Subpart 4 requires that Moderate areas that cannot or do not meet the Moderate area attainment date be reclassified as Serious nonattainment areas, in which case sources in the areas are then subject to BACM and BACT requirements. In the General Preamble, the EPA indicated that "it may be reasonable, in some limited circumstances, for states to consider the compatibility of RACM and RACT with the BACM and BACT that will ultimately be implemented under the Serious area plans for those areas." Furthermore, for such areas that do not meet the Moderate area attainment date, the EPA indicated that "in the case of RACM for area sources, EPA anticipates that any future implementation of BACM for these sources will be additive to, and hence compatible with, RACM. This is because BACM will generally consist of a more extensive implementation of the RACM measures... Since EPA anticipates that RACM and BACM for these sources will be compatible, the SIP's (sic) for these areas should reflect the application of available control measures to existing sources in moderate nonattainment areas as determined by the analysis described...for RACM." Thus, the state should assess the remaining set of technologically and economically feasible measures with regard to the compatibility of implementing RACM/RACT in the near term in a way that supports addressing BACM/BACT for such sources when the area is reclassified to Serious.
      The General Preamble also provided guidance for stationary source controls in this situation: "In many instances, the installation of pollution controls representing RACT may involve substantial capital expenditures. In the event that BACT is later required for those sources, this may require controls significantly incompatible with those recently installed as RACT, largely wasting those recent expenditures. Under such circumstances, the installation of controls in the first round of SIP planning would be unreasonable." Accordingly, SIPs for the Moderate areas that cannot practicably attain need not require major changes to the control systems for specific stack and process sources where a State reasonably demonstrates that such changes will be significantly incompatible with the application of BACT-level control systems. A State's demonstration should include, for example, showing what the State believes are RACT and BACT for the source, and why they are significantly incompatible.
       The EPA believes that in such cases, a state should consider selecting and implementing controls that may qualify as BACM or BACT in a Moderate nonattainment area as part of their RACM and RACT analysis. Early adoption of controls that would constitute BACM or BACT could be more efficient and could further the objectives of attaining the NAAQS expeditiously to protect public health and the environment.
      e. Comments and Responses.
      Any additional comments received on this section are addressed in the Response to Comments document found in the docket for this action.
 8. RACM and RACT and Additional Reasonable Measures Submission Requirements.
	a. Summary of Proposal.
      The proposal described a set of submission requirements for RACM/RACT and additional reasonable measures.
	b. Final Rule.
      The requirements in the final rule remain very similar to those that were included in the proposal. To ensure that attainment plan submissions contain the necessary supporting information to enable the EPA to review and approve a state's evaluation and selection of measures that constitute RACM and RACT in a given nonattainment area, the EPA requires under the authority of section 301(a) of the CAA that a state must submit the following information as part of its submission:
 A list of all sources and activities in the nonattainment area that emit direct PM2.5 or any PM2.5 precursor (for multi-state nonattainment areas, this would include source categories, sources and activities from all states which make up the area). See 40 CFR 51.1008(a)(1).
 For each source or activity in the nonattainment area, an inventory of direct PM2.5 emissions and emissions of all PM2.5 precursors, and a comprehensive list of potential control measures considered by the state for the nonattainment area. See 40 CFR 51.1009(a)(2). If the state submitted a precursor demonstration that was approved by the EPA, the state would not be required to submit a list of sources and potential measures to control emissions of the relevant precursor from the stationary sources addressed by the demonstration (i.e., all sources for a comprehensive precursor demonstration, or major sources for a major source precursor demonstration). See 40 CFR 51.1006(a)(1) and (2). Note that the emissions inventory would still need to include all sources of the relevant precursor. See 40 CFR 51.1008(a)(1).
 For each potential control measure considered by the state but eliminated from further consideration due to a determination by the state that the control measure or technology was not technologically feasible, a narrative explanation and quantitative or qualitative supporting documentation to justify the state's conclusion. See 40 CFR 51.1009(a)(3).
 For each technologically feasible emission control measure or technology, a determination of whether the measure is economically feasible must be included, with narrative explanation and quantitative supporting documentation to justify the state's conclusion. See 40 CFR 51.1009(a)(3). The following additional information relevant to economic feasibility should be included as necessary to justify the determination: (a) the control efficiency by pollutant; (b) the possible emissions reductions by pollutant; and, (c) the estimated cost per ton of pollutant reduced.

 For each technologically and economically feasible emission control measure or technology, the date by which the technology or measure could reasonably be implemented, in whole or in part. See 40 CFR 51.1009(a)(4)(i)-(ii).
	Each of these elements will provide information needed by the EPA to evaluate whether the state is meeting the statutory requirements for an attainment plan, and in particular meeting the statutory requirement for states to implement RACM and RACT on sources within the nonattainment area. The EPA recognizes that the base year emissions inventory for the area that the state submits in conjunction with its attainment plan will likely contain some of the information proposed to be required under the first two items in this list. However, the EPA is finalizing a requirement for emissions inventory information specifically relevant to the RACM and RACT element of the state's attainment plan in order to ensure that the EPA or any other party can appropriately evaluate the state's RACM and RACT analysis.
      c. Comments and Responses.
      Comment: Some commenters supported the general submission requirements because in some cases RACM/RACT demonstrations fail to provide the information necessary for the EPA to reasonably conclude that these requirements have been met and are supported by a systematic analysis.
      Response: The EPA agrees with the commenters and the final rule generally tracks the proposal.

9. Criteria for Effective Regulations to Implement RACM and RACT and Additional Reasonable Measures.
	a. Summary of Proposal.
      The preamble to the proposed rule described the four main criteria for effective control measure regulations: such regulations must be quantifiable, enforceable, replicable and accountable.
	b. Final Rule.
      The guidance in this preamble to the final rule remains very similar to what was proposed. After a state has identified a particular control measure as RACM or RACT or additional reasonable measure for a particular nonattainment area, it must then implement that measure through a legally enforceable mechanism that will be included in the SIP (e.g., a state rule that the EPA will approve as a part of the federally enforceable SIP for the state). The EPA is proposing that in order for the EPA to be able to approve any such measure as part of the SIP, the state would have to provide information to meet the following four criteria. These criteria are similar to the criteria finalized as part of the remanded 2007 PM2.5 Implementation Rule.
	First, the base year emissions from the source or group of sources to which the control measure applies and the future year projected emissions from those sources once controlled must be quantifiable so that the projected emissions reductions from the sources can be attributed to the specific measures being implemented. It is important that the emissions from the source category in question are accurately represented in the base year inventory so that emissions reductions are properly calculated. In particular, it is especially important to ensure that both the filterable and condensable components of direct PM2.5 emissions are accurately represented in the base year.
	Second, the control measures must be enforceable. This means that they must specify clear, unambiguous and measurable requirements. The measurable requirements for larger emitting facilities must include periodic source testing, monitoring or other viable means to establish whether the affected source meets the applicable emission limit. Additionally, to verify the continued performance of the control measure, specific emissions monitoring programs appropriate for the type of control measure employed and the level of emissions must be included to verify the continued performance of the control measure. The control measures and monitoring program must also have been adopted according to proper legal procedures. Note that if measures are found to be technically and economically feasible for reducing condensable PM2.5 emissions as well as filterable PM2.5 emissions from a source, the state will need to adopt a new emission limit for the source that accounts for both the filterable and condensable portions, and includes requirements for ensuring compliance using condensable PM2.5 source test methods updated in 2011.
	In response to a comment on this criterion, the EPA clarifies that an enforceable regulation for a CAA program must be enforceable by the EPA, the state, and citizens. By taking action to approve emissions limitations and related provisions into the SIP, the EPA thereby makes those emission limitations a federally enforceable component of the SIP that the state, the EPA, and citizens can enforce thereafter in the event of a violation. SIP provisions that effectively preclude enforcement of violations by the EPA or citizens, whether through impermissible exemptions or other SIP provisions that function to bar effective enforcement, are not acceptable.
	Third, the results of application of the control measures must be replicable. This means that where a rule contains procedures for interpreting, changing or determining compliance with the rule, the procedures are sufficiently specific and objective so that two independent entities applying the procedures would obtain the same result.
	Fourth, the control measures must be accountable. This means, for example, that source-specific emission limits must be permanent and must reflect the assumptions used in the attainment plan for the area, including the modeling conducted in conjunction with the attainment demonstration. It also means that the attainment plan must establish requirements to track emissions changes at sources and provide for corrective action if emissions reductions are not achieved according to the plan.
      c. Comments and Responses.
      Any additional comments received on this section are addressed in the Response to Comments document found in the docket for this action.
 10. Determination of RACM and RACT and additional reasonable measures in multi-state nonattainment areas.
	a. Summary of Proposal.
      The proposal included several proposed recommendations about the development of control measures by states with multi-state nonattainment areas.
	b. Final Rule.
      The guidance in the final preamble remains very similar to what was proposed. States in multi-state nonattainment areas will need to consult with each other on appropriate control measures for the shared nonattainment area. The agency anticipates that states could decide upon RACM and RACT and additional reasonable measures that differ from state to state in a shared nonattainment area, based upon each state's determination of the most effective strategies given the relevant mixture of sources and potential controls in the respective states' portions of a shared nonattainment area. As long as each state can adequately demonstrate that its chosen attainment strategy, including its selection and adoption of RACM and RACT and additional reasonable measures, will provide for meeting RFP requirements and for attainment of the NAAQS as expeditiously as practicable for the nonattainment area at issue, the EPA anticipates being able to approve individual state plans that may elect to control a different mix of sources or to implement different controls, under the proper circumstances. Nevertheless, in evaluating RACM and RACT and additional reasonable measures for a particular nonattainment area, states must consider potential reasonable control measures developed for other areas or other states, and particularly for other portions of an interstate nonattainment area. In addition, states in multi-state nonattainment areas must evaluate whether the reasonable measures each state may have identified as not being necessary for attainment could collectively advance the attainment date for the area by at least 1 year. The EPA may consider such measures in assessing the approvability of each state's individual attainment plan for a multistate nonattainment area.
      c. Comments and Responses.
      Any additional comments received on this section are addressed in the Response to Comments document found in the docket for this action.

 11. Environmental Justice Considerations in Developing the Attainment Plan Control Strategy for a Moderate PM2.5 Nonattainment Area.
	a. Summary of Proposal.
      The proposal provided guidance about environmental justice considerations in developing the attainment plan control strategy for a Moderate area.
	b. Final Rule.
      The guidance remains very similar to what was proposed. Current air quality data indicate that the more severe PM2.5 nonattainment areas contain a high population of people with low socio-economic status, who are among the most at-risk for adverse health effects from exposure to PM2.5. As part of its EJ2020 Action Agenda, the EPA is committed to making progress on improving air quality in communities with high particulate pollution. The EPA, therefore, strongly urges states to consider environmental justice concerns with respect to any control measures they have identified as potential RACM or RACT or additional reasonable measures in an area, particularly to the extent that control measures that a state may be considering are otherwise approximately equal (in terms of technological and economic feasibility) but unequal with respect to their direct or indirect impacts on overburdened
populations. In such cases, the EPA encourages the state to prioritize imposition of the control measures that will result in the least possible burden and greatest degree of health protection for overburdened populations in the nonattainment area. Section XI of this preamble discusses possible approaches for states to address environmental justice concerns associated with implementation of the PM2.5 NAAQS in their SIP development process and attainment plans.
      c. Comments and Responses.
