Efficient Ice Nuclei Research Articles

Observational evidence of the association between aerosol properties and lightning characteristics in the Indian subcontinent

Abstract Lightning is a leading cause of natural disaster-related mortality in the Indian Subcontinent. Currently, there is a lack of observational evidence supporting the role of aerosol composition in modulating lightning intensity and trends in this region. In this study, we analyzed satellite and reanalysis datasets to examine the association between aerosol properties and lightning intensity over three contrasting geographic regions of Indian subcontinent during the premonsoon season (1998-2022). By comparing lightning and non-lightning days, we found a 0.1 unit increase in total aerosol optical depth specifically on lightning days in all three study areas. Additionally, we observed that the aerosol composition of a region influences convective activity. The presence of dust alongside fine aerosols acts as efficient ice nuclei, thereby stimulating lightning activity in all three regions. Therefore, considering aerosol composition together with local meteorology is essential for accurately predicting lightning flash rates in a region.

Large-Scale Dust–Bioaerosol Field Observations in East Asia

Abstract 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.

On the correlation between hygroscopic properties and chemical composition of cloud condensation nuclei obtained from the chemical aging of soot particles with O3 and SO2

Soot particles released in the atmosphere have long been investigated for their ability to affect the radiative forcing. Although freshly emitted soot particles are generally considered to yield only positive contributions to the radiative forcing, atmospheric aging can activate them into efficient cloud condensation or ice nuclei, which can trigger the formation of persistent clouds and ultimately provide a negative contribution to the radiative forcing. Depending on their residence time in the atmosphere, soot particles can undergo several physical and chemical aging processes that affect their chemical composition, particle size distribution and morphology, and ultimately their optical and hygroscopic properties. The impact of the physical-chemical aging on the properties of soot particles is still difficult to quantify, as well as their effect on the radiative forcing of the atmosphere.This work investigates the hygroscopic properties of chemically aged soot particles obtained from the combustion of aviation fuel, and in particular the interplay between aging mechanisms initiated by two widespread atmospheric oxidizers (O3 and SO2). Activation is measured in water supersaturation conditions using a cloud condensation nuclei counter. Once particle morphology and size distribution are taken into account, the hygroscopicity parameter κ is derived using κ-Köhler theory and correlated to the change of the chemical composition of the particles aged in a simulation chamber. While fresh soot particles are poor cloud condensation nuclei (κ < 10−4) and are not significantly affected by either O3 or SO2 at the timescale of the experiments, rapid activation is observed when they are simultaneously exposed to both oxidizers. Activated particles become efficient cloud condensation nuclei, comparable to the highly hygroscopic particulate matter typically found in the atmosphere (κ = 0.2–0.6 at RH = 20 %). Statistical analysis reveals a correlation between the activation and sulfur-containing ions detected on the chemically aged particles that are absent from the fresh particles.

Dust effects on mixed-phase clouds and precipitation during a super dust storm over northern China

Dust aerosols have been generally regarded as efficient ice nuclei (IN), but their influences on mixed-phase clouds and precipitation are poorly quantified. In this study, combining satellite observations, reanalysis data and the Weather Research and Forecasting (WRF) model, we investigate the impacts of dust aerosols on mixed-phase clouds and precipitation during a super dust storm over northern China. The Suomi National Polar-Orbiting Partnership (S-NPP) satellite observed a super dust storm from March 15–19, 2021 in northern China, followed by heavy precipitation between March 17 and 19. The results showed that the occurrence of super dust storms may play an important role in cloud formation and precipitation processes. Furthermore, numerical modeling revealed that dust aerosols can increase the ice crystal number concentration (QNi) and decrease the cloud droplet number concentration (QNc) in mixed-phase clouds through the Wegener–Bergeron–Findeisen (WBF) process, in which the maximum increase in QNi can reach 21%. Simultaneously, the mass mixing ratios of rain, graupel and snow increased due to dust aerosols. Consequently, precipitation could increase by up to 9.8% in northern China. This study could provide evidence for understanding the mechanisms of dust effects on mixed-phase clouds over northern China.

When Do Subpollen Particles Become Relevant for Ice Nucleation Processes in Clouds?

