PNNL

Abstract

The partitioning between inorganic salts and organic compounds within individual particles is a key factor that influences particle water uptake. In this study, we investigated the aerosol hygroscopicity of ammonium sulfate (AS) and 2-methylglutaric acid (2-MGA) mixtures. 2-MGA is a known surface-active compound and dilute surface tension measurements of 2-MGA/AS mixtures were taken using a pendant drop goniometer at 297ºK. Hygroscopicity at subsaturated conditions was determined using a hygroscopicity tandem differential mobility analyzer (H-TDMA) and relative humidity was kept constant at 89% RH ± 5%. 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, ?, was derived from measurements and compared to theory. Mixtures predominantly composed of AS, up to a 50 wt% 2-MGA, exhibit ?-values close to pure AS. However, ? decreases significantly as the organic fraction increases (> 60 wt% 2-MGA). Previous predictions of ?-hygroscopicity assume full dissolution of both organic and inorganic compounds. In this work we show that the salting-out phenomenon can be parameterized and may lead to the formation of a surface-active monolayer at the droplet surface that will modify the propensity of water uptake for aerosol mixtures. Hygroscopicity predictions that account for the role of surface-active organic partitioning agree best with experimental results (R2 > 0.95). Therefore, this study helps enhance our understanding of cloud-forming properties of complex chemical mixtures containing surface-active organic and inorganic compounds.