[Federal Register: April 25, 2007 (Volume 72, Number 79)]
[Rules and Regulations]
[Page 20585-20667]
[DOCID:fr25ap07-10]

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Part II

Environmental Protection Agency

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40 CFR Part 51

Clean Air Fine Particle Implementation Rule; Final Rule

Agency Information Collection Activities: Proposed Collection; Comment
Request; PM2.5 Ozone National Ambient Air Quality Standard
Implementation Rule; EPA ICR No. 2258.01; Notice

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 51

[EPA-HQ-OAR-2003-0062; FRL-8295-2]
RIN 2060-AK74

Clean Air Fine Particle Implementation Rule

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: This final action provides rules and guidance on the Clean Air
Act (CAA) requirements for State and Tribal plans to implement the 1997
fine particle (PM2.5) national ambient air quality standards
(NAAQS). Fine particles and precursor pollutants are emitted by a wide
range of sources, including power plants, cars, trucks, industrial
sources, and other burning or combustion-related activities. Health
effects that have been associated with exposure to PM2.5
include premature death, aggravation of heart and lung disease, and
asthma attacks. Those particularly sensitive to PM2.5
exposure include older adults, people with heart and lung disease, and
children.
    Air quality designations became effective on April 5, 2005 for 39
areas (with a total population of 90 million) that were not attaining
the 1997 PM2.5 standards. By April 5, 2008, each State
having a nonattainment area must submit to EPA an attainment
demonstration and adopted regulations ensuring that the area will
attain the standards as expeditiously as practicable, but no later than
2015. This rule and preamble describe the requirements that States and
Tribes must meet in their implementation plans for attainment of the
1997 fine particle NAAQS. (Note that this rule does not include final
PM2.5 requirements for the new source review (NSR) program;
the final NSR rule will be issued at a later date.)

DATES: This rule is effective on May 29, 2007.

ADDRESSES: The EPA has established a docket for this action under
Docket ID EPA-HQ-OAR-2003-0062. All documents relevant to this action
are listed in the Federal docket management system at
http://www.regulations.gov. Although listed in the index, some information is

not publicly available (e.g. Confidential Business Information or other
information whose disclosure is restricted by statute). Certain other
material, such as copyrighted material, is not placed on the Internet
and will be publicly available only in hard copy form. Publicly
available docket materials are available either electronically through
http://www.regulations.gov or in hard copy format at the EPA Docket Center,

EPA/DC, EPA West, Room 3334, 1301 Constitution Avenue, 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<PHONE_NUMBER>, and the telephone number for the
Office of Air and Radiation Docket and Information Center is (202) 566-
1742. A variety of information and materials related to the fine
particle NAAQS and implementation program are also available on EPA's
Web site: http://www.epa.gov/air/particles.

FOR FURTHER INFORMATION CONTACT: For general information, contact Mr.
Richard Damberg, U.S. Environmental Protection Agency, Office of Air
Quality Planning and Standards, Mail Code C539-01, Research Triangle
Park, NC 27711, phone number (919) 54l-5592 or by e-mail at:
<EMAIL_ADDRESS>

SUPPLEMENTARY INFORMATION:

General Information

A. Does this action apply to me?

    Entities potentially regulated by this action are State and local
air quality agencies.

B. Where can I get a copy of this document and other related
information?

    In addition to being available in the docket, an electronic copy of
this final rule will also be available on the World Wide Web. Following
signature by the EPA Administrator, a copy of this final rule will be
posted at http://www.epa.gov/particles/actions.html.

C. How is the preamble organized?

I. Background
II. Elements of the Clean Air Fine Particle Implementation Rule
    A. Precursors and Pollutants Contributing to Fine Particle
Formation
    B. No Classification System
    C. Due Dates and Basic Requirements for Attainment
Demonstrations
    D. Attainment Dates
    E. Modeling and Attainment Demonstrations
    F. Reasonably Available Control Technology and Reasonably
Available Control Measures
    G. Reasonable Further Progress
    H. Contingency Measures
    I. Transportation Conformity
    J. General Conformity
    K. Emission Inventory Requirements
    L. Condensable Particulate Matter Test Methods and Related Data
Issues
    M. Improving Source Monitoring
    N. Guidance Specific to Tribes
    O. Enforcement and Compliance
    P. Emergency Episodes
    Q. Ambient Monitoring
III. Statutory and Executive Order Reviews
    A. Executive Order 12866: Regulatory Planning and Review
    B. Paperwork Reduction Act
    C. Regulatory Flexibility Act
    D. Unfunded Mandates Reform Act
    E. Executive Order 13132: Federalism
    F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
    G. Executive Order 13045: Protection of Children From
Environmental Health and Safety Risks
    H. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution, or Use
    I. National Technology Transfer Advancement Act
    J. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations
    K. Congressional Review Act
    L. Petitions for Judicial Review
    M. Judicial Review
IV. Statutory Authority

I. Background

    Fine particles in the atmosphere are comprised of a complex mixture
of components. Common constituents include: sulfate (SO4);
nitrate (NO3); ammonium; elemental carbon; a great variety
of organic compounds; and inorganic material (including metals, dust,
sea salt, and other trace elements) generally referred to as
``crustal'' material, although it may contain material from other
sources. Airborne particles generally less than or equal to 2.5
micrometers in diameter are considered to be ``fine particles'' (also
referred to as PM2.5). (A micrometer is one-millionth of a
meter, and 2.5 micrometers is less than one-seventh the average width
of a human hair.) ``Primary'' particles are emitted directly into the
air as a solid or liquid particle (e.g., elemental carbon from diesel
engines or fire activities, or condensable organic particles from
gasoline engines). ``Secondary'' particles (e.g., sulfate and nitrate)
form in the atmosphere as a result of various chemical reactions.
(Section II of the proposed rule included detailed technical discussion
on PM2.5, its precursors, formation processes, and emissions
sources.)
    The EPA established air quality standards for PM2.5
based on evidence from numerous health studies demonstrating that
serious health effects are associated with exposures to elevated levels
of PM2.5. Epidemiological studies have shown statistically
significant correlations between elevated PM2.5 levels and
premature mortality. Other important

effects associated with PM2.5 exposure include aggravation
of respiratory and cardiovascular disease (as indicated by increased
hospital admissions, emergency room visits, absences from school or
work, and restricted activity days), changes in lung function and
increased respiratory symptoms, as well as new evidence for more subtle
indicators of cardiovascular health. Individuals particularly sensitive
to PM2.5 exposure include older adults, people with heart
and lung disease, and children.
    On July 18, 1997, we revised the NAAQS for particulate matter (PM)
to add new standards for fine particles, using PM2.5 as the
indicator. We established health-based (primary) annual and 24-hour
standards for PM2.5 (62 FR 38652).\1\ The annual standard
was set at a level of 15 micrograms per cubic meter, as determined by
the 3-year average of annual mean PM2.5 concentrations. The
24-hour standard was set at a level of 65 micrograms per cubic meter,
as determined by the 3-year average of the 98th percentile of 24-hour
concentrations.
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    \1\ The original annual and daily standards for particles
generally less than or equal to 10 micrometers in diameter (also
referred to as PM10) were established in 1987. In the
1997 PM NAAQS revision, EPA also revised the standards for
PM10, but these revised PM10 standards were
later vacated by the court, and the 1987 PM10 standards
remained in effect. In the 2006 NAAQS revision, the 24-hour
PM10 standard was retained but the annual standard was
revoked. Today's implementation rule and guidance does not address
PM10.
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    Attainment of the 1997 PM2.5 standards is estimated to
lead to reductions in health impacts, including tens of thousands fewer
premature deaths each year, thousands fewer hospital admissions and
emergency room visits each year, hundreds of thousands fewer absences
from work and school, and hundreds of thousands fewer respiratory
illnesses in children annually. The EPA's evaluation of the science
concluded that there was not sufficient information to either support
or refute the existence of a threshold for health effects from PM
exposure.\2\
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    \2\ Environmental Protection Agency. (2004a). Air Quality
Criteria for Particulate Matter. Research Triangle Park, NC:
National Center for Environmental Assessment--RTP, Office of
Research and Development, U.S. Environmental Protection Agency,
Research Triangle Park, NC 27711; report no. EPA/600/P-99/002aF and
EPA/600/P-99/002bF. October 2004.
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    We subsequently completed in October 2006 another review of the
NAAQS for PM. With regard to the primary standards, the 24-hour
PM2.5 standard was strengthened to a level of 35 micrograms
per cubic meter, based on the 3-year average of the 98th percentile of
24-hour concentrations, and the level of the annual standard remained
unchanged.\3\ Attainment of the 2006 PM2.5 standards is
estimated to lead to additional reductions in health impacts, including
approximately 1,200 to 13,000 fewer premature deaths each year, 1,630
fewer hospital admissions and 1,200 fewer emergency room visits for
asthma each year, 350,000 fewer absences from work and school, and
155,300 fewer respiratory illnesses in children annually.\4\
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    \3\ The revised fine particle NAAQS were published on October
17, 2006 (71 FR 61144). See EPA's Web site for additional
information: http://www.epa.gov/pm/index.html.

    \4\ Regulatory Impact Analysis for Particulate Matter National
Ambient Air Quality Standards (September 2006), page ES-8. The
mortality range includes estimates based on the results of an expert
elicitation study, along with published epidemiological studies.
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    In both 1997 and 2006 EPA established welfare-based (secondary)
standards identical to the levels of the primary standards. The
secondary standards are designed to protect against major environmental
effects of PM2.5 such as visibility impairment, soiling, and
materials damage. The EPA also established the regional haze
regulations in 1999 for the improvement of visual air quality in
national parks and wilderness areas across the country. Because
regional haze is caused primarily by light scattering and light
absorption by fine particles in the atmosphere, EPA is encouraging the
States to integrate their efforts to attain the PM2.5
standards with those efforts to establish reasonable progress goals and
associated emission reduction strategies for the purposes of improving
air quality in our treasured natural areas under the regional haze
program.
    The scientific assessments used in the development of the
PM2.5 standards included a scientific peer review and public
comment process. We developed scientific background documents based on
the review of hundreds of peer-reviewed scientific studies. The Clean
Air Scientific Advisory Committee, a congressionally mandated group of
independent scientific and technical experts, provided extensive review
of these assessments, and found that EPA's review of the science
provided an adequate basis for the EPA Administrator to make a
decision. More detailed information on health effects of
PM2.5 can be found on EPA's Web site at: http://www.epa.gov/air/urbanair/pm/index.html.
 Additional information on EPA's scientific

assessment documents supporting the 1997 standards are available at
http://www.epa.gov/ttn/oarpg/t1cd.html; additional scientific assessment information on the 2006 standards is available at: http://

http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_cd.html.

    The EPA issued final PM2.5 designations for areas
violating the 1997 standards on December 17, 2004. They were published
in the Federal Register on January 5, 2005 (70 FR 944). On April 5,
2005, EPA issued a supplemental notice which changed the designation
status of eight areas from nonattainment to attainment based on newly
updated 2002-2004 air quality data (70 FR 19844; published in the
Federal Register on April 14, 2005). A total of 39 areas were
designated as nonattainment for the 1997 PM2.5 standards.
The population of these areas is estimated at about 90 million (or more
than 30% of the U.S. population). Most of these areas only violate the
annual standard, but a few violate both the annual and 24-hour
standards.
    The nonattainment designation for an area starts the process
whereby a State or Tribe must develop an implementation plan that
includes, among other things, a demonstration showing how it will
attain the ambient standards by the attainment dates required in the
CAA. Under section 172(b), States have up to 3 years after EPA's final
designations to submit their SIPs to EPA. These SIPs will be due on
April 5, 2008, 3 years from the effective date of the designations.
    Section 172(a)(2) of the Act requires States to attain the
standards as expeditiously as practicable but within 5 years of
designation (i.e. attainment date of April 2010 based on air quality
data for 2007-2009), or within up to 10 years of designation (i.e. to
April 2015) if the EPA Administrator extends an area's attainment date
by 1-5 years based upon the severity of the nonattainment problem or
the feasibility of implementing control measures.
    Virtually all nonattainment problems appear to result from a
combination of local emissions and transported emissions from upwind
areas. The structure of the CAA requires EPA to develop national rules
for certain types of sources which are also significant contributors to
local air quality problems, including motor vehicles and fuels. It also
provides for States to address emissions sources on an area-specific
basis through such requirements as RACT, RACM, and RFP.
    We believe that to attain the PM2.5 standards, it is
important to pursue emissions reductions simultaneously on the local,
regional, and national levels. The EPA issued the Clean Air Interstate

Rule (CAIR) \5\ on March 10, 2005 to address the interstate transport
of sulfur dioxide and nitrogen oxide emissions primarily from power
plants. Section 110 gives EPA the authority to require SIPs to
``prohibit * * * any source or other type of emission activity within
the State from emitting any air pollutant in amounts which will
contribute significantly to nonattainment in, or interfere with
maintenance by, any other State with respect to'' any NAAQS, and to
prohibit sources or emission activities from emitting pollutants in
amounts which will interfere with measures required to be included in
State plans to prevent significant deterioration of air quality or to
protect visibility (such as the protection of 156 mandatory Federal
class I areas under the regional haze rule \6\). CAIR employs the same
emissions trading approach used to achieve cost-effective emission
reductions under the acid rain program. It outlines a two-phase program
with increasingly tighter power plant emissions caps for 28 eastern
states and the District of Columbia: SO2 caps of 3.6 million
tons in 2010, and 2.5 million in 2015; NOX caps of 1.5 in
2009 and 1.3 in 2015; and NOX ozone season caps of 580,000
tons in 2009 and 480,000 tons in 2015. Emission caps are divided into
State SO2 and NOX budgets. By the year 2015, the
Clean Air Interstate Rule is estimated to result in:
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    \5\ See http://www.epa.gov/cair.

