Implementation of the New Source Review (NSR) Program for Particulate Matter Less Than 2.5 Micrometers (PM2.5, 28321-28350 [E8-10768]

Agencies

• ENVIRONMENTAL PROTECTION AGENCY

[Federal Register Volume 73, Number 96 (Friday, May 16, 2008)]
[Rules and Regulations]
[Pages 28321-28350]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: E8-10768]


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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Parts 51 and 52

[EPA-HQ-OAR-2003-0062; FRL-8566-1]
RIN 2060-AN86


Implementation of the New Source Review (NSR) Program for 
Particulate Matter Less Than 2.5 Micrometers (PM2.5)

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: The EPA is finalizing regulations to implement the New Source 
Review (NSR) program for fine particulate matter (that is, particles 
with an aerodynamic diameter less than or equal to a nominal 2.5 
micrometers,

[[Page 28322]]

generally referred to as ``PM2.5''). The NSR program was 
created by the Clean Air Act (Act or CAA) to ensure that stationary 
sources of air pollution are constructed or modified in a manner that 
is consistent with air quality goals in the area.
    The Clean Air Fine Particle Implementation Rule, which was proposed 
in the Federal Register on November 1, 2005, included requirements and 
guidance for State and local air pollution agencies to follow in 
developing State implementation plans (SIPs) and also the NSR 
provisions. The final implementation rule that was promulgated on April 
25, 2007, included all the SIPs related provisions. In this rulemaking, 
EPA is finalizing the NSR provisions of the November 1, 2005 proposed 
rule including the major source threshold, significant emissions rate, 
and offset ratios for PM2.5, interpollutant trading for 
offsets and applicability of NSR to PM2.5 precursors.

DATES: This final rule is effective on July 15, 2008.

ADDRESSES: The EPA has established a docket for this action under 
Docket ID No. EPA-HQ-OAR-2003-0062. All documents in the docket are 
listed on the www.regulations.gov Web site. Although listed in the 
index, some information may not be publicly available, e.g., CBI or 
other information whose disclosure is restricted by statute. Certain 
other material, such as copyrighted material, is not placed on the 
Internet and will be publicly available only in hard copy form. 
Publicly available docket materials are available either electronically 
through www.regulations.gov or in hard copy at the Air Docket, EPA/DC, 
EPA West, Room 3334, 1301 Constitution Avenue, Northwest, Washington, 
DC. The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday 
through Friday, excluding legal holidays. The telephone number for the 
Public Reading Room is (202) 566-1744, and the telephone number for the 
Air Docket is (202) 566-1742.

FOR FURTHER INFORMATION CONTACT: Mr. Raghavendra (Raj) Rao, Air Quality 
Policy Division, Office of Air Quality Planning and Standards (C504-
03), U.S. Environmental Protection Agency, Research Triangle Park, 
North Carolina 27711, telephone number: (919) 541-5344, facsimile 
number: (919) 541-5509, e-mail address: rao.raj@epa.gov; or Mr. Dan 
deRoeck, at the same address, telephone 919-541-5593, or e-mail at 
deroeck.dan@epa.gov.

SUPPLEMENTARY INFORMATION:

I. General Information

A. Does this action apply to me?

    Entities affected by this rule include sources in all industry 
groups. The majority of sources potentially affected are expected to be 
in the following groups:

------------------------------------------------------------------------
             Industry group                         NAICS \a\
------------------------------------------------------------------------
Electric services......................  221111, 221112, 221113, 221119,
                                          221121, 221122
Petroleum refining.....................  32411
Industrial inorganic chemicals.........  325181, 32512, 325131, 325182,
                                          211112, 325998, 331311, 325188
Industrial organic chemicals...........  32511, 325132, 325192, 325188,
                                          325193, 32512, 325199
Miscellaneous chemical products........  32552, 32592, 32591, 325182,
                                          32551
Natural gas liquids....................  211112
Natural gas transport..................  48621, 22121
Pulp and paper mills...................  32211, 322121, 322122, 32213
Paper mills............................  322121, 322122
Automobile manufacturing...............  336111, 336112, 336712, 336211,
                                          336992, 336322, 336312, 33633,
                                          33634, 33635, 336399, 336212,
                                          336213
Pharmaceuticals........................  325411, 325412, 325413, 325414
------------------------------------------------------------------------
\a\ North American Industry Classification System.

    Entities affected by this rule also include States, local reviewing 
authorities, and Indian country with new and modified major stationary 
sources.

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 in the regulations and standards section of our NSR home page 
located at https://www.epa.gov/nsr.

