Photochemistry of SO2 at the Air-Water Interface: A Source of OH and HOSO Radicals.

The photochemistry of sulfur dioxide in the near UV-vis energy range has been studied in aqueous environments. The combination of previously reported experimental measurements with accurate quantum chemical calculations achieved in this work reveals that the process represents an important source of OH radicals in the troposphere. It implicates the reaction of the lowest triplet excited state of SO2 with a water molecule. When the process occurs in the gas-phase, photochemical OH production is only significant under high humidity conditions and high SO2 concentrations as those measured in polluted urban areas. However, the OH production rate increases by several orders of magnitude when the process takes place at the surface of water droplets. The present study indicates therefore that the atmospheric importance of sulfur dioxide goes beyond its well-known role as acid rain and aerosol formation precursor.