These metrics represent the maximum contribution that project e 1 missions could make to the air

2 quality impact at any receptor, given the form of the NAAQS, and therefore provide an

3 appropriate part of the basis for determining whether a cumulative modeling analysis would be

4 needed.

5 For Case 3, where the source’s direct PM2.5 emissions and emissions of at least one

6 precursor exceed their respective SERs, the applicable SIL comparison would have to address

7 both primary and secondary PM2.5 ambient impacts associated with the proposed source. As with

8 Case 2, the ambient impacts due to direct PM2.5 emissions would be estimated using the preferred

9 AERMOD dispersion model (or acceptable alternative model). However, the comparison to the

10 applicable SIL will depend on the type of assessment conducted for the secondary PM2.5 impacts

11 from the source. As noted above, the assessment of the precursor emission impacts on secondary

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

13 quantitative assessments utilizing existing technical work; or c) a full quantitative photochemical

14 grid modeling exercise.

15 Since the applicable SIL represents a specific insignificant (or de minimis) ambient

16 concentration of PM2.5 that may be used to exempt the applicant from conducting a cumulative

17 impact assessment, basing the initial significant impact analysis for Case 3 on a qualitative

18 assessment (or a hybrid of qualitative and quantitative assessments) of secondary PM2.5 ambient

19 impacts may be difficult to justify in most cases. This is because there would be no specific

20 quantitative estimate of total PM2.5 impacts for comparison to the applicable SIL, unless a valid

21 argument can be made that secondary PM2.5 impacts associated with the source’s precursor

22 emissions will be very small (e.g., precursor emissions barely exceed the respective SER and/or

23 the


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