Frequently, a manufacturing plant or industrial operation may have processes that emit air pollutants into the atmosphere. In most cases, the operation will be required to calculate the potential, and sometimes actual, emissions and present the findings to their State’s environmental agency. Then, the type of emission sources and the potential and/or actual emissions rate will be used to determine if an air permit is required. The State will also review the sources and emissions and apply specific regulations to the plant to allow those emissions to be released into the environment. This information will typically be summarized and included in an air permit. Additionally, depending upon the amount or nature of the emissions, the permit may include requirements for controls or limits on emissions.
In some cases, a regulatory agency may ask an operation to present information beyond the amount of calculated air emissions released into the atmosphere. The plant may have to determine what the impact of their emissions is on the surrounding community. When an operation has the potential to release chemicals that, in high concentrations, could cause unhealthy conditions, State regulatory agencies may require an estimate of downwind effects of air pollution emissions. This is accomplished by using air quality modeling.
How do you calculate downwind air pollutant concentration?
Calculating the volume of pollutants being emitted from a stack is relatively straightforward, however, using air quality modeling to determine the concentration of pollutants downwind of the facility is a more complex process. Air quality modeling uses computer programs to calculate the release of a pollutant into the atmosphere and determine how it disperses as it travels downwind. The program will then calculate the eventual concentration of that pollutant at a certain location.
The U.S. EPA has developed many different air quality models to calculate downwind air pollutant concentrations, and these models vary in size and complexity. Simple air quality models, known as Screening Models, can quickly determine concentrations and require limited information about the emissions and the emission points. Screening Models are relatively quick to prepare, do not require a lot of site-specific information, and will provide general estimates of downwind concentrations.
Other more sophisticated models, known as Refined Models, require more detailed information to generate results. These models use site-specific information, such as local weather data, terrain elevation around the site, and other specifics about the emission sources. Due to the complexity and amount of information required, Refined Models require significantly more set-up time and effort to obtain proper results.
You may ask, “If the Screening Model is so easy to use, why would anyone want to use the Refined Model?” The simple answer is that Screening Models are not as accurate as Refined Models. Usually, Screening Models tend to estimate concentrations on the high end to prevent underestimating the downwind impacts. Screening Models are also very general and are not designed to estimate emissions from complex scenarios. Refined Models are able to model complex situations and are site-specific.
Which model should you use — Screening or Refined?
In most cases, Screening Models are used as a “first step” and, since they tend to overestimate emissions, provide a conservative estimate of concentrations from a source. If the calculated concentrations from a Screening Model show no adverse health effects, then one can assume the emission source will not adversely affect the surrounding environment. If, however, a Screening Model shows concentrations higher than allowable values, all is not lost. A Refined Model can be used to provide more accurate and often lower concentrations, possibly showing that the emission source would not have detrimental effects on the downwind area.
Further Information
Cornerstone can help you determine whether air quality modeling is necessary for your facility. Our team of experts has provided clients with both Screening and Refined Modeling analyses to assist them with State agency modeling requirements and other studies. For more information about our air quality services, please visit our website or contact us to discuss your specific needs.
John Nuckels, Sr. Environmental Engineer
John helps clients with the preparation of air permits and supports them in responding to inquiries by State permitting agency personnel. His expertise in air quality monitoring, stack tests, recordkeeping, and reporting assists clients in achieving and maintaining air quality compliance. Industry experience includes aerospace, aluminum, motor vehicle / automotive parts, government, plastics, military, and other manufacturing clients in both public and private sectors.