Amino acids in atmospheric droplets: perturbation of surface tension and critical supersaturation predicted by computer simulations

Abstract. Atmospheric amino acids constitute an important fraction of the water-soluble organic nitrogen compounds in both marine and continental aerosols, and have been confirmed as effective cloud condensation nuclei materials in laboratory tests. We here present a molecular dynamics study of amino acids representative for the remote marine atmospheric aerosol-cloud system, in order to investigate molecular distributions, orientations and induced changes in surface tension, and to evaluate their indirect effects on optical properties of clouds. These L-amino acids, including serine, glycine, alanine, valine, methionine and phenylalanine, are categorized as hydrophilic and hydrophobic according to their affinities to water. Different amino acids show distinct effects on the surface tension; even the same amino acid has different influence on the surface tension for planar and spherical interfaces. The curvature dependence of the surface tension is modelled by a quadratic polynomial function of the inverse of droplet radius, and such relationship is used to improve the Köhler equation in predicting the critical water vapour supersaturation of the droplet activation.