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Nitro-PAHs, regardless of whether directly emitted or formed in air, are of concern because many of them are animal, and possible human, carcinogens (IARC, 1989; see also review of environmental carcinogens by Tokiwa et al. (1998) and references therein).

The goal of this chapter is to illustrate the atmospheric chemistry of PAHs and PACs. However, because of their unique biological properties, we also provide some perspective on their relevance to air pollution toxicology and the development of sound scientific health risk assessments for specific carcinogenic PAHs such as benzo[a]pyrene (California Air Resources Board, 1994) and for complex combustion-generated emissions such as diesel exhaust (e.g., IARC, 1989; WHO, 1996; and California Air Resources Board, 1998). For discussions of such broad topics as research on analytical methods for the determination of PAHs in environmental samples, see, for example, the review of chromatographic methods by Poster, Sander, and Wise (1998) and references therein. Similarly, for reviews of their environmental chemistry and related carcinogenicities, see Neilson (1998) and Harvey (1997). The book Environmental Organic Chemistry by Schwarzenbach, Gschwend, and Imboden (1993) provides a useful perspective on the subject and contains helpful specific examples relating to PAHs.

A. NOMENCLATURE AND SELECTED PHYSICAL AND SPECTROSCOPIC PROPERTIES OF POLYCYCLIC AROMATIC HYDROCARBONS (PAHs) AND POLYCYCLIC AROMATIC COMPOUNDS (PACs)

1. Combustion-Generated PAHs and PACs

Historical, chemical, and toxicological interest in PAHs and PACs goes back over two centuries when Sir

Percival Pott proposed that the high rate of cancer of the scrotum incurred by London's chimney sweeps was due to the presence of certain chemicals in the fireplace soot (i.e., POM) to which they were heavily exposed (Pott, 1775). Some 150 years later, Passey (1925) reported that organic extracts of such "domestic soot" induced tumors in experimental animals.

In the early 1940s, Leiter et al. (1942) demonstrated that a similar phenomenon occurred with organic extracts of ambient air particles—that is, injection of "tars extracted from atmospheric dusts" collected at locations throughout New York City produced subcutaneous sarcomas in mice. Shortly thereafter, Leiter and Shear (1943) reported that "marginal doses of 3,4-benzpyrene" (known today as benzo[a]pyrene, BaP, I), the powerful carcinogen earlier isolated from coal tar and synthesized by Cook et al. (1933), also produced subcutaneous tumors in mice.

These observations, coupled with the discoveries of BaP in chimney soot (Goulden and Tipler, 1949) and in ambient air particles collected at 10 stations throughout Great Britain (Waller, 1952), and the tumori-genic/carcinogenic properties of extracts of ambient particles collected during episodes of Los Angeles photochemical smog (Kotin et al., 1954) were key factors in establishing the atmospheric chemistry of PAHs and PACs as a new field in air pollution research.

Sincc then, many monographs, handbooks, symposia proceedings, and specialized chapters (in addition to thousands of research papers) dealing with the chemistry and biological and toxicological aspects of PAHs, POM, and PACs have appeared in the scientific, engineering, and medical sciences literature. Examples are cited in Box 10.f.

2. Structures and IUPAC Rules for Nomenclature a. PAHs

Over the decades several significantly different PAH and PAC numbering/nomenclature systems have been proposed and widely used in the older literature, e.g., that of Clar (1964). Unfortunately, even today this can lead to confusion on the part of those unfamiliar with the history of different systems of nomenclature.

We follow the 1979 IUPAC recommendations summarized in Polynuclear Aromatic Hydrocarbons: Nomenclature Guide (Loening et al. 1990). The American Chemical Society also publishes the Ring Systems Handbook, which, ca. 1990, contained structural diagrams for over 70,000 unique ring systems (American Chemical Society, 1977 to present).

A detailed discussion of these rules and nomenclature is beyond the scope of this book. However, we

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