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Federal Register / Vol. 86, No. 114 / Wednesday, June 16, 2021 / Rules and Regulations
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its permitting exemption thresholds for minor sources in nonattainment areas and minor sources of PM2.5 in attainment areas under CAA section 110l and Appendix V to 40 CFR part 51.
First, we note that with the exception of the thresholds for PM2.5 sources, in our 2015 NSR action, the EPA
previously approved the ADEQs permitting thresholds for minor NSR as they apply in attainment areas, and, accordingly, those thresholds were not changed as part of the 2020 Minor NSR
submittal. The EPAs prior approval was based on the ADEQs demonstration that the emissions from the sources and projects to be exempted from its minor NSR program under these thresholds were inconsequential to attainment or maintenance of the NAAQS.21 However, in our 2015 NSR action, we also determined that the ADEQ had not provided a rationale for the PM2.5
permitting exemption threshold, nor had it provided an adequate rationale for why the permitting exemption thresholds were appropriate for nonattainment areas. 22 In this action, we are considering only the 2020 Minor NSR submittal and the ADEQs rationale for its permitting exemption thresholds as they apply to minor sources in nonattainment areas, and to minor sources of PM2.5 in attainment areas.
The commenters specifically take issue with the ADEQs comparing the percentage of emissions regulated by its NSR program to the percentage of emissions regulated by other NSR
programs, and assert that the ADEQs approach should focus more on future sources of emissions and ensuring that such sources do not jeopardize the NAAQS. As described below, the ADEQs approach did not rest solely on comparing its permitting thresholds to other programs, and we find that the approach ensures that the ADEQs minor NSR program reviews the necessary sources to ensure attainment and maintenance of the NAAQS.
Prior to 2012, the ADEQs minor NSR
program required permitting of nonmajor sources with potential emissions of a criteria pollutant at or above the SERs from the PSD program reflected in 40 CFR 51.166b23i. To address concerns raised by the EPA regarding 21 In reviewing the ADEQs minor NSR program under 40 CFR 51.160e, we considered it appropriate for the ADEQ to exclude emissions from its NSR program if such emissions would be inconsequential to attainment or maintenance of the NAAQS. 80 FR 67319, 67325. This was the same standard that the EPA used in developing the permitting thresholds for its minor NSR program for Indian country. 76 FR 38748, 38758 Jul. 1, 2011.
22 Id.

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these historic permitting thresholds, the ADEQ assessed other potential lower permitting thresholds for its minor NSR
program and ultimately selected revised, lower thresholds. In 2012, the ADEQ chose to use a method similar to the method that the EPA used to develop permitting thresholds under its minor NSR program applicable in Indian country, known as the Tribal Minor NSR rule. 23 To inform its selection of minor NSR permitting thresholds in developing the Tribal Minor NSR rule, the EPA conducted a source distribution analysis using data from the National Emissions Inventory.
The EPAs analysis concluded that the percentage of emissions that would be exempt from minor NSR under the Tribal Minor NSR rules thresholds would be small less than 1.5% of total emissions for each pollutant, while the programs permitting thresholds would require only 1458% of stationary sources varying based on the individual pollutant to obtain permits or register under the Tribal Minor NSR rule. The EPAs analysis determined that this approach provided evidence that sources with emissions below the proposed minor NSR thresholds will be inconsequential to attainment and maintenance of the NAAQS. 24 We stated that the permitting thresholds for the minor NSR program applicable in Indian country are not intended to establish a new set of minimum criteria that a Tribe or a state would need to follow in developing its own minor source permitting program. 25
Nevertheless, the approach taken by the EPA in developing the thresholds in the Tribal Minor NSR rule represents one approach that EPA has found to be appropriate in establishing such thresholds.
To assess potential thresholds for its minor NSR program, the ADEQ applied a similar approach to a local data set.
During the stakeholder process, the ADEQ proposed two alternative scenarios for its revised minor NSR
thresholds: One that generally used one half of the PSD SERs Scenario 1 and one that generally used one quarter of the PSD SERs Scenario 2. The ADEQs analysis looked at the percentage of emissions that would be regulated at the two thresholds and concluded that both scenarios result in a relatively large percentage of emissions being subject to regulation compared to the percentage of sources brought into the program. The results of the analysis showed that using Scenario 2 for the 23 76

FR 38748 July 1, 2011.
FR 48695, 4870148703 Aug. 21, 2006.
25 76 FR 38748, 38754.
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minor NSR emission thresholds rather than Scenario 1 would result in significantly more coverage of carbon monoxide CO and SO2 emissions under the ADEQs minor NSR program.
However, the ADEQ reasoned that stationary source emissions of CO are generally dwarfed by mobile source emissions and do not contribute significantly to nonattainment of the CO
NAAQS. Also, the ADEQ reasoned that in the areas within Arizona that are subject to its minor NSR program, the sources that could contribute to noncompliance with the SO2 NAAQS
are well-defined and consist of large industrial sources already subject to the permitting program. The ADEQ
concluded, based on the above considerations, that for purposes of minor NSR, use of the Scenario 2
thresholds would not offer any substantial benefits over Scenario 1, and set numerical exemption thresholds for the pollutants in its minor NSR program that equate to one half of the PSD
SERs.26
In response to the EPAs determination in our 2015 NSR action that the ADEQ needed to justify the chosen permitting thresholds for PM2.5
and to further justify the thresholds as they apply in nonattainment areas, in its 2020 Minor NSR submittal, the ADEQ
continued to build on its prior analyses supporting the current permitting thresholds in its minor NSR program.27
First, the ADEQ updated its prior source distribution analysis to use the National Emissions Inventory, the same data set that the EPA used for its analysis for the Tribal Minor NSR program, and to include PM2.5 emissions. The analysis shows that the ADEQs NSR program is expected to cover approximately 98% of PM2.5 emissions in counties where the ADEQ has minor source permitting jurisdiction and approximately 96% of PM2.5 emissions in PM2.5 nonattainment areas where the ADEQ has minor source permitting jurisdiction. Further, the 26 See Appendix A of the ADEQs 2012 NSR SIP
submittal at 15471549 for a detailed discussion of the ADEQs approach and analysis. See also, the Technical Support Document for the EPAs Notice of Proposed Rulemaking, Revision to the Arizona State Implementation Plan for the Arizona Department of Environmental Quality, March 2015
EPAs 2015 TSD at 2225. The ADEQs permitting exemption thresholds are found at R182101101. The thresholds are ton per year values set for various pollutants that determine when a permit or registration is required for new sources and when minor NSR review is triggered for modifications. If potential source-wide emissions from all regulated pollutants are below the permitting exemption thresholds, then the source is exempt from the ADEQs permitting and registration program.
27 See 2020 Minor NSR submittal at 1420 for the full discussion.

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