Theoretical and Experimental Structure - Reactivity Relationships

In their thoughtful 1983 review, Nielsen and coworkers noted that particles of diesel soot or wood smoke can absorb significant amounts of water. Thus, they suggested that the most plausible mechanism(s) for nitration (and possibly other electrophilic reactions) of particle-associated PAHs in ambient air may involve reactions both in a liquid film and on solid surfaces and that fundamental laboratory studies of the rates, products, and mechanisms of PAHs in polar solvents would be atmospherically relevant for reactions in the liquid films. Based on this, they proposed a classification scheme for the reactivities of key PAHs in electrophilic reactions, which was subsequently described in detail (Nielsen, 1984). 

Given the complexities of the system involved, it is interesting that this reactivity scale has proven to be a useful predictor not only for overall relative decay rates of PAHs associated with aerosols in ambient air but 

In developing the reactivity scale, Nielsen first investigated the transformation rates of 25 PAHs and four derivatives of anthracene in water-methanol-dioxane solutions, taken as a model of wet particles, and containing small amounts of dinitrogen tetroxide and nitric and nitrous acids. The measured half-lives and relative rates are shown in Table 10.29. The range of reactivities in solution for PAHs of different structures is remarkable, from 100,000 (arbitrarily set) for an-thanthrene (XXXII) to 0.2 for the least reactive compounds  

The half-lives of the powerful carcinogens increase from 200 min for BaP to 300 days for benzo[6]fluo-ranthene and 800 days for indeno[l,2,3-ot]pyrene, with the relative reactivities decreasing from 1100 to 0.5 and 0.2, respectively, an overall factor of -5500. 

Even taken qualitatively, these reactivity data have important toxicological as well as chemical implications regarding the composition of PAHs and PACs in and on the surfaces of aerosols in polluted air parcels, both near-source and during transport (downwind). Thus, under certain conditions (e.g., daytime, summer season, and high oxidant levels) over a period of hours BaP concentrations in ambient air could be expected to decay dramatically as a result of reactions, while those of the benzofluoranthenes and indeno[l,2,3-c i]pyrene would be expected to remain relatively constant. Of course, their absolute concentrations also change as a result of dilution of the air parcel caused by increased mixing depth over time and transport. However, impacts of such physical processes are minimized if one considers ratios of concentrations of reactive to nonre- 

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