Document ID: EPA-HQ-OAR-2007-0352-0109
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2009-12-17T05:00Z

6560-50-P

ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 50, 53, 58

 [EPA-HQ-OAR-2007-0352; FRL-]

Primary National Ambient Air Quality Standard for Sulfur Dioxide

AGENCY:  Environmental Protection Agency (EPA).

ACTION:  Proposed Rule 

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 [insert date 60 days after
date of publication in the Federal Register].  Under the Paperwork
Reduction Act, comments on the information collection provisions must be
received by OMB on or before {insert date thirty days after date of
publication in the Federal Register.}

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.

	The public hearing 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 10:00 a.m. and continue until 7:00
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 7:00 p.m.  A
lunch break is scheduled from 12:30 p.m. until 2:00 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 ( 
HYPERLINK "mailto:crabtree.tricia@epa.gov"  crabtree.tricia@epa.gov ). 
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 website for the proposal at   HYPERLINK
"http://www.epa.gov/ttn/naaqs/standards/so2/s_so2_index.html" 
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.

ADDRESSES:  Submit your comments, identified by Docket ID No.
EPA-HQ-OAR-2007-0352 by one of the following methods:

  HYPERLINK "http://www.regulations.gov"  www.regulations.gov : Follow
the on-line instructions for submitting comments.

	(	Email:    HYPERLINK "mailto:a-and-r-Docket@epa.gov" 
a-and-r-Docket@epa.gov  

	(	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.

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   HYPERLINK "http://www.regulations.gov" 
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  
HYPERLINK "http://www.regulations.gov"  www.regulations.gov  website 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   HYPERLINK
"http://www.epa.gov/epahome/dockets.htm" 
http://www.epa.gov/epahome/dockets.htm .  

Docket: All documents in the docket are listed in the   HYPERLINK
"http://www.regulations.gov"  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   HYPERLINK
"http://www.regulations.gov"  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; email:   HYPERLINK "mailto:stewart.michael@epa.gov" 
stewart.michael@epa.gov .

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  
HYPERLINK "http://www.regulations.gov"  www.regulations.gov  or email. 
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.

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.  

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 Exposure 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 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  

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.”  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.”  Section 109(b)(2) This proposal
concerns exclusively the primary NAAQS for oxides of sulfur.

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 (D.C. Cir 1980),
cert. denied, 449 U.S. 1042 (1980); American Petroleum Institute v.
Costle, 665 F.2d 1176, 1186 (D.C. 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 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 600 ppb ppm 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
subpopulation) 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 (D.C. 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 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).  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 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 risk and exposure assessment
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 2009).

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 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, 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). 

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 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-hour) 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, 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.  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.    

b.	Lung function decrements 

 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;).  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, approximately 5-30% percent 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.

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

ts > 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) 

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).

 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 representing the most SO2 sensitive individuals.  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 as 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-hour) 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 

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 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-hour) 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) 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

ave 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-α (TNF- α; 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 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).  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).	

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 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” and
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 the REA’s assessment of the adequacy of the current and
potential alternative primary standards (REA sections 10.3; 10.4; and
10.5).  

 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
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 actual
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, concentration-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.    

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 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 disadvantage 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) 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.

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
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 9.3 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). 

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 1950’s and
1960’s.  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 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 µg/m3 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. 

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.  The approach for considering this scientific
evidence and exposure/risk information is discussed below.  

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?

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
copollutants, including NO2 and O3, as well as particulate copollutants,
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 emergency
department 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 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 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, 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).

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 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).  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).  

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))., 
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)).  

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 10.3.3).  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 10.3.3).  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 10.3.3).  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 SO2 concentrations (REA, section
10.3.4).  

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 10.3.3).   

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 10.4.3).

c.	Summary of considerations from the REA regarding the annual standard 

.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).

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.  CASAC agreed 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.  

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 (D.C. 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 some degree of 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.  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.  However, as in
previous reviews, there was uncertainty as to whether these positive
associations are due to 24-hour average SO2 exposures, or exposure (or
multiple exposures) to short-term peaks of SO2 within a 24-hour period. 
More specifically, 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 - 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).   

The policy chapter of the REA found that 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).  However, it is worth noting again that the ISA
suggested effects observed in epidemiologic studies up to 24-hours, may
be due, at least in part, to shorter-term peaks of SO2.  Taken together,
the REA concluded that a primary concern with regard to averaging time
is the level of protection provided against 5 - 10 minute peak SO2
exposures (REA, section 10.5.2).  The REA also found that the evidence
described above suggested it would also be appropriate to consider the
degree of protection potential averaging times under consideration
provide against both 1-hour daily maximum and 24-hour average SO2
concentrations (REA, section 10.5.2).  

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 10.5.2.2).  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-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. 

ns ≥ 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).   

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).

ntration ≥ 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 1-hour 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 10.5.2.2).  

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, 374-75 (D.C. Cir.
2002).   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.  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 concentrations to provide increased
protection against effects associated with short-term (5-minutes to
24-hour) 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: 1) by definition, 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.   Finally, the Administrator notes that
the proposal to establish a new 1-hour averaging time is in agreement
with CASAC recommendations.  That is, 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 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).

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; 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.  Given this, 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 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 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 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 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.  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 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
(Figures 5-1 to 5-4 in the REA).  The REA noted that this information
provides the most direct 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
demonstrating 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 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 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.  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.  

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 conducted in Cleveland and Cincinnati, OH (Schwartz et., al.
1996 and Jaffe et., al. 2003) reported positive associations between ED
visits and hospital admissions when 99th percentile 1-hour daily maximum
SO2 concentrations 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).          

ncentrations ≥ 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). 

Figure 1.  Effect estimates for U.S. all respiratory ED visit studies
and associated 98th and 99th percentile 1-hour daily maximum SO2 levels.
 



 

Figure 2.  24-hour effect estimates for U.S. asthma ED visit studies and
associated 98th and 99th percentile 1-hour daily maximum SO2 levels.  

 

Figure 3.  1-hour effect estimates for U.S. asthma ED visit studies and
associated 98th and 99th percentile 1-hour daily maximum SO2 levels.