    \6\ See 64 FR 35714, July 1, 1999.

--$85 to $100 billion in annual health benefits, including preventing
17,000 premature deaths, millions of lost work and school days, and
tens of thousands of non-fatal heart attacks and hospital admissions
annually.
--Nearly $2 billion in annual visibility benefits in southeastern
national parks, such as Great Smoky and Shenandoah.
--Significant regional reductions in sulfur and nitrogen deposition,
reducing the number of acidic lakes and streams in the eastern U.S.

    Over the past several years, EPA has also issued a number of
regulations addressing emissions standards for new cars, trucks and
buses. These standards are providing reductions in motor vehicle
emissions of volatile organic compounds (VOCs, also referred to as
hydrocarbons), NOX, and direct PM emissions (such as
elemental carbon) as older vehicles are retired and replaced. Other
existing rules are designed to reduce emissions from several categories
of nonroad engines. The Tier 2 motor vehicle emission standards,
together with the associated requirements to reduce sulfur in gasoline,
are estimated to provide additional benefits nationally beginning in
2004.\7\ When the new tailpipe and sulfur standards are fully
implemented, Americans are estimated to benefit from the clean-air
equivalent of removing 164 million cars from the road. These new
standards require passenger vehicles to have emissions 77 to 95 percent
cleaner than those on the road today and require fuel manufacturers to
reduce the sulfur content of gasoline by up to 90 percent. In addition,
the 2001 heavy-duty diesel engine regulations \8\ will lead to
continued emissions reductions as older vehicles in that engine class
are retired and fleets turn over. New emission standards began to take
effect for model year 2007 and apply to heavy-duty highway engines and
vehicles. These standards are based on the use of high-efficiency
catalytic exhaust emission control devices or comparably effective
advanced technologies. Because these devices are damaged by sulfur, the
level of sulfur in highway diesel fuel was to be reduced by 97 percent
by mid-2006. We project a 2.6 million ton reduction of NOX
emissions in 2030 when the current heavy-duty vehicle fleet is
completely replaced with newer heavy-duty vehicles that comply with
these emission standards. By 2030, we estimate that this program will
reduce annual emissions of hydrocarbons by 115,000 tons and PM by
109,000 tons. These emissions reductions are on par with those that we
anticipate from new passenger vehicles and low sulfur gasoline under
the Tier 2 program.
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    \7\ See Tier II emission standards at 65 FR 6698, February 10,
2000.
    \8\ See heavy-duty diesel engine regulations at 66 FR 5002,
January 18, 2001.
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    The EPA also finalized national rules in May 2004 to reduce
significantly PM2.5 and NOX emissions from
nonroad diesel-powered equipment.\9\ These nonroad sources include
construction, agricultural, and industrial equipment, and their
emissions constitute an important fraction of the inventory for direct
PM2.5 emissions (such as elemental carbon and organic
carbon), and NOX. The EPA estimates that affected nonroad
diesel engines currently account for about 44 percent of total diesel
PM emissions and about 12 percent of total NOX emissions
from mobile sources nationwide. These proportions are even higher in
some urban areas. The diesel emission standards will reduce emissions
from this category by more than 90 percent, and are similar to the
onroad engine requirements implemented for highway trucks and buses.
Because the emission control devices can be damaged by sulfur, EPA also
established requirements to reduce the allowable level of sulfur in
nonroad diesel fuel by more than 99 percent by 2010. In 2030, when the
full inventory of older nonroad engines has been replaced, the nonroad
diesel program will annually prevent up to 12,000 premature deaths, one
million lost work days, 15,000 heart attacks and 6,000 children's
asthma-related emergency room visits.
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    \9\ For more information on the proposed nonroad diesel engine
standards, see EPA's Web site: http://www.epa.gov/nonroad/.

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    The EPA expects the implementation of regional and national
emission reduction programs such as CAIR and the suite of mobile source
rules described above to provide significant air quality improvements
for PM2.5 nonattainment areas. At the same time, analyses
for the final CAIR rule indicate that without implementation of local
measures, a number of PM2.5 areas are projected to remain in
nonattainment status in the 2010-2015 timeframe. Thus, EPA believes
that local and State emission reduction efforts will need to play an
important role in addressing the PM2.5 problem as well. The
EPA will work closely with States, Tribes, and local governments to
develop appropriate in-state pollution reduction measures to complement
regional and national strategies to meet the standards expeditiously
and in a cost-effective manner. States will need to evaluate
technically and economically feasible emission reduction opportunities
and determine which measures can be reasonably implemented in the near
term. Local and regional emission reduction efforts should proceed
concurrently and expeditiously.
    The promulgation of a revised 24-hour PM2.5 standard
effective on December 18, 2006 has initiated another process of State
recommendations, and the eventual designation by EPA of areas not
attaining the revised standard. The additional designations are to be
completed within two years from the effective date, although EPA may
take an additional year to complete the designations if it determines
it does not have sufficient information. State plans to attain the 24-
hour standard would then be due within three years of the final
designations. A number of areas, including some that are already
designated as not attaining the 1997 standards, may be exceeding the
revised 24-hour standard. The EPA encourages State and local
governments to be mindful of the strengthened 24-hour standard as they
adopt emission reduction strategies to attain the 1997 standards. Such
steps may help with

future attainment efforts, or even help some areas avoid a
nonattainment designation for the 24-hour standard in the first place.
    The public health benefits of meeting the PM2.5
standards are estimated to be significant. Even small reductions in
PM2.5 levels may have substantial health benefits on a
population level. For example, in a moderate-sized metropolitan area
with a design value of 15.5 [mu]g/m3, efforts to improve annual average
air quality down to the level of the standard (15.0 [mu]g/m3) are
estimated to result in as many as 25-50 fewer mortalities per year due
to air pollution exposure. In a smaller city, the same air quality
improvement from 15.5 to 15.0 [mu]g/m3 still are estimated to result in
a number of avoided mortalities per year. These estimates are based on
EPA's standard methodology for calculating health benefits as used in
recent rulemakings.\10\ In addition, because many different precursors
contribute to the formation of fine particles, reductions in pollutants
that contribute to PM2.5 also can provide concurrent
benefits in addressing a number of other air quality problems--such as
ground-level ozone, regional haze, toxic air pollutants, and urban
visibility impairment.
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    \10\ See: U.S. EPA 2006. Regulatory Impact Analysis for the
Particulate Matter National Ambient Air Quality Standards. Air
Benefits and Cost Group, Office of Air Quality Planning and
Standards, Research Triangle Park, N.C. October 6, 2006. Appendix A
provides an analysis of estimated benefits and costs of attaining
the 1997 PM NAAQS standards in 2015.
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    In order to assist States in developing effective plans to address
the local component of the PM2.5 nonattainment problem, EPA
is issuing this final fine particle implementation rule. The EPA is
issuing this rule to implement the 1997 PM2.5 NAAQS in
accordance with the statutory requirements of the CAA set forth in
Subpart 1 of Part D of Title 1, i.e., sections 171-179B of the Act. The
EPA believes that the CAA directs the Agency to implement new or
revised NAAQS in nonattainment areas solely in accordance with Subpart
1, unless another Subpart of the Act also applies to the particular
NAAQS at issue. In this case, EPA has concluded that Congress did not
intend the Agency to implement particulate matter NAAQS other than
those using PM10 as the indicator in accordance with Subpart
4 of Part D of Title 1, i.e., sections 188-190 of the CAA. Moreover,
EPA believes that implementation of the PM2.5 NAAQS under
the provisions of Subpart 1 is more appropriate, given the inherent
nature of the PM2.5 nonattainment problem. In contrast to
PM10, EPA anticipates that achieving the NAAQS for
PM2.5 will generally require States to evaluate different
sources for controls, to consider controls of one or more precursors in
addition to direct PM emissions, and to adopt different control
strategies. As a result, EPA has concluded that the provisions of
Subpart 1 will allow States and EPA to tailor attainment plans so that
they can be based more specifically upon the facts and circumstances of
each nonattainment area.
    The proposed clean air fine particle implementation rule was issued
on November 1, 2005 (70 FR 65984). About 100 comments were received
from private citizens and parties representing industry, state and
local governments, environmental groups, and federal agencies. Section
II of this document describes the primary elements of the fine particle
implementation program. Each section summarizes the relevant policies
and options discussed in the proposed rule, discusses the final policy
set forth by EPA in the final rule, and provides responses to the major
comments received on each issue.

II. Elements of the Clean Air Fine Particle Implementation Rule

A. Precursors and Pollutants Contributing to Fine Particle Formation

1. Introduction
    The main precursor gases associated with fine particle formation
are SO2, NOX, volatile organic compounds (VOC),
and ammonia. This section provides technical background on each
precursor, discusses the policy approach for addressing each precursor
under the PM2.5 implementation program, and responds to key
issues raised in the public comment process. A subsection is also
included on direct PM2.5 emissions to address key comments
received on this issue as well.
    Gas-phase precursors SO2, NOX, VOC, and
ammonia undergo chemical reactions in the atmosphere to form secondary
particulate matter. Formation of secondary PM depends on numerous
factors including the concentrations of precursors; the concentrations
of other gaseous reactive species; atmospheric conditions including
solar radiation, temperature, and relative humidity (RH); and the
interactions of precursors with preexisting particles and with cloud or
fog droplets. Several atmospheric aerosol species, such as ammonium
nitrate and certain organic compounds, are semivolatile and are found
in both gas and particle phases. Given the complexity of PM formation
processes, new information from the scientific community continues to
emerge to improve our understanding of the relationship between sources
of PM precursors and secondary particle formation.
    As an initial matter, it is helpful to clarify the terminology we
use throughout this notice to discuss precursors. We recognize
NOX, SO2, VOCs, and ammonia as precursors of
PM2.5 in the scientific sense because these pollutants can
contribute to the formation of PM2.5 in the ambient air. In
section II.K on emission inventory issues, we make the point that
because of the complex and variable interaction of multiple pollutants
and precursors in the formation of fine particles, it is important for
States and EPA to continue to characterize and improve the emissions
inventories for all PM2.5 precursors. The States and EPA
need to use the best available information available in conducting air
quality modeling and other assessments. At the same time, the
refinement of emissions inventories, the overall contribution of
different fine particle precursors to PM2.5 formation, and
the efficacy of alternative potential control measures will vary by
location. This requires that we further consider in this action how
States should address these PM2.5 precursors in their
PM2.5 attainment plan programs. Thus, we require emission
inventories to include the best available information on all pollutants
and precursors that contribute to PM2.5 concentrations, and
at same time we use the term ``PM2.5 attainment plan
precursor'' to describe only those precursors that are required to be
evaluated for control strategies in a specific PM2.5
nonattainment area or maintenance area plan.
    In this rule, EPA has not made a finding that all precursors should
be evaluated for possible controls in each specific nonattainment area.
The policy approach in the rule instead requires sulfur dioxide to be
evaluated for control measures in all areas, and describes general
presumptive policies for NOX, ammonia, and VOC for all
nonattainment areas. The rule provides a mechanism by which the State
and/or EPA can make an area-specific demonstration to reverse the
general presumption for these three precursors. States must also
consider any relevant information brought forward by interested parties
in the SIP planning and development process. (See section II.A.8 for
additional discussion on these issues.)
    In the following sections, we discuss how States must evaluate
PM2.5 precursors for nonattainment program issues in
PM2.5 implementation plans,

