Due to the potentially important contribution from secondary 1 formation of PM2.5, and the

2 more prominent role of monitored background concentrations of PM2.5 to account for

3 background levels of PM2.5 in the cumulative analysis, certain aspects of standard modeling

4 practices used for PM10 and other criteria pollutants may not be appropriate for PM2.5. Given

5 these issues, and especially the important contribution from secondary formation of PM2.5, which

6 is not explicitly accounted for by the current preferred dispersion model (i.e., AERMOD) used to

7 simulate dispersion of direct PM2.5 emissions, PSD modeling of secondarily formed PM2.5 should

8 currently be viewed as screening-level analyses under Appendix W, analogous to the screening

9 nature of the guidance in Section 5.2.4 of Appendix W regarding dispersion modeling for

10 nitrogen dioxide (NO2) impacts given the importance of chemistry in the conversion of nitric

11 oxide (NO) emissions to ambient NO2 and lack of a specified “refined” model.5 The

12 recommendations presented in this guidance for demonstrating compliance with the PM2.5

13 NAAQS through dispersion modeling and other techniques have been developed with the factors

14 listed above in mind.

15 As with any modeling analysis conducted under Appendix W, alternative models and

16 methods may be considered on a case-by-case basis, subject to approval by the EPA Regional

17 Office in accordance with the recommendations in Section 3.2. Additionally, Section 10.2.2 of

18 Appendix W could potentially be given consideration in select situations. The provisions of

19 Section 10.2.2 recognize that there are circumstances where there is no applicable model for a

20 particular compliance demonstration and that data from an array of ambient monitors

21 surrounding the permitted facility could be used in lieu of modeling if appropriately justified.


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