Polycyclic aromatic hydrocarbons (PAHs) are members of a unique class of air pollutants relevant to many scientific and societal issues having a variety of aspects: chemical, toxicological, engineering, technological, public health, economic and regulatory, and legislative. They are products of incomplete combustion formed during the burning or pyrolysis of organic matter and are released into ambient air as constituents of highly complex mixtures of polycyclic organic matter, POM. As defined in the U.S. Clean Air Act Amendments of 1990 (CAAA, 1990), POM "includes organic compounds with more than one benzene ring, and which have a boiling point greater than or equal to 212°F (100°C)." While we generally refer to specific PAHs and polycyclic aromatic compounds (PACs) throughout this chapter, it should be noted that the complex mixture represented by the term POM is the subject of many studies and regulatory designations.
Atmospheric PAHs occur in the form of gases (e.g., 2-ring, highly volatile naphthalene, III), solids adsorbed/absorbed to the surfaces of fine respirable aerosol particles (e.g., 5-ring benzo[a]pyrene, BaP, I), and 3- and 4-ring semivolatile compounds that are distributed between the gas- and the particle-phases (e.g., the semivolatile 3-ring phenanthrene, IV, and 4-ring pyrene, II, and fluoranthene, V); for gas-particle partitioning, see Chapter 9D and Section A.4 in this chapter.
The ubiquitous nature of these airborne PAHs is evident from the fact that the 16 U.S. Environmental Protection Agency "Priority Polycyclic Aromatic Hydrocarbon Pollutants" shown in Table 10.1 (U.S. EPA, 1988) are found, as we shall see in this chapter, in urban airsheds throughout the world. Their widespread
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