Cation Specific Effects on the Ice Nucleating Ability of Potassium-rich Feldspar

An important type of ice nucleating particle in the atmosphere is mineral dust. During transport in the atmosphere, mineral dust particles can be coated with solutes, which can affect the ice nucleating ability of mineral dust by modifying the surface properties of the mineral dust. Although previous studies have shown that solutes at low concentrations can modify the ice nucleation properties of mineral dust particles, our understanding of this topic is far from complete, especially at the molecular level. In the following we investigate the effect of a series of inorganic nitrates on the ice nucleating ability of potassium-rich feldspar (K-feldspar) using the droplet freezing technique. By holding the inorganic anion (i.e. nitrate anion) constant, we isolated the effect of the cation on the heterogeneous freezing properties of K-feldspar in the immersion mode. The change in the median freezing temperature caused by the solutes varied from -8 to 4 °C depending on the cation type and solute concentration. The dependence of median freezing temperature on the cation type could not be explained alone with the surface change density of cations, the viscosity B-coefficient of cations, or the cation radii. On the other hand, if only the alkali metals (excluding K⁺) were considered, the correlations between the freezing temperature and the surface charge density of cations, the cation radii, and the Gibbs free energy change for adsorption became statistically significant. These results are a step forward in understanding and predicting the cation specific effects of solutes on the ice nucleation ability of K-feldspar.