B Type II: Singlet oxygen mediated addition
FIGURE 10.29 Typical photoproducts observed (A) in the irradiation (A = 300 nm) in air of naphthalene and 1-methylnaphthalene adsorbed on silica and formed by a Type I electron transfer (superoxide) mechanism (Barbas et at., 1993) and (B) in the irradiation in air of acenaphthylene (A = 350 nm) adsorbed on silica formed by a Type II singlet oxygen mechanism (Barbas et ai, 1994) (adapted from Dabestani, 1997).
and pyrene were the most reactive 60-80% degraded at 1% RH) and, except for BaP, degradation was much less at 50% RH than at f% RH.
Fresh ambient particulate POM sampled near a freeway was also exposed in this passive system for 3 h in the dark to 200 ppb of 03 in air at f% RH. Concentrations of specific PAHs determined in the ambient particles and their percent reacted were similar for samples collected on both kinds of filters (glass fiber and TIGF). Again, under passive exposure conditions to approximately ambient levels of 03, BaP and BaA were found to be significantly more reactive than BeP.
Results of a study of the ozonolysis of primary combustion aerosols by Van Vaeck and Van Cauwen-berghe (1984a) are illustrated in Fig. 10.30. Shown are percent conversion profiles as a function of time for the decay of several 5- and 6-ring PAHs in diesel exhaust particulate matter (Z)p < 0.5 /jlm) collected on glass fiber filters and exposed in the dark to 1.5 ppm 03 in air under Hi-Vol sampling conditions. Half-lives for degradation range from ~f5-20 min for BaP to ~2-3 h for low-reactivity benzo[&]fluoranthene. The authors noted that experiments at ambient levels of 03 of fOO ppb also "include significant conversion." Similarly, Lindskog and co-workers (1985) reported several PAHs in soot particles that were generated in a smoke gas generator and collected on a glass fiber filter were "transformed" when exposed to ozone. Overall, such results suggest that in ambient aerosols, the highly reactive PAHs, e.g., BaP and cyclopenta[c<i]pyrene, could be susceptible to ozone degradation under certain atmospheric conditions, as well as during Hi-Vol collection of samples.
Van Cauwenberghe (1985) proposed two different mechanisms for the degradation of unsubstituted PAHs by ozone; one is a one-step electrophilic-nucleophilic attack on olefin bonds with high electron density. This forms an unstable primary ozonide that decomposes, with ring opening giving products with aldehyde and/or carboxylic acid groups. Peroxides or a-keto hydroperoxides may also be formed. The second mechanism is a two-step electrophilic attack involving initial formation of a a- complex and ultimately quinones.
Complex mixtures of oxidized products are produced in the ozonolysis of BaP on solid substrates. For example, BaP deposited on a glass fiber filter and exposed to 1 ppm 03 in air primarily formed products characteristic of the ring-opening mechanism, e.g., dialdehydes and dicarboxylic and ketocarboxylic acids. However, some quinones and phenols, were also formed. For details on the mechanisms and O-PAH products of these BaP-03, gas-solid substrate reactions, see Van Cauwenberghe et al. (1979) and Van Cauwenberghe and Van Vaeck (1983 and references therein).
Mixtures of products formed in the reactions of BaP deposited on glass fiber filters with f00-200 ppb of 03 in air are not only complex chemically, but their extracts are also directly mutagenic in the Ames assay (strain TA98, — S9). Using the method of bioassay-di-rected fractionation and chemical analysis, as illustrated in Figs. 10.3fa to 10.31d, Pitts and co-workers (f980) isolated, identified, and obtained the specific activity, 1600 rev /¿g"1 (TA98, -S9), of the major stable contributor to the activity of such mixtures, benzo[a]pyrene-4,5-oxide (Fig. 10.31d). It is a well-known BaP metabolite, a powerful direct-acting frameshift mutagen, and a weak carcinogen on mouse skin (Levin et al., 1976). The value of f600 rev /¿g~' is in good agreement with those of other researchers
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