Ion Partitioning at the Liquid/Vapor Interface of a Multicomponent Alkali Halide Solution: A Model for Aqueous Sea Salt Aerosols

The chemistry of Br species associated with sea salt ice and aerosols has been implicated in the episodes of ozone depletion reported at Arctic sunrise. However, Br- is only a minor component in sea salt, which has a Br-/Cl- molar ratio of similar to 0.0015. Sea salt is a complex mixture of many different species, with NaCl as the primary component. In recent years experimental and theoretical studies have reported enhancement of the large, more polarizable halide ion at the liquid/vapor interface of corresponding aqueous alkali halide solutions. The proposed enhancement is likely to influence the availability of sea salt Br- for heterogeneous reactions such as those involved in the ozone depletion episodes. We report here ambient pressure X-ray photoelectron spectroscopy studies and molecular dynamics simulations showing direct evidence of Br- enhancement at the interface of an aqueous NaCl solution doped with bromide. The experiments were carried out on samples with Br-/Cl- ratios in the range 0.1% to 10%, the latter being also the ratio for which simulations were carried out. This is the first direct measurement of interfacial enhancement of Br- in a multicomponent solution with particular relevance to sea salt chemistry.