Immersion Freezing Experiments Research Articles

Integrated Science Teaching in Atmospheric Ice Nucleation Research: Immersion Freezing Experiments.

This paper introduces hands-on curricular modules integrated with research in atmospheric ice nucleation, which is an important phenomenon potentially influencing global climate change. The primary goal of this work is to promote meaningful laboratory exercises to enhance the competence of students in the fields of science, technology, engineering, and math (STEM) by applying an appropriate methodology to laboratory ice nucleation measurements. To achieve this goal, three laboratory modules were developed with 18 STEM interns and tested by 28 students in a classroom setting. Students were trained to experimentally simulate atmospheric ice nucleation and cloud droplet freezing. For practical training, this work utilized a simple freezing assay device called the West Texas Cryogenic Refrigerator Applied to Freezing Test (WT-CRAFT) system. More specifically, students were provided with hands-on lessons to calibrate WT-CRAFT with deionized water and apply analytical techniques to understand the physicochemical properties of bulk water and droplet freezing. All procedures to implement the developed modules were typewritten during this process, and shareable read-ahead exploration materials were developed and compiled as a curricular product. Additionally, students conducted complementary analyses to identify possible catalysts of heterogeneous freezing in the water. The water analyses included: pH, conductivity, surface tension, and electron microscopy-energy-dispersive X-ray spectroscopy. During the data and image analysis process, students learned how to analyze droplet freezing spectra as a function of temperature, screen and interpret the data, perform uncertainty analyses, and estimate ice nucleation efficiency using computer programs. Based on the formal program assessment of learning outcomes and direct (yet deidentified) student feedback, we broadly achieved our goals to (1) improve their problem-solving skills by combining multidisciplinary science and math skills and (2) disseminate data and results with variability and uncertainty. The developed modules can be applied at any institute to advance undergraduate and graduate curricula in environmental science.

Comparing the ice nucleation properties of the kaolin minerals kaolinite and halloysite

Abstract. Heterogeneous ice nucleation on dust particles in the atmosphere is a key mechanism for ice formation in clouds. However, the conditions of a particle surface for efficient ice nucleation are poorly understood. In this study, we present results of immersion freezing experiments using differential scanning calorimetry on emulsified mineral dust suspensions, involving the two chemically identical, but morphologically different, kaolin minerals of kaolinite and halloysite. Kaolinite occurs in a platy morphology, while halloysites form predominantly tubular structures. We investigated six different halloysite and two different kaolinite samples. Our results show that, on average, the halloysite samples not only exhibit a higher ice nucleation (IN) activity than the kaolinite samples but also a higher diversity in terms of freezing onset temperatures and heterogeneously frozen fraction. Repeating the freezing experiments after shortly milling the samples led to a decrease in freezing onset temperatures and in the heterogeneously frozen fraction of the halloysite samples, bringing their IN activity closer to that of the kaolinites. To interpret these findings, the freezing experiments were complemented by dynamic vapor sorption (DVS), BET (Brunauer–Emmett–Teller) surface area measurements, pore ice melting experiments with slurries, and transmission electron microscopy (TEM) before and after milling. These measurements demonstrate an increase in surface area and the destruction of tubes by milling and provide evidence for the influence of the tubular structure of the halloysites on their IN activity. We identify the OH–Al–O–Si–OH functionalized edges as being the most likely site for ice nucleation, as the high geometric diversity of the edges best accounts for the high diversity in IN activity of halloysites. We hypothesize that the stacking of layers and the number of stacks in halloysite tubes and kaolinite platelets affect the freezing temperature, with thicker stacks having the potential to freeze water at higher temperatures. The notion that the edges constitute the IN-active part of kaolin minerals is further supported by comparing kaolin minerals with montmorillonites and feldspars, all of which exhibit enhanced IN activity in the presence of ammonia and ammonium-containing solutions. As OH–Al–O–Si–OH functionalized edge surfaces are the only surface type that kaolin particles have in common with montmorillonites and feldspars, the common feature of IN activity enhancement in ammoniated solutions can only be explained by ice nucleation occurring at the edges of kaolin minerals.