including issues such as RACT, RACM, and reasonable further progress.
This discussion in the final rule is linked to precursor policies for
the implementation of the new source review program, the transportation
conformity program, the general conformity program, and the regional
haze program. All of these programs take effect prior to approval of
SIPs for attaining the PM2.5 NAAQS. In the case of NSR, the
program applies on the effective date of the nonattainment area
designation. In the case of transportation conformity and general
conformity, the program takes effect 1 year from the effective date of
designation of the nonattainment area (i.e., April 5, 2006 for areas
designated nonattainment effective April 5, 2005). Thus, for each of
these programs there is an interim period between the date the program
becomes applicable to a given nonattainment area and the date the State
receives EPA approval of its overall PM2.5 implementation
plan.
2. Legal Authority to Regulate Precursors
 a. Background
    The CAA authorizes the Agency to regulate criteria pollutant
precursors. The term ``air pollutant'' is defined in section 302(g) to
include ``any precursors to the formation of any air pollutant, to the
extent the Administrator has identified such precursor or precursors
for the particular purpose for which the term `air pollutant' is
used.'' The first clause of this second sentence in section 302(g)
explicitly authorizes the Administrator to identify and regulate
precursors as air pollutants under other parts of the CAA. In addition,
the second clause of the sentence indicates that the Administrator has
discretion to identify which pollutants should be classified as
precursors for particular regulatory purposes. Thus, we do not
necessarily construe the CAA to require that EPA identify a particular
precursor as an air pollutant for all regulatory purposes where it can
be demonstrated that various CAA programs address different aspects of
the air pollutant problem. Likewise, we do not interpret the CAA to
require that EPA treat all precursors of a particular pollutant the
same under any one program when there is a basis to distinguish between
such precursors. For example, in a rule addressing PM2.5
precursors for purposes of the transportation conformity program, we
chose to adopt a different approach for one precursor based on the
limited emissions of that precursor from onroad mobile sources and the
degree to which it contributes to PM2.5 concentrations. (70
FR 24280; May 6, 2005).
    Other provisions of the CAA reinforce our reading of section 302(g)
that Congress intended precursors to NAAQS pollutants to be subject to
the air quality planning and control requirements of the CAA, but also
recognized that there may be circumstances where it is not appropriate
to subject precursors to certain requirements of the CAA. Section 182
of the CAA provides for the regulation of NOX and VOCs as
precursors to ozone in ozone nonattainment areas, but also provides in
section 182(f) that major stationary sources of NOX (an
ozone precursor) are not subject to emission reductions controls for
ozone where the State shows through modeling that NOX
reductions do not decrease ozone. Section 189(e) provides for the
regulation of PM10 precursors in PM10
nonattainment areas, but also recognizes that there may be certain
circumstances (e.g. if precursor emission sources do not significantly
contribute to PM10 levels) where it is not appropriate to
apply control requirements to PM10 precursors. The
legislative history of Section 189(e) recognized the complexity behind
the science of precursor transformation into PM10 ambient
concentrations and the need to harmonize the regulation of
PM10 precursors with other provisions of the CAA:

    The Committee notes that some of these precursors may well be
controlled under other provisions of the CAA. The Committee intends
that * * * the Administrator will develop models, mechanisms, and
other methodology to assess the significance of the PM10
precursors in improving air quality and reducing PM10.
Additionally, the Administrator should consider the impact on ozone
levels of PM10 precursor controls. The Committee expects
the Administrator to harmonize the PM10 reduction
objective of this section with other applicable regulations of this
CAA regarding PM10 precursors, such as NOX.
See H. Rpt. 101-490, Pt. 1, at 268 (May 17, 1990), reprinted in S.
Prt. 103-38, Vol. II, at 3292.

    In summary, section 302(g) of the CAA clearly calls for the
regulation of precursor pollutants, but the CAA also identifies
circumstances when it may not be appropriate to regulate precursors and
gives the Administrator discretion to determine how to address
particular precursors under various programs required by the CAA. Due
to the complexities associated with precursor emissions and their
variability from location to location, we believe that in certain
situations it may not be effective or appropriate to control a certain
precursor under a particular regulatory program or for EPA to require
similar control of a particular precursor in all areas of the country.
b. Final Rule
    The final rule maintains the same legal basis for regulating
precursors as was described in the proposal and in the background
section above. We also include a clarification of the term
``significant contributor.''
    In the proposal, when considering the impacts of the precursors
NOX, VOC and ammonia on ambient concentrations of
particulate matter, we referred to the possibility of reversing the
presumed approach for regulating or not regulating a precursor if it
can be shown that the precursor in question is or is not a
``significant contributor'' to PM2.5 concentrations within
the specific nonattainment area. ``Significant contribution'' in this
context is a different concept than that in Section 110(a)(2)(D).
Section 110(a)(2)(D) prohibits States from emitting air pollutants in
amounts which significantly contribute to nonattainment or other air
quality problems in other states. Consistent with the discussion of
sections 189(e) and 302(g) above, we are clarifying that the use in
this implementation rule of the term ``significant contribution'' to
the nonattainment area's PM2.5 concentration means that a
significant change in emissions of the precursor from sources in the
state would be projected to provide a significant change in
PM2.5 concentrations in the nonattainment area. For example,
if modeling indicates that a reduction in a state's NOX
emissions would reduce ambient PM2.5 levels in the
nonattainment area, but that a reduction in ammonia emissions would
result in virtually no change in ambient PM2.5 levels, this
would suggest that NOX is a significant contributor but that
ammonia is not. The EPA in this rule is not establishing a quantitative
test for determining whether PM2.5 levels in a nonattainment
area change significantly in response to reductions in precursor
emissions in a state. However, in considering this question, it is
relevant to consider that relatively small reductions in
PM2.5 levels are estimated to result in worthwhile public
health benefits.
    This approach to identifying a precursor for regulation reflects
atmospheric chemistry conditions in the area and the magnitude of
emissions of the precursor in the area or State. Assessments of which
source categories are more cost effective or technically feasible to
control should be part of the later RACT and RACM assessment, to

occur after the basic assessment of which precursors are to be
regulated is completed.
    In the proposed regulatory text, the provisions for reversing
presumptions for NOX, VOC and ammonia included consideration
of whether the precursor would significantly contribute to ``other
downwind air quality concerns.'' In the final rule we have removed that
language to clarify that identification of attainment plan precursors
involves evaluation of the impact on PM2.5 levels in a
nonattainment area of precursor emissions from sources within the
state(s) where the nonattainment area is located. Other parts of the
Act, notably section 110(a)(2)(D) and section 126, focus on interstate
transport of pollutants.
c. Comments and Responses
    Comment: The EPA received several comments supporting EPA's
interpretation of 302(g) to determine the appropriate regulatory status
of each precursor pollutant.
    Response: The EPA agrees with the commenters. In establishing
section 302(g), Congress intended that precursors to NAAQS pollutants
be subject to the air quality planning and control requirements of the
CAA. However, the CAA also recognizes that there may be circumstances
where it is not appropriate to subject precursors to certain
requirements of the CAA.
    Comment: The EPA received several comments regarding the
applicability of section 189(e), noting that it requires states to
presumptively control sources of PM10 precursors except
where the EPA ``determines that such sources [of precursors] do not
significantly contribute to PM10 levels which exceed the
standard in the area.'' Several commenters stated that EPA does not
have the legal authority to regulate PM2.5 precursors in a
different manner. Several commenters maintained that all
PM2.5 precursors presumptively should be subject to
regulation unless demonstrated by the State as not a significant
contributor to PM2.5 concentrations in a specific area.
    Response: As stated above, EPA believes that section 302(g) allows
the Administrator to presumptively not require certain precursors to be
addressed in PM2.5 implementation plans generally, while
allowing the State or EPA to make a finding for a specific area to
override the general presumption. In the following pollutant-specific
sections of this preamble, EPA finds that at this time there is
sufficient uncertainty regarding whether certain precursors
significantly contribute to PM2.5 concentrations in all
nonattainment areas such that the policy set forth in this rule does
not presumptively require certain precursors (ammonia, VOC) to be
controlled in each area. However, the State or EPA may reverse the
presumption and regulate a precursor if it provides a demonstration
showing that the precursor is a significant contributor to
PM2.5 concentrations in the area. In addition, if in the
State's SIP planning and adoption process a commenter provides
additional information suggesting an alternative policy for regulating
a particular precursor, the State will need to respond to this
information in its rulemaking action.
3. Policy for Ammonia
    [Section II.E.2 of November 1, 2005 proposed rule (70 FR 65999);
sec. 51.1002 in draft and final regulatory text.]
a. Background
    Ammonia (NH3) is a gaseous pollutant that is emitted by
natural and anthropogenic sources. Emissions inventories for ammonia
are considered to be among the most uncertain of any species related to
PM. Ammonia serves an important role in neutralizing acids in clouds,
precipitation and particles. In particular, ammonia neutralizes
sulfuric acid and nitric acid, the two key contributors to acid
deposition (acid rain). Deposited ammonia also can contribute to
problems of eutrophication in water bodies, and deposition of ammonium
particles may effectively result in acidification of soil as ammonia is
taken up by plants. The NARSTO Fine Particle Assessment \11\ indicates
that reducing ammonia emissions where sulfate concentrations are high
may reduce PM2.5 mass concentrations, but may also increase
the acidity of particles and precipitation. An increase in particle
acidity is suspected to be linked with human health effects and with an
increase in the formation of secondary organic compounds. Based on the
above information and further insights gained from the NARSTO Fine
Particle Assessment, it is apparent that the formation of particles
related to ammonia emissions is a complex, nonlinear process.
---------------------------------------------------------------------------

    \11\ NARSTO (2004) (Particulate Matter Assessment for Policy
Makers: A NARSTO Assessment. P. McMurry, M. Shepherd, and J.
Vickery, eds. Cambridge University Press, Cambridge, England. ISBN 0
52 184287 5.
---------------------------------------------------------------------------

    Though recent studies have improved our understanding of the role
of ammonia in aerosol formation, ongoing research is required to better
describe the relationships between ammonia emissions, particulate
matter concentrations, and related impacts. The control techniques for
ammonia and the analytical tools to quantify the impacts of reducing
ammonia emissions on atmospheric aerosol formation are both evolving.
Also, area-specific data are needed to evaluate the effectiveness of
reducing ammonia emissions on reducing PM2.5 concentrations
in different areas, and to determine where ammonia decreases may
increase the acidity of particles and precipitation.
    The proposal showed consideration for the uncertainties about
ammonia emissions inventories and about the potential efficacy of
ammonia control measures by providing for a case-by-case approach. It
was recommended that each State should evaluate whether reducing
ammonia emissions would lead to PM2.5 reductions in their
specific PM2.5 nonattainment areas. The proposed policy did
not require States to address ammonia as a PM2.5 attainment
plan precursor, unless a technical demonstration by the State or EPA
showed that ammonia emissions from sources in the State significantly
contribute to PM2.5 concentrations in a given nonattainment
area or to other downwind air quality concerns. Where the State or EPA
has determined that ammonia is a significant contributor to
PM2.5 formation in a nonattainment area, the State would be
required to evaluate control measures for ammonia emissions in its
nonattainment SIP due in 2008, in the implementation of the PM program,
and in other associated programs in that area.
b. Final Rule
    In the final rule, ammonia is presumed not to be a PM2.5
attainment plan precursor, meaning that the State is not required to
address ammonia in its attainment plan or evaluate sources of ammonia
emissions for reduction measures. This presumption can be reversed
based on an acceptable technical demonstration for a particular area by
the State or EPA. If a technical demonstration by the State or EPA
shows that ammonia emissions from sources in the State significantly
contribute to PM2.5 concentrations in a given nonattainment
area, the State must then evaluate and consider control strategies for
reducing ammonia emissions in its nonattainment SIP due in 2008, in the
implementation of the PM2.5 program. Technical
demonstrations on ammonia should also consider the potential for
atmospheric and particle acidity to increase with ammonia reductions.
Further discussion about technical demonstrations to