AbstractWhen exposed to sufficiently humid environments, pollen grains burst and release large quantities of small subpollen particles (SPPs) which carry ice nucleating macromolecules. In this study, for the first time we develop a physically based parameterization describing the bursting process of pollen by applying a turgor pressure parameterization and quantify the impact SPPs have on overall ice nucleation in clouds. SPPs are generated from simulated birch pollen emissions over Europe for a 10‐day case study in spring. We found SPP concentrations to surpass pollen grain concentrations by 4–6 orders of magnitude leading to an abundance of biological ice nuclei from SPPs in the range of 103−104 m−3. However, it is found that these concentrations lead to only small changes in hydrometeor number densities and precipitation. Addressing the question when SPPs become relevant for ice nucleation in clouds, we conducted a sensitivity investigation. We find that amplifying ice nucleation efficiency of biological particles by factors greater 100 increases the ice particle numbers by up to 25% (T ≈ 268 K). Strong reductions show in cloud droplet number concentration and water vapor at these temperatures while water vapor is increasing at 600 m. Overall, we found a net reduction of water in the atmosphere as liquid and particularly water vapor density is reduced, while frozen water mass density increases above 257 K. Findings indicate an alteration of mixed‐phase cloud composition and increased precipitation (up to 6.2%) when SPPs are considered as highly efficient biological ice nuclei.

Technical note: Identification of two ice-nucleating regimes for dust-related cirrus clouds based on the relationship between number concentrations of ice-nucleating particles and ice crystals

Abstract. Large amounts of dust aerosols are lifted to the upper troposphere every year and play a major role in cirrus formation by acting as efficient ice nuclei. However, the relative importance of heterogeneous nucleation and spontaneous homogenous nucleation in dust-related cirrus clouds is still not well evaluated globally. Here, based on spaceborne observations, we propose a method to identify two ice-nucleating regimes of dust-related cirrus clouds, i.e., (1) the sole presence of heterogeneous nucleation and (2) competition between heterogeneous and homogeneous nucleation, by characterizing the relationship between dust ice-nucleating particle concentrations (INPCs) calculated from the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) using the POlarization LIdar PHOtometer Networking (POLIPHON) method and in-cloud ice crystal number concentration (ICNC) from the DARDAR (lidar–radar) dataset. Two typical cirrus cases over central China are shown as a demonstration. In the first case, the upper part (near the cloud top) of a series of cirrus clouds successfully realized the INPC–ICNC closure, meaning that solely heterogeneous nucleation takes place, while the lower part of cirrus clouds showed the possible competition between heterogeneous and homogeneous nucleation. In the second case, the ICNCs in the cirrus cloud dramatically exceeded the dust INPCs in the vicinity by more than an order of magnitude, revealing that besides dust-induced heterogeneous nucleation, homogeneous nucleation also participated in ice formation and produced additional ice crystals. The proposed identification method is anticipated to apply in the evaluation of the influence of upper-troposphere dust on global cirrus formation and the investigation of the potential positive role of cirrus cloud thinning in the offset of climate warming.

Upper troposphere dust belt formation processes vary seasonally and spatially in the Northern Hemisphere

Dust aerosols impact global energy balance substantially by acting as efficient ice nuclei to alter cold cloud properties. However, the estimate of dust indirect effect remains uncertain due to simulating dust distributions poorly and lacking reliable dust observations, especially in the upper-troposphere. Here, we characterize and understand upper-troposphere dust sources and transport with an improved dust dataset derived from A-train satellite lidar and radar measurements and an air parcel trajectory model. The distinct upper-troposphere dust belt over the northern hemisphere has seasonally varying base and top heights of 3.65 ± 2.84 and 8.35 ± 1.50 km above mean sea level and its column loading is strongest during spring (March-April-May). The out-of-phase annual cycles of mid-level dust concentration and westerly wind over source regions control the seasonal upper-tropospheric dust loading variations. African deserts contribute the most (46.3%) to the upper-troposphere dust belt in spring and the synoptic trough is the leading (49%) dust lifting mechanism.