The Urmia playa as a source of airborne dust and ice-nucleating particles – Part 2: Unraveling the relationship between soil dust composition and ice nucleation activity

Abstract. Ice-nucleating particles (INPs) originating from deserts, semi-arid regions, and dried lakebeds may cause heterogeneous ice nucleation, impacting cloud properties. Recently, due to climate change and water scarcity, abandoned agricultural lands with little surficial crust and negligible vegetation cover have become an increasing source of atmospheric dust worldwide. Unlike deserts, these areas are rich in soluble salt and (bio-)organic compounds. Using soil samples from various sites of the Lake Urmia playa (LUP) in northwestern Iran and airborne dusts collected at nearby meteorological stations, we elucidate how minerals, soluble salts, and organic matter interact to determine the IN activity of saline soils and dust. X-ray powder diffraction shows that the mineralogical composition is dominated by K-feldspars (microcline), quartz, carbonates, and clay minerals. The samples were stripped stepwise of organic matter, carbonates, and soluble salts. After each removal step, the ice nucleation (IN) activity was quantified in terms of onset freezing temperatures (Thet) and heterogeneously frozen fractions (Fhet) by emulsion freezing experiments using differential scanning calorimetry (DSC). We examined the influence of soluble salts and pH on microcline and quartz in emulsion freezing experiments, comparing these with reference suspensions of microcline and quartz exposed to salt concentrations and pH levels characteristic of the LUP samples. These analyses, combined with correlations between Thet and Fhet, allow us to identify the components that contribute to or inhibit IN activity. The LUP dusts turn out to be very good INPs, with freezing onset temperatures around 248 K in immersion freezing experiments. Interestingly, their IN activity proves to be dominated by the relatively small share of (bio-)organic matter (1 %–5.3 %). After organic matter removal, the remaining IN activity (Thet≈244 K) can be traced back to the clay fraction, because Thet and Fhet correlate positively with the clay mineral content but negatively with quartz and microcline. We attribute the inability of quartz and microcline to act as INPs to the basic pH of the LUP samples as well as to the presence of soluble salts. After additionally removing soluble salts and carbonates, the IN activity of the samples increased again significantly (Thet≈249 K), and the negative correlation with quartz and microcline turned into a slightly positive one. Removing carbonates and salts from the natural samples leads to an increase in Thet and Fhet as well, indicating that their presence also suppresses the IN activity of the (bio-)organic INPs. Overall, this study demonstrates that mineral and organic INPs do not just add up to yield the IN activity of soil dust but that the freezing behavior is governed by inhibiting and promoting interactions between the components.

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.

Ice nucleation activity of iron oxides via immersion freezing and an examination of the high ice nucleation activity of FeO.

Heterogeneous ice nucleation is a common process in the atmosphere, but relatively little is known about the role of different surface characteristics on the promotion of ice nucleation. We have used a series of iron oxides as a model system to study the role of lattice mismatch and defects induced by milling on ice nucleation activity. The iron oxides include wüstite (FeO), hematite (Fe2O3), magnetite (Fe3O4), and goethite (FeOOH). The iron oxides were characterized by X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET) surface area measurements. The immersion freezing experiments were performed using an environmental chamber. Wüstite (FeO) had the highest ice nucleation activity, which we attribute to its low lattice mismatch with hexagonal ice and the exposure of Fe-OH after milling. A comparison study of MnO and wüstite (FeO) with milled and sieved samples for each suggests that physical defects alone result in only a slight increase in ice nucleation activity. Despite differences in the molecular formula and surface groups, hematite (Fe2O3), magnetite (Fe3O4), and goethite (FeOOH) had similar ice nucleation activities, which may be attributed to their high lattice mismatch to hexagonal ice. This study provides further insight into the characteristics of a good heterogeneous ice nucleus and, more generally, helps to elucidate the interactions between aerosol particles and ice particles in clouds.

Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – Part 3: Aluminosilicates

Abstract. Aluminosilicates and quartz constitute the majority of airborne mineral dust. Despite similarities in structures and surfaces they differ greatly in terms of their ice nucleation (IN) efficiency. Here, we show that determining factors for their IN activity include surface ion exchange, NH3 or NH4+ adsorption, and surface degradation due to the slow dissolution of the minerals. We performed immersion freezing experiments with the (Na-Ca)-feldspar andesine, the K-feldspar sanidine, the clay mineral kaolinite, the micas muscovite and biotite, and gibbsite and compare their IN efficiencies with those of the previously characterized K-feldspar microcline and quartz. Samples were suspended in pure water as well as in aqueous solutions of NH3, (NH4)2SO4, NH4Cl and Na2SO4, with solute concentrations corresponding to water activities aw equal to 0.88–1.0. Using differential scanning calorimetry (DSC) on emulsified micron-sized droplets, we derived onset temperatures of heterogeneous (Thet) and homogeneous (Thom) freezing as well as heterogeneously frozen water volume fractions (Fhet). Suspensions in pure water of andesine, sanidine and kaolinite yield Thet equal to 242.8, 241.2 and 240.3 K, respectively, while no discernable heterogeneous freezing signal is present in the case of the micas or gibbsite (i.e., Thet≈Thom≈237.0 K). The presence of NH3 and/or NH4+ salts as solutes has distinct effects on the IN efficiency of most of the investigated minerals. When feldspars and kaolinite are suspended in very dilute solutions of NH3 or NH4+ salts, Thet shifts to higher temperatures (by 2.6–7.0 K compared to the pure water suspension). Even micas and gibbsite develop weak heterogeneous freezing activities in ammonia solutions. Conversely, suspensions containing Na2SO4 cause the Thet of feldspars to clearly fall below the water-activity-based immersion freezing description (Δaw= const.) even in very dilute Na2SO4 solutions, while Thet of kaolinite follows the Δaw= constant curve. The water activity determines how the freezing temperature is affected by solute concentration alone, i.e., if the surface properties of the ice nucleating particles are not affected by the solute. Therefore, the complex behavior of the IN activities can only be explained in terms of solute-surface-specific processes. We suggest that the immediate exchange of the native cations (K+, Na+, Ca2+) with protons, when feldspars are immersed in water, is a prerequisite for their high IN efficiency. On the other hand, excess cations from dissolved alkali salts prevent surface protonation, thus explaining the decreased IN activity in such solutions. In kaolinite, the lack of exchangeable cations in the crystal lattice explains why the IN activity is insensitive to the presence of alkali salts (Δaw= const.). We hypothesize that adsorption of NH3 and NH4+ on the feldspar surface rather than ion exchange is the main reason for the anomalous increased Thet in dilute solutions of NH3 or NH4+ salts. This is supported by the response of kaolinite to NH3 or NH4+, despite lacking exchangeable ions. Finally, the dissolution of feldspars in water or solutions leads to depletion of Al and formation of an amorphous layer enriched in Si. This hampers the IN activity of andesine the most, followed by sanidine, then eventually microcline, the least soluble feldspar.

Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – Part 2: Quartz and amorphous silica

