The other extreme for combining primary and secondary PM2.5 impacts for comparison to

15 the applicable SIL for Case 3, relative to combining peak primary and peak secondary impacts

16 unpaired in time and space, would be full temporal and spatial pairing of estimated primary and

17 secondary PM2.5 impacts. Such an approach may not be feasible in many cases, given that the

18 dispersion modeling and photochemical grid modeling may be based on different data periods.

19 Furthermore, full temporal and spatial pairing of primary and secondary PM2.5 impacts may not

20 be appropriate in many cases due to the fact that photochemical grid modeling represents gridded

21 concentration estimates whereas dispersion modeling produces estimates at discrete receptor

22 locations and given the limitations in the skill of both the dispersion model and the

23 photochemical grid model to accurately predict impacts on a paired in time and space basis. On

the other hand, some degree of temporal pairing of primary 1 and secondary PM2.5 impacts on a

2 seasonal or monthly basis should be appropriate in most cases, recognizing the general lack of

3 correlation between primary and secondary impacts.

4 The permitting authority and the applicant should thoroughly discuss the details

5 regarding combining modeled primary and secondary PM2.5 impacts for Case 3 and should reach

6 agreement on a protocol during the initial review of the modeling protocol. It may be

7 appropriate for the protocol to specifically identify multiple tiers for combining the modeled

8 primary and secondary PM2.5 impacts with the more conservative approaches being easier to

9 justify. The permitting authority should ensure that any approach for combining estimated

10 primary and secondary PM2.5 impacts for comparison to the applicable SIL for Case 3 conforms

11 to the recommendations described above for Case 2 regarding the form of the modeled estimate.

12 Accordingly, the approach should be based on the highest of the multi-year averages of the

13 maximum modeled 24-hour or annual PM2.5 concentrations predicted each year at each receptor,


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