FIGURE 10.31 (a) Preparative HPLC separation (acetonitrile-water solvent) of products of the dark reaction of BaP deposited on a glass fiber filter with ~ 200 ppb 03 in air (asterisk marks scale change), (b) Specific direct-acting mutagenic activity of HPLC fractions (rev ¡j,g~') on TA98, - S9. (c) Preparative HPLC separation of mutagenic fraction 13. (d) Dose-response curve for fraction 13-4 (TA98, - S9), BaP-4,5-oxide, whose specific activity is 1600 rev (adapted from Pitts et al., 1980). N.A., no activity.
10 ppm N02. The 24-h nitration yields for pyrene rose from 0.02% with f ppm NOz in air to 2.85% when traces of HNO-, were present.
Jäger and Hanus (1980) found the order of reactivity for the reactions of PAHs adsorbed on several substrates with 1.3 ppm of N02 in air to be silica gel > fly ash > deactivated aluminum oxide > carbon. The qualitative composition of nitro-PAH products, however, was independent of the substrate.
Subsequently, Ramdahl and co-workers (1984b) adsorbed six PAHs on several substrates and exposed them to 0.5 ppm of NOz in air containing water vapor and traces of HN03. The highest reactivity was observed for PAHs adsorbed on silica. The yields of three nitro-PAHs detected on alumina were only 14-24% of those on silica. The relative PAH reactivity order was perylene > benzo[a]pyrene > pyrene > chrysene > fluoranthene = phenanthrene, similar to that found by other researchers in their solution-phase studies (e.g., see Dewar et al., 1956; and Nielsen, 1984). A similar order for relative reactivity was observed by Butler and Crossley (1981) for the loss of PAH deposited on soot particles and exposed to N02 in air.
Guo and Kamens (1991) describe a system for studying gas-particle reactions on the surfaces of combustion aerosols in which they report a half-life of ~80 h for "high loadings" of particle-bound BaP in wood smoke particles reacting with ~200 ppb of N02 in air.
Relatively small amounts of gaseous nitric acid in the N02-air mixtures used to nitrate particle-phase
PAHs on various substrates in simulated ambient air or combustion systems appear to play an important role in the reactivity (e.g., see Pitts et al., 1978; Pitts, 1979; Hughes et al., 1980; Lindskog et al., 1985; and Yokley et al., 1985). Grosjean and co-workers (1983) found no reaction of nitric acid free N02 in air with BaP, perylene, and 1-nitropyrene deposited on several substrates. Although Wu and Niki (1985) did not report an important role for HN03 in their spectroscopic study of the reaction of N02 with pyrene deposited on a silicon surface, acidity could have been provided to some extent by the silica surface. Thus, once again, substrate complexities may be involved.
The nitration of PAHs by N02/HN03 also occurs under laboratory conditions approximating plume gases, that is, higher concentrations of gases and deposition on coal fly ash as a substrate. Thus, Hughes and co-workers (1980) reported that BaP and pyrene reacted with 100 ppm of NOz; the presence of nitric acid (possibly on the surface of the fly ash) enhanced the rate of reaction. Reactions proceeded more rapidly on silica gel than fly ash substrates, and for pyrene, both mono and dinitro isomers were formed. At the fOO ppm plume gas level, neither NO nor S02 reacted with BaP or pyrene on the substrates studied; both PAHs reacted with S03, but products were not characterized.
In a study simulating stack gas sampling, Brorstrom-Lunden and Lindskog (1985) found that addition of N02 caused substantial degradation of the reactive PAHs present in soot generated from a propane flame. A strong enhancing effect was observed when gaseous HC1 was added to the laboratory stack gases, for example, 90% loss of BaP in f h with added HC1 compared to 20% in its absence. They suggested that, in addition to other processes that might occur in the hot effluent stream, under these acidic conditions, sampling artifacts may be a major problem in sampling stack gases, often in times as short as 15-30 min or less.
The possibility of artifactual formation of nitro-PAHs during the sampling of diesel exhaust was addressed soon after their discovery in diesel particles (e.g., Lee et al., 1980; Lee and Schuetzle, 1983). Schuet-zle (1983) concluded that artifactual formation of nitro-PAHs "is a minor problem" (between 10 and 20% of the measured f-nitropyrene) at short sampling times [e.g., 23 min, which is one federal test procedure (FTP) driving cycle], at low sampling temperatures (42°C), and in diluted exhaust containing NOz.
The question of formation of nitroarenes during Hi-Vol sampling of ambient POM was considered in early studies (Pitts et al., 1978; Pitts, 1979) and addressed in several of the studies of PAH nitrations discussed above. In a definitive evaluation, Arey and co-workers (1988a) coated several perdeuterated PAHs
(e.g., perdeuterofluoranthene, perdeuteropyrene, and perdeutero-BaP) onto Hi-Vol filters loaded with previously collected ambient POM and exposed them for 7-fO h to ambient air during a high-NOx episode in southern California. Less than 3% of the total 1-nitro-pyrene collected during the episode was formed in the sampling process, and no formation of nitrofluoran-thene was observed. Hence the authors concluded artifactual formation of nitroarenes during Hi-Vol sampling (e.g., the nitrofluorenes and nitropyrenes) is not significant (see Arey et al., 1988a, for references to other studies).
Overall, while the combinations of substrate effects, ambient NOz levels, and other gas-particle phenomena preclude a definitive answer, the formation of significant amounts of nitroarenes in heterogeneous particle-phase N02-PAH, atmospheric reactions seems unlikely, e.g., much slower than photooxidation or ozonolysis. This conclusion also applies to heterogeneous reactions of N2Os with particle-bound PAHs on diesel and wood soot (Kamens and co-workers, 1990; see also Pitts et al., 1985c, 1985d, 1985e).
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