law. The EPA intends to make determinations of whether an area attained
the relevant NAAQS pursuant to CAA section 188(b)(2) by notice-and-
comment rulemaking. See 40 CFR 51.1002(b)(2). Accordingly, the final
rule establishes a definition of ``date of reclassification'' to mean
the effective date of a PM2.5 area reclassification from
Moderate to Serious as promulgated by the Administrator. This
definition is then used, for example, to establish the due date for the
Serious area SIP. (See Section VI.A.1 of this preamble for more
information on mandatory reclassification area SIP due dates.)
3. 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.

VI. Requirements for PM2.5 Serious Nonattainment Area Plans

    Sections 189(b) and (c) of the CAA include the following
requirements for Serious area attainment plan submissions: (i) An
attainment demonstration (CAA section 189(b)(1)(A)); (ii) provisions
for the implementation of best available control measures (BACM) no
later than 4 years after reclassification of the area to Serious (CAA
section 189(b)(1)(B)); (iii) quantitative milestones that will be used
to evaluate compliance with the requirement to demonstrate RFP (CAA
section 189(c)); and (iv) regulation of PM2.5 precursors (in
general to meet attainment and control strategy requirements, and as
specifically required for major stationary sources by CAA section
189(e)). Other subpart 1 requirements for attainment plans not
otherwise superseded under subpart 4 also apply to Serious areas for
the PM2.5 NAAQS, including: (i) a description of the
expected annual incremental reductions in emissions that will
demonstrate RFP (CAA section 172(c)(2)); (ii) emissions inventories
(CAA section 172(c)(3)); (iii) other control measures (besides BACM and

BACT) needed for attainment (CAA section 172(c)(6)); and (iv)
contingency measures (CAA section 172(c)(9)).
    Additionally, CAA section 189(b)(1) requires that ``in addition''
to the attainment plan requirements specific to Serious areas, states
must also meet all Moderate area attainment plan requirements. The EPA
interprets the statutory language of CAA section 189(b)(1) to require
states with areas that are reclassified to Serious to meet Moderate
area attainment plan requirements, including all areas that the EPA
reclassifies through rulemaking under its discretionary authority, even
if that occurs before the area has met all of its Moderate area
attainment plan requirements.\151\ The following section describes the
EPA's final actions in this rule regarding Serious area attainment plan
requirements in greater detail.
---------------------------------------------------------------------------

    \151\ See Vigil v. Leavitt, 366 F.3d 1025, amended at 381 F.3d
826 (9th Cir. 2004).
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A. Plan Due Dates

    The proposal discussed the statutory provisions that informed the
options for the submission due dates for the various components of
Serious area attainment plans. The timing of Serious area attainment
plan elements is dictated by two provisions of the CAA: CAA section
189(b)(2) for certain subpart 4 elements and CAA section 172(b) for
subpart 1 elements not superseded by subpart 4 requirements. Section
189(b)(2) of the CAA addresses the due dates for Serious area
attainment demonstrations due under CAA section 189(b)(1)(A) and
provisions for BACM and BACT implementation under CAA section
189(b)(1)(B). Specifically, section 189(b)(2) stipulates two
alternative schedules for states to submit Serious area attainment
demonstrations, depending upon the statutory authority governing the
reclassification action. For an area reclassified to Serious by
operation of law under CAA section 188(b)(2) upon a determination by
the EPA that the area failed to attain the relevant NAAQS by the
applicable Moderate area attainment date, a state must submit a new
attainment demonstration for the area no later than 18 months after
reclassification. For an area reclassified to Serious pursuant to the
agency's discretionary authority provided under CAA section 188(b)(1),
a state must submit a new attainment demonstration no later than 4
years after reclassification of the area.\152\ For all Serious
nonattainment areas, CAA section 189(b)(2) requires a state to submit
within 18 months of an area's reclassification ``provisions to assure
that the best available control measures [BACM] for the control of
PM10 shall be implemented no later than 4 years after the
date the area is classified (or reclassified) as a Serious Area.''
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    \152\ Section V of this preamble provides a more detailed
discussion of the process for reclassifying areas with severe
nonattainment problems to Serious.
---------------------------------------------------------------------------

    When considering attainment plan due dates for areas that have been
discretionarily reclassified, it is also important to keep in mind the
requirements of CAA section 188(b)(1). Section 188(b)(1) of the CAA
generally states that: ``The Administrator may reclassify as a Serious
PM10 nonattainment area . . . any area that the
Administrator determines cannot practicably attain the [NAAQS] . . . by
the attainment date . . . for Moderate Areas.'' In addition, CAA
section 188(b)(1)(B) provides that ``the Administrator shall reclassify
appropriate areas within 18 months after the required date for the
state's submission of a SIP for the Moderate Area.'' Since all Moderate
area SIPs are due 18 months after designation, then this provision
contemplates that EPA will typically exercise its discretionary
reclassification authority within 3 years of the area's designation as
nonattainment. Taken together with CAA section 189(b)(2), which for
discretionary reclassifications requires the state to submit the
attainment demonstration within 4 years of reclassification to Serious,
subpart 4 contemplates that attainment plans for discretionary
reclassifications will be submitted no later than 7 years after
designation. However, as noted in the previous section, the EPA
believes it can discretionarily reclassify an area more than 18 months
after the Moderate area SIP due date under certain circumstances,
meaning that the Serious area attainment demonstration for such a plan
could be submitted to EPA more than 7 years after designation. (See
more discussion in Section V.A of this preamble on the timing of
discretionary reclassifications.)
    Lastly, because some of the Serious area plan requirements noted
earlier are established in subpart 1 of the Act (CAA section 172), the
proposal also noted that CAA section 172(b) provides the EPA discretion
to set a due date for submission of these subpart 1 attainment plan
elements that is no later than 3 years after designation of the area.
In the Addendum, the EPA interpreted the date of reclassification of a
Moderate area to Serious to be analogous to the date of designation of
the area to nonattainment. Accordingly, some of the options presented
in the proposal included 3 year SIP due dates for certain plan
requirements that stem from subpart 1.\153\
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    \153\ Addendum to the General Preamble, 59 FR 41998 (August 16,
1994), at page 42015.
---------------------------------------------------------------------------

1. Area Reclassified to Serious After Failing To Attain the
PM2.5 NAAQS
    a. Summary of Proposal. The proposal noted that for an area
reclassified to Serious after failing to attain the PM2.5
NAAQS by the Moderate area attainment date, CAA section 189(b)(2)
requires the state to submit both the attainment demonstration for an
area and provisions to ensure timely BACM and BACT implementation to
the EPA within 18 months after reclassification. The EPA proposed a
straightforward codification of this 18 month deadline. Assuming the
effective date of the Federal Register reclassification notice is
typically about 6 months after the end of the calendar year, this means
that the attainment demonstration and BACM/BACT provisions would be
required at about 8 years after designations. The notice also proposed
that (1) in addition to BACM/BACT and the attainment demonstration, the
emission inventory would also be required to be submitted within 18
months of the effective date of reclassification because it is
essential for the development of BACM/BACT determinations; and (2)
additional feasible measures (i.e., control measures that may be able
to help the area attain by the attainment date or advance the
attainment date by a year, and that may be implemented later than BACM/
BACT but before the attainment date) would also be required to be
submitted within 18 months of the effective date of reclassification
because such measures would be identified as part of the overall
control measures analysis. Regarding the SIP submission date for the
remaining required plan elements (i.e., RFP, quantitative milestones
and contingency measures), the proposal included two options: (1) no
later than 18 months after reclassification (i.e., at about 8 years
after designation, or 2 years prior to the Serious area attainment
date; or (2) within 3 years after reclassification (i.e., at about 9.5
years after designation, or 6 months prior to the Serious area
attainment date).
    b. Final Rule. Some commenters opposed the proposed requirements
for SIP elements other than BACM/BACT and the attainment demonstration
to be due within 18 months of the effective date of reclassification
because they favored providing states with as much time and flexibility
as possible to provide their submissions. Other commenters suggested
that having all

elements--including RFP, quantitative milestones, and contingency
measures--be due at the same time would be more administratively
efficient for states and would allow for EPA to conduct a single
coordinated review of these plans, and should therefore all be due
within 18 months of the effective date of reclassification. They also
indicated that the alternative would not make sense because RFP,
quantitative milestones, and contingency measures are all linked to the
attainment demonstration.
    After taking these comments into consideration, the EPA has decided
to require all Serious area plan elements to be due within 18 months of
the effective date of reclassification for any area reclassified due to
a failure to attain by the Moderate area attainment date. The EPA
believes that the proposed alternative 3 year deadline, which would
have allowed some elements to be submitted as late as 6 months prior to
the attainment date, would mean that the state would be required to
submit two different SIPs and would require greater state government
resources to conduct the administrative and public procedures required
to submit the separate plans to the EPA. This approach also would not
provide the EPA with sufficient time to appropriately review and take
action on the state's submission prior to the attainment date. It also
is appropriate to have the RFP, quantitative milestones, and
contingency measures elements be developed and submitted at the same
time as the attainment demonstration because they build from the
information in the attainment demonstration. The EPA also maintains
that requiring states to submit all elements of an attainment plan by
the same date is reasonable because it allows for a complete review of
the state submission by the EPA, regulated entities, and the general
public, and it also should prove to be most efficient for states and
the EPA. The EPA further agrees with commenters that a program
requiring two submissions rather than one can generally be expected to
be less administratively efficient because it will involve separate
public hearings and comment periods at the state level, and separate
proposed and final approval actions in the Federal Register by the EPA.
Thus, the final rule requires any area that has been reclassified to
Serious as a result of a failure to attain the standard by the Moderate
area attainment date to submit all the plan elements to the EPA within
18 months of reclassification: updated base year emission inventory
(described in more detail in the next section); BACM/BACT
determinations and adopted regulations; analysis of additional feasible
measures (i.e., control measures that may be able to help the area
attain by the attainment date or advance the attainment date by a year,
and that may be implemented later than BACM/BACT but before the
attainment date) and adopted regulations, as appropriate; attainment
demonstration; RFP; quantitative milestones; and contingency measures.
See 40 CFR 51.1003(b)(2)(ii).
    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.
2. Area Reclassified to Serious Because the EPA Finds in Its Discretion
That the Area Cannot Practicably Attain the NAAQS by the Statutory
Moderate Area Attainment Date
    a. Summary of Proposal. The proposal noted that for an area
reclassified to Serious because the area cannot practicably attain the
standard by the Moderate area attainment date, CAA section 189(b)(2)
requires the state to submit its BACM/BACT analyses and any adopted
regulations to the EPA within 18 months; and to submit the attainment
demonstration within 4 years of reclassification. Similar to the
proposal for mandatory reclassification areas, the notice also proposed
that an updated emission inventory (required under section 172(b) of
the CAA) be required to be submitted within 18 months of
reclassification because it is essential for the development of BACM/
BACT determinations.
    The notice also discussed a potential control measure option
(described in Section VI.D. of the proposal, Attainment Plan Control
Strategy, at page 15410) that would closely link the BACM/BACT
determinations to the attainment demonstrations (rather than consider
BACM/BACT as an independent requirement). Therefore, to facilitate this
linked approach to BACM/BACT, an alternative option was proposed for
submission of the attainment demonstration within 18 months of
reclassification, instead of within 4 years.
    The proposal also addressed the remaining plan elements: additional
feasible measures (i.e., control measures that may be able to help the
area attain by the attainment date or advance the attainment date by a
year, and that may be implemented later than BACM/BACT but before the
attainment date); RFP; quantitative milestones; and contingency
measures. Two SIP submission due date options were proposed for the
remaining plan elements: (1) No later than 3 years after
reclassification \154\; or (2) no later than 4 years after
reclassification. The proposal requested comments on all of the
proposed options for the various elements of a Serious area attainment
plan.
---------------------------------------------------------------------------

