Regulatory Dispersion Modeling Services

Exponent scientists have been influential in the development and evolution of regulatory air quality modeling. Exponent scientists have developed, contributed to, and applied many models recommended or referenced in the U.S. Environmental Protection Agency (EPA) Guideline on Air Quality Models.

The CALPUFF model, an advanced non-steady-state meteorological and air quality modeling system, was previously recommended by the EPA as the preferred model for assessing long-range transport of pollutants and their impacts on Federal Class I areas or for near-field applications involving complex meteorological conditions. Although EPA no longer recommends a specific model for long-range transport analyses, CALPUFF is currently identified by EPA as an accepted screening modeling technique for this purpose. CALPUFF is still recommended for use by EPA for the Bay Area Rapid Transport (BART)  determination process. CALPUFF is also widely used in other countries throughout the world for regulatory and other impact assessment purposes. Other EPA-recommended models that bear the imprint of the Exponent AQM staff include: the Buoyant Line and Point (BLP) source model, commonly used for aluminum reduction facilities; the Offshore and Coastal Dispersion (OCD) model, for emission sources in offshore or shoreline locations; and the Complex Terrain Dispersion Model (CTDM). Our staff also developed the PRIME building downwash model that is incorporated into the CALPUFF and AERMOD models.

As the developers of and contributors to many of the EPA-recommended models in the past 35 years, our scientists are experts on the capabilities and limitations of these models. This knowledge enables Exponent to provide solutions that go beyond typical or traditional modeling approaches.

Exponent’s AQM staff offer expertise in many regulatory areas, including:

  • Use of CALPUFF for long-range transport modeling for Class I studies
  • Best Available Retrofit Technology (BART) analyses
  • Modeling for near-field complex flows such as found in steep valleys and coastal environments
  • Deposition of particulates or gases
  • Prevention of Significant Deterioration (PSD) analyses
  • Compliance demonstrations for ambient air quality standards for criteria pollutants and acceptable ambient levels for air toxics. 
  • Modeling for risk management planning
  • Modeling for health and ecological risk assessment
  • Evaluating the effect of emissions on visibility and deposition in National Parks
  • Conducting expert third-part reviews of analyses conducted by others

Additionally, Exponent’s scientists have an in-depth knowledge of air quality regulations, which along with our expertise in air quality modeling, makes us well qualified to help clients obtain permits for their sources. In some cases, the interpretation of published regulations may not be clearly defined for a specific application. Our combined expertise allows us to develop appropriate and defensible solutions to complex regulatory problems.

Example Projects

Exponent developed and applied a methodology to calculate lateral and crosswind concentration gradients in order to address ambiguous EPA modeling guidance regarding the selection of off-site sources for inclusion in cumulative impact modeling assessments. This method was successfully applied to a PSD permitting project and resulted in the elimination of many facilities from the modeling analysis. The method has subsequently been used for other projects and has been cited by EPA as a precedent for addressing the issue. This project also involved special processing of meteorological data and output files to merge results of AERMOD and BLP model runs and obtain ranked values needed for comparison with ambient standards.

Exponent assisted a corporation with several large chemical manufacturing facilities using modeling analyses to identify potential configuration and control system changes needed to address requirements in a negotiated consent agreement with EPA. Exponent modeled numerous possible scenarios to help identify optimal solutions and provided modeling analyses to support required permit modifications. The modeling incorporated numerous enhancements, including the use of time-varying emission rates for various fugitive sources.

Exponent conducted modeling to assist a coal-fired electrical generation facility address issues arising from the filing of a Section 126 petition by a third party alleging that the facility was causing a violation of National Ambient Air Quality Standards (NAAQS) in a neighboring state. Exponent conducted modeling to demonstrate compliance and to address sulfur in fuel variability in a manner that resulted in more flexible emission limits.