United States; pertinent references are provided for those who desire more background information on general concepts of scientific and technical understanding of air pollution and its impacts.
The atmosphere is composed of a mixture of gases and particles. An air pollutant is generally defined as any substance in air that, in high enough concentrations, harms humans, ecosystems (other animals and vegetation), or materials (such as buildings and monuments) and reduces visibility. In this report, the committee uses the term air pollutant to denote the subset of harmful atmospheric substances that are present, at least in part, because of human activities rather than natural production and whose principal deleterious effects occur as a result of exposure at ground level. Greenhouse gases, as well as pollutants that cause depletion of ozone (O3) in the stratosphere, the layer of atmosphere extending from about 10 to 16 kilometers (km) up to 50 km altitude, are addressed only in the context of managing ground-level air quality.2
The science of air pollution is primarily concerned with quantitatively understanding the so-called “source-receptor relationships” that link specific pollutant emissions to the pollutant concentrations and deposition observed in the environment as a function of space and time. This quantitative understanding is developed through extensive field and laboratory measurements and analysis and is then tested and documented in air quality models that use mathematical and numerical techniques to simulate the physical and chemical processes that affect air pollutants as they disperse and react in the atmosphere. As illustrated in Figure 1-1, the pollutants at a particular time and place depend on the proximity to sources that emit pollutants or their precursors; the chemical reactions that pollutants or their precursors undergo once in the atmosphere; and the impact of mixing, dilution, transport, and removal or deposition processes (Seinfeld and Pandis 1998). The areas of air quality science and air quality management are closely coupled, because the tools developed by scientists and engineers to carry out the tasks described above are also widely used by the agencies tasked with controlling air pollution. For example, the instrumentation used by scientists in field experiments is also used by regulatory agencies to monitor air pollution exposures, trends, and compliance. Similarly, the models developed by scientists to simulate and better understand air pollution are used in AQM to help design effective strategies for air pollution mitigation.