The EPA is also clarifying its interpretation of the statutory language under CAA section 189(d) that requires a state to submit a new attainment plan to achieve annual reductions "from the date of such submission until attainment," to mean annual reductions beginning from the due date of such submission until the new projected attainment date for the area based on the new or additional control measures identified to achieve at least 5 percent emissions reductions annually. This clarification is intended to make clear that even if a state is late in submitting its CAA section 189(d) plan, the area must still achieve its annual 5 percent emissions reductions beginning from the date by which the state is required to make its CAA section 189(d) plan submission, not by some later date. Because attainment dates for PM2.5 nonattainment areas established under subpart 4 occur at the end of the calendar year, any CAA section 189(d) plan, which is required within 12 months of the missed attainment date for the area, would also be due by the end of the calendar year.
	c. Comments and Responses.
      Any additional comments received on this section are addressed in the Response to Comments document found in the docket for this action.
4. Additional Guidance on CAA Section 189(d) Control Measures.
	The EPA believes that an appropriate starting point for a state to identify measures to provide for attainment and to meet the requisite minimum 5 percent annual emissions reductions of direct PM2.5 and PM2.5 precursors is the list of potential control measures initially required to be identified as part of the RACM and RACT determination process, the BACM and BACT determination process, or the MSM determination process (if appropriate) for the area. The EPA anticipates that a state should be able to rely on much of the work it previously undertook to develop this list of potential control measures and analyze their technological and economic feasibility, and the time required to implement them. Control measures that the state identified but did not previously adopt may be likely measures for inclusion in an attainment plan to meet the requirements of section 189(d). However, for purposes of meeting the requirements of CAA section 189(d), the EPA recommends that the state first identify any additional potential measures not previously identified for the area, and then analyze any new or additional measures that the state has not already adopted in a previous attainment plan for the area.
	In addition, a state may include in the CAA section 189(d) plan control strategy for the area any control measures triggered as contingency measures after the area failed to attain the PM2.5 NAAQS by the applicable attainment date. In order to be included as control measures that will help the area meet its requisite minimum 5 percent reductions in direct PM2.5 emissions or in emissions of any one PM2.5 plan precursor, such measures would have to meet the same requirements as all other approvable control measures for being quantifiable, enforceable, replicable and accountable. The EPA believes that reliance on triggered contingency measures may be appropriate given the short timeline provided for in the statute for states to revise and submit their SIP revisions (12 months from the missed attainment date) and the fact that the contingency measures included in the prior attainment plan for the area under CAA section 172(c)(9) must be activated once the EPA publishes its finding of the area's failure to attain the NAAQS by the applicable attainment date. As explained previously, however, the EPA interprets the statute to require that any new 189(d) submission must meet all the statutory requirements applicable to all submissions, including the requirement to identify contingency measures. Thus, if contingency measures from the Serious area attainment plan are relied on in the new attainment demonstration as part of the control strategy, then the state must submit additional contingency measures for the CAA section 189(d) attainment plan. See 40 CFR 51.1003(c)(1)(vii).
5. Control Strategy Submission Requirements.
	To ensure that attainment plan submissions contain the necessary supporting information for the EPA to review and approve the state's new control strategy to achieve at least 5 percent annual reductions in emissions of direct PM2.5 or any PM2.5 plan precursor, the final rule requires that a state must submit information about the new control strategy for an area subject to section 189(d) in a manner consistent with the requirements described in section VI.D.3.
	As with other PM2.5 attainment plan submissions, the EPA believes that it is incumbent on the state to ensure that the information needed for the EPA to evaluate the state's analysis of new control measures  -  which in the case of 189(d) plans is also needed to achieve annual 5 percent reductions -- is presented separately as part of the control strategy analysis, and in a format that provides transparency, consistency and the ability for another party to evaluate the state's analysis effectively and to duplicate the state's results. For this reason, the EPA is including the CAA section 189(d) plan base year emissions inventory information as a necessary part of the control strategy submission and as one element of the state's CAA section 189(d) plan due 12 months after the missed attainment date for the area. In addition, the state must provide information as part of any attainment plan submitted to meet the requirements of CAA section 189(d) consistent with the criteria described in Section VI.D.5 of this preamble to ensure that a state adopts effective regulations to implement the control measures identified as being needed to meet those requirements. Specifically, all control measures must be quantifiable, enforceable, replicable and accountable.
E. Modeling for Attainment Demonstrations
	Section 189(d) of the CAA requires a state with a Serious nonattainment area that failed to attain the relevant NAAQS by the applicable Serious area attainment date to submit a new attainment plan for such area within 12 months after the missed attainment date. The same general requirements for attainment demonstrations and modeling that apply to Moderate area plans and Serious area plans due under CAA sections 189(a) and 189(b) should also apply to CAA section 189(d) attainment plans. However, the EPA is including additional requirements in the final rule specific to plans submitted pursuant to CAA section 189(d), as described in the following sections.

	1. Attainment Demonstrations for Serious Areas That Fail to Attain the NAAQS by the Applicable Attainment Date.
	a. Summary of Proposal.
      The EPA proposed attainment demonstration modeling requirements for Serious areas that fail to attain the NAAQS by the applicable attainment date. See Section VI.E of this preamble, for more details on Serious area attainment demonstrations.
	b. Final Rule.
      The final rule requirements are unchanged from the proposal with respect to this requirement. Attainment demonstrations for Serious areas subject to CAA section 189(d) requirements must consist of: (i) technical analyses such as base year and future year modeling of emissions that identify sources and quantify their emissions that are contributing to violations of the PM2.5 NAAQS; (ii) analyses of future year projected emissions reductions and air quality improvement resulting from national, regional and local programs already implemented as part of previous Moderate and/or Serious area attainment plans for the area (including reasonable control measures, BACM and BACT and additional feasible measures), and (iii) additional measures needed for expeditious attainment, including measures needed to achieve 5 percent emissions reductions on an annual basis. Each state with a nonattainment area subject to the requirements of CAA section 189(d) must submit an attainment plan with an attainment demonstration that includes analyses supporting the state's determination of its proposed new attainment date. In all cases, the state must show that the area will attain the NAAQS as expeditiously as practicable.

	c. Comments and Responses.
      Any comments received on this section are addressed in the Response to Comments document found in the docket for this action.
2. Air Quality Modeling Required for Serious Areas Subject to the Requirements of CAA Section 189(d).
	a. Summary of Proposal.
      The EPA proposed that states are required to submit air quality modeling in support of an attainment demonstration for a nonattainment area subject to the requirements of CAA section 189(d).
      b. Final Rule.
      The final rule requirements are unchanged from the proposal with respect to this issue. States are required to submit air quality modeling in support of an attainment demonstration for a Serious nonattainment area subject to the requirements of CAA section 189(d). The modeling demonstration must show how and when the area will attain the NAAQS. Other than the timing of plan submissions and requirement to achieve 5 percent emissions reductions in direct PM2.5 or any PM2.5 plan precursor, the relevant air quality modeling procedures and guidance for all PM2.5 nonattainment area plans are the same. See Sections IV.E. and VI.E of this preamble for more details on proposed modeling requirements and guidance for Moderate and Serious PM2.5 nonattainment areas, respectively.
	c. Comments and Responses.
      Any comments received on this section are addressed in the Response to Comments document found in the docket for this action.

3. Future Year(s) to be Modeled in Attainment Demonstrations.
	a. Summary of Proposal.
      The EPA proposed that a state performing a modeling analysis for a plan submitted under CAA section 189(d) must select a future modeling year such that all emissions control measures relied on for attainment will have been implemented by the beginning of that calendar year. To demonstrate attainment, the modeling results for the nonattainment area must predict that emissions reductions implemented by the beginning of the last calendar year preceding the attainment date will result in PM2.5 concentrations that meet the level of the standard.
	b. Final Rule.
	As discussed more fully in Section VII.I of this preamble, the EPA must establish a new attainment date for a PM2.5 nonattainment area subject to CAA section 189(d) and must do so according to the provisions of CAA sections 179(d)(3) and 172(a)(2), which require that the new attainment date must be as expeditious as practicable, but no later than 5 years from the date of publication in the Federal Register of the EPA's determination that the area failed to attain the relevant NAAQS. In addition, the EPA may extend the attainment date by up to 5 additional years (thus up to 10 years from the date of publication of the notice of finding of failure to attain by the applicable attainment date for the area) if the EPA deems it appropriate "considering the severity of nonattainment and the availability and feasibility of pollution control measures."
      For purposes of determining the attainment date that is as expeditious as practicable, the state must conduct future year modeling that takes into account emissions growth, known controls (including any controls that were previously determined to be RACM and RACT, , BACM and BACT, and MSM if appropriate, for the area), the 5 percent per year emissions reductions required by CAA section 189(d), plus any other emissions controls that are needed for expeditious attainment of the NAAQS. A state performing a modeling analysis for a plan submitted under CAA section 189(d) must select a future modeling year such that all emissions control measures relied on for attainment will have been implemented by the beginning of that year. To demonstrate attainment, the modeling results for the nonattainment area must predict that emissions reductions implemented by the beginning of the last calendar year preceding the attainment date will result in PM2.5 concentrations that meet the level of the standard.
      For a PM2.5 nonattainment area subject to CAA section 189(d), the state must adopt any control measures necessary to demonstrate expeditious attainment within 5 years of the area failing to attain the NAAQS by the applicable Serious area attainment date.
      c. Comments and Responses.
      Any comments received on this section are addressed in the Response to Comments document found in the docket for this action.
4. Attainment Year Motor Vehicle Emissions Budgets.
	As with all other PM2.5 NAAQS attainment plans, the transportation conformity rule requires that attainment plans for areas subject to CAA section 189(d) establish motor vehicle emissions budgets for the area's attainment year. Therefore, for such an area, the state would first determine the new attainment date as described in Section VII.I of this preamble. 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. 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 the EPA approves the state's use of an alternative model.
F. RFP Requirements
1. Specific Requirements.
      a. Summary of the Proposal.
      The EPA proposed to determine that a state has satisfied the RFP requirement if the state submits an approvable control strategy under CAA section 189(d) that demonstrates that the state will achieve at least 5 percent reductions in direct PM2.5 or PM2.5 precursor emissions from sources in the area annually until attainment. Additionally, the EPA proposed that motor vehicle emissions budgets must also be established as part of any RFP plan for direct PM2.5 and for any relevant PM2.5 plan precursor using the latest planning assumptions and the latest approved motor vehicle emissions model available at the time that the plan is developed for a Serious area subject to 189(d).
      b. Final Rule.
      The EPA is finalizing RFP requirements for attainment plans required pursuant to CAA section 189(d) that are similar to other Serious area RFP requirements discussed in section VI.F of this preamble. The EPA is providing similar guidance regarding how to prepare an RFP analysis, RFP projected emissions, geographic coverage of emission sources for RFP, and RFP requirements for multi-state nonattainment areas.
      The RFP analysis must contain appropriate information to demonstrate that the state will achieve the emissions reductions from the control strategy necessary to result in generally linear reductions in emissions and provide for expeditious attainment as discussed in Section VII.D of this preamble. As with other Serious area RFP analyses , the state may consider PM2.5 plan precursor emissions reductions in the aggregate for RFP purposes in a 189(d) area.
      The state's RFP analysis must include three components: (1) an implementation schedule for control measures on sources in the nonattainment area, (2) RFP projected emissions for each applicable quantitative milestone year (discussed in Section VII.G of this preamble), based on the anticipated control measure implementation schedule; and (3) an analysis that demonstrates that this schedule of aggregate emissions reductions achieves sufficient progress toward attainment between the applicable baseline year to the attainment year. For additional discussion of each of the components of the RFP analysis, refer to Section IV.F of this preamble. See 40 CFR 51.1012(a).
      In the proposal, the EPA proposed an option to require at least 5 percent emissions reductions in direct PM2.5 and all PM2.5 plan precursor from sources in the area annually until attainment to meet the separate RFP requirement for attainment plans. However, some commenters did not agree that EPA should consider an area meeting the 5 percent requirement under CAA section 189(d) to automatically have satisfied the RFP requirement. The EPA agrees with this comment and is therefore not finalizing an approach to the RFP requirement that is tied to the 5 percent requirement. Instead, the final RFP requirement will be tied to progress toward expeditious attainment (which the EPA recommends should be generally linear but may also be stepwise with appropriate justification), just as it is for all other types of Moderate and Serious area plans for PM2.5 as summarized in the previous paragraph. The emission reductions that a state achieves for purposes of meeting the 5% requirement may also be counted towards meeting the separate RFP requirement, but the EPA does not believe that meeting the 5% requirement would automatically equate to meeting the RFP requirement. That determination requires the separate evaluations required for the RFP analysis.
