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Exposure Research

EPA Scientists Advance Clean Air Research by Studying the Visible and the Invisible

Posted: November 30, 2009


Clouds, by definition are suspended, dense masses of water droplets, ice crystals, and solid particles that float in the atmosphere. As nature's vacuum cleaner, clouds also pull ground-level pollutants upward, and send rainfall downward to refresh ground-level air that people and ecosystems need for their continued survival.

Scientists at the National Exposure Research Laboratory (NERL) in the EPA's Office of Research and Development know that clouds also promote other transformations that naturally-occurring and manmade chemicals undergo in the atmosphere. Clouds and other characteristics of the atmosphere, such as the energy contained in sunlight, are of great interest to these scientists because they impact air quality, air pollution, and ultimately the health of people and ecosystems.

As nature's vacuum cleaner, clouds pull ground-level pollutants upward, and send rainfall downward to refresh ground-level air that people and ecosystems need for their continued survival.

In the 1970s, the prevailing hypothesis was that "the solution to pollution is dilution." This well-established perception shaped some of the earliest air pollution control technologies, such as industrial stacks that towered skyward to disperse pollution high into the atmosphere, diluting it, and thereby minimizing local air pollution problems.

In the late 1970s when scientists and the public learned about fish dying in national parks miles away from any major industry, the term "acid rain," or more precisely acid deposition, came to the forefront. The dilution solution became unsupportable.

Acid deposition was clear evidence that chemicals in the atmosphere were mixing, changing, and moving long distances before depositing back on Earth, causing harmful effects to wildlife and ecosystems, and possibly impacting human health.

"The acid deposition problem eclipsed other air quality problems at the time, including ozone and particle pollution," said Ken Schere, senior science advisor in NERL's Atmospheric Modeling and Analysis Division.

Modeling the Atmosphere

Schere began his relationship with EPA in 1975 as an employee of the National Oceanic and Atmospheric Administration (NOAA), when he and other modelers, meteorologists, and scientists from NOAA were assigned to EPA.

EPA was using computers to produce models of the atmosphere in a pollutant-by-pollutant fashion, and could predict the concentrations of many, single pollutants. For example, EPA had a regional acid deposition model. There was also a regional model for predicting ozone; and, a different regional model for particle pollution. It was a time when air quality modeling, as a whole, was somewhat fragmented.

The "one-atmosphere" reality, with all of its complexities, was yet-to-be incorporated into EPA's suite of air quality models. Doing so required developing tools and techniques that could reliably and accurately predict the impact of current or proposed air quality regulations at a national scale.

In 1994 Schere and other EPA meteorologists and modelers began work on a "super model" framework that would accept data from other related models, apply, digest, and interpret it to help scientists and policy makers at EPA, determine how changes in air emissions, instigated by proposed or existing regulations, would impact air quality and benefit public health.

This super model framework would need to harness the collective knowledge from numerous models throughout EPA and the research and regulatory community into a single modeling system. The framework would have to be efficient and consistent in calculating mathematical equations at millions of data points to meet the challenge.

The Community Multiscale Air Quality (CMAQ) modeling system — the "super model" framework — was first released for public use in 1998 after approximately four years of concentrated development effort by researchers in EPA, NOAA, and the academic and private sectors.

CMAQ was a boon to regulators at the federal, state, and local levels, and for the air-quality modeling community. It provided access to and improved utility of the best, available models that, when "talking" together, began to describe the entire process that natural and manmade chemicals go through as they move, transform and disperse in the atmosphere, and impact living things on land and in water.

Supporting CMAQ's World-wide Community of Users

After the initial release of CMAQ it became clear that long-term support for CMAQ's community of users, developers, and partners worldwide would be crucial to sustaining and improving it. Here is a map of the worldwide distribution of registered CMAS users (pdf, 2pp., 204KB). To support the CMAQ community, EPA entered into a cooperative research agreement with the Institute for the Environment at University of North Carolina at Chapel Hill, to start the Community Modeling and Analysis System exit epa (CMAS) Center.

"CMAS has contributed significantly toward the refinement and application of environmental models for research and regulatory purposes. It has been very effective in helping facilitate and leverage complementary talents and resources toward a common goal — cleaner air."

"CMAS has contributed significantly toward the refinement and application of environmental models for research and regulatory purposes," said Dr. S.T. Rao, director of NERL's Atmospheric Modeling and Analysis Division. "It has been very effective in helping facilitate and leverage complementary talents and resources toward a common goal — cleaner air."

Between 1998 and 2006, as CMAQ matured, EPA and the user community developed, and CMAS released new versions of CMAQ each year. The latest CMAQ release, Version 4.7, was delivered in late 2008.

Dr. Rohit Mathur, who leads EPA's current CMAQ model development team, explained the latest version of CMAQ as a numerical laboratory that houses state-of-the-science physical, numerical, and computational modeling capabilities that, more effectively than ever, simulate complex interactions occurring in today's atmosphere.

As EPA's premier air quality modeling system, CMAQ was designed and is dedicated to improving the agency's ability to evaluate the effectiveness of regulation and management practices in protecting human and ecological health.

CMAQ continues to evolve as a user-supported, multi-pollutant modeling system that is widely used by regulators and researchers alike to study and model ozone, particle pollution, visibility degradation, acid deposition, and air toxics.

The Future of CMAQ

What's next for CMAQ? The EPA team has a two-pronged approach they are planning to include with the next model release in 2011.

"First, we are working toward improving CMAQ's capability to model air quality at a smaller [finer] scale for use in urban and local applications," Mathur said. "The ability to model nearby air quality conditions — in areas as small as a few kilometers — helps equip air quality managers and researchers with more understanding of influences that may be impacting local air quality and related compliance with national ambient air quality standards."

On the other side of the scale, the team is working toward "enhancements to CMAQ that will provide users with models that describe the processes occurring in the atmosphere across hemispheric scales," Mathur said. Their current focus is on the northern hemisphere.

In 2009, EPA also reaffirmed its commitment to air quality modeling and research when the agency awarded a new seven-year contract to the CMAS Center for continued support of the CMAQ modeling system.

Something is working: recent reports indicate that U.S. air quality is improving. Comparisons between air quality data for 2007 and 1990 show that ozone has decreased by nine percent, lead by 80 percent, carbon monoxide by 67 percent, sulfur dioxide by 54 percent, and particulate matter (PM10) by 28 percent.

While clouds can spark the imagination and free the spirit, they also complicate air quality and air pollution. Nonetheless, scientists in EPA's laboratories study them, and many other factors associated with clean air research, because EPA is committed to protecting and improving human and ecosystem health.

For more information about air quality forecasts, see: "Airing on the Side of Protecting Human and Ecosystem Health."

For more information on CMAQ, visit:

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