Ice nucleation efficiency of natural dust samples in the immersion mode
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. Min...
Main Authors: | , , , , , |
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Format: | Article in Journal/Newspaper |
Language: | English |
Published: |
Copernicus
2016
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Subjects: | |
Online Access: | https://hdl.handle.net/20.500.11850/120332 https://doi.org/10.3929/ethz-b-000120332 |
Summary: | 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 2µm, containing different amounts of dust between 0.5 and 50wt% 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 2mg droplet consisting of a 5wt% aqueous suspension of the dusts/minerals. These measurements allow the investigation of the best ice-nucleating 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 250K, 25% between 242 and 246K, and 50% between 239 and 244K. Bulk freezing occurred between 255 and 265K. 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 245K for all tested suspension concentrations, and a microcline mineral showed ... |
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