      Any comments received on this section are addressed in the Response to Comments document found in the docket for this action.
E. Modeling for Attainment Demonstrations
1. Demonstrations for Moderate Areas.
      a. Summary of Proposal.
      Section 189(a) of the CAA generally requires a state with a designated Moderate nonattainment area to submit an attainment plan for such area. Section 189(a)(1)(B) of the CAA requires the state to submit an attainment demonstration including air quality modeling to establish either: (i) that the area will attain the relevant NAAQS by the applicable attainment date; or (ii) that it is impracticable for the area to attain the relevant NAAQS by the applicable attainment date. For Moderate nonattainment areas, the attainment date is as expeditiously as practicable, but no later than the end of the sixth calendar year after designation as nonattainment. The EPA therefore proposed to require all Moderate nonattainment areas to submit either an attainment demonstration which includes air quality modeling which establishes that the area will attain the PM2.5 NAAQS by the applicable attainment date, or an impracticability demonstration which documents that the area will not attain the NAAQS by the applicable attainment date. The EPA proposed that the impracticability demonstration must also include air quality modeling, but also asked for comments on an alternative option that would not require air quality modeling as part of an impracticability demonstration. The EPA also proposed to allow states to fulfill the statutory modeling requirement through either locally generated photochemical and/or dispersion modeling or, with proper justification, appropriate regional or national modeling.
      An attainment demonstration is a plan that provides an explanation of how a state will attain the PM2.5 NAAQS by the applicable attainment date in a particular nonattainment area.  The EPA proposed that the demonstration must contain: (i) technical analyses such as base year and future year modeling of emissions which identifies sources and quantifies their emissions that are contributing to violations of the PM2.5 NAAQS; and (ii) analyses of future year emissions reductions and air quality improvement resulting from existing (i.e., already-adopted or "on the books") national, regional and local programs, and potential new local measures needed for attainment, including RACM and RACT controls for the area.
      The EPA further proposed that each state with a Moderate nonattainment area must submit an attainment plan with an attainment demonstration that includes analyses supporting the state's determination of its proposed attainment date. In all cases, the state must show that the area will attain the NAAQS as expeditiously as practicable, but not later than the sixth calendar year after designation. In order to establish that the attainment date is as expeditious as practicable, the state must explain why the control measures adopted in the attainment plan provide for the most expeditious attainment and, in particular, must explain why any cumulative group of reasonable and available control measures that the state elected not to adopt will not collectively advance the attainment date by at least 1 year.
      b. Final Rule.
      As required by CAA section 189(a)(1)(B), the EPA is finalizing a requirement for states with Moderate nonattainment areas to submit a demonstration to establish either: (i) that the area will attain the relevant NAAQS by the applicable attainment date; or (ii) that it is impracticable for the area to attain the relevant NAAQS by the applicable attainment date.
      As proposed, attainment demonstrations must include analyses (including air quality modeling) supporting the state's determination of its proposed attainment date. In all cases, the state must show that the area will attain the NAAQS as expeditiously as practicable, but not later than the sixth calendar year after designation. The demonstration must include implementation of all measures identified as RACT/RACM plus additional reasonable measures, as necessary, for expeditious attainment. In order to establish that the attainment date is as expeditious as practicable, the state must explain why the control measures adopted in the attainment plan provide for the most expeditious attainment and, in particular, must explain why the cumulative group of reasonable and available control measures that the state elected not to adopt will not collectively advance the attainment date by at least 1 year. See 40 CFR 51.1009(a)(4).
      The EPA is not finalizing a regulatory requirement for air quality modeling to be included as part of an impracticability demonstration. See 40 CFR 51.1009(a)(4). Since all nonattainment areas will have modeling requirements associated with their attainment demonstration, the EPA believes it is likely that modeling will be submitted in support of impracticability demonstrations. However, it may be possible in some cases to support an impracticability demonstration with ambient PM2.5 data and other relevant non-modeling information. For example, the ambient data in a nonattainment area may be so far above the NAAQS, and the reasonable and available controls (i.e. RACM/RACT and additional reasonable measures) so limited, that it is clearly impossible (and thus also impracticable) for the area to attain by the Moderate area attainment date. In order to support this type of demonstration, the state must show that, even if all reasonable controls (i.e. RACM or RACT and additional reasonable measures) were implemented, the state could not attain the NAAQS within the statutory timeframe for a Moderate area.
	The EPA continues to assume that in most cases photochemical grid modeling will be required to demonstrate attainment with the PM2.5 NAAQS. However, the EPA recognizes that more simplistic modeling techniques (such as dispersion, receptor, and/or box models) may suffice to demonstrate that an area will attain the NAAQS, especially in areas that are dominated by primary PM2.5 emissions (e.g. residential wood smoke).
      c. Comments and Responses.
      Comment: Some commenters supported the EPA's proposal to require modeling to demonstrate that attainment is not practicable. The commenters stated that such an interpretation flows logically from the Act's requirement in section 189(a)(1)(B) that attainment demonstrations be supported by modeling. One commenter supported the alternative approach described in the proposal in which air quality modeling would not be required for a Moderate area impracticability demonstration.
      Response: After further consideration of this issue, the EPA has determined that modeling need not be a regulatory requirement to support an impracticability demonstration. We note that CAA section 189(a)(1)(B)(i) includes the parenthetical "including air quality modeling" which clearly makes modeling a statutory requirement for moderate area attainment demonstrations. However, the same parenthetical statement is absent from CAA section 189(a)(1)(B)(ii), which addresses an impracticability demonstration. While we believe that most impracticability demonstrations will indeed be supported by air quality modeling, there are cases where a modeling demonstration may not be needed. In addition, the EPA believes the burden of proof for an impracticability demonstration is logically lower than for an attainment demonstration because submission of an impracticability demonstration also requires reclassification to a serious nonattainment area and the accompanying more stringent regulatory requirements (e.g. BACT/BACM). The area is still required to meet RACT/RACM requirements and will also be required to submit a serious area attainment demonstration, which will necessarily need to include air quality modeling.
      Comment: Some commenters agreed with the EPA that states should be afforded flexibility to fulfill the statutory modeling requirement through appropriate regional or national modeling.
      Response: The EPA agrees that, where appropriate, regional and/or national scale air quality modeling could be sufficient to fulfill the statutory modeling requirement for attainment demonstration modeling. However, as with any attainment demonstration, the modeling must be shown to be appropriate for the nonattainment area, including good model performance, appropriate emissions and meteorological inputs, and consideration of emissions control strategies. It should be noted, however, that it may be difficult to fulfill other CAA requirements (such as emissions inventory, RACM, RFP, establishing motor vehicle emissions budgets for transportation conformity purposes, etc.) using regional or national modeling data. In order to fulfill those requirements, states may need more detailed data for sources in their nonattainment area compared to what is available through regional or national modeling.
      Comment: Some commenters stated that, as the proposal stands, if states wish to preclude RACT/RACM for any sources in the nonattainment area, they must do modeling for the year preceding the attainment year to demonstrate early attainment; this would require modeling for 3 years, rather than 2 years.
      Response: Although a RACM analysis is required, and eliminating potential control measures requires an assessment of whether the measures collectively could advance the attainment date by 1 year, EPA did not propose any specific modeling requirements for the RACM analysis. There are several components to the analysis. First, potential emissions reductions need to be assessed. Then, an assessment of whether those emissions reductions can advance attainment by at least a year needs to be completed. One way to minimize the number of future modeled years is to establish (through sensitivity modeling) a relationship between PM2.5 and PM2.5 precursor emissions reductions and PM2.5 concentrations in the nonattainment area. The established relationship can be used to estimate whether a particular set of emissions reductions will be able to advance the attainment date by at least a year. Also, in some cases, the emissions reductions identified through the RACM analysis may be relatively small (as a percentage of area-wide emissions) that a modeling analysis is not needed to show that the attainment date cannot be advanced.
2. Available Modeling Guidance for Demonstrating Attainment.
      a. Summary of Proposal.
      The EPA proposed that attainment demonstrations should be consistent with the procedures for modeling PM2.5 as described in the EPA's "Guidance on the Use of Models and Other Analyses for Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5, and Regional Haze" as well as the Guideline on Air Quality Models (40 CFR part 51, Appendix W).
      The PM2.5 attainment demonstration modeling guidance (hereafter referenced as the "modeling guidance") describes how states can apply air quality models to generate results needed to demonstrate attainment. Models are used to test whether control measures in an attainment plan to be adopted into a SIP are likely to result in attainment of the relevant standards. The attainment demonstration modeling guidance recommends a modeled attainment test for the annual and 24-hour PM2.5 NAAQS that uses a combination of ambient PM2.5 and PM2.5 species data and modeled PM2.5 concentrations to estimate future year air quality. In the recommended attainment test the state applies the test at each PM2.5 ambient monitor location within or near a designated nonattainment area. Models are used in a relative sense to estimate the response of measured air quality to future changes in emissions. Future air quality is estimated by multiplying recent monitored PM2.5 values by the modeled relative response (percent change) to projected future changes in emissions. If the future design value at all monitoring locations in the nonattainment area does not exceed the concentration of PM2.5 specified in the NAAQS, the area is projected to attain the NAAQS.
      b. Final Rule.
      In the final rule, EPA is continuing to recommend that attainment demonstrations should be consistent with the procedures for modeling PM2.5 as described in the PM2.5 attainment demonstration modeling guidance and Appendix W. The modeling guidance describes how states can apply air quality models to generate results needed to demonstrate attainment. These recommendations include developing a conceptual description of the problem to be addressed; developing a modeling/analysis protocol; selecting an appropriate model to support the demonstration; selecting appropriate meteorological episodes or time periods to model; choosing an appropriate area to model with appropriate horizontal/vertical resolution; generating meteorological and air quality inputs to the air quality model; generating emissions inputs to the air quality model; and evaluating performance of the air quality model. After these steps are completed, the state can apply a model to simulate effects of future year emissions and candidate control strategies.
      The EPA has updated the 2007 PM2.5 modeling guidance to include additional information related to the 2012 PM2.5 NAAQS and associated monitoring requirements. The main components of the modeling guidance and the modeled attainment test have not changed. Additional information has been added to address near-road monitoring sites and other information that was not available when the guidance was first released in 2007.
      The modeling guidance continues to recommend a relative attainment test for both the annual and 24-hour PM2.5 NAAQS. The EPA is not recommending a specific model for use in the attainment demonstration for the PM2.5 NAAQS. At present, there is no single model that has been extensively tested and shown to be clearly superior to other available models. The current modeling guideline, 40 CFR part 51, appendix W, does not identify a preferred model for use in attainment demonstrations of the NAAQS for PM2.5. Thus, states may choose from several alternatives.
      The EPA has developed software to perform both the annual and 24-hour PM2.5 attainment test (including interpolating PM species data where necessary). The current software is called the Software for the Modeled Attainment Test- Community Edition (SMAT-CE). The software is provided as a way to make it relatively easy for states to apply the recommended modeled attainment test(s). However, states are not required to use SMAT-CE and can develop their own post-processing software as necessary.
