scholarly journals Heterogeneous freezing of super cooled water droplets in micrometre range-freezing on a chip

2017 ◽  
Author(s):  
Thomas Häusler ◽  
Lorenz Witek ◽  
Laura Felgitsch ◽  
Regina Hitzenberger ◽  
Hinrich Grothe
Keyword(s):  
Active Sites ◽  
Birch Pollen ◽  
Freezing Temperature ◽  
Pure Gold ◽  
Ultrapure Water ◽  
Droplet Sizes ◽  
Temperature Window ◽  
Freezing Temperatures ◽  
Set Up ◽  
Washing Water

Abstract. A new setup to analyse the freezing behaviour of ice nucleation particles (INPs) dispersed in aqueous droplets has been developed with the aim to analyse ensembles of droplets with sizes in the micrometre range, in which INPs are immersed. Major disadvantages of conventional drop-freezing experiments like varying drop sizes or interactions between the water-oil mixture and the INP, were solved by introducing a unique freezing-chip consisting of an etched and sputtered 15 × 15 × 1 mm gold-plated silicon or pure gold film. Using this chip, isolated micrometre-sized droplets can be generated with sizes similar to droplets in real world clouds. The experimental set-up for drop-freezing experiments was revised and improved by establishing automated process control and image evaluation. The new set-up is economical, quick in handling the sample, precise in measurement and the results are more next to real conditions than former approaches. We were able to show the efficiency and accuracy of our setup by comparing measured freezing temperatures of different INPs (Snomax®, K-feldspar, birch pollen (Betula pendula) washing water, juniper pollen suspension (Juniperus communis) and ultrapure water) with already published results. The T50 values of ultrapure water (T50 = −37.2 °C), birch pollen washing water (T50 = −18 °C) and juniper pollen (T50 = −22.7 °C) match the data given in literature. Microcline shows higher freezing temperatures (T50 = −16.4 °C) than literature values from us and others, which can be explained by different preparing/milling parameters. The slightly lower freezing temperature of Snomax® (T50 = −8.9 °C) received by using the freezing-chip, compared to measurements already published, can be explained by different concentrations and droplet sizes. Our measurements and comparisons with the literature data show the important impact of droplet size, INP concentration and number of active sites on the T50 values. Here, the new set-up exhibits its strength in reproducibility and accuracy which is due to the defined and isolated droplets. Finally, it opens a temperature window down to −37 °C for freezing experiments which was not accessible with many former approaches and allows determination of IN also with weak nucleation activity.

scholarly journals Refreeze experiments of water droplets containing different types of ice nuclei interpreted by classical nucleation theory

2016 ◽  
Author(s):  
Lukas Kaufmann ◽  
Claudia Marcolli ◽  
Beiping Luo ◽  
Thomas Peter
Keyword(s):  
Contact Angle ◽  
Active Site ◽  
Energy Barrier ◽  
Active Sites ◽  
Birch Pollen ◽  
Ice Nucleation ◽  
Freezing Rate ◽  
Water Droplets ◽  
Washing Water ◽  
Heterogeneous Ice Nucleation

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. This reduction is conveniently parameterized in terms of a contact angle α between the ice phase immersed in liquid water and the heterogeneous surface area. 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 and Arizona Test Dust (ATD) and 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 parame-terizations 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 of freezing rate with decreasing temperature is too shallow to be described properly by CNT using the contact angle as the only fit parameter. Birch pollen washing water and small nonadecanol coated water droplets show the reverse behavior with temperature dependencies of freezing rates steeper than predicted by CNT formulations. Good agreement of observations and calculations can be obtained when a prefactor β is introduced to the rate coefficient as second fit parameter. Thus, the following microphysical picture emerges: Heterogeneous freezing occurs on 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 depending on sample, temperature, and CNT-based parameterization. Two fitting parameters are needed to characterize individual active sites. The contact angle lowers the energy barrier that has to be overcome to form the critical embryo on the site compared to the homogeneous case where the critical embryo develops in the volume of water. The prefactor β is needed to adjust the calculated slope of freezing rate increase with decreasing temperature to the measured one. When it is large, there are many nucleation attempts and nucleation occurs immediately when the temperature is low enough so that the active site can accommodate a critical embryo. This is the case for active sites of birch pollen washing water and the small droplets coated with nonadecanol. If the prefactor is low, the number of nucleation attempts is low and the increase of freezing rate with decreasing temperature is shallow. This is the case for Hoggar Mountain dust, the large droplets coated with nonadecanol, and ATD. Different hypotheses why the value of the prefactor depends on the nature of the active sites are discussed.

