Initial Ice Nucleation Research Articles

Modeling and Validation of Assisted Cold Start for a PEMFC Coupled to a Thermochemical Preheater

Cold start from sub-zero temperatures poses a challenge for automotive application of polymer electrolyte fuel cells (PEMFC), as product water freezes inside the cells hereby endangering operation safety. Therefore, ice formation in the cathode pores has to be avoided by preheating of the fuel cell above sub-zero temperatures. The concepts for such assisted cold start can be divided into internal and external heating strategies [1]. Amongst the latter, especially thermochemical preheating by metal hydrides presents a compelling yet efficient technology, as the heat is provided by exothermal hydrogen absorption without consumption of additional external energy.In this work, assisted PEMFC cold start is simulated with the transient 2D numerical model for a single fuel cell thermally coupled to a thermochemical preheater via a coolant fluid developed by Gießgen and Jahnke [2]. Here, the numerical model is validated against cold start experiments of the coupled system for both cell behavior and thermal balance of the heat transfer fluid. Starting from an initial temperature of -5°C, fuel cell behavior is then studied for 300 s of open circuit voltage (OCV) followed by a current density ramp up to 0.33 A/cm² in 150 s and a subsequent constant current period up to 500 s. Model validation is done with respect to temperature and voltage evolution as well as the energy balance of the coupled system. When the cold start is unassisted, a significant voltage drop down to 0.6 V is observed to occur already at 0.22 A/cm² and thus before reaching the maximum current density by the end of the ramp. Whereas difficult to deduce from experiments, this intermediate voltage drop can be linked to massive ice formation blocking up to 87% of the cathode catalyst pores, eventually causing a significant decrease of the electrochemically active surface (ECSA). For this ECSA reduction, an exponential dependence on the ice saturation is identified, suggesting that initial ice nucleation primarily occurs on the macroporous catalyst surface, thereby blocking the nanoporous surfaces of the primary particles within. When the cold start is assisted, a preheating phase of 41 s is performed prior to the current ramp. As the metal hydride reactor effectively heats up the heat transfer fluid, the fuel cell temperature in turn raises rapidly above the freezing point in less than 20 s. Consequently, ice formation is observed to be effectively reduced to saturations of less than 1%. As a result of this assisted cold start, the voltage drop is significantly attenuated and a minimum voltage of 0.65 V is now reached for a maximum of 0.33 A/cm² only at the end of the current ramp.

Ice nucleating behavior of different tree pollen in the immersion mode

Abstract Biological particles, including pollen, are released into the atmosphere in large amounts and carry components which can initiate ice nucleation (IN) under cloud conditions. In this study, we investigate in immersion mode the ice nucleation ability of eleven different tree pollen from deciduous and evergreen trees. Droplet freezing experiments are conducted, where we find the IN ability of filtered tree pollen aqueous suspensions to vary over more than 10 °C and ice nucleation to be triggered by subpollen particles that are smaller than 0.45 μm in size. The presence of full pollen grains in the sample does not significantly change the median freezing temperature. The IN activity of the pollen samples is complemented by total organic carbon (TOC) analysis, which allows calculation of the number of ice nucleating particles per mass of carbon. Infrared analysis of the bulk samples does not clearly indicate whether specific functional groups are correlated to the ability of the subpollen particles to nucleate ice.

Ice formation modes during flow freezing in a small cylindrical channel

Freezing of water flowing through a small channel can be used as a nonintrusive flow control mechanism for microfluidic devices. However, such ice valves have longer response times compared to conventional microvalves. To control and reduce the response time, it is crucial to understand the factors that affect the flow freezing process inside the channel. This study investigates freezing in pressure-driven water flow through a glass channel of 500 μm inner diameter using measurements of external channel wall temperature and flow rate synchronized with high-speed visualization. The effect of flow rate on the freezing process is investigated in terms of the external wall temperature, the growth duration of different ice modes, and the channel closing time. Freezing initiates as a thin layer of ice dendrites that grows along the inner wall and partially blocks the channel, followed by the formation and inward growth of a solid annular ice layer that leads to complete flow blockage and ultimate channel closure. A simplified analytical model is developed to determine the factors that govern the annular ice growth, and hence the channel closing time. For a given channel, the model predicts that the annular ice growth is driven purely by conduction due to the temperature difference between the outer channel wall and the equilibrium ice-water interface. The flow rate affects the initial temperature difference, and thereby has an indirect effect on the annular ice growth. Higher flow rates require a lower wall temperature to initiate ice nucleation and result in faster annular ice growth (and shorter closing times) than at lower flow rates. This study provides new insights into the freezing process in small channels and identifies the key factors governing the channel closing time at these small length scales commonly encountered in microfluidic ice valve applications.

Effect of Latent Heat Released by Freezing Droplets during Frost Wave Propagation.

Frost spreads on nonwetting surfaces during condensation frosting via an interdroplet frost wave. When a supercooled condensate water droplet freezes on a hydrophobic or superhydrophobic surface, neighboring droplets still in the liquid phase begin to evaporate. Two possible mechanisms govern the evaporation of neighboring water droplets: (1) The difference in saturation pressure of the water vapor surrounding the liquid and frozen droplets induces a vapor pressure gradient, and (2) the latent heat released by freezing droplets locally heats the substrate, leading to evaporation of nearby droplets. The relative significance of these two mechanisms is still not understood. Here, we study the significance of the latent heat released into the substrate by freezing droplets, and its effect on adjacent droplet evaporation, by studying the dynamics of individual water droplet freezing on aluminum-, copper-, and glass-based hydrophobic and superhydrophobic surfaces. The latent heat flux released into the substrate was calculated from the measured droplet sizes and the respective freezing times ( tf), defined as the time from initial ice nucleation within the droplet to complete droplet freezing. To probe the effect of latent heat release, we performed three-dimensional transient finite element simulations showing that the transfer of latent heat to neighboring droplets is insignificant and accounts for a negligible fraction of evaporation during microscale frost wave propagation. Furthermore, we studied the effect of substrate thermal conductivity on the transfer of latent heat transfer to neighboring droplets by investigating the velocity of ice bridge formation. The velocity of the ice bridge was independent of the substrate thermal conductivity, indicating that adjacent droplet evaporation during condensation frosting is governed solely by vapor pressure gradients. This study not only provides key insights into the individual droplet freezing process but also elucidates the negligible role of latent heat released into the substrate during frost wave propagation.

Oxidative Processing Lowers the Ice Nucleation Activity of Birch and Alder Pollen

AbstractPollen carry water extractable compounds with ice nucleating (IN) activity. This study investigates whether the hydroxyl radical, as the major atmospheric oxidant, can affect the IN activity of silver birch and grey alder subpollen particles under in‐cloud conditions for deposition freezing mode conditions at 234 K. It is found that oxidation increases the supersaturation ratio with respect to ice necessary for the onset of ice nucleation and decreases the fraction of particles which initiate ice nucleation. This reduction of IN activity under equivalent oxidation conditions does not occur with a mineral dust sample (Arizona Test Dust). Chemical analysis of fresh and oxidized pollen material indicates a change of molecular structure with a loss of conjugation and an increase in oxidized functional groups, such as carbonyls. This is the first demonstration that in‐cloud oxidation may lower the IN abilities of biological particles such as pollen.

Controlled ice nucleation using freeze-dried Pseudomonas syringae encapsulated in alginate beads

The control of ice nucleation is of fundamental significance in many process technologies related to food and pharmaceutical science and cryobiology. Mechanical perturbation, electromagnetic fields and ice-nucleating agents (INAs) have been known to induce ice nucleation in a controlled manner. But these ice-nucleating methods may suffer from cumbersome manual operations, safety concerns of external fields, and biocompatibility and recovery issues of INA particles, especially when used in living systems. Given the automatic ice-seeding nature of INAs, a promising solution to overcome some of the above limitations is to engineer a biocomposite that accommodates the INA particles but minimizes their interactions with biologics, as well as enabling the recovery of used particles. In this study, freeze-dried Pseudomonas syringae, a model ice-nucleating agent, was encapsulated into microliter-sized alginate beads. We evaluated the performance of the bacterial hydrogel beads to initiate ice nucleation in water and aqueous glycerol solution by investigating factors including the size and number of the beads and the local concentration of INA particles. In the aqueous sample of a fixed volume, the total mass of the INA particles (m) was found to be the governing parameter that is solely responsible for determining the ice nucleation performance of the bacterial hydrogel beads. The freezing temperature has a strong positive linear correlation with log10m. The findings in this study provide an effective, predictable approach to control ice nucleation, which can improve the outcome and standardization of many ice-assisted process technologies.