Figure 4.  24-hour effect estimates for U.S. hospitalization studies and
associated 98th and 99th percentile 1-hour daily maximum SO2 levels.

ons ≥ 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). 

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.  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 necessarily
represent the most SO2 sensitive individuals (e.g., 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.  Similarly, the REA noted that there is no evidence
to suggest that 200 ppb represents a threshold below which no adverse
respiratory effects occur.  In fact, limited evidence from two
mouthpiece exposure studies suggests that exposure to 100 ppb SO2 can
result in small decrements in lung function.  

 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.  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.

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.

Exposure Benchmarks

(5-minute exposures)	Air Quality Scenarios

	Just Meeting Current Standards	99th percentile 1-hour Daily Maximum
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).  

 ≥ 400 ppb per year.   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
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.

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.

Exposure Benchmarks

(5-minute exposures)	Air Quality Scenarios

	Just Meeting Current Standards	99th percentile 1-hour Daily Maximum
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%

≥ 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

Just Meeting Current Standards	99th percentile 1-hour Daily Maximum
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 provide protection against the
effects reported in these studies

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 significant weight is placed on the 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.

If significant weight is placed on the uncertainty that participants in
controlled human exposure studies do not represent the most SO2
sensitive individuals, standard levels down to 50 ppb could be
considered.   

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 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

In considering the appropriate level for an SO2 standard based on the
3-year average of the 99th percentile (or 4th highest) 1-hour daily
maximum SO2 concentration, the Administrator has considered the broad
body of scientific evidence and air quality, exposure, and risk
information.  As explained in the decision to propose an averaging time
of one-hour, she draws from that evidence and information the need to
protect at-risk populations from the array of health effects that have
been linked to short-term (5-minutes to 24-hour) SO2 exposures.  

Specifically, the Administrator has considered the extent to which a
variety of standard levels would be expected to protect at-risk
populations against decrements in lung function, respiratory symptoms,
and respiratory-related emergency department visits and hospital
admissions.  The Administrator notes that these health endpoints are
logically linked together in that the evidence for decrements in lung
function and respiratory symptoms in asthmatics is part of the body of
clinical evidence that the ISA recognized as supporting the plausibility
of associations between ambient SO2 and the respiratory morbidity
endpoints (i.e., respiratory symptoms, emergency department visits, and
hospital admissions) reported in epidemiologic studies.

In making judgments regarding the weight to place on the scientific
evidence and air quality, exposure, and risk information, the
Administrator notes the following: 

Based on epidemiologic and controlled human exposure evidence, the ISA
concluded that there is a causal relationship between short-term
(5-minutes to 24-hour) SO2 exposure and respiratory morbidity (ISA,
section 5.2).

Epidemiologic studies provide evidence of the most serious effects
following exposure to SO2 (i.e., ED visits and hospitalizations for all
respiratory causes and asthma), and the ISA reported generally
consistent associations were observed between these serious health
outcomes and ambient SO2 concentrations particularly among children and
older adults. 

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 copollutants, including NO2 and
O3, as well as particulate copollutants, particularly PM2.5 (ISA,
section 5.2; p. 5-9). Given this conclusion, along with conclusions from
the ISA regarding the consistency and the coherence of results across
the relatively large number of SO2 epidemiologic studies and the
evidence from controlled human exposure studies (ISA, section 5.2), the
Administrator has judged it appropriate to place weight on the
epidemiologic results.

Controlled human exposure studies provide direct evidence of a
relationship between 5-10 minute exposure to SO2 and respiratory
morbidity.  These studies report that 5-10 minute exposures to SO2 can
result in decrements in lung function and/or respiratory symptoms in
exercising asthmatics.  Given the potential public health importance of
this effect, due to the large size of the asthmatic population in the
U.S., the Administrator judges that it is also appropriate to place
weight on this evidence when identifying an appropriate range of levels
to propose.  

The results of the risk and exposure analyses presented in the REA
provide information on the potential public health implications of
setting the standard at different levels.  The Administrator
acknowledges the uncertainties associated with these analyses which, as
discussed in the REA, could result in either over- or underestimates of
SO2-associated health risks.  However, she also notes that those
uncertainties should be similar across different air quality simulations
within the air quality, exposure, and risk analyses.  Therefore, the
Administrator judges that these analyses are useful for considering the
relative levels of public health protection that could be provided by
specific standard levels.  

After considering the scientific evidence and the air quality, exposure,
and risk information (see sections II.B, II.C, and II.F.4.a, b), the
Administrator concludes that the strongest support is for a standard
level in the range of 50 - 100 ppb.  The Administrator’s rationale in
reaching this conclusion is provided below.    

In considering the epidemiologic evidence, the Administrator notes that
there have been more than 50 peer reviewed epidemiologic studies
evaluating SO2 published worldwide and 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
3.1.4.6).  The Administrator also finds that in assessing the extent to
which these studies and their associated air quality information can
inform the level of a new a 99th percentile 1-hour daily maximum
standard, U.S. and Canadian air quality information is most relevant. 
However 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 (see 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 the most direct evidence for effects in cities with
particular 99th percentile 1-hour SO2 levels, and hence, is of
particular relevance for identifying standard levels that could protect
against the SO2 concentrations observed in these studies.  These figures
report mostly positive 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 1-hour SO2 concentrations were in the range of
75 -150 ppb.  The Administrator particularly notes the cluster of three
epidemiologic studies between 78 - 150 ppb 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).  Taken
together, the Administrator concludes that the epidemiologic studies
described above provide support for consideration of a 99th percentile
1-hour daily maximum standard at or below 75 ppb.  

ions ≥ 400 ppb results in health effects that are clearly adverse. 
That is, exposure to ≥ 400 ppb SO2 for 5-10 minutes results in
moderate or greater decrements in lung function that are frequently
accompanied by respiratory symptoms.  The Administrator also focused on
exceedences of the 200 ppb benchmark because at 200 – 300 ppb (the
lowest SO2 concentrations tested in free-breathing chamber studies),
moderate or greater decrements in lung function occur in approximately
5-30% of exercising asthmatics.  The Administrator further notes that
while concentrations as low as 200 ppb have not been frequently
accompanied by respiratory symptoms, these effects are considered
adverse when considering CASAC advice and ATS guidelines.

