Collaborative Research: Why Does Oxidation Improve Freezing? Using Laboratory Measurements and Molecular Simulations to Investigate Heterogeneous Ice Nucleation on Carbon Surfaces

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The Environmental Chemical Sciences (ECS) program of the Division of Chemistry supports the collaborative research efforts of Sarah D. Brooks (Texas A&M University) and Valeria Molinero (University of Utah) to investigate the heterogeneous nucleation of ice on model organic surfaces and soot using an unprecedented combination of laboratory measurements and molecular simulations. The primary objective of this project is to evaluate which specific chemical and physical properties of carbonaceous surfaces modulate their ability to act as ice nuclei for water freezing. The second objective is to elucidate the heterogeneous mechanisms of formation of ice on these surfaces. Previous laboratory results suggest that oxidation processes representative of atmospheric aging improve the ice nucleating ability of aerosols, facilitating contact freezing at warmer temperatures. Oxidation, however, alters carbon surfaces not only by incorporation of hydrophilic groups but also through a modification of the nanostructure of the surface. Laboratory experiments will be conducted to determine the prerequisite temperature and relative humidity required for immersion and deposition freezing on hydrocarbon and oxidized hydrocarbon particles. Simulations will be used to determine the effect of nanostructure, chemical modifications, and hydrophilicity of the surface on the freezing temperatures, the heterogeneous mechanism of formation of ice, and the size and structure of the critical ice embryo. Abundant in the atmosphere due to anthropogenic and biogenic sources, carbonaceous particles display a wide range of effectiveness in promoting the nucleation of ice in clouds. This variability causes major uncertainties in predictions of the effect of aerosols on climate. The project will lead to an improved understanding of the phase of water in the atmosphere and assist in the parameterization of global climate models. A workshop to train high school teachers in the use of freely available molecular visualization software to develop classroom activities that complement and enhance the high-school curriculum will be developed at the University of Utah. The molecular visualization tools and the simulations that display the formation of ice will be incorporated into undergraduate courses taught by the PIs.