Preamble as stating that if a state contends that a source-specific
control level should not be established because the source(s) cannot
afford the control measure that is demonstrated to be economically
feasible for other sources in its source category, then the state must
support the claim with information regarding the impact of imposing the
identified control measure or technology on the several financial
indicators. The proposal also recommended that cost effectiveness
should generally be evaluated by assessing the cost per ton of
emissions reduced associated with a control measure, but the proposal
also requested comment on an alternative metric to assess cost
effectiveness in terms of the cost per unit of air quality improvement
(i.e., ``cost per microgram'').
b. Final Rule
    Based on a consideration of the comments received, the EPA has
determined that economic feasibility considerations should generally
align with the interpretation in the 1992 General Preamble. Note that
the proposal indicated that if it is claimed that a control approach is
not economically feasible for a specific source, the state needs to
provide information related to several financial indicators to support
the claim. We note that the original policy in the 1992 General
Preamble suggests that if a source desires to make such a claim, it
should provide such information to the state for its consideration.
This final rule characterizes the policy in a similar manner, where the
source would have the option of providing this financial information to
the state for its review. This approach should address the concerns of
some commenters that such financial information may not be readily
available to the state. Thus, the final policy for considering economic
feasibility of control measures is described in the following
paragraphs.
    The EPA has a longstanding interpretation of the term ``economic
feasibility'' in the context of evaluating potential RACM and RACT
which involves considering the cost of reducing emissions and the
difference between the cost of an emissions reduction measure at a
particular source and the cost of emissions reduction measures that
have been implemented at other similar sources in the same or other
areas.\91\ Absent other indications, the EPA presumes that it is
reasonable for similar sources to bear similar costs of emissions
reductions. Economic feasibility of RACM and RACT is thus largely
informed by evidence that other similar sources have implemented the
control technology, process change or measure in question.
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    \91\ See the Federal Register published on April 16, 1992 (57 FR
13498, 13540 and 13541).
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    For each technologically feasible control measure, a state should
evaluate the economic feasibility of the measure or control, through
consideration of factors such as the capital costs, operating and
maintenance costs, and cost effectiveness (i.e., cost per ton of
pollutant reduced by that measure or technology) associated with such
measure or control. A state should not reject a technologically
feasible control measure or technology as being economically infeasible
if such a measure or technology has been implemented at other similar
sources (i.e., at sources that would be included in the same source
category in the emissions inventory data collection process), unless
the state provides an adequate justification that clearly explains the
specific circumstances of the source or sources in the nonattainment
area that make such a measure or technology economically infeasible for
sources in the nonattainment area. See 40 CFR 51.1009(a)(3).
    The EPA believes that it is appropriate for states to give
substantial weight to cost effectiveness in evaluating the economic
feasibility of an emission reduction measure or technology. The cost
effectiveness of a measure is its annualized cost ($/year) divided by
the emissions reduced (tons/year) which yields a cost per amount of
emission reduction ($/ton). Cost effectiveness provides a relative
value for each emissions reduction option that is comparable with other
options and, in the case of control technologies, other facilities. In
considering what level of control is reasonable, the EPA does not
recommend a specific fixed dollar per ton cost threshold for economic
feasibility of controls identified as potential RACM and RACT.
    If a source contends that a source-specific control-level should
not be established because the source cannot afford the control measure
or technology that is demonstrated to be economically feasible for
other sources in its source category, the source should make its claim
known to the state and support the claim with information regarding the
impact of imposing the identified control measure or technology on the
following financial indicators, to the extent applicable:
    (1) Fixed and variable production costs ($/unit)
    (2) Product supply and demand elasticity
    (3) Product prices (cost absorption vs. cost pass-through)
    (4) Expected costs incurred by competitors
    (5) Company profits
    (6) Employment costs
    (7) Other costs (e.g., for RACM implemented by public sector
entities).\92\
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    \92\ These longstanding factors were established in the EPA
guidance in 1992 and are applicable to implementation programs for
all the NAAQS pollutants. See the appendices to the General
Preamble, 57 FR 18070 (April 28, 1992).
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c. Comments and Responses
    Comment: With regard to the presumption that it is reasonable for
similar sources to bear similar costs of emissions reductions, some
commenters supported returning to the approach described in the General
Preamble as EPA proposed, while other commenters suggested that based
on its experience with industry the EPA knows that just because a
technology will work at one source does not mean that it will
necessarily work at a similar source due to source configuration and
non-RACT reasons (e.g., enforcement proceedings) for the installation
of different technology at ``similar'' sources.
    Response: The latter commenters appear concerned that the rule
would require the imposition of all controls on similar sources without
allowing for consideration of whether such controls are technologically
and economically feasible. This is not what EPA proposed. Instead, the
EPA proposed, and is now finalizing, a requirement that the state first
identify potential control measures for sources in a nonattainment
area. The state should then identify which control measures are
economically and technologically feasible based on its review of
various factors. If the state determines that certain controls are not
reasonably available based on its review of these factors, it must
provide a written justification to the EPA explaining its rationale.
This review should at least evaluate the feasibility of all the
identified controls on similar sources to determine whether
implementation of such controls in the nonattainment area at issue is

reasonable. The EPA recognizes that there are a number of source-
specific factors that the state can take into account in making these
determinations. Factors such as the physical onsite configuration of a
facility may determine whether a particular control device or operation
can be feasibly implemented. Likewise, a state should take into account
information provided by the source on particular economic factors such
as those described earlier in making a case-by-case determination of
the economic feasibility of a control measure.
    Comment: Two commenters supported the EPA's proposal that cost-
effectiveness should consider capital costs, operating costs and
maintenance costs at the particular source in question.
    Some commenters supported using an alternative cost effectiveness
metric such as cost per microgram of air quality improvement where
appropriate air quality modeling has been developed for the area and
can reasonably characterize the relative importance of various
precursors. Some commenters opposed the proposal's alternative cost-
effectiveness metric because the approach is overly complex and the
impacts are rarely uniform across an area.
    Response: The EPA has decided to maintain its traditional
recommendation to use a cost per ton approach for evaluating the cost
effectiveness of particular control options. The EPA does not recommend
the cost per microgram alternative approach because there are a number
of technical and resource challenges associated with implementing it in
a technically rigorous manner based on detailed air quality modeling
information. The EPA believes that this policy approach would
unnecessarily add complication and extra burden to the state's process
for determining economic feasibility for subject sources in a
nonattainment area. Moreover, the EPA believes that the flexibility
described here to consider cost-effectiveness in assessing economic
feasibility, when coupled with the upcoming discussion of Step 6, and
with the major stationary source and comprehensive precursor
demonstrations previously described will ensure that unreasonable
application of measures (e.g., those that are not effective in reducing
PM2.5 concentrations) will not occur.
6. Step 5: Determine the Earliest Date by Which a Control Measure or
Technology Can be Implemented in Whole or in Part
a. Summary of Proposal
    In this section, the proposal discussed two main issues related to
the date by which control measures can be implemented. First, it
proposed that when a state is determining RACM/RACT, it must consider
whether a control measure can be implemented in part when full
implementation of the measure within 4 years of designation is not
feasible. The proposal also introduced the concept of ``additional
reasonable measures,'' meaning those measures that can only be
implemented after the fourth year but prior to the Moderate area 6-year
attainment date. It was proposed that a state must identify additional
reasonable measures and adopt those measures as needed for expeditious
attainment.
b. Final Rule
    This section remains relatively unchanged from the proposal. CAA
section 189(a)(1)(C) requires that the attainment plan for a Moderate
PM2.5 nonattainment area provide for the implementation of
RACM and RACT no later than 4 years after designation. The agency has
long interpreted the term ``implemented'' to mean that a control
measure or technology has not only been submitted to the EPA for
approval as part of a SIP but has also been built, installed and/or
otherwise physically manifested, and is achieving the intended
emissions reductions, and the EPA retains this definition in this rule.
See 40 CFR 51.1000.
    The EPA recognizes that a state may be able to implement a given
control measure only partially within 4 years after designation. The
EPA addressed this situation in the General Preamble, stating: ``It is
important to note that a State should consider the feasibility of
implementing measures in part when full implementation would be
infeasible.'' \93\ The EPA continues to interpret the RACM/RACT
definition to mean that a state should not reject an otherwise
technologically and economically feasible control measure or technology
as RACM or RACT even if it can be only partially implemented within the
statutory 4-year timeframe following designation of the area. Instead,
a state must adopt as RACM and RACT that portion of a control measure
or technology that can feasibly be implemented within 4 years of the
effective date of designation. See 40 CFR 51.1009(a)(4)(i)(A). For
instance, if paving unpaved roads is a control measure that is
technologically and economically feasible in a nonattainment area but a
state cannot pave all candidate roads within 4 years of designation,
the state must adopt as RACM a measure that requires paving of that
portion of roads that the state could feasibly accomplish within 4
years if such a measure is needed for timely attainment of the
PM2.5 NAAQS in the area.
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    \93\ 57 FR 13498 (April 16, 1992), at page 13541.
---------------------------------------------------------------------------

    Therefore, for the purposes of meeting the RACM/RACT requirement, a
state must identify those technologically and economically feasible
control measures and technologies that it can implement fully or
partially within 4 years of designation of its Moderate
PM2.5 nonattainment area. Depending on the severity of the
PM2.5 nonattainment problem in the area, some or all of
these measures identified as implementable within 4 years may be needed
in order to bring the area into attainment as expeditiously as
practicable. These candidate measures may constitute RACM and RACT if
the state determines, through its attainment demonstration that it
needs to implement them to achieve timely attainment for the area.
    In addition, a state must separately identify those technologically
and economically feasible control measures that can only be implemented
after the statutory window for implementing RACM and RACT, but before
the attainment date. The statutory 4-year timing requirement for
implementing RACM and RACT under CAA section 189(a)(1)(C) limits the
control measures and technologies that can qualify as RACM and RACT for
a Moderate PM2.5 nonattainment area. However, the statutory
requirement of CAA 172(c)(6) also requires states to implement ``other
measures'' necessary to provide for timely attainment in an area. The
EPA interprets this provision to include ``additional reasonable
measures,'' which are those measures and technologies that can be
applied at sources in the nonattainment area that are otherwise
technologically and economically feasible but can only be implemented
in whole or in part later than 4 years after designation.\94\
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    \94\ With respect to ``partial measures'' under this proposed
approach, the EPA would require that a state implement as RACM that
portion of any control measure determined to be technologically and
economically feasible and implementable within 4 years after
designation of a nonattainment area. The state would then be
required to implement as an additional reasonable measure that
portion of the same control measure that can be implemented starting
4 years from designation through the sixth calendar year following
designation.

