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Organic matter can initiate heterogeneous ice nucleation in supercooled water droplets, thereby influencing atmospheric cloud glaciation. Predicting the ice nucleation ability of organic matter-containing cloud droplets is challenging due to the unknown mechanism for templating ice. Here, we observed the presence of nanoparticles in aqueous samples of known ice-nucleating biopolymers cellulose and lignin, as well as in newly identified ice-nucleating biopolymers xylan and laminarin. Using our drop Freezing Ice Nuclei Counter (FINC), we measured the median ice nucleation temperature (T50) of xylan and of laminarin droplets of 2 μl to be −14.2 and −20.0 °C, respectively. Next, we characterized these samples using nanoparticle tracking analysis, and we detected and quantified nanoparticles with mean diameters between 132 and 267 nm. Xylan contained the largest nanoparticles and froze at higher temperatures. Xylan also dictated the freezing in a 1:1:1:1 mixture with cellulose, lignin, laminarin, and xylan. Filtration experiments down to 300 kDa with the xylan sample indicated that the presence of nanoparticles triggered freezing. Overall, only samples with mean diameters above 150 nm froze above −20 °C. Furthermore, we determined the ice-active site densities normalized to particle concentrations, surface area, and mass of the nanoparticles to show that the samples' nucleation site densities are similar to sea spray aerosols and nanometer-sized dust. The identification and characterization of xylan and laminarin as nanometer-sized ice-nucleating substances expands the growing list of organic matter capable of impacting cloud formation and thus climate. [ABSTRACT FROM AUTHOR]

Water freezing, initiated by ice nucleation, occurs widely in nature, ranging from cellular to global phenomena. Ice nucleation has been experimentally proven to require the formation of a critical ice nucleus, consistent with classical nucleation theory (CNT). However, the accuracy of CNT quantitative predictions of critical cluster sizes and nucleation rates has never been verified experimentally. In this study, we circumvent this difficulty by using molecular dynamics (MD) simulation. The physical properties of water/ice for CNT predictions, including density, chemical potential difference, and diffusion coefficient, are independently obtained using MD simulation, whereas the calculation of interfacial free energy is based on thermodynamic assumptions of CNT, including capillarity approximation among others. The CNT predictions are compared to the MD evaluations of brute-force simulations and forward flux sampling methods. We find that the CNT and MD predicted critical cluster sizes are consistent, and the CNT predicted nucleation rates are higher than the MD predicted values within three orders of magnitude. We also find that the ice crystallized from supercooled water is stacking-disordered ice with a stacking of cubic and hexagonal ices in four representative types of stacking. The prediction discrepancies in nucleation rate mainly arise from the stacking-disordered ice structure, the asphericity of ice cluster, the uncertainty of ice–water interfacial free energy, and the kinetic attachment rate. Our study establishes a relation between CNT and MD to predict homogeneous ice nucleation. [ABSTRACT FROM AUTHOR]

We use molecular dynamics simulations to examine the homogeneous nucleation of ice VII from metastable liquid water. An unsupervised machine learning classification identifies two distinct local structures composing Ice VII nuclei. The seeding method, combined with the classical nucleation theory (CNT), predicts the solid–liquid interfacial free energy, consistent with the value from the mold integration method. Meanwhile, the nucleation rates estimated from the CNT framework and brute force spontaneous nucleations are inconsistent, and we discuss the reasons for this discrepancy. Structural and dynamical heterogeneities suggest that the potential birthplace for an ice VII embryo is relatively ordered, although not necessarily relatively immobile. Moreover, we demonstrate that without the formation of hydrogen-bond links, ice VII embryos do not grow. [ABSTRACT FROM AUTHOR]

Various aerosols, including mineral dust, soot, and biological particles, can act as ice nuclei, initiating the freezing of supercooled cloud droplets. Cloud droplet freezing significantly impacts cloud properties and, consequently, weather and climate. Some biological ice nuclei exhibit exceptionally high nucleation temperatures close to 0 °C. Ice-nucleating macromolecules (INMs) found on pollen are typically not considered among the most active ice nuclei. Still, they can be highly abundant, especially for species such as Betula pendula, a widespread birch tree species in the boreal forest. Recent studies have shown that certain tree-derived INMs exhibit ice nucleation activity above - 10 °C, suggesting they could play a more significant role in atmospheric processes than previously understood. Our study reveals that three distinct INM classes active at - 8.7, - 15.7, and - 17.4 °C are present in B. pendula. Freeze drying and freeze–thaw cycles noticeably alter their ice nucleation capability, and the results of heat treatment, size, and chemical analysis indicate that INM classes correspond to size-varying aggregates, with larger aggregates nucleating ice at higher temperatures, in agreement with previous studies on fungal and bacterial ice nucleators. Our findings suggest that B. pendula INMs are potentially important for atmospheric ice nucleation because of their high prevalence and nucleation temperatures. [ABSTRACT FROM AUTHOR]

This study presents a robust and durable superhydrophobic photothermal coating featuring water droplets freezing from the outside in developed via a simple one-step spray method using acrylic resin (AR) combined with diatomaceous earth (DME) particles that encapsulate rigid Fe 3 O 4 nanoparticles (DME@Fe 3 O 4). [Display omitted] In low-temperature, high-humidity environments, the condensation of water vapor within microstructures can initiate a detrimental cycle of hydrophobic failure, high-adhesion ice formation, and microstructural degradation, thereby limiting the practical application of superhydrophobic coatings in anti-icing and de-icing technologies. Therefore, enhancing the hydrophobic stability and mechanical durability of these coatings under such conditions is imperative. This study presents a novel approach where rigid Fe 3 O 4 nanoparticles are encapsulated within porous diatomaceous earth (DME) and combined with high-adhesion acrylic resin (AR), resulting in a superhydrophobic photothermal coating that possesses both active and passive de-icing capabilities, fabricated through a straightforward one-step spraying technique. Nanosized Fe 3 O 4 particles, modified for hydrophobicity and smaller than the critical nucleation radius, are densely packed within the DME micropores, forming a micro-nano structured coating with a contact angle of 160.1° and a rolling angle of 2.1°. This dense nanoparticle distribution facilitates preferential nucleation of ice crystal nuclei at the gas–liquid interface, rapidly leading to the formation of a robust and uniform ice shell, which effectively reduces the ice-solid contact area, resulting in loose ice droplets that initiate melting within 18 s. Additionally, the self-removal of condensed droplets from the surface enhances the water repellency and ice-phobic performance in low-temperature and high-humidity environments. The protection afforded by the DME microstructure and the releasable action of Fe 3 O 4 nanoparticles allows the coating to maintain its superhydrophobic characteristics even after exposure to sandpaper abrasion, repeated de-icing, sand impact, and immersion in acid-base solutions. Thus, this robust and durable superhydrophobic photothermal coating, integrating active and passive de-icing functionalities, demonstrates substantial potential for application across various engineering domains. [ABSTRACT FROM AUTHOR]

The temperature for cloud glaciation importantly determines the initialization of precipitation and lifetime of clouds. The role of anthropogenic pollutants as ice nucleating particles (INPs) to determine the cloud glaciation remains uncertain. In this study, based on satellite radar and lidar observations, the clouds either in pure liquid or mixed‐phase with liquid top were statistically analyzed over China during 2006–2019, to obtain the transition freezing temperature (T*) of cloud top where mixed‐phase becomes more frequent than pure water, with further validation by the aircraft in situ measurements. Anthropogenic pollution was observed to raise T* up to −9°C, significantly increasing it by approximately 5°C per unit of aerosol optical depth. The results provide regional‐scale evidence that anthropogenic pollutants act as efficient INPs, increasing the freezing temperature of mixed‐phase clouds. Plain Language Summary: Cloud phase transition (from liquid water to ice) is of great importance to Earth's energy and water cycle. Forming ice crystals in mixed‐phase clouds requires ice nucleating particles (INPs). However, whether the anthropogenic pollution can be efficient INPs remains unclear. In this study, based on satellite and aircraft observations in China, the temperature when clouds start to freeze is estimated by analyzing the frequency of liquid water and glaciated cloud tops, under which the occurrence of glaciated cloud tops started to overtake that of pure liquid water cloud tops. We found that the clouds over the anthropogenically polluted region over China presented higher freezing temperature than other cleaner regions. The results suggest anthropogenic pollution can efficiently control cloud glaciation in regional scale by serving as INPs. Key Points: Transition freezing temperature is estimated by analyzing the frequency of mixed‐phase and liquid clouds over China using satellite dataAnthropogenic pollution can cause freezing temperature of clouds up to −9°CAnthropogenic pollutants can be efficient ice nuclei and increase the freezing temperature of mixed‐phase clouds [ABSTRACT FROM AUTHOR]