    \154\ Under the EPA's prior interpretation as described in the
Addendum at 42015, the EPA had suggested that states could submit
contingency measures no later than 3 years after reclassification of
an area to Serious because of the language of CAA section 172(b).
---------------------------------------------------------------------------

    b. Final Rule. Most commenters opposed the option requiring the
attainment demonstration to be due within 18 months, at the same time
as the BACM/BACT submission. Some of these commenters suggested that a
4-year due date for the attainment demonstration and other elements
would provide maximum flexibility to the states. While some commenters
acknowledged the reasoning behind requiring submittal of the attainment
demonstration and BACM/BACT at the same time if BACM/BACT is linked to
the attainment demonstration, most commenters favored an approach that
provided additional time for submittal of the attainment demonstration.
    Some commenters stated that for an area that is reclassified to
Serious because it cannot practicably attain the NAAQS by the Moderate
area attainment date, CAA section 188(b)(1)(B) requires the EPA to
reclassify the area within 3 years of designation (i.e. within 18
months of the Moderate area SIP due date), and then per CAA section
189(b)(2) the attainment demonstration for such area would be due 4
years later (i.e., 7 years from designation). The commenter stated
that, if the EPA finalizes any discretionary reclassifications beyond 3
years after designation, then it cannot allow the area to have the full
4 years for development of the attainment demonstration because it
would undermine the deadlines and schedules that Congress was plainly
trying to impose.
    For discretionary reclassification areas, just as for mandatory
reclassification areas, the EPA is finalizing the statutory due date of
18 months for the BACT/BACM submission. However, after considering
comments received on the timing options for submission of the
attainment demonstration, the EPA has determined that the attainment
demonstration should generally be due later than 18 months for areas
subject to discretionary reclassifications. Because the statutory
provision in CAA section 189(b)(2) provides up to 4 years, the EPA
believes that an appropriate default due date for

the attainment demonstration should be 4 years after reclassification
for areas reclassified within 3 years of initial designation. However,
after further consideration of this issue, the EPA also believes that a
due date of less than 4 years should be required for areas that are
reclassified closer to the Moderate area attainment date (i.e.,
reclassified between 4 and 6 years after initial designation). In
considering what would be a reasonable submission deadline for the
attainment demonstration in this situation, the EPA considered the
provisions applicable to areas that fail to attain by the attainment
date. Specifically, CAA section 189(b)(2) requires the attainment
demonstration (and the rest of the plan) to be submitted no later than
8 years after designation. As explained further, the EPA believes this
requirement provides a reasonable outer bound for submission of Serious
area plans for any area that is discretionarily reclassified to
Serious.
    The circumstance that one of the commenters identifies, where the
EPA reclassifies an area to Serious at a point in time more than 3
years after designation, raises an important timing issue that was not
explicitly addressed in the proposal. The EPA was aware that it might
need to reclassify an area to Serious beyond 3 years after designation
(e.g., for an area that fails to submit a Moderate area attainment plan
at all; or for an area that is discretionarily reclassified by the EPA
because it has very high air quality values). However, the proposal did
not address the issue of when the attainment demonstration and other
elements should be required for submission when this circumstance
occurs. The comment raises the question regarding whether, in the most
extreme example, it would be reasonable for an area to be reclassified
just before the Moderate area attainment date (end of the sixth
calendar year after designation) and then to have until just before the
Serious area attainment date (end of the tenth calendar year after
designation) to submit the attainment demonstration. This situation
would provide little meaningful time for the state and relevant
emissions sources to implement measures to reach attainment by the
attainment date, nor would it provide sufficient time for the EPA to
review and take action on the plan.
    The EPA maintains that the statutory authority to ``reclassify as a
Serious PM-10 nonattainment area . . . any area that the Administrator
determines cannot practicably attain [the NAAQS] by the attainment date
. . . for Moderate Areas'' includes the authority to make that
determination and issue a discretionary reclassification any time
before the Moderate area attainment date, as long as doing so does not
otherwise unreasonably frustrate the primary goals of the statute. For
example, the EPA must consider the timing for submission of Serious
area SIP requirements to ensure the state has sufficient time to
implement an effective plan and the agency has sufficient time to
review and act on the plan in advance of the outermost Serious area
attainment date (i.e., the end of the tenth calendar year after initial
designation as nonattainment). See CAA section 188(c)(2).
    The EPA interprets the statute to provide authority to require
submission of attainment plan requirements, including the attainment
demonstration, by a date less than 4 years from reclassification to
Serious when exercising its discretionary authority to reclassify an
area to serious nonattainment pursuant to CAA section 188(b)(1). While
the EPA generally prefers to give states as much time as possible to
develop and submit plans, the agency concluded that allowing 4 years
for submission of the attainment demonstration in all discretionary
reclassification actions would potentially frustrate the goals of the
statute.
    To resolve this issue, EPA is finalizing a specific schedule for
submission of the attainment demonstration following discretionary
reclassification. As discussed earlier, the terms of the statute
provide some guidance as to the appropriate schedule because, as
explained earlier, a state would have until the end of the 7th calendar
year to submit the attainment demonstration after a discretionary
reclassification that follows the timing in CAA section 188(b)(1)(B),
and a state would have until the end of the 8th calendar year after a
mandatory reclassification to submit the attainment demonstration. See
generally CAA sections 188(b) and 189(b).
    While not dispositive, these provisions indicate that Congress
believes that Serious area attainment plans should be submitted at
least 2 years in advance of the outermost statutory attainment date for
Serious areas to ensure expeditious attainment of the NAAQS. The EPA
finds that a minimum of 2 years is appropriate because (1) it provides
time for emission reduction measures adopted by the state to take
effect and improve air quality; (2) it will allow the agency sufficient
time to evaluate and act on the Serious area attainment demonstration;
and (3) for every other NAAQS, the CAA SIP submission dates are
generally 2 years or more prior to the attainment date. If for example
the plan is not submitted until just before year 10, and the agency
determines the plan will not lead to attainment, there will be no time
to take corrective action before the attainment date to ensure
attainment of the NAAQS. Such a result would not be reasonable.
    Therefore, the EPA believes that a reasonable attainment
demonstration due date for any discretionary reclassification to
Serious would be the earlier of (1) 4 years from the date of
reclassification, or (2) the end of the eighth calendar year after
designation. As an example, an area that is reclassified at the end of
year 5 would have 3 years rather than four years to submit the
attainment demonstration and other plan elements by the end of year 8.
An area that is reclassified no later than the end of year 4 would have
the full four years, and any area reclassified after this point would
have less than 4 years. At the outer extreme, in the unlikely event
that the EPA chooses to exercise its discretion to reclassify an area
in the sixth calendar year after designation (i.e., within a year of
the attainment date), the area would still have 2 years to submit the
attainment demonstration, which is still no less than the timeframe
Congress provided for a Moderate area that is reclassified because it
fails to attain. See 40 CFR 51.1003(b)(2)(i).
    Lastly, this section addresses appropriate SIP submission dates for
the other required plan elements. Regarding the base year emission
inventory, the EPA believes it is appropriate to require the updated
base year emissions inventory at the same time that the BACM/BACT
submission is due (18 months) because the updated inventory will be a
critical element relied on for making control measure determinations.
Regarding the remaining planning elements (i.e., additional feasible
measures, RFP, quantitative milestones, contingency measures, and
attainment projected inventory), the proposed options allowed for the
possibility of up to three separate submissions under certain policy
combinations, and we believe having such an outcome would be very
inefficient. Thus, the EPA has determined that the remaining elements
must be submitted at the same time as the attainment demonstration
(i.e., the earlier of 4 years from the date of reclassification, or the
end of the eighth calendar year after designation). This approach will
provide for the most efficient process and at the same time provide the
states with the maximum reasonable time when they are reclassified
pursuant to the EPA's

discretionary authority in CAA section 188(b)(2).
    With regard to the due date for submission of NNSR program
revisions that may be required when an area is reclassified to Serious,
such as revisions to meet nonattainment NSR program requirements to
lower the ``major stationary source'' threshold from 100 tons per year
(tpy) to 70 tpy (CAA section 189(b)(3)) and to address the control
requirements for major stationary sources of PM2.5
precursors [CAA section 189(e)], \155\ the Act does not specify a
deadline for the State's submission following reclassification of a
Moderate PM2.5 nonattainment area as Serious nonattainment
under subpart 4. Pursuant to EPA's gap-filling authority in CAA section
301(a) and to effectuate the statutory control requirements in section
189 of the Act, the final rule requires the state to submit these
nonattainment NSR SIP revisions no later than 18 months after the
effective date of final reclassification. This due date is also
consistent with the due date for submission of BACM and BACT provisions
and the emission inventory; thus, at most, a state will have two
required SIP submissions after being reclassified. See 40 CFR
51.1003(b)(2)(i) and (ii).
---------------------------------------------------------------------------

    \155\ Section 189(e) of the CAA requires that the control
requirements applicable to major stationary sources of
PM2.5 also apply to major stationary sources of
PM2.5 precursors, except where the state demonstrates to
the EPA's satisfaction that such sources do not contribute
significantly to PM2.5 levels that exceed the standard in
the 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.

B. Emissions Inventory Requirements

1. Summary of Proposal
    The EPA proposed that the inventory requirements for Serious areas
were the same as those for Moderate areas with some additions. In
addition to the Moderate area requirements, the EPA proposed that
Serious area inventory requirements would include using a major source
threshold of 70 tons/year for reporting sources as point sources for
both the base year inventory for the nonattainment area and the
attainment projected inventory for the nonattainment area.
    With regard to the due date for the attainment projected inventory
for the nonattainment area, the EPA proposed two cases. In the case
where the area is reclassified after failing to attain the NAAQS by the
Moderate area attainment date, the attainment projected inventory for
the nonattainment area was proposed to be submitted no later than 18
months after reclassification. In the case where the area is
reclassified by the EPA because the area cannot practicably attain the
NAAQS by the statutory attainment date, the EPA proposed that the
attainment projected inventory for the nonattainment area would be due
no later than 4 years after reclassification.
2. Final Rule
    a. What Emissions Inventory Requirements Apply to Serious Area
Attainment Plans? As with Moderate PM2.5 nonattainment
areas, Congress did not create a specific emissions inventory
requirement in subpart 4 that would supersede the emissions inventory
requirement under subpart 1 for Serious areas. Thus, the statutory
emissions inventory requirements that apply for Serious area attainment
plans continue to be those of section 172(c)(3), which explicitly
requires ``a comprehensive, accurate, current inventory of actual
emissions of the relevant pollutants'' in the nonattainment area. In
addition, the specific attainment plan requirements for the
PM2.5 NAAQS set forth in section 189(a) and associated
modeling requirements make an accurate and up-to-date emissions
inventory a critical element of any viable attainment plan. Finally,
the additional attainment plan requirements for the PM2.5
NAAQS for Serious areas contained in subpart 4 at section 189(b) have
additional requirements that affect the emissions inventory
requirements for Serious areas.\156\
---------------------------------------------------------------------------

