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Federal Register / Vol. 86, No. 183 / Friday, September 24, 2021 / Proposed Rules EPA a comprehensive precursor demonstration for a specific nonattainment area to show that emissions of a particular precursor from all existing sources located in the nonattainment area do not contribute significantly to PM2.5 levels that exceed the standard in the area.95 If the EPA
determines that the contribution of the precursor to PM2.5 levels in the area is not significant and approves the demonstration, then the state is not required to control emissions of the relevant precursor from sources in the attainment plan.96
In addition, in May 2019, the EPA
issued the Fine Particulate Matter PM2.5 Precursor Demonstration Guidance PM2.5 Precursor Guidance,97 which provides recommendations to states for analyzing nonattainment area PM2.5 emissions and developing such optional precursor demonstrations, consistent with the PM2.5 SIP Requirements Rule. The PM2.5
Precursor Guidance builds upon the draft version of the guidance, released on November 17, 2016 Draft PM2.5
Precursor Guidance, which CARB
referenced in developing its precursor demonstration in the SJV PM2.5 Plan.98
The EPAs recommendations in the PM2.5 Precursor Guidance are generally consistent with those in the Draft PM2.5
Precursor Guidance, with some exceptions, including that the EPAs recommended contribution threshold for the 24-hour PM2.5 NAAQS changed from 1.3 mg/m3 in the draft guidance to 1.5 mg/m3 in the final guidance.99
We are evaluating the 1997 24-hour PM2.5 NAAQS portion of the SJV PM2.5
Plan in accordance with the presumption embodied within subpart 4, that states address all PM2.5
precursors in the evaluation of potential control measures unless the state adequately demonstrates that emissions of a particular precursor or precursors 95 40

CFR 51.1006a1.

96 Id.
97 PM
2.5 Precursor Demonstration Guidance, EPA454/R19004, May 2019, including memorandum dated May 30, 2019 from Scott Mathias, Acting Director, Air Quality Policy Division and Richard Wayland, Director, Air Quality Assessment Division, Office of Air Quality Planning and Standards OAQPS, EPA to Regional Air Division Directors, Regions 110, EPA.
98 PM
2.5 Precursor Demonstration Guidance, Draft for Public Review and Comments, EPA454/
P16001, November 17, 2016, including memorandum dated November 17, 2016 from Stephen D. Page, Director, OAQPS, EPA to Regional Air Division Directors, Regions 110, EPA.
99 For the 24-hour PM
2.5 NAAQS, the EPA
generally expects that a precursor demonstration showing that the air quality impact of a given precursor at all relevant locations does not exceed a contribution threshold of 1.5 mg/m3 will be adequate to exempt sources of that precursor from control requirements. PM2.5 Precursor Guidance, 17.

