The EPA recognizes that with regard to Serious areas, only a nonattainment area that is reclassified under the agency's discretionary authority might have sufficient time between the date for implementing BACM and BACT and the statutory Serious area attainment date to implement additional measures beyond BACM and BACT. BACM and BACT must be implemented no later than 4 years after reclassification of the area; areas reclassified to Serious because they cannot practicably attain the relevant NAAQS by the applicable attainment date could potentially have significantly more than 4 years between the date of reclassification and the statutory Serious area attainment date, during which time the area could continue to implement additional feasible measures to bring the area into attainment.
      By way of illustration, for areas designated in the first round of designations for the 2012 PM2.5 NAAQS, the statutory Moderate area attainment date will be no later than December 31, 2021. If a state submits a Moderate area attainment plan by the statutory attainment plan due date (18 months after designation, or in this example, October 2016) and the plan demonstrates that the area cannot practicably attain the NAAQS by December 31, 2021, then the EPA has a statutory duty to reclassify such an area within 18 months of the attainment plan due date (i.e., by April 2018). The statutory Serious area attainment date would be the end of the tenth year following designation, or December 31, 2025. In such a case, the state would need to implement BACM for the area within 4 years of reclassification, or by April 2022, leaving over 3.5 years between the statutory deadline for implementing BACM and the statutory attainment date for the area. The requirement for the state to identify and adopt additional feasible measures for the area would mean that the state would need to identify those control measures and technologies that are feasible (according to the proposed BACM and BACT criteria described later in this section) and that can be implemented between April 2022 and December 2025. The EPA expects that while such a long span of time may be available only to a very few Serious nonattainment areas, it would be appropriate to require such areas to implement measures in addition to BACM and BACT if, taken together, they can provide for attainment by the attainment date or advance the attainment date for the area by at least 1 year. Accordingly the EPA has codified a definition of "additional feasible measures" and specified the conditions under which such measures would need to be included in a serious area plan submission. See 40 CFR 51.1000 and 40 CFR 51.1010(a)(4)(ii).
      v. Steps of the BACM/BACT Selection Process.
      In addition to the regulatory decisions earlier, the EPA summarized and sought comment on further guidance for states to follow in selecting BACM/BACT. The guidance was primarily derived from the Addendum. This section reviews that guidance, clarifies and updates it for purposes of PM2.5, and responds to significant comments on the guidance discussion included in the proposal.
      The BACM/BACT selection process for implementation of the PM2.5 NAAQS is designed to take into account the local facts and circumstances and the nature of the air pollution problem in a given nonattainment area. The following sections describe the steps of the process, including: (i) develop a comprehensive inventory of sources and source categories of directly emitted PM2.5 and PM2.5 precursors; (ii) identify existing and potential control measures for the sources in the inventory; (iii) evaluate the technological feasibility of potential control measures; (iv) evaluate the economic feasibility of potential control measures; and (v) determine the earliest date by which a control measure or technology can be implemented in whole or in part. These steps are described more fully in the following subsections.
      Step 1: Develop a comprehensive inventory of sources and source categories of directly emitted PM2.5 and PM2.5 precursors. As with any control strategy analysis for a nonattainment area, the EPA recommends that the state begin with a current detailed emissions inventory of the various sources that emit direct PM2.5 and PM2.5 precursors in the Serious area. The inventory should identify major stationary sources (i.e., sources with the potential to emit 70 tpy of direct PM2.5 or any precursor), nonmajor stationary sources, mobile sources, and area sources. The inventory also should identify both anthropogenic and nonanthropogenic emissions sources. The EPA expects the state to start with the base year emissions inventory submitted with the Moderate area attainment plan as required under CAA section 172(c)(3), and update it as necessary to reflect new source construction, facility shutdowns, growth in certain source categories, and any other relevant changes. This inventory should be the most comprehensive and accurate inventory available, and it should be consistent with the emissions inventory requirements for Serious area plans as described in Section VI.B of this preamble.
      Step 2: Identify potential control measures. The state should identify potential control measures for all sources and source categories in the latest base year emission inventory for the nonattainment area. The list of existing and potential control measures should include options not previously considered as RACM/RACT for the area, as well as additional measures not previously evaluated in the RACM/RACT analysis. For purposes of identifying new measures to consider in its BACM/BACT analysis, the EPA recommends that the state obtain and evaluate a wide range of sources of information on existing and potential control measures. Other nonattainment areas in the same state, and other states across the country are important sources of information about control measures that are currently being implemented. Regional planning
organizations, and state and local air quality consortiums have in the past developed summaries of control measures that should provide useful information for this process.
      The EPA's RBLC provides a central data base of air pollution technology information that may be highly relevant to states seeking information on stationary source control technology that may qualify as BACT for PM2.5 NAAQS implementation, and is available online at http://cfpub.epa.gov/RBLC/. There are also other resources available to assist states in identifying other potential control measures and control technologies for their BACM and BACT determinations. The EPA encourages states with Serious PM2.5 nonattainment areas to visit the agency's Web site to find links to other online sources of information on potential control measures for states to consider.
      The state must incorporate appropriate measures into the list of potential control measures for the source categories in the Serious nonattainment area. The EPA would expect the state to identify an array of existing and potential new measures at least as broad as the list identified for the same area as part of the RACM and RACT analysis, in order to ensure that the state has a sufficiently expansive and comprehensive set of potential measures to evaluate. The list of potential measures must include all measures identified as potential control measures for the nonattainment area when it was classified as Moderate or, for a given source category, one or more alternative control measures that would control emissions even more stringently than the measures included in the RACM/RACT analysis. In this way, the state will begin its BACM/BACT determination with a list of potential control options that is as complete and up-to-date as possible.
      Step 3: Determine whether an available control measure or technology is technologically feasible. After developing a list of existing and potential new measures to evaluate for BACM and BACT, the state would then need to determine the technological feasibility of each identified control measure in light of a number of considerations, including each measure's individual energy and environmental impacts.
      (1) Stationary sources. As described under the technological feasibility criteria for the control measures analysis for Moderate area attainment plans in Section IV.D of this preamble, the EPA's prior guidance on factors to consider for judging whether a particular control technology is technologically feasible should include a source's processes and operating procedures, raw materials, physical plant layout and potential environmental impacts such as increased water pollution, waste disposal and energy requirements. For example, the EPA recognizes that the process, operating procedures and raw materials used by a source can affect the feasibility of implementing process changes that reduce emissions and can also affect the selection of add-on emission control equipment. The feasibility of modifying processes or applying control equipment also can be influenced by the physical layout of the particular plant, if the physical space available in which to implement such changes limits the choices.
      (2) Area and mobile sources. With respect to determining whether a given control measure might not be technologically feasible as BACM for an area or mobile source, a state may consider factors in conducting its analysis that are similar to factors the state may have considered during the RACM and RACT determination process, such as local circumstances, the condition and extent of needed infrastructure, or population size or workforce type and habits, which may prohibit certain potential control measures from being implementable. However, in the instance where a given control measure has been applied in another NAAQS nonattainment area (for PM2.5 or other pollutant), the state will need to provide a detailed justification for rejecting any potential BACM measure as technologically infeasible. Furthermore, if the state identifies a certain control measure for area or mobile sources that has been implemented in another nonattainment area and may qualify as BACM or BACT, the state must provide a reasoned justification if it deems it technologically infeasible to implement the same control measure to the same extent or magnitude as it was applied in the other nonattainment area.
      Step 4: Determine whether an available control technology or measure is economically feasible. The fourth step of this process is to evaluate the costs of implementing each of the technologically feasible control measures and technologies in order to eliminate from further consideration any measures determined to be economically infeasible. In assessing "best" control measures and technologies, states with Serious PM2.5 nonattainment areas must identify a control strategy for the area that overall is more stringent than that identified for the area when the state considered only the "reasonableness" of potential control measures for purposes of the RACM/RACT analysis. States need to consider emission reduction measures with higher costs per ton when assessing the economic feasibility of BACM and BACT controls (and, where applicable, additional feasible measures) as compared to the economic feasibility criteria applied in their RACM and RACT analysis (and analysis for additional reasonable measures) for the same nonattainment area.
      Indeed, consistent with prior guidance on evaluating costs of a potential BACM/BACT, the EPA maintains that while the economic feasibility of a control measure is as important as its technological feasibility under the RACM and RACT determination process, economic feasibility is a less significant factor in the BACM and BACT determination process. In other words, a state must apply a higher standard for eliminating a technologically feasible control measure from further consideration as BACM due to cost alone.
      In the Addendum, the EPA stated that "for PM10 BACM purposes, it is reasonable for similar sources to bear similar costs of emission reduction." Additionally, the EPA indicated that "economic feasibility for PM10 BACM purposes should focus upon evidence that the control technology in question has previously been implemented at other sources in a similar source category without unreasonable economic impacts." Thus, a state may not eliminate a particular control measure from further consideration as potential BACM if similar sources have successfully implemented such a measure. That is, a state must at a minimum continue to consider as potential BACM any technologically feasible control measures or technologies implemented by similar sources.
      In addition, a state may not automatically eliminate a particular control measure merely because other sources have not implemented the measure. In other words, a state must continue to consider technologically feasible measures that have not been implemented by similar sources but that can nonetheless effectively reduce emissions from the source category in question at a cost that is not cost prohibitive.
      As with the EPA's approach for evaluating economic feasibility of potential reasonable measures for Moderate area attainment plans, for each technologically feasible control measure or technology, a state must evaluate the economic feasibility of the measure through consideration of the capital costs, operating and maintenance costs, and cost effectiveness (i.e., cost per ton of pollutant reduced by that measure or technology) associated with such measure or control. While the EPA is not establishing a fixed dollar per ton cost threshold for economic feasibility of controls identified as potential BACM and BACT, the cost per ton of an acceptable measure for the BACM and BACT analysis generally would be higher than it was for the RACM and RACT analysis for the same nonattainment area. In addition, if a source contends that a source-specific control level should not be established because the source cannot afford the control measure or technology that is demonstrated to be economically feasible for purposes of BACM for other sources in its source category, the source should make its claim known to the state and support the claim with information regarding the impact of imposing the identified control measure or technology on the following financial indicators, to the extent applicable:
      1. Fixed and variable production costs ($/unit);
      2. Product supply and demand elasticity;
      3. Product prices (cost absorption vs. cost pass-through);
      4. Expected costs incurred by competitors;
      5. Company profits;
      6. Employment costs;
      7. Other costs (e.g., for BACM implemented by public sector entities).
      Step 5: Determine the earliest date by which a control measure or technology can be implemented in whole or in part. Section 189(b)(1)(B) of the CAA requires that Serious area attainment plans provide for the implementation of BACM no later than 4 years after reclassification of the area to Serious. As with the EPA's proposed approach to RACM and RACT, the EPA proposes the term "implement" to mean that the control measure or technology has not only been adopted into the SIP for the area but has also been built, installed and/or otherwise physically manifested and the affected sources are required to comply. The EPA thus expects a state with a Serious nonattainment area to take timely action to implement BACM and BACT in the area.
      A state must identify those technologically and economically feasible control measures and technologies that it can implement fully or partially within 4 years of reclassification of its Serious PM2.5 nonattainment area. These measures will be considered BACM and BACT for the area. If a state evaluates a potential BACM or BACT measure and determines that it can be implemented only partially within 4 years after reclassification, the state must adopt the partial measure as BACM.
      Where the earliest date that a measure can be implemented is beyond the 4 year mark following reclassification to Serious, the measure may still be needed as an "additional feasible measure" if the 4 year mark occurs before the Serious area attainment date. "Additional feasible measures" would be "best"-level, feasible measures that a state could implement in whole or in part on sources in the area sometime after the fourth year following reclassification and prior to the statutory attainment date for the area.
      c. Comments and Responses.
