Gas-phase hydrolysis of triplet SO2: A possible direct route to atmospheric acid formation

Sulfur chemistry is of great interest to the atmospheric chemistry of several planets. In the presence of water, oxidized sulfur can lead to new particle formation, influencing climate in significant ways. Observations of sulfur compounds in planetary atmospheres when compared with model results suggest that there are missing chemical mechanisms. Here we propose a novel mechanism for the formation of sulfurous acid, which may act as a seed for new particle formation. In this proposed mechanism, the lowest triplet state of SO 2 ( 3 B 1 ), which may be accessed by near-UV solar excitation of SO 2 to its excited 1 B 1 state followed by rapid intersystem crossing, reacts directly with water to form H 2 SO 3 in the gas phase. For ground state SO 2 , this reaction is endothermic and has a very high activation barrier; our quantum chemical calculations point to a facile reaction being possible in the triplet state of SO 2 . This hygroscopic H 2 SO 3 molecule may act as a condensation nucleus for water, giving rise to facile new particle formation (NPF).