Abstract

Abstract. Homogeneous nucleation of ice in supercooled water droplets is a stochastic process. In its classical description, the growth of the ice phase requires the emergence of a critical embryo from random fluctuations of water molecules between the water bulk and ice-like clusters, which is associated with overcoming an energy barrier. For heterogeneous ice nucleation on ice-nucleating surfaces both stochastic and deterministic descriptions are in use. Deterministic (singular) descriptions are often favored because the temperature dependence of ice nucleation on a substrate usually dominates the stochastic time dependence, and the ease of representation facilitates the incorporation in climate models. Conversely, classical nucleation theory (CNT) describes heterogeneous ice nucleation as a stochastic process with a reduced energy barrier for the formation of a critical embryo in the presence of an ice-nucleating surface. The energy reduction is conveniently parameterized in terms of a contact angle α between the ice phase immersed in liquid water and the heterogeneous surface. This study investigates various ice-nucleating agents in immersion mode by subjecting them to repeated freezing cycles to elucidate and discriminate the time and temperature dependences of heterogeneous ice nucleation. Freezing rates determined from such refreeze experiments are presented for Hoggar Mountain dust, birch pollen washing water, Arizona test dust (ATD), and also nonadecanol coatings. For the analysis of the experimental data with CNT, we assumed the same active site to be always responsible for freezing. Three different CNT-based parameterizations were used to describe rate coefficients for heterogeneous ice nucleation as a function of temperature, all leading to very similar results: for Hoggar Mountain dust, ATD, and larger nonadecanol-coated water droplets, the experimentally determined increase in freezing rate with decreasing temperature is too shallow to be described properly by CNT using the contact angle α as the only fit parameter. Conversely, birch pollen washing water and small nonadecanol-coated water droplets show temperature dependencies of freezing rates steeper than predicted by all three CNT parameterizations. Good agreement of observations and calculations can be obtained when a pre-factor β is introduced to the rate coefficient as a second fit parameter. Thus, the following microphysical picture emerges: heterogeneous freezing occurs at ice-nucleating sites that need a minimum (critical) surface area to host embryos of critical size to grow into a crystal. Fits based on CNT suggest that the critical active site area is in the range of 10–50 nm2, with the exact value depending on sample, temperature, and CNT-based parameterization. Two fitting parameters are needed to characterize individual active sites. The contact angle α lowers the energy barrier that has to be overcome to form the critical embryo at the site compared to the homogeneous case where the critical embryo develops in the volume of water. The pre-factor β is needed to adjust the calculated slope of freezing rate increase with temperature decrease. When this slope is steep, this can be interpreted as a high frequency of nucleation attempts, so that nucleation occurs immediately when the temperature is low enough for the active site to accommodate a critical embryo. This is the case for active sites of birch pollen washing water and for small droplets coated with nonadecanol. If the pre-factor is low, the frequency of nucleation attempts is low and the increase in freezing rate with decreasing temperature is shallow. This is the case for Hoggar Mountain dust, the large droplets coated with nonadecanol, and ATD. Various hypotheses why the value of the pre-factor depends on the nature of the active sites are discussed.

Highlights

  • Freezing of liquid droplets and subsequent ice crystal growth affects optical cloud properties and precipitation (IPCC, 2013)

  • The glaciation of these clouds is ascribed to heterogeneous ice nucleation occurring on the foreign surfaces of ice-nucleating particles (INPs) present in the cloud droplets

  • We investigate the refreeze experiments for evidence against or in favor of ice nucleation at active sites

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Summary

Introduction

Freezing of liquid droplets and subsequent ice crystal growth affects optical cloud properties and precipitation (IPCC, 2013). Field measurements show that ice nucleation in relatively warm cumulus and stratiform clouds may begin at temperatures much higher than those associated with homogeneous ice nucleation in pure water droplets. The glaciation of these clouds is ascribed to heterogeneous ice nucleation occurring on the foreign surfaces of ice-nucleating particles (INPs) present in the cloud droplets. There is increasing evidence that preferred locations present on surfaces are responsible for ice nucleation (e.g., Vali, 2014; Vali et al, 2015) Such sites are thought to be special surface regions such as crystal defects (Vonnegut, 1947), pores, cracks, or ledges (Knight, 1979; Sear, 2011; Fletcher, 1969), direct evidence of the morphology, structure, and chemistry of active nucleation sites is lacking up to now. While observations of deposition nucleation on a crystal may provide evidence for preferred locations for ice nucleation, only indirect evidence from refreeze experiments exists for immersion freezing (Vali et al, 2015)

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