The Analysis of Four Factors for Hygroscopicity Parameterization - Surface Activity, Solubility, LLPS and O/C

NAHIN FERDOUSI, Kanishk Gohil, Kotiba A. Malek, Dewansh Rastogi, Kiran R. Pitta, Qishen Huang, Miriam Freedman, Akua Asa-Awuku, University of Maryland, College Park

     Abstract Number: 62
     Working Group: Aerosols, Clouds and Climate

Abstract
Aerosols and aqueous droplets can be composed of both inorganic salts and organic compounds; the composition impacts the droplet’s water uptake capability, also known as hygroscopicity. Prior studies focusing on internal binary mixtures focus on the influence of factors, such as oxygen to carbon (O/C) ratio, solubility, and liquid-liquid phase separation (LLPS) on hygroscopicity. Within these models, it is assumed that the surface tension of the droplet is equivalent to the surface tension of pure water. However, few studies have collectively discussed the presence of surface-active compounds, LLPS, solubility, and O/C influencing water-uptake (hygroscopicity).

In this study, we investigated the hygroscopicity of inorganic salt, ammonium sulfate (AS) and organic, surface-active compound, 2-methylglutaric acid (2-MGA), mixtures. AS/2-MGA compositions were varied by weight percentage in solution and then atomized. Hygroscopicity at subsaturated conditions was determined using a hygroscopicity tandem differential mobility analyzer (H-TDMA) and kept constant at a relative humidity of 89% ± 1.5%. Hygroscopicity was measured at supersaturated conditions using a cloud condensation nuclei counter (CCNC), at 0.4 to 1% SS. Measurements determined the single hygroscopicity parameter, κ, for each mixture. Mixtures predominantly composed of AS, up to a 1:1 AS/2-MGA composition, exhibit κ values close to pure AS at 0.57 ± 0.03. However, as the aerosol composition transitioned to being predominantly composed of the surface-active compound, hygroscopicity decreased significantly. Furthermore, as the composition of 2-MGA increases, a larger discrepancy between sub- and supersaturated estimated κ was observed. Hygroscopicity models utilizing variations of modified Köhler theory show that accounting for bulk surface tension presents a significant improvement in the agreement of measurement and prediction. Cryo-TEM droplet images demonstrate phase separation behavior and support the previous observations. Thus, the mixed inorganic/ organic surfactant composition is best described with uptake models that account for both liquid-liquid phase separation and surface tension depression. The findings of this study can help further enhance our understanding of cloud-forming properties of complex chemical mixtures containing surface-active organic and inorganic compounds. This work shows that accounting for surface tension partitioning via O/C can improve water uptake and CCN prediction of aerosol mixture.