Significant influence of water molecules on the SO + HCl reaction in the gas phase and at the air-water interface.

The products resulting from the reactions between atmospheric acids and SO have a catalytic effect on the formation of new particles in aerosols. However, the SO + HCl reaction in the gas-phase and at the air-water interface has not been considered. Herein, this reaction was explored exhaustively by using high-level quantum chemical calculations and Born Oppenheimer molecular dynamics (BOMD) simulations. The quantum calculations show that the gas-phase reaction of SO + HCl is highly unlikely to occur under atmospheric conditions with a high energy barrier of 22.6 kcal mol. HO and (HO) play obvious catalytic roles in reducing the energy barrier of the SO + HCl reaction by over 18.2 kcal mol. The atmospheric lifetimes of SO show that the (HO)-assisted reaction dominates over the HO-assisted reaction within the altitude range of 0-5 km, whereas the HO-assisted reaction is more favorable within an altitude range of 10-50 km. BOMD simulations show that HO-induced formation of the ClSO⋯HO ion pair and HCl-assisted formation of the HSO⋯HO ion pair were identified at the air-water interface. These routes followed a stepwise reaction mechanism and proceeded at a picosecond time scale. Interestingly, the formed ClSOH in the gas phase has a tendency to aggregate with sulfuric acids, ammonias, and water molecules to form stable clusters within 40 ns simulation time, while the interfacial ClSO and HO can attract HSO, NH, and HNO for particle formation from the gas phase to the water surface. Thus, this work will not only help in understanding the SO + HCl reaction driven by water molecules in the gas-phase and at the air-water interface, but it will also provide some potential routes of aerosol formation from the reaction between SO and inorganic acids.