In 1997, the U.S. Environmental Protection Agency (EPA) promulgated National Ambient Air Quality Standards (NAAQS) for PM2.5 (particles in air with a diameter of 2.5 microns or less). States in areas exceeding the Particulate Matter (PM) NAAQS are required to develop State Implementation Plans (SIP) that describe, with scientific justification, their approaches to reducing PM to below the level of the standards. In recognizing the need for improved information, Congress provided EPA with funds to support a major air quality program within the United States with one of its main objectives being to provide states and regional air quality organizations with additional scientific information for understanding the accumulation of PM on local and regional scales. Thus EPA's PM Supersites Program was developed as a multiyear, multi-institutional, multimillion dollar program of air quality methods development and evaluation, measurements, data analysis, and modeling with three main objectives: (1) to characterize PM in air to better understand source-receptor relationships, (2) to develop and evaluate methods in support of the first objective, and (3) to support health and exposure studies via air quality measurements performed in conjunction with the first objective. Additional information regarding the Supersites Program can be found at http://www.epa.gov/ttn/amtic/supersites.html.
 The major activities of the program included initial methods testing in two locations (Atlanta, Georgia, and Fresno, California) in 1999, followed by one to several years of methods development and evaluation and measurements in New York, New York; Baltimore, Maryland; Pittsburgh, Pennsylvania; St. Louis, Missouri; Houston, Texas; Fresno, California; and Los Angeles, California. These areas were selected on the basis of different atmospheric pollution conditions resulting from differences in sources and meteorological conditions. Given the wealth of information coming from the Supersites Project, EPA initiated a synthesis and integration of findings around 17 science/policy relevant questions (Synthesis) to ensure the most useful information from the program is provided directly to key stakeholders in support of new NAAQS development, SIP efforts, and future PM planning activities in both government and private sector organizations. The scope of the Synthesis goes beyond the Supersites Program information and includes related studies conducted during the last half decade or so since the Supersites Program projects integrated with many other projects conducted at the same time.
 To support the Synthesis, EPA in conjunction with the American Association for Aerosol Research organized an international specialty conference entitled “Particulate Matter (PM) Supersites Program and Related Studies,” in Atlanta, Georgia, 7–11 February 2005. The overall goal of the conference was to bring together atmospheric scientists, air quality managers, and policy makers to allow for enhanced communications and exchange of information among these groups. Specific objectives of the conference were to (1) advance the science in the area of PM pollution and to widely disseminate that information to key stakeholders, and thus support and provide guidance on policy planning and decisions for reducing PM concentrations in air; (2) provide important information on methods, measurements, modeling, and data analysis in support of EPA's PM Supersites Program and Related Studies Key and Policy Relevant Findings Synthesis; and (3) add significantly to the peer-reviewed literature by publishing research results presented at the conference in a variety of special issues of selected journals.
 Four peer-reviewed special journal issues are resulting from this conference helping to meet the objectives described above. The four special journal issues include the Journal of Geophysical Research–Atmospheres, Journal of the Air & Waste Management Association, Aerosol Science and Technology–the Journal of American Association for Aerosol Research, and Atmospheric Environment. We estimate that nearly 80 papers will be published in these journals and all four will be published during the first half of 2006.
 Papers in this special section of the Journal of Geophysical Research–Atmospheres can be grouped into three categories. The first group deals with gas phase composition. The second group of papers investigates particulate matter and individual contributions either focusing on chemical composition, spatial variability, temporal variability, or formation. The final group focuses on source apportionment using statistical techniques, molecular markers, isotope analysis, or back trajectory analysis.