Abstract. Divergent ice nucleation (IN) efficiencies of quartz, an important component of atmospheric mineral dust, have been reported in previous studies. We show here that quartz particles obtain their IN activity from milling and that quartz aged in water loses most of its IN efficiency relative to freshly milled quartz. Since most studies so far reported IN activities of commercial quartz dusts that were milled already by the manufacturer, IN active samples prevailed. Also, the quartz surface – much in contrast to that of feldspars – is not prone to ammonia-induced IN enhancement. In detail we investigate the influence of solutes on the IN efficiency of various silica (SiO2) particles (crystalline and amorphous) with special focus on quartz. We performed immersion freezing experiments and relate the observed variability in IN activity to the influence of milling, the aging time and to the exposure conditions since milling. Immersion freezing with silica particles suspended in pure water or aqueous solutions of NH3, (NH4)2SO4, NH4HSO4, Na2SO4 and NaOH, with solute concentrations corresponding to water activities aw=0.9–1.0, were investigated in emulsified droplets by means of differential scanning calorimetry (DSC) and analyzed in terms of the onset temperature of the heterogeneous freezing signal Thet and the heterogeneously frozen water volume fraction Fhet. Quartz particles, which originate from milling coarse samples, show a strong heterogeneous freezing peak in pure water with Thet equal to 247–251 K. This IN activity disappears almost completely after aging for 7 months in pure water in a glass vial. During this time quartz slowly grew by incorporating silicic acid leached from the glass vial. Conversely, the synthesized amorphous silica samples show no discernable heterogeneous freezing signal unless they were milled. This implies that defects provide IN activity to silica surfaces, whereas the IN activity of a natural quartz surface is negligible, when it grew under near-equilibrium conditions. For suspensions containing milled quartz and the solutes (NH4)2SO4, NH4HSO4 or Na2SO4, Thet approximately follows ThetΔawhet(aw), the heterogeneous freezing onset temperatures that obey Δawhet criterion, i.e., ThetΔawhet(aw)=Tmelt(aw+Δawhet) with Δawhet being a constant offset with respect to the ice melting point curve, similar to homogeneous IN. This water-activity-based description is expected to hold when the mineral surface is not altered by the presence of the solutes. On the other hand, we observe a slight enhancement in Fhet in the presence of these solutes, implying that the compliance with the Δawhet criterion does not necessarily imply constant Fhet. In contrast to the sulfates, dilute solutions of NH3 or NaOH (molality ≥5×10-4 mol kg−1) reveal Thet by 3–8 K lower than ThetΔawhet(aw), indicating a significant impact on the mineral surface. The lowering of Thet of quartz suspended in dilute NH3 solutions is opposite to the distinct increase in Thet that we found in emulsion freezing experiments with aluminosilicates, namely feldspars, kaolinite, gibbsite and micas. We ascribe this decrease in IN activity to the increased dissolution of quartz under alkaline conditions. The defects that constitute the active sites appear to be more susceptible to dissolution and therefore disappear first on a dissolving surface.

The Effect of Crystallinity and Crystal Structure on the Immersion Freezing of Alumina.

Determining the factors that constitute an efficient ice nucleus is an ongoing area of research in the atmospheric community. In particular, surface characteristics such as functional groups and surface defects impact the ice nucleation efficiency. Crystal structure has been proposed to be a possible factor that can dictate ice nucleation activity through the templating of water molecules on the surface of the aerosol particle. If the crystal structure of the surface matches that of the crystal structure of ice, it has been shown to increase ice nucleation activity. In this study, alumina was chosen as a model system because crystal structure and crystallinity can be tuned, and the effect on immersion freezing was explored. The nine alumina samples include polymorphs of AlOOH, Al(OH)3, and Al2O3, which have a range of crystal structures and crystallinities. The samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and Brunauer-Emmett-Teller (BET) analysis. From the immersion freezing experiments, corundum [α-Al2O3] was shown to have the highest ice nucleation activity likely because of its high lattice match and high degree of crystallinity. Crystal structure alone did not show a strong correlation with ice nucleation activity, but a combination of a hexagonal crystal structure and a highly crystalline surface was seen to nucleate ice at warmer temperatures than the other alumina samples. This study provides experimental results in the study of ice nucleation of a range of alumina samples, which have possible implications for alumina-based mineral dust particles. Our findings suggest that crystallinity and crystal structure are important to consider when evaluating the ice nucleation efficiency of aerosol particles in laboratory and modeling studies.

An improved approach for measuring immersion freezing in large droplets over a wide temperature range

Immersion freezing (ice nucleation by particles immersed in supercooled water) is a key process for forming ice in mixed-phase clouds. Immersion freezing experiments with particles in microliter-sized (millimeter-sized) water droplets are often applied to detecting very small numbers of ice nucleating particles (INPs). However, the application of such large droplets remains confined to the detection of INPs active at temperatures much higher than the homogeneous freezing limit, because of artifacts related to freezing of water droplets without added INPs at temperatures of −25 °C or higher on a supporting substrate. Here I report a method for measuring immersion freezing in super-microliter-sized droplets over a wide temperature range. To reduce possible artifacts, droplets are pipetted onto a thin layer of Vaseline and cooled in a clean booth. In the Cryogenic Refrigerator Applied to Freezing Test (CRAFT) system, freezing of pure (Milli-Q) water droplets are limited at temperatures above −30 °C. An intercomparison of various techniques for immersion freezing experiments with reference particles (Snomax and illite NX) demonstrates that despite the use of relatively large droplets, the CRAFT setup allows for evaluating the immersion freezing activity of the particles over almost the entire temperature range (about −30 °C to 0 °C) relevant for mixed-phase cloud formation.

