    (d) The network of SO2 monitors must be physically 
established no later than January 1, 2013, and at that time, must be 
operating under all of the requirements of this part, including the 
requirements of appendices A, C, D, and E to this part.
    22. Section 58.16 is amended by adding paragraph (g) to read as 
follows:


Sec.  58.16  Data submittal and archiving requirements.

* * * * *
    (g) Any State, or where applicable, local agency operating an 
SO2 monitor shall report the maximum 5-minute SO2 
block average of the twelve 5-minute block averages in each hour, in 
addition to the hourly SO2 average.
    23. Appendix A to Part 58 is amended by adding paragraph 2.3.1.6 to 
read as follows:

Appendix A to Part 58--Quality Assurance Requirements for SLAMS, SPMs 
and PSD Air Monitoring

* * * * *
    2.3.1.6 Measurement Uncertainty for SO2. The goal for 
acceptable measurement uncertainty for precision is defined as an 
upper 90 percent confidence limit for the coefficient of variation 
(CV) of 15 percent and for bias as an upper 95 percent confidence 
limit for the absolute bias of 15 percent.
* * * * *

    24. Appendix C to Part 58 is amended by adding paragraph 2.1.2 to 
read as follows:

Appendix C to Part 58--Ambient Air Quality Monitoring Methodology

* * * * *
    2.1.2 Any SO2 FRM or FEM used for making NAAQS 
decisions, as prescribed in 40 CFR Part 50 Appendix A-1, must be 
capable of providing 1-hour averaged and 5-minute averaged 
concentration data.
* * * * *
    25. Appendix D to Part 58 is amended by revising paragraph 4.4 to 
read as follows:

Appendix D to Part 58--Network Design Criteria for Ambient Air Quality 
Monitoring

* * * * *
    4.4 Sulfur Dioxide (SO2) Design Criteria.
    4.4.1 General Requirements. State and, where appropriate, local 
agencies must operate a minimum number of required SO2 
monitoring sites as described below.
    4.4.2 Requirement for Monitoring by the Population Weighted 
Emissions Index. (a) The population weighted emissions index (PWEI) 
shall be calculated by states for each CBSA they contain or share 
with another state or states for use in the implementation of or 
adjustment to the SO2 monitoring network. The PWEI shall 
be calculated by multiplying the population of each CBSA, using the 
most current census data, by the total amount of SO2 in 
tons per year emitted within the CBSA area, using an aggregate of 
the most recent county level emissions data available in the 
National Emissions Inventory for each county in each CBSA. The 
resulting product shall be divided by one million, providing a PWEI 
value, the units of which are million persons-tons per year. For any 
CBSA with a calculated PWEI value equal to or greater than 
1,000,000, a minimum of three SO2 monitors are required 
within that CBSA. For any CBSA with a calculated PWEI value equal to 
or greater than 10,000, but less than 1,000,000, a minimum of two 
SO2 monitors are required within that CBSA. For any CBSA 
with a calculated PWEI value equal to or greater than 5,000, but 
less than 10,000, a minimum of one SO2 monitor is 
required within that CBSA.
    (1) The SO2 monitoring site(s) required as a result 
of the PWEI in each CBSA shall be sited by states through a process 
of identifying locations within the boundaries of that CBSA where 
maximum ground-level 1-hour SO2 concentrations occur due 
to emissions that originate inside and/or outside of that CBSA. 
Where a state or local air monitoring agency identifies multiple 
acceptable candidate sites where maximum hourly SO2 
concentrations are expected to occur, the monitoring agency shall 
select the location with the greater population exposure. Where one 
CBSA is required to have more than one SO2 monitor, the 
monitoring sites shall not be oriented to measure maximum hourly 
concentrations from the same SO2 source or group of 
sources, but shall monitor a different source or group of sources. 
Any PWEI-triggered monitors shall not count toward satisfying any 
required monitors resulting from the state emissions triggered 
requirements described below.
    (2) The number of SO2 monitors operated as a result 
of the PWEI shall be reviewed and adjusted as needed as a part of 
the 5-year network assessment cycle required in Sec.  58.10 of this 
part.
    (b) [Reserved]
    4.4.3 Requirement for State Emission Triggered SO2 
Monitoring. (a) Each State shall operate a minimum number of 
monitors based on that state's contribution of SO2 
emissions to the national, anthropogenic SO2 inventory as 
identified in the most recent

[[Page 64881]]

