Response: After considering these comments and in an effort to simplify the rule, the EPA decided to eliminate the two proposed requirements for the quantitative milestone report as suggested by these commenters. As stated earlier, this decision was made because the EPA determined that the remaining components of the quantitative milestone report are sufficient to enable the EPA to assess whether the nonattainment area is meeting RFP.
      Comment: One commenter stated that, while they would not object to filing periodic reports, as part of their milestone report, the EPA should not insist on the state actually inspecting all covered facilities and indicating that RACT or RACM has not been implemented if a small subset of facilities is found in violation.
      Response: It is not the intent of the EPA to require states to physically inspect all covered sources to verify the implementation of required control measures. The intent is that, at the time of the milestone due date, all covered sources would be legally required to have implemented required control measures and the state has reasonably been assured that this occurred.

H. Contingency Measures
1. Summary of the Proposal.
      The Act requires Moderate PM2.5 nonattainment area plans to contain contingency measures consistent with CAA section 172(c)(9). Contingency measures are additional control measures to be implemented in the event that the EPA determines that an area failed to meet RFP requirements (including associated quantitative milestones) or failed to attain the PM2.5 primary standard by the applicable attainment date. These measures must be fully adopted rules or control measures that are ready to be implemented quickly upon failure to meet RFP or failure of the area to meet the standard by its attainment date, and such measures are required to take effect without further action by the state or the EPA. The EPA proposed and sought comment on general requirements for contingency measures for Moderate PM2.5 nonattainment areas. The EPA has longstanding interpretations of the statute with respect to the contingency measure requirement, both for PM and for other pollutants, in the General Preamble and Addendum. These documents provide guidance and recommendations for states to follow in submitting contingency measures, and the proposal did not contain any significant changes to the existing guidance and recommendations. However, the EPA sought comment on whether the guidance needed to be revised or expanded. Additionally, as discussed in the proposal, the EPA believes that the D.C. Circuit's decision in NRDC v. EPA does not affect the overall contingency measure requirements that were finalized in the remanded 2007 PM2.5 Implementation Rule. The EPA determined this because CAA section 172(c)(9) imposes the contingency measure requirement for attainment plans for the PM2.5 NAAQS and it is not superseded or subsumed by any specific contingency measure requirements under subpart 4. As a result, the proposal for this rule remained very similar to the final 2007 PM2.5 Implementation Rule.
2. Final Rule.
      Consistent with the proposal, the final rule codifies existing policies on contingency measures, but does not make significant changes to these policies. Although CAA section 172(c)(9) requires contingency measures, the provision does not specify exactly what parameters such measures must meet. The EPA is finalizing an approach to contingency measures for the PM2.5 NAAQS that is similar to the approach recommended in earlier EPA guidance. Specifically, in order for contingency measures to be approvable as part of a state's Moderate area attainment plan submission for the PM2.5 NAAQS, the state plan must meet the following general requirements (See 40 CFR 51.1014):
 Contingency measures must be fully adopted rules or control measures that are ready to be implemented quickly upon a determination by the Administrator of the nonattainment area's failure to meet RFP or failure to attain the standard by its attainment date.
 The state's attainment plan submission must contain trigger mechanisms for the contingency measures, specify a schedule for implementation, and indicate that the measures will be implemented with minimal further action by the state or by the EPA.
 The contingency measures shall consist of control measures that are not otherwise included in the control strategy or that achieve emission reductions not otherwise relied upon in the control strategy for the area.
 Contingency measures should provide for emissions reductions approximately equivalent to 1 year's worth of reductions needed for RFP, based on the overall level of reductions needed to demonstrate attainment divided by the number of years from the base year to the attainment year, or approximately equivalent to 1 year's worth of air quality improvement or emissions reductions proportional to the overall amount of air quality improvement or emissions reductions to be achieved by the area's attainment plan.
	Regarding the first two points, consistent with prior guidance, states must show that their contingency measures can be implemented with minimal further action on their part and with no additional rulemaking actions such as public hearings or legislative review. After the EPA determines that a moderate PM2.5 nonattainment area has failed to meet an RFP requirement or to attain the PM2.5 NAAQS, the EPA generally expects all actions needed to effect full implementation of the contingency measures to occur within 60 days after the EPA notifies the state of the area's failure. The EPA intends to notify the state of a failure to meet RFP or to attain the NAAQS by publication of its determination in the Federal Register. The state should ensure that the contingency measures are fully implemented as expeditiously as practicable after such notice.
	Regarding the third point, the EPA interprets the contingency measure requirement of CAA section 172(c)(9) to require control measures that are not otherwise included in the control strategy or that achieve emission reductions not otherwise relied upon in the control strategy for the area. However, suitable contingency measures may be measures that were technologically and economically feasible for the area, but did not qualify as RACM or RACT or additional reasonable measures for one or more reasons. For example, a candidate contingency measure may have been deemed technologically and economically feasible, but it was not needed to achieve expeditious attainment in a Moderate area for which the state could demonstrate attainment by the statutory attainment date and therefore was not included as part of the attainment demonstration for the area. It is important that states make decisions concerning contingency measures in conjunction with their determination of the overall control strategy for bringing the area into expeditious attainment, and that states first must identify those control measures needed in order to demonstrate expeditious attainment of the standards; any remaining measures should then be considered as candidates for contingency measures.