Comparing contact and immersion freezing from continuous flow diffusion chambers

Abstract. Ice nucleating particles (INPs) in the atmosphere are responsible for glaciating cloud droplets between 237 and 273 K. Different mechanisms of heterogeneous ice nucleation can compete under mixed-phase cloud conditions. Contact freezing is considered relevant because higher ice nucleation temperatures than for immersion freezing for the same INPs were observed. It has limitations because its efficiency depends on the number of collisions between cloud droplets and INPs. To date, direct comparisons of contact and immersion freezing with the same INP, for similar residence times and concentrations, are lacking. This study compares immersion and contact freezing efficiencies of three different INPs. The contact freezing data were obtained with the ETH CoLlision Ice Nucleation CHamber (CLINCH) using 80 µm diameter droplets, which can interact with INPs for residence times of 2 and 4 s in the chamber. The contact freezing efficiency was calculated by estimating the number of collisions between droplets and particles. Theoretical formulations of collision efficiencies gave too high freezing efficiencies for all investigated INPs, namely AgI particles with 200 nm electrical mobility diameter, 400 and 800 nm diameter Arizona Test Dust (ATD) and kaolinite particles. Comparison of freezing efficiencies by contact and immersion freezing is therefore limited by the accuracy of collision efficiencies. The concentration of particles was 1000 cm−3 for ATD and kaolinite and 500, 1000, 2000 and 5000 cm−3 for AgI. For concentrations < 5000 cm−3, the droplets collect only one particle on average during their time in the chamber. For ATD and kaolinite particles, contact freezing efficiencies at 2 s residence time were smaller than at 4 s, which is in disagreement with a collisional contact freezing process but in accordance with immersion freezing or adhesion freezing. With “adhesion freezing”, we refer to a contact nucleation process that is enhanced compared to immersion freezing due to the position of the INP on the droplet, and we discriminate it from collisional contact freezing, which assumes an enhancement due to the collision of the particle with the droplet. For best comparison with contact freezing results, immersion freezing experiments of the same INPs were performed with the continuous flow diffusion chamber Immersion Mode Cooling chAmber–Zurich Ice Nucleation Chamber (IMCA–ZINC) for a 3 s residence time. In IMCA–ZINC, each INP is activated into a droplet in IMCA and provides its surface for ice nucleation in the ZINC chamber. The comparison of contact and immersion freezing results did not confirm a general enhancement of freezing efficiency for contact compared with immersion freezing experiments. For AgI particles the onset of heterogeneous freezing in CLINCH was even shifted to lower temperatures compared with IMCA–ZINC. For ATD, freezing efficiencies for contact and immersion freezing experiments were similar. For kaolinite particles, contact freezing became detectable at higher temperatures than immersion freezing. Using contact angle information between water and the INP, it is discussed how the position of the INP in or on the droplets may influence its ice nucleation activity.

Immersion mode ice nucleation measurements with the new Portable Immersion Mode Cooling chAmber (PIMCA)