National Emissions Inventory. Each state shall operate one monitor 
for each percent that it contributes to the NEI. The percent 
contribution shall be rounded to the nearest whole integer value. 
Every state shall operate a minimum of one monitor under this 
requirement.
    (1) Each state emission triggered SO2 monitoring 
station shall be sited by states through a process of identifying 
locations within the boundaries of that state where maximum ground-
level 1-hour SO2 concentrations occur due to 
SO2 source emissions originate inside or outside the 
state. Where a state has CBSAs with PWEI-triggered monitoring, the 
PWEI-triggered monitors shall not count toward the emission-
triggered monitors. State emission-triggered monitors shall not be 
sited to measure maximum hourly concentrations from the same 
SO2 source or group of sources as another SO2 
monitor, but shall measure maximum hourly concentrations resulting 
from a different source or group of sources.
    (2) The number of SO2 monitors operated as a result 
of state-level emissions shall be reviewed and adjusted as needed as 
a part of the 5-year network assessment cycle required in Sec.  
58.10 of this part.
    (b) [Reserved]
    4.4.4 Regional Administrator Required Monitoring. The Regional 
Administrator may require additional SO2 monitoring 
stations above the minimum number of monitors required in 4.4.2 and 
4.4.3 of this appendix, where the minimum monitoring requirements 
are not sufficient to meet monitoring objectives. The Regional 
Administrator may require, at his/her discretion, additional 
monitors in situations where an area has the potential to have 
concentrations that may violate or contribute to the violation of 
the NAAQS and the area is not monitored under the minimum monitoring 
provisions described above. The Regional Administrator and the 
responsible State or local air monitoring agency shall work together 
to design and/or maintain the most appropriate SO2 
network to provide sufficient data to meet monitoring objectives.
    4.4.5 SO2 Monitoring Spatial Scales. (a) The 
appropriate spatial scales for SO2 SLAMS monitors are the 
microscale, middle, neighborhood, and possibly urban scales. 
Monitors sited at the microscale, middle, and neighborhood scales 
are suitable for determining maximum hourly concentrations for 
SO2 and can be used for compliance actions. Monitors 
sited at urban scales are useful for identifying SO2 
transport, trends, and, if sited upwind of local sources, background 
concentrations.
    (1) Microscale--This scale would typify areas in close proximity 
to SO2 point and area sources. Emissions from stationary 
point and area sources, and non-road sources may, under certain 
plume conditions, result in high ground level concentrations at the 
microscale. The microscale typically represents an area impacted by 
the plume with dimensions extending up to approximately 100 meters.
    (2) Middle scale--This scale generally represents air quality 
levels in areas up to several city blocks in size with dimensions on 
the order of approximately 100 meters to 500 meters. The middle 
scale may include locations of expected maximum short-term 
concentrations due to proximity to major SO2 point, area, 
and/or non-road sources.
    (3) Neighborhood scale--The neighborhood scale would 
characterize air quality conditions throughout some relatively 
uniform land use areas with dimensions in the 0.5 to 4.0 kilometer 
range. Emissions from stationary point and area sources may, under 
certain plume conditions, result in high SO2 
concentrations at the neighborhood scale. Where a neighborhood site 
is located away from immediate SO2 sources, the site may 
be useful in representing typical air quality values for a larger 
residential area, and therefore suitable for population exposure and 
trends analyses.
    (4) Urban scale--Measurements in this scale would be used to 
estimate concentrations over large portions of an urban area with 
dimensions from 4 to 50 kilometers. Such measurements would be 
useful for assessing trends in area-wide air quality, and hence, the 
effectiveness of large scale air pollution control strategies. Urban 
scale sites may also support other monitoring objectives of the 
SO2 monitoring network such as identifying trends, and 
when monitors are sited upwind of local sources, background 
concentrations.
    (b) [Reserved]
    4.4.6 NCore Monitoring. SO2 measurements are included 
within the NCore multipollutant site requirements as described in 
paragraph (3)(b) of this appendix. NCore-based SO2 
measurements are primarily used to characterize SO2 
trends and assist in understanding SO2 transport across 
representative areas in urban or rural locations and are also used 
for comparison with the SO2 NAAQS.
* * * * *

    26. Appendix G to Part 58 is amended as by revising Table 2 to read 
as follows:

Appendix G to Part 58--Uniform Air Quality Index (AQI) and Daily 
Reporting

* * * * *

                                                            Table 2--Breakpoints for the AQI
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                              These breakpoints                                                            Equal these AQIs
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                       PM10
      O3 (ppm) 8-hour        O3 (ppm) 1-    PM2.5    ([mu]g/  CO (ppm)   SO2 (ppm) 1-hour   NO2 (ppm) 1-hour    AQI                 Category
                               hour 1    ([mu]g/m3)    m3)
--------------------------------------------------------------------------------------------------------------------------------------------------------
0.000-0.059................  ..........    0.0-15.4     0-54   0.0-4.4    0-(0.025-0.050)    0-(0.040-0.053)     0-50  Good.
0.060-0.075................  ..........   15.5-40.4   55-154   4.5-9.4  (0.026-0.051)-(0.  (0.041-0.054)-(0.   51-100  Moderate.
                                                                               050-0.100)         080-0.100)
0.076-0.095................  0.125-0.16   40.5-65.4  155-254  9.5-12.4  (0.051-0.101)-(.1  (0.081-0.101)-(0.  101-150  Unhealthy for Sensitive Groups.
                                      4                                          75-.200)         360-0.370)
0.096-0.115................  0.165-0.20   \3\ 65.5-  255-354  12.5-15.  (0.176-0.201)-(.3  (0.361-0.371)-0.6  151-200  Unhealthy.
                                      4       150.4                  4                04)                  4
0.116-0.374................  0.205-0.40  \3\ 150.5-  355-424  15.5-30.        0.305-0.604          0.65-1.24  201-300  Very Unhealthy.
                                      4       250.4                  4
(2)........................  0.405-0.50  \3\ 250.5-  425-504  30.5-40.        0.605-0.804          1.25-1.64  301-400  .................................
                                      4       350.4                  4
(2)........................  0.505-0.60  \3\ 350.5-  505-604  40.5-50.        0.805-1.004          1.65-2.04  401-500  Hazardous.
                                      4       500.4                  4
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI based on 1-hour
  ozone values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be
  calculated, and the maximum of the two values reported.
\2\ 8-hour O3 values do not define higher AQI values (>= 301). AQI values of 301 or greater are calculated with 1-hour O3 concentrations.
\3\ If a different SHL for PM2.5 is promulgated, these numbers will change accordingly.

* * * * *
[FR Doc. E9-28058 Filed 12-7-09; 8:45 am]
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