A theoretical study of the H2SO4+H2O → HSO-4+H3O+ reaction at the surface of aqueous aerosols

The surface region of sulfate aerosols (supercooled aqueous concentrated sulfuric acid solutions) is the likely site of a number of important heterogeneous reactions in various locations in the atmosphere, but the surface region ionic composition is not known. As a first step in exploring this issue, the first acid ionization reaction for sulfuric acid, H2SO4 + H2O --> HSO4- + H3O+, is studied via electronic structure calculations at the Hartree-Fock level on an H2SO4 molecule embedded in the surface region of a cluster containing 33 water molecules. An initial H2SO4 configuration is selected which could produce H3O+ readily available for heterogeneous reactions, but which involves reduced solvation and is consistent with no dangling OH bonds for H2SO4. It is found that at 0 K and with zero-point energy included, the proton transfer is endothermic by 3.4 kcal/mol. This result is discussed in the context of reactions on sulfate aerosol surfaces and, further, more complex calculations.