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Support Center for Regulatory Atmospheric Modeling (SCRAM)

Photochemical Air Quality Modeling

Photochemical air quality models have become widely recognized and routinely utilized tools for regulatory analysis and attainment demonstrations by assessing the effectiveness of control strategies. These photochemical models are large-scale air quality models that simulate the changes of pollutant concentrations in the atmosphere using a set of mathematical equations characterizing the chemical and physical processes in the atmosphere. These models are applied at multiple spatial scales from local, regional, national, and global.

There are two types of photochemical air quality 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. This site provides links to several photochemical air quality models as follows:

Community Multiscale Air Quality (CMAQ) – The CMAQ modeling system includes state-of-the-science capabilities for conducting urban-to-regional-to-hemispheric scale simulations of multiple air quality issues, including tropospheric ozone, fine particles, toxics, acid deposition, and visibility degradation. CMAQ is distributed as a “git” repository on EPA’s GitHub site ( User support is provided by the Community Modeling and Analysis System (CMAS) Center at UNC-Chapel Hill.

Comprehensive Air quality Model with extensions (CAMx) Exit- 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 PM2.5/PM10, and mercury and other toxics. CAMx also has plume-in-grid and source apportionment capabilities.

Regional Modeling System for Aerosols and Deposition (REMSAD) Exit- REMSAD was designed to calculate the concentrations of both inert and chemically reactive pollutants by simulating the physical and chemical processes in the atmosphere that affect pollutant concentrations over regional scales. It includes those processes relevant to regional haze, particulate matter and other airborne pollutants, including soluble acidic components and mercury.

Urban Airshed Model Variable Grid (UAM-V ®) Exit- The UAM-V Photochemical Modeling System 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. It is a three-dimensional photochemical grid model designed to calculate the concentrations of both inert and chemically reactive pollutants by simulating the physical and chemical processes in the atmosphere that affect pollutant concentrations. This model is typically applied to model air quality "episodes" - periods during which adverse meteorological conditions result in elevated ozone pollutant concentrations.

Over the last two decades, EPA has devoted significant efforts to developing photochemical air quality models for the assessment of air pollution issues and evaluation of control strategies. The EPA's Air Quality Modeling Group has used photochemical models as part of its modeling analyses to support policy and regulatory decisions in OAR for which information can be found at Modeling Applications & Tools and provides guidance on the use of these models for attainment demonstrations available at Modeling Guidance & Support. Additional information about photochemical models can be found at Related Links.