Key Roles of Bulk Viscosity and Acidity on Liquid–Liquid Phase Separation of Atmospheric Organic–Inorganic Mixed Aerosols

Liquid-liquid phase separation (LLPS) and the resulting particle morphologies in atmospheric organic-inorganic mixed aerosols are key regulators of aerosol chemistry and climate forcing. However, the influence of coexisting viscous water-soluble organic compounds (WSOCs) on the LLPS behavior in complex multicomponent aerosol systems remains poorly understood. In this study, we introduced three representative WSOCs, i.e., sucrose (SUC), glycerol (GLY), and citric acid (CA), to increase the bulk viscosity of a model LLPS system composed of 1,2,6-hexanetriol (HXT) and ammonium sulfate (AS). Using microscopic imaging techniques and viscosity model predictions, we examined the impact of mass transfer limitations on LLPS. As WSOC fractions increased, both the phase separation relative humidity (SRH) and the efflorescence relative humidity (ERH) progressively decreased. For the HXT/AS/SUC and HXT/AS/CA mixed systems with molar ratios of 1:1:0.5 and 1:1:0.75, LLPS was completely suppressed, although efflorescence still occurred. In the 1:1:1 mixtures, neither LLPS nor efflorescence was observed. In contrast, the addition of GLY caused minimal changes to phase transitions due to its minor effect on the aqueous-phase viscosity. Additionally, reducing bulk acidity, along with the transformation of CA into its salts, alleviated molecular transport limitations, leading to increased SRH and ERH values for the HXT/AS/CA mixtures. These findings are critical for advancing high-resolution phase state modeling of multicomponent aerosols and assessing the atmospheric implications of particle morphologies in the presence or absence of LLPS.