      Any additional comments received on this section are addressed in the Response to Comments document found in the docket for this action.
3. BACM and BACT Submission Requirements.
      a. Summary of Proposal.
      The proposal further specified the submission requirements once a state has determined the BACT/BACM requirements in its plan. The proposal required the state to submit a list of emissions sources, an emissions inventory for such sources, and several pieces of information regarding potential control measures for these sources.
      b. Final Rule.
      The final rule remains relatively unchanged from the proposal. To ensure that attainment plan submissions contain the necessary supporting information for EPA review and approval of the state's selected BACM and BACT and additional feasible measures as applicable, 40 CFR 51.1010(a)(1)-(5) require the state to submit the following information as part of its Serious area attainment plan submission:
 A list of all emissions source categories, sources and activities in the nonattainment area that emit direct PM2.5 or any PM2.5 precursor (for multi-state nonattainment areas, this would include source categories, sources and activities from all states which make up the area);
 For each source category, source or activity in the nonattainment area, an inventory of direct PM2.5 and all PM2.5 precursor emissions;
 For each source category, source or activity in the nonattainment area, a comprehensive list of potential control measures considered by the state for the nonattainment area;,
 For each potential control measure considered by the state but eliminated from further consideration due to a determination by the state that the control measure or technology was not technologically feasible, a narrative explanation and quantitative or qualitative supporting documentation to justify the state's conclusion;
 For each technologically feasible emission control measure or technology, the state must provide the following information relevant to economic feasibility: (i) the control efficiency by pollutant; (ii) the possible emissions reductions by pollutant; (iii) the estimated cost per ton of pollutant reduced; and, (iv) a determination of whether the measure is economically feasible, with narrative explanation and quantitative supporting documentation to justify the state's conclusion;
 For each technologically and economically feasible emission control measure or technology, the date by which the technology or measure can be implemented.
      As with a Moderate area attainment plan submission, the EPA recognizes that the base year emissions inventory that the state submits for the area in conjunction with its Serious area attainment plan will likely contain the information required under the first two items in this list. However, the EPA believes that it is incumbent on the state to ensure that the information needed for the EPA to evaluate the state's BACM and BACT and additional feasible measures analysis is presented as part of that analysis and in a format that provides transparency, consistency and the ability for another party to evaluate the state's analysis effectively and to duplicate the state's results. For this reason, the EPA is requiring the state to include the base year emissions inventory information with the BACM and BACT submission and as one element of the state's attainment plan due 18 months after reclassification of the area to Serious.
      c. Comments and Responses.
      Any additional comments received on this section are addressed in the Response to Comments document found in the docket for this action.
4. Criteria for Effective Regulations to Implement BACM and BACT and Additional Feasible Measures.
	a. Summary of Proposal.
      The proposal described the four main criteria for effective control measure regulations: such regulations must be quantifiable, enforceable, replicable, and accountable.
      b. Final Rule.
      Guidance on effective control measure regulations is provided in the control strategy discussion for Moderate areas. See section IV.D.9 of this preamble, Criteria for effective regulations to implement RACM and RACT and additional reasonable measures.

5. Relevance of prior BACT, LAER and BART determinations.
      a. Summary of Proposal.
      The preamble of the proposed rule stated that it should not be assumed that past control technology determinations would automatically be deemed to meet the Serious area control measure requirements (BACM, BACT, or additional feasible measures) for an area.
      b. Final Rule.
      The guidance on this issue in the preamble to the final rule remains largely unchanged. The EPA believes that BACT or lowest achievable emission rate (LAER) provisions for new sources (as distinct from BACT for existing sources), or best available retrofit technology (BART) for existing sources, could potentially qualify as BACM or BACT for purposes of meeting the Serious area attainment plan requirements. However, the EPA does not believe it is appropriate for a state to assume that just because a certain control technology was determined to meet BACT, LAER, or BART criteria for a new source sometime in the past, that such a control will also automatically meet the criteria for BACM or BACT or additional feasible measures for attainment planning purposes because the regulated pollutant or source applicability may differ and the analyses may be conducted many years apart. Thus, a state may not simply rely on prior BACT, LAER or BART analyses for the purposes of showing that a source has also met BACT for the relevant PM2.5 NAAQS. Rather, the EPA expects that in Step 2 of the BACM and BACT determination process, the state would identify such measures as "existing measures" that should be further evaluated as potential BACM or BACT or additional feasible measures. At the same time, the EPA notes that the presence of previously installed control technology, and the technical and economic considerations that would be associated with upgrading to a measure that achieves greater reductions, is something that should be considered in the assessments of technological and economic feasibility of the newer measure.
      c. Comments and Responses.
      Any additional comments received on this section are addressed in the Response to Comments document found in the docket for this action.
6. Multi-state Nonattainment Areas.
      a. Summary of Proposal.
      The preamble to the proposed rule provided general guidance on coordination between states in multi-state nonattainment areas to ensure they adopt sufficient BACM/BACT and additional feasible measures to ensure expeditious attainment of the standard.
      b. Final Rule.
      The guidance in the final rule remains largely unchanged. States that share a multi-state Serious PM2.5 nonattainment area must consult with one another on BACM and BACT and additional feasible measures that will be required for the nonattainment area in the different states. This requirement would be consistent with the overall requirements for BACM and BACT and additional feasible measures determinations, as all states with Serious areas need to consider implementing BACM and BACT-level measures that have been implemented in other states, even if those measures incur higher costs. The EPA anticipates that states may potentially adopt controls that differ from state to state, based upon each state's determination of what qualifies as "best" given the mixture of sources and potential controls in the state portions of relevant nonattainment areas, subject to EPA approval. If the state can adequately demonstrate that its chosen BACM and BACT and additional feasible measures fully meet the EPA's proposed criteria for such measures, then the agency may consider approving individual state plans that differ in implementation of control measures.
      c. Comments and Responses.
      Any additional comments received on this section are addressed in the Response to Comments document found in the docket for this action.
7. Environmental Justice Considerations for Developing the Attainment Plan Control Strategy for a Serious PM2.5 Nonattainment Area.
      a. Summary of Proposal.
      The proposal provided general guidance for ensuring that overburdened populations are appropriately protected.
      b. Final Rule.
      The guidance in the final rule remains largely unchanged. The EPA strongly urges states to consider the environmental justice aspects of any control measures they have identified as BACM and BACT or additional feasible measures in order to provide health protection for overburdened populations. Please see Section XI of this preamble, which discusses possible approaches for states to address environmental justice concerns associated with implementation of the PM2.5 NAAQS in their SIP development process and attainment plans.
      c. Comments and Responses.
      Any additional comments received on this section are addressed in the Response to Comments document found in the docket for this action.

E. Modeling for Attainment Demonstrations
1. Due Dates for Submission of Serious Area Attainment Demonstrations.
	a. Summary of Proposal.
      Section IV.E. of this preamble describes the EPA's attainment demonstration and modeling requirements for Moderate area plans. The EPA proposed that the same general attainment demonstration and modeling requirements for Moderate area plans should apply to Serious area attainment demonstrations. However, Serious area plans have additional statutory requirements.
      Attainment demonstrations are due 18 months after reclassification if the EPA reclassifies the area to Serious after failure of the area to attain the applicable Moderate area deadline. Alternatively, CAA section 189(b)(2) requires states with designated Serious nonattainment areas to submit attainment demonstrations no later than 4 years after reclassification of the area to Serious if the reclassification occurs before the Moderate area attainment deadline. The EPA proposed an approach for determining an appropriate attainment plan control strategy for a Serious PM2.5 nonattainment area that requires the state to submit the attainment demonstration for the area within 18 months after reclassification, regardless of when or the authority under which an area was reclassified to Serious.
      b. Final Rule.
      The statutory attainment demonstration provisions for Serious areas are as follows: Section 189(b) of the CAA requires a state with a designated Serious nonattainment area to submit an attainment plan for such area. As discussed earlier, CAA section 189(b)(1)(A) more specifically requires the state to submit an attainment demonstration including air quality modeling to establish either: (i) that the area will attain the relevant NAAQS by the applicable attainment date, or (ii) if the state is seeking an extension of the attainment date, that it is impracticable for the area to attain the relevant NAAQS by the statutory Serious area attainment date. For Serious nonattainment areas, the attainment date is as expeditiously as practicable, but no later than the end of the tenth calendar year after designation as nonattainment. A demonstration that shows that it is impracticable for the area to attain within this timeframe must also provide for attainment of the NAAQS by the most expeditious alternative date practicable, but no later than 5 years after the maximum statutory Serious area attainment date (based on the criteria specified in CAA section 188(e)).
	The EPA is not finalizing the proposed approach of requiring all Serious area attainment demonstrations to be due 18 months after reclassification. If the EPA reclassifies the area to Serious after failure of the area to attain the applicable Moderate area deadline, the attainment demonstration will be due in 18 months. States with Serious nonattainment areas that were reclassified before the Moderate area attainment deadline must submit attainment demonstrations the earlier of 4 years after reclassification of the area to Serious or the end of the eighth calendar year after initial designation. However, these areas are still required to submit BACT/BACM measures within 18 months of being reclassified as Serious. Sections VI.A and VI.D of this preamble describe more fully the EPA's approach for plan due dates and control strategy analyses for all elements of a Serious area attainment plan. Section VI.J of this preamble provides a complete discussion of the EPA's criteria for granting a Serious area attainment date extension.
      c. Comments and Responses.
      Any comments received on this section are addressed in the Response to Comments document found in the docket for this action.
2. Attainment Demonstration Requirements for Serious Areas.
      a. Summary of Proposal.
      The proposal described the attainment demonstration and impracticability demonstration requirements for Serious nonattainment areas. The EPA proposed that a serious area plan must include an attainment demonstration that demonstrates how a state will attain the PM2.5 NAAQS by the applicable attainment date, must include analyses supporting the state's determination of its proposed attainment date, and must show that the area will attain the NAAQS as expeditiously as practicable, but not later than the tenth calendar year after designation. The proposal indicated that in order to establish that the attainment date is as expeditious as practicable, the state must explain why the control measures adopted in the attainment plan provide for the most expeditious attainment and must include all BACM and BACT controls in the analysis.
      b. Final Rule.
      The final rule requirements for Serious area attainment demonstrations are generally unchanged from the proposal. As described in Section IV.E of this preamble, an attainment demonstration is a plan that demonstrates how a state will attain the PM2.5 NAAQS by the applicable attainment date. The EPA is finalizing a requirement that the demonstration for Serious areas must consist of: (i) technical analyses such as base year and future year modeling of emissions which identify sources and quantify emissions that are contributing to violations of the PM2.5 NAAQS; and, (ii) analyses of future year projected emissions reductions and air quality improvement resulting from existing (i.e. already-adopted or "on the books") national, regional and local programs, and potential new local measures needed for attainment, including RACM and RACT and BACM and BACT controls for the area, as well as other measures either inside the nonattainment area or outside the nonattainment area but within the state that could potentially accelerate attainment. Each state with a Serious nonattainment area must submit an attainment plan with an attainment demonstration that includes analyses supporting the state's determination of its proposed attainment date. In all cases, the state must show that the area will attain the NAAQS as expeditiously as practicable, but not later than the tenth calendar year after designation. In order to establish that the attainment date is as expeditious as practicable, the state must explain why the control measures adopted in the attainment plan provide for the most expeditious attainment and must include all BACM and BACT controls in the analysis.
	A state with a Serious nonattainment area can also submit an impracticability demonstration (under CAA section 189(b)(1)(A)(ii)) as part of seeking an extension of the attainment date under CAA section 188(e). The impracticability demonstration for a Serious area would be similar to an impracticability demonstration for Moderate areas because it must show that the area will not be able to attain the PM2.5 NAAQS by the latest possible statutory attainment date, which in this case is by the end of the tenth calendar year following designation.
