Heterogeneous Freezing Nucleation Research Articles

Exploratory experiments on pre-activated freezing nucleation on mercuric iodide

Abstract. Pre-activation of freezing nucleation (PFN) with mercuric iodide was first reported by Edwards, Evans, and Zipper (Edwards et al., 1970). They found that freezing, followed by melting just a few degrees Celsius above the melting point, leads to subsequent freezing of the sample more than 10 ∘C above the temperature of the initial nucleation temperature. Results presented in this paper are from laboratory experiments that followed the procedure designed by Edwards, Evans, and Zipper (1970) but employed multiple sample drops and many repetitions of the pre-activation cycle. The results obtained confirm the basic findings of the earlier work and refine them. It is shown that the pre-activation effect is lost gradually as the sample is heated above the melting point and that some effect is still seen with heating above +5 ∘C. Instrumental limitation in these experiments precluded detection of pre-activated freezing above −2 ∘C, but that possibility is not excluded. Some PFN was noted down to at least −6 ∘C. By also drawing on the results of Seeley and Seidler (2001), PFN is analyzed in search of constraints that help define the process responsible for it. No firm conclusions are reached, but the accumulated evidence points quite clearly to the role of surface sites in leading to PFN. Thus, sites are seen to play the same role as they do in heterogeneous freezing nucleation in general. PFN differs from pore condensation and freezing described by Marcolli (2020) and David et al. (2020), in that PFN is observed in liquid water while that process takes place in the vapor phase. Further explorations of the process leading to PFN can help in understanding ice nucleation and its practical manifestations at a basic level. The results call attention to an ice nucleation pathway hitherto barely explored that can be expected to have consequences in how ice nucleation occurs in atmospheric clouds and in other systems. PFN is also a potential tool for deliberate initiation of freezing in clouds and other systems.

Interpretation of freezing nucleation experiments: singular and stochastic; sites and surfaces

Abstract. Publications of recent years dealing with laboratory experiments of immersion freezing reveal uncertainties about the fundamentals of heterogeneous freezing nucleation. While it appears well accepted that there are two major factors that determine the process, namely fluctuations in the size and configuration of incipient embryos of the solid phase and the role of the substrate to aid embryo formation, views have been evolving about the relative importance of these two elements. The importance of specific surface sites is being established in a growing number of experiments and a number of approaches have been proposed to incorporate these results into model descriptions. Many of these models share a common conceptual basis yet diverge in the way random and deterministic factors are combined. The divergence can be traced to uncertainty about the permanence of nucleating sites, to the lack of detailed knowledge about what surface features constitute nucleating sites, and to the consequent need to rely on empirical or parametric formulas to define the population of sites of different effectiveness. Recent experiments and models, consistent with earlier work, demonstrate the existence and primary role of permanent nucleating sites and the continued need for empirically based formulations of heterogeneous freezing. In order to clarify some aspects of the processes controlling immersion freezing, the paper focuses on three identifiably separate but interrelated issues: (i) the combination of singular and stochastic factors, (ii) the role of specific surface sites, and (iii) the modeling of heterogeneous ice nucleation.

The role of time in heterogeneous freezing nucleation

A small fraction of particles in the atmosphere can catalyze ice formation in cloud water drops through heterogeneous freezing nucleation at temperatures warmer than the homogeneous freezing temperature of approximately −38°C. The rate for heterogeneous freezing nucleation is dependent on several factors, including the type and surface area of dust that is immersed inside the drop. Although nucleation is an inherently stochastic process resulting from size fluctuations of the incipient ice germ, there is a growing body of literature that suggests that quasi‐deterministic models of ice nucleation can describe laboratory experiments. Here we present new experiments and simulations that aim to better constrain theoretical models fitted to laboratory data. We collected ice nucleation data for Arizona Test Dust aerosol immersed in water using a droplet freezing assay setup that allows for the cooling rates to be changed between 10 and 0.01 K min−1. Discrete event simulations based on a variant of the multiple‐component stochastic model of heterogeneous freezing nucleation were used to simulate different experimental procedures. The nucleation properties of the dust are specified by four material‐dependent parameters that accurately describe the time dependence of the freezing process. We anticipate that the combination of discrete event simulations and a spectrum of experimental procedures described here can be used to design more meaningful laboratory experiments probing ice nucleation and will aid the development of better parameterizations for use in models.