scholarly journals Refreeze experiments with water droplets containing different types of ice nuclei interpreted by classical nucleation theory

2017 ◽  
Vol 17 (5) ◽  
pp. 3525-3552 ◽  
Author(s):  
Lukas Kaufmann ◽  
Claudia Marcolli ◽  
Beiping Luo ◽  
Thomas Peter
Keyword(s):  
Contact Angle ◽  
Active Site ◽  
Energy Barrier ◽  
Active Sites ◽  
Birch Pollen ◽  
Ice Nucleation ◽  
Freezing Rate ◽  
Water Droplets ◽  
Washing Water ◽  
Heterogeneous Ice Nucleation

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.

scholarly journals Efficiency of immersion mode ice nucleation on surrogates of mineral dust

2007 ◽  
Vol 7 (19) ◽  
pp. 5081-5091 ◽  
Author(s):  
C. Marcolli ◽  
S. Gedamke ◽  
T. Peter ◽  
B. Zobrist
Keyword(s):  
Contact Angle ◽  
Active Sites ◽  
Quantitative Agreement ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Aqueous Suspensions ◽  
Freezing Temperatures ◽  
Good Agreement ◽  
Heterogeneous Ice Nucleation

Abstract. A differential scanning calorimeter (DSC) was used to explore heterogeneous ice nucleation of emulsified aqueous suspensions of two Arizona test dust (ATD) samples with particle diameters of nominally 0–3 and 0–7 μm, respectively. Aqueous suspensions with ATD concentrations of 0.01–20 wt% have been investigated. The DSC thermograms exhibit a homogeneous and a heterogeneous freezing peak whose intensity ratios vary with the ATD concentration in the aqueous suspensions. Homogeneous freezing temperatures are in good agreement with recent measurements by other techniques. Depending on ATD concentration, heterogeneous ice nucleation occurred at temperatures as high as 256 K or down to the onset of homogeneous ice nucleation (237 K). For ATD-induced ice formation Classical Nucleation Theory (CNT) offers a suitable framework to parameterize nucleation rates as a function of temperature, experimentally determined ATD size, and emulsion droplet volume distributions. The latter two quantities serve to estimate the total heterogeneous surface area present in a droplet, whereas the suitability of an individual heterogeneous site to trigger nucleation is described by the compatibility function (or contact angle) in CNT. The intensity ratio of homogeneous to heterogeneous freezing peaks is in good agreement with the assumption that the ATD particles are randomly distributed amongst the emulsion droplets. The observed dependence of the heterogeneous freezing temperatures on ATD concentrations cannot be described by assuming a constant contact angle for all ATD particles, but requires the ice nucleation efficiency of ATD particles to be (log)normally distributed amongst the particles. Best quantitative agreement is reached when explicitly assuming that high-compatibility sites are rare and that therefore larger particles have on average more and better active sites than smaller ones. This analysis suggests that a particle has to have a diameter of at least 0.1 μm to exhibit on average one active site.

scholarly journals An instrument for quantifying heterogeneous ice nucleation in multiwell plates using infrared emissions to detect freezing

2018 ◽  
Vol 11 (10) ◽  
pp. 5629-5641 ◽  
Author(s):  
Alexander D. Harrison ◽  
Thomas F. Whale ◽  
Rupert Rutledge ◽  
Stephen Lamb ◽  
Mark D. Tarn ◽  
...  
Keyword(s):  
Active Sites ◽  
Freezing Temperature ◽  
Equilibrium Temperature ◽  
Ice Nucleation ◽  
Ir Camera ◽  
Low Concentrations ◽  
Immersion Freezing ◽  
Mixed Phase Clouds ◽  
The City ◽  
Good Agreement

Abstract. Low concentrations of ice-nucleating particles (INPs) are thought to be important for the properties of mixed-phase clouds, but their detection is challenging. Hence, there is a need for instruments where INP concentrations of less than 0.01 L−1 can be routinely and efficiently determined. The use of larger volumes of suspension in drop assays increases the sensitivity of an experiment to rarer INPs or rarer active sites due to the increase in aerosol or surface area of particulates per droplet. Here we describe and characterise the InfraRed-Nucleation by Immersed Particles Instrument (IR-NIPI), a new immersion freezing assay that makes use of IR emissions to determine the freezing temperature of individual 50 µL droplets each contained in a well of a 96-well plate. Using an IR camera allows the temperature of individual aliquots to be monitored. Freezing temperatures are determined by detecting the sharp rise in well temperature associated with the release of heat caused by freezing. In this paper we first present the calibration of the IR temperature measurement, which makes use of the fact that following ice nucleation aliquots of water warm to the ice–liquid equilibrium temperature (i.e. 0 ∘C when water activity is ∼1), which provides a point of calibration for each individual well in each experiment. We then tested the temperature calibration using ∼100 µm chips of K-feldspar, by immersing these chips in 1 µL droplets on an established cold stage (µL-NIPI) as well as in 50 µL droplets on IR-NIPI; the results were consistent with one another, indicating no bias in the reported freezing temperature. In addition we present measurements of the efficiency of the mineral dust NX-illite and a sample of atmospheric aerosol collected on a filter in the city of Leeds. NX-illite results are consistent with literature data, and the atmospheric INP concentrations were in good agreement with the results from the µL-NIPI instrument. This demonstrates the utility of this approach, which offers a relatively high throughput of sample analysis and access to low INP concentrations.

