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Federal Register / Vol. 86, No. 183 / Friday, September 24, 2021 / Proposed Rules percent emissions reductions, and in both the 2013 base year and 2020 and 2024 future year, it is not necessary to distinguish between the timing and scale of emissions reductions with respect to the response of ambient PM2.5
levels as in the ammonia evaluation where the results diverged according to scale and timing of modeled emissions reductions. The EPAs February 2020
Precursor TSD contains additional detail on the EPAs evaluation of SOX as a PM2.5 precursor, including the disbenefit associated with a reduction in SOX emissions. Accordingly, we find that the States decision to rely on the 2013 sensitivity modeling results for a 30 percent SOX reduction is acceptable.
Therefore, on the basis of the modeled ambient PM2.5 response to both a 30
percent and 70 percent reduction in SOX emissions in 2013, and on the facts and circumstances of the area, the EPA
proposes to approve the States demonstration that SOX emissions do not contribute significantly to ambient PM2.5 levels that exceed the 1997 24hour PM2.5 NAAQS in the San Joaquin Valley.
c. VOC
For VOC, the State found that the ambient PM2.5 response to VOC
emissions reductions were generally below the EPAs recommended contribution threshold of 1.3 mg/m3 in the Draft PM2.5 Precursor Guidance and below the EPAs recommended threshold of 1.5 mg/m3 in the final PM2.5
Precursor Guidance, and often predicted an increase in ambient PM2.5 levels in response to such reductions i.e., a disbenefit, except for a 70 percent emissions reduction for the 2013 base year, where the State predicted the ambient PM2.5 response to be above both recommended thresholds at a majority of sites. The EPA has evaluated and agrees with the States determination that the modeling for future years is more representative of conditions in the San Joaquin Valley than the 2013
modeling for sensitivity-based analyses and the States resulting conclusion as to whether the contribution from VOC
emissions is significant.
Regarding emissions trends, the EPA
agrees that the 8.6 percent decrease in VOC emissions from 2013 to 2020 and the 9.2 percent projected decrease from 2013 to 2024 favors reliance on the future year modeling results.
Furthermore, there is a large decrease in NOX emissions over this period, as discussed in the EPAs evaluation of ammonia, which affects the atmospheric chemistry with respect to ambient PM2.5
formation from VOC emissions. The 9
percent VOC emissions reductions and
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the NOX emissions reductions are projected to result from implementation of existing baseline measures. We therefore find it reasonable to rely on future year 2020 or 2024 modeled responses to VOC emissions reductions, and both years show a disbenefit from VOC emissions reductions. The EPA
also finds that the State provided a reasonable explanation for the VOC
reduction disbenefit and evidence that it occurs in the San Joaquin Valley.
For these reasons, we propose to approve the States demonstration that VOC emissions do not contribute significantly to ambient PM2.5 levels that exceed the 1997 24-hour PM2.5 NAAQS
in the San Joaquin Valley.
C. Attainment Plan Control Strategy 1. Statutory and Regulatory Requirements Section 189b1B of the Act requires for any Serious PM2.5
nonattainment area that the state submit provisions to assure that BACM for the control of PM2.5 and PM2.5 precursors shall be implemented no later than four years after the date the area is reclassified as a Serious area. The EPA
has defined BACM in the PM2.5 SIP
Requirements Rule to mean any technologically and economically feasible control measure that . . . can achieve greater permanent and enforceable emissions reductions of direct PM2.5 emissions and/or emissions of PM2.5 plan precursors from sources in the area than can be achieved through the implementation of RACM on the same sources. BACM includes best available control technology BACT. 142
Because the 2015 Serious area attainment date has passed, and the EPA
found that the area failed to attain by the Serious area attainment date, we are evaluating the submission for compliance with the BACM/BACT
requirements now, in conjunction with the States SIP submission intended to meet both the Serious area plan and section 189d plan requirements.
The EPA generally considers BACM a control level that goes beyond existing RACM-level controls, for example by expanding the use of RACM controls or by requiring preventative measures instead of remediation.143 Indeed, as 142 40

CFR 51.1000 definitions. In longstanding guidance, the EPA has similarly defined BACM to mean, among other things, the maximum degree of emissions reduction achievable for a source or source category, which is determined on a case-bycase basis considering energy, environmental, and economic impacts. General Preamble Addendum, 42010, 42013.
143 81 FR 58010, 58081 and General Preamble Addendum, 42011, 42013.

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implementation of BACM and BACT is required when a Moderate nonattainment area is reclassified as Serious due to its inability to attain the NAAQS through implementation of reasonable measures, it is logical that best control measures should represent a more stringent and potentially more costly level of control.144 If RACM and RACT level controls of emissions have been insufficient to reach attainment, the CAA contemplates the implementation of more stringent controls, controls on more sources, or other adjustments to the control strategy are necessary to attain the NAAQS in the area.
Under the PM2.5 SIP Requirements Rule, those control measures that otherwise meet the definition of BACM/
BACT but can only be implemented in whole or in part beginning four years after reclassification are referred to as additional feasible measures. 145 In accordance with the requirements of CAA section 172c6, a Serious area plan must include any additional feasible measures to control emissions of direct PM2.5 and PM2.5 precursors that are necessary and appropriate to provide for attainment of the relevant NAAQS as expeditiously as practicable and no later than the applicable attainment date.146
Consistent with longstanding guidance provided in the General Preamble Addendum, the preamble to the PM2.5 SIP Requirements Rule discusses the following steps for determining BACM and BACT and additional feasible measures:
1 Develop a comprehensive emissions inventory of the sources of PM2.5 and PM2.5 precursors;
2 Identify potential control measures;
3 Determine whether an available control measure or technology is technologically feasible;
4 Determine whether an available control measure or technology is economically feasible; and 5 Determine the earliest date by which a control measure or technology can be implemented in whole or in part.147
The EPA allows consideration of factors such as physical plant layout, 144 Id. and General Preamble Addendum, 42009
42010.
145 40 CFR 51.1000, 40 CFR 51.1010a4ii.
146 Because the Serious area attainment year has passed and the area failed to attain by the Serious area attainment date, we will evaluate the BACM/
BACT and additional feasible measure analysis for the Serious area plan with respect to the current section 189d projected attainment date of December 31, 2020.
147 81 FR 58010, 5808358085.

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