Aerosol Hygroscopicity and Surface-Active Coverage for the Droplet Growth of Aerosol Mixtures

The partitioning between inorganic salts and organic compounds within individual particles is a key factor that influences the uptake of water by particles. In this study, we investigated the aerosol hygroscopicity of ammonium sulfate (AS) and 2-methylglutaric acid (2-MGA) mixtures. 2-MGA is a moderately surface-active compound. Dilute surface tension measurements of 2-MGA/AS mixtures were taken by using a pendant drop goniometer. Hygroscopicity at subsaturated conditions was determined using a hygroscopicity tandem differential mobility analyzer (H-TDMA) and relative humidity was kept constant at 89 ± 0.9% RH. The droplet activation was also measured at supersaturated conditions using a cloud condensation nuclei counter (CCNC) from 0.4 to 1% supersaturation (SS). The single-hygroscopicity parameter κ was derived from measurements. Mixtures predominantly composed of AS, up to a 60 wt% 2-MGA, exhibit κ-values close to pure AS. However, κ decreases significantly as the organic fraction increases (>60 wt% 2-MGA). Previous predictions of κ-hygroscopicity assume full dissolution of both the organic and inorganic compounds. However, organic partitioning can influence the κ-hygroscopicity. A coverage-based parametrization, ϕ, assumes the probability of surface-active organics at the droplet surface. By estimation of the bulk and surface organic contribution, overall κ-hygroscopicity can be calculated. The model is computationally efficient, and the results indicate that organic solute depletion should be considered for fully soluble surface-active organics. Hygroscopicity predictions that account for the role of organic surface-active partitioning agree best with experimental results (R ² > 0.95). Therefore, this study helps to enhance our understanding of cloud-forming properties of complex chemical mixtures containing surface-active organic and inorganic compounds.