The Influence of Saturation on the Surface Structure of Mixed Fatty Acid-on-Water Aerosol: A Molecular Dynamics Study

The reactivity of C=C groups in aerosols towards incoming species is highly influential to their chemical evolution and thus plays an important role in determining the properties of these aerosols and their impact on a variety of atmospheric processes. Reactions between aerosol components and gas-phase radical species often occur at the gas-liquid interface of the aerosol and thus the availability of different groups at this interface is central to determining their reactivities towards these species and the rates of these reactions. Here we look at model aerosol systems, C18 fatty acids on water, and carry out molecular dynamics simulations to determine how the presence of ‘inert’, fully saturated stearic acid molecules affects the accessibility of alkene groups within oleic acid molecules to ozone radicals. A range of stearic acid : oleic acid ratios have been studied, and a methodology has been developed in order to grow the organic layer in a random and stepwise manner that as closely as possible mimics growth in the atmosphere. The surface presence of HC=CH was found to undergo a near-linear decrease as the stearic component in the slab increased, however, the coverage of other groups was found to vary in a less linear fashion and there was an increase in the overall ordering of the organic components as the stearic acid concentration increased. It was concluded that the presence of fully saturated fatty acids is unlikely to significantly alter the rate of oxidation of unsaturated species at the surface of atmospheric aerosols, however, it cannot be ruled out that differences in the overall structure of the aerosols with varying compositions could affect the rate of ozonolysis of these within the bulk of the organic layer.