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7. Step 6: Evaluate the Collective Impact of Potential Control Measures
To Determine Whether the Area Can Attain Expeditiously or Whether it is
Impracticable to Attain by the Attainment Date, and Adopt the
Appropriate Set of Control Measures
a. Summary of Proposal
    The proposal described the control measure requirements for two
situations: The case where the state can demonstrate attainment by the
attainment date; and the case where the state demonstrates the area
cannot practicably attain by the attainment date. If a state determines
that a Moderate nonattainment area can attain the PM2.5
NAAQS by the statutory attainment date, the state must adopt and
implement any technologically and economically feasible control
measures that are necessary to ensure that the area will attain the
NAAQS as expeditiously as practicable. Those technologically and
economically feasible measures needed for attainment that can be
implemented within 4 years of the date of designation would be
considered to be RACM/RACT. Those measures needed for attainment that
cannot be implemented within 4 years but can be implemented no later
than the attainment date would be considered to be ``additional
reasonable measures.'' The proposal stated that, consistent with
longstanding policy, this means that the state may choose to not adopt
certain measures if collectively they would not advance the attainment
date by 1 year.\95\
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    \95\ In the context of the PM10 NAAQS, the EPA has
concluded that ``advancement of the attainment date'' should mean an
advancement of at least 1 calendar year. See ``State Implementation
Plans; General Preamble for the Implementation of Title I of the CAA
Amendments of 1990,'' 57 FR 13498 (April 16, 1992). See also Sierra
Club v. EPA, 294 F.3d 155 (D.C. Cir. 2002).
---------------------------------------------------------------------------

    For the situation where a state determines that it is impracticable
to attain by the Moderate area attainment date, the proposal included
two policy options for describing what control measures must be adopted
and implemented. One option would have required the state to adopt all
technologically and economically feasible control measures, as stated
in past guidance in the General Preamble. The other option would have
required adoption of technologically and economically feasible control
measures with an explicit exception for those measures that
collectively are determined to be ``ineffective in reducing ambient
PM2.5 levels.'' The proposal also reviewed the proposed
options for demonstrating that a precursor does not make a significant
contribution to PM2.5 levels that exceed the standard, and
discussed how the final precursor policy may be an important
consideration in deciding upon a control measure policy approach for
Moderate areas that demonstrate they cannot practicably attain.
b. Final Rule (General)
    For an area that can demonstrate that it will attain by the
attainment date, the final rule maintains the same approach as
described in the proposal regarding the collective evaluation of
potential control measures to determine whether the area can advance
the attainment date by 1 year. For an area that demonstrates that it
would be impracticable to attain by the attainment date, the final rule
does not include an explicit exception for those measures that
collectively are determined to be ``ineffective in reducing ambient
PM2.5 levels.'' More details are provided in sections (c)
and (d) that follow.
    Section 189(a)(1) of the CAA establishes a requirement that the
attainment plan for a Moderate PM2.5 nonattainment area must
demonstrate either that an area can attain the relevant NAAQS by the
applicable attainment date, or that it is impracticable for the area to
do so. As noted previously, for Moderate PM2.5 nonattainment
areas, the ``applicable attainment date'' is as expeditious as
practicable, but no later than the end of the sixth calendar year after
designation as nonattainment. A complete discussion of the requirements
for attainment demonstration modeling is presented in Section IV.E of
this preamble. However, one of the key features of attainment
demonstration modeling and related analysis is that they provide a
means of synthesizing the effects of emissions reductions from all
existing and potential new control measures identified for sources in a
given nonattainment area on overall air quality in that area. States
will use the results of their analyses to identify the appropriate
combination of reasonable control measures for sources in their
Moderate PM2.5 nonattainment area and any other control
measures needed on sources outside the nonattainment area to ensure
expeditious attainment of the relevant NAAQS in the area and to meet
the statutory requirements of CAA sections 189(a)(1)(B) and 172(c)(6)
as explained later.\96\
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    \96\ Note that under section 110(l) of the CAA, after a state
has adopted a control measure into the SIP for an attainment
demonstration, it may remove or modify a measure if the state
demonstrates to the satisfaction of the EPA that such removal or
modification will not interfere with any applicable requirement of
the CAA, such as attainment of the PM2.5 NAAQS or meeting
RFP requirements.
---------------------------------------------------------------------------

    Section 188 establishes the attainment dates for Moderate and
Serious PM10 nonattainment areas, and this rule also applies
such dates to Moderate and Serious PM2.5 nonattainment
areas. As described in Sections IV.D and IV.E of this preamble, in the
case of a Moderate PM2.5 nonattainment area for which a
state can demonstrate attainment by the end of the sixth calendar year
following designation, the state must follow a two-step process for
determining the appropriate attainment date for the area. First, the
state must demonstrate through air quality modeling that the area can
attain the relevant NAAQS by the latest statutory attainment date and
determine which control measures and technologies are needed for the
area to attain by that date. Second, the state must determine whether
implementing other reasonable controls (i.e., those not needed for
attainment by the latest possible date but that are technologically and
economically feasible) can cumulatively advance the attainment date for
the area by at least 1 year. In the event that a state determines that
the area can attain the relevant NAAQS earlier through the application
of other measures, the state must propose the earlier date as part of
the attainment plan submission for the area. When the EPA takes action
to approve the different elements of the attainment plan for the area,
one of the elements that the agency will take action on will be the
state's proposed attainment date for the area. If the EPA approves an
attainment date for the area that is earlier than the latest date
allowed by statute, then the applicable attainment date for the area
will be the approved date. See 40 CFR 51.1004(a)(1)(i). If the area
ultimately needs additional time to attain the relevant NAAQS, the
state may request an attainment date extension for the Moderate
nonattainment area under section 188 as long as certain conditions are
met, as described in Section IV.J of this preamble.
c. Final Rule--Step 6 (Attainment Case): If the State Can Demonstrate
Attainment in the Area by the Statutory Attainment Date for a Moderate
Area, Then the State Must Implement Those Control Measures Needed for
Expeditious Attainment of the NAAQS in the Area
    If a state determines that a Moderate nonattainment area can attain
the PM2.5 NAAQS by the statutory attainment date, the state
must adopt and

implement any technologically and economically feasible control
measures that are necessary to ensure that the area will attain the
NAAQS as expeditiously as practicable. The EPA will consider any such
measures that can be implemented within 4 years of designation of the
area to fulfill the RACM and RACT requirements for the area. In
addition, the EPA will consider any such measures that can only be
implemented between 4 years and the sixth calendar year after
designation to meet the requirements of CAA section 172(c)(6) as
``additional reasonable measures'' for the area and necessary to
demonstrate timely attainment under CAA section 189(a)(1)(B).
    For this type of situation, the state may reject any otherwise
technologically or economically feasible measures that are not needed
to demonstrate attainment or that will not advance the attainment date
by at least 1 year. That is, for a Moderate area that can demonstrate
attainment by the statutory Moderate area attainment date, the EPA
defines as ``reasonable'' only those technologically and economically
feasible measures that are necessary for expeditious attainment of the
NAAQS, as the CAA does not require a state to adopt measures that are
not needed for expeditious attainment in a Moderate PM2.5
nonattainment area. Thus, a state may exclude those otherwise
reasonably available measures that, if adopted and considered
collectively, would not advance the attainment date for the area by at
least 1 year, so long as the state can demonstrate attainment as
expeditiously as practicable and no later than the statutory Moderate
area attainment date. The EPA maintains that identifying a complete set
of measures that qualify as RACM/RACT and additional reasonable
measures but that are not necessary for attainment within 6 years is
imperative to adequately demonstrate that such measures will not
collectively advance the attainment date for a Moderate area by at
least 1 year. The EPA will require a robust analysis and explanation by
the state when such determinations are made. See 40 CFR
51.1009(a)(4)(i).
d. Final Rule--Step 6 (Impracticability Case): If the State Cannot
Demonstrate Attainment by the Statutory Attainment Date for a Moderate
Area, Then the State Must Adopt All Reasonably Available Control
Measures
    Section 189(a)(1)(B) of the CAA anticipates that not all Moderate
nonattainment areas will be able to demonstrate attainment by the
attainment date, and it incorporates the concept of an
``impracticability demonstration'' for such areas.\97\ Commenters on
this issue stated that nowhere in the statute is there an explicit
exception for those measures that collectively are determined to be
``ineffective in reducing ambient PM2.5 levels.'' Further,
they suggested that such an approach would enable the most polluted
areas to exempt sources that individually are small by some arbitrary
test when in other cleaner areas such sources would be required to
reduce emissions because they collectively would advance attainment.
Other commenters emphasized that sources located in such Moderate areas
should still be subject to the regular review process for determining
whether potential control measures are not technologically or
economically feasible.
---------------------------------------------------------------------------

    \97\ The concept of an ``impracticability demonstration'' is
established in CAA section 188(b), which addresses reclassifying
Moderate PM2.5 areas to Serious. Section 188(b)(1) of the
CAA describes the EPA's discretionary authority to reclassify an
area upon a determination that an area cannot practicably attain by
the Moderate area attainment date. More relevant to this
determination, however, CAA section 189(a)(1)(B) specifically
provides for submission of a demonstration addressing this concept
in the case of Moderate areas that cannot attain the NAAQS by the
applicable attainment date.
---------------------------------------------------------------------------

    After considering comments on the two options described in the
proposal, the EPA has decided to keep the policy in this final rule
consistent with past guidance in the General Preamble. This guidance
stated that ``the EPA believes it is reasonable for all available
control measures that are technologically and economically feasible to
be adopted for areas that do not demonstrate attainment [by the
applicable attainment date].'' \98\ The EPA believes that this
interpretation is compelled by the language of CAA section
189(a)(1)(C), which separately requires a state to submit a Moderate
area attainment plan and meet the RACM and RACT requirement, even if
the state submits a demonstration showing that with those measures it
cannot attain the NAAQS by the applicable attainment date.
---------------------------------------------------------------------------

    \98\ 57 FR 13498 (April 16, 1992), at page 13544.
---------------------------------------------------------------------------

    Under this approach, if the state had an approved precursor
demonstration (as described in Section III of this preamble) showing
that a particular precursor does not have a significant contribution on
PM2.5 levels that exceed the standard, then it would not
need to adopt control measures for that particular precursor. The state
then would be required to identify potential control measures for
sources in the area that emit direct PM2.5 and any remaining
significant precursors. Of these potential measures, the state would
determine which would be technologically feasible to implement. Then
the state would identify which of the technologically feasible measures
are economically feasible to implement.
    Subpart 4 requires that Moderate areas that cannot or do not meet
the Moderate area attainment date be reclassified as Serious
nonattainment areas, in which case sources in the areas are then
subject to BACM and BACT requirements. In the General Preamble, the EPA
indicated that ``it may be reasonable, in some limited circumstances,
for states to consider the compatibility of RACM and RACT with the BACM
and BACT that will ultimately be implemented under the Serious area
plans for those areas.'' \99\ Furthermore, for such areas that do not
meet the Moderate area attainment date, the EPA indicated that ``in the
case of RACM for area sources, EPA anticipates that any future
implementation of BACM for these sources will be additive to, and hence
compatible with, RACM. This is because BACM will generally consist of a
more extensive implementation of the RACM measures . . . Since EPA
anticipates that RACM and BACM for these sources will be compatible,
the SIP's (sic) for these areas should reflect the application of
available control measures to existing sources in moderate
nonattainment areas as determined by the analysis described . . . for
RACM.'' \100\ Thus, the state should assess the remaining set of
technologically and economically feasible measures with regard to the
compatibility of implementing RACM/RACT in the near term in a way that
supports addressing BACM/BACT for such sources when the area is
reclassified to Serious.
---------------------------------------------------------------------------

    \99\ Ibid. at 13544.
    \100\ Ibid.
---------------------------------------------------------------------------

    The General Preamble also provided guidance for stationary source
controls in this situation: ``In many instances, the installation of
pollution controls representing RACT may involve substantial capital
expenditures. In the event that BACT is later required for those
sources, this may require controls significantly incompatible with
those recently installed as RACT, largely wasting those recent
expenditures. Under such circumstances, the installation of controls in
the first round of SIP planning would be unreasonable.'' Accordingly,
SIPs for the Moderate areas that cannot practicably attain need not
require major changes to the control systems for specific stack and
process sources where a State reasonably demonstrates that such changes
will be significantly

incompatible with the application of BACT-level control systems. A
State's demonstration should include, for example, showing what the
State believes are RACT and BACT for the source, and why they are
significantly incompatible.
    The EPA believes that in such cases, a state should consider
selecting and implementing controls that may qualify as BACM or BACT in
a Moderate nonattainment area as part of their RACM and RACT analysis.
Early adoption of controls that would constitute BACM or BACT could be
more efficient and could further the objectives of attaining the NAAQS
expeditiously to protect public health and the environment.
e. Comments and Responses
    Any additional comments received on this section are addressed in
the Response to Comments document found in the docket for this action.
8. RACM and RACT and Additional Reasonable Measures Submission
Requirements
a. Summary of Proposal
    The proposal described a set of submission requirements for RACM/
RACT and additional reasonable measures.
b. Final Rule
    The requirements in the final rule remain very similar to those
that were included in the proposal. To ensure that attainment plan
submissions contain the necessary supporting information to enable the
EPA to review and approve a state's evaluation and selection of
measures that constitute RACM and RACT in a given nonattainment area,
the EPA requires under the authority of section 301(a) of the CAA that
a state must submit the following information as part of its
submission:
    (1) A list of all sources and activities in the nonattainment area
that emit direct PM2.5 or any PM2.5 precursor
(for multi-state nonattainment areas, this would include source
categories, sources and activities from all states which make up the
area). See 40 CFR 51.1008(a)(1).
    (2) For each source or activity in the nonattainment area, an
inventory of direct PM2.5 emissions and emissions of all
PM2.5 precursors, and a comprehensive list of potential
control measures considered by the state for the nonattainment area.
See 40 CFR 51.1009(a)(2). If the state submitted a precursor
demonstration that was approved by the EPA, the state would not be
required to submit a list of sources and potential measures to control
emissions of the relevant precursor from the stationary sources
addressed by the demonstration (i.e., all sources for a comprehensive
precursor demonstration, or major sources for a major source precursor
demonstration). See 40 CFR 51.1006(a)(1) and (2). Note that the
emissions inventory would still need to include all sources of the
relevant precursor. See 40 CFR 51.1008(a)(1).
    (3) For each potential control measure considered by the state but
eliminated from further consideration due to a determination by the
state that the control measure or technology was not technologically
feasible, a narrative explanation and quantitative or qualitative
supporting documentation to justify the state's conclusion. See 40 CFR
51.1009(a)(3).
    (4) For each technologically feasible emission control measure or
technology, a determination of whether the measure is economically
feasible must be included, with narrative explanation and quantitative
supporting documentation to justify the state's conclusion. See 40 CFR
51.1009(a)(3). The following additional information relevant to
economic feasibility should be included as necessary to justify the
determination: (a) The control efficiency by pollutant; (b) the
possible emissions reductions by pollutant; and, (c) the estimated cost
per ton of pollutant reduced.
    (5) For each technologically and economically feasible emission
control measure or technology, the date by which the technology or
measure could reasonably be implemented, in whole or in part. See 40
CFR 51.1009(a)(4)(i)-(ii).
    Each of these elements will provide information needed by the EPA
to evaluate whether the state is meeting the statutory requirements for
an attainment plan, and in particular meeting the statutory requirement
for states to implement RACM and RACT on sources within the
nonattainment area. The EPA recognizes that the base year emissions
inventory for the area that the state submits in conjunction with its
attainment plan will likely contain some of the information proposed to
be required under the first two items in this list. However, the EPA is
finalizing a requirement for emissions inventory information
specifically relevant to the RACM and RACT element of the state's
attainment plan in order to ensure that the EPA or any other party can
appropriately evaluate the state's RACM and RACT analysis.
c. Comments and Responses
    Comment: Some commenters supported the general submission
requirements because in some cases RACM/RACT demonstrations fail to
provide the information necessary for the EPA to reasonably conclude
that these requirements have been met and are supported by a systematic
analysis.
    Response: The EPA agrees with the commenters and the final rule
generally tracks the proposal.
9. Criteria for Effective Regulations to Implement RACM and RACT and
Additional Reasonable Measures
a. Summary of Proposal
    The preamble to the proposed rule described the four main criteria
for effective control measure regulations: Such regulations must be
quantifiable, enforceable, replicable and accountable.
b. Final Rule
    The guidance in this preamble to the final rule remains very
similar to what was proposed. After a state has identified a particular
control measure as RACM or RACT or additional reasonable measure for a
particular nonattainment area, it must then implement that measure
through a legally enforceable mechanism that will be included in the
SIP (e.g., a state rule that the EPA will approve as a part of the
federally enforceable SIP for the state). The EPA is proposing that in
order for the EPA to be able to approve any such measure as part of the
SIP, the state would have to provide information to meet the following
four criteria. These criteria are similar to the criteria finalized as
part of the remanded 2007 PM2.5 Implementation Rule.
    First, the base year emissions from the source or group of sources
to which the control measure applies and the future year projected
emissions from those sources once controlled must be quantifiable so
that the projected emissions reductions from the sources can be
attributed to the specific measures being implemented. It is important
that the emissions from the source category in question are accurately
represented in the base year inventory so that emissions reductions are
properly calculated. In particular, it is especially important to
ensure that both the filterable and condensable components of direct
PM2.5 emissions are accurately represented in the base year.
    Second, the control measures must be enforceable. This means that
they must specify clear, unambiguous and measurable requirements. The
measurable requirements for larger emitting facilities must include
periodic source testing, monitoring or other viable means to establish
whether the

affected source meets the applicable emission limit. Additionally, to
verify the continued performance of the control measure, specific
emissions monitoring programs appropriate for the type of control
measure employed and the level of emissions must be included to verify
the continued performance of the control measure. The control measures
and monitoring program must also have been adopted according to proper
legal procedures. Note that if measures are found to be technically and
economically feasible for reducing condensable PM2.5
emissions as well as filterable PM2.5 emissions from a
source, the state will need to adopt a new emission limit for the
source that accounts for both the filterable and condensable portions,
and includes requirements for ensuring compliance using condensable
PM2.5 source test methods updated in 2011.\101\
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    \101\ See 75 FR 80118 (December 21, 2010), revisions to test
methods for measuring condensable PM emissions from stationary
sources (Method 202).
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    In response to a comment on this criterion, the EPA clarifies that
an enforceable regulation for a CAA program must be enforceable by the
EPA, the state, and citizens. By taking action to approve emissions
limitations and related provisions into the SIP, the EPA thereby makes
those emission limitations a federally enforceable component of the SIP
that the state, the EPA, and citizens can enforce thereafter in the
event of a violation. SIP provisions that effectively preclude
enforcement of violations by the EPA or citizens, whether through
impermissible exemptions or other SIP provisions that function to bar
effective enforcement, are not acceptable.
    Third, the results of application of the control measures must be
replicable. This means that where a rule contains procedures for
interpreting, changing or determining compliance with the rule, the
procedures are sufficiently specific and objective so that two
independent entities applying the procedures would obtain the same
result.
    Fourth, the control measures must be accountable. This means, for
example, that source-specific emission limits must be permanent and
must reflect the assumptions used in the attainment plan for the area,
including the modeling conducted in conjunction with the attainment
demonstration. It also means that the attainment plan must establish
requirements to track emissions changes at sources and provide for
corrective action if emissions reductions are not achieved according to
the plan.
c. Comments and Responses
    Any additional comments received on this section are addressed in
the Response to Comments document found in the docket for this action.
10. Determination of RACM and RACT and Additional Reasonable Measures
in Multi-State Nonattainment Areas
a. Summary of Proposal
    The proposal included several proposed recommendations about the
development of control measures by states with multi-state
nonattainment areas.
b. Final Rule
    The guidance in the final preamble remains very similar to what was
proposed. States in multi-state nonattainment areas will need to
consult with each other on appropriate control measures for the shared
nonattainment area. The agency anticipates that states could decide
upon RACM and RACT and additional reasonable measures that differ from
state to state in a shared nonattainment area, based upon each state's
determination of the most effective strategies given the relevant
mixture of sources and potential controls in the respective states'
portions of a shared nonattainment area. As long as each state can
adequately demonstrate that its chosen attainment strategy, including
its selection and adoption of RACM and RACT and additional reasonable
measures, will provide for meeting RFP requirements and for attainment
of the NAAQS as expeditiously as practicable for the nonattainment area
at issue, the EPA anticipates being able to approve individual state
plans that may elect to control a different mix of sources or to
implement different controls, under the proper circumstances.
Nevertheless, in evaluating RACM and RACT and additional reasonable
measures for a particular nonattainment area, states must consider
potential reasonable control measures developed for other areas or
other states, and particularly for other portions of an interstate
nonattainment area. In addition, states in multi-state nonattainment
areas must evaluate whether the reasonable measures each state may have
identified as not being necessary for attainment could collectively
advance the attainment date for the area by at least 1 year. The EPA
may consider such measures in assessing the approvability of each
state's individual attainment plan for a multistate nonattainment area.
c. Comments and Responses
    Any additional comments received on this section are addressed in
the Response to Comments document found in the docket for this action.
11. Environmental Justice Considerations in Developing the Attainment
Plan Control Strategy for a Moderate PM2.5 Nonattainment
Area
a. Summary of Proposal
    The proposal provided guidance about environmental justice
considerations in developing the attainment plan control strategy for a
Moderate area.
b. Final Rule
    The guidance remains very similar to what was proposed. Current air
quality data indicate that the more severe PM2.5
nonattainment areas contain a high population of people with low socio-
economic status, who are among the most at-risk for adverse health
effects from exposure to PM2.5. As part of its EJ2020 Action
Agenda, the EPA is committed to making progress on improving air
quality in communities with high particulate pollution. The EPA,
therefore, strongly urges states to consider environmental justice
concerns with respect to any control measures they have identified as
potential RACM or RACT or additional reasonable measures in an area,
particularly to the extent that control measures that a state may be
considering are otherwise approximately equal (in terms of
technological and economic feasibility) but unequal with respect to
their direct or indirect impacts on overburdened populations.\102\ In
such cases, the EPA encourages the state to prioritize imposition of
the control measures that will result in the least possible burden and
greatest degree of health protection for overburdened populations in
the nonattainment area. Section XI of this preamble discusses possible
approaches for states to address environmental justice concerns
associated with implementation of the PM2.5 NAAQS in their
SIP development process and attainment plans.
---------------------------------------------------------------------------

    \102\ The term ``overburdened populations'' is defined in the
EPA's ``Plan EJ 2014'' to describe the minority, low-income, tribal,
and indigenous populations or communities in the U.S. that
potentially experience disproportionate environmental harms and
risks as a result of greater vulnerability to environmental hazards.
This increased vulnerability may be attributable to an accumulation
of both negative and lack of positive environmental, health,
economic or social conditions within these populations or
communities. For more information on Plan EJ 2014, see https://www.epa.gov/environmentaljustice.

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c. Comments and Responses
    Any comments received on this section are addressed in the Response
to Comments document found in the docket for this action.

E. Modeling for Attainment Demonstrations

1. Demonstrations for Moderate Areas
a. Summary of Proposal
    Section 189(a) of the CAA generally requires a state with a
designated Moderate nonattainment area to submit an attainment plan for
such area. Section 189(a)(1)(B) of the CAA requires the state to submit
an attainment demonstration including air quality modeling to establish
either: (i) That the area will attain the relevant NAAQS by the
applicable attainment date; or (ii) that it is impracticable for the
area to attain the relevant NAAQS by the applicable attainment date.
For Moderate nonattainment areas, the attainment date is as
expeditiously as practicable, but no later than the end of the sixth
calendar year after designation as nonattainment. The EPA therefore
proposed to require all Moderate nonattainment areas to submit either
an attainment demonstration which includes air quality modeling which
establishes that the area will attain the PM2.5 NAAQS by the
applicable attainment date, or an impracticability demonstration which
documents that the area will not attain the NAAQS by the applicable
attainment date. The EPA proposed that the impracticability
demonstration must also include air quality modeling, but also asked
for comments on an alternative option that would not require air
quality modeling as part of an impracticability demonstration. The EPA
also proposed to allow states to fulfill the statutory modeling
requirement through either locally generated photochemical and/or
dispersion modeling or, with proper justification, appropriate regional
or national modeling.
    An attainment demonstration is a plan that provides an explanation
of how a state will attain the PM2.5 NAAQS by the applicable
attainment date in a particular nonattainment area.\103\ The EPA
proposed that the demonstration must contain: (i) Technical analyses
such as base year and future year modeling of emissions which
identifies sources and quantifies their emissions that are contributing
to violations of the PM2.5 NAAQS; and (ii) analyses of
future year emissions reductions and air quality improvement resulting
from existing (i.e., already-adopted or ``on the books'') national,
regional and local programs, and potential new local measures needed
for attainment, including RACM and RACT controls for the area.
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    \103\ An area is designated nonattainment for either the annual
PM2.5 NAAQS or the 24-hr PM2.5 NAAQS or both.
The attainment demonstration should show that the area is attaining
the form of the NAAQS for which they have been designated
nonattainment.
---------------------------------------------------------------------------

