A Conceptual Framework for the Influence of Water Activity Parameterization on Hygroscopic Growth and Cloud Droplet Activation in Aerosols Containing Strong Nonionic Surfactants

Abstract

Discrepancies between observations of cloud droplet number concentrations observed during field studies and what are predicted from Köhler theory suggest surface active organics may impact hygroscopic growth. Surface active material that partitions to the droplet-air interface can reduce the surface tension, lowering the barrier to cloud droplet activation. However, when surface-active material partitions to the interface, it also increases the droplet water activity, having the opposite effect on cloud droplet activation. Although much effort has been focused on accurately describing the size- and composition-dependent surface tension, the influence of surface-active organics on water activity has received far less attention. Here, the impact of the description of water activity during hygroscopic growth and eventual cloud droplet activation is investigated. In AIOMFAC (Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients), two subgroup descriptions (ether-alkyl and oxyethylene) of the strong nonionic surfactants Triton X-100 and Tween20 and their mixtures with NaCl are used to calculate water activity for Köhler calculations. Although for low and medium organic content aerosol, the two AIOMFAC descriptions of the surfactants provide equivalent estimates of the critical supersaturation and radius, under high surfactant conditions, the oxyethylene subgroup description predicts a lower critical supersaturation than the ether-alkyl subgroup description. Under these high surfactant conditions, the oxyethylene subgroup also provides better agreement with available literature data for critical supersaturation.