Chapter 7. Secondary Ice Production - current state of the science and recommendations for the future

AbstractMeasured ice crystal concentrations in natural clouds at modest supercooling (temperature ~>−10°C) are often orders of magnitude greater than the number concentration of primary ice nucleating particles. Therefore, it has long been proposed that a secondary ice production process must exist that is able to rapidly enhance the number concentration of the ice population following initial primary ice nucleation events. Secondary ice production is important for the prediction of ice crystal concentration and the subsequent evolution of some types of clouds, but the physical basis of the process is not understood and the production rates are not well constrained. In November 2015 an international workshop was held to discuss the current state of the science and future work to constrain and improve our understanding of secondary ice production processes. Examples and recommendations for in situ observations, remote sensing, laboratory investigations, and modeling approaches are presented.

Cloud chamber experiments on the origin of ice crystal complexity in cirrus clouds

Abstract. This study reports on the origin of small-scale ice crystal complexity and its influence on the angular light scattering properties of cirrus clouds. Cloud simulation experiments were conducted at the AIDA (Aerosol Interactions and Dynamics in the Atmosphere) cloud chamber of the Karlsruhe Institute of Technology (KIT). A new experimental procedure was applied to grow and sublimate ice particles at defined super- and subsaturated ice conditions and for temperatures in the −40 to −60 °C range. The experiments were performed for ice clouds generated via homogeneous and heterogeneous initial nucleation. Small-scale ice crystal complexity was deduced from measurements of spatially resolved single particle light scattering patterns by the latest version of the Small Ice Detector (SID-3). It was found that a high crystal complexity dominates the microphysics of the simulated clouds and the degree of this complexity is dependent on the available water vapor during the crystal growth. Indications were found that the small-scale crystal complexity is influenced by unfrozen H2SO4 / H2O residuals in the case of homogeneous initial ice nucleation. Angular light scattering functions of the simulated ice clouds were measured by the two currently available airborne polar nephelometers: the polar nephelometer (PN) probe of Laboratoire de Métérologie et Physique (LaMP) and the Particle Habit Imaging and Polar Scattering (PHIPS-HALO) probe of KIT. The measured scattering functions are featureless and flat in the side and backward scattering directions. It was found that these functions have a rather low sensitivity to the small-scale crystal complexity for ice clouds that were grown under typical atmospheric conditions. These results have implications for the microphysical properties of cirrus clouds and for the radiative transfer through these clouds.

A Hypothesis for Salt Frost Scaling in Cementitious Materials

In this study freezing characteristics was investigated experimentally on adequately air-entrained cementitious materials (Powers’ spacing factor is between 60-160 µm) with variables including water-binder (w/b) ratio, slag cement addition and a wide range of air content. Results confirm cryogenic suction as the primary transport mode of the external moisture into the surface region to amplify continuous ice growth in capillary pores therein. This leads to pore pressure build-up and eventually surface scaling. Three key factors are identified to govern this cryogenic suction mechanism: (I) the initial ice nucleation in capillary pores and (II) the presence of unfrozen moisture transported through (III) the capillary network. Air entrainment, although needed for protecting internal frost damage, yields no added gain in improving scaling resistance in concrete and is consistent with the capillary suction dominated scaling mechanism. Addition of salt in the pore solution suppresses the ice nucleation and subsequent growth which may explain the pessimum effect.

