Reaction of SO 3 with H 2 SO 4 and its implications 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 particles. 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 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 using the Atmospheric Cluster Dynamics Code (ACDC) kinetic model and QC calculations. The present findings show that the gas-phase reactions of SO3 + H2SO4 without and with water molecules are both low-energy-barrier processes. With the involvement of interfacial water molecules, H2O induced the formation of the S2O72-⋯H3O+ ion pair, HSO4- mediated the formation of the HSO4-⋯H3O+ ion pair, and the deprotonation of H2S2O7 was observed and proceeded on the picosecond timescale. The present findings suggest the potential contribution of the SO3–H2SO4 reaction to NPF and aerosol particle growth, showing that (i) although H2S2O7 is easily hydrolyzed with water to form H2SO4, it can directly participate in H2SO4–NH3-based cluster formation and can present a more obvious enhancement effect on SA–A-based cluster formation, and (ii) the formed interfacial S2O72- can attract candidate species from the gas phase to the water surface and, thus, accelerate particle growth.