C. How is this preamble organized?

    The information presented in this preamble is organized as follows:

I. General Information
    A. Does this action apply to me?
    B. Where can I get a copy of this document and other related 
information?
    C. How is this preamble organized?
II. Purpose
III. Background
    A. New Source Review (NSR) Program
    B. Fine Particulate Matter and the NAAQS for PM2.5
    C. Implementation of NSR for PM2.5
IV. Overview of This Final Action
V. Rationale for Final Actions
    A. Applicability of NSR to Precursors of PM2.5 in the 
Ambient Air
    1. What is EPA's legal authority to regulate precursors?
    2. What is EPA's approach for addressing precursors?
    3. Final Action on SO2
    4. Final Action on NOX
    5. Final Action on VOC
    6. Final Action on Ammonia
    B. Major Stationary Source Threshold for PM2.5
    C. Significant Emissions Rate for Direct Emissions of 
PM2.5
    D. Significant Emissions Rates for PM2.5 Precursors
    E. Condensable PM Emissions
    F. Prevention of Significant Deterioration (PSD) Program 
Requirements
    1. How must BACT be implemented for PM2.5?
    2. How does EPA plan to address PM2.5 Increments, 
Significant Impact Levels (SILs), and Significant Monitoring 
Concentrations (SMCs)?
    3. What is the ambient air quality analysis requirement for 
PM2.5?
    4. How must the PSD preconstruction monitoring requirement be 
implemented for PM2.5?
    G. Nonattainment New Source Review (NA NSR) Requirements
    1. What is the required offset ratio for direct PM2.5 
emissions?
    2. Which precursors are subject to the offset requirement?
    3. What is the required offset ratio for PM2.5 
precursors?
    4. Is interpollutant trading allowable to comply with offset 
requirements?
    H. How will the transition to the PM2.5 PSD 
requirements occur?
    1. Background
    2. Transition for ``Delegated States''
    3. Transition for ``SIP-Approved States''
    I. How will the transition to the PM2.5 NA NSR 
requirements occur?
    1. Background
    2. Transition

[[Page 28323]]

    3. Implementation of NSR Under the ``Emissions Offset 
Interpretative Ruling'' (40 CFR part 51, appendix S) with Revisions
    J. Does major NSR apply to PM2.5 precursors during 
the SIP development period?
    K. Are there any Tribal concerns?
    L. What are the requirements for minor NSR for PM2.5?
    M. Rural Transport Areas
VI. 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 and 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. Determination Under Section 307(d)
VII. Statutory Authority

II. Purpose

    The purpose of this rulemaking is to finalize the major NSR program 
provisions for PM2.5. This final rule supplements the final 
implementation rule for PM2.5 (excluding the NSR provisions) 
that we \1\ promulgated on April 25, 2007 at 72 FR 20586. This final 
action on the bulk of the major NSR program for PM2.5 along 
with our proposed rule on increments, SILs, and SMC, when final, will 
represent the final elements necessary to implement a PM2.5 
PSD program. When both rules are promulgated and in effect, the 
PM2.5 PSD program will no longer use a PM10 
program as a surrogate, as has been the practice under our existing 
guidance.
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    \1\ In this proposal, the terms ``we,'' ``us,'' and ``our,'' 
refer to the EPA and the terms ``you,'' and ``your,'' refer to the 
owners or operators of stationary sources of air pollution.
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III. Background

A. New Source Review (NSR) Program

    The existing regulations require both major and minor NSR programs 
to address any pollutant for which there is a National Ambient Air 
Quality Standard (NAAQS) and precursors to the formation of such 
pollutant when identified for regulation by the Administrator. This 
final rule amends the NSR regulations to establish the minimum elements 
for State, local, and Tribal agency programs implementing NSR for the 
PM2.5 NAAQS. This preamble also explains what interim 
provisions would apply with respect to PM2.5 during the 
State Implementation Plan (SIP) development period.
    The NSR program is a preconstruction permitting program that 
applies when a source is constructed or modified. The NSR program is 
composed of three different programs:
     Prevention of Significant Deterioration (PSD);
     Nonattainment NSR (NA NSR); and
     Minor NSR.
We often refer to the PSD and NA NSR programs together as the major NSR 
program because these programs regulate only major sources.\2\
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    \2\ The Act uses the terms ``major emitting facility'' to refer 
to sources subject to the PSD program, and ``major stationary 
source'' to refer to sources subject to NA NSR. See CAA sections 
165, 169, 172(c)(5), and 302(j). For ease of reference, we use the 
term ``major source'' to refer to both terms.
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    The PSD program applies when a major source that is located in an 
area that is designated as attainment or unclassifiable for any 
criteria pollutant is constructed or undergoes a major 
modification.3 4 The NA NSR program applies when a major 
source that is located in an area that is designated as nonattainment 
for any criteria pollutant is constructed or undergoes a major 
modification. The minor NSR program addresses both major and minor 
sources that undertake construction or modification activities that do 
not qualify as major, and it applies regardless of the designation of 
the area in which a source is located.
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    \3\ The term ``criteria pollutant'' means a pollutant for which 
we have set a NAAQS.
    \4\ In addition, the PSD program applies to most noncriteria 
regulated pollutants.
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    The national regulations that apply to each of these programs are 
located in the Code of Federal Regulations (CFR) as shown in the 
following table:

