Southern Alaska as a source of atmospheric mineral dust and ice-nucleating particles.

Ice-nucleating particles (INPs) influence cloud radiative properties and climate; however, INP sources and concentrations are poorly constrained, particularly in high-latitude regions. Southern Alaska is a known source of high-latitude dust, but its contribution to atmospheric mineral dust and INP concentrations has not been quantified. We show that glacial dust collected in southern Alaska is an effective ice-nucleating material under conditions relevant for mixed-phase clouds and is more active than low-latitude dust because of a biological component that enhances its activity. We use dispersion modeling to show that this source contributes to the regional INP population and that the dust emitted is transported over a broad area of North America, reaching altitudes where it could cause cloud glaciation. Our results highlight the importance of quantifying emissions and ice-nucleating characteristics of high-latitude dusts and suggest that the ice-nucleating ability of emitted dust in these regions should be represented in models using different parametrizations to low-latitude dust.

A highly active mineral-based ice nucleating agent supports in situ cell cryopreservation in a high throughput format.

Cryopreservation of biological matter in microlitre scale volumes of liquid would be useful for a range of applications. At present, it is challenging because small volumes of water tend to supercool, and deep supercooling is known to lead to poor post-thaw cell viability. Here, we show that a mineral ice nucleator can almost eliminate supercooling in 100 µl liquid volumes during cryopreservation. This strategy of eliminating supercooling greatly enhances cell viability relative to cryopreservation protocols with uncontrolled ice nucleation. Using infrared thermography, we demonstrate a direct relationship between the extent of supercooling and post-thaw cell viability. Using a mineral nucleator delivery system, we open the door to the routine cryopreservation of mammalian cells in multiwell plates for applications such as high throughput toxicology testing of pharmaceutical products and regenerative medicine.

Highly Active Ice-Nucleating Particles at the Summer North Pole.

The amount of ice versus supercooled water in clouds is important for their radiative properties and role in climate feedbacks. Hence, knowledge of the concentration of ice-nucleating particles (INPs) is needed. Generally, the concentrations of INPs are found to be very low in remote marine locations allowing cloud water to persist in a supercooled state. We had expected the concentrations of INPs at the North Pole to be very low given the distance from open ocean and terrestrial sources coupled with effective wet scavenging processes. Here we show that during summer 2018 (August and September) high concentrations of biological INPs (active at >-20°C) were sporadically present at the North Pole. In fact, INP concentrations were sometimes as high as those recorded at mid-latitude locations strongly impacted by highly active biological INPs, in strong contrast to the Southern Ocean. Furthermore, using a balloon borne sampler we demonstrated that INP concentrations were often different at the surface versus higher in the boundary layer where clouds form. Back trajectory analysis suggests strong sources of INPs near the Russian coast, possibly associated with wind-driven sea spray production, whereas the pack ice, open leads, and the marginal ice zone were not sources of highly active INPs. These findings suggest that primary ice production, and therefore Arctic climate, is sensitive to transport from locations such as the Russian coast that are already experiencing marked climate change.

Volcanic ash ice nucleation activity is variably reduced by aging in water and sulfuric acid: the effects of leaching, dissolution, and precipitation.

Volcanic ash nucleates ice when immersed in supercooled water droplets, giving it the potential to influence weather and climate from local to global scales. This ice nucleation activity (INA) is likely derived from a subset of the crystalline mineral phases in the ash. The INA of other mineral-based dusts can change when exposed to various gaseous and aqueous chemical species, many of which also interact with volcanic ash in the eruption plume and atmosphere. However, the effects of aqueous chemical aging on the INA of volcanic ash have not been explored. We show that the INA of two mineralogically distinct ash samples from Fuego and Astroni volcanoes is variably reduced following immersion in water or aqueous sulfuric acid for minutes to days. Aging in water decreases the INA of both ash samples by up to two orders of magnitude, possibly due to a reduction in surface crystallinity and cation availability accompanying leaching. Aging in sulfuric acid leads to minimal loss of INA for Fuego ash, which is proposed to reflect a quasi-equilibrium between leaching that removes ice-active sites and dissolution that reveals or creates new sites on the pyroxene phases present. Conversely, exposure to sulfuric acid reduces the INA of Astroni ash by one to two orders of magnitude, potentially through selective dissolution of ice-active sites associated with surface microtextures on some K-feldspar phases. Analysis of dissolved element concentrations in the aged ash leachates shows supersaturation of certain mineral species which could have precipitated and altered the INA of the ash. These results highlight the key role that leaching, dissolution, and precipitation likely play in the aqueous aging of volcanic ash with respect to its INA. Finally, we discuss the implications for understanding the nature and reactivity of ice-active sites on volcanic ash and its role in influencing cloud properties in the atmosphere.