support reversing a PM2.5 precursor presumption is included
in section II.A.8 below.
    This approach was retained from the proposal because of continued
uncertainties regarding ammonia emission inventories and the effects of
ammonia emission reductions. Ammonia emission inventories are presently
very uncertain in most areas, complicating the task of assessing
potential impacts of ammonia emissions reductions. In addition, data
necessary to understand the atmospheric composition and balance of
ammonia and nitric acid in an area are not widely available across
PM2.5 nonattainment areas, making it difficult to predict
the results of potential ammonia emission reductions. Ammonia
reductions may be effective and appropriate for reducing
PM2.5 concentrations in selected locations, but in other
locations such reductions may lead to minimal reductions in
PM2.5 concentrations and increased atmospheric acidity.
Research projects continue to expand our collective understanding of
these issues, but at this time EPA believes this case-by-case policy
approach is appropriate. In light of these uncertainties, we encourage
States to continue efforts to better understand the role of ammonia in
its fine particle problem areas.
c. Comments and Responses
    Comment: One commenter stated that scientific understanding of the
complexities of PM formation from ammonia is limited. The commenter
claimed that the reduction of ammonia will not reduce PM in many areas,
and speciated PM data to investigate the potential decrease in PM from
ammonia emissions reductions is not available in all areas.
    Response: The final rule takes these uncertainties into
consideration by allowing ammonia to be addressed on a case-by-case
basis. For any area about which enough information is available to
determine that ammonia emission reductions would lead to a beneficial
reduction in PM2.5, the State can develop a technical
demonstration justifying the control of ammonia. If the State chooses
to develop such a demonstration, preferably it should be completed as
part of the SIP development process and prior to the adoption of
control measures, in consultation with the appropriate EPA regional
office.
    Comment: Some commenters claimed that requiring no action on some
precursors is counter to the requirement in sections 172(a)(2) and 188
to attain the NAAQS as expeditiously as practicable. They also asserted
that presuming that ammonia is not a PM2.5 attainment plan
precursor violates 302(g) by improperly delegating authority to the
States.
    Response: In many areas, reducing ammonia emissions could have
little effect on PM2.5 concentrations and could lead to the
potentially harmful effect of increased atmospheric acidity. While
States are not required to take action on ammonia sources under this
policy, States would be required to address information on ammonia
brought to their attention during the planning and rule adoption
process. Under this approach, States should assess whether ammonia
reductions would lead to reduced PM2.5 concentrations in
specific nonattainment areas. If the State decides that ammonia
reductions could yield beneficial reductions in PM2.5, the
State should complete a technical demonstration supporting a reversal
of the presumption. The EPA does not believe that this approach
improperly delegates authority to the States. It establishes a general
presumption for all areas through this rulemaking process, and allows
for the presumption to be modified by the State or EPA on a case-by-
case basis. EPA still retains the ability to make a technical
demonstration for any area if appropriate to reverse the presumption
and require ammonia to be addressed in its attainment plan.
    Comment: Some commenters stated that the results of a large study
on air emissions from concentrated animal feeding operations (CAFOs)
should be evaluated before requiring control of ammonia in areas where
agriculture is alleged to be a major source.
    Response: The $15 million national CAFO consent agreement study
coordinated by Purdue University will greatly improve ammonia and VOC
emissions inventories and our understanding of the impacts of
agricultural emissions on particle formation. The EPA recognizes that
the agricultural emissions study is expected to provide data for future
planning purposes, and we expect that some of the results of the study
will not be available in time to be considered in the development of
PM2.5 State Implementation Plans dues in April 2008.
However, if a State believes it has sufficient technical information to
warrant regulation of ammonia emissions in their 2008 implementation
plans, it may include in its plan a demonstration to reverse the
presumption as well as emission reduction measures. The EPA will review
each submittal on a case-by-case basis.
    Comment: A presumption to not address ammonia will impede certain
states (i.e. those that have provisions requiring their regulations to
be ``no stricter than Federal'' provisions) from regulating ammonia.
    Response: This presumptive approach to ammonia will not restrict
States from addressing ammonia in their PM2.5 attainment
plans. If a State has information indicating that reductions in ammonia
emissions would cause beneficial reductions in PM2.5
concentrations, the State can make a technical demonstration to reverse
the presumption. In such cases, inclusion of ammonia as a
PM2.5 attainment plan precursor would not be considered
stricter than Federal requirements. Under the policy in the final rule,
the Federal government or the State may assess the impact of ammonia in
a particular area and determine whether the presumption of
insignificance is appropriate or whether ammonia is in fact a
significant contributor to the PM2.5 problem in the area.
4. Policy for VOC
    [Section II.E.2 of November 1, 2005 proposed rule (70 FR 65999);
sec. 51.1002 in draft and final regulatory text.]
a. Background
    The VOC policy in this rule addresses volatile and semivolatile
organic compounds, generally up to 24 carbon atoms. High molecular
weight organic compounds (typically 25 carbon atoms or more) are
emitted directly as primary organic particles and exist primarily in
the condensed phase at ambient temperatures. Accordingly, high
molecular weight organic compounds are to be regulated as primary
PM2.5 emissions for the purposes of the PM2.5
implementation program.
    The organic component of ambient particles is a complex mixture of
hundreds or even thousands of organic compounds. These organic
compounds are either emitted directly from sources (i.e. primary
organic aerosol) or can be formed by reactions in the ambient air (i.e.
secondary organic aerosol, or SOA). Volatile organic compounds are key
precursors in the formation processes for both SOA and ozone. The
relative importance of organic compounds in the formation of secondary
organic particles varies from area to area, depending upon local
emissions sources, atmospheric chemistry, and season of the year.
    The lightest organic molecules (i.e., molecules with six or fewer
carbon atoms) occur in the atmosphere mainly as vapors and typically do
not directly

form organic particles at ambient temperatures due to the high vapor
pressure of their products. However, they participate in atmospheric
chemistry processes resulting in the formation of ozone and certain
free radical compounds (such as the hydroxyl radical [OH]) which in
turn participate in oxidation reactions to form secondary organic
aerosols, sulfates, and nitrates. These VOCs include all alkanes with
up to six carbon atoms (from methane to hexane isomers), all alkenes
with up to six carbon atoms (from ethene to hexene isomers), benzene
and many low-molecular weight carbonyls, chlorinated compounds, and
oxygenated solvents.
    Intermediate weight organic molecules (i.e., compounds with 7 to 24
carbon atoms) often exhibit a range of volatilities and can exist in
both the gas and aerosol phase at ambient conditions. For this reason
they are also referred to as semivolatile compounds. Semivolatile
compounds react in the atmosphere to form secondary organic aerosols.
These chemical reactions are accelerated in warmer temperatures, and
studies show that SOA typically comprises a higher percentage of
carbonaceous PM in the summer as opposed to the winter. The production
of SOA from the atmospheric oxidation of a specific VOC depends on four
factors: Its atmospheric abundance, its chemical reactivity, the
availability of oxidants (O3, OH, HNO3), and the
volatility of its products. In addition, recent work suggests that the
presence of acidic aerosols may lead to an increased rate of SOA
formation. Aromatic compounds such as toluene, xylene, and trimethyl
benzene are considered to be the most significant anthropogenic SOA
precursors and have been estimated to be responsible for 50 to 70
percent of total SOA in some airsheds. Man-made sources of aromatics
gases include mobile sources, petrochemical manufacturing and solvents.
Some of the biogenic hydrocarbons emitted by trees are also considered
to be important precursors of secondary organic particulate matter.
Terpenes (and b-pinene, limonene, carene, etc.) and the sesquiterpenes
are expected to be major contributors to SOA in areas with significant
vegetation cover, but isoprene is not. Terpenes are very prevalent in
areas with pine forests, especially in the southeastern U.S. The rest
of the anthropogenic hydrocarbons (higher alkanes, paraffins, etc.)
have been estimated to contribute 5-20 percent to the SOA concentration
depending on the area.
    The contribution of the primary and secondary components of organic
aerosol to the measured organic aerosol concentrations remains a
complex issue. Most of the research performed to date has been done in
southern California, and more recently in central California, while
fewer studies have been completed on other parts of North America. Many
studies suggest that the primary and secondary contributions to total
organic aerosol concentrations are highly variable, even on short time
scales. Studies of pollution episodes indicate that the contribution of
SOA to the organic particulate matter can vary from 20 percent to 80
percent during the same day.
    Despite significant advances in understanding the origins and
properties of SOA, it remains probably the least understood component
of PM2.5. The reactions forming secondary organics are
complex, and the number of intermediate and final compounds formed is
voluminous. Some of the best efforts to unravel the chemical
composition of ambient organic aerosol matter have been able to
quantify the concentrations of hundreds of organic compounds
representing only 10-20 percent of the total organic aerosol mass. For
this reason, SOA continues to be a significant topic of research and
investigation.
    Current scientific and technical information clearly shows that
carbonaceous material is a significant fraction of total
PM2.5 mass in most areas, that certain VOC emissions are
precursors to the formation of secondary organic aerosol, and that a
considerable fraction of the total carbonaceous material is likely from
local as opposed to regional sources. However, while significant
progress has been made in understanding the role of gaseous organic
material in the formation of organic PM, this relationship remains
complex. We recognize that further research and technical tools are
needed to better characterize emissions inventories for specific VOC
compounds, and to determine the extent of the contribution of specific
VOC compounds to organic PM mass.
    In light of these factors, the proposed rule did not require States
to address VOCs as PM2.5 attainment plan precursors and
evaluate them for control measures, unless the State or EPA makes a
finding that VOCs significantly contribute to a PM2.5
nonattainment problem in the State or to other downwind air quality
concerns. Many PM2.5 nonattainment areas are also
nonattainment areas for the 8-hour ozone standard; control measures for
VOCs will be implemented in some of these areas, potentially providing
a co-benefit for PM2.5 concentrations.
b. Final Rule
    The final rule maintains the same policy as proposed.\12\ States
are not required to address VOC in PM2.5 implementation
plans and evaluate control measures for such pollutants unless the
State or EPA makes a technical demonstration that emissions of VOCs
from sources in the State significantly contribute to PM2.5
concentrations in a given nonattainment area. Technical demonstrations
are discussed in section II.A.8 below. If a State chooses to make a
technical demonstration, it should be developed in advance of the
attainment demonstration.
---------------------------------------------------------------------------

    \12\ The policy is the same as proposed, with the clarification
regarding downwind areas discussed above (Section A.2.b).
---------------------------------------------------------------------------

c. Comments and Responses
    Comment: One commenter stated that our understanding of the
complexities of PM2.5 formation from VOCs is limited, that
speciated PM data are not available in all areas, and that VOC
reductions will not reduce PM2.5 in many areas.
    Response: The EPA acknowledges the uncertainties regarding the role
of VOC in secondary organic aerosol formation. For this reason the
final rule does not presumptively include VOC as a regulated pollutant
for PM planning. However, if available data demonstrates that control
of VOC would reduce PM2.5 concentrations in an area, the
State or EPA may include VOC as an attainment plan precursor.
    Comment: One commenter stated that the rationale that VOC should
not be considered a PM2.5 attainment plan precursor because
most PM areas are also ozone areas is not appropriate because many
ozone areas will attain soon and VOC reductions will still be needed
for PM.
    Response: The primary rationale for not including VOC as a
PM2.5 attainment plan precursor in every nonattainment area
is the uncertainty regarding the contribution of anthropogenic VOCs to
the formation of the organic carbon portion of fine particles. In
certain areas, EPA expects that VOC control measures will have some co-
benefits in the reduction of fine particulates. However, this reason
should not be considered the principal reason for the policy in the
final rule that VOCs presumptively should not be considered
PM2.5 attainment plan precursors. If a State or EPA
determines that VOCs do contribute significantly to PM2.5
concentrations in an area, the State will be required to evaluate
control measures for VOC as a PM2.5 attainment plan

