An exception may be particle-associated cyclo-penta[«/]pyrene (XXVIII), which, with its exocyclic double bond (analogous to acenaphthalene), may react at a significant rate with the gas-phase N03 radical (or N2Os) and even faster in heterogeneous photooxida-tions or ozonolysis. Certainly, by analogy with acenaphthylene, whose atmospheric lifetimes under typical atmospheric conditions are 6 min for N03 radicals, 43 min for 03, and 1.6 h for OH radicals (see Atkinson and Arey, 1994), gas-phase cyclopenta[c<f]pyrene would also be expected to decay rapidly by these reactions in nighttime and daytime polluted air environments. Recall that Fraser and co-workers (1998) found that over 50% of the cyclopenta[c<i]pyrene was in the gas phase (see Figs. 10.2 and 10.27) during hot weather. As noted earlier, the rates of formation, mechanisms, chemical structures, and yields of products, as well as the mutagenic activities, bacterial and human cell, of such cy-clopenta[c<f]pyrene reactions are currently unknown but constitute an interesting and relevant research challenge to atmospheric chemists and toxicologists alike.
6. Atmospheric Fates of Particle-Associated Nitroarenes
As we have seen, key nitroarenes found in extracts of ambient particulate matter are f-nitropyrene (1-N02-Py), predominant in primary combustion emissions, and 2-nitrofluoranthene and 2-nitropyrene, major products of gas-phase atmospheric reactions. Here we focus simply on their atmospheric fates as particle-bound species participating in heterogeneous decay processes. Formation of such nitro-PAHs in gas-phase reactions is addressed in Section F.
a. Gas-Particle Reactions
Heterogeneous reactions in sunlight of particle-bound nitro-PAHs with ambient 03 or N02 are generally believed to be minor relative to photodegradation. For example, based on their experimental results and modeling study of the formation and decay of nitro-PAHs in diesel exhaust emissions aged in the light and dark in an outdoor smog chamber, Fan and co-workers (1995) concluded that "photodecomposition was the main loss pathway for nitro-PAH in the atmosphere."
Subsequently, Fan and co-workers (1996a) conducted an experimental and modeling study of the reactions of 03 and N02 with nitro-PAHs on heterogeneous soot particles. They concluded that while photodegradation is the major daytime loss process for the nitro-PAH, during the night, particle oxidation by 03 may be the most important decay pathway.
b. Photochemical Reactions
In 1966, Chapman and co-workers proposed a nitro-nitrite photorearrangement as an efficient primary photochemical process for nitroarenes in which the nitro group is out of the plane of the aromatic rings. This is followed by dissociation into NO and a phenoxy-type radical; ultimately quinones and other oxy products are formed (Chapman et al., 1966).
Subsequently, Ioki (1977) used ESR spectroscopy to confirm the production of the benzo[a]pyrene-6-oxyl radical in the nitro-nitrite photorearrangement of 6-N02-BaP irradiated in benzene solution:
NO -> Products
Products slow reaction
1 peri hydrogen
NO -> Products slow reaction
Based on the Chapman mechanism, Pitts (1983) proposed that 6-N02-BaP, with two peri hydrogens and the N02 "out of plane," should be less stable photo-chemically than the 1- and 3-N02 isomers with only one peri hydrogen. This proved to be the case. Thus, in solution-phase irradiations of these isomers, 6-N02-BaP decomposed rapidly whereas the 1- and 3-isomers were much more stable (Zielinska, 1985). A key question then was whether or not these results could be extrapolated to give their relative photodecomposition rates when irradiated as particle-bound species on the surfaces of primary combustion products and ambient aerosols (vide infra; see also Feilberg and Nielsen, 1999b).
Subsequently, Benson (1985) reported 1-nitropyrene deposited on glass photodecomposed in sunlight with a half-life of 14 h. The reaction was accompanied by loss of the nitro group, formation of a phenolic derivative and possibly quinones, and a significant reduction in mutagenicity, consistent with the Chapman mechanism and previous results on nitro-BaP isomers (Finlayson-Pitts and Pitts, 1986).
Stark and co-workers (1985) reported that irradiation of a 0.1 mM solution of f-nitropyrene in 2-pro-panol with light from 320 to 4f 8 nm changes its absorption spectrum and concurrently results in almost total loss of its direct ( —S9) or activatable ( + S9) mutagen activity in the Ames Salmonella assay.
A similar concomitant loss of mutagenicity with loss of compound was observed when Holloway and coworkers (1987) irradiated (A > 310 nm) 1-nitropyrene, 1,8-dinitropyrene, and 3-nitrofluoranthene coated onto silica or in a dimethyl sulfoxide solution. Half-lives for photodecomposition of f-nitropyrene in solution compared to those on silica were 1.2 and 6 days, respectively; for 3-nitrofluoranthene the half-lives were 12.5 days in solution and >20 days on silica. Interestingly, 1,8-dinitropyrene photodecomposed with half-lives of 0.7 day in dimethyl sulfoxide compared to 5.7 days on silica; a major photodecomposition product was 1-nitropyren-8-ol.
Van den Braken-van Leersum and co-workers (1987) reported that on irradiation in methanol solutions (A > 300 nm), 1-nitropyrene undergoes a rapid conversion via the nitro-nitrite rearrangement, forming f-hydroxy-pyrene (88%) and f-hydroxy-2-nitropyrene (7%). Under the same conditions, the 4-nitro isomer is more stable than the 1-nitro isomer and 2-nitropyrene is very stable; it does not react either with or without oxygen present.
In a study of substrate effects on the photodecomposition of several key nitroarenes, Fan and co-workers (1996b) added several key nitro-PAHs and their deuterated analogs, along with particles of diesel exhaust or wood smoke, to a 190-m3 outdoor smog chamber. Rapid degradation was observed when they were aged in natural sunlight at temperatures from - f 9 to + 38°C. For example, the half-lives on diesel soot particles at noon on June 15 were 0.8 h for both f-nitropyrene and 2-nitropyrene and f.2 h for 2-nitrofluoranthene. The half-life was 0.5 h for f-nitro-pyrene-dy on particles of wood soot. Overall, the authors concluded that the photodecomposition rates of these nitro-PAHs are strongly influenced by the physical and chemical nature of the substrates as seen in the photooxidation of PAHs discussed earlier, and on the solar radiation.
This is consistent with a study by Feilberg and Nielsen (1999b), who investigated the influence of other aerosol components on the photodegradation rates of representative particle-associated nitro-PAHs in a model system consisting of the nitro-PAH dissolved in cyclohexane along with various known constituents of diesel exhaust and wood smoke particles. These "coso-lutes" included PAHs, substituted phenols, hydroxy-PAHs, oxy-PAHs, and substituted benzaldehydes.
In the absence of cosolutes, the photodegradation rates depended on the orientation of the nitro group. Thus f-nitropyrene decayed relatively fast by the nitro-nitrite primary intramolecular photorearrange-ment process, followed by secondary radical reactions. However, 2-nitropyrene and 2-nitrofluoranthene were stable toward photolysis, consistent with the N02 group being in the same plane as the aromatic rings.
However, when H-atom-donating cosolutes, e.g., certain phenols, were added, the photodegradation rates of both 1-nitropyrene and 3-nitrofluoranthene increased. In this case, the reaction occurred via H-atom abstraction from the phenol by the electronically excited nitro-PAHs. Feilberg and Nielsen concluded that the photodegradation of nitro-PAHs on both diesel particles and wood smoke proceeds primarily by radical formation. However, H-atom abstraction by the excited triplet states of 1-nitropyrene and 2-nitrofluoranthene may also contribute.
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