14 In the preamble to the 2008 final rule (73 FR 28321), the EPA included preferred or presumptive offset ratios,

applicable to specific PM2.5 precursors that state/local air agencies may adopt in conjunction with the new

interpollutant offset provisions for PM2.5, and for which the state could rely on the EPA's technical work to

demonstrate the adequacy of the ratios for use in any PM2.5 nonattainment area.

The EPA also notes that the NACAA Workgroup “1 considered, but rejected, other

2 methods for assessing secondary PM2.5 impacts, including use of a simple emissions divided by

3 distance (Q/D) metric and use of AERMOD with 100 percent conversion of SO2 and NOx

4 concentrations to (NH4)2SO4 and (NH4)NO3.” The EPA has reviewed the detailed discussion

5 provided in Appendix E of the NACAA Workgroup final report and agrees with these

6 conclusions.

III.2.3 Full Quantitative Photochemical Grid Modeling

9 In those rare cases where it is deemed necessary to estimate secondary PM2.5 impacts

10 with full quantitative photochemical grid modeling, the candidate model for use in estimating

11 single source impacts on secondarily formed PM2.5 should meet the general criteria for an

12 “alternative model” outlined in Section 3.2.2 of 40 CFR 51.112 and 40 CFR part 51, Appendix

13 W, for condition (3) where “the preferred model is less appropriate for the specific application,

14 or there is no preferred model,” i.e.,

15 i. The model has received a scientific peer review;

16 ii. The model can be demonstrated to be applicable to the problem on a theoretical

17 basis;

18 iii. The data bases that are necessary to perform the analysis are available and

19 adequate;

20 iv. Appropriate performance evaluations of the model have shown that the model is

21 not biased toward underestimates; and

22 v.


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