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.
---------------------------------------------------------------------------

    \98\ In addition, the PSD program applies to most noncriteria
regulated pollutants.
---------------------------------------------------------------------------

    The national regulations that apply to each of these programs are
located in the Code of Federal Regulations (CFR) as shown below:

------------------------------------------------------------------------
                                            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 Emissions 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,
     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 the [NAAQS] are achieved.''
    This proposed rule on the implementation of NSR for
PM2.5 does not supersede existing PM10 NSR
requirements. EPA is not planning to revoke the original
PM10 standards at this time. Accordingly, sources are
subject to NSR for both PM10 and PM2.5.
2. What are the principal elements of the proposed major NSR program
for PM2.5?
    The table below summarizes the main elements of the existing major
NSR program that EPA is proposing to address for PM2.5 as a
regulated NSR pollutant. The EPA's proposal for each element, or where
appropriate, explanation of implementation under existing regulations,
is explained in detail in the referenced sections of this preamble.

------------------------------------------------------------------------
  Major NSR program element       EPA proposal             Section
------------------------------------------------------------------------
PSD Major Source Threshold..  100/250 TPY (no       IV.M.4.
                               change).
NA NSR Major Source           100 TPY (no change).  IV.M.4.
 Threshold.
Significant Emissions Rate..  PM2.5 Direct          IV.M.5 & 6.
                               Emissions--10 TPY;
                               SO2--40 TPY.
                              If other precursors
                               are included:.
                              NOX--40 TPY (no
                               change).
                              VOC & Ammonia--
                               determined by SIP.
Control technology: BACT and  Applies for PM2.5     IV.M.9 & 13.
 LAER.                         direct, SO2 and
                               other precursors,
                               if included.
Air quality impact analysis.  Applies for PM2.5...  IV.M.11.
Preconstruction monitoring..  Applies for PM2.5...  IV.M.12.
                              Proposing five
                               options to address..
NA NSR Statewide compliance.  Applies for PM2.5     IV.M.13.
                               direct and
                               precursors, if
                               included.
NA NSR offsets..............  Applies for PM2.5     IV.M.14.
                               direct.
                              Considering for
                               precursor emissions.
Interprecursor Offsetting...  Allowed with          IV.M.14.c.
                               modeling
                               demonstration (no
                               change).
Transition for PSD..........  Continues to apply    IV.M.16.
                               with limited
                               provisions for use
                               of PM10 as a
                               surrogate.
Transition for NA NSR.......  Applies at            IV.M.17.
                               designation through
                               an approved SIP or
                               through 40 CFR part
                               51, appendix S.
Minor NSR...................  Clarifies that State  IV.M.20.
                               and local
                               regulatory programs
                               must include PM2.5
                               requirements for
                               minor sources.
NSR Transport Option........  Flexible              IV.M.21.
                               implementation for
                               areas granted a
                               transport
                               classification.
------------------------------------------------------------------------

    The proposed provisions of the PM2.5 major NSR program
will be codified in the regulatory text as revisions to 40 CFR 51.165;
51.166; 52.21; and 40 CFR part 51, Appendix S. We have made two
assumptions in developing the proposed regulatory text for this rule.
    The first assumption is that the ozone phase II rule will be
promulgated prior to the promulgation of this proposed PM2.5
rule. Thus, this proposed PM2.5 rule includes language
related to ozone precursors and offsets that make the format of the
ozone rule consistent with the PM2.5 language. The ozone
provisions contained in the regulatory text set forth below are
consistent with what we expect to finalize in the ozone rule, and this
PM2.5 proposal is not intended to alter the substance of the
ozone phase II rule. To the extent there are changes to the ozone phase
II regulatory language when promulgated or the ozone rule is not
promulgated prior to the final PM2.5 rule, we would need to
make changes to the proposed regulatory text in this PM2.5
rule at promulgation.
    The paragraphs in the revisions to appendix S of this proposed
PM2.5 rule have not been numbered at this time, based on the
second assumption that both of the appendix S rule revisions, appendix
S changes in the ozone phase II rule (incorporating the 1990
amendments) and the revisions to appendix S (incorporating NSR reform),
will be promulgated prior to the final PM2.5 rule. Depending
on the status of these appendix S rule revisions at the time of
promulgation of the PM2.5 rule, the paragraphs would be
numbered accordingly.
3. Should precursors to the formation of ambient concentrations of
PM2.5 be subject to regulation under NSR?
a. Background
    Certain NAAQS pollutants such as ozone and PM2.5, are
partially or entirely formed by precursors. Precursors are currently
regulated under parts C and D of the Act based on either statutory
presumptions or a scientific determination that the pollutants must be
regulated to achieve attainment. The following table shows precursors
that we have identified for regulation under the NSR program because of
their ability

to cause or contribute to violations of the ozone NAAQS.

------------------------------------------------------------------------
           Criteria pollutant                  Precursor pollutants
------------------------------------------------------------------------
Ozone:
    Nonattainment Areas................  Volatile Organic Compounds
                                          (VOC) and Nitrogen Oxides
                                          (NOX).
    Attainment Areas...................  VOC \99\.
------------------------------------------------------------------------

Individual SIPs may identify additional precursors as regulated NSR
pollutants.
---------------------------------------------------------------------------

    \99\ We have proposed to amend the PSD regulations to expressly
include NOx as an ozone precursor. 68 FR 32802 (June 2,
2003).
---------------------------------------------------------------------------

    Scientific research has shown that various pollutants can
contribute to ambient PM2.5 concentrations, including the
following:
     PM2.5 (direct emissions)
     SO2 (as a precursor)
     NOX (as a precursor)
     VOC (as a precursor)
     Ammonia (as a precursor)
b. Should NSR cover precursor emissions in addition to direct emissions
of PM2.5?
    Contribution of precursors to PM2.5 nonattainment. As
discussed in Section II, precursors contribute significantly to ambient
PM2.5 concentrations, producing approximately half of the
concentration. In most areas of the country, PM2.5 precursor
emissions are the major contributors to ambient PM2.5
concentrations. However, it is technically difficult to determine
impacts of source-specific precursor emissions on ambient air quality
levels. 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.
    PM2.5 precursors already addressed under NSR. Some
PM2.5 precursors are already subject to major NSR under
other NAAQS as shown below:

PM2.5 precursor........................  Existing Program coverage for
                                          major NSR applicability.
NOX....................................  NA NSR for NO2 and Ozone PSD
                                          for NO2.
SO2....................................  NA NSR and PSD for SO2.
VOC....................................  NA NSR and PSD for Ozone.
Ammonia................................  No coverage for NSR (Some areas
                                          regulate ammonia for other air
                                          quality purposes.)

    The PM2.5 NSR program could include some, all or none of
these precursors of PM2.5.
    Legal Authority. As discussed earlier in section II.E. of this
preamble, we interpret the Clean Air Act to provide explicit authority
for EPA to regulate precursors but also to grant the Administrator
discretion to determine how to address precursors for particular
regulatory purposes. This reading is based on section 302(g) of the
Clean Air 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
Administrator's discretion to determine how to address precursors under
specific programs is also supported by the language in sections 182(f)
and 189(e) which identifies circumstances where the Administrator may
determine that it is not appropriate to regulate certain precursors. We
discuss these provisions in more detail in section II.E.
    Thus, we interpret section 302(g) of the Act to require that the
Administrator consider how to address precursors under the NSR program.
The term ``air pollutant'' is incorporated into the NSR provisions for
various purposes. Sections 182(f) and 189(e) apply to State
implementation plan provisions and control requirements, which include
NSR programs.
    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 the Agency has
identified for this purpose. Section 165(a)(4) of the Act states that a
new or modified source must apply the Best Available Control Technology
(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 nonattainment 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 the Agency as
subject to regulation for that purpose. Treatment of Precursors for
Purposes of NSR. As discussed in section II.E., where there is a basis
to do so, we believe EPA may treat precursors of the same pollutant
differently under the same program. In this action, we propose
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 from stationary sources 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 from stationary sources 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 also request comment on whether, despite
reasonable scientific certainty associated with the effect of a

pollutant's emissions from stationary sources on ambient
PM2.5 concentrations, there are circumstances that would
support a finding that the Administrator should not identify the
pollutant as a precursor for the purposes of the NSR program even if
the pollutant is so identified for other programs.
    For the purposes of the NSR program, the EPA proposes the following
options for addressing SO2, NOX, VOCs, and
ammonia as precursors to PM2.5, and requests comment on
these options. Commenters should provide detailed technical information
supporting their comments. Sulfur Dioxide. We are proposing to regulate
SO2 as a precursor to PM2.5 for purposes of NSR
in all attainment, unclassifiable and nonattainment areas. We believe
that the technical discussion and analysis of speciated air quality
data described in Section II provide an appropriate basis for requiring
States to address SO2 as a precursor to PM2.5 for
NSR purposes. The fact that sulfate is a significant contributor (e.g.
ranging from 9 percent to 40 percent) to PM2.5 nonattainment
and other air quality problems in all regions of the country is a
critical piece of evidence supporting this approach. Additionally,
sulfates are a major contributor to ambient PM2.5
concentrations in the Eastern United States, roughly equaling the
concentration of carbonaceous particles.
    EPA does 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 in these programs as part of the
NSR program for the SO2 NAAQS. The EPA requests comments on
this approach to regulate SO2 as a precursor to
PM2.5 and a ``regulated NSR pollutant'' for purposes of NSR
in all attainment, unclassifiable and nonattainment areas[boxur]
Nitrogen Oxides. We are proposing to regulate NOX as a
precursor to PM2.5 for the NSR program. Under this approach,
a State or EPA would presume that NOX is a significant
contributor to an area's ambient PM2.5 concentration. This
presumption is warranted based on the well-known transformation of
NOX into nitrates, as discussed in more detail in Section
II. Nitrates are a significant component of PM2.5 mass in
northern regions, such as the Midwest and East Coast, and are a main
contributor to urban PM2.5 mass in California (35-40
percent). However, as described in Section II, nitrate concentrations
vary significantly in other regions of the country.
    Thus, a State could exempt NOX from its PM2.5
NSR program in a specific area by demonstrating to the Administrator's
satisfaction that NOX emissions from stationary sources in
that area are not a significant contributor to that area's ambient
PM2.5 concentrations and the area is not in a State
identified by EPA as a source of a PM2.5 interstate
transport problem. Hence, for such an area, the State would not need to
regulate construction and modification of stationary sources that
increase emissions of NOX 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.
Otherwise, this option would make NOX a precursor for the
PSD, NA NSR and minor source programs for PM 2.5. EPA does
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.
    Volatile Organic Compounds. The consideration of VOC for NSR
applicability is complicated by the variations in reactions of the
different species of VOC in the atmospheric transformation into
PM2.5. Scientific analysis demonstrates that, while the
transformation of VOC into particles is a complex and uncertain
process, all VOC potentially play a role in the formation of
PM2.5. However some specific compounds play a more direct
role than others. These transformations are discussed in Section II. In
light of the complexity in assessing the role of VOC in PM
2.5 formation, we are not proposing to regulate VOC as a
precursor to PM2.5 for the NSR program.
    However, if a State demonstrates to the Administrator's
satisfaction that VOC emissions from stationary sources in a specific
area are a significant contributor to that area's ambient
PM2.5 concentrations, then the State would regulate VOC (or
a subset of VOC) as a PM2.5 precursor for the NSR program in
that area. Therefore, for such an area, 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. Under
either scenario, as discussed in Section II, we would still regulate
high molecular weight VOC (with 25 carbon atoms or more and low vapor
pressure) as PM2.5 direct emissions because they are emitted
directly as primary organic particles and exist primarily in the
condensed phase at ambient temperatures.
    Ammonia. As discussed in section II.E., in some areas of the
country, ammonia plays a significant role in the formation of ambient
PM2.5 concentrations. In other areas, ammonia plays a less
significant role. Our understanding of emissions inventories, and the
impact that reducing ammonia emissions has on ambient PM2.5
concentrations, is evolving. In some cases, undesired consequences may
result from reductions of ammonia, such as increased acidity levels for
particles and deposition. For these reasons, EPA proposes that ammonia
would only be identified as a precursor to PM2.5 NAAQS in a
nonattainment area for purposes of NSR on a case-by-case basis. If the
State demonstrates to the Administrator's satisfaction that ammonia
emissions from stationary sources in a specific nonattainment area are
a significant contributor to that area's ambient PM2.5
concentrations, then the State would regulate ammonia as a
PM2.5 precursor under the NSR program in that nonattainment
area. Therefore the State would need to regulate construction and
modification of stationary sources that increase emissions of ammonia
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. However, in other nonattainment
areas, we would not require States to include ammonia in their NSR
programs. We are not proposing to identify ammonia as a regulated NSR
pollutant for purposes of PSD in any attainment or unclassifiable
areas.
    The EPA requests comments on this approach for addressing ammonia
emissions under the NSR programs.
4. What is a major stationary source (major source) under the major NSR
program for PM2.5?
a. Background
    The major NSR program applies to construction of major stationary
sources and major modifications at major stationary sources. A
stationary source is a ``major source'' if its actual emissions or its
potential to emit for a specific pollutant equals or exceeds the major
source threshold for that pollutant established in the CAA. Different
pollutants are not summed to determine applicability.
b. Proposed Option
    Sections 169 and 302(j) of the Act contain definitions of ``major
emitting facility'' and ``major stationary source'' that apply to
programs implemented under part C and subpart 1 of part D of the Act.
Accordingly, we are proposing to follow these definitions for purposes
of defining a major emitting facility or

