14 which represents the maximum contribution that the source’s emissions could make in a

15 cumulative impact assessment.

16 For Case 4, where the source’s precursor emissions exceed the respective SER but direct

17 PM2.5 emissions do not, the SIL comparison would only address secondary PM2.5 ambient

18 impacts associated with the proposed source. The assessment of the precursor emission impacts

19 on secondary PM2.5 formation may be: a) qualitative in nature; b) based on a hybrid of

20 qualitative and quantitative assessments utilizing existing technical work; or c) a full quantitative

21 photochemical grid modeling exercise. As discussed above for Case 3, since the applicable SIL

22 represents a specific insignificant (or de minimis) ambient concentration of PM2.5 that exempts

23 the applicant from conducting a cumulative impact assessment, basing the significant impact

1 ambient impacts or a hybrid of

2 qualitative and quantitative assessments, utilizing existing technical work for Case 4 may be

3 difficult to justify in most cases unless a demonstrably conservative estimate of the secondary

4 PM2.5 contribution can be made that is below the applicable SIL. As such, when using either of

5 these approaches, it may be appropriate for the permitting authority to recommend the applicant

6 to forego the SIL assessment and focus on the NAAQS compliance demonstration using a

7 cumulative impact analysis. However, it may be more feasible for the permitting authority to

8 allow the applicant to apply the applicable SIL to full photochemical grid model estimates of

9 secondary PM2.5 for Case 4 than for Case 3 since the issues associated with combining modeled

10 estimates of primary and secondary PM2.5 would not apply for Case 4. In these cases the highest

11 of the multi-year averages of the maximum modeled 24-hour or annual PM2.5 concentrations


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