Multi-thermals and high concentrations of secondary ice: a modelling study of convective clouds during the Ice in Clouds Experiment – Dust (ICE-D) campaign

Abstract. This paper examines the mechanisms responsible for the production of ice in convective clouds influenced by mineral dust. Observations were made in the Ice in Clouds Experiment – Dust (ICE-D) field campaign which took place in the vicinity of Cape Verde during August 2015. Measurements made with instruments on the Facility for Airborne Atmospheric Measurements (FAAM) aircraft through the clouds on 21 August showed that ice particles were observed in high concentrations at temperatures greater than about −8 ∘C. Sensitivity studies were performed using existing parameterization schemes in a cloud model to explore the impact of the freezing onset temperature, the efficiency of freezing, mineral dust as efficient ice nuclei, and multi-thermals on secondary ice production by the rime-splintering process. The simulation with the default Morrison microphysics scheme (Morrison et al., 2005) that involved a single thermal produced a concentration of secondary ice that was much lower than the observed value of total ice number concentration. Relaxing the onset temperature to a higher value, enhancing the freezing efficiency, or combinations of these increased the secondary ice particle concentration but not by a sufficient amount. Simulations that involved only dust particles as ice-nucleating particles produced a lower concentration of secondary ice particles, since the freezing onset temperature is low. The simulations implicate that a higher concentration of ice-nucleating particles with a higher freezing onset temperature may explain some of the observed high concentrations of secondary ice. However, a simulation with two thermals that used the original Morrison scheme without enhancement of the freezing efficiency or relaxation of the onset temperature produced the greatest concentration of secondary ice particles. It did so because of the increased time that graupel particles were exposed to significant cloud liquid water in the Hallett–Mossop temperature zone. The forward-facing camera and measurements of the vertical wind in repeated passes of the same cloud suggested that these tropical clouds contained multiple thermals. It is possible of course that several mechanisms, some of them only recently discovered, may be responsible for producing the ice particles in clouds. This study highlights the fact that the dynamics of the clouds likely play an important role in producing high concentrations of secondary ice particles in clouds.

Bioaerosols and atmospheric ice nuclei in a Mediterranean dryland: community changes related to rainfall

Abstract. Certain biological particles are highly efficient ice nuclei (IN), but the actual contribution of bioparticles to the pool of atmospheric IN and their relation to precipitation are not well characterized. We investigated the composition of bioaerosols, ice nucleation activity, and the effect of rainfall by metagenomic sequencing and freezing experiments of aerosol samples collected during the INUIT 2016 campaign in a rural dryland on the eastern Mediterranean island of Cyprus. Taxonomic analysis showed community changes related to rainfall. For the rain-affected samples, we found higher read proportions of fungi, particularly of Agaricomycetes, which are a class of fungi that actively discharge their spores into the atmosphere in response to humidity changes. In contrast, the read proportions of bacteria were reduced, indicating an effective removal of bacteria by precipitation. Freezing experiments showed that the IN population in the investigated samples was influenced by both rainfall and dust events. For example, filtration and heat treatment of the samples collected during and immediately after rainfall yielded enhanced fractions of heat-sensitive IN in the size ranges larger than 5 µm and smaller than 0.1 µm, which were likely of biological origin (entire bioparticles and soluble macromolecular bio-IN). In contrast, samples collected in periods with dust events were dominated by heat-resistant IN active at lower temperatures, most likely mineral dust. The DNA analysis revealed low numbers of reads related to microorganisms that are known to be IN-active. This may reflect unknown sources of atmospheric bio-IN as well as the presence of cell-free IN macromolecules that do not contain DNA, in particular for sizes &lt; 0.1 µm. The observed effects of rainfall on the composition of atmospheric bioaerosols and IN may influence the hydrological cycle (bioprecipitation cycle) as well as the health effects of air particulate matter (pathogens, allergens).