Active sites for ice nucleation differ depending on nucleation mode.

The nucleation of ice crystals in clouds is poorly understood, despite being of critical importance for our planet's climate. Nucleation occurs largely at rare "active sites" present on airborne particles such as mineral dust, but the nucleation pathway is distinct under different meteorological conditions. These give rise to two key nucleation pathways where a particle is either immersed in a supercooled liquid water droplet (immersion freezing mode) or suspended in a supersaturated vapor (deposition mode). However, it is unclear if the same active sites are responsible for nucleation in these two modes. Here, we directly compare the sites that are active in these two modes by performing immersion freezing and deposition experiments on the same thin sections of two atmospherically important minerals (feldspar and quartz). For both substrates, we confirm that nucleation is dominated by a limited number of sites and show that there is little correlation between the two sets of sites operating in each experimental method: across both materials, only six out of 73 sites active for immersion freezing nucleation were also active for deposition nucleation. Clearly, different properties determine the activity of nucleation sites for each mode, and we use the pore condensation and freezing concept to argue that effective deposition sites have size and/or geometry requirements not of relevance to effective immersion freezing sites. Hence, the ability to nucleate is pathway dependent, and the mode of nucleation has to be explicitly considered when applying experimental data in cloud models.

Homogeneous Freezing of Water Using Microfluidics.

The homogeneous freezing of water is important in the formation of ice in clouds, but there remains a great deal of variability in the representation of the homogeneous freezing of water in the literature. The development of new instrumentation, such as droplet microfluidic platforms, may help to constrain our understanding of the kinetics of homogeneous freezing via the analysis of monodisperse, size-selected water droplets in temporally and spatially controlled environments. Here, we evaluate droplet freezing data obtained using the Lab-on-a-Chip Nucleation by Immersed Particle Instrument (LOC-NIPI), in which droplets are generated and frozen in continuous flow. This high-throughput method was used to analyse over 16,000 water droplets (86 µm diameter) across three experimental runs, generating data with high precision and reproducibility that has largely been unrepresented in the microfluidic literature. Using this data, a new LOC-NIPI parameterisation of the volume nucleation rate coefficient (JV(T)) was determined in the temperature region of -35.1 to -36.9 °C, covering a greater JV(T) compared to most other microfluidic techniques thanks to the number of droplets analysed. Comparison to recent theory suggests inconsistencies in the theoretical representation, further implying that microfluidics could be used to inform on changes to parameterisations. By applying classical nucleation theory (CNT) to our JV(T) data, we have gone a step further than other microfluidic homogeneous freezing examples by calculating the stacking-disordered ice-supercooled water interfacial energy, estimated to be 22.5 ± 0.7 mJ m-2, again finding inconsistencies when compared to theoretical predictions. Further, we briefly review and compile all available microfluidic homogeneous freezing data in the literature, finding that the LOC-NIPI and other microfluidically generated data compare well with commonly used non-microfluidic datasets, but have generally been obtained with greater ease and with higher numbers of monodisperse droplets.

On-chip density-based sorting of supercooled droplets and frozen droplets in continuous flow.

The freezing of supercooled water to ice and the materials which catalyse this process are of fundamental interest to a wide range of fields. At present, our ability to control, predict or monitor ice formation processes is poor. The isolation and characterisation of frozen droplets from supercooled liquid droplets would provide a means of improving our understanding and control of these processes. Here, we have developed a microfluidic platform for the continuous flow separation of frozen from unfrozen picolitre droplets based on differences in their density, thus allowing the sorting of ice crystals and supercooled water droplets into different outlet channels with 94 ± 2% efficiency. This will, in future, facilitate downstream or off-chip processing of the frozen and unfrozen populations, which could include the analysis and characterisation of ice-active materials or the selection of droplets with a particular ice-nucleating activity.

On-chip analysis of atmospheric ice-nucleating particles in continuous flow.