      The EPA requires that the applicable baseline year for the RFP analysis must be the same year as that represented by the latest base year inventory for the Serious area. The projected attainment date should be as expeditiously as practicable and is discussed further in Section VII.I of this preamble. The RFP analysis must clearly convey how the schedule for implementing the control strategy will provide for generally linear or step-wise progress towards attainment. If step-wise progress is more appropriate for the specific nonattainment area, the state is required to submit a clear rationale and supporting information to explain why generally linear progress towards attainment in the area is not appropriate (e.g., due to the nature of the nonattainment problem, the types of sources contributing to PM2.5 levels in the area, and the ability to perform timely implementation of control measures). Further, if a step-wise approach is needed, this does not relieve the state of the requirements of CAA section 189(d). As stated earlier, the EPA requires that a section 189(d) plan must include in its RFP analysis the anticipated emissions reductions expected to be achieved through the implementation of control measures required by the control strategy described in Section VII.D of this preamble. Further, the optional air quality analysis discussed in Section IV.F of this preamble is also available for use by a state preparing a section 189(d) plan.
      Additionally, the EPA requires states to establish motor vehicle emissions budgets for direct PM2.5 and PM2.5 plan precursors using the latest planning assumptions and the latest approved motor vehicle emissions model available at the time that the 189(d) plan is developed. See 40 CFR 51.1012(a).It is also important to note that if a section 189(d) area is multi-state or multi-jurisdictional, the states or jurisdictions comprising the area must provide a coordinated approach to meeting the RFP requirement for the shared area For further information, see Section IV.F.5 of this preamble. See 40 CFR 51.1012(b).
      c. Comments and Responses.
      Any additional comments received on RFP are addressed in the Response to Comments document found in the docket for this action.

G. Quantitative Milestones
1. Specific Requirements.
      a. Summary of the Proposal.
      The proposal indicated that quantitative milestones would need to be achieved every 3 years until the area attains the relevant NAAQS, similar to proposed requirements for Moderate area plans and other types of Serious area plans. In the proposal, the EPA stated that, at a minimum, quantitative milestones selected for an attainment plan submitted under CAA section 189(d) would need to demonstrate a reduction of at least 15 percent (i.e., 5 percent for each year in the 3-year period) in emissions of direct PM2.5 and PM2.5 plan precursors below those emissions reported in the most recent inventory for the area. The proposal identified requirements for direct PM2.5 and precursors, to be consistent with the RFP proposal. The EPA proposed that attainment plans developed pursuant to CAA section 189(d) would have to contain quantitative milestones beginning at 13.5 years or 16.5 years from designation (depending on whether the section 189(d) plan would be due before or after the 13.5 year mark), and every 3 years thereafter until the attainment date for the area.
	The EPA also proposed that the requirements for quantitative milestones, described in Section VI.G of this preamble, should also apply to quantitative milestones submitted with any revised Serious area attainment plan pursuant to CAA section 189(d).
      b. Final Rule.
      The revised attainment plan for any Serious nonattainment area that fails to attain the relevant PM2.5 NAAQS by the applicable attainment date must include quantitative milestones pursuant to CAA section 189(c). These quantitative milestones should track the progress being made in the nonattainment area in the implementation of specific control measures in the SIP, and may potentially be in the form of metrics for tracking air quality improvement or emissions reductions over time. The EPA wishes to clarify that the quantitative milestones for a section 189(d) plan are designed to track RFP, not solely to track progress in achieving the minimum 5 percent annual emission reduction requirement in this section of the CAA. The RFP discussion in the previous section noted that in some cases, the state may need to adopt additional emission reduction measures (beyond those existing or new measures that will meet the 5 percent emission reduction requirement) in order for the plan to meet the overarching requirement to attain the standard as expeditiously as practicable. Thus, the RFP plan and quantitative milestones must be designed to track progress based on the overall set of control measures needed for expeditious attainment.
      The quantitative milestones need to be achieved every 3 years until the area attains the relevant NAAQS. Therefore, at a minimum, the final rule requires that quantitative milestones selected for an attainment plan submitted under CAA section 189(d) need to track progress in the implementation of control measures required to achieve RFP in emissions reductions of direct PM2.5 and/or all PM2.5 plan precursors described in the previous section. The CAA section 189(d) plan must contain quantitative milestones to be achieved every 3 years, beginning with a milestone at either 13.5 years or 16.5 years from the area's date of designation. If the attainment plan is due prior to a date 13.5 years from designation of the area, then the plan shall contain milestones to be achieved by no later than a milestone date of 13.5 years from the date of designation of the area, and every 3 years thereafter, with the final milestone being the first 3-year milestone date falling after the applicable attainment date. If the attainment plan is due later than a date 13.5 years from designation of the area, then the plan shall contain milestones to be achieved by no later than a milestone date of 16.5 years from the date of designation of the area, and every 3 years thereafter, with the final milestone being the first 3-year milestone date falling after the applicable attainment date. See 40 CFR 51.1013(a)(3).
      The EPA is also finalizing that the requirements for quantitative milestones described in Section VI.G of this preamble shall also apply to quantitative milestones submitted with any revised attainment plan pursuant to CAA section 189(d), including but not limited to, the construction, content, reporting requirements and a quantitative milestone that specifically tracks implementation of control measures identified in the plan to attach the standard as expeditiously as practicable. See 40 CFR 51.1013(a)(3).
      c. Comments and Responses.
      Any additional comments received on quantitative milestones are addressed in the Response to Comments document found in the docket for this action.
 H. Contingency Measures
1. Summary of the Proposal.
      The EPA proposed that contingency measures for attainment plans under CAA section 189(d) for Serious areas that fail to attain the NAAQS by the applicable attainment date must meet the same criteria as contingency measures for a Serious area attainment plan outlined in Section VI.H of this preamble. The EPA also proposed that the contingency measures should achieve approximately 1 year's worth of emissions reductions.
2. Final Rule.
      All PM2.5 attainment plans, including plans for areas subject to CAA section 189(d), must contain contingency measures that are consistent with CAA section 172(c)(9). Section VI.H of this preamble describes the EPA's criteria for contingency measures for a Serious area attainment plan, and contingency measures for a section 189(d) plan must meet the same criteria. The final rule reiterates the EPA's longstanding policy that contingency measures should provide for emission reductions approximately equivalent to 1 year's worth of reductions needed for RFP.
	The statutory contingency measure requirement at CAA section 172(c)(9) is not superseded or subsumed by any requirement under subpart 4. Thus, contingency measures are required as part of a state's attainment plan submission under section 189(d). Accordingly, the final rule requires the criteria for identifying and selecting contingency measures for a section 189(d) submission that are the same as for Moderate or Serious area attainment plans. 	Specifically, the EPA is finalizing that the following requirements must be met in order for contingency measures to be approvable as part of a state's attainment plan submission for purposes of section 189(d):
	1) Contingency measures must be fully adopted rules or control measures that are ready to be implemented quickly upon a determination by the Administrator of the nonattainment area's failure to meet RFP or failure to meet the standard by its attainment date.
	2) The SIP must contain trigger mechanisms for the contingency measures, specify a schedule for implementation, and indicate that the measures will be implemented without significant further action by the state or by the EPA.
	3) Contingency measures should consist of control measures that are not otherwise included in the control strategy for the SIP, or that achieve emission reductions not otherwise relied upon in the control strategy for the area.
	4) Contingency measures should provide for emissions reductions equivalent to 1 year's share of reductions needed to demonstrate attainment (i.e., the overall needed reductions divided by the number of years from the base year to the attainment year), or equivalent to 1 year's worth of air quality improvement or emissions reductions proportional to the overall amount of air quality improvement or emissions reductions to be achieved by the area's attainment plan.
	The EPA is also finalizing its proposal to allow a state to rely on contingency measures that achieve emissions reductions on sources located outside the nonattainment area, but within the state provided that the measures on sources outside the designated nonattainment area are demonstrated to produce the appropriate air quality impact within the nonattainment area.
	As with contingency measures for Moderate or Serious areas, the EPA allows a state under these circumstances to rely on additional reductions in the years following a failure to meet RFP requirements or a failure to attain the NAAQS by the applicable attainment date from federal or local measures already scheduled for implementation as part or all of their contingency measures. The EPA could consider such measures as meeting the contingency measure requirement as long as they produce emissions reductions in excess of those required to meet other statutory nonattainment provisions (e.g., such as to meet BACM/BACT requirements) and they can be relied on to achieve a sufficient portion of the actual emissions reductions necessary to reduce emissions in the area while the state develops a new plan to bring the area into attainment. As with contingency measures for Moderate area or Serious area attainment plans, the EPA requires that the emissions reductions associated with contingency measures for attainment plans under section 189(d) should be equal to approximately 1 year's worth of emissions reductions necessary to achieve RFP for the area, unless the state adequately demonstrates that some smaller amount of reductions is appropriate while the state is revising its attainment plan for the area. See 40 CFR 51.1014(b)(2).
      The EPA recognizes that identifying contingency measures for a Serious PM2.5 nonattainment area that failed to attain the relevant NAAQS by the applicable attainment date may be challenging for a state that should already have fully implemented all control measures identified as "reasonable" and "best," and potentially "most stringent," in addition to any new control measures to achieve the requisite minimum 5 percent reductions in direct PM2.5 or PM2.5 plan precursor emissions necessary for expeditious attainment. However, for an area that has not implemented MSM, states could identify potential contingency measures by reviewing attainment plans for other nonattainment areas. The state should also reevaluate control measures that were identified previously as technologically or economically infeasible for the area, or otherwise removed from consideration as part of the RACM/RACT or BACM/BACT process. Additionally, states can review other sources of control measure information, such as the RBLC ((a central database of air pollution control technology information) and the EPA's Menu of Control Measures document available at http://www3.epa.gov/ttn/naaqs/pdfs/MenuOfControlMeasures.pdf. See 40 CFR 51.1014.
3. Comments and Responses.
      Comment: Commenters stated that because 189(d) requires annual emissions reductions of not less than 5 percent, then the EPA cannot assume that 1 year's worth of emissions reductions will be no greater than 5 percent. The commenter stated that only if an area shows that the 5 percent reduction requirement of CAA section 189(d) is greater than what would be necessary to demonstrate RFP annually may an area assume that contingency measures must achieve only the 5 percent target.
      Response: The EPA agrees with the commenters. As discussed earlier and in Sections IV.H and VI.H of this preamble, contingency measures should equal approximately 1 year's worth of emissions reductions necessary to achieve RFP for the area. The EPA notes that RFP might require more than the 5 percent emissions reductions required by CAA section 189(d). Therefore, if contingency measures should equal approximately 1 year's worth of emissions reductions necessary to achieve RFP, then those contingency measures should provide more than 5 percent of emissions reductions in direct PM2.5 or aggregate PM2.5 plan precursors.
      Comment: Commenters suggested that, similar to the ozone program, the EPA should consider whether the contingency measures for an area that failed to attain by the deadline for Serious areas could anticipate the development of proven new technology, with a requirement to add further contingency measures if such technology does not develop as anticipated.
      Response: The EPA disagrees with the commenters, noting that CAA section 182(e)(5) provides this flexibility for Extreme areas that are nonattainment for the ozone NAAQS. That section of the Act falls within subpart 2 of part D, which identifies additional contingency measure provisions applicable only in ozone nonattainment areas. Subpart 4 does not contain a provision similar to that in subpart 2. Therefore, CAA section 172(c)(9) applies and, as explained earlier, that provision requires contingency measures be included in the attainment plan.
      Comment: Commenters requested the EPA to allow a state to demonstrate, in the alternative, that its contingency measures will achieve a 5 percent reduction in PM2.5 ambient concentrations, and that such reductions can be obtained by reducing direct PM2.5 emissions, emissions of one or more precursors, or both.
      Response: The EPA is finalizing the optional air quality analysis as an additional component of the RFP plan, as previously discussed in Section IV.F of the preamble. Therefore, although the state could demonstrate that its contingency measures will achieve a 5 percent reduction in PM2.5 ambient concentrations, the EPA notes that this optional analysis does not relieve the requirements of 189(d). Specifically, the area remains required to achieve an emissions reduction of not less than 5 percent of direct PM2.5 or any PM2.5 plan precursor.
I. Attainment Dates
1. Summary of the proposal.
      The proposed rule indicated that the new attainment date for an area that failed to attain by the Serious area attainment date would be governed by sections 172(a)(2) and 179(d)(3) of the CAA. Under the proposal, the attainment date would be as expeditiously as practicable, but no later than 5 years from the date of publication in the Federal Register of the EPA's determination that the area failed to attain the relevant NAAQS. The EPA may extend the attainment date by up to 5 additional years based on certain criteria.