      As discussed in Section IV.D of this preamble, the RACM/RACT provisions in this rule require that, for Moderate areas that cannot practicably attain the NAAQS by the statutory attainment date, states must implement all control measures that they determine to be reasonable (i.e., all technologically and economically feasible measures) for sources in the area. In such cases, the contingency measures for such nonattainment areas would necessarily exceed the criteria for determining whether a measure is reasonable for purposes of RACM/RACT and additional reasonable measures. For example, contingency measures could consist of reasonable controls on sources outside the nonattainment area, early implementation of BACM/BACT on select sources inside the area, other measures identified by the state, or a combination thereof, that collectively provide approximately 1 year's worth of emissions reductions/air quality improvement. Such contingency measures would only be triggered in the event the area fails to meet RFP; the EPA does not interpret the requirement for contingency measures for failing to attain the NAAQS by the applicable attainment date to apply to a Moderate area that a state demonstrates cannot practicably attain the NAAQS by the statutory attainment date. Rather, the EPA believes it is appropriate for the state to identify and adopt contingency measures for failing to attain the NAAQS in a timely way as part of the Serious area attainment plan that it will develop once the EPA reclassifies such an area. However, if a Moderate area that cannot practicably attain the NAAQS fails to meet RFP when reviewed as part of the quantitative milestone either 4.5 or 7.5 years after designation, the requirement to implement contingency measures would be triggered as required by CAA section 172(c)(9). For any Moderate PM2.5 nonattainment area, contingency measures can include measures that achieve emissions reductions on sources located outside the nonattainment area as well as from sources within the nonattainment area, provided that the measures offer reasonable assurance that the appropriate air quality impact will result within the nonattainment area.
      The final rule continues to allow states to rely on federal measures (e.g. federal mobile source measures based on the incremental turnover of the motor vehicle fleet each year) and local measures already scheduled for implementation that provide emissions reductions in excess of those needed to provide for RFP or expeditious attainment. The key is that the statute requires that contingency measures provide for additional emission reductions that are not relied on for RFP or attainment and that are not included in the RFP or attainment demonstrations as meeting part or all of the contingency measure requirements. The purpose is "to provide a cushion while the plan is being revised to meet the missed milestone." Nothing in the statute precludes a State from implementing such measures before they are triggered. Additionally, the EPA determined that the court ruling upholding contingency measures that were previously required and implemented where they were in excess of the attainment demonstration and RFP for ozone attainment plans necessitates similar treatment for PM2.5 NAAQS. The EPA has approved numerous SIPs under this interpretation-- i.e., SIPs that use as contingency measures one or more federal or local measures that are in place and provide reductions that are in excess of the reductions required by the attainment demonstration or RFP plan.
      For these reasons, the EPA concluded that this approach is reasonable for Moderate PM2.5 nonattainment areas that can demonstrate attainment by the statutory attainment date, as the state would calculate the emissions reductions needed for RFP separately from the control strategy determination for such an area. However, crediting an area for "excess" emissions reductions to satisfy the contingency measure requirement is not allowable for Moderate areas that cannot practicably attain by the statutory attainment date. Under the EPA's approach for calculating RFP for such areas, RFP would be calculated directly from the projected emissions reductions from all control measures identified for the area (as RACM and RACT or additional reasonable measures), such that there should be no difference between emissions reductions estimated from control measures and those estimated for demonstrating RFP.
	Finally, consistent with the EPA's past approach for contingency measures for PM2.5 nonattainment areas, the EPA expects that the emissions reductions from contingency measures should be approximately equivalent to 1 year's worth of emissions reductions while the state is revising its attainment plan for the area. States should explain the amount of anticipated emissions reductions to be accomplished by the contingency measures outlined in the plan. In the rare event that an area is unable to identify contingency measures to account for approximately 1 year's worth of emissions reductions, the state should provide a reasoned justification why the smaller amount of emissions reductions is appropriate. As described in Section IV.F of this preamble, the EPA requires an approach for interpreting the statutory RFP requirement that would require demonstrating RFP based on reductions from sources located inside the nonattainment area. Keeping with the historic linkage between RFP and contingency measures, the EPA is also finalizing a similar approach for calculating 1 year's worth of emissions reductions for purposes of adopting appropriate contingency measures. That is, the EPA's approach for determining the level of emissions reductions for contingency measure purposes is to calculate the annual reductions in emissions of direct PM2.5 and PM2.5 plan precursors needed from sources located inside the nonattainment area. As explained earlier, however, some or all of the contingency measures reductions can come from outside the area if they are demonstrated to produce the appropriate air quality impact within the nonattainment area.
	The CAA requires that states must implement contingency measures after the EPA determines that the area has either failed to meet RFP requirements, or failed to attain the standards by the applicable attainment date. The purpose of the contingency measure provision is to ensure that corrective measures are put in place automatically at the time that the EPA makes its determination that an area has either failed to meet RFP or failed to meet the standard by its attainment date. The EPA is required to determine within 90 days after receiving a state's quantitative milestone demonstration, and within 6 months after the attainment date for an area, whether these requirements have been met. The additional consequences for states with areas that fail to attain the NAAQS or to meet RFP are described in section 179(d) of the CAA and discussed in Section V of this preamble.
	See Section IV.A. of this preamble for a discussion of the due dates for submission of contingency measures and other attainment plan elements.
3. Comments and Responses.
      Comment: Commenters stated that requiring contingency measures in areas with mature air pollution control programs is very challenging because they already have developed aggressive control measures to meet CAA requirements and support expeditious attainment. Commenters asserted that it would be extremely difficult to develop further control measures to meet any contingency measure requirements. Commenters objected to the proposed requirement that contingency measures must be approximately equivalent to 1 year's worth of emissions reductions because it is a departure from existing guidance which states the contingency emissions reductions "should be" approximately equal and because sometimes identifying control measures for this level of reductions is just not possible. Commenters advocated that EPA should provide a more reasonable approach to the contingency measure requirement, but did not provide specific recommendations. Other commenters stated that contingency measures should provide 1 year's worth of emissions reductions needed for RFP.
      Response: The EPA acknowledges that states containing areas with more longstanding and pervasive nonattainment problems may already have implemented many control measures for purposes of attaining the NAAQS, and there may be fewer sources and measures available to meet the contingency measure requirements of the statute. However, the EPA notes that section 172(c)(9) of the CAA explicitly requires states to adopt contingency measures to apply in the event of failure to meet RFP or failure to attain the NAAQS as a required component of all attainment plans. Typically, contingency measures will be comprised of measures that a state and the EPA have determined are not required to meet RACM/RACT or other requirements, e.g., on the grounds that they are more technologically or economically challenging. As a result, such measures may not be required as RACM/RACT, but are nevertheless available for use as contingency measures. Another approach to contingency measures, if appropriate, would be to rely on control measures imposed on sources outside the boundaries of the designated nonattainment area. Such contingency measures require adequate support to establish that the reductions would have the intended impacts within the nonattainment area, but can be a source of additional measures for this purpose.