------------------------------------------------------------------------
                Program                       Applicable regulations
------------------------------------------------------------------------
PSD....................................  40 CFR 52.21, 40 CFR 51.166, 40
                                          CFR 51.165(b).
NA NSR.................................  40 CFR 52.24, 40 CFR 51.165, 40
                                          CFR part 51, appendix S.
Minor NSR..............................  40 CFR 51.160-164.
------------------------------------------------------------------------

    The PSD requirements include but are not limited to:
     Installation of Best Available Control Technology (BACT);
     Air quality monitoring and modeling analyses to ensure 
that a project's emissions will not cause or contribute to a violation 
of any NAAQS or maximum allowable pollutant increase (PSD increment);
     Notification of Federal Land Manager of nearby Class I 
areas; and
     Public comment on the permit.
    Nonattainment NSR requirements include but are not limited to:
     Installation of Lowest Achievable Emission Rate (LAER) 
control technology;
     Offsetting new emissions with creditable emissions 
reductions;
     Certification that all major sources owned and operated in 
the State by the same owner are in compliance with all applicable 
requirements under the Act;
     An alternative siting analysis demonstrating that the 
benefits of the proposed source significantly outweigh the 
environmental and social costs imposed as a result of its location, 
construction, or modification; and
     Public comment on the permit.
    Minor NSR programs must meet the statutory requirements in section 
110(a)(2)(C) of the Act, which requires ``* * * regulation of the 
modification and construction of any stationary source * * * as 
necessary to assure that national ambient air quality standards are 
achieved.''

B. Fine Particulate Matter and the NAAQS for PM2.5

    Fine particles in the atmosphere are made up 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 particulate matter (PM) with a nominal aerodynamic 
diameter of 2.5 micrometers or less (a micrometer is

[[Page 28324]]

one-millionth of a meter, and 2.5 micrometers is less than one-seventh 
the average width of a human hair) are considered to be ``fine 
particles,'' and are also known as PM2.5. ``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.
    The health effects associated with exposure to PM2.5 are 
significant. Epidemiological studies have shown a significant 
correlation 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), lung 
disease, decreased lung function, asthma attacks, and certain 
cardiovascular problems. 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 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). We set an annual standard at a level of 
15 micrograms per cubic meter ([mu]g/m\3\) and a 24-hour standard at a 
level of 65 [mu]g/m\3\. At the time we established the primary 
standards in 1997, we also established welfare-based (secondary) 
standards identical to 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.
    On October 17, 2006, we revised the primary and secondary NAAQS for 
PM2.5 and PM10. In that rulemaking, we reduced 
the 24-hour NAAQS for PM2.5 to 35 [mu]g/m\3\ and retained 
the existing annual PM2.5 NAAQS of 15 [mu]g/m\3\. In 
addition, we retained PM10 as the indicator for coarse PM, 
retained the existing PM10 24-hour NAAQS of 150 [mu]g/m\3\, 
and revoked the annual PM10 NAAQS (which had previously been 
set at 50 [mu]g/m\3\). See 71 FR 61236.