2. Final Rule.
      As described in the proposal, the final rule includes the overarching requirement for a Serious area that failed to attain by the previous attainment date to establish a new date for attaining the standard as expeditiously as practicable. However, neither CAA section 189(d) nor other sections in subpart 4 explicitly establish or provide the authority to establish a new attainment date for the area. Therefore, once an area is beyond the attainment dates that Congress specified in subpart 4 for the PM10 NAAQS, the EPA must look to other provisions of part D of the CAA to provide authority for a new attainment date. Sections 179(d)(3) and 172(a)(2) of the CAA provide generally applicable attainment dates that fill the gap in the statute left for areas subject to the requirements of CAA section 189(d). Thus, for a PM2.5 nonattainment area subject to CAA section 189(d) requirements, the EPA must establish a new attainment date according to the provisions of CAA section 179(d)(3) and 172(a)(2). The EPA has followed this same approach in the past for PM10 nonattainment areas governed by subpart 4 nonattainment requirements.
      Applying these provisions, the final rule therefore provides that the new attainment date in a CAA section 189(d) plan must be as expeditious as practicable, but no later than 5 years from the date of publication in the Federal Register of the EPA's determination that the area failed to attain the relevant NAAQS. The EPA may extend the attainment date by up to 5 additional years (thus to 10 years from the date of publication of the notice of finding of failure to attain by the applicable attainment date for the area) if the agency deems it appropriate "considering the severity of nonattainment and the availability and feasibility of pollution control measures." For a PM2.5 nonattainment area subject to CAA section 189(d), the EPA expects that the state will adopt any control measures necessary to demonstrate expeditious attainment within 5 years of the area failing to attain the NAAQS by the applicable Serious area attainment date. The EPA will consider the state's proposed attainment date for the area based on its revised attainment demonstration and modeling of its updated control strategy, and other relevant facts and circumstances for the area, in order to identify the most expeditious attainment date practicable for the area.
3. Comments and Responses.
	Comment: Some commenters stated that the EPA should set a date that is as expeditious as practicable, but if it takes longer than 10 more years to attain, the EPA may approve such a plan, as long as the minimum 5 percent reduction requirement is met. The commenter stated that this is the plain meaning of CAA section 189(d)'s reference that the plan shall provide for at least 5 percent reductions "from the date of such submission until attainment."
	Response: The EPA does not agree with the commenter. As indicated earlier, the EPA's longstanding interpretation is that the statutory provisions of CAA sections 172(c)(2) and 179(d)(3) govern the attainment date for new plans required under CAA section 189(d) for Serious areas that previously fail to attain by the Serious area attainment date. Under certain circumstances, these provisions would allow for an attainment date up to 10 years from the effective date of a finding of failure to attain, but would not allow for an attainment date longer than that.
VIII. NNSR Requirements for PM2.5 Nonattainment Areas
A. Background
1. Statutory Requirements for NSR.
	Section 110(a)(2)(C) of the CAA requires states to include in their SIPs a preconstruction review permitting program that regulates the construction and modification of stationary sources as necessary to ensure that NAAQS are achieved. To address the regulation of the larger pollutant-emitting sources (defined as major stationary sources), Congress provided specific permitting requirements in the CAA in parts C and D of title I. The requirements for preconstruction permits under parts C and D of the CAA are commonly known collectively as the major NSR program because they apply specifically to the preconstruction review and permitting of new major stationary sources and major modifications at existing sources. As explained in Sections VIII.A.1.a and b of this preamble, the preconstruction review of each proposed new major stationary source and major modification generally is carried out on a pollutant-specific basis and the permitting requirements with regard to each pollutant apply based on whether the area in which the proposed major source or major modification would locate is designated attainment (or unclassifiable) or nonattainment for that pollutant at the time the permit is issued.
	a. Prevention of Significant Deterioration.
	Part C of title I of the CAA (hereafter referred to simply as part C) contains implementation plan requirements that apply to new major stationary sources and major modifications locating in areas designated attainment or unclassifiable for any NAAQS. These requirements constitute the Prevention of Significant Deterioration (PSD) program. Pursuant to part C, the EPA has adopted PSD regulations at 40 CFR 51.166 (minimum requirements for an approvable state PSD program in the SIP) and 40 CFR 52.21 (the federal PSD program, applicable in areas where the state does not have an EPA-approved PSD program in its SIP). The EPA last amended the PSD regulations for PM2.5 on January 15, 2013, in the final rule revising the PM2.5 NAAQS. This final rule does not relate to the PSD program, nor does it amend the PSD regulations.
      b. Nonattainment New Source Review.
      Part D of title I of the CAA (hereafter referred to as part D) contains implementation plan requirements for nonattainment areas, which include the requirements for permitting new major stationary sources and major modifications locating in designated nonattainment areas, referred to as the Nonattainment New Source Review (NNSR) program. As noted earlier, part D contains several subparts that include various requirements for addressing nonattainment areas. Subpart 1 addresses plan requirements for nonattainment areas generally, including CAA section 172(c)(5), which requires preconstruction and operating permits for new major stationary sources and major modifications locating in nonattainment areas. Section 173 of the CAA outlines the minimum statutory requirements for a state's NNSR permit program and serves as the basis for the EPA's NNSR regulations for PM2.5 as promulgated in the 2008 PM2.5 NSR Rule published at 73 FR 28321, May 16, 2008. Subpart 4 was added to part D as part of the 1990 CAA Amendments and includes additional plan provisions for designated PM10 nonattainment areas. Relevant here, CAA section 189(a)(1)(A) of subpart 4 requires states to include in their implementation plan a permit program addressing major stationary sources of PM10 that meets the requirements under CAA section 173 of subpart 1. Subpart 4 also includes some additional preconstruction review requirements, which, until the court's decision in NRDC v. EPA, the EPA has only applied to major sources of PM10 located in PM10 nonattainment areas. The specific NNSR requirements contained in both subparts 1 and 4 are described later, including the changes that we are making in this final rule to the NNSR regulations to address these requirements with respect to PM2.5.
2. Federal NNSR Regulations.
	The EPA has adopted numerous NNSR regulations in 40 CFR parts 51 and 52, including §51.165; part 51 Appendix S; and §52.24. An approvable NNSR program in a state's implementation plan must, at a minimum, meet the applicable program requirements set forth in the federal NNSR provisions at 40 CFR 51.165, which for PM2.5 have been based on changes to that section made by the 2008 PM2.5 NSR Rule. States with designated nonattainment areas for a particular pollutant are required to adopt regulations consistent with those applicable plan requirements, including any subsequent rule changes that the EPA may make, and submit them to the EPA for approval as part of their SIP within a period of time consistent with the schedule prescribed by the CAA or the EPA, as appropriate.
      The EPA interprets the requirement established under section 110(a)(2)(C) of the CAA that states regulate the construction and modification of sources to apply as of the effective date of an area's designation to nonattainment for a given pollutant. Although CAA section 110(a)(2)(C) does not contain specific requirements a state must follow for issuing major source permits during the interim period between effective date of designation and the date when the EPA approves a state's NNSR program to address a given pollutant, the EPA regulations at 40 CFR 52.24(k) authorize states to apply 40 CFR part 51, Appendix S, known as the Emission Offset Interpretative Ruling or simply the Offset Ruling, during the interim period. ,
      Accordingly, states with newly designated nonattainment areas for the revised primary PM2.5 NAAQS have two possible means by which they can implement NNSR requirements for PM2.5 following the effective date of designations and until the EPA approves a SIP submission meeting the NNSR requirements for PM2.5 promulgated in this rule. First, any state that already has a SIP-approved NNSR program for PM2.5 (e.g., where the state has had other PM2.5 nonattainment areas for which the EPA has approved an NNSR program) should continue to apply those permitting requirements in the interim. Second, any state that lacks an approved NNSR program for PM2.5 may rely upon the NNSR provisions in Appendix S until the EPA approves that state's SIP submission in order to ensure that proposed new major stationary sources and major modifications for PM2.5 in newly designated PM2.5 nonattainment areas undergo the appropriate type of preconstruction review in the interim.
      a. General Applicability.
      Stationary sources are considered to be "major" sources based on the amount of a given pollutant (or a precursor for that pollutant) the source emits or has the potential to emit, as defined by the statute. New major stationary sources are subject to the NNSR requirements when they are major for the pollutant (or a precursor for that pollutant) for which an area is designated nonattainment. See 40 CFR 51.165(a)(2)(i). With regard to major modifications, NNSR applies to proposed physical changes or changes in the method of operation of an existing stationary source that (1) is major for the nonattainment pollutant (or a precursor for that pollutant) and (2) results in both a significant emissions increase and a significant net emissions increase of that same nonattainment pollutant (or same precursor for that pollutant).
      For each proposed new major stationary source and major modification, the general NNSR requirements to be included in a state's SIP include the following: (i) the installation and continuous operation of pollution control technology that complies with the Lowest Achievable Emission Rate (LAER); (ii) the acquisition of creditable emissions reductions to adequately offset the proposed emissions increase of the nonattainment pollutant; and (iii) a certification that all other sources owned by the applicant are complying with all applicable requirements of the
CAA. These NNSR requirements apply as of the effective date of the designation of an area as nonattainment for the pollutant and must be satisfied by a proposed major new source or major modification locating in such area as a prerequisite for obtaining a NNSR permit.
      b. Historical Overview of NNSR for PM10 and PM2.5 National Ambient Air Quality Standards (NAAQS).
The EPA revised the PM NAAQS in 1997, establishing new annual and 24-hour NAAQS using PM2.5 particles as a new indicator, while retaining the NAAQS for PM10. In 2006, the EPA again revised the suite of PM NAAQS by tightening the 24-hour PM2.5 standards and retaining the level of the annual PM2.5 standards. In 2008, the EPA issued the PM2.5 NSR Rule that established various provisions ensuring that proposed new major stationary sources or major modifications of sources of direct PM2.5 emissions or emissions of applicable PM2.5 precursors would be required to undergo preconstruction review. The EPA included specific provisions in the 2008 PM2.5 NSR Rule that apply when such sources are located in a designated PM2.5 nonattainment area. Unlike the NNSR requirements for PM10 developed under subpart 4, the EPA determined that the applicable implementation requirements for the PM2.5 NAAQS were limited to the general nonattainment provisions under subpart 1.
With regard to NSR applicability for PM2.5 precursors in the 2008 PM2.5 NSR Rule, the EPA recognized that, under the appropriate conditions, NOx, SO2, VOC and ammonia could each contribute to the formation of PM2.5 in the ambient air. However, the EPA issued regulations that did not require states to subject all of these precursors to regulation as part of the attainment plan or NSR permitting requirements applicable in a given nonattainment area. Instead, the EPA established the initial presumptions for nonattainment areas that SO2 and NOx should be regulated precursors for PM2.5, but VOC and ammonia need not be regulated precursors.
As described in Section II.B of this preamble, in January 2013 the court's decision in NRDC v. EPA held that the EPA erred in implementing the PM2.5 NAAQS under the general implementation requirements in subpart 1, rather than relying on the implementation requirements specific to PM10 in subpart 4 of the CAA. Accordingly, the court directed the EPA to comply with the requirements of subpart 4 when developing implementing regulations for PM2.5 nonattainment areas.
The NRDC decision has specific implications for implementing the NNSR program for PM2.5. Two provisions of subpart 4 impose additional requirements on NNSR plans developed to address sources locating in areas designated nonattainment for PM2.5. The first relates to the definition of "major stationary source" that applies to areas initially designated as Moderate nonattainment areas and subsequently reclassified as Serious. In such areas, CAA section 189(b)(3) of the CAA defines the major source threshold as 70 tpy of PM10. The second relevant subpart 4 provision governs the treatment of major sources of PM10 precursors. As previously described in Section III of this preamble, section 189(e) of the CAA requires that the control requirements applicable to major stationary sources of PM10 also apply to major stationary sources of PM10 precursors, unless the Administrator determines that such sources of PM10 precursors do not contribute significantly to PM10 levels that exceed the standard in that area. The EPA's proposed amendments to address the subpart 4 requirements with respect to PM2.5 and the EPA's responses to comments received on its proposal are summarized in the relevant subsections later.
      It is worth noting that the 2008 PM2.5 NSR Rule promulgated new NSR requirements for implementation of PM2.5 in both nonattainment areas (NNSR) and attainment/unclassifiable areas (PSD). As subpart 4 includes requirements only pertinent to nonattainment areas, the EPA does not consider the portions of the 2008 PM2.5 NSR Rule that address requirements for PM2.5 attainment and unclassifiable areas to be affected by the court's opinion in NRDC v. EPA. Therefore, the EPA did not propose to revise any PSD requirements promulgated in the 2008 PM2.5 NSR Rule in order to comply with the court's decision.