      The modeling guidance continues to describe the opportunity for states to supplement their modeling with a "weight of evidence" demonstration. States may use other information and analyses, in addition to the modeled attainment test to estimate whether future attainment of the NAAQS in an area is likely. Other analyses may include, but are not limited to, emissions trends, ambient data trends and analyses, other modeling analyses, and documentation of other non-modeled emissions control strategies, including voluntary programs.
      The application of air quality models requires a substantial effort by state and local agencies. Therefore, states should work closely with their EPA regional office in executing each step of the modeling process. By doing so, it will increase the likelihood of the EPA's approval of the state demonstration submitted at the end of the modeling and overall attainment plan development process.

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

	b. Final Rule.
	For the final rule, the EPA maintains the policy of not requiring the use of ambient air quality data in a modeled PM2.5 attainment demonstration unless there is at least one complete design value of data available (generally 3 complete years of data). This applies to both near-road and other PM2.5 ambient monitoring data. Some states may have installed their monitors well before the January 1, 2015 deadline and may therefore have complete data before the SIP deadline. In addition, some attainment demonstrations may be submitted after the statutory deadline. Because of the varying monitor installation dates, the use of near-road monitoring data in attainment demonstrations depends on the timing of the attainment demonstration submission relative to the installation date of the monitor(s). Ambient data with sufficient completeness to calculate a design value may not be ignored in an attainment demonstration. Such data can be addressed either in the attainment demonstration analysis of ambient monitors or as part of an unmonitored area analysis (see the next section), as appropriate.
	The revised PM2.5 modeling guidance document includes procedures for applying a dispersion model or a combination of photochemical grid models and dispersion modeling to demonstrate attainment at monitors with large primary PM2.5 concentration gradients. Depending on the nature of the ambient data in a particular area, it may be appropriate to treat near-road monitors as high concentration gradient locations. However, in other cases, near-road monitors may have little or no gradient compared to other nearby monitors. Therefore, the appropriate treatment of near-road monitors in attainment demonstrations should be evaluated on a case-by-case, depending on the facts and circumstances in each nonattainment area.

	c. Comments and Responses.
	Comment: Some commenters stated the EPA's proposal to excuse areas from having to include projected design values for near-road monitoring locations promises to undermine the likelihood of success for attainment demonstrations. The commenters stated the EPA's blanket waiver for near-road data has no rational basis and that just because such monitors were not required before January 1, 2015, does not mean that areas did not have them in place before then. The commenters stated the EPA should at least clarify that if an area has 3 years of near-road monitoring data, it should use such data in its attainment modeling. The commenters stated this would be particularly important, for example, if an area is late in preparing its demonstration.
	Response: The EPA agrees there should not be a "blanket waiver" for the use of near-road monitoring data in attainment demonstrations that are due in 2016 or thereafter. The statements in the proposal referenced the fact that the near-road monitors were not required to be in place before January 1, 2015. This makes it unlikely that sufficient data from these monitors will be available to be considered in attainment demonstrations that are due in 2016. However, if complete data are available at near-road monitors during the development of the attainment demonstration, the data should be considered as appropriate (similar to any other PM2.5 monitoring data). Since the near-road PM2.5 monitoring network is relatively new, there may not be 3 years of complete data in time to be considered in the upcoming attainment demonstrations. In addition, the base modeling year of the attainment demonstration may predate the startup date of the near-road monitor(s). In this case, it may be possible to consider the near-road data in the attainment demonstration, but the recommended default projection methodology may not be applicable (since the time period of the near-road data may not correspond to the 5 year time period centered about the base modeling year, as recommended in the modeling guidance). Additionally, near-road PM2.5 monitors are only required in the 27 largest metropolitan areas of the country. Some PM2.5 nonattainment areas may not have any near-road monitoring sites. States should consult with the appropriate EPA regional office to determine the best way to treat near-road data in their attainment demonstration.
4. Demonstrating Attainment in Unmonitored Areas.
      a. Summary of Proposal.
      The 2007 PM2.5 modeling guidance describes a recommended "relative" attainment test that is based on showing attainment at ambient monitoring locations. The guidance also recommends that states conduct further analyses based on modeling results to determine whether there are unmonitored areas that merit additional analysis or investigation. In order to clarify the statutory and rule requirements of a modeled attainment demonstration, the EPA proposed four options for demonstrating attainment in unmonitored areas in an attainment demonstration.
      Option 1 would require the attainment demonstration modeling to demonstrate attainment at ambient monitoring locations. There would be no requirement to specifically examine attainment in unmonitored areas. Option 2 would require modeling to demonstrate attainment at ambient monitoring locations and in unmonitored areas within the nonattainment area. Enforceable emissions reductions would be required to eliminate any potential future year NAAQS violations in all locations within the nonattainment area (including unmonitored areas). Option 3 would require modeling to demonstrate attainment at ambient monitoring locations and in unmonitored areas within the nonattainment area. However, rather than requiring states to impose additional enforceable emissions reductions in the SIP to address potential violations in unmonitored areas, states would be required to use the unmonitored area analysis results to develop an assessment of the likelihood of violations in unmonitored areas. The assessment would be used to evaluate the need for additional controls and/or could be used to inform the ambient monitoring plan (the need to add additional monitors or move existing monitors). Option 4 would require modeling to demonstrate attainment at ambient monitoring locations and recommend the analysis of unmonitored areas within the nonattainment area. This differs from Option 3 in that there would be no rule requirement to perform an unmonitored area analysis. But the submission of an unmonitored area analysis would be still be recommended, especially in areas with a relatively sparse PM2.5 monitoring network or in locations where information such as modeling data, emissions inventories or non-FEM monitoring data (such as from special purpose monitors or saturation monitoring studies) may indicate potential high PM2.5 concentrations in areas that are currently unmonitored.
      b. Final Rule.
      The EPA is finalizing proposed Option 4. This option requires states to show attainment at all current and recent monitoring locations. States will not be required to provide an unmonitored area analysis as a mandatory element of each attainment demonstration. However, an unmonitored area analysis can provide useful information about PM2.5 concentrations and gradients in the nonattainment area and therefore the EPA recommends that all attainment demonstrations should contain an unmonitored area analysis. The EPA encourages states to use information available to them to consider what, if any, impacts may be occurring in unmonitored areas. States can evaluate the need to perform an unmonitored area analysis by using available information such as modeling data, emissions inventories, or non-FEM monitoring data (such as from special purpose monitors or saturation monitoring studies) to indicate the potential high PM2.5 concentrations in areas that are currently unmonitored. An unmonitored area analysis is strongly recommended where the state and/or the EPA has reason to believe that potential violations may be occurring in unmonitored areas, or other available information indicates that further analysis is warranted. The EPA will consider whether the state has adequately addressed all available information about potential exceedances of the NAAQS in unmonitored areas when determining whether the plan can be approved.
      The EPA is requiring an attainment demonstration approach that relies primarily on existing monitoring sites and modeling to project attainment in future years. This approach to evaluating monitored and unmonitored areas is consistent with how EPA determines whether an area meets the PM2.5 NAAQS for purposes of designations and redesignations. As discussed in Section II of this preamble, the EPA promulgates designations for PM2.5 NAAQS nonattainment areas based primarily on ambient data measured at FRM and FEM monitors. Although the EPA considers other information for purposes of evaluating areas with sources that contribute to those monitored violations for inclusion within the nonattainment area boundaries, the fundamental basis for designating an area as nonattainment for a PM2.5 NAAQS is the presence of one or more FRM or FEM monitors with data showing violations of the NAAQS. Similarly, determinations of attainment of the PM2.5 NAAQS for purposes of redesignation actions are based primarily on monitored data. When all FRM and FEM monitors in a nonattainment area measure attainment of the PM2.5 NAAQS, the state is eligible to submit a redesignation request for the area, assuming that it has complied with all other applicable requirements for purposes of redesignation. Specifically, the EPA's approval of a redesignation request is subject to meeting the requirements of CAA section 107(d)(3)(E). Among those requirements is that the area has attained the NAAQS. For the PM2.5 NAAQS, this determination is based on ambient data measured at the FRM and FEM monitors in the area in question.
      In addition, the ``relative'' attainment test for PM2.5 attainment demonstrations uses FRM or FEM ambient monitoring data, combined with future year modeled percentage changes in PM2.5 concentrations to project future year design values. Since the attainment test relies on ambient monitoring data, an analysis of future year concentrations in unmonitored areas can only be accomplished by interpolating ambient data to a particular location where there is no existing monitor or recent monitoring data. Therefore, in the context of an attainment demonstration, the projection of future year PM2.5 concentrations in unmonitored locations is inherently more uncertain than projections in monitored locations due to the fact that the ambient concentrations from which these projections are developed are unknown in the unmonitored locations.
      While the unmonitored area analysis is not a regulatory requirement, and states are not required to identify enforceable emissions reductions to eliminate potential violations in unmonitored areas, an unmonitored area analysis has the potential to provide additional important information about PM2.5 levels and gradients in the nonattainment area. The results of the analysis can be used to provide information to inform future monitoring plans, to examine the need for potential emissions controls, to evaluate potential environmental justice concerns, and to provide additional information to the public. The EPA believes that Option 4 provides the best balance between the regulatory requirements of the attainment demonstration and additional analyses which could provide helpful information to inform future regulatory activities.
      Where information is available, states and the EPA have obligations to address potential violations in unmonitored areas, and, although we expect this to be relatively rare, attainment plans need to address air quality in unmonitored areas where information exists suggesting the potential for such violations. Where an unmonitored area analysis is performed, states should use model results and available ambient data to develop an assessment of the likelihood of violations in unmonitored areas. The nature of the assessment depends on the available information and the nature of the local PM2.5 problem, but could include, as appropriate, elements such as an evaluation of the emissions inventory (particularly for local direct PM2.5 sources), the existing ambient data for the area, and meteorological model inputs to evaluate the accuracy of the modeled violations in unmonitored areas. If potential violations are determined to be likely, additional steps could include imposition of emissions reductions at nearby emission sources or a commitment to deploy special purpose monitors and/or saturation monitors in the area (in order to further evaluate the problem). The state should document the assessment, including analyses of emissions, meteorological inputs and ambient data.
      The PM2.5 modeling guidance recommends a default procedure for applying an unmonitored area analysis, which combines gridded model data with interpolated ambient data. States can apply the default recommended approach or develop their own analysis which may be more appropriate for the specific area or situation. States are expected to consult with the appropriate EPA Regional Office to evaluate available information to determine if an unmonitored area analysis is needed for a particular area and how the analysis should be performed.
	c. Comments and Responses.
      Comment: Some commenters stated that, of the options for addressing unmonitored areas, only Option 2 is technically and legally defensible (80 FR 15382). The commenters stated the Act requires that ambient concentrations in all areas meet the applicable NAAQS and cited 42 U.S.C. section 7407(a) as requiring states to assure "air quality within the entire geographic area comprising such State" will achieve the national standards and requiring "an implementation plan [to] . . . specify the manner in which national primary and secondary ambient air quality standards will be achieved and maintained"). The commenters also cited 42 U.S.C. section 7410(a)(1) as requiring implementation plans to provide for implementation of the NAAQS "in each air quality control region (or portion thereof) within such State"). The commenters stated it is insufficient to suggest that an area need only show attainment at monitored locations and need only adopt controls that will address those locations.