      Finally, the EPA notes that its longstanding guidance is that contingency measures should provide approximately 1 year's worth of RFP, but this amount may vary based upon appropriate facts and circumstances of each unique nonattainment area. As discussed, states should explain the amount of anticipated emissions reductions to be accomplished by the contingency measures outlined in the plan. In the rare event that an area is unable to identify contingency measures to account for approximately 1 year's worth of emissions reductions, the state should provide a reasoned justification why the smaller amount of emissions reductions is appropriate.
      Comment: Commenters stated that "excess" emission reductions (i.e., emission reduction measures that are included in a modeled attainment demonstration indicating that the area will improve air quality to well below the standard) should not be used as contingency measures in the event an area actually fails to attain. Commenters asserted that the failure to attain should be considered a demonstration that excess emissions reductions do not exist. Further, the commenters stated that excess reductions do not provide the public health benefit intended by Congress. Other commenters stated that Moderate areas that cannot attain by the statutory attainment date could also have excess emissions reductions creditable as contingency measures.
      Response: In keeping with longstanding practice, the final rule allows excess emissions reductions to be credited as contingency measures in plans that demonstrate attainment but not for plans that demonstrate an impracticability to attain. This allows nonattainment areas to credit emissions reductions beyond those planned to satisfy attainment plan requirements as meeting part or all of the contingency measure requirements. This allowance is further supported by the court's ruling in LEAN v. EPA, which found that emissions reductions in excess of what were needed for the attainment demonstration and RFP in ozone attainment plans are creditable for contingency measures. Because the contingency measures requirement for both ozone and PM2.5 originates in CAA section 172(c)(9), it is applicable for all areas designated nonattainment for any NAAQS. Therefore, the EPA concluded that the same approach is appropriate for Moderate PM2.5 nonattainment areas that can demonstrate attainment by the statutory attainment date. Allowing "excess" emissions reductions affords proper credit for these areas as they continue to make progress toward attainment while the new SIP is developed for the area. Additionally, in support of the overarching goal of the CAA, public health will benefit from the excess emissions reductions. However, such an allowance for a Moderate area that cannot practicably attain is not acceptable because all emissions reductions anticipated from control measures while developing the attainment plan should be accounted for in the RFP plan. With all of these reductions accounted for in the RFP plan, there are no excess reductions beyond the attainment planning period to be credited as contingency measures.
I. Attainment Dates
1. Summary of Proposal.
      The proposal described the CAA section 188(c)(1) requirement for Moderate areas to attain the standard as expeditiously as practicable, but no later than the end of the sixth calendar year after the "area's designation as nonattainment." For purposes of clarity, the EPA proposed to interpret the term "area's designation" as meaning "the area's effective date of designation," consistent with the agency's past approach for implementing the 1997 and 2006 PM2.5 NAAQS, and with its approach for implementing the NAAQS for other criteria pollutants under part D, title I of the CAA. The EPA requested comment on this interpretation. The preamble to the proposal also described the process for determining whether an area has attained the NAAQS.
2. Final Rule.
      The final rule maintains the requirement interpreting of CAA section 188(c)(1) to mean that the attainment date must be as expeditiously as practicable, but no later than the end of the sixth calendar after the effective date of an area's designation. See 51.1004(a)(1). Thus, as an example, for areas designated nonattainment in the first round of designations for the 2012 PM2.5 NAAQS, the effective date of designation is April 15, 2015, and the Moderate area attainment date would be as expeditious as practicable, but no later than December 31, 2021 (i.e., the end of the sixth calendar year after designation). Serious area attainment dates are discussed fully in Section VI.I of this preamble.
	The EPA's approach to approving an attainment date for a PM2.5 nonattainment area will be different for a Moderate area that cannot practicably attain the relevant PM2.5 NAAQS by the end of the sixth calendar year after designation. Given that the agency will reclassify any such area to Serious and thereby trigger additional Serious area requirements for the area, the EPA will approve an attainment date for the area when it takes action on the Serious area attainment plan submitted for the area. In the interim, before the EPA takes action to reclassify the area, the statutory Moderate area attainment date will continue to apply to such an area. See 40 CFR 51.1000 and 51.1004(a)(1)(ii). As discussed more fully in Section VI.H of this preamble, when the EPA reclassifies the area, then the presumptive attainment date for the area will be as expeditious as practicable, but no later than the end of the tenth calendar year following designation.
	Once an area has an approved attainment date and has implemented its plan, the EPA has the responsibility for determining whether the nonattainment area has attained the standard by its applicable attainment date. Section 179(c)(1) of the CAA requires the EPA to make determinations of attainment no later than 6 months following the attainment date for the area. Under CAA section 179(c)(2), the EPA must publish a notice in the Federal Register identifying those areas that failed to attain by the applicable attainment date. The statute further provides that the EPA may revise or supplement its determination of attainment for the affected areas based upon more complete information or analysis concerning the air quality for the area as of the area's attainment date.
	Section 179(c)(1) of the CAA provides that the EPA is to base the attainment determination for an area upon an area's "air quality data as of the attainment date." The EPA will make the determination of whether an area's air quality is meeting the PM2.5 NAAQS by the applicable attainment date based upon data gathered from the air quality monitoring sites that have been entered into the EPA's Air Quality System (AQS) database. The state is not required to make any special or additional submission in order for EPA to make a determination of attainment.
	A Moderate PM2.5 nonattainment area's air quality status is determined in accordance with Appendix N of 40 CFR part 50. To show attainment of the current 24-hour and annual standards for PM2.5, data from the most recent 3 consecutive years prior to the area's attainment date must show that PM2.5 concentrations over the prior 3 year period are at or below the levels of the standards. A complete year of air quality data, as described in part 50, Appendix N, is comprised of all 4 calendar quarters with each quarter containing data from at least 75 percent of the scheduled sampling days.
