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Zusammenfassung:
Immersion freezing experiments were performed utilizing two distinct single-droplet levitation methods. In the Mainz vertical wind tunnel (M-WT) supercooled droplets of 700 μm diameter were freely floated in a vertical air stream at constant temperatures ranging from −5 °C to −30 °C where heterogeneous freezing takes place. These investigations under isothermal conditions allow applying the stochastic approach to analyze and interpret the results in terms of the freezing or nucleation rate. In the Mainz acoustic levitator (M-AL) 2 mm diameter drops were levitated while their temperature was continuously cooling from +20 °C to −28 °C by adapting to the ambient temperature. Therefore, in this case the singular approach was used for analysis. From the experiments, the densities of ice nucleating active sites (INAS) were obtained as function of temperature. The direct comparison of the results from two different instruments indicates a shift of the freezing temperatures towards lower values that was material dependent. As ice nucleating particles, seven materials were investigated, two representatives of biological species (fibrous and microcrystalline cellulose), four mineral dusts (feldspar, illite NX, montmorillonite, and kaolinite), and natural Sahara dust. Based on detailed analysis of our results we determined a material dependent temperature correction factor for each investigated particle type. The analysis allowed further classifying the investigated materials as single- or multiple-component. From our experiences during the present synergetic studies, we listed a number of suggestions for future experiments regarding cooling rates, determination of the drop temperature, purity of the water used to produce the drops, and characterization of the ice nucleating material. The observed freezing temperature shift is significantly important not only for the intercomparison of ice nucleation instruments with different cooling rates but also for cloud model simulations with high speed ascents of air masses.