Emerging techniques for assisting and accelerating food freezing processes: A review of recent research progresses

ABSTRACTFreezing plays an important role in food preservation and the emergence of rapid freezing technologies can be highly beneficial to the food industry. This paper reviews some novel food freezing technologies, including high-pressure freezing (HPF), ultrasound-assisted freezing (UAF), electrically disturbed freezing (EF) and magnetically disturbed freezing (MF), microwave-assisted freezing (MWF), and osmo-dehydro-freezing (ODF). HPF and UAF can initiate ice nucleation rapidly, leading to uniform distribution of ice crystals and the control of their size and shape. Specifically, the former is focused on increasing the degree of supercooling, whereas the latter aims to decrease it. Direct current electric freezing (DC-EF) and alternating current electric freezing (AC-EF) exhibit different effects on ice nucleation. DC-EF can promote ice nucleation and AC-EF has the opposite effect. Furthermore, ODF has been successfully used for freezing various vegetables and fruit. MWF cannot control the nucleation temperature, but can decrease supercooling degree, thus decreasing the size of ice crystals. The heat and mass transfer processes during ODF have been investigated experimentally and modeled mathematically. More studies should be carried out to understand the effects of these technologies on food freezing process.

Laboratory Study of Effects of Atmospheric Pollutants on Ice Nucleation Activity of Bacteria

Ice nuclei of some bacterial origin as ice catalysts can initiate ice nucleation at temperatures as warm as –2°C in certain laboratory experiments. The ice nucleation activities of airborne bacteria in the real atmosphere may be different from those experiments. To estimate the impact of typical atmospheric pollutants including monocarboxylic acids (MCAs), dicarboxylic acids (DCAs) and ammonia sulfate on ice nucleation activity of P.syringae pv lachrymans and P.syringae pv.panici, we have conducted some experiments by means of the modified Vali’s droplet freezing testing method in the immersion freezing mode with the mixture of the pure water and the polluted water. Our results show that the ice nucleation activity of bacterial origins can be regulated by such pollutant compounds even though the onset freezing temperatures of water droplets mainly depend on the concentrations of ice nucleation-active bacteria. Atmospheric acids can decrease ice nucleation activity of P.syringae pv lachrymans and P.syringae pv.panici. However, the onset freezing temperatures of water droplets immersed with ice nucleation-active P.syringae pv lachrymans will be enhanced by the low concentration of such atmospheric pollutants.

High ice nucleation activity located in blueberry stem bark is linked to primary freeze initiation and adaptive freezing behaviour of the bark.

Controlled ice nucleation is an important mechanism in cold-hardy plant tissues for avoiding excessive supercooling of the protoplasm, for inducing extracellular freezing and/or for accommodating ice crystals in specific tissues. To understand its nature, it is necessary to characterize the ice nucleation activity (INA), defined as the ability of a tissue to induce heterogeneous ice nucleation. Few studies have addressed the precise localization of INA in wintering plant tissues in respect of its function. For this purpose, we recently revised a test tube INA assay and examined INA in various tissues of over 600 species. Extremely high levels of INA (-1 to -4 °C) in two wintering blueberry cultivars of contrasting freezing tolerance were found. Their INA was much greater than in other cold-hardy species and was found to be evenly distributed along the stems of the current year's growth. Concentrations of active ice nuclei in the stem were estimated from quantitative analyses. Stem INA was localized mainly in the bark while the xylem and pith had much lower INA. Bark INA was located mostly in the cell wall fraction (cell walls and intercellular structural components). Intracellular fractions had much less INA. Some cultivar differences were identified. The results corresponded closely with the intrinsic freezing behaviour (extracellular freezing) of the bark, icicle accumulation in the bark and initial ice nucleation in the stem under dry surface conditions. Stem INA was resistant to various antimicrobial treatments. These properties and specific localization imply that high INA in blueberry stems is of intrinsic origin and contributes to the spontaneous initiation of freezing in extracellular spaces of the bark by acting as a subfreezing temperature sensor.