The aerosol ice‐nucleus (IN) effect, affecting heterogeneous ice nucleation and ultimately shaping cloud properties and precipitation efficiency, remains poorly understood and is overlooked in most operational numerical weather models. Therefore, this study aims to investigate the role of aerosol IN effect in the "21⋅7" Henan extreme precipitation event using two simulations based on an aerosol‐aware configuration of the WRF model. One prescribed spatially uniform ice nucleation particles (INP) concentration (labeled CTL), whereas the other incorporated MERRA2 aerosol concentration fields to represent a more realistic INP spatial distribution (labeled M2), which is marked by a west‐east gradient with higher INP concentration in the west. Results show that M2 simulated heavier precipitation near Zhengzhou closer to the observation regarding both spatial distribution and intensity than the CTL simulation. This improvement is attributed to an INP‐induced feedback akin to the aerosol lifetime effect. Specifically, the M2 simulation had higher INP concentration in the west, causing increased rates of heterogeneous ice nucleation as the convection progressed from west to east. This released additional latent heat, intensifying the convection and subsequently enhancing homogeneous ice nucleation, yielding more but smaller cloud ice particles in M2. These ice particles had reduced sedimentation speeds and lower aggregation rates, suppressing precipitation upstream of Zhengzhou. The intensified convection, carrying more water, propagated eastward, ultimately resulting in extremely heavy precipitation near Zhengzhou. This study underscores the aerosol IN effect on cloud microphysics, convection intensity, and regional rainfall distribution, elucidating the crucial role of aerosol‐meteorology coupling in accurately simulating moderate deep convective cloud precipitation. Plain Language Summary: Aerosols can directly or indirectly influence the formation of precipitation by serving as cloud condensation nuclei (CCN) and ice nucleation particles (INP). However, current numerical weather predictions often overlook the impact of aerosols, especially for INP. To capture the spatiotemporal distribution and relationship between aerosol fields and extreme precipitation, we coupled the dust aerosol reanalysis from MERRA2 with the WRF model, using realistic INP concentration to simulate the "21⋅7" event in Henan on 20 July 2021. Results show that a realistic INP concentration can improve the precipitation simulation in terms of spatial distribution and intensity. This paper proposes a mechanism of aerosol IN effect similar to the CCN effect. The precipitation was delayed and became heavier near Zhengzhou. This study emphasizes aerosol IN effect on cloud microphysics and precipitation and also elucidates the critical role of aerosol‐meteorological coupling in accurately modeling precipitation from moderate deep convective clouds. Key Points: The aerosol ice‐nucleus effect may play an important role in the "21⋅7" Henan extreme precipitation eventRealistic ice nuclei concentration improves the simulation of precipitation intensity and spatial distribution in deep convective systemHigher ice nuclei concentration may suppress early stage precipitation by forming more ice particles and enhance late‐stage precipitation [ABSTRACT FROM AUTHOR]

Extreme events in the Mediterranean have increased in frequency and intensity during the last decade, and this trend is expected to continue in the future. Characterising the features of extremes using observations and well-established datasets is critical for understanding the processes and development of extreme weather, as well as improving forecast. This study uses observational data collected during the Mediterranean Experiment for Sea Salt And Dust Ice Nuclei (MESSA-DIN) from July to September 2021 to characterise the synoptic analysis of the severe summer of 2021. The analysis focuses primarily on water vapour fluxes, humidity, convective parameters, and the role of aerosols in cloud formation. Furthermore, we compare the findings to the widely known atmospheric reanalysis ERA5, pointing out agreements and inconsistencies with observations, as well as discussing aspects that can improve modelling activities for addressing and forecasting extreme weather, with a focus on the extreme flooding that affected Central Europe in July 2021. The findings show the crucial role of water vapour fluxes in regional climate events, emphasizing the need for high- resolution data from microwave radiometers and atmospheric profilers to verify and improve predictions under complex atmospheric conditions. ERA5 performed rather well in synoptic representation, but it exhibited a dry bias in RH values, which affected the accurate representation of water vapour fluxes. By comparing the ERA5 RH bias with upper-air measurements available during the campaign period at the Potenza GRUAN (GCOS Reference Upper-Air Network) site, in South Italy, the bias was further examined, showing to exhibit an irregular behavior at different sites. The findings also emphasize the need for improving reanalysis model performance in complex terrain conditions, particularly near coastal areas, as well as the use of km-scale models for mesoscale research and dealing with extreme weather. [ABSTRACT FROM AUTHOR]

Ice formation mechanisms in generating cells near stratiform cloud tops, where mixing and entrainment occurs in the presence of supercooled water droplets, remain poorly understood. Supercooled cloud droplet temperature and lifetime may impact heterogeneous ice nucleation through contact and immersion freezing; however, modeling studies normally assume the droplet temperature to be spatially uniform and equal to the ambient temperature. Here, we present a first-of-its-kind quantitative investigation of the temperature and lifetime of evaporating droplets, considering internal thermal gradients within the droplet, as well as thermal and vapor density gradients in the surrounding air. Our approach employs solving Navier–Stokes and continuity equations, coupled with heat and vapor transport, using an advanced numerical model. For typical ranges of cloud droplet sizes and environmental conditions, the droplet internal thermal gradients dissipate quickly (≤ 0.3 s) when droplets are introduced to new subsaturated environments. However, the magnitude of droplet cooling is much greater than estimated from past studies of droplet evaporation, especially for drier environments. For example, for an environment with 500 hPa pressure, and ambient temperature far from the droplet of - 5 °C , the droplet temperature reduction can be as high as 24, 11, and 5 °C for initial ambient relative humidities of 10 %, 40 %, and 70 %, respectively. Droplet lifetimes are found to be tens of seconds longer compared to previous estimates, due to weaker evaporation rates because of lower droplet surface temperatures. Using these new end-of-lifetime droplet temperatures, the enhancement in the activation of ice-nucleating particles predicted by current ice nucleation parameterization schemes is discussed. [ABSTRACT FROM AUTHOR]

The aerosol particles serving as cloud condensation and ice nuclei contribute to key cloud processes associated with cold-air outbreak (CAO) events but are poorly constrained in climate models due to sparse observations. Here we retrieve aerosol number size distribution modes from measurements at Andenes, Norway, during the Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) and at Zeppelin Observatory, approximately 1000 km upwind from Andenes at Svalbard. During CAO events at Andenes, the sea-spray-mode number concentration is correlated with strong over-ocean winds with a mean of 8±4 cm−3 that is 71 % higher than during non-CAO conditions. Additionally, during CAO events at Andenes, the mean Hoppel minimum diameter is 6 nm smaller than during non-CAO conditions, though the estimated supersaturation is lower, and the mean number concentration of particles that likely activated in-cloud is 109±61 cm−3 with no statistically significant difference from the non-CAO mean of 99±66 cm−3. For CAO trajectories between Zeppelin Observatory and Andenes, the upwind-to-downwind change in number concentration is the largest for the accumulation mode with a mean decrease of 93±95 cm−3, likely attributable primarily to precipitation scavenging. These characteristic properties of aerosol number size distributions during CAO events provide guidance for evaluating CAO aerosol–cloud interaction processes in models. [ABSTRACT FROM AUTHOR]

The present study investigates the impact of the electrode surface roughness on the electrofreezing of water. This research focuses on how the electrode microstructure induced by a laser treatment affects the nucleation and growth of ice crystals under controlled electric fields. For this, electrofreezing experiments of deionized water over electrodes with varying surface roughnesses and crystalline textures were conducted. The electrodes of the Al6061 T6 alloy were microstructured via the Laser Shock Processing (LSP) method. For this purpose, the pulse densities during the LSP process were varied (900, 1600, and 2500 pulses/cm2). The increase in pulse density was correlated to the microstructural features and average roughness of the LSP-treated Al6061 alloy. A wave-like microstructure was induced upon the LSP treatment, with roughnesses between 3.5 and 6 µm at the selected pulse densities. The results indicate that electrode roughness significantly influences the electrofreezing process. Rougher electrodes were found to increase the nucleation temperature, suggesting enhanced ice nucleation activity. These findings are attributed to the increased electric field concentration at the asperities of the rough surfaces and the (111) planes of the Al6061 alloy, which may facilitate the alignment of water molecules and the formation of critical ice nuclei. [ABSTRACT FROM AUTHOR]

Homogeneous nucleation is the prominent mechanism of glaciation in cirrus and other high-altitude clouds. Ice nucleation rates can be studied in laboratory assays that gradually lower the temperature of pure water droplets. These experiments can be performed with different cooling rates, with different droplet sizes, and often with a distribution of droplet sizes. We combine nucleation theory, survival probability analysis, and published data on the fraction of frozen droplets as a function of temperature to understand how the cooling rate, droplet size, and size dispersity influence the nucleation rates. The framework, implemented in the Python code AINTBAD (Analysis of Ice nucleation Temperature for B and A Determination), provides a temperature-dependent nucleation rate on a per volume basis, in terms of approximately temperature-independent prefactor (A) and barrier (B) parameters. We find that dispersion in droplet diameters of less than an order of magnitude, if not properly included in the analysis, can cause apparent nucleation barriers to be underestimated by 50 %. This result highlights the importance of droplet size dispersion in efforts to model glaciation in the polydisperse droplets of clouds. We also developed a theoretical framework, implemented in the Python code IPA (Inhomogeneous Poisson Analysis), to predict the fraction of frozen droplets at each temperature for arbitrary droplet size dispersions and cooling rates. Finally, we present a sensitivity analysis for the effect of temperature uncertainty on the nucleation spectrum. Our framework can improve models for ice nucleation in clouds by explicitly accounting for droplet polydispersity and cooling rates. [ABSTRACT FROM AUTHOR]

Aviation soot surrogates processed in contrails are believed to become potent ice nuclei at cirrus temperatures. This is not verified for real aviation soot, which can have vastly different physico-chemical properties. Here, we sampled soot particles from in-use commercial aircraft engines and quantified the effect of contrail processing on their ice nucleation ability at T< 228 K. We show that aviation soot becomes compacted upon contrail processing, but that does not change their ice nucleation ability in contrast to other soot types. The presence of H 2 SO 4 condensed in soot pores, the highly fused nature of the soot primary particles and their arrangement are what limit the volume of pores generated upon contrail processing, in turn limiting sites for ice nucleation. Furthermore, we hypothesized that contrail-processed aviation soot particles emitted from alternative jet fuel would also be poor ice-nucleating particles if their emission sizes remain small (< 150 nm). [ABSTRACT FROM AUTHOR]

Abstract Taklimakan dust is known to be transported over long distances to the Tibetan Plateau, significantly impacting its climates and ecosystems. However, information on how dust aerosols affect airborne microorganisms is still lacking. In the summer of 2019, we simultaneously collected air samples from the Tibetan Plateau (Ali) and the Taklimakan Desert (Minfeng and Tazhong) and analyzed them using high‐throughput DNA sequencing techniques. We found that the diversity of airborne bacteria decreased by 42.2% during dust events. Taklimakan dust contributed 85.4% ± 2.1% of airborne bacteria on the Plateau. Moreover, the potential pathogens and bacterial ice nuclei (IN) in the Plateau increased by 16.4% and 73.8%, respectively. Our study provides novel insights into the impact of dust‐coated bacteria on the climate and ecosystem across the Tibetan Plateau.