    \156\ All definitions described in Section III.B of this
preamble for areas classified as Moderate apply in this section.
---------------------------------------------------------------------------

    As noted earlier in this preamble, states must use the best
available, current emissions inventory information for attainment plan
development, because complete, high quality emissions inventory data
are essential for the development of an effective control strategy. To
assist states in preparing complete, high quality inventories, the EPA
provides guidance for developing emissions inventories in its SIP
Emissions Inventory Guidance, available at https://www.epa.gov/air-emissions-inventories/emissions-inventory-guidance-documents. The EPA
recommends that states consult this guidance while developing emissions
inventories to meet requirements for Serious area attainment plans.
    b. How do States Meet the Inventory Requirements for the
PM2.5 NAAQS for Areas Classified as Serious? As with
Moderate PM2.5 nonattainment areas, neither section
172(c)(3) nor the provisions specifically applicable to attainment
plans for the PM2.5 NAAQS in subpart 4 specify how states
should meet statutory emissions inventory requirements for Serious
PM2.5 nonattainment areas. Section 172(c)(3) requires that
states submit ``a comprehensive, accurate, current inventory of actual
emissions from all sources of the relevant pollutant or pollutants in
such area, including such periodic revisions as the Administrator may
determine necessary to assure that the requirements of this part are
met'' (emphasis added). The EPA interprets this provision to authorize
the agency to require states to revise their base year emissions
inventories whenever the state is required to submit a new attainment
plan because of a change in the nonattainment area's status (e.g.
failure to attain by the applicable attainment date resulting in
reclassification). In addition, pursuant to CAA section 301, the EPA
has additional authority to promulgate regulations as necessary for the
implementation of the PM2.5 NAAQS, including requirements
pertaining to emissions inventories. Accordingly, this rule includes
specific emissions inventory requirements that the EPA considers
necessary to effectuate the attainment plan requirements of the CAA for
the PM2.5 NAAQS.
    Like Moderate areas, there are three key facets of the emissions
inventory requirements: (i) The types of inventories required; (ii) the
content of these inventories; and, (iii) the timing of submission of
these inventories. The three facets are addressed in the following
paragraphs.
    First, the same two types of inventories required for Moderate
areas are also required for Serious areas. While these inventories are
the same types and names of inventories as for Moderate areas, they
must be created specifically for Serious area attainment plans in
accordance with the applicable Serious area requirements. The first
type of inventory is called the ``base year inventory for the
nonattainment area,'' and the second type of inventory is called the
``attainment projected inventory for the nonattainment area.'' See 40
CFR 51.1000. The attainment projected inventory is necessary to
implement the attainment demonstration requirement of section
189(a)(1)(B), and it also may be used as part of the RFP requirement
(see Section VI.F). For these reasons, this rule establishes a
regulatory requirement that Serious area attainment plans must include
a base year inventory for the nonattainment area and an attainment

projected inventory for the nonattainment area.
    Second, the content of the inventories will follow the content
requirements for Moderate area inventories, with two exceptions needed
to meet the requirements of section 189(b)(3). The first exception for
Serious areas stems from the Section 189(b)(3) definition of a separate
emissions threshold for major sources in Serious nonattainment areas
(70 tpy potential to emit of PM10). This threshold is lower
than the 100 tpy potential to emit general requirement for major
sources of PM10, PM2.5 or one of its precursors
that is used for Moderate area emissions inventories. Inventories for
Serious area attainment plans must include these smaller sources as
major stationary sources (rather than the nonmajor stationary source
category that would apply for these in Moderate area plans) using the
lower threshold specified in the CAA. Also as described earlier and in
40 CFR part 51, subpart A, this means that all other smaller stationary
sources within the nonattainment area must be included in the base year
inventory and projected attainment year inventory as nonpoint sources.
    As described previously for Moderate areas, Appendix A of Table 1
of 40 CFR part 51, subpart A (the AERR) is required by this rule to
define which sources must be reported as point sources for inventories
associated with this rule (base year and projected attainment year
inventories). To be consistent with the 70 tpy threshold finalized in
this rule, this rule is also amending Table 1 of Appendix A of the AERR
to include the 70 tpy threshold for PM2.5, SO2,
NOX, VOC and ammonia for point sources within nonattainment
areas.
    The second difference between the Serious area and Moderate area
inventory requirements is a minor wording difference for the year that
should be chosen for the base year inventory. The year should be one of
the 3 years used for reclassification (rather than designation for
Moderate areas) or another technically appropriate inventory year.
Another inventory year may be chosen under specific circumstances
(e.g., to account for a change in sources in the nonattainment area,
changes in nonattainment area boundaries, or significant time lag
between designations and preparation of the inventory) with
consultation from the appropriate EPA Regional Office. This requirement
is intended to ensure that the inventory will represent the emissions
sources whose contributions resulted in a nonattainment designation for
the area.
    The third facet of the Serious area inventory requirements is the
timing, which is somewhat different than for Moderate areas. Section
VI.A of this preamble describes the requirement that states submit the
base year inventory for a Serious nonattainment area at the same time
that it submits provisions to implement BACM and BACT on sources in the
area (due no later than 18 months from reclassification of the area
pursuant to section 189(b)(2)). This is because the base year inventory
serves as the starting point for conducting a BACM and BACT
determination. In contrast to the base year inventory, the attainment
projected inventory is more closely related to the Serious area
attainment demonstration. Thus, the attainment projected inventory is
most appropriately submitted with the attainment demonstration for a
given Serious area to allow effective evaluation of the attainment plan
as a whole.
    Consequently, this rule requires that attainment projected
emissions inventories be submitted at the same time as the Serious area
attainment demonstration. This requirement gives rise to two possible
deadlines for Serious areas to submit the attainment projected
emissions inventory for the nonattainment area. For areas that are
reclassified after failing to attain the NAAQS by the applicable
Moderate area attainment date, the deadline is no later than 18 months
after reclassification (same time period as for Moderate areas). For
areas reclassified by the EPA because the area cannot practicably
attain the NAAQS by the statutory Moderate area attainment date, the
deadline is the earlier of 4 years from the date of reclassification,
or the end of the eighth calendar year after designation.
3. Comments and Responses
    Comment: Some commenters noted that the proposal was unclear with
regard to the inventory year for areas that are reclassified from
Moderate to Serious, and whether the terms ``reclassification'' and
``designation'' are interchangeable in this regard.
    Response: In the final rule, the EPA clarifies that for areas that
are redesignated to Serious, the inventory year must be one of the 3
years used for reclassification. Any additional comments received on
this section are addressed in the Response to Comments document found
in the docket for this action.

C. Pollutants To Be Addressed in the Plan

    All PM2.5 precursors are presumptively required to be
addressed in any Serious area attainment plan. Section III of this
preamble includes a detailed discussion about optional analyses that a
state may provide to demonstrate that sources of a precursor do not
significantly contribute to PM2.5 concentrations in a
particular nonattainment area. These demonstrations may be conducted
for all sources of a precursor in an area (i.e., comprehensive
precursor demonstration), or just for major sources of the precursor
(i.e., major source precursor demonstration). It also discussed a
similar demonstration that may be conducted for NNSR (i.e., NNSR
precursor demonstration). These demonstrations may be used to justify
the exclusion of certain types of precursor sources from certain SIP
requirements in Serious area plans, just as in Moderate area plans.
However, the expeditious attainment demonstration is not available for
Serious area plans.
    As noted in Section III of this preamble, if the EPA approves a
state's precursor demonstration for the Moderate area plan, the state
would need to re-evaluate whether the precursor contributes
significantly to PM2.5 levels that exceed the standard for
the Serious area plan. The reason for this is that precursor emissions
and air quality concentrations will have changed since the submission
of the demonstration for the Moderate area, and precursor emissions
technical information and scientific understanding of precursor
emissions and interactions in the area should be better understood
several years later, and the Serious area plan needs to be based on the
best available information to date. If the state reevaluates a
precursor for potential exclusion from one or more of the Serious area
plan requirements, it should take into account factors such as
increases or decreases in emissions since the last precursor
demonstration; new ambient monitoring data for fine particle
composition and concentrations of important gases (such as ammonia);
and improved air quality modeling programs that reflect improved
understanding of the role of precursors in atmospheric transformation
processes. To the extent appropriate, this precursor demonstration can
build off the analyses conducted for the Moderate area precursor
demonstration.
    If the EPA approves a comprehensive precursor demonstration for the
Serious area plan, then the state would not be

obligated to evaluate BACM/BACT measures for reducing that precursor in
the nonattainment area, nor would it need to account for that precursor
in the RFP plan, quantitative milestones, and contingency measures. If
a major stationary source precursor demonstration is approved, then the
state would not be obligated to evaluate BACM/BACT measures for
reducing that precursor from major sources in the nonattainment area,
nor would it need to account for emissions of that precursor from major
sources in the RFP plan, quantitative milestones, and contingency
measures. If a NNSR precursor demonstration is approved, then the state
would not be obligated to address LAER and emission offset requirements
for that precursor in the NNSR program for that nonattainment area.

