Photochemical Modeling

Photochemical models are typically used in regulatory or policy assessments to simulate the impacts from all sources by estimating pollutant concentrations and deposition of both inert and chemically reactive pollutants.  Photochemical models have become widely recognized and routinely utilized tools for regulatory analysis and attainment demonstrations (e.g., ozone and PM2.5) by assessing the effectiveness of control strategies and are applied at multiple spatial scales (e.g., local, regional, national, and global).

There are two types of photochemical models commonly used in air quality assessments: the Lagrangian trajectory model that employs a moving frame of reference, and the Eulerian grid model that uses a fixed coordinate system with respect to the ground. Earlier generation modeling efforts often adopted the Lagrangian approach to simulate the pollutants formation because of its computational simplicity. The disadvantage of Lagrangian approach, however, is that the physical processes it can describe are somewhat incomplete. Most of the current operational photochemical air quality models have adopted the three-dimensional Eulerian grid modeling mainly because of its ability to better and more fully characterize physical processes in the atmosphere and predict the species concentrations throughout the entire model domain. 

Air Quality Models

Community Multi-scale Air Quality System (CMAQ) - The CMAQ modeling system is used by EPA and is supported by the Community Modeling and Analysis System (CMAS) Center. The CMAQ model includes state-of-the-science capabilities for conducting urban to regional scale simulations of multiple air quality issues, including ozone, fine particles, toxics, acid deposition, and visibility degradation.

Comprehensive Air Quality Model with Extensions (CAMx) - The CAMx model 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 particles, and mercury and other toxics. CAMx also has plume-in-grid and source apportionment capabilities.

Emission Data and Models

The Sparse Matrix Operator Kernel Emissions (SMOKE) Modeling System - SMOKE is primarily an emissions processing system designed to create gridded, speciated, hourly emissions for input into a variety of air quality models such as CMAQ and CAMx. SMOKE supports area, biogenic, mobile (both onroad and nonroad), and point source emissions processing for criteria, particulate, and toxic pollutants.  The model is now under active development at the Institute for the Environment and is partially supported by the Community Modeling and Analysis System (CMAS).

Model of Emissions of Gases and Aerosols from Nature (MEGAN) - MEGAN is a modeling system for estimating the net emission of gases and aerosols from terrestrial ecosystems into the atmosphere. Driving variables include landcover, weather, and atmospheric chemical composition. MEGAN is a global model with a base resolution of ~1 km and is suitable for regional and global models. Current model development and support is through Washington State University.

Biogenic Emissions Inventory System (BEIS) - BEIS estimates volatile organic compound (VOC) emissions from vegetation and nitric oxide (NO) emissions from soils. Because of resource limitations, recent BEIS development has been restricted to versions that are compatible with the Sparse Matrix Operational Kernel Emissions (SMOKE) system.

Motor Vehicle Emission Simulator (MOVES) - EPA's Office of Transportation and Air Quality (OTAQ) developed MOVES to estimate emissions for on-road mobile sources, cover a broad range of pollutants, and allow multiple scale analysis.

MOBILE6 - MOBILE6 is an emission factor model for predicting gram per mile emissions of hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), carbon dioxide (CO2), particulate matter (PM), and toxics from cars, trucks, and motorcycles under various conditions. MOBILE6 has been replaced by MOVES as the EPA approved on-road mobile source emission model.

National Mobile Inventory Model (NMIM) - NMIM was developed by EPA to help develop estimates of current and future emission inventories for on-road motor vehicles and nonroad equipment. NMIM uses current versions of MOBILE6 and NONROAD to calculate emission inventories, based on multiple input scenarios that you enter into the system. It can be used to calculate national, individual state or county inventories. Note: MOBILE6, used in NMIM, has been replaced by MOVES as EPA's official model for estimating emissions from cars, trucks, and motorcycles. Thus, NMIM cannot be used for official modeling of these emissions.

NONROAD Model - NONROAD is primarily used for estimation of air pollution inventories from non-road equipment, including farm and construction machinery, lawn and garden, recreational vehicles, among other categories. NONROAD2008 has been incorporated into MOVES2014 and MOVES2014a.

