The Kelvin versus the Raoult Term in the Kohler Equation

A comprehensive sensitivity study was carried out examining the sensitivity of hygroscopic growth and activation as modeled with the Kohler equation. Different parameters in the Kohler equation were varied within the range of their currently known uncertainties. The parameters examined include not only those describing the nature of the soluble substances in a particle/droplet and the surface tension sigma of the droplet solution, but also the recently proposed representation of parameters coupling the Raoult and Kelvin terms (i.e., partitioning of solute between the surface and bulk phases, although the recently proposed adsorption to wettable but insoluble material was not considered). The examined variations cause significant changes in both hygroscopic growth and activation. Whereas the hygroscopic growth regime below 95% RH is insensitive toward the surface tension sigma,sigma has a large influence on the activation, increasing with decreasing particle size. This implies that a cloud condensation nuclei (CCN) closure, connecting particle hygroscopic growth to activation, has to account for an influence of the examined substance on sigma of the particle, especially for smaller particles in the size range from 50 to 100 nm. A simple estimate showed that a lowering of sigma by only 10% can cause a change in the activated fraction (i.e., in the cloud droplet number concentration) of at least 10%-20%. Where organic molecules are present in sufficient concentration to reduce sigma, surface tension may be an important factor in determining the activation of aerosol particles to cloud droplets.