Atmospheric dispersion models are computer programs that use mathematical algorithms to simulate how pollutants in the ambient atmosphere disperse and, in some cases, how they react in the atmosphere. The dispersion models are used to estimate or to predict the downwind concentration of air pollutants emitted from sources such as industrial plants and and vehicular traffic. Such models are important to governmental agencies tasked with protecting and managing the ambient air quality. The models are typically used to determine whether existing or proposed new industrial facilites are or will be in compliance with national ambient air quality standards. The models may also be used assist in the design of effective control strategies to reduce emissions of harmful air pollutants.
The dispersion models require the input of data which includes:
- 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 and the height to the bottom of any inversion aloft that may be present.
- Emissions parameters, such as source location and height, source vent stack diameter and exit velocity, exit temperature and mass flow rate.
- Terrain elevations at the source location and at the receptor location.
- The location, height and width of any obstructions (such as buildings or other structures) in the path of the emitted gaseous plume
The atmospheric dispersion models are also known as atmospheric diffusion models, air dispersion models, air quality models, and air pollution dispersion models. This compilation of atmospheric dispersion models lists and, where possible, very briefly describes the models currently in use by the U.S. EPA.
U.S. Environmental Protection Agency Models
Many of the dispersion models developed by or accepted for use by the U.S. Environmental Protection Agency (EPA) are accepted for use in many other countries as well. Those EPA models are grouped below into four categories.
Preferred and recommended models
- AERMOD - A steady-state plume model that incorporates air dispersion based on planetary boundary layer turbulence structure and scaling concepts, including treatment of both surface and elevated sources, and both simple and complex terrain.
- CALPUFF - A non-steady-state puff dispersion model that simulates the effects of time- and space-varying meteorological conditions on pollution transport, transformation, and removal. CALPUFF can be applied for long-range transport and for complex terrain.
- BLP - A Gaussian plume dispersion model designed to handle unique modeling problems associated with industrial sources where plume rise and downwash effects from stationary line sources are important.
- CALINE3 - A steady-state Gaussian dispersion model designed to determine air pollution concentrations at receptor locations downwind of highways located in relatively uncomplicated terrain.
- CAL3QHC and CAL3QHCR - CAL3QHC is a CALINE3 based model with queuing and hot spot calculations and with a traffic model to calculate delays and queues that occur at signalized intersections. CAL3QHCR is a more refined version based on CAL3QHC that requires local meteorological data.
- CTDMPLUS - A Complex Terrain Dispersion Model Plus algorithms for unstable situations (CTDMPLUS) is a refined point source Gaussian air quality model for use in all stability conditions for complex terrain.
- OCD - Offshore and Coastal Dispersion Model (OCD) is a Gaussian model developed to determine the impact of offshore emissions from point, area or line sources on the air quality of coastal regions. It incorporates overwater plume transport and dispersion as well as changes that occur as the plume crosses the shoreline.
- ADAM - Air Force Dispersion Assessment Model (ADAM) is a modified box and Gaussian dispersion model which incorporates thermodynamics, chemistry, heat transfer, aerosol loading, and dense gas effects.
- ADMS-3 - Atmospheric Dispersion Modeling System (ADMS-3) is an advanced dispersion model developed in England for calculating concentrations of pollutants emitted both continuously from point, line, volume and area sources, or discretely from point sources.
- AFTOX - A Gaussian dispersion model that handles continuous or instantaneous, liquid or gas, elevated or surface releases from point or area sources.
- SLAB - A model for denser-than-air pollutant releases that utilizes the one-dimensional equations of momentum, conservation of mass and energy, and the equation of state. SLAB handles ground-level and elevated jet releases, liquid pool evaporation releases, and releases from volume sources.
- DEGADIS - Dense Gas Dispersion (DEGADIS) is a model that simulates the dispersion at ground level of area source clouds of denser-than-air gases or aerosols released with zero momentum into the atmosphere over flat, level terrain.
- HGSYSTEM - A collection of computer programs designed to predict the source-term and subsequent dispersion of accidental chemical releases with an emphasis on dense gas behavior.
- HOTMAC and RAPTAD - HOTMAC is a model for weather forecasting used in conjunction with RAPTAD which is a puff model for pollutant transport and dispersion. These models are used for complex terrain, coastal regions, urban areas, and around buildings where other models fail.