We here review mostly experimental and some computational work devoted to nucleation in amorphous ices. In fact, there are only a handful of studies in which nucleation and growth in amorphous ices are investigated as two separate processes. In most studies, crystallization temperatures Tx or crystallization rates RJG are accessed for the combined process. Our Review deals with different amorphous ices, namely, vapor-deposited amorphous solid water (ASW) encountered in many astrophysical environments; hyperquenched glassy water (HGW) produced from μm-droplets of liquid water; and low density amorphous (LDA), high density amorphous (HDA), and very high density amorphous (VHDA) ices produced via pressure-induced amorphization of ice I or from high-pressure polymorphs. We cover the pressure range of up to about 6 GPa and the temperature range of up to 270 K, where only the presence of salts allows for the observation of amorphous ices at such high temperatures. In the case of ASW, its microporosity and very high internal surface to volume ratio are the key factors determining its crystallization kinetics. For HGW, the role of interfaces between individual glassy droplets is crucial but mostly neglected in nucleation or crystallization studies. In the case of LDA, HDA, and VHDA, parallel crystallization kinetics to different ice phases is observed, where the fraction of crystallized ices is controlled by the heating rate. A key aspect here is that in different experiments, amorphous ices of different "purities" are obtained, where "purity" here means the "absence of crystalline nuclei." For this reason, "preseeded amorphous ice" and "nuclei-free amorphous ice" should be distinguished carefully, which has not been done properly in most studies. This makes a direct comparison of results obtained in different laboratories very hard, and even results obtained in the same laboratory are affected by very small changes in the preparation protocol. In terms of mechanism, the results are consistent with amorphous ices turning into an ultraviscous, deeply supercooled liquid prior to nucleation. However, especially in preseeded amorphous ices, crystallization from the preexisting nuclei takes place simultaneously. To separate the time scales of crystallization from the time scale of structure relaxation cleanly, the goal needs to be to produce amorphous ices free from crystalline ice nuclei. Such ices have only been produced in very few studies. [ABSTRACT FROM AUTHOR]

Temperatures and pressures from high explosive detonations far exceed atmospheric conditions in typical combustion reactions, and consequently, detonation soot forms with physiochemical properties distinct from soot formed by combustion. In this study, samples of detonation soot from two high explosives, PBX 9502 and Composition B‐3, were analyzed. Ice nucleation experiments on soot collected after controlled detonations were conducted in the laboratory to probe immersion and contact mode freezing. Samples nucleated ice at temperatures warmer than commercially available nanodiamonds, which has a mean nucleation temperature of −20.7°C. Ice nucleation rate coefficients increase rapidly by two to three orders of magnitude below −20°C for every sample. Size‐selected 137 μm diameter particles produced during detonation in an ambient air atmosphere yield bimodal distributions of freezing temperature with primary and secondary nucleation modes centered at −20°C and −13°C, respectively. The presence of a secondary mode allows for enhanced ice nucleation rate coefficients (one to two orders of magnitude greater than samples without a secondary mode) at temperatures outside the influence of the primary mode (>−17°C). Given the observed onset nucleation temperatures of −9.2°C, our results imply that detonation soot of the type studied here would only need to reach an altitude of approximately 4 km to facilitate ice formation. Plain Language Summary: Tiny suspended particles can modify the earth's energy balance. Some of these particles influence cloud formation through a process called the indirect effect of aerosol. One aspect of the indirect effect involves a subset of aerosol known as ice nucleating particles (INPs) that enhance ice formation in cold regions of the atmosphere. Because of the impact of INPs on climate, it is important to identify the types of aerosols that efficiently form ice. One understudied aerosol is a unique soot with distinct properties that is formed by the extreme temperatures and pressures achieved within high explosive detonations. This detonation soot can be lofted high into the atmosphere where it could impact ice formation. The ice nucleation activity of detonation soot is examined here for the first time. Through experimentation, we measured the freezing temperature of detonation soot originating from two different high explosives, PBX‐9502 and Composition B‐3. Detonation soot does act as an INP with a small portion of efficient particles. Our results indicate that detonation soot lofted into the atmosphere could increase the number of INPs around the globe and promote ice nucleation at warmer temperatures and lower altitudes in regions lacking more efficient INPs. Key Points: High explosive detonation soot particles act as ice nucleating particles with a small percentage activating as warm as −9.2°CDetonations in oxygen‐rich environments generate more effective ice nucleating particles than those generated in oxygen‐poor environmentsCuprous oxide and diamond are moderately effective ice nucleating particles but less effective than the detonation soot samples [ABSTRACT FROM AUTHOR]

Ice nucleating particles (INPs) are rare particles that initiate primary ice formation, a critical step required for subsequent important cloud microphysical processes that ultimately govern cloud phase and cloud radiative properties. Laboratory studies have found that organic‐rich dusts, such as those found in soils, are more efficient INPs compared to mineral dust. However, the atmospheric relevance of these organic‐rich dusts are not well understood, particularly in regions with significant agricultural activity. The Agricultural Ice nuclei at the Southern Great Plains field campaign (AGINSGP) was conducted in rural Oklahoma to investigate how soil dusts contribute to INP populations in the Great Plains. We present chemical characterization of ambient and ice crystal residual particles from a single day of sampling, using single particle mass spectrometry (SPMS) and scanning microscopy. Ambient particles were primarily carbonaceous or secondary aerosol, while the fraction of dust particles was higher in the residual particles. We also observed an unusual particle type consisting of a carbonaceous core mixed with dust fragments on the surface, which was found in higher proportion in residuals. Dust particles measured during residual sampling contained greater proportions of phosphate (63PO2− ${\text{PO}}_{2}^{-}$ and 79PO3− ${\text{PO}}_{3}^{-}$) and lead (206Pb+). Strong sulfate signals were not seen in the residual dust particles measured by the SPMS, while nitrate was slightly depleted relative to ambient dust. This study shows that organic‐rich soils may be important contributors to the ambient INP population in agricultural regions. Plain Language Summary: Ice nucleating particles (INPs) affect climate by causing clouds to freeze at warmer temperatures. However, they are very rare which this means that they are challenging to characterize. Agricultural soils have exhibited high ice nucleation activity in lab studies though their relevance to ambient INP populations is unknown. In this work, we directly measure INP composition in a field campaign at a site in the middle of farmlands to determine whether they are an important source of INPs. We show that dust particles, along with carbonaceous particles mixed with dust, are enriched in INP samples relative to ambient particles. We also show that phosphate (presumed to come from biological fragments) and lead may enhance the IN activity of dust particles. Key Points: Ice nucleating particle composition was directly measured with the ice crystal residual method using two characterization techniquesDust residual particles contained greater phosphate and lead relative to ambient dust particlesCarbonaceous particles internally mixed with dust fragments were found in residual particles measured by SEM/EDX [ABSTRACT FROM AUTHOR]

Ice‐templated porous biomaterials possess transformative potential in regenerative medicine; yet, scaling up ice‐templating processes for broader applications—owing to inconsistent pore formation—remains challenging. This study reports an innovative semi‐solid freeze‐casting technique that draws inspiration from semi‐solid metal processing (SSMP) combined with ice cream‐production routines. This versatile approach allows for the large‐scale assembly of various materials, from polymers to inorganic particles, into isotropic 3D scaffolds featuring uniformly equiaxed pores throughout the centimeter scale. Through (cryo‐)electron microscopy, X‐ray tomography, and finite element modeling, the structural evolution of ice grains/pores is elucidated, demonstrating how the method increases the initial ice nucleus density by pre‐fabricating a semi‐frozen slurry, which facilitates a transition from columnar to equiaxed grain structures. For a practical demonstration, as‐prepared scaffolds are integrated into a bilayer tissue patch using biodegradable waterborne polyurethane (WPU) for large‐scale oral mucosal reconstruction in minipigs. Systematic analyses, including histology and RNA sequencing, prove that the patch modulates the healing process toward near‐scarless mucosal remodeling via innate and adaptive immunomodulation and activation of pro‐healing genes converging on matrix synthesis and epithelialization. This study not only advances the field of ice‐templating fabrication but sets a promising precedent for scaffold‐based large‐scale tissue regeneration. [ABSTRACT FROM AUTHOR]

