Ice Cloud Formation Research Articles

A statistically significant increase in ice supersaturation in the atmosphere in the past 40 years

Ice saturation (and supersaturation) is a frequent phenomenon in cold regions of the upper troposphere. Its existence is essential for the formation of ice clouds and a necessary condition for the persistence of contrails. Its spatial and temporal evolution is important for weather and climate. The ice saturation and supersaturation values are found in the upper tail of the probability density function (pdf) of upper tropospheric humidity with respect to ice (UTHi). Here, we analyse the changes in the frequency of occurrence of ice saturation and supersaturation from 1979 to 2020 and compare them to changes in the mean UTHi. Our results show that while the mean UTHi increases near-globally with a rate of about 0.15% per decade, high UTHi values exceeding the 70%, 80%, 90% and 100% thresholds increase faster than the mean, at rates of about 0.7%, 0.6%, 0.4% and 0.3% per decade, respectively. The increasing rates of values found in the upper tail of the UTHi pdf suggest that the ambient conditions for cirrus and contrail formation and persistence will be more favourable in the future and this is expected to further enhance the impact of aviation on climate.

Decreased dust particles amplify the cloud cooling effect by regulating cloud ice formation over the Tibetan Plateau.

Ice-nucleating particles (INPs) can initiate cloud ice formation, influencing cloud radiative effects (CRE) and climate. However, the knowledge of INP sources, concentrations, and their impact on CRE over the Tibetan Plateau (TP)-a highly climate-sensitive region-remains largely hypothetical. Here, we integrated data from multisource satellite observations and snowpack samples collected from five glaciers to demonstrate that dust particles constitute primary INP sources over the TP. The springtime dust influxes lead to seasonally elevated ice concentrations in mixed-phase clouds. Furthermore, the decadal reduction in dustiness from 2007 to 2019 results in decreased springtime dust INPs, thereby amplifying the cooling effect of clouds over the TP, with a 1.98 ± 0.39-watt per square meter reduction in surface net CRE corresponding to a 0.01 decrease in dust optical depth. Our findings elucidate previously unidentified pathways of climate feedback from an atmospheric INP perspective, especially highlighting the crucial role of dust in aerosol-cloud interactions.

Microplastic Particles Contain Ice Nucleation Sites That Can Be Inhibited by Atmospheric Aging.

Recent research has shown that microplastics are widespread in the atmosphere. However, we know little about their ability to nucleate ice and their impact on ice formation in clouds. Ice nucleation by microplastics could also limit their long-range transport and global distribution. The present study explores the heterogeneous ice-nucleating ability of seven microplastic samples in immersion freezing mode. Two polypropylene samples and one polyethylene terephthalate sample froze heterogeneously with median freezing temperatures of -20.9, -23.2, and -21.9 °C, respectively. The number of ice nucleation sites per surface area, ns(T), ranged from 10-1 to 104 cm-2 in a temperature interval of -15 to -25 °C, which is comparable to that of volcanic ash and fungal spores. After exposure to ozone or a combination of UV light and ozone, simulating atmospheric aging, the ice nucleation activity decreased in some cases and remained unchanged in others. Our freezing data suggest that microplastics may promote ice formation in cloud droplets. In addition, based on a comparison of our freezing results and previous simulations using a global transport model, ice nucleation by microplastics will impact their long-range transport to faraway locations and global distribution.

Quantifying Downward Radiative Fluxes From Nighttime Martian Water Ice Clouds: Applications to Thermal Modeling of Surface Temperatures

AbstractDuring the first part of the Martian year (Ls = 50°–160°) a phenomenon occurs on Mars in the tropical and equatorial regions (30°N–10°S) known as the Aphelion Cloud Belt (ACB). During this time, there is prominent formation and diurnal variability of water ice clouds. Limited empirical attempts have been made to characterize the magnitude of radiative flux contributions from clouds to nighttime surface temperatures. In this work, we estimated the infrared (IR) flux contribution at ground level from the clouds by comparing surface temperature data from the Thermal Emission Spectrometer (TES) onboard Mars Global Surveyor (MGS) to calculated temperatures using the KRC numerical thermal model. We then generated a database of IR fluxes at the ground contributed by clouds spanning the entirety of the tropical and equatorial regions as a function of Solar Longitude (Ls) on Mars in one degree bins. We compared results with work presented elsewhere in the literature and found good agreement. We also found that temporal trends followed the general established range for the ACB but our analysis demonstrated the peak ACB values occurred at later times (Ls = 100°–140°) than previously published data sets using water ice opacity retrievals (Ls = 90°–110°). This database may be used in comparison to calculated Global Climate Model fluxes as well as a lookup tool for more precise estimation of surface and subsurface thermal environments in these regions.

