Abstract. 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 nm particles using the dimer relaxation method are presented. These measurements extend the available data of temperature- and humidity-dependent viscosity to −28 ∘C. Predicted relationships agree well with observations at room temperature and with measured isopleths of constant viscosity at ∼107 Pa s at temperatures warmer than −28 ∘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 (-80∘C<T<40∘C, temperature, and 0%<RH<100%, relative humidity) for sucrose and citric acid are constructed and associated equilibration timescales are identified.

Atmospheric aerosols can assume liquid, amorphous semi-solid or glassy, and crystalline phase states. Particle phase state plays a critical role in understanding and predicting aerosol impacts on human health, visibility, cloud formation, and climate. Melting point depression increases with decreasing particle diameter and is predicted by the Gibbs–Thompson relationship. This work reviews existing data on the melting point depression to constrain a simple parameterization of the process. The parameter $$\xi $$ ξ describes the degree to which particle size lowers the melting point and is found to vary between 300 and 1800 K nm for a wide range of particle compositions. The parameterization is used together with existing frameworks for modeling the temperature and RH dependence of viscosity to predict the influence of particle size on the glass transition temperature and viscosity of secondary organic aerosol formed from the oxidation of $$\alpha $$ α -pinene. Literature data are broadly consistent with the predictions. The model predicts a sharp decrease in viscosity for particles less than 100 nm in diameter. It is computationally efficient and suitable for inclusion in models to evaluate the potential influence of the phase change on atmospheric processes. New experimental data of the size-dependence of particle viscosity for atmospheric aerosol mimics are needed to thoroughly validate the predictions.

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Cirrus clouds play an important role in Earth's energy budget by reflecting the incoming solar radiation into space which leads to cooling, and by reducing the infrared longwave coming from the Earth surface which leads to warming (Chen et al., 2000;Heymsfield et al., 2017;Storelvmo & Herger, 2014). The radiative properties of cirrus clouds depend on the ice crystal number concentration, which in turn depends on the aerosol that nucleate ice at these conditions. Cirrus cloud ice crystals form either through homogeneous processes, requiring no ice nucleating particle (INP) or active site, or heterogeneously where an INP is involved (Hoose & Möhler, 2012;Kanji et al., 2017;Vali et al., 2015). Heterogeneous ice nucleation (IN) can occur via deposition of ice on particle surfaces, condensation in pores and subsequent freezing (Campbell et al., 2017;David et al., 2019;Marcolli, 2014), through immersion freezing of droplets as they cool, or condensation freezing as particles simultaneously uptake water and cool. Another freezing path is contact freezing where a particle approaches the air-water interface from either the outside of the droplet or from inside of the droplet (Durant & Shaw, 2005;Kanji et al., 2017). Homogeneous freezing of liquid aerosol is predominantly controlled by water activity (or ambient relative humidity (RH)), temperature, particle size, and hygroscopicity (Baumgartner et al., 2022;Koop et al., 2000;Schneider et al., 2021). Among these, temperature and RH dominate. Chemical composition influences homogeneous IN by modulating hygroscopicity (Junge, 1953;M. D. Petters & Kreidenweis, 2007). However, the overall influence of chemical composition on homogeneous freezing nucleation is small (Kreidenweis et al., 2009). In contrast, heterogeneous IN strongly depends on the chemical composition and morphology (Hiranuma et al., 2014). Ambient measurements of cirrus ice crystal residuals show a wide range of chemical composition, including organic particles, black carbon, mineral dust, lead and other metal bearing particles, sulfate particles, and salt particles