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Exploration of factors that influence heterogeneous ice nucleation : laboratory experiments and molecular dynamics simulations Ren, Yi


Pure water does not freeze homogeneously at temperatures above -35°C. Heterogeneous ice nucleation induced by ice nucleating particles (INPs) occurs at warmer temperatures than homogeneous nucleation. Atmospherically relevant INPs can significantly affect ice formation in mixed-phase and ice clouds, and therefore influence the Earth’s climate system. Several factors, including inorganic ions, acidic or basic solutes, and pressure variations influence the ice nucleating ability of INPs. At present, little is known about the microscopic mechanisms by which these factors affect heterogeneous ice nucleation and the ice nucleating ability of INPs. We examine the influences of inorganic ions and pH changes on kaolinite’s ice nucleating ability employing direct molecular dynamics (MD) simulations. We demonstrate that ionic solutes decrease the ice nucleating ability of the Al (001) kaolinite surface. The hydroxy groups exposed on the Al (001) surface can become deprotonated in basic solution or dual-protonated in acidic solution. We find that the mono-protonated Al (001) surface, which is expected to be stable at near-neutral pH, is the most effective ice nucleating surface. We perform droplet freezing experiments with kaolinite samples under acidic and basic conditions, covering a pH range of 0.18 - 13.26. Our MD simulations and experimental results indicate that the Al (001) surface may be important for ice nucleation by kaolinite. Employing MD simulations and varying the protonation state of the α-alumina (0001) surface, we find that the ice nucleating efficiency of this surface decreases nearly symmetrically with acidic and basic pH changes away from the neutral point. This is in qualitative agreement with previous droplet freezing experiments performed on a single (0001) surface. We carry out droplet freezing experiments on two α-alumina powder samples, covering a pH range of 0.59 - 13.19. The freezing results for the powder differ significantly from those of the single surface. We use MD simulations to probe the influence of pressure on heterogeneous ice nucleation by a finite-size β-AgI disk. We find that negative pressures enhance ice nucleation at high temperature, and we demonstrate how water diffusion can significantly influence ice nucleation and growth at low temperatures and negative pressures.

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