22 v. A protocol on methods and procedures to be followed has been established.

Public Review Draft 03/04/2013


Section 3.2.1 of Appendix W also discusses appropriate 1 methodologies for evaluating

2 performance of models for regulatory applications, including the EPA’s Protocol for

3 Determining the Best Performing Model (EPA-454/R-92-025). The determination of

4 acceptability of a particular model and approach for such an alternative model application is an

5 EPA Regional Office responsibility (Section 3.2.2(a)) that may also include consultation with the

6 EPA Headquarters, if appropriate.

7 As noted in the NACAA Workgroup final report, photochemical grid models provide a

8 complete characterization of emissions, chemical transformation, transport, and deposition using

9 time and space variant meteorology. The EPA’s modeling guidance for PM2.5 attainment

10 demonstrations (U.S. EPA, 2007a) identifies both the Comprehensive Air Quality Model with

11 Extensions (CAMx) (ENVIRON, 2011; Nobel et al., 2001; Russell, 2008) and the Community

12 Multiscale Air Quality (CMAQ) model (Byun and Schere, 2006; Foley et al., 2010). These

13 state-of-the-science photochemical grid models have been used by the EPA for air quality

14 modeling to support federal rulemaking and by state/local air permitting agencies for their air

15 quality planning efforts. Some photochemical grid models have been instrumented with

16 extensions that allow for the identification of impacts from specific sources to important receptor

17 locations. These extensions generally fall in the categories of source apportionment and source

18 sensitivity, and of sub-grid plume treatment and sampling, as described below.


20 III.2.3.1 Use of Photochemical Grid Models for Secondary PM2.5 Impacts

21 Based on the current capabilities of photochemical grid models and consistent with the

22 NACAA Workgroup report, the EPA recommends the following approaches be considered to

23 estimate


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