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.

US Environmental Protection Agency models Many of the dispersion models developed by or accepted for use by the U.S. Environmental Protection Agency (U.S. 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 – An atmospheric dispersion model based on atmospheric boundary layer turbulence structure and scaling concepts, including treatment of multiple ground-level and elevated point, area and volume sources. It handles flat or complex, rural or urban terrain and includes algorithms for building effects and plume penetration of inversions aloft. It uses Gaussian dispersion for stable atmospheric conditions (i.e., low turbulence) and non-Gaussian dispersion for unstable conditions (high turbulence). Algorithms for plume depletion by wet and dry deposition are also included in the model. This model was in development for approximately 14 years before being officially accepted by the U.S. EPA.

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 modelling 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 pollution concentrations at receptor locations downwind of highways located in relatively uncomplicated terrain. CAL3QHC and CAL3QHCR – CAL3QHC is a CALINE3 based model with queuing calculations and 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 (CTDM) plus algorithms for unstable situations (i.e., highly turbulent atmospheric conditions). It is a refined point source Gaussian air quality model for use in all stability conditions (i.e., all conditions of atmospheric turbulence) 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.

Alternative models 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 5 – Atmospheric Dispersion Modelling System (ADMS 5) is an advanced dispersion model developed in the United Kingdom 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 puff, liquid or gas, elevated or surface releases from point or area 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 developed by Shell Research Ltd. and 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 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. PANACHE – Fluidyn-PANACHE is an Eulerian (and Lagrangian for particulate matter), 3-dimensional finite volume fluid mechanics model designed to simulate continuous and short-term pollutant dispersion in the atmosphere, in simple or complex terrain. 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. SLAB – A model for denser-than-air gaseous plume releases that utilizes the one-dimensional equations of momentum, conservation of mass and energy, and the equation of state. SLAB handles point source ground-level releases, elevated jet releases, releases from volume sources and releases from the evaporation of volatile liquid spill pools.

Screening models

These are models that are often used before applying a refined air quality model to determine if refined modelling 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. The U.S. EPA released version 11060 of AERSCREEN on 11 March 2010 with a subsequent update, version 11076, on 17 March 2010. The U.S. EPA published the "Clarification memorandum on AERSCREEN as the recommended screening model" on 11 April 2011.Clarification memorandum on AERSCREEN as the recommended screening model 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 models

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 modelling in the early 1970s and has been used widely for air quality studies focusing on ozone.

Other models developed in the United States

CHARM – A model capable of simulating dispersion of toxics and particles. It can calculate impacts of thermal radiation from fires, overpressures from mechanical failures and explosions, and nuclear radiation from radionuclide releases. CHARM is capable of handling effects of complex terrain and buildings. A Lagrangian puff screening version and Eulerian full-function version are available. More information is available here. HYSPLIT – Hybrid Single Particle Lagrangian Integrated Trajectory Model. Developed at NOAA's Air Resources Laboratory. The HYSPLIT model is a complete system for computing simple air parcel trajectories to complex dispersion and deposition simulations. More information about this model can be found at PUFF-PLUME – A Gaussian chemical/radionuclide dispersion model that includes wet and dry deposition, real-time input of meteorological observations and forecasts, dose estimates from inhalation and gamma shine, and puff or plume dispersion modes. It is the primary model for emergency response use for atmospheric releases of radioactive materials at the Savannah River Site of the United States Department of Energy. It was first developed by the Pacific Northwest National Laboratory (PNNL) in the 1970s. Puff model – Puff is a volcanic ash tracking model developed at the University of Alaska Fairbanks. It requires NWP wind field data on a geographic grid covering the area over which ash may be dispersed. Representative ash particles are initiated at the volcano's location and then allowed to advect, diffuse, and settle within the atmosphere. The location of the particles at any time after the eruption can be viewed using the post-processing software included with the model. Output data is in netCDF format and can also be viewed with a variety of software. More information on the model is available here.

Models developed in the United Kingdom