US EPA released the “Guidance for PM2.5 Permit Modeling” document on May 20, 2014. Key points include:
- The guidance addresses PM2.5 compliance demonstration steps for new major sources, existing major source modifications, and PSD increment analysis for minor sources located in counties that have established PM2.5 minor source baseline dates.
- In the past EPA was unable to specify which model/method to be used for addressing secondary formation of PM2.5 from a single source due to the complex chemistry and variability between different types of sources and its emissions. This guidance document provides recommendations on the choice of methods/models to address secondary formation of PM2.5. However, the document makes it clear that the final decision on the choice of method/model to demonstrate compliance is dependent on the respective permitting agency and should be discussed in advance via a modeling protocol.
- EPA’s recommended assessment of primary and secondary impacts of direct PM2.5 and precursors (NOx and SO2) depend on the project’s emissions of each pollutant as compared to the PSD significant emission rate (SER).
- AERMOD dispersion model is recommended to perform source impact from direct (primary) emissions.
- Secondary PM2.5 impacts from precursor emissions can be analyzed in 3 different methods:
Qualitative assessment: This can include – detailed conceptual description of the pollutant concentrations in the area, assessment of speciated PM2.5 composition in the area, species background concentrations, meteorological factors etc.
Hybrid qualitative/quantitative: This can include – qualitative discussion similar to above, quantitative data from previous sensitivity studies conducted for the region, precursor emissions converted to equivalent amounts of direct PM2.5 emissions using “pollutant offset ratios” etc.
Full quantitative using photochemical modeling exercise (e.g., CMAQ, CAMx). EPA expects only few sources to require this.
- Wet and/or dry deposition of PM2.5 is still not recommended due to significant deposition variances associated with constituent elements of PM2.5.
- If the Significant Impact Level (SIL) is exceeded then a cumulative impact analysis will need to be conducted. Cumulative analysis will include nearby sources and monitored background concentration values in the assessment. The monitored background concentration will take account of regional transport, secondary PM2.5 formation impacts from nearby sources, and primary PM2.5 impacts from nearby sources not included in the modeled inventory.
The guidance may be accessed here: http://www.epa.gov/ttn/scram/guidance/guide/Guidance_for_PM25_Permit_Modeling.pdfPlease contact your EPS consultant for more information.