21 estimates based on the project’s and other nearby source’s direct PM2.5 emissions), the modeled

22 design value for secondary PM2.5 impacts (from photochemical grid model estimates accounting

23 for the project’s precursor PM2.5 emissions), and the monitored design value (from a

Public Review Draft 03/04/2013


representative monitor accounting for the contribution of secondary 1 PM2.5 formation associated

2 with existing sources representative of the modeling domain, in addition to the background levels

3 of primary PM2.5 associated with background sources that are not included in the modeled

4 inventory). The resulting cumulative PM2.5 concentrations would then be compared to the

5 annual PM2.5 NAAQS of 12 μg/m3 and 24-hour PM2.5 NAAQS of 35 μg/m3.

6 The recommendations provided above constitute a First Tier modeling analysis for PM2.5

7 compliance demonstrations that should be acceptable without further justification in most cases.

8 For applications where impacts from primary PM2.5 emissions are not temporally correlated with

9 background PM2.5 levels, as discussed above, combining the modeled and monitored

10 contributions as described above may be overly conservative. In these cases, a Second Tier

11 modeling analysis may be considered that would involve combining the monitored and modeled

12 PM2.5 concentrations on a seasonal or quarterly basis. The use of a seasonally-varying monitored

13 background component is likely to be a more important factor for the 24-hour NAAQS analysis

14 than for the annual NAAQS.

15 The AERMOD model provides several options for specifying the monitored background

16 concentration for inclusion in the cumulative impact assessment. The options that are most

17 relevant to PM2.5 analyses include an option to specify a single annual background concentration

18 that is applied to each hour of the year (appropriate for the First Tier described above), and an


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