Considering the results of the air quality, exposure, and risk analyses
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 1-hour standards up to 100 ppb to protect against 5-minute
SO2 exposures.  She notes that the 40 county air quality analysis
estimates that a 100 ppb standard would allow at most 2 days per year on
average when statistically 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 100 ppb standard level, most
counties in the air quality analysis would experience 0 and ≤ 3 days
per year on average when 5-minute SO2 concentrations exceed the 400 and
200 ppb benchmarks respectively (see REA, Tables 7-14 and 7-12).  In
addition, the St. Louis exposure analysis estimates that a 1-hour
standard at 100 ppb would 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.  Finally the Administrator notes that the
St. Louis risk assessment estimates that a 1-hour standard at 100 ppb
would 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). 

 concentrations ≥ 400 and ≥ 200 ppb.  

In considering the lower or upper ends of the proposed range further,
the Administrator notes that if it is emphasized that the most severe
effects associated with SO2 exposure are those reported in epidemiologic
studies (i.e. respiratory-related ED visits and hospitalizations), then
consideration could be given to a standard level as low as 50 ppb.  The
Administrator finds that a 99th percentile 1-hour daily maximum standard
at 50 ppb would provide a margin of safety against the air quality
levels observed in the cluster of three epidemiologic studies observing
statistically significant positive associations between SO2 and
respiratory-related ED visits and hospitalizations in multi-pollutant
models with PM (i.e. 99th percentile 1-hour daily maximum SO2
concentrations ≥ 78 ppb).  It also recognizes limited evidence from
two epidemiologic studies that reported positive associations between
ambient SO2 and ED visits in cities when SO2 concentrations were
approximately 50 ppb, while also recognizing that these studies did not
look for potential confounding by co-pollutants through multi-pollutant
models.

≥ 100 ppb per year (Table 3; REA, Figure 8-19). 

ions ≥ 200 ppb.  However, it should be noted that although a 1-hour
standard at 100 ppb would be estimated to protect about 97% of asthmatic
children at moderate or greater exertion in St. Louis from experiencing
at least one 5-minute SO2 exposure ≥ 200 ppb, a 1-hour standard at 50
ppb is estimated to be more effective, protecting > 99% of these
children from 5-minute SO2 exposures ≥ 200 ppb.  

As stated above, the Administrator proposes to set the level of a new
1-hour standard between 50 ppb and 100 ppb.  In so doing, the
Administrator proposes to place emphasis on reported findings from both
epidemiologic and controlled human exposure studies.   The Administrator
solicits comment on the appropriateness of 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, and the uncertainties associated with each of these.  

The most appropriate part of the proposed range in which to set the
standard level 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 proposed range is consistent with CASAC advice that there
is clearly sufficient evidence for consideration of standard levels
starting at 50 ppb (Samet 2009).  Moreover, the proposed range is in
agreement with the broader CASAC indication 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). 

 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 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 website
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 §53.16, or (as in this case)
EPA can originate the process itself as provided in §53.7.  If, after
reviewing a new methodology, the Administrator determines that the new
methodology is substantially superior, §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 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, 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 §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 §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 §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 §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 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 §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 §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 1970’s, there were 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 (Watkins and Thompson, 2009)
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 (  HYPERLINK "http://www.epa.gov/ttn/chief/net/2005inventory.html" 
http://www.epa.gov/ttn/chief/net/2005inventory.html ) indicates that SO2
emissions from EGUs contribute approximately 70% of the anthropogenic
non-fire 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 is reporting hourly data to EPA’s Air Quality
System (AQS) database, as required in 40 CFR §58.12, since the network
utilizes the continuous UVF FEM, which can provide aggregated data on
the order of minutes, hours, or longer.  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 (Watkins and
Thompson, 2009).  Although there are 488 SO2 monitors operating in the
network (2008 estimate), 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) that any
ongoing SO2 monitoring must have at least one monitor sited to measure
the maximum concentration of SO2 in that area.  These current CFR
requirements regarding the existing monitors are intended to ensure that
any ongoing measurements are for maximum concentrations relative to both
the 24-hour and the annual standards. 

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 created the
NCore multi-pollutant network 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 area 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 and the data from the
NCore sites will be compared to the NAAQS.  The NCore network, which
will be fully implemented by January 1, 2012, will result in
approximately 83 sites, each with SO2 monitors, with approximately 60
sites being located in urban areas. 

As set out in detail in section II.B of the preamble, 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 was 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 - 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 be proposed 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 ( 
HYPERLINK "http://www.epa.gov/ttn/chief/net/2005inventory.html" 
http://www.epa.gov/ttn/chief/net/2005inventory.html ), electrical
generating units (EGUs) emit approximately 70% of the anthropogenic
non-fire SO2 emissions in the U.S.  The 2005 NEI indicates that the
total anthropogenic non-fire 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
exposures 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).  Given that maximum ground-level
concentrations of SO2 are usually directly traceable to specific
sources, or a cluster of sources, the proposed network design would
support implementation of the proposed 1-hour SO2 NAAQS by targeting
maximum ground-level concentrations in areas of both higher population
and higher emissions.

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 no minimum
monitoring requirements for SO2, except for 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
(Watkins and Thompson, 2009) 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 short-term, peak
concentrations, such as those locations nearer to stationary sources or
in areas of higher emissions densities, where maximum hourly and 5-10
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) -- 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, 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, non-fire SO2 emissions inventory.  EPA notes that although we
propose that the network include a minimum number of required monitors,
State, local, and tribal agencies always have the right to conduct
additional monitoring above the minimum requirements.  If those 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 to
become source- oriented and sited at locations of expected maximum
short-term concentrations of SO2.  

We solicit comment on whether the estimated 345 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 Exposure 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 exposure
to peak hourly SO2 concentrations.  In order to do this, EPA has
developed a Population Weighted Exposure Index (PWEI) that uses
population and emissions inventory data at the CBSA level to assign
required monitoring for a given CBSA (population and exposure 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 in 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).  

