Determination of the influence of water on the SO 3 + CH 3 OH 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 SO 3 with CH 3 OH to form CH 3 OSO 3 H reduces the amount of H 2 SO 4 produced by the hydrolysis of SO 3 in regions polluted with CH 3 OH. However, the influence of the water molecule has not been fully considered yet, which will limit the accuracy of calculating the loss of SO 3 in regions polluted with CH 3 OH. Here, the influence of water molecules on the SO 3 + CH 3 OH 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 CH 3 OSO 3 H and H 2 SO 4 with water molecules have greatly lowered energy barriers compared to the naked SO 3 + CH 3 OH reaction. The effective rate coefficients reveal that H 2 O-catalyzed CH 3 OSO 3 H formation (a favorable route for CH 3 OSO 3 H formation) can be competitive with H 2 O-assisted H 2 SO 4 formation (a favorable process for H 2 SO 4 formation) at high altitudes up to 15 km. BOMD simulations found that H 2 O-induced formation of the CH 3 OSO 3 - ⋯H 3 O + ion pair and CH 3 OH-assisted formation of HSO 4 - and H 3 O + 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 SO 3 losses in the polluted areas of CH 3 OH.