Cloud drop activation of insoluble aerosols aided by film forming surfactants

Abstract. Cloud droplet activation of insoluble aerosols covered by insoluble surfactant films has been studied theoretically by combining the FHH activation theory and an equation of state suitable for surfactant films that are in an expanded state. The key parameters governing the surfactant's ability to suppress critical supersaturations are its partial molecular area at the water surface, and the size of the molecule. For a fixed size, molecules with larger molecular area are more efficient, whereas with a fixed area-to-volume ratio, smaller molecules are more efficient. Calculations made for stearic acid films on black carbon and illite aerosols indicate that the critical supersaturations are significantly lower than with pure particles, especially when the dry particle sizes are several hundred nanometers and larger. Furthermore, the reductions of the critical supersaturation are similar when stearic acid is replaced by water-soluble organics with hygroscopicity parameter (κ) values up to 0.1. However, mixtures of surfactant and water-soluble organics are relatively weaker in reducing critical supersaturations than either of these compounds alone.