major stationary source that would be subject to major NSR based on
direct PM2.5 emissions or emissions of pollutants identified
as PM2.5 precursors for the NSR program. This approach is
also consistent with how we treat other criteria pollutants that are
covered by subpart 1 of part D of the Clean Air Act and thus are not
subject to a tiered classification system such as the one required for
ozone nonattainment areas under subpart 2 of the Clean Air Act. EPA
does not interpret subpart 4 of part D of the Act (creating ``serious''
and ``moderate'' classifications for PM10 nonattainment
areas) to apply to PM2.5.
    This means the major source thresholds would be:

PSD...............................  100 tpy for source categories listed
                                     in 40 CFR 51.166(b)(1)(i)(a) and
                                     52.21(b)(1)(i)(a).
                                    250 tpy for all other source
                                     categories.
NA NSR............................  100 tpy for all source categories.

Thus, no regulatory change would be required. See Sec. Sec.
51.165(a)(1)(iv)(a); 51.166(b)(1)(i); 52.21(b)(1)(i); Appendix S,
Section II.A.4.
    We request comment on this approach for establishing the major
source threshold for purposes of the major NSR program for the
PM2.5 NAAQS. We also request comment on whether the
definitions in Section 169 and 302(j) are controlling for purposes of
establishing the definition of major stationary source for the
PM2.5 NAAQS, which is being implemented under part C and
subpart 1 of part D of the Act.
c. What is the effect of this proposed option?
    Although our proposed approach is consistent with Sections 169 and
302(j) and Subpart 1 of part D of the Act, this approach results in a
higher major source threshold in PM2.5 nonattainment areas
than the major source threshold that applies in some PM10
nonattainment areas under Subpart 4 of part D of the Act. This is
because Section 189(b) of the Act establishes a 70 tpy major source
threshold for ``serious'' PM10 nonattainment areas while
``moderate'' PM10 nonattainment areas apply a 100 tpy major
source threshold based on the definition in section 302(j). We do not
believe the Act gives us the discretion to promulgate a lower major
source threshold for pollutants such as PM2.5 that are only
subject to Subpart 1 of part D of the Act.
    Nevertheless, we do not believe this situation will adversely
impact attainment of the PM2.5 NAAQS. Data from EPA's
emissions inventory indicate that a significant number of sources have
actual PM2.5 emissions in the 100 to 250 tpy range.
Additionally, the more current inventory data shows that the number of
sources that would be covered as major sources by a lower major source
threshold would not increase substantially unless the threshold were
lowered to 20 tpy or below. Thus, even if EPA had the discretion to
adopt a 70 tpy major source threshold for PM2.5
nonattainment areas, we do not believe that many additional sources
would be subject to the major NSR program in PM2.5
nonattainment areas.
    States should consider this information in developing their own
SIP-approved NSR programs. For example, if construction of
PM2.5 sources emitting 99 tpy with no major NSR controls and
without mitigation would undermine a State's ability to achieve
reasonable further progress or attain the PM2.5 NAAQS, then
the State should consider imposing emissions controls or other
requirements on these sources through the State's minor NSR program.
5. What should the significant emissions rate be for direct emissions
of PM2.5?
a. Background
    The determination of what should be classified as a modification
subject to major NSR is based, in part, on a significant emissions
rate.\100\ The NSR regulations define this term as a rate above which a
net emissions increase will trigger major NSR permitting requirements
if such increase results from a major modification. Sources are exempt
from major NSR requirements if an emissions increase resulting from a
modification is below this rate because EPA considers such lower
emissions increase to be de minimis for purposes of the NSR program.
The significant emissions rates for criteria pollutants are given
below:
---------------------------------------------------------------------------

    \100\ For additional background on EPA's interpretation of
modification and rationale for including significant emissions rates
in defining major modifications, see 61 FR 38253-54 (Dec. 31, 2002).

------------------------------------------------------------------------
        Criteria pollutant            Significant emissions rate (tpy)
------------------------------------------------------------------------
Ozone.............................  VOC: Any increase--40 tpy (dependent
                                     on NA classification).
                                    NOX: Any increase--40 tpy (dependent
                                     on NA classification).
NO2...............................  NOX: 40 tpy.
PM10..............................  15 tpy.
CO................................  100 tpy.
SO2...............................  40 tpy.
Lead..............................  .6 tpy.
------------------------------------------------------------------------

    The significant emissions rates listed in the above table apply to
the direct and precursor pollutants listed in the table in section
III.M.3.a. Significant emissions rates for additional pollutants that
are subject to the PSD program are contained in the following
provisions of our regulations:
     40 CFR 51.166(b)(23) and
     40 CFR 52.21(b)(23)
    The EPA performed some preliminary modeling analyses to determine
an appropriate significant emissions rate for direct emissions of
PM2.5. Several typical stack heights (ranging from 5 to 200
meters in height) were modeled using meteorological data from
Pittsburgh and Oklahoma City. Modelers ran ISCST (Industrial Source
Complex Short Term model) to assess the impact of emissions increases
on ambient PM2.5 concentrations. EPA ran models for a
variety of source types with varying meteorology, release heights,
building shapes, and receptor locations.
    The modeling produced the following results that we considered
further in developing the options below:
     Shorter stacks had much more impact in the local area than
taller stacks.
     Increases of about 5 tons per year from facilities with
short stacks were shown to cause a measurable increase in ambient
PM2.5 concentrations.
     Emissions increases from tall stacks, 100 meters or
greater, were associated with a small increase in ambient
PM2.5 concentrations in the immediate area.
b. Proposed Options
    Preferred option 1: For direct emissions of PM2.5, EPA
is proposing to define the significant emissions rate as 10 tons per
year. This proposal is based fundamentally on the same approach as we
used in setting the significant emissions rate for total suspended
particulate matter (TSP) and PM10.
    Historically, the significant emissions rate for TSP (equal or
exceeding 25 tons per year) was set by analyzing the source size that
would be unlikely to cause impacts above 4 percent of the standard (4
percent of 260 [mu]g/m3 or 10.4 [mu]g/m3 as a 24-
hour average). Although a range of source configurations can yield a
wide range of impacts per ton per year of emissions, EPA reviewed
typical configurations of major TSP sources and concluded that a major
modification that increased emissions by 25 tons per year or more would
be unlikely to increase 24-hour average TSP concentrations by more than
10.4 [mu]g/m3.
    When EPA set the significant emissions rate for PM10, we
first determined the ratio between the

controlling standards for PM10 and TSP, i.e. (150 [mu]g/
m3)/(260 [mu]g/m3) or about 3/5. Both of these
standards are based on a year's second highest 24-hour average
concentration. The EPA then set the PM10 significant
emissions rate at about 3/5 of 25 tons per year, which (with rounding)
is 15 tons per year. This reflects the fact that a source emitting 25
tons of TSP per year that has an impact of 4 percent of the TSP
standard would show an impact from 15 tons PM10 per year of
approximately 4 percent of the PM10 standard (i.e., 6 [mu]g/
m3).
    Conceptually, EPA is proposing a significant emissions rate for
PM2.5 based on the same approach. However, the comparison of
the PM2.5 standard with earlier particulate matter standards
is complicated by the difference in the averaging times of the
controlling standards, which are 24-hour average values for TSP and
PM10 but an annual average value for PM2.5.
Because the annual standard is the generally controlling standard for
lowering both short-term and long-term ambient PM2.5
concentrations (62 FR at 38669), EPA proposes using the annual standard
to determine the significant emissions rate.
    We conducted additional modeling using the ISC3 model to compare
annual average and 24-hour average impacts of a fixed emissions rate
for a variety of source configurations. Several typical stack heights
(ranging from 5 to 200 meters in height) were modeled using
meteorological data from Pittsburgh and Oklahoma City and both with and
without downwash from different building types.
    Our analysis of these modeling results shows that a major
modification that increases direct PM2.5 emissions by less
than 10 tons per year would be unlikely to increase annual average
ambient PM2.5 concentrations by more than 4 percent of the
annual PM2.5 standard. This finding relies on EPA's
comparison of annual average versus 24-hour average concentrations. As
noted above, EPA previously concluded that a source that increases
PM10 emissions by 15 tons per year would likely cause an
increase in the 24-hour average PM10 concentration by 6
[mu]g/m3 or less. Based on the ratios between annual and 24-
hour average concentrations found in EPA's recent modeling, a source
having that impact would typically increase annual average
PM10 concentrations by about 0.8 [mu]g/m3 or
less. The EPA is using a target PM2.5 impact of 4 percent of
the annual PM2.5 standard or 0.6 [mu]g/m3. This
target impact is (0.6 [mu]g/m3)/(0.8 [mu]g/m3) or
3/4 of the potential impact of a 15 ton per year emissions increase.
This suggests a significant emissions rate of 3/4 of 15 tons per year.
By rounding the result, we determined that an emissions increase below
10 tons per year increase in direct PM2.5 emissions would be
unlikely to increase ambient PM2.5 concentrations by more
than 4 percent of the annual PM2.5 standard.
    Option 2: The EPA recognizes that a range of source configurations
can have a range of impacts, that the PM2.5 source
population differs in some respects from the TSP and PM10
source population and that the acceptable stationary source impact on
ambient PM2.5 concentrations may warrant being defined
differently from the acceptable impact for TSP or PM10. The
EPA specifically solicits comments on a range of potential thresholds
ranging from 5 to 15 tons per year for the significant emissions rate
for PM2.5 direct emissions. The upper bound is a set rate of
15 tons per year because that is the significant emissions rate for
PM10. The lower bound is a set rate of 5 tons per year
because our modeling indicates that an increase in ambient
PM2.5 concentrations above the target de minimis impact
level can occur where facilities with short stacks have
PM2.5 emissions increases of about 5 tons per year.
    We solicit comments on the proposed significant emissions rate
level and on any other approaches for determining this value.
6. What should be the significant emissions rates for PM2.5
precursors?
a. Background
    It is difficult to determine the ambient air quality effects that
result from a single source of emissions of PM2.5
precursors. There are conservative screening models for predicting
impacts of large NOX and SO2 sources on ambient
PM2.5 concentrations. We conducted a range of modeling
analyses to determine the amount of PM2.5 precursor
emissions needed to show an increase in ambient PM2.5
concentrations. These analyses showed that precursor emissions probably
have some localized impacts, but that most impact is farther downwind
as precursors have the time to convert to PM2.5. In
addition, the modeling available at this time does not provide
sufficient information to estimate impacts of single source emissions
of ammonia and VOC on ambient PM2.5 concentrations.
    Although we have not finally determined which pollutants (if any)
will be regulated as PM2.5 precursors under the NSR program,
we are proposing significant emissions rates in the event that the
precursors under consideration are identified as such for the major NSR
program. In the event that EPA adopts an ``opt-in'' approach--that is,
the presumption that a precursor is not subject to NSR unless a State
demonstrates to the Administrator's satisfaction that it should be
included--the State opting in would be required to adopt the
significant emissions rate for the precursor as set forth below, in the
absence of demonstrating that another significant emissions rate is
more appropriate.
b. Proposed Options
    Preferred Option 1: The EPA proposes the use of existing
significant emissions rates for those pollutants already included in
major NSR programs as shown below:

------------------------------------------------------------------------
                                     Significant emissions rate  (equal
             Pollutant                          or exceeding)
------------------------------------------------------------------------
NOX...............................  40 tpy.
SO2...............................  40 tpy.
VOC...............................  40 tpy.
------------------------------------------------------------------------

    The use of existing significant emission rates where the
PM2.5 precursor is also regulated under NSR for a separate
criteria pollutant harmonizes the NSR program for PM2.5 with
the NSR programs for those other criteria pollutants. This enables a
source to determine the NSR impacts of proposed modifications by
reference to a single significant emissions rate for each pollutant,
and enables streamlining of determinations regarding the applicable
control technology and analysis of air quality impacts into a single
and comprehensive decision making process for both PM2.5 and
other criteria pollutants that also cover PM2.5 precursors.
This also follows precedent. When ozone became a criteria pollutant EPA
used the NOX significant emissions rate from the
NO2 program.
    EPA has never set a significant emissions rate for ammonia to
determine major NSR applicability. A necessary component of our
approach to NSR applicability for ammonia is that those States who
determine in their SIPs that control of ammonia is necessary will set
the significant emissions rate for ammonia based on the information
presented in each attainment demonstration.
    Option 2: Set the precursor levels at the same level as the
significant emission rate for PM2.5 direct emissions, that
is, 10 TPY. This would make more modifications subject to
PM2.5 permitting requirements and therefore could provide
more protection to the environment. This does not, however, follow the
precedent in the ozone NSR program. Having several different
significant emissions rates for the same

pollutant would add additional complexity to an already complex program
without necessarily providing additional environmental benefits.
    We request comment on the options listed above and on any other
approaches for establishing precursor significant emissions rates.
7. What is the role of condensible emissions in determining major NSR
applicability?
    Condensible emissions commonly make up a significant component of
PM2.5 emissions. As discussed in Sections IV.I. and IV.P,
certain sources utilizing high temperature processes emit gaseous
pollutants into the ambient air which rapidly condense into particle
form. The constituents of these condensed particles include, but are
not limited to, organic material, sulfuric acid, nitrates, and metals.
    The EPA has issued guidance clarifying that PM10
includes condensible particles and that, where condensible particles
are expected to be significant, States should use methods that measure
condensible emissions.\101\ States are already required under the
consolidated emissions reporting rule to report condensible emissions
in each inventory revision (see 67 FR 39602, June 10, 2001), and Method
202 in Appendix M of 40 CFR part 51 quantifies condensible particulate
matter.
---------------------------------------------------------------------------

    \101\ Memo. from Thompson G. Pace, Acting Chief, Particulate
Matter Programs Branch, to Sean Fitzsimmons, Iowa Department of
Natural Resources, (Mar. 31, 1994) (copy avaiable at http://www.epa.gov/Region7/programs/artd/air/nsr.nsrmemos/cpm.pdf
).

---------------------------------------------------------------------------

    However, because of the flexibility incorporated into EPA's
approach to the issue and the inconsistent implementation of the
existing guidance, there have been some misconceptions as to whether
condensible emissions must be included in a source's PM10
emissions under the PM10 standard in determining NSR
applicability. The rules at 40 CFR 51.100 define ``PM emissions'' and
``PM10 emissions'' by reference to the PM measured by
applicable reference methods, an equivalent or alternative method
specified in part 51, or by a test method specified in an approved SIP.
See 40 CFR 51.100(pp), (rr), and Sec.  52.01 (incorporating Sec.
51.100 definitions by default). As discussed in Section III.P.,
different test methods measure condensible emissions with varying
levels of accuracy. In addition, sources often project their emissions
increases from new construction and modifications based on emissions
factors, such as AP-42 factors, that in some cases have not accounted
for condensible emissions. Sources have used other methods to project
their PM emissions that do not account for condensible emissions (e.g.,
projecting PM10 impacts based on an analysis of existing TSP
limits without adding condensible emissions).
    We are proposing to clarify in this rule that condensible emissions
must be included when determining whether a source is subject to the
major NSR program. The inclusion of condensible emissions in a source's
PM2.5 emissions is of increasing importance with the change
in the indicator for particulate matter to PM2.5.
Condensible emissions are essentially fine particles, and thus are a
larger fraction of PM2.5 emissions than of TSP or
PM10 emissions. Condensible emissions commonly make up a
significant component of PM2.5 emissions, and the failure to
include them may result in adverse consequences to the environment.
    While EPA has always included condensible emissions in its
definition of particulate matter emissions, insofar as these emissions
are measured by applicable test methods or included in emissions
factors, we believe that the greater significance of condensible
emissions in addressing PM2.5 warrants greater emphasis on
including these emissions in implementing the major NSR program. A key
aspect of this issue is the development of the new test method
discussed in Section III.P., which quantifies and can be used to
characterize the constituents of PM2.5 emissions, including
both the filterable and condensible portion of the emissions stream.
8. What are the requirements of the Prevention of Significant
Deterioration (PSD) program for attainment areas?
    Background. Sources subject to PSD must:
     Install Best Available Control Technology (BACT),
     Conduct air quality modeling analyses to ensure that the
project's emissions will not cause or contribute to:
     A violation of any NAAQS or maximum allowable pollutant
increase (PSD increment),
     Any impact on any Class I area air quality related value,
and
     As required, perform preconstruction monitoring.

    Each of these elements is discussed below.
9. How should BACT be implemented?
    We are not proposing any change to our current policy for
implementing BACT requirements at a major source that is subject to the
requirements of the PSD program. Accordingly, if a physical or
operational change at the source will result in a significant emissions
increase and a significant net emissions increase of a regulated NSR
pollutant, then the major source must apply BACT (for that pollutant)
to the emissions unit(s) that will be physically or operationally
changed as a part of that project. Under the PM2.5 major NSR
program, BACT will be required at an emissions unit if a physical or
operational change at the unit causes a significant emissions increase
and significant net emissions increase of PM2.5 direct
emissions, or a PM2.5 precursor, if applicable at the major
stationary source.
10. What is EPA's plan for preventing significant deterioration of air
quality for PM2.5?
    Background. The PSD provisions of the CAA limit the degradation of
ambient air concentrations of certain pollutants. The CAA does not
dictate the mechanism to achieve this result for pollutants other than
PM10 and SO2. One mechanism involves a system of
``increments'' and area classifications that define significant
deterioration for individual pollutants. The PSD increments are the
maximum allowable increase in ambient air concentrations above a
baseline concentration for a criteria pollutant. The current increments
are:

----------------------------------------------------------------------------------------------------------------
       Pollutant/averaging time                Class I                  Class II                Class III
----------------------------------------------------------------------------------------------------------------
PM10
    Annual average...................  4 [mu]g/m3               17 [mu]g/m3              34 [mu]g/m3
    8-Hour average...................  8 [mu]g/m3               30 [mu]g/m3              60 [mu]g/m3
SO2
    Annual average...................  2 [mu]g/m3               20 [mu]g/m3              40 [mu]g/m3
    24-hour average..................  5 [mu]g/m3               91 [mu]g/m3              182 [mu]g/m3
    3 Hour Average...................  25 [mu]g/m3              512 [mu]g/m3             700 [mu]g/m3

    NO2--Annual Average..............  2.5 [mu]g/m3             25 [mu]g/m3              50 [mu]g/m3
----------------------------------------------------------------------------------------------------------------

    We are in the process of developing an approach for preventing
significant deterioration of air quality which may include
PM2.5 increments. The EPA has placed this action on a
separate administrative track due to the additional time necessary to
fully develop any potential proposal. In the interim period, States
must continue to implement the PM10 increments in 40 CFR
51.166, 52.21 and/or their SIPs, as applicable.
11. How will the air quality analysis required under section 165(a)(3)
be implemented?
    Scope of the Requirement. All sources subject to PSD review must
perform an ambient air quality impact analysis to show that the
emissions from the source do not cause or contribute to a PSD increment
or NAAQS violation. See CAA Section 165(a)(3); 40 CFR 51.166(k),
52.21(k). Accordingly, sources will be required to perform this
analysis for the PM2.5 NAAQS. Such analyses would consider
how a source impacts air quality at existing PM2.5 monitor
locations as well as at other locations which are appropriate to allow
the comparison of predicted PM2.5 concentrations to the
NAAQS, based on PM2.5 monitor siting requirements and
recommendations.
    Sources also will remain under an obligation to perform the air
quality impact analysis for the PM10 increments and the
PM10 NAAQS.
    Plan for Development of Significant Impact Levels for PM2.5. The
Agency has had a practice of exempting sources from the cumulative air
quality impact analyses where their level of contribution is below a
significant impact level (SIL). If the maximum ambient impacts from the
proposed project are less than a SIL, the source
     Is presumed to not cause or significantly contribute to a
PSD increment or NAAQS violation, and
     Is not required to perform multiple source cumulative
impact assessments.
    The EPA has long interpreted the ``significant contribution'' test
set forth in Sec.  51.165(b)(2) to apply to the PSD program since the
provision applies to major new sources and major modifications located
in attainment and unclassifiable areas. We have proposed codifying this
exemption in the PSD regulations in a separate Federal Register notice.
See 61 FR 38249, 38293 (July 23, 1996). This exemption is based on the
de minimis nature of the source's contribution.
    The SIL (in [mu]g/m3) have been established for other
criteria pollutants with PSD increments and are given below:

----------------------------------------------------------------------------------------------------------------
                                                                                Class I SIL
                                                                                [mu]g/m3
             Criteria pollutant                      Averaging time           (proposed 7/23/   Class II and III
                                                                                96, not         SIL [mu]g/m3
                                                                               promulgated)
----------------------------------------------------------------------------------------------------------------
SO2........................................  3 hour........................               1.0               25.0
                                             24-hour.......................                .2                5.0
                                             Annual........................                .1                1.0
CO.........................................  1 hour........................               N/A             2000
                                             8 hours.......................               N/A              500
NO2........................................  Annual........................                .1                1.0
PM10.......................................  24-hour.......................                .3                5.0
                                             Annual........................                .2                1.0
----------------------------------------------------------------------------------------------------------------

    Because the SIL benefits the NSR permitting program by exempting
sources with de minimis impacts from the cumulative air quality
analysis, EPA is considering establishing PM2.5 SIL for
emissions of PM2.5 direct. This would enable sources with
impacts below the SIL to avoid the cumulative air quality impact
analysis with respect to their potential contribution to a
PM2.5 NAAQS violation, and create a de minimis ``cause or
contribute'' definition for violations. Direct PM2.5
emissions can be evaluated with current models. Therefore, the
development of SIL for impact evaluations of direct PM2.5
emissions is technically achievable. The EPA is soliciting comments on
this question and on methods for the development of PM2.5
SIL.
    The limited capabilities of existing models make it difficult to
establish and implement SIL for PM2.5 precursors. Current
models are only able to accurately address individual source impacts
associated with direct PM2.5 emissions and, to a lesser
degree, SO2 and NOX. They can not accurately
predict single source impacts on ambient PM2.5
concentrations from other precursors. Without including formation of
PM2.5 from precursor emissions, the complete impact cannot
be assessed.
    EPA solicits comments and ideas on the direction to take and
possible approaches to setting PM2.5 SIL for direct and
precursor emissions. The EPA intends to use these comments in
developing SIL on a separate administrative track.
12. How should the PSD pre-construction monitoring requirement be
implemented for PM2.5?
    EPA solicits comment on what preconstruction monitoring
requirements should be required by the PM2.5 PSD program.
a. Background
    Sources subject to PSD are subject to pre-construction ambient air
quality monitoring requirements. See Sections 165(a)(7) and 165(e) of
the CAA and 40 CFR 51.166(m), Sec.  52.21(m). The PSD permitting
requirements currently provide that continuous pre-construction ambient
air quality monitoring must be conducted for any criteria pollutant
emitted in significant amounts. Under 40 CFR 51.166(i)(5), and 40 CFR
52.21(i)(5) the reviewing authority has the discretion to exempt an
applicant from this monitoring requirement if:
     The maximum modeled concentration for the applicable
averaging period caused by the proposed significant emissions increase
(or net emissions increase) is less than

the prescribed significant monitoring concentration (SMC); or
     The existing monitored ambient concentrations are less
than the prescribed SMC. The following are the SMC for criteria
pollutants:

------------------------------------------------------------------------
                                         Ambient
            Pollutants                concentration    Averaging  period
                                       ([mu]g/m3)
------------------------------------------------------------------------
CO................................               575  8 hours.
NO2...............................                14  Annual.
SO2...............................                13  24 hours.
PM10..............................                10  24 hours.
------------------------------------------------------------------------

    A source may also use existing data as a surrogate for pre-
construction monitoring if the existing monitored data record is
determined to be representative of the project's location. For
information on representative monitoring see ``Ambient Monitoring
Guideline for Prevention of Significant Deterioration (PSD),'' EPA-450/
4-87-007. Under the current regulatory approach, the need for pre-
construction monitoring by an applicant depends on the spatial and
temporal coverage of the current monitoring program. The expected
gradients of concentration between existing monitors also need to be
considered in deciding whether there is a need for pre-construction
ambient monitoring.
    The PM2.5 ambient monitoring data are used in the PSD
program to:
     Establish current PM2.5 NAAQS compliance status
in the project's impact area;
     Determine a representative background ambient
PM2.5 concentration which will be included with modeled
estimates to assess NAAQS compliance.
    The PM2.5 ambient monitoring measurements include
particulate matter from PM2.5 direct emissions and those
formed by PM2.5 precursors. If required of a particular
source, pre-construction monitoring could add one year to the
permitting process and increase the cost of the permit. Such a
requirement could have the effect of delaying or preventing sources
from undertaking environmentally beneficial projects. Accordingly,
today, we are reconsidering our current approach for satisfying the
pre-construction monitoring requirements for the purposes of the
PM2.5 standard. While we are proposing to retain the current
approach, we are also soliciting comments on innovative options that
could provide better solutions for satisfying the preconstruction
monitoring requirements.
b. Options for PSD Preconstruction Monitoring
    Preferred Option 1: Require preconstruction monitoring for all
major sources of PM2.5 direct and the precursors identified
as regulated NSR pollutants for PM2.5, but on a case-by-case
basis allow sources to satisfy this requirement by demonstrating the
existing PM2.5 network is sufficient. This option will
provide information on effects of new construction on the
PM2.5 NAAQS and increments. This option would not require a
change to the preconstruction monitoring regulations. Concerns about
this option include:
     It is challenging to find an appropriate location for any
monitor because PM2.5 direct emissions typically affect
nearby locations while precursor emissions affect areas farther away.
     The existing monitors can either measure total
PM2.5 mass or can provide data on the mass of different
PM2.5 components. The latter type, a speciation monitor, is
more expensive to operate but provides useful information on the
contribution of sources of precursor and PM2.5 direct
emissions.
    In cases where ambient PM2.5 concentration gradients
between existing monitors are small with little likelihood of local
site-specific ``hot spots,'' interpolation between existing monitored
values may be appropriate for determining that the existing
PM2.5 monitoring network is sufficient. We request comment
on this approach.
    Option 2: Exempt all PM2.5 sources from doing monitoring
by determining the existing PM2.5 network is sufficient.
    The use of the acquired PM2.5 monitored data record in
place of applicant performed pre-construction monitoring would follow
the current trend in PSD permitting activities. This procedure would
have the advantage of reducing the time required for permit preparation
and reduce the costs of the permit application. If ambient
PM2.5 concentration gradients between monitoring stations
are small there may be little need for additional monitoring data. The
need to make discretionary decisions on whether to perform pre-
construction monitoring would be eliminated.
    However, EPA favors the continued use of the case-by-case
determination as to the need to perform ambient PM2.5 pre-
construction monitoring because of the following limitations to using
the existing PM2.5 monitoring data record:
     The PM2.5 monitoring data record would require
spatial interpolation between monitors for the determination of
appropriate concentrations at the project's location.
     Use of existing monitored data will not increase the
PM2.5 monitoring data record to confirm or contradict
conventional perceptions.
     The PM2.5 monitoring data record assumes that
local hot spots of high PM2.5 concentrations do not exist or
are already being monitored, which may not be true in all cases.
     Automatic acceptance of existing measurements does not
follow EPA's current policy that a case-by-case determination needs to
be made to determine whether pre-construction ambient monitoring is
necessary.
     When used with the impact modeling, separate
concentrations of direct and precursor formed particulate matter is
needed.
    Because of these limitations, existing PM2.5 monitoring
data must be reviewed for applicability and representativeness before
being judged appropriate for use in lieu of project acquired ambient
data. The current PM2.5 network may not be sufficient for
all applicants. The EPA is soliciting comments and suggestions on this
issue.
    Option 3: Use Significant Monitoring Concentrations (SMC) to exempt
sources from pre-construction monitoring requirements. The reviewing
authority has the discretion to exempt an applicant from the pre-
construction monitoring requirement if the modeled impacts from the
proposed source are less than the prescribed SMC.
    Similar to the significant impact levels used in modeled impact
analysis, the PSD process will become simpler through the use of SMC.
It provides a definitive means for applicants with little impact to opt
out of the resource intensive, costly, and time consuming pre-
construction ambient air quality monitoring requirement. Therefore, it
is an important component of the PSD program.
    The form of the SMC will be defined by the form of the impact
modeling. SMC must be developed for direct PM2.5 emissions
if the impact modeling only addresses direct emissions of
PM2.5. This may require different direct and precursor SMC.
    Because of the advantages SMC provide to the NSR permitting
program, EPA is considering the development of PM2.5 SMC.
The EPA is soliciting comments on the development and use of
PM2.5 SMC in the PSD program. This option could be used in
combination with the other options described.
    Option 4: Use of the available large PM10 data record,
combined with the recent PM2.5 acquired ambient
measurements, may provide a monitoring data base that is sufficiently
distributed to provide representative

ambient measurements for most applicants. This would alleviate the need
for pre-construction monitoring and make the PSD program less
burdensome. This would also provide an interim means to estimate
ambient PM2.5 concentrations until more extensive monitoring
data record can be developed.
    However, the differences in characteristics between
PM2.5 and PM10, and our limited understanding of
their relationships, presents a problem.
     PM10 conversion factors may not sufficiently
reflect important industry specific and spatially related
characteristics of PM2.5.
     Removing the obligation to provide pre-construction
ambient monitoring data would eliminate industry's contribution to the
ambient PM2.5 data record.
    This may not be a viable substitute to satisfy the need to provide
representative PM2.5 ambient measurements. The EPA requests
comments on these options on pre-construction monitoring.
    Option 5: Existing Sec.  52.21(i)(5)(ii) and Sec.
51.166(i)(5)(iii) could be interpreted to allow a reviewing authority
to exempt an applicant from pre-construction monitoring for any
pollutant for which we have not established a SMC. These provisions
state that a source may be exempted from preconstruction monitoring
``if * * * the pollutant is not listed in'' the list of pollutants for
which SMC have been set.\102\ The original rationale for this exemption
is based on the lack of adequate methods for measuring ambient
concentrations of pollutants not on the list. 45 FR at 52709, 52723-
52724. We request comment on this interpretation and any other legal or
policy rationale that could support applying the text of these
provisions to exempt sources from preconstruction monitoring if we do
not define a SMC for PM2.5.
---------------------------------------------------------------------------

    \102\ These sections actually cross-reference the list at Sec.
51.166(i)(8)(i) and 52.21(i)(8)(i), however we renumbered those
sections to subsection (i)(5)(i) of those provisions in December
2002 and inadvertently overlooked correcting the cross-references in
subsections (i)(5)(ii) and (i)(5)(iii). See 67 FR 80186. It is
apparent from the rule as originally promulgated in 1980 that
subsection (i)(5)(i) is now the correct cross-reference. See 45 FR
52676, 52739 (Aug. 7, 1980). We propose to correct this misnumbering
and others in this section when we finalize today's proposal.
---------------------------------------------------------------------------

13. Nonattainment New Source Review (NA NSR) Requirements
    Background. Sources subject to NA NSR must:
     Install Lowest Achievable Emissions Rate (LAER) control
technology;
     Offset new emissions with creditable emissions reductions.
     Certify that all sources owned and operated by the same
owner within the State are in compliance; and,
     Conduct an alternative siting analysis demonstrating that
the benefits of the proposed source significantly outweigh the
environmental and social costs.
14. What are the offset requirements for NA NSR?
    Background. Under Section 173 of the Act, all major sources and
major modifications at existing sources within a nonattainment area
must obtain emissions reductions to offset any emissions increases
resulting from the project in an amount that is at least equal to the
emissions increase, and that is consistent with reasonable further
progress towards attainment. In addition, these offsets must be:
     From the same nonattainment area or a different
nonattainment area that impacts the area where the source is located
(as long as the other area has the same or higher classification);
     Federally enforceable; and
     Affect air quality in the area where the emissions
increases from the new major source or modification are occurring.
    We refer to the proportional difference between the amount of the
required offsets to the amount of emissions increase as the ``offset
ratio.'' The offset ratios for the other criteria pollutants are:

------------------------------------------------------------------------
                 Pollutant                          Offset ratio
------------------------------------------------------------------------
Ozone.....................................  At least 1:1 to 1.5:1
                                             depending on ozone
                                             nonattainment
                                             classification.
PM10......................................  At least 1:1.
NOX.......................................  At least 1:1.
SO2.......................................  At least 1:1.
Lead......................................  At least 1:1.
CO........................................  At least 1:1.
------------------------------------------------------------------------

a. What is the required offset ratio for PM2.5 direct
emissions?
    The Act specifies an offset ratio for several situations. In ozone
nonattainment areas subject to subpart 2, the ratio is set between
1.1:1 and 1.5:1 depending on the area's level of classification
pursuant to subpart 2 of the Act. For other nonattainment areas, the
Act establishes a minimum offset ratio of 1:1 pursuant to Subpart 1 of
the Act. Since the PM2.5 program is being implemented under
subpart 1, the applicable ratio is at least 1:1 on a mass basis. We
request comment on establishing a required offset ratio of at least
1:1, and on any other option for establishing the required offset ratio
for PM2.5 direct emissions.
b. Which precursors shall be subject to the offset requirement?
    If we identify a precursor as a regulated NSR pollutant in our
final action, then that pollutant will be subject to the offset
requirement. Accordingly, consistent with our preferred approach for
identifying SO2 as a national precursor and NOX
as a presumptive national precursor, we propose that SO2 and
NOX would be subject to the offset requirement. VOCs and
ammonia would be subject to the offset requirement if we designated
these pollutants as PM2.5 precursors for the purposes of
major NSR. If we adopt an approach under which the precursors are
presumptively excluded from major NSR unless and until a State NA NSR
program specifically includes such a pollutant, then the precursor
would not be subject to the offset requirement until such time.
c. What is the required offset ratio for PM2.5 precursors?
    The Act requires that a source obtain offsets for emissions
increases that occur in a nonattainment area. As with PM2.5
direct emissions, the minimum offset ratio permitted under the Act
would be at least 1:1. We believe this ratio should apply where a
source seeks to offset an increase in emissions of a PM2.5
precursor with creditable reductions of the same precursor. We request
comment on requiring an offset ratio of at least 1:1 for any precursor
identified by the Administrator as a regulated NSR pollutant for
PM2.5 nonattainment areas. We also request comment on
whether this mandatory offset ratio should apply to any other precursor
identified by a State for regulation through its SIP-approved
nonattainment major NSR program, or whether the State should have the
option to establish a different offset ratio for such pollutant.
d. Should EPA allow interprecursor trading to comply with the offset
requirement?
    Because several different pollutants contribute to ambient
PM2.5 concentrations, we are proposing to allow flexibility
in how major sources may satisfy the offset requirement. Specifically,
we are proposing to allow increases in emissions of direct
PM2.5 to be offset by a decrease in PM2.5
precursor emissions; and we are proposing to allow an increase in a
PM2.5 precursor to be offset by a decrease in emissions of a
different PM2.5 precursor or with PM2.5 direct