We propose that the first prong of the SO2 network design require
monitors in CBSAs, according to the following criteria. Those CBSAs with
a PWEI value of 1,000,000 or more would be required to operate three
monitors within that CBSA.  Those CBSAs with a PWEI value between
1,000,000 and 10,000 would be required to operate two monitors within
that CBSA. Those CBSAs with a PWEI value between 10,000 and 5,000 would
be required to operate one monitor within that CBSA.  EPA believes that
the monitors required within these breakpoints provide a reasonable
minimum number of monitors in a CBSA that consider 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,
due to the fact that the greater population creates increased potential
for exposure to those moderate sources.  Alternatively, this proposed
requirement 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, within the boundaries
of a given CBSA. EPA also proposes that when state, local, or tribal
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
believes that states will likely need to use some form of quantitative
analysis, such as modeling, data analysis, or saturation studies to aid
in determining where ground-level SO2 maxima may occur in a given CBSA. 
We also propose that the selection of these sites must be approved by
the EPA Regional Administrators, again based on objective quantitative
criteria which would necessarily vary case-by-case (and which therefore
would not be appropriate to further specify by rule). See further
discussion of this issue in section c. below.   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) requiring that the monitors shall be
sited in locations of expected maximum 1-hour concentration, and 5)
requiring that the monitoring sites must be approved by the EPA Regional
Administrators based on quantitative objective criteria. 

b. 	State-level Emissions Triggered Monitoring 

As the second prong of the SO2 network, we are proposing to require an
additional monitor or monitors in each state, allocated by state-level
SO2 emissions.  In this prong, EPA proposes to distribute approximately
114 sites, based on the corresponding percent contribution of each
individual state to the national anthropogenic, non-fire 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 states 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 out of one hundred 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, non-fire 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, non-fire national emissions inventory. As a
result, approximately 114 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 (~114 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: 

State or Territory	Percent Contribution to the National SO2 Inventory
Proposed

Number of Required Monitors

Alabama	4.02%	4

Alaska	0.46%	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

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

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

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

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. 

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 in or out of CBSAs, or on
tribal lands.  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.  We
also propose that the selection of these sites must be approved by the
EPA Regional Administrators, as with the first prong.  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, 3)
requiring all monitors to be sited in locations of expected
maximum1-hour concentration inside or outside of CBSAs or on tribal
land, and 5) requiring that monitoring sites must be approved by the EPA
Regional Administrators based on quantitative objective criteria. 

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 under
section 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, EPA will be in a
position to review emissions distributions from updated NEIs to assess
whether the monitoring requirements remain appropriate.  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 the cycle of the next SO2 NAAQS review. 

c. 	Monitor Placement and Siting

In the proposed network design, we have proposed that all sites are to
be placed in locations of expected maximum 1-hour concentrations, which
will also likely discern 5-minute peaks as well.  EPA expects that in
general, these locations will be in relative 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 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, 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.  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.   For example, sources emitting from a relatively
higher stack compared to another source will have increased variability
in the location of where an emission plume may fumigate an area and in
the resultant ground level concentration.  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. 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.  

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, local, or tribal 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 more generalized 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 such required monitor would
also be vetted, publicly, through a state or local monitoring agency’s
annual monitoring plan in accordance to 40 CFR Part 58, §58.10 (a)(1). 
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 restrict which existing monitors
can be shut down.  These requirements in 40 CFR Part 58 §58.14 (c)
indicate that any SO2 monitor that has not shown attainment during the
previous five years, or has a probability of greater than 10 percent of
exceeding 80 percent of the applicable NAAQS during the next three years
shall not be shut down.  EPA is not reopening or otherwise reconsidering
this provision.  However, this requirement is summarized here because
EPA does not intend for any monitor meeting the above requirements to be
moved by a state, local, or tribal agency in their efforts to meet the
monitoring requirements of prongs 1 or 2 proposed above.

e. 	Alternative Network Design

EPA solicits comments on an alternative network design, 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 SCREEN 3, 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 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 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, as this
alternative approach would not distinctly use population as a factor for
where monitors should be placed.  For example, our NEI shows that 2,407
sources emit 50 tons per year or more of SO2, and 1,928 sources emit 100
tons per year or more of SO2.  If the state screening approach found
that a substantial fraction of those had a significant probability of
violating the NAAQS, states could be required to model, evaluate, and
potentially monitor a corresponding number of sources.  EPA solicits
comment on the resource implications for state, local, and tribal
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 continue 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 was 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) because there was only a
voluntary request, very few state and local agencies in the U.S.
reported ambient 5-minute SO2 concentrations since such monitoring 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 its exposure
and risk analyses. Thus given the demonstrated importance of 5-minute
SO2 concentrations, EPA proposes that State and local agencies shall
report the maximum 5-minute block average of SO2 each hour, out of the
twelve 5-minute blocks in the hour, in addition to the existing
requirement to report the hourly average. EPA solicits comment on the
proposed requirement for State and local monitoring agencies to report
both hourly average and maximum 5-minute average values for each hour. 
EPA also solicits comment on the advantages and disadvantages of
requiring state, local, and tribal agencies to report all twelve
5-minute SO2 values for each hour.