Pollen can serve as an effective ice‐nuclei (IN), altering cloud microphysical and radiative properties, thus precipitation and cloud life cycles. Here, a nationwide pollen emission inventory with a horizontal resolution of 5 km was established based on a parameterization scheme of mass balance of pollen grain fluxes surrounding the plant crowns, and using satellite observational data sets (including leaf area index and fractional vegetation cover) as well as pollen emission rates. The potential emission is then implemented in RegCM‐pollen model which treated pollen as aerosol tracers. Besides, pollen‐IN parameterization schemes were incorporated in RegCM‐pollen to simulate the interactions between pollen and ice clouds. Investigations show that the mean annual pollen emission in China is 2.65 × 107 grains m−2 yr−1, mainly distributed in the south and northeast of China. The IN magnitude is mainly determined by a combination of temperature and pollen concentration. Notably, an increasing number concentration of pollen grains produces opposite effects in Southern China (SC) and Northern China (NC). The weakened upward motion and vertical transport of water vapor in NC made ice clouds hardly form, resulting in cloud forcing (CF) of +0.86 W/m2. In contrast, it generates a CF of −0.84 W/m2 in SC mainly owing to expanded cloud cover. The changes in shortwave radiative forcing is more significant compared to longwave radiative forcing in the two regions. At the surface, the net radiative forcing in NC is +0.74 W/m2, while it is a −0.51 W/m2 in SC. Among them, downward shortwave radiative forcing is approximately twice that of upward longwave radiative forcing in SC and 1.4 times in NC. Surface temperature shows rising over NC, ranging from 0.05 to 0.25 K. In SC, it is primarily decreasing by −0.12 to −0.03 K. The pollen‐IN effect also causes a decline of precipitation in NC (−0.17 mm/day) and a rise in SC (0.09 mm/day). Our results suggest that the pollen effect on ice clouds is complex, yet significant in understanding its impact on radiation and climate of the atmosphere. Plain Language Summary: Bioaerosols such as pollen are emitted into the atmosphere from the terrestrial biosphere. They can affect cloud formation as ice nuclei, thus influencing radiation and climate. Whereas, up to now little is understood about the ice‐nucleating properties of pollen and how they impact cloud and precipitation formation. In this work, the RegCM‐pollen was utilized to quantitatively estimate vegetation pollen emissions in China and their induced radiative forcing and climate responses. We found that higher airborne pollen is chiefly concentrated in Southern China (SC) and Northern China (NC). The ice nucleation effect of pollen generates contrasting effects in the two areas. A positive effect on cloud forcing (CF) in NC is mainly owing to the inhibition of upward motion and reduced vertical transport of water vapor not conducive to the formation of ice clouds. On the opposite, the negative effect on CF in SC is mostly influenced by the enhanced water vapor, thus, expanding the coverage of the ice cloud. Key Points: Model simulations show that airborne pollen is mainly concentrated in Southern China (SC) and Northern China (NC) with seasonal variationsPollen‐induced IN concentrations decrease with altitude, exhibiting large gradient at a given height, particularly above 7 kmOpposite cloud forcing caused by pollen ice nuclei effect was found in SC (−0.84 W/m2) and NC (+0.86 W/m2) mainly due to water vapor difference in these two regions [ABSTRACT FROM AUTHOR]

The representation of Arctic clouds and their phase distributions, i.e., the amount of ice and supercooled water, influences predictions of future Arctic warming. Therefore, it is essential that cloud phase is correctly captured by models in order to accurately predict the future Arctic climate. Ice crystal formation in clouds happens through ice nucleation (primary ice production) and ice multiplication (secondary ice production). In common weather and climate models, rime splintering is the only secondary ice production process included. In addition, prescribed number concentrations of cloud condensation nuclei or cloud droplets and ice-nucleating particles are often overestimated in Arctic environments by standard model configurations. This can lead to a misrepresentation of the phase distribution and precipitation formation in Arctic mixed-phase clouds, with important implications for the Arctic surface energy budget. During the Ny-Ålesund Aerosol Cloud Experiment (NASCENT), a holographic probe mounted on a tethered balloon took in situ measurements of number and mass concentrations of ice crystals and cloud droplets in Svalbard, Norway, during fall 2019 and spring 2020. In this study, we choose one case study from this campaign that shows evidence of strong secondary ice production and use the Weather Research and Forecasting (WRF) model to simulate it at a high vertical and spatial resolution. We test the performance of different microphysical parametrizations and apply a new state-of-the-art secondary ice parametrization. We find that agreement with observations highly depends on the prescribed cloud condensation nuclei/cloud droplet and ice-nucleating particle concentrations and requires an enhancement of secondary ice production processes. Lowering mass mixing ratio thresholds for rime splintering inside the Morrison microphysics scheme is crucial to enable secondary ice production and thereby match observations for the right reasons. In our case, rime splintering is required to initiate collisional breakup. The simulated contribution from collisional breakup is larger than that from droplet shattering. Simulating ice production correctly for the right reasons is a prerequisite for reliable simulations of Arctic mixed-phase cloud responses to future temperature or aerosol perturbations. [ABSTRACT FROM AUTHOR]

Identifying how aerosol particles interact with atmospheric water is critical to understand climate and precipitation. Ice‐nucleating particles (INP) trigger the formation of atmospheric ice crystals at higher temperatures than pure water. They are difficult to characterize because of their scarce occurrence, and variability, in the atmosphere, especially at temperatures above −20°C. It has been demonstrated that at these temperatures, biological aerosol particles can contribute significantly to INP number concentration. This study incorporates a series of offline, size‐segregated measurements of INPs collected at the Puy de Dôme station (PUY, 1,465 m a.s.l.) over a 6 month period from October to May, covering the transitions from autumn, winter, to spring. These measurements show a general trend of decreasing particle number concentrations during the winter months and higher concentration during autumn and spring. INP concentrations measured in the range of −5 and −18°C, had concentrations of 0.001 INP/Lair at the warmest temperatures, and between 0.01 and 0.1 INP/Lair at the coldest temperatures. The majority of INP measured at temperatures warmer than −15°C were heat labile, suggesting a biological or organic origin. The INP variability was compared with collocated aerosol physical and chemical properties, allowing us to associate highest INP concentrations with local and marine origins. Following these comparisons, we use aerosol total number concentration to develop a new parameterization. In addition, this parameterization is specifically optimized for warmer temperature INP measurements, and demonstrated a good performance when tested on independent data sets. Plain Language Summary: Understanding how tiny particles in the air (aerosol particles) interact with water in the atmosphere is crucial for studying climate and precipitation. Ice Nuclei Particles (INP) play a role in forming ice crystals in the atmosphere, especially at higher temperatures. This is important because it's been found that biological aerosol particles can significantly contribute to INP at temperatures above −20°C. The study conducted offline measurements of INPs over 6 months at the Puy de Dôme station, covering the transition from autumn to spring. Results showed a general trend of fewer particles during winter and more during autumn and spring. The INP concentrations were highest at temperatures between −5 and −18°C, with warmer temperatures having 0.001 INP per liter of air and colder temperatures having 0.01 to 0.1 INP per liter of air. Most INPs at temperatures above −15°C were heat‐sensitive, indicating a biological or organic origin. This study linked high INP concentrations to local and marine sources by comparing them with aerosol properties. A new parameterization based on total aerosol number concentration, specifically optimized for warmer temperatures, performed well when tested on independent data sets. Key Points: Size‐segregated airborne ice‐nucleating particles (INP) concentrations were measured at an altitude station in Central France over 6 monthsSeasonal variations are observed with a minimum in winter. The majority of INPs is biological and activated above −18°CWe developed a parameterization for predicting INP concentrations at warm temperatures based on total aerosol concentrations [ABSTRACT FROM AUTHOR]

A study has been conducted to analyse the impact of an ice sheet on the flexural-gravity wave force acting on a neutrally buoyant, spherical object immersed in a sea in accordance with the linear wave hypothesis. The bottom of the sea is presumed to be protected by a surface composed of some sort of flexibility, and the top is enclosed by an ice sheet. The solution to a diffraction problem is established using the multipole method. The estimated vertical and horizontal flexural-gravity wave forces are visualized in relation to different immersion depths of the spherical body, depths of the fluid and various flexural rigidities of the ice plate and the flexible seabed. The approach adopted here is likely to be of immense significance in the construction of various spherical gadgets like a seabed pressure detector device for tsunami recognition or communication in crisis for a submarine in earthquake zones. [ABSTRACT FROM AUTHOR]

Atmospheric ice nucleation particles (INPs) play a crucial role in influencing cloud formation and microphysical properties, which in turn impact precipitation and Earth's radiation budget. However, the influence of anthropogenic activities on the properties and concentrations of INPs remains an area of significant uncertainty. This study investigated the physical and chemical characteristics of atmospheric ice nucleation particles in Huangshan, China during the May Day labor holiday period (spanning 8 days, from April 27th to May 5th). INP concentrations were measured at temperatures from −17 °C to −26 °C and relative humidities (RHw) from 95% to 101%. Average INP concentrations reached 13.7 L−1 at −26 °C and 101% RH, 137 times higher than at −17 °C and 95% RH. INP concentrations showed exponential increases with decreasing temperature and exponential increases with increasing RH. Concentration fluctuations were observed over time, with a peak of ~30 L−1 (t = −26 °C, RHw = 101%) around the start and end of the holiday period. Aerosol number concentrations were monitored simultaneously. The peak in aerosols larger than 0.5 μm aligned with the peak in INP concentrations, suggesting a link between aerosol levels and INPs. Chemical composition analysis using SEM–EDX revealed the distinct elemental makeup of INPs based on the activation temperature. INPs active at warmer temperatures contained N, Na, and Cl, indicating possible biomass and sea salt origins, while those active at colder temperatures contained crustal elements like Al and Ca. [ABSTRACT FROM AUTHOR]

