Chemical Transformation of Polycyclic Aromatic Hydrocarbons via Heterogeneous Reactions

Abstract

The heterogeneous reactions of atmospheric oxidants with aerosol particles are of central importance to air quality. Polycyclic aromatic hydrocarbons (PAHs) are efficiently oxygenated upon exposure to ozone, which leads to an enhancement in toxicity. For a wide range of substrates, including solid and liquid organic and inorganic substances, the concentration and time-dependence of the heterogeneous reaction between PAHs and O_3 can be well described by a Langmuir-Hinshelwood mechanism. Competitive adsorption and chemical transformation of the surface lead to a strong non-linear dependence of uptake coefficient of O_3 on time and gas phase composition, with different characteristics under dilute atmospheric and concentrated laboratory conditions. Long-lived reactive oxygen intermediates (ROIs) are formed upon decomposition of O_3 on PAH surface. The ROIs reconcile apparent discrepancies between earlier quantum mechanical calculations and kinetic experiments. They play a key role in the chemical transformation and adverse health effects of toxic and allergenic air particulate matter. Moreover, ROIs may contribute to the coupling of atmospheric and biospheric multiphase processes.

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