As a part of the larger data quality performance requirements of the
ambient monitoring program, we are proposing to develop 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 goals for developing 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 section 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 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 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 section 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 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

NAAQS Pollutant/

Standard/(Level)/

Promulgation Date	Air Quality Data Collected for Calendar Year	Event
Flagging & Initial Description Deadline	Detailed Documentation
Submission Deadline

PM2.5/24-Hr Standard (35 µg/m3)  Promulgated October 17, 2006	2004-2006
October 1, 2007a	April 15, 2008a

Ozone/8-Hr Standard (0.075 ppm)Promulgated March 12, 2008	2005-2007	June
18, 2009a	June 18, 2009a

	2008	June 18, 2009a	June 18, 2009a

	2009	60 Days after the end of the calendar quarter in which the event
occurred or February 5, 2010, whichever date occurs firstb	60 Days after
the end of the calendar quarter in which the event occurred or February
5, 2010, whichever date occurs firstb 

NO2/1-Hour Standard(80-100 PPB, final level TBD)	2008	July 1, 2010a
January 22, 2011a

	2009	July 1, 2010a 	January 22, 2011a

	2010	 April 1, 2011a	July 1, 2011 a

SO2/1-Hour Standard (50-100 PPB, final level TBD)	2008	October 1, 2010b
June 1, 2011b

	2009	October 1, 2010b	June 1, 2011 b

	2010	June 1, 2011 b	June 1, 2011 b

	2011	60 Days after the end of the calendar quarter in which the event
occurred or March 31, 2011, whichever date occurs firstb	60 Days after
the end of the calendar quarter in which the event occurred or March 31,
2011, whichever date occurs firstb

aThese 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.

bIndicates 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 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 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 based on 3 years of quality assured,
certified air quality monitoring data from the current monitoring
network, where the monitors would be retained in the new network.  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.  Thus, potentially
one third to one half of the current monitors could be used to generate
the necessary data for purposes of initial area designations.  In areas
where the current network would not generate data that are adequate to
determine whether an area is attaining or not attaining the revised SO2
NAAQS, EPA would designate such areas as "unclassifiable."   

	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.  The EPA believes that there are
sufficient guidance documents and regulations currently in place to
fully implement the proposed revision to the SO2 NAAQS.  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. 

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
C.F.R. Part 49.  See 63 Fed. Reg. 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 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 C.F.R. §
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
C.F.R. § 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 C.F.R. § 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.  The list of
section 110(a)(2) NAAQS implementation requirements are the following: 

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
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.  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.  

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.  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. 

	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 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 of 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 (ug/m3), 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
(ug/m3), 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 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:  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.

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 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 D.C. Circuit Court opinion on the Phase I ozone implementation
rule.  See South Coast Air Quality Management Dist. v. EPA, 472 F.3d 882
(D.C. 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. 

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 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 (to be determined).  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 [Insert date of publication in the Federal Register], a comment to
OMB is best assured of having its full effect if OMB receives it by
[Insert date 30 days after publication in the Federal Register.].  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 (D.C.
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 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.  

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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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 AMBIENT AIR QUALITY STANDARDS 

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

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

Subpart A—General Provisions

2.  Section 50.14 is amended by revising paragraph (c)(2)(vi) to read as
follows:

§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

NAAQS Pollutant/

Standard/(Level)/

Promulgation Date	Air Quality Data Collected for Calendar Year	Event
Flagging & Initial Description Deadline	Detailed Documentation
Submission Deadline

PM2.5/24-Hr Standard (35 µg/m3)  Promulgated October 17, 2006	2004-2006
October 1, 2007a	April 15, 2008a

Ozone/8-Hr Standard (0.075 ppm)Promulgated March 12, 2008	2005-2007	June
18, 2009a	June 18, 2009a

	2008	June 18, 2009a	June 18, 2009a

	2009	60 Days after the end of the calendar quarter in which the event
occurred or February 5, 2010, whichever date occurs firstb	60 Days after
the end of the calendar quarter in which the event occurred or February
5, 2010, whichever date occurs firstb 

NO2/1-Hour Standard(80-100 PPB, final level TBD)	2008	July 1, 2010a
January 22, 2011a

	2009	July 1, 2010a 	January 22, 2011a

	2010	 April 1, 2011a	July 1, 2011 a

SO2/1-Hour Standard (50-100 PPB, final level TBD)	2008	October 1, 2010b
June 1, 2011b

	2009	October 1, 2010b	June 1, 2011 b

	2010	June 1, 2011 b	June 1, 2011 b

	2011	60 Days after the end of the calendar quarter in which the event
occurred or March 31, 2011, whichever date occurs firstb	60 Days after
the end of the calendar quarter in which the event occurred or March 31,
2011, whichever date occurs firstb

aThese 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.

bIndicates 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.

*****

3.  A new Section 50.16 is added to read as follows:

§ 50.16   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 

*****

4. EPA proposes to add Appendix A-1 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 §50.4 and
§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.  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.

oroethylene (PTFE Teflon™), 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 µmole/mole) or SRM 1694a (100 µmole/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

with the pollutant are of glass, Teflon™, 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:

 

where:

Fc = corrected flow rate (L/min at 25° 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:

 

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 SO 2 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 www.epa.gov/ttn/amtic/qabook.html.)

Figure 1.  UVF SO2 analyzer schematic diagram.



Figure 2. Calibration system using a compressed gas standard.

5. EPA proposes to recode Appendix A to read as follows:

Appendix A-2 to Part 50—Reference Method for the Determination of
Sulfur Dioxide in the Atmosphere (Pararosaniline Method)

6. If EPA finalizes the 4th highest daily maximum form, an Appendix T
would be added as follows: 

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 § 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 §§
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 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”). 

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.

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 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 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.

(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.

If EPA finalizes the 99th percentile form, an Appendix T would be added
as follows: 

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 § 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 §§
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 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
values will be based on the number of days with at least 75 percent of
the hourly values reported. 

(i) For the year, from only the days with at least 75 percent of the
hourly values reported, select from each day the highest hourly value. 

(ii) Sort all the valid daily 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. 

Table 1

Annual number of days with valid data for year “y” (cny)	P0.99, y is
the nth maximum value of the year, where n is the listed number

1–100	1

101-200	2

201-300	3

301-366	4

(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.

7.  EPA proposes to amend several sections and Table A-1 in Subpart A,
as follows:

8.  Section 53.2(a)(1) would be amended to read as follows:

(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 [insert
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 §53.2(b).  Except
as provided in §53.16, other manual methods for lead will not be
considered for a reference method determination under this part.

9.  Section 53.2(b) would be amended to read as follows:

(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 after [insert effective date of
appendix A-1]) and must have been shown in accordance with this part to
meet the requirements specified in this subpart A and subpart B of this
part.

