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Molecular dynamics simulations of heterogeneous ice nucleation by atmospheric aerosols Zielke, Stephen Alexander

Abstract

Water droplets in the atmosphere do not freeze homogeneously until -38ºC. Freezing at warmer temperatures requires heterogeneous ice nuclei (IN). Despite the importance of ice in the atmosphere, little is known about the microscopic mechanisms of heterogeneous ice nucleation. This thesis employs molecular dynamics simulations to investigate ice nucleation by silver iodide, kaolinite, potassium feldspar, gibbsite, and a protein. Silver iodide is one of the best known ice nucleating agents. We examined seven surfaces of silver iodide and observed ice nucleation on three surfaces. The surfaces that nucleated ice organized the first layer of water molecules into a configuration resembling ice, such as chair conformed hexagonal rings. Surfaces that do not nucleate ice do not organize water into icelike configurations, such as planar rings. Results suggest lattice mismatch is insufficient in predicting ice nucleation, and a finer atomistic match is required. Finite silver iodide disks and plates were used to probe the relationship between the size of a nucleating surface and maximum temperature of ice nucleation. Larger disks nucleated ice at warmer temperatures than smaller disks by forming larger initial cluster of ice which could reach the critical size easier than homogeneously formed clusters. Kaolinite is a common clay known to nucleate ice. Our simulations investigated both sides of the (001) surface and found both sides able to nucleate ice. The Al-surface was simulated with varying degrees of freedom of motion. An optimum amount of movement was required to nucleate ice as the surface needs to adapt into a configuration favorable to ice. Ice nucleated on the Si-surface via the formation of a novel composite surface structure which facilitated bulk ice nucleation. Potassium feldspar simulations explored three variations of the two primary cleavage planes. All surfaces failed to nucleate ice and density profiles suggest that the surfaces are unlikely to nucleate ice. We succeeded in nucleating ice on gibbsite with prepared surface conformations compatible with ice. Biological IN, such as ice nucleation proteins, are among the most efficient IN. We attempted to simulate ice nucleation via a protein, but were unable to achieve ice nucleation.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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