B. Final NNSR Requirements for PM2.5 Nonattainment Areas
	This section provides a description of the changes that the EPA is making to the NNSR requirements for PM2.5 that are contained in 40 CFR 51.165, which provides the minimum requirements for a NNSR program under an approved implementation plan, and in Appendix S, which serves as an interim NNSR permitting program pending approval of a state's SIP to address NNSR requirements for a particular pollutant.
For both sets of regulations, we will describe the changes that were proposed, the final requirements, the comments received, and the EPA's responses to them.
1. 40 CFR 51.165.
In this final rule, as explained in more detail later, the EPA is making the following revisions that affect the NNSR regulations for PM2.5 at 40 CFR 51.165: (a) amending the definition of "regulated NSR pollutant" with regard to PM2.5 precursors; (b) amending the definition of "major stationary source" with regard to major sources of direct PM2.5 emissions and PM2.5 precursors locating in PM2.5 nonattainment areas classified as Moderate and Serious; (c) amending the definition of "significant" with regard to emissions of PM2.5 precursors; and (d) codifying the EPA's policy for determining whether a source is "major" for PM2.5 with regard to emissions of direct PM2.5 and its precursors.
Also, the EPA explains in this section that it is codifying the schedule for states to submit NNSR SIP revisions for PM2.5 that meet the requirements of 40 CFR 51.165. The schedules for submitting revised NNSR programs for PM2.5 for Moderate and Serious areas are not contained in 40 CFR 51.165 NNSR regulations but in new 40 CFR 51.1003(a) and (b), respectively.
a. Definition of "regulated NSR pollutant" -- PM2.5 Precursors.
i. Summary of Proposal.
CAA section 189(e) requires that the control requirements applicable to major stationary sources of PM10 also apply to major stationary sources of PM10 precursors, unless the Administrator determines that such sources of PM10 precursors do not contribute significantly to PM10 levels that exceed the standard in that area. In order to align the NNSR regulations for PM2.5 with the requirements of CAA section 189(e), the EPA proposed several amendments to certain definitions within 40 CFR 51.165, as explained in the subsections that follow, in order to require regulation of all four identified PM2.5 precursors consistent with the statute.The EPA proposed to revise the NNSR definition of "regulated NSR pollutant" to ensure that SO2, NOx, VOC and ammonia are all listed as regulated PM2.5 precursors.
 	The EPA also proposed to add language to the definition of "regulated NSR pollutant" to address the provision of CAA section 189(e) that allows an exemption from the NNSR permit requirements for major stationary sources or major modifications of a particular precursor if the state demonstrates to the satisfaction of the EPA that major stationary sources of such precursor do not contribute significantly to PM2.5 levels that exceed the PM2.5 ambient standards in a particular nonattainment area. In Section III of the preamble of the proposal, the EPA proposed and sought comment on several policy approaches that a state could use to make the necessary demonstration that would enable the state to exempt a particular precursor from being regulated under the attainment plan for a particular PM2.5 nonattainment area altogether or from individual components of that plan, including the NNSR permitting requirements. See 80 FR 15350-62.
ii. Final Rule.
The EPA is amending the definition of "regulated NSR pollutant" at 40 CFR 51.165 to include a new provision stating that SO2, NOx, VOC and ammonia are PM2.5 precursors in any PM2.5 nonattainment area, except where specifically exempted via a demonstration approved by the EPA showing that a particular precursor does not contribute significantly to levels of PM2.5 that exceed the standard in a particular nonattainment area. See 40 CFR 51.165(a)(1)(xxxvii)(C)(2).
The EPA has set forth in this final rule several types of demonstrations for showing that a precursor does not contribute significantly to PM2.5 levels that exceed the standard in a particular PM2.5 nonattainment area. As explained in Section III of this preamble, the EPA has defined a precursor demonstration specifically for exempting major sources of a particular precursor from regulation under the NNSR program. This demonstration involves a sensitivity-based analysis that evaluates the sensitivity of ambient PM2.5 concentrations in a nonattainment area to increases of precursor emissions resulting from potential major source growth in the area. The EPA will issue a technical assistance document that provides details on how to design appropriate sensitivity-based analyses for this purpose. A more complete description of this and the other types of precursor demonstrations is contained in Section III of this preamble. The EPA recommends that the state consult with the appropriate EPA Regional Office as early as possible to discuss appropriate analyses for the precursor demonstration. If the appropriate precursor demonstration is submitted to and approved by the EPA, the state would not be required to regulate major sources of the insignificant precursor under the state's approved NNSR program in a particular nonattainment area. Such exemption from controls would include an exemption from the requirement to implement LAER and to obtain emissions offsets for the insignificant precursor.
iii. Comments and Responses.
Comments: Some commenters supported revising the definition of "regulated NSR pollutant" consistent with the NRDC decision and subpart 4 to establish SO2, NOx, VOC and ammonia as regulated PM2.5 precursors, unless a state demonstrates that major stationary sources of a particular precursor do not contribute significantly to PM2.5 levels that exceed the standard in the nonattainment area.
      On the other hand, several commenters urged the EPA not to begin regulating VOC and ammonia as PM2.5 precursors under the NNSR program at this time. Some of the commenters cited ongoing technical challenges related to evaluating the contribution of PM2.5 precursor gases to ambient PM2.5 concentrations, while some commenters stated that the EPA should provide an up-front rebuttable presumption that a state is not required to regulate VOC and ammonia as PM2.5 precursors under NNSR. A commenter stated that requiring NNSR to apply to sources of these precursors that would have an insignificant impact on the nonattainment issue is a waste of resources and will unnecessarily lead to burdensome over-regulation for affected sources.
      Response: We do not agree with the commenters who oppose the EPA's proposal to require regulation of all four technical and scientific precursors in PM2.5 nonattainment areas. Specifically, we do not agree that a delay in regulating VOC and ammonia under a state's attainment plan is reasonable or permissible. Similarly, the EPA does not agree that it has the authority to provide a rebuttable presumption to exempt VOC and ammonia from NNSR permitting requirements. CAA section 189(e) explicitly requires the regulation of major sources of PM2.5 precursors unless the state demonstrates to the EPA's satisfaction that such regulation is unnecessary.  Thus, CAA section 189(e) does not allow the EPA to unilaterally exempt an identified scientific and technical PM2.5 precursor from regulation, nor does it permit the EPA to establish a rebuttable presumption exempting any PM2.5 precursor from regulation. The EPA's use of a rebuttable presumption exempting certain precursors from regulation in its prior PM2.5 implementation rules was directly at issue in NRDC v. EPA, wherein the court made it clear that it would be inappropriate for the EPA to establish such presumptions pursuant to the requirements of subpart 4.
      In certain PM2.5 nonattainment areas, the minimization of new precursor emissions from major source growth in the area could be an important component of a state's attainment plan for the PM2.5 NAAQS. Where it is not, CAA section 189(e) provides states with the opportunity to make an area-by-area opt-out demonstration that would enable the state to exempt sources of any PM2.5 precursor from regulation if it is shown that the precursor does not contribute significantly to the PM levels that exceed the standard in a particular nonattainment area. Accordingly, consistent with CAA section 189(e), this final rule provides guidance to assist states in preparing a NNSR precursor demonstration, as described in Section III of this preamble, by which VOC, ammonia or any other PM2.5 precursor may be exempted from the NNSR requirements for PM2.5 if the state shows that emissions of such precursor do not contribute significantly to PM2.5 concentrations in a PM2.5 nonattainment area.
      Moreover, as described in Section VIII.B.2 of this preamble, the EPA is not requiring regulation of VOC and ammonia for those states relying on Appendix S to issue NNSR permits during the interim SIP development period. This provides states with an opportunity to evaluate the contribution of VOC and ammonia emissions to ambient PM2.5 concentrations and determine whether an exemption of either of these precursors from NNSR permitting requirements is justified before such sources will be subject to regulation.
      b. Definition of "major stationary source" in Moderate and Serious PM2.5 Nonattainment Areas -- Direct PM2.5 Emissions and PM2.5 Precursors.
      i. Summary of Proposal.
      Subpart 4, as described earlier in this notice, contains provisions for the classifications of PM10 nonattainment areas as either Moderate or Serious areas. However, the NNSR regulations for PM2.5 set forth in the 2008 PM2.5 NSR Rule were developed pursuant to subpart 1, which does not provide for the classification of designated nonattainment areas as Moderate and Serious areas. Accordingly, in the proposal for this final rule, the EPA proposed to amend its definition of "major stationary source" in the 40 CFR 51.165 NNSR regulations for PM2.5 to address subpart 4 requirements concerning the regulation of direct PM2.5 and PM2.5 precursors in both Moderate and Serious nonattainment areas for PM2.5. First, the EPA proposed to revise the definition of "major stationary source" by qualifying the term "regulated NSR pollutant" with the new phrase "(as defined in paragraph (a)(1)(xxxvii) of this section)." The new phrase explicitly cross-references the definition of "regulated NSR pollutant," which also defines each of the PM2.5 precursors. Hence, sources of both direct PM2.5 emissions and emissions of each PM2.5 precursor would clearly be included in the definition of a "major stationary source."
      Second, the EPA proposed to amend the definition of "major stationary source" consistent with section 189(b)(3) of the CAA to establish a lower major source threshold for new major stationary sources and major modifications locating in PM2.5 nonattainment areas classified as Serious. CAA section 302(j) generally defines a "major stationary source" as a source that emits or has the potential to emit 100 tpy or more of any air pollutant. The provision explicitly states that this definition applies for purposes of the CAA except as otherwise expressly provided in the statute. Thus, for sources locating in PM2.5 nonattainment areas classified as Moderate, where no CAA provision provides otherwise, the applicable major source threshold for direct PM2.5 emissions and for each PM2.5 precursor is 100 tpy. Subpart 4, meanwhile, establishes a major source threshold for PM10 nonattainment areas classified as Serious at 70 tpy in CAA section 189(b)(3). Therefore, the EPA proposed to set the major source threshold for direct PM2.5 emissions and for each PM2.5 precursor at 70 tpy of direct PM2.5 emissions and each individual precursor. The alternative proposed for consideration was to promulgate a PM2.5 major source threshold lower than 70 tpy of direct PM2.5 emissions, recognizing that PM2.5 is a subset of PM10. The EPA sought comment on possible ways in which a PM2.5 emissions rate different from the statutory 70 tpy rate for PM10 emissions could be established, taking into account variations in the PM10/PM2.5 ratio for different source categories and activities. Nevertheless, the agency indicated that the proposed option (i.e., a major source threshold of 70 tpy of direct PM2.5 emissions for stationary sources proposing to construct or modify in PM2.5 nonattainment areas classified as Serious) represented the preferred approach.
      In its effort to ensure that major sources of PM2.5 precursors locating in Serious areas are regulated in the same manner as major sources of direct PM2.5 emissions locating in Serious areas, the EPA proposed major source thresholds for PM2.5 precursors would be consistent with the threshold already defined for direct PM2.5 emissions in PM2.5 nonattainment areas reclassified as Serious. Consistent with the EPA's preferred approach for direct PM2.5 emissions, the EPA proposed to define "major" for each PM2.5 precursor as 70 tpy. However, the EPA also solicited comments on the appropriateness of setting the precursor major source thresholds at a different rate, particularly if, as alternatively proposed, the agency defined "major stationary source" for sources of direct PM2.5 emissions in Serious PM2.5 nonattainment areas at a rate lower than 70 tpy of PM2.5 emissions. For example, if the agency had set the major source threshold at 60 tpy of direct PM2.5 emissions in Serious PM2.5 nonattainment areas, the agency would have also considered setting the major source threshold for each PM2.5 precursor at 60 tpy of that particular precursor.
      Regardless of whether the major source threshold for direct PM2.5 emissions was set at 70 tpy or some lower rate, the EPA indicated in the proposal that it believed a reasonable technical argument could be made that the threshold set for direct PM2.5 emissions would be too low to be regarded as "major" for each precursor when considering the effects that any precursor sources could have on ambient PM2.5 concentrations. In support of higher emissions rates for defining "major" for PM2.5 precursors, the EPA cited a previous analysis that it had undertaken to examine the relationship between emissions of SO2 and NOx and the formation of secondary PM2.5 in the ambient air. However, the agency also identified potential legal impediments to setting a major source threshold for precursors at a rate higher than those statutorily prescribed for direct emissions of a pollutant. Accordingly, the agency solicited comments on the general appropriateness of setting higher major source thresholds for one or more PM2.5 precursors in PM2.5 nonattainment areas, and asked commenters to include legal and technical considerations that should be made as part the EPA's future analysis of NNSR requirements with respect to PM2.5 precursors.
      ii. Final Rule.
