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 

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). 

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- 

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