Air Dispersion Modeling 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