The importance of feldspar for ice nucleation by mineral dust in mixed-phase clouds

Abstract

Figure 1a. In experiments with similar dust surface areas, the temperature at which 50% of droplets were frozen was 250.5 K for K-feldspar, followed by 247 K for Na/Ca-feldspar, 242.5 K for quartz, and below 237.5 K for the clay minerals and calcite. These results suggest that it is the minerals of the feldspar group, in particular K-feldspar, that make mineral dust an effective immersion mode IN in the atmosphere. This data contrasts with the prevailing view 1,2 that clay minerals are the most important component of atmospheric mineral dust for ice nucleation.Droplet freezing temperatures are dependent on experimental parameters such as droplet volume and mineral surface area and are therefore of limited value 2 . In order to normalise the efficiency with which a material nucleates ice we determine the nucleation sites per unit surface area 2,11,14 (n s ; Figure 1b; see supplementary online material). This method of quantifying ice nucleation efficiency neglects the role of time dependence in nucleation, on the basis that IN particle-to-particle variability is more important than the time dependence of nucleation 2,11,14,15 . Our derived n s values for 9 -19 μm size droplets are shown in Figure 1b.This data shows that the feldspar minerals, in particular K-feldspar, are the most efficient mineral dust IN per unit surface area.In airborne dusts the abundance of clay minerals tends to be greater than the feldspars, hence it is not clear which minerals dominate ice nucleation in the atmosphere. The n s values presented in Figure 1b were combined with the average mineralogical composition of atmospheric dust to estimate the temperature-dependent IN concentration (shown in Figure 2). We have assumed that all particles are spherical in order to estimate their surface area and have made two limiting calculations, one assuming that dust particles are internally mixed (i.e. each particle contains all eight minerals) and the other assuming they are externally mixed (each particle is composed of an individual mineral). The mixing state of atmospheric dust is poorly constrained but atmospheric dust falls between these two limiting cases 16 .Despite only accounting for 3% of atmospheric dust by mass, K-feldspar dominates the number of IN above 248 K in both the internally and externally mixed cases. One potential caveat to this conclusion is that clay mineral particles may have a smaller particle size than feldspar or quartz 13 , and therefore may have a greater surface area per unit mass which would increase the concentration of clay IN. However, even if the surface area of the clays was 100 times higher (likely an overestimate 7 ), the feldspars remain the dominant ice nucleating minerals (Supplementary Figure 4). contains the most K-feldspar (20 wt%). In general, the more feldspar a sample contains the higher the freezing temperature. We hypothesise that that the feldspar component controlled the nucleation of ice in these experiments, highlighting the need to characterise sample mineralogy in such work.The mineralog...

Refreeze experiments with water droplets containing different types of ice nuclei interpreted by classical nucleation theory

“…performed refreeze experiments with ATD and observed jumps similar to the ones that we observed for the Hoggar Mountain dust sample, but they did not mention a memory effect. ATD is a complex mixture of minerals with a considerable share of microcline (20-30 %) (Atkinson et al, 2013), which is a K feldspar with exceptionally high heterogeneous ice nucleation temperatures. Microcline samples showed high freezing temperatures from T = 264-272 K in bulk freezing experiments (Kaufmann et al, 2016) similar to the ones performed in this study.…”

“…ATD is a commercial dust sample that has been used by many groups as a proxy of natural atmospheric mineral dust (Murray et al, 2012). It is a mixture of minerals with a considerable share of microcline, a K feldspar with a high ice nucleation ability as demonstrated in laboratory experiments (Atkinson et al, 2013;Zolles et al, 2015;Harrison et al, 2016). Pollen is among the primary biological aerosol particles that nucleate ice (Hader et al, 2014).…”

Single Particle Measurements of the Optical Properties of Small Ice Crystals and Heterogeneous Ice Nuclei

“…The only heterogeneous ice crystals which showed variation in the total backscatter intensity were those nucleated on Arizona test dust. Results based on the scattering intensities of three of the four sampled dusts suggest that while IN type may influence nucleation conditions and relative rates (Archuleta et al 2005;Atkinson et al 2013), the resulting ice crystals possess similar shapes and optical properties. One exception to this is that ice crystals nucleated on Arizona test dust have total backscatter intensities approximately twice as high as the other three ice crystal samples.…”

“…Previous studies have shown the ability of various mineral dusts to act as IN (Archuleta et al 2005;Koehler et al 2010;Murray et al 2012;Atkinson et al 2013).…”

“…Mineral dust represents a population of large atmospheric aerosols that exhibit exceptional ice nucleation abilities at warmer temperatures and lower RH (Chen et al 1998;Kanji and Abbatt 2009;Rosenfeld et al 2011;Creamean et al 2013). For example, a recent study reports that higher feldspar content promotes ice nucleation efficiency (Atkinson et al 2013). Above » ¡26 C, the nucleation per surface area for feldspar is higher than for real world dust samples reported by Niemand et al (2012).…”