emissions. However, such trades would only be permissible if the State
shows that the trade is beneficial in reducing overall ambient
concentrations of PM2.5, and the Administrator approves of
the trade.
    This additional flexibility might make it difficult to ensure that
the ambient air concentration of PM2.5 continues to
decrease. It may also be administratively difficult to manage.
Nonetheless, we are proposing to allow interprecursor trading to
generate creditable emissions reductions for use as offsets, because we
believe that reductions of a different PM2.5 precursor may
have an equal or better impact in reducing ambient PM2.5
concentrations if an appropriate offset ratio is determined.
Additionally, interprecursor trading may provide a reliable source of
offset emissions in areas where availability may otherwise be limited.
    There are several ways in which interprecursor trading for offsets
could be implemented. Under one approach, a State would develop its own
interprecursor trading rule for inclusion in its SIP, based on a
modeling demonstration for a specific nonattainment area. The EPA would
review a State interprecursor trading rule during the SIP approval
process. Once approved, the State could follow this approach on all
future NSR permits issued. Another approach would be to review
individual trades as part of the major NSR permitting process. The EPA
and the public would have an opportunity to comment on whether the
modeling or other technical evidence presented by a particular State is
sufficient to support interprecursor offsets for that specific permit
application. Under either approach, a State could not allow
interprecursor trading without EPA approval. The EPA is requesting
comment on whether, States should be required to demonstrate the
adequacy of offset ratio(s) using modeling as part of a State rule, in
demonstrations for specific nonattainment areas, and/or on a permit-by-
permit basis, and/or on some other basis. While EPA believes that such
interprecursor trading flexibility is more appropriate for offsets
which are statutorily required, we are seeking comment on whether this
flexibility should also apply to netting analysis for a source.
15. What are the implementation and transition issues associated with
this rule?
    Implementation. Implementation of NSR for PM2.5 is
dependent on:
     Who implements the program and
     What regulations are used to implement NSR.
    The components of the NSR programs are implemented by the
following:
     PSD: States or EPA
     Nonattainment NSR: State or EPA
     Minor NSR: States only
     NSR in Indian country: Tribes or EPA
    Transition. The requirements applicable to NSR SIPs for and the
obligation to subject sources to NSR permitting for PM2.5
direct and precursor emissions are codified in the existing federal
regulations, and can be implemented without specific regulatory
changes. The existing regulations require NSR for any NAAQS pollutant
for which an area is designated attainment or nonattainment. See 40 CFR
51.160(b); 51.165(a)(2)(i); 51.166(a)(7); 52.21(a)(2); 52.24(k); 40 CFR
part 51, Appendix S, Section IV. A. Thus, the obligation to implement
PSD for the NAAQS was triggered upon the effective date of the NAAQS,
as explained in prior guidance.\103\ (In that guidance, EPA also
explained that PSD permitting for PM10 would be accepted as
a surrogate approach for this obligation, as discussed in more detail
below.) For nonattainment areas, permits must comply with the
nonattainment NSR requirements for PM2.5, either in a
State's approved part D program or, where that is lacking, as set forth
in 40 CFR part 51, Appendix S, pursuant to Sec.  52.24(k). To clarify
how these requirements are to be implemented for PM2.5, we
are proposing to add provisions to:
---------------------------------------------------------------------------

    \103\ See ``Interim Implementation for New Source Review
Requirements for PM2.5,'' J. Seitz, EPA (Oct. 23, 1997).
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     40 CFR 51.166--implementation plan requirements for major
new or modified sources in attainment or unclassifiable areas;
     40 CFR 51.165--implementation plan requirements for
addressing major new or modified sources in nonattainment areas and
sources located in attainment or unclassifiable areas that would impact
a nonattainment area;
     40 CFR 52.21--the federal implementation plan for areas
lacking an approved SIP or TIP program to regulate construction or
modification of major stationary sources in an attainment or
unclassifiable area.
     40 CFR part 51, Appendix S--provisions for issuing permits
before a State has an approved implementation plan regulating
construction or modification of major stationary sources.
16. Implementation of PSD provisions during the SIP Development period
a. Background
    On October 23, 1997, we issued a guidance document entitled
``Interim Implementation for the New Source Review Requirements for
PM2.5,'' John Seitz, EPA. 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. As discussed in this preamble,
those difficulties have been resolved in most respects, and where they
have not been, the proposal contains appropriate provisions to account
for it. These issues will be finally resolved by the Agency upon
promulgation of these proposed revisions. When final, these revisions
will take effect immediately on the effective date in States that issue
permits under a delegation from EPA. However, States with a SIP-
approved PSD program requiring amendments to incorporate these rule
changes will need additional time to incorporate the final NSR rule
change for PM2.5 into their SIPs. For example, a State may
need to amend their existing regulations to add the specific
significant emissions rate for PM2.5 or a designated
precursor. We propose to require that States with SIP-approved PSD
programs submit revised PSD programs for PM2.5 at the same
time that they must submit nonattainment NSR programs for
PM2.5 (April 5, 2008).\104\ However, during the SIP-

development period, the PM2.5 NAAQS must still be protected
under the PSD program in such States.
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    \104\ We note that we requested that States submit
certifications that their SIPs were adequate with respect to certain
infrastructure elements, including PSD, for the PM2.5
NAAQS, by July 2000, consistent with Section 110(a)(1) and (2). See
Re-issue of the Early Planning Guidance for the Revised Ozone and
Particulate Matter (PM) National Ambient Air Quality Standards
(NAAQS) (June 16, 1998). In accordance with a Consent Decree in
Environmental Defense and American Lung Ass'n v. Johnson, No.
1:05CV00493 (D.D.C. June 15, 2005), EPA must determine by October 4,
2008 whether each State has submitted SIP revisions for
PM2.5 required under section 110(a)(2) of the Clean Air
Act.
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b. Proposed Options
    Upon promulgation of this rule, States that accept delegation would
implement the PM2.5 program in 40 CFR 52.21 from the
effective date of this rule. However, for SIP-approved States, we seek
comment on the following options to address implementation of the PSD
program from the time this rule is final until EPA approves a State's
PSD program for PM2.5:
Option 1--Continue Implementing the 1997 Guidance To use
PM10 Program as a Surrogate for PM2.5
    We are proposing that if a SIP-approved State is unable to
implement a PSD program for the PM2.5 NAAQS upon
promulgation of these proposed revisions, then the State may continue
to implement a PM10 program as a surrogate to meet the PSD
program requirements for PM2.5 pursuant to the 1997 guidance
mentioned above. However, to assure that use of PM10 is
protective of the PM2.5 NAAQS, the State must assure that
two requirements are met. First, States must require sources to
demonstrate that emissions from construction or operation of the
facility will not cause or contribute to a violation of the
PM2.5 NAAQS. We believe that States have the authority to
implement this requirement through existing SIP-approved programs.
Second, States will be required to include condensible particulate
matter emissions in determining major NSR applicability and control
requirements. As discussed elsewhere, PM10 already includes
condensible emissions, but many States have not regulated condensible
emissions in implementing the PM10 NAAQS because EPA has not
consistently implemented its guidance on this issue. Because
condensible emissions are essentially fine particles and a larger
fraction of PM2.5 emissions in comparison to
PM10, EPA believes inclusion of condensible emissions during
the SIP development period for PSD programs is necessary to ensure that
the PM10 indicator acts as an adequate surrogate for
PM2.5.
Option 2--Update the 1997 Guidance to Include Proposed Provisions of
this Rule or Amend 40 CFR part 51, Appendix S to State That 40 CFR
52.21 Would Apply
    Another option would be to update the 1997 guidance to reflect the
provisions in this proposed rule and allow States to run a
PM2.5 program pursuant to this updated guidance.
Alternatively, we would amend Appendix S and 40 CFR 52.24 so that the
PSD requirements of 40 CFR 52.21 would govern the issuance of major NSR
permits during the period between the time we finalize this
implementation rule and when we approve changes to the State's PSD
program to include PM2.5 as a regulated NSR pollutant. This
provision would not apply to sources located in Indian Country because
they are already directly subject to the requirements of 40 CFR 52.21.
    If a State does not believe it has the authority to issue PSD
permits consistent with Appendix S, then EPA would issue the permit. We
specifically seek comment on whether we should update the 1997 guidance
or amend Appendix S to allow States to run a PSD program for
PM2.5 in attainment areas during the SIP development period.
Option 3--State Requests Delegation of 40 CFR 52.21
    A third option would be for EPA to allow a State to request
delegation of just the federal PM2.5 program (reflected in
Sec.  52.21 of our regulations) in that State. A State that otherwise
has a SIP-approved PSD program could request delegation for
PM2.5 by informing EPA that it does not intend to submit a
PSD SIP for PM2.5 in the immediate future.
    After promulgation of a new NAAQS, EPA may allow States up to three
years to submit a State implementation plan containing a PSD program
for that pollutant. 42 U.S.C. 7410(a). EPA's PSD regulation at Sec.
51.166 gives SIP-approved States up to three years to submit a revision
to their PSD program after EPA amends Sec.  51.166. (Sec.
51.166(a)(6)). Under section 110(c) of the Act, EPA must promulgate a
federal implementation plan (FIP) upon finding that a State has failed
to make a required plan submission or that a required submission is
inadequate.
    If a State notifies EPA prior to the close of the customary three-
year period that the State does not intend to submit a PSD SIP for
PM2.5 in the immediate future and requests delegation, we
believe EPA could find that the State has failed to submit the
requisite PSD SIP for PM2.5, promulgate a PSD FIP for
PM2.5 based on 40 CFR 52.21, and delegate implementation of
the federal PSD program to the State. The State would then be able to
implement a PSD program for PM2.5 in accordance with the
terms of section 52.21, as amended in this rulemaking action. However,
such a State would still have the option to obtain EPA approval of a
PSD SIP for PM2.5 if it submitted the SIP revision at a
later date.
c. Rationale
    We believe option 1 is reasonable for the following reasons. First,
PM10 will act as an adequate surrogate for PM2.5
in most respects, because all new major sources and major modifications
that would trigger PSD requirements for PM2.5 would also
trigger PM10 requirements because PM2.5 is a
subset of PM10. The one situation where this would not be
true is where a source emitted significant amounts of condensible
emissions that would not otherwise be counted under a State's
PM10 PSD program. This is the reason EPA would ensure that
States include condensible emissions in determining major NSR
applicability as a condition of using PM10 as a surrogate.
Second, both of the precursors proposed for regulation in this
preamble--SO2 and NOX--are already regulated
under State NSR programs for other criteria pollutants. Thus, those
precursors will be subject to NSR through those other programs. Third,
requiring immediate implementation of the Section 165(a)(3) air quality
analysis for the PM2.5 NAAQS will adequately cover the
remaining gap that results from using PM10 as a surrogate
for PM2.5.
    Upon promulgation of these rules, except in SIP-approved States
which would be running a PM10 program as a surrogate for a
PM2.5 program as stated in option 1 above, a
PM2.5 program would apply in attainment areas of delegated
States and in nonattainment areas. Hence to avoid this imbalance, we
are seeking comment on option 2 which addresses whether there is a need
to update the 1997 interim policy to reflect these rules in SIP-
approved States or whether we should amend Appendix S to allow these
States to run a PM2.5 program for PSD based on the
requirements of 40 CFR 52.21 during the SIP development period in
attainment areas.
    Option 3 would also address this imbalance by allowing a State to
request delegation of only the PSD program for PM2.5 prior
to the deadline for submitting a PSD SIP for PM2.5. (April
5, 2008 as discussed in the background discussion of section M16.)
Because we need to allow a State enough time to submit a PSD SIP for
PM2.5, we do not