	The EPA will begin processing and analyzing data related to the attainment of Moderate PM2.5 nonattainment areas after the applicable attainment date for the affected areas. Current EPA regulations, under 40 CFR part 58, set the deadline for the state to submit air quality data into the AQS database as no later than 90 days after the end of the calendar year.
	While the EPA may determine that an area's air quality data indicate that an area may be meeting the PM2.5 NAAQS for a specified period of time, this does not eliminate the state's responsibility under the Act to adopt and implement an approvable attainment plan unless the area also has been granted a clean data determination. If the EPA determines that an area has attained the standard as of its attainment date, the area will remain designated as nonattainment until the state has submitted an acceptable redesignation request and maintenance plan, and EPA has approved them.
	In order for an area to be redesignated as attainment, the state must comply with the five requirements listed under section 107(d)(3)(E) of the CAA. Briefly, this section requires that:
 The EPA has determined that the area has attained the PM2.5 NAAQS;
 The EPA has fully approved the applicable state implementation plan;
 The improvement in air quality is due to permanent and enforceable reductions in emissions;
 The EPA has fully approved a maintenance plan for the area; and
 The state(s) containing the area or portions of the area have met all applicable requirements under CAA section 110 and part D.
J. Attainment Date Extensions
1. Attainment Date Extension Criteria.
	a. Summary of Proposal.
      Subpart 4 of title I of the CAA provides the EPA with authority to grant up to two 1-year extensions of the attainment date for a Moderate area that otherwise could be found to have failed to attain the relevant PM2.5 NAAQS, if the area can meet specific statutory criteria related to monitored air quality in the area and the implementation of measures in the attainment plan. Under CAA section 188(d), a state may apply to the EPA for an extension of a Moderate area's attainment date of 1 additional year (the "Extension Year") if "(1) the state has complied with all requirements and commitments pertaining to the area in the applicable implementation plan; and (2) no more than one exceedance of the 24-hour [NAAQS] level for PM10 has occurred in the area in the year preceding the Extension Year, and the annual mean concentration of PM10 in the area for such year is less than or equal to the standard level." Section 188(d) of the CAA also provides for the possibility that the EPA may grant a second 1-year extension if the Moderate area meets specific criteria. The proposal took comment on two ambiguous aspects of this language that warrant further interpretation through this rule.
	First, the proposal addressed the statutory language explicitly setting ambient air quality conditions for an attainment date extension in terms that relate factually to the 24-hour PM10 NAAQS that was in effect at the time of the 1990 Amendments of the CAA, which has a statistical form that is substantially different from the 24-hour PM2.5 NAAQS. The requirement in CAA section 188(d)(2) states that an extension may be granted if "no more than one exceedance of the 24-hour national ambient air quality standard level for PM10 has occurred in the area in the year preceding the Extension Year, and the annual mean concentration of PM10 in the area for such year is less than or equal to the standard level." The proposal noted that the form of the 2006 24-hour PM2.5 NAAQS is a percentile-based form and not a "one expected exceedance" form as is the PM10 NAAQS, and therefore the statutory language requires some interpretation with regard to how it applies to the PM2.5 NAAQS.
      The EPA included a proposed option and requested comment on two other alternatives. The preferred proposed approach would only require a state to demonstrate that in the year prior to the applicable attainment date for the area, a Moderate area did not exceed the level of (i.e., had clean data for) the specific PM2.5 NAAQS for which the area is designated nonattainment (the "applicable NAAQS") and for which the state is seeking the extension of the attainment date. The second approach would require that a state demonstrate that in the year prior to the applicable attainment date for an area, the Moderate area did not exceed the level of the specific PM2.5 NAAQS for which the area is designated nonattainment (the applicable NAAQS), and did not exceed the most stringent level of any other PM2.5 NAAQS in effect nationally at the time the area was designated for the applicable NAAQS. The third approach would require that a state demonstrate that in the year prior to the applicable attainment date for an area, the Moderate area did not have more than one exceedance of the level of the 24-hour PM2.5 standard, and that the annual mean concentration of PM2.5 in the area for the attainment year was less than or equal to the annual standard, regardless of the NAAQS for which the state is seeking an attainment date extension.
      Second, the proposal addressed how the language of CAA section 188(d)(2) should apply to the PM2.5 NAAQS to the extent that it does not specify whether the air quality criteria for an attainment date extension apply equally for a Moderate area designated nonattainment for both the 24-hour and annual standards, or for just one of the standards. In practice, most areas designated nonattainment for the PM10 NAAQS following passage of the 1990 CAA Amendments were designated nonattainment only for the 24-hour PM10 NAAQS, with a few designated for only the annual PM10 NAAQS or for both the 24-hour and the annual PM10 NAAQS. The 24-hour NAAQS has served as the "controlling" (i.e., functionally more stringent) PM10 standard, such that the agency's experience to date in granting PM10 Moderate area attainment date extension requests has been limited to extending the attainment date for the 24-hour PM10 NAAQS. The situation is distinctly different for PM2.5 nonattainment areas, as the specific facts and circumstances of a particular area may warrant a nonattainment designation for either the 24-hour standard or the annual standard, but often not both. In most cases, for instance, the current nonattainment areas for PM2.5 are designated either for the 1997 annual NAAQS or for the 2006 24-hour NAAQS, but not both. For example, the EPA recently promulgated designations for areas violating only the annual PM2.5 NAAQS revised in 2012, not the 24-hour NAAQS, which was retained at the level established during the 2006 PM NAAQS review. If a PM2.5 nonattainment area is designated only for the 24-hour or only for the annual PM2.5 NAAQS, this situation raises the question of how CAA section 188(d)(2) air quality criteria for both standards should apply to such a PM2.5 NAAQS nonattainment area if the state seeks an extension of the applicable attainment date for such area.
	Regarding the "requirements and commitments" criterion, the EPA proposed to interpret this provision to mean that the state has adopted and is implementing the control measures in the SIP submission it made to address the attainment plan requirements for the applicable PM2.5 NAAQS. The proposal also described a second potential interpretation, in which the state would not be eligible for an attainment date extension unless it has adopted and submitted its Moderate area SIP and has received full approval from the EPA.
	b. Final Rule.
