Ice nucleation efficiency of natural dust samples in the immersion mode

Abstract

Abstract. A total of 12 natural surface dust samples, which were surface-collected on four continents, most of them in dust source regions, were investigated with respect to their ice nucleation activity. Dust collection sites were distributed across Africa, South America, the Middle East, and Antarctica. Mineralogical composition has been determined by means of X-ray diffraction. All samples proved to be mixtures of minerals, with major contributions from quartz, calcite, clay minerals, K-feldspars, and (Na, Ca)-feldspars. Reference samples of these minerals were investigated with the same methods as the natural dust samples. Furthermore, Arizona test dust (ATD) was re-evaluated as a benchmark. Immersion freezing of emulsion and bulk samples was investigated by differential scanning calorimetry. For emulsion measurements, water droplets with a size distribution peaking at about 2 µm, containing different amounts of dust between 0.5 and 50 wt % were cooled until all droplets were frozen. These measurements characterize the average freezing behaviour of particles, as they are sensitive to the average active sites present in a dust sample. In addition, bulk measurements were conducted with one single 2 mg droplet consisting of a 5 wt % aqueous suspension of the dusts/minerals. These measurements allow the investigation of the best icenucleating particles/sites available in a dust sample. All natural dusts, except for the Antarctica and ATD samples, froze in a remarkably narrow temperature range with the heterogeneously frozen fraction reaching 10 % between 244 and 250 K, 25 % between 242 and 246 K, and 50 % between 239 and 244 K. Bulk freezing occurred between 255 and 265 K. In contrast to the natural dusts, the reference minerals revealed ice nucleation temperatures with 2-3 times larger scatter. Calcite, dolomite, dolostone, and muscovite can be considered ice nucleation inactive. For microcline samples, a 50 % heterogeneously frozen fraction occurred above 245 K for all tested suspension concentrations, and a microcline mineral showed bulk freezing temperatures even above 270 K. This makes microcline (KAlSi 3 O 8 ) an exceptionally good ice-nucleating mineral, superior to all other analysed K-feldspars, (Na, Ca)-feldspars, and the clay minerals. In summary, the mineralogical composition can explain the observed freezing behaviour of 5 of the investigated 12 natural dust samples, and partly for 6 samples, leaving the freezing efficiency of only 1 sample not easily explained in terms of its mineral reference components. While this suggests that mineralogical composition is a major determinant of ice-nucleating ability, in practice, most natural samples consist of a mixture of minerals, and this mixture seems to lead to remarkably similar ice nucleation abilities, regardless of their exact composition, so that global models, in a first approximation, may represent mineral dust as a single species with respect to ice nucleation activity. However, more sophisticated representations of ice nucleation by mineral dusts...

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

“…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. Therefore, microcline is most probably the mineral component responsible for freezing of bulk samples.…”

“…The best nucleation sites probed in the refreeze experiments with bulk samples are active from 260 to 268 K, i.e., at distinctly higher temperatures than the average sites probed in the emulsion experiments, which nucleate ice below 252 K. In contrast to the bulk measurements, no memory effect was observed for ATD emulsions. Hoggar Mountain dust is a mixture of various minerals which nucleate ice at quite different temperatures (Pinti et al, 2012;Kaufmann et al, 2016), giving rise to the broad freezing signal starting below 257 K with the freezing of single large emulsion droplets as shown in panel (a). Again, there is no overlap in freezing temperatures between emulsion measurements and the refreeze experiments performed with large single droplets which froze from 258 to 265 K. With an onset of 255 K, the heterogeneous freezing peak of the emulsion made from the birch pollen washing water exhibits a clear overlap with the freezing temperatures observed for bulk measurements, which indicates that the ice nucleation active macromolecules present in the birch pollen washing water contain nucleation sites of quite uniform quality.…”

“…Hoggar Mountain dust consists of a mixture of minerals with high shares of the clay minerals smectite and montmorillonite, illite, and kaolinite and minor contributions of quartz and the feldspars sanidine and plagioclase (Kaufmann et al, 2016). However, nucleation at the best sites present in bulk samples (Pinti et al, 2012) does not need to be closely related to the prevailing minerals in the sample.…”

Online differentiation of mineral phase in aerosol particles by ion formation mechanism using a LAAP-TOF single-particle mass spectrometer

“…Field studies show that most dust in the atmosphere is dominated by varying quantities of quartz, K-feldspar, plagioclase, calcite, hematite, kaolinite and the illite-smectite group of clay minerals (Kaufman et al, 2005;Kandler et al, 2007Kandler et al, , 2009Kandler et al, , 2011Formenti et al, 2008;Jeong, 2008;Kaufmann et al, 2016). At a single-particle level, particle size dependence is observed with an abundance of quartz and feldspar grains in the coarse fraction and a fine fraction (< 2 µm) dominated by clay minerals.…”

Enhanced ice nucleation efficiency of microcline immersed in dilute NH&lt;sub&gt;3&lt;/sub&gt; and NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;-containing solutions

“…kaolinite, illite and montmorillonite in varying proportions, are a well-established class of INPs (Pruppacher and Klett, 1994;Zobrist et al, 2008;Lüönd et al, 2010;Murray et al, 2011;Hoose and Möhler, 2012;Pinti et al, 2012;Atkinson et al, 2013;Cziczo et al, 2013;Kaufmann et al, 2016). A study by Atkinson et al (2013) performed on minerals from the clay group and also on K-feldspar, Na/Ca-feldspar, quartz, and calcite showed that the IN efficiency in the immersion mode of K-feldspar is exceptionally high compared to the 65 other minerals.…”