Abstract
Abstract. At the Leipzig Aerosol Cloud Interaction Simulator (LACIS) experiments investigating homogeneous and heterogeneous nucleation of ice (particularly immersion freezing in the latter case) have been carried out. Here both the physical LACIS setup and the numerical model developed to design experiments at LACIS and interpret their results are presented in detail. Combining results from the numerical model with experimental data, it was found that for the experimental parameter space considered, classical homogeneous ice nucleation theory is able to predict the freezing behavior of highly diluted ammonium sulfate solution droplets, while classical heterogeneous ice nucleation theory, together with the assumption of a constant contact angle, fails to predict the immersion freezing behavior of surrogate mineral dust particles (Arizona Test Dust, ATD). The main reason for this failure is the compared to experimental data apparently overly strong temperature dependence of the nucleation rate coefficient. Assuming, in the numerical model, Classical Nucleation Theory (CNT) for homogeneous ice nucleation and a CNT-based parameterization for the nucleation rate coefficient in the immersion freezing mode, recently published by our group, it was found that even for a relatively effective ice nucleating agent such as pure ATD, there is a temperature range where homogeneous ice nucleation is dominant. The main explanation is the apparently different temperature dependencies of the two freezing mechanisms. Finally, reviewing the assumptions made during the derivation of the CNT-based parameterization for immersion freezing, it was found that the assumption of constant temperature during ice nucleation and the chosen ice nucleation time were justified, underlining the applicability of the method to determine the fitting coefficients in the parameterization equation.
Highlights
Ice containing clouds have an impact on the Earth’s radiative balance by scattering and absorbing solar and terrestrial radiation (Zuberi et al, 2002; Hung et al, 2003)
For determining the homogeneous and heterogeneous ice nucleation rate coefficients to be used in FLUENT/Fine Particle Model (FPM), two different model approaches are adopted: (a) Classical Nucleation Theory is applied for both homogeneous and heterogeneous ice nucleation, and (b) CNT is used for modeling homogeneous ice nucleation, but immersion freezing is described by implementing a CNT-based parameterization derived from prior Leipzig Aerosol Cloud Interaction Simulator (LACIS) measurements (Niedermeier et al, 2010)
The numerical model FLUENT/FPM as described above is a suitable tool for exploring LACIS’ behavior for a given set of boundary conditions, testing assumptions made during the interpretation of experimental data, and evaluating the feasibility of different theoretical approaches for modeling homogeneous and heterogeneous ice nucleation
Summary
Ice containing clouds have an impact on the Earth’s radiative balance by scattering and absorbing solar and terrestrial radiation (Zuberi et al, 2002; Hung et al, 2003). The three main foci of the present paper are the description of (1) the physical setup and operating principle of LACIS for investigating homogeneous and heterogeneous ice nucleation (especially immersion freezing in the latter case), (2) the introduction of the numerical model developed to design and interpret the experiments at LACIS, and (3) the interpretation of actual experimental results by comparing with ice nucleation theory (Classical Nucleation Theory and a CNTbased parameterization). This rather theoretical paper and that of Niedermeier et al (2010) are linked closely. The validity of assumptions made for the CNT-based parameterization of immersion freezing in Niedermeier et al (2010) is discussed
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