Experiments and Simulations of Ion-Enhanced Interfacial Chemistry on Aqueous NaCl Aerosols

TLDR

The study combines experiments, molecular dynamics, and kinetic modeling of NaCl aerosol exposed to ozone and UV to show that chloride ions at the air‑water interface are readily available and strongly attract hydroxyl radicals. Gaseous chlorine product measurements can only be explained by dominant interfacial reactions, and model predictions of daytime Cl atom concentrations agree with field estimates, indicating ion‑enhanced gas‑surface chemistry is a significant atmospheric process.

Abstract

A combination of experimental, molecular dynamics, and kinetics modeling studies is applied to a system of concentrated aqueous sodium chloride particles suspended in air at room temperature with ozone, irradiated at 254 nanometers to generate hydroxyl radicals. Measurements of the observed gaseous molecular chlorine product are explainable only if reactions at the air-water interface are dominant. Molecular dynamics simulations show the availability of substantial amounts of chloride ions for reaction at the interface, and quantum chemical calculations predict that in the gas phase chloride ions will strongly attract hydroxl radicals. Model extrapolation to the marine boundary layer yields daytime chlorine atom concentrations that are in good agreement with estimates based on field measurements of the decay of selected organics over the Southern Ocean and the North Atlantic. Thus, ion-enhanced interactions with gases at aqueous interfaces may play a more generalized and important role in the chemistry of concentrated inorganic salt solutions than was previously recognized.