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

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 condensed phases remain controversial. This is particularly important in respect of benzo(a)pyrene, which is responsible for a substantial proportion of the carcinogenic risk associated with airborne PAHs and is used as a marker for the PAH mixture in air quality standards. There is substantial evidence from field measurements that B(a)P is less reactive in the atmosphere than low molecular weight PAHs, but many uncertainties remain regarding the rate and determinants of its atmospheric decay. Using novel methodology, changes in the ratios of PAHs during atmospheric transport are used to infer the concentrations of OH and NO3 radicals responsible for the gas phase oxidation of low molecular weight PAHs. These are then used to examine the reactivity of B(a)P during atmospheric transport, and it is concluded that the loss of B(a)P can plausibly be described by the vapour phase reaction with the OH radical. These results offer an alternative mechanism to some others involving heterogeneous chemistry proposed in the literature.