Abstract
Abstract. Immersion freezing experiments were performed utilizing two distinct single-droplet levitation methods. In the Mainz vertical wind tunnel, supercooled droplets of 700 µm diameter were freely floated in a vertical airstream at constant temperatures ranging from −5 to −30 ∘C, where heterogeneous freezing takes place. These investigations under isothermal conditions allow the application of the stochastic approach to analyze and interpret the results in terms of the freezing or nucleation rate. In the Mainz acoustic levitator, 2 mm diameter drops were levitated while their temperature was continuously cooling from +20 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 nucleation active sites were obtained as a function of temperature. The direct comparison of the results from two different instruments indicates a shift in the mean freezing temperatures of the investigated drops 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 parameter for calculating the freezing-temperature shift due to a change in cooling rate for each investigated particle type. The analysis allowed further classification of the investigated materials to be described by a single- or a multiple-component approach. 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 for the intercomparison of ice nucleation instruments with different cooling rates.
Highlights
Immersion freezing is considered to be the most effective nucleation process for ice particle production in mixed-phase clouds (Diehl and Grützun, 2018)
We present the results of Mainz vertical wind tunnel (M-WT) and measurement techniques: an acoustic levitator (M-AL) experiments on immersion freezing using the clay mineral kaolinite
The INAS densities of different types of aerosol particles obtained by the M-AL and M-WT revealed a shift in the freezing temperatures to lower values
Summary
Immersion freezing is considered to be the most effective nucleation process for ice particle production in mixed-phase clouds (Diehl and Grützun, 2018). INAS density is used to represent the number of ice-active sites on the particles that are active between 0 ◦C and the subzero temperature T (DeMott, 1995; Connolly et al, 2009; Murray et al, 2012; Hoose and Möhler, 2012). Another important parameter employed for describing the INP nucleation ability is the nucleation rate coefficient, i.e., the probability of nucleation at a certain temperature per unit time per unit surface area of the particle
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