Effect of adsorption on the uptake of organic trace gas by cloud droplets

Adsorption can greatly influence the partitioning of organic trace species between the gas and liquid phases. Here we investigate how adsorption of organic species at the airwater interface in clouds can increase the amount of trace gas found in cloud water. For atmospherically relevant cloud surface areas and realistic ranges of adsorption parameters, we show that a significant amount of organic molecules can reside at the cloud droplet surface. Two different physical situations are considered corresponding to the cases where the surfactant is water-soluble and where it is water-insoluble. For both cases, numerical calculations were carried out for a wide range of adsorption parameters, effective Henry's law constants (the former case only), and cloud surface areas. For a soluble surfactant, our calculations show that the "overall" Henry's law constant, which takes into account for both adsorption at the droplet surface and partitioning into the droplet interior, can significantly differ from the standard Henry's law constant. For a moderately soluble species the overall uptake can strongly depend on the surface coverage if the species has a strong affinity for partitioning into the surface layer. Our results also suggest that even insoluble and/or scarcely soluble species can be scavenged by clouds if they are surface-active.