D. Attainment Plan Control Strategy

1. General Approach to Designing a Control Strategy for a Serious
Nonattainment Area
    The statutory attainment planning requirements of subparts 1 and 4
were established to ensure that states meet the following goals of the
CAA: (i) Implement measures that provide for attainment of the
PM2.5 NAAQS as expeditiously as practicable, and (ii) adopt
emission reduction strategies that will be effective at reducing
PM2.5 levels in nonattainment areas. A state has discretion
to require reductions from any source inside or outside of a
PM2.5 nonattainment area (but within the state's boundaries)
in order to fulfill its obligation to demonstrate attainment in a
PM2.5 nonattainment area as expeditiously as practicable, in
addition to having an obligation to meet the statutory requirements for
specific control measures on sources located within a nonattainment
area (e.g., BACM and BACT). A state may need to require emissions
reductions on sources located outside of a PM2.5
nonattainment area if such reductions are needed in order to provide
for expeditious attainment of the PM2.5 NAAQS.
    The following sections describe the recommended approach for a
state to follow in order to identify and select the complete suite of
measures needed for an approvable attainment plan submission for a
Serious PM2.5 nonattainment area.
2. Identification and Selection of BACM/BACT and Additional Feasible
Measures
    a. Summary of Proposal. The proposal provided background
information on statutory requirements and existing guidance regarding
Serious area control strategies, and then presented two broad
approaches describing the steps for determining BACM/BACT and
additional feasible measures (i.e. control measures that may be able to
help the area attain by the attainment date or advance the attainment
date by a year, and that may be implemented later than BACM/BACT but
before the attainment date). The first approach is consistent with
current guidance for PM10 NAAQS implementation in the
Serious Area Addendum. Under the first approach, the emphasis of the
analysis would be on the identification of feasible control measures.
The analysis would be considered to be ``generally independent'' of
whether such measures are needed for expeditious attainment of the
relevant NAAQS. However, this approach also would allow the state to
identify de minimis source categories before conducting any further
analysis of technologically feasible or economically feasible control
measures. The proposal requested comment on inclusion of an ambient
impact threshold of 3 percent for determining whether a source category
impact would be de minimis. This proposed threshold level was similar
to the de minimis ambient levels included in the Serious Area Addendum
for implementation of the PM10 NAAQS, and the state would
likely need to conduct air quality modeling to demonstrate de minimis
impacts below a particular threshold. The proposal noted the challenges
associated with providing a nationally consistent definition of what
would be a ``source category.'' For source categories found to be de
minimis, the state would not be obligated to evaluate potential control
measures. The basic analytical steps for proposed option 1 were
presented as follows: (1) Update base year emissions inventory for the
area; (2) evaluate source category impacts; (3) identify existing and
potential control measures; (4) determine whether an available control
measure or technology is technologically feasible; (5) determine
whether an available control measure or technology is economically
feasible; (6) determine the earliest date by which a control measure or
technology can be implemented in whole or in part.
    Under the second proposed option, there would be a greater emphasis
on linking the control strategy evaluation process with the attainment
needs for the area. Accordingly, this option would not include a ``de
minimis'' step 2 early in the process. However, at the end of the
process, the state would be able to choose to not adopt certain
measures that would otherwise meet the criteria for BACM/BACT if those
measures collectively would not be necessary to bring the area into
attainment or to advance the attainment date by 1 year (similar to the
approach that EPA uses, and has historically used, for RACM/RACT). The
EPA requested comment on all aspects of these options, and indicated
the agency may finalize either approach or various elements of each
approach after evaluating the comments that had been received.
    b. Final Rule. The EPA has considered the comments that were
submitted on the two proposed options for determining BACM/BACT (and
additional feasible measures), and has determined that the final rule
should include aspects of each option. The following sections provide
background information and guidance on the steps of the process for
determining Serious area control measures for PM2.5
nonattainment areas.
i. BACM and BACT
    A Serious area attainment plan must include provisions to implement
BACM on sources in a Serious nonattainment area, as provided by section
189(b)(1)(B), no later than 4 years after reclassification. Under
section 189(b)(2), a state has 18 months following reclassification to
submit these BACM provisions.
    Section 189(b)(1)(B) refers only to BACM, but the EPA has long
interpreted this term to include BACT, just as the analogous term for
RACM includes RACT for Moderate areas. For implementation of the
PM2.5 NAAQS, the EPA finds it reasonable to maintain the
same interpretation. The legislative history for the 1990 Amendments to
the CAA supports this interpretation, as the EPA has explained in past
guidance.\157\ Additionally, the requirement for BACT for existing
sources in the context of PM2.5 NAAQS implementation in
nonattainment areas is separate and distinct from the requirement for
BACT for new and modified sources under the Prevention of Significant
Deterioration (PSD) permitting program for new stationary sources in
areas designated as attainment or unclassifiable for the
PM2.5 NAAQS. As described later in this section, however,
the process and criteria that states have historically used to
determine BACT for new and modified sources under the PSD program have
also been referenced and applied to the process for determining BACT
for PM10 NAAQS implementation, but these requirements

are otherwise unrelated. Consistent with past policy, BACT
determinations for PM2.5 NAAQS implementation are to follow
the same process and criteria that are applied to the BACT
determination process for the PSD program.
---------------------------------------------------------------------------

    \157\ Ibid. at 42008-09.
---------------------------------------------------------------------------

    Longstanding guidance in the General Preamble and Addendum,
together with past practice associated with implementing the
PM10 NAAQS under subpart 4, has helped to establish a
general approach for states and the EPA to determine BACM and BACT for
Serious PM10 nonattainment areas. This approach has served
as the basis for developing a more stringent control strategy for a
Serious PM10 nonattainment area than that developed for such
area when it was classified as Moderate. Indeed, as BACM and BACT are
required to be implemented when a Moderate nonattainment area is
reclassified as Serious due to its actual or projected inability to
attain the relevant NAAQS by the Moderate area attainment date through
the implementation of ``reasonable'' measures, it is logical that
``best'' control measures should represent a more stringent and
potentially more costly level of control.\158\ The level of stringency
generally refers to the overall level of emissions reductions of a
control measure or technology, or of such measures and technologies
combined.
---------------------------------------------------------------------------

    \158\ Ibid. at 42009.
---------------------------------------------------------------------------

    Congress first defined BACT in CAA section 169(3) for the PSD
permitting program as: ``an emission limitation based on the maximum
degree of reduction of each pollutant . . . which the permitting
authority, on a case-by-case basis, taking into account energy,
environmental, and economic impacts and other costs, determines is
achievable for such facility through application of production
processes and available methods, systems, and techniques . . .''
    In the Addendum, the EPA provided guidance concerning the
requirements for BACM and BACT for Serious area attainment plan
requirements for the PM10 NAAQS.\159\ The EPA discussed in
the Addendum that when Congress amended the CAA, Congress selected the
same ``best'' terminology for PM10 nonattainment areas as it
did for the language selected for the PSD program in 1977. The EPA
interpreted this word choice at the time to mean that PSD BACT and
PM10 nonattainment area BACM should be generally analogous
in definition and implementation, but with some differences due to
different end policy goals between the PSD and nonattainment area
programs.\160\ The EPA thus defined BACM for PM10 Serious
nonattainment area planning to be the maximum degree of emission
reduction achievable from a source or source category which is
determined on a case-by-case basis, considering energy, economic and
environmental impacts and other costs.\161\
---------------------------------------------------------------------------

    \159\ Ibid. at 42009.
    \160\ Ibid. at 42010. ``EPA will interpret PSD BACT and PM-10
BACM as generally similar because, despite the similarity in
terminology, certain key differences exist between control measures
applicable in the PSD and PM-10 serious nonattainment area programs.
The BACT under the PSD program applies only in areas already meeting
the NAAQS, while PM-10 BACM applies in areas which are seriously
violating the NAAQS. The difference in policy goals, arguably,
suggests that the PM-10 BACM control standard should be more
stringent than that for PSD BACT . . . EPA considers it reasonable
to use the approach adopted in the PSD BACT program as defined in
section 169(3) of the Act as an analogue for determining appropriate
PM-10 nonattainment control measures in serious areas, while at the
same time retaining the discretion to depart from that approach on a
case-by-case basis as particular circumstances warrant.''
    \161\ Ibid.
---------------------------------------------------------------------------

ii. BACM/BACT ``Generally Independent'' of Attainment
    As noted earlier, the issue of whether BACM/BACT should be
considered generally independent of attainment or more closely tied to
attainment for purposes of implementing the PM2.5 NAAQS was
a central issue distinguishing the two proposed options for determining
BACM/BACT. Some commenters suggested that the overarching requirement
of the CAA is to attain the standard expeditiously, and therefore the
benefits of a ``generally independent'' BACM/BACT requirement are not
clear. On the other hand, some other commenters supported maintaining
the longstanding policy from the Serious Area Addendum that the BACM/
BACT requirement is generally independent of attainment, citing the
emphasis on ``best'' control measures and the statutory provision
requiring BACM/BACT well before the attainment demonstration for
certain reclassified areas. For the reasons discussed later in this
section, the EPA has decided to maintain the policy that BACM/BACT
determinations are to be ``generally independent'' of attainment for
purposes of implementing the PM2.5 NAAQS.
    In the Serious Area Addendum, the EPA described BACM as a generally
independent requirement, to be determined without regard to the
specific attainment analysis (i.e., attainment demonstration) for the
area.\162\ The EPA established that such an interpretation is in
accordance with the structural scheme of the CAA, which by its
definition requires that when an area is classified as Serious, BACM
are implemented in addition to RACM. Because of the two types of
measures employed, the EPA found it reasonable to interpret the statute
as requiring a different analysis for determining BACM, i.e., that
while RACM emphasizes the attainment needs of the area, BACM has a
greater emphasis on identifying measures that are feasible to
implement. Keeping in mind that the overall objective of the
implementation of BACM and BACT and additional feasible measures is to
bring a Serious PM2.5 nonattainment area into attainment as
expeditiously as practicable, the General Preamble noted that the test
for BACM puts a ``greater emphasis on the merits of the measure or
technology alone,'' rather than on ``flexibility in considering other
factors,'' in contrast to the approach for determining RACM and
RACT.\163\
---------------------------------------------------------------------------

    \162\ Ibid. at 42011.
    \163\ Ibid at 42011.
---------------------------------------------------------------------------

    The view that BACM and BACT measures are generally independent of
the attainment needs of the area is also consistent with the
statutorily specified submission date for BACM and BACT control
measures, contrasted against the statutorily specified submission date
for the attainment demonstration for Serious areas. Specifically,
states with Serious nonattainment areas must submit BACM and BACT
measures within 18 months of reclassification of areas to Serious,
whereas they are given up to 4 years from reclassification (for areas
where it is impracticable to attain by the attainment date) to submit
the attainment demonstration for such areas.
    Additionally, the EPA believes that interpreting the Serious
PM2.5 nonattainment area BACM/BACT requirements to be
``generally independent'' of attainment is consistent with the
structure and substance of the CAA control measure requirements for
ground-level ozone nonattainment areas with more serious air quality
problems. In the CAA ozone implementation requirements, an area that is
reclassified to a more serious category because it failed to attain the
standard or because it is impracticable to attain by the attainment
date is then subject to additional specific control measure
requirements that are considered to be generally independent of
attainment (for example, see CAA section 182(b) through (e)). The
statute includes these specific requirements in order to ensure
continued progress toward attainment for these areas with

