Boosting aerosol surface effects: strongly enhanced cooperative surface propensity of atmospherically relevant organic molecular ions in aqueous solution

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 relevant for atmospheric aerosols, as they can affect crucial processes, such as chemical aging and water accommodation, as well as properties such as the surface tension, which drives droplet formation. Two exemplary types of atmospherically relevant organics are carboxylic acids and alkyl amines, and often both are found together within aerosols. In the most atmospherically significant 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 anions and the alkyl-ammonium cations of alkyl chain lengths of 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 of 1 : 1 molar ratio 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. These changes can significantly influence radiative forcing via aerosol growth, cloud condensation nuclei activity, and aerosol chemical aging. Our results demonstrate the principal feasibility of a more advanced input of molecular details for creating parameterized descriptions of aerosol surface composition needed to properly account for their impacts in climate models.