C. Implementation of NSR for PM2.5

    After we promulgated the NAAQS for PM2.5 in 1997, we 
issued a guidance document entitled ``Interim Implementation for the 
New Source Review Requirements for PM2.5'' (John S. Seitz, 
EPA, October 23, 1997).\5\ As noted in that guidance, section 165 of 
the Act suggests that PSD requirements become effective for a new NAAQS 
upon the effective date of the NAAQS. Section 165(a)(1) of the Act 
provides that no new or modified major source may be constructed 
without a PSD permit that meets all of the section 165(a) requirements 
with respect to the regulated pollutant. Moreover, section 165(a)(3) 
provides that the emissions from any such source may not cause or 
contribute to a violation of any NAAQS. Also, section 165(a)(4) 
requires BACT for each pollutant subject to PSD regulation. The 1997 
guidance stated that sources would be allowed to use implementation of 
a PM10 program as a surrogate for meeting PM2.5 
NSR requirements until certain difficulties were resolved, primarily 
the lack of necessary tools to calculate the emissions of 
PM2.5 and related precursors, the lack of adequate modeling 
techniques to project ambient impacts, and the lack of PM2.5 
monitoring sites.
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    \5\ Available in the docket for this rulemaking, ID No. EPA-HQ-
OAR-2003-0062, and at https://www.epa.gov/region07/programs/artd/air/
nsr/nsrmemos/pm25.pdf.
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    On April 5, 2005, we issued a guidance document entitled 
``Implementation of New Source Review Requirements in PM-2.5 
Nonattainment Areas'' (Stephen D. Page, EPA).\6\ This memorandum 
provides guidance on the implementation of the nonattainment major NSR 
provisions in PM2.5 nonattainment areas in the interim 
period between the effective date of the PM2.5 NAAQS 
designations (April 5, 2005) and the promulgation date of the final NSR 
regulations reflected in this action. Besides affirming the 
continuation of the Seitz guidance memo in PM2.5 attainment 
areas, the April 5, 2005 memo recommends that until we promulgate the 
PM2.5 major NSR regulations, States should use a 
PM10 nonattainment major NSR program as a surrogate to 
address the requirements of nonattainment major NSR for 
PM2.5.
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    \6\ Available in the docket for this rulemaking, ID. No. EPA-HQ-
OAR-2003-0062, and at https://www.epa.gov/nsr/documents/nsrmemo.pdf.
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    On November 1, 2005, we proposed a rule to implement the 1997 
PM2.5 NAAQS, including proposed revisions to the NSR program 
(70 FR 65984). As discussed above, this action finalizes the portion of 
that proposal related to NSR. The other portions of that proposal, 
concerning attainment dates, SIP submittals, reasonable further 
progress (RFP) requirements, etc., were finalized on April 25, 2007 (72 
FR 20586).
    On September 21, 2007, we proposed additional elements for the PSD 
program for PM2.5 including PM2.5 ``increments,'' 
significant impact levels (SILs), and significant monitoring 
concentrations (SMCs) (72 FR 54112). Increments are the maximum 
allowable increases over baseline concentrations that can be permitted 
to occur when a major source is constructed or modified. This is one 
mechanism by which the PSD program prevents significant deterioration 
in air quality. A SIL defines the level of ambient air impact that is 
considered a ``significant contribution'' to air quality. If the 
modeled maximum ambient impacts of a new source or modification are 
below the SILs, the source: (1) Is presumed not to cause or contribute 
significantly to a PSD increment or NAAQS violation, and (2) is not 
required to perform the multiple-source, cumulative impacts assessments 
that are otherwise required under PSD. An SMC defines the level of 
modeled ambient air impact below which the reviewing authority may 
exempt a new or modified source from conducting the preconstruction 
monitoring that may otherwise be required under PSD. The reviewing 
authority may also exempt the source from preconstruction monitoring if 
the existing monitored ambient concentration is less than the SMC. This 
final action on the bulk of the major NSR program for PM2.5 
along with our proposed rule on increments, SILs, and SMC, when final, 
will represent the final elements necessary to implement a 
PM2.5 PSD program. When both rules are promulgated and in 
effect, the PM2.5 PSD program will no longer use a 
PM10 program as a surrogate, as has been the practice under 
our existing guidance.

IV. Overview of This Final Action

    The table below summarizes the main elements of the existing NSR 
program that this action addresses for PM2.5 as a regulated 
NSR pollutant. The table indicates our final position on an issue and 
whether our position has changed based on comments received. Our final 
action for each element, or where appropriate, explanation of 
implementation under existing

[[Page 28325]]

regulations, is addressed in detail in the referenced sections of this 
preamble.

----------------------------------------------------------------------------------------------------------------
         NSR program element                             Final action                            Section
----------------------------------------------------------------------------------------------------------------
Applicability to PM2.5 precursors....  SO2--Must be regulated as precursor, NOX--        V.A
                                        Presumed regulated, VOC--Presumed not
                                        regulated, Ammonia--Presumed not regulated.
PSD major source threshold...........  100/250 tons per year (tpy).....................  V.B
NA NSR major source threshold........  100 tpy.........................................  V.B
Significant emissions rate...........  Direct PM2.5 emissions--10 tpy, SO2 precursor--   V.C & V.D
                                        40 tpy, NOX precursor--40 tpy, if regulated.
Condensable PM2.5 emissions..........  Included in direct PM2.5 emissions for major NSR  V.E
                                        applicability determinations after the end of
                                        the transition period (changed based on
                                        comments received).
Control technology: BACT and LAER....  Applies for direct PM2.5 emissions, SO2, and      V.F.1 & V.G
                                        other precursors if regulated.
Prevention of significant              Increments, SILs and SMCs covered in a separate   V.F.2
 deterioration.                         rulemaking.
Air quality impact analysis..........  Applies for PM2.5...............................  V.F.3
Preconstruction monitoring...........  Applies for PM2.5 (finalizing options 1 & 3)....  V.F.4
NA NSR Statewide compliance and        Applies for direct PM2.5 emissions and            V.G
 alternative siting analyses.           precursors, if regulated.
NA NSR offsets.......................  Applies for direct PM2.5 emissions and            V.G.1-3
                                        precursors, if regulated.
Interpollutant offsetting............  Allowed on a regional or statewide basis; EPA is  V.G.4
                                        issuing guidance with recommended regional
                                        hierarchies and trading ratios (changed based
                                        on comments received).
Transition for PSD...................  Continues to use PM10 as a surrogate............  V.H
Transition for NA NSR................  Applies through an approved SIP or through 40     V.I
                                        CFR part 51, appendix S.
SIP development period...............  Clarifies that major NSR does not apply to        V.J
                                        precursors during the SIP development period in
                                        attainment areas (changed based on comments
                                        received).
Tribal concerns......................  Cross references to proposed NSR rules for        V.K
                                        Indian country.
Minor NSR............................  Clarifies that State and local regulatory         V.L
                                        programs must include PM2.5 requirements for
                                        minor sources.
NSR transport option.................  Transport classification not available..........  V.M
----------------------------------------------------------------------------------------------------------------