Biological Ice Nucleators in Snow Samples from Greece

The formation of precipitation in clouds is initiated by inorganic and organic/biological ice nuclei. Certain species of bacteria and fungi are known to act as efficient biological ice nuclei at temperatures between −10 and 0 °C. Biological ice nuclei have been found and characterized in precipitation samples (snow, rain, and hail). We investigated the presence of warm temperature biological ice nuclei in 17 fresh snow samples from Greece and isolated and partially characterized ice nucleation active bacteria from these. All snow samples contained particles or other material active as ice nuclei at −9 °C in concentrations ranging from 3 to 943 nuclei/L. The numbers of this class of ice nuclei were reduced or eliminated after incubating snowmelt concentrates at 100 °C for 15 min and by treatment with lysozyme, a bacterial cell wall-degrading enzyme. These findings indicate the presence of microbial ice nuclei in snow samples from Greece. We also isolated ice nucleation active bacteria from some of the samples. These bacteria belong to genus Pseudomonas and are common on plants and soil. This is the first report on biological ice nuclei in precipitation samples from Greece.

Cloud icing by mineral dust and impacts to aviation safety

Ice particles in high-altitude cold clouds can obstruct aircraft functioning. Over the last 20 years, there have been more than 150 recorded cases with engine power-loss and damage caused by tiny cloud ice crystals, which are difficult to detect with aircraft radars. Herein, we examine two aircraft accidents for which icing linked to convective weather conditions has been officially reported as the most likely reason for catastrophic consequences. We analyze whether desert mineral dust, known to be very efficient ice nuclei and present along both aircraft routes, could further augment the icing process. Using numerical simulations performed by a coupled atmosphere-dust model with an included parameterization for ice nucleation triggered by dust aerosols, we show that the predicted ice particle number sharply increases at approximate locations and times of accidents where desert dust was brought by convective circulation to the upper troposphere. We propose a new icing parameter which, unlike existing icing indices, for the first time includes in its calculation the predicted dust concentration. This study opens up the opportunity to use integrated atmospheric-dust forecasts as warnings for ice formation enhanced by mineral dust presence.

Sensitivities to biological aerosol particle properties and ageing processes: potential implications for aerosol–cloud interactions and optical properties

Abstract. Primary biological aerosol particles (PBAPs), such as bacteria, viruses, fungi, and pollen, represent a small fraction of the total aerosol burden. Based on process model studies, we identify trends in the relative importance of PBAP properties, e.g., number concentration, diameter, hygroscopicity, surface tension, and contact angle, for their aerosol–cloud interactions and optical properties. While the number concentration of PBAPs likely does not affect total cloud condensation nuclei (CCN) concentrations globally, small changes in the hygroscopicity of submicron PBAPs might affect their CCN ability and thus their inclusion into clouds. Given that PBAPs are highly efficient atmospheric ice nuclei (IN) at T &gt; −10 ∘C, we suggest that small changes in their sizes or surface properties due to chemical, physical, or biological processing might translate into large impacts on ice initiation in clouds. Predicted differences in the direct interaction of PBAPs with radiation can be equally large between different species of the same PBAP type and among different PBAP types. Our study shows that not only variability of PBAP types but also their physical, chemical, and biological ageing processes might alter their CCN and IN activities to affect their aerosol–cloud interactions and optical properties. While these properties and processes likely affect radiative forcing only on small spatial and temporal scales, we highlight their potential importance for PBAP survival, dispersion, and transport in the atmosphere.

Concentration and variability of deposition-mode ice nucleating particles from Mt. Tai of China in the early summer

The concentration and chemical composition of aerosol particles were measured and sampled at two altitudes of Mt. Tai, North China Plain in May and June 2017. Concentrations of ice nucleating particles (INP) were analyzed with a static vacuum water vapor diffusion chamber (FRIDGE-NUIST) in deposition mode after being sampled using an electrostatic aerosol collector. The average INP concentration (1.19 to 3.45 L−1) at Mt. Tai matches with the same measured at Tai’ An (foot of the Mt. Tai, 1.57–3.49 L−1, at RHw = 95%, 97%, 99% and t = −20 °C). Correlations between concentrations of aerosol particles and INP were below 0.5 and less than simultaneous measurements made at Tai'An. Clear diurnal variations at the height of the planetary boundary layer (PBL) resulted in changes in aerosol and INP concentrations at the top of the mountain. We found that EC-Aged (aged elemental carbon) and mineral type particles are clearly associated with ice nuclei when the height of the PBL exceeds the altitude of the mountaintop. Furthermore, we found significant correlations between INP concentrations and concentrations of OC (organic carbon) and NaK (sodium/potassium salt) type aerosol particles at night (in the free troposphere). Ice nucleation active surface site densities are higher at night (22:00 LST, local standard time) than during the day (11:00 LST) due to the change in aerosol characteristics. This study shows that OC and NaK type particles may serve as sources of efficient ice nuclei at night.

