The ice-nucleating ability of quartz immersed in water and its atmospheric importance compared to K-feldspar

Mineral dust particles are thought to be an important type of ice-nucleating particle (INP) in the mixed phase cloud regime around the globe. While K-feldspar has been identified as being a particularly important component of mineral dust for ice nucleation, it has been shown that quartz is also relatively ice nucleation active. Given quartz typically makes up a substantial proportion of atmospheric desert dust it could potentially be important for cloud glaciation. Here, we survey the ice-nucleating ability of 10 α-quartz samples (the most common quartz polymorph) when immersed in microlitre supercooled water droplets. Despite all samples being α-quartz, the temperature at which they induce freezing varies by around 12°C for a constant active site density. We find that some quartz samples are very sensitive to ageing in both aqueous suspension and air, resulting in a loss of ice-nucleating activity, while other samples are insensitive to exposure to air and water over many months. The sensitivity to water and air is perhaps surprising as quartz is thought of as a chemically resistant material, but this observation suggests that the active sites responsible for nucleation are less stable than the bulk of the material. We find that the quartz group of minerals are generally less active than K-feldspars, although the most active quartz samples are of a similar activity to some K-feldspars. We also find that the quartz samples are generally more active than the plagioclase feldspar group of minerals and the albite end-member has an intermediate activity. Using both the new and literature data, active site density parameterisations have been proposed for quartz, K-feldspar, plagioclase and albite. Combining these parameterisations with the typical atmospheric abundance of each mineral and comparing the results with atmospheric ice-nucleating particle concentrations, supports previous work that suggests that K-feldspar dominates, rather than quartz (or other minerals), the ice nucleation particle population in desert dust aerosol.