Surfactants in cloud droplet activation: mixed organic-inorganic particles

Organic compounds with surfactant properties are commonly found in atmospheric aerosol particles. Surface activity can significantly influence the cloud droplet forming ability of these particles. We have studied the cloud droplet formation by two-component particles comprising one of the organic surfactants sodium octanoate, sodium decanoate, sodium dodecanoate, and sodium dodecyl sulfate, mixed with sodium chloride. Critical supersaturations were measured with a static diffusion cloud condensation nucleus counter (Wyoming CCNC-100B). Results were modeled from Köhler theory applying three different representations of surfactant properties in terms of surfactant surface partitioning and reduced droplet surface tension. We here confirm previous results for single-component organic surfactant particles, that experimental critical supersaturations are greatly underpredicted, if reduced surface tension is used while ignoring the effects of surface partitioning in droplets. Furthermore, disregarding surfactant properties by ignoring surface partitioning and assuming the constant surface tension of pure water can also lead to significant underpredictions of experimental critical supersaturations. For the mixed particles comprising less than 50% by mass of surfactant, this approach however still provides a good description of the observed droplet activation. A comprehensive account for surfactant properties, including both surface tension reduction and effects of surface partitioning in activating droplets, generally predicts experimental critical supersaturations well

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), respectively, 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 modeled 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. It was found that Köhler theory using the reduced surface tension with no account for surfactant partitioning under predicts experimental critical supersaturations significantly, whereas Köhler theory modified to account for surfactant partitioning reproduced the experimental data well