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Air Research

Atmospheric Science



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Atmospheric scientists study the atmosphere's physical characteristics, its weather patterns or meteorology, and its chemistry. The best known application of this collective knowledge is forecasting the weather.

Atmospheric science is also important to the study of air pollution. At the U.S. Environmental Protection Agency, the Clean Air Research Program in the Office of Research and Development is leading a research effort to better understand how air pollutants are formed, how they interact with one another, and where they come from and go.

Research in atmospheric science provides information to EPA, states and others to develop strategies to reduce air pollution and to evaluate the effectiveness of various control measures.

Describing or characterizing the often complex composition of outdoor (ambient) air pollution and explaining where various pollutants come from is challenging. The complex and dynamic chemistry is often difficult to capture as pollutants vary during different times of the day and by their location.

Integrated science that includes laboratory and field studies is needed to better understand air pollution. Laboratory studies offer scientists the opportunity to understand the processes associated with the formation and distribution of air pollutants under controlled circumstances. This knowledge is used to develop theory for model development. Field studies are also important in that they provide measurements in real-time that can be used to assess models and develop other scientific tools.

Scientific Objective:

Scientists in the Clean Air Research Program explore every aspect of the "life" of an air pollutant – from formation and transformation with co-pollutants, to how they are transported and come in contact with humans. Specifically, researchers work to answer questions such as:

  • What are the various sources of air pollutants?
  • How do the types of air pollutants form, age, and change in the atmosphere?
  • How are air pollutants transformed and transported in the atmosphere?
  • What chemical processes are important in the formation and transformation of air pollution, especially ozone, particulate matter, and air toxics?
  • How can air pollution best be monitored? What instruments and techniques are needed to monitor pollutants?
  • What tools can be developed to support efforts to reduce and control air pollution? How can computer models be improved to better represent air pollutants and their related processes in the atmosphere?
  • How can the information about pollutant concentrations, fate and transport be included in models to estimate future concentrations?

Research areas include:

  • Developing and evaluating the Community MultiScale Air Quality Model (CMAQ) for a variety of time and space scales for PM, ozone and air toxics.
  • Understanding the chemistry of pollutants, particularly the formation of secondary organic aerosols (SOAs) by learning about their physical properties, and determining their exact chemical composition.
  • Understanding factors and parameters of air pollutants such as concentration, chemical composition, size distribution, and source contribution.
  • Developing and evaluating monitoring methods for implementation of the National Ambient Air Quality Standards.
  • Identifying the various sources of a pollutant and determining the amount each source contributes to the outdoor concentration.

Application and Impact:

The Clean Air Research Program's air quality research provides a strong scientific foundation for protecting the public from adverse impacts of air pollutants.

The methods, models, tools, and techniques are used by EPA to make decisions about air quality standards, provide information for conducting health studies and assist states to develop control plans for minimizing pollutants within their air sheds.

Atmospheric research enables air quality managers and regulators to determine what air pollutants are potentially harmful, what emissions should be targeted for reduction and how they can be controlled in the most cost effective and efficient way.

With the growing evidence of climate change and its expected impacts on air quality, atmospheric science will also provide key information to health effects research. This science will be used to better understand potential health implications of the evolving complexity of ambient air environment.


David Kryak (kryak.david@epa.gov), National Exposure Research Laboratory, EPA's Office of Research and Development, 919-541-1457.

Sherri Hunt (hunt.sherri@epa.gov), National Center for Environmental Research, EPA's Office of Research and Development, 202-343-9644.

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US EPA: Air Quality Models Used to Reduce Pollution and Improve Air Quality (PDF) (2 pp, 73KB, About PDF)


The U.S. Environmental Protection Agency (EPA) sets and enforces national air quality standards to protect public health. Pollutants are generated by many sources from factories and power plants to motor vehicles and wildfires. These pollutants can travel thousands of miles across state and international borders.

Pollutants mixed from various sources in the atmosphere may interact with one another, especially in sunlight, causing the formation of new chemicals. Air pollution can even be exacerbated by the interaction of man-made chemicals with naturally occurring organic emissions from trees, crops, and other vegetation.