Macromolecular fungal ice nuclei in &amp;lt;i&amp;gt;Fusarium&amp;lt;/i&amp;gt;: effects of physical and chemical processing

Abstract. Some biological particles and macromolecules are particularly efficient ice nuclei (IN), triggering ice formation at temperatures close to 0 ∘C. The impact of biological particles on cloud glaciation and the formation of precipitation is still poorly understood and constitutes a large gap in the scientific understanding of the interactions and coevolution of life and climate. Ice nucleation activity in fungi was first discovered in the cosmopolitan genus Fusarium, which is widespread in soil and plants, has been found in atmospheric aerosol and cloud water samples, and can be regarded as the best studied ice-nucleation-active (IN-active) fungus. The frequency and distribution of ice nucleation activity within Fusarium, however, remains elusive. Here, we tested more than 100 strains from 65 different Fusarium species for ice nucleation activity. In total, ∼11 % of all tested species included IN-active strains, and ∼16 % of all tested strains showed ice nucleation activity above −12 ∘C. Besides Fusarium species with known ice nucleation activity, F. armeniacum, F. begoniae, F. concentricum, and F. langsethiae were newly identified as IN-active. The cumulative number of IN per gram of mycelium for all tested Fusarium species was comparable to other biological IN like Sarocladium implicatum, Mortierella alpina, and Snomax®. Filtration experiments indicate that cell-free ice-nucleating macromolecules (INMs) from Fusarium are smaller than 100 kDa and that molecular aggregates can be formed in solution. Long-term storage and freeze–thaw cycle experiments revealed that the fungal IN in aqueous solution remain active over several months and in the course of repeated freezing and thawing. Exposure to ozone and nitrogen dioxide at atmospherically relevant concentration levels also did not affect the ice nucleation activity. Heat treatments at 40 to 98 ∘C, however, strongly reduced the observed IN concentrations, confirming earlier hypotheses that the INM in Fusarium largely consists of a proteinaceous compound. The frequency and the wide distribution of ice nucleation activity within the genus Fusarium, combined with the stability of the IN under atmospherically relevant conditions, suggest a larger implication of fungal IN on Earth’s water cycle and climate than previously assumed.

Survival and ice nucleation activity of Pseudomonas syringae strains exposed to simulated high-altitude atmospheric conditions

Pseudomonas syringae produces highly efficient biological ice nuclei (IN) that were proposed to influence precipitation by freezing water in clouds. This bacterium may be capable of dispersing through the atmosphere, having been reported in rain, snow, and cloud water samples. This study assesses its survival and maintenance of IN activity under stressing conditions present at high altitudes, such as UV radiation within clouds. Strains of the pathovars syringae and garcae were compared to Escherichia coli. While UV-C effectively inactivated these cells, the Pseudomonas were much more tolerant to UV-B. The P. syringae strains were also more resistant to radiation from a solar simulator, composed of UV-A and UV-B, while only one of them suffered a decline in IN activity at −5 °C after long exposures. Desiccation at different relative humidity values also affected the IN, but some activity at −5 °C was always maintained. The pathovar garcae tended to be more resistant than the pathovar syringae, particularly to desiccation, though its IN were found to be generally more sensitive. Compared to E. coli, the P. syringae strains appear to be better adapted to survival under conditions present at high altitudes and in clouds.