more difficult air quality problems. The EPA believes it is appropriate
to have a similar interpretation of the PM2.5 Serious area
control measure requirements. In a similar manner, interpreting BACM/
BACT to be generally independent of the attainment needs of a Serious
PM2.5 area will ensure continued progress toward attainment
for those areas with more difficult air quality problems. The EPA also
believes this more rigorous ``independent control measure'' approach
for implementing the PM2.5 standards in a manner similar to
ozone is appropriate because the health effects of both standards are
very significant (including premature mortality), and robust emission
reduction programs are needed to bring about expeditious attainment and
public health protection for citizens in these nonattainment areas.
iii. No de Minimis Source Category Analysis for PM2.5 NAAQS
Implementation
    Another central issue distinguishing the two proposed options for
how to determine BACM/BACT was the issue of whether, before analyzing
any potential BACM/BACT, the state should conduct technical analyses to
identify whether there are any source categories having a de minimis
contribution to PM2.5 levels in the PM2.5
nonattainment area. This de minimis analysis is part of the process
described in the Serious Area Addendum for implementation of the
PM10 standards. Under the proposal, for source categories
found to be de minimis, the state would not be obligated to evaluate
potential control measures.
    As noted previously, the proposal requested comment on inclusion of
an ambient impact threshold of 3 percent for determining whether a
source category impact would be de minimis. This proposed threshold
level was similar to the de minimis ambient levels included in the
Serious Area Addendum for implementation of the PM10 NAAQS,
and the state would likely need to conduct air quality modeling to
demonstrate de minimis impacts below a particular threshold. The
proposal noted the challenges associated with providing a nationally
consistent definition of what would be a ``source category.''
    The EPA also proposed a similar de minimis source category concept
for the RACM/RACT process for Moderate area plans, and many of the
comments received on the proposed Moderate area ``upfront'' de minimis
source category analysis are also applicable when considering whether
to include a de minimis source category analysis concept for Serious
areas in the final rule. A number of commenters expressed concern about
the analytical resources that might be needed to conduct air quality
modeling to identify whether all the sources in a particular source
category have an ambient air quality contribution exceeding an air
quality threshold. Some commenters suggested that a de minimis source
category approach for either Moderate or Serious areas would allow the
state to ignore a set of control measures that later in the control
measure evaluation process could be determined to provide for a more
expeditious attainment date. They believe that allowing the exemption
of de minimis source categories would undermine any analysis to
evaluate whether a collection of measures could advance the attainment
date by a year. For example, it would be possible for a state to
identify multiple de minimis source categories at the beginning of the
process, and then after all potential control measures are identified,
the state and the EPA would be unable to determine whether the
collective reductions and air quality impact of the exempted categories
could actually be sufficient to advance the attainment date. Other
commenters noted that providing a source category exemption in one
nonattainment area would lead to inconsistent treatment within a state
or across states because it would give the exempted companies a
competitive advantage over the same types of sources in other areas.
    A number of commenters supported the de minimis source category
concept because they believed it could result in a reduced burden in
the control measure evaluation stage and help avoid regulating sources
with limited impact on PM2.5 levels. Some commenters
supported the de minimis concept only if controls on the source are not
needed for expeditious attainment. Some commenters suggested that the
EPA include an emissions-based threshold (e.g. tons per day) rather
than an air quality based threshold to reduce potential analytical
burden associated with de minimis source category analyses. However, in
their comments they did not address the fact that the air quality
impact of a specific tons per day rate could vary greatly from one
pollutant to another within a particular nonattainment area, or across
different nonattainment areas. One state commenter noted that the NAICS
system does not provide categories for nonpoint sources, and that this
issue would need to be addressed if the NAICS approach were to be
included in the final rule. Other commenters suggested that the rule
not have a de minimis threshold at all but include the ability for the
state to propose de minimis source categories to the EPA on a case-by-
case basis.
    After taking the range of comments on the de minimis source
category concept into consideration, the EPA has decided to not
finalize a de minimis source category approach for the purposes of
implementing the PM2.5 NAAQS. The EPA is persuaded by
commenters who argued it is not necessary, and believes that without
this concept the final rule will nevertheless provide sufficient
flexibility in the Serious area control measure analysis and attainment
demonstration process, due to the availability of provisions enabling
states to identify sources that should not be subject to control
measures, including the ability to develop precursor demonstrations to
exclude certain precursors from control requirements, and to consider
case-specific factors in determining technical and economic feasibility
of potential control measures. If the final rule were to include an
explicit step to conduct a de minimis source category analysis on the
entire inventory early in the control measure identification process,
the EPA believes that there is a risk that such an analysis may bring
about investment of scarce time and analytical resources on analysis of
categories to exclude rather than on the identification of the most
beneficial control measures for reducing PM2.5 and its
precursors to achieve expeditious attainment of the standard. In
addition, the EPA finds merit in comments suggesting that an upfront
exemption of multiple de minimis source categories in an area would
undermine the ability of the state (or other interested parties) to
evaluate, after the identification of potential control measures,
whether the area could advance the attainment date in order to attain
``as expeditiously as practicable.''
    Moreover, as noted in Section IV.D of this preamble on Moderate
areas, the EPA also finds that from a technical perspective, it would
be very challenging to implement a de minimis source category process
in a consistent manner nationally without clear guidelines describing
how narrowly or how broadly a de minimis exemption could apply to a
``source category,'' or how the technical analysis would need to be
performed. For example, should a source category consist of all
industrial boilers? Or all industrial boilers that burn a particular
fuel? Or all industrial boilers that burn a particular fuel and

are within a specific size range? The NAICS codes do not provide an
appropriately comprehensive approach for defining source categories for
all stationary, mobile, and area sources for this purpose. It has been
noted that a de minimis source category exemption process is described
in the 1994 PM10 NAAQS implementation guidance (the Serious
Area Addendum). In PM10 areas, however, it may have been
relatively straightforward to identify what were the predominant source
categories contributing to the NAAQS violations (such as direct
PM2.5 emissions from dust or wood smoke), and therefore to
be able to identify what categories might be considered as not
predominant contributors (or de minimis). However, implementation of
the PM2.5 NAAQS presents much more complex challenges.
Precursors and their contribution to secondarily formed PM play a much
greater role in PM2.5 nonattainment areas than in
PM10 nonattainment areas. In addition, the relative impact
of each precursor to local PM2.5 concentrations varies from
area to area, and even within sections of the same area. To
appropriately implement an approach allowing for de minimis source
category impacts, the EPA believes that a nationally consistent source
category definition would be needed, along with sophisticated air
quality modeling to evaluate the relative impacts of precursors emitted
from different ``source categories.'' The resources needed to conduct
such analyses could be substantial, and would ultimately not help
identify what control measures would be needed to solve the air quality
problem. For all of these reasons, a de minimis source category concept
is not included in the final rule for Serious areas.
iv. Additional Feasible Measures
    While the proposed approaches and criteria for identifying
appropriate control measures for a Serious area are necessarily
different than for a Moderate area, it is important to note two
similarities: First, that the EPA interprets the requirement under CAA
section 172(c)(6) for a state to adopt ``other measures'' needed for
attainment to apply to sources located inside and outside of any
PM2.5 nonattainment area (but within the state's
boundaries), whether the area is classified as Moderate or Serious; and
second, similar to the RACM requirement for Moderate nonattainment
areas under subpart 4, CAA section 189(b)(1)(B) requires that BACM must
be implemented no later than 4 years after a Moderate area is
reclassified to Serious.
    Taking these two statutory provisions together, the EPA proposed
that the other measures required under CAA section 172(c)(6) must
include ``additional feasible measures,'' which would be those measures
and technologies that otherwise meet the criteria for BACM/BACT but
that can only be implemented in whole or in part beginning 4 years
after reclassification of an area, but no later than the statutory
attainment date for the area. See proposed 40 CFR 51.1000.
    Some commenters agreed that an area must also consider adopting
control measures that cannot be implemented within the 4-year deadline
for implementation of BACM and BACT. Some commenters suggested that
additional feasible measures should only be tied to expeditious
attainment.
    In the final rule, additional feasible measures would necessarily
be implemented by sources in the nonattainment area, and a state is
required to implement them if they are needed in addition to BACM and
BACT to bring the area into expeditious attainment. The state must also
adopt other emission reduction measures for sources within the state
but outside the nonattainment area if such measures in conjunction with
other control measures would enable the area to attain the standard by
the attainment date, or enable the area to advance the attainment date
by at least 1 year.
    These ``additional feasible measures'' would be analogous to the
``additional reasonable measures'' in the RACM and RACT analysis
process, which are technologically and economically feasible measures
that cannot qualify as RACM or RACT because they cannot be implemented
within 4 years of designation of a Moderate nonattainment area. Under
the approach for determining BACM and BACT for sources in a Serious
nonattainment area described later in this section, a state would
identify additional feasible measures as part of the BACM and BACT
determination process, just as additional reasonable measures would be
identified as part of the state's RACM and RACT determination process.
    The EPA recognizes that with regard to Serious areas, only a
nonattainment area that is reclassified under the agency's
discretionary authority might have sufficient time between the date for
implementing BACM and BACT and the statutory Serious area attainment
date to implement additional measures beyond BACM and BACT. BACM and
BACT must be implemented no later than 4 years after reclassification
of the area; areas reclassified to Serious because they cannot
practicably attain the relevant NAAQS by the applicable attainment date
could potentially have significantly more than 4 years between the date
of reclassification and the statutory Serious area attainment date,
during which time the area could continue to implement additional
feasible measures to bring the area into attainment.
    By way of illustration, for areas designated in the first round of
designations for the 2012 PM2.5 NAAQS, the statutory
Moderate area attainment date will be no later than December 31, 2021.
If a state submits a Moderate area attainment plan by the statutory
attainment plan due date (18 months after designation, or in this
example, October 2016) and the plan demonstrates that the area cannot
practicably attain the NAAQS by December 31, 2021, then the EPA has a
statutory duty to reclassify such an area within 18 months of the
attainment plan due date (i.e., by April 2018). The statutory Serious
area attainment date would be the end of the tenth year following
designation, or December 31, 2025. In such a case, the state would need
to implement BACM for the area within 4 years of reclassification, or
by April 2022, leaving over 3.5 years between the statutory deadline
for implementing BACM and the statutory attainment date for the area.
The requirement for the state to identify and adopt additional feasible
measures for the area would mean that the state would need to identify
those control measures and technologies that are feasible (according to
the proposed BACM and BACT criteria described later in this section)
and that can be implemented between April 2022 and December 2025. The
EPA expects that while such a long span of time may be available only
to a very few Serious nonattainment areas, it would be appropriate to
require such areas to implement measures in addition to BACM and BACT
if, taken together, they can provide for attainment by the attainment
date or advance the attainment date for the area by at least 1 year.
Accordingly the EPA has codified a definition of ``additional feasible
measures'' and specified the conditions under which such measures would
need to be included in a serious area plan submission. See 40 CFR
51.1000 and 40 CFR 51.1010(a)(4)(ii).
v. Steps of the BACM/BACT Selection Process
    In addition to the regulatory decisions earlier, the EPA summarized
and sought comment on further guidance for states

to follow in selecting BACM/BACT. The guidance was primarily derived
from the Addendum. This section reviews that guidance, clarifies and
updates it for purposes of PM2.5, and responds to
significant comments on the guidance discussion included in the
proposal.
    The BACM/BACT selection process for implementation of the
PM2.5 NAAQS is designed to take into account the local facts
and circumstances and the nature of the air pollution problem in a
given nonattainment area. The following sections describe the steps of
the process, including: (i) Develop a comprehensive inventory of
sources and source categories of directly emitted PM2.5 and
PM2.5 precursors; (ii) identify existing and potential
control measures for the sources in the inventory; (iii) evaluate the
technological feasibility of potential control measures; (iv) evaluate
the economic feasibility of potential control measures; and (v)
determine the earliest date by which a control measure or technology
can be implemented in whole or in part.\164\ These steps are described
more fully in the following subsections.
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    \164\ For additional information, see ibid. at 42012-13.
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    Step 1: Develop a comprehensive inventory of sources and source
categories of directly emitted PM2.5 and PM2.5
precursors. As with any control strategy analysis for a nonattainment
area, the EPA recommends that the state begin with a current detailed
emissions inventory of the various sources that emit direct
PM2.5 and PM2.5 precursors in the Serious area.
The inventory should identify major stationary sources (i.e., sources
with the potential to emit 70 tpy of direct PM2.5 or any
precursor), nonmajor stationary sources, mobile sources, and area
sources. The inventory also should identify both anthropogenic and
nonanthropogenic emissions sources.\165\ The EPA expects the state to
start with the base year emissions inventory submitted with the
Moderate area attainment plan as required under CAA section 172(c)(3),
and update it as necessary to reflect new source construction, facility
shutdowns, growth in certain source categories, and any other relevant
changes. This inventory should be the most comprehensive and accurate
inventory available, and it should be consistent with the emissions
inventory requirements for Serious area plans as described in Section
VI.B of this preamble.
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    \165\ Addendum to the General Preamble, 59 FR 41998 (August 16,
1994), at page 42012.
---------------------------------------------------------------------------