Heterogeneous nucleation of ice from supercooled water is the process responsible for triggering nearly all ice formation in the natural environment. Understanding of heterogeneous ice nucleation is particularly key for understanding the formation of ice in clouds, which impacts weather and climate. While many effective ice nucleators are known, the mechanisms of their actions remain poorly understood. Some inorganic nucleators have been found to nucleate ice at warmer temperatures in dilute ammonium solution than in pure water. This is surprising, analogous to salty water melting at a warmer temperature than pure water. Here, the magnitude of this effect is rationalized as being due to thermodynamically favorable ammonium-induced disordering of the hydrogen bond network of ice critical clusters formed on inorganic ice nucleators. Theoretical calculations are shown to be consistent with new experimental measurements aimed at finding the maximum magnitude of the effect. The implication of this study is that the ice-nucleating sites and surfaces of many inorganic ice nucleators are either polar or charged and, therefore, tend to induce formation of hydrogen-ordered ice clusters. This work corroborates various literature reports, indicating that some inorganic ice nucleators are most effective when nominally neutral, and implies a commonality in mechanism between a wide range of inorganic ice nucleators. [ABSTRACT FROM AUTHOR]

We investigate the microscopic pathway of spontaneous crystallization in the ST2 model of water under deeply supercooled conditions via unbiased classical molecular dynamics simulations. After quenching below the liquid–liquid critical point, the ST2 model spontaneously separates into low-density liquid (LDL) and high-density liquid phases, respectively. The LDL phase, which is characterized by lower molecular mobility and enhanced structural order, fosters the formation of a sub-critical ice nucleus that, after a stabilization time, develops into the critical nucleus and grows. Polymorphic selection coincides with the development of the sub-critical nucleus and favors the formation of cubic (Ic) over hexagonal (Ih) ice. We rationalize polymorphic selection in terms of geometric arguments based on differences in the symmetry of second neighbor shells of ice Ic and Ih, which are posited to favor formation of the former. The rapidly growing critical nucleus absorbs both Ic and Ih crystallites dispersed in the liquid phase, a crystal with stacking faults. Our results are consistent with, and expand upon, recent observations of non-classical nucleation pathways in several systems. [ABSTRACT FROM AUTHOR]

Suspended dust plays a critical role in regulating the Martian climate by influencing the atmospheric thermal gradient, altering the amount of infrared and visible energy absorbed and scattered by the atmosphere, and acting as condensation nuclei for CO 2 ice-clouds particles. There are many well demonstrated, dust-related effects on martian climate which depend on dustsize, concentration, chemical and bulk composition. Currently, an accurate estimation of these parameters is lacking as they are derived from indirect measurement of the optical depth acquired from the surface and orbital data. These indirect measurements require a priori assumptions on the grain distribution curve and are hence subject to possible biases. To overcome these limitations, an optical particles counter called MicroMED has been developed under the National Institute of Astrophysics (INAF) leadership. MicroMED can measure the sizes of individual dust grains in a specific volume of air thus providing the grain size distribution and concentration. MicroMED has been selected to join the Dust Complex payload on board the ExoMars 2022. The Flight Model of the instrument has been developed and optimized starting from two prototype BreadBoard versions. Here we present the sub-systems that constitute MicroMED and their testing and characterization process, performed using the Breadboard models. We discuss the optimizations and improvements introduced on the basis of the prototype results and the overall performances of the final design. [ABSTRACT FROM AUTHOR]

Various cloud systems responsible for snowfall along the western coast of Japan are formed over the Sea of Japan. In the present study, stratiform snow clouds associated with a mid-level trough were investigated using an instrumented aircraft and dual Doppler radars. The snow clouds exhibited a double-layer structure with thermodynamically and kinematically different characteristics. The top height and base height of the clouds were 4.5 km and 0.9 km at temperatures of -29 °C and -5 °C, respectively. The layer below 2 km mean sea level (MSL) had turbulent air, which reflected its convectively unstable stratification. The maximum updraft exceeded 4 m s-1 at approximately 1 km MSL, and the maximum cloud water content was 0.6 g m-3. The layer above 2 km was less turbulent and characterized by a weak updraft of < 2 m s-1 and maximum cloud water content of 0.1 g m-3. These values were considerably lower than those in the lower layer. The weak updraft was likely caused by an approaching mid-level trough. Ice crystal and precipitation particle concentrations, measured using two-dimensional cloud and precipitation optical array probes, respectively, were almost constant with height and measured several tens of particles L-1 and several particles L-1, respectively. Precipitation particles grew by the seeder-feeder mechanism in the two-layer stratiform snow cloud. In the upper layer (2 - 4.5 km), precipitation particles increased in size by vapor deposition and showed a remarkable broadening of size distributions toward large sizes. In the lower layer (0.9 - 2 km MSL), the precipitation particles grew further via accretion of supercooled cloud droplets and produced denser particles like graupel with no substantial change in the size distribution. Below 0.9 km MSL, particle concentrations decreased at all sizes due to sublimation and melting. [ABSTRACT FROM AUTHOR]

Undesirable icing is likely to pose a threat to the running of devices and personal security; hence, numerous studies are focusing on seeking efficient and low-consumption strategies to hinder the ice formation and accumulation. In this work, a black Ni3S2 coating that integrates superhydrophobicity and photothermal conversion property was developed. Owing to the combination of surface nanorods and myristic acid, the nickel surface was endowed with excellent superhydrophobicity to reduce the contact area between water droplets and the surface. This allowed the water droplets to delay freezing; ultimately the freezing time on the superhydrophobic Ni3S2 coating was extended about 20 times compared to that on the nickel substrate. In addition, while exposed to the sunlight, the Ni3S2 coating could harvest sufficient solar energy and facilitate a surface temperature increase up to 77.06 ℃. This response to sunlight was repeatable, and the generated heat was conducive to reducing the ice melting time. This work is expected to provide a reliable idea for designing and fabricating multifunctional coatings for efficient anti-icing application. [ABSTRACT FROM AUTHOR]

Mixed-phase clouds (MPCs) are key players in the Arctic climate system due to their role in modulating solar and terrestrial radiation. Such radiative interactions rely, among other factors, on the ice content of MPCs, which is regulated by the availability of ice-nucleating particles (INPs). While it appears that INPs are associated with the presence of primary biological aerosol particles (PBAPs) in the Arctic, the nuances of the processes and patterns of INPs and their association with clouds and moisture sources have not been resolved. Here, we investigated for a full year the abundance of and variability in fluorescent PBAPs (fPBAPs) within cloud residuals, directly sampled by a multiparameter bioaerosol spectrometer coupled to a ground-based counterflow virtual impactor inlet at the Zeppelin Observatory (475 m a.s.l.) in Ny-Ålesund, Svalbard. fPBAP concentrations (10 -3 –10 -2 L -1) and contributions to coarse-mode cloud residuals (0.1 to 1 in every 10 3 particles) were found to be close to those expected for high-temperature INPs. Transmission electron microscopy confirmed the presence of PBAPs, most likely bacteria, within one cloud residual sample. Seasonally, our results reveal an elevated presence of fPBAPs within cloud residuals in summer. Parallel water vapor isotope measurements point towards a link between summer clouds and regionally sourced air masses. Low-level MPCs were predominantly observed at the beginning and end of summer, and one explanation for their presence is the existence of high-temperature INPs. In this study, we present direct observational evidence that fPBAPs may play an important role in determining the phase of low-level Arctic clouds. These findings have potential implications for the future description of sources of ice nuclei given ongoing changes in the hydrological and biogeochemical cycles that will influence the PBAP flux in and towards the Arctic. [ABSTRACT FROM AUTHOR]

Supercooled liquid clouds are common at higher latitudes (especially over the Southern Ocean) and are critical for constraining climate projections. We take advantage of the Macquarie Island Cloud and Radiation Experiment (MICRE) to perform an analysis of observed and simulated cloud processes over the Southern Ocean in a region and season dominated by supercooled liquid clouds. Using a single-column version of the European Centre for Medium-range Weather Forecast's (ECMWF) Integrated Forecast System (IFS), we compare two different cloud microphysical schemes to ground based observations of cloud, precipitation and radiation over a 2.5 month period (January 1 – March 17, 2017). Both schemes are able to reproduce aspects of the cloud and radiation observations during MICRE to within the uncertainty of the data, when the thermodynamic profile is prescribed with relaxation. There are differences in water mass and representation of reflectivity between the schemes. A sensitivity study of the cloud microphysics schemes, one a bulk one-moment scheme, and the other a two-moment scheme with prediction of mass and number, indicates that several key processes create differences between the schemes. Surface radiative fluxes and total water path are highly sensitive to the formation and fall speed of precipitation. The prediction of hydrometeor number with the two-moment scheme yields a better comparison with observed reflectivity and radiative fluxes, despite predicting higher liquid water contents than observed. With the two-moment scheme, we are also able to test the sensitivity of the results to the input of liquid Cloud Condensation Nuclei (CCN) and Ice Nuclei (IN). The cloud properties and resulting radiative effects are found to be sensitive to the CCN and IN concentrations. More CCN and IN increase liquid and ice water paths respectively. Thus both the dynamic environment and aerosols, integrated through the cloud microphysics, are important for properly representing Southern Ocean cloud radiative effects. [ABSTRACT FROM AUTHOR]

