Mechanisms of reactivity of benzo(a)pyrene and other PAH inferred from field measurements

Özet

Semi-volatile polycyclic aromatic hydrocarbons (PAHs) partition between the vapour and condensed phases,both of which undergo oxidation. However, the relative contributions of oxidation in the vapour and condensedphases remain controversial. This is particularly important in respect of benzo(a)pyrene, which is responsible fora substantial proportion of the carcinogenic risk associated with airborne PAHs and is used as a marker for thePAH mixture in air quality standards. There is substantial evidence from field measurements that B(a)P is lessreactive in the atmosphere than low molecular weight PAHs, but many uncertainties remain regarding the rateand determinants of its atmospheric decay. Using novel methodology, changes in the ratios of PAHs duringatmospheric transport are used to infer the concentrations of OH and NO 3 radicals responsible for the gas phaseoxidation of low molecular weight PAHs. These are then used to examine the reactivity of B(a)P during atmo-spheric transport, and it is concluded that the loss of B(a)P can plausibly be described by the vapour phasereaction with the OH radical. These results offer an alternative mechanism to some others involving hetero-geneous chemistry proposed in the literature.