    The provisions of the PM2.5 major NSR program finalized 
in this action are codified as revisions in the previously existing 
regulatory text. The revisions to NA NSR are codified in 40 CFR 51.165 
and appendix S to 40 CFR part 51. The PSD revisions are codified in 40 
CFR 51.166 and 52.21.

V. Rationale for Final Actions

    In this section we discuss each element of our proposal for this 
rulemaking, explain our final action, discuss the rationale for our 
final action, and summarize the major public comments we received. The 
full summary of public comments on the proposal, along with our 
responses, can be found in the docket for this rulemaking.\7\
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    \7\ See ``Implementation of the New Source Review (NSR) Program 
for Particulate Matter Less Than 2.5 Micrometers in Diameter 
(PM2.5); Response to Comments,'' U.S. Environmental 
Protection Agency. It can be viewed or downloaded at 
www.regulations.gov, Docket ID No. EPA-HQ-OAR-2003-0062.
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A. Applicability of NSR to Precursors of PM2.5 in the 
Ambient Air

    Scientific research has shown that various pollutants can 
contribute to ambient PM2.5 concentrations. In addition to 
direct PM2.5 emissions, these include the following 
precursors:
     Sulfur dioxide (SO2);
     Oxides of nitrogen (NOX);
     Volatile organic compounds (VOC); and
     Ammonia.
    These gas-phase precursors undergo chemical reactions in the 
atmosphere to form secondary PM. 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; 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 semi-
volatile 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.
    Precursors contribute significantly to ambient PM2.5 
concentrations, producing approximately half of the concentration 
nationally. In most areas of the country, PM2.5 precursor 
emissions are major contributors to ambient PM2.5 
concentrations. The relative contribution to ambient PM2.5 
concentrations from each of these pollutants varies by area. The 
relative effect of reducing emissions of these pollutants is also 
highly variable.
    Some PM2.5 precursors are already subject to major NSR 
under other NAAQS, as shown in the following table:

------------------------------------------------------------------------
                                          Existing program coverage for
            PM2.5  precursor                 major NSR applicability
------------------------------------------------------------------------
NOX....................................  NA NSR and PSD for NO2 and
                                          Ozone.
SO2....................................  NA NSR and PSD for SO2.
VOC....................................  NA NSR and PSD for Ozone.
Ammonia................................  No coverage for NSR.
------------------------------------------------------------------------

    In the subsections that follow, we first discuss our legal 
authority under the Act for regulating precursors to the formation of 
criteria pollutants, and then discuss our final action for each of the 
PM2.5 precursors.
1. What is EPA's legal authority to regulate precursors?
    As we discussed in the November 1, 2005 proposal, we interpret the 
Act to not only provide explicit authority for EPA to regulate 
precursors, but also to grant us discretion to determine how to address 
precursors for particular regulatory purposes. This reading is based on 
section 302(g) of the Act, which defines the term ``air pollutant'' 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

[[Page 28326]]

to identify and regulate precursors as air pollutants under other parts 
of the Act. 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 Act to require that EPA 
identify a particular precursor as an air pollutant for all regulatory 
purposes where it can be demonstrated that various programs under the 
Act address different aspects of the air pollutant problem. Likewise, 
we do not interpret the Act 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 within that program. For 
example, in a recent 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 Act reinforce our reading of section 302(g) 
to mean that Congress intended precursors to NAAQS pollutants to be 
subject to the air quality planning and control requirements of the 
Act, but also recognized that there may be circumstances where it is 
not appropriate to subject precursors to certain requirements of the 
Act. Section 182 of the Act provides for the regulation of 
NOX and VOC 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 reduction requirements 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 
Act:

    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 Act clearly calls for the 
regulation of precursor pollutants, but the Act 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 Act. 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.
    The term ``air pollutant,'' as defined in section 302(g), is 
incorporated into the NSR provisions for various purposes. Thus, we 
interpret section 302(g) of the Act to require us to consider how to 
address precursors under the NSR program.
    With regard to PSD, section 165(a)(3) of the Act states that new or 
modified major sources must demonstrate that emissions ``will not 
cause, or contribute to, air pollution in excess of any * * * NAAQS in 
any air quality control region * * *.'' A source could not reasonably 
make this demonstration without considering precursors that EPA has 
identified for this purpose. Section 165(a)(4) of the Act states that a 
new or modified source must apply BACT ``for each pollutant subject to 
regulation under this Act emitted from, or which results from, such 
facility.'' The phrase ``emitted from, or which results from'' 
indicates that the statute is not limited to direct emissions, but 
rather extends to precursors as well.
    With regard to NA NSR, sections 172(c)(4) and 173 require States to 
demonstrate, among other things, that emissions from new or modified 
major sources are consistent with the achievement of ``reasonable 
further progress.'' Reasonable further progress is further defined as 
reductions of the relevant air pollutant, which is defined in section 
302(g) to include precursors identified by EPA as subject to regulation 
for that purpose.
2. What is EPA's approach for addressing precursors?
    As proposed, we are finalizing different approaches for addressing 
the individual precursors to PM2.5 under the Act's NSR 
provisions. Generally, where the scientific data and modeling analyses 
provide reasonable certainty that the pollutant's emissions are a 
significant contributor to ambient PM2.5 concentrations, we 
believe that pollutant should be identified as a ``regulated NSR 
pollutant'' and subject to the PM2.5 NSR provisions. 
Conversely, where the effect of a pollutant's emissions on ambient 
PM2.5 concentrations is subject to substantial uncertainty, 
such that in some circumstances the pollutant may not result in 
formation of PM2.5, or control of the pollutant may have no 
effect or may even aggravate air quality, we generally believe it is 
unreasonable to establish a nationally-applicable presumption that the 
pollutant is a regulated NSR pollutant subject to the requirements of 
NSR for PM2.5. We discuss our final action with respect to 
each of the PM2.5 precursors and the basis for that action 
in sections V.A.2 through 5.
    For those precursors that are either presumed to be regulated or 
not regulated (NOX, VOC, and ammonia), a State program need 
not follow the presumed approach if it can be demonstrated that the 
precursor in question is not, or is, a ``significant contributor'' to 
PM2.5 concentrations within the specific area. ``Significant 
contribution'' in this context is a different concept than that in 
section 110(a)(2)(D) of the Act. Section 110(a)(2)(D) of the Act 
prohibits States from emitting air pollutants in amounts which 
significantly contribute to nonattainment or other air quality problems 
in other States. Consistent with the previous discussion of sections 
189(e) and 302(g), we are clarifying that the use in this NSR 
implementation rule of the term ``significant contribution'' to the 
area's PM2.5 concentration means that a significant change 
in emissions of the precursor from sources in the area would be 
projected to provide a significant change in PM2.5 
concentrations in the area. For example, if modeling indicates that a 
reduction in an area's NOX emissions would reduce ambient 
PM2.5 levels in the 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. We are not

[[Page 28327]]

establishing in this rule a quantitative test for determining whether 
PM2.5 levels in an area change significantly in response to 
reductions in precursor emissions in the area. 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 as a regulated NSR 
pollutant reflects atmospheric chemistry conditions in the area and the 
magnitude of emissions of the precursor in the area. Assessments of 
whether it is technically feasible and cost effective to control 
particular emissions units at a source should be part of the later BACT 
determination within a permit action, and should occur after the basic 
assessment of which precursors are to be regulated NSR pollutants in an 
area is completed.
    Most commenters did not question our legal authority to identify 
and regulate PM2.5 precursors. However, some commenters 
argued, based on the language of sections 302(g) and 189(e) of the Act, 
that once we have designated a compound as a precursor, we do not have 
discretion to presumptively exclude it from NSR requirements. Other 
commenters on this issue indicated that we do have such discretion, 
based on the de minimis doctrine of the Alabama Power decision or on 
practical implementation considerations such as the uncertainty in 
measuring and modeling the effect of PM2.5 precursors.
    We do not agree with the comment that the Act does not give us 
discretion to presumptively exclude a PM2.5 precursor from 
NSR requirements. As stated previously, we believe that section 302(g) 
allows the Administrator to presumptively not require certain 
precursors to be addressed in PM2.5 NSR programs 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, we find that at this time there is 
sufficient uncertainty regarding whether certain precursors 
significantly contribute to PM2.5 concentrations in all 
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 NSR program 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.
    Hence, we continue to believe that the Act provides us the 
authority not only to identify and regulate precursors to 
PM2.5, but also to treat precursors of the same pollutant 
differently under the same program.
3. Final Action on SO2
    Sulfur dioxide is emitted mostly from the combustion of fossil 
fuels in boilers operated by electric utilities and other industrial 
sources. 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 rainwater), 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 these 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, sulfur trioxide (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 component (e.g., ranging from 9 
percent to 40 percent) of PM2.5 concentrations, and contributes to 
other air quality problems in all regions of the country, we proposed 
to require States to treat SO2 as a PM2.5 precursor in all areas. We 
are retaining the same approach for SO2 in this final rule. Sulfate is 
an important precursor to PM2.5 formation in all areas, and has a 
strong regional impact on PM2.5 concentrations. This approach is 
consistent with past EPA regulations, such as the Clean Air Interstate 
Rule (CAIR), the Clean Air Visibility Rule, the Acid Rain rules, and 
the Regional Haze rule, each of which require SO2 reductions to address 
fine particle pollution and related air quality problems. Finally, we 
do not believe that regulating SO2 as a precursor to PM2.5 is likely to 
add a major burden to sources, as SO2 is already regulated as part of 
the NSR program for the SO2 NAAQS.
    Most commenters who addressed this issue agreed that SO2 should be 
regulated as a PM2.5 precursor, although one only supported regulation 
of SO2 as a precursor in NA NSR, and not under PSD. Two commenters 
disagreed that SO2 acts as precursors to PM2.5 in all cases and 
indicated that it should not be regulated as an ``always-in'' 
precursor.
    We find the commenters' arguments against regulating SO2 as a 
precursor unpersuasive. 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, in both attainment and 
nonattainment areas. Sulfur dioxide emissions also lead to sulfate 
formation on both regional and local scales.