       In this final rule, the EPA has followed its preferred approach and has made the changes necessary to ensure that "major" is defined for direct PM2.5 emissions as well as all PM2.5 precursors in Moderate and Serious PM2.5 nonattainment areas. For Moderate areas, the major source threshold of 100 tpy applies individually to direct PM2.5 emissions and to each PM2.5 precursor; in Serious areas, the major source thresholds for direct PM2.5 emissions and emissions of each PM2.5 precursor are individually defined as 70 tpy. As explained in Section VIII.B.1.d of this preamble, the determination of whether sources of direct PM2.5 emissions or each PM2.5 precursor are "major" is to be made separately for each pollutant. That is, emissions rates for individual precursors should not be added together to determine a source's major source status with regard to PM2.5. See 40 CFR 51.165(a)(2)(i).

      iii. Comments and Responses.
      Comment: Most commenters generally supported the preferred approach of setting a major source threshold at 70 tpy of direct PM2.5 emissions in Serious areas, agreeing with the EPA that establishing a PM2.5 equivalency to PM10 emissions would be problematic. Some commenters specifically opposed any effort to set a threshold for PM2.5 that is lower than the threshold for PM10. A commenter stated that, if PM2.5 is legally subject to subpart 4 because it is a subset of PM10, and Congress meant to subject all sources of PM10 emissions to subpart 4, then Congress meant to have the major source threshold for PM10 apply to PM2.5 as well. No commenter advocated that the EPA set a major source threshold lower than 70 tpy for direct PM2.5 emissions.
      Response: The EPA agrees with the commenters that it is reasonable to set the major source threshold at 70 tpy of direct PM2.5 emissions for sources locating in PM2.5 nonattainment areas classified as Serious. While CAA section 189(b)(3) does not explicitly define a "major source" and "major stationary source" as 70 tpy of PM2.5 for PM2.5 nonattainment areas reclassified as Serious (because it refers to PM10), the most straightforward and consistent application of the statutory provision is to establish the same numerical threshold for sources of PM2.5 in Serious PM2.5 nonattainment areas as the threshold for sources of PM10 emissions in Serious PM10 nonattainment areas. Sources locating in Moderate nonattainment areas are already subject to the same numerical major source threshold (100 tpy) under CAA section 302(j) for direct emissions of both PM10 and PM2.5, so the EPA believes that it is also reasonable to establish the threshold for PM2.5 in Serious areas at the same numerical rate as the threshold that applies to PM10 in Serious areas.
      We also agree that it would be difficult to establish a lower uniform major source threshold for PM2.5 that would represent a rate that is equivalent to 70 tpy of PM10 emissions at all sources subject to NNSR permitting requirements. With regard to the commenter who stated that "Congress meant to have the major source threshold for PM10 apply to PM2.5 as well," it is not clear whether the commenter advocates that proposed sources of PM2.5 be subjected to NNSR permitting using a major source threshold of 70 tpy of PM10 emissions or a major source threshold of 70 tpy of PM2.5 emissions for sources of PM2.5 locating in PM2.5 nonattainment areas. The former is not the EPA's interpretation of the CAA. While PM2.5 is a subset of PM10, to assume that a source emitting major amounts of PM10 will also emit a substantial amount of PM2.5 is not always reasonable. The relative amounts of PM10 and PM2.5 emitted by various source categories is known to vary significantly and we do not believe that it would be reasonable to subject sources to major source review for PM2.5 on the basis of the level of PM10 emissions as this could mean that sources are subject to NNSR based on different levels of PM2.5 emissions on an area-by-area basis. We do not believe that Congress intended such a lack of uniformity in the application of the major source threshold to sources of direct PM2.5. Moreover, even if it were permissible to interpret CAA section 189(b)(3) in this manner, we have determined that the most reasonable and straightforward approach is to establish a separate major source threshold for direct emissions of PM2.5 at 70 tpy for sources locating in PM2.5 nonattainment areas classified as Serious.
      Comment: With regard to the definition of "major stationary source" for PM2.5 precursors in Moderate and Serious areas, several commenters supported using the same major source threshold value for direct PM2.5 emissions and PM2.5 precursors. One of these commenters expressly opposed any alternative approach that would set a different threshold for PM2.5 precursors than for direct PM2.5 emissions because the commenter asserted that it would be impossible to set a uniform national ratio reflecting the effect of the various precursors on ambient PM2.5 concentrations relative to direct PM2.5 emissions.
      On the other hand, some commenters wanted the EPA to include a provision in the NNSR regulations allowing states to make a case-by-case demonstration to use higher major source thresholds for PM2.5 precursors for permit reviews. These commenters expressed concern that the 100 tpy major source threshold for Moderate areas, and the 70 tpy threshold for Serious, are both too low for the PM2.5 precursors and do not realistically reflect the effect that each precursor has on ambient PM2.5 concentrations. These commenters suggested the EPA should conduct further analyses to determine what higher quantity of emissions of each regulated precursor would be equivalent to 100 tpy (for Moderate areas) and 70 tpy (for Serious areas) of direct PM2.5 emissions in terms of contribution to PM2.5 concentrations in ambient air. These commenters recommended that the EPA use the information gained from the recommended analyses to determine appropriate thresholds and make its proposed thresholds available for public comment.
      Response: In setting the major source threshold for each PM2.5 precursor at 100 tpy for Moderate areas, the EPA is following the precedent established in the 2008 PM2.5 NSR Rule in which the agency set the same 100 tpy major source threshold for direct PM2.5 emissions and each of the regulated precursors (at that time SO2 and NOx). Setting the same 100 tpy major source thresholds for sources of PM2.5 emissions and regulated PM2.5 precursor emissions is also consistent with the way in which we have historically interpreted the requirements of CAA section 189(e) as they applied to emissions of PM10 and PM10 precursors.
      Moreover, section 302(j) of the CAA contains a definition of "major emitting facility" and "major stationary source" that applies to, among other things, programs implemented under subpart 1 such as the general NNSR program requirements in CAA section 173. This definition also applies to programs implemented under subpart 4 to the extent that they regulate PM2.5 nonattainment areas classified as Moderate, as subpart 4 does not establish a different definition of major sources for such areas. That definition defines a source as "major" whenever a facility or source "emits, or has the potential to emit, one hundred tons per year or more of any air pollutant." This provision does not clearly provide the EPA with the authority to set a major source threshold higher than 100 tpy for a pollutant merely because it is a precursor for another pollutant. Rather, CAA section 302(g) clearly defines the term "air pollutant" to "include[] any precursors to the formation of any air pollutant."
      With regard to the setting of the major source thresholds for PM10 precursors in Serious areas, a House of Representatives Report accompanying the 1990 amendments to the CAA described the effects of adding CAA section 189(b)(3), defining "major" sources locating in PM nonattainment areas classified as Serious as those sources that emit or have the potential to emit 70 tpy of PM10. The report specifically notes that "new or modified sources emitting 70 tons or more per year of VOC [a PM2.5 precursor] will be subject to new source review requirements." Thus, Congress contemplated that the same major source threshold would apply to sources of direct PM10 emissions and PM10 precursors in Serious PM10 nonattainment areas. The same approach logically applies when applying the provision to sources locating in areas designated as Serious PM2.5 nonattainment areas.
      Since the EPA may not have the legal authority to establish major source thresholds for PM2.5 precursors at levels higher than the statutory threshold applied to sources of direct PM2.5 emissions, it would be inappropriate to allow states discretion for setting major source thresholds for PM2.5 precursors that exceed the statutory thresholds. Moreover, while we acknowledge that PM2.5 precursors will not likely form ambient PM2.5 in the nonattainment area on a ton-per-ton basis, there is not currently sufficient technical basis that would enable the agency to propose uniform higher major source thresholds for any of the four PM2.5 precursors. As stated in the proposal, the EPA intends to continue its analysis of the relationship between each precursor and ambient PM2.5 concentrations.
      Comment: Some commenters questioned the EPA's interpretation of the 2006 court decision in South Coast Air Quality Mgmt. Dist. v. EPA  as precluding higher major source thresholds because the court determined that NSR provisions, including major source thresholds, were control requirements subject to anti-backsliding provisions of the statute. The commenter argued that the South Coast decision did not address setting a major source threshold for a precursor pollutant that is as stringent as, or more stringent than, the major source threshold for the pollutant when the pollutant is directly emitted. The commenter stated the statutory provision on which the court in South Coast relied [CAA section 172(e)] is applicable on its face only when the EPA relaxes the NAAQS, which the commenter claimed is not relevant to the current situation here, where the EPA has promulgated progressively more stringent NAAQS for PM.
      Response: CAA section 189(e) requires the control requirements that are applicable to major stationary sources of PM2.5 to also apply to major stationary sources of PM2.5 precursors. The court in South Coast held that the term "controls" under section 172(e) of the CAA includes NSR requirements, and in particular includes major source thresholds specified by the statute. The commenter did not explain why the term "control" in CAA section 189(e) of the statute should be interpreted differently than the term "control" in other parts of the statute. Section 172(e) of the CAA is a provision in subpart 1 of part D of the statute concerning anti-backsliding requirements in designated nonattainment areas. It is reasonable for the EPA to conclude that the term "control" in one part of the statute pertaining to nonattainment area requirements should be interpreted consistent with the use of that term in other provisions of part D pertaining to nonattainment area requirements, particularly where both provisions apply to designated PM2.5 nonattainment areas. Thus, consistent with the holding of South Coast regarding the definition of "controls," the EPA interprets the use of the term "control requirements" in CAA section 189(e) to require the same major source threshold be applied to PM2.5 precursors as applies to direct PM2.5 emissions.
      The commenter also did not explain, and it is not clear, how a relaxation versus a strengthening of the NAAQS would bear on whether the EPA has authority to set different control requirements (e.g., major source thresholds) for sources of direct emissions of a pollutant and sources of precursors of that pollutant. The EPA notes that Congress, in adding additional particulate matter requirements in subpart 4 of the CAA, decided that more stringent requirements were required to address air quality in particulate matter nonattainment areas. Hence, it would be inconsistent with that intention for Congress to allow higher major source thresholds to apply to sources of precursors than apply to direct PM2.5 emissions.
      The EPA therefore believes that at this time the most reasonable approach for defining the major source threshold for PM2.5 precursors in both Moderate and Serious areas is to use the same threshold that is being defined for direct PM2.5 emissions. As explained earlier, the EPA currently has studies underway to better understand the effects of emissions of each precursor on the secondary formation of ambient PM2.5 concentrations. However, even if such studies support the commenters' recommendation for higher precursor thresholds, the EPA must consider the potential legal restrictions on setting thresholds for precursors above the statutory requirements for direct emissions of an air pollutant.
c. Significant Emissions Rates (SERs) for PM2.5 Precursors.
i. Summary of Proposal.
As noted earlier, stationary sources locating in nonattainment areas are subject to the NNSR permitting requirements to the extent construction at the source qualifies as a major modification with respect to a pollutant for which the area is designated nonattainment. A major modification of a stationary source is defined in the NNSR regulations at 40 CFR 51.165(a)(1)(v)(A) as "any physical change in or change in the method of operation of a major stationary source" that would result in (1) a significant emissions increase of a regulated NSR pollutant, and (2) a significant net emissions increase of that pollutant. The term "significant" is separately defined at 40 CFR 51.165(a)(1)(x)(A) to mean a rate of emissions specified for each pollutant or precursor for that pollutant. This is known as a significant emissions rate (SER). In the 2008 PM2.5 NSR Rule, the EPA defined "significant" for SO2 and NOx as PM2.5 precursors with an emissions rate of 40 tpy for each precursor. Also, in the preamble to the 2008 PM2.5 NSR Rule, the EPA indicated that it would consider 40 tpy of VOC emissions to be "significant" in any state regulating VOC as a PM2.5 precursor; however, that significant emissions rate was not codified in any of the NSR regulations because the regulations governing both NNSR and PSD permitting programs provided that VOC was generally presumed not to be a precursor to PM2.5. Instead, the agency explained that any state making a demonstration that VOC should be treated as a PM2.5 precursor in a particular nonattainment area "would be required to adopt the 40 tpy SER unless it demonstrated that a more stringent SER (lower rate) is more appropriate."
