Surface functionality of sub- to full-monolayer organic coverage of water aerosols determined by molecular dynamics simulations

Abstract

The role that organic aqueous aerosols play within the atmosphere is highly dependent on their structures. This is influenced not only by the components that make up the organic layer of the aerosol, but also on the way that they pack together, as this affects the accessibility of key functional groups to incoming gaseous species. This is in turn impacted by the total coverage of organic molecules on the aerosol core. Here we present the results of a molecular dynamics study into how the surface coverage of three C18 fatty acids, stearic, oleic and linoleic acid, affects the structure of the resulting aerosol. Surface coverages ranging from significantly submonolayer to above monolayer coverages have been investigated. Acid chains are found to pack more tightly at higher coverages, and to be directed more along the surface normal. The surfaces are dominated by CH₃ and CH₂ groups at all coverages, however, changes in surface presence of HCCH and COOH can be used as a marker of when the monolayer coverage has been exceeded. The organic layers have been grown in a random and stepwise manner designed to mimic natural growth of aerosols in the atmosphere. Analysis focusses on the accessibility of different groups to an incoming ozone molecule and thus gives a measure of how the reactivity of aerosols may be affected by the organic surface coverage.