Sulfur Dioxide Accelerates the Heterogeneous Oxidation Rate of Organic Aerosol by Hydroxyl Radicals

There remains considerable uncertainty in how anthropogenic gas phase emissions alter the oxidative aging of organic aerosols in the troposphere. Here we observe a 10–20 fold acceleration in the effective heterogeneous OH oxidation rate of organic aerosol in the presence of SO₂. This acceleration originates from the radical chain reactions propagated by alkoxy radicals, which are formed efficiently inside the particle by the reaction of peroxy radicals with SO₂. As the OH approaches atmospheric concentrations, the radical chain length increases, transforming the aerosol at rates predicted to be up to 10 times the OH-aerosol collision frequency. Model predictions, constrained by experiments over orders of magnitude changes in [OH] and [SO₂], suggest that in polluted regions the heterogeneous processing of organic aerosols by OH ([SO₂] ≥ 40 ppb) occur on similar time scales as analogous gas-phase oxidation reactions. These results provide evidence for a previously unidentified mechanism by which organic aerosol oxidation is enhanced by anthropogenic gas phase emissions.