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 (Frenkel–Halsey–Hill) activation theory and an equation of state suitable for surfactant films that are in an expanded state. The key parameters governing the ability of the surfactant 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 a 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 in critical supersaturation are similar when stearic acid is replaced by water-soluble organics with values of the hygroscopicity parameter (κ) of up to 0.5. However, mixtures of surfactant and water-soluble organic compounds are relatively weaker at reducing critical supersaturations than either of these compounds alone, which is caused by dilution of the surfactant film as the dissolved organic causes increased uptake of water vapor in the critical droplets. The theory has also been tested against experimental results on the impact of oleic acid films on the activation of calcite particles.