    The EPA further proposed that each state with a Moderate
nonattainment area must submit an attainment plan with an attainment
demonstration that includes analyses supporting the state's
determination of its proposed attainment date. In all cases, the state
must show that the area will attain the NAAQS as expeditiously as
practicable, but not later than the sixth calendar year after
designation. In order to establish that the attainment date is as
expeditious as practicable, the state must explain why the control
measures adopted in the attainment plan provide for the most
expeditious attainment and, in particular, must explain why any
cumulative group of reasonable and available control measures that the
state elected not to adopt will not collectively advance the attainment
date by at least 1 year.
    b. Final Rule. As required by CAA section 189(a)(1)(B), the EPA is
finalizing a requirement for states with Moderate nonattainment areas
to submit a demonstration to establish either: (i) That the area will
attain the relevant NAAQS by the applicable attainment date; or (ii)
that it is impracticable for the area to attain the relevant NAAQS by
the applicable attainment date.
    As proposed, attainment demonstrations must include analyses
(including air quality modeling) supporting the state's determination
of its proposed attainment date. In all cases, the state must show that
the area will attain the NAAQS as expeditiously as practicable, but not
later than the sixth calendar year after designation. The demonstration
must include implementation of all measures identified as RACT/RACM
plus additional reasonable measures, as necessary, for expeditious
attainment. In order to establish that the attainment date is as
expeditious as practicable, the state must explain why the control
measures adopted in the attainment plan provide for the most
expeditious attainment and, in particular, must explain why the
cumulative group of reasonable and available control measures that the
state elected not to adopt will not collectively advance the attainment
date by at least 1 year. See 40 CFR 51.1009(a)(4).
    The EPA is not finalizing a regulatory requirement for air quality
modeling to be included as part of an impracticability demonstration.
See 40 CFR 51.1009(a)(4). Since all nonattainment areas will have
modeling requirements associated with their attainment demonstration,
the EPA believes it is likely that modeling will be submitted in
support of impracticability demonstrations. However, it may be possible
in some cases to support an impracticability demonstration with ambient
PM2.5 data and other relevant non-modeling information. For
example, the ambient data in a nonattainment area may be so far above
the NAAQS, and the reasonable and available controls (i.e. RACM/RACT
and additional reasonable measures) so limited, that it is clearly
impossible (and thus also impracticable) for the area to attain by the
Moderate area attainment date.\104\ In order to support this type of
demonstration, the state must show that, even if all reasonable
controls (i.e. RACM or RACT and additional reasonable measures) were
implemented, the state could not attain the NAAQS within the statutory
timeframe for a Moderate area.
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    \104\ Pursuant to CAA section 188(b)(1)(B), upon the EPA
determination that attainment by the Moderate date is impracticable,
the EPA shall reclassify the area as Serious within 18 months after
the Moderate area SIP due date.
---------------------------------------------------------------------------

    The EPA continues to assume that in most cases photochemical grid
modeling will be required to demonstrate attainment with the
PM2.5 NAAQS. However, the EPA recognizes that more
simplistic modeling techniques (such as dispersion, receptor, and/or
box models) may suffice to demonstrate that an area will attain the
NAAQS, especially in areas that are dominated by primary
PM2.5 emissions (e.g. residential wood smoke).
    c. Comments and Responses. Comment: Some commenters supported the
EPA's proposal to require modeling to demonstrate that attainment is
not practicable. The commenters stated that such an interpretation
flows logically from the Act's requirement in section 189(a)(1)(B) that
attainment demonstrations be supported by modeling. One commenter
supported the alternative approach described in the proposal in which
air quality modeling would not be required for a Moderate area
impracticability demonstration.
    Response: After further consideration of this issue, the EPA has
determined that modeling need not be a regulatory requirement to
support an impracticability demonstration. We note that CAA section
189(a)(1)(B)(i) includes the parenthetical ``including air quality
modeling'' which clearly

makes modeling a statutory requirement for moderate area attainment
demonstrations. However, the same parenthetical statement is absent
from CAA section 189(a)(1)(B)(ii), which addresses an impracticability
demonstration. While we believe that most impracticability
demonstrations will indeed be supported by air quality modeling, there
are cases where a modeling demonstration may not be needed. In
addition, the EPA believes the burden of proof for an impracticability
demonstration is logically lower than for an attainment demonstration
because submission of an impracticability demonstration also requires
reclassification to a serious nonattainment area and the accompanying
more stringent regulatory requirements (e.g. BACT/BACM). The area is
still required to meet RACT/RACM requirements and will also be required
to submit a serious area attainment demonstration, which will
necessarily need to include air quality modeling.
    Comment: Some commenters agreed with the EPA that states should be
afforded flexibility to fulfill the statutory modeling requirement
through appropriate regional or national modeling.
    Response: The EPA agrees that, where appropriate, regional and/or
national scale air quality modeling could be sufficient to fulfill the
statutory modeling requirement for attainment demonstration modeling.
However, as with any attainment demonstration, the modeling must be
shown to be appropriate for the nonattainment area, including good
model performance, appropriate emissions and meteorological inputs, and
consideration of emissions control strategies. It should be noted,
however, that it may be difficult to fulfill other CAA requirements
(such as emissions inventory, RACM, RFP, establishing motor vehicle
emissions budgets for transportation conformity purposes, etc.) using
regional or national modeling data. In order to fulfill those
requirements, states may need more detailed data for sources in their
nonattainment area compared to what is available through regional or
national modeling.
    Comment: Some commenters stated that, as the proposal stands, if
states wish to preclude RACT/RACM for any sources in the nonattainment
area, they must do modeling for the year preceding the attainment year
to demonstrate early attainment; this would require modeling for 3
years, rather than 2 years.
    Response: Although a RACM analysis is required, and eliminating
potential control measures requires an assessment of whether the
measures collectively could advance the attainment date by 1 year, EPA
did not propose any specific modeling requirements for the RACM
analysis. There are several components to the analysis. First,
potential emissions reductions need to be assessed. Then, an assessment
of whether those emissions reductions can advance attainment by at
least a year needs to be completed. One way to minimize the number of
future modeled years is to establish (through sensitivity modeling) a
relationship between PM2.5 and PM2.5 precursor
emissions reductions and PM2.5 concentrations in the
nonattainment area. The established relationship can be used to
estimate whether a particular set of emissions reductions will be able
to advance the attainment date by at least a year. Also, in some cases,
the emissions reductions identified through the RACM analysis may be
relatively small (as a percentage of area-wide emissions) that a
modeling analysis is not needed to show that the attainment date cannot
be advanced.
2. Available Modeling Guidance for Demonstrating Attainment
    a. Summary of Proposal. The EPA proposed that attainment
demonstrations should be consistent with the procedures for modeling
PM2.5 as described in the EPA's ``Guidance on the Use of
Models and Other Analyses for Demonstrating Attainment of Air Quality
Goals for Ozone, PM2.5, and Regional Haze'' \105\ as well as
the Guideline on Air Quality Models (40 CFR part 51, Appendix W).
---------------------------------------------------------------------------

    \105\ The PM2.5 attainment demonstration modeling
guidance can be found at the following Web site: http://www3.epa.gov/ttn/scram/guidance_sip.htm.
---------------------------------------------------------------------------

    The PM2.5 attainment demonstration modeling guidance
(hereafter referenced as the ``modeling guidance'') describes how
states can apply air quality models to generate results needed to
demonstrate attainment. Models are used to test whether control
measures in an attainment plan to be adopted into a SIP are likely to
result in attainment of the relevant standards. The attainment
demonstration modeling guidance recommends a modeled attainment test
for the annual and 24-hour PM2.5 NAAQS that uses a
combination of ambient PM2.5 and PM2.5 species
data and modeled PM2.5 concentrations to estimate future
year air quality. In the recommended attainment test the state applies
the test at each PM2.5 ambient monitor location within or
near a designated nonattainment area. Models are used in a relative
sense to estimate the response of measured air quality to future
changes in emissions. Future air quality is estimated by multiplying
recent monitored PM2.5 values by the modeled relative
response (percent change) to projected future changes in emissions. If
the future design value at all monitoring locations in the
nonattainment area does not exceed the concentration of
PM2.5 specified in the NAAQS, the area is projected to
attain the NAAQS.
    b. Final Rule. In the final rule, EPA is continuing to recommend
that attainment demonstrations should be consistent with the procedures
for modeling PM2.5 as described in the PM2.5
attainment demonstration modeling guidance and Appendix W. The modeling
guidance describes how states can apply air quality models to generate
results needed to demonstrate attainment. These recommendations include
developing a conceptual description of the problem to be addressed;
developing a modeling/analysis protocol; selecting an appropriate model
to support the demonstration; selecting appropriate meteorological
episodes or time periods to model; choosing an appropriate area to
model with appropriate horizontal/vertical resolution; generating
meteorological and air quality inputs to the air quality model;
generating emissions inputs to the air quality model; and evaluating
performance of the air quality model. After these steps are completed,
the state can apply a model to simulate effects of future year
emissions and candidate control strategies.
    The EPA has updated the 2007 PM2.5 modeling guidance to
include additional information related to the 2012 PM2.5
NAAQS and associated monitoring requirements.\106\ The main components
of the modeling guidance and the modeled attainment test have not
changed. Additional information has been added to address near-road
monitoring sites and other information that was not available when the
guidance was first released in 2007.
---------------------------------------------------------------------------

    \106\ See updated guidance at https://www3.epa.gov/ttn/scram/guidance/guide/Draft_O3-PM-RH_Modeling_Guidance-2014.pdf: ``Draft
Modeling Guidance for Demonstrating Attainment of Air Quality Goals
for Ozone, PM2.5, and Regional Haze,'' December 2014.
---------------------------------------------------------------------------

    The modeling guidance continues to recommend a relative attainment
test for both the annual and 24-hour PM2.5 NAAQS. The EPA is
not recommending a specific model for use in the attainment
demonstration for the PM2.5 NAAQS. At present, there is no
single model that has been extensively tested and shown to be clearly
superior to other available models. The current

modeling guideline, 40 CFR part 51, appendix W, does not identify a
preferred model for use in attainment demonstrations of the NAAQS for
PM2.5. Thus, states may choose from several alternatives.
    The EPA has developed software to perform both the annual and 24-
hour PM2.5 attainment test (including interpolating PM
species data where necessary). The current software is called the
Software for the Modeled Attainment Test--Community Edition (SMAT-
CE).\107\ The software is provided as a way to make it relatively easy
for states to apply the recommended modeled attainment test(s).
However, states are not required to use SMAT-CE and can develop their
own post-processing software as necessary.
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    \107\ SMAT-CE replaced the Modeled Attainment Test Software
(MATS) in January 2016. SMAT-CE performs the same functionality as
MATS, but is open source, runs faster, and is more stable than its
predecessor.
---------------------------------------------------------------------------