      Response: The EPA does not agree that Option 2 is the only technically and legally defensible option. The CAA requires states with nonattainment areas to submit an attainment demonstration as part of their PM2.5 SIP. States must show that they will attain the NAAQS by their attainment date. The CAA also requires states to use air quality modeling in their attainment demonstration. But other than those general requirements, the EPA believes states have discretion to interpret how and where to show attainment of the NAAQS through modeling in support of an attainment demonstration.
      In addition, the EPA believes that a monitor based attainment demonstration satisfies the CAA requirement to show that "the entire geographic area" will attain the NAAQS. The EPA's monitoring requirements for PM2.5 are designed to ensure a robust nationwide monitoring network in both nonattainment and attainment areas. States have achieved this by maintaining their PM2.5 networks in accordance with EPA's network design criteria. Historically, these criteria provided that CBSAs have at least one PM2.5 monitoring site located in an ``area-wide'' location of expected maximum concentration (within the CBSA). Thus, by assuring compliance with the NAAQS at the location of the expected highest area-wide concentration in the CBSA, air quality is protected throughout each CBSA. The EPA has identified recommended procedures for PM2.5 modeled attainment demonstrations. These recommendations are contained in modeling guidance. The recommended attainment test relies on the ambient monitors to provide the "anchor point" for future year air quality projections. This ensures that future year concentration predictions are grounded by "real world" measurements. Since the attainment test relies on ambient monitoring data, the projection of future year PM2.5 concentrations in unmonitored locations is inherently more uncertain than projections in monitored locations due to the fact that the ambient concentrations from which these projections are developed are unknown in the unmonitored locations. Therefore, the EPA continues to believe that for PM2.5 attainment demonstrations, modeling results in unmonitored areas are too uncertain to use in this manner. For the reasons stated earlier, in the final rule, the EPA does not believe that it is necessary to require states to submit an unmonitored area analysis and to show that any potential violations of the NAAQS in unmonitored areas have been eliminated through enforceable controls.
      Comment: Several commenters supported Option 1 where only monitored grid cells are included in the attainment plan. Commenters stated that, due to their concerns related to the accuracy of air dispersion modeling tools and protocols, it is not appropriate to use air dispersion modeling to predict receptor impacts in unmonitored areas. Some commenters stated that Option 1 is the approach that most closely describes the current EPA rule.
      Response: The EPA agrees that the modeling results are too uncertain in unmonitored areas to require an unmonitored area analysis as part of the attainment demonstration (for the reasons enumerated earlier). However, the EPA disagrees that Option 1 is the approach that most closely describes the attainment demonstration requirements in the 2007 PM2.5 implementation rule. An unmonitored area analysis has never been an implementation rule requirement, but was a recommended analysis in the PM2.5 modeling guidance. Therefore, the EPA believes that Option 4 is closer to the current status quo. This final rule clearly states the continued recommendation to perform an unmonitored area analysis and the benefits of doing so.
      Comment: Some commenters stated that an "unmonitored area analysis" is essential since speciation monitoring is conducted at a limited number of sites. The commenters stated that, however, given the inherent uncertainty from modeling analysis in unmonitored areas, results from such analysis should only be used to inform additional actions. The commenters stated that, while modeling analysis in unmonitored areas can be used as a reference for additional studies, it should not be used for the attainment demonstration in the SIP. The commenters stated that, under any of the options, the EPA should specify the recommended level of detail for an unmonitored area analysis, especially if it is required. The commenters recommended that the analysis need not require modeled results at finer spatial scales than those specified in the modeling protocol.
      Response: The EPA agrees that an unmonitored area analysis is important and continues to recommend development of unmonitored area analyses to support attainment demonstrations. The EPA also agrees that due to uncertainty, the results from such analysis should only be used to inform additional actions. As stated earlier, the PM2.5 modeling guidance contains a default recommended unmonitored area analysis technique which combines gridded modeling data and interpolated ambient data (including PM2.5 speciation data). But the exact nature of the unmonitored area analysis can be considered based on the information relevant to each nonattainment area. The EPA also agrees that where an unmonitored area analysis is conducted, it should be at the same spatial scale (model resolution) as the modeled attainment demonstration at monitoring locations. For example, if the gridded modeling analysis is performed at 4km resolution (model grids that are 4km on a side), then the unmonitored areas should be examined at the same resolution. Similarly, if near road monitors are examined with a dispersion model at a finer resolution (compared to the other monitors) as part of the attainment demonstration, the unmonitored area analysis could also examine unmonitored near-road areas at a finer resolution.
      Comment: Several commenters disagreed with the proposal to require states to perform the attainment test at "recent" monitoring locations. Commenters stated that, within the EPA's description of Option 1, the proposal indicates that the attainment test required under Option 1 would also apply to locations that have "recent" FRM and/or FEM monitoring data. Commenters stated the current FRM/FEM monitoring data should be sufficient to demonstrate attainment.
      Response: States must demonstrate that they will attain the PM2.5 NAAQS in the nonattainment area as expeditiously as practicable, and no later than the moderate area attainment date. The recommended attainment test in the modeling guidance uses recent ambient data that encompass a 5-year period that is dependent on the base modeling year. For example, for a base modeling year of 2014, the guidance recommends using ambient PM2.5 data from the 2012-2016 period. The guidance also recommends only using ambient data from a particular monitoring site if it has at least one complete design value period during the relevant 5-year period. With these recommendations in mind, there are numerous cases where a monitoring site may have only partial data from the relevant 5-year period or may be a new monitor that started collecting data after the 5 year period or may have been shut down before the 5-year period. The EPA agrees that it is generally not necessary to examine the modeling results where monitors were shut down before the base modeling period. These monitors will not be used to make future decisions relating to attainment status of the area. However, monitors that are new or were operating during the base modeling period are still relevant and can be used to provide additional information in the attainment demonstration. The data from these monitors may serve as the basis for examining potential violations in the area as part of an unmonitored area analysis. This is especially the case for new monitors, which may not have enough data to provide a robust future year concentration estimate in the attainment demonstration. But the monitoring data, combined with modeled information, can provide information about the likelihood of future violations in the area surrounding the monitor location.
5. Future Year(s) to be Modeled in Attainment Demonstrations.
      a. Summary of Proposal.
      A state performing a modeling analysis for an attainment demonstration or impracticability analysis must select a future year for the analysis. For an attainment demonstration, a state should select the future modeling year such that all emissions control measures relied on for attainment will have been fully implemented by the beginning of that year. The EPA proposed that to demonstrate attainment, the modeling results for the nonattainment area must predict that emissions controls implemented no later than the beginning of the last calendar year preceding the attainment date will result in PM2.5 concentrations that meet the level of the standard. While states should choose the future modeling year based on a number of factors, the EPA proposed the last possible year permitted under the statute as a starting point for modeling.
      b. Final Rule.
      The EPA is finalizing the recommendation that the last possible year permitted under the statute is an appropriate starting point for modeling. See 40 CFR 51.1011(a)(6). For a state that is submitting an attainment demonstration, modeling the sixth calendar year is a logical starting point to determine if attainment by that year is likely. Even though attainment is determined by averaging 3 years' worth of ambient data, states do not have to model 2 years before the attainment date to show modeled attainment. Since the design value is an average of 3 years' worth of data, attainment can still be shown even if concentrations exceed the NAAQS in one or more of the 3 years used to determine attainment (as long as the 3 year average is less than the NAAQS). Therefore, it can be appropriate to model any of the 3 years used to determine attainment. In addition, if ambient data show attainment-level concentrations in the final statutory attainment year, a state may be eligible for up to two 1-year extensions of the attainment date, if the area meets the criteria for such extensions. Therefore, modeling attainment-level concentrations for the last year permitted by statute is acceptable.
      States with Moderate areas that submit an impracticability demonstration must show that the area cannot attain the NAAQS by the end of the sixth calendar year following designation of the area. Therefore, the appropriate future modeling year for such a demonstration is also the sixth calendar year after designation.
      For the reasons stated earlier, it is both acceptable, and will in fact be most efficient, for a state to begin the attainment demonstration process by modeling the last year permitted under the statute to determine future year modeled PM2.5 concentrations in the sixth year after designations. For example, since designations for the 2012 PM2.5 NAAQS were effective in March 2015, it is appropriate for states to model air quality for 2021 in the attainment demonstrations for designated nonattainment areas.
	Because an area must attain "as expeditiously as practicable," additional considerations are necessary before an attainment date can be established. For purposes of determining the attainment date that is as expeditious as practicable, the state must conduct future year modeling which takes into account expected growth and known controls that are already in effect or that are adopted and will be in effect by January 1 of the future year. For example, for a Moderate nonattainment area for the 2012 PM2.5 NAAQS, a future base case scenario for the year 2021 would project future air quality given implementation of existing federal, state and local measures. If this future base case scenario demonstrates attainment, then the state must determine if attainment can be achieved in an earlier year through the application of additional measures. Therefore, the state must conduct an analysis of RACM and RACT and additional reasonable measures to determine if, collectively, all technologically and economically feasible measures identified by the state that can be implemented by the beginning of the sixth calendar year following designations can advance the attainment date by at least 1 year (note that RACM and RACT controls must be implemented within 4 years of an area being designated nonattainment, but additional reasonable measures for an area for which a state can demonstrate attainment by the end of the sixth calendar year following designation of the area are those technologically and economically feasible measures that can be implemented by the beginning of the last year prior to the projected attainment date). Results of this analysis may indicate attainment can be achieved earlier, through implementation of all reasonable control measures (i.e., RACM and RACT and additional reasonable measures).
      If, on the other hand, the future base case scenario does not demonstrate attainment, then a control case scenario is needed to examine whether the reasonable, technically and economically feasible measures identified by the state would result in attainment in the analysis year (i.e., in 2021 for purposes of this example based on the 2012 PM2.5 NAAQS). The control case scenario would add potential control measures -- e.g. RACM and RACT (which must be implemented in 4 years) and additional reasonable measures, plus any measures on sources outside of the nonattainment area that the state has identified as feasible to implement by the attainment date. This modeling, along with other relevant information, would inform a judgment as to whether attainment of the relevant NAAQS is practicable by the end of the sixth year after designation or earlier. In the case of areas designated for the 2012 PM2.5 NAAQS, if the analysis does not demonstrate attainment by December 31, 2021, then the analysis could serve as the technical basis for the state to submit a demonstration that attainment by the outermost statutory attainment date for Moderate areas is impracticable. This in turn could serve as a technical basis for the Administrator to reclassify the area to Serious. If the analysis does demonstrate attainment, then the remaining step is to assess whether the attainment date can be advanced by 1 year.
