Cloud droplet activation: solubility revisited

Abstract

Soluble compounds present in atmospheric aerosol facilitate their transformation into cloud droplets by depressing the equilibrium vapor pressure required for activation. Their impact depends on the amount of dissolved substance in the aerosol aqueous phase, which in turn is controlled by its solubility. This study explores the impact of particle curvature on solubility, expressed in terms of a Kelvin enhancement. The augmented solubility, termed 'Curvature Enhanced Solubility' (CES), is then introduced into Köhler theory for assessment of its impact on CCN activity for several organic compounds with a wide range of aqueous solubility. The interfacial energy between solute and aqueous phase required for quantification of CES is determined from existing correlations based on bulk solubility, and concurrent measurements of contact angle and surface tension. A number of important findings arise from this study: i) CES can substantially increase solubility and impact CCN activity but only if the aerosol is initially wet, ii) CES can stabilize highly supersaturated solutions, and provide a mechanism for retention of an aerosol aqueous phase even at very low relative humidity (RH), and, iii) trace amounts of surfactant impurities can magnify the impact of CES.