Erroneous Use of the Modified Kohler Equation in Cloud and Aerosol Physics Applications

Abstract

The phase equilibrium equation between water vapor and a liquid droplet is an important tool of cloud physics. Although several different forms of the modified Kohler equation were derived in the past, the authors show that a thermodynamically consistent treatment leads to the original Kohler equation. Recent experimental measurements of the condensational growth of a single levitated ammonium sulfate solution droplet are compared with theoretical calculations using 1) the Kohler equation with a constant van?t Hoff factor, 2) using the Kohler equation with the van?t Hoff factor as a function of ammonium sulfate concentration, and 3) using the recently derived equilibrium equation containing an additional term proportional to the first derivative of the van?t Hoff factor with respect to solute concentration. It was found that the Kohler equation with van?t Hoff factor as a function of solute concentration provides the best agreement with experimental data.