      In conducting this assessment, the EPA believes that it is not reasonable to require states to model each and every calendar year to determine the appropriate attainment date. Developing and modeling future year inventories is a time-consuming and resource intensive process. Multiple emissions models are needed in order to generate year specific emissions for the various emissions sectors (e.g. mobile, non-road, non-EGU point and EGU point). In some cases it may be reasonable to model one additional interim year before the maximum statutory attainment date. However, in most cases, the air quality benefits of an identified set of RACM and RACT and additional reasonable measures can be estimated through model sensitivity analyses and the development of sensitivity factors (i.e., factors to relate tons of emissions reductions in the area to PM2.5 concentration changes in the area). For example, states can model across the board percentage reductions in direct PM2.5 and/or precursor emissions (in separate model runs or using advanced modeling techniques such as DDM) to determine the impact of emissions reductions on PM2.5 concentrations in the area. This modeling can be performed with a single attainment year modeling platform, which is much less resource intensive than modeling additional future years. The identified potential emissions reductions available from RACT and RACM and additional reasonable measures can be compared to the magnitude of the modeled PM2.5 reductions from the sensitivity analyses to determine if all such controls will advance attainment by a year. The EPA strongly recommends that states discuss the selection of the future year(s) to model with their EPA Regional Office as part of the modeling protocol development process and before embarking on the modeling.
      c. Comments and Responses.
      Comment: Some commenters disagreed with the proposal that the future year should reflect when all control measures relied on have been fully implemented by the beginning of that year and it should be no later than the beginning of the last calendar year preceding the attainment date. The commenters stated the CAA provides attainment must be achieved as expeditiously as practicable but no later than the end of the 6th calendar year (except RACT/RACM are required within four years) and States should be given the full period to demonstrate attainment and to require control of emissions.
      Response: The proposal to require the modeling to reflect control measures that have been fully implemented no later than the beginning of the last calendar year preceding the attainment date does give nonattainment areas "the full period" to demonstrate attainment and to require control of emissions. Since the design value is an average of 3 years' worth of data, it could be argued that modeling and related emissions controls should be in place 3 years before the attainment date. However, if ambient data show attainment level concentrations in the final statutory attainment year, a state may be eligible for up to two 1-year extensions of the attainment date, if the area meets the criteria for such extensions. Therefore, modeling attainment level concentrations for the last year permitted by statute is acceptable. But in order to measure attainment level concentrations in the final year, controls must be in place for the full year (at the beginning of the year). Implementation of emissions controls at the end of the year would not be consistent with modeling attainment level or measuring attainment level concentrations during the year.
6. Attainment Year Motor Vehicle Emissions Budgets.
	The transportation conformity rule requires that attainment plans establish motor vehicle emissions budgets for the area's attainment year. Therefore, once an area's attainment date has been established, the state would establish motor vehicle emissions budgets for direct PM2.5 and any relevant PM2.5 precursor for the attainment year. It should be noted that states that submit impracticability demonstrations for Moderate areas under CAA section 189(a)(1)(B)(ii) are not required to submit motor vehicle emissions budgets for attainment purposes because the submitted SIP does not demonstrate attainment. A motor vehicle emissions budget for the purposes of a PM2.5 attainment plan is that portion of the total allowable emissions within the nonattainment area allocated to on-road sources as defined in the submitted attainment plan. Such motor vehicle emissions budgets would be calculated using the latest planning assumptions and the latest approved motor vehicle emissions model available at the time that the attainment plan is developed, unless EPA approves the state's use of an alternative model.
F. RFP Requirements
1. Background on Statutory Requirements and Existing Guidance.
	Reasonable further progress (RFP) is a concept included in the CAA under part D, title I to assure that states make steady, incremental progress toward attaining air quality standards in the years prior to the attainment date for a nonattainment area, rather than merely deferring implementation of control measures and therefore emissions reductions until the date by which the standards are to be attained. As discussed elsewhere in this preamble, section 172 of the CAA addresses attainment plan provisions in general. Section 172(c)(2) of the CAA requires attainment plans to provide for RFP, which is defined in CAA section 171(l) as "such annual incremental reductions in emissions of the relevant air pollutant as are required by [part D of title I] or may reasonably be required by the Administrator for the purpose of ensuring attainment of the applicable national ambient air quality standard by the applicable date." Section 189(c) of the CAA requires that "[P]lan revisions demonstrating attainment submitted to the Administrator for approval under this subpart shall contain quantitative milestones which are to be achieved every 3 years until the area is redesignated attainment and which demonstrate reasonable further progress, as defined in CAA section 171(1), toward attainment by the applicable date." Quantitative milestones are discussed later in Section IV.G of the preamble.
	Section 172(c)(3) of the CAA requires the state plan to include "a comprehensive, accurate, current inventory of actual emissions from all sources of the relevant pollutant or pollutants in such area..." Section 172(c)(1) of the CAA requires the state plan to include "all reasonably available control measures as expeditiously as practicable (including such reductions in emissions from existing sources in the area as may be obtained through the adoption, at a minimum, of reasonably available control technology)..." Section 172(c)(9) requires the state plan to "provide for the implementation of specific measures to be undertaken if the area fails to make reasonable further progress... Such measures shall be included in the plan revision as contingency measures to take effect in any such case without further action by the State or the Administrator." For additional background on statutory requirements and existing guidance, refer to preamble Section IV.F of the proposal for this rule.
2. General Approach to RFP.
      a. Summary of Proposal. To satisfy the statutory requirements for RFP at CAA section 172(c)(2), the EPA proposed that a state must submit an RFP plan as part of its Moderate area attainment plan submission. The EPA proposed the following two options for developing an RFP plan.
	Under the first option, the EPA proposed that the RFP analysis for any Moderate PM2.5 nonattainment area that can demonstrate attainment by the statutory attainment date must demonstrate either: (i) generally linear progress toward attainment; or (ii) step-wise progress toward attainment. Step-wise emissions reductions would be slower than "generally linear" reductions for certain periods, and then would decline sharply (due to implementation of a new emission reduction program, or new operation of control technology on one or more stationary sources). The EPA proposed that a state must follow one primary approach for conducting the RFP analysis, but that they also have an option to conduct a secondary analysis that will provide greater flexibility in setting RFP goals with alternative emissions reductions and air quality improvement scenarios. The primary approach would be to show that nonattainment area emissions of each pollutant decline from the base year to the attainment year, either in a generally linear manner or in a step-wise manner. In the optional secondary analysis, the state could show that emissions of the various pollutants would change in a manner that would provide a change in air quality during the attainment period that is equivalent or more expeditious than the air quality change that would be estimated to occur under the primary approach. This optional analysis was referred to as an equivalency determination.
	Under the second proposed option, the state would provide the control strategy implementation schedule and estimate the emissions reductions anticipated from the control measures (i.e., RACM/RACT and additional reasonable measures) for sources in the nonattainment area. The state then would employ modeling or another quantitative method to predict the overall PM2.5 concentrations in the nonattainment area for each milestone year. The milestone years would correspond to the years for which the state would be required to provide quantitative milestones pursuant to the requirement in section 189(c) of the Act.
      b. Final Rule.
      The EPA is finalizing RFP requirements that allow the state flexibility to demonstrate RFP under CAA section 172(c)(2) using any of the general approaches included in the proposed rule. As part of its Moderate area attainment plan submission, the state must submit an RFP plan that includes three components: (1) an implementation schedule for control measures on sources in the nonattainment area, (2) RFP projected emissions for each applicable quantitative milestone year determined in Section IV.G of this preamble, based on the anticipated control measure implementation schedule; and (3) an analysis that demonstrates that this schedule of aggregate emissions reductions achieves sufficient progress toward attainment between the applicable baseline year to the attainment year. See 40 CFR 51.1012(a).
	The first component of the RFP plan is the implementation schedule for all required control measures contained in the control strategy. The schedule should describe which measures will be implemented within the first 4 years following designation (and therefore would meet the statutory requirement for RACM and RACT). It should also describe the implementation schedule of additional reasonable measures (to be implemented more than 4 years following designation but before the attainment date) that have been adopted to help provide for expeditious attainment of the standard. Any Moderate area that cannot demonstrate attainment by the statutory Moderate area attainment date is required to provide an implementation schedule for all of the control measures identified as RACM/RACT and additional reasonable measures, in the same manner as an area that can demonstrate attainment.
	The second component of the RFP plan is an analysis by the state identifying the RFP projected emissions by pollutant that are expected to be achieved by the control measures implemented within the nonattainment area according to the implementation schedule. The EPA requires the state to estimate these RFP projected emissions for each quantitative milestone year (i.e., for a Moderate area, at 4.5 years and 7.5 years after designation of the area) by sector on a pollutant-by-pollutant basis. These milestone year projected emissions are discussed further in Section IV.F.3 of the preamble. This information will be used by the state to show that the area is complying with quantitative milestone and RFP requirements for the area (discussed in Section IV.G of this preamble).
      The final component of the RFP plan is an analysis demonstrating that the schedule of emissions changes achieves reasonable progress toward attainment between the applicable baseline year to the attainment year. This demonstration can be expressed in the form of emissions reductions only, or emissions reductions converted to air quality concentrations. This optional air quality RFP analysis is discussed later in this section.
	Because the statute does not clearly establish the applicable baseline year from which to begin calculating annual emissions reductions for purposes of demonstrating RFP, the EPA is finalizing a requirement that states use the same year as the base year inventory used for developing the control strategy and associated air quality modeling demonstrating that the area will attain expeditiously.
      A demonstration based on only emissions reductions must show that the implementation schedule achieves either: (i) generally linear progress toward the projected attainment date; or (ii) stepwise progress toward the projected attainment date. For example, in one area new emission standards for mobile sources may achieve reductions in a generally linear manner over time, as a portion of the existing vehicle fleet is replaced each year with new vehicles meeting the more stringent standards. In another area, regulations to reduce emissions from certain stationary source sectors could have a single compliance date by which controls must be in place, which could result in a significant drop in emissions in a "stepwise" manner over a relatively short period.
      In the first case, the EPA expects that, so long as the attainment date is as expeditious as practicable, then generally linear progress toward attainment by that date would satisfy the RFP requirement. In the second case, where progress is slower than generally linear, the state is required to submit a clear rationale and supporting information to explain why generally linear progress is not appropriate (e.g., due to the nature of the nonattainment problem, the types of sources contributing to PM2.5 levels in the area and the implementation schedule for control requirements at such sources).
      Similarly, for areas that cannot demonstrate attainment within the Moderate area statutory deadline in CAA section 188(c)(1), the state must demonstrate either generally linear or stepwise emissions reductions toward the full amount of reductions that will be achieved by that deadline, i.e., the amount that reflects implementation of all of the control measures identified as RACM and RACT and additional reasonable measures for the entire period of the applicable attainment plan. Generally linear progress toward this full amount would meet the RFP requirement, but progress that is slower than that would require further justification.
      In some circumstances, the EPA expects that a state could develop an approvable RFP plan even if emissions of one or more PM2.5 plan precursors are not decreasing. In this scenario, the state must demonstrate that the emissions reductions of direct PM2.5 combined with the aggregate emissions reductions of PM2.5 plan precursors support expeditious attainment of the applicable PM2.5 NAAQS. To accomplish this, the EPA expects that a state could use the relative air quality impacts of the different PM2.5 plan precursors identified in the attainment modeling to demonstrate that the emissions reductions of direct PM2.5 and aggregate PM2.5 plan precursors constitute an acceptable RFP plan. For example, the state could demonstrate that even if one or more PM2.5 plan precursor is not decreasing, the emissions reductions of direct PM2.5 and the remaining PM2.5 plan precursors are the dominant factors in reducing ambient PM2.5 levels and are therefore adequate to support expeditious attainment. In providing this flexibility, the EPA recognizes that control measures for certain pollutants may be more effective at reducing PM2.5 concentrations than others, and that states may be able to implement some measures more quickly than others while still achieving reasonable overall progress toward attainment.
