Similarly, Jang and McDow (1995, 1997a) investigated the rates, products, and mechanisms of the pho-tooxidation of BaA carried out in three solvents (toluene, benzene, and benzene-<i6) that contained three common constituents of ambient aerosols that are known accelerators of the photodegradation process: 9,10-anthraquinone, 9-xanthone, and the substituted methoxyphenol, vanillin. On the basis of this and previous studies, the researchers proposed "that there were at least two mechanisms" (see also Jang and McDow, 1995). One is singlet oxygen mediated and observed with BaA dissolved in benzene and benzene-d(t solvents; it involves '02 addition to an aromatic ring of BaA followed by ring opening and products such as phthalic acid. The second mechanism is most important in toluene; it features H-atom abstraction from the solvent by aromatic carbonyl compounds, leading to complex free radical initiated oxidations. Jang and McDow (1995) have also suggested as other possible mechanisms for the photooxidation of PAH triplet-triplet energy transfer to the PAH by a photosensitizer (e.g., polycyclic aromatic quinones or ketones and aromatic aldehydes) or free radical chain oxidations initiated by electronically excited PAH. In short, efficiencies of PAH photooxidation processes may depend in part on the H-atom-donating properties of some of the organic constituents (e.g., methoxyphenols) and on properties such as oxygen solubility and 'Oz lifetimes in the liquid-like layer of organic combustion aerosols.

b. Photooxidations on Inorganic Solid-Air Surfaces

Just as with organic combustion aerosols, the chemical and physical nature of inorganic solid substrates can have a dramatic impact on the photoreactivity of adsorbed PAH. In 1980, Korfmacher and co-workers reported that BaP, pyrene, and anthracene all pho-tolyzed efficiently in liquid solution but were resistant to photodegradation when adsorbed on coal fly ash. Subsequent studies confirmed this observation and revealed that the carbon content of the ash (and the associated darkening of color) is a key factor in establishing the photostability of these PAHs. Indeed, they were stabilized at relatively small percentages of carbon, e.g., 5% or less (Behymer and Hites, 1985, 1988; Yokley et al., 1986; Dunstan et al., 1989; Miller et al., 1990).

PAHs adsorbed on particles of carbon black were also photostabilized (Behymer and Hites, 1988). However, Barofsky and Baum (1976) demonstrated that BaP, anthracene, BaA, and pyrene deposited on carbon microneedle field desorption emitters and exposed to UV radiation were all photooxidized to carbonyl compounds. Similarly, PAHs can photodegrade efficiently in air when adsorbed to substrates of silica gel, alumina, or glass plates (e.g., see Lane and Katz, 1977; Kormacher et al., 1980; Behymer and Hites, 1985; Yokely et al., 1986).

The effect of the physical state of BaP and perylene adsorbed on fused-silica plates on their reaction rates with ozone in air was studied by Wu and co-workers (1984), who measured the fluorescence of the two PAHs

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