Chemical Dispersion Modeling

Exponent has been influential in the evolution of air quality modeling. Exponent scientists have developed and contributed to five of the seven U.S. EPA models recommended in the Guideline on Air Quality Models. Most notable of these is the CALPUFF model, an advanced non-steady-state meteorological and air quality modeling system 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.

Other EPA-recommended models that bear the imprint of the Atmospheric Sciences 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 models’ capabilities and limitations. This knowledge enables Exponent to provide solutions beyond the typical or traditional modeling approaches.

Exponent Specializes In:

  • Expertise in applying CALPUFF to 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 or coastal environments
  • Deposition of particulates or gases
  • Real-time air quality forecast systems
  • Prevention of Significant Deterioration (PSD) analyses
  • Compliance with National Ambient Air Quality (NAAQS) or New Source Review (NSR) requirements
  • Risk management planning
  • Evaluating visibility or deposition issues in National Parks
  • Developing emissions inventories
  • Addressing nuisance issues such as odor.

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.

Beyond regulatory and industrial permitting applications, our staff routinely applies air quality models to a wide range of other problems. Examples of specialized model applications include: odor evaluation and mitigation, mobile sources, forest fires, explosions, pesticide drift, and radioactive releases. We also offer deep experience in dense-gas dispersion modeling. Applications have included emergency planning, and accidental spills or upset conditions releasing ammonia, hydrogen sulfide, and chlorine from pipelines, rail cars, storage tanks, and flares. Models used include DEGADIS, HGSYSTEM, SLAB, ALOHA, PHAST, SCIPUFF, and FLUENT.