Physical Properties and Atmospheric Reactivity of Aqueous Sea Salt Micro-Aerosols

Summary

Aqueous sea salt micro-aerosols play an important role in the heterogenous chemistry of the lower marine troposphere both in polluted and in remote areas. In particular, reactions with gases such as ozone or OH radicals leading to the release of molecular chlorine have been intensely studied, both experimentally and theoretically. Moreover, thin layers of sea water deposited on Arctic ice packs have been discovered to be a major source of reactive bromine species which destroy the surface ozone layer during polar sunrise. There is increasing evidence that the air-water interface is of a key importance in these chemical processes. Despite this, little has been known about the structure and physical properties of aqueous sea salt aerosols at a detailed, molecular level. Here, we summarize results of classical molecular dynamics, Car-Parrinello molecular dynamics and ab initio quantum chemistry calculations on concentrated aqueous sodium chloride and bromide solutions confined to cluster and slab geometries. The main questions addressed by the simulations concern the onset of NaC1 ionic solvation in water clusters, transition from clusters to slabs, structure of solvation layers and degree of ion pairing in concentrated solutions with confined geometries. A key result of the simulations is the observation that polarizable halogen anions (chloride and bromide) are present at the air-water interface of bulk solutions in amounts sufficient for the heterogenous atmospheric chemistry to take place. The calculations also reveal that bromide actually exhibits surfactant activity, i.e. its concentration at the interface is higher than in the bulk. This is in accord with the observed enhanced atmospheric reactivity of aqueous bromide compared to chloride and with SEM experiments on wetting and re-drying of NaCl/NaBr co-crystals.