Reactions of sulfur trioxide with hypochlorous acid catalyzed by water in gas phase and at the air-water nanodroplet interface in the atmosphere: An important sink for hypochlorous acid

The atmospheric chemistry of sulfur trioxide (SO3) mediated by organic acids is crucial for the formation of sulfuric acid and aerosol particles. Herein, high-level quantum chemistry combined with ab initio molecular dynamics has been used to study the reactions of SO3 with hypochlorous acid (HOCl) in gas phase and at the air-water nanodroplet interface. The results show that water plays a strong catalytic role in the reaction between HOCl and SO3 in the gas phase. Moreover, kinetics calculation indicates that the HOCl + SO3 + H2O reaction can compete well with the HOCl + OH reaction during the nighttime. We also find that the reaction between SO3 and HOCl occurs rapidly, resulting in the formation of an ion pair particle SO4Cl-…H3O+ with the free energy barrier of about 0.51 kcal·mol-1 at the air-water nanodroplet interface. Specifically, the process begins with the formation of a complex between HOCl and water, which then attach to SO3 through an O-S bond formation, and finally undergoes a proton transfer process to form ion pairs particles. The present findings also show that the air-water nanodroplet interface of HOCl significantly dominates over the corresponding gas phase reactions reported here. Furthermore, the products of this reaction can be further converted into organic sulfates, thereby promoting the formation and growth of aerosol particles.