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Ice nucleation properties of mineral dusts
Ice nucleation in clouds has a significant impact on the global hydrological cycle as well as on the radiative budget of the Earth. The AIDA cloud chamber was used to investigate the ice nucleation efficiency of various atmospherically relevant mineral dusts. From experiments with Arizona Test Dust (ATD) a humidity and temperature dependent ice nucleation active surface site density parameterization was developed to describe deposition nucleation at temperatures above 220 K. Based on these results, a parameterization for deposition nucleation initiated by desert dusts and clay minerals is proposed. The time dependence observed during the deposition nucleation experiments with ATD seems to be relevant only at very small cooling rates. Experiments with atmospherically relevant dust samples revealed large differences in the ice nucleation efficiencies among the different dusts. Volcanic ash emitted during the Eyjafjallajökull eruption in 2010 proved to be slightly less ice-active than mineral dust particles from desert areas. In contrast, soil dust particles rich in organic matter and fossile diatomite were very ice-active particles. For the soil dust particles, the soil organic matter is probably the reason for the enhanced ice nucleation efficiency compared to desert dusts which is, however, most likely not directly determined by viable organisms such as fungi or bacteria. Coatings with secondary organic compounds led to a significant deterioration of the ice nucleation efficiency observed for fossil diatomite in the immersion freezing and the deposition nucleation mode. Deposition mode nucleation initiated by mineral dust particles was suppressed by coatings with sulfuric acid whereas there was no effect on the immersion freezing properties. The immersion freezing properties of several substrates used as ice nuclei substitutes were investigated with a cold stage setup. Muscovite initiated heterogeneous ice nucleation at approximately 250 K but was much less ice-active than mineral dusts. Droplets placed on silicon wafers froze only close to the homogeneous freezing threshold. The freezing properties of these silicon wafers could be changed signicantly by modifying the wafer surface through adding regular structures such as trenches which suggests that the surface morphology may have an influence on the ice nucleation efficiencies of atmospheric particles.
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500 Naturwissenschaften und Mathematik
500 Natural sciences and mathematics