To assist states in implementing air pollution standards, scientific tools are needed such as atmospheric computer models that can be used to determine how best to reduce or mitigate pollution and improve air quality. Such state-of-the-art modeling is critical to understanding the impacts of pollutants in today's complex atmosphere.

Air quality models are needed to help risk assessors determine if the actions they take today will result in reduced pollution tomorrow. They are used to protect people from air pollution by forecasting what may happen to the quality of the air under different potential emissions-control strategies and varying weather conditions.

Scientific Objective

The Clean Air Research Program in EPA's Office of Research and Development is advancing air quality models that can simulate the transport and fate (or outcome) of pollutants in the atmosphere. Research is conducted to develop, evaluate, and improve models for use by EPA and states in implementing the air quality standards for ozone and fine particulate matter.

The major focus of model development is to improve EPA's Community Multistage Air Quality (CMAQ) modeling system for use by EPA as well as its research and regulatory partners. CMAQ combines current knowledge in atmospheric science and air quality modeling with multi-processor computing techniques in an open-source framework to deliver fast, technically sound estimates of air pollutants.

This multistage and multipollutant chemistry-transport model (CTM) permits the assessment not only of a target pollutant but also its interactions with other environmental contaminants. It also enables EPA and its partners to test how well emissions-control strategies reduce air pollutants.

CMAQ workgroups are focused in several areas of research to improve the chemical and physical process representations in the model and to develop refined versions of the model on a regular basis for particular applications. Some of the research focus areas are:

  • Atmospheric chemistry and aerosols
  • Two-way, meteorology-chemical transport modeling
  • Weather research and forecast model
  • Multipollutant modeling
  • Air quality and climate change interactions
  • Modeling resolution to depict local community levels

Application and Impact

Air quality models are being used by EPA to help formulate national emissions control programs, while states use air quality models to examine the impacts of local emissions control policies on air pollution. In addition, models are being used to examine the longer-range potential impacts of global climate change and rising levels of Asian industrialization on air quality in the United States.

The CMAQ model, made publicly available through the Community Modeling and Analysis System Center, is used by over 500 groups nationally and internationally to study air pollution issues from both research and air quality management perspectives.

The model has been used effectively by EPA regulators to assess the impacts of potential new nationwide or regional emissions-control rules to improve air quality.

The National Weather Service is using the CMAQ model to produce operational daily forecasts for ozone air quality over the continental United States. The model is also used by states to assess implementation actions to attain the air quality standards set by EPA and to assess the interstate transport of air pollution.

The CMAQ model has been used by scientists to better probe, understand, and simulate chemical and physical interactions in the atmosphere. The complex emissions and chemical interactions of air pollutants from man-made and natural sources have been studied with the model to provide a better understanding on reducible and irreducible pollution levels.

The CMAQ modeling system is also being used to understand the separate and combined impacts of source emissions and weather/climate impacts on air quality over the United States.


Byun, D., and K.L. Schere, 2006: Review of the Governing Equations, Computational Algorithms, and Other Components of the Models-3 Community Multiscale Air Quality (CMAQ) Modeling System. Applied Mechanics Reviews 59:51-77.

Binkowski, F.S. and S.J. Roselle, 2003: Models-3 Community Multiscale Air Quality (CMAQ) model aerosol component. 1. Model description. Journal of Geophysical Research, 108: 4183, doi:10.1029/2001JD001409.

CMAQ model Web site (www.cmaq-model.org)


Rohit Mathur (mathur.rohit@epa.gov), National Exposure Research Laboratory, EPA's Office of Research and Development, 919-541-1483.

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US EPA: Researchers Study Chemical Reactions That Lead to Particle Pollution (PDF) (2 pp, 73KB, About PDF)


Particulate matter (PM) is an air pollutant that is made up of many different chemical compounds. The specific chemicals in the PM are determined by the pollutant source and even geographic location. While ozone, for example, has the same chemical composition or formula no matter where you find it, PM does not.

PM can be composed of many compounds, including:

  • Inorganic compounds such as sulfate, nitrate, and ammonium, and different elements;
  • Organic compounds from the incomplete burning of fuels such as wood, gasoline, and diesel fuel; and
  • Secondary organic aerosols, or SOAs.

