Significant influence of water molecules on the SO 3 + HCl reaction in the gas phase and at the air-water interface.

The products resulting from the reactions between atmospheric acids and SO 3 have a catalytic effect on the formation of new particles in aerosols. However, the SO 3 + 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 3 + HCl is highly unlikely to occur under atmospheric conditions with a high energy barrier of 22.6 kcal mol -1 . H 2 O and (H 2 O) 2 play obvious catalytic roles in reducing the energy barrier of the SO 3 + HCl reaction by over 18.2 kcal mol -1 . The atmospheric lifetimes of SO 3 show that the (H 2 O) 2 -assisted reaction dominates over the H 2 O-assisted reaction within the altitude range of 0-5 km, whereas the H 2 O-assisted reaction is more favorable within an altitude range of 10-50 km. BOMD simulations show that H 2 O-induced formation of the ClSO 3 - ⋯H 3 O + ion pair and HCl-assisted formation of the HSO 4 - ⋯H 3 O + 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 ClSO 3 H 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 3 - and H 3 O + can attract H 2 SO 4 , NH 3 , and HNO 3 for particle formation from the gas phase to the water surface. Thus, this work will not only help in understanding the SO 3 + 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 3 and inorganic acids.