scholarly journals Effect of particle surface area on ice active site densities retrieved from droplet freezing spectra

2016 ◽  
Vol 16 (20) ◽  
pp. 13359-13378 ◽  
Author(s):  
Hassan Beydoun ◽  
Michael Polen ◽  
Ryan C. Sullivan
Keyword(s):  
Surface Area ◽  
Active Sites ◽  
Particle Surface ◽  
Active Surface ◽  
Freezing Temperature ◽  
Ice Nucleation ◽  
Critical Surface ◽  
Cold Plate ◽  
Particle Surface Area ◽  
Immersion Freezing

Abstract. Heterogeneous ice nucleation remains one of the outstanding problems in cloud physics and atmospheric science. Experimental challenges in properly simulating particle-induced freezing processes under atmospherically relevant conditions have largely contributed to the absence of a well-established parameterization of immersion freezing properties. Here, we formulate an ice active, surface-site-based stochastic model of heterogeneous freezing with the unique feature of invoking a continuum assumption on the ice nucleating activity (contact angle) of an aerosol particle's surface that requires no assumptions about the size or number of active sites. The result is a particle-specific property g that defines a distribution of local ice nucleation rates. Upon integration, this yields a full freezing probability function for an ice nucleating particle. Current cold plate droplet freezing measurements provide a valuable and inexpensive resource for studying the freezing properties of many atmospheric aerosol systems. We apply our g framework to explain the observed dependence of the freezing temperature of droplets in a cold plate on the concentration of the particle species investigated. Normalizing to the total particle mass or surface area present to derive the commonly used ice nuclei active surface (INAS) density (ns) often cannot account for the effects of particle concentration, yet concentration is typically varied to span a wider measurable freezing temperature range. A method based on determining what is denoted an ice nucleating species' specific critical surface area is presented and explains the concentration dependence as a result of increasing the variability in ice nucleating active sites between droplets. By applying this method to experimental droplet freezing data from four different systems, we demonstrate its ability to interpret immersion freezing temperature spectra of droplets containing variable particle concentrations. It is shown that general active site density functions, such as the popular ns parameterization, cannot be reliably extrapolated below this critical surface area threshold to describe freezing curves for lower particle surface area concentrations. Freezing curves obtained below this threshold translate to higher ns values, while the ns values are essentially the same from curves obtained above the critical area threshold; ns should remain the same for a system as concentration is varied. However, we can successfully predict the lower concentration freezing curves, which are more atmospherically relevant, through a process of random sampling from g distributions obtained from high particle concentration data. Our analysis is applied to cold plate freezing measurements of droplets containing variable concentrations of particles from NX illite minerals, MCC cellulose, and commercial Snomax bacterial particles. Parameterizations that can predict the temporal evolution of the frozen fraction of cloud droplets in larger atmospheric models are also derived from this new framework.

The Spontaneous Freezing Temperatures of Melted Snow and of Small Water Drops

1954 ◽  
Vol 35 (2) ◽  
pp. 52-55 ◽  
Author(s):  
Harry C. Vaughan
Keyword(s):  
Drop Size ◽  
Freezing Temperature ◽  
Bulk Sample ◽  
Bulk Water ◽  
Lower Atmosphere ◽  
Obvious Effect ◽  
Small Water ◽  
Melted Snow ◽  
Freezing Temperatures ◽  
Snow Crystals

The spontaneous freezing temperatures of samples of melted snow collected at hourly intervals during several storms are reported. The freezing temperatures were found to range from −8.3 to −16.8°C. In most cases the freezing temperature remained fairly constant over periods of several hours. Gross variations in the pollution of the lower atmosphere caused no obvious effect on the observed freezing temperatures. It is suggested that the observed freezing temperatures represent the temperatures of initial formation of the snow crystals. A few experiments on the freezing temperatures of small drops were made with a technique such that all drops were formed from the same bulk sample without any danger of contamination. In all cases the freezing temperatures of the drops were well below that of the bulk water. The freezing temperature decreased with decreasing drop size but there was a relatively large variation for a given drop size.

