From O2--Initiated SO2 Oxidation to Sulfate Formation in the Gas Phase.

We have investigated the chemical fate of O2SOO-, the immediate product of the reaction between sulfur dioxide (SO2) and the superoxide ion (O2-), by reactions with nitrogen oxides (NO and NO2) using high-level theoretical calculations. Both reactions with NO and NO2 lead to exergonic formation of adducts, which subsequently overcome low energy barriers to form SO3 + NO3- and SO4- + NO, with rate constants of 6.9 × 10-10 and 6.3 × 10-10 cm3 molecule-1 s-1, respectively. Reactions with water are ∼15-23 times slower than corresponding naked reactions at ambient conditions, hence not slow enough to be prevented at these conditions. The studied reactions not only are useful for understanding ionic SO2 oxidation in the atmosphere and in chamber experiments but also provide a new mechanism for the gas-phase formation of sulfate from an ion-induced mechanism. These reactions may enhance our understanding of the early stages of secondary aerosol formation.