SOAs are created when chemicals from different air pollutants mix and react with one another to form new organic compounds. These new compounds eventually attach themselves to preexisting air particles.

Scientists at the U.S. Environmental Protection Agency are studying the chemistry of PM, particularly the formation of secondary organic aerosols to protect public health. The knowledge can be incorporated into models that are used to estimate air quality and, in turn, the amount of PM that people are exposed to. Researchers are working to learn more about the formation of SOAs, describe or characterize their physical properties, and determine their exact chemical composition when possible.

There are substantial scientific and technical obstacles to studying the formation of organic matter by chemical processes in the atmosphere. Some of the chemical reactions themselves are poorly understood. Because such a large number of organic products form, many have not yet been identified.

In addition, there are hundreds of chemicals present from hydrocarbons emitted into the atmosphere. When they react with one another, they produce thousands of different secondary organic aerosols, making it a challenge to determine even a small fraction of these products. Even measuring the total organic mass without regard to chemical identity represents a significant challenge.

Scientific Objective:

The Clean Air Research Program in EPA's Office of Research and Development is conducting atmospheric chemistry research on the small particles known as PM2.5, which are regulated by EPA. The research objective is to understand the major processes that produce SOAs and other important components of PM2.5.

Atmospheric chemistry also provides critical information to support health studies and research to understand the sources of particle pollution.

Key scientific questions being addressed include:

  • What are the main emitted compounds which lead to SOA formation?
  • What fraction of the organic matter in PM comes from natural versus human-made sources?
  • What atmospheric factors such as temperature and season influence SOA formation and to what extent?

Application and Impact:

For more than a decade, the Clean Air Research Program has provided innovative science in the field of atmospheric chemistry to understand the properties and chemical components of many SOAs.

EPA funded research is changing the way scientists understand and model organic particles and the formation of SOA in the atmosphere. By identifying the largest sources contributing to PM2.5 and its precursors, the work enables regulators and air quality managers to create more effective control strategies.

Researchers have also developed instruments and techniques to measure particles and their composition. This leads to an improved understanding of particle sources and formation processes. Health scientists value the new information about particle composition as it guides their own studies.

Atmospheric chemistry research has been used to improve air quality models such as EPA's CMAQ model. One of the model features allows regulators and air quality managers to predict secondary organic aerosol concentrations.

The discoveries are also helping states to develop and implement plans to control particle emissions.


Edney, E. O., T. E. Kleindienst, M. Lewandowski, and J. H. Offenberg. Updated SOA chemical mechanism for the Community Multi-Scale Air Quality Model. (2008) U.S. Environmental Protection Agency, Washington, DC, EPA/600/X-07/025.

Kleindienst, T. E., M. Jaoui, M. Lewandowski, J. H. Offenberg, C. W. Lewis, P. Bhave, and E. O. Edney. (2007) Estimates of the contributions of biogenic and anthropogenic hydrocarbons to secondary organic aerosol at a Southeastern U.S. location. Atmos. Environ., 41, 8288-8300 (doi: 10.1016/j.atmosenv 2007.06.045).

Kroll, J. H. and Seinfeld, J. H. (2008) Chemistry of secondary organic aerosol: Formation and evolution of low-volatility organics in the atmosphere, Atmos. Environ., 42, 3593-3624.

Lewandowski, M., M. Jaoui, J.H. Offenberg, T.E. Kleindienst, E.O. Edney, R.J. Sheesley, J.J. Schauer. (2008) Primary and secondary contributions to ambient PM2.5 in the Midwestern United States. Environ. Sci. Technol. 42, 3303-3309 (doi: 10.1021/es0720412).

Robinson, A.L., N.M. Donahue, M.K. Shrivastava, E.A. Weitkamp, A.M. Sage, A.P. Grieshop, T.E. Lane, J. R. Pierce, S.N. Pandis. (2007) Rethinking Organic Aerosols: Semivolatile Emissions and Photochemical Aging. Science, 315, 1259-1262.


Tad Kleindienst (kleindienst.tad@epa.gov), National Exposure Research Laboratory, EPA's Office of Research and Development, 919- 541-2308.