[[Page 28328]]

4. Final Action on NOX
    The sources of NOX are numerous and widespread. The combustion of 
fossil fuel generates the majority of NOX emissions, with large 
contributions from power generation and mobile sources. Nitrates are 
formed from the oxidation of NOX into nitric acid (HNO3) 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. Reductions in NOX 
emissions 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 these factors, we are finalizing our proposed approach 
to NOX as a precursor to PM2.5 for the NSR program. Under this 
approach, NOX is presumed to be a significant contributor to ambient 
PM2.5 concentrations in all PSD and NA NSR areas. However, a State or 
EPA may rebut this presumption for a specific area if the State 
demonstrates to the Administrator's satisfaction or EPA demonstrates 
that NOX emissions in that area are not a significant contributor to 
that area's ambient PM2.5 concentrations. If a State or EPA makes such 
a demonstration, NOX would not be considered a PM2.5 precursor under 
the NSR program in that area. If a State or EPA does not make such a 
demonstration, NOX must be regulated as a precursor under the PSD, NA 
NSR, and minor source programs for PM2.5. As discussed previously, this 
``presumed-in'' approach is warranted based on the well-known 
transformation of NOX into nitrates, coupled with the fact that nitrate 
concentrations vary significantly around the country. This approach is 
consistent with other recent EPA regulations requiring NOX reductions, 
which will reduce fine particle pollution, such as the CAIR and a 
number of rules targeting onroad and nonroad engine emissions.
    We had proposed that NOX be presumed to be a precursor in any State 
that EPA has identified as a source of the PM2.5 interstate transport 
problem. In the final rule, we have dropped this requirement to be 
consistent with EPA's Clean Air Fine Particle Implementation Rule 
published on April 25, 2007. 72 FR 20586. Such a requirement is not 
necessary in this rule because States that contribute to downwind 
nonattainment for PM2.5 are otherwise required to address transported 
NOX emissions under the CAIR.
    In areas where NOX is regulated as a precursor to PM2.5, we do not 
believe that this is likely to add a major burden to sources, as NOX is 
already a regulated NSR pollutant. This is because NOX is an identified 
precursor for the ozone NAAQS and an indicator for the NO2 NAAQS.
    Several commenters agreed that NOX should be regulated under major 
NSR as a precursor to PM2.5. Some of these commenters believe that 
States should not have the opportunity to demonstrate otherwise, or 
indicated that a waiver for exclusion of NOX as a precursor should be 
allowed only if downwind States approve such a waiver. A few commenters 
stated that NOX should not be regulated as a precursor to PM2.5 in the 
major NSR program, either on grounds of scientific uncertainty 
regarding the impact of NOX emissions on ambient PM2.5 concentrations 
or on policy grounds (i.e., because NOX is already regulated under NSR 
for other NAAQS).
    We are not persuaded by the argument that NOX should not be 
regulated as a PM2.5 precursor because it is a regulated pollutant 
under other NAAQS. We do not find the degree of scientific uncertainty 
regarding PM2.5 formation from NOX to be great enough to preclude 
regulation of NOX as a precursor with an opportunity for a case-by-case 
demonstration that NOX is not a significant contributor. Furthermore, 
the fact that we regulate NOX for other NAAQS under the NSR program 
does not by itself justify declining to regulate NOX as a PM2.5 
precursor in circumstances where NOX also significantly contributes to 
PM2.5 formation. As noted earlier, the regulation of NOX as a precursor 
for PM2.5 is not expected to add a major burden to regulated sources 
that are already required to limit NOX emission to meet other NAAQS.
    We disagree with the commenters who believe that emissions of NOX 
cannot be correlated to PM2.5 formation, or that it is unclear when NOX 
acts as a precursor. As discussed previously, our decision to regulate 
NOX as a precursor to PM2.5 is based on the well-known transformation 
of NOX into nitrates. Nevertheless, nitrate concentrations vary 
significantly across the country. As a result, we believe that the 
``presumed-in'' approach is appropriate for NOX since a State can 
demonstrate that NOX should not be a precursor in a given area or 
region.
    While we recognize that NOX emissions can affect PM2.5 
concentrations in downwind areas, we disagree that approval from 
downwind States should be required for a State to exclude NOX as a 
PM2.5 precursor for a particular area. This is because States that 
contribute to downwind nonattainment for PM2.5 are otherwise required 
to address transported NOX emissions under the CAIR.
5. Final Action on VOC
    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