      The EPA is also providing an additional optional RFP analysis that evaluates the collective changes in emissions of multiple pollutants during the attainment period in terms of changes in air quality concentration. Under this optional approach, a state would have to show that the air quality improvement that is anticipated by milestone dates due to the identified control measures in the implementation schedule supports expeditious attainment of the PM2.5 NAAQS. For an area that can demonstrate attainment within the Moderate area statutory deadline, a state using this option could rely upon attainment demonstration modeling results that link emissions reductions with air quality improvements. For areas that cannot demonstrate attainment within the Moderate area statutory deadline, the state may have to conduct modeling or employ another quantitative method to predict the overall PM2.5 concentrations in the nonattainment area in each milestone year. The state would compare these air quality target values to certified ambient air quality monitoring data as part of the quantitative milestone report due after the area reaches each quantitative milestone date. The EPA recommends that states estimate air quality targets by establishing the relationship between modeled emissions reductions and air quality changes in the attainment plan (for the attainment year) and interpolating to the intermediate year(s) based on the same relationship.
	The EPA recognizes that because atmospheric processes are complex, a specific percent change in emissions of PM2.5 precursors does not lead to an equivalent percent change in air quality, potentially creating uncertainty when determining air quality targets based upon predicted emissions reductions. Nevertheless, the EPA recognizes the importance of providing the flexibility to address different pollutants on different timetables so long as the plan can reasonably be expected to achieve the intended air quality benefits represented by the RFP analysis.
	As previously noted, submission of the air quality-based RFP plan is optional. However, in certain circumstances, the applicable Regional Administrator may strongly recommend that a state or local agency submit an RFP plan with air quality targets for milestone years in order to satisfy the statutory RFP requirement. This approach could be appropriate when one or more pollutants is not decreasing over the attainment planning period or for areas that have experienced longstanding and persistent PM2.5 pollution problems despite the prior implementation of required control measures. The EPA will review each RFP plan on a case-by-case basis to determine whether it provides for such annual incremental reductions in emissions of the relevant air pollutant(s) as are necessary for the purpose of ensuring attainment by the applicable attainment date. See 40 CFR 51.1012. An additional RFP analysis will be required as part of a Serious area attainment plan if EPA reclassifies the area to Serious.

      c. Comments and Responses.
      Comment: Some commenters generally supported the equivalency determination concept because they noted that different precursors are more or less effective in reducing atmospheric concentrations of PM2.5. Some commenters stated that, because the goal is timely attainment regardless of when controls are implemented for each precursor that is to be controlled, the EPA should allow both options, including the option for states to determine whether to approach the demonstration on a pollutant-by-pollutant basis, or overall.
      Response: The final rule allows for emissions from one or more PM2.5 plan precursors to increase over the attainment planning period, as long as the emissions of direct PM2.5 and aggregate PM2.5 plan precursors decrease consistent with RFP and the state can demonstrate that the emissions increase does not delay expeditious attainment. This approach recognizes the fact that different precursors have different impacts on PM2.5 concentrations depending upon the specific atmospheric chemistry of each area. As previously noted, submission of the air quality-based RFP plan is optional but may be strongly recommend by the applicable Regional Administrator depending upon the unique circumstances of the nonattainment area.
      Comment: One commenter suggested that the equivalency determination must be based on modeling of ambient concentrations, not simply on inter-pollutant equivalency ratios.
      Response: The EPA recognizes the importance of modeling and notes that there are potential benefits of using modeling when providing the optional air quality analysis. Additionally, in the preamble, the EPA described when these types of tools could be useful in the development the air quality targets. Because the development of air quality targets is optional and complements the emissions reductions analysis, the EPA does not require modeling of ambient conditions for this purpose.
      Comment: A few commenters supported the proposed Option 1 and the allowance for either generally linear or stepwise progress toward attainment. These commenters stated that allowing both methods is consistent with the pattern of many federal emissions reduction measures and it provides the most flexibility to states. Other commenters stated that existing guidance in the Addendum failed to recognize that, in many cases, more can be accomplished during one given year than in another. The commenters suggested the EPA provide states with the flexibility to manage their resources for rulemaking such that emissions reductions are obtained to attain generally linear progress averaged over the 3-year period rather than in each individual year.
      Response: As stated earlier, this rule requires that the RFP analysis must demonstrate either generally linear or stepwise emissions reduction progress toward attainment. If there are significant differences between emissions reductions in different years, which make the emissions reductions no longer generally linear, then the state would have to provide a justification for the stepwise progress as discussed earlier. Therefore, the suggestion of averaging the emissions reductions to obtain generally linear progress over a 3-year period is not an acceptable way to demonstrate RFP. In this example, the state would have to submit a justification of why stepwise emissions reductions are more appropriate for their area. However, the EPA notes that if stepwise emissions reductions are achieved more rapidly than expected and consistent with the amount necessary to demonstrate RFP toward timely attainment, this would be in line with the overall principles of the CAA and would not require the aforementioned justification.

3. RFP Projected Emissions for RFP Analyses.
      a. Summary of Proposal.
      The EPA proposed that a state with a Moderate PM2.5 nonattainment area must submit RFP projected emissions for sources within the nonattainment area as part of the RFP plan. The EPA also proposed that these RFP projected emissions would, at a minimum, include projected emissions of each pollutant by different source types corresponding to the quantitative milestone dates for the area.
      b. Final Rule.
      The EPA is finalizing that a state with a Moderate PM2.5 nonattainment area must submit RFP projected emissions for sources within the nonattainment area as part of the RFP plan. These RFP projected emissions shall, at a minimum, include projected emissions of each pollutant (i.e., direct PM2.5 and PM2.5 plan precursors) by different source types corresponding to the quantitative milestone dates for the area (quantitative milestone dates are described in greater detail in Section IV.G of this preamble). Specifically, the EPA requires that the RFP plan for any Moderate area must contain RFP projected emissions for each calendar year in which quantitative milestones for a Moderate nonattainment area must be met. As explained in Section IV.G of this preamble, a state must identify as part of the attainment plan submission for a Moderate nonattainment area quantitative milestones to be achieved every 3 years from the Moderate area attainment plan due date, or 4.5 years from the effective date of designation of the area. For example, the first round of designations for the 2012 PM2.5 NAAQS became effective in April 2015; Moderate area attainment plans for these areas will thus be due 18 months later, or in October 2016. The first quantitative milestones for each of these areas will then have to be met in October 2019; the second quantitative milestones, in October 2022; and so on, until the area attains the NAAQS. Under this approach, the state will be required to submit such RFP projected emissions as part of the Moderate area attainment plan due in October 2016 that project emissions from sources in the nonattainment area for the same calendar years as those for which quantitative milestones will be due (i.e., 2019 and 2022 inventories in this example).
	The transportation conformity rule requires that RFP plans establish motor vehicle emissions budgets. RFP plans would therefore be required to establish motor vehicle emissions budgets for direct PM2.5 and any relevant PM2.5 precursor as determined under the transportation conformity rule. Precursors that are relevant for transportation conformity purposes would be limited to the PM2.5 plan precursors but may not include all of the PM2.5 plan precursors. For example, it is likely that many PM2.5 plans will include SO2 as a plan precursor. However, emissions of SO2 from on-road sources are usually low compared to stationary sources. The transportation conformity rule allows for the state to determine through its SIP development process if it is necessary to establish motor vehicle emissions budgets for SO2. See 40 CFR 93.102(b)(2)(v). On the other hand, if VOCs are not identified as a PM2.5 plan precursor, motor vehicle emissions budgets for VOCs are not required to be established for transportation conformity purposes. A motor vehicle emissions budget for the purposes of a PM2.5 RFP plan is that portion of the total allowable emissions allocated to on-road sources as defined in the submitted RFP plan for the relevant years as described earlier. Such motor vehicle emissions budgets will 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 the EPA approves the state's use of an alternative model.
      c. Comments and Responses.
      Comment: Some commenters stated that since RFP is one of the general attainment plan provisions listed in CAA section 172(c), the EPA's proposal to require motor vehicle emissions budgets as part of RFP plans extends beyond just the implementation of the PM2.5 NAAQS and, as a result, this proposal should be presented within the context of a revision to the conformity rule itself and not just this PM2.5 implementation rule.
      Response: The EPA disagrees with the commenters. The transportation conformity rule already states that motor vehicle emissions budgets come from control strategy SIPs. Additionally, the transportation conformity rule defines control strategy SIPs as RFP plans and attainment demonstrations. It goes further to say that control strategy SIPs include the SIPs required by CAA sections 172(c), 189(a)(1)(B) and 189(b)(1)(A). The requirement in this PM2.5 SIP Requirements Rule does not amend the transportation conformity rule; it merely explains what is already required.
4. Geographic Coverage of Emission Sources for RFP.
      a. Summary of Proposal.
      The EPA proposed that the RFP demonstration to be included with a state's PM2.5 nonattainment area plan must include emissions only for sources located in the nonattainment area, and not from an area larger than the nonattainment area. This proposed policy approach differed from the remanded 2007 PM2.5 implementation rule. As explained in the proposal, the difference was due to the evolution of policy on a similar RFP issue in the ozone NAAQS implementation program that stemmed in part from a petition for reconsideration and a D.C. Circuit decision on the November 2005 Phase 2 Ozone Implementation Rule. The EPA received a similar petition for reconsideration of the 2007 PM2.5 Implementation Rule, which dealt with the EPA's interpretation of the statutory RFP requirements to allow a state to take "credit" for emissions reductions from outside the nonattainment area when addressing RFP in its attainment plan. The EPA granted the petition for reconsideration on this issue in 2010, after the D.C. Circuit issued its decision on litigation on the Phase 2 Ozone Implementation Rule.,  In light of these developments, the proposal indicated that the EPA now believes the best reading of the statute is that the CAA does not allow for a state to include emissions reductions from sources outside a nonattainment area when developing the plan to meet the CAA section 172(c)(2) RFP requirements for a PM2.5 nonattainment area.
      b. Final Rule.
      The EPA is finalizing that the RFP demonstration to be included with a state's PM2.5 nonattainment area plan must include emissions only for sources located in the nonattainment area, and not from an area larger than the nonattainment area. Commenters disagreed with limiting the RFP demonstration to include emissions only for sources located in the nonattainment area stating that some areas are so dominated by upwind emissions and local sources over which they have no control, such as motor vehicles, that they cannot demonstrate RFP. One of these commenters noted they have provided extensive comments on this issue in connection with the EPA's proposal for the recent ozone implementation rule and incorporated by reference their prior comments. Other commenters agreed with the proposal and stated the EPA's conclusion is compelled by CAA sections 172(c)(1), 172(c)(3) and 189(d), which all focus on emissions and reductions in the area.
