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Federal Register / Vol. 86, No. 183 / Friday, September 24, 2021 / Proposed Rules
0.5 mg/m3 while for 2024, the responses ranged from 1.1 mg/m3 to 0.6 mg/m3;
these are also all below the contribution threshold, with most sites showing a disbenefit from SOX reductions.126 The Stockton, Manteca, and Tranquillity sites showed the same pattern of slight benefits as for 2013.127 For further detail, please see the EPAs February 2020 Precursor TSD, Table 3 and the 2018 PM2.5 Plan, Appendix G, tables 8
and 9 and Appendix K, tables 46, 48, and 50.
CARB also included additional information regarding emissions trends and an evaluation of the SOX emissions reduction disbenefit. We summarize this additional information below and provide a more detailed evaluation in the EPAs February 2020 Precursor TSD.
In terms of emissions trends, the State found that SOX emissions decreased from 2013 to 2014 and then were expected to very gradually rise to 7.8
tpd in 2020 and 8.0 tpd in 2024.128
Given that projected SOX emissions are very similar in 2020 and 2024, the State concluded that the 2020 and 2024
sensitivity results were redundant.
Comparing the ambient responses in 2013 and 2024, the State found that the responses were slightly less negative or, for a small number of sites, slightly higher in 2024, but still no more than 0.6 mg/m3 in response to a 70 percent SOX emissions reduction.129 This supports the States conclusion as to the overall disbenefit of reducing SOX
emissions.
To explain the SOX emissions reduction disbenefit that is observed in some cases, CARB refers to the nonlinearity of inorganic aerosol thermodynamics, as described in a study by West et al.130 That paper discusses how, under certain conditions, reducing SOX could free ammonia to combine with nitrate, increasing overall PM2.5 mass. To investigate this issue further, CARB
conducted simulations with the ISORROPIA inorganic aerosol thermodynamic equilibrium model used within the CMAQ model and provided 126 CARBs September 2019 Precursor Clarification, 2020 analysis tables 15 and 16, and 2024 analysis tables 15 and 16.
127 2018 PM
2.5 Plan, Appendix K, Table 48 and Table 50.
128 2018 PM
2.5 Plan, Appendix G, Figure 4.
129 CARBs September 2019 Precursor Clarification, 2013 analysis Table 16 and 2024
analysis Table 16.
130 2018 PM
2.5 Plan, Appendix K, section 5.6
PM2.5 Precursor Sensitivity Analysis; and West, J.J., Ansari, A.S., Pandis, S.N., 1999, Marginal PM2.5:
Nonlinear aerosol mass response to sulfate reductions in the eastern United States, Journal of the Air & Waste Management Association, 49, 14151424. https doi.org/10.1080/
10473289.1999.10463973.