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do not contribute significantly to ambient PM2.5 levels that exceed the PM2.5 NAAQS in the nonattainment area and are not necessary for attainment. In reviewing any determination by a state to exclude a PM2.5 precursor from the required evaluation of potential control measures, we consider both the magnitude of the precursors contribution to ambient PM2.5
concentrations in the nonattainment area and the sensitivity of ambient PM2.5
concentrations in the area to reductions in emissions of that precursor.
2. Summary of the States Submission The State presents a brief summary of its PM2.5 precursor analysis in Chapter 5 of the 2018 PM2.5 Plan and the full precursor demonstration in Appendix G
Precursor Demonstration of the 2018
PM2.5 Plan.100 CARB presents additional modeling results in Appendix K
Modeling Attainment Demonstration, section 5.6 PM2.5
Precursor Sensitivity Analysis. CARB
also provided clarifying information on its precursor assessment, including an Attachment A to its letter transmitting the 2018 PM2.5 Plan to the EPA 101 and further clarifications in five email transmittals.102 The CARB Staff Report contains additional discussion of the role of ammonia in the formation of ammonium nitrate and the role of VOC
in the formation of ammonium nitrate and secondary organic aerosol.103
100 A copy of the contents of Appendix G appears in the CARB Staff Report, Appendix C4 Precursor Demonstrations for Ammonia, SOX, and ROG.
101 Letter dated May 9, 2019, from Richard Corey, Executive Officer, CARB, to Michael Stoker, Regional Administrator, EPA Region 9, Attachment A Clarifying information for the San Joaquin Valley 2018 Plan regarding model sensitivity related to ammonia and ammonia controls.
102 Email dated June 20, 2019, from Jeremy Avise, CARB, to Scott Bohning, EPA Region IX, Subject:
RE: SJV model disbenefit from SOX reduction, with attachment CARBs June 2019 Precursor Clarification; email dated September 19, 2019, from Jeremy Avise, CARB, to Scott Bohning, EPA
Region IX, Subject: FW: SJV species responses, with attachments CARBs September 2019
Precursor Clarification; email dated October 18, 2019, from Laura Carr, CARB, to Scott Bohning, Jeanhee Hong, and Rory Mays, EPA Region IX, Subject: Clarifying information on ammonia, with attachment Clarifying Information on Ammonia CARBs October 2019 Precursor Clarification;
email dated April 19, 2021, from Laura Carr, CARB, to Rory Mays, EPA Region IX, Subject: Ammonia update, with attachment Update on Ammonia in the San Joaquin Valley CARBs April 19, 2021
Precursor Clarification; and email dated April 26, 2021, from Laura Carr, CARB, to Scott Bohning, EPA Region IX, Subject: RE: Ammonia update, with attachment Ammonia in San Joaquin Valley CARBs April 26, 2021 Precursor Clarification.
103 CARB Staff Report, Appendix C, 916. The CARB Staff Report, Appendix C4 Precursor Demonstrations for Ammonia, SOX, and ROG is very similar to the contents of Appendix G of the 2018 PM2.5 Plan.

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The 2018 PM2.5 Plan provides both concentration-based and sensitivitybased analyses of precursor contributions to ambient PM2.5
concentrations in the San Joaquin Valley. The State supplemented the sensitivity analysis, particularly for ammonia, with additional information, including factors identified in the PM2.5
Precursor Guidance, such as emissions trends, the appropriateness of future year versus base year sensitivity, available emissions controls, and the severity of nonattainment.104 These analyses led CARB to conclude that direct PM2.5 and NOX emissions contribute significantly to ambient PM2.5 levels that exceed the PM2.5
NAAQS in the San Joaquin Valley while ammonia, SOX, and VOC do not contribute significantly to such exceedances.105 We summarize the States analysis and conclusions below.
For a more detailed summary of the precursor demonstration in the Plan, please refer to the EPAs Technical Support Document, EPA Evaluation of PM2.5 Precursor Demonstration, San Joaquin Valley PM2.5 Plan for the 2006
PM2.5 NAAQS, February 2020 EPAs February 2020 Precursor TSD.
For direct PM2.5 and NOX, CARB
modeled the sensitivity of ambient PM2.5 in the San Joaquin Valley to a 30
percent reduction in anthropogenic emissions of each pollutant in 2013, 2020, and 2024.106 The State concluded that direct PM2.5 and NOX emissions reductions will continue to have a significant impact on 24-hour PM2.5
design values in the San Joaquin Valley, with NOX reductions being particularly important.107 Consistent with this conclusion, the State focused the control strategy and attainment demonstration on these two pollutants, 104 PM
2.5 Precursor Guidance, 1819
consideration of additional information, 31
available emissions controls, and 3536
appropriateness of future year versus base year sensitivity.
105 Direct PM
2.5 emissions are considered a primary source of ambient PM2.5 i.e., no further formation in the atmosphere is required, and therefore is not considered a precursor pollutant under subpart 4, which may differ from a more generalized understanding of what contributes to ambient PM2.5.
106 2018 PM
2.5 Plan, Chapter 5, 57 to 58. CARB
modeled the effects of both NOX reductions and direct PM2.5 reductions but the direct PM2.5 results were used only as a point of comparison, as direct PM2.5 emissions must be regulated in all PM2.5
nonattainment areas.
107 Id. at 58; and 2018 PM
2.5 Plan, Appendix G, 2. CARB presents its sensitivity analysis for emissions reductions in direct PM2.5 and NOX in the Plans attainment demonstration appendix. 2018
PM2.5 Plan, Appendix K, Table 47 annual average design values and Table 48 24-hour average design values.

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