10.  Section 53.8 would be amended to read as follows: 

(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 would be amended 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

Pollutant	Reference or 

equivalent	Manual or automated	Applicable part 50 appendix	Applicable
subparts of part 53

A	B	C	D	E	F

SO2 	Reference	Manual	A-2

Automated	A-1	(	(

	Equivalent	Manual	A-1	(

(

	Automated	A-1	(	(	(

	CO	Reference	Automated	C	(	(

	Equivalent	Manual	C	(

(

	Automated	C	(	(	(

	O3	Reference	Automated	D	(	(

	Equivalent	Manual	D	(

(

	Automated	D	(	(	(

	NO2	Reference	Automated	F	(	(

	Equivalent	Manual	F	(

(

	Automated	F	(	(	(

	Pb	Reference	Manual	G

	Equivalent	Manual	G	(

(

	Automated	G	(

(

	PM10-Pb	Reference	Manual	Q

	Equivalent	Manual	Q	(

(

	Automated	Q	(

(

	PM10	Reference	Manual	J	(

	(

	Equivalent	Manual	J	(

(	(

Automated	J	(

(	(

PM2.5	Reference	Manual	L	(

(

Equivalent Class I	Manual	L	(

(

(

Equivalent Class II	Manual	L1	(

(2

(	(1, 2

	Equivalent Class III	Automated	L1	(

(

(	(1

PM10-2.5	Reference	Manual	L, O	(

(

Equivalent Class I	Manual	L, O	(

(

(

Equivalent Class II	Manual	L, O	(

(2

(	(1, 2

	Equivalent Class III	Automated	L1, O1	(

(

(	(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.

II.  EPA proposes to amend several sections and tables in Subpart B, as
follows:

12.  Section 53.20(b) would be revised to read as follows:

(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).

13.  Section 53.21(a) would be read as follows:

(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 §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.  Table B-1 would be revised to read as follows:

Table B-1—Performance Specifications for Automated Methods

Performance parameter	Units1	SO2 	O3	CO	NO2	Definitions and test
procedures

Std. range3	Lower range2,3

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

      Total, all interferents	

ppm

ppm	

±0.005

  0.020	

±0.005

  0.020	

±0.02

  0.06	

±1.0

  1.5	

±0.02

  0.04	

Sec. 53.23(d)

Sec. 53.23(d)

5. Zero drift, 12 and 24 hour	ppm	±0.004	±0.002	±0.02	±1.0	±0.02
Sec. 53.23(e)

7. Span drift, 24 hour

     20 % of upper range limit

     80 % of upper range limit	

Percent

Percent	

 ---

±5.0	

  ---

±5.0	

±20.0

  ±5.0	

±10.0

  ±2.5	

±20.0

  ±5.0	

Sec. 53.23(e)

Sec. 53.23(e)

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

     80 % of upper range limit

     	

ppm

Percent

ppm

Percent	

 ---

2

 ---

2	

 ---

2

 ---

2	

0.010

0.010

 ---	

0.5

0.5

 ---	

0.020

0.030

 ---	

Sec. 53.23(e)

Sec. 53.23(e)

Sec. 53.23(e)

Sec. 53.23(e)

1. To convert from parts per million (ppm) to μg/m3 at 25 °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.

15.  Table B-2 would be revised to read as follows: 

Table B-2—Test Atmospheres

Test gas	Generation	Verification

Ammonia	Permeation device. Similar to system described 

in references 1 and 2	Indophenol method, reference 3.

Carbon dioxide

	Cylinder of zero air or nitrogen containing CO2 as required to obtain
the concentration specified in table B-3.	Use NIST-certified standards
whenever possible.  If NIST standards are not available, obtain 2
standards from 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 air or nitrogen containing CO as
required to obtain the concentration specified in table B-3.	Use a FRM
CO analyzer as described in reference 8.

Ethane

	Cylinder of zero air or nitrogen containing ethane as required to
obtain the concentration specified in table B-3.	Gas chromatography,
ASTM D2820, reference 10.  Use NIST-traceable gaseous methane or propane
standards for calibration.

Ethylene

	Cylinder of pre-purified nitrogen containing ethylene as required to
obtain the concentration specified in table B-3.	Do. 

Hydrogen chloride

	Cylinder1 of pre-purified nitrogen containing approximately 100 ppm of
gaseous HCL. Dilute with zero air to concentration specified in table
B-3	Collect samples in bubbler containing distilled water and analyze by
the mercuric thiocyante method, ASTM (D612), p. 29, reference 4.

Hydrogen sulfide

	Permeation device system described in references 1 and 2.

	Tentative method of analysis for H2S content of the atmosphere, p. 426,
reference 5.

Methane

	Cylinder of zero air containing methane as required to obtain the
concentration specified in table B-3.

	Gas chromatography ASTM D2820, reference 10.  Use NIST-traceable
methane standards for calibration.

Nitric oxide

	Cylinder1 of pre-purified nitrogen containing approximately 100 ppm NO.
Dilute with zero air to required concentration.	Gas phase titration as
described in reference 6, section 7.1

Nitrogen dioxide

	1. Gas phase titration as described in reference 6.

2. Permeation device, similar to system described in reference 6.	1. Use
an FRM NO2 analyzer calibrated with a gravimetrically calibrated
permeation device.

2. Use an FRM NO2 analyzer calibrated by gas-phase titration as
described in reference 6.

Ozone

	Calibrated ozone generator as described in reference 9.	Use an FEM
ozone analyzer calibrated as described in reference 9.

Sulfur dioxide

	1. Permeation device as described in references 1 and 2.

2. Dynamic dilution of a cylinder containing approximately 100 ppm SO2
as described in Reference 7.

	Use an SO2 FRM or FEM analyzer as described in reference 7.

Table B-2—Test Atmospheres (Continued)

Test gas	Generation	Verification

Water

	Pass zero air through distilled water at a fixed known temperature
between 20° and 30° C such that the air stream becomes saturated.
Dilute with zero air to concentration specified in table B-3.	Measure
relative humidity by means of a dew-point indicator, calibrated
electrolytic or piezo electric hygrometer, or wet/dry bulb thermometer.

