Viscosity and physical state of sucrose/ammonium sulfate/H2O droplets

To explore aerosol chemistry and climate change, information of the physical state of aerosol particles is essential. Herein, we measured viscosities of binary mixtures of sucrose/H₂O and ammonium sulfate (AS)/H₂O, and ternary mixtures of sucrose/AS/H₂O with different organic-to-inorganic dry mass ratios. For sucrose droplets, the viscosity gradually enhanced from ~4 × 10^-1 to > ~1 × 10⁸ Pa‧s as the relative humidity (RH) decreased from ~81% to ~24%. This corresponds from liquid to semisolid or solid state. For AS droplets, the viscosity dramatically enhanced at ~ 50% RH upon dehydration; to be < 10² Pa‧s for RH > ~50% (liquid state), and > ~1 × 10¹² Pa‧s for RH ≤ ~50% (solid state). In case of the ternary mixtures, remarkable enhancement in viscosity was observed as the inorganic ratio increased at a given RH. All particles studied in this work were observed to exist as a liquid, semi-solid or solid depending on the organic-to-inorganic dry mass ratios and RH. Moreover, the measured viscosities of the binary and ternary mixtures were compared with the calculated viscosities using the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients Viscosity model (AIOMFAC-VISC) predictions with the Zdanovskii–Stokes–Robinson (ZSR)-style organic–inorganic mixing model. It showed excellent model–measurement agreement. The result will be discussed.