      In order to support a Serious area impracticability demonstration, the state must show (through modeling) that attainment cannot be reached by the latest statutory Serious area attainment date, even if all RACM and RACT and BACM and BACT controls, as well as other measures either inside the nonattainment area or outside the nonattainment area but within the state (as may be necessary to meet the requirements of 172(c)(6)), were implemented before the attainment date. Moreover, in addition to the Serious area impracticability demonstration, to support an extension of the attainment date, the Serious area plan must demonstrate (again, using air quality modeling) that it provides for attainment by the most expeditious alternative date practicable employing MSM, as specified in CAA section 188(e). (MSM are discussed in more detail in Section VI.J of this preamble). As a result, the required plan in the case of a Serious area that cannot attain by the statutory attainment date is both an impracticability demonstration (to justify an extension beyond the statutory attainment date) and an attainment demonstration that serves as the basis for proposing an appropriate alternative attainment date. Note that this is different from a Moderate area impracticability demonstration, which is not required to serve as the basis for proposing a new area attainment date.
      c. Comments and Responses.
      Any comments received on this section are addressed in the Response to Comments document found in the docket for this action.
3. Air Quality Modeling Required for Serious Area Attainment Demonstrations and Impracticability Demonstrations.
      a. Summary of Proposal.
      The EPA proposed to require air quality modeling in support of both a Serious area attainment demonstration and a Serious area impracticability demonstration.
	b. Final Rule.
      The EPA is finalizing a requirement for states to submit air quality modeling in support of both attainment demonstrations and impracticability demonstrations for Serious PM2.5 nonattainment areas. Unlike the impracticability demonstration for Moderate areas described in CAA section 189(a)(1)(B)(ii), the impracticability demonstration for Serious areas in CAA section 189(b)(1)(A)(ii) also requires air quality modeling establishing the most expeditious alternative attainment date practicable. Therefore, air quality modeling is a required element in all attainment demonstrations for Serious areas.
      Some commenters believed that both Moderate and Serious area impracticability demonstrations must include air quality modeling. The EPA does not agree and believes the statute only requires air quality modeling for Serious area impracticability demonstrations. This stems from the slightly different statutory construction in CAA section 189(b)(1)(A) compared to CAA section 189(a)(1)(B). Section 189(b)(1)(A) of the CAA specifies an air quality modeling requirement as a parenthetical, which the EPA interprets to apply to both the requirements in CAA section 189(b)(1)(A)(i) [attainment demonstrations] and CAA section 189(b)(1)(A)(ii) [impracticability demonstrations]. Additionally, the fact that a Serious area impracticability demonstration must also include an attainment demonstration with an alternative attainment date logically supports the final rule conclusion that a Serious area impracticability demonstration must include air quality modeling. Modeling is needed to demonstrate attainment and to propose an alternative attainment date for the Serious area. This differs from a Moderate area impracticability demonstration, which only serves to demonstrate that attainment cannot be reached by the Moderate area attainment date. A Moderate area impracticability demonstration does not require a demonstration of attainment or setting of an alternative future attainment date. It merely starts the process of reclassifying an area to Serious and the eventual required submission of a Serious area implementation plan.
	Other than the timing of plan submissions and additional required elements of a Serious area plan (such as BACM and BACT), the relevant air quality modeling procedures and guidance for Moderate and Serious area plans are the same. See Section IV.E. of this preamble for more details on the modeling requirements and guidance for all PM2.5 nonattainment areas.

      c. Comments and Responses.
      Any additional comments received on this section are addressed in the Response to Comments document found in the docket for this action.
4. Attainment Demonstrations Required to be Submitted by an Area Reclassified to Serious.
	a. Summary of Proposal.
      The proposal discussed the attainment demonstration requirements for Moderate nonattainment areas that subsequently are reclassified to Serious nonattainment. The EPA proposed that states with Moderate nonattainment areas that get reclassified to Serious nonattainment areas must first submit a Moderate area plan and then a separate Serious area plan.
	b. Final Rule.
      The EPA is finalizing requirements for states to submit a Moderate area attainment demonstration (or impracticability demonstration) and then if reclassified to Serious nonattainment, a separate Serious area attainment demonstration. Under CAA section 189(a)(1)(B), a state with a Moderate nonattainment area is required to submit a demonstration that the area either will attain or cannot practicably attain the NAAQS by the statutory attainment date. Regardless of whether the state submits an attainment demonstration or an impracticability demonstration for a Moderate area, if such an area is reclassified to Serious prior to or after failing to attain the applicable NAAQS, the state is required under CAA section 189(b)(1)(A) to submit a new attainment demonstration as part of an area's Serious area attainment plan. The separate statutory requirements for Moderate and Serious nonattainment areas anticipate two separate attainment plan submissions, and the EPA's existing guidance in the General Preamble and Addendum further support this expectation. While the state is required to submit a separate Serious area attainment plan, the EPA anticipates that certain control strategies may build upon those previously adopted and implemented as part of the Moderate area plan. For example, an area dominated by woodsmoke emissions may not attain the standard by the statutory Moderate area attainment date because all necessary woodstove change-outs could not occur in that timeframe, but additional woodstove change-outs could occur by the statutory Serious area attainment date.
	c. Comments and Responses.
	Comment: Some commenters agreed with the EPA that areas seeking to be reclassified from moderate to serious must submit two separate attainment plan submissions. The commenter stated the Act promises that all areas, even the most polluted, will implement reasonably available controls and provide at least some interim health protections while preparing a serious area plan containing more protective requirements.
      Response: The EPA agrees with the comment. In the final rule, an area that is reclassified to Serious must submit both Moderate and Serious area plans, and all statutory requirements for a Moderate area (including RACT and RACM) must be met by the statutory deadline.
5. Future Year(s) to be Modeled in Attainment Demonstrations.
      a. Summary of proposal.
      A state performing a modeling analysis for an attainment demonstration or a Serious area impracticability analysis must select a future year for the analysis. The EPA proposed that for an attainment demonstration, a state should select the future modeling year such that all emissions control measures relied on for attainment will have been implemented by the beginning of that year. The EPA recommended the last year of the statutory attainment date as a starting point for Serious nonattainment area modeling demonstrations.

      b. Final Rule.
      The EPA is finalizing a requirement that all emissions control measures relied on for attainment must have been implemented by the beginning of the attainment year. See 40 CFR 51.1011(b)(6). To demonstrate attainment, the modeling results for the nonattainment area must predict that emissions reductions implemented by the beginning of the last calendar year preceding the attainment date will result in PM2.5 concentrations that meet the level of the standard.
	While states should choose the future modeling year based on a number of factors, the EPA recommends the last year of the statutory attainment date as a starting point for modeling for two reasons. First, a state with a Serious area for which it submits an attainment date extension request under CAA section 188(e) must show that the area cannot practicably attain the NAAQS by the end of the tenth calendar year following designation of the area. Therefore, the appropriate future modeling year for making such a demonstration is the tenth year after designations. Even if a state does not submit (or does not intend to submit) a Serious area attainment date extension request, modeling the tenth year is a logical starting point to determine if attainment by year ten is likely. If attainment-level concentrations of PM2.5 are not expected in the tenth calendar year after designations, then the area must also, as a requirement to receive an extension of the Serious area attainment date, submit a demonstration (using air quality modeling) that provides for attainment by the most expeditious alternative date practicable, but no later than the end of the fifteenth year after designation, with the implementation of MSM (see Section VI.J of this preamble for details about MSM determinations).
	Second, even though attainment of any PM2.5 NAAQS is determined by averaging 3 years of ambient data, states do not have to model 2 years before the attainment date to show modeled attainment. Since the design value is an average of the annual or 98[th] percentile value for three consecutive years, attainment can still be shown even if concentrations exceed the NAAQS in one or more of the 3 years used to determine attainment (as long as the average of the three annual values is less than the NAAQS). Therefore, it is appropriate to model any of the 3 years used to determine attainment. For these reasons, it is acceptable, and may in fact be most efficient, for a state to begin the Serious area attainment demonstration process by modeling the final year of the statutory attainment date to determine future year modeled PM2.5 concentrations in the tenth year after designations.
	Because an area must attain "as expeditiously as practicable," additional considerations are necessary before an attainment date can be established. Criteria for establishment of the Serious area attainment date are discussed in Section VI.I of this preamble. In evaluating such considerations, the question arises as to whether additional future modeling is required beyond the recommended final year modeling just discussed. For purposes of determining the attainment date that is as expeditious as practicable, the state must conduct future year modeling that takes into account growth and known controls (including any controls that were previously determined to be RACM and RACT for the area). For example, for an area designated nonattainment for the 2012 PM2.5 NAAQS in 2015 and subsequently reclassified to Serious in 2021, a future case scenario for the year 2025 (10 years after the initial nonattainment designation) would be needed to examine whether existing federal, state, and local measures (including previously identified and implemented RACT/RACM controls for the area) plus the BACM and BACT identified by the state would result in attainment. Since the EPA is finalizing the requirement that BACM and BACT must be determined independent of the attainment demonstration for the area, the future case scenario must include BACM and BACT controls in the analysis plus any additional measures on sources inside and outside of the nonattainment area (but within the state) that the state has identified as feasible to implement by the attainment date. Note that similar to RACM and RACT, BACM and BACT controls must be implemented within 4 years after reclassification to Serious nonattainment. In order to justify an extension of the attainment date beyond the end of the tenth year after designation, the state must show that attainment by that date (including the anticipated emissions reductions from RACM and RACT and additional reasonable measures, and BACM and BACT and additional feasible measures) would be impracticable. Any proposed attainment date after the 10 year period must include modeling of BACM and BACT controls plus the most stringent measures that are included in the implementation plan of any state and can be feasibly implemented in the area. The attainment date extension beyond 10 years can be for up to 5 additional years, but the proposed attainment date must also be shown to be as expeditious as practicable. Section VI.J of this preamble provides a complete discussion of the EPA's proposed interpretation of the statutory requirements for a Serious area attainment date extension under CAA section 188(e).
	As with Moderate area attainment demonstrations, the EPA believes that it is not necessary or reasonable to require states to model each and every year to determine the appropriate attainment date for a Serious PM2.5 nonattainment area given the resource demands associated with modeling. In some cases it may be reasonable to model one additional interim year before the maximum statutory attainment date. However, in most cases, the air quality benefits of an identified set of reasonable control measures, BACM and BACT and additional feasible control measures can be estimated through model sensitivity analyses and the development of sensitivity factors (factors to relate tons of emissions reductions in the area to PM2.5 concentration changes in the area). For example, states can model across the board percentage reductions in direct PM2.5 and/or precursor emissions (in separate model runs or using advanced modeling techniques such as DDM) to determine the impact of emissions reductions on PM2.5 concentrations in the area. This modeling can be performed with a single attainment year modeling platform, which is much less resource intensive than modeling multiple additional future years. The EPA strongly recommends that states discuss the selection of the future year(s) to model with their respective EPA Regional Office as part of the modeling protocol development process prior to embarking on the modeling.
      c. Comments and Responses.
      Any comments received on this section are addressed in the Response to Comments document found in the docket for this action.
