A comparative study of K-rich and Na/Ca-rich feldspar ice nucleating particles in a nanoliter droplet freezing assay

Abstract

Abstract. A recently designed droplet freezing assay was used to study the freezing of up to 1500 identical 0.2 nL water droplets containing suspensions of three K-rich and one Na/Ca-rich feldspar particles. Three types of experiments have been conducted: cooling ramp, isothermal freezing at constant temperature, and freeze-thaw cycles. The observed freezing behavior have been interpreted with the help of a model based on the classical nucleation theory (Soccer Ball Model, SBM, Niedermeier, (2015). Applying the model to the different freezing experiments conducted with the same ice nucleating material allowed to constrain the parameter space and to derive the unique sets of model parameters for specific feldspar suspensions. The SBM was shown to adequately describe the shift of the freezing curves towards the lower temperature with dilution, the cooling rate dependence and the ice nucleating active sites (INAS) surface density ns (T) in a wide temperature range. Moreover, the SBM was capable of reproducing the variation of INAS surface density ns (T) with concentration of IN in the suspension droplets and correctly predicting the leveling-off of the ns (T) at low temperature. The freeze-thaw experiments have clearly shown that the heterogeneous freezing induced even by very active ice nucleating species still possesses a stochastic nature, with the degree of randomness increasing towards homogeneous nucleation. A population of the high temperature INAS has been identified in one of the K-rich feldspar samples (FS04). The freezing of 0.8 wt% suspension droplets of this particular feldspar was observed already at −5 °C. These high temperature active sites could be completely deactivated by treatment the sample with hydrogen peroxide but survived heating up to 90 °C. Although the mass density of the high temperature IN sites is comparable to that of the typical bacterial or fungal INPs, the possibility of biological contamination of the sample have been ruled out. The freezing efficacy of all feldspar samples have been shown to reduce only slightly after suspending in water for over 5 months.