Ice-nucleating particles (INPs) are of atmospheric importance because they catalyse the freezing of supercooled cloud droplets, strongly affecting the lifetime and radiative properties of clouds. There is a need to improve our knowledge of the global distribution of INPs, their seasonal cycles and long-term trends, but our capability to make these measurements is limited. Atmospheric INP concentrations are often determined using assays involving arrays of droplets on a cold stage, but such assays are frequently limited by the number of droplets that can be analysed per experiment, often involve manual processing (e.g. pipetting of droplets), and can be susceptible to contamination. Here, we present a microfluidic platform, the LOC-NIPI (Lab-on-a-Chip Nucleation by Immersed Particle Instrument), for the generation of water-in-oil droplets and their freezing in continuous flow as they pass over a cold plate for atmospheric INP analysis. LOC-NIPI allows the user to define the number of droplets analysed by simply running the platform for as long as required. The use of small (∼100 µm diameter) droplets minimises the probability of contamination in any one droplet and therefore allows supercooling all the way down to homogeneous freezing (around -36 °C), while a temperature probe in a proxy channel provides an accurate measure of temperature without the need for temperature modelling. The platform was validated using samples of pollen extract and Snomax®, with hundreds of droplets analysed per temperature step and thousands of droplets being measured per experiment. Homogeneous freezing of purified water was studied using >10 000 droplets with temperature increments of 0.1 °C. The results were reproducible, independent of flow rate in the ranges tested, and the data compared well to conventional instrumentation and literature data. The LOC-NIPI was further benchmarked in a field campaign in the Eastern Mediterranean against other well-characterised instrumentation. The continuous flow nature of the system provides a route, with future development, to the automated monitoring of atmospheric INP at field sites around the globe.

Cryopreservation of primary cultures of mammalian somatic cells in 96-well plates benefits from control of ice nucleation.

Cryopreservation of mammalian cells has to date typically been conducted in cryovials, but there are applications where cryopreservation of primary cells in multiwell plates would be advantageous. However excessive supercooling in the small volumes of liquid in each well of the multiwell plates is inevitable without intervention and tends to result in high and variable cell mortality. Here, we describe a technique for cryopreservation of adhered primary bovine granulosa cells in 96-well plates by controlled rate freezing using controlled ice nucleation. Inducing ice nucleation at warm supercooled temperatures (less than 5 °C below the melting point) during cryopreservation using a manual seeding technique significantly improved post-thaw recovery from 29.6% (SD = 8.3%) where nucleation was left uncontrolled to 57.7% (9.3%) when averaged over 8 replicate cultures (p < 0.001). Detachment of thawed cells was qualitatively observed to be more prevalent in wells which did not have ice nucleation control which suggests cryopreserved cell monolayer detachment may be a consequence of deep supercooling. Using an infra-red thermography technique we showed that many aliquots of cryoprotectant solution in 96-well plates can supercool to temperatures below -20 °C when nucleation is not controlled, and also that the freezing temperatures observed are highly variable despite stringent attempts to remove contaminants acting as nucleation sites. We conclude that successful cryopreservation of cells in 96-well plates, or any small volume format, requires control of ice nucleation.

High-speed imaging of ice nucleation in water proves the existence of active sites.

Understanding how surfaces direct nucleation is a complex problem that limits our ability to predict and control crystal formation. We here address this challenge using high-speed imaging to identify and quantify the sites at which ice nucleates in water droplets on the two natural cleavage faces of macroscopic feldspar substrates. Our data show that ice nucleation only occurs at a few locations, all of which are associated with micron-size surface pits. Similar behavior is observed on α-quartz substrates that lack cleavage planes. These results demonstrate that substrate heterogeneities are the salient factor in promoting nucleation and therefore prove the existence of active sites. We also provide strong evidence that the activity of these sites derives from a combination of surface chemistry and nanoscale topography. Our results have implications for the nucleation of many materials and suggest new strategies for promoting or inhibiting nucleation across a wide range of applications.

Unravelling the origins of ice nucleation on organic crystals.

