AERMOD is the latest generation air dispersion model designed for short-range (up to 50 kilometers) dispersion of air pollutant emissions from stationary industrial sources. It is a steady-state plume model incorporating dispersion based on planetary boundary layer (PBL) turbulence structure and scaling, and it accomodates surface and elevated emission sources as well as simple or complex terrain.
As of December 9, 2005, the U.S. EPA designated AERMOD as the preferred model to be used for compliance with any federal and state air pollution dispersion modeling requirements. As of November 9, 2006, AERMOD completely replaces the previous preferred model known as the Industrial Source Complex (ISC) model.
The AERMOD system integrates three modules:
- A steady-state Gaussian dispersion model designed for short-range dispersion of continuous emissions from stationary industrial sources.
- A meteorological data preprocessor (AERMET) that accepts surface meteorological data, upper air soundings, and optionally, data from on-site instrument towers. It then calculates parameters needed by the dispersion model, such as atmospheric turbulence characteristics, mixing heights, friction velocity, Monin-Obukov length and surface heat flux.
- A terrain preprocessor (AERMAP) whose main purpose is to provide a physical relationship between terrain features and the behavior of air pollution plumes. It generates location and height data for each receptor location. It also provides information that allows the dispersion model to simulate the effects of air flowing over hills or splitting to flow around hills.
AERMOD also includes PRIME (Plume Rise Model Enhancements) which is an algorithm for modeling the effects of downwash created by the pollution plume flowing over nearby buildings.
History of the development of AERMOD
In 1991, the American Meteorological Society (AMS) and the U.S. EPA initiated a formal collaboration with the goal of incorporating planetary boundary layer (PBL) concepts into regulatory dispersion models. A working group called the AMS / EPA Regulatory Model Improvement Committee (AERMIC), comprised of AMS and U.S. EPA scientists, was formed for this effort.
The AERMIC developed AERMOD in seven steps:
- Initial model formulation
- Developmental evaluation
- Internal peer review and beta testing
- Revised model formulation
- Performance evaluation and sensitivity testing
- External peer review
- Submission to the U.S. EPA for consideration as a regulatory model.
On April 21st of 2000, the U.S. EPA proposed that AERMOD be adopted as their preferred regulatory model for both simple and complex terrain. About 5 years later, AERMOD was adopted by the U.S. EPA and promulgated as their preferred regulatory model, effective as of December 9th of 2005. The entire developmental and adoption process took 14 years (from 1991 to 2005).
Features and capabilities of AERMOD
Some of the primary features and capabilities of AERMOD are:
- Source types: Multiple point, area and volume sources
- Source releases: Surface, near surface and elevated emission sources
- Source locations: Urban or rural locations. Urban effects are scaled by population.
- Plume types: Continuous, buoyant plumes
Plume deposition: Dry or wet deposition
- Plume dispersion treatment: Gaussian model treatment in horizontal and in vertical for stable atmospheres. Non-Gaussian treatment in vertical for unstable atmospheres
- Terrain types: Simple or complex topography
- Building effects: Handled by PRIME downwash algorithms
- Meteorology data height levels: Accepts meteorology data from multiple heights
- Meteorological data profiles: Vertical profiles of wind, turbulence and temperature are created
- 40 CFR Part 51, Revision to the Guideline on Air Quality Models: Adoption of a Preferred General Purpose (Flat and Complex Terrain) Dispersion Model and Other Revisions; Final Rule, published in the Federal Register, November 9, 2005
- AERMOD: Description of Model Formulation., EPA-454/R-03-004, September 2004
- E.T. Prater and C. Midgley, "A new air dispersion modeling system is helping create more accurate industrial source models", Environmental Protection, Volume 17, Issue 3, April 2006.
- R.W. Brode, AERMOD Technical Forum, EPA R/S/L Modelers Workshop, San Diego, California, May 16, 2006
- Lloyd L. Schulman, David G. Strimaitis and Joseph S. Scire, Development and Evaluation of the PRIME Plume Rise and Building Downwash Model, 7th Conference on Air Quality Modeling, June 28 - 29, 2000, Washington, D.C.
- AERMOD: Latest Features and Evaluation Results., EPA-454/R-03-003, June 2003.
For those who would like to learn more about this topic, it is suggested that any of the following books be consulted:
- D.B. Turner (1994), Workbook of Atmospheric Dispersion Estimates , 2nd Edition, CRC Press, ISBN 1-56670-023-X.
- Milton R. Beychok (2005), Fundamentals of Stack Gas Dispersion, 4th Edition, author-published, ISBN 0-9644588-0-2.
- Karl B. Schnelle, Jr. and Partha R. Dey (2000), Atmospheric Dispersion Modeling Compliance Guide, McGraw-Hill, ISBN 0-07-058059-6.