GEOS–Chem Model - GEOS–Chem is a global 3-D chemical transport model (CTM) for atmospheric composition. It principally uses meteorological input from the Goddard Earth Observing System (GEOS) of the NASA Global Modeling and Assimilation Office, but can use other meteorological inputs as well. It is applied by research groups around the world to a wide range of atmospheric composition problems.  It serves as an outer nest (e.g., boundary conditions) for several regional air quality models and has interfaces developed for that purpose.

Hourly Continuous Emissions Monitoring (CEM) Data in SMOKE-ready Format - This site contains hourly CEM data prepared for SMOKE version 2.3 and later for all available CEM data (under the "Prepackaged Data" tab).

Clearinghouse for Inventories and Emissions Factors (CHIEF) - CHIEF has been designed to support and promote emission modeling activities both internal and external to EPA. Through this site the EPA distributes emissions model input formatted inventories based on the latest versions of its National Emission Inventory databases.  In addition to the emissions data, this site is used to document and distribute the Agency's latest versions of the ancillary files used to support the temporal, spatial, speciation, and projection of these emissions.

Meteorological Models

The Weather Research and Forecasting (WRF) Model - WRF is a next-generation mesoscale numerical weather prediction system designed to serve both operational forecasting and atmospheric research needs (e.g., air quality models).  WRF is suitable for a broad spectrum of applications across scales ranging from meters to thousands of kilometers.  The effort to develop WRF has been a collaborative partnership, principally among the National Center for Atmospheric Research (NCAR), the National Oceanic and Atmospheric Administration (the National Centers for Environmental Prediction (NCEP) and the Forecast Systems Laboratory (FSL), the Air Force Weather Agency (AFWA), the Naval Research Laboratory, the University of Oklahoma, and the Federal Aviation Administration (FAA).

The Fifth-Generation NCAR / Penn State Mesoscale Model (MM5) - MM5 is a limited-area, nonhydrostatic, terrain-following sigma-coordinate model designed to simulate or predict mesoscale atmospheric circulation. The model is supported by several pre- and post-processing programs, which are referred to collectively as the MM5 modeling system. The MM5 modeling system was developed at Penn State and NCAR. 

Modeling Guidance

Guidance on the Use of Models and Other Analyses for Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5, and Regional Haze (Adobe pdf) - Current version of EPA's ozone, PM2.5, and regional haze modeling guidance document.

EPA Support Center for Regulatory Atmospheric Modeling (SCRAM) - This website is maintained by EPA's Air Quality Modeling Group (AQMG).  The AQMG conducts modeling analyses to support policy and regulatory decisions in the Office of Air and Radiation (OAR) and provides leadership and direction on the full range of air quality models and other mathematical simulation techniques used in assessing control strategies and source impacts.  Documentation and guidance for these air quality models can be found on this website, including downloadable computer code, input data, and model processors.

Guideline on Air Quality Models (40 CFR 51, Appendix W) (Adobe pdf) - EPA’s Guideline on Air Quality Models (‘‘Guideline’’) addresses the regulatory application of air quality models for assessing criteria pollutants under the Clean Air Act. The Guideline was revised in January 2017.

The Office of Air Quality Planning and Standards (OAQPS) - The mission of OAQPS is to preserve and improve the quality of our nation's air.  One facet of accomplishing this goal requires that new and existing air pollution sources be modeled for compliance with the National Ambient Air Quality Standards (NAAQS).

Other Documents and Links

Virginia Mercury Study - The 2006 General Assembly of Virginia enacted legislation (HB1055/SB651) that required the Virginia Department of Environmental Quality (VADEQ) to conduct a detailed assessment of mercury deposition in Virginia. The final Virginia Mercury Study Report is now available.

openair Project - analysis tools for air pollution measurement data and modeling output

State Implementation Plans

Regional Planning Organizations

  • SESARM (Metro 4, Inc. and Southeastern States Air Resource Managers, Inc.)
  • MANE-VU (Mid-Atlantic/Northeast Visibility Union)
  • OTC (Ozone Transport Commission)
  • Midwest RPO/LADCO (Midwest RPO and Lake Michigan Air Directors Consortium)
  • CENRAP/CenSARA (Central Regional Air Planning Association)
  • WRAP (Western Regional Air Partnership)


Air Emissions Inventory Training

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Virginia Department of
Environmental Quality
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