EPA updates air-quality model
The U.S. EPA is retiring the air-quality model it has used for the past 25 years, replacing it with a state-of-the-science tool.
After 25 years, the U.S. EPA is recommending that state and local regulators use a new air-quality model as of December 9, according to a notice published in the Federal Register. The AERMOD model is being phased in as the tool that regulators use for assessing their progress in reducing criteria air pollutants under the Clean Air Act (CAA).
AERMOD was designed to meet requirements identified by a committee that included scientists from EPA and the American Meteorological Society (AMS). The committee’s charge was to “introduce state-of-the-art modeling concepts into the EPA’s local-scale air-quality models,” according to the agency’s website. AERMOD is a steady-state plume model that incorporates air dispersion on the basis of planetary boundary-layer turbulence structure and scaling concepts.
According to the Federal Register [notice], AERMOD “provides better characterization of plume dispersion than” Industrial Source Complex (ISC3), which has been EPA’s preferred model since 1980. AERMOD is able to simulate elevated air pollution sources as well as those at the surface, and it can incorporate both simple and complex terrain.
On November 9, 2006, AERMOD will replace ISC3 as the agency’s “preferred general-purpose” model. At that time, it will become the model that regulators use to estimate local levels of ozone, volatile organic compounds, carbon monoxide, NO2, SO2, particulate matter, and lead. Areas that have been unable to attain CAA standards for these criteria pollutants will be required to use AERMOD to revise their State Implementation Plans (SIPs) for existing air pollution sources. States will also need to use AERMOD for their New Source Review (NSR) and Prevention of Significant Deterioration (PSD) programs.
AERMOD’s predictions of ground-level pollutant concentrations are more accurate because they incorporate up-to-date physics to simulate air emissions plume transport and dispersion, says Robert Paine, technical director of ENSR, an international environmental services company. Paine was a member of EPA’s AERMOD design team. “In many cases, AERMOD predicts lower concentrations than the ISC[3] model,” he adds. This means that some facilities’ operational restrictions may be relaxed, he explains.
The agency’s peer-review process verified that the AERMOD model is widely considered to represent the “state of the science” and confirmed that it “represents sound and significant advances over” ISC3, according to the notice.
