CMAQ Anniversary

EM – January 2024: This month, EM looks back on the storied history of the U.S. Environmental Protection Agency's Community Multiscale Air Quality (CMAQ) modeling system for studying air pollution from global to local scales as it turns 25.
by Tanya L. Spero and Golam Sarwar

The Community Multiscale Air Quality (CMAQ) modeling system ( was initially released to the public on June 30, 1998. CMAQ was built as a third-generation air quality modeling system, meant to replace a collection of air pollution modeling tools that were tailored for specific pollutants and applications. Instead, CMAQ would simultaneously and holistically simulate the fate and transport of multiple species within the same model and evolution of atmospheric conditions while leveraging new computing technologies. The new system would provide a consistent platform for both research and applications. It took nearly seven years to design, build, test, document, and launch the initial release of CMAQ.

Today, CMAQ is a powerful computational tool that is used to translate fundamental atmospheric science principles to air quality policy scenarios. CMAQ combines current knowledge in atmospheric science and air quality modeling, advanced computing techniques, and an open-source framework to deliver fast, technically sound estimates of airborne ozone, particulates, toxics, and atmospheric acid and nutrient deposition—that is, concurrent simulation of species that are regulated under the U.S. Clean Air Act. EPA and states nationwide have used CMAQ to support air quality management.

Over the past quarter century, CMAQ has been continually updated to incorporate advances in scientific knowledge and computing power to characterize air quality more accurately and efficiently to protect human health and the environment. CMAQ boasts a community of thousands of users across six continents who use the modeling system for air quality management, forecasting, and research. CMAQ has been the backbone of the National Air Quality Forecasting System ( at the National Weather Service since 2004, and it was featured in the inaugural Air Quality Chapter in the National Climate Assessment ( in 2018. CMAQ source code is now freely available via GitHub (, which broadens the accessibility of the model.

The concept of “community” has been integral to the success of the modeling system. Although the routine development and evaluation of CMAQ occurs at EPA, the community of CMAQ users has contributed scientific ideas and novelties that have advanced the modeling system over the years. In addition, the worldwide userbase for CMAQ has tested the limits of the system, identified areas for improvement, and increased the overall robustness of the science and utility of the system. Furthermore, the interactions within the CMAQ community have facilitated its expansion from simulating week-long episodes of poor air quality in a region of the United States to simulating full annual, decadal, and multi-decadal evolutions of air quality throughout the Northern Hemisphere (with critical supporting emissions estimates) to scenarios of the impacts of climate change on air quality and human health, and much more!

As federal resources for science have gradually declined in the United States, the reliance on collaboration—or community—has become increasingly important, not just for sustaining the CMAQ modeling system. In this issue, we highlight the value of modeling communities to strengthen atmospheric science, using the CMAQ community as a microcosm to explore this concept. We contend that a modeling system as complex as CMAQ could not exist in today's world without a community to nurture it.

Continue reading the full January 2024 issue of EM.


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