believe we can unilaterally issue a FIP for the PSD PM2.5
program right away. However, if a State informs EPA prior to April 5,
2008 that it does not intend to submit a PM2.5 SIP, we would
then have cause to issue a FIP addressing the PSD program for
PM2.5 and then delegate that program to the State.
17. Implementation of the Nonattainment NSR Provisions During the SIP
Development Period
a. Background
    EPA interprets section 172(c)(5) of the Clean Air Act to require
that States issue major NSR permits for construction and major
modifications of major stationary sources in any nonattainment area.
Thus, since the PM2.5 nonattainment designations became
effective on April 5, 2005, States are now required to issue major NSR
permits that address the Section 173, nonattainment major NSR
requirements for PM2.5. On the date that the
PM2.5 non-attainment designations took effect (April 5,
2005), we issued a guidance to address implementation of the NA NSR
program pending the completion of this action to develop implementation
rules for PM2.5. See memorandum from Stephen D. Page,
Director, Office of Air Quality Planning and Standards to Regional Air
Directors, ``Implementation of New Source Review Requirements in
PM2.5 Nonattainment Areas'' (Apr. 5, 2005).
    Our current guidance permits States to implement a PM10
nonattainment major NSR program as a surrogate to address the
requirements of nonattainment major NSR for the PM2.5 NAAQS.
A State's surrogate major NSR program in PM2.5 nonattainment
areas may consist of either the implementation of the State's SIP-
approved nonattainment major NSR program for PM10 or
implementation of a major NSR program for PM10 under the
authority in 40 CFR part 51, Appendix S. Appendix S generally applies
where a State lacks a nonattainment major NSR program covering a
particular pollutant.
    Once this PM2.5 implementation rule is finalized, States
will have the necessary tools to implement a major NSR program for
PM2.5 States will no longer be permitted to implement a
nonattainment major NSR program for PM10 as a surrogate for
the PM2.5 nonattainment major NSR program. Most States will
then need to implement a transitional PM2.5 nonattainment
major NSR program under Appendix S (as amended in this rulemaking
action) until EPA approves changes to a State's SIP-approved major NSR
program to reflect the requirements of this rule.
    The NA NSR provisions in a State's existing SIP-approved NA NSR
program would only apply in areas designated nonattainment for the
PM2.5 NAAQS if the SIP-approved regulations contain a
generic requirement to issue part D permits in areas designated as
nonattainment for any criteria pollutant and do not otherwise need to
be amended to incorporate the changes proposed in this rule. In the
situations described below, the States will need to revise their NA NSR
regulations and submit them to EPA for incorporation into the SIP by
the date the new implementation plans for PM2.5 are due
(April 5, 2008):
     States that have nonattainment regulations that need to be
amended to incorporate the new PM2.5 requirements.
     States that have newly designated nonattainment areas for
PM2.5 and nonattainment NSR regulations that specifically
list the areas in which NA NSR applies (i.e., the list does not include
the newly designated areas).
     States that currently have no nonattainment areas but have
newly designated nonattainment areas for PM2.5.
    States in the categories listed above will have to implement a
transitional major NSR permitting program for PM2.5 pursuant
to 40 CFR 52.24(k) and Appendix S until their existing part D SIPs are
revised to meet these new PM2.5 NSR regulations.
b. Implementation of NSR Under the Emissions Offset Interpretative
Ruling (40 CFR part 51, Appendix S) With Revisions
    In general, Appendix S requires new or modified major sources to
meet LAER and obtain sufficient offsetting emissions reductions to
assure that a new major source or major modification of an existing
major source will not interfere with the area's progress toward
attainment. Readers should refer to 40 CFR part 51, Appendix S for a
complete understanding of these and other existing Appendix S
requirements. In this action, we propose to revise Appendix S to
include provisions necessary to implement a transitional major NSR
program for PM2.5, including significant emissions rates
applicable to major modifications for PM2.5 and, as
appropriate, precursors.\105\
---------------------------------------------------------------------------

    \105\ In a separate Federal Register notice, we will be revising
Appendix S to incorporate changes that conform Appendix S with the
minimum requirements for implementation plans that are set forth in
40 CFR 51.165.
---------------------------------------------------------------------------

    As currently written, Appendix S applies directly to major
stationary sources. In accordance with the requirements of Section
110(a)(2)(c) of the Act, we believe that the majority of States have
the legal authority to issue permits consistent with these requirements
under an existing SIP-approved permitting program. Nonetheless, at
least one State has reported that it lacks the legal authority to issue
permits implementing the requirements of Appendix S under its existing
permitting rules. If a State is unable to apply the requirements of
Appendix S, EPA will act as the reviewing authority for the relevant
portion of the permit.
    We believe that it is appropriate for EPA to issue the pre-
construction permits in such circumstances. As discussed earlier,
Congress amended the Act in 1990 to remove the requirements that would
have applied a construction ban in area's that lacked a SIP-approved
part D permit program. Thus, we believe that it is consistent with
Congressional intent that either the State or EPA issue permits to
construct during the interim period.
c. Legal Basis for Requiring States To Issue Nonattainment NSR Permits
During the SIP-Development Period
    Section 110(a)(2)(C) of the CAA establishes a general duty on
States to include a program in their SIP that regulates the
modification and construction of any stationary source as necessary to
assure that NAAQS are achieved. This general duty exists during all
periods, including before a State has an approved NA NSR permit
program.
    Section 110(a)(2)(c) of the Act does not define specific
requirements States must follow for issuing major source permits during
the period between nonattainment designation and EPA approval of a
nonattainment NSR SIP (the ``SIP-development'' period). However, EPA
has historically recognized that the SIP development period provided
under Section 172(b) of the CAA leaves a gap in part D major NSR
permitting and has determined that this gap is to be filled, in
general, with a transitional major NSR program that includes the LAER
and offset requirements from part D. 57 FR 18070, 18076 (Apr. 28,
1992). This transitional NSR program has been implemented, to date,
through the Emissions Offset Interpretative Ruling at 40 CFR part 51,
Appendix S. The EPA's regulations at 40 CFR 52.24(k) require that
Appendix S govern permitting during this time.\106\

In addition, Congress indicated in the 1977 CAA Amendments that major
NSR permitting should apply during the SIP development period. See
Public Law 95-95, section 129(a), 91 Stat. 685 (1977). Specifically, in
1977, when Congress enacted a moratorium on construction in any area
lacking an approved part D SIP, with a delayed effective date of July
1, 1979, Congress directed that Appendix S govern permitting of sources
constructing in such areas prior to that date. Id. section 108(b),
section 129(a).
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    \106\ EPA has interpreted this requirement to require States to
issue permits that are consistent with the requirements in Appendix
S. We believe that many States have the authority to issue permits
that are consistent with Appendix S for example, through State minor
NSR permit programs. However, if a State lacks authority to issue a
permit, then EPA will issue the permit.
---------------------------------------------------------------------------

    The EPA subsequently codified the use of Appendix S as the
transitional major NSR program in 40 CFR 52.24(k), reasoning (in the
context of implementing a delay in the construction ban for then-
recently designated nonattainment areas) that Congress had directed
that Appendix S remain in effect to protect air quality while State
plans were being designed. 45 FR 91604 (Oct. 2, 1980). When Congress
removed the construction ban (except as provided in Section 110(n)(3)),
it left 40 CFR 52.24(k) in place, implementing the transitional major
NSR program under Appendix S.
    The continued application of appendix S through Sec.  52.24(k) is
also supported by one of the purposes of the Clean Air Act ``to protect
and enhance the quality of the Nation's air resources so as to promote
the public health and welfare and the productive capacity of its
population.'' 42 U.S.C. 7401(b)(1). This provision was the basis for
the original judicial finding that the Act imposed an obligation to
prevent significant deterioration in areas that meet the NAAQS, prior
to Congress' enactment of the PSD program at part C of the Act.\107\
This policy of nondegradation applies with even greater force in areas
that fail to meet the NAAQS. Thus, we believe that an interim major NSR
program for the SIP development period--as codified at appendix S and
updated to reflect CAA amendments and the promulgation of the
PM2.5 NAAQS--is supported by section 110(a)(2)(C), section
101(b)(1), Congressional intent, and our gap-filling authority under
section 301(a).
---------------------------------------------------------------------------

    \107\ See Alabama Power Co. v. Costle, 636 F.3d 323, 346-047 (DC
Cir. 1980) (discussing Sierra Club v. Ruckelshaus, 344 F. Supp. 253
(D.D.C. 1972), aff'd per curiam 4 ERC 1815 (DC Cir. 1972), aff'd by
an equally divided court, sub nom Fri v. Sierra Club, 412 U.S. 541
(1973).
---------------------------------------------------------------------------

    Although EPA omitted Sec.  52.24(k) from the regulatory text
accompanying a proposed rulemaking in 1996 (see 61 FR 38250, 38305
(July 23, 1996)), the preamble indicated that the change was intended
only to update and clarify the regulation with regard to the changes to
the construction ban made by the 1990 Amendments.\108\ The proposal did
not in any manner indicate that EPA believed that NSR permits complying
with Appendix S, or otherwise satisfying Section 110(a)(2)(C), were not
required during the interim period. We have discussed the continued
applicability of Sec.  52.24(k) and Appendix S in implementation of the
8-hour ozone NAAQS. 68 FR at 32846.
---------------------------------------------------------------------------

    \108\ The actual language at 40 CFR 52.24(k) arguably allows
States to issue permits under Appendix S for a maximum period of 18
months after designation. After this time, if the nonattainment area
does not have an approved NA NSR permit program, a construction ban
would apply. However, in 1990, Congress altered the provisions of
the construction ban such that it would not apply when a State/Local
lacked an approved NA NSR permit program in the future. The EPA
believes that Congress' removal of the construction ban from the Act
supersedes the regulatory language at 52.24(k) and EPA has
reinterpreted this language to allow States to issue permits under
Appendix S from designation until the SIP is approved even if this
exceeds 18 months. See 1991 memo, ``New Source Review (NSR) program
Transitional Guidance, John S. Seitz, March 11, 1991. The EPA
anticipates revising the language at section 52.24(k) to properly
reflect this interpretation.
---------------------------------------------------------------------------

18. NSR Applicability to Precursors During the Interim Period
    As discussed in Section M.2, EPA has proposed several options for
NSR applicability to the potential PM2.5 precursors
(SO2, NOX, VOC, and ammonia). EPA has proposed
that SO2 is a national precursor to PM2.5. EPA
has also proposed that if NOX emissions are subject to NSR
as PM2.5 precursor, States could exempt NOX from
its PM2.5 NSR program in a specific area by demonstrating
that NOX emissions from stationary sources in that area are
not a significant contributor to that area's ambient PM2.5
concentrations and the area is not in a State identified by EPA as a
source of a PM2.5 interstate transport problem. However,
during the SIP development period, States face substantial hurdles in
making such a demonstration because they are in the initial stages of
gathering information and analyses necessary to prepare their
attainment demonstrations. Thus, during this period, a presumption that
a precursor is a regulated NSR pollutant for PM2.5 may
amount to an irrebuttable presumption for many States. Because of the
challenges posed by the SIP development period, EPA is considering
whether NSR applicability to precursors should be stayed for one or
more precursors during the SIP development period. The EPA is
soliciting comments on the applicability of NSR to precursors during
the SIP development period.
19. Are there any Tribal concerns?
    We expect that some Tribal areas will be designated as
nonattainment in part because of pollution that is transported from
surrounding State lands. Tribal representatives have advocated for
additional flexibility to address nonattainment problems caused by
transported pollution, such as the provision of NSR offset set-asides
(which we expect would come from State offset pools or banks), because
they have limited ability to generate offsets on their own. Tribal
representatives have raised these and other concerns in discussions on
implementation of the 8-hour ozone and PM2.5 standards, and
in comments on the 8-hour ozone implementation rule.\109\ We request
comment on whether emissions offset set-asides, possibly generated by
innovative measures to promote additional emissions reductions, are an
appropriate method to help level the playing field for the Tribes and
support economic development in Tribal areas. We also request comment
on ways in which States may help provide the Tribes access to offsets
from non-Tribal areas.
---------------------------------------------------------------------------

    \109\ 109 Letter from Bill Grantham, National Tribal
Environmental Council, to docket 2003-0079, providing comments on
the proposed 8-hour ozone implementation rule (66 FR 32802).
---------------------------------------------------------------------------

    In addition, to address these and other issues related to
implementation of the NSR program in Indian country, EPA is evaluating
the impact of the NSR program on Tribes in Indian country. The EPA
plans to address these concerns in a future Tribal NSR rule.
20. What must a State or local agency do about minor sources of
PM2.5?
    Pursuant to Section 110(a)(2)(C), States must have a minor source
permitting program. This applies to new and modified stationary sources
that are not considered major for a criteria pollutants or a precursor
for a criteria pollutant. At this time States must include the
following pollutants in their minor NSR program:
     VOC,
     SO2,
     NOX,
     CO,
     PM10, and
     Lead (Pb)
    States must now amend their minor source programs to include
     PM2.5 direct emissions, and
     Precursor emissions as included in PM2.5 major
NSR.

