23 up to 5 complete years) of site-specific meteorological data, using the same procedures used for

Public Review Draft 03/04/2013

50

the initial significant impact analysis. The memo cited several issues, 1 especially the importance

2 of the contribution from secondary formation of PM2.5 from precursor emissions and the fact that

3 such contributions are not explicitly accounted for by the dispersion model, as the basis for

4 viewing modeling of PM2.5 as screening-level analyses, analogous to the screening nature of the

5 guidance in Section 5.2.4 of Appendix W regarding dispersion modeling for NO2 impacts, given

6 the importance of chemistry in the conversion of NO emissions to ambient NO2.

7 Recognizing that the primary focus and motivation for this guidance document is to

8 provide recommendations on appropriate tools and methodologies to account for the potential

9 contribution from a new or modifying source’s precursor emissions on ambient PM2.5 levels, it is

10 appropriate to reassess the March 23, 2010, guidance under this broadened paradigm. Since each

11 of the four cases outlined above, based on comparisons of the project’s direct PM2.5 and

12 precursor emissions with their respective SER, involves some assessment of the source’s

13 potential secondary PM2.5 impacts, we recommend as a new First Tier that the modeled design

14 value (based on the multi-year average of the 98th-percentile of 24-hour values) be added to the

15 monitored design value from a representative monitor. For Case 2, where only the project’s

16 direct PM2.5 emissions exceed the SER, the modeled design value would be based solely on

17 AERMOD (or other acceptable preferred or alternative model) estimates of primary PM2.5

18 impacts. For Case 3, where both the project’s direct PM2.5 emissions and precursor emissions

19 exceed the respective SER, the cumulative impact for comparison to the NAAQS should be

20 based on the sum of the modeled design value for primary PM2.5 impacts (from dispersion model

 

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