Micro-PINGUIN: microtiter-plate-based instrument for ice nucleation detection in gallium with an infrared camera

Abstract. Ice nucleation particles play a crucial role in atmospheric processes; for example, they can trigger ice formation in clouds and thus influence their lifetime and optical properties. The quantification and characterization of these particles require reliable and precise measurement techniques. In this publication, we present a novel droplet freezing instrument to measure the immersion freezing of biotic and abiotic ice-nucleating particles within the temperature range of 0 to −25 °C. Immersion freezing of the samples is investigated using 384-well PCR plates with a sample volume of 30 µL. Nucleation events are detected with high precision using a thermal camera that records the increase in infrared emission due to the latent heat release. To maximize the thermal contact between the PCR plate and the surrounding cooling unit, we use a gallium bath as a mount for the PCR plate. The instrument was validated relative to a calibrated temperature standard and through reproducibility measurements employing the same suspension. We find that the combination of good thermal connectivity and precise temperature recording enables accurate (±0.81 °C at −10 °C) and reproducible (±0.20 °C) detection of the nucleation temperatures. Consequently, the results that are produced using the MICROtiter-Plate-based instrument for Ice Nucleation detection in GalliUm with an INfrared camera (micro-PINGUIN) are of good quality and the instrument can be used to study the immersion freezing of various ice-nucleating particles. For comparison with already existing instruments, Snomax® (hereafter Snomax) and illite NX suspensions are measured with the new ice nucleation instrument, micro-PINGUIN. Further, we investigated the reproducibility of experiments using Snomax suspensions and found poor reproducibility when suspensions were prepared freshly even if the same batch of Snomax is used. This could be attributed to substrate heterogeneity, aging effects, and dilution errors. The reproducibility of the measurements is greatly improved for Snomax suspensions that are prepared in advance and stored frozen in aliquots. Thus, we suggest the use of suspensions frozen in aliquots for further reproducibility measurements and intercomparison studies.

Bulk cloud microphysical properties as seen from numerical simulation and remote sensing products: case study of a hailstorm event over the La Plata Basin

Hailstorms develop over the La Plata Basin, in south-eastern South America, more often during later winter and early austral spring, between September and October. These systems have significant socioeconomic impacts over the region. Thus, a better understanding of how atmospheric drivers modulate the formation of hailstorms is important to improve the forecast of such phenomena. In this study, we selected a hailstorm event observed over the eastern La Plata Basin during 14–15 July 2016 to evaluate the performance of the Brazilian developments on the Regional Atmospheric Modelling System (BRAMS) model. The ability of the model in simulating cloud microphysical properties was evaluated by comparing simulations driven by different global forcings against in situ and remote sensing observations. The model results showed good skill in capturing the basic characteristics of the thunderstorm, particularly in terms of the spatial distribution of hydrometeors. The simulated spatial distribution of hail covers locations where hail fall was reported. The BRAMS simulations suggest that, despite relatively low values of the convective available potential energy (CAPE) (700–1000 J kg−1), environments with strong 0–8-km bulk shear (60–70 kt, ~30.9–36.0 m s–1) can promote the formation of ice clouds and hail fall over the eastern La Plata Basin. To be more conclusive, however, further research is needed to understand how different combinations of CAPE and shear affect hail formation over the region.

Soot aerosols from commercial aviation engines are poor ice-nucleating particles at cirrus cloud temperatures

Abstract. Ice-nucleating particles catalyze ice formation in clouds, affecting climate through radiative forcing from aerosol–cloud interactions. Aviation directly emits particles into the upper troposphere where ice formation conditions are favorable. Previous studies have used proxies of aviation soot to estimate their ice nucleation activity; however, investigations with commercial aircraft soot from modern in-use aircraft engines have not been quantified. In this work, we sample aviation soot particles at ground level from different commercial aircraft engines to test their ice nucleation ability at temperatures ≤228 K as a function of engine thrust and soot particle size. Additionally, soot particles were catalytically stripped to reveal the impact of mixing state on their ice nucleation ability. Particle physical and chemical properties were further characterized and related to the ice nucleation properties. The results show that aviation soot nucleates ice at or above relative humidity conditions required for homogeneous freezing of solution droplets (RHhom). We attribute this to a mesopore paucity inhibiting pore condensation and the sulfur content which suppresses freezing. Only large soot aggregates (400 nm) emitted under 30 %–100 % thrust conditions for a subset of engines (2 out of 10) nucleate ice via pore condensation and freezing. For those specific engines, the presence of hydrophilic chemical groups facilitates the nucleation. Aviation soot emitted at thrust ≥ 100 % (sea level thrust) nucleates ice at or above RHhom. Overall, our results suggest that aviation soot will not contribute to natural cirrus formation and can be used in models to update impacts of soot–cirrus clouds.