      In the preamble to the remanded 2007 PM2.5 Implementation Rule, the EPA allowed states to incorporate reductions of NOx and SO2 emissions up to 200 km from outside the nonattainment area (and potentially for reductions of VOC or ammonia) into their RFP plan when certain conditions were met. This policy was included in the 2007 PM2.5 Implementation Rule in part to be consistent with a similar RFP policy for NOx and VOC that was included in the November 2005 Phase 2 ozone NAAQS implementation rule, which provided guidance for states on implementing the 1997 ozone NAAQS. Under this policy, if a state intended to include emissions reductions from outside the nonattainment area in the RFP plan, the state would need to take on the additional accounting work associated with developing: (i) an expanded baseline emissions inventory for the entire geographic area and, (ii) a projected attainment year inventory for this expanded area outside the boundaries of the designated nonattainment area. Development of these more extensive inventories would likely have involved a substantial amount of additional time and resources. In addition, the state would have needed to provide information supporting its decision regarding how far outside the nonattainment area the RFP inventory should extend. While this "outside the nonattainment area" RFP approach was theoretically available to states in developing their PM2.5 attainment plans due in 2008, there were no states to the agency's knowledge that elected to follow this approach.
	Both the Phase 2 ozone implementation rule and the 2007 PM2.5 Implementation Rule were challenged on several issues. With regard to the Phase 2 ozone implementation rule, the EPA granted a petition for reconsideration and ultimately issued a final notice of reconsideration in June 2007. In November 2008, the U.S. Court of Appeals for the D.C. Circuit heard oral argument concerning multiple petitions for judicial review of the Phase 2 ozone rule and the notice of reconsideration. One of the issues in that case involved whether compliance by EGUs with a regional emissions trading program could be considered to meet the RACT requirement for those sources located in a nonattainment area. In its July 2009 decision, the court emphasized that: ``the RACT requirement calls for reductions in emissions from sources in the area; reductions from sources outside the nonattainment area do not satisfy the requirement. Accordingly, participation in the NOx SIP call would constitute RACT only if participation entailed at least RACT-level reductions in emissions from sources within the nonattainment area."
	In light of this court decision, the EPA has determined that the best reading of the statute is that the term "sources in the area" should be interpreted in the same manner as ozone. The term appears in CAA section 182 (requirements for ozone nonattainment areas) with regard to RFP as well as RACT. The decision on the Phase 2 ozone rule found that CAA section 182(b)(2) requires that a SIP must provide for implementation of RACT (under CAA section 172(c)) for emissions sources "in the area," meaning in the nonattainment area. Similarly, the EPA position is that when CAA section 182(b)(1)(A)-(B) defines baseline emissions for RFP as "the total amount of actual VOC or NOx emissions from all anthropogenic sources in the area," this also means sources in the nonattainment area.
	 Turning to PM2.5, the EPA has determined that the D.C. Circuit's interpretation of the phrase "sources in the area" should apply to RACT and RFP requirements for both the ozone NAAQS and the PM2.5 NAAQS. In particular, for PM2.5, the statutory language at CAA section 171(1) defines RFP in terms of "reductions in emissions" required in an attainment plan, which the EPA interprets as being directly linked to the baseline emissions inventory for sources located in a PM2.5 nonattainment area. The baseline emissions inventory is the foundation for the attainment plan. The emissions inventory requirement of CAA section 172(c)(3) explicitly requires that the attainment plan inventory include all sources of the relevant pollutants "in such area," which is a clear reference to the designated nonattainment area. Given that the baseline inventory must reflect the emissions "in such area," and that this inventory provides the starting point for a state's RFP analysis, in which the state must calculate generally linear progress in emissions reductions that will lead to attainment of the NAAQS in the area, the EPA believes it is appropriate that a state should consider only sources located within the nonattainment area when conducting its analysis to determine the annual emissions reductions necessary for demonstrating RFP.
	Beyond the Court's interpretation, the EPA believes that the most appropriate approach with regard to the geographic area required to be covered for demonstrating RFP in a PM2.5 attainment plan also should be limited to the nonattainment area for two other reasons. First, the EPA believes that it makes policy sense for the PM2.5 implementation rule approach to be consistent with the approach finalized in the 2008 NAAQS for Ozone: SIP Requirements rule. Second, a policy allowing the geographic area of the RFP plan to be larger than the nonattainment area would conflict with a key provision of subpart 4 that requires annual incremental reductions in emissions from sources within the nonattainment area. Under subpart 4, an area that fails to attain the standard by the Serious area attainment date is then subject to the provisions of CAA section 189(d). Section 189(d) of the CAA specifies that the state must submit a plan revision within 12 months which 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 as reported in the most recent inventory prepared for such area" (emphasis added). Therefore, the EPA is finalizing an RFP policy approach that is consistent with CAA section 189(d).
      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.
5. Other RFP Considerations.
      a. Summary of Proposal.
      The proposal outlined the statutory requirements and existing guidance for RFP. During this discussion, the following guidance from the Addendum was referenced, "Additionally, the EPA believes that it is appropriate to require early implementation of the most cost-effective control measures... while phasing in the more expensive control measures." The proposal also discussed other RFP considerations, including PM2.5 nonattainment areas that are shared by more than one state or tribe.
      b. Final Rule.
      The EPA is finalizing that, although early implementation of the most cost-effective control measures is often appropriate, states should consider both cost-effectiveness and pollution reduction effectiveness when developing implementation schedules for their control measures and may implement measures that are more effective at reducing PM2.5 earlier to provide greater public health benefits. This increased flexibility enables states to develop a more effective implementation schedules for their control measures while efficiently using their resources.
      For a multi-state or multi-jurisdictional nonattainment area, the RFP plans for each state represented in the nonattainment area shall 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. Each state must ensure that the sources within its boundaries comply with enforceable emission levels and other requirements that in combination with the reductions planned in other states within the nonattainment area will provide for attainment as expeditiously as practicable and demonstrate RFP consistent with these regulations. In general, the EPA seeks to ensure that PM2.5 nonattainment areas that are shared by more than one state or tribe meet RFP requirements as a whole. States and tribes that share a nonattainment area should therefore consult with one another to develop the RFP analysis and control strategy implementation schedule for the area as a whole. Such states and tribes should work with the EPA region or regions that oversee them to confirm that their collective approach is appropriate for RFP.
	The EPA's approach for states to meet the RFP requirement is designed to ensure emissions reductions will yield incremental improvements in air quality on the path to attainment, while being sufficiently flexible to accommodate the range of control strategies necessary to address the complex mixtures of pollutants comprising PM2.5 in different areas.
      c. Comments and Responses.
      Comment: Some commenters asserted that the EPA should not "require" implementing the most cost-effective measures first since states should have the flexibility to implement the more effective but less cost-effective measure earlier, thus providing earlier and greater public health benefits.
      Response: In this final rule, the EPA is providing states with the flexibility to implement measures that are more effective at reducing PM2.5 earlier to provide greater public health benefits, but is not requiring it. This increased flexibility is in keeping with the overall requirement of expeditious attainment of the NAAQS.
G. Quantitative Milestones
1. General Approach to Quantitative Milestones.
	a. Summary of the Proposal.
      The proposal built from the statutory language of 189(c)(1), which requires quantitative milestones that (1) demonstrate RFP, and (2) must be achieved every 3 years until the area is redesignated attainment. The proposal first addressed the issue of the starting date for counting the 3-year periods. For a Moderate area that cannot practicably attain the relevant PM2.5 NAAQS within the statutory timeframe for a Moderate area, the EPA proposed that a state must submit two sets of quantitative milestones  -  the first set to be achieved at year 4.5 from designation and the second set to be achieved at year 7.5 from designation. The EPA also proposed that the quantitative milestones contained in the attainment plan for a Moderate nonattainment area must be constructed such that they can be tracked, quantified and/or measured adequately in order for the state to meet its milestone reporting obligations, which come due 90 days after a given milestone date. The EPA therefore proposed to require that states select the quantitative milestones that are appropriate and quantifiable and that will provide for objective evaluation of progress toward attainment in their Moderate PM2.5 nonattainment area, whether the area can practicably attain the PM2.5 NAAQS by the statutory attainment date or not. In addition to this general proposed approach for selecting quantitative milestones for a Moderate nonattainment area, the EPA proposed a requirement that, at a minimum, states must include in all attainment plans for Moderate PM2.5 nonattainment areas a metric to confirm that all control measures identified and adopted as RACM and RACT for the area have been fully implemented within 4 years of designation.
      b. Final Rule.
      Section 189(c) of the Act explicitly requires that quantitative milestones must be achieved every 3 years, but does not specify the starting date for counting the 3 year periods. In the General Preamble and Addendum, the agency stated that quantitative milestones must be achieved every 3 years starting from the due date for the plan submission (i.e., because the Moderate area attainment plan is due no later than 18 months after designation of the area, the first set of milestones would need to be achieved 4.5 years after the area's designation) until the area is redesignated attainment. The EPA is finalizing this approach for the PM2.5 NAAQS. The EPA interprets this requirement to be the most appropriate reading of CAA section 189(c)(1) which requires "quantitative milestones which are to be achieved every 3 years until the area is redesignated attainment." This approach is also consistent with the longstanding approach outlined in the General Preamble. These timeframes for the quantitative milestones apply to all areas designated nonattainment for a PM2.5 NAAQS on or after January 15, 2015, including all areas designated nonattainment effective April 15, 2015 for the 2012 PM2.5 NAAQS. See 40 CFR 51.1013(a).
      For all areas designated nonattainment for the 1997 and/or 2006 PM2.5 NAAQS before January 15, 2015, the EPA is establishing December 31, 2014 as the starting point for the first 3 year period for quantitative milestones under CAA section 189(c). This is because
December 31, 2014, was the due date for states to submit additional SIP elements necessary to satisfy the subpart 4 Moderate area requirements for the 1997 and 2006 PM2.5 standards. Establishing December 31, 2014 as the starting point for the first 3 year period under CAA section 189(c) for the 1997 and 2006 PM2.5 standards is in keeping with the EPA's historical approach to quantitative milestone dates (i.e., using the due date for the Moderate area plan submission as the starting point for the first 3 year milestone period). Thus, for any area designated nonattainment for the 1997 and/or 2006 PM2.5 NAAQS that has not yet attained these NAAQS and therefore continues to have attainment planning obligations for these NAAQS, the state must submit quantitative milestones to be achieved by December 31, 2017 (the first milestone date), at minimum. Additional milestone dates may also apply to such an area depending on the relevant attainment planning period. See 40 CFR 51.1013(a)(4).
      Regardless of whether or not an attainment plan demonstrates attainment by the statutory attainment date, the EPA requires that all Moderate area PM2.5 attainment plans must define appropriate quantitative milestones to be achieved by 4.5 years and 7.5 years following designation of the area. Although it occurs after the Moderate area attainment date, the EPA is requiring Moderate area plans to contain this 7.5 year milestone, even where those plans demonstrate attainment before the milestone would arrive. Where the EPA uses its discretionary authority in CAA section 188(b)(1) to reclassify an area before the Moderate area attainment date applicable to the area, CAA section 189(b)(2) allows the state up to 4 years to submit the required Serious area attainment demonstration and related plan elements, such as quantitative milestones. For example, if the reclassification occurs 4 years after designation, the state may be allowed to submit the Serious area quantitative milestones up to 8 years after designation, well after the second milestone date (7.5 years after designation) has passed. Without the 7.5 year milestone requirement, this circumstance would undermine the purpose of the quantitative milestone requirement, which is to ensure that states will report to EPA on each nonattainment area's progress toward attainment at regular intervals, even following reclassification.
