Voglio sapere a riguardo di…

Federal Register / Vol. 86, No. 183 / Friday, September 24, 2021 / Proposed Rules 3. The EPAs Review of the States Submission The EPA has evaluated the States precursor demonstration consistent with the PM2.5 SIP Requirements Rule and the recommendations in the PM2.5
Precursor Guidance. Based on this evaluation, the EPA agrees that NOX
emissions contribute significantly to ambient PM2.5 levels that exceed the 1997 24-hour PM2.5 NAAQS in the San Joaquin Valley and that NOX emissions sources, therefore, remain subject to control requirements under subparts 1
and 4 of the part D, title I of the Act.
For the reasons provided below, the EPA proposes to approve the States demonstration that ammonia, SOX, and 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.
Regarding the States analytical approach, the EPA finds that the State based its analyses on the latest available data and studies concerning ambient PM2.5 formation in the San Joaquin Valley from precursor emissions.
Regarding the required concentrationbased analysis, the EPA finds that the State assessed the absolute annual average contribution of each precursor in ambient PM2.5 i.e., in 2015. On the basis of the absolute concentrations being well above the EPAs recommended contribution thresholds for both the 24-hour and annual average NAAQS, the State proceeded with its sensitivity-based analysis, which is the recommended sequence under the final PM2.5 Precursor Guidance.137
With respect to the sensitivity-based analysis, we find that the State performed its analyses following the steps of the EPAs recommended approachi.e., for each modeled year and percent precursor emissions reduction, the State estimated the ambient PM2.5 response using the procedure recommended in the PM2.5
Precursor Guidance and compared the result to the recommended contribution threshold. The EPA also finds that the performance of the photochemical model was adequate for use in estimating the ambient PM2.5 responses, as discussed in section J Air Quality Model Performance of the EPAs Technical Support Document, EPA
Evaluation of Air Quality Modeling, San Joaquin Valley PM2.5 Plan for the 2006
PM2.5 NAAQS, February 2020 EPAs 137 For
further discussion of the EPAs evaluation of the States concentration-based analysis, see the EPAs February 2020 Precursor TSD, sections entitled Concentration-based analysis within the EPAs evaluation for each of ammonia, SOX, and VOC.

VerDate Sep<11>2014

17:10 Sep 23, 2021

Jkt 253001

February 2020 Modeling TSD. The State considered the EPAs recommended range of emissions reductions 30 percent to 70 percent for the 2013 base year, the projected 2020
attainment year for the 1997 24-hour PM2.5 NAAQS, and the projected 2024
attainment year for the 2006 PM2.5
NAAQS, and quantified the estimated response of ambient PM2.5
concentrations to precursor emissions changes for the first time in a PM2.5 SIP
submission for the San Joaquin Valley.
The EPA finds that such quantification and CARBs consideration of additional information provide an informed basis on which to make a determination as to whether ammonia, SOX, and VOC do or do not contribute significantly to ambient PM2.5 levels that exceed the 1997 24-hour PM2.5 NAAQS in the San Joaquin Valley.138 Therefore, we turn to our evaluation of the States determination for each of these three precursor pollutants.
a. Ammonia For ammonia, as detailed above, CARB estimated the ambient PM2.5
response to both a 30 percent and a 70
percent emissions reduction. We find that it was appropriate for the State to consider additional information to interpret those results to determine whether the ammonia contribution is significant. The primary conclusion demonstrated by the States analysis of additional information is that ammonium nitrate formation is NOXlimited. As discussed in more detail below, we agree with this conclusion.
We have evaluated CARBs determination that a projected future year is more representative of conditions in the San Joaquin Valley for sensitivity-based analyses and that 30
percent is a reasonable upper bound for ammonia emissions reductions to assess the precursor contribution, as discussed below.
The State provided ample information from scientific studies based on ambient measurements to help assess the estimated sensitivity of ambient PM2.5 to ammonia reductions. Conclusions based on ambient data are particularly relevant because they provide direct evidence of the chemical state of the atmosphere and are not dependent on modeled estimates of emissions or modeled ambient PM2.5 concentrations.
Measurements represent the real 138 The State did not evaluate the 2015 Serious area attainment year. Because the year has passed and the area failed to attain by the Serious area attainment date, we will evaluate the precursor analysis for the Serious area plan based on the current section 189d projected attainment date of December 31, 2020.

PO 00000

Frm 00015

Fmt 4701

Sfmt 4702

53163

world result of the pollutants differing geographic distributions, the various meteorological and chemical factors influencing their conversion to particulate, and their removal from the atmosphere by deposition and other processes. The observed abundance of ammonia relative to nitric acid, and the positive amount of chemically excess ammonia, both provide strong evidence that ammonia is not the limiting pollutant for particulate ammonium nitrate formation. They also support the States conclusion that PM2.5
concentrations are insensitive to ammonia emissions reductions.
The relative amount of ammonia and NOX emissions is one of the most critical factors in determining the sensitivity of ambient PM2.5 to ammonia reductions. We note that the model response to precursor reductions may be unrealistically large due to the underestimation of ammonia emissions and therefore of the ratio of ammonia to NOX emissions. There is evidence that ammonia emissions may be underestimated based on direct measurements of ammonia emissions flux during two measurement campaigns, as discussed in the EPAs February 2020 Precursor TSD. If ammonia emissions were higher in the modeling, then ammonia would be more abundant relative to nitrate and particulate nitrate formation would be more NOX-limited and less sensitive to ammonia reductions. This would make the model response more consistent with the ambient measurement studies, which suggest a very low sensitivity to ammonia. This evidence indicates that ammonia contribution to PM2.5 levels above the standard is likely to be less than estimated by the States modeling in each of the three years. In comparison to the 2013 and 2020 modeling, the modeling for the year 2024 incorporates lower NOX emissions and so has a larger abundance of ammonia relative to nitrate, more similar to the studies ambient measurements. Thus, the 2024
response to ammonia reductions is likely to be more reliable than the 2013
and 2020 responses and appears to be more representative of current atmospheric conditions despite the use of emissions projections for a future year.
The relative sizes of the ammonia and NOX precursor emissions inventories after accounting for their differing molecular weights are a rough indicator of which pollutant is the limiting pollutant for production of ammonium nitrate because ammonium nitrate forms from a one-to-one ratio of molecules derived from each precursor i.e., one ammonium nitrate forms from one
E:FRFM24SEP2.SGM

24SEP2