Seasonal vertical distributions of diurnal variation of ice cloud frequency by CATS measurements over a global region (51°S-51°N)

Ice clouds can strongly affect the radiation budget through the reflection (absorption) of solar shortwave (longwave) radiation in the Earth-atmosphere system. Unlike water clouds, ice clouds present a longer lifetime in the upper troposphere. Therefore, the vertical patterns of diurnal variation of ice clouds are of very importance to understand climate change in the globe. The present paper investigates the diurnal variation of ice cloud frequency (ICF) from March 2015 to October 2017 over the global scale (51 ° S and 51 ° N) by CATS (Cloud-Aerosol Transport System) LiDAR onboard the International Space Station (ISS) and CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) LiDAR measurements. Our results suggest that: (1) ICF derived from the CATS and CALIPSO LiDAR presents the similar geographical variations. (2) The vertical distribution of ICF in land area over the North Hemisphere (NH) is found larger than in the South Hemisphere (SH) likely due to the more dust aerosols as ice nuclei to influence ice cloud formation over the NH. (3) ICF at nighttime (00:00–06:00 and 18:00–23:00) exhibits higher value than daytime (06:00–18:00), especially in high altitude (more than 12 km) ice clouds in the tropics. (4) The number of ice cloud samples is higher in spring and winter compared to summer and autumn, and the highest amount of ice cloud samples in spring(winter) occurs during nighttime(daytime) for most hours. These findings provide significant insights that may help improving the ice cloud's representation in climate models.

Aerosol-related effects on the occurrence of heterogeneous ice formation over Lauder, New Zealand ∕ Aotearoa

Abstract. The presented study investigates the efficiency of heterogeneous ice formation in natural clouds over Lauder, New Zealand / Aotearoa. Aerosol conditions in the middle troposphere above Lauder are subject to huge contrasts. Clean, pristine air masses from Antarctica and the Southern Ocean arrive under southerly flow conditions, while high aerosol loads can occur when air masses are advected from nearby Australia. This study assesses how these contrasts in aerosol load affect the ice formation efficiency in stratiform midlevel clouds in the heterogeneous freezing range (−40 to 0 ∘C). For this purpose, an 11-year dataset was analyzed from a dual-wavelength polarization lidar system operated by National Institute of Water and Atmospheric Research (NIWA), Taihoro Nukurangi, at Lauder in collaboration with the National Institute for Environmental Studies in Japan and the Meteorological Research Institute of the Japan Meteorological Agency. These data were used to investigate the efficiency of heterogeneous ice formation in clouds over the site as a function of cloud-top temperature as in previous studies at other locations. The Lauder cloud dataset was put into context with lidar studies from contrasting regions such as Germany and southern Chile. The ice formation efficiency found at Lauder is lower than in polluted midlatitudes (i.e., Germany) but higher than, for example, in southern Chile. Both Lauder and southern Chile are subject to generally low free-tropospheric aerosol loads, which suggests that the low ice formation efficiency at these two sites is related to low ice-nucleating-particle (INP) concentrations. However, Lauder sees episodes of continental aerosol, more than southern Chile does, which seems to lead to the moderately increased ice formation efficiency. Trajectory-based tools and aerosol model reanalyses are used to relate this cloud dataset to the aerosol load and the air mass sources. Both analyses point clearly to higher ice formation efficiency for clouds which are more strongly influenced by continental aerosol and to lower ice formation efficiency for clouds which are more influenced by Antarctic/marine aerosol and air masses.

Effect of Acidity on Ice Nucleation by Inorganic-Organic Mixed Droplets.

