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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules
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on this analysis, no precursor emissions from any source category exceeded the 5 mg/m3 threshold.32 The largest contributing source category was Onroad Mobile, contributing less than 3.5
mg/m3 of PM10 from precursor emissions to either monitor on the design days.
3. The EPAs Review of the States Submission We identified several issues with the analysis that ADEQ presented. First, SO2, NOX, VOC, and ammonia are precursors to the formation of PM10.
ADEQ does not address VOC emissions in its analysis; therefore, we cannot evaluate whether sources of VOC
emissions contribute significantly to PM10 levels that exceed the NAAQS in the West Pinal County PM10
nonattainment area.
Second, it is unclear whether the chemical composition values presented in Appendix B, Exhibit BXVI, Table BXVI1 reflect one maximum day sample, or the maximum chemical composition measured for each individual component during the entire CMBC study. If the latter approach was used, then the resulting percentages would not reflect percentages measured on any actual exceedance day and could overrepresent or underrepresent the various chemicals when compared to actual exceedance days. If the chemical composition values represent one maximum day sample, then: a The individual components listed in Table BXVI1 when totaled together should equal that days total mass, which they do not; and b this would only represent a single daytherefore, a single type of exceedance day. ADEQ
modeled two meteorological scenarios causing exceedances, high wind conditions and stagnant or low wind conditions. The emission sources affecting the PM10 composition would vary between these two scenarios, making use of a single maximum value at each site for each chemical component likely insufficient.
Therefore, ADEQs approach of assuming the chemical composition of the two design days match those reported in the study likely does not address all conditions affecting nonattainment for the area.
Third, ADEQ applied the 5 mg/m3
threshold from the Serious PM10
nonattainment area addendum to the General Preamble.33 The Serious area addendum states that, for purposes of evaluating best available control measures BACM, a source category 32 Appendix 33 Appendix
B, Exhibit BXVI, Table BXVI3.
B, Exhibit BXVI, BXVI2. See 59 FR

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will be presumed to contribute significantly to a violation of the NAAQS if its PM10 impact at the location of the expected violation would exceed 5 mg/m3. This guidance is not precursor guidance and was intended to apply to the total impact of a source category including direct PM and precursor emissions. It is not clear from the States submission why the application of this threshold to the impact of precursor emissions from individual source categories is an appropriate method of evaluating the significance of PM10 precursor emissions for the West Pinal County PM10 nonattainment area.34
Finally, ADEQ used an annual inventory to partition the source category contribution to PM10. High wind affected days would likely have a different composition of sources than what would be reflected in an annual inventory, potentially by a substantial margin. ADEQ did not address this issue, and used the annual inventory composition to represent all exceedance days.
The State has not adequately shown that PM10 precursors do not contribute significantly to concentrations above the NAAQS in the West Pinal County PM10
nonattainment area. As described elsewhere in this notice, due to the deficiencies with the States precursor analysis, the State has not shown that it was unnecessary to regulate emissions of precursors in its RACM and modeled attainment demonstration. As explained in Section III.C., the State has only evaluated sources of direct PM
emissions within the West Pinal County PM10 Plan. The EPA anticipates that ADEQ could develop an improved precursor analysis for the area, and this analysis may ultimately confirm that it is not necessary to regulate one or all of the PM10 precursors; however, we find that the precursor analysis submitted with the Plan does not provide a sufficient basis for that conclusion.
In conclusion, because of the omissions and uncertainties in ADEQs PM10 precursor analysis, we are unable to determine whether precursor emissions contribute significantly to PM10 levels that exceed the NAAQS in the West Pinal County nonattainment area. Consequently, we are proposing to disapprove the precursor demonstration in the West Pinal County PM10 Plan because the demonstration is inadequate to show that emissions reductions from 34 For more recent guidance on precursor significance, see Memorandum from Scott Mathias, Acting Director, Air Quality Policy Division and Richard Wayland, Director, Air Quality Assessment Division, Fine Particulate Matter PM2.5 Precursor Demonstration Guidance May 30, 2019.

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all PM10 precursors do not contribute significantly to PM10 levels exceeding the NAAQS, as required by CAA
Section 189e. As explained in section III.C, the deficiencies in the States precursor analysis mean that the State failed to establish in its RACM/RACT
analysis that it was unnecessary to regulate PM10 precursor emissions.
C. Reasonably Available Control Measures Demonstration 1. Statutory and Regulatory Requirements CAA section 172c1 requires that each attainment plan provide for the implementation of all RACM/RACT as expeditiously as practicable including such reductions in emissions from sources in the area through implementation of reasonably available control technology and for attainment of the NAAQS. Consistent with section 189a1C, each state with a Moderate PM10 nonattainment area is required to submit provisions to assure implementation of reasonably available control measures no later than 4 years after the date of designation of the area.
Taken together, these CAA provisions require that Moderate area attainment plans must provide for the implementation of RACM and RACT in the nonattainment area as expeditiously as practicable but no later than 4 years after designation.
Section 189a1B of the CAA
requires states to demonstrate attainment of the PM10 standard by the applicable attainment date or demonstrate that attainment by such date is impracticable and Section 188c1 requires that the attainment date for a Moderate area shall be as expeditiously as practicable, but no later than the end of the sixth calendar year after the year of the nonattainment areas designation.
To address this requirement to adopt all RACM/RACT and meet the PM10
NAAQS as expeditiously as practicable, states should consider all potentially reasonable control measures for source categories in the nonattainment area to determine whether they are reasonably available for implementation in that area and whether they would, if implemented individually or collectively, advance the areas applicable attainment date by one year or more.35 Any measures that are necessary to meet these requirements that are not either federally promulgated, or part of the states SIP, must be submitted in enforceable form 35 44 FR 20372 April 4, 1979 and 57 FR 13498
April 16, 1992.

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