Quantum Chemical Study of Potential Energy Surface in the Formation of Atmospheric Sulfuric Acid

A new potential energy surface (PES) for the atmospheric formation of sulfuric acid from OH+SO2 is investigated using density functional theory and high-level ab initio molecular orbital theory. A pathway focused on the new PES assumes the reaction to take place between the radical complex SO3 center dot HO2 and H2O. The unusual stability of SO3 center dot HO2 is the principal basis of the new pathway, which has the same final outcome as the current reaction mechanism in the literature but it avoids the production and complete release of SO3. The entire reaction pathway is composed of three consecutive elementary steps: (1) HOSO2+O-2 -> SO3 center dot HO2, (2) SO3 center dot HO2+H2O -> SO3 center dot H2O center dot HO2, (3) SO3 center dot H2O center dot HO2 -> H2SO4+HO2. All three steps have small energy barriers, under 10 kcal/mol, and are exothermic, and the new pathway is therefore favorable both kinetically and thermodynamically. As a key step of the reactions, step (3), HO2 serves as a bridge molecule for low-barrier hydrogen transfer in the hydrolysis of SO3. Two significant atmospheric implications are expected from the present study. First, SO3 is not released from the oxidation of SO2 by OH radical in the atmosphere. Second, the conversion of SO2 into sulfuric acid is weakly dependent on the humidity of air.