Toward closure between predicted and observed particle viscosity over a wide range of temperatures and relative humidity

Atmospheric aerosols can exist in amorphous semi-solid or glassy phase states whose viscosity varies with atmospheric temperature and relative humidity. The temperature and humidity dependence of viscosity has been hypothesized to be predictable from the combination of a water–organic binary mixing rule of the glass transition temperature, a glass-transition-temperature-scaled viscosity fragility parameterization, and a water uptake parameterization. This work presents a closure study between predicted and observed viscosity for sucrose and citric acid. Viscosity and glass transition temperature as a function of water content are compiled from literature data and used to constrain the fragility parameterization. New measurements characterizing viscosity of sub-100 inline-formulanm particles using the dimer relaxation method are presented. These measurements extend the available data of temperature- and humidity-dependent viscosity to inline-formula−28 inline-formula^∘C. Predicted relationships agree well with observations at room temperature and with measured isopleths of constant viscosity at inline-formula∼10⁷ inline-formulaPa s at temperatures warmer than inline-formula−28 inline-formula^∘C. Discrepancies at colder temperatures are observed for sucrose particles. Simulations with the kinetic multi-layer model of gas–particle interactions suggest that the observed deviations at colder temperature for sucrose can be attributed to kinetic limitations associated with water uptake at the timescales of the dimer relaxation experiments. Using the available information, updated equilibrium phase-state diagrams (inline-formula $M8inlinescrollmathml-\mathrm{normal\; 80}unit{}^{\circ }\mathrm{normal\; C}<T<\mathrm{normal\; 40}unit{}^{\circ }\mathrm{normal\; C}$ 92pt11ptsvg-formulamathimga8a5c5d34161296918d48810867ab97f acp-21-1127-2021-ie00001.svg92pt11ptacp-21-1127-2021-ie00001.png , temperature, and inline-formula $M9inlinescrollmathml\mathrm{normal\; 0}unit\mathrm{normal\; \%}<\text{RH}<\mathrm{normal\; 100}unit\mathrm{normal\; \%}$ 89pt10ptsvg-formulamathimge604a1ecdeb3f856b232be1639e45ed7 acp-21-1127-2021-ie00002.svg89pt10ptacp-21-1127-2021-ie00002.png , relative humidity) for sucrose and citric acid are constructed and associated equilibration timescales are identified.