Reaction of SO3 with H2SO4 and Its Implication for Aerosol Particle Formation in the Gas Phase and at the Air-Water Interface

Abstract. The reactions between SO3 and atmospheric acids are indispensable in improving the formation of aerosol particle. However, relative to those of SO3 with organic acids, the reaction of SO3 with inorganic acids has not received much attention. Here, we explore the atmospheric reaction between SO3 and H2SO4, a typical inorganic acid, in the gas phase and at the air-water interface by using quantum chemical (QC) calculations and Born-Oppenheimer molecular dynamics simulations. We also report the effect of H2S2O7, the product of the reaction between SO3 and H2SO4, on new particle formation (NPF) in various environments by using the Atmospheric Cluster Dynamics Code kinetic model and the QC calculation. The present findings show that the gas phase reactions of SO3 + H2SO4 without and with water molecule are both low energy barrier processes. With the involvement of interfacial water molecules, H2O-induced the formation of S2O72-⋅⋅⋅H3O+ ion pair, HSO4- mediated the formation of HSO4-⋅⋅⋅H3O+ ion pair and the deprotonation of H2S2O7 were observed and proceeded on the picosecond time-scale. The present findings suggest the potential contribution of SO3-H2SO4 reaction to NPF and aerosol particle growth as the facts that i) H2S2O7 can directly participate in H2SO4-NH3-based cluster formation and can facilitate the fastest possible rate of NPF from H2SO4-NH3-based clusters by about a factor of 6.92 orders of magnitude at 278.15 K; and ii) the formed interfacial S2O72- can attract candidate species from the gas phase to the water surface, and thus, accelerate particle growth.