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Draft Guidance on Significant Impact Levels (SIL) for O3 and PM2.5 under the PSD Program

On August 1st, 2016 (revised August 18th, 2016) EPA released a draft guidance memo on Significant Impact Levels (SIL) for Ozone (O3) and particulate matter less than 2.5 microns in aerodynamic diameter (PM2.5). The salient points from the draft memo are listed below:

The following NAAQS SIL concentrations are recommended:
Ozone 8-hour:  1.0 ppb
PM2.5 24-hour:  1.2 ug/m3
PM2.5 Annual:  0.2 ug/m3
Source: EPA Webinar held on August 24, 2016.

PSD Increments
Currently, there is no PSD increment for 8-hr O3 NAAQS. Therefore, an O3 increment SIL is not recommended in this guidance.

PM2.5 increment SIL recommendations are also provided.

General points
EPA has left the adoption of these SILs to the discretion of permitting authorities.
Final guidance memo is expected by end of 2016.
EPA is accepting informal comments until September 30, 2016.

EPA Website:

Proposed Revisions to EPA’s Guideline on Air Quality Models

On 07/29/2015, EPA’s proposed revision to the air quality modeling guideline (Appendix W to 40 CFR Part 51) was published.

Appendix W Factsheet:


  • Proposed enhancements to the scientific formulation of the preferred nearfield dispersion model, AERMOD, including modeling techniques to address the secondary chemical formation of fine particle and ozone pollution from direct, single source emissions of pollutants that form them such as sulfur dioxide, oxides of nitrogen, volatile organic compounds. (See details below)
  • Proposed streamlining of resources necessary to conduct regulatory modeling with AERMOD by incorporating model algorithms from the Buoyant Line and Point Source (BLP) model and replacing the CALINE3 model used for mobile source applications including fine particle pollution (PM2.5, PM10), and carbon monoxide (CO) hot-spot analyses.
  • Remove CALPUFF as a preferred model for long range air quality assessments and recommending its use as a screening technique along with other Lagrangian models for addressing PSD increment beyond 50 km from a new or modifying source. This proposed change does not affect the EPA’s recommendation in the 2005 BART Guidelines to use CALPUFF in the BART determination process.

Read More …

AERMOD Dispersion Model Version 15181 Released

On 07/24/2015, EPA released an updated version of the AERMOD model. Key technical changes in the new version are as follows:

  • Includes option in AERMET to adjust the surface friction velocity (u*) to address issues with AERMOD model over prediction under stable, low wind speed conditions.
  • Includes a low wind option in AERMOD to address issues with model over prediction under low wind speed conditions. The low wind option will increase the minimum value of the lateral turbulence intensity (sigma-v) from 0.2 to 0.3 and adjusts the dispersion coefficient to account for the effects of horizontal plume meander on the plume centerline concentration.
  • Modifications to AERMOD formulation to address issues with over prediction for applications involving relatively tall stacks located near relatively small urban areas.
  • Includes regulatory default options to address plume rise for horizontal and capped stacks.
  • Includes buoyant line source option based on the BLP model.
  • Updates to the NO2 Tier 2 and Tier 3 screening techniques coded within AERMOD for assessments related to 1 hour NO2 standards. Tier 2 technique will be ARM2 which is based on hourly measurements of the NO2 to NOx ratios and provides more detailed estimates of this ratio based on the total NOx present. For Tier 3 technique, the existing detailed screening options of the Ozone Limiting Method (OLM) and PVMRM which have been  available as non-regulatory, non-default options in AERMOD for many years is formally incorporated as regulatory options.

Please contact your EPS consultant for more information.

PM2.5 Permit Modeling Guidance Issued

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: contact your EPS consultant for more information.



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