    The modeling guidance continues to describe the opportunity for
states to supplement their modeling with a ``weight of evidence''
demonstration. States may use other information and analyses, in
addition to the modeled attainment test to estimate whether future
attainment of the NAAQS in an area is likely. Other analyses may
include, but are not limited to, emissions trends, ambient data trends
and analyses, other modeling analyses, and documentation of other non-
modeled emissions control strategies, including voluntary programs.
    The application of air quality models requires a substantial effort
by state and local agencies. Therefore, states should work closely with
their EPA regional office in executing each step of the modeling
process. By doing so, it will increase the likelihood of the EPA's
approval of the state demonstration submitted at the end of the
modeling and overall attainment plan development process.
    c. Comments and Responses. Comment: Several commenters questioned
the ability of the current most common photochemical models to
accurately model how the PM2.5 precursors impact overall
PM2.5 concentrations. They raise particular concerns about
ammonia emissions and the ability of models to predict PM2.5
formation from ammonia precursor emissions. The commenters stated that
emissions inventories necessary for such modeling, as well as the tools
used to measure those emissions, remain uncertain and are sometimes
inaccurate; e.g., emission rates are too often based on unreliable
data, due to either lack of representative information or technical
issues associated with test methods. Some commenters stated that these
concerns are particularly salient here because the PM2.5 SIP
Requirements Proposal requires that states account for new precursors,
including VOCs and ammonia.
    Response: The EPA disagrees with the commenters' assertion that
emissions inventory and modeling tools are insufficient to estimate
PM2.5 concentrations and the predicted change in
PM2.5 due to changes in PM2.5 emissions and
PM2.5 precursors. While there will always be uncertainty in
emissions inventories and modeling, photochemical models of
PM2.5 concentrations, including secondary formation through
chemistry, have been used in the scientific and regulatory community
for over 30 years. State attainment demonstration modeling has been
performed by numerous states over the last 10+ years to support the 1st
round of PM2.5 SIPs that were due in 2007. In addition, the
EPA has used photochemical modeling of PM2.5 to support
numerous regulatory rulemakings over the last decade.
    The technical tools to perform photochemical modeling are well
established and have been improved almost continuously over many years.
New versions of the CMAQ and CAMx models with numerous science updates
are released every 1 to 2 years. National emissions inventories that
include primary PM2.5 and all scientific precursors
(SO2, NOX, VOC and ammonia) have existed since
the NEI for 2002. The NEI is released every 3 years with methodological
improvements with every release.
    In addition, the commenters refer to VOC and ammonia as ``new
precursors,'' which is not accurate. VOC and ammonia have always been
``scientific'' PM2.5 precursors, and as such have always
been inventoried and modeled with chemistry in PM2.5
photochemical models. The only thing ``new'' is that VOC and ammonia
are now assumed to be presumptive PM2.5 precursors. However,
even though the previous implementation rule did not assume that VOC
and ammonia were default precursors, all photochemical modeling of
PM2.5 has always included VOC and ammonia emissions and the
resultant chemical formation of ammonium sulfate, ammonium nitrate, and
secondary organic carbon.
    The commenters were concerned that model errors in the formation of
PM2.5 from ammonia sources would impose an unreasonable
regulatory burden on sources of ammonia such as animal agriculture.
Even though there may be general uncertainty in ammonia inventories, it
is not clear how those uncertainties would lead to an unreasonable
regulatory burden on any emissions sources in particular. Every
modeling application in support of an attainment demonstration must be
shown to adequately represent the emissions, chemistry, and
PM2.5 concentrations in the nonattainment area. Ambient
measurements of PM2.5 and precursors are used in a model
performance evaluation to demonstrate that the modeling system is
appropriate to use to determine the sensitivity of PM2.5
mass to emissions changes. In addition, all SIPs are required to
undergo a public comment process where specific emissions and/or
modeling concerns can be raised to the state. And then, after review of
the SIP submission by EPA, all approvals or disapprovals of attainment
SIPs go through a notice and comment rulemaking process. There are
therefore numerous opportunities for both industry and the general
public to participate in the SIP development process. States are
expected to use the appropriate tools and the best information
available to demonstrate how they will attain the PM2.5
NAAQS by the attainment date. The EPA believes that the appropriate
tools are available to perform the modeling needed for an attainment
demonstration.
3. Demonstrating Attainment at Near-Road Monitors
    a. Summary of Proposal. The 2012 PM2.5 NAAQS final rule
contains new requirements for operating near-road monitors in the
largest metropolitan areas.\108\ The first set of monitors was required
to be in place by January 1, 2015. Some of the near-road monitors began
operation prior to 2015. However, none of the monitors will have the
requisite 3 years of monitoring data that can be used to calculate a
PM2.5 design value until 2017 at the earliest. Therefore,
these data were not used for the initial designations for the 2012
PM2.5 NAAQS (finalized in December 2014) and in most
nonattainment areas, there will be less than 3 years' worth of data
available when the initial attainment demonstrations for Moderate
nonattainment areas are due in 2016. As a result of this timing, the
EPA proposed that the initial set of Moderate area attainment
demonstrations will not need to include projected design values for
near-road monitors. But when 3 or more years' worth of complete ambient
data are available at near-road monitors,

states will need to address those monitors in their attainment
demonstrations and will need to include a demonstration that those
monitors will attain the NAAQS by the applicable statutory attainment
date.
---------------------------------------------------------------------------

    \108\ 78 FR 3283.
---------------------------------------------------------------------------

    b. Final Rule. For the final rule, the EPA maintains the policy of
not requiring the use of ambient air quality data in a modeled
PM2.5 attainment demonstration unless there is at least one
complete design value of data available (generally 3 complete years of
data). This applies to both near-road and other PM2.5
ambient monitoring data. Some states may have installed their monitors
well before the January 1, 2015 deadline and may therefore have
complete data before the SIP deadline. In addition, some attainment
demonstrations may be submitted after the statutory deadline. Because
of the varying monitor installation dates, the use of near-road
monitoring data in attainment demonstrations depends on the timing of
the attainment demonstration submission relative to the installation
date of the monitor(s). Ambient data with sufficient completeness to
calculate a design value may not be ignored in an attainment
demonstration. Such data can be addressed either in the attainment
demonstration analysis of ambient monitors or as part of an unmonitored
area analysis (see the next section), as appropriate.
    The revised PM2.5 modeling guidance document includes
procedures for applying a dispersion model or a combination of
photochemical grid models and dispersion modeling to demonstrate
attainment at monitors with large primary PM2.5
concentration gradients. Depending on the nature of the ambient data in
a particular area, it may be appropriate to treat near-road monitors as
high concentration gradient locations. However, in other cases, near-
road monitors may have little or no gradient compared to other nearby
monitors. Therefore, the appropriate treatment of near-road monitors in
attainment demonstrations should be evaluated on a case-by-case,
depending on the facts and circumstances in each nonattainment area.
    c. Comments and Responses. Comment: Some commenters stated the
EPA's proposal to excuse areas from having to include projected design
values for near-road monitoring locations promises to undermine the
likelihood of success for attainment demonstrations. The commenters
stated the EPA's blanket waiver for near-road data has no rational
basis and that just because such monitors were not required before
January 1, 2015, does not mean that areas did not have them in place
before then. The commenters stated the EPA should at least clarify that
if an area has 3 years of near-road monitoring data, it should use such
data in its attainment modeling. The commenters stated this would be
particularly important, for example, if an area is late in preparing
its demonstration.
    Response: The EPA agrees there should not be a ``blanket waiver''
for the use of near-road monitoring data in attainment demonstrations
that are due in 2016 or thereafter. The statements in the proposal
referenced the fact that the near-road monitors were not required to be
in place before January 1, 2015. This makes it unlikely that sufficient
data from these monitors will be available to be considered in
attainment demonstrations that are due in 2016. However, if complete
data are available at near-road monitors during the development of the
attainment demonstration, the data should be considered as appropriate
(similar to any other PM2.5 monitoring data). Since the
near-road PM2.5 monitoring network is relatively new, there
may not be 3 years of complete data in time to be considered in the
upcoming attainment demonstrations. In addition, the base modeling year
of the attainment demonstration may predate the startup date of the
near-road monitor(s). In this case, it may be possible to consider the
near-road data in the attainment demonstration, but the recommended
default projection methodology may not be applicable (since the time
period of the near-road data may not correspond to the 5 year time
period centered about the base modeling year, as recommended in the
modeling guidance). Additionally, near-road PM2.5 monitors
are only required in the 27 largest metropolitan areas of the country.
Some PM2.5 nonattainment areas may not have any near-road
monitoring sites. States should consult with the appropriate EPA
regional office to determine the best way to treat near-road data in
their attainment demonstration.
4. Demonstrating Attainment in Unmonitored Areas
    a. Summary of Proposal. The 2007 PM2.5 modeling guidance
describes a recommended ``relative'' attainment test that is based on
showing attainment at ambient monitoring locations. The guidance also
recommends that states conduct further analyses based on modeling
results to determine whether there are unmonitored areas that merit
additional analysis or investigation. In order to clarify the statutory
and rule requirements of a modeled attainment demonstration, the EPA
proposed four options for demonstrating attainment in unmonitored areas
in an attainment demonstration.
    Option 1 would require the attainment demonstration modeling to
demonstrate attainment at ambient monitoring locations. There would be
no requirement to specifically examine attainment in unmonitored areas.
Option 2 would require modeling to demonstrate attainment at ambient
monitoring locations and in unmonitored areas within the nonattainment
area. Enforceable emissions reductions would be required to eliminate
any potential future year NAAQS violations in all locations within the
nonattainment area (including unmonitored areas). Option 3 would
require modeling to demonstrate attainment at ambient monitoring
locations and in unmonitored areas within the nonattainment area.
However, rather than requiring states to impose additional enforceable
emissions reductions in the SIP to address potential violations in
unmonitored areas, states would be required to use the unmonitored area
analysis results to develop an assessment of the likelihood of
violations in unmonitored areas. The assessment would be used to
evaluate the need for additional controls and/or could be used to
inform the ambient monitoring plan (the need to add additional monitors
or move existing monitors). Option 4 would require modeling to
demonstrate attainment at ambient monitoring locations and recommend
the analysis of unmonitored areas within the nonattainment area. This
differs from Option 3 in that there would be no rule requirement to
perform an unmonitored area analysis. But the submission of an
unmonitored area analysis would still be recommended, especially in
areas with a relatively sparse PM2.5 monitoring network or
in locations where information such as modeling data, emissions
inventories or non-FEM monitoring data (such as from special purpose
monitors or saturation monitoring studies) may indicate potential high
PM2.5 concentrations in areas that are currently
unmonitored.
    b. Final Rule. The EPA is finalizing proposed Option 4. This option
requires states to show attainment at all current and recent monitoring
locations. States will not be required to provide an unmonitored area
analysis as a mandatory element of each attainment demonstration.
However, an unmonitored area analysis can provide useful information
about PM2.5

concentrations and gradients in the nonattainment area and therefore
the EPA recommends that all attainment demonstrations should contain an
unmonitored area analysis. The EPA encourages states to use information
available to them to consider what, if any, impacts may be occurring in
unmonitored areas. States can evaluate the need to perform an
unmonitored area analysis by using available information such as
modeling data, emissions inventories, or non-FEM monitoring data (such
as from special purpose monitors or saturation monitoring studies) to
indicate the potential high PM2.5 concentrations in areas
that are currently unmonitored. An unmonitored area analysis is
strongly recommended where the state and/or the EPA has reason to
believe that potential violations may be occurring in unmonitored
areas, or other available information indicates that further analysis
is warranted. The EPA will consider whether the state has adequately
addressed all available information about potential exceedances of the
NAAQS in unmonitored areas when determining whether the plan can be
approved.
    The EPA is requiring an attainment demonstration approach that
relies primarily on existing monitoring sites and modeling to project
attainment in future years. This approach to evaluating monitored and
unmonitored areas is consistent with how EPA determines whether an area
meets the PM2.5 NAAQS for purposes of designations and
redesignations. As discussed in Section II of this preamble, the EPA
promulgates designations for PM2.5 NAAQS nonattainment areas
based primarily on ambient data measured at FRM and FEM monitors.\109\
Although the EPA considers other information for purposes of evaluating
areas with sources that contribute to those monitored violations for
inclusion within the nonattainment area boundaries, the fundamental
basis for designating an area as nonattainment for a PM2.5
NAAQS is the presence of one or more FRM or FEM monitors with data
showing violations of the NAAQS. Similarly, determinations of
attainment of the PM2.5 NAAQS for purposes of redesignation
actions are based primarily on monitored data. When all FRM and FEM
monitors in a nonattainment area measure attainment of the
PM2.5 NAAQS, the state is eligible to submit a redesignation
request for the area, assuming that it has complied with all other
applicable requirements for purposes of redesignation. Specifically,
the EPA's approval of a redesignation request is subject to meeting the
requirements of CAA section 107(d)(3)(E). Among those requirements is
that the area has attained the NAAQS. For the PM2.5 NAAQS,
this determination is based on ambient data measured at the FRM and FEM
monitors in the area in question.
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    \109\ A monitor must have 3 years of quality assured ambient
data available to be used to calculate a PM2.5 design
value and determine compliance with the NAAQS.
---------------------------------------------------------------------------