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clarifications to the EPA.131 In essence, CARB states that for some conditions typical of San Joaquin Valley, ISORROPIA switches to a different chemical regime in which the disbenefit occurs. CARB states that it is not known how well this model behavior reflects the actual atmosphere, but CARB
accepts the results because it is a wellknown and widely used chemical model.
Based on the small and mostly negative modeled response of ambient PM2.5 to SOX emissions reductions, and based on its scientific understanding of sulfate interactions with other molecules in the air, the State concludes that SOX does not contribute significantly to ambient PM2.5 levels that exceed the 1997 24-hour PM2.5 NAAQS
in the San Joaquin Valley.
c. VOC
For VOC, CARB compared the 24hour precursor contributions to the EPAs recommended draft contribution threshold of 1.3 mg/m3. For a modeled 30 percent VOC emissions reduction, the ambient PM2.5 responses in 2013
ranged from 0.1 to 1.9 mg/m3 across 15
monitoring sites, with two sites above the 1.3 mg/m3 draft contribution threshold.132 133 The 2020 responses ranged from 0.1 to 0.6 mg/m3, with all monitoring sites below the 1.3 mg/m3
draft contribution threshold, and hence also below the contribution threshold of 1.5 mg/m3 that was finalized in the final PM2.5 Precursor Guidance. The 2024
responses ranged from 0.4 to 0.0 mg/
m3, with all monitoring sites below both the draft and final contribution thresholds. For a 70 percent VOC
emissions reduction, the PM2.5
responses in 2013 ranged from 0.2 to 4.8
mg/m3, including responses above both contribution thresholds at a majority of sites. The 2020 response ranged from 0.2 to 1.5 mg/m3, with one site at the final contribution threshold. The 2024
response ranged from 1.0 to 0.0 mg/m3
with monitoring sites below both the contribution thresholds. In other words, in response to either a 30 percent or a 70 percent reduction in VOC emissions, CARB models a decrease in ambient PM2.5 levels at all sites for 2013, whereas for 2020, there were just small decreases in ambient PM2.5 levels at most sites and an increase at one site, and for 2024 there were increases in
PM2.5 at all sites, i.e., a disbenefit. For further detail, please see the EPAs February 2020 Precursor TSD, Table 4, and the 2018 PM2.5 Plan, Appendix G, tables 10 through 15.
CARB then considered additional information to assess whether these PM2.5 responses constituted a significant contribution to ambient PM2.5 in the San Joaquin Valley, including emissions trends and an assessment of the modeled disbenefit of VOC emissions reductions. Regarding emissions trends, CARB found that VOC emissions would decrease approximately 30 tpd or 9
percent from 2013 to 2024, with approximately 28 out of the 30 tpd reduction taking place by 2020.134 The State concludes that the formation of ambient PM2.5 from VOC may therefore differ in base and future years and that the sensitivity analysis for 2013 is not representative of current or future conditions.
CARB explained the modeled disbenefit of VOC reductions as follows:
Emissions of VOC and NOX react in the atmosphere to form organic nitrate species, such as peroxyacetyl nitrate PAN, meaning that some portion of the NOX emissions is not available to react with ammonia to form ammonium nitrate. In other words, VOC emissions are a sink for NOX emissions.
Reducing VOC emissions therefore reduces the formation of organic nitrates, so the sink is smaller and nitrate molecules are freed to react with ammonia to form particulate ammonium nitrate.135 The State further explored the VOC disbenefit based on a 2016 CARB
modeling assessment provided in Appendix A Air Quality Modeling of the 2016 Moderate Area Plan for the 2012 PM2.5 Standard for the San Joaquin Valley 2016 PM2.5 Plan, which CARB submitted to the EPA as a SIP revision on May 10, 2019.136
Based on its sensitivity-based analysis of VOC emissions reductions, VOC
emissions trends, and the scientific understanding of VOC chemistry in the San Joaquin Valley, CARB concludes that VOC emissions do not contribute significantly to PM2.5 levels that exceed the 1997 24-hour PM2.5 NAAQS in the San Joaquin Valley.
134 2018

PM2.5 Plan, Appendix G, 19 and Figure
5.
131 CARBs
June 2019 Precursor Clarification.
PM2.5 Plan, Appendix G, Table 10.
133 We note that one site Visalia has a modeled response above the EPAs final recommended contribution threshold of 1.5 mg/m3 and one additional site Bakersfield-California Avenue has a modeled response below the 1.5 mg/m3 threshold but above the EPAs draft threshold of 1.3 mg/m3.
132 2018

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135 2018 PM
2.5 Plan, Appendix K, 72 citing Meng, Z., D. Dabdub, D., Seinfeld, J.H., Chemical Coupling Between Atmospheric Ozone and Particulate Matter, Science 277, 116 1997. DOI: 10.1126/
science.277.5322.116.
136 2016 PM
2.5 Plan, Appendix A, A57. See also 2018 PM2.5 Plan, Appendix K, section 5.6 PM2.5
Precursor Sensitivity Analysis, 7172.

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