precursor for that area. This approach will provide for regulation of
VOCs in locations where it is most appropriate.
    Comment: One commenter suggested that EPA wait for the results of
the pending agricultural emissions study before requiring control of
VOCs in agricultural areas.
    Response: The $15 million national CAFO consent agreement study
coordinated by Purdue University will greatly improve ammonia and VOC
emissions inventories and our understanding of the impacts of
agricultural emissions on particle formation. The EPA recognizes that
the agricultural emissions study is expected to provide data for future
planning purposes, and we expect that some of the results of the study
will not be available in time to be considered in the development of
PM2.5 State Implementation Plans dues in April 2008.
However, if a State believes it has sufficient technical information to
warrant regulation of VOC emissions in their 2008 implementation plans,
it may include in its plan a demonstration to reverse the presumption
as well as emission reduction measures. The EPA will review each
submittal on a case-by-case basis.
5. Policy for NOX
    [Section II.E.2 of November 1, 2005 proposed rule (70 FR 65999);
sec. 51.1002 in draft and final regulatory text.]
a. Background
    The sources of NOX are numerous and widespread. The
combustion of fossil fuel in boilers for commercial and industrial
power generation and in mobile source engines each account for
approximately 30 percent of NOX emissions in
PM2.5 nonattainment areas (based on 2001 emission inventory
information). Nitrates are formed from the oxidation of oxides of
nitrogen into nitric acid either during the daytime (reaction with OH)
or during the night (reactions with ozone and water). Nitric acid
continuously transfers between the gas and the condensed phases through
condensation and evaporation processes in the atmosphere. However,
unless it reacts with other species (such as ammonia, sea salt, or
dust) to form a neutralized salt, it will volatilize and not be
measured using standard PM2.5 measurement techniques. The
formation of aerosol ammonium nitrate is favored by the availability of
ammonia, low temperatures, and high relative humidity. Because ammonium
nitrate is semivolatile and not stable in higher temperatures, nitrate
levels are typically lower in the summer months and higher in the
winter months. The resulting ammonium nitrate is usually in the sub-
micrometer particle size range. Reactions with sea salt and dust lead
to the formation of nitrates in coarse particles. Nitric acid may be
dissolved in ambient aerosol particles.
    Based on a review of speciated monitoring data analyses, it is
apparent that nitrate concentrations vary significantly across the
country. For example, in some southeastern locations, annual average
nitrate levels are in the range of 6 to 8 percent of total
PM2.5 mass, whereas nitrate comprises 40 percent or more of
PM2.5 mass in certain California locations. Nitrate
formation is favored by the availability of ammonia, low temperatures,
and high relative humidity. It is also dependent upon the relative
degree of nearby SO2 emissions because ammonia reacts
preferentially with SO2 over NOX. NOX
reductions are expected to reduce PM2.5 concentrations in
most areas. However, it has been suggested that in a limited number of
areas, NOX control would result in increased
PM2.5 mass by disrupting the ozone cycle and leading to
increased oxidation of SO2 to form sulfate particles, which
are heavier than nitrate particles. Because of the above factors, the
proposed rule presumed that States must evaluate and implement
reasonable controls on sources of NOX in all nonattainment
areas, but allowed for the State and EPA to develop a technical
demonstration to reverse this presumption.
b. Final Rule
    The EPA is retaining the proposed approach in the final rule.\13\
Under this policy, States are required to address NOX as a
PM2.5 attainment plan precursor and evaluate reasonable
controls for NOX in PM2.5 attainment plans,
unless the State and EPA make a finding that NOX emissions
from sources in the State do not significantly contribute to
PM2.5 concentrations in the relevant nonattainment area.
This presumptive policy is consistent with other recent EPA regulations
requiring NOX reductions which will reduce fine particle
pollution, such as the Clean Air Interstate Rule and a number of rules
targeting onroad and nonroad engine emissions.
---------------------------------------------------------------------------

    \13\ The policy is the same as proposed, with the clarification
regarding downwind areas discussed above (Section A.2.b).
---------------------------------------------------------------------------

    Technical demonstrations that would reverse the presumption should
be developed in advance of the attainment demonstration and are
discussed in section II.A.8 below.
c. Comments and Responses
    Comment: Most commenters generally agreed with the proposed
inclusion of NOX as a presumptive PM2.5
attainment plan precursor.
    Response: The EPA agrees with these commenters.
    Comment: Some commenters requested guidance on what would
constitute an acceptable demonstration to reverse the presumption that
NOX is a PM2.5 attainment plan precursor.
    Response: Guidance on technical demonstrations to reverse the
presumptive inclusion of NOX in all state implementation
plans is discussed in section II.A.8 below.
    Comment: One commenter raised concerns that the proposed policy for
NOX would allow a State to find NOX to be an
insignificant contributor to an area's PM2.5 nonattainment
problem and effectively keep the State from controlling the area's
NOX emissions for other purposes, such as to address
interstate transport under section 110 of the CAA. Section 110 requires
SIPs to prohibit emissions within the State that would contribute
significantly to another State's nonattainment problem or interfere
with another State's maintenance plan.
    Response: The identification of precursors for regulation under
this rule is for purposes of PM2.5 nonattainment and
maintenance plans under Part D of the CAA. The PM2.5
implementation rule does not prevent a State from regulating
NOX sources under any other Federal or State rule, including
interstate transport rules under Section 110.
6. Policy for SO2
    [Section II.E.2 of November 1, 2005 proposed rule (70 FR 65999);
sec. 51.1002 in draft and final regulatory text.]
a. Background
    Sulfur dioxide is emitted mostly from the combustion of fossil
fuels in boilers operated by electric utilities and other industry.
Less than 20 percent of SO2 emissions nationwide are from
other sources, mainly other industrial processes such as oil refining
and pulp and paper production. The formation of sulfuric acid from the
oxidation of SO2 is an important process affecting most
areas in North America. There are three different pathways for this
transformation.
    First, gaseous SO2 can be oxidized by the hydroxyl
radical (OH) to create sulfuric acid. This gaseous SO2
oxidation reaction occurs slowly and only in the daytime. Second,
SO2 can

dissolve in cloud water (or fog or rain water), and there it can be
oxidized to sulfuric acid by a variety of oxidants, or through
catalysis by transition metals such as manganese or iron. If ammonia is
present and taken up by the water droplet, then ammonium sulfate will
form as a precipitate in the water droplet. After the cloud changes and
the droplet evaporates, the sulfuric acid or ammonium sulfate remains
in the atmosphere as a particle. This aqueous phase production process
involving oxidants can be very fast; in some cases all the available
SO2 can be oxidized in less than an hour. Third,
SO2 can be oxidized in reactions in the particle-bound water
in the aerosol particles themselves. This process takes place
continuously, but only produces appreciable sulfate in alkaline (dust,
sea salt) coarse particles. Oxidation of SO2 has also been
observed on the surfaces of black carbon and metal oxide particles.
During the last 20 years, much progress has been made in understanding
the first two major pathways, but some important questions still remain
about the smaller third pathway. Models indicate that more than half of
the sulfuric acid in the eastern United States and in the overall
atmosphere is produced in clouds.
    The sulfuric acid formed from the above pathways reacts readily
with ammonia to form ammonium sulfate,
(NH4)2SO4. If there is not enough
ammonia present to fully neutralize the produced sulfuric acid (one
molecule of sulfuric acid requires two molecules of ammonia), part of
it exists as ammonium bisulfate, NH4HSO4 (one
molecule of sulfuric acid and one molecule of ammonia) and the
particles are more acidic than ammonium sulfate. In certain situations
(in the absence of sufficient ammonia for neutralization), sulfate can
exist in particles as sulfuric acid, H2SO4.
Sulfuric acid often exists in the plumes of stacks where
SO2, SO3, and water vapor are in much higher
concentrations than in the ambient atmosphere, but these concentrations
become quite small as the plume is cooled and diluted by mixing.
    Because sulfate is a significant contributor (e.g. ranging from 9
percent to 40 percent) to PM2.5 concentrations in
nonattainment areas and to other air quality problems in all regions of
the country, EPA proposed that States would be required to address
sulfur dioxide as a PM2.5 attainment plan precursor in all
areas.
b. Final Rule
    The final rule includes the same policy for sulfur dioxide as in
the proposal. States are required to address sulfur dioxide as a
PM2.5 attainment plan precursor and evaluate SO2
for possible control measures in all areas. Sulfate is an important
precursor to PM2.5 formation in all areas, and has a strong
regional impact on PM2.5 concentrations. This policy is
consistent with past EPA regulations, such as the CAIR, the Clean Air
Visibility Rule, the Acid Rain rules, and the Regional Haze rule, that
require SO2 reductions to address fine particle pollution
and related air quality problems.
    Under the transportation conformity program, sulfur dioxide is not
required to be addressed in transportation conformity determinations
before a SIP is submitted unless either the state air agency or EPA
regional office makes a finding that on-road emissions of sulfur
dioxide are significant contributors to the area's PM2.5
problem. Sulfur dioxide would be addressed after a PM2.5 SIP
is submitted if the area's SIP contains an adequate or approved motor
vehicle emissions budget for sulfur dioxide. EPA based this decision on
the de minimis level of sulfur dioxide emissions from on-road vehicles
currently, and took into consideration the fact that sulfur dioxide
emissions from on-road sources will decline in the future due to the
implementation of requirements for low sulfur gasoline (which began in
2004) and for low sulfur diesel fuel (beginning in 2006). For more
information, see the May 6, 2005 transportation conformity rule on
PM2.5 precursors at 70 FR 24283.
c. Comments and Responses
    Comment: Most commenters agreed with the proposed policy for
SO2. One commenter stated, ``* * * requiring states to
address sulfur dioxide in attainment planning in all areas is
consistent with the science of PM2.5 formation and essential
to effective implementation of the PM2.5 NAAQS.'' Another
commenter concluded that EPA's proposal ``* * * is justified based on
the fact that SO2 has been found to be a significant
contributor to PM2.5 nonattainment in all areas.''
    Response: The EPA agrees with these comments.
    Comment: Some commenters believe States should be able to make a
demonstration that SO2 not be addressed as an attainment
plan precursor. The commenters claim that the urban increment of
sulfate is generally small, and SO2 control will not matter
in many areas. Commenters also note that a large percentage of the
SO2 emission inventory is being reduced and will be reduced
further through existing programs, and that if attainment can be
demonstrated without additional SO2 controls, a State should
be allowed to make that demonstration in its SIP. One commenter stated
that whether SO2 emissions from a given source located in a
nonattainment area in fact contribute significantly to ambient
concentrations of sulfate and PM2.5 in that nonattainment
area likely will depend on a range of factors, including source type,
stack height, location, and meteorology. The commenter asserted that
sulfate forms over significant geographic distances from the source of
the SO2 emissions and may not form significant
concentrations of PM2.5 in the local nonattainment area.
    Response: As in the proposal, the final rule requires
SO2 to be considered a PM2.5 attainment plan
precursor in all cases. Sulfate is a significant fraction of
PM2.5 mass in all nonattainment areas currently, and
although large SO2 reductions are projected from electric
generating units with the implementation of the CAIR program, sulfate
is still projected to be a key contributor to PM2.5
concentrations in the future. SO2 emissions also lead to
sulfate formation on both regional and local scales. The EPA agrees
that the extent of the contribution from a particular source in a
nonattainment area to PM2.5 concentrations in the area will
depend on a number of factors, and that at times the reaction of
SO2 emissions in the atmosphere to form sulfate particles
may occur less rapidly and extend over a significant distance. However,
at other times the conversion of SO2 to sulfate can occur
rapidly and local impacts from a particular source can be more
significant. States are required to develop plans to attain as
expeditiously as practicable through the identification of technically
and economically feasible control measures from the full range of
source categories contributing to PM2.5 nonattainment areas.
In developing these plans, each State will need to consider whether
controls on local SO2 sources would be cost-effective and
would be needed to attain expeditiously.
7. Policy for Direct PM
    [Section II.E.2 of November 1, 2005 proposed rule (70 FR 65999);
sec. 51.1002 in draft and final regulatory text.]
a. Background
    This section addresses inorganic and organic forms of directly
emitted PM. Although these direct emissions are by definition not
precursors to PM2.5, this section is included to provide
information on the full range of