    In addition, the ``relative'' attainment test for PM2.5
attainment demonstrations uses FRM or FEM ambient monitoring data,
combined with future year modeled percentage changes in
PM2.5 concentrations to project future year design values.
Since the attainment test relies on ambient monitoring data, an
analysis of future year concentrations in unmonitored areas can only be
accomplished by interpolating ambient data to a particular location
where there is no existing monitor or recent monitoring data.
Therefore, in the context of an attainment demonstration, the
projection of future year PM2.5 concentrations in
unmonitored locations is inherently more uncertain than projections in
monitored locations due to the fact that the ambient concentrations
from which these projections are developed are unknown in the
unmonitored locations.
    While the unmonitored area analysis is not a regulatory
requirement, and states are not required to identify enforceable
emissions reductions to eliminate potential violations in unmonitored
areas, an unmonitored area analysis has the potential to provide
additional important information about PM2.5 levels and
gradients in the nonattainment area. The results of the analysis can be
used to provide information to inform future monitoring plans, to
examine the need for potential emissions controls, to evaluate
potential environmental justice concerns, and to provide additional
information to the public. The EPA believes that Option 4 provides the
best balance between the regulatory requirements of the attainment
demonstration and additional analyses which could provide helpful
information to inform future regulatory activities.
    Where information is available, states and the EPA have obligations
to address potential violations in unmonitored areas, and, although we
expect this to be relatively rare, attainment plans need to address air
quality in unmonitored areas where information exists suggesting the
potential for such violations. Where an unmonitored area analysis is
performed, states should use model results and available ambient data
to develop an assessment of the likelihood of violations in unmonitored
areas. The nature of the assessment depends on the available
information and the nature of the local PM2.5 problem, but
could include, as appropriate, elements such as an evaluation of the
emissions inventory (particularly for local direct PM2.5
sources), the existing ambient data for the area, and meteorological
model inputs to evaluate the accuracy of the modeled violations in
unmonitored areas. If potential violations are determined to be likely,
additional steps could include imposition of emissions reductions at
nearby emission sources or a commitment to deploy special purpose
monitors and/or saturation monitors in the area (in order to further
evaluate the problem). The state should document the assessment,
including analyses of emissions, meteorological inputs and ambient
data.
    The PM2.5 modeling guidance recommends a default
procedure for applying an unmonitored area analysis, which combines
gridded model data with interpolated ambient data. States can apply the
default recommended approach or develop their own analysis which may be
more appropriate for the specific area or situation. States are
expected to consult with the appropriate EPA Regional Office to
evaluate available information to determine if an unmonitored area
analysis is needed for a particular area and how the analysis should be
performed.
    c. Comments and Responses. Comment: Some commenters stated that, of
the options for addressing unmonitored areas, only Option 2 is
technically and legally defensible (80 FR 15382). The commenters stated
the Act requires that ambient concentrations in all areas meet the
applicable NAAQS and cited 42 U.S.C. 7407(a) as requiring states to
assure ``air quality within the entire geographic area comprising such
State'' will achieve the national standards and requiring ``an
implementation plan [to] . . . specify the manner in which national
primary and secondary ambient air quality standards will be achieved
and maintained''). The commenters also cited 42 U.S.C. 7410(a)(1) as
requiring implementation plans to provide for implementation of the
NAAQS ``in each air quality control region (or portion thereof) within
such State''). The commenters stated it is insufficient to suggest that
an area need only show attainment at monitored locations and need only
adopt controls that will address those locations.
    Response: The EPA does not agree that Option 2 is the only
technically and

legally defensible option. The CAA requires states with nonattainment
areas to submit an attainment demonstration as part of their
PM2.5 SIP. States must show that they will attain the NAAQS
by their attainment date. The CAA also requires states to use air
quality modeling in their attainment demonstration. But other than
those general requirements, the EPA believes states have discretion to
interpret how and where to show attainment of the NAAQS through
modeling in support of an attainment demonstration.
    In addition, the EPA believes that a monitor based attainment
demonstration satisfies the CAA requirement to show that ``the entire
geographic area'' will attain the NAAQS. The EPA's monitoring
requirements for PM2.5 are designed to ensure a robust
nationwide monitoring network in both nonattainment and attainment
areas. States have achieved this by maintaining their PM2.5
networks in accordance with EPA's network design criteria.
Historically, these criteria provided that CBSAs have at least one
PM2.5 monitoring site located in an ``area-wide'' location
of expected maximum concentration (within the CBSA).\110\ Thus, by
assuring compliance with the NAAQS at the location of the expected
highest area-wide concentration in the CBSA, air quality is protected
throughout each CBSA. The EPA has identified recommended procedures for
PM2.5 modeled attainment demonstrations. These
recommendations are contained in modeling guidance. The recommended
attainment test relies on the ambient monitors to provide the ``anchor
point'' for future year air quality projections. This ensures that
future year concentration predictions are grounded by ``real world''
measurements. Since the attainment test relies on ambient monitoring
data, the projection of future year PM2.5 concentrations in
unmonitored locations is inherently more uncertain than projections in
monitored locations due to the fact that the ambient concentrations
from which these projections are developed are unknown in the
unmonitored locations. Therefore, the EPA continues to believe that for
PM2.5 attainment demonstrations, modeling results in
unmonitored areas are too uncertain to use in this manner. For the
reasons stated earlier, in the final rule, the EPA does not believe
that it is necessary to require states to submit an unmonitored area
analysis and to show that any potential violations of the NAAQS in
unmonitored areas have been eliminated through enforceable controls.
---------------------------------------------------------------------------

    \110\ See fine particulate (PM2.5) design criteria at
40 CFR part 58-Appendix D to part 58.
---------------------------------------------------------------------------

    Comment: Several commenters supported Option 1 where only monitored
grid cells are included in the attainment plan. Commenters stated that,
due to their concerns related to the accuracy of air dispersion
modeling tools and protocols, it is not appropriate to use air
dispersion modeling to predict receptor impacts in unmonitored areas.
Some commenters stated that Option 1 is the approach that most closely
describes the current EPA rule.
    Response: The EPA agrees that the modeling results are too
uncertain in unmonitored areas to require an unmonitored area analysis
as part of the attainment demonstration (for the reasons enumerated
earlier). However, the EPA disagrees that Option 1 is the approach that
most closely describes the attainment demonstration requirements in the
2007 PM2.5 implementation rule. An unmonitored area analysis
has never been an implementation rule requirement, but was a
recommended analysis in the PM2.5 modeling guidance.
Therefore, the EPA believes that Option 4 is closer to the current
status quo. This final rule clearly states the continued recommendation
to perform an unmonitored area analysis and the benefits of doing so.
    Comment: Some commenters stated that an ``unmonitored area
analysis'' is essential since speciation monitoring is conducted at a
limited number of sites. The commenters stated that, however, given the
inherent uncertainty from modeling analysis in unmonitored areas,
results from such analysis should only be used to inform additional
actions. The commenters stated that, while modeling analysis in
unmonitored areas can be used as a reference for additional studies, it
should not be used for the attainment demonstration in the SIP. The
commenters stated that, under any of the options, the EPA should
specify the recommended level of detail for an unmonitored area
analysis, especially if it is required. The commenters recommended that
the analysis need not require modeled results at finer spatial scales
than those specified in the modeling protocol.
    Response: The EPA agrees that an unmonitored area analysis is
important and continues to recommend development of unmonitored area
analyses to support attainment demonstrations. The EPA also agrees that
due to uncertainty, the results from such analysis should only be used
to inform additional actions. As stated earlier, the PM2.5
modeling guidance contains a default recommended unmonitored area
analysis technique which combines gridded modeling data and
interpolated ambient data (including PM2.5 speciation data).
But the exact nature of the unmonitored area analysis can be considered
based on the information relevant to each nonattainment area. The EPA
also agrees that where an unmonitored area analysis is conducted, it
should be at the same spatial scale (model resolution) as the modeled
attainment demonstration at monitoring locations. For example, if the
gridded modeling analysis is performed at 4km resolution (model grids
that are 4km on a side), then the unmonitored areas should be examined
at the same resolution. Similarly, if near road monitors are examined
with a dispersion model at a finer resolution (compared to the other
monitors) as part of the attainment demonstration, the unmonitored area
analysis could also examine unmonitored near-road areas at a finer
resolution.
    Comment: Several commenters disagreed with the proposal to require
states to perform the attainment test at ``recent'' monitoring
locations. Commenters stated that, within the EPA's description of
Option 1, the proposal indicates that the attainment test required
under Option 1 would also apply to locations that have ``recent'' FRM
and/or FEM monitoring data. Commenters stated the current FRM/FEM
monitoring data should be sufficient to demonstrate attainment.
    Response: States must demonstrate that they will attain the
PM2.5 NAAQS in the nonattainment area as expeditiously as
practicable, and no later than the moderate area attainment date. The
recommended attainment test in the modeling guidance uses recent
ambient data that encompass a 5-year period that is dependent on the
base modeling year. For example, for a base modeling year of 2014, the
guidance recommends using ambient PM2.5 data from the 2012-
2016 period. The guidance also recommends only using ambient data from
a particular monitoring site if it has at least one complete design
value period during the relevant 5-year period. With these
recommendations in mind, there are numerous cases where a monitoring
site may have only partial data from the relevant 5-year period or may
be a new monitor that started collecting data after the 5-year period
or may have been shut down before the 5-year period. The EPA agrees
that it is generally not necessary to examine the modeling results
where monitors were shut down before the base modeling period. These
monitors will not be used to make future

decisions relating to attainment status of the area. However, monitors
that are new or were operating during the base modeling period are
still relevant and can be used to provide additional information in the
attainment demonstration. The data from these monitors may serve as the
basis for examining potential violations in the area as part of an
unmonitored area analysis. This is especially the case for new
monitors, which may not have enough data to provide a robust future
year concentration estimate in the attainment demonstration. But the
monitoring data, combined with modeled information, can provide
information about the likelihood of future violations in the area
surrounding the monitor location.
5. Future Year(s) To Be Modeled in Attainment Demonstrations
    a. Summary of Proposal. A state performing a modeling analysis for
an attainment demonstration or impracticability analysis must select a
future year for the analysis. For an attainment demonstration, a state
should select the future modeling year such that all emissions control
measures relied on for attainment will have been fully implemented by
the beginning of that year. The EPA proposed that to demonstrate
attainment, the modeling results for the nonattainment area must
predict that emissions controls implemented no later than the beginning
of the last calendar year preceding the attainment date will result in
PM2.5 concentrations that meet the level of the
standard.\111\ While states should choose the future modeling year
based on a number of factors, the EPA proposed the last possible year
permitted under the statute as a starting point for modeling.
---------------------------------------------------------------------------

