Secondary Organic Aerosol (SOAs) Research

EPA’s smog simulator used to study health effects of air mixtures that include secondary organic aerosols and other reaction products in the atmosphere.

What are Secondary Organic Aerosols (SOAs)?

Secondary Organic Aerosols (SOAs) are air pollutants emitted from natural and man-made sources. They are produced through a complex interaction of sunlight, volatile organic compounds from trees, plants, cars or industrial emissions, and other airborne chemicals. SOAs are a major component of fine particle pollution (PM.2.5), which has been found to cause lung and heart problems and other health effects. As a result, EPA has established levels for these particles in the atmosphere to protect public health. 

Why are scientists studying SOAs?

SOAs are under investigation because there is much that is not known about how they contribute to air pollution overall or the possible public health and environmental effects directly related to SOAs.

Researchers are investigating SOAs to answer questions that can be used to develop air quality management policies. They include:

  • What are the most common sources of SOAs in the atmosphere?
  • How are SOAs formed in the atmosphere?
  • What is their chemical composition?
  • What happens to SOAs as they travel through the atmosphere?
  • What are their health effects?
  • What are their contributions to climate change?

What is EPA doing?

EPA scientists and their partners are studying SOAs in a multidisciplinary approach to advance understanding of the formation, atmospheric fate and transport, health effects and climate change interactions of SOAs. Research highlights are described below.

  • Southern Oxidant and Aerosol Study (SOAS)

    The Southern Oxidant and Aerosol Study (SOAS) is a major collaborative effort by scientists from more than 60 organizations to learn more about how man-made sources of air pollution interact with volatile organic compounds from plants to affect air quality and climate in the southeastern United States. The field research was conducted from June 1 through July 15, 2013. Data analysis is being conducted.

    Data gathered from multiple locations across the southeastern United States will help to understand:

    • The extent to which airborne particles play a role in regional temperature trends
    • How man-made and natural pollution sources interact and affect air quality and climate
    • How air chemistry changes as an air mass ages and is transported across a region
    • How much SOA comes from various sources (trees, motor vehicles, industry, wildfires, etc
    • Whether carbon found in airborne particles comes from natural sources (trees and other vegetation), or from man-made sources (oil, coal, or natural gas that has been burned in combustion processes by cars, trucks, trains, industry, or coal-fired power plants)

    Pollutants measured by EPA scientists during the SOAS study included ozone, nitrogen oxides, volatile organic compounds (VOCs), semi-volatile organic compounds, sulfur dioxide, and airborne particles (particulate matter).

    An important and novel contribution of EPA scientists is the use of a SOA tracer method to determine the sources of SOA by measuring for specific chemical “marker” compounds. This allows scientists to differentiate man-made SOA sources from natural sources. Scientists want to gain a better understanding of SOA sources since previous research has found that emissions from vehicles, industry, and power plants interact with natural volatile organic compound emissions from trees to impact air quality.

    EPA participated in partnership with the National Science Foundation (NSF), the National Oceanic and Atmospheric Administration (NOAA), and researchers at colleges, universities and institutions. Southern Company, and Electric Power Research Institute provided additional support for SOAS. SOAS is part of a larger study — the Southeast Atmosphere Study (SAS) — one of the largest studies of the North American atmosphere in decades.

    EPA-supported research for SOAS study

    In 2013, EPA awarded more than $4.4 million in grants to 14 academic and research institutions that took part in the SOAS study. These awards involve extensive field, laboratory and modeling work to improve understanding of the formation, transformation and radiative properties of organic aerosols.

    Related Resources:

    Partner Links

    SOAS Study partner data are available at:

  • Health and Environmental Effects Research

    EPA has new capabilities to study urban air mixtures, including SOAs, with a new photochemical smog simulator, which can create different atmospheres to assess the health impacts of relevant multipollutant atmospheres and identify relative contributions of specific sources on these processes.

    The research objectives are to:
    • Generate novel atmospheres containing secondary organic aerosols and other reaction products
    • Study multipollutant health effects, including types, classes and activity that affect various health indicators.
    • Perform cell (in vitro) screening for mutagenicity, cytotoxicity and oxidative stress markers
    • Conduct acute cardiopulmonary health testing of atmospheres using animal models of cardiac stress, hypertension, metabolic syndrome, respiratory infections and allergic asthma
    • Assess effects of temperature changes on smog formation
    • Support EPA’s evaluation of air mixtures for setting the National Ambient Air Quality Standards (NAAQS)

    The chamber is designed to create complex atmospheres at a wide range of controllable conditions and reaction times. The steady-state mode of operation enables extended testing times from weeks to months.

  • Atmospheric Chemistry Research

    To determine the effects that emissions in outdoor air have on humans and ecosystems, critical data on atmospheric gas phase and particulate phase chemistry are necessary. Generation of these data is needed for air quality model development and improvements, as well as for global climate models.

    Scientists simulate atmospheric conditions under highly controlled conditions in the laboratory to study the interactions of various atmospheric chemicals and determine their transformation rates, products and fates. The atmospheric simulators can test chemical reactions that occur from sunlight or photochemical reactions.

    The atmospheric simulators are used to study precursor-specific secondary organic aerosol mechanisms, and the interplay between several common air pollutants regulated by the Clean Air Act. In addition, research has been conducted to study the impact of nano-scale fuel borne catalysts and biofuel emissions on photochemical transformations (i.e., the interaction of chemicals with nitrogen oxides and light).

    Results are incorporated into EPA’s Community Multi-scale Air Quality model (CMAQ,) which is used by states to assess implementation actions needed to attain National Ambient Air Quality Standards that are protective of human and ecosystem health. Study data is also incorporated into international air quality and global climate change models and used to improve, refine and evaluate the accuracy of air quality models.

    Atmospheric chemistry research is also focused on studying potential future atmospheric conditions to enable EPA to take a proactive approach in addressing major atmospheric chemistry issues such as the impact of climate change on regional air quality.

  • Modeling Research

    Scientists are using a variety of modeling approaches including EPA’s Community Multi-scale Air Quality Modeling System (CMAQ) to simulate the concentrations of the chemical compounds that form SOA. The modeled results are being compared to data collected as part of the SOAS study.

    The model is used by states to meet National Ambient Air Quality Standards, and by the National Weather Service to produce daily U.S. air quality forecasts for ozone. The model allows scientists and air quality managers to simulate different air quality scenarios and quantify the environmental, human health, and climate-related benefits of reducing emissions from different sources.

    Related Resources: