Ice nucleation efficiency of natural dust samples in the immersion mode
Authors: Kaufmann Lukas , Marcolli Claudia , Hofer Julian , Pinti Valeria , Hoyle Christopher R. , Peter Thomas 
Published: May 2016

Abstract. Twelve natural dust samples from eight dust source regions on four continents were investigated with respect to their ice nucleation activity, revealing no significant differences between source regions. Dust collection sites were distributed across Africa, South America, the Middle East and Antarctica. Mineralogical compositions have 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 diameter of about 2 µm, containing different amounts of dust between 0.5 wt% 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 1 mm diameter droplet consisting of a 5 wt% aqueous suspension of the dusts/minerals. These measurements allow the investigation of the best particles/sites available in a dust. 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 reveal ice nucleation temperatures with 2–3 times larger scatter. Calcite, dolomite, dolostone and muscovite can be considered inactive as ice nuclei (IN). For microcline samples 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 (KAlSi3O8) an exceptionally good IN, superior to all other analysed K-feldspars, (Na, Ca)-feldspars and the clay minerals. However, microcline is not abundant in the investigated natural dust samples. While K-feldspars were identified in five of the seven investigated natural source regions, only one sample contained microcline, and then only as a minor fraction. In summary, the mineralogical composition can explain the observed freezing behaviour of five of the investigated 12 natural dust samples, and partly for six samples, leaving the freezing efficiency of only one sample not easily explained in terms of its mineral reference components. While this suggests that mineralogical composition is a major determinant of ice nucleation 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 should rely on the mineralogical composition based on a source scheme of dust emissions.