organic pollutant contamination) as well as hemispheric and global scales (greenhouse gas warming, mercury contamination, and stratospheric ozone depletion) (NRC, 1998). As we begin the 21st century, strong links between climate change and global air quality are becoming apparent, and the possibility that intercontinental transport of air pollutants can have a significant impact on local and regional air quality is more widely recognized (NRC, 2001; HTAP-TF, 2007; Stohl, 2004).

Air pollution, once thought of as purely a local issue, now is recognized as a complex problem that is also subject to regional, hemispheric, and even global influences. Although domestic sources are the primary contributors to most of our nation’s air quality problems, the United States is both a source and a receptor for pollutants transported great distances. Pollutants not only flow across our borders with Canada and Mexico but also travel between North America and Asia, Africa, and Europe. These pollutants contribute to public health threats, degraded visibility, agricultural and native vegetation injury, decreased domestic and wild animal viability, infrastructure materials damage, poorer water quality, degraded aquatic ecosystems, and climate change.

Long-range transport of air pollution from international sources is receiving increased attention in both the scientific literature and popular media. It is also an increasing concern for managers charged with meeting air quality standards. Air quality management stakeholders have advanced differing positions on how localities should address the impact of international pollutant transport in air quality planning. There also is concern about international pollution transport as a source of continuing exposure to chemicals that have been banned in the United States and other countries (e.g., certain persistent organic pollutants, POPs). The U.S. government is under increasing pressure from European countries to mitigate the transport of pollution across the Atlantic, and to address concerns about the contamination of pristine Arctic environments. Although there is now qualitative evidence of long-range transport of pollution, we need a more quantitative understanding of the true magnitude and dynamics of these flows, in order to assess their impacts on human health, ecosystem viability, and other concerns, and to design effective cooperative international control strategies. Some basic concepts needed to discuss the long-range transport of air pollutants are defined in Box 1.1.

To help address these issues the National Research Council (NRC) of the U.S. National Academies was asked to convene an ad hoc study committee to assess and summarize current understanding of the long-range transport of four key classes of pollutants: ozone (O3) and its precursors, particulate matter (PM) and its precursors, persistent organic pollutants



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