6. Attainment Year Motor Vehicle Emissions Budgets.
	As with Moderate areas, the transportation conformity rule requires that Serious area attainment plans establish motor vehicle emissions budgets for the area's attainment year. Therefore, once a Serious area's attainment date has been established, the state is required to establish motor vehicle emissions budgets for direct PM2.5 and any relevant PM2.5 precursor for the attainment year. If a state's SIP submission demonstrates that a Serious area cannot attain by the end of the tenth calendar year after the area's designation, motor vehicle emissions budgets are not required for that tenth calendar year, but are required for the year that the state demonstrates to be the area's attainment year. A motor vehicle emissions budget for the purposes of a Serious area PM2.5 attainment plan is that portion of the total allowable emissions within the nonattainment area allocated to on-road sources as defined in the submitted attainment plan. Such motor vehicle emissions budgets would be calculated using the latest planning assumptions and the latest approved motor vehicle emissions model available at the time that the attainment plan is developed, unless EPA approves the state's use of an alternative model.
F. RFP Requirements
1. General Approach to RFP.
      a. Summary of the Proposal.
      The EPA generally proposed that a state must submit an RFP plan as part of any attainment plan submission for a Serious nonattainment area in order to satisfy the statutory requirements for RFP, similar to a Moderate area attainment plan. The EPA proposed that the applicable baseline year must be the same year as that represented by the latest base year inventory for the Serious area. The EPA proposed that the state must include in its RFP analysis the anticipated emissions reductions expected to be achieved through the implementation of control measures required by the control strategy explained in Section VI.D of this preamble (BACM and BACT, additional feasible measures and MSM if applicable). As with RFP plans for Moderate areas, the EPA proposed that a state must submit RFP projected emissions as part of the RFP plan for any Serious PM2.5 nonattainment area following the same guidance that applies to emissions inventories for attainment plans (see Section VI.B of this preamble for a complete discussion of emissions inventories for Serious area attainment plans). The EPA also proposed that motor vehicle emissions budgets must also be established for direct PM2.5 and any PM2.5 plan precursor using the latest planning assumptions and the latest approved motor vehicle emissions model available at the time that the Serious area attainment plan is developed. It was not necessary to propose that RFP plans for Serious areas include motor vehicle emissions budgets for direct PM2.5 and any PM2.5 plan precursor because, as stated in the section of this rule that addresses RFP requirements for Moderate PM2.5 areas, the transportation conformity rule already requires that RFP plans establish motor vehicle emissions budgets. RFP plans would therefore be required to establish motor vehicle emissions budgets for direct PM2.5 and any relevant PM2.5 precursor. The EPA also proposed that guidance found in the Moderate nonattainment areas RFP section of the proposal should also apply to Serious nonattainment areas.

      b. Final Rule.
      The EPA is finalizing rule provisions for Serious areas that essentially mirror the approach to Moderate areas found in Section IV.F of this preamble. The EPA is further clarifying application of those provisions by providing guidance that closely follows the Moderate area guidance regarding how to prepare an RFP plan, RFP projected emissions, geographic coverage of emission sources for RFP, and RFP requirements for multi-state nonattainment areas.
      As with a Moderate area attainment plan, the EPA is finalizing that a state must submit an RFP plan as part of any Serious area attainment plan in order to satisfy the statutory requirements for RFP. The plan must contain appropriate information to demonstrate that adequate emissions reductions will be achieved through control measures in the attainment plan in order to meet the statutory definition of RFP. The plan must include three components: (1) an implementation schedule for control measures on sources in the nonattainment area, (2) RFP projected emissions for each applicable quantitative milestone year determined in Section VI.G of this preamble, based on the anticipated control measure implementation schedule; and (3) an analysis that demonstrates that this schedule of aggregate emissions reductions achieves sufficient progress toward attainment between the applicable baseline year to the attainment year. For additional discussion of each of the components of the RFP plan, refer to Section IV.F of this preamble. See 40 CFR 51.1012(a).
      The EPA requires that the applicable baseline year must be the same year as that represented by the latest base year inventory for the Serious area. The projected attainment year may be up to the end of the tenth year following designation for a Serious area that can demonstrate attainment pursuant to CAA section 189(b)(1)(A), or up to the end of the fifteenth year following designation for a Serious area that sought an extension of the statutory attainment date pursuant to CAA section 188(e). As with Moderate areas, the RFP analysis must clearly convey how the schedule for implementing the control strategy will provide for generally linear or step-wise progress towards attainment. If step-wise progress is more appropriate for the specific nonattainment area, the state is required to submit a clear rationale and supporting information to explain why generally linear progress towards attainment in the area is not appropriate (e.g., due to the nature of the nonattainment problem, the types of sources contributing to PM2.5 levels in the area, and the ability to perform timely implementation of control measures). For a Serious area, the EPA requires that the state must include in its RFP analysis the anticipated emissions reductions expected to be achieved through the implementation of control measures required by the control strategy described in Section VI.D of this preamble (BACM and BACT, additional feasible measures and MSM, if applicable). Similar to Moderate areas, the optional air quality analysis discussed in Section IV.F of this preamble is also available for use by a state preparing a Serious area RFP plan.
      Additionally, the EPA requires that motor vehicle emissions budgets must also be established for direct PM2.5 and PM2.5 plan precursors using the latest planning assumptions and the latest approved motor vehicle emissions model available at the time that the Serious area attainment plan is developed. See 40 CFR 51.1012(a).
      Finally, similar to Moderate areas, Serious areas that are multi-state or multi-jurisdictional shall provide RFP plans for each state represented in the nonattainment area that demonstrate RFP on the basis of common multi-state inventories. The states or jurisdictions within which the area is located must provide a coordinated RFP plan. For further information, see Section IV.F.5 of this preamble. See 40 CFR 51.1012(b).
      c. Comments and Responses.
      Any additional comments received related to RFP are addressed Section IV.F of this preamble or in the Response to Comments document found in the docket for this action.
G. Quantitative Milestones
1. Summary of the Proposal.
      The EPA proposed that a Serious area plan for an area that can demonstrate attainment by the statutory Serious area attainment date must also include quantitative milestones to be reached 7.5 and 10.5 years from designation, to help assess the state's progress toward attaining the PM2.5 NAAQS in the event the area fails to attain by the applicable attainment date. For a Serious area that cannot demonstrate attainment by the statutory Serious area attainment date, the EPA proposed that the state must include in the Serious area attainment plan quantitative milestones to be achieved at 7.5, 10.5 and 13.5 years from the area's date of designation.
      The EPA proposed that the general approach to selecting quantitative milestones outlined in the Moderate nonattainment area section of the proposal should apply to any attainment plan for a PM2.5 nonattainment area, independent of its classification. Specifically, the EPA proposed that states be allowed to select the quantitative milestones that they identify as appropriate and quantifiable and that will provide for objective evaluation of progress toward attainment in their Serious PM2.5 nonattainment area, and that the EPA, in its attainment plan approval process, will determine if they satisfy the statutory requirements of CAA section 189(c). Additionally, the EPA proposed to require that, at a minimum, states must include in all attainment plans for Serious PM2.5 nonattainment areas a measure to confirm that some specific portion of BACM and BACT for the area has been implemented as appropriate in order to comply with the statutory requirement at CAA section 189(b)(1)(B).
2. Final Rule.
      The final rule provisions for Serious area quantitative milestones are similar to such provisions for Moderate areas discussed in Section IV.G of this preamble. As required for Moderate areas, Serious area attainment plans must include quantitative milestones that demonstrate RFP towards attainment to be achieved every 3 years until the area is redesignated to attainment. To account for variations in the timing of possible additional plans that may be required beyond the Serious area attainment plan (such as a plan revision under CAA section 189(d) for a Serious area that fails to attain) the EPA is also clarifying, consistent with the requirements discussed in Section IV.G of this preamble for Moderate areas, that all Serious area attainment plans must contain one additional quantitative milestone to be met in the 3-year period beyond the applicable Serious area attainment date. This will provide the EPA with appropriate tools necessary to continue to monitor the area's continued progress toward attainment in the event that the area fails to attain and develops a new attainment plan.
      For an area that is discretionarily reclassified to Serious under the provisions of CAA section 188(b)(1), the Serious area plan must contain quantitative milestones to be achieved by 7.5 years from the area's date of designation as nonattainment. In this case, the 7.5 year quantitative milestone that was submitted with the Moderate area plan may still be sufficient to demonstrate RFP or may have to be adjusted to reflect the difference in actual progress from the projections of the Moderate area plan. For an area that is reclassified to Serious under CAA section 188(b)(2) due to failure to attain, the 7.5 year quantitative milestones that were submitted with the Moderate area plan are still required and would be sufficient for the EPA to evaluate the area's progress toward attaining the NAAQS while the Serious area plan is being developed. All Serious area plans must also include quantitative milestones to be achieved 10.5 years from designation, to help assess the state's progress toward attaining the PM2.5 NAAQS in the event the area fails to attain by the applicable attainment date. Finally, for a Serious area that cannot demonstrate attainment by the statutory Serious area attainment date, the state must include quantitative milestones to be achieved every 3 years, such that the final milestone falls within the 3 years after the applicable Serious area attainment date. For example, if a state requests an attainment date extension to 14 years after designation pursuant to CAA section 188(e), the attainment plan should contain not only the 7.5 and 10.5 year milestones, but also milestones to be achieved 13.5 and 16.5 years from designation.
	The Addendum included guidance that recommended milestones "should be addressed by quantifying and comparing the annual incremental emission reductions which result from implementation of BACM and BACT (required within 4 years after the area is reclassified as serious) and from additional measures included in the final serious area SIP to those reductions which were identified in the SIP as quantitative milestones necessary to achieve the NAAQS by the applicable attainment date." The final rule does not specify that the milestones must be expressed in terms of emission reductions. While the EPA notes that the Addendum contains this fundamental concept, it is impractical to expect that a state will always be able to quantify and compare real and projected emission reductions, and submit a report to the EPA within 90 days of a given milestone, as required under CAA section 189(c)(2). Therefore, the final rule requires that states selecting quantitative milestones for a Serious area plan should use the approach outlined for Moderate areas, as described in Section IV.G of this preamble. This approach applies to any attainment plan for a PM2.5 nonattainment area, independent of its classification. Specifically, the final rule requires that states be allowed to select the quantitative milestones that they identify as appropriate and quantifiable and that will provide for objective evaluation of progress toward attainment in their Serious PM2.5 nonattainment area, and that the EPA, in its attainment plan approval process, will determine if they satisfy the statutory requirements of CAA section 189(c). See 40 CFR 51.1013(a)(2).
	In addition to this general approach for selecting quantitative milestones and similar to what the final rule requires for Moderate area attainment plans, the final rule requires that, at a minimum, states must ensure that the quantitative milestones for Serious PM2.5 nonattainment areas assure RFP is being met by demonstrating that BACM and BACT have been implemented, as appropriate considering the timing of the milestone report, in order to comply with the statutory requirement at CAA section 189(b)(1)(B). The agency is further finalizing a corresponding requirement that states must ensure that quantitative milestones for Serious PM2.5 nonattainment areas that receive an attainment date extension. For these areas, the quantitative milestone should assure that RFP is being met by demonstrating that MSM for the area has been implemented as required pursuant to CAA section 188(e). This requirement is not specified by the statute, nor was it specifically outlined in the proposal. However, while considering the requirements that were proposed for Serious areas, the EPA determined that including this additional provision within quantitative milestones would enable the agency to better evaluate progress toward attainment in areas that receive a Serious area extension. The EPA acknowledges that the precise quantifiable metric for a quantitative milestone (e.g., 50 percent of BACM and BACT measures implemented by milestone date 7.5 years from designation) would need to be determined on a case-by-case basis, as it would depend upon the date of reclassification of the area, which quantitative milestone (i.e., 7.5 or 10.5 years from designation), and the anticipated implementation timing and nature of the BACM and BACT controls themselves. Nonetheless, the EPA believes it is appropriate to include confirmation that such control measures and technologies are implemented as a metric that any state with a Serious nonattainment area must adopt as a quantitative milestone to demonstrate RFP (and thus must demonstrate compliance with when they submit their milestone report), as it derives from a statutory provision that applies to all Serious areas and thus represents a milestone that all Serious nonattainment areas must meet.
	Additional provisions discussed in the Moderate area quantitative milestones requirements in Section IV.G of the preamble also apply to Serious areas. Specifically, if a Serious area submitted the optional air quality targets with the RFP plan then an air quality based milestone (i.e., one that is expressed in terms of an ambient PM2.5 level) is strongly recommended to be included in order to confirm that the air quality target has been met for the quantitative milestone year. If used, this milestone will be compared to the most recently certified monitored ambient air data as part of the milestone report due after the area reaches each quantitative milestone date. For additional details on this optional provision, refer to Section IV.G of this preamble.
      Finally, the quantitative milestone report requirements outlined in Section IV.G of this preamble apply to Serious areas as well. Specifically, the requirements associated with the timing and contents of the quantitative milestone report submission for a Moderate area also requirements in a Serious area. For additional details on these requirements, refer to Section IV.G of this preamble. See 40 CFR 51.1013(b).
3. Comments and Responses.
      Any additional comments received on this section are addressed in Section IV.G of this preamble or in the Response to Comments document found in the docket for this action.
H. Contingency Measures
1. Summary of the Proposal.
      In the proposal, the EPA proposed that the criteria for identifying and selecting contingency measures for a Serious area attainment plan should be the same as those for Moderate area plans. The EPA also proposed that, as with Moderate areas, a state may elect to rely on contingency measures that achieve emissions reductions not only from sources within the nonattainment area, but also from sources located outside the nonattainment area but within the state, provided that the measures on sources outside the designated nonattainment area are demonstrated to produce the appropriate air quality impact within the nonattainment area. As with contingency measures for Moderate area attainment plans, the EPA proposed that the emissions reductions associated with contingency measures for Serious area plans must be equal to approximately 1 year's worth of emissions reductions necessary to achieve RFP for the area, unless the state adequately demonstrates that some smaller amount of reductions is appropriate while the state is revising its attainment plan for the area. The agency also proposed options for submission deadlines for Serious area contingency measures.

2. Final Rule.
   As noted in Section IV.G of this preamble, all PM2.5 nonattainment areas must include in their attainment plans contingency measures consistent with CAA section 172(c)(9). Contingency measures are additional control measures to be implemented in the event that an area fails to meet RFP requirements or fails to attain the PM2.5 standard by the applicable attainment date. These measures must be fully adopted rules or control measures that are ready to be implemented quickly upon a determination by the EPA that the area failed to meet RFP or failed to meet the standard by the applicable attainment date, and such measures are required to take effect without significant further action by the state or the EPA.
	The statutory contingency measure requirement at CAA section 172(c)(9) is not superseded or subsumed by any requirement under subpart 4, nor does it apply only to Moderate area attainment plans. Thus, contingency measures are required for Serious PM2.5 nonattainment areas as part of a state's Serious area attainment plan submission. Accordingly, the final rule requires the criteria for identifying and selecting contingency measures for a Serious area attainment plan that are the same as those for Moderate area plans. Specifically, the EPA is finalizing that the following requirements must be met in order for contingency measures to be approvable as part of a state's Serious area attainment plan submission:
 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 meet the standard by its attainment date.
 The SIP must contain trigger mechanisms for the contingency measures, specify a schedule for implementation, and indicate that the measures will be implemented without significant further action by the state or by the EPA.
 Contingency measures should consist of control measures that are not otherwise included in the control strategy for the SIP, or that achieve emission reductions not otherwise relied upon in the control strategy for the area.
 Contingency measures should provide for emissions reductions equivalent to 1 year's share of reductions needed to demonstrate attainment (i.e., the overall needed reductions divided by the number of years from the base year to the attainment year), or 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.
	The EPA is also finalizing its proposal to allow a state to rely on contingency measures that achieve emissions reductions on sources located outside the nonattainment area, but within the state provided that the measures on sources outside the designated nonattainment area are demonstrated to produce the appropriate air quality impact within the nonattainment area.
	As with contingency measures for Moderate nonattainment areas, the EPA allows a state under these circumstances to rely on additional reductions in the years following a failure to meet RFP requirements or a failure to attain the NAAQS by the applicable attainment date from federal or local measures already scheduled for implementation as part or all of their contingency measures. The EPA could consider such measures as meeting the contingency measure requirement as long as they produce emissions reductions in excess of those required to meet other statutory nonattainment provisions (such as to meet BACM/BACT requirements) and they can be relied on to achieve a sufficient portion of the actual emissions reductions necessary to reduce emissions in the area while the state develops a new plan to bring the area into attainment. As with contingency measures for Moderate area attainment plans, the EPA requires that the emissions reductions associated with contingency measures for Serious area plans should be equal to approximately 1 year's worth of emissions reductions necessary to achieve RFP for the area, unless the state adequately demonstrates that some smaller amount of reductions is appropriate while the state is revising its attainment plan for the area. See 40 CFR 51.1014(b)(2).
	The Addendum provided guidance related specifically to the selection and implementation of contingency measures for Serious nonattainment areas. First, the guidance indicated that "for those moderate areas reclassified as serious, if all or part of the moderate area plan contingency measures become part of the required serious area control measures (i.e., BACM), then additional contingency measures must be submitted whether or not the previously submitted contingency measures had already been implemented. Further, the affected states must ensure that serious areas have adequate contingency measures considering, among other things, new information about the potential attainment shortfall for the newly reclassified serious area." The EPA continues to believe that this approach to the statutory contingency measure requirement is appropriate and is finalizing it for purposes of implementing the PM2.5 NAAQS in Serious nonattainment areas. See 40 CFR 51.1014.
	With regard to the timing for implementing contingency measures, the EPA reiterates that the purpose of contingency measures is to ensure that corrective measures are put in place automatically at the time that the EPA makes a determination that an area has failed to meet RFP or failed to meet the NAAQS by the applicable attainment date. For any nonattainment area, the EPA is required to determine within 90 days after receiving a state's RFP demonstration, and within 6 months after the attainment date for an area, whether the state has met their statutory obligations for demonstrating RFP or attaining the standard, as appropriate. As with Moderate areas, the EPA expects that contingency measures should become effective for Serious areas within 60 days of the EPA making its determination that the area failed to meet RFP or attain the NAAQS.
3. Comments and Responses.
      Comment: One commenter supported the proposal that contingency measures may be approved if they will result in the equivalent air quality improvement as would be obtained by implementing measures obtaining 1 year's worth of emissions reductions needed to demonstrate attainment.
	Response: In the case where a state selected the optional RFP analysis that includes air quality targets, the EPA expects that an area contingency measures may be approved if they will result in approximately 1 year's worth of air quality improvement.
I. Attainment Dates
1. Summary of Proposal.
      Section 188(c) of the CAA states that the attainment date for a Serious area is to be the end of the tenth calendar year after designation. The EPA proposed to interpret the reference to "designation" in section 188(c) as meaning the "effective date of designation."
2. Final Rule.
      As explained earlier, section 188 establishes the attainment dates for both Moderate and Serious areas. For a Serious area, CAA section 188(c)(2) provides that "the attainment date shall be as expeditiously as practicable but no later than the end of the tenth calendar year beginning after the area's designation as nonattainment."  For example, for an area initially designated as a Moderate nonattainment area effective in April 2015 that is reclassified to Serious at some future date, the Serious area attainment date, absent any approved Serious area attainment date extension, would be no later than December 31, 2025 (the end of the tenth calendar year after designation). As discussed in Section IV.I of this preamble, the EPA interprets the references to "designation" in CAA section 188(c) as meaning "effective date of designation," consistent with the agency's prior approach for implementing the previous PM2.5 NAAQS under subpart 1 and other NAAQS.
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.
J. Attainment Date Extensions
	Background. Section 188(e) of the CAA provides that the EPA may grant a Serious area one attainment date extension of no more than 5 years "upon application by any state . . . if attainment by the [original Serious area attainment date] would be impracticable, the state has complied with all requirements and commitments pertaining to that area in the implementation plan, and the state demonstrates to the satisfaction of the Administrator that the plan for that area includes the most stringent measures that are included in the implementation plan of any state or are achieved in practice in any state, and can feasibly be implemented in the area."
	The statute also includes factors that the EPA may consider in determining whether to grant the extension and the length of the extension, including "the nature and extent of nonattainment, the types and numbers of sources or other emitting activities in the area (including the influence of uncontrollable natural sources and transboundary emissions from foreign countries), the population exposed to concentrations in excess of the standard, the presence and concentrations of potentially toxic substances in the mix of particulate emissions in the area, and the technological and economic feasibility of various control measures."
	The proposal described the four main elements the state must submit when requesting a Serious area attainment date extension: 1) a demonstration that attainment by the statutory Serious area attainment date is impracticable; 2) a demonstration that the area is complying with all requirements and commitments in the applicable attainment plan; 3) a demonstration that the plan includes the MSM that are included in the implementation plan of any state, or are achieved in practice in any state; and 4) a demonstration of attainment by the most expeditious alternative date practicable. The proposal also included a discussion about the timing of extension request submissions, and how to interpret the second element in cases where the extension request is submitted after the state has already submitted an initial Serious area attainment plan. These topics are addressed in the following sections.

	1. Demonstration That Attainment by the Statutory Serious Area Attainment Date is Impracticable.
	a. Summary of Proposal.
      The proposed rule discussed the requirements for a demonstration to show that it is impracticable for a Serious area to attain by the attainment date. This demonstration involves evaluating through air quality modeling whether all best available control measures will enable the area to attain the standard by the attainment date.
	b. Final Rule.
      This section remains relatively unchanged from the proposal. In order to demonstrate that it is impracticable for an area to attain by the attainment date, the state would have to show that the implementation of all BACM/BACT (and additional feasible measures) will not bring the area into attainment by the statutory Serious area attainment date (i.e., by no later than the end of the tenth calendar year after designation). The statutory provision for demonstrating that it is impracticable to attain by the Serious area attainment date requires that the demonstration be based on air quality modeling (see CAA section 189(b)(1)(A)). Additional guidance on this demonstration is provided in Section VI.E of this preamble.
	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. Demonstration That the Area is Complying With all Requirements and Commitments in the Applicable Implementation Plan.
      a. Extension Request Submitted at the Same Time as the Serious Area Attainment Plan.
      i. Summary of Proposal.
      The EPA proposed to interpret the criterion under CAA section 188(e) that requires a state to have "complied with all requirements and commitments pertaining to that area in the implementation plan" simply to mean that the state has implemented the control measures in the SIP revisions it has submitted to address the applicable requirements in CAA sections 172 and 189. For a Serious area attainment date extension request being submitted contemporaneously with the "original" Serious area attainment plan for the area, the EPA proposed to read CAA section 188(e) not to require the area to have a fully approved attainment plan that meets the CAA's requirements for Moderate areas. The EPA also proposed 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 as a FIP, provided the area has complied with all of the requirements in the applicable FIP, or in the applicable SIP and FIP.
      ii. Final Rule.
      Some commenters stated that an area should only be able to receive an extension if the Moderate area plan had been fully approved by the EPA. Other commenters agreed with the EPA's proposed approach. They suggested that if a part of the Moderate plan had been disapproved, but it was clear that the area could not practicably attain by the Serious area attainment date, then the area should be able to receive an extension. Other commenters suggested that an area should not be deprived an extension if the approval of all or part of the Moderate area attainment plan is delayed due to logistical reasons or the EPA's inability to take final action in a timely manner.