[[Page 28329]]

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 OH) which in turn participate in 
oxidation reactions to form SOA, 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 SOA. 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 (ozone, 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 PM. 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 United States. The rest of the anthropogenic hydrocarbons 
(higher alkanes, paraffins, etc.) have been estimated to contribute 5 
to 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 PM 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 resulted in quantifying the concentrations 
of hundreds of organic compounds representing only 10 to 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 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 SOA, and that a considerable fraction of the total carbonaceous 
material originates 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, and to determine the extent of the contribution of 
specific VOC to organic PM mass.
    As a result, this final rule does not, in general, require 
regulation of VOC as a precursor to PM2.5 for the NSR program. However, 
a State may demonstrate to the Administrator's satisfaction or EPA may 
demonstrate that VOC emissions in a specific area are a significant 
contributor to that area's ambient PM2.5 concentrations. After such a 
demonstration, the State would regulate VOC (or a subset of VOC) as a 
PM2.5 precursor for the NSR program in that area. That is, the State 
would need to regulate construction and modification of stationary 
sources that increase emissions of VOC in that area to assure that 
these emissions do not interfere with reasonable further progress or 
the ability of that area to attain or maintain the PM2.5 NAAQS.
    We believe that this ``presumed-out'' approach is appropriate for 
VOC because of the complexity in assessing the role of VOC in PM2.5 
formation, as discussed previously. Where the effect of a pollutant's 
emissions on ambient PM2.5 concentrations is subject to this degree of 
uncertainty, we do not have justification to establish a nationally-
applicable presumption that the pollutant is a regulated NSR pollutant 
subject to the requirements of NSR for PM2.5. Under the circumstances, 
we believe the best policy is to continue to regulate VOC under NSR as 
a precursor to ozone in all areas, which will potentially provide a co-
benefit for PM2.5 concentrations despite the uncertainty in PM2.5 
formation from VOC. As discussed previously, we do not find it 
appropriate to utilize the same approach for NOX because the scientific 
data and modeling analyses provide more certainty that NOX emissions 
are a significant contributor to ambient PM2.5 concentrations.
    Note that we intend to regulate high molecular weight VOC (with 25 
carbon atoms or more and low vapor pressure) as direct PM2.5 emissions 
because they are emitted directly as primary organic particles and 
exist primarily in the condensed phase at ambient temperatures. See 
section V.E following for more on the regulation of such 
``condensables.''
    Most commenters agreed with the ``presumed-out'' approach for VOC. 
One commenter said that the role of VOC in the formation of PM2.5 is 
sufficiently understood to recommend a ``waiver'' approach for this 
pollutant in the same way as NOX is treated for PM2.5 in the rule.
    As discussed previously, 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 merely been able to 
quantify the concentrations of hundreds of organic compounds 
representing only 10 to 20 percent of the total organic aerosol mass. 
For this reason, SOA continues to be a significant topic of research 
and investigation. Accordingly, we do not agree with the commenter who 
suggested a waiver or ``presumed-in'' approach for VOC. We continue to 
believe that our ``presumed-out'' approach is most appropriate for VOC 
and have included this approach in the final rules.

[[Page 28330]]

6. Final Action on Ammonia
    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 \8\ 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 adverse human health effects and with an 
increase in the formation of secondary organic compounds. Based on this 
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.
---------------------------------------------------------------------------

    \8\ 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. See the docket for this rulemaking, Docket ID No. EPA-
HQ-OAR-2003-0062, or https://www.narsto.org/section.src?SID=6.
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    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, PM 
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 decrea