Observations of Ice Nucleation and Propagation in Flowers of Japanese Pear (Pyrus pyrifolia Nakai) using Infrared Video Thermography

Ice formation and propagation in the flower and young fruit of Japanese pear 'Kosui' were investigated with infrared (IR) video thermography imaging system. The changes of thermal response (exotherm) in freezing events could be detected using the infrared thermography camera with exotherm rise of 2-5 °C accompanied by a marked color change in the tissues. With first bloom flowers, the temperature of peduncles decreased faster than that of the receptacle; the initial ice nucleation occurred primarily in petals, sepals and/or receptacle. Subsequently, ice was propagated along the peduncle. The temperature of receptacles was above that of peduncles during the freezing test at full bloom; the sepals and/or receptacle were initially frozen, then ice nucleation was extended to the petals along the peduncle. These results suggest that the sepals and/or receptacle where the initial nucleation occurred may be an intrinsic or extrinsic source of high ice nucleation activity in flowers of the Japanese pear 'Kosui'. In contrast with first bloom and full bloom stage, the exotherms during freezing could not be detected clearly in young fruits. However, cold injury appeared as a spotty stain on the skin of young fruits immediately upon being subjected to chilling.

Inhibition of bacterial ice nucleation by polyglycerol polymers

The simple linear polymer polyglycerol (PGL) was found to apparently bind and inhibit the ice nucleating activity of proteins from the ice nucleating bacterium Pseudomonas syringae. PGL of molecular mass 750 Da was added to a solution consisting of 1 ppm freeze-dried P. syringae 31A in water. Differential ice nucleator spectra were determined by measuring the distribution of freezing temperatures in a population of 98 drops of 1 μL volume. The mean freezing temperature was lowered from −6.8 °C (control) to −8.0,−9.4,−12.5, and −13.4 °C for 0.001, 0.01, 0.1, and 1% w/w PGL concentrations, respectively (SE < 0.2 °C). PGL was found to be an ineffective inhibitor of seven defined organic ice nucleating agents, whereas the general ice nucleation inhibitor polyvinyl alcohol (PVA) was found to be effective against five of the seven. The activity of PGL therefore seems to be specific against bacterial ice nucleating protein. PGL alone was an ineffective inhibitor of ice nucleation in small volumes of environmental or laboratory water samples, suggesting that the numerical majority of ice nucleating contaminants in nature may be of nonbacterial origin. However, PGL was more effective than PVA at suppressing initial ice nucleation events in large volumes, suggesting a ubiquitous sparse background of bacterial ice nucleating proteins with high nucleation efficiency. The combination of PGL and PVA was particularly effective for reducing ice formation in solutions used for cryopreservation by vitrification.

Segregation of solutes and gases in experimental freezing of dilute solutions: implications for natural glacial systems

Low ionic strength waters containing significant calcium and bicarbonate are common in nature, but little literature exists on their behaviour during freezing. Modelling indicates that freezing-induced concentration of solutes (in a closed-system) would lead to progressive increase in calcite saturation index, despite rising partial pressure of CO 2 (PCO 2), but the consequences of CaCO 3 precipitation for the distribution of matter between solid, liquid, and gas phases required experimental investigation. We studied the effects of variations in the rate of advance of an ice-water interface and in the initial degree of saturation for calcite on the behaviour of the system. Downward growth of ice in a 24-cm diameter cylindrical vessel was achieved at a constant linear rate of 3 or 8 mm/h by the progressive cooling of an overlying alcohol reservoir, and the expansion of volume accommodated by regular water sampling through side ports, together with a small expansion chamber. Initial air-saturated solutions (initial PCO 2 in the range 10 −3 to 10 −3.2) were prepared to reflect a range from strongly undersaturated to supersaturated for calcite. Comparative blank experiments were run using deionized water. Ice growth led to enrichment in solutes at the ice-water interface and the creation of a diffusive boundary layer, calculated to be 0.6 mm thick, truncated below by convecting fluid. The first-formed ice (stage 1), was relatively solute-rich because of initial rapid ice nucleation. Where solutions were not strongly supersaturated for calcite this was followed by formation of a solute-poor (stage 2) ice. Ice-interface water segregation coefficients of stage 2 ice were calculated to be 0.0004–0.003 for various solute ions. The relative magnitude of segregation coefficients (Mg 2+ > Ca 2+ > Sr 2+) is attributed to interstitial incorporation (coupled with HCO 3 −) in the ice lattice, and controlled by ion size. Air bubbles nucleated once nitrogen supersaturation had reached values of 2–2.5 in the boundary layer and were incorporated into ice. These gas inclusions had dissolved air compositions modified by the differential diffusion of O 2, N 2, and CO 2 out of the boundary layer, an O 2/N 2 ratio of 0.4 being characteristic. Freezing of solutions strongly supersaturated for calcite led to formation of impure (stage 3) ice in which ions are incorporated in similar proportions to those of the parent aqueous solution. Stage 3 ice contains both solid CaCO 3 and aqueous (solute-rich) inclusions, associated with an irregular ice-water interface. Gas inclusions were invariably rich in CO 2, up to 63% by volume, yet represented only a small proportion of the CO 2 generated as a by-product of CaCO 3 precipitation. These data allow a better understanding of the expected chemical characteristics of ice that has formed from freezing of bulk water, including river icings, basal ice of glaciers, and local refrozen layers in firn and glacier ice. Generation of CO 2-rich gas bubbles by re-freezing is a powerful mechanism for modification of CO 2 compositions of bulk gaseous inclusions in ice.