      The final rule does not require the area to have a fully approved Moderate area plan when the attainment date extension request is submitted at the same time as the Serious area plan. An extension is allowed if the area is complying with all Moderate area requirements and commitments pertaining to that area in the state's submitted Moderate area implementation plan, but the plan does not need to be fully approved by EPA. The EPA considers this to be a reasonable interpretation of the statute because, as noted by commenters, there may be various reasons why an area may not have a fully approved Moderate area SIP by the time an extension request may be granted.
	iii. 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. Extension Request Submitted After Submission of an "Original" Serious Area Attainment Plan.
      i. Summary of Proposal.
      For a Serious area extension request that was submitted after submission of an "original" Serious area attainment plan that contained an attainment demonstration meeting the requirements of CAA section 189(b)(1)(A)(i), the EPA proposed to read CAA section 188(e) not to require the area to have a fully approved attainment plan that meets the CAA's requirements for Serious areas, but to have a fully approved Moderate area attainment plan. The EPA stated that this proposed interpretation of this criterion would apply whether the area was reclassified to Serious under the EPA's discretionary authority (CAA section 188(b)(1)) or by operation of law upon failing to attain by the Moderate area attainment date (CAA section 188(b)(2)).
	The proposal also requested comment on an "alternative interpretation" that, as pointed out by Some commenters, appears to also have mistakenly required the same thing as the first option: that the state would need to have a fully approved Moderate area attainment plan in order to receive an extension.
	The EPA notes that Section VI.C of this preamble, Timing of Extension Request Submission, also discusses this issue. It requested comment on whether, for areas that had already submitted Serious area attainment plans, it would be appropriate that the state must have complied with all requirements and commitments in the area's initial Serious area plan (the EPA's preferred option), or in the Moderate area plan.
	ii. Final Rule.
      After considering the comments received on this issue, the EPA is finalizing an approach that requires that, where a Serious area attainment date extension is being submitted after the initial Serious area attainment plan has been submitted, the state would need to demonstrate that it was complying with all Serious area requirements and commitments pertaining to the area in the plan it had initially submitted. However, it would not need a fully approved Serious area attainment plan. The EPA believes the state should not be prevented from obtaining an attainment date extension in the event the EPA is unable to take final action on a submitted plan in a timely manner. The original proposal did not specify Serious area provisions implementing this approach, but commenters noted the proposed analogous provisions for Moderate areas seeking 1-year extensions, and suggested that EPA should adopt a similar approach for Serious areas. Under this approach, the state would not need a fully approved Serious area plan; it would be able to receive an extension if it had already submitted the Serious area plan but had not received EPA approval yet, and if it was complying with all Serious area requirements and commitments pertaining to the area in the state's implementation plan. The EPA also considered an alternative option wherein the state would be able to receive an extension only if it had a fully approved Serious area attainment plan. The commenters did not favor this option, nor does the EPA.
	iii. Comments and Responses.
      Any additional comments received on this section are addressed in the Response to Comments document found in the docket for this action.
3. Demonstration That the Plan Includes the MSM That are Included in the Implementation Plan of Any State, or are Achieved in Practice in Any State.
	To qualify for any extension of a Serious area attainment date, CAA section 188(e) requires a state to "demonstrate to the satisfaction of the Administrator that the plan for the area includes the most stringent measures that are included in the implementation plan of any state, or are achieved in practice in any state, and can feasibly be implemented in the area." In its prior guidance in the Addendum, the EPA interpreted the term ``most stringent measures'' (MSM) to mean the maximum degree of emission reduction that has been required or achieved from a source or source category in any other attainment plans or in practice in any other states and that can feasibly be implemented in the area seeking the extension, such as what LAER represents for new or modified sources under the NNSR permit program.
	a. Summary of Proposal.
      The proposal suggested that a state would need to follow a process for determining MSM for a Serious nonattainment area that is generally similar to proposed Option 2 for BACM/BACT described in Section VI.D of this preamble, which would include exemptions from MSM for sources in de minimis source categories if such measures did not collectively advance the attainment date for the area by at least 1 year. The EPA also proposed an alternative approach for determining MSM for a Serious nonattainment area that would provide for de minimis source category exemptions for MSM only for those source categories that do not contribute significantly to ambient PM2.5 concentrations in the Serious nonattainment area, an approach more closely aligned with proposed Option 1 for determining BACM/BACT.
	For each approach, the proposal described a five step process for determining MSM: 1) update the emissions inventories for the nonattainment area; 2) identify de minimis source categories through modeling; 3) identify potential MSM; 4) compare MSM to control measures already adopted in the SIP for the nonattainment area; and 5) adopt and implement any MSM that are more stringent than any measures that are already approved into the SIP. The proposal requested comment on whether the two proposed approaches are sufficiently consistent with the agency's respective proposed approaches to BACM/BACT determinations.
	b. Final Rule.
      Almost all comments received on this section involved the issue of whether the rule should allow for de minimis source categories to be exempted in the process of determining MSM. A few commenters supported the identification of de minimis source categories and their exemption from the MSM requirement. These commenters were split in terms of their preference for the two de minimis approaches that were presented. Some commenters suggested that under any approach, an area could still exclude a measure from MSM based on the inability to feasibly implement the measure in the area. Some commenters stated that it would be too burdensome to require a state to evaluate whether a particular source category had a "significant" or de minimis impact on air quality, while others supported the approach. One group of commenters disagreed with the notion that a Serious area could exempt de minimis sources from the MSM requirement in the first place. They stated that de minimis exemptions would not be appropriate for MSM, for which the CAA has expansive language requiring the most stringent measures required in any SIP or achieved in practice in any state.
	After considering the comments received on the de minimis source category issue, the EPA is adopting a final rule that does not include an explicit de minimis source category exemption in determining MSM. The agency's reasons for not allowing a de minimis source category or de minimis impact concept, articulated in prior sections on determining RACM/RACT (Section IV.D) and BACM/BACT (Section VI.D), apply equally here. Moreover, the EPA believes it would be particularly inappropriate to allow for a de minimis source category approach for MSM. The statute requires MSM to be implemented because the area is unable to attain the standard within 10 years of designation and has a more severe air quality problem. Congress clearly intended for such areas to more widely explore potential control measure possibilities, and a de minimis source category exclusion would be contrary to that intent.
      The EPA believes the rule provides sufficient flexibility in the MSM area control measure analysis and attainment demonstration process enabling states to identify sources that should not be subject to control measures, including the ability to develop precursor demonstrations to exclude precursors from control requirements, and to consider case-specific factors in determining technological 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 MSM process, the EPA also believes that 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 stringent control measures necessary to attain the standard. As noted in Section IV.D of this preamble on Moderate areas, and again in [serious area section] 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 all of these reasons, a de minimis source category concept is not included in the final rule for MSM.
	Process for determining MSM. The following sections describe the process for determining MSM that is finalized in this rule: a) update emissions inventories; b) identify potential MSM; c) compare MSM to control measures already adopted in the SIP for the nonattainment area; and d) adopt and implement any MSM that are more stringent than any measures that are already approved into the SIP. (See 40 CFR 51.1010(b)(1)-(4).)
	i. Update Emissions Inventories.
      The first step would be for the state to update as needed the emissions inventory of direct PM2.5 and PM2.5 precursor sources and source categories in the Serious nonattainment area required under CAA section 172(c)(3) for any attainment plan submission. The EPA expects that the state would meet this inventory requirement as part of its Serious area attainment plan submission without any additional work if the state submits the Serious area attainment date extension request simultaneously with the plan itself. However, in the event the attainment date extension request is submitted after the "original" Serious area attainment plan for the area (i.e., toward the end of the Serious area attainment period), then the state must submit a more recent, complete and accurate emissions inventory that meets the same emissions inventory requirements for Moderate and Serious PM2.5 nonattainment areas pursuant to CAA section 172(c)(3), as well as an attainment projected inventory as part of the new Serious area attainment plan for the area. The inventories submitted to support a Serious area attainment plan must also include point sources meeting the lower major stationary source threshold in 40 CFR part 51, subpart A.
	ii. Identify Potential MSM.
      The second step in determining MSM involves identifying the potentially MSM in other state implementation plans for PM2.5 or other NAAQS, or that are used in practice in other states for controlling emissions from sources similar to those listed in the emissions inventory. This information can be obtained from a number of sources, including state regulations on the books, state summaries of control measures, state permitting databases, the RACT/BACT/LAER Clearinghouse, and control measure compilations developed by regional or state/local organizations. Elsewhere in this preamble, the EPA recommends that a state identify potential measures for consideration as RACM/RACT or BACM/BACT by evaluating control measures implemented by other states to meet PM2.5 NAAQS or other NAAQS. Thus, a state seeking to identify MSM should be able to start its process using the work already undertaken for the nonattainment area's RACM and BACM determinations and to make updates to the list of potential control measures accordingly.
	For each measure, the state is required to determine its technological and economic feasibility for sources in the area. States should apply more stringent criteria for determining the feasibility of potential MSM than that described for BACM and BACT in Section VI.D of this preamble. In some situations, MSM could involve increasing the coverage of measures that were already adopted and implemented as BACM and BACT (for example, changing out an even greater percentage of woodstoves in an area, if such sources were major contributors to the air quality problem in the nonattainment area).
	However, because BACM and BACT represent the "best" level of control feasible for an area, in some cases it may be possible for the MSM requirement to result in no more controls and no more emissions reductions in an area than result from the implementation of BACM and BACT. Stated another way, there may be sources or categories for which no other feasible controls exist beyond what a state has already adopted as BACM or BACT. Given the strategy in the nonattainment provisions of the CAA to offset longer attainment timeframes with more stringent control requirements, the EPA therefore interprets the MSM provision so as to increase the potential that it will result in additional controls beyond the set of measures adopted as BACM and BACT. In the MSM analysis, in addition to identifying additional candidate MSM, the state is required to reanalyze any measures that were rejected during the state's BACM and BACT analysis for the area to see if they are now feasible for the area given the potentially longer attainment date (up to 5 years after the statutory Serious area attainment date), or given the changes that have occurred in the interim that improve the feasibility of previously rejected measures.
	iii. Compare MSM to Control Measures Already Adopted in the SIP for the Nonattainment Area.
      The third step requires the state to compare the potential MSM that have been identified for each source type or source category against the measures, if any, already adopted into the Serious area SIP for that source category to determine if such MSM would provide any additional reductions. This comparison will be used in determining what measures to adopt in the next step.
	iv. Adopt and Implement Any MSM That are More Stringent Than Any Measures That are Already Approved Into the SIP.
      The fourth step requires the adoption of any MSM that are more stringent than existing measures as a regulation, and requires submission of the regulation as part of the SIP, as well as expeditious implementation of the regulation. For any measures that the state determines cannot be feasibly implemented in the area, it should provide a reasoned justification for rejecting the potential MSM.
	The EPA notes that CAA section 188(e) does not identify a deadline for a state to implement MSM, whereas elsewhere the statute establishes a deadline for implementing RACM and RACT and BACM and BACT [see CAA sections 189(a)(1)(C) and 189(b)(1)(A)], respectively). However, because the clear intent of CAA section 188(e) is to minimize the length of a Serious area attainment date extension, the EPA requires that the implementation of MSM must be as expeditious as practicable but no later than 1 year prior to the alternate Serious area attainment date identified by the state in its extension request.
	c. Comments and Responses.
      Any additional comments received on this section are addressed in the Response to Comments document found in the docket for this action.