AbstractThe new Portable Immersion Mode Cooling chAmber (PIMCA) has been developed for online immersion freezing of single‐immersed aerosol particles. PIMCA is a vertical extension of the established Portable Ice Nucleation Chamber (PINC). PIMCA immerses aerosol particles into cloud droplets before they enter PINC. Immersion freezing experiments on cloud droplets with a radius of 5–7 μm at a prescribed supercooled temperature (T) and water saturation can be conducted, while other ice nucleation mechanisms (deposition, condensation, and contact mode) are excluded. Validation experiments on reference aerosol (kaolinite, ammonium sulfate, and ammonium nitrate) showed good agreement with theory and literature. The PIMCA‐PINC setup was tested in the field during the Zurich AMBient Immersion freezing Study (ZAMBIS) in spring 2014 in Zurich, Switzerland. Significant concentrations of submicron ambient aerosol triggering immersion freezing at T > 236 K were rare. The mean frozen cloud droplet number concentration was estimated to be 7.22·105 L−1 for T < 238 K and determined from the measured frozen fraction and cloud condensation nuclei (CCN) concentrations predicted for the site at a typical supersaturation of SS = 0.3%. This value should be considered as an upper limit of cloud droplet freezing via immersion and homogeneous freezing processes. The predicted ice nucleating particle (INP) concentration based on measured total aerosol larger than 0.5 μm and the parameterization by DeMott et al. (2010) at T = 238 K is INPD10=54 ± 39 L−1. This is a lower limit as supermicron particles were not sampled with PIMCA‐PINC during ZAMBIS.

Analysis of isothermal and cooling-rate-dependent immersion freezing by a unifying stochastic ice nucleation model

Abstract. Immersion freezing is an important ice nucleation pathway involved in the formation of cirrus and mixed-phase clouds. Laboratory immersion freezing experiments are necessary to determine the range in temperature, T, and relative humidity, RH, at which ice nucleation occurs and to quantify the associated nucleation kinetics. Typically, isothermal (applying a constant temperature) and cooling-rate-dependent immersion freezing experiments are conducted. In these experiments it is usually assumed that the droplets containing ice nucleating particles (INPs) all have the same INP surface area (ISA); however, the validity of this assumption or the impact it may have on analysis and interpretation of the experimental data is rarely questioned. Descriptions of ice active sites and variability of contact angles have been successfully formulated to describe ice nucleation experimental data in previous research; however, we consider the ability of a stochastic freezing model founded on classical nucleation theory to reproduce previous results and to explain experimental uncertainties and data scatter. A stochastic immersion freezing model based on first principles of statistics is presented, which accounts for variable ISA per droplet and uses parameters including the total number of droplets, Ntot, and the heterogeneous ice nucleation rate coefficient, Jhet(T). This model is applied to address if (i) a time and ISA-dependent stochastic immersion freezing process can explain laboratory immersion freezing data for different experimental methods and (ii) the assumption that all droplets contain identical ISA is a valid conjecture with subsequent consequences for analysis and interpretation of immersion freezing. The simple stochastic model can reproduce the observed time and surface area dependence in immersion freezing experiments for a variety of methods such as: droplets on a cold-stage exposed to air or surrounded by an oil matrix, wind and acoustically levitated droplets, droplets in a continuous-flow diffusion chamber (CFDC), the Leipzig aerosol cloud interaction simulator (LACIS), and the aerosol interaction and dynamics in the atmosphere (AIDA) cloud chamber. Observed time-dependent isothermal frozen fractions exhibiting non-exponential behavior can be readily explained by this model considering varying ISA. An apparent cooling-rate dependence of Jhet is explained by assuming identical ISA in each droplet. When accounting for ISA variability, the cooling-rate dependence of ice nucleation kinetics vanishes as expected from classical nucleation theory. The model simulations allow for a quantitative experimental uncertainty analysis for parameters Ntot, T, RH, and the ISA variability. The implications of our results for experimental analysis and interpretation of the immersion freezing process are discussed.

Immersion Freezing of Kaolinite: Scaling with Particle Surface Area

Abstract This study presents an analysis showing that the freezing probability of kaolinite particles from Fluka scales exponentially with particle surface area for different atmospherically relevant particle sizes. Immersion freezing experiments were performed at the Leipzig Aerosol Cloud Interaction Simulator (LACIS). Size-selected kaolinite particles with mobility diameters of 300, 700, and 1000 nm were analyzed with one particle per droplet. First, it is demonstrated that immersion freezing is independent of the droplet volume. Using the mobility analyzer technique for size selection involves the presence of multiply charged particles in the quasi-monodisperse aerosol, which are larger than singly charged particles. The fractions of these were determined using cloud droplet activation measurements. The development of a multiple charge correction method has proven to be essential for deriving ice fractions and other quantities for measurements in which the here-applied method of size selection is used. When accounting for multiply charged particles (electric charge itself does not matter), both a time-independent and a time-dependent description of the freezing process can reproduce the measurements over the range of examined particle sizes. Hence, either a temperature-dependent surface site density or a single contact angle distribution was sufficient to parameterize the freezing behavior. From a comparison with earlier studies using kaolinite samples from the same provider, it is concluded that the neglect of multiply charged particles and, to a lesser extent, the effect of time can cause a significant overestimation of the ice nucleation site density of one order of magnitude, which translates into a temperature bias of 5–6 K.

