Oxygen Vacancies as Features of Ice-Nucleating Metal Oxides

RYAN MITCH, Ayat Tassanov, Brendan Troesch, Mikyung Hwang, Konstantinos Alexopoulos, James Hodges, Miriam Freedman, The Pennsylvania State University

     Abstract Number: 338
     Working Group: Aerosols, Clouds and Climate

Abstract
Metal oxides comprise a fraction of mineral dust aerosol and are susceptible to the formation of oxygen vacancies on their surfaces. However, the role that these sites play in inducing ice nucleation has not been previously investigated, despite their prevalence and propensity to enhance heterogeneous chemistry. To explore how oxygen vacancies influence the ice nucleation activity of metal oxides, we perform droplet immersion freezing assays on zinc aluminate (ZnAl₂O₄) and magnesium aluminate (MgAl₂O₄) spinels annealed at 900°C under air, N₂, and O₂ atmospheres. We observe that substituting Zn²⁺ cations for Mg²⁺ cations enhances the spinel ice nucleation activity, and that this enhancement is greatest upon switching from an oxidizing atmosphere to a reducing atmosphere. To better understand these results, we investigate the ice nucleation activity of zinc oxide (ZnO) and magnesium oxide (MgO), observing that ice nucleation occurs at warmer temperatures for ZnO than MgO after annealing under reducing conditions. Using density functional theory (DFT) calculations, we investigate the relationship between the thermodynamics of oxygen vacancy formation of the spinels and metal oxides studied and their ice nucleation activity. Our study is the first to characterize the role of oxygen vacancies on ice nucleation activity, which has impacts for understanding the fate and climate effects of mineral dust aerosol in the atmosphere.