Heterogeneous Reactions of SO3 on Ice: An Overlooked Sink for SO3 Depletion

The reaction between SO3 and water in the gas phase has always been of great interest, as it has important implications in atmospheric and environmental science. Compared to gas-phase water, however, heterogeneous hydration of SO3 on the surfaces of condensed phases of water/ice is relatively less explored. Here, we present a systematic study of the reactions between SO3 and three different phases of water, namely, water vapor, the surface of a liquid water droplet, and the {11-20} plane of hexagonal ice (Ih). The computational results show that, contrary to the gas-phase water, the surface of a water droplet and the {11-20} plane of Ih ice play distinctly different roles in the reaction, in which the HSO4-/H3O+ is formed within a few picoseconds. Moreover, the SO3 hydration exhibits multiple reaction pathways on the surface of a water droplet and the {11-20} plane of Ih ice, including a newly observed chemical mechanism without the formation of water-loop structures. Considering temperature effects (winter vs summer), SO3/water vapor concentrations in the atmosphere, and the effective surface areas of water droplets in the atmosphere and ice on the ground, the reaction rates of SO3 with water in the gas phase, in aerosols, in clouds, and on snowpack are estimated and compared. Consistent with previous experimental studies, the loss rates of SO3 due to aerosols and clouds are less important compared to that due to water vapor. Surprisingly, the ice snowpack is shown to be an efficient sink for SO3 depletion, especially in the winter season.