“pollutant offset ratios” and then use a dispersion model to assess the impacts of the combination

Public Review Draft 03/04/2013


of direct PM2.5 emissions and the equivalent direct PM2.5 emissions. The 1 “pollutant offset ratios”

2 referenced in the final report were those put forth by the EPA in the 2008 Implementation of the

3 New Source Review (NSR) Program for Particulate Matter Less Than 2.5 Micrometers (PM2.5)

4 (73 FR 28321) final rule concerning the development and adoption of interpollutant trading

5 (offset) provisions for PM2.5 under state nonattainment area NSR programs for PM2.5.14 The

6 EPA’s July 23, 2007, technical analysis entitled “Details on Technical Assessment to Develop

7 Interpollutant Trading Ratios for PM2.5 Offsets,” describes the method used to establish the

8 original "preferred" precursor offset ratios (U.S. EPA, 2007b).

9 We do not expect that the specific results from the EPA's 20

We do not expect that the specific results from the EPA's 2007 technical assessment

10 could be used generally in this context without additional technical demonstration specific to the

11 source(s) and area(s) for which the ratios would be applied. However, we expect that the EPA

12 Regional Offices, with assistance from the OAQPS, may assist state/local air permitting

13 agencies, as necessary, to structure appropriate technical demonstrations leading to the

14 development of appropriate source and area-specific offset ratios for PM2.5 that may be

15 appropriate for the purposes of estimating potential secondary PM2.5 impacts. As described in

16 the July 21, 2011, Gina McCarthy memorandum addressing reconsideration of the interpollutant

17 trading provisions for the 2008 final rule, the EPA acknowledged that existing models and

18 techniques are adequate to “conduct local demonstrations leading to the development of area19

specific ratios for PM2.5 nonattainment areas” and provided a general framework for efforts that

20 may be relevant in developing appropriate “pollutant offset ratios” for use in hybrid

21 qualitative/quantitative assessment of secondary PM2.5 impacts (U.S. EPA, 2011f).


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