The EPA did not include any changes to the existing SERs for SO2 and NOx as PM2.5 precursors in the proposal. Nor did we propose a SER for ammonia, citing a lack of adequate technical support. However, the EPA proposed to codify a SER of 40 tpy for VOC in the NNSR permitting regulations. See 55 FR 15434. The EPA further stated that, as a result, only the ammonia SER would remain to be defined by each state that needs to control major stationary sources of ammonia as part of its NNSR program for PM2.5. While not proposing to revise the existing 40 tpy SER values for SO2 and NOx, the EPA indicated it believed that, when more data are available, such data might provide a reasonable basis for considering subsequent changes to the SER for each PM2.5 precursor for purposes of implementing the PM2.5 NAAQS. Moreover, the EPA indicated that a separate rulemaking might be used to propose a new SER for each PM2.5 precursor. See 80 FR 15434.
ii. Final Rule.
The EPA is finalizing its proposed approach with some changes to the final regulatory language. The final rule amends the definition of "significant" in the NNSR regulations at 40 CFR 51.165(a)(x)(A) to add a SER for VOC. Thus, the revised definition contains individual SERs for direct PM2.5 emissions (10 tpy), SO2 emissions (40 tpy), NOx emissions (40 tpy), and VOC emissions (40 tpy). The revised definition does not contain a SER for ammonia emissions. Instead, a new subparagraph has been added to the definition of "significant" to require that an implementation plan define the term for ammonia in cases where ammonia is not otherwise exempted from regulation as a PM2.5 precursor (either because the state did not submit a NNSR precursor demonstration for ammonia or the EPA disapproved the state's demonstration). See 40 CFR 51.165(a)(1)(x)(F). Such definition of "significant" for ammonia would need to be established by the state for a particular nonattainment area as part of its SIP submission for NNSR. The EPA's rationale for not establishing an ammonia SER in this action is provided in greater detail in the following section.
iii. Comments and Responses.
Comment: Several commenters generally recommended that the EPA establish higher SERs for the PM2.5 precursors. These commenters expressed the need for values that more accurately represented each precursor's relative effect on ambient PM2.5 concentrations. One of these commenters stated that in the absence of such higher SERs in the NNSR regulations, the EPA should allow states to demonstrate the appropriateness of a higher SER for a particular precursor on either a statewide or area-by-area basis in a SIP submission, or through the NNSR program on a case-by-case basis.
      Another of the commenters supporting higher significance thresholds for each precursor stated that that the CAA's definitions of "major source" and "major emitting facility" trigger the statutory control requirements and its permit requirements for affected sources, but they do not define how much of a pollutant is regulated after the control or permit requirement is triggered by the CAA. The commenter stated that the EPA would appear to have ample authority to require that precursors be regulated based on different thresholds once a major source triggers a particular control or permit requirement, provided there is adequate technical basis for doing so.
      Response: The EPA did not propose to reconsider or revise the SERs for SO2 and NOx; therefore, revisions to these rates are outside the scope of this action. Even if the EPA were to consider such a revision, it would provide little relief to new or modified sources subject to NSR. Because SO2 and NO2 are pollutants for which the EPA has established NAAQS and because NOx and VOC are a precursors for ozone, modifications with emissions increases above the current SERs for SO2, NOx or VOC would still be subject to some form of new source review (PSD if the area is attainment for the NAAQS pollutant or nonattainment NSR if the area is nonattainment) even if the SERs for these pollutants as PM2.5 precursors were revised to a higher value. Moreover, we do not believe that the statute would permit the EPA or states to adopt a definition of "significant" for purposes of identifying modifications subject to NSR permitting with rates greater than the statutory and regulatory definitions of a major source in a nonattainment area, as defined in CAA section 302(j) for sources locating in PM2.5 nonattainment areas classified as Moderate (100 tpy) and as defined in CAA section 189(b)(3) for sources locating in PM2.5 nonattainment areas classified as Serious (70 tpy). Consequently, we do not believe that there would be substantial cost savings to many sources if we were to revise the SERs for these pollutants specifically as PM2.5 precursors.
      Comment: Some commenters directed specific attention to the definition of a SER for ammonia. These commenters urged the EPA to set a significance threshold for ammonia as soon as possible. These commenters stated that, without a SER, any significant emission increase greater than zero tons per year would result in a major source review for NNSR. One of the commenters stated that unforeseen results such as the imposition of new requirements for the agriculture industry could occur.
      Some commenters stated that, while the EPA indicates that a SER for ammonia may be developed in a subsequent rulemaking, if that rulemaking is not timely, the state would need to develop a SER for ammonia to reduce the burden on permit applicants and avoid permit issuance delays related to major source applicability determinations and permit development for ammonia and PM2.5. Two of the commenters noted that ammonia is used in many industry and source types to control NOx emissions though the implementation of selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) control devices. These commenters stated that, without a SER for ammonia, it's very possible that many NNSR reviews will be initiated simply because of any ammonia increase at a major source. To address this problem, some commenters stated that, until the EPA completes its analysis for ammonia, states need the ability to conduct case-by-case reviews for NNSR permits by requiring applicants to submit a technical demonstration showing that emissions of a particular precursor do not significantly contribute to PM2.5 levels that exceed the standard in an area, thus exempting the precursor from being controlled by that source.
      Some commenters recommended that the EPA propose a SER for ammonia before finalizing the March 23, 2015, proposal and suggested the EPA should also provide definitive guidance for state and local agencies on how to conduct permitting of major sources of ammonia until a SER is established. Other commenters stated that, at the time the EPA proposes new significant emission rates for precursors, the EPA should also establish the significant emission rate for ammonia. Yet another commenter stated that any precursor analyses conducted by the EPA should be done in close coordination with designated nonattainment areas to reduce duplication of efforts and conflicting outcomes that could in turn lead to more costly impacts on sources and on agencies' limited resources. Finally, some commenters stated that the EPA should at least provide guidance for states to develop a SER for ammonia as a PM2.5 precursor.
      Response: The EPA did not propose a SER for ammonia and, therefore, this rule does not finalize a SER for ammonia. The EPA's initial plan, as explained in the proposal, was to establish a SER for ammonia in a separate rulemaking, which was also intended to establish significant impact levels (SILs) for Ozone and PM2.5 in order to streamline the air quality impact analysis under the PSD permitting program. However, based on the imminent need for the SILs (especially for ozone) for PSD permitting, the agency has decided to issue guidance in lieu of a rulemaking for the PSD-based SILs. After due consideration, the EPA has also concluded that a separate rulemaking solely for the purpose of developing a SER for ammonia is not warranted. We anticipate that very few states will actually need to regulate ammonia as a PM2.5 precursor under their NNSR programs for PM2.5 since (1) stationary sources of ammonia generally are not one of the primary causes of ambient PM2.5 concentrations in most PM2.5 nonattainment areas, and (2) according to information in the EPA's NEI database, most existing PM2.5 nonattainment areas do not have an existing major stationary source of ammonia to which the ammonia SER would be applied to determine whether a proposed modification of such major source would be "major" for ammonia.
      Unlike the EPA's PSD regulations, the definition of "significant" in the NNSR regulations at 40 CFR 51.165 does not include a provision stipulating that, for any pollutant that does not have a listed emissions rate, "any increase" must be considered significant. Therefore, contrary to the concerns of some commenters, the absence of an ammonia SER in the EPA's NNSR regulations at 40 CFR 51.165 does not result in a default "any increase" interpretation of "significant" that must be contained in state NNSR programs. Accordingly, for the above reasons and due to the time, resources and process investment associated with a national rulemaking, the EPA believes that a national rulemaking to develop a SER for ammonia is neither warranted nor effective. As explained above, the EPA is finalizing a provision that requires any state that must regulate ammonia as a PM2.5 precursor to develop and submit a definition of "significant," such as an appropriate SER, for ammonia to be included, subject to the EPA's approval in the state's SIP. See 40 CFR 51.165(a)(1)(x)(F). The EPA recommends that any state that must regulate ammonia as a PM2.5 precursor consult with the appropriate EPA Regional Office if the state chooses to develop an ammonia SER as a means of defining "significant" for a particular nonattainment area. As a general rule, the EPA believes that the ammonia SER in a Moderate nonattainment area should be an emissions rate no greater than 100 tpy of ammonia. Any SER that exceeds 100 tpy could not be approved by the EPA because any higher emissions rate would exceed the major source threshold established in the CAA. In the event that a nonattainment area is classified Serious for PM2.5, the maximum acceptable ammonia SER would be a rate no greater than 70 tpy in accordance with the major source thresholds being finalized in this rule for major stationary sources of direct PM2.5 emissions and PM2.5 precursors locating in Serious PM2.5 nonattainment areas. States that regulate ammonia as a PM2.5 precursor should also include a technical justification for any ammonia SER for a nonattainment area that the state includes as a part of its NNSR SIP rules submission for EPA approval.
d. NNSR Applicability Determinations.
      i. Summary of Proposal.
      In setting SERs and major source thresholds for emissions of direct PM2.5 and PM2.5 precursors, the EPA explained in the preamble to the proposal that it intended for direct PM2.5 emissions and each individual PM2.5 precursor to be treated separately for determining the applicability of the NNSR requirements to a proposed new source or modification. The EPA stated that such individual treatment of direct emissions and precursors was consistent with its policy as explained in previous rulemakings. In particular, the preamble to the 2008 PM2.5 NSR Rule explained that this applicability interpretation applied to both PSD and NNSR. However, at that time, we did not codify this interpretation in any of the NSR regulations. See 73 FR 28231, May 16, 2008, at page 28331. In the proposal, the EPA proposed language in the NNSR regulations at 40 CFR 51.165(a)(2)(i) to explicitly codify the policy.
      ii. Final Action.
      The EPA is revising the NNSR regulations at 40 CFR 51.165(a)(2)(i) to codify the EPA's policy that direct emissions of a pollutant and emissions of any applicable precursor are to be considered independently for purposes of determining the applicability of the NNSR requirements for PM2.5 sources. For example, in order for a source to be subject to the NNSR requirements for PM2.5 with respect to NOx as a PM2.5 precursor, the source must be either (1) a new stationary source that emits or has the potential to emit major amounts of NOx (new major source of NOx); or (2) an existing major source of NOx that proposes to increase its emissions of NOx by a significant amount and also results in a significant net emissions increase.
      iii. Comments and Responses.
      Comment: A commenter requested that the EPA clarify in its NSR rules how to evaluate major source applicability for NNSR and PSD with respect to PM2.5 precursors. The commenter agreed that major source applicability determinations should be based on individual precursor pollutant emissions, and that different pollutants, including individual precursors, should not be summed to determine applicability for NNSR major stationary source or major modification. The commenter also raised various questions pertaining to how the precursors would trigger major source applicability for other pollutants.
      Response: This final rule contains the following statement within the NNSR regulations at 40 CFR 51.165(a)(2)(i), "Different pollutants, including individual precursors, are not summed to determine applicability of a major stationary source or major modification." The commenter's specific precursor-related applicability questions and the EPA's responses are included in the Response to Comment document contained in the Docket for this rulemaking.

      e. NNSR Plan Due Dates.
i. Summary of Proposal.
In the proposal, the EPA explained that CAA section 189(a)(2)(B) requires states to submit to the EPA an attainment plan satisfying the applicable requirements within 18 months of an area being designated nonattainment pursuant to a new or revised PM2.5 NAAQS. See 80 FR 15437. Section 189(a)(1)(A) of the CAA specifically requires that such plans include the NNSR permitting requirements under CAA section 173. Thus, the EPA indicated that states would be required to submit the applicable NNSR program requirements for PM2.5 within 18 months from the effective date of area designations for the 2012 PM2.5 NAAQS. See 80 FR 15437.
The EPA also noted that the CAA does not specify a deadline for the states' submittal of NNSR program revisions in the event that a Moderate PM2.5 nonattainment area is subsequently reclassified as Serious like the CAA establishes a deadline for other plan provisions. Accordingly, the EPA used its gap-filling authority under CAA section 301(a) to propose a similar 18-month deadline, from the effective date of a final reclassification of the area as Serious, for states to submit a plan prescribing the more stringent NNSR requirements required by the statute for Serious areas. However, in light of the fact that such revisions would generally be straightforward to make, and to assure that new major sources and major modifications in the area would be subject to the more stringent NNSR requirements contained in subpart 4 for Serious areas, the EPA sought comments on an alternative 12-month timeframe for submittal of the NNSR plan revisions for Serious areas.
ii. Final Rule.
The EPA is finalizing an 18-month deadline for states to submit plan revisions for NNSR requirements for a PM2.5 after an area is designated to nonattainment or reclassified to Serious. See 40 CFR 51.1003(a)(1)(viii) and (a)(2). As explained elsewhere in this Section VIII of the preamble, such plans revisions must address the more stringent major source thresholds for direct PM2.5 emissions and each PM2.5 precursor that apply in Serious areas. Moreover, states will need to reevaluate which scientific precursors must be subject to the NNSR control requirements regardless of any previous precursor exemption that may have applied for the Moderate area plan.
      iii. Comments and Responses.