The aerosol particles that provide cloud condensation and ice nuclei contribute to key cloud processes associated with cold-air outbreak (CAO) events but are poorly constrained in climate models due to sparse observations. Here we retrieve aerosol size distribution modes from measurements at Andenes, Norway during the Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) and at Zeppelin Observatory, approximately 1000 km upwind in Svalbard. During CAO events at Andenes, the sea spray mode number concentration is correlated to strong over-ocean winds with a mean of 8±4 cm-3 that is 71 % higher than during non-CAO conditions. Additionally during CAO events at Andenes, the mean Hoppel minimum diameter is 6 nm smaller than during non-CAO conditions though the estimated supersaturation is lower and the number concentration of particles that likely activated in-cloud is 109±61 cm-3 (similar to non-CAO conditions). For CAO trajectories between Zeppelin Observatory and Andenes, the upwind-to-downwind change in number concentration is largest for the accumulation mode with a mean decrease of 93±95 cm-3, likely attributable primarily to precipitation scavenging. These characteristic properties of aerosol size distributions during CAO events provide guidance for evaluating CAO aerosol-cloud interaction processes in models. [ABSTRACT FROM AUTHOR]

To improve cloud simulation in a general circulation model, we develop a new cloud macrophysics scheme that treats detrained cumulus generated by convective detrainment process separately from pure stratus; prognoses two symmetric triangular probability density functions of total specific humidity for each detrained cumulus and pure stratus; and diagnoses both cloud fraction and cloud condensate in all liquid, ice, and mixed‐phases in a consistent and unified way without any adjustment to remove empty or very dense cloud. Supersaturation is allowed within ice cloud. The new scheme ("NEW") is compared with the previous model ("OLD") using single‐column simulations for subtropical marine stratocumulus (DYCOMS2) and continental deep convection (ARM97) cases. In DYCOMS2, both NEW and OLD produce vertical profiles of grid‐mean cloud condensate similar to large‐eddy simulation. In ARM97, compared with OLD, NEW simulates less sporadic vertical profiles of in‐cloud condensates, due to consistent diagnosis of cloud fraction and cloud condensate; more continuously‐varying detrained cumulus with time, due to prognostic treatment of detrained cumulus; and ice cloud fraction and ice condensate similar to those of OLD, in spite of completely different treatment of ice cloud processes. The global performance of NEW is similar to OLD with improved relative humidity. Compared to OLD, NEW simulates more and improved cloud condensate, but less and degraded cloud fraction, particularly, in the lower troposphere. Detrained cumulus is moister and colder and has a larger moisture variance than pure stratus. Overall, NEW simulates stronger condensation‐deposition rates than OLD, due in part to the separate treatment of detrained cumulus and pure stratus. Plain Language Summary: This study suggests a new cloud parameterization scheme in a general circulation model that can represent clouds in nature in a realistic way. Our new cloud scheme separates two types of stratus, called detrained cumulus and pure stratus, by using a set of prognostic equations. Moreover, both liquid, ice, and mixed‐phase clouds are treated in a consistent way. We found that the new scheme successfully operates for two single‐column simulations for subtropical marine stratocumulus and continental deep convection cases. In addition, we performed the global simulation and found that the new scheme simulated more cloud condensate but less cloud fraction than the old scheme with some improvements. Key Points: We develop a new cloud macrophysics scheme by prognosing two triangular probability density functions for each detrained cumulus and pure stratusThe new scheme computes both cloud fraction and cloud condensate in all liquid, ice, and mixed‐phases in a consistent and unified wayThe new scheme simulates more cloud condensate but less cloud fraction than the old scheme with improved relative humidity and global performance [ABSTRACT FROM AUTHOR]

Airborne particles alter the radiative forcing of climate and have further consequences on air visibility, atmospheric chemistry, and human health. Recent studies reported the existence of highly viscous semisolid and even solid amorphous organic aerosol (OA) particles. Particle viscosity has an impact on the heterogeneous chemistry, gas-particle partitioning, and ice nucleation properties. Consequently, variations in particle viscosity must be considered when predicting the atmospheric impact of OA. Here, we use scanning electron microscopy (SEM) and scanning transmission X-ray microscopy (STXM) to estimate the viscosity of individual particles deposited on substrates based on their characteristic height-to-width ratios, which are affected by changes in morphology upon deposition. The height-to-width ratios obtained from SEM and STXM exhibit a strong correlation, demonstrating that both imaging approaches can be applied separately for viscosity assessment of the substrate-deposited particles. While these metrics are largely qualitative, this method enables rapid assessment of particle viscosity ranges, distinguishing between semisolid (>1010 Pa·s), viscous (104–108 Pa·s), and liquid (10°–101 Pa·s) particles within ensembles of ambient particles collected for microscopy studies. Copyright © 2024 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Besides a well-adapted structure, proteins often require a specific dynamical flexibility to undergo conformational changes in order to carry out their function. The latter dynamics can be directly measured by quasielastic neutron scattering as demonstrated here for three variants of the orange carotenoid protein (OCP), which plays a pivotal role in the protection of the cyanobacterial photosynthetic apparatus against photodamage. We investigate the dynamics of the structurally compact, dark-adapted wild type of OCP (OCPwt) in comparison with that of two mutant forms. The latter two mutants differ preferentially in their structures. The orange mutant OCP-W288A is assumed to have a compact structure and to preferentially bind the pigment echinenone, while the pink mutant OCP-W288A appears to represent the more elongated structure of the red active state of OCP binding the carotenoid canthaxanthin, respectively. The study reveals three major findings: (a) the dynamics of the red active state of OCP is significantly enhanced due to a larger number of protein residues being exposed to the solvent at the surface of the protein; (b) the dynamics of all OCP forms appear to be suppressed upon the freezing of the solvent, which is most likely due to an ice-induced aggregation of the proteins; and (c) the wild type and the compact mutant exhibit different dynamics attributed to a missing H-bond between the pigment and protein, resulting a destabilization of the surrounding protein. [ABSTRACT FROM AUTHOR]

Atmospheric soot and organic particles from fossil fuel combustion and biomass burning modify Earth's climate through their interactions with solar radiation and through modifications of cloud properties by acting as cloud condensation nuclei and ice nucleating particles. Recent advancements in understanding their individual properties and microscopic composition have led to heightened interest in their microphysical properties. This review article provides an overview of current advanced microscopic measurements and offers insights into future avenues for studying microphysical properties of these particles. To quantify soot morphology and ageing, fractal dimension (Df) is a commonly employed quantitative metric which allows to characterize morphologies of soot aggregates and their modifications in relation to ageing factors like internal mixing state, core-shell structures, phase, and composition heterogeneity. Models have been developed to incorporate Df and mixing diversity metrics of aged soot particles, enabling quantitative assessment of their optical absorption and radiative forcing effects. The microphysical properties of soot and organic particles are complex and they are influenced by particle sources, ageing process, and meteorological conditions. Furthermore, soluble organic particles exhibit diverse forms and can engage in liquid–liquid phase separation with sulfate and nitrate components. Primary carbonaceous particles such as tar balls and soot warrant further attention due to their strong light absorbing properties, presence of toxic organic constituents, and small size, which can impact human health. Future research needs include both atmospheric measurements and modeling approaches, focusing on changes in the mixing structures of soot and organic particle ensembles, their effects on climate dynamics and human health. [ABSTRACT FROM AUTHOR]

Ice pellets can form when supercooled raindrops collide with small ice particles that can be generated through secondary ice production processes. The use of atmospheric models that neglect these collisions can lead to an overestimation of freezing rain. The objective of this study is therefore to understand the impacts of collisional freezing and secondary ice production on simulations of ice pellets and freezing rain. We studied the properties of precipitation simulated with the microphysical scheme Predicted Particle Properties (P3) for two distinct secondary ice production processes. Possible improvements to the representation of ice pellets and ice crystals in P3 were analyzed by simulating an ice pellet storm that occurred over eastern Canada in January 2020. Those simulations showed that adding secondary ice production processes increased the accumulation of ice pellets but led to unrealistic size distributions of precipitation particles. Realistic size distributions of ice pellets were obtained by modifying the collection of rain by small ice particles and the merging criteria of ice categories in P3. [ABSTRACT FROM AUTHOR]

The long-range transport of bioaerosols by dust events significantly impacts ecological and meteorological networks of the atmosphere, biosphere, and anthroposphere. Bioaerosols not only cause significant public health risks, but also act as efficient ice nuclei for inducing cloud formation and precipitation in the hydrological cycle. To establish risk management for bioaerosol impacts on the Earth system, a large-scale investigation of bioaerosols must be performed under different environmental conditions. For this purpose, a Dust–Bioaerosol (DuBi) field campaign was conducted to investigate the distribution of bioaerosols by collecting ∼950 samples at 39 sites across East Asia from 2016 to 2021. Concentrations and community structures of bioaerosols were further analyzed using fluorescence microscopic observations and high-throughput DNA sequencing, and these factors were compared to environmental factors, such as PM10 and aridity. The results indicated that microbial concentrations at dryland sites were statistically higher than those at humid sites, while the microbe-to-total-particle ratio was statistically lower in drylands than in humid regions. Microbial cells per microgram of PM10 decreased when PM10 increased. The proportion of airborne particles at each site did not vary substantially with season. The richness and diversity of airborne bacteria were significantly higher in drylands than in semiarid regions, while the community structures were stable among all sampling sites. The DuBi field campaign improves our understanding of bioaerosol characteristic variations along the dust transport pathway in East Asia and the changes of bioaerosols under the trend of climate warming, supporting the efforts to reduce public health risks. SIGNIFICANCE STATEMENT: We first conducted large-scale Dust–Bioaerosol (DuBi) field observations in East Asia from 2016 to 2021. We used unified sampling and analysis methods to collect 950 samples at 39 sites and analyzed bioaerosols' concentration and community structure. This study aims to find the relationship between bioaerosols with atmospheric particles and aridity, which fills gaps in the field. These results improve our understanding of bioaerosols' impacts on ecological and meteorological networks of the atmosphere, biosphere, and anthroposphere. The comprehensive observations will promote prediction of bioaerosol change under the trend of climate warming, supporting the efforts to reduce global climate, ecological security, and public health risks in the future. [ABSTRACT FROM AUTHOR]

