Hygroscopicity of Isoprene-Derived Secondary Organic Aerosol Mixture Proxies: The Importance of Solute Diffusion and Salting-In Effects

Abstract. Isoprene-derived secondary organic aerosol (SOA) components, such as the 2-methyltetrols (2-MT) and 2-methyltetrol sulfates (2-MTS), have been readily detected in atmospheric aerosols. SOA commonly exist in aerosol mixtures containing inorganic salts, such as ammonium sulfate (AS). Despite its prevalence in the atmosphere, the water uptake of 2-MT, 2-MTS, and their mixtures are not well understood. In this study, we determine the physicochemical properties of 2-MT, 2-MTS, and their mixtures with AS. 2-MT and 2-MTS are viscous and dynamic surface tension measurements were taken to determine organic diffusion coefficients. The droplet growth was measured and both subsaturated and supersaturated hygroscopicity are parameterized by the single hygroscopicity parameter κ.  Furthermore, aerosol phase state and morphology were analysed using atomic force microscopy. Results show that solute diffusion and salting-in influence the water uptake of 2-MT and 2-MTS with AS. The diffusion for 2-MTS/AS becomes an order of magnitude greater than for the organic alone but 2-MT diffusivity remains unchanged in the presence of AS. 2-MT/AS aerosols present a plateau in sub- and supersaturated κ-values close to pure AS. 2-MTS/AS aerosols exhibit a similar behavior under subsaturated conditions. However, under supersaturated conditions, 2-MTS/AS behaves as an ideal well-mixed aerosol that can be described by traditional κ-Köhler theory. 2-MT and 2-MTS are abundant globally, and thus the impact from biogenic sources and non-ideal droplet activation properties must be considered in aerosol-cloud interactions.