Xylene

	Cylinder of pre-purified nitrogen containing 100 ppm xylene.  Dilute
with zero air to concentration specified in table B-3.	Use
NIST-certified standards whenever possible.  If NIST standards are not
available, obtain 2 standards from 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.

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 FIuorscence)."

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.

16. Table B-3 would be revised to read as follows:

Table B-3—Interferent Test Concentration,1 Parts Per Million

Pollu-

tant	

Analyzer Type	Hydro-

chloric

acid	Ammo-

nia	Hydro-

gen

sulfide	Sulfur

dioxide	Nitrogen

dioxide	Nitric

oxide	Carbon

dioxide	Ethy-

lene	Ozone	M-

xylene

	Water

vapor	Carbon

mon-oxide	Meth-

ane	Ethane	Naphthalene

SO2 	Ultraviolet fluorescence

	0.15	0.144	0.5	0.5

	0.5	0.2	20,000

0.056

SO2 	Flame photometric

	0.01	0.144

	750

20,0003	50

	SO2 	Gas chromatography

	0.1	0.144

	750

20,0003	50

	SO2 	Spectrophotometric-wet chemical (pararosanaline)	0.2	0.1	0.1	0.144
0.5

750

0.5

SO2 	Electrochemical	0.2	0.1	0.1	0.144	0.5	0.5

0.2	0.5

20,0003

SO2 	Conductivity	0.2	0.1

0.144	0.5

750

SO2 	Spectrophotometric-gas phase, including DOAS

0.144	0.5

0.5	0.2

	O3	Chemiluminescent

	0.13

750

0.084

20,0003

O3	Electrochemical

0.13

0.5	0.5

0.084

O3	Spectrophotometric-wet chemical (potassium iodide)

0.13

0.5	0.5	0.53

	0.084

O3	Spectrophotometric-gas phase, including ultraviolet absorption and
DOAS)

0.5	0.5	0.5

	0.084	0.02	20,000

CO	Infrared

	750

20,000	104

	CO	Gas chromatography with flame ionization detector

	20,000	104

0.5

	CO	Electrochemical

0.5

0.2

	20,000	104

	CO	Catalytic combustion-thermal detection

0.1

	750	0.2

	20,000	104	5.0	0.5

	CO	IR fluorescence

	750

20,000	104

0.5

	CO	Mercury replacement-UV photometric

0.2

104

0.5

	Table B-3—Interferant Test Concentration,1 Parts Per Million
(Continued)

Pollu-

tant	Analyzer Type	Hydro-

chloric

acid	Ammo-

nia	Hydro-

gen

sulfide	Sulfur

dioxide	Nitrogen

dioxide	Nitric

oxide	Carbon

dioxide	Ethy-

lene	Ozone	M-

xylene	Water

vapor	Carbon

mon-oxide	Meth-

ane	Ethane	Naphthalene

NO2	Chemiluminescent

0.13

0.5	0.14	0.5

	20,000

NO2	Spectrophotometric-wet chemical (azo-dye reaction)

0.5	0.14	0.5	750

0.5

NO2	Electrochemical	0.2	0.13

0.5	0.14	0.5	750

0.5

20,000	50

	NO2	Spectrophotometric-gas phase

0.13

0.5	0.14	0.5

	0.5

20,000	50

	

Concentrations of interferent listed must be prepared and controlled to
±10 percent of the stated value.

Analyzer types not listed will be considered by the Administrator as
special cases.

Do not mix with the pollutant.

Concentration of pollutant used for test.  These pollutant
concentrations must be prepared to ±10 percent of the stated value.

If candidate method utilizes an elevated-temperature scrubber for
removal of aromatic hydrocarbons, perform this interference test.

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.III. EPA proposes to revise section 53.32(e)(2) and table
C-1 in Subpart C to read as follows:

17.  Section 53.32(e)(2) would be amended to read as follows:

(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.

18. Table C-1 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

Pollutant	

Concentration range, parts per million (ppm)	

Simultaneous measurements required	Maximum discrepancy specification,
parts per million

1-hour	24-hour

	First set	Second set	First set	Second set

	

Ozone

	Low  0.06 to 0.10

Med. 0.15 to 0.25

High 0.35 to 0.46	5

5

4	6

6

6	--

--

--	--

--

--	0.02

0.03

0.04

	Total….	14	18	--	--	--

Carbon monoxide	Low    7 to 11

Med. 20 to 30

High 25 to 45	5

5

4	6

6

6	--

--

--	--

--

--	1.5

2.0

3.0

	Total….	14	18	--	--	--

Sulfur dioxide	Low  0.02 to 0.05

Med. 0.10 to 0.15

High 0.30 to 0.50	5

5

4	6

6

6	3

2

2	3

3

2	0.02

0.03

0.04

	Total….	14	18	7	8	--

Nitrogen dioxide	Low  0.02 to 0.08

Med. 0.10 to 0.20

High 0.25 to 0.35	--

--

--	--

--

--	3

2

2	3

2

2	0.02

0.02

0.03

	Total….	--	--	7	8	--

19.

§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 §50.16.  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 §50. 16;  except that for
areas designated nonattainment for the SO2 NAAQS set forth in this
section as of the effective date of §50. 16, 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 §50.16.

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 58--AMBIENT AIR QUALITY SURVEILLANCE

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]

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

§ 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.  The plan shall provide for all
required SO2 monitoring sites to be operational by January 1, 2013.

* * * * *

21. Section 58.12 is amended by adding paragraph (g) to read as follows:

§ 58.12   Operating Schedules

* * * * *

	(g) For continuous SO2 analyzers, the maximum 5-minute block-average
for each hour out of the twelve 5-minute blocks in the hour must be
collected except as noted in § 58.12 (a)(1) – (a)(3).

22. Section 58.13 is amended by adding paragraph (d) to read as follows:

§ 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, E, and G to this part. 

* * * * *

23. Section 58.16 is amended by adding paragraph (g) to read as follows:

§ 58.16   Data submittal and archiving requirements.

* * * * *

(g) Any State, or where applicable, local agency or tribal agency
operating an SO2 monitor shall report the maximum 5-minute block average
of SO2 of the twelve available 5-minute blocks for each hour in addition
to the hourly SO2 average.  

