Ice nucleation activity of alkali feldspar in aerosol

• Applicant: Dr. Alexei Kiselev
• Subject Area: Atmospheric Science
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 515700978

Aerosols foster the formation and glaciation of clouds and thus are key to atmospheric processes and climate. Alkali feldspar is the most ice nucleating active phase in airborne mineral dust. This has been ascribed to crystal structure similarities between ice and certain lattice planes and specific surface features of feldspar serving as ice nucleating sites. A systematic study of ice nucleating surface features and of the mechanisms underlying ice nucleation on alkali feldspar is, however, still incomplete. Ice nucleation on alkali feldspar is likely facilitated by surface topography related to exsolution lamellae, twin boundaries, and secondary porosity generated by mineral reactions that alkali feldspar typically undergoes during cooling from formation temperatures. Associated open cracks and pores expose patches of favorably oriented surfaces providing templates for epitaxial ice nucleation and growth and, due to confined space geometry, facilitate condensation of water and ice nucleation at relative humidity below water saturation. These hypotheses should be tested experimentally. Favorably oriented surface cracks and secondary porosity will be induced to gem quality alkali feldspar by reaction with alkali halide melts and solutions, and feldspars containing exsolution lamellae will be etched to enhance nano-porosity at lamellar boundaries. The pristine and conditioned feldspars will then be used for immersion freezing experiments in droplet assay setup mounted on optical and environmental scanning electron microscopes (ESEM). The high spatial resolution will allow for correlation of surface features and ice nucleation. Deposition freezing experiments will be done in the ESEM and in an environmental chamber installed at a synchrotron facility to obtain phase, structural, strain and orientation information of adsorbed water and mineral phase at high spatial (ESEM) and sub-second time resolution (synchrotron-based diffraction). Comparison of results from pristine and custom conditioned alkali feldspar will reveal the ice nucleation active sites and the underlying mechanisms. The experiments will be complemented by atomistic modelling combining existing neural network potentials for alkali feldspar, water and ice. Enhanced rare event sampling methods such as replica exchange umbrella sampling and transition path sampling will be employed for simulating ice nucleation and growth. Level of originality: Comparison of ice nucleation on pristine and custom conditioned alkali feldspar, droplet freezing assay experiments under the ESEM and real time monitoring of water condensation and freezing in synchrotron beamline are novel by themselves. Fundamental new insights are expected from the combination of these advanced experimental and simulation techniques.

• DFG Programme: Research Grants
• International Connection: Austria
• Cooperation Partners: Professor Dr. Christoph Dellago; Dr. Elena Petrishcheva
• Co-Investigator: Dr. Bärbel Krause