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USEPA: Research Characterizes Particulate Matter (PM) and Improves Monitoring and Pollution Control (PDF) (2 pp, 74KB, About PDF)


Under the Clean Air Act, states are required to operate and maintain air monitoring networks to determine if they are meeting the federal standards for high-priority air pollutants, known as criteria pollutants. One of these pollutants, particulate matter (PM), offers unique challenges to monitoring and regulation because it is the only criteria pollutant that is not defined by its chemical composition.

Unlike an ozone molecule, which is the same wherever it is measured across the country, PM's characteristics vary greatly. Airborne PM comes in many different sizes, ranging from the size of the smallest viruses to larger than the diameter of a human hair. It also comes in a variety of chemical "flavors," meaning chemical properties vary from one particle to the next.

PM's qualities are also defined by the source of the pollutant such as an automobile or industry, and by the changes of seasons, geographic location, or even local meteorological conditions. As a result, particle pollution is different depending on where you live.

Research to better characterize PM is needed by the U.S. Environmental Protection Agency and others to more accurately identify and define the many different types of PM across the country and to advance the technology to measure, monitor, and control the pollutant.

Scientific Objective

Scientists in the Clean Air Research Program in EPA's Office of Research and Development (ORD) are working to understanding the chemical composition, size, and concentration of PM at different locations across the country. They are also studying the origin of the source of PM which will assist state governments with regulating the pollutant and developing more targeted control measures.

While some pollutants can be studied in a laboratory, researchers studying the many characteristics of PM must conduct research in outdoor settings across the country to assess the many unique and differing factors that contribute to the creation of particle pollution.

Key questions being addressed include:

  • What air pollutants need to be routinely monitored and controlled to protect public health?
  • What methods are needed to ensure sufficient measurement accuracy and precision of air monitors for each regulated pollutant?
  • What testing requirements and measurement criteria are needed to ensure methods used to obtain data from monitors are accurate?

Research is under way to:

  • Develop and evaluate methods that characterize coarse PM, i.e., particles ranging in size from 2.5-10 micrometers (m) in diameter.
  • Develop and evaluate methods for fine PM2.5 and coarse PM10-2.5 to enable PM measurement with high-time resolution of periods of one hour or less.
  • Develop, evaluate, and apply advanced air monitoring methods to identify air pollution sources contributing to non-attainment of the air quality standards in certain areas of the country.

Application and Impact

The Clean Air Research Program is uncovering the secrets of PM by defining the many different properties and qualities of the pollutant. Research has led to improved understanding of the characteristics of PM at different locations across the country and is providing critical regulatory support to implement the national air quality standards for PM.

Health researchers can use the discoveries about the characteristics of PM to study in more detail how the pollutant causes adverse heart and lung effects. In this way, the Clean Air Research Program is providing a source-to-health outcome approach to protecting the public from air pollutants.


  • ORD science supported the adoption of testing specifications and acceptance criteria for PM2.5 and PM10-2.5 monitors.
  • ORD developed a method for use by EPA's regulatory programs to measure coarse PM.
  • Since 2003, ORD science has supported the designation of 19 monitor types that can be used by states to determine if their air is in compliance with federal standards.
  • ORD science led to the designation of new test methods and equivalent methods for PM2.5 and PM10-2.5 monitoring instruments.


Vanderpool, R.; Hanley, T.; Dimmick, F.; Solomon, P.; McElroy, F.; Murdoch, R.; Natarajan, S., Multi-Site Evaluations of Candidate Methodologies for Determining Coarse Particulate Matter (PM10-2.5) Concentrations: August 2005 Updated Report Regarding Second-Generation and New PM10-2.5 Samplers.


Robert Vanderpool (vanderpool.robert@epa.gov), National Exposure Research Laboratory, EPAs Office of Research and Development, 919-541-7877.

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Source Apportionment

US EPA: Research Identifying Sources of Airl Pollutants to Improve Control Stratagies (PDF) (2 pp, 75KB, About PDF)


Humans are exposed to a number of air pollution sources. Major sources include motor vehicle exhaust, both large and small industries, power plants, agricultural and forest fires, and domestic activities (e.g., lawn mowing).  