Aerosol acidity significantly influences heterogeneous chemical reactions and human health. Additionally, acidity may play a role in cloud formation by modifying the ice nucleation properties of inorganic and organic aerosols. In this work, we combined our well-established ice nucleation technique with Raman microspectroscopy to study ice nucleation in representative inorganic and organic aerosols across a range of pH conditions (pH -0.1 to 5.5). Homogeneous nucleation was observed in systems containing ammonium sulfate, sulfuric acid, and sucrose. In contrast, droplets containing ammonium sulfate mixed with diethyl sebacate, poly(ethylene glycol) 400, and 1,2,6-hexanetriol were found to undergo liquid-liquid phase separation, exhibiting core-shell morphologies with observed initiation of heterogeneous freezing in the cores. Our experimental findings demonstrate that an increased acidity reduces the ice nucleation ability of droplets. Changes in the ratio of bisulfate to sulfate coincided with shifts in ice nucleation temperatures, suggesting that the presence of bisulfate may decrease the ice nucleation efficiency. We also report on how the morphology and viscosity impact ice nucleation properties. This study aims to enhance our fundamental understanding of acidity's effect on ice nucleation ability, providing context for the role of acidity in atmospheric ice cloud formation.

Interactions between Gravity Waves and Cirrus Clouds: Asymptotic Modeling of Wave-Induced Ice Nucleation

Abstract We present an asymptotic approach for the systematic investigation of the effect of gravity waves (GWs) on ice clouds formed through homogeneous nucleation. In particular, we consider high- and midfrequency GWs in the tropopause region driving the formation of ice clouds, modeled with a double-moment bulk ice microphysics scheme. The asymptotic approach allows for identifying reduced equations for self-consistent description of the ice dynamics forced by GWs including the effects of diffusional growth and nucleation of ice crystals. Further, corresponding analytical solutions for a monochromatic GW are derived under a single-parcel approximation. The results provide a simple expression for the nucleated number of ice crystals in a nucleation event. It is demonstrated that the asymptotic solutions capture the dynamics of the full ice model and accurately predict the nucleated ice crystal number. The present approach is extended to allow for superposition of GWs, as well as for variable ice crystal mean mass in the deposition. Implications of the results for an improved representation of GW variability in cirrus parameterizations are discussed.

Ice Cloud Formation Related to Oceanic Supply of Ice‐Nucleating Particles: A Case Study in the Southern Ocean Near an Atmospheric River in Late Summer

AbstractThis study investigated ice cloud formation associated with marine bioaerosols over the Southern Ocean (SO) using a combination of cloud particle sensor (CPS) sonde observations, satellite products, reanalysis data, and backward trajectory analysis. The CPS sonde detected ice clouds at temperatures higher than −10°C in the mid‐troposphere near an atmospheric river at high‐latitudes over the SO. Backward trajectory analyses indicated that a mid‐latitude air mass with a high concentration of atmospheric dimethylsulfide (DMS) in the atmospheric boundary layer (<1 km) arrived at the ice cloud formation layer over the high‐latitudes. The DMS in the boundary layer began to increase under high wave conditions, coincident with the highest chlorophyll‐a concentrations in the ocean. These results suggest that bioaerosols emitted from the ocean over the mid‐latitudes acted as ice‐nucleating particles for ice cloud formation over high‐latitudes.

Retrievals of vertical distribution of Martian water vapor from combined day and night MGS TES nadir spectra

Vertical distribution of water vapor in the Martian lower atmosphere (0–40 km) is an important characteristic of the Martian water cycle. It reflects a complex interplay between various processes shaping the contemporary water cycle, including atmospheric transport, formation of water ice clouds, sublimation of surface frost, and possible interaction with the subsurface. Despite the growing dataset of water vapor observations, there is yet no comprehensive climatology of the vertical distribution of vapor. Limb and solar occultation observations that are typically used to retrieve vapor vertical distribution are much less numerous than nadir observations and are available for limited seasons and locations. We propose a new methodology to retrieve vertical distribution of water vapor in the Martian lower atmosphere using only nadir data. The methodology is based on the observation that infrared spectra collected over same location during different parts of day are sensitive to water vapor in different parts of the atmosphere. We validate our new methodology by applying it to infrared spectra simulated for realistic Mars environments created with Mars General Circulation Model (GCM). These tests show that vertical profiles of vapor can be recovered with accuracy ~20–40%, which is comparable to accuracy of limb and occultation observations. Importantly, the new retrieval algorithm can constrain near-surface (0–10 km) vapor abundances that cannot be observed by limb and occultation techniques. Application of this new methodology to a large dataset on Martian nadir spectra, such as the one collected by the Thermal Emission Spectrometer (TES) onboard Mars Global Surveyor (MGS) during its mission in 1999–2004, can result in the development of the detailed climatology of vapor vertical distribution on Mars. As an initial step toward this goal, we apply the new retrieval methodology to a subset of nadir spectra collected by MGS TES during one Martian year. The retrieved vapor profiles are qualitatively consistent with vapor distributions predicted by the GCM for different seasons. Differences between retrieved and simulated profiles can be interpreted as reflecting wetter Martian atmosphere and weaker atmospheric transport, during some seasons. Retrieved vapor distribution during southern summer suggests disruption of atmospheric vapor transport into southern extra-tropics following a global dust storm.