components that commonly make up fine particulate matter.
    The main anthropogenic sources of inorganic (or crustal) particles
are: entrainment by vehicular traffic on unpaved or paved roads;
mechanical disturbance of soil by highway, commercial, and residential
construction; and agricultural field operations (tilling, planting and
harvesting). Industrial processes such as quarries, minerals
processing, and agricultural crop processing can also emit crustal
materials. While much of these emissions are coarse PM, the size
distribution can have a tail of particles smaller than
PM2.5.
    In general, coarse PM is most important close to the source, and
not generally a significant contributor to regional scale PM problems.
Even so, during certain high wind events, fine crustal PM has been
shown to be transported over very long distances.
    Emission estimates of mechanically suspended crustal PM from
sources within the U.S. are often quite high. However, this PM is often
released very close to the ground, and with the exception of windblown
dust events, thermal or turbulent forces sufficient to lift and
transport these particles very far from their source are not usually
present. Thus, crustal material is only a minor part of
PM2.5 annual average concentrations.
    Primary carbonaceous particles are largely the result of incomplete
combustion of fossil or biomass fuels. This incomplete combustion
usually results in emissions of both black carbon and organic carbon
particles. High molecular weight organic molecules (i.e., molecules
with 25 or more carbon atoms) are either emitted as solid or liquid
particles, or as gases that rapidly condense into particle form. These
heavy organic molecules sometimes are referred to as volatile organic
compounds, but because their characteristics are most like direct PM
emissions, they will be considered to be primary emissions for the
purposes of this regulation. Primary organic carbon also can be formed
by condensation of semi-volatile compounds on the surface of other
particles.
    The main combustion sources emitting carbonaceous PM2.5
are certain industrial processes, managed burning, wildland fires, open
burning of waste, residential wood combustion, coal and oil-burning
boilers (utility, commercial and industrial), and mobile sources (both
onroad and nonroad). Certain organic particles also come from natural
sources such as decomposition or crushing of plant detritus. Most
combustion processes emit more organic particles than black carbon
particles. A notable exception to this is diesel engines, which
typically emit more black carbon particles than organic carbon. Because
photochemistry is typically reduced in the cooler winter months for
much of the country, studies indicate that the carbon fraction of PM
mass in the winter months is likely dominated by direct PM emissions as
opposed to secondarily formed organic aerosol.
    Particles from the earth's crust may contain a combination of
metallic oxides and biogenic organic matter. The combustion of surface
debris will likely entrain some soil. Additionally, emissions from many
processes and from the combustion of fossil fuels contain elements that
are chemically similar to soil. Thus, a portion of the emissions from
combustion activities may be classified as crustal in a compositional
analysis of ambient PM2.5. The proposed rule required that
States address the direct emissions of particulate matter in their
PM2.5 attainment plans. During the comment period, EPA
received several comments regarding the definition of what should be
regulated as ``direct PM2.5.''
b. Final Rule
    This rule defines direct PM2.5 emissions as ``air
pollutant emissions of direct fine particulate matter, including
organic carbon, elemental carbon, direct sulfate, direct nitrate, and
miscellaneous inorganic material (i.e. crustal material).'' Development
of attainment plans will include direct PM2.5 emissions and
specific PM2.5 attainment plan precursors.
c. Comments and Responses
    Comment: A few commenters noted that 40 CFR 51.1000 of the proposed
rule includes definitions for both ``direct PM2.5
emissions'' and for ``PM2.5 direct emissions.'' They
recommend including just one definition in the final rule.
    Response: The EPA acknowledges this oversight and has included in
the final rule a single definition for ``direct PM2.5
emissions.'' It reads: ``Direct PM2.5 emissions means solid
particles emitted directly from an air emissions source or activity, or
gaseous emissions or liquid droplets from an air emissions source or
activity which condense to form particulate matter at ambient
temperatures. Direct PM2.5 emissions include elemental
carbon, directly emitted organic carbon, directly emitted sulfate,
directly emitted nitrate, and other inorganic particles (including but
not limited to crustal material, metals, and sea salt).''
8. Optional Technical Demonstrations for NOX, VOC, and
Ammonia
    [Section II.E.2 of November 1, 2005 proposed rule (70 FR 65999);
sec. 51.1002 in draft and final regulatory text.]
a. Background
    The proposed rule required States to evaluate and consider control
strategies for sources of SO2 and direct PM2.5
emissions in all nonattainment areas. For the precursors
NOX, VOC, and ammonia, the proposed rule included
presumptive policies that could be reversed with an acceptable
technical demonstration by the State or EPA. (The policy in the
proposal presumptively required that NOX emissions must be
addressed in all areas, and that VOC and ammonia emissions do not need
to be addressed in all areas.) A number of commenters requested
additional guidance on the criteria for an acceptable technical
demonstration.
b. Final Rule
    The final rule retains provisions for the State or EPA to conduct a
technical demonstration to reverse the presumptive inclusion of
NOX or to reverse the presumptive exclusions of ammonia and
VOC as PM2.5 attainment plan precursors. Demonstrations to
reverse the presumptions for ammonia, VOC, or NOX are to be
based on the weight of evidence of available information, and any
demonstration by the State must be approved by EPA. The State must
demonstrate that based on the sum of available technical and scientific
information, it would be appropriate for a nonattainment area to
reverse the presumptive approach for a particular precursor. The
demonstration should include information from multiple sources,
including results of speciation data analyses, air quality modeling
studies, chemical tracer studies, emission inventories, or special
intensive measurement studies to evaluate specific atmospheric
chemistry in an area.
    Because of the variation among nonattainment areas in terms of such
factors as local emissions sources, growth patterns, topography, and
severity of the nonattainment problem, EPA believes that it would not
be appropriate to define a prescriptive set of analyses that must be
included in all PM2.5 precursor technical demonstrations.
The key criterion is that any technical demonstration must fairly
represent available information.
    In developing the implementation plan for a nonattainment area, the
State should use all relevant information

available (from EPA, the State, or other sources) to determine the
scientifically most appropriate approach to regulating NOX,
ammonia, and VOC emissions in the area. As required under any State
rulemaking process, the State must consider and provide a response in
the record to any information or evidence brought forward by commenters
during the SIP planning, development and review process which indicates
that the presumption for a precursor should be reversed. In its review
of the forthcoming State implementation plan submittal, EPA will review
the State's proposed precursor policies in light of all currently
available information. If information brought forward by commenters or
the State in the SIP development process shows that the presumption in
this rule for ammonia, VOC or NOX is not technically
justified for a particular nonattainment area, the State must conduct a
technical demonstration to reverse the presumption. In the case of
ammonia or VOC, the State then would evaluate control measures and
implement those measures that are technically and economically feasible
and that will contribute to expeditious attainment of the standards.
    In the section below we suggest examples of the types of analyses
that would be appropriate to use in developing such a demonstration.
States are encouraged to consult with EPA in formulating appropriate
technical demonstrations.
    i. Emission Inventory Information: An analysis might show that a
precursor composes a significant fraction of the emissions inventory in
an area and therefore requires greater consideration.

    Example: Several stationary sources emitting particular VOCs
known to contribute to SOA formation make up a significant portion
of the area's VOC inventory. This analysis may be useful in
conjunction with other analyses included in a weight of evidence
demonstration.

    ii. Speciation Data Information: Analysis of data from speciation
networks might lead a State to determine the relative importance of a
precursor to seasonal or yearly average PM concentrations. Individual
precursors require different approaches. Collection of new data could
be used to understand the impacts of precursors in an area.

    Example: Nitrate ion is a large portion of winter average
PM2.5 mass. Nitrate ion is a major portion of
PM2.5 mass on the 10 highest PM2.5 days in
winter in the past 3 years. The days with the highest mass
concentrations might be indicative of inversion conditions and/or
local impacts, rather than large-scale transport processes. For
these reasons, nitrate should be addressed in the PM2.5
attainment plan.
    Example: Ammonium ion data combined with total calculated
nitrate data indicates that reductions in ammonia would reduce PM
concentrations without a sharp related increase in particle acidity.
PM speciation data shows that PM in the area is generally within 10%
of calculated neutralization. In places for which the needed
atmospheric data are available to determine whether increased
acidity is estimated to lead to negative environmental effects,
analysis showing that increased acidity of particles and
precipitation would likely result from ammonia reductions would
support the presumption against ammonia regulation. Analysis showing
that ammonia reductions would be unlikely to increase the acidity of
particles and precipitation, and that potential reductions in
ammonia would significantly reduce PM2.5 levels, would
support a technical demonstration to reverse the presumption.

    iii. Modeling Information: Results of atmospheric modeling may help
a State characterize the impacts of potential precursor emission
reductions on PM2.5 concentrations in an area.

    Example: Modeling of SO2, NOX, and VOC
emission reductions result in lower sulfate and nitrate levels but
not lower secondary organic aerosol levels. This likely indicates
that VOC reductions are not as vital as reductions of the other
precursors.
    Example: Modeled reductions of NOX show a potential
increase in sulfate formation through disruption of the ozone cycle.
SO2 reductions may be a better choice than NOX
reductions.
    Example: Modeled ammonia reductions show a projected reduction
in PM2.5 concentrations in selected areas. Although
dependant on good quality inventory data, this type of an analysis
would indicate that the area is ammonia-limited and that ammonia
reductions may be beneficial.
    Example: Modeling shows that reductions in SO2 in the
absence of NOX reductions in an area will not result in a
significant PM2.5 reduction because more nitrate
particles form when less SO2 is available for particle
formation. However, PM2.5 reductions are significant when
both SO2 and NOX are reduced concurrently.
This analysis would indicate that NOX reductions should
be included in the PM2.5 attainment plan for the area.

    iv. Monitoring, Data Analysis, or Other Special Studies: Could
include monitoring of gases and compounds not typically monitored under
the PM2.5 speciation network, receptor modeling analysis, or
special monitoring studies.

    Example: Data from specialized monitoring studies can provide
insights about concentrations of ammonia gas and nitric acid in an
area and whether the area is ammonia-limited or not. Ammonia
reductions in ammonia-limited areas typically yield reductions in
PM2.5 concentrations. Specialized monitoring and
laboratory studies can also assess the relative concentrations of
organic compounds and provide insights into the contributions of
different anthropogenic and biogenic VOCs to secondary organic
aerosol formation.
    Example: Receptor modeling and statistical analysis
PM2.5 speciation monitoring data can indicate relative
contributions to PM2.5 mass from sources with different
chemical ``fingerprints.''
    Example: Additional analysis of organic compounds on filters
collected through speciation monitoring may reveal insights about
the relative degree of carbonaceous material considered to be from
fossil fuel combustion as opposed to combustion of ``modern''
material (such as wood or biomass).
c. Comments and Responses
    Comment: A number of commenters requested that the final rule
include guidance on acceptable technical demonstrations.
    Response: The above section includes examples designed to help
States formulate appropriate demonstrations. Prescribing specific
technical indicators to be used in all areas would ignore the
scientific uncertainty inherent in the relationships between precursor
emissions and the responses of atmospheric concentrations of
PM2.5. Therefore, States are encouraged to review available
information and consult with EPA in formulating technical
demonstrations appropriate to a particular area.

B. No Classification System

1. No Classification System
a. Background
    Section 172 of subpart 1 contains the general requirements for SIPs
for all nonattainment areas. Section 172(a)(1) states that on or after
the date of designation, the Administrator may classify an area for the
purpose of applying an attainment date or for some other purpose. Thus,
a classification system is allowed under section 172 of the CAA, but is
not required for the purposes of implementing a national ambient air
quality standard. The CAA also states that EPA may consider certain
factors in making a decision concerning classification for areas, such
as the severity of nonattainment in such areas, and the availability
and feasibility of the pollution control measures that may be needed to
achieve attainment. In the proposed rule, EPA provided two
implementation approaches for classifying PM2.5
nonattainment areas. Under the first approach, there would be no
classification system. Under the second approach, a two-tiered
classification system would apply, with areas classified as either
``moderate'' or ``serious'' based on specific criteria.

