Surfactant partitioning in cloud droplet activation: a study of C8, C10, C12 and C14 normal fatty acid sodium salts

We have measured critical supersaturations of dried single-component particles of sodium caprylate [CH3(CH2)6COONa], sodium caprate [CH3(CH2)8COONa], sodium laurate [CH3(CH2)10COONa] and sodium myristate [CH3(CH2)12COONa] in the diameter range 33–140 nm at 296 K using a static thermal gradient diffusion cloud condensation nucleus counter. These fatty acid sodium salts are surface active molecules which have all been identified in atmospheric aerosol particles. Experimental critical supersaturations increased systematically with increasing carbon chain length and were in the range 0.96–1.34% for particles with a dry diameter of 40 nm. The experimental data were modelled using Köhler theory modified to account for partitioning of the surface active fatty acid sodium salts between the droplet bulk and surface as well as Köhler theory including surface tension reduction without accounting for surfactant partitioning and Köhler theory using the surface tension of pure water. It was found that Köhler theory using the reduced surface tension with no account for surfactant partitioning underpredicts experimental critical supersaturations significantly, whereas Köhler theory modified to account for surfactant partitioning and Köhler theory using the surface tension of pure water reproduced the experimental data well.