    Step 2: Identify potential control measures. The state should
identify potential control measures for all sources and source
categories in the latest base year emission inventory for the
nonattainment area. The list of existing and potential control measures
should include options not previously considered as RACM/RACT for the
area, as well as additional measures not previously evaluated in the
RACM/RACT analysis. For purposes of identifying new measures to
consider in its BACM/BACT analysis, the EPA recommends that the state
obtain and evaluate a wide range of sources of information on existing
and potential control measures. Other nonattainment areas in the same
state, and other states across the country are important sources of
information about control measures that are currently being
implemented. Regional planning organizations, and state and local air
quality consortiums have in the past developed summaries of control
measures that should provide useful information for this process.\166\
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    \166\ Add cite to 2006 STAPPA ALAPCO document and other control
measure summaries. Add cite to menu of measures. Specific to
potential control measures for mobile source emissions, the EPA's
past guidance has indicated that where mobile sources contribute
significantly to PM2.5 violations, ``the state must, at a
minimum, address the transportation control measures listed in CAA
section 108(f) to determine whether such measures are achievable in
the area considering energy, environmental and economic impacts and
other costs.''
---------------------------------------------------------------------------

    The EPA's RBLC provides a central data base of air pollution
technology information that may be highly relevant to states seeking
information on stationary source control technology that may qualify as
BACT for PM2.5 NAAQS implementation, and is available online
at http://cfpub.epa.gov/RBLC/. There are also other resources available
to assist states in identifying other potential control measures and
control technologies for their BACM and BACT determinations. The EPA
encourages states with Serious PM2.5 nonattainment areas to
visit the agency's Web site to find links to other online sources of
information on potential control measures for states to consider.\167\
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    \167\ Links are provided to a number of national, state and
local air quality agency sites from the EPA's PM2.5 Web
site: http://www3.epa.gov/pm/measures.html.
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    The state must incorporate appropriate measures into the list of
potential control measures for the source categories in the Serious
nonattainment area. The EPA would expect the state to identify an array
of existing and potential new measures at least as broad as the list
identified for the same area as part of the RACM and RACT analysis, in
order to ensure that the state has a sufficiently expansive and
comprehensive set of potential measures to evaluate. The list of
potential measures must include all measures identified as potential
control measures for the nonattainment area when it was classified as
Moderate or, for a given source category, one or more alternative
control measures that would control emissions even more stringently
than the measures included in the RACM/RACT analysis. In this way, the
state will begin its BACM/BACT determination with a list of potential
control options that is as complete and up-to-date as possible.
    Step 3: Determine whether an available control measure or
technology is technologically feasible. After developing a list of
existing and potential new measures to evaluate for BACM and BACT, the
state would then need to determine the technological feasibility of
each identified control measure in light of a number of considerations,
including each measure's individual energy and environmental
impacts.\168\
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    \168\ Ibid. at 42012.
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    (1) Stationary sources. As described under the technological
feasibility criteria for the control measures analysis for Moderate
area attainment plans in Section IV.D of this preamble, the EPA's prior
guidance on factors to consider for judging whether a particular
control technology is technologically feasible should include a
source's processes and operating procedures, raw materials, physical
plant layout and potential environmental impacts such as increased
water pollution, waste disposal and energy requirements. For example,
the EPA recognizes that the process, operating procedures and raw
materials used by a source can affect the feasibility of implementing
process changes that reduce emissions and can also affect the selection
of add-on emission control equipment. The feasibility of modifying
processes or applying control equipment also can be influenced by the
physical layout of the particular plant, if the physical space
available in which to implement such changes limits the choices.\169\
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    \169\ Ibid. at 42013.
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    (2) Area and mobile sources. With respect to determining whether a
given control measure might not be technologically feasible as BACM for
an area or mobile source, a state may consider factors in conducting
its analysis that are similar to factors the state may have considered
during the RACM and RACT determination process, such as local
circumstances, the condition and extent of needed infrastructure, or
population size or workforce type and habits, which may

prohibit certain potential control measures from being implementable.
However, in the instance where a given control measure has been applied
in another NAAQS nonattainment area (for PM2.5 or other
pollutant), the state will need to provide a detailed justification for
rejecting any potential BACM measure as technologically infeasible.
Furthermore, if the state identifies a certain control measure for area
or mobile sources that has been implemented in another nonattainment
area and may qualify as BACM or BACT, the state must provide a reasoned
justification if it deems it technologically infeasible to implement
the same control measure to the same extent or magnitude as it was
applied in the other nonattainment area.
    Step 4: Determine whether an available control technology or
measure is economically feasible. The fourth step of this process is to
evaluate the costs of implementing each of the technologically feasible
control measures and technologies in order to eliminate from further
consideration any measures determined to be economically infeasible. In
assessing ``best'' control measures and technologies, states with
Serious PM2.5 nonattainment areas must identify a control
strategy for the area that overall is more stringent than that
identified for the area when the state considered only the
``reasonableness'' of potential control measures for purposes of the
RACM/RACT analysis. States need to consider emission reduction measures
with higher costs per ton when assessing the economic feasibility of
BACM and BACT controls (and, where applicable, additional feasible
measures) as compared to the economic feasibility criteria applied in
their RACM and RACT analysis (and analysis for additional reasonable
measures) for the same nonattainment area.
    Indeed, consistent with prior guidance on evaluating costs of a
potential BACM/BACT, the EPA maintains that while the economic
feasibility of a control measure is as important as its technological
feasibility under the RACM and RACT determination process, economic
feasibility is a less significant factor in the BACM and BACT
determination process. In other words, a state must apply a higher
standard for eliminating a technologically feasible control measure
from further consideration as BACM due to cost alone.
    In the Addendum, the EPA stated that ``for PM10 BACM
purposes, it is reasonable for similar sources to bear similar costs of
emission reduction.'' \170\ Additionally, the EPA indicated that
``economic feasibility for PM10 BACM purposes should focus
upon evidence that the control technology in question has previously
been implemented at other sources in a similar source category without
unreasonable economic impacts.'' \171\ Thus, a state may not eliminate
a particular control measure from further consideration as potential
BACM if similar sources have successfully implemented such a measure.
That is, a state must at a minimum continue to consider as potential
BACM any technologically feasible control measures or technologies
implemented by similar sources.
---------------------------------------------------------------------------

    \170\ Ibid.
    \171\ Ibid.
---------------------------------------------------------------------------

    In addition, a state may not automatically eliminate a particular
control measure merely because other sources have not implemented the
measure. In other words, a state must continue to consider
technologically feasible measures that have not been implemented by
similar sources but that can nonetheless effectively reduce emissions
from the source category in question at a cost that is not cost
prohibitive.
    As with the EPA's approach for evaluating economic feasibility of
potential reasonable measures for Moderate area attainment plans, for
each technologically feasible control measure or technology, a state
must evaluate the economic feasibility of the measure through
consideration of 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. While
the EPA is not establishing a fixed dollar per ton cost threshold for
economic feasibility of controls identified as potential BACM and BACT,
the cost per ton of an acceptable measure for the BACM and BACT
analysis generally would be higher than it was for the RACM and RACT
analysis for the same nonattainment area. In addition, 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 purposes of BACM 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,\172\ to the extent applicable:
---------------------------------------------------------------------------

    \172\ These longstanding factors were established in the EPA
guidance in 1992 and are applicable to implementation programs for
all of the NAAQS pollutants. See the appendices to the General
Preamble, 57 FR 18070 (April 28, 1992).
---------------------------------------------------------------------------

    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 BACM implemented by public sector
entities).

    Step 5: Determine the earliest date by which a control measure or
technology can be implemented in whole or in part. Section 189(b)(1)(B)
of the CAA requires that Serious area attainment plans provide for the
implementation of BACM no later than 4 years after reclassification of
the area to Serious. As with the EPA's proposed approach to RACM and
RACT, the EPA proposes the term ``implement'' to mean that the control
measure or technology has not only been adopted into the SIP for the
area but has also been built, installed and/or otherwise physically
manifested and the affected sources are required to comply. The EPA
thus expects a state with a Serious nonattainment area to take timely
action to implement BACM and BACT in the area.
    A state must identify those technologically and economically
feasible control measures and technologies that it can implement fully
or partially within 4 years of reclassification of its Serious
PM2.5 nonattainment area. These measures will be considered
BACM and BACT for the area. If a state evaluates a potential BACM or
BACT measure and determines that it can be implemented only partially
within 4 years after reclassification, the state must adopt the partial
measure as BACM.
    Where the earliest date that a measure can be implemented is beyond
the 4 year mark following reclassification to Serious, the measure may
still be needed as an ``additional feasible measure'' if the 4 year
mark occurs before the Serious area attainment date. ``Additional
feasible measures'' would be ``best''-level, feasible measures that a
state could implement in whole or in part on sources in the area
sometime after the fourth year following reclassification and prior to
the statutory attainment date for the 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.
3. BACM and BACT Submission Requirements
    a. Summary of Proposal. The proposal further specified the
submission requirements once a state has determined the BACT/BACM
requirements in its plan. The proposal required the state to submit a
list of emissions sources, an emissions inventory for such sources, and
several pieces of information regarding potential control measures for
these sources.
    b. Final Rule. The final rule remains relatively unchanged from the
proposal. To ensure that attainment plan submissions contain the
necessary supporting information for EPA review and approval of the
state's selected BACM and BACT and additional feasible measures as
applicable, 40 CFR 51.1010(a)(1)-(5) require the state to submit the
following information as part of its Serious area attainment plan
submission:
    (1) A list of all emissions source categories, 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);
    (2) For each source category, source or activity in the
nonattainment area, an inventory of direct PM2.5 and all
PM2.5 precursor emissions;
    (3) For each source category, source or activity in the
nonattainment area, a comprehensive list of potential control measures
considered by the state for the nonattainment area; 173 174
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    \173\ The EPA believes that it is not necessary to identify
every possible variation of every type of control measure, or all
possible combinations of technologies and measures that would apply
to a given source or activity, as long as the state has properly
characterized the potentially available emissions reductions and
their costs. For example, the EPA believes that the state can
conduct a thorough analysis of VMT reduction measures without
including every possible level or stringency of implementation of
certain possible measures or combinations of measures for reducing
VMT, so long as those measures would not affect the overall
assessment of VMT reduction capabilities and the associated costs.
    \174\ The Menu of Control Measures document is available at:
http://www3.epa.gov/ttn/naaqs/pdfs/MenuOfControlMeasures.pdf.
---------------------------------------------------------------------------