Repeatability and randomness in heterogeneous freezing nucleation

Abstract. This study is aimed at clarifying the relative importance of the specific character of the nuclei and of the duration of supercooling in heterogeneous freezing nucleation by immersed impurities. Laboratory experiments were carried out in which sets of water drops underwent multiple cycles of freezing and melting. The drops contained suspended particles of mixtures of materials; the resulting freezing temperatures ranged from −6°C to −24°C. Rank correlation coefficients between observed freezing temperatures of the drops in successive runs were >0.9 with very high statistical significance, and thus provide strong support for the modified singular model of heterogeneous immersion freezing nucleation. For given drops, changes in freezing temperatures between cycles were relatively small (<1°C) for the majority of the events. These frequent small fluctuations in freezing temperatures are interpreted as reflections of the random nature of embryo growth and are associated with a nucleation rate that is a function of a temperature difference from the characteristic temperatures of nuclei. About a sixth of the changes were larger, up to ±5°C, and exhibited some systematic patterns. These are thought to arise from alterations of the nuclei, some being permanent and some transitory. The results are used to suggest ways of describing ice initiation in cloud models that account for both the temperature and the time dependence of freezing nucleation.

Ice nucleation in industrial wastewater

Freezing characteristics of pulp mill effluent, piggery wastewater and oil sands tailings pond water droplets was investigated. The ice nucleus concentrations of the wastewater droplets were calculated with the nucleus spectra model of Vali [Vali, G., 1971. Quantitative evaluation of experimental results on the heterogeneous freezing nucleation of supercooled liquids. J. Atmos. Sci. 28 (1971) 402–409]. The “freezing ability” of the wastewaters was compared based on the derived ice nucleus concentrations. Variation in freezing was found to be dependent on pH, impurity concentration and droplet volume.

The role of ammoniated aerosols in cirrus cloud nucleation

Recent results obtained during the SUCCESS experiment show that aerosols containing ammonium ion are commonly found in the upper troposphere and therefore it is of great importance to investigate the ice freezing properties of such aerosols. For this purpose we present mathematical expressions, derived from recent model calculations and laboratory data, for calculating the deliquescence relative humidity of (NH4)2SO4, NH4HSO4 and NH4NO3 under upper tropospheric conditions. Using these functions we define the temperature and relative humidity requirements for the deliquescence of dry particles in the upper troposphere. Further using a thermodynamic model of the H2SO4/HNO3/NH3/ H2O system, we calculate the variation in the supercooled aerosol composition and saturation ratios of various ionic salts in solution as a function of temperature and relative humidity. Using these values we show salt crystallization in the upper troposphere may occur in two distinct type of ammoniated aerosol solutions. First (NH4)3H(SO4)2 (letovicite) may crystallize in acidic solutions of (NH4)2SO4. Second double salts of (NH4)2SO4 and NH4NO3 may crystallize in nearly neutralized solutions of (NH4)2SO4 and NH4NO3. Finally we discuss how the presence of crystalline salt surfaces in aqueous solutions may alter the mode of cirrus cloud formation from homogeneous to heterogeneous freezing nucleation.

The role of heterogeneous freezing nucleation in upper tropospheric clouds: Inferences from SUCCESS

A temperature spectrum of heterogeneous freezing nuclei concentrations in continental air in the upper troposphere was determined based on airborne measurements. Numerical model simulations incorporating ice formation by heterogeneous and homogeneous freezing of deliquesced soluble aerosol particles were performed to investigate the effect of the heterogeneous process on the microphysics of upper tropospheric clouds. Heterogeneous freezing nuclei were predicted to cause lower maximum concentrations of ice particles formed in clouds. These nuclei also initiate the first ice formation and act to broaden ice crystal size distributions in upper tropospheric clouds. Observations of ice formation in an orographic wave cloud supported these predictions.