- HYROAD - The HYbrid ROADway Model (HYROAD) integrates three individual modules simulating the pollutant emissions from vehicular traffic and the dispersion of those emissions. The dispersion module is a puff model that determines concentrations of carbon monoxide (CO) or other gaseous pollutants and particulate matter (PM) from vehicle emissions at receptors within 500 meters of the roadway intersections.
- ISC3 - A Gaussian model used to assess pollutant concentrations from a wide variety of sources associated with an industrial complex. This model accounts for: settling and dry deposition of particles; downwash; point, area, line, and volume sources; plume rise as a function of downwind distance; separation of point sources; and limited terrain adjustment. ISC3 operates in both long-term and short-term modes.
- OBODM - A model for evaluating the air quality impacts of the open burning and detonation (OB/OD) of obsolete munitions and solid propellants. It uses dispersion and deposition algorithms taken from existing models for instantaneous and quasi-continuous sources to predict the transport and dispersion of pollutants released by the open burning and detonation operations.
- PLUVUEII - A model that estimates atmospheric visibility degradation and atmospheric discoloration caused by plumes resulting from the emissions of particles, nitrogen oxides, and sulfur oxides. The model predicts the transport, dispersion, chemical reactions, optical effects and surface deposition of such emissions from a single point or area source.
- SCIPUFF - A puff dispersion model that uses a collection of Gaussian puffs to predict three-dimensional, time-dependent pollutant concentrations. In addition to the average concentration value, SCIPUFF predicts the statistical variance in the concentrations resulting from the random fluctuations of the wind.
- SDM - Shoreline Dispersion Model (SDM) is a Gaussian dispersion model used to determine ground-level concentrations from tall stationary point source emissions near a shoreline.
These are models that are often used before applying a refined air quality model to determine if refined modeling is needed.
- AERSCREEN - The screening version of AERMOD. It produces estimates of concentrations without the need for meteorological data that are equal to or greater than the estimates produced by AERMOD with a full set of meteorological data.
- CTSCREEN - The screening version of CTDMPLUS.
- SCREEN3 - The screening version of ISC3.
- TSCREEN - Toxics Screening Model (TSCREEN) is a Gaussian model for screening toxic air pollutant emissions and their subsequent dispersion from possible releases at superfund sites. It contains 3 modules: SCREEN3, PUFF, and RVD (Relief Valve Discharge).
- VALLEY - A screening, complex terrain, Gaussian dispersion model for estimating 24-hour or annual concentrations resulting from up to 50 point and area emission sources.
- COMPLEX1 - A multiple point source screening model with terrain adjustment that uses the plume impaction algorithm of the VALLEY model.
- RTDM3.2 - Rough Terrain Diffusion Model (RTDM3.2) is a Gaussian model for estimating ground-level concentrations of one or more co-located point sources in rough (or flat) terrain.
- VISCREEN - A model that calculates the impact of specified emissions for specific transport and dispersion conditions.
Photochemical air quality models have become widely utilized tools for assessing the effectiveness of control strategies adopted by regulatory agencies. These models are large-scale air quality models that simulate the changes of pollutant concentrations in the atmosphere by characterizing the chemical and physical processes in the atmosphere. These models are applied at multiple geographical scales ranging from local and regional to national and global.
- Models-3/CMAQ - The latest version of the Community Multi-scale Air Quality (CMAQ) model has state-of-the-science capabilities for conducting urban to regional scale simulations of multiple air quality issues, including tropospheric ozone, fine particles, toxics, acid deposition, and visibility degradation.
- CAMx - The Comprehensive Air quality Model with extensions (CAMx) simulates air quality over many geographic scales. It handles a variety of inert and chemically active pollutants, including ozone, particulate matter, inorganic and organic PM2.5/PM10, and mercury and other toxics.
- REMSAD - The Regional Modeling System for Aerosols and Deposition (REMSAD) calculates the concentrations of both inert and chemically reactive pollutants by simulating the atmospheric processes that affect pollutant concentrations over regional scales. It includes processes relevant to regional haze, particulate matter and other airborne pollutants, including soluble acidic components and mercury.
- UAM-V - The Urban Airshed Model was a pioneering effort in photochemical air quality modeling in the early 1970s and has been used widely for air quality studies focusing on ozone.