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Federal Register / Vol. 86, No. 183 / Friday, September 24, 2021 / Proposed Rules
as described in section IV.C of this preamble.
For ammonia, SOX, and VOC, CARB
assessed the 2015 annual average concentration of each precursor in ambient PM2.5 at Bakersfield, for which the necessary speciated PM2.5 data are available and where the highest PM2.5
design values have been recorded in most years, and compared those concentrations to the recommended annual average contribution threshold of 0.2 mg/m3 from the Draft PM2.5
Precursor Guidance, which was available at the time the State developed the SIP.108 The contributions of ammonia, SOX, and VOC were 5.2 mg/
m3, 1.6 mg/m3, and 6.2 mg/m3, respectively. Given that these levels are well above the EPAs 0.2 mg/m3
recommended contribution threshold, the State proceeded with a sensitivitybased analysis.
CARBs sensitivity-based analysis used the same Community Multiscale Air Quality CMAQ modeling platform as that used for the Plans attainment demonstration. The State modeled the sensitivity of ambient PM2.5
concentration in the San Joaquin Valley to 30 percent and 70 percent emissions reductions in 2013, 2020, and 2024 for each of ammonia, SOX, and VOC. The State estimated baseline 2013, 2020, and 2024 design values for PM2.5 using relative response factors RRFs and calculated the ammonia, SOX, and VOC
precursor contribution for a given year and for each sensitivity scenario 30
percent and 70 percent emissions reductions as the difference between its baseline design value and the design value for each sensitivity scenario.109
We summarize the States sensitivitybased analysis and additional information in the sections that follow for ammonia, SOX, and VOC.
a. Ammonia For ammonia, the State compared the 24-hour precursor contributions to 1.3
mg/m3, the recommended contribution threshold in the Draft PM2.5 Precursor 108 2018 PM
2.5 Plan, Appendix G, 3. The Plan does not present a concentration-based analysis for the 24-hour average concentrations in the San Joaquin Valley. Instead, CARB relied on the annual average concentration-based analysis as an interim step to the sensitivity-based analysis, for which CARB assessed the sensitivity of both 24-hour average and annual average ambient PM2.5
concentrations to precursor emissions reductions.
Separately, the Plan presents a graphical representation of annual average ambient PM2.5
components i.e., crustal particulate matter, elemental carbon, organic matter, ammonium sulfate, and ammonium nitrate for 20112013 for Bakersfield, Fresno, and Modesto. 2018 PM2.5 Plan, Chapter 3, 33 to 34.
109 This procedure is the procedure recommended by the EPA. PM2.5 Precursor Guidance, 37.

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Guidance. For a modeled 30 percent ammonia emissions reduction, the ambient PM2.5 responses in 2013 ranged from 0.9 to 3.3 mg/m3 across 15
monitoring sites, with a majority of sites above the 1.3 mg/m3 contribution threshold and also above the 1.5 mg/m3
contribution threshold in the final PM2.5
Precursor Guidance. PM2.5 responses in 2020 ranged from 0.5 to 1.9 mg/m3, with four sites at or above the 1.3 mg/m3
contribution threshold, including one site above the 1.5 mg/m3 contribution threshold in the final PM2.5 Precursor Guidance. In 2024, all modeled responses were below both recommended contribution thresholds.
For a modeled 70 percent ammonia emissions reduction, the ambient PM2.5
responses in 2013 ranged from 3.5 to 12.4 mg/m3, with all monitoring sites above the 1.3 mg/m3 threshold and above the 1.5 mg/m3 threshold, the PM2.5 responses in 2020 ranged from 1.6
to 6.4 mg/m3, and the PM2.5 responses in 2024 ranged from 1.2 to 3.0 mg/m3, with most sites above both recommended thresholds. For further detail, please see the EPAs February 2020 Precursor TSD, Table 2, and the 2018 PM2.5 Plan, Appendix G, tables 2 through 7. In summary, for a 30 percent ammonia reduction, a majority of sites have PM2.5
responses above the contribution threshold in the 2013 modeling, decreasing to a single site above the contribution threshold for 2020, and no sites above the contribution threshold for 2024. For a 70 percent reduction, all sites are above the contribution threshold in the 2013 and 2020
modeling, and a majority of sites are above the contribution threshold in 2024.
The State based its ammonia precursor determination on the sensitivity analysis for the future years, using a 30 percent ammonia emissions reduction. These choices respectively reflect its assessment of research studies and the Plans projected emissions reductions, and on its assessment of available emissions controls. As explained in the PM2.5 Precursor Guidance, precursor responses may be above the recommended contribution threshold and yet not contribute significantly to levels that exceed the standard in the area. Therefore, as recommended by the EPA, the State considered additional information to examine whether the identified PM2.5
responses constituted a significant contribution to ambient PM2.5 in the San Joaquin Valley. The additional information included research studies, emissions trends, and information to support the States conclusion that a 30

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percent ammonia emissions reduction represented a reasonable upper bound on the ammonia emissions reductions to model in estimating its contribution to ambient PM2.5 levels. We summarize this additional information below and provide a more detailed evaluation in the EPAs February 2020 Precursor TSD.
The State describes previous research that supports its finding that ammonium nitrate PM2.5 formation is the San Joaquin Valley is NOX-limited rather than ammonia-limited.110 Essentially, ammonia is so abundant in the San Joaquin Valley that even with large ammonia emissions reductions there would still be enough ammonia to combine with the available NOX to readily form particulate ammonium nitrate. Therefore, ammonia emissions reductions would lead to only small decreases in PM2.5 concentrations. In contrast, because emissions of NOX are less abundant in the San Joaquin Valley i.e., more limited relative to emissions of ammonia after normalizing for their differing molecular weights, the PM2.5
concentrations in the atmosphere are more responsive to reductions in NOX
than to reductions of ammonia. Thus, these analyses indicate that the area is NOX-limited.
The State also points to the conclusions of a study conducted by Lurmann et al., based on ambient measurements during the winter 2000
2001 California Regional Particulate Air Quality Study intensive field study.111
That study found that most areas of the San Joaquin Valley were NOX-limited with respect to ammonium nitrate formation. Since that time, large additional NOX emissions reductions have occurred, which would increase the degree to which ammonium nitrate formation in the San Joaquin Valley is NOX-limited. Based on more recent aircraft-borne measurements during the 2013 DISCOVERAQ campaign,112 the State similarly concluded that ammonium nitrate formation is NOXlimited based on the large amount of excess ammonia, which is defined as the amount of measured ammonia left over if all the nitrate and sulfate present 110 2018 PM
2.5 Plan, Appendix G, 910; CARB
Staff Report, Appendix C, 1215; and Attachment A to CARBs May 9, 2019 submittal letter.
111 Frederick W. Lurmann, Steven G. Brown, Michael C. McCarthy, and Paul T. Roberts, Processes Influencing Secondary Aerosol Formation in the San Joaquin Valley during Winter, Journal of the Air & Waste Management Association, 2006, 56:12, 16791693, DOI:
10.1080/10473289.2006.10464573.
112 Deriving Information on Surface conditions from COlumn and VERtically Resolved Observations Relevant to Air Quality, https
www.nasa.gov/mission_pages/discover-aq/
index.html.

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