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

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 S₂O₇^2-⋅⋅⋅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 S₂O₇^2- can attract candidate species from the gas phase to the water surface, and thus, accelerate particle growth. [ABSTRACT FROM AUTHOR]