Models We Use
Our extensive experience with a wide range of air dispersion models gives us deep insight into each model's strengths and limitations. We understand that not all models fit every situation, and in some cases, combining multiple models offers the best approach. This expertise enables us to provide tailored recommendations, helping clients choose the most suitable air quality model for their specific project needs.
Select a tab below to find out more about each model type.
CALPUFF
Is an advanced non-steady-state meteorological and Lagrangian Puff air quality modeling system. CALPUFF was introduced into the Guideline on Air Quality Models (Appendix W to 40 CFR Part 51) in 2003 to assess impacts for distances beyond 50-km and complex winds on a case-by-case basis and was adopted by the Federal Land Management community to assess visibility and deposition impacts. EPA removed the model from the Guideline as a preferred model in 2015, but it remains the recommended model of the Federal Land Management community for Class I air quality analyses required under PSD.
AERMOD
The American Meteorological Society/Environmental Protection Agency Regulatory Model Improvement Committee (AERMIC) was formed to introduce state-of-the-art modeling concepts into the EPA's air quality models. Through AERMIC, the Gaussian model AERMOD, was introduced in 2005. AERMOD 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. The workhorse model of EPA, AERMOD has been continuously since its inception, incorporating features from other regulatorily approved models such as ISC3, BLP, and CAL3QHC.
CTDMPLUS
The Complex Terrain Dispersion Model Plus Algorithms for Unstable Situations (CTDMPLUS) is a refined air quality model for use in all stability conditions to assess pollutant impacts in complex terrain. CTDMPLUS uses the critical dividing streamline height (H c ) concept to separate flow in the vicinity of a hill, flow around a hill, and flow above the H c. CTSCREEN is the screening version of CTDMPLUS.DEGADIS
The Dense Gas Dispersion Model models the transport of toxic chemical releases into the atmosphere. It was originally developed for the U.S. Coast Guard and the Gas Research Institute primarily for simulation of the dispersion of cryogenic flammable gases. The DEGADIS model accounts for the three regimes of heavy gas dispersion processes and can be used to simulate instantaneous, steady state, and transient releases.
TSCREEN
Is a Gaussian model used for estimating toxic air pollutant concentrations from various release scenarios, particularly at Superfund sites. It implements procedures to analyze toxic emissions and their dispersion, incorporating models like SCREEN3, PUFF, and RVD (Relief Valve Discharge). The model is designed to assist in consequence assessments and can simulate release scenarios to calculate dispersion characteristics and pollutant concentrations.
ALOHA
Areal Locations of Hazardous Atmospheres model is used to model chemical releases for emergency responders. It can estimate how a toxic cloud might disperse after a chemical release, fire, or explosion. ALOHA is part of the CAMEO software suite developed jointly by the National Oceanic and Atmospheric Administration (NOAA) and the US Environmental Protection Agency.
SLAB
Is used to simulate the atmospheric dispersion of a denser-than-air vapor release. The types of releases treated by the model include an evaporating pool (ground level area source), an elevated horizontal jet, and an instantaneous volume source. Subsequent dispersion of the released vapor is modeled as either a steady state plume, a transient puff, or a combination of the two depending upon the duration of the release.
In the case of a finite duration evaporating pool or horizontal jet release, cloud dispersion is initially described using the plume mode while the source is active. Once the source is shut off, subsequent dispersion is calculated using the puff mode. For an instantaneous release, the puff dispersion mode is used for the entire calculation.Comprehensive Air Quality Model with Extensions (CAMx)
Is a eulerian chemical transport model that simulates air quality over many geographic scales. The model treats a wide variety of inert and chemically active pollutants, including ozone, particulate matter, inorganic and organic PM2.5/PM10, and mercury and other toxics. CAMx also has plume-in-grid and source apportionment capabilities.
