A predictive thermodynamic framework of cloud droplet activation for chemically unresolved aerosol mixtures, including surface tension, non-ideality, and bulk–surface partitioning

Abstract. This work presents a thermodynamically consistent framework that enables self-contained, predictive Köhler calculations of droplet growth and activation with considerations surface adsorption, tension reduction, non-ideal water activity for chemically complex unresolved surface-active aerosol mixtures. The common presence species in atmospheric aerosols is now well-established. However, the impacts different effects driven by activity, particular bulk–surface partitioning resulting bulk depletion and/or on hygroscopic cloud remain to be generally established. Because specific characterization key properties, including tension, remains exceedingly challenging finite-sized activating droplets, self-contained model needed resolve individual during activation. Previous frameworks have achieved this simple mixtures, comprising at most few well-defined chemical species. mixtures more realistic laboratory systems are typically not (unresolved). Therefore, which require knowledge concentrations all mixture their composition-dependent interactions cannot applied. For or where between components unknown, analytical models based retrofitting can applied, represented proxy compound well-known properties. evaluated such independently verified. presented couples theory Gibbs adsorption Szyszkowski-type equations. Contrary previous thermodynamic frameworks, it formulated mass basis obtain quantitative description properties Application illustrated calculating condensation nuclei (CCN) particles Nordic aquatic fulvic acid (NAFA), strongly humic-like substance (HULIS), NaCl, dry diameters 30–230 nm compositions spanning full range relative NAFA NaCl mixing ratios. comparison presented, several other simplified treatments NAFA, also Effects gauged via suite growing droplets. predicts similar influence CCN as was previously shown single, strong surfactants. Comparison experimental data shows well-represented thermodynamics. recent studies, no evidence significantly reduced point found. Calculations show throughout activation, finite amounts microscopic submicron droplets depleted from bulk, due partitioning, because areas given volume very large. As result, both effective hygroscopicity ability reduce lower than macroscopic aqueous solutions same overall composition. actual samples, systematically explored. Thermodynamic input parameters constrained measurements, instead being either approximated determined retrofitting, potentially confounding mechanisms influenced activity.