PNNL

Abstract

The propensity of certain atomic and small molecular ions for the water/vapor interface has important implications for heterogeneous chemistry at aqueous surfaces. In particular, important tropsopheric reactions involving polluting gases such as ozone or OH and leading to the production of reactive halogen compounds take place at the surface of aqueous sea salt aerosols in the lower marine troposphere(1,2) or salt solution films sprayed over the polar snowpack(3). There is mounting computational and experimental evidence that heavier halides (iodine, bromide, and, to lesser extent, chloride) penetate the aqueous surface, where they can react with atmospheric gases. Among small molecular ions, nitrate and azide were also shown to exhibit a similar surface behavior, Note that the propensity for the water/vapor interface of these ions cannot be attributed to hydrophobicity since they lack any hydrophobic center. Results of molecular dynamics (MD) simulations indicate that in these caseds the main force that drives the above anions to the aqueous surface is their large polarizability (softness).