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EPA Issues Draft Guidance for PM2.5 Permit Modeling


On March 4, 2013, EPA issued for public comment its Draft Guidance for PM2.5 Permit Modeling.  The document reflects EPA’s preliminary recommendations for how a stationary source seeking a PSD Permit may demonstrate that it will not cause or contribute to a violation of the PM2.5 NAAQS, specifically taking into account secondary formation of PM2.5 due to emission of PM2.5 precursors (i.e., SO2 and/or NOx).  In issuing its revised PM2.5 NAAQS on December 14, 2012, EPA noted that it intended to issue the revised guidance on PM2.5 modeling to address various technical concerns with demonstrating compliance.  The draft guidance for PM2.5 permit modeling incorporates and builds upon EPA’s March 23, 2010 guidance memorandum, Modeling Procedures for Demonstrating Compliance for PM2.5 NAAQS.

The 60-page draft guidance contains detailed discussion of proposed modeling protocols and approaches.  The draft guidance addresses significant impact analyses, cumulative impact analyses, and PM2.5 increment analyses.  It also reflects EPA’s views as to the use of SILs and SMCs for PM2.5 that are presented in the companion Question and Answer document that is discussed elsewhere in this issue of the Air Quality Letter.  EPA cautions that due to the fact that the current preferred dispersion model (i.e., AERMOD), does not explicitly address secondary formation of PM2.5, any modeling protocol developed by a permit applicant for approval by the permitting authority should be reviewed with the regional EPA office.

The draft guidance contains four assessment case scenarios for determining the extent of ambient air impact analyses that will be required. 

In Case 1, no air quality analysis would be required.  In this situation, direct PM2.5 emissions are less than the 10 tpy significant emission rate (SER) and NOx and SO2 emissions are also each below the 40 tpy SER for those precursors.

In Case 2, only a primary air quality impact of direct PM2.5 emissions is required.  This situation arises where direct PM2.5 emissions are greater than or equal to the 10 tpy SER, but NOx and SO2 emissions are each less than the 40 tpy SER for those precursors.

Case 3 would require both a primary and secondary air quality impact analysis.  This would be triggered where both direct PM2.5 emissions and precursor emissions are greater than or equal to the respective SERs.

In Case 4, only a secondary air quality impact analysis is required.  This scenario arises where direct PM2.5 emissions are below the SER, but either NOx or SO2 emissions are greater than or equal to the 40 tpy SER.

Of importance, the draft guidance delineates potential approaches for evaluating secondarily formed PM2.5.  Secondary PM2.5 impacts may be assessed through: a qualitative assessment; a hybrid of a qualitative and a quantitative assessment; or a full quantitative photochemical grid modeling exercise.  These concepts are explained in detail in the draft guidance which includes examples of qualitative assessments of the potential impacts of secondary PM2.5 formation.  EPA also notes that, in those rare cases where it is deemed necessary to estimate secondary PM2.5 impacts with a full quantitative photochemical grid modeling, the candidate model for use in estimating single source impacts should meet the general criteria for an alternative model as outlined in Appendix W.  The draft guidance provides that PSD modeling of secondarily formed PM2.5 should be viewed as a screening level analysis under Appendix W, analogous to the screening nature of the guidance in Appendix W regarding dispersion modeling for nitrogen dioxide impacts.

EPA has also clarified and revised its recommended approach for combining the modeled and monitored concentrations of PM2.5 for comparison to the PM2.5 NAAQS.  Because the proposed modeling guidance takes into account secondary PM2.5 formation, EPA has proposed a departure from the March 23, 2010 clarification memo to allow as a First Tier that the modeled design value (based on the multi-year average of the 98th-percentile of 24-hour values) be added to the monitored design value from a representative monitor.  The modeled design value would include, as appropriate, the primary and secondary projected PM2.5 impacts.  (Monitored design values already account for secondary PM2.5 formation.)

Any entity that is considering constructing or modifying a source that may be major for PM2.5 should closely study EPA’s new guidance for PM2.5 impact modeling.  The express consideration of secondary PM2.5 formation in conjunction with the more stringent primary annual PM2.5 standard that was recently adopted creates significant incentives to evaluate project approaches that can avoid PSD review for PM2.5 emissions.  In many areas, background ambient PM2.5 concentrations are already very close to the revised annual PM2.5 NAAQS.  In such cases, for projects that are major for PM2.5 due to either the emission of direct PM2.5 or precursors, it may not be possible to avoid a cumulative impact analysis, and demonstrating compliance with the NAAQS may be very difficult.

To view a complete PDF of the First Quarter 2013 issue of the Air Quality Letter, click HERE.

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