Cloud droplet activation of organic–salt mixtures predicted from two model treatments of the droplet surface

Abstract The droplet surface plays important roles in the interaction between organic aerosols with clouds and climate. Surface active organic compounds can partition to the droplet surface, depleting solute from the droplet bulk or depressing the droplet surface tension. This may in turn affect the shape of the droplet growth curve, threshold of aerosol activation into cloud droplets, as well as activated droplet size distributions, and cloud radiative effects. In this work, a new monolayer model along with a traditional Gibbs adsorption isotherm model were used in conjunction with equilibrium Köhler theory to predict CCN activation of both simple and complex surface active model aerosol systems. For the surface active aerosol considered, the monolayer droplet model produces similar results to the Gibbs model as well as comparable results to CCN measurements from literature, even for systems where specific molecular identities and aqueous properties are unknown. The monolayer model is self-contained, fully prognostic, and provides a versatile, conceptually simple, yet physically-based model for understanding the role of organic surfactants in cloud droplet formation