    (4) 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;
    (5) For each technologically feasible emission control measure or
technology, the state must provide the following information relevant
to economic feasibility: (i) The control efficiency by pollutant; (ii)
the possible emissions reductions by pollutant; (iii) the estimated
cost per ton of pollutant reduced; and, (iv) a determination of whether
the measure is economically feasible, with narrative explanation and
quantitative supporting documentation to justify the state's
conclusion;
    (6) For each technologically and economically feasible emission
control measure or technology, the date by which the technology or
measure can be implemented.
    As with a Moderate area attainment plan submission, the EPA
recognizes that the base year emissions inventory that the state
submits for the area in conjunction with its Serious area attainment
plan will likely contain the information required under the first two
items in this list. However, the EPA believes that it is incumbent on
the state to ensure that the information needed for the EPA to evaluate
the state's BACM and BACT and additional feasible measures analysis is
presented as part of that analysis and in a format that provides
transparency, consistency and the ability for another party to evaluate
the state's analysis effectively and to duplicate the state's results.
For this reason, the EPA is requiring the state to include the base
year emissions inventory information with the BACM and BACT submission
and as one element of the state's attainment plan due 18 months after
reclassification of the area to Serious.
    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.
4. Criteria for Effective Regulations to Implement BACM and BACT and
Additional Feasible Measures
    a. Summary of Proposal. The proposal described the four main
criteria for effective control measure regulations: such regulations
must be quantifiable, enforceable, replicable, and accountable.
    b. Final Rule. Guidance on effective control measure regulations is
provided in the control strategy discussion for Moderate areas. See
section IV.D.9 of this preamble, criteria for effective regulations to
implement RACM and RACT and additional reasonable measures.
5. Relevance of Prior BACT, LAER and BART Determinations
    a. Summary of Proposal. The preamble of the proposed rule stated
that it should not be assumed that past control technology
determinations would automatically be deemed to meet the Serious area
control measure requirements (BACM, BACT, or additional feasible
measures) for an area.
    b. Final Rule. The guidance on this issue in the preamble to the
final rule remains largely unchanged. The EPA believes that BACT or
lowest achievable emission rate (LAER) provisions for new sources (as
distinct from BACT for existing sources), or best available retrofit
technology (BART) for existing sources, could potentially qualify as
BACM or BACT for purposes of meeting the Serious area attainment plan
requirements. However, the EPA does not believe it is appropriate for a
state to assume that just because a certain control technology was
determined to meet BACT, LAER, or BART criteria for a new source
sometime in the past, that such a control will also automatically meet
the criteria for BACM or BACT or additional feasible measures for
attainment planning purposes because the regulated pollutant or source
applicability may differ and the analyses may be conducted many years
apart. Thus, a state may not simply rely on prior BACT, LAER or BART
analyses for the purposes of showing that a source has also met BACT
for the relevant PM2.5 NAAQS. Rather, the EPA expects that
in Step 2 of the BACM and BACT determination process, the state would
identify such measures as ``existing measures'' that should be further
evaluated as potential BACM or BACT or additional feasible measures. At
the same time, the EPA notes that the presence of previously installed
control technology, and the technical and economic considerations that
would be associated with upgrading to a measure that achieves greater
reductions, is something that should be considered in the assessments
of technological and economic feasibility of the newer measure.
    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.
6. Multi-state Nonattainment Areas
    a. Summary of Proposal. The preamble to the proposed rule provided
general guidance on coordination between states in multi-state
nonattainment areas to ensure they

adopt sufficient BACM/BACT and additional feasible measures to ensure
expeditious attainment of the standard.
    b. Final Rule. The guidance in the final rule remains largely
unchanged. States that share a multi-state Serious PM2.5
nonattainment area must consult with one another on BACM and BACT and
additional feasible measures that will be required for the
nonattainment area in the different states. This requirement would be
consistent with the overall requirements for BACM and BACT and
additional feasible measures determinations, as all states with Serious
areas need to consider implementing BACM and BACT-level measures that
have been implemented in other states, even if those measures incur
higher costs. The EPA anticipates that states may potentially adopt
controls that differ from state to state, based upon each state's
determination of what qualifies as ``best'' given the mixture of
sources and potential controls in the state portions of relevant
nonattainment areas, subject to EPA approval. If the state can
adequately demonstrate that its chosen BACM and BACT and additional
feasible measures fully meet the EPA's proposed criteria for such
measures, then the agency may consider approving individual state plans
that differ in implementation of control measures.
    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.
7. Environmental Justice Considerations for Developing the Attainment
Plan Control Strategy for a Serious PM2.5 Nonattainment Area
    a. Summary of Proposal. The proposal provided general guidance for
ensuring that overburdened populations are appropriately protected.
    b. Final Rule. The guidance in the final rule remains largely
unchanged. The EPA strongly urges states to consider the environmental
justice aspects of any control measures they have identified as BACM
and BACT or additional feasible measures in order to provide health
protection for overburdened populations. Please see Section XI of this
preamble, which 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.
    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.

E. Modeling for Attainment Demonstrations

1. Due Dates for Submission of Serious Area Attainment Demonstrations
    a. Summary of Proposal. Section IV.E of this preamble describes the
EPA's attainment demonstration and modeling requirements for Moderate
area plans. The EPA proposed that the same general attainment
demonstration and modeling requirements for Moderate area plans should
apply to Serious area attainment demonstrations. However, Serious area
plans have additional statutory requirements.
    Attainment demonstrations are due 18 months after reclassification
if the EPA reclassifies the area to Serious after failure of the area
to attain the applicable Moderate area deadline. Alternatively, CAA
section 189(b)(2) requires states with designated Serious nonattainment
areas to submit attainment demonstrations no later than 4 years after
reclassification of the area to Serious if the reclassification occurs
before the Moderate area attainment deadline. The EPA proposed an
approach for determining an appropriate attainment plan control
strategy for a Serious PM2.5 nonattainment area that
requires the state to submit the attainment demonstration for the area
within 18 months after reclassification, regardless of when or the
authority under which an area was reclassified to Serious.
    b. Final Rule. The statutory attainment demonstration provisions
for Serious areas are as follows: Section 189(b) of the CAA requires a
state with a designated Serious nonattainment area to submit an
attainment plan for such area. As discussed earlier, CAA section
189(b)(1)(A) more specifically 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) if the state is seeking an
extension of the attainment date, that it is impracticable for the area
to attain the relevant NAAQS by the statutory Serious area attainment
date. For Serious nonattainment areas, the attainment date is as
expeditiously as practicable, but no later than the end of the tenth
calendar year after designation as nonattainment. A demonstration that
shows that it is impracticable for the area to attain within this
timeframe must also provide for attainment of the NAAQS by the most
expeditious alternative date practicable, but no later than 5 years
after the maximum statutory Serious area attainment date (based on the
criteria specified in CAA section 188(e)).
    The EPA is not finalizing the proposed approach of requiring all
Serious area attainment demonstrations to be due 18 months after
reclassification. If the EPA reclassifies the area to Serious after
failure of the area to attain the applicable Moderate area deadline,
the attainment demonstration will be due in 18 months. States with
Serious nonattainment areas that were reclassified before the Moderate
area attainment deadline must submit attainment demonstrations the
earlier of 4 years after reclassification of the area to Serious or the
end of the eighth calendar year after initial designation. However,
these areas are still required to submit BACT/BACM measures within 18
months of being reclassified as Serious. Sections VI.A and VI.D of this
preamble describe more fully the EPA's approach for plan due dates and
control strategy analyses for all elements of a Serious area attainment
plan. Section VI.J of this preamble provides a complete discussion of
the EPA's criteria for granting a Serious area attainment date
extension.
    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.
2. Attainment Demonstration Requirements for Serious Areas
    a. Summary of Proposal. The proposal described the attainment
demonstration and impracticability demonstration requirements for
Serious nonattainment areas. The EPA proposed that a serious area plan
must include an attainment demonstration that demonstrates how a state
will attain the PM2.5 NAAQS by the applicable attainment
date, must include analyses supporting the state's determination of its
proposed attainment date, and must show that the area will attain the
NAAQS as expeditiously as practicable, but not later than the tenth
calendar year after designation. The proposal indicated that 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 must
include all BACM and BACT controls in the analysis.
    b. Final Rule. The final rule requirements for Serious area
attainment demonstrations are generally unchanged from the proposal. As
described in Section IV.E of this

preamble, an attainment demonstration is a plan that demonstrates how a
state will attain the PM2.5 NAAQS by the applicable
attainment date. The EPA is finalizing a requirement that the
demonstration for Serious areas must consist of: (i) Technical analyses
such as base year and future year modeling of emissions which identify
sources and quantify emissions that are contributing to violations of
the PM2.5 NAAQS; and, (ii) analyses of future year projected
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 and BACM and BACT controls for the
area, as well as other measures either inside the nonattainment area or
outside the nonattainment area but within the state that could
potentially accelerate attainment. Each state with a Serious
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 tenth 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 must include all BACM and BACT controls in
the analysis.
    A state with a Serious nonattainment area can also submit an
impracticability demonstration (under CAA section 189(b)(1)(A)(ii)) as
part of seeking an extension of the attainment date under CAA section
188(e). The impracticability demonstration for a Serious area would be
similar to an impracticability demonstration for Moderate areas because
it must show that the area will not be able to attain the
PM2.5 NAAQS by the latest possible statutory attainment
date, which in this case is by the end of the tenth calendar year
following designation.
    In order to support a Serious area impracticability demonstration,
the state must show (through modeling) that attainment cannot be
reached by the latest statutory Serious area attainment date, even if
all RACM and RACT and BACM and BACT controls, as well as other measures
either inside the nonattainment area or outside the nonattainment area
but within the state (as may be necessary to meet the requirements of
172(c)(6)), were implemented before the attainment date. Moreover, in
addition to the Serious area impracticability demonstration, to support
an extension of the attainment date, the Serious area plan must
demonstrate (again, using air quality modeling) that it provides for
attainment by the most expeditious alternative date practicable
employing MSM, as specified in CAA section 188(e). (MSM are discussed
in more detail in Section VI.J of this preamble). As a result, the
required plan in the case of a Serious area that cannot attain by the
statutory attainment date is both an impracticability demonstration (to
justify an extension beyond the statutory attainment date) and an
attainment demonstration that serves as the basis for proposing an
appropriate alternative attainment date. Note that this is different
from a Moderate area impracticability demonstration, which is not
required to serve as the basis for proposing a new area attainment
date.
    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.
3. Air Quality Modeling Required for Serious Area Attainment
Demonstrations and Impracticability Demonstrations
    a. Summary of Proposal. The EPA proposed to require air quality
modeling in support of both a Serious area attainment demonstration and
a Serious area impracticability demonstration.
    b. Final Rule. The EPA is finalizing a requirement for states to
submit air quality modeling in support of both attainment
demonstrations and impracticability demonstrations for Serious
PM2.5 nonattainment areas. Unlike the impracticability
demonstration for Moderate areas described in CAA section
189(a)(1)(B)(ii), the impracticability demonstration for Serious areas
in CAA section 189(b)(1)(A)(ii) also requires air quality modeling
establishing the most expeditious alternative attainment date
practicable. Therefore, air quality modeling is a required element in
all attainment demonstrations for Serious areas.
    Some commenters believed that both Moderate and Serious area
impracticability demonstrations must include air quality modeling. The
EPA does not agree and believes the statute only requires air quality
modeling for Serious area impracticability demonstrations. This stems
from the slightly different statutory construction in CAA section
189(b)(1)(A) compared to CAA section 189(a)(1)(B). Section 189(b)(1)(A)
of the CAA specifies an air quality modeling requirement as a
parenthetical, which the EPA interprets to apply to both the
requirements in CAA section 189(b)(1)(A)(i) [attainment demonstrations]
and CAA section 189(b)(1)(A)(ii) [impracticability demonstrations].
Additionally, the fact that a Serious area impracticability
demonstration must also include an attainment demonstration with an
alternative attainment date logically supports the final rule
conclusion that a Serious area impracticability demonstration must
include air quality modeling. Modeling is needed to demonstrate
attainment and to propose an alternative attainment date for the
Serious area. This differs from a Moderate area impracticability
demonstration, which only serves to demonstrate that attainment cannot
be reached by the Moderate area attainment date. A Moderate area
impracticability demonstration does not require a demonstration of
attainment or setting of an alternative future attainment date. It
merely starts the process of reclassifying an area to Serious and the
eventual required submission of a Serious area implementation plan.
    Other than the timing of plan submissions and additional required
elements of a Serious area plan (such as BACM and BACT), the relevant
air quality modeling procedures and guidance for Moderate and Serious
area plans are the same. See Section IV.E of this preamble for more
details on the modeling requirements and guidance for all
PM2.5 nonattainment areas.
    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.
4. Attainment Demonstrations Required To Be Submitted by an Area
Reclassified to Serious
    a. Summary of Proposal. The proposal discussed the attainment
demonstration requirements for Moderate nonattainment areas that
subsequently are reclassified to Serious nonattainment. The EPA
proposed that states with Moderate nonattainment areas that get
reclassified to Serious nonattainment areas must first submit a
Moderate area plan and then a separate Serious area plan.
    b. Final Rule. The EPA is finalizing requirements for states to
submit a Moderate area attainment demonstration (or impracticability
demonstration) and then if reclassified to Serious nonattainment, a
separate Serious area attainment demonstration. Under CAA section
189(a)(1)(B), a state with a

