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ENVIRONMENTAL CHEMISTRY, SOURCES, AND FATE <br />PROPERTIES <br />Polycyclic aromatic hydrocarbons (PAHs), also known as polynuclear <br />aromatic hydrocarbons (PNAs) and polycyclic organic matter (POM), are composed <br />of hydrogen and carbon arranged in the form of two or more fused benzene rings <br />in linear, angular, or cluster arrangements, which may or may not have <br />substituted groups attached to one or more rings (Sims and Overcash 1983). In <br />some cases, the newly defined substituted PAH has strikingly greater <br />toxicological effects than does the parent compound (Cooke and Dennis 1984). <br />The nomenclature of PAH compounds has been ambiguous in the past due to <br />different peripheral numbering systems. The currently accepted nomenclature <br />is shown in Figure 1. <br />Of major environmental concern are mobile PAHs that vary in molecular <br />weight from 128.16 (naphthalene, C H ) to 300.36 (coronene, C2 H ). Higher <br />molecular weight PAHs are relativei9 immobile because of their fa?je molecular <br />volumes and their extremely low volatility and solubility. Among the mobile <br />forms are thousands of compounds that differ in the number and position of <br />aromatic rings, and in the position of substituents on the basic ring system. <br />The lower molecular weight unsubstituted PAH compounds, containing 2 to 3 <br />rings, such as naphthalenes, fluorenes, phenanthrenes, and anthracenes (Figure <br />2), have significant acute toxicity to some organisms, whereas the higher <br />molecular weight 4- to 7-ring aromatics do not. However, all known PAH <br />carcinogens, cocarcinogens, and tumor producers are in the high molecular <br />weight PAH group (Figure 3). <br />Physical and chemical characteristics of PAHs generally vary with <br />molecular weight. With increasing molecular weight, aqueous solubility <br />decreases, and melting point, boiling point, and the log Kow (octanol/water <br />partition coefficient) increases (Table 1), suggesting_ increased solubility in <br />fats, a decrease in resistance to oxidation and reduction, and a decrease in <br />vapor pressure. Accordingly, PAHs of different molecular weight vary <br />substantially in their behavior and distribution in the environment and in <br />3