      The EPA received a number of comments on the attainment date extension criteria. With respect to the criterion requiring compliance with all requirements and commitments in the applicable implementation plan, several commenters agreed with the EPA's proposed approach that the state must have adopted and submitted its Moderate area SIP but does not need to have full approval of the plan by the EPA in order to receive an extension. These commenters indicated that a state should not be penalized for a failure by the EPA to take timely action on the implementation plan. Some commenters opposed the proposed approach, stating that an area's attainment date is not predetermined as the end of the sixth calendar year after designation, but instead is to be "as expeditiously as practicable," and no later than the end of the sixth calendar year. For this reason, the commenters stated that the actual attainment date to be extended would not be known until after approval of the SIP by the EPA.
	After considering the comments received on this issue, the EPA is finalizing an approach similar to the preferred option in the proposal. This interpretation is based on the plain language of CAA section 188(d) that does not explicitly require that the state comply with all requirements pertaining to the area in the CAA, but merely requires that the state comply with all requirements in the applicable SIP. In other words, the EPA believes that CAA section 188(d)(1) should be interpreted to mean that so long as the state has submitted the necessary attainment plan for the area for the applicable PM2.5 NAAQs and is implementing the control measures in the submission, the fact that the EPA has not yet acted on such submission to make it an approved part of the applicable SIP should not be a barrier to the state obtaining an extension of the attainment date under CAA section 188(d)(1). See section 51.1005(a)(1) of the CAA. For the same reason, the EPA also proposes to read this provision not to bar an extension if all or part of an area's Moderate area plan is disapproved or has been promulgated by the EPA as a FIP. In the case that the "applicable implementation plan" is a FIP (or combination of SIP and FIP), then the EPA requires the state to have implemented the control measures contained therein in order to meet the statutory criteria at CAA section 188(d)(1) for a Moderate area attainment date extension.
	With respect to the air quality criterion, several commenters supported the EPA's preferred option because it would require an area to show clean data only for the specific standard for which it is seeking an extension year. Some commenters acknowledged that a literal reading of the statute may seem to require a showing of clean data for both the annual and 24-hour PM2.5 standards in order to receive an extension, but suggested that this interpretation would not make sense under the circumstance where the two standards have different attainment dates. The commenter believed it would lead to absurd results if, in order to receive an extension for one standard, an area were required to show clean data for the other standard for which the attainment date had not yet passed. On the other hand, other commenters favored the option that would require clean data for both standards in order to obtain an extension for one standard because they believed that only requiring clean data for one standard would allow the area to avoid or delay achieving additional emissions reductions.
	After considering the comments on the air quality criterion, the EPA has decided to finalize the approach that would require an area to show clean data during the attainment year only for the specific standard for which it is seeking an extension. See 40 CFR 51.1005(a)(1). Under this approach, the EPA interprets the requirement to demonstrate that the area had "no more than one exceedance" of the 24-hour PM2.5 NAAQS to mean that the state must simply demonstrate that the area had "clean data" in the attainment year. Thus, a state seeking an attainment date extension for a Moderate nonattainment area for a 24-hour PM2.5 NAAQS would be required to demonstrate that the area had clean data with respect to the statistical form of that particular standard (i.e., for the 2006 PM2.5 NAAQS, the 98[th] percentile value did not exceed 35 ug/m[3]) in the calendar year prior to the applicable attainment date for the area. The state would not have to demonstrate that the area also had clean data for any other PM2.5 NAAQS, including any annual PM2.5 NAAQS or later revision of the 24-hour PM2.5 NAAQS. Likewise, a state seeking an attainment date extension for an annual PM2.5 NAAQS would be required to demonstrate that the area had clean data for that particular standard (i.e., for the 2012 annual PM2.5 NAAQS, the annual mean value did not exceed 12.0 ug/m3) in the calendar year prior to the applicable attainment date for the area, but would not have to demonstrate that the area had clean data for any other PM2.5 NAAQS.
	The EPA believes this interpretation of CAA section 188(d)(2) is appropriate for two main reasons. First, while most PM10 nonattainment areas were designated nonattainment for either just the 24-hour PM10 NAAQS or for both the 24-hour and annual PM10 NAAQS, the majority of current PM2.5 nonattainment areas are, in contrast, designated for either the 24-hour or the annual PM2.5 NAAQS, and should arguably only need to demonstrate clean data for the NAAQS for which the area is designated nonattainment. For those few PM2.5 nonattainment areas designated for both 24-hour and annual PM2.5 NAAQS, the EPA believes it also is appropriate that a state must only demonstrate clean data for the specific NAAQS for which the state is seeking an attainment date extension because such an approach is consistent with the statute's overall approach to designating nonattainment areas and implementing control strategies for each separate PM2.5 NAAQS. Second, if an area is designated as nonattainment for both the 24-hour and annual PM2.5 standards and receives an extension for one standard while still working toward a later attainment date for the other standard, public health protection would not be delayed because the state would still be subject to the ongoing mandate to adopt and implement measures to ensure expeditious attainment of the other standard.
	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. Process for Attainment Date Extension Request Submissions.
	a. Summary of Proposal.
      The proposal recognized that CAA section 188(d) does not specify the process by which the state should submit a Moderate area attainment date extension request, nor how the EPA should evaluate and act upon such a request. The proposal described the elements that the state would be required to submit for the various options proposed regarding the CAA section 188(d) extension criteria for 1) compliance with requirements and commitments in the applicable SIP, and 2) air quality data. The proposal suggested that any Moderate area extension request should be submitted to the EPA by the attainment date for the area (i.e., by December 31 of the attainment year), and it proposed requiring the state to submit certified air quality data for the attainment year to the EPA by February 28 of the following year in order for the EPA to issue a determination within 6 months of the attainment date regarding whether the area attained or failed to attain. The proposal stated that an attainment date extension should be granted only after the agency provides notice in the Federal Register and an opportunity for the public to comment. Lastly, the proposal clarified that any 1-year extension would extend from January 1 to December 31 for the year following the year including the December 31 attainment date.
	b. Final Rule.