4. Demonstration of Attainment by the Most Expeditious Alternative Date Practicable.
      Section 189(b)(1)(A) of the CAA requires that a Serious area plan demonstrate attainment, using air quality modeling, by the most expeditious date practicable after the statutory Serious area attainment date. This demonstration is the final criterion that must be met before the EPA may consider granting an extension. The agency's determination of whether the plan provides for attainment by the most expeditious date practicable would depend on whether the plan provides for implementation of BACM and BACT by the statutory implementation deadline and MSM as expeditiously as practicable. In no case would a state be able to seek an extension of a Serious area attainment date to a date more than 5 years past the statutory attainment date for Serious areas. Section VI.E of this preamble describes the EPA's proposed requirements for attainment demonstration modeling for Serious area attainment plans.
5. Apply for an Attainment Date Extension.
      The state would have to apply to the EPA for any extension of a Serious area attainment date. The request would have to accompany an attainment plan submission containing an attainment demonstration showing attainment by the most expeditious alternative date practicable, and the state would need to submit modeling as part of the attainment demonstration in accordance with Section VI.E of this preamble. Furthermore, the state would have to provide the public reasonable notice and a public hearing on the attainment date extension request before submitting it to the EPA, as the EPA would consider it an integral part of the attainment demonstration and part of the revised SIP submission which is subject to the requirements of the CAA and federal regulations for public notice and hearing on SIP revisions.
6. Timing of Extension Request Submission.
	The EPA has identified two potential Serious area attainment date extension scenarios: (1) the more straightforward scenario where the attainment date extension is included with the initial Serious area plan, and (2) the scenario where a state may prepare and fully implement a timely Serious area plan that includes a modeling analysis that demonstrates the area would attain no later than the statutory Serious area attainment date (the end of the tenth calendar year following designation), and yet the state may see as the attainment date nears that the Serious area will in fact fail to attain by its projected attainment date. While the statute provides a remedy to be instituted immediately upon failure of a Serious area to attain the standard (through contingency measures and other measures stipulated in CAA section 189(d)), the EPA also believes that the criteria of CAA section 188(e) could be applied after a state submits a Serious area attainment plan but prior to the area failing to attain (as long as the area had not already been granted a prior Serious area attainment date extension under CAA section 188(e)).
	In the first scenario, there is no need to specify any further timing requirements beyond those previously described for Serious area plan submission. However, for the second scenario the final rule needs to specify a due date for the request. The EPA believes that it would be acceptable for a state to submit a Serious area attainment date extension request (as described earlier) together with a new Serious area attainment plan meeting all of the statutory requirements that apply to such plans. The state should submit the extension request and new implementation plan to EPA as early as possible, but the final rule requires that it must be submitted no later than 60 days prior to the approved attainment date for the area or, in the absence of an approved attainment date, no later than 60 days prior to the applicable statutory attainment date for Serious areas (i.e., the end of the tenth year after designation). See 40 CFR 51.1005(b)(6). The EPA believes that this deadline is necessary due to its statutory obligation to determine whether the area attained by the attainment date. In order to preserve the possibility that EPA could review and take action on the new attainment plan for the area and the accompanying attainment date extension request prior to its deadline for making the attainment determination the EPA estimates that the 60-day deadline provides the minimum amount of necessary time. The EPA notes that during this time, it would have to ascertain the status of compliance with all requirements and commitments in the Moderate and initial Serious area attainment plans for the area, evaluate the state's justification for the selection of the alternate attainment date (including modeling), and review provisions for the implementation of MSM).
VII. Requirements under CAA Section 189(d) for PM2.5 Serious Areas that Fail to Attain the NAAQS by the Applicable Attainment Date
	Background. In the event that a Serious area fails to attain the PM2.5 NAAQS by the applicable attainment date, CAA section 189(d) requires that "the state in which such area is located shall, after notice and opportunity for public comment, submit within 12 months after the applicable attainment date, plan revisions which provide for attainment of the ...standard and, from the date of such submission until attainment, for an annual reduction in PM10 or PM10 precursor emissions within the area of not less than 5 percent of the amount of such emissions as reported in the most recent inventory prepared for such area."
	In addition to the requirement for the submission of control measures providing for a 5 percent reduction in emissions of certain pollutants on an annual basis, the EPA interprets CAA section 189(d) as requiring the State to submit an attainment plan that includes the same basic statutory plan elements that are required for other attainment plans. Because section 189(d) does not include a specific provision specifying a new attainment date, the EPA relies on sections 179(d)(3) and 172(a)(2) of the CAA to establish the attainment date for such plans to be as expeditiously as practicable, and no later than five years from the effective date of the EPA's determination that the area failed to attain. Pursuant to those provisions, the Administrator may also extend the attainment date to the extent the Administrator deems appropriate, for a period no greater than 10 years from the effective date of the EPA's determination that the area failed to attain, considering the severity of nonattainment and the availability and feasibility of pollution control measures. The state must submit as part of the new attainment plan a justification explaining that it represents an attainment date that is as expeditious as practicable.
	A state must submit to the EPA its plan to meet the requirements of CAA section 189(d) in the form of a complete attainment plan submission that includes the following elements:
 (i) Base year and attainment projection year emissions inventory requirements; (ii) additional attainment plan control strategy requirements, including control measures and a demonstration that each year the area will achieve at least a 5 percent reduction in emissions of direct PM2.5 or a 5 percent reduction in emissions of a PM2.5 plan precursor based on the most recent emissions inventory for the area; (iii) attainment demonstration and modeling; (iv) RFP plan and quantitative milestones; and (v) contingency measures. A state with a Serious PM2.5 nonattainment area that fails to attain the NAAQS by the applicable Serious area attainment date must also address any statutory requirements relevant to Moderate nonattainment areas and Serious nonattainment areas under CAA sections 172 and 189 of the CAA that have not already been satisfied. These elements are discussed in more detail in the following sections.
A. Plan Due Dates
1. Summary of Proposal.
      The proposed rule indicated that under CAA section 189(d), the state would be required to submit the attainment plan for a Serious area that failed to attain the NAAQS by the Serious area attainment date within 12 months after the applicable attainment date.
2. Final Rule.
      The final rule remains unchanged from the proposal. Section 189(d) of the CAA requires a state with a Serious PM10 nonattainment area that failed to attain the NAAQS by the applicable Serious area attainment date to submit a new attainment plan submission for the area within 12 months after the missed "applicable attainment date." The EPA finds that the most straightforward interpretation of the statutory language is that the state must submit a new attainment plan for the area  -  with all required elements  -  within 12 months after the missed applicable attainment date. Although the EPA may take up to 6 months to make a determination that the area failed to attain, the text of the statute ties the 12-month SIP due date to the missed attainment date, not to the date that the EPA determines that the area failed to attain. Because all attainment dates for implementation of the PM2.5 NAAQS under subpart 4 are expressed in terms of the end of a calendar year, the new due date for a SIP required under CAA section 189(d) also would be due on December 31  -  of the year following the area's Serious area attainment date. This requirement is consistent with the manner in which the CAA section 189(d) SIP submission date has been interpreted for implementation of the PM10 NAAQS in the past. The EPA recognizes that this statutory timeline is shorter than for Moderate or Serious area attainment plans, but expects that, given the prior planning history for such areas, much of the analyses to support these new attainment plan submissions will be based on updates to previous analyses, which would require less time than generating new analyses. In any event, it is clear from the face of the statute that Congress intended that states with areas that fail to attain the NAAQS by the outermost statutory attainment date for Serious areas must proceed more quickly to revise their SIPs to provide for attainment of the NAAQS.
3. Comments and Responses.
      Any 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 also proposed that the inventory requirements under section 189(d) for Serious areas that fail to attain by the attainment date should be the same as those for Moderate and Serious areas, but with a change to the appropriate year for the inventory. The EPA proposed that for these areas, the inventory year must be one of the 3 years from which monitored data was used to determine that the area failed to attain the PM2.5 NAAQS by the applicable Serious area attainment date. In addition, the EPA proposed an alternative approach that would allow the state to use an earlier year than one of the 3 years used to determine that the area failed to attain. As proposed, this alternative approach would require written justification that included an explanation of how the inventory modifications adequately incorporate emissions reductions projected to be achieved through the implementation of BACM and BACT, and additional feasible control measures submitted with the original Serious area attainment plan for the area, and through implementation of MSM if appropriate.
2. Final Rule.
      The statute requires states to use an emissions inventory that meets the requirements of section 172(c)(3). The final rule recommends using an inventory for one of the 3 years for which air quality data were used to determine that the area failed to attain in order to meet this requirement. However it also allows the state to use an earlier inventory year under certain circumstances.
      As with all other attainment plan submissions required for Moderate and Serious PM2.5 nonattainment areas, a state must develop its submission to meet CAA section 189(d) based on "the most recent inventory prepared for such [nonattainment] area." This inventory must meet the same requirements that would apply to any other emissions inventory submitted for a PM2.5 nonattainment area to meet the requirements of CAA section 172(c)(3), which requires "a comprehensive, accurate, current inventory of actual emissions of the relevant pollutants" in the nonattainment area. Therefore this rule requires that the inventory submitted with an attainment plan to meet CAA section 189(d) requirements must also meet the EPA's regulatory requirements for such emissions inventories as described earlier in this preamble under Section IV.B of this preamble (for Moderate area attainment plans) and Section VI.B of this preamble (for Serious area attainment plans).
      One important aspect of the emissions inventory required to be submitted with an attainment plan under CAA section 189(d) is its role as the basis for calculating the emissions reductions of direct PM2.5 or any PM2.5 precursor necessary to satisfy the 5 percent annual reduction criterion of CAA section 189(d). For this reason, the "most recent inventory" for the area must not only meet the criteria described for a base year inventory submitted pursuant to CAA section 172(c)(3) and in Section VI.B of this preamble, but it also must fully account for emissions reductions achieved to date through the implementation of all RACM and RACT, BACM and BACT, additional reasonable and feasible measures, and MSM (as applicable) submitted with the Moderate and original Serious area attainment plans for the area. In this way, the state will calculate the additional reductions that the nonattainment area will need beyond those already required in order to fulfill the requirements of CAA section 189(d) and bring the area into attainment as expeditiously as practicable.
	To ensure that the inventory is representative of the nonattainment problem in the area current at the time of the CAA section 189(d) submission, the EPA strongly recommends that the inventory year be one of the 3 years from which monitored air quality data were used to determine that the area failed to attain the PM2.5 NAAQS by the applicable Serious area attainment date. The EPA believes that associating the inventory with one of these 3 years is reasonable in light of the fact that some control measures (e.g., BACM and BACT controls and additional feasible measures) for sources in the area may not be implemented until the beginning of the attainment year. Thus, using an emissions inventory for one of those 3 years will help ensure that the inventory adequately captures the emissions reductions already achieved through the prior implementation of control measures for Moderate and Serious areas.
	The EPA recognizes that the timing and resource requirements for inventory preparation may make it challenging in some cases for a state to use an inventory for a year that is one of the 3 years from which monitored data were used to determine that the area failed to attain the NAAQS by the applicable attainment date. To address such cases, the final rule allows states to use an earlier inventory year in the plan, provided that (1) the year is selected in consultation with the appropriate EPA Regional Office, and (2) the state provides a written justification for selecting the earlier year in its SIP submission. See 51.1008(c)(1). At a minimum, the inventory must adequately incorporate emissions reductions projected to be achieved through the implementation of BACM and BACT, and additional feasible control measures submitted with the original Serious area attainment plan for the area, and MSM if appropriate. Because these emissions reductions may have occurred after the inventory year the state intends to use, adjustments to the original inventory for that year would need to be made to reflect those reductions. The written justification must also include an explanation of how those reductions have been incorporated into the inventory. In considering use of an "older" inventory, the EPA recommends that states weigh the possible impact of using an older inventory that could have higher emissions than a more current inventory. The state may be obligated to achieve a larger annual emissions reduction to satisfy the 5 percent annual reduction criteria of CAA section 189(d) than would otherwise be required if a newer inventory were used with lower emissions.
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.