    \111\ Note that for purposes of the PM2.5 NAAQS, a
determination of attainment (or failure to attain), which the EPA is
required to make after the attainment date has passed, is based on
an average of the most recent 3 years of ambient data prior to the
area's attainment date.
---------------------------------------------------------------------------

    b. Final Rule. The EPA is finalizing the recommendation that the
last possible year permitted under the statute is an appropriate
starting point for modeling. See 40 CFR 51.1011(a)(6). For a state that
is submitting an attainment demonstration, modeling the sixth calendar
year is a logical starting point to determine if attainment by that
year is likely. Even though attainment is determined by averaging 3
years' worth of ambient data, states do not have to model 2 years
before the attainment date to show modeled attainment. Since the design
value is an average of 3 years' worth of data, attainment can still be
shown even if concentrations exceed the NAAQS in one or more of the 3
years used to determine attainment (as long as the 3 year average is
less than the NAAQS). Therefore, it can be appropriate to model any of
the 3 years used to determine attainment. In addition, if ambient data
show attainment-level concentrations in the final statutory attainment
year, a state may be eligible for up to two 1-year extensions of the
attainment date, if the area meets the criteria for such extensions.
Therefore, modeling attainment-level concentrations for the last year
permitted by statute is acceptable.
    States with Moderate areas that submit an impracticability
demonstration must show that the area cannot attain the NAAQS by the
end of the sixth calendar year following designation of the area.
Therefore, the appropriate future modeling year for such a
demonstration is also the sixth calendar year after designation.
    For the reasons stated earlier, it is both acceptable, and will in
fact be most efficient, for a state to begin the attainment
demonstration process by modeling the last year permitted under the
statute to determine future year modeled PM2.5
concentrations in the sixth year after designations. For example, since
designations for the 2012 PM2.5 NAAQS were effective in
March 2015, it is appropriate for states to model air quality for 2021
in the attainment demonstrations for designated nonattainment areas.
    Because an area must attain ``as expeditiously as practicable,''
additional considerations are necessary before an attainment date can
be established. For purposes of determining the attainment date that is
as expeditious as practicable, the state must conduct future year
modeling which takes into account expected growth and known controls
that are already in effect or that are adopted and will be in effect by
January 1 of the future year. For example, for a Moderate nonattainment
area for the 2012 PM2.5 NAAQS, a future base case scenario
for the year 2021 would project future air quality given implementation
of existing federal, state and local measures. If this future base case
scenario demonstrates attainment, then the state must determine if
attainment can be achieved in an earlier year through the application
of additional measures. Therefore, the state must conduct an analysis
of RACM and RACT and additional reasonable measures to determine if,
collectively, all technologically and economically feasible measures
identified by the state that can be implemented by the beginning of the
sixth calendar year following designations can advance the attainment
date by at least 1 year (note that RACM and RACT controls must be
implemented within 4 years of an area being designated nonattainment,
but additional reasonable measures for an area for which a state can
demonstrate attainment by the end of the sixth calendar year following
designation of the area are those technologically and economically
feasible measures that can be implemented by the beginning of the last
year prior to the projected attainment date). Results of this analysis
may indicate attainment can be achieved earlier, through implementation
of all reasonable control measures (i.e., RACM and RACT and additional
reasonable measures).
    If, on the other hand, the future base case scenario does not
demonstrate attainment, then a control case scenario is needed to
examine whether the reasonable, technically and economically feasible
measures identified by the state would result in attainment in the
analysis year (i.e., in 2021 for purposes of this example based on the
2012 PM2.5 NAAQS). The control case scenario would add
potential control measures--e.g. RACM and RACT (which must be
implemented in 4 years) and additional reasonable measures, plus any
measures on sources outside of the nonattainment area that the state
has identified as feasible to implement by the attainment date. This
modeling, along with other relevant information, would inform a
judgment as to whether attainment of the relevant NAAQS is practicable
by the end of the sixth year after designation or earlier. In the case
of areas designated for the 2012 PM2.5 NAAQS, if the
analysis does not demonstrate attainment by December 31, 2021, then the
analysis could serve as the technical basis for the state to submit a
demonstration that attainment by the outermost statutory attainment
date for Moderate areas is impracticable. This in turn could serve as a
technical basis for the Administrator to reclassify the area to
Serious.\112\ If the analysis does demonstrate attainment, then the
remaining step is to assess whether the attainment date can be advanced
by 1 year.
---------------------------------------------------------------------------

    \112\ A demonstration that the area cannot attain by the
moderate area attainment date would not be the only trigger for a
reclassification to serious nonattainment. The Administrator
maintains wide discretion in making a determination that an area
cannot practicably attain the NAAQS by their attainment date.
---------------------------------------------------------------------------

    In conducting this assessment, the EPA believes that it is not
reasonable to require states to model each and every calendar year to
determine the

appropriate attainment date. Developing and modeling future year
inventories is a time-consuming and resource intensive process.
Multiple emissions models are needed in order to generate year specific
emissions for the various emissions sectors (e.g. mobile, non-road,
non-EGU point and EGU point). In some cases it may be reasonable to
model one additional interim year before the maximum statutory
attainment date.\113\ However, in most cases, the air quality benefits
of an identified set of RACM and RACT and additional reasonable
measures can be estimated through model sensitivity analyses and the
development of sensitivity factors (i.e., factors to relate tons of
emissions reductions in the area to PM2.5 concentration
changes in the area). For example, states can model across the board
percentage reductions in direct PM2.5 and/or precursor
emissions (in separate model runs or using advanced modeling techniques
such as DDM) to determine the impact of emissions reductions on
PM2.5 concentrations in the area. This modeling can be
performed with a single attainment year modeling platform, which is
much less resource intensive than modeling additional future years. The
identified potential emissions reductions available from RACT and RACM
and additional reasonable measures can be compared to the magnitude of
the modeled PM2.5 reductions from the sensitivity analyses
to determine if all such controls will advance attainment by a year.
The EPA strongly recommends that states discuss the selection of the
future year(s) to model with their EPA Regional Office as part of the
modeling protocol development process and before embarking on the
modeling.
---------------------------------------------------------------------------

    \113\ If several future modeling years are available, it may in
some cases be appropriate for states to interpolate PM2.5
concentrations between years.
---------------------------------------------------------------------------

    c. Comments and Responses. Comment: Some commenters disagreed with
the proposal that the future year should reflect when all control
measures relied on have been fully implemented by the beginning of that
year and it should be no later than the beginning of the last calendar
year preceding the attainment date. The commenters stated the CAA
provides attainment must be achieved as expeditiously as practicable
but no later than the end of the sixth calendar year (except RACT/RACM
are required within 4 years) and states should be given the full period
to demonstrate attainment and to require control of emissions.
    Response: The proposal to require the modeling to reflect control
measures that have been fully implemented no later than the beginning
of the last calendar year preceding the attainment date does give
nonattainment areas ``the full period'' to demonstrate attainment and
to require control of emissions. Since the design value is an average
of 3 years' worth of data, it could be argued that modeling and related
emissions controls should be in place 3 years before the attainment
date. However, if ambient data show attainment level concentrations in
the final statutory attainment year, a state may be eligible for up to
two 1-year extensions of the attainment date, if the area meets the
criteria for such extensions. Therefore, modeling attainment level
concentrations for the last year permitted by statute is acceptable.
But in order to measure attainment level concentrations in the final
year, controls must be in place for the full year (at the beginning of
the year). Implementation of emissions controls at the end of the year
would not be consistent with modeling attainment level or measuring
attainment level concentrations during the year.
6. Attainment Year Motor Vehicle Emissions Budgets
    The transportation conformity rule requires that attainment plans
establish motor vehicle emissions budgets for the area's attainment
year. Therefore, once an area's attainment date has been established,
the state would establish motor vehicle emissions budgets for direct
PM2.5 and any relevant PM2.5 precursor for the
attainment year.\114\ It should be noted that states that submit
impracticability demonstrations for Moderate areas under CAA section
189(a)(1)(B)(ii) are not required to submit motor vehicle emissions
budgets for attainment purposes because the submitted SIP does not
demonstrate attainment. A motor vehicle emissions budget for the
purposes of a PM2.5 attainment plan is that portion of the
total allowable emissions within the nonattainment area allocated to
on-road sources as defined in the submitted attainment plan. Such motor
vehicle emissions budgets would be calculated using the latest planning
assumptions and the latest approved motor vehicle emissions model
available at the time that the attainment plan is developed, unless EPA
approves the state's use of an alternative model.\115\
---------------------------------------------------------------------------

    \114\ For more information on PM2.5 precursor
requirements, see CAA section 93.102(b)(2)(iv) and (v) of the
transportation conformity rule. See also the May 6, 2005, final
transportation conformity rule that addressed requirements for
PM2.5 precursors. (70 FR 24280).
    \115\ If an area includes re-entrained road dust in the motor
vehicle emissions budget, the latest approved version of AP-42
should be used unless the EPA has approved an alternative model for
the area.
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F. RFP Requirements

1. Background on Statutory Requirements and Existing Guidance
    Reasonable further progress (RFP) is a concept included in the CAA
under part D, title I to assure that states make steady, incremental
progress toward attaining air quality standards in the years prior to
the attainment date for a nonattainment area, rather than merely
deferring implementation of control measures and therefore emissions
reductions until the date by which the standards are to be attained. As
discussed elsewhere in this preamble, section 172 of the CAA addresses
attainment plan provisions in general. Section 172(c)(2) of the CAA
requires attainment plans to provide for RFP, which is defined in CAA
section 171(l) as ``such annual incremental reductions in emissions of
the relevant air pollutant as are required by [part D of title I] or
may reasonably be required by the Administrator for the purpose of
ensuring attainment of the applicable national ambient air quality
standard by the applicable date.'' Section 189(c) of the CAA requires
that ``[P]lan revisions demonstrating attainment submitted to the
Administrator for approval under this subpart shall contain
quantitative milestones which are to be achieved every 3 years until
the area is redesignated attainment and which demonstrate reasonable
further progress, as defined in CAA section 171(1), toward attainment
by the applicable date.'' Quantitative milestones are discussed later
in Section IV.G of the preamble.
    Section 172(c)(3) of the CAA requires the state plan to include ``a
comprehensive, accurate, current inventory of actual emissions from all
sources of the relevant pollutant or pollutants in such area . . .''
Section 172(c)(1) of the CAA requires the state plan to include ``all
reasonably available control measures as expeditiously as practicable
(including such reductions in emissions from existing sources in the
area as may be obtained through the adoption, at a minimum, of
reasonably available control technology) . . .'' Section 172(c)(9)
requires the state plan to ``provide for the implementation of specific
measures to be undertaken if the area fails to make reasonable further
progress . . . Such measures shall be

included in the plan revision as contingency measures to take effect in
any such case without further action by the State or the
Administrator.'' For additional background on statutory requirements
and existing guidance, refer to preamble Section IV.F of the proposal
for this rule.\116\
---------------------------------------------------------------------------

