In response to epidemiological studies published over 20 years ago, at least three research communities have been intensively studying airborne particulate matter (PM). These efforts have been coordinated by approaching the source-atmospheric accumulation/receptor-exposure-dose-health effects paradigm (adopted from National Research Council (NRC) ) from different perspectives or along different parts of the paradigm. The atmospheric sciences communities consider the emissions of particles and precursors from sources, their transport and transformation in air to receptor locations, and finally removal from the atmosphere. The exposure communities' interest is to examine the pathways by which pollution, or particulate matter in this case, approaches and enters the body, typically by trying to relate PM concentrations at a central location(s) to exposure and perhaps dose. Both the atmospheric sciences and exposure communities approach the paradigm from left to right. In contrast, the health effects communities have studied health outcomes, including hospital admissions, school absences, disease rates, and deaths in human populations, and potential mechanisms of biological actions in laboratory settings. In general, the health effects communities approach the paradigm from right to left, attempting to correlate an observed adverse health effect with dose or exposure measures. For the most part, research results are reported in scientific publications and conferences for each community respectively. Over the years, there has been little effort to integrate information from these diverse groups in a substantive way. While a major attempt took place in 1998 at the Chapel Hill workshop [Albritton and Greenbaum, 1998], little has occurred since.
 In April 2003 the American Association for Aerosol Research held its first specialty conference entitled “Particulate Matter: Atmospheric Sciences, Exposure, and the Fourth Colloquium on PM and Human Health,” which was intended to bridge this gap. The conference was co-chaired by Cliff Davidson (Carnegie Mellon University, Pittsburgh, Pennsylvania), Robert Phalen (University of California, Irvine, California), and Paul Solomon (U.S. Environmental Protection Agency, Las Vegas, Nevada). Over 550 scientists, air quality managers, and policy makers from the public and private sectors participated in the meeting, representing science and policy investments from 20 countries. The program consisted of a series of workshops with three concurrent sessions the first day, a series of plenary sessions that included over 70 invited speakers, and poster sessions where 388 posters were presented during the course of the meeting.
 The overall goal of the conference was to bring together health and exposure scientists with 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. Facilitate the dissemination of findings from among the major PM health, exposure, and atmospheric science research programs worldwide to the scientific, regulatory, and regulated communities in a timely manner and to enable rigorous debate of the findings.
 2. Showcase national assessments (e.g., the NARSTO North American PM Assessment and Integrated Critical Review, Canadian Assessment, and others) and to evaluate progress on national PM strategies (e.g., the NRC Committee on Research Priorities for Airborne Particulate Matter).
 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.
 Six peer-reviewed special journal issues are resulting from this conference, helping to meet the last of the three specific objectives described above. The first two specific objectives were successfully achieved during the meeting. Four special journal issues are appearing in atmospheric sciences–related journals and include: Journal of Geophysical Research, Aerosol Science and Technology, The Journal of the American Association for Aerosol Research, Atmospheric Environment, and Journal of the Air and Waste Management Association. We estimate that nearly 60 papers will be published in these journals, and all four will be published during the last half of 2004. Two dedicated issues in Inhalation Toxicology allowed for the publication of health-related papers, one focusing on the impacts of ambient environmental particulate air pollution on the respiratory tract (Inhalation Toxicology, 16, special supplement, 2004) and the other on the nonpulmonary effects of such particles (Inhalation Toxicology, 16(6–7), 2004). Thirty papers resulting from presentations at this meeting are being published in these issues of Inhalation Toxicology.
 Papers published in these journals have been subject to the normal peer-review process of their respective journals. Papers in this special section of Journal of Geophysical Research report on the optical and chemical properties of the urban aerosol, methodologies for measurement of related gas-phase constituents, and particle removal processes. Most of these studies focus on urban areas of the eastern United States, where many regions are expected to exceed new federal standards designed to protect public health against particulate matter exposure. Two of the papers address the optical properties of the aerosol: one in Baltimore, Maryland [Adam et al., 2004], and one in Pittsburgh, Pennsylvania [Cabada et al., 2004]. Detailed measurements of the composition of wood smoke [Hays et al., 2004] and particles from urban and regional sources in New York state [Schwab et al., 2004] illustrate the complexity of particle compositions and the challenges associated with identifying the sources and properties of particles from various local and regional sources. Two modeling studies focus on different aspects of particle formation and loss. Takahama et al.  discuss the sources and sinks of particulate nitrate leading to pronounced diurnal variations in the urban atmosphere of Pittsburgh, while Andronache  focuses on the removal of recently formed ultrafine particles by precipitation before they can grow to sizes of climate and health significance. Finally, Li et al.  report on intercomparisons of gas-phase techniques for the gases nitrogen dioxide, formaldehyde, and sulfur dioxide, which are relevant to aerosol studies because of their roles in photochemical oxidation processes and as precursors for condensable species.
 The American Association for Aerosol Research (AAAR) was the primary professional society sponsoring the AAAR PM Meeting. The conference was cosponsored by the Air and Waste Management Association. The primary financial sponsor was the U.S. Environmental Protection Agency. Other sponsors include the National Science Foundation, American Chemistry Council, American Petroleum Institute, Electric Power Research Institute, Ford Motor Company, Health Effects Institute, International Society for Aerosols in Medicine, Mid-Atlantic Region Air Management Association, NARSTO, National Institute of Environmental Health Science, New York State Energy Research and Development Authority, South Coast Air Quality Management District, Southern Company, U.S. Department of Energy (Office of Fossil Energy, National Energy Technology Laboratory), and the University of California, Irvine, Center for Occupational and Environmental Health. The sizable financial sponsorship allowed for a reduced general registration fee, numerous travel grants, and further reduced registration fees for students attending the conference, as well as enhanced the overall character of the meeting.
 Many people contributed to the success of the AAAR PM Meeting, and AAAR's first specialty conference and their efforts are greatly appreciated. These include members of the Organizing Committee and Science Advisory Committee, who provided guidance throughout the development of the meeting and acted as session chairs during the meeting. Actively working on publications from the conference are the members of the publications committee, whose work began after the conference and continues with diligence. Finally, thanks are given to the authors who presented high-quality technical papers across a multidisciplinary topic and to all who attended the conference, for without you the Particulate Matter: Atmospheric Sciences, Exposure and the Fourth Colloquium on PM and Human Health conference would not have been a huge success.