Air Dispersion Modeling: The Basics
What is Air Dispersion Modeling?
In simple terms, dispersion modeling is a mathematical simulation of how air pollutants disperse in the atmosphere. Using computer programs, dispersion modeling predicts ground-level concentrations from facilities to be compared with national, state, and local air pollution standards. The U.S. Environmental Protection Agency (EPA) has developed computer models for dispersion modeling. A common model used in the U.S. is the American Meteorological Society (AMS)/ (EPA) Regulatory Model (AERMOD).
What is the Importance of Dispersion Modeling Results?
- It helps protect public health and welfare. By requiring facilities to perform modeling before starting a new, large source of air pollutants, the public is protected from pollutant concentrations that exceed health standards.
- It ensures consistency in enforcing regulations. By having uniform rules on how and when dispersion modeling is performed, all facilities are treated equally.
- It aids million-dollar decisions by Industry. By modeling emission source scenarios before they are built, industry can save millions of dollars by finding cost-effective solutions and only installing necessary equipment.
What are some Reasons for Dispersion Modeling?
- PSD Requirements. If facilities plan on emitting pollutants above certain criteria, the U.S. EPA requires a Prevention of Significant Deterioration (PSD) dispersion modeling analysis, including evaluating Significant Impact Level (SIL – your facility only), PSD increment, National Ambient Air Quality Standards (NAAQS), Visibility, and possibly Class I analyses for National Parks and protected wilderness areas.
- NAAQS Requirements. The U.S. EPA has NAAQS for all sources, not just PSD. Many states require a modeling analysis for NAAQS if the facility’s emissions are large enough.
- Nonattainment Rules. If counties or parishes within a state have values above the NAAQS, then there are additional dispersion modeling requirements in those areas to ensure that the air quality does not degrade any further.
- State and Local Permits. Each state and local agency has their own rules that may require a dispersion modeling analysis.
- Emergency Planning. If a facility stores toxic materials above certain criteria, they will need to perform an Off-Site Consequence Analysis (OCA) as part of a Risk Management Plan (RMP). While there are look-up tables that simplify this process, the tables are designed to be extremely conservative. Many facilities choose to use dispersion modeling instead of look-up tables to more accurately depict their theoretical emissions.
- Risk Assessment. If a facility burns or detonates hazardous material, the U.S. EPA requires a risk assessment to ensure that human health and the environment are protected. Dispersion modeling is used to determine ground-level concentrations and depositions which are then used in a toxicological analysis to determine risk.
- Quick Results. While a monitor sampling study can be used to determine actual ground-level concentrations, such studies are expensive and can take a long time to collect and analyze data. A Dispersion modeling analysis is much more cost-effective and time saving.
- Safe – No Actual Exceedances. A modeling analysis is all done as a computer simulation. If exceedances are discovered, changes can be made to a facility before it is built or modified such that no exceedances ever occur as a real-life event.
What are the Basic Components of Air Dispersion Modeling?
- Source Definition – Is it a stack? Area? Volume? Line?
- Building Downwash – This is a study of nearby structures which determines if the wind flow field is disturbed by the presence of large structures.
- Pollutant Averaging Period – This is determined by each pollutant that is modeled and the form of the standard it is to be compared to (i.e., 1-hour, annual, etc.).
- Receptor Grid – A receptor is a geographical point where the model will make a calculation.
- Terrain Data – Elevation data is used to determine if and how terrain will affect air flow.
- Meteorological Data – Refined models use recorded meteorological data as input to the model. This is typically the most recent 5 years from the nearest National Weather Station (NWS). Screening models simulate worst-case meteorology.
- Urban/Rural Land Use – Air dispersion is different in urban versus rural areas. Part of a modeling analysis is a review of the local land use to determine if the area is rural or urban.
Liesa Elliot
Sr. Air Quality Meteorologist
Need Support, Reach Out
If you have any questions regarding air dispersion modeling or need support with an air modeling project, please contact Carter Venable at Carter@StevensEHS.com.
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