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Federal Register / Vol. 86, No. 183 / Friday, September 24, 2021 / Proposed Rules were to combine with available ammonia to form particulate.113 The CARB Staff Report describes these conclusions in more detail and lists results from multiple other recent studies with similar conclusions.114
Finally, in a supplemental submittal, CARB described the results of two analyses confirming the likely underestimation of ammonia emissions in the modeled emissions inventory inputs.115 CARB compared CMAQ
model predictions of ammonia with the 2013 DISCOVERAQ aircraft measurements and found ammonia was underpredicted, and noted that this would result in the response to ammonia reductions being overpredicted. CARB also compared 2017 satellite measurements of ammonia with CMAQ model predictions and found that modeled ammonia concentrations were half of the magnitude of the satellite observations at some locations, and the modeled valley-wide average was about 25 percent less than observed. Because the modeling performs well for the various PM2.5 components, as well as for ozone and NO2,116 the CARB finding of CMAQ model underpredictions for ammonia is consistent with an underestimation of ammonia emissions inventory input to the model.
Regarding emissions trends, the CARB
Staff Report presents an emissions inventory-based argument on the relative insensitivity of PM2.5 to ammonia reductions.117 CARB
compared the size of the ammonia and NOX emissions inventories in tons per day, after normalizing for their differing molecular weights, and found that ammonia was roughly three times as abundant as NOX in 2013 and is projected to be about six times as abundant in 2025, due to the continuing decline in NOX emissions while ammonia emissions are generally constant into the future.118 While the State recognized that this is only a first-level assessment, it provides additional support for the States conclusion that NOX, and not ammonia, is the limiting precursor for ammonium nitrate formation, and that the ammonium nitrate portion of ambient PM2.5 would be expected to be relatively insensitive to ammonia emissions reductions. This is also consistent with 113 2018

PM2.5 Plan, Appendix G, Figure 2.
Staff Report, Appendix C, 12.
115 CARBs April 26, 2021 Precursor Clarification.
116 EPAs February 2020 Modeling TSD, 21.
117 CARB Staff Report, Appendix C, 15.
118 Annual average ammonia emissions are projected to decrease 4.6 tpd 1.4 percent from 2013 to 2024. 2018 PM2.5 Plan, Appendix B, Table B5.
114 CARB

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the ammonia sensitivity modeling for the San Joaquin Valley, which showed that PM2.5 concentrations will be less sensitive to ammonia reductions as NOX
emissions go down in the future i.e., the PM2.5 impacts were much smaller in the 2020 and 2024 future modeled cases compared to the 2013 base year.
The State projected that NOX
emissions in the San Joaquin Valley would decrease by 36 percent from 2013
to 2020, and by 53 percent from 2013 to 2024, while ammonia emissions would remain relatively flat, thereby increasing the relative abundance of ammonia.119
Based on the Plans emissions reduction projections combined with the research study conclusions, the State relies on the modeled responses for the future years, rather than the 2013 base year, stating that the future year NOX
emissions are more representative of San Joaquin Valley emissions conditions.120 The State references the Draft PM2.5 Precursor Guidance, which notes that it may be appropriate to model future conditions that are more representative of current atmospheric conditions and those conditions expected closer to the attainment date.
The State concludes that this in fact applies to the San Joaquin Valley.121
With respect to the States selection of 30 percent as an upper bound on the ammonia reductions to model, the State described its review of the most important ammonia source categories in the San Joaquin Valley, existing control measures that affect ammonia emissions from these sources, additional mitigation options for these sources, and information provided in the PM2.5
Precursor Guidance about ammonia reductions achieved nationwide from 2011 to 2017.122 The primary sources of ammonia emissions identified in the 2018 PM2.5 Plan are: 1 Confined animal facilities CAFs; 2 agricultural fertilizer; 3 biosolids, animal manure, and poultry litter operations; and 4
organic material composting operations.123 CAFs are subject to District Rule 4570; biosolids, animal manure, and poultry litter operations are subject to District Rule 4565; and organic material composting operations are subject to District Rule 4566.
Although these District rules explicitly apply only to VOC emissions from these sources, the State concludes that these 119 2018

PM2.5 Plan, Appendix G, 89.
at 9.
121 Id referencing Draft PM
2.5 Precursor Guidance, 33. See also PM2.5 Precursor Guidance, 35.
122 2018 PM
2.5 Plan, Appendix G and Appendix C, section C25, and CARBs October 2019
Precursor Clarification.
123 2018 PM
2.5 Plan, Appendix C, section C25.
120 Id.

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rules also reduce ammonia emissions.
Appendix C of the 2018 PM2.5 Plan cites several scientific studies that address the correlation between VOC and ammonia emissions from these emissions sources.124 Based on these evaluations, the State concludes that ammonia control measures achieving even the low end of the range 30
percent are not feasible for implementation in the San Joaquin Valley and that it is therefore reasonable to treat a 30 percent ammonia reduction as an upper bound for modeling in the precursor demonstration.
In summary, the States sensitivity analysis presents a range of PM2.5
responses to ammonia emissions reductions depending on base year versus future year, and on the scale of emissions reductions that may be possible. The Plan provides the States bases for finding that the future year sensitivity results better represent conditions in the San Joaquin Valley than the 2013 base year and for finding a 30 percent ammonia reduction to be a reasonable upper bound for modeled ammonia emissions reductions in assessing the ammonia contribution.
Based on these analyses, the State concludes that ammonia does not contribute significantly to ambient PM2.5 levels above the 1997 24-hour PM2.5 NAAQS in the San Joaquin Valley.
b. SOX
For SOX, the State compared the 24hour precursor contributions to the recommended draft contribution threshold of 1.3 mg/m3 in the Draft PM2.5
Precursor Guidance. For modeled SOX
emissions reductions of 30 percent and 70 percent, the ambient PM2.5 responses in 2013 ranged from 1.4 to 0.5 mg/m3
across 15 monitoring sites, which all fall below the 1.3 mg/m3 draft contribution threshold, and hence also below the contribution threshold of 1.5 mg/m3 in the final version of the PM2.5 Precursor Guidance.125 The response was below zero at most monitoring sites, indicating an increase, rather than a decrease, in ambient PM2.5 in response to SOX
emissions reductions i.e., a disbenefit.
Only the Stockton and Manteca sites had slightly positive responses to 30
percent and 70 percent emissions reductions, and the Tranquillity site also had a slightly positive response only to a 30 percent reduction. For the 15 sites, in 2020, the responses to 30
percent and 70 percent emissions reductions ranged from 1.3 mg/m3 to 124 Id.

at C314 and following.
PM2.5 Plan, Appendix G, Table 8 and
125 2018

Table 9.

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