      Comment: Some commenters supported allowing states at least 18 months to make the required SIP submission for NNSR. A commenter who supported the longer submission period stated that, although it is easy to write the rule language to make this change, it is likely to be quite difficult to perform the environmental and socio-economic analyses required by state law if the lowering of the threshold for a Serious area does indeed have a significant effect on the building of new or the repowering of existing power plants.
      Response: Although the types of revisions needed to an existing NNSR program to address the new subpart 4 requirements for PM2.5 are relatively straightforward, the EPA acknowledges that such changes nevertheless often involve related analyses as well as state legislative review and approval. In addition, some states will be submitting NNSR regulations for PM2.5 for the first time and, as such, could need more than 12 months to obtain the necessary legislative review and approval. Accordingly, the EPA believes that the most reasonable approach for establishing the plan due date for revised plans for PM2.5 is to establish an 18-month deadline for submission of plans both upon initial designation to nonattainment for a particular PM2.5 standard and upon any subsequent reclassification to Serious.

2. Offset Ruling at 40 CFR part 51 Appendix S.
	In this final rule, as explained later, the EPA is making the following revisions for PM2.5 in the Emission Offset Interpretive Ruling (40 CFR part 51, Appendix S): (a) amending the definition of "regulated NSR pollutant" with regard to PM2.5 precursors; (b) amending the definition of "major stationary source" with regard to major sources of direct PM2.5 emissions and PM2.5 precursors in Serious areas; and (c) amending the definition of "significant" with regard to identifying major modifications of sources of PM2.5 precursors.
      a. Appendix S definition of "regulated NSR pollutant" -- PM2.5 precursors.
      i. Summary of Proposal.
      The definition of "regulated NSR pollutant" contained in Appendix S at section II.A.31(ii)(b)(2) has, to date, only required regulation of SO2 as a PM2.5 precursor for states relying on Appendix S to issue permits to sources locating in PM2.5 nonattainment areas. The EPA proposed to revise the definition in Appendix S of "regulated NSR pollutant" to also require regulation of NOx as a PM2.5 precursor.  The EPA noted that this proposed approach would ensure that states using the permitting requirements contained in Appendix S to issue permits to major new and modified sources in PM2.5 nonattainment areas will regulate the same precursors that have been subject to regulation in states that have already adopted NNSR requirements for PM2.5 based on the 2008 PM2.5 NSR Rule,
      The EPA also proposed an alternative approach based on similar logic that would initially require only SO2 and NOx to be regulated as PM2.5 precursors, while later phasing in VOC and ammonia after states have prepared and the EPA has had sufficient time to evaluate any pending demonstrations to opt out certain precursors. See 80 FR 15436-37. Finally, the EPA also sought comments on an alternative to require the immediate regulation of all four scientific PM2.5 precursors (SO2, NOx, VOC and ammonia) in Appendix S.
      ii. Final Rule.
      The EPA is amending Appendix S in this final rule to provide for the immediate regulation of SO2 and NOx as PM2.5 precursors and for the subsequent conditional phasing in of the regulation of VOC and ammonia on the date 24 months from the effective date of the nonattainment designation in each area. The EPA was persuaded by the comments received expressing concerns that states may delay NNSR SIP development to instead rely on a less-inclusive Appendix S for NNSR permitting if only SO2 and NOx were regulated.
      The alternative proposal featuring the phase-in approach balances the opportunity for states to demonstrate in the short-term that certain precursors need not be regulated with the need to ensure that the appropriate precursors are controlled in a manner consistent with the CAA. NNSR is unique among the nonattainment area requirements in that sources seeking a construction permit must comply with NNSR requirements for a particular pollutant as soon as an area is designated nonattainment for that pollutant and not some months or years later, when the EPA formally approves a state plan and the sources comply with the remaining plan provisions. With respect to precursors in particular, this means that new and modified major sources of direct PM2.5 or a regulated PM2.5 precursor would be subject to NNSR regulation upon the effective date of the area designation to nonattainment. If the EPA required the immediate regulation of all four scientific PM2.5 precursors in Appendix S, states issuing permits pursuant to those provisions during the interim SIP development period would need to require regulation of certain precursors that the state may later be able to demonstrate through a SIP submission do not significantly contribute to PM2.5 levels that exceed the standard in a particular area. As state plans making such a NNSR precursor demonstration are not due until 18 months after the effective date of the area designation, and as the statute allows the EPA up to 18 months to act on such submissions, sources seeking permits to locate in such areas during this interim period might for several years be subject to more stringent controls than necessary to address PM2.5 nonattainment in that area.
      The EPA is also cognizant, however, that some states have relied on Appendix S to conduct NNSR permitting well beyond the statutory SIP development period. In such cases, it would be inequitable if states could indefinitely rely on Appendix S that requires little to no regulation of some of the scientific PM2.5 precursors when other states are fulfilling their statutory duty to submit a SIP revision addressing all PM2.5 precursors. In particular, states that have submitted NNSR SIPs addressing PM2.5 requirements for the 1997 and 2006 standards have to date regulated SO2 and NOx as PM2.5 precursors. These SIP provisions will continue to apply with respect to any areas designated nonattainment as to the 2012 standard in those states until the states submit SIP revisions to address the 2012 NAAQS, including provisions necessary to comply with the precursor requirements in CAA section 189(e). States either continuing to rely on Appendix S by virtue of a nonattainment area designation under a prior PM2.5 standard or states newly relying on Appendix S by virtue of a nonattainment area designation under the 2012 standard have to date, only been required to regulate SO2.
      In order to balance these competing interests and concerns, the EPA has determined in this final rule to revise Appendix S in order to require that any state relying on Appendix S initially regulate both SO2 and NOx as PM2.5 precursors for NNSR permits, thereby aligning the requirements of Appendix S with the prevailing requirements of SIP-approved NNSR permitting provisions for PM2.5 in other states. See Appendix S, section II.A.31.(ii)(b)(2). Further, the final rule provides that VOC and ammonia will be phased in as regulated PM2.5 precursors according to a prescribed schedule based on existing and future nonattainment area designations for PM2.5, unless the EPA has determined, prior to the scheduled phase-in, that the state submitted a complete proposed NNSR program for PM2.5 that includes a NNSR precursor demonstration. The EPA believes it is reasonable not to require regulation of VOC and ammonia in Appendix S during the interim SIP development period because we expect that, in many cases, states will submit SIPs that include as part of their proposed NNSR rules for PM2.5 a NNSR precursor demonstration indicating that they do not need to regulate emissions of ammonia (and in some cases emissions of VOC) under their NNSR programs in order to provide for attainment of the PM2.5 NAAQS.
      Under the phase-in schedules being finalized in Appendix S, permits issued by states under the requirements in Appendix S will not be required to address VOC and ammonia as PM2.5 precursors until the state has had an opportunity to show that, as part of a proposed NNSR program for PM2.5, a particular precursor does not significantly contribute to PM2.5 concentrations that exceed the standard in a given nonattainment area. If a state submits such a NNSR precursor demonstration as to either VOC or ammonia as part of a complete SIP submission that includes the state's proposed NNSR program for PM2.5, the state would not be required to regulate the applicable precursor pursuant to the provisions of Appendix S, unless the EPA reviews that proposed NNSR program for PM2.5 and the NNSR precursor demonstration and either determines that the SIP submission is incomplete or disapproves both the NNSR program and the NNSR precursor demonstration. Thus, the regulation of VOC and ammonia as PM2.5 precursors pursuant to Appendix S will occur in three circumstances. First, in the absence of a plan submission that includes the appropriate NNSR precursor demonstration, VOC and ammonia will be phased in as regulated precursors pursuant to Appendix S 24 months after the effective date of area designations for PM2.5. This will prevent states that fail to make a complete plan submission from continuing to rely on Appendix S to regulate only SO2 and NOx as PM2.5 precursors indefinitely. Second, if the EPA determines that the portion of the SIP containing the NNSR precursor demonstration submitted by the state is incomplete within the time allowed under CAA section 110(k)(1)(B), all precursors must be regulated upon EPA's determination of incompleteness or by the prescribed phase-in date, whichever date is later. The EPA believes it is important to condition the phase-in of VOC and ammonia regulation on the completeness of the SIP submission in order to deter the submission of plans that do not meet certain minimum criteria simply to avoid the regulation of these additional precursors. Finally, if the EPA disapproves both the proposed NNSR program for PM2.5 and the accompanying NNSR precursor

demonstration, the relevant precursors will be phased in to be regulated under Appendix S as of
the effective date of the disapproval or by the prescribed phase-in date, whichever date is later.
      The EPA chose this 24-month period for phase-in of VOC and ammonia as PM2.5 precursors in accordance with (1) the requirement under CAA section 189(a)(2)(B) of subpart 4 that plan revisions for PM2.5 attainment plans be submitted to the EPA within 18 months of area designations, and (2) the requirement under CAA section 110(k)(1)(B) that the EPA determine no later than six months after the date by which a state is required to submit a SIP whether a state has made a submission that meets the minimum completeness criteria established per CAA
section 110(k)(1)(A). In order to provide an appropriate balance between the EPA's interests in providing states with the opportunity to develop precursor demonstrations prior to regulation of those precursors and in encouraging states to submit SIPs in a timely manner, the EPA believes it is reasonable to align the conditional phase-in of VOC and ammonia regulation with the statutory timeframe for states to make SIP submissions addressing precursor regulation for NNSR and for the EPA to evaluate whether a state has made a complete submission. Thus, if by this 24-month deadline, a state has not submitted a precursor demonstration that VOC and/or ammonia need not be regulated, which has been determined to be complete by the EPA or deemed complete by the operation of law by this 24-month deadline, Appendix S will require regulation of these precursors going forward.
      The EPA has specifically included the 6-month period for EPA's completeness review because we believe it is important to discourage states from submitting SIPs that do not meet the minimum completeness criteria found in 40 CFR Part 51, Appendix V. Conditioning the phase-in on a completeness review will not only encourage states to make timely SIP submissions addressing the NNSR requirements, but also ensure that those submissions contain the minimum information necessary to enable the Administrator to determine whether the SIP complies with the statute. If a state with a designated PM2.5 nonattainment area that is currently relying on Appendix S makes a submission addressing NNSR program requirements, including a NNSR precursor demonstration, within 18 months of the designation (as required by CAA section 189(a)(2)(B)), either EPA must evaluate the submission for completeness within 6 months or the SIP will become complete by operation of law, pursuant to CAA section 110(k)(1)(B). The latest date that a timely-submitted implementation plan would be determined to be complete by the EPA or deemed complete by operation of law is 24 months from the effective date of the PM2.5 nonattainment area designation. In other words, in the absence of EPA action to evaluate completeness, a state that submits a timely SIP addressing NNSR and including an NNSR precursor demonstration can be confident the submission will become complete by operation of law by the 24-month conditional phase-in date, and such states will not be required to regulate the precursor addressed by its demonstration (VOC or ammonia) in the PM2.5 nonattainment area pursuant to Appendix S during the period EPA's review of the SIP. States that submit untimely SIPs, after the 18-month SIP submission deadline, cannot rely on the SIP becoming complete by operation of law before the 24-month conditional phase-in date. If the EPA has not acted to evaluate the completeness of the state's untimely SIP by the 24-month conditional phase-in, control of VOC and ammonia are automatically phased in in the PM2.5 nonattainment area under Appendix S, regardless of whether such SIP submission might later be determined complete, whether by EPA or by operation of law. Thus, if a state submits an untimely SIP addressing NNSR for a particular PM2.5 nonattainment area, including an NNSR precursor demonstration, such state can only avoid the conditional phase-in of VOC and ammonia control pursuant to Appendix S if the EPA affirmatively determines the submission to be complete by 24 months from the date of the area designation. In such circumstances, states are encouraged to coordinate with the appropriate EPA Regional Office.
      The timing of the phase-in for a particular area will depend upon the effective date of the designation to nonattainment for PM2.5. Because this rule establishes requirements that apply in both present and future nonattainment areas, the regulations address the timing of the precursor phase-in for both for areas already designated nonattainment for PM2.5 and for areas that may be so designated in the future.