    For example, the two classification tiers could be based on the
severity of nonattainment (e.g., serious areas would be those with a
design value above a specific threshold), or on the attainment date for
the area (e.g., serious areas would be those with attainment dates
after April 2010). However, any moderate area that needs an attainment
date longer than 5 years would be reclassified as serious. This would
ensure that areas with a more persistent PM2.5 problem are
subject to more stringent requirements, even if they are not one of the
areas with the highest current design values. For such areas, the State
would be required to request reclassification for an area and ensure
that the 2008 attainment SIP submission for the area includes all
measures needed to meet the serious area requirements. Under the two
tiered classification approach, we proposed that serious
PM2.5 nonattainment areas would be required to meet the more
stringent requirements than moderate areas that would be defined in
this rulemaking action (e.g., lower thresholds for RACT, fixed
percentage reduction for RFP, etc.). For serious areas, the attainment
date would be as expeditious as practicable, but no later than 10 years
after designation, depending on the year in which the area would be
projected to attain considering existing control requirements and the
effect of RACM, RACT and RFP.
b. Final Rule
    The EPA believes that in the case of PM2.5, the no-
classification approach is the most appropriate approach. An advantage
of this approach is that it provides a relatively simple implementation
structure for State implementation of the PM2.5 standards,
and avoids the need to define a classification system and determine
classifications for each area. Without classifications, this rule still
requires that that SIPs include all reasonable measures that contribute
to achieving attainment as expeditiously as practicable. (Further
detail is provided in sections D. and F. below.) Because of differences
in the nature and sources of the PM2.5 problem in different
parts of the country, EPA did not find it appropriate to establish a
tiered classification system with increasing control measure
requirements. The no-classifications approach provides States with
greater flexibility to determine the control strategies that will be
most effective and efficient in bringing specific areas into attainment
as expeditiously as practicable. In addition, EPA believes that States
requesting additional time to attain the standard beyond the initial 5
year attainment date, provided for under Subpart I, will need to adopt
additional or more stringent measures to meet their obligations for
RACT, RACM and attainment that is as expeditious as practicable. We
believe that this addresses the main concerns of those commenters who
contend that a two tiered classification system should be implemented.
c. Comments and Responses
    Comment: The majority of the commenters who commented on this issue
stated that they agreed with EPA's preferred no classification
approach. These commenters generally stated that they believed that EPA
has the authority not to establish a classification system for
PM2.5 nonattainment areas. Some commenters stated that it
would also be unreasonable, at this point in the process, for EPA to
implement a classification scheme for the PM2.5 standard.
Many commenters support the no classification approach because it
provides for a simple implementation structure and/or allows greater
implementation flexibility to States, including flexibility to address
specific problems related to individual nonattainment areas in the most
cost-effective and expeditious manner, rather than through a one size
fits all approach. Other commenters stated that they believe that a
classification system is not needed because nonattainment areas in the
Eastern United States are likely to attain the standard within a
timeframe that is consistent with the timeframe established under
Subpart 1.
    Response: The EPA agrees with these commenters.
    Comment: Several commenters disagreed with EPA's preferred approach
and agreed with the two tiered classification approach featuring a
``moderate'' and a ``serious'' area classification. These commenters
also stated that the threat of reclassification or ``bump up'' to a
higher classification was a powerful incentive for areas to attain as
expeditiously as practicable. Commenters also indicated that areas
needing more time to attain the standard should be required to
implement more stringent measures or mandatory measures.
    Response: The EPA agrees that areas with more severe nonattainment
problems will need to implement more stringent measures to attain.
However, EPA does not believe that a classification system is needed to
ensure that such measures are implemented. The EPA believes that on
balance the no classification approach is the most appropriate
classification option for the implementation of the PM2.5
standard because of the difference in contributing sources from area to
area.
    Comment: Several commenters stated that under EPA's preferred
approach, each State would be required to submit an attainment
demonstration proposing an attainment date that is ``as expeditious as
practicable'' for each area. They asserted that to allow States to
propose their own attainment dates would invite delay in the process of
cleaning up fine particle pollution. These commenters further stated
that States would have no incentive to set an attainment date earlier
than the outer limit set by EPA, even if it would be practicable to
attain the NAAQS sooner.
    Response: Section 172 of the CAA requires SIPs to demonstrate
attainment as expeditiously as practicable regardless of whether there
is a classification system, and under this rule states must justify
that their attainment date is as expeditious as practicable considering
all reasonable measures. As noted above, EPA believes that States
requesting additional time to attain the standard beyond the initial 5
year attainment date will need to adopt additional or more stringent
measures to meet their obligations for RACT and RACM and to attain as
expeditiously as practicable. More details on the analytical process
required for an attainment demonstration is included in section II.F.
    Comment: Several commenters stated that the CAA requires regulation
of the PM2.5 standard under Subpart 4 of Part D. These
commenters state that EPA takes the position that it must regulate
PM2.5 under Subpart 1 of the CAA, which applies to
nonattainment areas in general. The commenters state that section 7513,
in Subpart 4 of Part D of the CAA, contains specific provisions for
classification of particulate matter nonattainment areas, and that EPA
must therefore regulate PM2.5 under Subpart 4, which
requires a moderate and serious area classification system. Other
commenters argued that implementation of the PM2.5 standard
must proceed under Subpart 1 of Part D of Title I of the CAA and cannot
be governed by Subpart 4 of Part D, which addresses the implementation
of the PM10 standard which is a different pollutant than
PM2.5.
    Response: The EPA finds that the PM2.5 standard should
be implemented under subpart I of the CAA, which is the general
provision of the CAA related to NAAQS implementation. Part D of Title I
of the CAA sets forth the requirements for SIPs needed to attain the
national ambient air quality standards. Part D also includes a general
provision under

Subpart I which applies to all NAAQS for which a specific subpart does
not exist. Because the PM2.5 standards were not established
until 1997, the plan provisions found in section 172 of subpart 1
pertaining to plans for nonattainment areas apply. The EPA further
agrees with comments stating that subpart 4 on its face applies only to
the PM10 standard. In general, the emphasis in subpart 4 on
reducing PM10 concentrations from certain sources of direct
PM2.5 emissions can be somewhat effective in certain
PM2.5 nonattainment areas but not in all. Contributions to
PM2.5 concentrations are typically from a complex mix of
sources of primary emissions and sources of precursor emissions which
form particles through reactions in the atmosphere. PM2.5
also differs from PM10 in terms of atmospheric dispersion
characteristics, chemical composition, and contribution from regional
transport.
2. Rural Transport Classification Option
a. Background
    The 8-hour ozone implementation program includes a ``rural
transport classification'' for subpart 1 nonattainment areas. In the
proposal for this rule we discussed whether an area classification of
this type would be appropriate for the PM2.5 implementation
program in light of the fact that no currently designated
PM2.5 nonattainment area met the criteria similar to those
that apply to rural transport areas under the ozone implementation
program.
    As addressed in the proposal, a PM2.5 nonattainment area
would qualify for the ``rural transport'' classification if it met
criteria similar to those specified for rural transport areas for the
1-hour ozone standard under section 182(h). Section 182(h) defines
``rural transport'' areas as those areas that do not include, and are
not adjacent to, any part of a Metropolitan Statistical Area (MSA) or,
where one exists, a Consolidated Metropolitan Statistical Area (CMSA).
Section 182(h) further limits the category to those areas whose own
emissions do not make a significant contribution to pollutant
concentrations in those areas, or in other areas.
    As discussed in the preamble to the proposed rule, potential
criteria for a State to identify an area for a rural transport
classification under the PM2.5 program could be similar to
the criteria used in the ozone implementation program: A State with a
PM2.5 ``rural transport'' area would need to (1) demonstrate
that the area meets the above criteria, (2) demonstrate using EPA
approved attainment modeling that the nonattainment problem in the area
is due to the ``overwhelming transport'' of emissions from outside the
area, and (3) demonstrate that sources of PM2.5 and its
precursor emissions within the boundaries of the area do not contribute
significantly to PM2.5 concentrations that are measured in
the area or in other areas.
    An area which qualifies for the ``rural transport'' classification
would only be required to adopt local control measures sufficient to
demonstrate that the area would attain the standard by its attainment
date ``but for'' the overwhelming transport of emissions emanating from
upwind States. RFP requirements under subpart 1 would still apply to
these areas. As with other nonattainment areas, rural transport
nonattainment areas would be subject to NSR, transportation conformity,
and general conformity requirements. In the proposal we solicited
comments on whether it would be appropriate to establish less
burdensome NSR requirements in the event that a classification for
rural transport areas is adopted in the final rule. The EPA requested
comment on whether this type of classification option is needed at all
under the PM2.5 implementation program.
b. Final Rule
    The final rule does not include a rural transport classification.
This type of classification was included in the CAA for purposes of
implementing the ozone standards because of the phenomenon of the
formation of high ozone levels far downwind in very rural locations,
including on high elevation mountain peaks. In reviewing the currently
designated PM2.5 nonattainment areas, it appears that all
areas but one are within or adjacent to a metropolitan area (i.e. core-
based statistical area or consolidated statistical area), and thus
would not meet the criteria discussed above. Although PM2.5
concentrations are greatly affected by long-range transport of air
pollution, it appears that nonattainment areas typically are located in
urban areas and include significant local pollutant sources.
c. Comments and Responses
    Comment: Several commenters stated that they do not support the
adoption of a rural transport classification because it is not needed.
Commenters stated that given the criteria for the rural transport
classification, which greatly limits its applicability, few if any
PM2.5 nonattainment areas can qualify for the option. One
commenter stated that EPA modeled the rural transport classification
after the ``rural transport areas'' provision contained in subpart 2 of
the CAA, which applies only to the ozone standard. The commenter
further states that neither Subpart 1 nor 4 contain any statutory
authority for such a classification.
    Response: The EPA believes that it has sufficient statutory
authority under the CAA to establish a rural transport classification,
but we do not believe that such a classification is needed.
    Comment: One commenter generally supported the rural transport
concept and the proposed associated requirements, with the addition
that data analysis be included as appropriate in the required technical
demonstrations in addition to modeling. While no PM2.5 area
currently meets the requirements for the rural transport classification
option, several commenters recommended that it be maintained for
potential cases in which the PM2.5 standards are made more
stringent, or measured air quality in areas change in such a way that
areas would qualify for the rural transport classification at a later
date.
    Response: The EPA does not agree that a rural transport
classification is needed. The EPA will re-evaluate the need for such a
classification as appropriate.

C. Due Dates and Basic Requirements for Attainment Demonstrations

a. Background
    Part D of Title I of the CAA sets forth the requirements for SIPs
needed to attain the national ambient air quality standards. Part D
includes a general subpart 1 which applies to all NAAQS for which a
specific subpart does not exist. The 1990 CAA Amendments do not include
any subpart for PM2.5 because the PM2.5 standards
were not yet established. The EPA has determined that for
PM2.5, the nonattainment area plan provisions found in
section 172 of subpart 1 apply.
    Section 172(b) of the CAA requires that at the time the Agency
promulgates nonattainment area designations, the EPA must also
establish a schedule for States to submit SIPs meeting the applicable
requirements of section 172(c) and of section 110(a)(2) of the CAA.
Nonattainment area designations were finalized in December 2004, and a
supplemental notice was issued in April 2005. Consistent with section
172(b) of the CAA, 40 CFR 51.1002 of the proposed rule requires the
State to submit its attainment demonstration and SIP revision within 3
years, or by April 2008.
    Section 51.1006 of the proposed rule addresses the situation in
which an area

is initially designated as attainment/unclassifiable but is later
designated as nonattainment based on air quality data after the 2001-
2003 period. Under such circumstances, the SIP submittal date would be
3 years from the effective date of the redesignation, and the
attainment date would be as expeditiously as practicable but no later
than 5 years from the effective date of the redesignation.
    The section 172(c) requirements that States are to address under
section 172(c) (including RACT, RACM, RFP, contingency measures,
emission inventory requirements, and NSR) are discussed in later
sections of this document. Section 110(a)(2) of the CAA requires 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, 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 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). In order
to assist States in addressing their obligations regarding regionally
transported pollution, EPA has finalized the CAIR to reduce
SO2 and nitrogen oxide emissions from large electric
generating units.\14\
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    \14\ More information on the Clean Air Interstate Rule (CAIR) is
available at: http://www.epa.gov/cair.

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    To date, few states have submitted a SIP revision addressing the
section 110(a)(2) requirements for the purposes of implementing the
PM2.5 standards. The EPA recognizes that this situation is
due in part to the fact that there were a series of legal challenges to
the PM standards which were not resolved until March 2002, at which
time the standards and EPA's decision process were upheld (see section
I.B. for further discussion of past legal challenges to the standards).
To address the States' continuing obligation to address the
requirements of section 110(a), 40 CFR 51.1002 of the proposed rule
also required each State to address the required elements of section
110(a)(2) of the CAA as part of the SIP revision adopting its
attainment plan, if it has not already done so. On March 10, 2005, EPA
entered into a consent decree with Environmental Defense and American
Lung Association concerning EPA's failure to find that States failed to
submit SIPs to address the section 110(a)(2) requirements. As a part of
that consent decree, by no later than October 8, 2008, EPA is required
to publish a notice in the Federal Register related to its
determinations of whether each State has submitted SIPs for
PM2.5 that meet the requirements as stated under section
110(a)(2) of the CAA.
b. Final Rule
    The final rule maintains the regulatory approach described above.
c. Comments and Responses
    There were no comments on this portion of the proposal.