Sensitivity of cloud-phase distribution to cloud microphysics and thermodynamics in simulated deep convective clouds and SEVIRI retrievals

Abstract. The formation of ice in clouds is an important process in mixed-phase clouds, and the radiative properties and dynamical developments of clouds strongly depend on their partitioning between the liquid and ice phases. In this study, we investigated the sensitivities of the cloud phase to the ice-nucleating particle (INP) concentration and thermodynamics. Moreover, passive satellite retrieval algorithms and cloud products were evaluated to identify whether they could detect cloud microphysical and thermodynamical perturbations. Experiments were conducted using the ICOsahedral Nonhydrostatic (ICON) model at the convection-permitting resolution of about 1.2 km on a domain covering significant parts of central Europe, and they were compared to two different retrieval products based on Spinning Enhanced Visible and InfraRed Imager (SEVIRI) measurements. We selected a day with multiple isolated deep convective clouds, reaching a homogeneous freezing temperature at the cloud top. The simulated cloud liquid pixel fractions were found to decrease with increasing INP concentration, both within clouds and at the cloud top. The decrease in the cloud liquid pixel fraction was not monotonic and was stronger in high-INP cases. Cloud-top glaciation temperatures shifted toward warmer temperatures with an increasing INP concentration by as much as 8 ∘C. Moreover, the impact of the INP concentration on cloud-phase partitioning was more pronounced at the cloud top than within the cloud. Furthermore, initial and lateral boundary temperature fields were perturbed with increasing and decreasing temperature increments from 0 to ±3 and ±5 K between 3 and 12 km, respectively. Perturbing the initial thermodynamic state was also found to systematically affect the cloud-phase distribution. However, the simulated cloud-top liquid pixel fraction, diagnosed using radiative transfer simulations as input to a satellite forward operator and two different satellite remote-sensing retrieval algorithms, deviated from one of the satellite products regardless of perturbations in the INP concentration or the initial thermodynamic state for warmer subzero temperatures while agreeing with the other retrieval scheme much better, in particular for the high-INP and high-CAPE (convective available potential energy) scenarios. Perturbing the initial thermodynamic state, which artificially increases the instability of the mid- and upper-troposphere, brought the simulated cloud-top liquid pixel fraction closer to the satellite observations, especially in the warmer mixed-phase temperature range.

Experimental investigation of performance and soot emissions of oxygenated fuel blends in a small aero engine

Recent studies show that mixing jet fuel with oxygenated fuels has an impact on exhaust gas soot formation. Soot particles are an environmental hazard with an impact on air quality around airports, and possibly influencing ice crystal nucleation, leading to contrail and ice cloud (cirrus) formation. These ice clouds significantly warm up the atmosphere by reflecting heat radiation back to Earth and, at the same time, being transparent to incoming sunlight. Many investigations concentrate on reducing aviation’s CO2 footprint, but only a few account for reducing soot emissions in aero engines. This study examines the potential of blending Jet A-1 with oxygenated fuels to decrease soot particle formation in aero engines. For this, blends with 5 vol% and 20 vol% of ethanol, and 5 vol% of a polyoxymethylene dimethyl ether 3–5 mix (OME3-5 mix) are investigated in an Allison 250-C20B turboshaft engine with the help of a condensation particle counter (CPC). The results show tendencies in soot particle reduction, which, in most cases, is larger than the volumetric percentage of the oxygenated fuel within the blend.

Crystal Polymorph Selection Mechanism of Hard Spheres Hidden in the Fluid.

