HPLC fraction number Increasing polarity —>-


Lewtas et al., 1990a). Similarly, Greenberg and coworkers (1990, 1993) found that the polar neutral compounds in inhalable particle samples at Newark, New Jersey, were the greatest contributors to ambient mutagenicities, regardless of the season of the year. The importance of more polar compounds in the direct-acting TA98 mutagenicity has also been seen in the emissions of POM from heavy-duty diesel trucks and wood smoke (e.g., Schuetzle and Lewtas, 1986). (Note, however, that the structures, concentrations, and mutagenicity polarity profiles of diesel exhaust emissions can be strongly impacted by such parameters as fuel composition, engine type, age, operating conditions, control devices, etc.)

A breakthrough in identifying possible particle-phase polar compounds responsible for the direct mutagenicity of fraction 6 in the Claremont, California, study of Harger et al. (Fig. f0.25) came with the isolation and identification by Helmig and co-workers (f992a, f992b) of a new class of direct mutagens in particulate matter collected in Riverside, California, the nitrolactones of phenanthrene, specifically, the 2- and 4-nitro-6 H-dibenzo[6,<i]pyran-6-ones. The 2-nitro isomer (structure XI) is a very powerful, direct mutagen (240,000 rev Ijlg~') and Harger et al. (1992) ascribe a major portion of the mutagenicity in their fraction 6 to this widely distributed product of the gas-phase OH radical attack on phenanthrene (see Atkinson and Arey (1994), Sasaki et al. (1995), Arey (1998a), and references therein; and Section F).

Based on results from concurrent environmental chamber studies of the photooxidations of naphthalene, fluorene, and phenanthrene by Arey and coworkers (f992), Harger and co-workers attributed a significant fraction of the vapor-phase direct activity of fraction 4 (Fig. 10.25) to 2-nitronaphthalene (2-NN) present in these 12-h daytime samples as a result of an OH-initiated homogeneous gas-phase reaction of naphthalene in ambient air. Although f-nitronaphthalene (f-NN) is also present at significant concentrations in both ambient and laboratory systems, it is a much weaker, direct mutagen (see Table f0.20; Arey et al., 1992).

Harger and co-workers noted that methylnitronaph-thalenes (MNN) were also abundant in southern California air, but their mutagenic activities in the microsuspension were unknown. Subsequently, Gupta and co-workers (1996) determined them for the 14 methyl-nitronaphthalenes (see Table 10.18) and used these data to quantify contributions of nitronaphthalenes and methylnitronaphthalenes to the vapor-phase mutagenicity of 12-h daytime and 12-h nighttime samples collected on PUF plugs at an air monitoring site in

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