Abstract
Abstract. 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-rich feldspar (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. For example, the ice-nucleation temperatures for one quartz sample shift down by ∼2 ∘C in 1 h and 12 ∘C after 16 months in water. The sensitivity to water and air is perhaps surprising, as quartz is thought of as a chemically resistant mineral, but this observation suggests that the active sites responsible for nucleation are less stable than the bulk of the mineral. We find that the quartz group of minerals is generally less active than K-feldspars by roughly 7 ∘C, although the most active quartz samples are of a similar activity to some K-feldspars with an active site density, ns(T), of 1 cm−2 at −9 ∘C. We also find that the freshly milled quartz samples are generally more active by roughly 5 ∘C 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 parameterizations have been proposed for freshly milled quartz, K-feldspar, plagioclase and albite. Combining these parameterizations with the typical atmospheric abundance of each mineral supports previous work that suggests that K-feldspar is the most important ice-nucleating mineral in airborne mineral dust.
Highlights
The formation of ice in supercooled clouds strongly affects hydrometeor size, which in turn impacts cloud lifetime, precipitation and radiative properties (Kanji et al, 2017)
Homogeneous freezing of cloud droplets becomes increasingly important below −33 ◦C (Herbert et al, 2015), but clouds commonly glaciate at much warmer temperatures (Kanitz et al, 2011; Ansmann et al, 2009). Freezing at these warmer temperatures can occur through secondary ice production (Field et al, 2017) or heterogeneous freezing on ice-nucleating particles (INPs) (Murray et al, 2012; Hoose and Möhler, 2012)
We have studied 10 quartz samples for their ice-nucleating ability in order to better understand and define the ice-activity of this abundant mineral
Summary
The formation of ice in supercooled clouds strongly affects hydrometeor size, which in turn impacts cloud lifetime, precipitation and radiative properties (Kanji et al, 2017). Homogeneous freezing of cloud droplets becomes increasingly important below −33 ◦C (Herbert et al, 2015), but clouds commonly glaciate at much warmer temperatures (Kanitz et al, 2011; Ansmann et al, 2009). Freezing at these warmer temperatures can occur through secondary ice production (Field et al, 2017) or heterogeneous freezing on ice-nucleating particles (INPs) (Murray et al, 2012; Hoose and Möhler, 2012). Our understanding of which type of aerosol particles serve as effective INPs is incomplete (Vergara-Temprado et al, 2017; Kanji et al, 2017)
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