Moderate nonattainment area is required to submit a demonstration that
the area either will attain or cannot practicably attain the NAAQS by
the statutory attainment date. Regardless of whether the state submits
an attainment demonstration or an impracticability demonstration for a
Moderate area, if such an area is reclassified to Serious prior to or
after failing to attain the applicable NAAQS, the state is required
under CAA section 189(b)(1)(A) to submit a new attainment demonstration
as part of an area's Serious area attainment plan. The separate
statutory requirements for Moderate and Serious nonattainment areas
anticipate two separate attainment plan submissions, and the EPA's
existing guidance in the General Preamble and Addendum further support
this expectation. While the state is required to submit a separate
Serious area attainment plan, the EPA anticipates that certain control
strategies may build upon those previously adopted and implemented as
part of the Moderate area plan. For example, an area dominated by wood
smoke emissions may not attain the standard by the statutory Moderate
area attainment date because all necessary woodstove change-outs could
not occur in that timeframe, but additional woodstove change-outs could
occur by the statutory Serious area attainment date.
c. Comments and Responses
    Comment: Some commenters agreed with the EPA that areas seeking to
be reclassified from moderate to serious must submit two separate
attainment plan submissions. The commenter stated the Act promises that
all areas, even the most polluted, will implement reasonably available
controls and provide at least some interim health protections while
preparing a serious area plan containing more protective requirements.
    Response: The EPA agrees with the comment. In the final rule, an
area that is reclassified to Serious must submit both Moderate and
Serious area plans, and all statutory requirements for a Moderate area
(including RACT and RACM) must be met by the statutory deadline.
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 a Serious area impracticability analysis
must select a future year for the analysis. The EPA proposed that 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 implemented by the beginning of that year. The EPA
recommended the last year of the statutory attainment date as a
starting point for Serious nonattainment area modeling demonstrations.
    b. Final Rule. The EPA is finalizing a requirement that all
emissions control measures relied on for attainment must have been
implemented by the beginning of the attainment year. See 40 CFR
51.1011(b)(6). To demonstrate attainment, the modeling results for the
nonattainment area must predict that emissions reductions implemented
by the beginning of the last calendar year preceding the attainment
date will result in PM2.5 concentrations that meet the level
of the standard.\175\
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    \175\ 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
ambient data from the most recent 3 years prior to the attainment
date for the area.
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    While states should choose the future modeling year based on a
number of factors, the EPA recommends the last year of the statutory
attainment date as a starting point for modeling for two reasons.
First, a state with a Serious area for which it submits an attainment
date extension request under CAA section 188(e) must show that the area
cannot practicably attain the NAAQS by the end of the tenth calendar
year following designation of the area. Therefore, the appropriate
future modeling year for making such a demonstration is the tenth year
after designations. Even if a state does not submit (or does not intend
to submit) a Serious area attainment date extension request, modeling
the tenth year is a logical starting point to determine if attainment
by year ten is likely. If attainment-level concentrations of
PM2.5 are not expected in the tenth calendar year after
designations, then the area must also, as a requirement to receive an
extension of the Serious area attainment date, submit a demonstration
(using air quality modeling) that provides for attainment by the most
expeditious alternative date practicable, but no later than the end of
the fifteenth year after designation, with the implementation of MSM
(see Section VI.J of this preamble for details about MSM
determinations).
    Second, even though attainment of any PM2.5 NAAQS is
determined by averaging 3 years 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 the annual or 98th percentile
value for 3 consecutive years, 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 average of the three annual values
is less than the NAAQS). Therefore, it is appropriate to model any of
the 3 years used to determine attainment. For these reasons, it is
acceptable, and may in fact be most efficient, for a state to begin the
Serious area attainment demonstration process by modeling the final
year of the statutory attainment date to determine future year modeled
PM2.5 concentrations in the tenth year after designations.
    Because an area must attain ``as expeditiously as practicable,''
additional considerations are necessary before an attainment date can
be established. Criteria for establishment of the Serious area
attainment date are discussed in Section VI.I of this preamble. In
evaluating such considerations, the question arises as to whether
additional future modeling is required beyond the recommended final
year modeling just discussed. For purposes of determining the
attainment date that is as expeditious as practicable, the state must
conduct future year modeling that takes into account growth and known
controls (including any controls that were previously determined to be
RACM and RACT for the area). For example, for an area designated
nonattainment for the 2012 PM2.5 NAAQS in 2015 and
subsequently reclassified to Serious in 2021, a future case scenario
for the year 2025 (10 years after the initial nonattainment
designation) would be needed to examine whether existing federal,
state, and local measures (including previously identified and
implemented RACT/RACM controls for the area) plus the BACM and BACT
identified by the state would result in attainment. Since the EPA is
finalizing the requirement that BACM and BACT must be determined
independent of the attainment demonstration for the area, the future
case scenario must include BACM and BACT controls in the analysis plus
any additional measures on sources inside and outside of the
nonattainment area (but within the state) that the state has identified
as feasible to implement by the attainment date. Note that similar to
RACM and RACT, BACM and BACT controls must be implemented within 4
years after reclassification to Serious nonattainment. In order to
justify an extension of the attainment date beyond the end of the tenth
year after designation, the state must show that attainment by that
date (including the anticipated emissions reductions from

RACM and RACT and additional reasonable measures, and BACM and BACT and
additional feasible measures) would be impracticable. Any proposed
attainment date after the 10 year period must include modeling of BACM
and BACT controls plus the most stringent measures that are included in
the implementation plan of any state and can be feasibly implemented in
the area. The attainment date extension beyond 10 years can be for up
to 5 additional years, but the proposed attainment date must also be
shown to be as expeditious as practicable. Section VI.J of this
preamble provides a complete discussion of the EPA's proposed
interpretation of the statutory requirements for a Serious area
attainment date extension under CAA section 188(e).
    As with Moderate area attainment demonstrations, the EPA believes
that it is not necessary or reasonable to require states to model each
and every year to determine the appropriate attainment date for a
Serious PM2.5 nonattainment area given the resource demands
associated with modeling.\176\ In some cases it may be reasonable to
model one additional interim year before the maximum statutory
attainment date. However, in most cases, the air quality benefits of an
identified set of reasonable control measures, BACM and BACT and
additional feasible control measures can be estimated through model
sensitivity analyses and the development of sensitivity factors
(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 multiple
additional future years. The EPA strongly recommends that states
discuss the selection of the future year(s) to model with their
respective EPA Regional Office as part of the modeling protocol
development process prior to embarking on the modeling.
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    \176\ States with Serious areas that request an attainment date
extension beyond 10 years must model the tenth year after
designation of the area as part of an impracticability
demonstration, plus an additional year beyond that which represents
the attainment date.
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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.
6. Attainment Year Motor Vehicle Emissions Budgets
    As with Moderate areas, the transportation conformity rule requires
that Serious area attainment plans establish motor vehicle emissions
budgets for the area's attainment year. Therefore, once a Serious
area's attainment date has been established, the state is required to
establish motor vehicle emissions budgets for direct PM2.5
and any relevant PM2.5 precursor for the attainment
year.\177\ If a state's SIP submission demonstrates that a Serious area
cannot attain by the end of the tenth calendar year after the area's
designation, motor vehicle emissions budgets are not required for that
tenth calendar year, but are required for the year that the state
demonstrates to be the area's attainment year. A motor vehicle
emissions budget for the purposes of a Serious area 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.\178\ 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.\179\
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    \177\ 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).
    \178\ A state would also establish motor vehicle emissions
budgets for an area's attainment year. Those budgets would be the
motor vehicle emissions that the SIP establishes as being necessary
to attain the NAAQS.
    \179\ 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. General Approach to RFP
    a. Summary of the Proposal. The EPA generally proposed that a state
must submit an RFP plan as part of any attainment plan submission for a
Serious nonattainment area in order to satisfy the statutory
requirements for RFP, similar to a Moderate area attainment plan. The
EPA proposed that the applicable baseline year must be the same year as
that represented by the latest base year inventory for the Serious
area. The EPA proposed that the state must include in its RFP analysis
the anticipated emissions reductions expected to be achieved through
the implementation of control measures required by the control strategy
explained in Section VI.D of this preamble (BACM and BACT, additional
feasible measures and MSM if applicable). As with RFP plans for
Moderate areas, the EPA proposed that a state must submit RFP projected
emissions as part of the RFP plan for any Serious PM2.5
nonattainment area following the same guidance that applies to
emissions inventories for attainment plans (see Section VI.B of this
preamble for a complete discussion of emissions inventories for Serious
area attainment plans). The EPA also proposed that motor vehicle
emissions budgets must also be established for direct PM2.5
and any PM2.5 plan precursor using the latest planning
assumptions and the latest approved motor vehicle emissions model
available at the time that the Serious area attainment plan is
developed.\180\ It was not necessary to propose that RFP plans for
Serious areas include motor vehicle emissions budgets for direct
PM2.5 and any PM2.5 plan precursor because, as
stated in the section of this rule that addresses RFP requirements for
Moderate PM2.5 areas, the transportation conformity rule
already 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 plan precursor. The EPA also proposed that guidance
found in the Moderate nonattainment areas RFP section of the proposal
should also apply to Serious nonattainment areas.
---------------------------------------------------------------------------

    \180\ Ibid.
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    b. Final Rule. The EPA is finalizing rule provisions for Serious
areas that essentially mirror the approach to Moderate areas found in
Section IV.F of this preamble. The EPA is further clarifying
application of those provisions by providing guidance that closely
follows the Moderate area guidance regarding how to prepare an RFP
plan, RFP projected emissions, geographic coverage of emission sources
for RFP, and RFP requirements for multi-state nonattainment areas.
    As with a Moderate area attainment plan, the EPA is finalizing that
a state must submit an RFP plan as part of any Serious area attainment
plan in order to satisfy the statutory requirements for RFP. The plan
must contain appropriate information to demonstrate that adequate
emissions reductions will be achieved through control measures in the
attainment plan in order to meet the

statutory definition of RFP. The plan must include 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 VI.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.
For additional discussion of each of the components of the RFP plan,
refer to Section IV.F of this preamble. See 40 CFR 51.1012(a).
    The EPA requires that the applicable baseline year must be the same
year as that represented by the latest base year inventory for the
Serious area. The projected attainment year may be up to the end of the
tenth year following designation for a Serious area that can
demonstrate attainment pursuant to CAA section 189(b)(1)(A), or up to
the end of the fifteenth year following designation for a Serious area
that sought an extension of the statutory attainment date pursuant to
CAA section 188(e).\181\ As with Moderate areas, the RFP analysis must
clearly convey how the schedule for implementing the control strategy
will provide for generally linear or stepwise progress towards
attainment. If stepwise progress is more appropriate for the specific
nonattainment area, the state is required to submit a clear rationale
and supporting information to explain why generally linear progress
towards attainment in the area is not appropriate (e.g., due to the