Numerous health studies have demonstrated an association between air pollution sources and adverse health and environmental effects. Therefore, it is important to know all we can about the sources we are exposed to and the characteristics and concentration of those sources. 

For example, if we know which specific sources contribute to air pollution in a given area, and where people are most likely to be exposed, strategies such as changing fuels or installing air pollution control equipment can be used to reduce the impact of those sources.

Source-related research informs the air pollution control strategies of EPA, state, and local governments for specific sources or categories of sources.

Scientific Objective

Scientists and engineers in EPA’s Clean Air Research Program in the Office of Research and Development (ORD) are identifying and quantifying more clearly the various sources of air pollution to improve EPA’s understanding of the links between sources and health effects. 

The focus of this “source–apportionment” research is on several sources of air pollutants: fine particulate matter (PM2.5), coarse PM, regulated gaseous pollutants, volatile organic compounds, and mercury.  Research is being conducted across the country and in EPA laboratories to evaluate source emissions, determine where they travel, and learn how people and ecosystems are exposed.

Efforts are underway to advance the sampling and analytical methods to measure specific pollutants in air.  The routine measurements collected in areas that are not meeting air quality regulations are typically not sufficient to identify local source contributions. 

Work is also underway to improve modeling tools that can be used to identify and quantify the local, urban, and regional sources.  The modeling tools are publically available and are used by EPA, state and local governments, as well as academic and international environmental researchers.  These models are:

Application and Impact

The science developed by the Clean Air Research Program provides information and tools to EPA, states, and local agencies for developing effective air pollution regulations. 

Recent advances in the understanding of source contributions to air pollution include:

  • Studies in Tampa, Fla., showed a reduction in mercury impact when a major power plant changed its fuel from coal to natural gas.  In addition, Steubenville, Ohio research demonstrated the large impact of regional domestic coal combustion on the deposition of mercury in rain water.

  • A study in Baltimore, Md., found that 30 percent of particulate matter (PM) from motor vehicles infiltrated inside a retirement home.  The indoor level reflected the PM pollution from local coal fire power plants (sulfate). Another study in Research Triangle Park, N.C., found around 50 percent of PM from motor vehicle exhaust was present inside homes and that cooking was a major contributor to PM personal exposure.

  • World Trade Center research results showed differences in air pollution sources during the different stages of the recovery effort after 9/11.

  • In St. Louis, studies at a major steel facility helped quantify the impact of various industrial sources on local areas.

This research underlines the importance of tracking specific sources in exposure assessments and using the results to improve control strategies.


“Chemical Characterization of Ambient Particulate Matter near the World Trade Center: Source Apportionment using Organic and Inorganic Source Markers,” Atmos. Environ., in press.

Olson, David A.; Norris, Gary A.; Seila, Robert L.; Landis, Matthew S.; and Vette, Alan F. (2007) “Chemical Characterization of Volatile Organic Compounds near the World Trade Center: Ambient Concentrations and Source Apportionment,” Atmos. Environ., 41(27) 5673-5683.

Keeler, G.J.; Landis, M.S.; Norris, G.A.; Christianson, E. M.; Dvonch, J.T. (2006) Sources of Mercury Wet Deposition in Eastern Ohio, USA, Environ. Sci. Technol., 40(19) 5874-5881.

Zhao, W.; Hopke, P.K.; Norris, G.; Williams, R.; Paatero, P. (2006) Source apportionment and analysis on ambient and personal exposure samples with a combined receptor model and an adaptive blank estimation strategy.  Atmos. Environ. 40 (20) 3788-3801.

Hopke, P.K.; Ramadan, Z.; Paatero, P.; Norris, G.A.; Landis, M.S.; Williams, R.W.; Lewis, C.W. (2003) Receptor modeling of ambient and personal exposure samples: 1998 Baltimore Epidemiology-Exposure Study Atmos. Environ. 37: 3289 – 3302.


Gary Norris (norris.gary@epa.gov), Ph.D., EPA’s Office of Resaearch and Development, National Exposure Research Laboratory, 919-541-1519.

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