as functions of concentration, reaction time, and 03 concentration. Interestingly, their reaction rates in aggregated states (inferred from eximer formation) were much slower than those in the dispersed state (only monomer fluorescence). They concluded that the reaction of 03 with aggregated PAH molecules was much slower than with monomers on the surface. Alebic-Juretic and co-workers (1990) reported a similar observation for the PAH-silica gel-03 reaction.

Such a "surface/bulk" reactivity phenomenon may in part be responsible for the low (or zero) reactivity reported for BaP deposited on, or present in, a variety of substrates and exposed to ambient levels of 03 (e.g., 100 ppb) in air (see Grosjean et al., 1983; and Coutant et al., 1988). These observations can be rationalized by assuming that, while BaP in fact does react rapidly with 03 in ambient particles, not all of it is at (or close enough to) the surface to be available for reaction (Atkinson et al., 1988a; Arey, 1998a).

In the early 1990s, a series of articles appeared that described the rates, mechanisms, and products of the photooxidations of several simple gas-phase PAHs adsorbed on the surfaces of Si02 and A1203 particles. These are models for some of the inorganic particles found in air (see Chapter 9). Studies of naphthalene and I-methylnaphthalene (Barbas et al., 1993), ace-naphthylene (Barbas et al., f994), anthracene (Dabestani et al., 1995), phenanthrene (Barbas et al., 1996), and 1-methoxynaphthalene on Si02 (Sigman et al., 1996; see also below and the reviews by Dabestini (1997) and Sigman (1997)) in the presence and absence of air and using different photolytic wavelengths were carried out.

Sigman and co-workers (1997) and Dabestini (1997) reported studies of the photochemistry of 2- and 3-ring PAHs adsorbed on Si02. Photooxidations were shown to proceed by two distinct mechanisms. Type I involves electron transfer from the photoexcited singlet state (S,) of the PAH to atmospheric oxygen, forming superoxide, 02" (or transfer to "surface-active sites"). Indeed, Barbas and co-workers (1993) reported the room temperature EPR spectrum of 02~ when naphthalene adsorbed on the surface of Si02 is irradiated. The type II mechanism is a singlet oxygen mediated, photooxida-tion process such as that described earlier for BaP in liquids—that is, triplet-triplet energy transfer from the T, state of an electronically excited PAH molecule to the triplet ground state of an 02 molecule (3Sg), producing 02('A), which then adds to the ground-state PAH, ultimately forming oxidized photoproducts.

PAHs photooxidized exclusively by the type I electron transfer-superoxide mechanism include naphthalene and 1-methylnaphthalene (Barbas et al., 1993), fluorene (Barbas et al., 1997), and acenaphthene (Re yes et al., 1998). Typical products formed in the type I photooxidation of naphthalene and f-methylnaph-thalene are shown in Fig. 10.29A.

PAHs photooxidized by the type II singlet oxygen mediated mechanism include acenaphthylene, whose oxidized products and yields are shown in Fig. 10.29B (Barbas et al., 1994), phenanthrene (Barbas et al., 1996), anthracene (Dabestini et al., 1995), and tetracene (Dabestini et al., 1996). An additional photochemical process, the formation of photodimers, is also observed for acenaphthylene, anthracene, and tetracene.

Interestingly, Barbas and co-workers (1996) noted that, once sorbed to the Si02 surface, phenanthrene did not desorb into the gas phase even under high vacuum. As in previous studies of the PAHs cited above, no dark reactions were observed at the Si02-phenanthrene interface. The authors suggested that the photostability of phenanthrene adsorbed on carbon black (half-life > f000 h) and fly ash (half-life ~49 h) reported by Behymer and Hites (1985) may be due to a number of factors, including competitive absorption of incident radiation, energy transfer from excited phenanthrene to other PAHs in carbon black and "more facile reactions of singlet oxygen with the substrate."

5. Gas-Particle Reactions a. Ozonolysis

Lane and Katz reported in f977 that the dark reaction of BaP deposited on the surface of glass Petri dishes with air containing 200 ppb of ozone was fast, with a half-life of ~38 min. Katz and co-workers (f979) exposed nine PAHs on thin-layer chromatography plates of cellulose in the dark to 200 ppb of 03 in air and found pronounced differences in their reactivities, e.g., half-lives of 36 min for BaP, 2.9 h for BaA, 7.6 h for BeP, and 53 h for benzo[6]fluoranthene. Subsequently, in good agreement with Lane and Katz, a half-life of ~ f h was determined for BaP deposited on glass fiber filters and exposed (passively in a controlled atmosphere) to 200 ppb of 03 in the dark (Pitts et al., f980).

Pitts et al. (1986) exposed five individual PAHs, pyrene, fluoranthene, benz[a]anthracene, BeP, and BaP, deposited on glass fiber and Teflon-impregnated glass fiber filter (TIGF) substrates "passively" for 3 h in the dark in a 360-L Teflon environmental chamber to 50-300 ppb of 03 in air at several relative humidities. These experimental conditions more nearly resemble the actual exposure of ambient particles to 03 (in the dark) during transport than do exposures in Hi-Vol flow systems. Consistent with earlier studies, BaP, BaA,

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