Abstract

The crystallization of water in clouds plays a key role in weather and climate through its effect on albedo and precipitation. The unassisted, homogeneous nucleation of ice occurs only at temperatures lower than -32oC, which are only achieved in high altitude clouds. Ice formation in lower lying, warmer clouds is promoted by atmospheric aerosols. These aerosols include minerals, organics, and biological molecules. Bacterial proteins are the most potent ice-nucleating agents in the atmosphere and the biosphere, able to nucleate ice at temperatures as high as -1oC. Some organics reach reach comparably high ice nucleation potencies, and have been used for the seeding of clouds to promote precipitation. The molecular mechanisms by which these potent nucleants interact with ice, and the mechanisms by which they promote crystallization have long been debated, but only recently it has been possible to access them through molecular simulations. In this presentation, I will discuss our work to elucidate the mode of binding and nucleation of ice by proteins and organics, how ice nucleation is modulated by the size and interactions of these particles, and our quest to find even stronger ice nucleants that can form ice without supercooling. I will discuss how to achieve the latter through prefreezing of water above the melting temperature, and present an integrated view of ice premelting, ice nucleation, and prefreezing in water. The enticing possibility of accessing the prefreezing regime by small modifications of existing surfaces opens the doors to the design of materials for energy-efficient formation of artificial snow and the induction of precipitation through cloud glaciation.