The susceptibility of ice formation in upper tropospheric clouds to insoluble aerosol components

Ice may form by both homogeneous and heterogeneous freezing nucleation processes in clouds at temperatures below −35°C. Most investigations have focused on the former process. This paper presents results from adiabatic parcel model calculations that include the effects of both freezing processes in unactivated solution droplets. Uncertainties in predicting the homogeneous freezing rates are discussed and used to select solution drop composition and freezing characteristics that bracket those expected in the upper troposphere. The heterogeneous freezing rates of insoluble atmospheric aerosols are parameterized based on published freezing rates of carbonaceous particles. Process model simulations show that the potential variability in ice formation in cirrus clouds is much greater if heterogeneous freezing nucleation is considered in addition to homogeneous freezing. The impact of insoluble aerosols on ice formation is inferred to increase with insoluble particle size and with the fraction of soluble aerosols containing insoluble components. The maximum impact of heterogeneous nucleation is indicated for vertical motions less than 0.2 m s−1 and for insoluble components being associated with at least 10% of all soluble aerosols. The wide range of ice crystal concentrations observed in cirrus is most consistent with the occurrence of both heterogeneous and homogeneous ice formation processes. These conclusions are partially supported by existing observations of aerosols and cloud microphysical characteristics in upper tropospheric clouds but require new measurements for confirmation.

Ice nucleation of AgI — CuBr nucleants in the presence of electric field

Heterogeneous freezing nucleation in an electric field was studied on six samples of water containing silver iodide and copper bromide. Electric fields of 100 V cm −1 (dc) were applied with different time and temperature sequences. Considerable improvement in the nucleation temperature was observed on the application of the electric field. The application of an electric field causes the production and collapse of cavity bubbles on the electrode during which the ice nucleation was found to be enhanced.

Ice Nucleation by Aerosol Particles. Theory of Condensation-Freezing Nucleation

Abstract A probabillstic theory of heterogeneous condensation-freezing nucleation is developed for monodisperse aerosol particles. An assumption is made that the physical process is heterogeneous condensation nucleation followed by heterogeneous freezing nucleation. The theory is expanded to a polydisperse system. Computed results based on the theory show a striking resemblance particularly in shape with experimental data. Using the theory and an empirical expression for deposition nucleation, the ice nucleation probability of pure AgI particles is reconstructed on the supersaturation-supercooling plane. Importance of proper environmental conditions free from transient supersaturation for condensation-freezing nuclei measurement and necessity of condensation-freezing nucleation modeling are pointed out.

Freezing nucleation in aqueous electrolytes

Experiments were performed to determine the extent to which the nucleating abilities of artificial and natural freezing nuclei are influenced by the presence of salts dissolved in water. Four different nucleants (at fixed concentrations) were tested in combinations with 10–20 soluble salts using the drop freezing technique. Changes in freezing temperatures were determined for each nucleant–salt combination at salt concentrations of 0, 0.01, 0.1 and 1 molal. Major differences were noted in the responses of the different nucleants and it was concluded that the influence of dissolved salts on heterogeneous freezing nucleation is not due to changes in the bulk water structure, but results from modifications at the nucleant–embryo interface. The measurements indicate that the salt contents of cloud droplets can be expected to alter the ice nucleating activities of suspended particles by as much as ±4°C. DOI: 10.1111/j.2153-3490.1975.tb01692.x

Heterogeneous Freezing Nucleation in Electric Fields

Abstract Heterogeneous freezing nucleation in electric fields was studied on samples of water containing organic nuclei or silver iodide. Electric fields of 6000 V cm−1 (dc) were applied over sets of supercooled drops supported on a silicone varnish coated surface during different time-temperature sequences. In no case was a significant difference in the nucleation rates observed due to application of the field. It is concluded from these experiments that electric fields of up to 6000 V cm−1 have no intrinsic effect on the heterogeneous freezing process and that the probability of enhanced ice nucleation in the atmosphere due to natural electric fields is quite remote.

Quantitative Evaluation of Experimental Results an the Heterogeneous Freezing Nucleation of Supercooled Liquids

Freezing experiments using large numbers of small drops are frequently used for the study of both homogeneous and heterogeneous nucleation of water and of other substances. For heterogeneous nucleation, the spread in the observed freezing temperatures of drops has been shown to arise from the presence of nuclei of different activities in the sample. In the past no quantitative assessment of the nucleus content could be given independent of the drop sizes used. It is shown in this paper that from the observed freezing temperatures of the drops one can derive both a differential and a cumulative nucleus spectrum. The differential spectrum represents the concentrations of nuclei which are active at specific temperatures and the cumulative spectrum represents the concentrations of nuclei active at all temperatures warmer than the selected temperature. The accuracies of the derived spectra were examined by Monte Carlo simulation and are shown to be such that the concentrations are reliable to within factors of 2–4. The dependence of the average freezing temperature on drop volume is shown in general to be determined by the shape of the nucleus spectrum but is approximately exponential for many spectra.