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T. Zolles, H. Grothe, B. G. Pummer, K. Hradil, K. Whitmore:
"Immersion Freezing On Mineral Dust Particles";
Vortrag: 15th Austrian Chemistry Days (Chemietage), Graz; 23.09.2013 - 26.09.2013; in: "15th Austrian Chemistry Days, Graz Universtiy of Technology", (2013).

Mineral dust is considered to play a major role in ice cloud nucleation in the troposphere. More than 1.000 Tg of mineral dust are aerosolized from the ground every year, 1-20% of these reach the upper troposphere [1]. At an altitude of about 8 km ice residual particle analysis has shown that about 50% of all ice nuclei (IN) are mineral dust [2]. In principle, natural occurring dusts may either be ice nucleation active themselves or are carriers of organic and/or biological IN. Up to now the ice nucleation, i.e. cloud glaciation, has not been quantized. However, different authors report a high ice nucleation activity for many mineral dust samples, although a systematic comparison between different minerals is still missing. Therefore, we studied selected mineral samples which were characterized by X-ray powder diffraction, FTIR spectroscopy, and electron microscopy before use.

Oil immersion measurements were performed on the most common minerals, clay materials and volcanic ash. The median freezing temperatures range from -21°C up to homogenous freezing at 38°C. Even though quite a few dust samples show a reasonable high ice nucleation activity, their median freezing temperatures are low compared to biological samples [3, 4]. The samples with a high ice nucleation activity were further treated with different enzymes to decompose or denaturize organic and/or biological surfactants. In K-feldspar and quartz an activity loss could be observed. This turned out to be due to a blocking of the active sites by these enzymes. Heat treatment of the dust particles has almost no effect on the ice nucleation activity. The mineral dusts did show an intrinsic ice nucleation activity. A relatively high activity of the pure mineral dusts was only observed in quartz, kaolinite, K-feldspar, and mixed natural dusts (ATD), which are mainly composed of SiO2 and clays. K-feldspar is the only IN active above -25°C within the feldspar family. The exact reason is still under investigation. TEM and SEM and the respective EDX analysis suggest no morphological and crystallographic influence on the ice nucleation in the feldspars. Different surface reorientation in solution and charging of the surface might play a role. However, with the enzymatic treatment we could show that the nucleation takes place at specific sites on the particles and these sites can be blocked. Therefore, the history of atmospheric aerosol particles has an important influence on their ice nucleation activity. We hope to get more information about these sites by manipulating them (e.g. by changing the pH value) in order to further understand the process of heterogeneous ice nucleation.

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[1] C. S. Zender, R. L. Miller and I. Tegen, Eos Trans. AGU, 2004,85, 509.
[2] K. A. Pratt, P. J. DeMott, J. R. French, Z. Wang, D. L. Westphal, A. J. Heymsfield, C. H. Twohy, A. J. Prenni, K. A. Prather, Nat. Geosci., 2009, 2, 397-400.
[3] B. Pummer, H. Bauer, J. Bernardi, S. Bleicher and H. Grothe, Atmos. Chem. Phys., 2012, 12, 2541-2550.
[4] V. T. J. Phillips, C. Andronache, B. Christner, C. E. Morris, D. C. Sands, A. Bansemer, A. Lauer, C. McNaughton and C. Seman, Biogeosciences, 2009, 6, 987-1014.