Viscosity and physical state of sucrose mixed with ammonium sulfate droplets

Abstract

Although knowledge of the physical state of aerosol particles is essential to understand atmospheric chemistry model and measurements, information on the viscosity and physical state of aerosol particles consisting of organic and inorganic salts is still rare. Herein, we quantified viscosities at 293 ± 1 K upon dehydration for the binary systems, sucrose–H2O and ammonium sulfate (AS)–H2O, and the ternary systems, sucrose–AS–H2O for organic-to-inorganic dry mass ratios (OIRs) = 4:1, 1:1, and 1:4 using bead-mobility and poke-and-flow techniques. Based on the viscosity value of the aerosol particles, we defined the physical states of the total aerosol particles studied in this work. For binary systems, the viscosity of sucrose–H2O particles gradually increased from ∼ 4 × 10−1 to > ∼ 1 × 108 Pa s when the relative humidity (RH) decreased from ∼ 81 % to ∼ 24 %, ranging from liquid to semisolid or solid state, which agrees with previous studies. The viscosity of AS–H2O particles remained in the liquid state (< 102 Pa s) for RH > ∼ 50 %, while for RH ≤∼ 50 %, the particles showed a viscosity of > ∼ 1 × 1012 Pa s, corresponding to a solid state. In case of the ternary systems, the viscosity of organic-rich particles (OIR = 4:1) gradually increased from ∼ 1 × 10−1 to ∼ 1 × 108 Pa s for a RH decrease from ∼ 81 % to ∼ 18 %, similar to the binary sucrose–H2O particles. This indicates that the sucrose–AS–H2O particles range from liquid to semisolid or solid across the RH. In the ternary particles for OIR = 1:1, the viscosities ranged from less than ∼ 1 × 102 for RH > 34 % to > ∼ 1 × 108 Pa s at ∼ 27 % RH. The viscosities correspond to liquid for RH > ∼ 34 %, semisolid for ∼ 34 % < RH < ∼ 27 %, and semisolid or solid for RH < ∼ 27 %. Compared to the organic-rich particles, in the inorganic-rich particles (OIR = 1:4), drastic enhancement in viscosity was observed as RH decreased; the viscosity increased by approximately 8 orders of magnitude during a decrease in RH from 43 % to 25 %, resulting in liquid to semisolid or solid in the RH range. Overall, all particles studied in this work were observed to exist as a liquid, semisolid, or solid depending on the RH. Furthermore, we compared the measured viscosities of ternary systems with OIRs of 4:1, 1:1, and 1:4 to the predicted viscosities using the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients Viscosity model (AIOMFAC-VISC) predictions with the Zdanovskii–Stokes–Robinson (ZSR) organic–inorganic mixing model, with excellent model–measurement agreement for all OIRs.