less than the applicable SIL, and the record demonstrates that no further 1 air quality assessment

2 would be needed to demonstrate that the source would not cause or contribute to a violation of

3 any NAAQS or increment. Otherwise, a cumulative impact assessment would be necessary,

4 which is discussed in Section IV.

5 To inform a hybrid qualitative/quantitative assessment, the existing air quality model6

based information would need to be appropriate in terms of representing the type of source, its

7 precursor emissions, and geographic location, in addition to those elements of the conceptual

8 description discussed above for the qualitative assessment. The quantitative modeling

9 information may be available from past or current SIP attainment demonstration modeling,

10 published modeling studies, or peer-review literature with estimates of model responsiveness to

11 precursor emissions in contexts that are relevant to the new or modifying source. The estimates

12 of model responsiveness, such as impact on PM2.5 concentrations per ton of SO2 emissions,

13 would then be used in conjunction with the precursor emissions estimates for the proposed new

14 or modifying source to provide a quantitative estimate of the impact of such precursor emissions

15 on the formation of secondary PM2.5 concentrations. The estimates should be technically

16 credible in representing such impacts and it may be advisable for the estimate to reflect an upper

17 bound of potential impacts.

18 The NACAA Workgroup final report (NACAA, 2011) provides details on potential

19 approaches to quantify the secondary PM2.5 impacts from a proposed new or modifying source

20 that may be appropriate to inform a hybrid qualitative/quantitative assessments of PM2.5 impacts

21 (See Appendix C and D of NACAA, 2011). One suggested method in the final report is to

22 convert emissions of precursors into equivalent amounts of direct PM2.5 emissions using



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