Nucleation plays a critical role in the birth of crystals and is associated with a vast array of phenomena, such as protein crystallization and ice formation in clouds. Despite numerous experimental and theoretical studies, many aspects of the nucleation process, such as the polymorph selection mechanism in the early stages, are far from being understood. Here, we show that the hitherto unexplained excess of particles in a face-centered-cubic (fcc)-like environment, as compared to those in a hexagonal-close-packed (hcp)-like environment, in a crystal nucleus of hard spheres can be explained by the higher order structure in the fluid phase. We show using both simulations and experiments that in the metastable fluid phase, pentagonal bipyramids, clusters with fivefold symmetry known to be inhibitors of crystal nucleation, transform into a different cluster, Siamese dodecahedra. These clusters are closely similar to an fcc subunit, which explains the higher propensity to grow fcc than hcp in hard spheres. We show that our crystallization and polymorph selection mechanism is generic for crystal nucleation from a dense, strongly correlated fluid phase.

Ice-nucleating particles in northern Greenland: annual cycles, biological contribution and parameterizations

Abstract. Ice-nucleating particles (INPs) can initiate ice formation in clouds at temperatures above −38 ∘C through heterogeneous ice nucleation. As a result, INPs affect cloud microphysical and radiative properties, cloud lifetime, and precipitation behavior and thereby ultimately the Earth's climate. Yet, little is known regarding the sources, abundance and properties of INPs, especially in remote regions such as the Arctic. In this study, 2-year-long INP measurements (from July 2018 to September 2020) at Villum Research Station in northern Greenland are presented. A low-volume filter sampler was deployed to collect filter samples for offline INP analysis. An annual cycle of INP concentration (NINP) was observed, and the fraction of heat-labile INPs was found to be higher in months with low to no snow cover and lower in months when the surface was well covered in snow (> 0.8 m). Samples were categorized into three different types based only on the slope of their INP spectra, namely into summer, winter and mix type. For each of the types a temperature-dependent INP parameterization was derived, clearly different depending on the time of the year. Winter and summer types occurred only during their respective seasons and were seen 60 % of the time. The mixed type occurred in the remaining 40 % of the time throughout the year. April, May and November were found to be transition months. A case study comparing April 2019 and April 2020 was performed. The month of April was selected because a significant difference in NINP was observed during these two periods, with clearly higher NINP in April 2020. In parallel to the observed differences in NINP, also a higher cloud-ice fraction was observed in satellite data for April 2020, compared to April 2019. NINP in the case study period revealed no clear dependency on either meteorological parameters or different surface types which were passed by the collected air masses. Overall, the results suggest that the coastal regions of Greenland were the main sources of INPs in April 2019 and 2020, most likely including both local terrestrial and marine sources.

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.

Atmospheric oxidation impact on sea spray produced ice nucleating particles

Ice nucleating particles (INPs) in sea spray aerosol (SSA) are important for ice formation in clouds over oceans. We found that SSA INP concentrations during a phytoplankton bloom were degraded with exposure to 3 to 8 days of atmospheric oxidation.

Observational evidence for the non-suppression effect of atmospheric chemical modification on the ice nucleation activity of East Asian dust

Airborne mineral dust triggers ice formation in clouds and alters cloud microphysical properties by acting as ice-nucleating particles (INPs), potentially influencing weather and climate at regional and global scales. Anthropogenic pollution would modify natural mineral dust during the atmospheric transport process. However, the effects of anthropogenic pollution aging on the ice nucleation activity (INA) of mineral dust remain not well-understood. In this study, we investigated the immersion mode ice nucleation properties and particle chemical characterizations of collected size-resolved Asian dust samples (eight particle size classes ranging from 0.18 to 10.0 μm), and testified the chemical modification of aged dust particles via particle chemistry and morphology analyses including the mass concentrations of particulate matter, the water-soluble ion concentrations, the mental element concentrations, and single-particle morphology. The mass fraction of Ca2+ in element Ca and the mean relative mass proportions of supermicron Ca2+ increased by 67.0 % and 3.5–11.2 % in aged Asian dust particles, respectively, suggesting the occurrence of heterogeneous reactions. On the other hand, the total INP concentrations (total NINP) and total ice nucleation active site densities (total ns(T)) were consistent between aged and normal dust particles (0.62–1.18 times) without a statistically significant difference. And the NINP and ns(T) of chemically aged supermicron dust (1.0–10.0 μm) in each particle size class were nearly equal to or slightly higher than those of normal Asian dust, which were 0.70–2.45 times and 0.64–4.34 times at −18 °C, respectively. These results reveal that anthropogenic air pollution does not notably change the INP concentrations and does not impair the INA of Asian dust. Our work provides direct observational evidence and clarifies the non-suppression effect of anthropogenic air pollution on the INA of East Asian dust, advancing the understanding of the ice nucleation of airborne aged mineral dust.