D. Attainment Dates

1. Background on Statutory Requirements
    Establishing attainment dates. Section 172(a)(2) states that an
area's attainment date ``shall be the date by which attainment can be
achieved as expeditiously as practicable, but no later than 5 years
from the date such area was designated nonattainment * * *, except that
the Administrator may extend the attainment date to the extent the
Administrator determines appropriate, for a period no greater than 10
years from the date of designation as nonattainment considering the
severity of nonattainment and the availability and feasibility of
pollution control measures.''
    Since PM2.5 designations have an effective date of April
5, 2005, the initial 5-year attainment date for PM2.5 areas
would be no later than April 5, 2010. For an area with an attainment
date of April 5, 2010, EPA would determine whether it had attained the
standard by evaluating air quality data from the three previous
calendar years (i.e. 2007, 2008, and 2009).
    For any areas that are granted the full 5 year attainment date
extension under section 172, the attainment date would be no later than
April 5, 2015. For such areas, EPA would determine whether they have
attained the standard by evaluating air quality data from 2012, 2013,
and 2014. Section 51.1004 of the proposed regulations addressed the
attainment date requirement. Section 51.1004(b) provided that in their
attainment demonstrations, States would propose an attainment date
representing attainment as expeditiously as practicable based upon
implementation of existing Federal and State measures, and all new
reasonable local and intrastate measures. The EPA would approve a
particular attainment date based on its review of the attainment
demonstration.
    Determining Whether an Area Has Attained. The EPA has the
responsibility for determining whether a nonattainment area has
attained the standard by its applicable attainment date. Section
179(c)(1) of the Act requires EPA to make determinations of attainment
no later than 6 months following the attainment date for the area.
Under section 179(c)(2), EPA must publish a notice in the Federal
Register identifying those areas which failed to attain by the
applicable attainment date. The statute further provides that EPA may
revise or supplement its determination of attainment for the affected
areas based upon more complete information or analysis concerning the
air quality for the area as of the area's attainment date.
    Section 179(c)(1) of the Act provides that the attainment
determination for an area is to be based upon an area's ``air quality
data as of the attainment date.'' The EPA will make the determination
of whether an area's air quality is meeting the PM2.5 NAAQS
by the applicable attainment date primarily based upon data gathered
from the air quality monitoring sites which have been entered into
EPA's Air Quality System (AQS) database. No special or additional SIP
submittal will be required from the State for this determination.
    A PM2.5 nonattainment area's air quality status is
determined in accordance with appendix N of 40 CFR part 50. To show
attainment of the 24-hour and annual standards for PM2.5,
the most recent three consecutive years of data prior to the area's
attainment date must show that PM2.5 concentrations over a
three-year period are at or below the levels of the standards. A
complete year of air quality data, as described in part 50, Appendix N,
comprises of all 4 calendar quarters with each quarter containing data
from at least 75 percent of the scheduled sampling days. The annual
standard for PM2.5 is attained when the 3-year average
annual mean concentration is less than or equal to 15.05 [mu]g/
m3. The 24-hour standard for PM2.5 is met when
the average of 98th percentile values for three consecutive calendar
years at each monitoring site is less than or equal to 65.5 [mu]g/
m3.
    The EPA will begin processing and analyzing data related to the
attainment of PM2.5 areas immediately after the applicable
attainment date for the affected areas. Current EPA policy, under 40
CFR part 58, sets the deadline for submittal of air quality data into
the AQS database for no later than 90 days after the end of the
calendar year.

    While EPA may determine that an area's air quality data indicates
that an area may be meeting the PM2.5 NAAQS for a specified
period of time, this does not eliminate the State's responsibility
under the Act to adopt and implement an approvable SIP. If EPA
determines that an area has attained the standard as of its attainment
date, the area will remain classified as nonattainment until the State
has requested, and EPA has approved, redesignation to attainment for
the area.
    In order for an area to be redesignated as attainment, the State
must comply with the five requirements listed under section
107(d)(3)(E) of the Act. This section requires that:

--EPA has determined that the area has met the PM2.5 NAAQS;
--EPA has fully approved the state's implementation plan;
--The improvement in air quality is due to permanent and enforceable
reductions in emissions;
--EPA has fully approved a maintenance plan for the area;
--The State(s) containing the area have met all applicable requirements
under section 110 and part D.
2. Establishing Attainment Dates
a. Background
    The EPA proposed rule language on attainment dates that closely
tracks the statutory language. In the preamble, EPA noted that the
attainment date that is as expeditious as practicable should reflect
the projected impact of existing national and State programs (e.g.
partial implementation of the CAIR rule, final Acid Rain Program, motor
vehicle tier II standards and heavy-duty diesel engine standards,
NOX SIP call, State legislation such as Clean Smokestacks
bill in North Carolina) as well as additional reasonable measures
required for the PM2.5 nonattainment SIP.
    With respect to its authority to extend an area's date beyond 5
years, EPA stated in the preamble that the State can submit a SIP
demonstrating that it is impracticable to attain by the 5-year
attainment date: ``As stated previously, under section 172(a)(2)(A),
EPA may grant an area an extension of the initial attainment date for a
period of one to 5 years. States that request an extension of the
attainment date under this provision of the CAA must submit a SIP by
April 5, 2008 that includes, among other things, an attainment
demonstration showing that attainment within 5 years of the designation
date is impracticable. It must also show that the area will attain the
standard by an alternative date that is as expeditious as practicable,
but in no case later than 10 years after the designation date for the
area (i.e. by April 5, 2015 for an area with an effective designation
date of April 5, 2005). An appropriate extension in some cases may be
only 1 or 2 years--a 5-year extension is not automatic upon request.
    The attainment demonstration must provide sufficient information to
show that attainment by the initial attainment date is impracticable
due the severity of the nonattainment problem in the area, the lack of
available control measures, and any other pertinent information related
to these statutory criteria. States requesting an extension of the
attainment date must also demonstrate that all local control measures
that are reasonably available and technically feasible for the area are
currently being implemented to bring about expeditious attainment of
the standard by the alternative attainment date for the area. The
State's plan will need to project the emissions reductions expected due
to Federally enforceable national standards, State regulations, and
local measures such as RACT and RACM, and then conduct modeling to
project the level of air quality improvement in accordance with EPA's
modeling guidance. The EPA will not grant an extension of the
attainment date beyond the initial 5 years required by section
172(a)(2)(A) for an area if the State has not considered the
implementation of all RACM and RACT local control measures for the area
(see section III.I for a more detailed discussion of RACT and RACM).
The EPA also will examine whether the State has adequately considered
measures to address intrastate transport of pollution from sources
within its jurisdiction. In attainment planning, States have the
obligation and authority to address the transport of pollution from one
area of the State to another. Any decision made by EPA to extend the
attainment date for an area beyond its original attainment date will be
based on facts specific to the nonattainment area at issue and will
only be made after providing notice in the Federal Register and an
opportunity for the public to comment.''
b. Final Rule
    We are adopting the approach described above from the proposed
rule. We also wish to clarify language that was in the preamble to the
proposed rule regarding the criteria for an extension. The preamble
stated that attainment date extensions would be based on the two
statutory extension criteria--``the severity of nonattainment, and the
availability and feasibility of pollution control measures,''--as well
as ``other pertinent information which shows that additional time is
required for the area to attain the standard.'' The CAA does not
include this third clause and the regulatory text for the final rule
does not include this third clause. The intent of this language in the
preamble to the proposal was that States could include ``other
pertinent information'' related to the two statutory criteria.
c. Comments and Responses
    Comment: Some commenters expressed concern that EPA's preamble
language appeared to assert a new basis for granting extensions not
provided by the statute. They said EPA has authority to extend the
attainment date under Section 7502(a)(2) based solely on consideration
of two enumerated factors: the severity of nonattainment, and the
availability and feasibility of control measures.
    Response: The EPA agrees that extensions must be based upon the two
factors in the statute, which are quite broad. A clarification of the
preamble phrase cited by the commenter is provided above. The phrase in
question--``any other pertinent information which shows that additional
time is required for the area to attain the standard''--refers to
information that relates to the two statutory factors.
    Comment: One commenter stated that an area should qualify for an
extension only if the area will implement stringent local controls, yet
still cannot practicably attain by the five-year deadline. The
commenter stated that at a minimum, EPA must require states to adopt
RACM for both mobile and stationary sources before granting an
extension. Another commenter said that given the difficulty many areas
will have in meeting the five-year deadline for attainment of the
PM2.5 NAAQS (and especially in light of the fact that the
deadline occurs only 2 years after states are to submit attainment
SIPs), EPA should provide maximum flexibility in allowing extensions to
the full 10-year period.
    Response: The EPA agrees that extensions should be granted only if
an area cannot practicably attain within 5 years despite application of
all reasonable measures, including RACM. Although some measures can be
implemented within a year or two, many measures require a longer period
for installation of controls or full program implementation. In light
of the limited time period between the SIP submittal deadline and the
5-year date, EPA believes that a significant number of areas may
warrant extensions ranging from one to 5 years, with the length of

extension depending on the factors described above.
    Comment: One commenter advocated that EPA include in this final
rule a determination of those areas for which attainment within 5 years
is impracticable. Another commenter advocated that EPA establish
guidance based on EPA national modeling conducted last year to
establish 2015 as constituting expeditious attainment for certain
areas.
    Response: The EPA is not determining in this rulemaking the areas
that should receive extensions or should receive the maximum 10-year
attainment date, for several reasons. First, EPA did not propose such
an approach. Therefore, the public has not had the opportunity to
comment on the approach or on the technical information on which EPA
would make such judgments.
    Second, EPA believes that modeling being conducted by the states,
with updated inventories and finer grids, will generally provide a more
reliable basis for projecting future PM2.5 base case levels
than national modeling conducted by EPA with older information. State
modeling of future year PM2.5 levels that has been conducted
to date indicates that some areas will start closer or farther from the
standard than EPA had projected.
    Third, the SIP process provides a forum for states to identify
reasonable controls and conduct analyses to determine the appropriate
attainment date for an area. This process provides for input from
expert stakeholders, the general public, other states which may share
the same multi-State nonattainment area, and EPA on decisions regarding
controls and attainment dates. At this time, EPA does not have the
benefit of this process to inform a judgment as to when areas can
practicably attain. States are responsible for developing RACM
demonstrations; at this time, EPA lacks the information to conduct a
credible RACM demonstration for all PM2.5 nonattainment
areas.
    Fourth, no State commenter advocated that EPA attempt to make these
judgments on attainment dates in advance of the State SIP process. The
statute gives the states the lead in developing State implementation
plans.
    Comment: Another commenter recommends that an area should receive
an attainment date extension when collectively the following conditions
have been met:
     It is proven through modeling that the region is adversely
effected by transport of PM2.5 emissions from up wind
sources beyond that State's control;
     A State has submitted and committed to implementing all
Federal PM2.5 emission reduction requirements in a timely
manner; and,
     The extension concept is approved through the State air
agency or through the MPO Interagency Consultation Process at the MPO
level if applicable.
    Response: This commenter advocates for attainment date extensions
without any consideration of reasonable local measures. As stated
above, EPA believes that extensions should be granted only if an area
cannot practicably attain within 5 years despite application of all
reasonable measures, including RACM. Although some measures can be
implemented within a year or two, many measures may require a longer
period for installation of controls or full program implementation. In
light of the limited time period between the SIP submittal deadline and
the 5-year date, EPA believes that a significant number of areas may
warrant extensions ranging from one to 5 years, with the length of
extension depending on the factors described above.
3. Attainment Dates: 1-Year Extensions
a. Background
    Subpart 1 provides for States to request two 1-year extensions of
the attainment date for a nonattainment area under limited
circumstances. Section 172(a)(2)(C) of the CAA provides that EPA
initially may extend an area's attainment date for 1 year, provided
that the State has complied with all the requirements and commitments
pertaining to the area in the applicable implementation plan, and
provided that the area has had no more than a minimal number of
``exceedances'' of the relevant standard in the preceding year. Because
the PM2.5 standards do not have exceedance-based forms but
are based on 3-year averaging periods, we interpret the air quality
test in 40 CFR 51.1005 to mean that the area would need to have ``clean
data'' for the third of the 3 years that are to be evaluated to
determine attainment.\15\ By this we mean that for the third year, the
air quality for all monitors in the area as analyzed in accordance with
Appendix N to 40 CFR part 50 each must have an annual average of 15.0
[mu]g/m\3\ or less, and a 98th percentile of 24-hour monitoring values
of 65 [mu]g/m\3\ or less in order to qualify for a 1-year extension.
(Given the rounding provisions specified in 40 CFR part 50, Appendix N,
these criteria would be satisfied if the concentrations before final
rounding are less than an annual average of 15.05 [mu]g/m\3\ and a 24-
hour value of 65.5 [mu]g/m\3\.)
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    \15\ See section 51.1005 of the proposed regulation.
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    For example, suppose an area in violation of the annual standard
has an attainment date of April 2010, and its annual average for 2007
was 15.8 and for 2008 was 15.6. If the annual average for the area in
2009 is 14.9, then the 3-year average would be 15.4, and it would not
have attained the standard. We interpret section 172(a)(2)(C) as
allowing the area to submit a request to EPA for a 1-year extension of
its attainment date to 2011 (provided the State has also complied with