      As discussed in the previous section, in order for the EPA to make a decision on whether to grant a 1-year attainment date extension, the state needs to submit sufficient information to demonstrate that it has both complied with applicable requirements and commitments in the applicable implementation plan, and that it has clean data for the attainment year. Under the final rule, a state would have to demonstrate that control measures have been submitted in the form of a SIP revision, and that RACM and RACT and additional reasonable measures for sources in the area have been implemented. The SIP revision would need to have been adopted and submitted by the state, but it would not need to have been approved by the EPA in order for the state to qualify for an extension. See 40 CFR 51.1005(a)(1)(i). The state also would need to have "clean" air quality data in the attainment year, as explained in the previous section. See 40 CFR 51.1005(a)(1)(ii)-(iii). Any decision made by the EPA to extend the attainment date for an area would be based on facts specific to the nonattainment area at issue.
	Some commenters suggested that in some cases a state will not know if it should seek an extension request until after the attainment date has passed, particularly for areas that commonly have higher air quality levels in the cooler months at the end of the calendar year. The commenter recommended that states should have until February 28 of the following year to submit an extension request along with certified air quality data. Other commenters stated that there is no legal basis for requiring the certification of monitoring data by February 28[th] of the following year, and therefore it should not be a requirement that could potentially disqualify a state from having an extension request be approved.
	The EPA considered these comments in light of the EPA's obligation under the CAA to issue a determination of attainment or failure to attain within 6 months of the original attainment date. After considering these comments, the EPA strongly recommends that a state should submit a Moderate area 1-year extension request to the appropriate EPA Regional Office by February 28 of the following year. In addition, the EPA strongly recommends that the state provide certified air quality data for the previous calendar year by this date or as close to this date as possible. The EPA understands that there may be certain situations that prevent the full certification of filter-based PM2.5 monitoring data by this date. If air quality data for the previous full calendar year has not been fully certified by February 28, the extension request should include any available preliminary data the state can provide. Submission of the necessary air quality data must occur as soon as possible after the attainment date to enable the EPA to review the state's request expeditiously and take appropriate action on the request prior to the date by which the EPA is required to make a determination that the area failed to attain by its Moderate area attainment date, i.e., within 6 months of the applicable attainment date (see the discussion of reclassification in Section V of this preamble).
	As indicated in the proposal, the EPA believes that an attainment date extension should only be granted after the agency provides notice in the Federal Register and an opportunity for the public to comment. A notice-and-comment rulemaking allows for the EPA to adequately evaluate whether the area meets the air quality and program implementation criteria, and to consider other relevant facts and information presented by the state and the public in determining whether the extension request should be granted or denied. This process also is consistent with past practice by the EPA in granting attainment date extensions, most recently for ozone nonattainment areas.
	Regarding the extension period, the EPA interprets CAA section 188(d) to authorize the EPA to stipulate that any extension would begin on January 1 and end on December 31 of the extension year, and these dates would not depend on when the state submitted its request for an extension or was granted the extension by the EPA. The EPA is finalizing this interpretation at 40 CFR 51.1005(a)(4). The EPA believes this is a reasonable approach, as the original attainment date for the area will either be the end of the sixth calendar year following designation of the area, or the end of an earlier calendar year if the state demonstrated that it could advance attainment by at least 1 year. In addition, compliance with the relevant NAAQS will be evaluated based on monitored data collected over a full calendar year (i.e., over the period beginning January 1 and ending December 31), so starting the extension year on January 1 is logical.
	As noted earlier in this discussion of Moderate area attainment date extensions, CAA section 188(d) provides that a state may seek up to two 1-year extensions of the Moderate area attainment date if it meets the applicable criteria of CAA sections 188(d)(1) and 188(d)(2). The statute makes no distinction between the criteria that must be met for the first 1-year extension and the criteria for the second 1-year extension. Therefore, for a second 1-year attainment date extension request, the EPA intends to apply the same interpretations of the statutory criteria as described earlier in this section, including the recommended deadlines for the state to submit the extension request and the certified air quality data.

	c. Comments and Responses.
	Comment: Some commenters described the situation where the EPA has approved a Moderate area attainment date that is earlier than the latest date allowed by the statute (for example, assume the approved attainment date is the end of the 5[th] calendar year after designation). The commenter suggested that if the area was unable to attain by its "earlier" approved attainment date, CAA section 188(d) should be interpreted in a way that would not require the state to submit a request for an attainment date extension. The commenter suggested that the state should only be required to meet the CAA section 188(d) requirements if the area is seeking an extension beyond the latest Moderate area attainment date allowed by statute (i.e. the end of the 6[th] calendar year after designations).
	Response: The EPA does not agree with the commenter because the statute appears to address this situation clearly. Section 188(c)(1) of the CAA states that the Moderate area attainment date is "as expeditiously as practicable but no later than the end of the sixth calendar year after the area's designation as nonattainment." If the area had provided an attainment demonstration supporting the approval of an earlier attainment date by the EPA, then that approved attainment date is then regarded as the "applicable attainment date" for that area. Section 188(d)(1) of the CAA of the statute then enables the EPA to grant a 1-year extension for the "date specified in paragraph (c)(1)," which in this case would be the earlier attainment date.
V. Reclassification of a PM2.5 Moderate Nonattainment Area to Serious
	As discussed elsewhere in this preamble, subpart 4, part D of title I of the CAA establishes a two-tier classification system for areas designated nonattainment for the PM2.5 NAAQS. While all areas designated nonattainment are initially classified as Moderate, CAA section 188(b) describes two pathways by which the EPA has the authority and/or the duty to reclassify a Moderate nonattainment area to a Serious nonattainment area. Pursuant to CAA section 188(b)(1), the EPA has general discretionary authority to reclassify from Moderate to Serious any area that the Administrator determines cannot practicably attain the NAAQS by the applicable Moderate area attainment date. Pursuant to CAA section 188(b)(2), the EPA has a mandatory duty to reclassify from Moderate to Serious any area that fails to attain the NAAQS by the applicable Moderate area attainment date. Both of these pathways are more fully described in the following sections.