    \116\ 80 FR 15385.
---------------------------------------------------------------------------

2. General Approach to RFP
    a. Summary of Proposal. To satisfy the statutory requirements for
RFP at CAA section 172(c)(2), the EPA proposed that a state must submit
an RFP plan as part of its Moderate area attainment plan submission.
The EPA proposed the following two options for developing an RFP plan.
    Under the first option, the EPA proposed that the RFP analysis for
any Moderate PM2.5 nonattainment area that can demonstrate
attainment by the statutory attainment date must demonstrate either:
(i) Generally linear progress toward attainment; or (ii) stepwise
progress toward attainment. Stepwise emissions reductions would be
slower than ``generally linear'' reductions for certain periods, and
then would decline sharply (due to implementation of a new emission
reduction program, or new operation of control technology on one or
more stationary sources). The EPA proposed that a state must follow one
primary approach for conducting the RFP analysis, but that they also
have an option to conduct a secondary analysis that will provide
greater flexibility in setting RFP goals with alternative emissions
reductions and air quality improvement scenarios. The primary approach
would be to show that nonattainment area emissions of each pollutant
decline from the base year to the attainment year, either in a
generally linear manner or in a stepwise manner. In the optional
secondary analysis, the state could show that emissions of the various
pollutants would change in a manner that would provide a change in air
quality during the attainment period that is equivalent or more
expeditious than the air quality change that would be estimated to
occur under the primary approach. This optional analysis was referred
to as an equivalency determination.
    Under the second proposed option, the state would provide the
control strategy implementation schedule and estimate the emissions
reductions anticipated from the control measures (i.e., RACM/RACT and
additional reasonable measures) for sources in the nonattainment area.
The state then would employ modeling or another quantitative method to
predict the overall PM2.5 concentrations in the
nonattainment area for each milestone year. The milestone years would
correspond to the years for which the state would be required to
provide quantitative milestones pursuant to the requirement in section
189(c) of the Act.
    b. Final Rule. The EPA is finalizing RFP requirements that allow
the state flexibility to demonstrate RFP under CAA section 172(c)(2)
using any of the general approaches included in the proposed rule. As
part of its Moderate area attainment plan submission, the state must
submit an RFP plan that includes three components: (1) An
implementation schedule for control measures on sources in the
nonattainment area, (2) RFP projected emissions for each applicable
quantitative milestone year determined in Section IV.G of this
preamble, based on the anticipated control measure implementation
schedule; and (3) an analysis that demonstrates that this schedule of
aggregate emissions reductions achieves sufficient progress toward
attainment between the applicable baseline year to the attainment year.
See 40 CFR 51.1012(a).
    The first component of the RFP plan is the implementation schedule
for all required control measures contained in the control strategy.
The schedule should describe which measures will be implemented within
the first 4 years following designation (and therefore would meet the
statutory requirement for RACM and RACT). It should also describe the
implementation schedule of additional reasonable measures (to be
implemented more than 4 years following designation but before the
attainment date) that have been adopted to help provide for expeditious
attainment of the standard. Any Moderate area that cannot demonstrate
attainment by the statutory Moderate area attainment date is required
to provide an implementation schedule for all of the control measures
identified as RACM/RACT and additional reasonable measures, in the same
manner as an area that can demonstrate attainment.
    The second component of the RFP plan is an analysis by the state
identifying the RFP projected emissions by pollutant that are expected
to be achieved by the control measures implemented within the
nonattainment area according to the implementation schedule. The EPA
requires the state to estimate these RFP projected emissions for each
quantitative milestone year (i.e., for a Moderate area, at 4.5 years
and 7.5 years after designation of the area) by sector on a pollutant-
by-pollutant basis. These milestone year projected emissions are
discussed further in Section IV.F.3 of the preamble. This information
will be used by the state to show that the area is complying with
quantitative milestone and RFP requirements for the area (discussed in
Section IV.G of this preamble).
    The final component of the RFP plan is an analysis demonstrating
that the schedule of emissions changes achieves reasonable progress
toward attainment between the applicable baseline year and the
attainment year. This demonstration can be expressed in the form of
emissions reductions only, or emissions reductions converted to air
quality concentrations. This optional air quality RFP analysis is
discussed later in this section.
    Because the statute does not clearly establish the applicable
baseline year from which to begin calculating annual emissions
reductions for purposes of demonstrating RFP, the EPA is finalizing a
requirement that states use the same year as the base year inventory
used for developing the control strategy and associated air quality
modeling demonstrating that the area will attain expeditiously.
    A demonstration based on only emissions reductions must show that
the implementation schedule achieves either: (i) Generally linear
progress toward the projected attainment date; or (ii) stepwise
progress toward the projected attainment date. For example, in one area
new emission standards for mobile sources may achieve reductions in a
generally linear manner over time, as a portion of the existing vehicle
fleet is replaced each year with new vehicles meeting the more
stringent standards. In another area, regulations to reduce emissions
from certain stationary source sectors could have a single compliance
date by which controls must be in place, which could result in a
significant drop in emissions in a ``stepwise'' manner over a
relatively short period.
    In the first case, the EPA expects that, so long as the attainment
date is as expeditious as practicable, then generally linear progress
toward attainment by that date would satisfy the RFP requirement. In
the second case, where progress is slower than generally linear, the
state is required to submit a clear rationale and supporting
information to explain why generally linear progress is not appropriate
(e.g., due to the nature of the nonattainment problem, the types of
sources contributing to PM2.5 levels in the area and the
implementation schedule for control requirements at such sources).
    Similarly, for areas that cannot demonstrate attainment within the

Moderate area statutory deadline in CAA section 188(c)(1), the state
must demonstrate either generally linear or stepwise emissions
reductions toward the full amount of reductions that will be achieved
by that deadline, i.e., the amount that reflects implementation of all
of the control measures identified as RACM and RACT and additional
reasonable measures for the entire period of the applicable attainment
plan. Generally linear progress toward this full amount would meet the
RFP requirement, but progress that is slower than that would require
further justification.
    In some circumstances, the EPA expects that a state could develop
an approvable RFP plan even if emissions of one or more
PM2.5 plan precursors are not decreasing. In this scenario,
the state must demonstrate that the emissions reductions of direct
PM2.5 combined with the aggregate emissions reductions of
PM2.5 plan precursors support expeditious attainment of the
applicable PM2.5 NAAQS. To accomplish this, the EPA expects
that a state could use the relative air quality impacts of the
different PM2.5 plan precursors identified in the attainment
modeling to demonstrate that the emissions reductions of direct
PM2.5 and aggregate PM2.5 plan precursors
constitute an acceptable RFP plan. For example, the state could
demonstrate that even if one or more PM2.5 plan precursor is
not decreasing, the emissions reductions of direct PM2.5 and
the remaining PM2.5 plan precursors are the dominant factors
in reducing ambient PM2.5 levels and are therefore adequate
to support expeditious attainment. In providing this flexibility, the
EPA recognizes that control measures for certain pollutants may be more
effective at reducing PM2.5 concentrations than others, and
that states may be able to implement some measures more quickly than
others while still achieving reasonable overall progress toward
attainment.
    The EPA is also providing an additional optional RFP analysis that
evaluates the collective changes in emissions of multiple pollutants
during the attainment period in terms of changes in air quality
concentration. Under this optional approach, a state would have to show
that the air quality improvement that is anticipated by milestone dates
due to the identified control measures in the implementation schedule
supports expeditious attainment of the PM2.5 NAAQS. For an
area that can demonstrate attainment within the Moderate area statutory
deadline, a state using this option could rely upon attainment
demonstration modeling results that link emissions reductions with air
quality improvements. For areas that cannot demonstrate attainment
within the Moderate area statutory deadline, the state may have to
conduct modeling or employ another quantitative method to predict the
overall PM2.5 concentrations in the nonattainment area in
each milestone year. The state would compare these air quality target
values to certified ambient air quality monitoring data as part of the
quantitative milestone report due after the area reaches each
quantitative milestone date. The EPA recommends that states estimate
air quality targets by establishing the relationship between modeled
emissions reductions and air quality changes in the attainment plan
(for the attainment year) and interpolating to the intermediate year(s)
based on the same relationship.
    The EPA recognizes that because atmospheric processes are complex,
a specific percent change in emissions of PM2.5 precursors
does not lead to an equivalent percent change in air quality,
potentially creating uncertainty when determining air quality targets
based upon predicted emissions reductions. Nevertheless, the EPA
recognizes the importance of providing the flexibility to address
different pollutants on different timetables so long as the plan can
reasonably be expected to achieve the intended air quality benefits
represented by the RFP analysis.
    As previously noted, submission of the air quality-based RFP plan
is optional. However, in certain circumstances, the applicable Regional
Administrator may strongly recommend that a state or local agency
submit an RFP plan with air quality targets for milestone years in
order to satisfy the statutory RFP requirement. This approach could be
appropriate when one or more pollutants is not decreasing over the
attainment planning period or for areas that have experienced
longstanding and persistent PM2.5 pollution problems despite
the prior implementation of required control measures. The EPA will
review each RFP plan on a case-by-case basis to determine whether it
provides for such annual incremental reductions in emissions of the
relevant air pollutant(s) as are necessary for the purpose of ensuring
attainment by the applicable attainment date. See 40 CFR 51.1012. An
additional RFP analysis will be required as part of a Serious area
attainment plan if EPA reclassifies the area to Serious.
    c. Comments and Responses. Comment: Some commenters generally
supported the equivalency determination concept because they noted that
different precursors are more or less effective in reducing atmospheric
concentrations of PM2.5. Some commenters stated that,
because the goal is timely attainment regardless of when controls are
implemented for each precursor that is to be controlled, the EPA should
allow both options, including the option for states to determine
whether to approach the demonstration on a pollutant-by-pollutant
basis, or overall.
    Response: The final rule allows for emissions from one or more
PM2.5 plan precursors to increase over the attainment
planning period, as long as the emissions of direct PM2.5
and aggregate PM2.5 plan precursors decrease consistent with
RFP and the state can demonstrate that the emissions increase does not
delay expeditious attainment. This approach recognizes the fact that
different precursors have different impacts on PM2.5
concentrations depending upon the specific atmospheric chemistry of
each area. As previously noted, submission of the air quality-based RFP
plan is optional but may be strongly recommend by the applicable
Regional Administrator depending upon the unique circumstances of the
nonattainment area.
    Comment: One commenter suggested that the equivalency determination
must be based on modeling of ambient concentrations, not simply on
inter-pollutant equivalency ratios.
    Response: The EPA recognizes the importance of modeling and notes
that there are potential benefits of using modeling when providing the
optional air quality analysis. Additionally, in the preamble, the EPA
described when these types of tools could be useful in the development
the air quality targets. Because the development of air quality targets
is optional and complements the emissions reductions analysis, the EPA
does not require modeling of ambient conditions for this purpose.
    Comment: A few commenters supported the proposed Option 1 and the
allowance for either generally linear or stepwise progress toward
attainment. These commenters stated that allowing both methods is
consistent with the pattern of many federal emissions reduction
measures and it provides the most flexibility to states. Other
commenters stated that existing guidance in the Addendum failed to
recognize that, in many cases, more can be accomplished during one
given year than in another. The commenters suggested the EPA provide
states with the flexibility to manage their resources for rulemaking
such that emissions reductions are obtained to attain

generally linear progress averaged over the 3-year period rather than
in each individual year.
    Response: As stated earlier, this rule requires that the RFP
analysis must demonstrate either generally linear or stepwise emissions
reduction progress toward attainment. If there are significant
differences between emissions reductions in different years, which make
the emissions reductions no longer generally linear, then the state
would have to provide a justification for the stepwise progress as
discussed earlier. Therefore, the suggestion of averaging the emissions
reductions to obtain generally linear progress over a 3-year period is
not an acceptable way to demonstrate RFP. In this example, the state
would have to submit a justification of why stepwise emissions
reductions are more appropriate for their area. However, the EPA notes
that if stepwise emissions reductions are achieved more rapidly than
expected and consistent with the amount necessary to demonstrate RFP
toward timely attainment, this would be in line with the overall
principles of the CAA and would not require the aforementioned
justification.
3. RFP Projected Emissions for RFP Analyses
    a. Summary of Proposal. The EPA proposed that a state with a
Moderate PM2.5 nonattainment area must submit RFP projected
emissions for sources within the nonattainment area as part of the RFP
plan. The EPA also proposed that these RFP projected emissions would,
at a minimum, include projected emissions of each pollutant by
different source types corresponding to the quantitative milestone
dates for the area.
    b. Final Rule. The EPA is finalizing that a state with a Moderate
PM2.5 nonattainment area must submit RFP projected emissions
for sources within the nonattainment area as part of the RFP plan.
These RFP projected emissions shall, at a minimum, include projected
emissions of each pollutant (i.e., direct PM2.5 and
PM2.5 plan precursors) by different source types
corresponding to the quantitative milestone dates for the area
(quantitative milestone dates are described in greater detail in
Section IV.G of this preamble). Specifically, the EPA requires that the
RFP plan for any Moderate area must contain RFP projected emissions for
each calendar year in which quantitative milestones for a Moderate
nonattainment area must be met. As explained in Section IV.G of this
preamble, a state must identify as part of the attainment plan
submission for a Moderate nonattainment area quantitative milestones to
be achieved every 3 years from the Moderate area attainment plan due
date, or 4.5 years from the effective date of designation of the
area.\117\ For example, the first round of designations for the 2012
PM2.5 NAAQS became effective in April 2015; Moderate area
attainment plans for these areas will thus be due 18 months later, or
in October 2016. The first quantitative milestones for each of these
areas will then have to be met in October 2019; the second quantitative
milestones, in October 2022; and so on, until the area attains the
NAAQS. Under this approach, the state will be required to submit such
RFP projected emissions as part of the Moderate area attainment plan
due in October 2016 that project emissions from sources in the
nonattainment area for the same calendar years as those for which
quantitative milestones will be due (i.e., 2019 and 2022 inventories in
this example).
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    \117\ According to CAA section 189(a)(2)(B), Moderate area
attainment plans are due to the EPA 18 months after designation.
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    The transportation conformity rule requires that RFP plans
establish motor vehicle emissions budgets. RFP plans would therefore be
required to establish motor vehicle emissions budgets for direct
PM2.5 and any relevant PM2.5 precursor as
determined under the transportation conformity rule.\118\ Precursors
that are relevant for transportation conformity purposes would be
limited to the PM2.5 plan precursors but may not include all