scholarly journals Cleaning up our water: reducing interferences from nonhomogeneous freezing of “pure” water in droplet freezing assays of ice-nucleating particles

2018 ◽  
Vol 11 (9) ◽  
pp. 5315-5334 ◽  
Author(s):  
Michael Polen ◽  
Thomas Brubaker ◽  
Joshua Somers ◽  
Ryan C. Sullivan
Keyword(s):  
Pure Water ◽  
Freezing Temperature ◽  
Cloud Microphysics ◽  
Ice Nucleation ◽  
Great Promise ◽  
Water Control ◽  
Temperature Spectra ◽  
Droplet Sizes ◽  
Mixed Phase Clouds ◽  
Insight Into

Abstract. Droplet freezing techniques (DFTs) have been used for half a century to measure the concentration of ice-nucleating particles (INPs) in the atmosphere and determine their freezing properties to understand the effects of INPs on mixed-phase clouds. The ice nucleation community has recently adopted droplet freezing assays as a commonplace experimental approach. These droplet freezing experiments are often limited by contamination that causes nonhomogeneous freezing of the “pure” water used to generate the droplets in the heterogeneous freezing temperature regime that is being measured. Interference from the early freezing of water is often overlooked and not fully reported, or measurements are restricted to analyzing the more ice-active INPs that freeze well above the temperature of the background water. However, this avoidance is not viable for analyzing the freezing behavior of less active INPs in the atmosphere that still have potentially important effects on cold-cloud microphysics. In this work we review a number of recent droplet freezing techniques that show great promise in reducing these interferences, and we report our own extensive series of measurements using similar methodologies. By characterizing the performance of different substrates on which the droplets are placed and of different pure water generation techniques, we recommend best practices to reduce these interferences. We tested different substrates, water sources, droplet matrixes, and droplet sizes to provide deeper insight into what methodologies are best suited for DFTs. Approaches for analyzing droplet freezing temperature spectra and accounting and correcting for the background “pure” water control spectrum are also presented. Finally, we propose experimental and data analysis procedures for future homogeneous and heterogeneous ice nucleation studies to promote a more uniform and reliable methodology that facilitates the ready intercomparison of ice-nucleating particles measured by DFTs.

scholarly journals Effects of storage temperature and repeated freeze–thaw cycles on stability of bovine plasma concentrations of haptoglobin and ceruloplasmin

2017 ◽  
Vol 29 (5) ◽  
pp. 738-740 ◽  
Author(s):  
Paulo G. M. A. Martins ◽  
Philipe Moriel ◽  
John D. Arthington
Keyword(s):  
Whole Blood ◽  
Storage Temperature ◽  
Plasma Concentrations ◽  
Freezing Temperature ◽  
Plasma Samples ◽  
Blood Samples ◽  
Freeze Thaw ◽  
Bovine Plasma ◽  
Whole Blood Samples ◽  
Freezing Temperatures

We evaluated the effects of storage temperature (−20 or −80°C) and handling procedure on plasma concentrations of bovine haptoglobin and ceruloplasmin. Within each temperature, whole blood samples were: centrifuged within 2 h of collection and plasma kept frozen until analysis (control); refrigerated at 4°C for 24 h before plasma harvest and freezing (24H); or plasma harvested and frozen within 2 h after collection, but then plasma samples were thawed and refrozen 1 wk (1X), 1 and 2 wk (2X), or 1, 2 and 3 wk (3X) before analyses. Haptoglobin concentrations were greatest at 24H, but similar among remaining treatments. Ceruloplasmin concentrations were not affected by the handling procedures. Storage temperature did not affect haptoglobin concentrations, but ceruloplasmin concentrations decreased when stored at −20 versus −80°C. Except for greater concentrations after 24 h storage at 4°C, haptoglobin concentrations remained stable at either freezing temperature and through freeze–thaw cycles. Ceruloplasmin concentrations decreased after 3 freeze–thaw cycles and required lower freezing temperatures to remain stable.

scholarly journals Immersion freezing of water and aqueous ammonium sulfate droplets initiated by humic-like substances as a function of water activity

2013 ◽  
Vol 13 (13) ◽  
pp. 6603-6622 ◽  
Author(s):  
Y. J. Rigg ◽  
P. A. Alpert ◽  
D. A. Knopf
Keyword(s):  
Water Activity ◽  
Active Sites ◽  
Melting Curve ◽  
Contact Angles ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Surface Areas ◽  
Immersion Freezing ◽  
Freezing Temperatures

