Co-condensation and co-evaporation of levoglucosan onto and from deliquesced ammonium sulfate particles – influence of relative humidity, particle mass and size, and presence of a surfactant

Abstract. Co-condensation is the process by which condensable vapors condense alongside water vapor onto growing aerosols, and it can significantly alter the properties of clouds. Semi-volatile species partition dynamically between the gas and the condensed phases, and their co-condensation together with water vapor may increase the condensed mass and amplify water uptake. Although this process is based on thermodynamics and has been simulated in models, it has been scarcely investigated experimentally. In this study, the ability of levoglucosan to co-condense and co-evaporate from inorganic particles together with water was investigated by monitoring, under ambient-like conditions, its gas-particle partitioning on ammonium sulfate (AS) monodispersed particles in the Experimental Multiphasic Atmospheric Simulation Chamber (CESAM). The net evaporation flux of levoglucosan depending on particle size and the presence of a surfactant was explored for relative humidities (RH) from 100 % to dry conditions as particle concentration in the chamber was reduced. Due to the high deliquescence point of AS, wet experiments were initialized at RH above 80 %. Co-condensation of levoglucosan was observed when RH increased up to 100 %, while co-evaporation of levoglucosan occurred when RH decreased. It was shown that gas-particle partitioning of levoglucosan was sensitive to all the investigated parameters, but the main drivers were the levoglucosan-to-sulfate mass ratios and RH. The effect of the levoglucosan-to-sulfate mass ratio was interpreted as salting-out, and the significant influence of RH provides experimental proof of co-evaporation or co-condensation of levoglucosan from or onto AS particles.