CMAQ Overview

The Community Multi-scale Air Quality (CMAQ) modeling system has been designed to approach air quality as a whole by including state-of-the-science capabilities for modeling multiple air quality issues, including tropospheric ozone, fine particles, toxics, acid deposition, and visibility degradation. In this way, the development of CMAQ involves the scientific expertise from each of these areas and combines the capabilities to enable a community modeling practice. CMAQ was also designed to have multi-scale capabilities so that separate models were not needed for urban and regional scale air quality modeling.

The target grid resolutions and domain sizes for CMAQ range spatially and temporally over several orders of magnitude. With the temporal flexibility of the model, simulations can be performed to evaluate longer term (annual to multi-year) pollutant climatologies as well as short term (weeks to months) transport from localized sources. With the model's ability to handle a large range of spatial scales, CMAQ can be used for urban and regional scale model simulations. By making CMAQ a modeling system that addresses multiple pollutants and different spatial scales, CMAQ has a "one atmosphere" perspective that combines the efforts of the scientific community. Improvements will be made to the CMAQ modeling system as the scientific community further develops the state-of-the-science.

To implement multi-scale capabilities in CMAQ, several issues, such as scalable atmospheric dynamics and generalized coordinates, that depend on the desired model resolution are addressed. Meteorological models may assume hydrostatic conditions for large regional scales, where the atmosphere is assumed to have a balance of vertical pressure and gravitational forces with no net vertical acceleration on larger scales. However, on smaller scales such as urban scales, this assumption cannot be made. A set of governing equations for compressible non-hydrostatic atmospheres is available to better resolve atmospheric dynamics at smaller scales. These non-hydrostatic equations are more appropriate for finer regional scale and urban scale meteorology. Because CMAQ is designed to handle scale dependent meteorological formulations and a large amount of flexibility, CMAQ's governing equations are expressed in a generalized coordinate system. This approach ensures consistency between CMAQ and the meteorological modeling system. The generalized coordinate system determines the necessary grid and coordinate transformations, and it can accommodate various vertical coordinates and map projections.

The CMAQ modeling system simulates various chemical and physical processes that are thought to be important for understanding atmospheric trace gas transformations and distributions. The CMAQ modeling system contains three types of modeling components: a meteorological modeling system Exit EPA Disclaimer for the description of atmospheric states and motions, emission models for man-made and natural emissions that are injected into the atmosphere, and a chemistry-transport modeling system for simulation of the chemical transformation and fate. The emissions model and CMAQ science codes are available from the Community Modeling and Analysis System (CMAS) Exit EPA Disclaimer center.

The CMAQ modeling system consists of several processors and the chemical-transport model:

• Meteorology-chemistry interface processor (MCIP)
• Photolysis rate processor (JPROC)
• Initial conditions processor (ICON)
• Boundary conditions processor (BCON)
• CMAQ chemical-transport model (CCTM)

The CMAQ system was designed to have a flexible community modeling structure based on modular components. The CCTM includes the following major processes:

• Horizontal advection
• Vertical advection
• Mass conservation adjustments for advection processes
• Horizontal diffusion
• Vertical diffusion
• Emissions injection
• Deposition
• Gas-phase chemical reactions
• Aqueous-phase reactions and cloud mixing
• Aerosol dynamics, thermodynamics, and chemistry
• Plume chemistry effects
• Photolytic rate computation
• Process analysis