      For any existing nonattainment area that was first designated nonattainment for PM2.5 effective on or before April 15, 2015 (which includes areas designated for the 1997, 2006 and 2012 PM2.5 NAAQS), VOC and ammonia will be required to be controlled as PM2.5 precursors for any NNSR permit issued on or after April 15, 2017 (24 months from the date of area designations for the 2012 PM2.5 NAAQS), unless the state has submitted before the phase-in date a complete SIP revision that includes the state's proposed NNSR program for PM2.5 and a NNSR precursor demonstration showing that VOC, ammonia, or both do not contribute significantly to PM2.5 concentrations that exceed the standard in a given PM2.5 nonattainment area, in which case the control of the precursors addressed by the submitted demonstration will not be required to be controlled at the 24-month mark. See Appendix S, section II.A.31.(ii)(b)(3). In order to satisfy this condition, such demonstration must be submitted in the form of a SIP revision and must either be determined to be complete by the EPA or deemed to be complete by operation of law pursuant to the provisions in CAA section 110(k)(1)(B).
      Although areas were designated nonattainment for the 1997, 2006, and 2012 standards at different times, the EPA believes it is reasonable to apply the same phase-in date for all areas designated nonattainment as of the date of the designations for the 2012 standard. Area designations for the 2012 PM2.5 standards were finalized on April 15, 2015, and plans addressing the nonattainment area requirements as to that standard are due October 15, 2016. Therefore, states evaluating their NNSR programs in light of the subpart 4 requirements with respect to the 2012 standard will have some, if limited, opportunity to consider the requirements of this rule and EPA's technical guidance before submitting a plan revision addressing the statutory and regulatory requirements. By contrast, area designations for the 1997 standards were finalized many years ago. As to those areas, after the court's decision in NRDC v. EPA, the EPA promulgated a rule setting a deadline of December 31, 2014, for states to submit any attainment plan provisions that may be necessary to satisfy the subpart 4 requirements. 79 FR 31566
(June 2, 2014) at 31570. This included any submissions necessary to address NNSR permitting such as the CAA section 189(e) requirement that states regulate all PM2.5 precursors absent a demonstration that such regulation is unnecessary. This deadline superseded previous SIP submission deadlines initially established by application of the subpart 1 requirements. As that deadline has passed, if the EPA were to apply the 24-month phase-in policy strictly, states relying upon Appendix S to issue NNSR permits in these areas would have had to commence regulating VOC and ammonia as PM2.5 precursors in June 2015 -- 6 months after the SIP submission deadline. The EPA believes it is reasonable to provide states that have areas designated nonattainment with respect to the 1997 and 2006 PM2.5 standards with at least some limited opportunity to consider the requirements of this rule and EPA's technical guidance and submit a plan revision addressing the statutory and regulatory requirements before the state will be required to regulate sources of VOC and ammonia in such areas. Accordingly, the EPA finds that it is reasonable to subject all areas designated nonattainment as to any standard as of April 15, 2015, to the same Appendix S requirements in this final rule.
      For any area that is first designated nonattainment for any PM2.5 NAAQS after April 15, 2015 (that is, the area was not already designated nonattainment with respect to another PM2.5 NAAQS immediately prior to such date), any state relying on Appendix S to issue a NNSR permit on or after the effective date of such area designation must require control of SO2 and NOx as PM2.5 precursors. Beginning on the date 24 months from the effective date of such area designation, a state relying on Appendix S to issue a NNSR permit must also require control of VOC and ammonia as PM2.5 precursors in that area, unless by that date the state has submitted a complete SIP revision that includes the state's proposed NNSR program for PM2.5 and an accompanying NNSR precursor demonstration that VOC, ammonia, or both ammonia do not contribute significantly to the PM2.5 concentrations that exceed the standard in the PM2.5 nonattainment area. See Appendix S, section II.A.31.(ii)(b)(4). As explained earlier, such demonstration must be submitted as part of a SIP revision that is determined to be complete by the EPA or deemed to be complete by operation of law by the conditional phase-in date.
      As noted earlier, the second phase-in provision applies to PM2.5 nonattainment areas that were not already designated as nonattainment for PM2.5 immediately prior to that date. If at the time of a new designation, an area was already designated nonattainment as to any prior PM2.5 NAAQS, and Appendix S applied and continues to apply for NNSR permitting with respect to that existing nonattainment area, all PM2.5 precursors would likely already be required to be regulated in accordance with a prior phase-in schedule prescribed under Appendix S for that existing nonattainment designation. In such cases, all precursors would continue to be subject to regulation for NNSR permitting under Appendix S, even as to the new nonattainment designation. That is, once Appendix S applies to all PM2.5 precursors in a given nonattainment area, it is not possible to later defer regulation of any precursors so long as the state continues to rely on Appendix S for NNSR permitting in that area. Once the state submits a SIP including an NNSR program and any appropriate NNSR precursor demonstration, and the EPA approves the SIP, Appendix S will no longer apply for the issuance of NNSR permits for PM2.5.
      iii. Comments and Responses.
      Comment: Several commenters generally supported the EPA's preferred approach in the proposal that would require only SO2 and NOx as PM2.5 precursors for NNSR permits issued pursuant to Appendix S. One commenter supported the alternative approach to phase in VOC and ammonia as PM2.5 precursors, while another commenter expressly opposed the EPA's preferred approach and the phase-in alternative, claiming that any approach that does not regulate all four scientific precursors of PM2.5 is contrary to CAA subpart 4 and unlawful.
      Commenters supporting the preferred approach did not believe that it was appropriate to require NSR permitting during an interim period for sources that may be exempted from control requirements if a state can demonstrate that these sources do not contribute significantly to nonattainment in a particular area. These commenters stated that, since most, if not all, areas will not be able to demonstrate that SO2 and NOx do not contribute significantly to nonattainment levels of PM2.5, the EPA's approach to include these two precursors in the interim is reasonable.
      One commenter who supported the EPA's alternative approach to phase in VOC and ammonia as PM2.5 precursors stated that there are many unanswered questions and the science is not adequate to justify regulation of secondary formation precursors at this time. The commenter further stated that Appendix S should initially require sources issued a NNSR permit to control only SO2 and NOx as PM2.5 precursors, and only later, after a prescribed date (e.g., the date on which SIP revisions based on subpart 4 requirements are due), require sources to control emissions of VOC and ammonia, if applicable.
      A commenter who opposed any approach that did not immediately require the control of all four scientific precursors of PM2.5 stated that such approaches are unlawful and must be rejected. The commenter stated that the EPA must require immediate regulation of all four precursors, as only that alternative follows the plain language of the CAA and the NRDC decision. The commenter objected to the presumptive exemption of VOC and ammonia emissions as being identical to the "gamesmanship" that both Congress intended to curtail with subpart 4, and that the D.C. circuit found illegal in the NRDC decision. The commenter stated that the scope of the statutory definition, and consequently the application of subpart 4, did not change when the EPA subdivided PM10 by regulation. The commenter stated that only this option would conform Appendix S to the requirements of subpart 4, and in so doing, align Appendix S with forthcoming state obligations to harmonize the PM2.5 portions of their SIPs with the obligations of subpart 4. The commenter stated that this approach would encourage states to submit SIPs in a timely fashion, rather than to rely on Appendix S for an extended period of time. The commenter stated that, in contrast, were the EPA to adopt illegally lax provisions into Appendix S, states might delay submitting replacement SIPs, particularly in those parts of the country with high VOC or ammonia precursor emissions.
      Response: The EPA took each of these comments into consideration in concluding that the proposed phase-in alternative is a reasonable approach that balances competing factors regarding the regulation of PM2.5 precursors for NNSR permits issued pursuant to Appendix S. While CAA section 189(e) generally requires state plans to control all PM2.5 precursors, it also affords states an opportunity to demonstrate that a particular precursor does not contribute significantly to levels of PM2.5 that exceed the standard in a PM2.5 nonattainment area. Section 189(e) of the CAA clearly addresses how PM2.5 precursors must be regulated in the state's plan, the statute does not address exactly when precursors are to be regulated pursuant to the NNSR requirements of Appendix S prior to the submission of the state's plan. As noted earlier, the NNSR provisions are unique among the nonattainment area requirements in that sources are required to address NNSR immediately upon the effective date of an area's designation to nonattainment, rather than upon the EPA's approval of the state's SIP, which could be as much as 3 years after the nonattainment area designation (e.g., states have 18 months to submit attainment plans and the EPA may have up to 18 months from the date of the SIP submission to finalize action on such plans). Given this ambiguity in the statute and the unique application of the NNSR requirements, we believe a reasonable and balanced approach to the Appendix S requirements would allow states a time-limited period to submit a NNSR program for PM2.5 that includes a NNSR precursor demonstration that certain precursors do not contribute significantly to PM2.5 levels in a PM2.5 nonattainment area. The time limit will discourage states from unreasonably delaying regulation of such precursors where otherwise required to do so.
      Moreover, the EPA believes it is reasonable to construct the Appendix S provisions regulating PM2.5 precursors in a manner that closely follows the way in which the precursors are being regulated in most state NSR programs based on EPA's 2008 NSR regulation. For areas that were designated attainment or unclassifiable prior to a new nonattainment designation, the PSD permit program was in effect and required that, at minimum, SO2 and NOx be regulated as PM2.5 precursors. It is therefore reasonable to ensure that those precursors continue to be regulated as part of the interim NNSR permit program via Appendix S. Moreover, in areas that were already designated nonattainment for a pre-existing PM2.5 NAAQS, and an approved plan containing NNSR permit requirements for PM2.5 is in effect, sources are required to control SO2 and NOx as PM2.5 precursors, as required under the 2008 PM2.5 NSR rule, until the EPA approves a SIP revision conforming those NNSR programs to the requirements of CAA subpart 4. Similarly, the EPA believes it is reasonable to not require the regulation of VOC and ammonia immediately upon designation of an area to nonattainment because it result in more regulation in newly designated nonattainment areas relying on Appendix S than is required in most states with approved programs. All states will ultimately be required to address the regulation of ammonia and VOC at the time their state plans are due or, failing submission of such plan by states relying on Appendix S to issue NNSR permits, Appendix S will require such regulation.
      The phase-in schedule contained in this final rule requires that VOC and ammonia be phased in as PM2.5 precursors 24 months from the effective date of area designations for PM2.5; however, states will not be required to control VOC and ammonia as PM2.5 precursors as part of a NNSR permit issued under Appendix S so long as the state submits a plan revision that includes the state's NNSR program for PM2.5 and a NNSR precursor demonstration to show that such precursor does not contribute significantly to PM2.5 levels that exceed the standard in a PM2.5 nonattainment area. See Appendix S, sections II.A.31.(ii)(b)(3) and (4).
      In initially requiring sources to control SO2 and NOx as PM2.5 precursors, states that rely on Appendix S to issue NNSR permits generally will implement NNSR consistent with those states that issue NNSR permits for PM2.5 under the NNSR program in their approved SIP. The EPA believes that it is reasonable and appropriate to assure this consistency in the issuance of NNSR permits during the interim period when all states must revise their plans to address the 2012 PM2.5 NAAQS. Moreover, this final rule allows states to submit a SIP revision that contains a NNSR precursor demonstration showing that either SO2 or NOx should be exempted where an analysis of increases in emissions of the particular precursor shows that the precursor does not contribute significantly to PM2.5 levels that exceed the standard in the PM2.5 nonattainment area. In this case, the opportunity to opt out of controlling either SO2 or NOx as a PM2.5 precursor is addressed in the NNSR rules at 51.165. See 40 CFR 51.165(a)(1)(xxxvii)(C)(2). Hence, where the EPA approves a state's plan revision containing the NNSR program for PM2.5 and a NNSR precursor demonstration showing an insignificant contribution, either SO2 or NOx will not be required to be controlled going forward in a NNSR permit issued to address PM2.5, which would then be issued in accordance with the NNSR requirements in the approved plan, rather than the NNSR requirements in Appendix S.
      With regard to the commenter's concern that states might delay submitting NNSR programs as part of their PM2.5 SIPs if Appendix S regulates only SO2 and NOx, the phase-in approach in this final rule will negate any incentive that a state may have to delay submitting an NNSR program for PM2.5 addressing the CAA section 189(e) requirement to regulate all four precursors, absent a showing that such regulation is unnecessary. In fact, the phase-in requirement should actually encourage states to timely submit their NNSR programs for PM2.5. Given CAA section 189(e) does not directly speak to its application to the Appendix S requirements, the EPA believes this approach represents a reasonable and equitable application of the CAA section 189(e) requirements regarding regulation of PM2.5 precursors to states applying Appendix S.
      b. Appendix S Definition of "major stationary source" in Serious PM2.5 Nonattainment Areas.
      i. Summary of Proposal.
      The EPA proposed to amend Appendix S by revising the definition of "major stationary source" to include a separate PM2.5 major source threshold applicable to new major stationary sources and major modifications of direct PM2.5 and PM2.5 emissions in PM2.5 nonattainment areas reclassified as Serious. This proposed amendment to Appendix S was similar to one that was proposed to the definition of "major stationary source" in 40 CFR 51.165.

      ii. Final Rule.