A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of 17 ice nucleation measurement techniques

Abstract. Immersion freezing is the most relevant heterogeneous ice nucleation mechanism through which ice crystals are formed in mixed-phase clouds. In recent years, an increasing number of laboratory experiments utilizing a variety of instruments have examined immersion freezing activity of atmospherically relevant ice-nucleating particles. However, an intercomparison of these laboratory results is a difficult task because investigators have used different ice nucleation (IN) measurement methods to produce these results. A remaining challenge is to explore the sensitivity and accuracy of these techniques and to understand how the IN results are potentially influenced or biased by experimental parameters associated with these techniques. Within the framework of INUIT (Ice Nuclei Research Unit), we distributed an illite-rich sample (illite NX) as a representative surrogate for atmospheric mineral dust particles to investigators to perform immersion freezing experiments using different IN measurement methods and to obtain IN data as a function of particle concentration, temperature (T), cooling rate and nucleation time. A total of 17 measurement methods were involved in the data intercomparison. Experiments with seven instruments started with the test sample pre-suspended in water before cooling, while 10 other instruments employed water vapor condensation onto dry-dispersed particles followed by immersion freezing. The resulting comprehensive immersion freezing data set was evaluated using the ice nucleation active surface-site density, ns, to develop a representative ns(T) spectrum that spans a wide temperature range (−37 °C < T < −11 °C) and covers 9 orders of magnitude in ns. In general, the 17 immersion freezing measurement techniques deviate, within a range of about 8 °C in terms of temperature, by 3 orders of magnitude with respect to ns. In addition, we show evidence that the immersion freezing efficiency expressed in ns of illite NX particles is relatively independent of droplet size, particle mass in suspension, particle size and cooling rate during freezing. A strong temperature dependence and weak time and size dependence of the immersion freezing efficiency of illite-rich clay mineral particles enabled the ns parameterization solely as a function of temperature. We also characterized the ns(T) spectra and identified a section with a steep slope between −20 and −27 °C, where a large fraction of active sites of our test dust may trigger immersion freezing. This slope was followed by a region with a gentler slope at temperatures below −27 °C. While the agreement between different instruments was reasonable below ~ −27 °C, there seemed to be a different trend in the temperature-dependent ice nucleation activity from the suspension and dry-dispersed particle measurements for this mineral dust, in particular at higher temperatures. For instance, the ice nucleation activity expressed in ns was smaller for the average of the wet suspended samples and higher for the average of the dry-dispersed aerosol samples between about −27 and −18 °C. Only instruments making measurements with wet suspended samples were able to measure ice nucleation above −18 °C. A possible explanation for the deviation between −27 and −18 °C is discussed. Multiple exponential distribution fits in both linear and log space for both specific surface area-based ns(T) and geometric surface area-based ns(T) are provided. These new fits, constrained by using identical reference samples, will help to compare IN measurement methods that are not included in the present study and IN data from future IN instruments.

The immersion mode ice nucleation behavior of mineral dusts: A comparison of different pure and surface modified dusts

AbstractIn this study we present results from immersion freezing experiments with size‐segregated mineral dust particles. Besides two already existing data sets for Arizona Test Dust (ATD), and Fluka kaolinite, we show two new data sets for illite‐NX, which consists mainly of illite, a clay mineral, and feldspar, a common crustal material. The experiments were carried out with the Leipzig Aerosol Cloud Interaction Simulator. After comparing the different dust samples, it became obvious that the freezing ability was positively correlated with the K‐feldspar content. Furthermore, a comparison of the composition of the ATD, illite‐NX, and feldspar samples suggests that within the K‐feldspars, microcline is more ice nucleation active than orthoclase. A coating with sulfuric acid leads to a decrease in the ice nucleation ability of all mineral dusts, with the effect being more pronounced for the feldspar sample.