Atmospheric ice nucleation plays an important role in modulating the global hydrological cycle and atmospheric radiation balance. To date, few comprehensive field observations of ice nuclei have been carried out at high-altitude sites, which are close to the height of mixed-phase cloud formation. In this study, we measured the concentration of ice-nucleating particles (INPs) in the immersion freezing mode at the summit of the Changbai Mountains (2623 m above sea level), northeast Asia, in summer 2021. The cumulative number concentration of INPs varied from 1.6 × 10 -3 to 78.3 L -1 over the temperature range of - 5.5 to - 29.0 °C. Proteinaceous-based biological materials accounted for the majority of INPs, with the proportion of biological INPs (bio-INPs) exceeding 67 % across the entire freezing-temperature range, with this proportion even exceeding 90 % above - 13.0 °C. At freezing temperatures ranging from - 11.0 to - 8.0 °C, bio-INPs were found to significantly correlate with wind speed (r = 0.5–0.8, p < 0.05) and Ca 2+ (r = 0.6–0.9), and good but not significant correlation was found with isoprene (r = 0.6–0.7) and its oxidation products (isoprene × O 3) (r = 0.7), suggesting that biological aerosols may attach to or mix with soil dust and contribute to INPs. During the daytime, bio-INPs showed a positive correlation with the planetary boundary layer (PBL) height at freezing temperatures ranging from - 22.0 to - 19.5 °C (r > 0.7, p < 0.05), with the valley breezes from southern mountainous regions also influencing the concentration of INPs. Moreover, the long-distance transport of air mass from the Japan Sea and South Korea significantly contributed to the high concentrations of bio-INPs. Our study emphasizes the important role of biological sources of INPs in the high-altitude atmosphere of northeastern Asia and the significant contribution of long-range transport to the INP concentrations in this region. [ABSTRACT FROM AUTHOR]

Ice accumulation on solid surfaces is a severe problem for safety and functioning of a large variety of engineering systems, and its control is an enormous challenge that influences the safety and reliability of many technological applications. The use of molecular dynamics (MD) simulations is popular, but as ice nucleation is a rare event when compared to simulation timescales, the simulations need to be accelerated to force ice to form on a surface, which affects the accuracy and/or applicability of the results obtained. Here, we present an alternative seeded MD simulation approach, which reduces the computational cost while still ensuring accurate simulations of ice growth on surfaces. In addition, this approach enables, for the first time, brute-force all-atom water simulations of ice growth on surfaces unfavorable for nucleation within MD timescales. Using this approach, we investigate the effect of surface wettability and structure on ice growth in the crucial surface–ice interfacial region. Our main findings are that the surface structure can induce a flat or buckled overlayer to form within the liquid, and this transition is mediated by surface wettability. The first overlayer and the bulk ice compete to structure the intermediate water layers between them, the relative influence of which is traced using density heat maps and diffusivity measurements. This work provides new understanding on the role of the surface properties on the structure and dynamics of ice growth, and we also present a useful framework for future research on surface icing simulations. [ABSTRACT FROM AUTHOR]

Aviation soot surrogates processed in contrails are believed to become potent ice nuclei at cirrus temperature. This is not verified for real aviation soot, that can have vastly different physico-chemical properties. Here, we sampled soot particles from in-use commercial aircraft engines and quantified the effect of contrail processing on their ice nucleation ability at T < 228 K. We show that aviation soot becomes compacted upon contrail processing but this does not change their ice nucleation ability in contrast to other soot types. The presence of H2SO4 condensed in soot pores, the highly fused nature of the soot primary particles and their arrangement limit the volume of pores generated upon contrail processing, limiting sites for ice nucleation. Furthermore, we hypothesized that contrail processed aviation soot particles emitted from alternative jet fuel would also be poor ice nucleating particles if their emission sizes remain small (< 150 nm). [ABSTRACT FROM AUTHOR]

Sea spray aerosols (SSAs) represent one of the most abundant aerosol types on a global scale and have been observed at all altitudes including the upper troposphere. SSA has been explored in recent years as a source of ice-nucleating particles (INPs) in cirrus clouds due to the ubiquity of cirrus clouds and the uncertainties in their radiative forcing. This study expands upon previous works on low-temperature ice nucleation of SSA by investigating the effects of atmospheric aging of SSA and the ice-nucleating activity of newly formed secondary marine aerosols (SMAs) using an oxidation flow reactor. Polydisperse aerosol distributions were generated from a marine aerosol reference tank (MART) filled with 120 L of real or artificial seawater and were dried to very low relative humidity to crystallize the salt constituents of SSA prior to their subsequent freezing, which was measured using a continuous flow diffusion chamber (CFDC). Results show that for primary SSA (pSSA), as well as aged SSA and SMA (aSSA + SMA) at temperatures >220 K, homogeneous conditions (92 %–97 % relative humidity with respect to water – RH w) were required to freeze 1 % of the particles. However, below 220 K, heterogeneous nucleation occurs for both pSSA and aSSA + SMA at much lower RH w , where up to 1 % of the aerosol population freezes between 75 % and 80 % RH w. Similarities between freezing behaviors of the pSSA and aSSA + SMA at all temperatures suggest that the contributions of condensed organics onto the pSSA or alteration of functional groups in pSSA via atmospheric aging did not hinder the major heterogeneous ice nucleation process at these cirrus temperatures, which have previously been shown to be dominated by the crystalline salts. Occurrence of a 1 % frozen fraction of SMA, generated in the absence of primary SSA, was observed at or near water saturation below 220 K, suggesting it is not an effective INP at cirrus temperatures, similar to findings in the literature on other organic aerosols. Thus, any SMA coatings on the pSSA would only decrease the ice nucleation behavior of pSSA if the organic components were able to significantly delay water uptake of the inorganic salts, and apparently this was not the case. Results from this study demonstrate the ability of lofted primary sea spray particles to remain an effective ice nucleator at cirrus temperatures, even after atmospheric aging has occurred over a period of days in the marine boundary layer prior to lofting. We were not able to address aging processes under upper-tropospheric conditions. [ABSTRACT FROM AUTHOR]

Zygnematophyceae, a class of freshwater green algae, exhibit distinctive seasonal dynamics. The increasing frequency of cold snaps during the growing season might challenge the persistence of some populations. The present study explored the frost hardiness of two Mougeotia species isolated from different elevations and habitats. Additionally, a phylogenetic (rbcL sequence), ultrastructural and physiological characterization was performed. Both species, grown under standard culture conditions and cold acclimated cultures (+4°C), were exposed to freezing temperatures down to −9°C. Furthermore, ultrastructural‐, hydrogen peroxide (H2O2)‐ and photosynthetic pigment analysis were performed on cells exposed to −2°C, with and without induced ice nucleation. The alpine M. disjuncta showed a higher frost hardiness (LT50 = −5.8°C), whereas the lowland M. scalaris was susceptible to ice. However, frost hardiness did not improve after cold acclimation in either species but rather decreased significantly in M. disjuncta (LT50 = −4.7°C). Despite darkness, prolonged sub‐zero temperatures or freezing induced the activation of the xanthophyll (VAZ) cycle in M. scalaris. Our results demonstrate that frost hardiness varies within the genus Mougeotia and that the VAZ cycle can be activated in the dark under subzero temperature‐ and freezing stress but does not necessarily increase frost hardiness. As highly frost hardy cell types are usually formed at the end of the growing season, the ability of young cells to survive ice formation in the upper subzero temperature range represents a crucial survival strategy in populations exposed to late spring frosts. [ABSTRACT FROM AUTHOR]

Anthropogenic (industrial) snowfalls in the UK have hitherto been regarded as rare and extremely localised. This article analyses the distribution and possible causes of three 'stripes' of anthropogenic snow extending up to 10 km from the vicinity of London's Heathrow airport on 23 January 2023. The early morning had been exceptionally cold at the airport, partly due to the relatively late arrival of freezing fog compared to other parts of the lower Thames valley. This is likely to have facilitated the formation of the snowfall by means of ice nuclei. [ABSTRACT FROM AUTHOR]

