Viscosity and phase state of aerosol particles consisting of sucrose mixed with inorganic salts

Abstract

Research on the viscosity and phase state of aerosol particles is essential because of their significant influence on the particle growth rate, equilibration times, and related evolution of mass concentration as well as heterogeneous reactions. So far, most studies of viscosity and phase state have been focused on organic aerosol particles, yet data on how viscosity can vary when the organic materials are mixed with inorganic salts remain scarce. Herein, using bead-mobility and poke-and-flow techniques, we quantified viscosities at 293 ± 1 K for binary mixtures of organic material / H2O and inorganic salts / H2O, as well as ternary mixtures of organic material / inorganic salts / H2O over the atmospheric relative humidity (RH) range. Sucrose as the organic species and calcium nitrate (Ca(NO3)2) or magnesium nitrate (Mg(NO3)2) as the inorganic salts were examined. For binary sucrose / H2O particles, the viscosities gradually increased from ∼ 3 × 10−2 to ≳1 × 108 Pa s as RH decreased from ∼ 75 % to ∼ 25 %. Compared with the results for the sucrose / H2O particles, binary Ca(NO3)2/H2O and Mg(NO3)2/H2O particles showed drastic enhancements to ≳1 × 108 Pa s at low RH close to the efflorescence RH. For ternary mixtures of sucrose / Ca(NO3)2 / H2O or sucrose / Mg(NO3)2 / H2O, with organic-to-inorganic mass ratios of 1:1, the viscosities of the particles gradually increased from ∼ 3 × 10−2 to greater than ∼ 1 × 108 Pa s for RH values from ∼ 75 % to ∼ 5 %. Compared to the viscosities of the Ca(NO3)2/H2O particles, higher viscosities were observed for the ternary sucrose / Ca(NO3)2 / H2O particles, with values increased by about 1 order of magnitude at 50 % RH and about 6 orders of magnitude at 35 % RH. Moreover, we applied a thermodynamics-based group-contribution model (AIOMFAC-VISC, Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients Viscosity) to predict aerosol viscosities for the studied systems. The model predictions and viscosity measurements show good agreement within ∼ 1 order of magnitude in viscosity. The viscosity measurements indicate that the studied mixed organic–inorganic particles range in phase state from liquid to semi-solid or even solid across the atmospheric RH range at a temperature of 293 K. These results support our understanding that organic / inorganic / H2O particles can exist in a liquid, semisolid, or even a solid state in the troposphere.