Boosting aerosol surface effects: Strongly Enhanced Cooperative Surface Propensity of Atmospherically Relevant Organic Molecular Ions in Aqueous Solution

Abstract. The effects of atmospheric aerosols are key uncertainties in climate models. One reason is the complex aerosol composition which includes a relatively large fraction of organics. Another reason is the small size of aerosols, which makes surface effects and processes important. These two factors make surface-active organics important for atmospheric aerosols, as they can affect important processes, such as chemical aging and water accommodation, as well as properties such as the surface tension, which drives droplet formation. Two important types of atmospherically relevant organics are carboxylic acids and alkyl amines, and often both are found together within aerosols. In the most atmospherically relevant pH range, these exist as alkyl carboxylate ions and alkyl ammonium ions, Using liquid-jet photoelectron spectroscopy, tuned to high surface sensitivity, we measured the alkyl carboxylate cations and the alkyl ammonium anions of alkyl chain lengths 1 to 6 carbon atoms, both as single-component and mixed-component aqueous solutions. This enabled us to systematically study how their surface propensity is affected by the length of the alkyl chains, and how cooperative ion–ion interactions result in strongly increased surface propensity. An exponential increase in surface propensity is found for the single-species solutions, with cooperative solute-solute effects in mixed solutions drastically increasing the number of molecules present at the solutions' surfaces up to a factor of several hundred. This cooperative surface propensity is shown to strongly affect the amounts of organics at the surface, with pronounced chain length-dependent variations. Our results demonstrate that the surface composition of these water-organics systems can be very different from the bulk composition, and that the surface compositions of organic mixtures cannot be directly inferred from the behaviour of the single components. For aerosols containing these or similar species, this means that all surface-related properties and processes will be enhanced, and implications for atmospherically relevant processes such as water accommodation, droplet formation, and chemical aging, are discussed.