Abstract. Immersion freezing of water and aqueous (NH4)2SO4 droplets containing leonardite (LEO) and Pahokee peat (PP) serving as surrogates for humic-like substances (HULIS) has been investigated. Organic aerosol containing HULIS are ubiquitous in the atmosphere; however, their potential for ice cloud formation is uncertain. Immersion freezing has been studied for temperatures as low as 215 K and solution water activity, aw, from 0.85 to 1.0. The freezing temperatures of water and aqueous solution droplets containing LEO and PP are 5–15 K warmer than homogeneous ice nucleation temperatures. Heterogeneous freezing temperatures can be represented by a horizontal shift of the ice melting curve as a function of solution aw by Δaw = 0.2703 and 0.2466, respectively. Corresponding hetrogeneous ice nucleation rate coefficients, Jhet, are (9.6 ± 2.5)×104 and (5.4 ± 1.4)×104 cm−2 s−1 for LEO and PP containing droplets, respectively, and remain constant along freezing curves characterized by Δaw. Consequently predictions of freezing temperatures and kinetics can be made without knowledge of the solute type when relative humidity and ice nuclei (IN) surface areas are known. The acquired ice nucleation data are applied to evaluate different approaches to fit and reproduce experimentally derived frozen fractions. In addition, we apply a basic formulation of classical nucleation theory (α(T)-model) to calculate contact angles and frozen fractions. Contact angles calculated for each ice nucleus as a function of temperature, α(T)-model, reproduce exactly experimentally derived frozen fractions without involving free-fit parameters. However, assigning the IN a single contact angle for the entire population (single-α model) is not suited to represent the frozen fractions. Application of α-PDF, active sites, and deterministic model approaches to measured frozen fractions yield similar good representations. Furthermore, when using a single parameterization of α-PDF or active sites distribution to fit all individual aw immersion freezing data simultaneously, frozen fraction curves are not reproduced. This implies that these fitting formulations cannot be applied to immersion freezing of aqueous solutions, and suggests that derived fit parameters do not represent independent particle properties. Thus, from fitting frozen fractions only, the underlying ice nucleation mechanism and nature of the ice nucleating sites cannot be inferred. In contrast to using fitted functions obtained to represent experimental conditions only, we suggest to use experimentally derived Jhet as a function of temperature and aw that can be applied to conditions outside of those probed in laboratory. This is because Jhet(T) is independent of time and IN surface areas in contrast to the fit parameters obtained by representation of experimentally derived frozen fractions.

scholarly journals Ice nucleation efficiency of AgI: review and new insights

2016 ◽  
Vol 16 (14) ◽  
pp. 8915-8937 ◽  
Author(s):  
Claudia Marcolli ◽  
Baban Nagare ◽  
André Welti ◽  
Ulrike Lohmann
Keyword(s):  
Water Adsorption ◽  
Water Saturation ◽  
Experimental Studies ◽  
Freezing Temperature ◽  
Nucleating Agent ◽  
Ice Nucleation ◽  
Lattice Match ◽  
Complex Picture ◽  
Good Lattice ◽  
Freezing Temperatures

Abstract. AgI is one of the best-investigated ice-nucleating substances. It has relevance for the atmosphere since it is used for glaciogenic cloud seeding. Theoretical and experimental studies over the last 60 years provide a complex picture of silver iodide as an ice-nucleating agent with conflicting and inconsistent results. This review compares experimental ice nucleation studies in order to analyze the factors that influence the ice nucleation ability of AgI. The following picture emerges from this analysis: the ice nucleation ability of AgI seems to be enhanced when the AgI particle is on the surface of a droplet, which is indeed the position that a particle takes when it can freely move in a droplet. The ice nucleation by particles with surfaces exposed to air depends on water adsorption. AgI surfaces seem to be most efficient at nucleating ice when they are exposed to relative humidity at or even above water saturation. For AgI particles that are completely immersed in water, the freezing temperature increases with increasing AgI surface area. Higher threshold freezing temperatures seem to correlate with improved lattice matches as can be seen for AgI–AgCl solid solutions and 3AgI·NH4I·6H2O, which have slightly better lattice matches with ice than AgI and also higher threshold freezing temperatures. However, the effect of a good lattice match is annihilated when the surfaces have charges. Also, the ice nucleation ability seems to decrease during dissolution of AgI particles. This introduces an additional history and time dependence for ice nucleation in cloud chambers with short residence times.

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