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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules
tkelley on DSKBCP9HB2PROD with PROPOSALS

is agricultural emissions. Within the human activity-based emissions inventory, the largest source is unpaved roads, followed by construction fugitive emissions.
3. The EPAs Review of the States Submission We have reviewed the 2008 base year emissions inventory for direct PM10 in the West Pinal County PM10 Plan and emissions inventory estimation methodologies used by ADEQ for consistency with CAA requirements and EPA guidance. We address the States analysis for PM10 precursors in Section III.B.
First, we find that although the 2008
base year inventory reports annual PM10
emissions estimates, the Plan also provides and uses daily emissions estimates within the attainment demonstration modeling and the related modeling domain micro-emissions inventories; therefore, the Plan is consistent with the requirement that ADEQ must use an emissions inventory in a form consistent with the 24-hour PM10 standard.25 ADEQ has provided adequate documentation explaining how it calculated the 2008 base year emissions estimates, both as annual and daily inventories.26
Second, we find that the 2008 base year emissions inventory in the West Pinal County PM10 Plan used emissions models, emission factors, and methodologies for estimating PM10
emissions that were accurate and appropriate to the time that the Plan was written. Also, the 2008 base year inventory for direct PM10 is comprehensive in scope and coverage.
Therefore, the submitted emissions inventory represents a comprehensive, accurate, and current inventory of actual emissions of direct PM10 during that year in the West Pinal County Area.
Third, we find that ADEQs selection of 2008 for the base year emissions inventory is appropriate because it is chosen from one of the three years, 20062008, in which the area was designated nonattainment. The 2008
emissions inventory is representative of the sources of direct PM10 pollution contributing to exceedances of the PM10
NAAQS that caused the area to be designated nonattainment.
Consequently, the EPA is proposing to approve the 2008 base year emissions inventory for direct PM10 in the West 25 As discussed in Section III.G Motor Vehicle Emissions Budgets and Transportation Conformity, an annual emissions inventory introduces difficulties with determining and presenting a motor vehicle emissions budget.
26 Chapter 5 and Appendix B of the West Pinal County PM10 Plan.

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Pinal County PM10 Plan as meeting the requirements for a base year inventory set forth in CAA section 172c3.
B. PM10 Precursors 1. Statutory and Regulatory Requirements Section 189e of the Act requires that the control requirements for major stationary sources of direct PM10 also apply to major stationary sources of PM10 precursors, except where the Administrator determines that such sources do not contribute significantly to PM10 levels that exceed the standard in the area. While CAA section 189e expressly requires control of precursors from major stationary sources, subpart 4
and other CAA provisions collectively require the control of direct PM10 and PM10 precursors from all types of sources i.e., stationary sources, area sources and mobile sources as may be needed for the purposes of demonstrating attainment as expeditiously as practicable in a given nonattainment area.27
The provisions of subpart 4 of part D, title I of the CAA do not define the term precursor for purposes of PM10, nor do they explicitly require the control of any specific PM precursor. The statutory definition of air pollutant in CAA
section 302g, however, provides that the term includes any precursors to the formation of any air pollutant, to the extent the Administrator has identified such precursor or precursors for the particular purpose for which the term air pollutant is used. EPA has identified sulfur dioxide SO2, oxides of nitrogen NOX, volatile organic compounds VOC, and ammonia NH3
as precursors to the formation of PM.28
Accordingly, a state must include emissions of direct PM emissions and these four precursors in emissions inventories and must control emissions from sources of all of these pollutants, unless the state demonstrates to EPAs satisfaction that control of one or more of these pollutants is not needed for expeditions attainment of the NAAQS
in the nonattainment area at issue.
2. Summary of the States Submission Appendix B, Exhibit BXVI contains ADEQs demonstration that emissions of SO2, NOX, and NH3 from existing sources in the West Pinal County nonattainment area do not contribute significantly to PM10 levels that exceed the NAAQS. For this analysis, ADEQ
estimated the impact of these three PM10
27 See CAA requirements for states to demonstrate attainment as expeditiously as practicable, CAA
section 188c1 and section 172a1.
28 See 81 FR 58010, 58018.

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precursors on PM10 concentrations at two sites, Cowtown CWT and Pinal County Housing PCH, using worst impact day monitored data from a year-long chemical mass balance characterization CMBC study Desert Southwest Coarse Particulate Matter Study and emissions data from the 2008 National Emission Inventory NEI.29 ADEQ evaluated these data to determine which, if any, source categories had precursor emissions that contribute more than 5 micrograms per cubic meter mg/m3 on specific design days. 30
Appendix B, Exhibit BXVI, Table BXVI1 provides the maximum particle mass concentration and chemical composition i.e., crustal, organic material, nitrate, sulfate, ammonium, other species, and unidentified particle fractions measured during the CMBC
study for CWT and PCH. ADEQ then calculated the percentage of each chemical constituent to the summed total of the chemical constituent parts.
ADEQ assumed the design days for each monitor had the same relative chemical composition as the worst impact day identified in the CMBC study. The State calculated design day concentrations for each chemical constituent by multiplying the study-derived percentages by a design day concentration for CWT 244.5 mg/m3
and PCH 178.0 mg/m3. The CMBC
study estimated that summed nitrate, sulfate, and ammonium impacts on the CWT and PCH monitors were 3.4
percent and 4.0 percent, respectively.
These percentages suggest that 8.4 mg/
m3 and 7.2 mg/m3 of the design day ambient PM10 concentrations at the CWT and PCH monitors resulted from emissions of the three PM10 precursors examined.
Next, ADEQ processed Pinal County 2008 EPA NEI reported emissions for NOX, SO2, and NH3 to determine the percent contribution of each source sector to the total emissions of these pollutants for the county. 31 ADEQ
apportioned the precursor concentrations derived above to individual source sectors based on the relative contribution of each sector to the annual emission inventory. Based 29 Clements, A.L., Fraser, M.P., Upadhyay, N., Herckes, P., Sundblom, M., Lantz, J., and Solomon, P.A., Chemical characterization of coarse particulate matter in the Desert SouthwestPinal County Arizona, USA, Atmospheric Pollution Research, 5 2014 5261.
30 ADEQ focused their attainment demonstration on a set of design days and monitors that have experienced, or are conducive to, the highest concentrations. See EPA TSD, p. 11. Two design days were examined in ADEQs PM10 precursor demonstration.
31 Appendix B, Exhibit BXVI, Table BXVI2.

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