The supersaturation equation for a vertically moving adiabatic cloud parcel is analyzed. The effects of turbulent updrafts are incorporated in the shape of a stochastic Lagrangian model, with spatial and time correlations expressed in terms of turbulent kinetic energy. Using the Fokker–Planck equation, the steady-state probability distributions of supersaturation are analytically computed for a number of approximations involving the time-scale separation between updraft fluctuations and phase relaxation, and droplet or ice particle size fluctuations. While the analytical results are presented in general for single-phase clouds, the calculated distributions are used to compute mixed-phase cloud properties—mixed fraction and mean liquid water content in an initially icy cloud—and are argued to be useful for generalizing and constructing new parameterization schemes. Significance Statement: Supersaturation is the fuel for the development of clouds in the atmosphere. In this paper, our goal is to better understand the supersaturation budget of clouds embedded in a turbulent environment by analyzing the basic equations of cloud microphysics. It is found that the turbulent characteristics of an air parcel substantially affect the cloud's supersaturation budget and hence its life cycle. This is also shown in the context of mixed-phase clouds where, depending on the turbulent regime, different liquid-to-ice ratios are found. Consequently, the theoretical approach of this paper is crucial to develop tools to parameterize small-scale atmospheric features, like clouds, into global circulation models to improve climate projections for the future. [ABSTRACT FROM AUTHOR]

We report the highest altitude detection of water vapor on Mars to date. The daytime limb observations by the Imaging Ultraviolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft are of hydroxyl (OH) prompt emission near 308 nm, which is excited directly from the photodissociation of water vapor by the solar Lyman‐α flux. Average IUVS daytime water vapor densities near 130 km are 3 × 107 cm−3 around perihelion. The water vapor densities diurnally vary with a peak near midday and no detection at sunrise and sunset. To evaluate the large daytime water vapor densities for self‐consistency, we also report the simultaneous observation of OH solar fluorescence emission near 308 nm in the thermosphere, which enables the retrieval of OH densities. Using a one‐dimensional photochemical model initialized with the daytime IUVS water vapor densities, modeled peak OH densities are in good agreement with the observed IUVS peak OH densities. Because the observed thermospheric temperatures are controlled by solar insolation and cross the water frost point during the day, we suggest that the IUVS observed water vapor is created by the daily sublimation of water ice particles supplied from below. We discuss the implications of the IUVS observations on the present day loss of water vapor from Mars in the form of atomic hydrogen. Plain Language Summary: The loss of water from Mars is a compelling topic of study that helps trace its transformation from a warm and wet planet to the cold and dry planet that exists today. Water loss is greatly facilitated by water vapor transport to altitudes above 100 km during the dust storm season on Mars. We report the highest altitude detection of water vapor on Mars to date. Daytime observations between 110 and 150 km altitude indicate a strong diurnal variation of water vapor there, with a peak near midday. The observations are made by the Imaging Ultraviolet Spectrograph (IUVS) on NASA's Mars Atmosphere and Volatile Evolution spacecraft currently orbiting Mars. Simultaneous IUVS observations of hydroxyl, a product of water vapor destruction by solar ultraviolet radiation, reveal a self‐consistent picture of daytime water vapor variability. We suggest that the water is lofted to these high altitudes from below as ice particles before they reach altitudes where the temperatures are high enough to release the water in the vapor phase. The IUVS daytime water vapor densities in the upper atmosphere indicate that its escape to space is more efficient than previously thought. Key Points: OH prompt emission observations provide the highest altitude detection of water vapor on Mars to dateThermospheric water vapor concentrations are diurnally dependent, with a peak near midday at perihelionDiurnal temperature variations are consistent with nighttime sequestration of water as ice, followed by its daytime sublimation to vapor [ABSTRACT FROM AUTHOR]

Quantifying surface–atmosphere exchange rates of particles is important for understanding the role of suspended particulate matter in radiative transfer, clouds, precipitation, and climate change. Emissions of coarse-mode particles with a diameter greater than 0.5 µm provide giant cloud condensation nuclei and ice nuclei. These emissions are critical for understanding the evolution of cloud microphysical properties yet remain poorly understood. Here we introduce a new method that uses lidar retrievals of the elastic backscatter and Doppler velocity to obtain surface number emissions of particles with a diameter greater than 0.53 µm. The technique is applied to study particle number fluxes over a 2-month period from 1 June to 10 August 2022 during the TRACER campaign at an urban site near Houston, TX, USA. We found that all the observed fluxes were positive (upwards), indicating particle emission from the surface. The fluxes followed a diurnal pattern and peaked near noon local time. Flux intensity varied through the 2 months with multi-day periods of strong fluxes and multi-day periods of weak fluxes. Emission particle number fluxes peaked near ∼ 100 cm -2 s -1. The daily averaged emission fluxes correlated with friction velocity and were anticorrelated with surface relative humidity. The emission flux can be parameterized as F= 3000 u*4 , where u* is the friction velocity in m s -1 and the emission flux F is in cm -2 s -1. The u* dependence is consistent with emission from wind-driven erosion. Estimated values for the mass flux are in the lower range of literature values from non-urban sites. These results demonstrate that urban environments may play an important role in supplying coarse-mode particles to the boundary layer. We anticipate that quantification of these emissions will help constrain aerosol–cloud interaction models that use prognostic aerosol schemes. [ABSTRACT FROM AUTHOR]

The Arctic is the fastest warming environment on Earth and the role of clouds in this warming is undisputed. The representation of Arctic clouds and their phase distribution, i.e. the amount of ice and supercooled water, influences predictions of future Arctic warming. Therefore, it is essential that cloud phase is correctly captured by models in order to accurately predict the future Arctic climate. Ice crystal formation in clouds happens through ice nucleation (primary ice production) and ice multiplication (secondary ice production). In common weather and climate models, rime-splintering is the only secondary ice production process included. In addition, prescribed number concentrations of cloud condensation nuclei or cloud droplets and ice-nucleating particles are often overestimated in Arctic environments by standard model configurations. This can lead to a misrepresentation of the phase distribution and precipitation formation in Arctic mixed-phase clouds, with important implications for the Arctic surface energy budget. During the Ny-Ålesund Aerosol Cloud Experiment (NASCENT) a holographic probe mounted on a tethered balloon took in-situ measurements of ice crystal and cloud droplet number and mass concentrations in Svalbard, Norway, during fall 2019 and spring 2020. In this study, we choose one case study from this campaign showing evidence of strong secondary ice production and use the Weather Research and Forecasting (WRF) model to simulate it at a high vertical and spatial resolution. We test the performance of different microphysical parametrizations and apply a new state-of-the-art secondary ice parametrization. We find that the agreement with observations highly depends on the prescribed cloud condensation nuclei/cloud droplet and ice-nucleating particle concentration and requires an enhancement of secondary ice production processes. Lowering mass mixing ratio thresholds for rime splintering inside the Morrison microphysics scheme is crucial for enabling secondary ice production and thereby matching observations for the right reasons. The latter is a prerequisite for reliable simulations of Arctic mixed-phase cloud responses to future temperature- or aerosol perturbations. [ABSTRACT FROM AUTHOR]

A significant share of aviation's climate impact is due to persistent contrails. Thus, avoiding the creation of contrails that exert a warming impact is a crucial step in approaching the goal of sustainable air transportation. For this purpose, a reliable forecast of when and where persistent contrails are expected to form is needed (i.e. a reliable prediction of ice supersaturation). With such a forecast at hand, it would be possible to plan aircraft routes on which the formation of persistent contrails can be avoided. One problem on the way to these forecasts is the current systematic underestimation of the frequency and degree of ice supersaturation at cruise altitudes in numerical weather prediction due to the practice of "saturation adjustment". In this common parameterisation, the air inside cirrus clouds is assumed to be exactly at ice saturation, while measurement studies have found cirrus clouds to be quite often out of equilibrium. In this study, we propose a new ice-cloud scheme that overcomes saturation adjustment by explicitly modelling the decay of the in-cloud humidity after nucleation, thereby allowing for both in-cloud super- and subsaturation. To achieve this, we introduce the in-cloud humidity as a new prognostic variable and derive the humidity distribution in newly generated cloud parts from a stochastic box model that divides a model grid box into a large number of air parcels and treats them individually. The new scheme is then tested against a parameterisation that uses saturation adjustment, where the stochastic box model serves as a benchmark. It is shown that saturation adjustment underestimates humidity, both shortly after nucleation, when the actual cloud is still highly supersaturated, and also in aged cirrus if the temperature keeps decreasing, as the actual cloud remains in a slightly supersaturated state in this case. The new parameterisation, on the other hand, closely follows the behaviour of the stochastic box model in any considered case. The improvement in comparison with saturation adjustment is largest if slow updraughts occur in relatively clean air in models with a high spatial and temporal resolution. We conclude that our parameterisation is promising but needs further testing in more realistic frameworks. [ABSTRACT FROM AUTHOR]

Microplastic pollution is occurring in most ecosystem, yet their presence in high altitude clouds and their influence on cloud formation and climate change are poorly known. Here we analyzed microplastics in cloud water sampled at the summits of Japan mountains at 1300–3776 m altitude by attenuated total reflection imaging and micro-Fourier transform infrared spectroscopy. We observed nine microplastics including polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyamide 6, polycarbonate, ethylene–propylene copolymer or polyethylene–polypropylene alloy, polyurethane, and epoxy resin. Microplastic were fragmented, with mean concentrations ranging from 6.7 to 13.9 pieces per liter, and with Feret diameters ranging from 7.1 to 94.6 μm. Microplastics bearing hydrophilic groups such as carbonyl and/or hydroxyl groups were abundant, suggesting that they might have acted as condensation nuclei of cloud ice and water. Overall, our finding suggest that high-altitude microplastics cloud influence cloud formation and, in turn, might modify the climate. [ABSTRACT FROM AUTHOR]