Organic molecules such as steroids or amino acids form crystals that can facilitate the formation of ice - arguably the most important phase transition on earth. However, the origin of the ice nucleating ability of organic crystals is still largely unknown. Here, we combine experiments and simulations to unravel the microscopic details of ice formation on cholesterol, a prototypical organic crystal widely used in cryopreservation. We find that cholesterol - which is also a substantial component of cell membranes - is an ice nucleating agent more potent than many inorganic substrates, including the mineral feldspar (one of the most active ice nucleating materials in the atmosphere). Scanning electron microscopy measurements reveal a variety of morphological features on the surfaces of cholesterol crystals: this suggests that the topography of the surface is key to the broad range of ice nucleating activity observed (from -4 to -20 °C). In addition, we show via molecular simulations that cholesterol crystals aid the formation of ice nuclei in a unconventional fashion. Rather than providing a template for a flat ice-like contact layer (as found in the case of many inorganic substrates), the flexibility of the cholesterol surface and its low density of hydrophilic functional groups leads to the formation of molecular cages involving both water molecules and terminal hydroxyl groups of the cholesterol surface. These cages are made of 6- and, surprisingly, 5-membered hydrogen bonded rings of water and hydroxyl groups that favour the nucleation of hexagonal as well as cubic ice (a rare occurrence). We argue that the phenomenal ice nucleating activity of steroids such as cholesterol (and potentially of many other organic crystals) is due to (i) the ability of flexible hydrophilic surfaces to form unconventional ice-templating structures and (ii) the different nucleation sites offered by the diverse topography of the crystalline surfaces. These findings clarify how exactly organic crystals promote the formation of ice, thus paving the way toward deeper understanding of ice formation in soft and biological matter - with obvious reverberations on atmospheric science and cryobiology.

Correction: The enhancement and suppression of immersion mode heterogeneous ice-nucleation by solutes.

[This corrects the article DOI: 10.1039/C7SC05421A.].

Is Black Carbon an Unimportant Ice-Nucleating Particle in Mixed-Phase Clouds?

It has been hypothesized that black carbon (BC) influences mixed-phase clouds by acting as an ice-nucleating particle (INP). However, the literature data for ice nucleation by BC immersed in supercooled water are extremely varied, with some studies reporting that BC is very effective at nucleating ice, whereas others report no ice-nucleating ability. Here we present new experimental results for immersion mode ice nucleation by BC from two contrasting fuels (n-decane and eugenol). We observe no significant heterogeneous nucleation by either sample. Using a global aerosol model, we quantify the maximum relative importance of BC for ice nucleation when compared with K-feldspar and marine organic aerosol acting as INP. Based on the upper limit from our laboratory data, we show that BC contributes at least several orders of magnitude less INP than feldspar and marine organic aerosol. Representations of its atmospheric ice-nucleating ability based on older laboratory data produce unrealistic results when compared against ambient observations of INP. Since BC is a complex material, it cannot be unambiguously ruled out as an important INP species in all locations at all times. Therefore, we use our model to estimate a range of values for the density of active sites that BC particles must have to be relevant for ice nucleation in the atmosphere. The estimated values will guide future work on BC, defining the required sensitivity of future experimental studies.

The study of atmospheric ice-nucleating particles via microfluidically generated droplets.

Ice-nucleating particles (INPs) play a significant role in the climate and hydrological cycle by triggering ice formation in supercooled clouds, thereby causing precipitation and affecting cloud lifetimes and their radiative properties. However, despite their importance, INP often comprise only 1 in 103-106 ambient particles, making it difficult to ascertain and predict their type, source, and concentration. The typical techniques for quantifying INP concentrations tend to be highly labour-intensive, suffer from poor time resolution, or are limited in sensitivity to low concentrations. Here, we present the application of microfluidic devices to the study of atmospheric INPs via the simple and rapid production of monodisperse droplets and their subsequent freezing on a cold stage. This device offers the potential for the testing of INP concentrations in aqueous samples with high sensitivity and high counting statistics. Various INPs were tested for validation of the platform, including mineral dust and biological species, with results compared to literature values. We also describe a methodology for sampling atmospheric aerosol in a manner that minimises sampling biases and which is compatible with the microfluidic device. We present results for INP concentrations in air sampled during two field campaigns: (1) from a rural location in the UK and (2) during the UK's annual Bonfire Night festival. These initial results will provide a route for deployment of the microfluidic platform for the study and quantification of INPs in upcoming field campaigns around the globe, while providing a benchmark for future lab-on-a-chip-based INP studies.

The enhancement and suppression of immersion mode heterogeneous ice-nucleation by solutes.