21. Supplemental Program Option: Rural Transport Areas
a. What flexible implementation options should be available for
Transport areas?
    EPA is considering flexible implementation for Nonattainment NSR
for areas that qualify for the transport classification. These areas
are designated nonattainment due to overwhelming transport, for
example, areas where pollution is from surrounding jurisdictions but
where there are few or no sources of PM2.5 in the area.
Under the current program no flexibility is available under NA NSR for
sources in these areas overwhelmed by transport. As mentioned earlier,
in this rule, we are proposing a transport classification to provide
some flexibility to address some of the fairness issues associated with
transport. This transport classification can be used by States and
Tribes if they meet the criteria discussed below. If there is no
transport classification then this option will not be available in the
near-term. However, EPA intends to develop a separate proposed rule on
flexible implementation of nonattainment NSR for areas designated
nonattainment for any criteria pollutant, where transport is the
primary cause of the area's nonattainment. Such a proposal would not be
dependent on the incorporation of a transport classification in a
classification system for a NAAQS.
b. Which nonattainment areas would be eligible for the transport
program?
    In order to be eligible for the transport option the State/Local
with jurisdiction over a nonattainment area must:
    (1) Have submitted an attainment plan which demonstrates, through
modeling, that the area is designated nonattainment due to overwhelming
transport from an upwind area(s); and
    (2) Have submitted an attainment plan containing any additional
local control measures needed for attainment of the PM2.5
standard; and
    (3) Have submitted the attainment plan that commits the State/Local
to implement a program that meets the requirements for transport areas
discussed below.
    As described earlier in the classification section, an area will
not be reclassified as a ``transport'' area until after the SIP is
approved by the Regional Office. A transport area could apply for
single or multi-state/Local nonattainment areas. Such areas will not be
able to implement the nonattainment NSR transport program until the
area is reclassified as a ``transport'' area. Until an area is
reclassified, States must continue to apply the nonattainment NSR
program.
c. What would be the basic requirements of a transport nonattainment
NSR program?
    EPA is requesting comment on what type of regulatory flexibility
would be beneficial for transport areas while providing equal
environmental protection. Specific examples of needed flexibility for
areas which the commenter suggests would qualify as transport areas
would be helpful. As noted above, we anticipate proposing a separate
rulemaking on the details of the NSR requirements.

N. How will EPA ensure that the 8-hour ozone standard will be
implemented in a way which allows an optimal mix of controls for PM2.5,
ozone, and regional haze?

1. Could an area's PM2.5 strategy affect its 8-hour ozone
and/or regional haze strategy?
    Based on current data, many areas are violating both the 8-hour
ozone and the PM2.5 NAAQS. Thus, many cities will have ozone
and PM2.5 nonattainment areas with overlapping boundaries.
Requirements for regional haze apply to all areas. Each State is
responsible for developing SIP revisions to meet all the requirements
relevant to each nonattainment area for each pollutant as well as
developing a regional haze plan. In some cases, ozone control measures
may also be useful for a PM2.5 control strategy or a
regional haze plan. Similarly, controls for PM2.5 may lead
to reductions in ozone or regional haze. For example, considered in
isolation, a metropolitan area's ozone strategy might be based on
additional VOC emissions reductions; if the area needs NOX
reductions for PM2.5 attainment, however, an optimal
approach might include a more complex ozone strategy using both
NOX and VOC reductions. We believe integration of ozone and
PM2.5 attainment planning will reduce overall costs of
meeting multiple air quality goals.
    Many of the factors affecting concentrations of ozone also affect
concentrations of PM2.5. Emissions of NOX and/or
VOC will lead to formation of organic particles and the precursors of
particulate nitrate, as well as ozone. The presence of ozone is an
important factor affecting PM2.5 formation; as ozone builds
up, so do hydroxyl (OH-) radicals which are instrumental in
oxidizing gas phase SO2 to sulfuric acid. The sulfuric acid
may be converted to sulfate particles, increasing the PM2.5
concentration. Further, the local ozone concentrations may be decreased
by the reaction of ozone with nitric oxide; thus, in some large urban
areas, a decrease in local NOX emissions can result in
higher local ozone concentrations, leading to higher OH radical
concentrations and increases in secondary PM2.5. Because the
precursors for ozone and PM2.5 may be transported hundreds
of kilometers, regional scale impacts must also be considered.
2. What guidance has EPA provided regarding ozone, PM2.5 and
regional haze interaction?
    States must develop 8-hour ozone and PM2.5 attainment
demonstrations for most nonattainment areas. General criteria for
attainment demonstrations are contained in 40 CFR part 51, appendix W
(i.e., ``EPA's Guideline on Air Quality Models''). The EPA's May 1999
draft ``Guidance on the Use of Models and Other Analyses in Attainment
Demonstrations for the 8-Hour Ozone NAAQS'' provides a set of general
requirements that an air quality model should meet to qualify for use
in an attainment demonstration for the 8-hour ozone NAAQS. The draft
guidance encourages States to integrate PM2.5 control
strategies with strategies designed to attain the 8-hour ozone NAAQS
and to meet reasonable progress goals for regional haze. In addition,
the draft guidance presents some modeling/analysis principles to help
States develop databases and capabilities for considering joint effects
of control strategies for ozone, PM2.5 and regional haze.
Because emissions and meteorological conditions vary seasonally, the
guidance recommends assessing the effects of an ozone control strategy
on annual PM2.5 concentrations by estimating effects on mean
PM2.5 for each season and using the resulting information to
estimate annual impacts. Emission estimates for VOC, NOX,
primary PM2.5, SO2 and ammonia will be needed. In
addition, the modeling should separately estimate the effects of the
ozone strategy on the major components of PM2.5: Mass
associated with sulfates, nitrates, organic carbon, elemental carbon,
and all other species. We believe that this approach is adequate to
ensure that the 8-hour ozone standard will be implemented by States in
a way that allows an optimal mix of controls for ozone,
PM2.5, and regional haze.
    Similarly, EPA's draft attainment demonstration guidance for
PM2.5 and regional haze states that models intended to
address secondary PM problems should also be capable of simulating
ozone formation and transport (January 2, 2001 (draft),

``Guidance for Demonstrating Attainment of Air Quality Goals for
PM2.5 and Regional Haze''). The formation and transport of
secondary PM are closely related to processes that are important in the
formation and transport of ozone. Thus, it makes sense for programs
designed to control ozone to be cognizant of programs to reduce
PM2.5 and improve visibility and vice versa. The
PM2.5 guidance suggests conducting a ``mid-course review''
of an approved PM2.5 plan to review changes in air quality
resulting from implementation of plans to reduce PM2.5,
regional haze, and ozone (see section E).
3. What is EPA proposing?
    Today, we propose to continue the policy of encouraging each State
with a PM2.5 nonattainment area which overlaps, is near to,
or otherwise affects an 8-hour ozone nonattainment area to take all
reasonable steps to coordinate the required control measures needed to
attain the standards in nonattainment areas and meet reasonable
progress goals for regional haze. Specifically, States conducting
modeling analyses for PM2.5 should evaluate the concurrent
effects of control strategies on estimated ozone levels. In addition,
we encourage States conducting modeling analyses for ozone to estimate
separately the effects of ozone control strategies on PM2.5
and its precursors.

O. What emission inventory requirements should apply under the PM2.5
NAAQS?

    Emission inventories are critical for the efforts of State, local,
tribal and federal agencies to attain and maintain the NAAQS that EPA
has established for criteria pollutants including PM2.5.
Pursuant to its authority under section 110 of Title I of the CAA, EPA
has long required States to submit emission inventories containing
information regarding the emissions of criteria pollutants and their
precursors. The EPA codified these requirements in 40 CFR part 51,
subpart Q in 1979 and amended them in 1987.
    The 1990 CAAA revised many of the provisions of the CAA related to
attainment of the NAAQS and the protection of visibility in mandatory
Class I Federal areas (certain national parks and wilderness areas).
These revisions established new emission inventory requirements
applicable to certain areas that were designated nonattainment for
certain pollutants. In the case of particulate matter, the emission
inventory provisions are in the general provisions under Section
172(c)(3).
    In June 2002, EPA promulgated the Consolidated Emissions Reporting
Rule (CERR)(67 FR 39602, June 10, 2002). The CERR consolidates the
various emissions reporting requirements that already exist into one
place in the CFR, establishes new reporting requirements for
PM2.5 and ammonia, and establishes new requirements for the
statewide reporting of area source and mobile source emissions.
    The CERR establishes two types of required emission inventories:
     Annual inventories
     3-year cycle inventories
    The annual inventory requirement is limited to reporting statewide
emissions data from the larger point sources. For the 3-year cycle
inventory, States will need to report data from all of their point
sources plus all of the area and mobile sources on a statewide basis. A
special case exists for the first 3-year cycle inventory for the year
2002 which is due on June 1, 2004. The EPA has designated 2002 as the
new Base Year for 8-hour ozone, PM2.5 and regional haze
(November 18, 2002 EPA memorandum ``2002 Base Year Emission Inventory
SIP Planning: 8-Hour Ozone, PM2.5 and Regional Haze
Programs'' http://www.epa.gov/ttn/chief/eidocs/2002baseinven_102502new.pdf
).

    States would estimate mobile source emissions by using the latest
emissions models and planning assumptions available at the time the SIP
is developed. The latest approved version of the MOBILE model should be
used to estimate emissions from on-road transportation sources, in
combination with the latest available estimates of vehicle miles
traveled (VMT). The current version of the MOBILE model, MOBILE6.2, is
used for areas outside California.\110\ The model EMFAC2002 is used for
California. The latest information on MOBILE6.2 is available at: http://www.epa.gov/otaq/m6.htm.
 The NONROAD model is currently available in

draft form and can be used for estimates of non-road mobile source
emissions: http://www.epa.gov/otaq/nonrdmdl.htm. By merging the

information on point sources, area sources and mobile sources into a
comprehensive emission inventory, State, local and Tribal agencies may
do the following:
---------------------------------------------------------------------------

    \110\ As in the past, EPA will provide sufficient time for state
and local agencies to transition to any new motor vehicle emissions
factor model, if one becomes available during the development of
PM2.5 SIPs.
---------------------------------------------------------------------------