C. Pollutants to be Addressed in the Plan
1. Summary of Proposal.
      The proposed rule indicated that to determine what pollutants need to be addressed in the attainment plan and 5 percent requirement in CAA section 189(d), the state could provide a demonstration to the EPA showing that a particular precursor does not significantly contribute to PM2.5 levels that exceed the standard. The proposal suggested that if the precursor demonstration is deemed adequate by the EPA, then the state would not be required to evaluate or adopt control measures for that precursor, nor would the state need to address the precursor in meeting the 5 percent annual emissions reduction requirement in section 189(d). The proposal indicated that Section III of the preamble further discussed options describing optional precursor demonstrations.
2. Final Rule.
      The final rule remains relatively unchanged with respect to this issue. Section 189(d) of the CAA requires states to develop a new attainment plan for an area that failed to attain by the applicable Serious area attainment date that provides for "an annual reduction in PM10 or PM10 precursor emissions within the area of not less than 5 percent of the amount of such emissions" reported in the latest emissions inventory for the area. In Section III of this preamble, the EPA describes optional approaches by which a state could demonstrate that a PM2.5 precursor does not contribute significantly to PM2.5 levels that exceed the standard in the area, and thereby would not need to evaluate potential control measures for that precursor in the area. The EPA also interprets the CAA generally to allow a state to provide such a "precursor demonstration" for the attainment plan required under section 189(d), even if the area has previously failed to attain the relevant NAAQS by the applicable Serious area attainment date. If the state has provided a demonstration with the previous Serious area attainment plan to establish that a precursor does not significantly contribute to PM2.5 levels for purposes of the attainment plan for the area, and it seeks to maintain the status of that precursor as not significantly contributing to PM2.5 levels in the area, the state would still need to provide an updated precursor demonstration for the new section 189(d) SIP because emissions and atmospheric conditions will have changed since the previous demonstration was submitted, and the conclusions from any previous precursor demonstration may no longer be appropriate. See Section III of this preamble for more information about potential precursor demonstrations that could be conducted to show that a particular precursor does not contribute significantly to PM2.5 levels that exceed the standard.
3. Comments and Responses.
      Any comments received on this section are addressed in the Response to Comments document found in the docket for this action.
D. Attainment Plan Control Strategy
1. Background.
      As for other PM2.5 NAAQS attainment demonstrations, the overarching requirement for the CAA section 189(d) control strategy is that it needs to provide for attainment of the standard as expeditiously as practicable. The strategy must include any additional measures (beyond those already adopted in previous SIPs for the area as RACM/RACT, BACM/BACT, MSM (if applicable), for example) that are needed for the area to attain expeditiously. The plan must also demonstrate that the new attainment plan will at a minimum achieve an annual 5 percent reduction in emissions of direct PM2.5 or any PM2.5 plan precursor from sources in the area, based on the most recent emissions inventory for the area. However, it is important to emphasize that a CAA section 189(d) plan must require other control measures (even if beyond those sufficient to meet the annual 5 percent reduction requirement) that are needed in order to meet the overarching goal of attaining the standard as expeditiously as practicable.
2. 5 Percent Annual Reduction in Direct PM2.5 or Any PM2.5 Plan Precursor.
	a. Summary of Proposal.
Section 189(d) of the CAA requires an "annual reduction in PM10 or PM10 precursor emissions within the area of not less than 5 percent of the amount of such emissions as reported in the most recent inventory prepared for such area." Because the statute is ambiguous with regard to how this language should apply for the PM2.5 NAAQS, the EPA proposed two options for interpreting this provision. One option interpreted this language to require a 5 percent annual reduction in all pollutants that contribute to PM2.5, meaning direct PM2.5 and the four precursors (SO2, NOx, VOC, and ammonia), or those precursors that are necessary to control in the area. A second option interpreted the language more literally, meaning that it required a 5 percent annual reduction of either direct PM2.5 or PM2.5 precursor emissions on an annual basis, and that a state could elect to control either direct PM2.5 or PM2.5 precursor emissions in a given year . (Note that under either proposed option, a precursor still could be excluded from control requirements if the state submitted a new precursor demonstration as part of the revised CAA section 189(d) implementation plan showing that the precursor does not contribute significantly to levels that exceed the relevant PM2.5 NAAQS, and such demonstration is deemed adequate by the EPA).
	b. Final Rule.
      One group of commenters supported the inclusion of direct PM2.5 and all precursors in the calculation of the annual emission reduction requirement because precursors typically play a significant role in PM2.5 formation, and they believed that allowing states to be able to pick and choose which pollutants to reduce would undermine efforts to attain most expeditiously. Other commenters supported the second option because they believe it follows a plain reading of the statute (i.e., it uses the word "or"), and because it would allow a state to devote resources toward achieving emissions reductions in those pollutants that are most effective in reducing PM2.5 concentrations and thus in attaining the NAAQS most expeditiously.
	After considering comments on this issue, the EPA agrees that the second option is the more appropriate reading of the statute. When paired with the overarching requirement for the area to reach attainment of the NAAQS as expeditiously as practicable, and with provisions in the rule allowing a state to demonstrate that a precursor does not provide a significant contribution to PM2.5 levels, the EPA believes that such an interpretation is reasonable and would authorize states to focus emission reduction efforts on those pollutants that will be most effective for purposes of attainment in a given area. For example, interpreting the statutory provision to require emission reductions in a specific precursor merely for purposes of meeting a 5 percent requirement, without regard to whether the reductions would be effective for purposes of attainment, could be counterproductive to reducing the emissions of other pollutants that could result in earlier attainment. This interpretation of CAA section 189(d) is also consistent with past EPA actions for an area that failed to attain the PM10 Serious area attainment date.
	Thus, in applying the statutory language to implementation of the PM2.5 NAAQS in the final rule, the EPA interprets an "annual reduction in PM10 or PM10 precursor emissions within the area of not less than 5 percent of the amount of such emissions" to mean that an attainment demonstration for a Serious area that failed to attain by the attainment date must include control measures providing for a 5 percent annual reduction in direct PM2.5 emissions or in the emissions of any PM2.5 plan precursor. The EPA considered whether the statutory phrase "precursor emissions" requires a 5 percent reduction of each individual plan precursor in each year, but determined that such an interpretation was unnecessarily restrictive in light of the overarching requirement for states to adopt the control measures that will result in attainment as expeditiously as practicable, and is not compelled by the wording of the 5 percent requirement in the statute. Accordingly, the final rule requires an annual reduction of either direct PM2.5 or any single PM2.5 precursor.
      Because this requirement is an annual one, the final rule also authorizes the state to meet the 5 percent requirement to vary between direct PM2.5 and PM2.5 precursors, or among precursors, from year to year throughout the duration of the section 189(d) attainment plan, so long as the attainment plan provides for expeditious attainment and meets the other applicable attainment plan requirements. For example, in year 1 a state could provide for a 5 percent reduction of direct PM2.5, and in year 2 could provide for a 5 percent reduction in a precursor, and so on.
	c. Comments and Responses.
	Comment: Some commenters suggested that a more appropriate approach would be to require a 5 percent annual reduction in PM2.5 ambient concentrations (rather than in pollutant emissions), and allow the state to meet this air quality target with any combination of emissions reductions.
	Response: The EPA does not find that this approach would be consistent with the statutory language in CAA section 189(d), which clearly expresses the requirement in terms of emissions reductions (i.e., "annual reduction in PM10 or PM10 precursor emissions within the area of not less than 5 percent of the amount of such emissions as reported in the most recent inventory prepared for such area.") Moreover, the EPA is concerned that this approach would necessitate, on an annual basis, a complex modeling analysis or at the very least some other analytical approach to translate emissions to ambient concentrations. The burdens of such analysis could be significant, and it is unclear what benefit would be realized from such an approach. States are already obligated to provide a modeled attainment demonstration as part of the new SIP submission to meet the requirements of section 189(d), and the 5 percent requirement is a separate requirement that the statute explicitly imposes in addition to that modeled attainment demonstration. For these reasons, the EPA is not adopting the commenter's suggested air quality approach in the final rule.

3. Calculating the 5 Percent Annual Reductions.
	a. Summary of Proposal.
	The proposed rule provided an example of how annual reductions would be tracked under this provision, and it also provided another example describing how reductions in excess of the 5 percent requirement in 1 year could be "carried forward" to help meet the requirement in a future year.
	b. Final Rule.
      The previous section 2 explains that the EPA interprets the statute to require a 5 percent annual reduction in direct PM2.5 emissions or in the emissions of any one PM2.5 plan precursor in each year, until attainment. The requisite minimum 5 percent emissions reduction level for any pollutant must be calculated from the total emissions of the pollutant contained in the most recent inventory for the area, as described earlier in this section. The requirement for a 5 percent annual reduction in any one pollutant, calculated based on the emissions levels in the most recent inventory, must then be achieved every year between the CAA section 189(d) plan submission date and the new projected attainment date for the area.
	For example, assume it is 2026, and based on monitoring data from years 2023-2025, a Serious area has failed to attain the 2012 PM2.5 NAAQS within 10 years of designation. Assume also that the most recent inventory available for an area subject to CAA section 189(d) is for the year 2023. This inventory would serve as the base inventory for determining the 5 percent emissions reduction requirement under CAA section 189(d). If the state elects to reduce direct PM2.5 emissions each year of the plan (i.e., instead of choosing to reduce a precursor), and the most recent inventory ("base inventory") indicates that emissions of direct PM2.5 from all sources in the area are 10,000 tons/year, then the area at a minimum would need to reduce emissions of direct PM2.5 by 5 percent of the 2023 base inventory, or 500 tons, each year until the area attains the NAAQS. Thus, in the first year following submission of the CAA section 189(d) plan for the area, emissions of direct PM2.5 could not exceed 9500 tons/year; in the second year, emissions could not exceed 9000 tons/year; and so forth. Note that if the area needs emission reductions beyond this amount (i.e., in direct PM2.5 or in PM2.5 plan precursors) in order to meet the overarching requirement of attaining the standard as expeditiously as practicable, then it must adopt and implement such control measures.
	Although CAA section 189(d) requires that a state develop measures that will obtain annual emissions reductions of "not less than 5 percent" from the most recent inventory, the EPA interprets this language to authorize states to maximize emissions reductions in earlier years and still meet the 5 percent per year requirement for subsequent years. The EPA notes that interpreting the statute in this way will encourage states to implement measures earlier, where possible, rather than delay implementation of measures merely to assure that the 5 percent requirement can be met in later years. Thus, using the example described earlier, the annual reduction requirement for the area would be 500 tons/year from a base year emissions level of 10,000 tons/year. The required level after year 1 would be 9500 tons/year, after year 2 the level would be 9000 tons/year, and so on. If the area reached a level of 8100 tons/year by the end of year 3, then by the end of year 4 it would only need to reduce emissions by 100 tons/year to yield an emissions level of 8000 tons/year. Thus, this approach will allow states to carry forward any emissions reductions beyond the required minimum 5 percent in a given year to the next year as a means to encourage states to achieve emissions reductions as quickly as possible, as long as those emissions reductions are realized after the Serious area attainment date.
	The previous example addresses a situation where the state chooses to reduce only direct PM2.5. In that example, the 5 percent annual reduction amount for any year would be 5 percent of the 2023 PM2.5 emission inventory amount of 10,000 tons. The final rule allows the state to meet its 5 percent reduction each year in terms of reducing direct PM2.5 or any PM2.5 plan precursor. Thus, if the area had a 2023 emission inventory that included 5000 tons of each of the four PM2.5 precursors, and if the state chose to meet its "5% reduction" obligation in a particular year by reducing SO2, it would need to achieve emissions reductions of 250 tons of SO2 in that year.