Atmospheric Dispersion Modeling

Atmospheric dispersion modeling is the mathematical simulation of how air pollutants disperse in the ambient atmosphere. It is performed with computer programs that include algorithms to solve the mathematical equations that govern the pollutant dispersion. The dispersion models are used to estimate the downwind ambient concentration of air pollutants or toxins emitted from sources such as industrial plants, vehicular traffic, or accidental chemical releases. They can also be used to predict future concentrations under specific scenarios (i.e. changes in emission sources). Therefore, they are the dominant type of model used in air quality policy‐making. They are most useful for pollutants that are dispersed over large distances and that may react in the atmosphere. For pollutants that have a very high spatiotemporal variability (i.e. have very steep distance to source decay such as black carbon) and for epidemiological studies, statistical land‐use regression models are also used.

Dispersion models are important to governmental agencies tasked with protecting and managing the ambient air quality. The models are typically employed to determine whether existing or proposed new industrial facilities are or will be in compliance with the NAAQS in the United States and other nations. The models also serve to assist in the design of effective control strategies to reduce emissions of harmful air pollutants. During the late 1960s, the Air Pollution Control Office of the USEPA initiated research projects that would lead to the development of models for the use by urban and transportation planners (Fensterstock et al. 1971).

Air dispersion models are also used by public safety responders and emergency management personnel for emergency planning of accidental chemical releases. Models are used to determine the consequences of accidental releases of hazardous or toxic materials. Accidental releases may result from fires, spills, or explosions that involve hazardous materials, such as chemicals or radionuclides. The results of dispersion modeling, using worst‐case accidental release source terms and meteorological conditions, can provide an estimate of location impacted areas, ambient concentrations, and be used to determine protective actions appropriate in the event a release occurs. Appropriate protective actions may include evacuation or shelter in place for persons in the downwind direction. At industrial facilities, this type of consequence assessment or emergency planning is required under the CAA codified in Part 68 of Title 40 of the Code of Federal Regulations.

The dispersion models vary depending on the mathematics used to develop the model, but all require the input of data that may include the following:

• Meteorological conditions such as wind speed and direction, the amount of atmospheric turbulence (as characterized by what is called the “stability class”), the ambient air temperature, the height to the bottom of any inversion aloft that may be present, cloud cover, and solar radiation
• Source term (the concentration or quantity of toxins in emission or accidental release source terms) and temperature of the material
• Emissions or release parameters such as source location and height, type of source (i.e. fire, pool, or vent stack) and exit velocity, exit temperature, and mass flow rate or release rate
• Terrain elevations at the source location and at the receptor location(s), such as nearby homes, schools, businesses, and hospitals
• The location, height, and width of any obstructions (such as buildings or other structures) in the path of the emitted gaseous plume, surface roughness, or the use of a more generic parameter “rural” or “city” terrain

Many of the modern, advanced dispersion modeling programs include a preprocessor module for the input of meteorological and other data, and many also include a post‐processor module for graphing the output data and/or plotting the area impacted by the air pollutants on maps. The plots of areas impacted may also include isopleths showing areas of minimal to high concentrations that define areas of the highest health risk. The isopleths plots are useful in determining protective actions for the public and responders.

The atmospheric dispersion models are also known as atmospheric diffusion models, air dispersion models, air quality models, and air pollution dispersion models.