Immersion Freezing Efficiency of ZnAl2O4 and MgAl2O4 Spinels, ZnO, and MgO: The Role of Oxygen Vacancies.

Aerosol particles that catalyze ice nucleation alter the optical properties and precipitation cycles of clouds. Although mineral dust aerosol particles containing metal oxides are susceptible to the formation of oxygen vacancies (VO) on their surfaces, the impact of these defects on ice nucleation activity has not been addressed. To investigate the impact of VO sites, we conducted a droplet immersion freezing assay on zinc aluminate (ZnAl2O4) and magnesium aluminate (MgAl2O4) spinels annealed under air, nitrogen, and oxygen atmospheres. We observe that samples annealed under nitrogen promote ice nucleation at warmer temperatures compared to those treated in oxidizing atmospheres, with the effect being most pronounced for ZnAl2O4. To further understand these results, we investigated the immersion freezing of zinc oxide (ZnO) and magnesium oxide (MgO). Here, we observe that ZnO nucleates ice at substantially warmer temperatures than MgO after annealing under nitrogen. We hypothesize that the trends in ice nucleation activity are due to the varying concentrations of VO that form during the annealing process on the oxide surfaces, which tend to be higher in the absence of O2. Density functional theory (DFT) calculations support our hypothesis, indicating that VO is more stable on the surfaces of the Zn-containing oxides. The study suggests that oxygen vacancies, which are common defects on metal oxide surfaces that affect their adsorption and catalytic properties, can influence the efficiency with which mineral dust aerosol particles activate ice formation and affect cloud radiative forcing.