meteorological characteristics of the region during periods of precursor 1 pollutant emissions; the

2 level of conservatism associated with the modeling of the primary PM2.5 component and other

3 elements of conservatism built into the overall NAAQS compliance demonstration; aspects of

4 the precursor pollutant emissions in the context of limitations of other chemical species

5 necessary for the photochemical reactions to form secondary PM2.5; and an additional level of

6 NAAQS protection through a post-construction monitoring requirement. While each of the

7 components of the EPA Region 10 example may or may not be necessary, this example should

8 provide a useful template for other qualitative assessments under this guidance, recognizing that

9 additional components may be essential in other qualitative assessments of the potential for

10 secondary PM2.5 formation.

11

III.2.2 Hybrid Qualitative/Quantitative Assessment

13 The qualitative assessment discussed above is largely focused on a determination that the

14 proposed new or modifying source precursor emissions, in combination with the estimated

15 primary PM2.5 impacts (if applicable for Case 3), will not cause or contribute to a violation of the

16 24-hour and/or annual PM2.5 NAAQS. However, it may not always be possible to provide such a

17 justification without some quantification of the potential secondary PM2.5 impacts from the

18 proposed new or modifying source’s precursor emissions. In such cases, the EPA expects that

19 existing air quality model-based information regarding the potential for SO2 and NOx precursor

20 emissions to form secondary PM2.5 concentrations may be used to establish an appropriate

21 estimate of secondary PM2.5 impacts for comparison of the proposed new or modifying source’s

22 total ambient impact (i.e., primary and secondary impacts) to the applicable SIL. As described

23 above, there may be situations where the proposed new or modifying source’s ambient impact is

 

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