To the extent a permitting authority wishes to use any of the SILs values in the vacated

14 Sections 51.166(k)(2) or 52.21(k)(2) as a screening tool to determine whether it is necessary to

15 conduct a cumulative analysis, the permitting authority must first examine background air

16 quality concentrations to determine whether a substantial portion of the NAAQS has been

17 consumed.10 For this purpose, the EPA recommends using the preconstruction monitoring data

18 compiled to meet the requirements of Section 51.166(m) or 52.21(m) of the EPA’s regulations.

9 The EPA has previously observed that the absence of an EPA-promulgated SIL does not preclude PSD permitting

authorities from developing and applying SILs to support permitting decisions. See, Response to Comments,

Implementation of New Source Review (NSR) Program for Particulate Matter Less Than 2.5 Micrometers in

Diameter (PM2.5) at 82 (March 2008) [EPA-HQ-OAR-2003-0062-0278]. However, the EPA has also observed that,

“[t]he application of any SIL that is not reflected in a promulgated regulation should be supported by a record in

each instance that shows the value represents a de minimis impact.” See, NO2 NAAQS Guidance at 13; and

Mississippi Lime at 41 (granting the petition for review where the permitting authority failed to substantiate in the

record which SIL it applied and its reasons for doing so).

10 The recent court decision vacating the PM2.5 SMC from the PSD regulations will mean that each PSD application

must include ambient monitoring data representative of the area of concern. This data need not be collected by the

PSD applicant if existing data is determined by the permitting authority to represent the air quality in the area of

concern over the 12-month period prior to the submittal of a complete PSD application.


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