23 the chemical environment is not conducive to secondary formation). As such, when using either

Public Review Draft 03/04/2013


of these approaches, it may be appropriate to forego the SIL 1 assessment and focus on the

2 NAAQS compliance demonstration using a cumulative impact analysis.

3 For cases where a full quantitative photochemical grid modeling assessment of secondary

4 PM2.5 is conducted, the SIL comparison for Case 3 should be based on the combined ambient

5 impacts of primary and secondary PM2.5. However, the primary and secondary PM2.5 impacts

6 may be combined in various ways which may entail greater or lesser degrees of conservatism.

7 For example, combining the peak estimated primary PM2.5 impact with the peak estimated

8 secondary PM2.5 impact, unpaired in time and space would likely result in a conservative

9 estimate of combined impacts since, as noted above, peak impacts associated with a source’s

10 direct PM2.5 and precursor emissions are not likely well-correlated in time or space. On the other

11 hand, the conservatism associated with combining peak estimated primary and secondary

12 impacts for comparison to the applicable SIL would likely make such an approach easier to

13 justify than other approaches for combining estimated primary and secondary PM2.5 impacts.


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