SCICHEM
SCICHEM 3.1 is a reactive puff model that can be used to calculate single or multi-source impacts of emissions at downwind locations. The model can be used for both short-range calculations or long-range calculations for primary and secondary pollutant impacts. For long-range applications or near-field PM2.5/ozone, the full chemistry option can be used to calculate downwind ozone and PM2.5 concentrations. The full chemistry modules include a gas-phase chemistry module based on the latest version of the Carbon Bond mechanism (CB6r2), while the aerosol and aqueous-phase chemistry modules are based on those found in the Community Multiscale Air Quality (CMAQ) Model version 4.7.1.HYSPLIT
The Hybrid-Single Particle Lagrangian Integrated Trajectory model is a complete system for computing simple air parcel trajectories, as well as complex transport, dispersion, chemical transformation, and deposition simulations. The model calculation method is a hybrid between the Lagrangian approach, using a moving frame of reference for the advection and diffusion calculations as the trajectories or air parcels move from their initial location, and the Eulerian methodology, which uses a fixed three-dimensional grid as a frame of reference to compute pollutant air concentrations. The model was developed and is maintained by the National Oceanic and Atmospheric Administration.
FLEXPART
The FLEXible PARTicle dispersion model is a Lagrangian Particle Dispersion Model (LPDM) used to simulate air parcel trajectories. It can be run in either forward or backward mode. The forward mode is typically used to determine the downwind concentration or mixing ratio of pollutants. The backward mode can be used to estimate footprint areas, to determine the origin of observed emissions. The Comprehensive Nuclear Test-Ban Treaty Organization (CTBTO) runs FLEXPART in backward mode based on operational deterministic European Centre for Medium-Range Weather Forecasts-Integrated Forecasting System (ECMWF-IFS) and on National Centers for Environmental Prediction-Global Forecast System (NCEP-GFS) input data.
SCIPUFF
The Second-Order Closure Integrated Puff (SCIPUFF) model is a Lagrangian puff dispersion model using Gaussian puffs to represent an arbitrary, three-dimensional, time-dependent concentration. The diffusion parameterization is based on turbulence closure theory, which gives a prediction of the dispersion rate in terms of the measurable turbulent velocity statistics of the wind field. This generalized approach provides a unified parameterization for modeling dispersion at all ranges from laboratory to continental scales.ISC3
The Industrial Source Complex (ISC3) is a steady-state Gaussian plume dispersion model developed by the U.S. Environmental Protection Agency (EPA) to estimate ground-level concentrations of air pollutants emitted from various industrial sources. It is utilized for regulatory assessments, including compliance demonstrations and air quality planning.
However, the EPA has since adopted AERMOD as the preferred model for air quality dispersion modeling. While ISC3 is no longer the preferred model, it remains available for use in regulatory applications with appropriate justification.
BLP
The BLP (Buoyant Line and Point source) model is an air dispersion model developed by the U.S. Environmental Protection Agency (EPA) specifically to estimate the impact of buoyant plumes from stationary line or point sources, such as those commonly found in industrial settings like aluminum reduction plants or smelters.
The BLP model's algorithms have been incorporated into AERMOD, allowing users to model buoyant line sources using the "BOUYLINE" source type and the "BLAVGVAL" keyword for composite line sources.
VALLEY
The VALLEY model is a Gaussian plume air dispersion model developed to estimate pollutant concentrations in complex terrain, particularly in valley settings where terrain features significantly influence airflow and dispersion patterns.
SCREEN3
Is a simplified, user-friendly air dispersion model developed by the U.S. Environmental Protection Agency (EPA) for evaluating the short-term impacts of air pollution from point sources, such as industrial facilities or power plants. It is the screening version of the ISC3 model.
The model estimates the plume centerline concentration of pollutants at various distances downwind of a single source, based on factors like meteorological conditions, terrain, and stack parameters (e.g., height, diameter, and exhaust velocity). SCREEN3 was typically used for preliminary assessments to quickly determine whether a detailed modeling study is necessary. SCREEN3 was replaced with the AERSCREEN model, the screening version of the EPA AERMOD model.
COMPLEX1
Is a multiple point source screening technique with terrain adjustment that incorporates the plume impaction algorithm of the VALLEY model. The COMPLEX plume half-height adjustment algorithm was incorporated into ISC3 but remains the preferred complex terrain algorithm choice within CALPUFF by the Federal Land
Management community.