Application of linear polarized light for the discrimination of frozen and liquid droplets in ice nucleation experiments

Abstract. We report on the development and test results of the new optical particle counter TOPS-Ice (Thermo-stabilized Optical Particle Spectrometer for the detection of Ice). The instrument uses measurements of the cross-polarized scattered light by single particles into the near-forward direction (42.5° ± 12.7°) to distinguish between spherical and non-spherical particles. This approach allows the differentiation between liquid water droplets (spherical) and ice particles (non-spherical) having similar volume-equivalent sizes and therefore can be used to determine the fraction of frozen droplets in a typical immersion freezing experiment. We show that the numerical simulation of the light scattered on non-spherical particles (spheroids in random orientation) considering the actual scattering geometry used in the instrument supports the validity of the approach, even though the cross-polarized component of the light scattered by spherical droplets does not vanish in this scattering angle. For the separation of the ice particle mode from the liquid droplet mode, we use the width of the pulse detected in the depolarization channel instead of the pulse height. Exploiting the intrinsic relationship between pulse height and pulse width for Gaussian pulses allows us to calculate the fraction of frozen droplets even if the liquid droplet mode dominates the particle ensemble. We present test results obtained with TOPS-Ice in the immersion freezing experiments at the laminar diffusion chamber LACIS (Leipzig Aerosol Cloud Interaction Simulator) and demonstrate the excellent agreement with the data obtained in similar experiments with a different optical instrument. Finally, the advantages of using the cross-polarized light measurements for the differentiation of liquid and frozen droplets in the realistic immersion freezing experiments are discussed.

The ice nucleating ability of pollen: Part III: New laboratory studies in immersion and contact freezing modes including more pollen types

Based on earlier experimental studies, the ice nucleating abilities of further pollen types were investigated in the immersion and contact freezing modes. The studies were carried out at the Mainz vertical wind tunnel with freely floating supercooled droplets down to − 28 °C. The pollen had diameters between 26 and 28 μm and correspondingly low sink velocities around 2.5 cm s − 1. The radii of the studied drops were calculated from the recorded wind velocity and for both freezing modes the radii of the observed droplets varied between 315 and 380 μm. Immersion freezing experiments were conducted with pollen particles added to the droplets while in contact freezing experiments supercooled droplets were subjected to a burst of pollen particles. The median freezing temperatures found in the immersion freezing mode were: − 13.5 °C (alder), − 21.5 °C (lombardy poplar), − 21.0 °C (redtop grass) and − 19.8 °C (kentucky blue). The median freezing temperatures in the contact freezing mode were found as: − 12.6 °C (alder), − 17.9 °C (lombardy poplar), − 18.7 °C (redtop grass) and − 16.1 °C (kentucky blue). The results show that the ice nucleating ability of pollen is not restricted to single pollen types but seems to be a general pollen property.

The ice nucleating ability of pollen:: Part II. Laboratory studies in immersion and contact freezing modes

Laboratory tests were conducted of the ice nucleating ability of four kinds of pollen in the immersion and the contact freezing modes. The diameters of the selected pollen were between 25 and 70 μm. The experiments were carried out at the Mainz vertical wind tunnel with freely suspended supercooled droplets at temperatures down to −28 °C. The immersion freezing experiments were conducted with drops of radii between 250 and 375 μm formed from distilled water with a defined amount of pollen added. The drops were freely floated in the wind tunnel while being supercooled. For the contact freezing experiments, a short burst of pollen was allowed to collide with freely suspended, supercooled pure water drops of 360-μm radius. The results showed that particle-free water drops in particle-free air in the wind tunnel did not freeze at temperatures above −28 °C while water drops containing pollen froze at temperatures as high as −9 °C, and water drops colliding with pollen froze at temperatures −5 °C and lower. Combined with earlier results about the ice nucleating ability of some bacteria, marine plankton, and leaf litters, the present results confirm the importance of biological aerosol particles as potential ice nuclei at relatively warm temperatures.