A. Discretionary Authority
1. Summary of Proposal.
      The proposal provided background on the EPA's discretionary authority to reclassify a Moderate area to Serious. It proposed to interpret the statute to give EPA broad authority to reclassify based on available information, noting that the EPA could base this determination upon whatever factors are pertinent. The proposal sought comment on whether EPA should discretionarily reclassify an area without a request or submission from the affected state. The proposal also addressed the mandatory statutory timing for discretionary reclassification (i.e., within 18 months of the moderate area SIP due date), and took comment on the appropriateness of EPA acting to reclassify an area beyond 18 months after the Moderate area SIP due date, including right up to the Moderate area attainment date.
2. Final Rule.
      The final rule remains largely unchanged with regard to this issue. The EPA's discretionary authority to reclassify a Moderate area to Serious derives from language in section 188(b)(1) of the CAA, which provides 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." The use of this discretionary authority thus would be triggered by the EPA making a determination that the Moderate area in question cannot practicably attain by its statutory attainment date.
	The CAA does not specify the basis on which the EPA may make a determination that the area cannot practicably attain by the applicable attainment date. In the General Preamble, the EPA explained that the agency could base this determination upon whatever facts are pertinent, and could do so whether or not the state in question has submitted a Moderate area attainment plan, and whether or not the state has made the demonstration contemplated in CAA section 189(a)(1)(B). The EPA may make such a determination based on evaluation of the attainment plan for the Moderate area in question, or based on other facts known to the agency. As discussed earlier in this preamble, the attainment plan that a state would submit for a Moderate nonattainment area must include either a demonstration that the area will attain the NAAQS by the statutory Moderate area attainment date or a demonstration that attaining by the statutory Moderate area attainment date is impracticable. If the state makes and the EPA concurs with an impracticability demonstration submitted as part of the attainment plan, then the demonstration could serve as the basis for the EPA initiating a notice-and-comment rulemaking to reclassify the area to Serious.
 	However, the CAA does not specify the basis for the EPA's exercise of its discretionary authority and does not require the EPA to make its determination based on a submission from the state. Indeed, such a prerequisite would be illogical in the case of a state that fails to make any attainment plan submission or fails to address the issue of the need for reclassification in such submission. The EPA believes that while a Moderate area impracticability demonstration as contemplated in CAA section 189(a)(1)(B) is desirable in order to help the agency make a determination that the area cannot practicably attain by its attainment date, such a demonstration is not necessary to trigger action by the EPA to reclassify a Moderate area to Serious. The statute does not prohibit the EPA from using the weight of available evidence, including information available in the public record of a state, to make such a determination, even in the absence of a complete attainment plan submission.
	Regarding the timing of discretionary reclassifications, CAA section 188(b)(1)(B) establishes timeframes by which EPA is to act if it intends to exercise its discretionary authority to reclassify areas as appropriate following the Moderate area attainment plan due date, stating 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." In the case of areas designated nonattainment for the 2012 PM2.5 NAAQS in the first round of designations, states will be required by statute to submit a Moderate area attainment plan within 18 months of the date of designation (April 2015), or no later than October 2016. Pursuant to CAA section 188(b)(1)(B), the EPA would then have until April 2018 (18 months following the Moderate area attainment plan submission deadline) to use its discretionary authority to reclassify any area that the EPA determines at that time cannot practicably attain by the Moderate area attainment date of December 2021.
	However, as noted earlier, there may be situations in which it may be appropriate to reclassify an area at a point in time more than 18 months after the SIP due date. On this issue, the General Preamble stated that:
            "...under the plain meaning of the terms of section 188(b)(1), EPA has general discretion to reclassify at any time before the applicable attainment date any area EPA determines cannot practically attain the standards by such date. Accordingly, CAA section 188(b)(1) is a general expression of delegated rulemaking authority. In addition, subparagraphs (A) and (B) of CAA section 188(b)(1) mandate that the EPA reclassify at specified timeframes any areas it determines appropriate for reclassification at those dates. These subparagraphs do not restrict the general authority but simply specify that, at a minimum, it must be exercised at certain times."

      The EPA continues to consider this the correct interpretation of the statutory requirements concerning its authority to reclassify a Moderate nonattainment area to Serious at any time prior to the area's Moderate area attainment date, if the agency determines that the area cannot practicably attain the relevant PM2.5 NAAQS by that date. See Section VI.A.2 of this preamble for a discussion of the due dates for submission of attainment plan elements for areas that receive a discretionary reclassification.
	The EPA emphasizes that a state with an area designated as nonattainment for the PM2.5 NAAQS is required to meet all Moderate area attainment plan requirements, even after the EPA reclassifies the area to Serious. Section 189(b)(1) of the CAA states clearly that "in addition to" the Moderate area attainment plan requirements, states with areas reclassified to Serious must also meet Serious area attainment plan requirements, i.e., the reclassification does not eliminate the statutory obligation to meet Moderate area attainment plan requirements. Thus, the EPA believes that reclassifying Moderate areas to Serious at any time under its discretionary authority does not provide incentives to delay development and implementation of control measures by excusing states from meeting substantive Moderate area attainment plan requirements or by extending the applicable attainment date. The EPA articulated this position in the General Preamble, explaining that this interpretation:
            ... creates an incentive for the timely submittal and effective implementation of moderate area SIP requirements and facilitates the PM10 attainment objective. For example, if an area that fails to submit a timely moderate area SIP is reclassified, this does not obviate the requirement that the area submit and implement RACM consistent with the moderate area schedule. Accordingly, the area could be subject to sanctions for its delay in submitting the RACM SIP requirement... Further, reclassification before the applicable attainment date will ensure that additional control measures (i.e., in addition to RACM, serious areas must implement best available control measures (BACM)), are implemented sooner and will expedite the application of more stringent new source review requirements to the area... Similarly, where an area submits a timely moderate area SIP, EPA may not discover that the area cannot practicably attain until sometime after it begins implementing its moderate area control measures. The EPA then may want to reclassify the area in order to facilitate the development and implementation of BACM.

