Determination of the influence of water on the SO3 + CH3OH reaction in the gas phase and at the air–water interface

Abstract

Liu et al. (Proc. Natl. Acad. Sci. U. S. A, 2019, 116, 24966–24971) showed that at an altitude of 0 km, the reaction of SO₃ with CH₃OH to form CH₃OSO₃H reduces the amount of H₂SO₄ produced by the hydrolysis of SO₃ in regions polluted with CH₃OH. However, the influence of the water molecule has not been fully considered yet, which will limit the accuracy of calculating the loss of SO₃ in regions polluted with CH₃OH. Here, the influence of water molecules on the SO₃ + CH₃OH reaction in the gas phase and at the air–water interface was comprehensively explored by using high-level quantum chemical calculations and Born–Oppenheimer molecular dynamics (BOMD) simulations. Quantum chemical calculations show that both pathways for the formation of CH₃OSO₃H and H₂SO₄ with water molecules have greatly lowered energy barriers compared to the naked SO₃ + CH₃OH reaction. The effective rate coefficients reveal that H₂O-catalyzed CH₃OSO₃H formation (a favorable route for CH₃OSO₃H formation) can be competitive with H₂O-assisted H₂SO₄ formation (a favorable process for H₂SO₄ formation) at high altitudes up to 15 km. BOMD simulations found that H₂O-induced formation of the CH₃OSO₃⁻⋯H₃O⁺ ion pair and CH₃OH-assisted formation of HSO₄⁻ and H₃O⁺ ions were observed at the droplet surface. These interfacial routes followed a loop-structure or chain reaction mechanism and proceeded on a picosecond time scale. These results will contribute to better understanding of SO₃ losses in the polluted areas of CH₃OH.