Correlations of atmospheric dynamics with solar activity evidence for a connection via the solar wind, atmospheric electricity, and cloud microphysics

We respond to several criticisms of the view that there is a physical linkage between solar activity and the dynamics of the troposphere and lower stratosphere, and we provide further evidence in support of a mechanism for such a linkage involving atmospheric electricity and cloud microphysics. The main criticisms are (1) that the decadal time scale variations in stratified data result from aliasing introduced by the sampling process and are not responses to a decadal time scale physical input; (2) that the observed correlations are due to chance coincidence or an atmospheric periodicity that is not uniquely related to solar variability; and (3) that there are no plausible mechanisms that can amplify one of the weak solar‐varying inputs in the region where the correlations are found. We show that the aliasing criticism is inadequate because the real quasi‐biennial oscillation departs from an ideal sine wave in a way that reduces aliasing effects to insignificant levels. The nonuniqueness of identification of the 11‐year solar cycle as the period of the arctic forcing for the Arctic winter stratospheric temperatures is a problem only for the short 33‐year record of polar temperatures; in much longer time series of unstratified climate data the periods of 11 and 22 years are prominent. Highly unique signatures of solar wind forcing of tropospheric dynamics exist on the day‐to‐day time scale via two independent inputs to atmospheric electricity. These are (1) through changes in tropospheric ion production as a result of solar wind modulation of galactic cosmic rays and (2) through changes in the potential difference between the polar ionospheres and the surface, forced by the solar wind By component. The product of the cosmic ray flux and the ionospheric potential determines the vertical air‐earth electrical current. In the presence of clouds of large horizontal extent, this current determines the rate of polarization charging of the clouds via the accumulation of positive electrostatic charges on droplets near cloud tops. The observed correlations, and theoretical and laboratory results for the effects of electrostatic charges on droplets and aerosols on the rates of ice nucleation, are consistent with the postulate that for certain regions and seasons and atmospheric levels the large‐scale atmospheric electrical parameters have significant effects on the rates of initial ice nucleation. In such cases the chain of consequences includes changes in the rates of precipitation, net latent heat release, vertical motions, atmospheric vorticity, and ultimately in the general circulation. Much more work is required before the mechanism can be considered to have a secure basis in laboratory experiment and quantitative atmospheric modeling.

