USEPA Revises Modeling Guidance for Predicting Secondary Impacts from Emission Sources

Though it’s been widely understood for years that nitrogen oxides (NOx) and volatile organic compounds (VOCs) can react in the atmosphere to form ground level ozone, and that NOx and sulfur dioxide (SO2) can react to form fine particulate matter (PM2.5), we have not had a great way to predict those secondary impacts from individual emission sources. The EPA’s recent revision to its Guideline on Air Quality Models, which is just finishing up its year-long transition period, provides a new two-tiered approach for facilities to use in predicting their potential impact on ozone and secondary PM2.5 formation.

Owners or operators who want to build a major new source of air pollution—power plants or manufacturing facilities, for example—or make a major modification to an existing facility must demonstrate to the EPA that emissions from the facility will not cause or contribute a violation of any National Ambient Air Quality Standard (NAAQS) or Prevention of Significant Deterioration (PSD) increment standard. Estimates of ambient pollutant concentrations are determined using approved air quality models outlined in the Guideline, which are used to regulate pollutant sources appropriately. The revision incorporates specific guidance identifying models and analytical techniques for assessing single-source impacts on ground level ozone and the secondary formation of PM2.5, both of which generally occur downwind of emission sources as a result of chemical reactions in the atmosphere.

New screening tool enables more cost-effective analysis

In support of the Guideline revision, the EPA has released its Modeled Emission Rates for Precursors (MERP) tool for use in Tier 1 compliance demonstrations. Prior to the revision of the Guideline, facility owners or operators were able to conduct a qualitative or quasi-quantitative analysis of the estimated impact of any “precursor” pollutants on secondary pollutant formation. The actual formation of these substances is dependent on physical location, topography, and meteorology, making it difficult to meaningfully predict what impact these precursors would have on local levels of ozone and PM2.5 without using more sophisticated and expensive photochemical modeling techniques.

The MERP tool provides a simple way to evaluate the impact of precursor emissions on the formation of ozone and secondary PM2.5, and new major sources or major modifications that potentially exceed the significant emission rates for any of the precursor emission species (SO2, NOx, or VOCs) are required to evaluate the combined effects of all precursor emissions for potential impacts. Facilities that emit precursors at annual rates below the lowest MERPs for a specific area of the country can, with some exceptions, use the MERP tool to demonstrate that the sum of the precursor impacts from a project will be below the critical air quality thresholds and therefore will not cause or contribute to a violation of the NAAQS or PSD standard. For facilities with predicted impacts that exceed the critical threshold values, additional analytical techniques can be applied using the results of the EPA’s recent photochemical grid modeling (conducted in support of the MERP tool development) to potentially demonstrate compliance. If compliance still can’t be demonstrated using these additional techniques, a more refined Tier 2 demonstration involving the use of chemical transport models will likely be required.

The use of the MERP tool and any additional techniques based on EPA’s photochemical grid modeling should be detailed in the air dispersion modeling protocol and reviewed by the appropriate regulatory agency as part of the pre-application review process. Once those procedures have been approved, the results of the analyses should be detailed in the final air dispersion modeling report submitted as part of the PSD permit application process. Until now, facilities have not typically been required to address the impacts of ozone due to precursor emissions as part of air dispersion modeling assessments; in light of this revision, it is important to fully address the potential effects on ozone and secondary PM2.5 formation in the modeling demonstration to avoid delays in the PSD permitting process.

While the new MERP tool provides a great new capability for analyzing your facility’s potential impacts on secondary pollutant formation, understanding the full picture can still be quite complex. It’s important to work with someone who understands the underlying components of this tool to allow you to utilize it to better meet your facility’s specific needs.


Kelley Begin Practice Leader Air Services

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