* * * * *

24. Appendix A to Part 58 is amended as 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.

* * * * *

25. Appendix C to Part 58 is amended as 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 primary NAAQS decisions, as
prescribed in 40 CFR Part 50 Appendix A-1, must be capable of providing
hourly averaged and five minute averaged concentration data.

* * * * *

26. Appendix D to Part 58 is amended as by replacing 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 Requirement s. (a) State and, where appropriate, local
agencies must operate a minimum number of required SO2 monitoring sites
as described below. Global comment; suggest replacing “given” with
“particular” or “each”

4.4.2 Requirement for Monitoring by the Population Weighted Emissions
Index. (a)  The population weighted emissions index (PWEI) shall be
calculated for each CBSA in the country. 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 with the unit million persons-tons per year.  For
any CBSA with a calculated PWEI value equal to or greater than 1,000,000
million persons-tons per year, 3 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 million persons-tons per year, 2 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 million
persons-tons per year, 1 SO2 monitor is required within that CBSA.  

(1) The SO2 monitoring site required as a result of the PWEI in each
CBSA shall be sited by identifying locations within the boundaries of
that CBSA where SO2 source emissions that originate inside and/or
outside of a given CBSA result in or contribute to maximum hourly
concentrations within 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 short-term 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.

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, non-fire
SO2 inventory as identified in the 2005 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 with a minimum value of 1 percent.  If a
state contributes to the anthropogenic, non-fire SO2 inventory, those
states shall be required to operate a minimum of one monitor.  

(1) Each state emission triggered SO2 monitoring stations shall be sited
by identifying locations inside or outside of CBSAs, and also
considering tribal lands, where SO2 source emissions that originate
inside or outside the state result in or contribute to maximum
short-term concentrations within that state. Where a state or local air
monitoring agency identifies multiple acceptable candidate sites where
maximum short-term SO2 concentrations are expected to occur, the
monitoring agency shall select the location with the greater population
exposure. Where a state had CBSAs with population weighted emissions
index-triggered monitoring, the population weighted emissions
index-triggered monitors may not count toward the emission-triggered
monitors.  State emission triggered monitors shall not be sited to
measure maximum short-term concentrations from the same SO2 source or
group of sources as another SO2 monitor, but shall measure maximum
short-term concentrations resulting from a different source or group of
sources. 

  4.4.4 Regional Administrator Required Monitoring. (a) The Regional
Administrator may use his/her discretion to require additional SO2
monitoring stations above the minimum number of monitors required above,
where the minimum monitoring requirements are not sufficient to meet
monitoring objectives.  The Regional Administrator may use this
discretion in situations where an area that is suspected of having
concentrations that may violate the NAAQS is not monitored as a result
of the minimum monitoring requirements described above.  The Regional
Administrator and the responsible State or local air monitoring agency
should work together to design and/or maintain the most appropriate SO2
network to provide sufficient data to protect public health.

 4.4.5 (a) Continued operation of an existing SO2 monitor is required if
data from an individual monitor exceeds the proposed NAAQS, has exceeded
80% of the proposed NAAQS over the prior 5 year period, or has greater
than 10% chance of exceeding 80% of the proposed NAAQS.  

4.4.6 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 the
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(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. 

4.4.7 NCore Monitoring. (a) 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 although they can also
be compared with the SO2 NAAQS.  

* * * * *

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 AQI’s

O3 (ppm)

8-hour	O3 (ppm)

1-hour1	PM2.5

(µg/m3)	PM10

(µg/m3)	CO (ppm)	SO2 (ppm) 1-hour	NO2 (ppm)

1-hour	AQI	Category

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) – (0.050 – 0.100)	(0.041 – 0.054) – (0.080 – 0.100)
51-100	Moderate.

0.076-0.095	0.125-0.164	40.5-65.4	155-254	9.5-12.4	(0.051 – 0.101) –
(.175 - .200)	(0.081 – 0.101) – (0.360 –  0.370)	101-150	Unhealthy
for Sensitive Groups.

0.096-0.115	0.165-0.204	365.5-150.4	255-354	12.5-15.4	(0.176 – 0.201)
– (.304)	(0.361 – 0.371) – 0.64	151-200	Unhealthy.

0.116-0.374	0.205-0.404	3150.5-250.4	355-424	15.5-30.4	0.305 – 0.604
0.65 – 1.24	201-300	Very Unhealthy.

(2)...............	0.405-0.504	3250.5-350.4	425-504	30.5-40.4	0.605 –
0.804	1.25 – 1.64	301-400

	(2)...............	0.505-0.604	3350.5-500.4	505-604	40.5-50.4	0.805 –
1.004	1.65 – 2.04	401-500	Hazardous.

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.

 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)

 EPA is currently conducting a separate review of the secondary SO2
NAAQS jointly with a review of the secondary NO2 NAAQS (see   HYPERLINK
"http://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.html" 
http://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.html  for more
information).

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.  

 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.

 FEV1 and sRaw are measures of bronchoconstriction.  Decreases in FEV1
or increases in sRaw can result in difficulty breathing.   

 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)  

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

 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.  

 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.

 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).

 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.  

 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.

 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. "

 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).

 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.

  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.   

 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.

 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.

 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 at set an appropriate level could effectively
control these 5-minute peaks was the air quality, exposure, and risk
analyses discussed in section II.F.4. 

 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)  

 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.     

 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. 

 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.

 

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

 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).

  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 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.  

 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. 

 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. 

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

 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]

 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.

  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.

 See SO2 Guideline Document, Office of Air Quality Planning and
Standards, Research Triangle Park, NC 27711, EPA-452/R-94-008, February
1994.  

  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.  

 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.

 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).

 Criteria pollutants are those pollutants for which EPA has established
a NAAQS under section 109 of the CAA.

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40 CFR 93.102(b)(1).

 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  
HYPERLINK "http://www.epa.gov/oar/oaqps/greenbk/lnc.html" 
http://www.epa.gov/oar/oaqps/greenbk/lnc.html ).  The Billings/Laurel,
MT, area is the only area currently subject to a SIP call.

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