O Xi

2-Nitro-6//-dibenzo[6,fii]pyran-6-one

For example, of the three possible electronic structures for naphthalene, the symmetrical structure (a) with two benzenoid rings (aromatic sextets) would be the more stable, and hence the preferable representation.

[Note that when the Clar convention is used for Kekule structures, the benzenoid rings are often shown as circles (e.g., see Moyano and Paniagua, 1991).]

Finally, since our focus is on the atmospheric chemistry of unsubstituted (or methyl substituted) PAHs, we do not discuss major classes of PAHs from natural sources that are not found at significant levels in ambient air but may be important in soil and water environments, e.g., alkylated PAHs (see the review by Si-moneit, 1998).

b. PACs

Despite the generally lower emission factors and ambient levels of PACs compared to PAHs (there are some exceptions), interest in certain airborne N-, O-, and S-atom PACs has increased significantly, in large part because of their mutagenic and carcinogenic properties. For example, in 1976, Cautreels and Van Cauwenberghe used GC-MS to identify more than fOO compounds in three different fractions (neutral, acidic,

A. NOMENCLATURE ANL) SELECTED PHYSICAL AND SPECTROSCOPIC PROPERTIES OF PAHs AND PACs

TABLE 10.3 Structures, Common Names, Empirical Formulas, and Molecular Weights of Selected Nitrogen-Containing Mono- and Polycyclic Aromatic Compounds (N-PACs) Identified in Ambient Air

Structure

Common name

Empirical formula

Reference

Pyridine c5h5n

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