A Method for the Cryopreservation of Liver Parenchymal Cells for Studies of Xenobiotics

An optimized computer-controlled freezing protocol for the cryopreservation of rat liver parenchymal cells was developed. The best survival rates were obtained when a slow cooling rate was used and when the supercooling was interrupted with a shock cooling to initiate ice nucleation. Ten percent dimethyl sulfoxide was added and removed gradually for best results. Thawed rat liver parenchymal cells had a viability, as judged by trypan blue exclusion, of 69% (SD = 6) versus 82% (SD = 7) for freshly isolated cells. The content and activities of the xenobiotic metabolizing enzymes, cytochrome P450. UDP-glucuronosyl transferase, and microsomal and cytosolic epoxide hydrolase, were not affected, whereas a slight reduction of glutathione S -transferase and sulfotransferase occurred. If cryopreserved cells were purified by a Percoll centrifugation after thawing the enzyme activities were not significantly different from those of freshly isolated parenchymal cells and also the viability was 86% (SD = 3). Cryopreserved rat liver parenchymal cells only metabolized about 50% of benzo(a)pyrene compared to freshly isolated cells. It is less likely that the reduction in enzyme activities was due to the cryopreservation procedure than that it was due to the loss of NADPH as a cofactor for cytochrome P450 which then resulted in the decreased xenobiotic metabolism. This cryopreservation protocol was also suitable for a variety of liver parenchymal cells from other species when trypan blue exclusion was used as a viability marker.

Nature of ice-forming nuclei in marine air masses

Aerosol particles collected over the Pacific Ocean between 7°N and 5°S latitude and 110° to 142°W longitude during the period from 23 May to 19 June 1985 were examined for their ability to nucleate ice by sorption, freezing, and condensation-followed-by-freezing. Ice-forming nuclei (IFN) active by sorption in the temperature range between − 4° and − 17°C were not found in the air masses of pure marine origin. The highest temperatures of freezing of water drops placed on aerosol particles larger than 0.3 μm and below 0.3 μm in diameter were − 12° and − 4°C, respectively. Maximum concentrations of IFN active by condensation-followed-by-freezing at initial ice nucleation temperatures of − 4°, − 6°, and − 7°C were found over some areas to be 40 ± 5, 10, and 401 −1, respectively; these concentrations were of a patchy character over the Pacific Ocean where upwelling regions were present, and they were found to be independent of temperature for the temperature range from the initial ice nucleation temperatures down to − 17°C. Aerosol particles active as IFN were in the 0.1–0.3 μm diameter size range; they were hydrophobic particles and consisted of a chemical compound or compounds that completely vaporized in 10 −6 mm Hg vacuum, indicating that they were neither bacteria nor proteins. It was found that the IFN always existed in the presence of sulfate-bearing aerosol particles smaller than 0.3 μm in diameter and that the sulfate ion is an integral part of the ice nucleating particle. A relation was also found between SO 4 2− ion concentration, the temperature of ice nucleation (the temperature-concentration curve has a maximum), and the nature of the hydrophobic surface. The result of this relationship is that some of the cloud droplets growing in an updraft can be immune to freezing when the concentration of the SO 4 2− ion in solution is too low. Upon evaporation of droplets, the increasing concentration of SO 4 2− ions reaches the critical concentration at the ice nucleation (freezing) curve and causes evaporating droplets to freeze; this supports the Hobbs-Rangno observations. The freezing of condensing and evaporating droplets is time-dependent and therefore depends on the dynamics of clouds.

Purification and composition of an ice nucleating protein from queens of the hornet,Vespula maculata

Hemolymph ice nucleating factors are found in many freeze tolerant insects. These factors function to initiate ice nucleation in the extracellular fluid at fairly high subzero temperatures thereby minimizing the possibility of lethal intracellular ice formation. An ice nucleating protein was purified from the hymolymph of pupal bald faced hornets,Vespula maculata. This is the first ice nucleating protein to be purified. The protein has a molecular weight of 74,000, as determined by SDS-PAGE, and is quite hydrophilic. Glutamate and/or glutamine accounts for 20% of the amino acid residues. It is likely that the hydrophilic nature of the protein is involved in the ability of the protein to function as an ice nucleator.

A quantum-chemical study of the interaction of the water molecule with silver iodide surface clusters

The interaction between a water molecule and silver iodide clusters has been studied by means of the CNDO method. Two different adsorption forms obtained, one of which may initiate ice nucleation. The formation of a six-membered water molecule ring above the surface requires less energy than its formation in the gaseous phase.