      A similar issue would result in the event that a Moderate area that did demonstrate attainment in the original attainment plan fails to attain by the statutory attainment date. In this case, the area would have failed to meet the attainment date which is as expeditiously as practicable but no later than the end of the sixth calendar year after designation as required by CAA section 188(c)(1). Section 188(b)(2) of the Act allows the Administrator up to 6 months to determine that a Moderate failed to attain and reclassify that area to Serious, which would be at least 6.5 years after designation. As described in Section VI.A.1 of this preamble, the Serious area would have 18 months from reclassification due to a failure to attain (8 years after designation) in order to submit an attainment plan. The EPA has therefore determined that, in order to avoid gaps of greater than three years in the implementation of quantitative milestones, all Moderate area attainment plans must contain quantitative milestones to be achieved 4.5 years and 7.5 years after designation and which demonstrate continued progress toward timely attainment of the relevant PM2.5 NAAQS. In the event that the area fails to attain, this will provide the EPA with appropriate tools necessary to continue to monitor the area's continued progress toward attainment while the state develops the Serious area attainment plan.
	The quantitative milestones contained in the attainment plan for a Moderate nonattainment area should be constructed such that they can be tracked, quantified and/or measured adequately in order for the state to meet its milestone reporting obligations, which come due 90 days after a given milestone date. In the Addendum, the EPA suggested some possible metrics that "support and demonstrate how the overall quantitative milestones identified for an area may be met," such as percent implementation of control strategies, percent compliance with implemented control measures, and adherence to a compliance schedule. This list was not exclusive or exhaustive but reflected the EPA's view that the purpose of the quantitative milestone requirement is to provide an objective way to determine whether the area is making the necessary progress towards attainment by the applicable attainment date. The EPA interprets Section 189(c) of the Act to allow states to identify milestones that are suitable for the specific facts and circumstances of the attainment plan for a particular area, so long as they provide an objective means to measure RFP.
	This rule requires that each attainment plan for a Moderate PM2.5 nonattainment area contain quantitative milestones that provide for objective evaluation of RFP toward attainment in the PM2.5 nonattainment area, whether the plan provides for attainment of the PM2.5 NAAQS by the statutory attainment date or demonstrates that attainment by such date is impracticable. For this approach, the EPA does not require that such quantitative milestones take any particular form, merely that they provide a means to evaluate progress (i.e., demonstrate RFP) meaningfully. The EPA will review each attainment plan submission on a case-by-case basis to determine whether the quantitative milestones contained in the plan are specific enough to provide an objective means for evaluating the area's progress toward attainment, consistent with the statutory requirements of CAA section 189(c). The EPA recommends that states confer with their respective EPA regional office to develop appropriate quantitative milestones. See 40 CFR 51.1013(a).
	The Addendum stated that the Moderate area quantitative milestones "will be met by showing that emission reductions scheduled to be made between the SIP due date and the attainment date for these moderate areas were actually achieved. Most of these emission reductions will result from implementation of RACM (including RACT) as part of the moderate area SIP." However, this rule does not specify that quantitative milestones must be expressed in terms of emissions reductions. The EPA recognizes that it is impractical to expect that a state will always be able to quantify and compare real and projected emissions 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, at a minimum, states must include in all attainment plans for Moderate PM2.5 nonattainment areas a milestone that all control measures identified and adopted as RACM and RACT for the area have been fully implemented within 4 years of designation. This milestone specifically derives from section 189(a)(1)(C) of the Act, that applies to all Moderate areas and thus represents a milestone that all Moderate areas must meet regardless of whether it is listed explicitly as an individual milestone. See 40 CFR 51.1013(a)(1)(iii).
      For an area that submitted air quality targets with the RFP plan under the optional provision that was described in Section IV.F of this preamble, 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. The EPA recognizes that certified monitored ambient air data are not available for some period after a calendar year ends. As a result, the EPA expects that this quantitative milestone may sometimes be satisfied with data that are over a year old. For example, for Moderate areas having an effective date of designations in April 2015, the first quantitative milestone date will be in October 2019 (3 years after the 18 month SIP due date), and the associated report will be due 90 days later, in January 2020. In this example, the state would likely have to rely upon certified air quality data for 2018 because data for the 2019 calendar year would not yet be fully certified. Additionally, this milestone should normally be reported in the same form as the applicable PM2.5 NAAQS. However, the EPA expects that in some circumstances, it may be appropriate to use annual averages instead of the 3 year average to help justify variations due to meteorological occurrences.
      c. Comments and Responses.
      Comment: Some commenters stated the Act may be read to conclude that the requirement to include any quantitative milestones in a Moderate area plan does not apply to a plan demonstrating the impracticability of attaining the NAAQS by the attainment date.
      Response: The EPA's longstanding interpretation of CAA section 189(c) as a requirement that applies to all PM10 nonattainment area plans, including those demonstrating that attainment by the applicable attainment date is impracticable, is consistent with the purpose and structure of subpart 4. The design of the CAA for PM nonattainment areas combines the requirements of subpart 1 and subpart 4 to support expeditious attainment of the applicable NAAQS. RFP is the prescribed tool available to a state to plan their emission reduction progress toward expeditious attainment. Quantitative milestones are a critical aspect of the CAA and the attainment plan in order for the EPA to monitor the area's RFP toward expeditious attainment and trigger the appropriate response if RFP is not maintained. The EPA thus determined that allowing an area to simply not submit any quantitative milestones would not afford the EPA the necessary tools to monitor RFP toward expeditious attainment.
2. Milestone Report Submission.
      a. Summary of the Proposal.
      Because the statute does not define the parameters of quantitative milestone demonstrations, the EPA has discretion to determine the components of the required demonstration and the form and manner for submission. The proposal took comment on options for doing this. The EPA proposed to require that the milestone report submission must include the following four components: i) a certification by the Governor or Governor's designee that the state's attainment plan control strategy is being implemented as described in the applicable attainment plan, ii) technical support sufficient to document completion statistics for appropriate milestones and to demonstrate that the quantitative milestones have been satisfied and how the emissions reductions achieved to date compare to those required or scheduled to meet RFP, iii) as applicable, an air quality screening analysis to determine if measured air quality progress is consistent with the expected air quality improvement target correlated with the RFP emissions reductions for the previous 3 year period, and iv) an evaluation of whether the PM2.5 NAAQS will be attained by the projected attainment date for the area. In addition, the EPA proposed that the milestone report must include a description and schedule for any remedial actions the state has taken or will take to address any failure to meet a quantitative milestone, including the implementation status of contingency measures for failing to meet RFP in the area.
	The EPA also sought comment on how electronic reporting could facilitate a state's submission of the required milestone report, how it could accommodate the various narrative and data-dependent components that the EPA proposed be part of such a submission, and what particular system features might be desirable to accommodate milestone report submissions through the eSIP system.
      b. Final Rule.
      The final rule, mirroring section 189(c)(2) of the Act, requires that each state containing a PM2.5 nonattainment area submit to EPA, within 90 days after each milestone date applicable to the area, a demonstration that all measures in the approved plan (including the RFP plan) for the area have been implemented and that the milestone has been met. This rule outlines the content required by the EPA for the quantitative milestone report. The EPA must then determine whether or not a state's demonstration is adequate within 90 days after receiving a demonstration which contains the required information and analysis. The EPA intends to promptly inform the relevant state of any determination that the state has failed to submit a timely quantitative milestone report and any determination that a submitted milestone report is not adequate.
	The EPA will work with a state to assist them in meeting the reporting deadline, and expects that, because the report is to be fairly low burden and may be submitted electronically through eSIP, in most cases the state will submit it on time, especially if they have implemented the programs required to meet their milestones.  If, however, a state fails to submit a milestone demonstration report by the due date or the EPA determines that a milestone was not met, the final rule requires the state to submit a SIP revision within 9 months of either the missed reporting deadline or the EPA's determination of the state's failure to meet a milestone. According to the statutory requirements of CAA section 189(c)(3), the new SIP revision must assure "that the State will achieve the next milestone (or attain the national ambient air quality standard ..., if there is no next milestone) by the applicable date." If a state fails to make a SIP submission to correct a failure to meet RFP expeditiously, sanctions under CAA sections 110(m) and 179(b) may apply. If a state is unable to correct a failure to meet RFP, this may be evidence that the state cannot practicably attain the NAAQS by the applicable attainment date and may serve as a basis for reclassification of the area to Serious under CAA section 188(b)(1).
	As previously noted, the EPA has offered guidance about what the milestone report should contain. The Addendum says, "This report must contain technical support sufficient to document completion statistics for appropriate milestones. For example, the demonstration should graphically display RFP over the course of the relevant 3 years and indicate how the emissions reductions achieved to date compare to those required or scheduled to meet RFP and the required [quantitative] milestones. The calculations (and any assumptions made) necessary to determine the emissions reductions to date should also be submitted. The demonstration should also contain an evaluation of whether the PM10 NAAQS will be attained by the projected attainment date." This guidance is still appropriate for states demonstrating compliance with RFP and quantitative milestones for PM2.5 NAAQS. The EPA requires that the milestone report submission must include the following components. See 40 CFR 51.1013(b).
      First, the report must include a certification by the Governor or Governor's designee that the SIP control strategy is being implemented consistent with the RFP plan, as described in the applicable attainment plan. Second, the report must contain technical support, including calculations, sufficient to document completion statistics for each quantitative milestone and to demonstrate that the quantitative milestones have been satisfied and how the emissions reductions achieved to date compare to those required or scheduled to meet RFP. Additionally, the report must include a discussion of whether the PM2.5 NAAQS will be attained by the projected attainment date for the area. See 40 CFR 51.1013(b). The EPA decided not to finalize the proposed requirements to include an air quality screening analysis or the description and schedule for remedial actions taken by the state to address a failure to meet a quantitative milestone. This decision was made because the remaining components of the quantitative milestone report are sufficient to enable the EPA to assess whether the nonattainment area is meeting RFP.
	As stated in the Addendum, the milestone report must be submitted from the Governor or Governor's designee to the Regional Administrator of the respective EPA Regional Office serving the submitting state. The EPA will notify the state of its determination (regarding whether or not the state's report is adequate) by sending a letter to the appropriate Governor or Governor's designee. The EPA encourages states to submit milestone reports, including supporting documents, through the agency's electronic SIP (eSIP) submission system in order to simplify the process and reduce resource burden on all sides.
      c. Comments and Responses.
      Comment: Some commenters did not support the proposal and stated that requiring this level of documentation is unnecessary and puts an excessive workload burden on states and local agencies.
      Response: The EPA recognizes that there is some level of resources required to address the requirements prescribed by every rule. However, the EPA concluded that the benefit offered to the public by reviewing quantitative milestone reports while assessing whether nonattainment areas are making reasonable further progress toward attaining the PM2.5 NAAQS and the associated public health benefits outweigh the anticipated workload burden for states.
      Comment: Some commenters stated that the first two components of the quantitative milestone report described in the proposal are sufficient to comply with the requirements of CAA section 189(c)(2). The commenters stated that the proposed air quality screening analysis is not supported by the statute and is unnecessary if the second component is fulfilled. The commenters stated that the proposed description and schedule for remedial actions the state has taken or will take to address any failure to meet a quantitative milestone is more than what is necessary to demonstrate compliance with RFP milestones and could require revisions to the SIP.