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

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-MIDLINE HORIZONTAL ELLIPSISH3O+ 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.