	The EPA considers this longstanding interpretation of CAA section 188(b)(1) to be the correct interpretation of the statutory requirements governing the discretionary reclassification of Moderate areas. The EPA will reclassify any area it determines cannot practicably attain by the Moderate area attainment date through notice-and-comment rulemaking. See 40 CFR 51.1002(b)(1).
3. Comments and Responses.
	Comment: Some commenters stated that while it may be desirable for a state or local agency to provide an impracticability demonstration to the EPA, the EPA is not prohibited from using the weight of available evidence to reclassify an area to Serious even before the Moderate area plan is due if it has a particularly challenging air quality situation. Other commenters did not agree with the EPA's interpretation of the statute, and believed that the EPA's authority should be limited to reclassification of areas that submit an impracticability demonstration.
	Response: For the reasons described earlier, the EPA agrees with the first commenter and does not believe its authority is limited in the manner suggested by the second commenter.
B. Mandatory Duty
1. Summary of Proposal.
      The proposal provided background on the EPA's mandatory duty pursuant to CAA section 188(b)(2) to reclassify a Moderate area to Serious when the area fails to attain the standard by the attainment date. The CAA directs the EPA to reclassify an area from Moderate to Serious if the area fails to attain the relevant NAAQS by the applicable Moderate area attainment date (including any attainment date that had been extended by one or 2 years pursuant to CAA section 188(d)). Reclassification occurs by operation of law when the EPA determines that the area failed to attain the NAAQS by the applicable attainment date, in accordance with CAA section 188(b)(2)(A). Section 188(b)(2) of the CAA requires that "within six months following the applicable attainment date for a PM10 nonattainment area, the Administrator shall determine whether the area attained the standard by that date" and publish its determination in the Federal Register.
 	The EPA proposed that the date of reclassification would be the effective date of the Federal Register notice issued by the EPA that determines the area failed to attain by the attainment date. Thus, for example in the case of the 2012 PM2.5 NAAQS, assuming a Moderate PM2.5 nonattainment area fails to attain the standard by its approved attainment date of
December 31, 2021, the EPA would be required to publish in the Federal Register no later than June 30, 2022 its determination that the area failed to attain the NAAQS and is therefore reclassified as Serious by operation of law. The actual date of reclassification for the area would be the effective date of the Federal Register document (e.g. in July or August 2022). To meet the requirements of CAA section 189(b)(2), the Serious area attainment plan for the area would be due within 18 months of the actual reclassification date (i.e., in early 2024).
	The proposal also discussed a possible alternative option, which would be to consider the date of reclassification to Serious to be the same as the Moderate area attainment date. Applying this approach in the example earlier would yield an earlier date of reclassification of
December 31, 2021, and an earlier Serious area attainment plan due date of June 30, 2023.
2. Final Rule.
      Several commenters supported the EPA's proposed approach to interpret the date of reclassification as the effective date of the Federal Register notice announcing the area had failed to attain the standard by the Moderate area attainment date because this approach would allow adequate time for the EPA to evaluate air quality data and any exceptional events claims before making the determination that the area failed to attain. Some commenters opposed the proposed approach and supported interpreting the date of reclassification as being the same as the missed attainment date for the Moderate area. This commenter suggested that the proposed approach could introduce additional delay because the EPA does not always issue determinations of failure to attain promptly. They also claimed that the term "reclassified by operation of law" in CAA section 188(b)(2)(A) would have no meaning (i.e., surplusage) if the proposed approach was adopted.
	After taking the comments received under consideration, the EPA has decided to retain the proposed approach. The date of reclassification is the effective date of the Federal Register notice issued by the EPA that determines the area failed to attain by the attainment date.
For practical reasons, the EPA does not believe that as a general matter it can be expected to make a determination on December 31 that an area failed to attain. Because the PM2.5 ambient monitoring method requires laboratory analysis of filters prior to determining the ambient mass for each day, adequate time is needed after December 31 to ensure that the filter-based measurements have been evaluated and quality-assured in an accurate manner.
	Although CAA section 188(b)(2) does not explicitly address this issue, the EPA believes that this approach is a reasonable interpretation of statutory ambiguity in CAA section 188(b)(2) and preferable over the alternative approach for two additional reasons. First, the statute at CAA section 189(b)(2) gives a state 18 months from the date of reclassification of an area to submit for the EPA's approval an attainment demonstration with air quality modeling and provisions to assure timely implementation of BACM and BACT on sources in the nonattainment area. The workload associated with developing a Serious area plan can be substantial, and the EPA believes that it is reasonable to resolve the statutory ambiguity in favor of providing the state with the full 18 months from the effective date of reclassification to develop and submit a thorough, complete and accurate Serious area attainment plan that will provide for expeditious attainment of the NAAQS.
	Second, the statutory attainment date for a Serious area reclassified under any circumstances is as expeditious as practicable but no later than the end of the tenth year following designation of the area, and is thus independent of the date of reclassification of the area. Allowing a state some additional amount of time beyond 18 months from the missed attainment date to develop and submit a complete Serious area attainment plan, including adopting BACM and BACT, will not change the statutory obligation on the state for the area to attain the relevant NAAQS by the applicable attainment date. On the contrary, the EPA believes that the extra time may in fact help the area timely attain the relevant NAAQS by allowing the state to develop a more effective attainment plan for the area.
	Thus, the EPA interprets the CAA such that the date of reclassification for an area reclassified under the EPA's mandatory duty is to be considered the effective date of the Federal Register document announcing that the area had not attained the relevant PM2.5 NAAQS and is therefore reclassified by operation of 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. The following section describes the EPA's final actions in this rule regarding Serious area attainment plan requirements in greater detail.
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. 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."
	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.
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 the 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; 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.
	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 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 10[th] 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 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 7[th] 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 8[th] 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 6[th] 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)], 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).
      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.
      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 deems a state's precursor demonstration as adequate 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. 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.
	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. 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.
      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. 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. 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.

	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. 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.
	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.