     Set a baseline for SIP development.
     Measure their progress in reducing emissions.
     Have a tool to support future trading programs.
     Answer the public's request for information.
    EPA uses the data submitted by the States to develop the National
Emission Inventory (NEI). The NEI is used by EPA to show national
emission trends, as modeling input for analysis of potential
regulations, and other purposes.
    Most importantly, States need these inventories to help
nonattainment areas develop and meet SIP requirements to attain the
annual and 24-hour PM2.5 NAAQS. There is a special situation
regarding emission inventories from Tribal areas that should be
considered. In the past, there have been instances where portions of
tribal areas have been included in designated nonattainment areas, but
when the baseline emission inventory was prepared, emissions from the
tribal lands were not included. This has had the effect of preventing
the tribes from generating emissions reductions from existing sources
to develop emission offsets, as well as impairing the ability of the
State to model as accurately as possible. We are encouraging the States
and Tribes to work together to ensure that the information used in
developing the baseline emission inventory is inclusive of all
emissions from the nonattainment area.
    In April 1999, EPA published the ``Emissions Inventory Guidance for
Implementation of Ozone and Particulate Matter National Ambient Air
Quality Standards (NAAQS) and Regional Haze Regulations,'' EPA-454/R-
99-006. The EPA updated this guidance in August 2005.\111\ The current
version of this guidance is available at: http://www.epa.gov/ttn/chief/eidocs/eiguid/index.html.
 The EPA developed this guidance document to

complement the CERR and to provide specific guidance to State and local
agencies and Tribes on how to develop emissions inventories for 8-hour
ozone, PM2.5, and regional haze SIPs. While the CERR sets
forth requirements for data elements, EPA guidance complements these
requirements and indicates how the data should be prepared for SIP
submissions. The SIP inventory also must be approved by EPA as a SIP
element and is subject to public hearing requirements where the CERR is
not. Because of the regulatory significance of the SIP inventory, EPA
will need more documentation on how the SIP inventory was developed by
the State as

opposed to the documentation required for the CERR inventory. In
addition, the geographic area encompassed by some aspects of the SIP
submission inventory will be different from the statewide area covered
by the CERR emissions inventory. If a State's 2005 emission inventory
(or a later one) becomes available in time to use for an area
subsequently redesignated nonattainment, then that inventory should be
used. We also encourage the cooperation of the Tribes and the State and
local agencies in preparing their emissions inventories.
---------------------------------------------------------------------------

    \111\ Emissions Inventory Guidance for Implementation of Ozone
and Particulate Matter National Ambient Air Quality Standards
(NAAQS) and Regional Haze Regulations,'' EPA-454/R-05-001, August
2005.
---------------------------------------------------------------------------

    Therefore, the basis for EPA's emission inventory program is
specified in the CERR and the related guidance document. The EPA is
interested in receiving comments on whether or not additional emission
inventory requirements or guidance are needed to implement the
PM2.5 NAAQS. For example, do any of the following issues
need to be defined through additional requirements or guidance?
     An important difference between inventories submitted in
response to the CERR and SIP inventories is the issue of approvability.
While it is likely that an inventory submitted under the CERR would be
identical to the inventory submitted as part of a SIP, the SIP
inventory will need to go through public hearing and formal approval by
EPA as a SIP element. This approval process can be combined with other
SIP elements. Should EPA specify an inventory approval process?
     Are the data elements specified within the CERR sufficient
to develop adequate SIPs? For example, in the determination of RACT
should more information on existing control devices be required?
     Currently the CERR requires the reporting of
SO2, VOC, NOX, CO, Pb, PM10,
PM2.5, and NH3. VOC and PM are speciated by the
emissions processing models based on speciation profiles for specific
source categories. Is this approach sufficient, or should EPA require
more specific emission component reporting such as specific organic
compounds or groups of compounds or reporting of elemental carbon and
organic carbon?
     The CERR allows states to adopt EPA developed emission
estimates from area and mobile sources in lieu of making these
estimates themselves if they accept these estimates for their emission
inventory. Since 2002 has been designated as the new base year, should
EPA require that States develop their own estimates for area and mobile
sources?
     Are there other inventory issues that EPA should define
through either regulation or guidance?

P. What stationary source test methods should States use under the
PM2.5 implementation program?

1. Will the existing stationary source test methods for particulate
matter (PM) be acceptable for use in PM2.5 SIPs?
    We believe that states that need to adopt local control measures
for primary particulate matter in nonattainment areas will need to
revise their stationary source test methods. However, the acceptability
of existing stationary source test methods for PM2.5 SIPs
depends upon what is measured under the State's current test methods
for particulate matter. Information available to the Agency indicates
that the majority of existing SIPs currently specify the use of
stationary source test methods that quantify only filterable
particulate matter. We believe that test methodologies that measure
only filterable particulate matter would be acceptable in areas where
no additional reductions of primary PM2.5 and particulate
precursor emissions are required to project attainment of the
PM2.5 NAAQS. The use of these existing stationary source
test methods provide verification that PM2.5 emissions are
consistent with the levels emitted as a result of existing applicable
requirements for filterable particulate matter. However, for areas
where additional local control of primary particulate matter emissions
are required as part of the attainment demonstration, we believe that
existing test methodologies that measure only filterable particulate
matter would not be acceptable. The use of existing source test methods
potentially would limit the control measures available for developing
cost effective strategies to achieve attainment of the PM2.5
NAAQS. In addition, the existing test methods may not be acceptable for
demonstrating compliance with emission limitations to achieve the
PM2.5 NAAQS under certain circumstances:
    (1) Where the attainment demonstration includes control
methodologies for PM precursors which are likely to result in a
significant increase in the direct emissions of fine particulate matter
(for example, ammonia injection to reduce NOX emissions).
    (2) Where the attainment demonstration includes control
methodologies for PM precursors which are likely to result in a
significant decrease in the direct emissions of fine particulate matter
(for example, alkaline scrubbers to reduce SO2 emissions)
and incorporate these direct emissions reductions in their attainment
demonstration or allow for the use of these reductions as credits for
other programs.
2. Why are the existing stationary source test methods for PM
deficient?
    Most stationary source test methods specified in State rules do not
adequately quantify either total PM emissions or PM2.5
emissions. Additionally, some of the current stationary source test
methods will not adequately provide a uniform indication of the
sources' performance in controlling PM2.5 emissions. Most
source test methods referenced in SIPs provide a measurement of the
particulate matter that is solid or liquid at a temperature specified
in the method or applicable standard. Filtration temperatures of 250
[deg]F and 320 [deg]F are typical although other temperatures may be
specified in a few test methods or applicable standards. Generally,
these filterable particulate matter test methods are either identical
or very similar to one of the ten Federal test methods published in
Appendix A of 40 CFR Part 60 and used to determine compliance with New
Source Performance Standards (NSPS). These test methods are adequate to
evaluate the compliance status of a source for emissions of that
component of particulate matter evaluated when the applicable rule was
developed. However, these test methods do not provide a measurement of
total particulate matter emissions, or PM2.5 emissions.
    The test method proposed to determine compliance with the first
group of NSPS (36 FR 15713) determined the sum of the mass of material
collected on or prior to the filters maintained at 250 [deg]F and the
material collected in the cooled impingers that followed the filter.
While the material collected prior to the filter provided a measure of
the filterable particulate material, the material collected in the
impingers was stated to measure vapors in the stack that would become
particulate matter at 70 [deg]F (36 FR 15495). When combined, the
method provided a measurement of the total particulate matter emissions
from the facility tested. The promulgated test method (36 FR 24888) did
not include the analysis of the impinger portion of the sampling train.
To accommodate the change in the test method, EPA made adjustments in
the promulgated emission limits to reflect the change in the test
method. The EPA made adjustments of up to 50 percent in the promulgated
emission limitations to

reflect the measurement of only the filterable portion of the
emissions.
    EPA recognized in setting several subsequent NSPS that the source
test method used to determine compliance with the particulate matter
emissions limits measured only part of the total particulate matter
emitted by the applicable sources. This recognition was published on
October 6, 1975, in the promulgated Revisions to Performance Testing
Methods (40 FR 46250). Similarly, EPA acknowledged this in the proposal
preamble to Subpart CC--Standards of Performance for Glass
Manufacturing Plants (6/15/79) in the section ``Selection of
Performance Test Methods: The use of EPA Reference Method 5--
Determination of Particulate Emissions from Stationary Sources.''
    In developing the NSPS emission limitations, it is evident that
only a portion of the particulate matter emissions were considered. As
a result, the test methods that EPA selected for determining compliance
with these emission limitations measured only that same portion of the
particulate matter emissions. It was recognized that these test methods
were not suitable for quantifying the total emissions to the atmosphere
and that the impinger portion of the sampling train contained the
missing portion of the particulate matter emissions.
    On December 17, 1990, EPA promulgated Method 202 in Appendix D of
40 CFR Part 51 (56 FR 65433) to provide a method for States to use to
analyze the impinger (or ``back half'') content of PM emissions and
provide a measure of the condensable particulate emissions. The
principal procedures in Method 202 improved upon the original Method 5
back half analysis proposed in 1971. In developing this measurement
method, EPA consulted with several State and local agencies and
incorporated several options to simplify or accommodate existing
policies and source testing methodologies for condensable particulate
matter. We believe that by excluding the optional components, the use
of EPA Method 202, combined with EPA Method 5 or EPA Method 17,
provides a reasonable indication of total particulate matter emissions
for the majority of stationary emission sources.
    However, the combination of EPA Method 5 and Method 202 measures
particulate matter that is larger than 2.5 micrometers in aerodynamic
diameter, and will not provide a reasonable measurement of the
emissions of PM2.5. Methods are available that can separate
particulate matter by aerodynamic size. On April 17, 1990, EPA
promulgated EPA Method 201 and Method 201A to provide a source test
method that separated filterable particulate matter greater than 10
micrometers from filterable particulate matter equal or smaller than 10
micrometers. The single cyclone used in these methods replaced the
nozzle of EPA Method 17 to separate the two size classes of filterable
particulate. This method allows sources to determine their emissions of
filterable PM10 when there are size specific emission limits
or when there is a need for size specific emission inventories. With
the addition of a second smaller cyclone following the single cyclone
of EPA Method 201A, the filterable particulate can be separated into
three size classifications. These classifications include filterable
particulate matter greater than 10 micrometers, filterable particulate
matter equal or smaller than 10 micrometers but greater than 2.5
micrometers, and filterable particulate matter equal or smaller than
2.5 micrometers. This method is posted as Conditional Method 40 (CTM
40) on EPA's Emission Measurement Centers web page at http://www.epa.gov/ttn/emc/ctm.html.
 Of the methods mentioned previously, the

most reliable measurement of total direct PM2.5 emissions
would combine the use of Conditional Method 40 with EPA Method 202.
    Conditional Method 40 has been used at several facilities in the
U.S. and the hardware required to implement this method has been
readily available since the mid-1980's. The acceptability of a source
using the existing SIP test methods for filterable particulate matter
as an indication of the source's relative performance in controlling
PM2.5 emissions would depend on the source's level of
condensable particulate matter emissions in relation to filterable PM
emissions, the proportion of filterable particulate matter that is
smaller than 2.5 micrometers, the add-on PM control device
effectiveness, and the need to consider limiting the emissions of the
condensable material. In areas where there is no need to reduce
stationary source particulate matter emission levels to demonstrate
attainment of the PM2.5 NAAQS, the use of total filterable
particulate test methods may be adequate to insure that existing levels
of PM control are being maintained. However, in areas where a reduction
of stationary source particulate matter emissions is incorporated into
the attainment demonstration, the use of a test method that measures
total PM2.5 emissions would be more appropriate than
existing test methods that measure only total filterable PM.
3. If the stationary source test methods are changed, will the existing
emission limitations incorporated in SIPs need to be changed?
    Changes in the source test method will require reevaluations of the
emission limitations. The reevaluation will need to consider the
interrelated impacts due to differences in the test method,
characteristics of the particulate matter emissions from the sources,
and intended changes in the stringency of the emission limitations. The
following three examples provide a range of the relationships that can
occur between the source test method and the characteristics of the
particulate matter emissions. For sources with no condensable
particulate matter emissions, a change from a total particulate matter
test method (using the same particle size cutoff) to a PM2.5
test method will result in lower measured emissions. The difference in
mass measured by the two test methods depends on the size distribution
of the filterable particulate matter emissions from the source. For
sources with condensable particulate matter emissions, a change from a
filterable particulate matter test method to a total particulate matter
test method will result in higher measured emissions. The difference in
mass measured by the two test methods depends on the relative emissions
of filterable and condensable emissions from the source. For sources
with condensable particulate matter emissions, a change from a total
filterable particulate matter test method to a total PM2.5
test method may increase or decrease the measured emissions. The
increase or decrease and the magnitude of any change would depend on
the particle size distribution of the filterable particulate matter and
the magnitude of the condensable particulate matter emission.
    As can be inferred from these three examples, the application of a
single multiplier to convert existing emission limitations to a total
PM2.5 emission limitation would result in a variable change