Significant influence of water molecules on the SO₃ + HCl reaction in the gas phase and at the air-water interface

The products resulting from the reactions between atmospheric acids and SO₃ have a catalytic effect on the formation of new particles in aerosols. However, the SO₃ + 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 SO₃ + HCl is highly unlikely to occur under atmospheric conditions with a high energy barrier of 22.6 kcal mol^-1. H₂O and (H₂O)₂ play obvious catalytic roles in reducing the energy barrier of the SO₃ + HCl reaction by over 18.2 kcal mol^-1. The atmospheric lifetimes of SO₃ show that the (H₂O)₂-assisted reaction dominates over the H₂O-assisted reaction within the altitude range of 0-5 km, whereas the H₂O-assisted reaction is more favorable within an altitude range of 10-50 km. BOMD simulations show that H₂O-induced formation of the ClSO₃^-⋯H₃O⁺ ion pair and HCl-assisted formation of the HSO₄^-⋯H₃O⁺ 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 ClSO₃H 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 ClSO₃^- and H₃O⁺ can attract H₂SO₄, NH₃, and HNO₃ for particle formation from the gas phase to the water surface. Thus, this work will not only help in understanding the SO₃ + 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 SO₃ and inorganic acids.