Heterogeneous nucleation of ice from aqueous solutions is an important yet poorly understood process in multiple fields, not least the atmospheric sciences where it impacts the formation and properties of clouds. In the atmosphere ice-nucleating particles are usually, if not always, mixed with soluble material. However, the impact of this soluble material on ice nucleation is poorly understood. In the atmospheric community the current paradigm for freezing under mixed phase cloud conditions is that dilute solutions will not influence heterogeneous freezing. By testing combinations of nucleators and solute molecules we have demonstrated that 0.015 M solutions (predicted melting point depression <0.1 °C) of several ammonium salts can cause suspended particles of feldspars and quartz to nucleate ice up to around 3 °C warmer than they do in pure water. In contrast, dilute solutions of certain alkali metal halides can dramatically depress freezing points for the same nucleators. At 0.015 M, solutes can enhance or deactivate the ice-nucleating ability of a microcline feldspar across a range of more than 10 °C, which corresponds to a change in active site density of more than a factor of 105. This concentration was chosen for a survey across multiple solutes-nucleant combinations since it had a minimal colligative impact on freezing and is relevant for activating cloud droplets. Other nucleators, for instance a silica gel, are unaffected by these 'solute effects', to within experimental uncertainty. This split in response to the presence of solutes indicates that different mechanisms of ice nucleation occur on the different nucleators or that surface modification of relevance to ice nucleation proceeds in different ways for different nucleators. These solute effects on immersion mode ice nucleation may be of importance in the atmosphere as sea salt and ammonium sulphate are common cloud condensation nuclei (CCN) for cloud droplets and are internally mixed with ice-nucleating particles in mixed-phase clouds. In addition, we propose a pathway dependence where activation of CCN at low temperatures might lead to enhanced ice formation relative to pathways where CCN activation occurs at higher temperatures prior to cooling to nucleation temperature.

Strong control of Southern Ocean cloud reflectivity by ice-nucleating particles.

Large biases in climate model simulations of cloud radiative properties over the Southern Ocean cause large errors in modeled sea surface temperatures, atmospheric circulation, and climate sensitivity. Here, we combine cloud-resolving model simulations with estimates of the concentration of ice-nucleating particles in this region to show that our simulated Southern Ocean clouds reflect far more radiation than predicted by global models, in agreement with satellite observations. Specifically, we show that the clouds that are most sensitive to the concentration of ice-nucleating particles are low-level mixed-phase clouds in the cold sectors of extratropical cyclones, which have previously been identified as a main contributor to the Southern Ocean radiation bias. The very low ice-nucleating particle concentrations that prevail over the Southern Ocean strongly suppress cloud droplet freezing, reduce precipitation, and enhance cloud reflectivity. The results help explain why a strong radiation bias occurs mainly in this remote region away from major sources of ice-nucleating particles. The results present a substantial challenge to climate models to be able to simulate realistic ice-nucleating particle concentrations and their effects under specific meteorological conditions.

The role of phase separation and related topography in the exceptional ice-nucleating ability of alkali feldspars.

Our understanding of crystal nucleation is a limiting factor in many fields, not least in the atmospheric sciences. It was recently found that feldspar, a component of airborne desert dust, plays a dominant role in triggering ice formation in clouds, but the origin of this effect was unclear. By investigating the structure/property relationships of a wide range of feldspars, we demonstrate that alkali feldspars with certain microtextures, related to phase separation into Na and K-rich regions, show exceptional ice-nucleating abilities in supercooled water. We found no correlation between ice-nucleating efficiency and the crystal structures or the chemical compositions of these active feldspars, which suggests that specific topographical features associated with these microtextures are key in the activity of these feldspars. That topography likely acts to promote ice nucleation, improves our understanding of ice formation in clouds, and may also enable the design and manufacture of bespoke nucleating materials for uses such as cloud seeding and cryopreservation.

Rate of Homogenous Nucleation of Ice in Supercooled Water.

The homogeneous freezing of water is of fundamental importance to a number of fields, including that of cloud formation. However, there is considerable scatter in homogeneous nucleation rate coefficients reported in the literature. Using a cold stage droplet system designed to minimize uncertainties in temperature measurements, we examined the freezing of over 1500 pure water droplets with diameters between 4 and 24 µm. Under the assumption that nucleation occurs within the bulk of the droplet, nucleation rate coefficients fall within the spread of literature data and are in good agreement with a subset of more recent measurements. To quantify the relative importance of surface and volume nucleation in our experiments, where droplets are supported by a hydrophobic surface and surrounded by oil, comparison of droplets with different surface area to volume ratios was performed. From our experiments it is shown that in droplets larger than 6 µm diameter (between 234.6 and 236.5 K), nucleation in the interior is more important than nucleation at the surface. At smaller sizes we cannot rule out a significant contribution of surface nucleation, and in order to further constrain surface nucleation, experiments with smaller droplets are necessary. Nevertheless, in our experiments, it is dominantly volume nucleation controlling the observed nucleation rate.