15 may also be applicable for increment analyses. For example, the increment assessment for Case

16 2, where only direct PM2.5 emissions exceed the SER, should be based on application of

17 AERMOD (or other appropriate preferred or approved alternative model), using the allowable

18 increase in emissions from the project source plus actual emission changes associated with any

19 increment-consuming or increment-expanding nearby sources. The AERMOD model allows for

20 inclusion of increment-consuming and increment-expanding sources (represented as negative

21 emissions) in the same model run, and will output the net cumulative concentrations (although

22 the “maximum” cumulative impacts will be output as zero if the cumulative impacts computed in

23 the model are less than zero).

As discussed above in relation to the NAAQS, for Cases 1 3 and 4, where PM2.5 precursor

2 emissions from the proposed source exceed the respective SERs, the assessment of potential

3 impacts on secondary PM2.5 due to those emissions may be: a) qualitative in nature; b) based on

4 a hybrid of qualitative and quantitative assessments utilizing existing technical work; or c) a full

5 source-specific quantitative photochemical modeling exercise. Aside from the use of actual

6 emission increases (or decreases) from increment-affecting sources instead of maximum

7 allowable emissions from all nearby sources, the guidance provided under Sections III and IV in

8 relation to NAAQS compliance for hybrid qualitative/quantitative and full source-specific

9 quantitative photochemical modeling is largely applicable for increment analyses.

10 Several existing rules, including the current PM2.5 NAAQS, have resulted in reductions in

11 precursor emissions in most areas in recent years. As a result, in many cases the potential

12 increment-consumption due to secondary PM2.5 impacts from background sources may easily be

13 addressed through a qualitative assessment, supported by trends in available precursor emissions

14 data and ambient PM2.5 monitored concentrations that net secondary PM2.5 impacts associated


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