The Community Multiscale Air Quality (CMAQ) model is an active open-source development project of the U.S. EPA that consists of a suite of programs for conducting air quality model simulations. CMAQ combines current knowledge in atmospheric science and air quality modeling, multi-processor computing techniques, and an open-source framework to deliver fast, technically sound estimates of ozone, particulates, toxics and acid deposition. For further information please visit the EPA website for the CMAQ system: www.epa.gov/cmaq CMAQv5.3.1 System Updates CMAQ model version 5.3.1 is a minor update to CMAQv5.3 that includes multiple bug fixes to both CMAQ and MCIP (released as MCIP version 5.1), as well as a feature addition to the Detailed Emission Scaling Isolation and Diagnostic (DESID) module to allow for the definition of chemical, region and stream families. CMAQv5.3 System Updates Incorporated updated instrumented models: Sulfur Tacking, Integrated Source Apportionment (ISAM) consistent with science process updates. Improvements in computational efficiency of these instrumented techniques have led to substantially faster run times to support their practical applications. A new emissions interface allows for substantial flexibility in the way emissions are mapped, scaled, and checked for quality and can greatly simplify the task of assessing air quality improvements resulting from emission changes. Incorporated updates (new data sources, updated vertical coordinate system) to CMAQ and the Meteorology-Chemistry Interface Processor (MCIPv5.0) to increase scientific consistency between the atmospheric dynamics and chemistry calculations. New Features and Processes in v5.3 Updated marine chemistry to represent impacts of (1) halogen chemistry on ozone depletion and sulfate formation and (2) dimethyl sulfide on aerosol sulfate. Expanded the representation of secondary pollutant formation in clouds (AQCHEM-KMT2). Updated aerosol module (AERO7) that explicitly tracks 84 particulate species . Updated pathways for secondary organic aerosol formation from biogenic VOCs . Harmonized treatment of water uptake to aerosol organic phase to improve representation of aerosol chemistry, mixing, state and optical properties. Improved the representation of bi-directional exchange of ammonia at the surface Improved representation of O3 dry deposition to snow. Incorporated a new deposition module – the Surface Tiled Aerosol and Gaseous Exchange (STAGE) model to estimate land-use specific deposition.

For up-to-date documentation, source code, and sample run scripts, please clone or download the CMAQ git repository available through GitHub: https://github.com/USEPA/CMAQ