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2 <br />3 <br />Figure 1. Nomenclature of PAHs (modified from Lee and Grant 1981, and Grimmer <br />1983) The PAH formula is oriented so that the greatest number of rings are <br />in a horizontal row and a maximum number of rings are above and to the right <br />of the horizontal row. The first carbon atom that belongs to the uppermost <br />ring and is not engaged in ring fusion with another ring is given the number <br />C-1; numbering continues in a clockwise direction omitting those carbon atoms <br />which do not carry a hydrogen atom. The bond between C-1 and C-2 is <br />designated as side "a"; other peripheral sides continue in clockwise direction <br />in alphabetical order. Examples are: (1) pyrene (correctly oriented, <br />numbered, and lettered), (2) benzo(a)pyrene (not oriented correctly), (3) <br />benzo(a)pyrene (correctly oriented, numbered, and lettered). <br />4