with the EPA’s commitments in the Gina McCarthy, January 4, 2012, administrative grant of the

16 July 28, 2012, Sierra Club petition.

17 Because of the complex chemistry of secondary formation of PM2.5, the EPA's judgment

18 in the past has been that it was not technically sound to assign with particularity specific models

19 that must be used to assess the impacts of a single source on PM2.5 concentrations. Instead, the

20 EPA has determined it was appropriate to satisfy the requirements of Section 165(e)(3)(D) of the

21 Clean Air Act (CAA) by recommending that the “[c]hoice of methods used to assess the [PM2.5]

22 impact of an individual source depends on the nature of the source and its emissions,” as stated

23 in Section of Appendix W. As such, the appropriate methods for assessing PM2.5

impacts are determined as part of the normal consultation process 1 with the appropriate permit

2 reviewing authority. A modeling protocol should be developed by the permit applicant and

3 approved by the appropriate permitting authority to ensure that the analysis conducted will

4 conform to the recommendations, requirements, and principles of Section 10.2.1 of Appendix W.

5 This guidance is intended to inform that process through recommendations regarding appropriate

6 methods to assess secondary PM2.5 impacts from the precursor emissions from the new or

7 modifying source by providing the permit applicant and the appropriate permit reviewing

8 authority with both focus and flexibility. As experience is gained with these NAAQS

9 compliance demonstrations, this guidance will likely evolve such that the EPA will be able to

10 provide further specificity on assessing the impacts of a single source on PM2.5 concentrations.


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