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

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

Effects of Ice Nucleation Protein Repeat Number and Oligomerization Level on Ice Nucleation Activity

AbstractIce nucleation active bacteria have attracted particular attention due to their unique ability to produce specific ice nucleation proteins (INpros), which are the most efficient ice nuclei known as they induce nucleation at temperatures close to 0°C. Our model bacterium Pseudomonas syringae strain R10.79 produced INpros containing 67 tandem repeats, forming the proposed ice‐binding surface. To understand the role of the INpro repeats as well as the role of intermolecular interactions between INpros for their ice nucleation behavior, we produced a truncated version of the protein with only 16 tandem repeats (INpro16R). The purified INpro16R produced oligomers of varying sizes. Immersion freezing ice nucleation behavior of purified INpro16R was characterized by droplet‐freezing assays and in the Leipzig Aerosol Cloud Interaction Simulator. Predominant INpro16R oligomers introduced into Leipzig Aerosol Cloud Interaction Simulator as single particles with diameters of 50 nm or 70 nm were ice nucleation active at temperatures of −26°C and −24°C, respectively. These are much lower temperatures compared to that of intact INpros (−12°C). The data clearly indicated that the number of repeats determines the ice nucleation temperature. In addition, ice nucleation between −9°C and −10°C, comparable to the activity of intact INpro, was caused by higher‐order INpro16R oligomers. This supported previous observations that INpro oligomerization increases the ice‐binding surface, thereby affecting ice nucleation activity. In conclusion, both repeat number and oligomerization contribute in a seemingly independent manner to the nucleation mechanism of INpros.

Evidence for a missing source of efficient ice nuclei

It has been known for several decades that some bioaerosols, such as ice-nucleation-active (INA) bacteria, especially Pseudomonas syringae strains, may play a critical potential role in the formation of clouds and precipitation. We investigated bacterial and fungal ice nuclei (IN) in rainwater samples collected from the Hulunber temperate grasslands in North China. The median freezing temperatures (T50) for three years’ worth of unprocessed rain samples were greater than −10 °C based on immersion freezing testing. The heat and filtration treatments inactivated 7–54% and 2–89%, respectively, of the IN activity at temperatures warmer than −10 °C. We also determined the composition of the microbial community. The majority of observed Pseudomonas strains were distantly related to the verified ice-nucleating Pseudomonas strains, as revealed by phylogenetic analysis. Here, we show that there are submicron INA particles <220 nm in rainwater that are not identifiable as the known species of high-INA bacteria and fungi and there may be a new potential type of efficient submicroscale or nanoscale ice nucleator in the regional rainwater samplers. Our results suggest the need for a reinterpretation of the source of high-INA material in the formation of precipitation and contribute to the search for new methods of weather modification.

Role of Organic Hydrocarbons in Atmospheric Ice Formation via Contact Freezing.

An optical ice microscope apparatus equipped with a sealed cooling stage and a CCD camera was used to examine contact freezing events between a water droplet and ice nucleating particles (INP) containing organic hydrocarbons including octacosane, squalane, and squalene. Sample viscosities were measured with a capillary viscometer and compositions were characterized using Fourier transfer infrared spectroscopy with horizontal attenuated total reflectance and Raman microspectroscopy. All of the samples proved to be moderately efficient ice nuclei that induced freezing between -23 and -26 °C, regardless of whether the INP was solid or liquid. At their ice nucleating temperatures, the viscosity of the liquid samples (squalane and squalene) was 0.6 P or greater. Oxidation increased the viscosity of squalene to over 1330 P, but decreased the viscosity of squalane to 0.07 P at room temperature. Most importantly, our results demonstrate that even moderately viscous liquids in contact with water droplets can act to catalyze freezing, plausibly by providing a flexible template which decreases the energy barrier to ice nucleation. The simple soccer ball model of nucleation theory was used to derive the probability of freezing and nucleation rate coefficients as a function of temperature for each type of INP.

Cloud ice caused by atmospheric mineral dust – Part 1: Parameterization of ice nuclei concentration in the NMME-DREAM model

Abstract. Dust aerosols are very efficient ice nuclei, important for heterogeneous cloud glaciation even in regions distant from desert sources. A new generation of ice nucleation parameterizations, including dust as an ice nucleation agent, opens the way towards a more accurate treatment of cold cloud formation in atmospheric models. Using such parameterizations, we have developed a regional dust-atmospheric modelling system capable of predicting, in real time, dust-induced ice nucleation. We executed the model with the added ice nucleation component over the Mediterranean region, exposed to moderate Saharan dust transport, over two periods lasting 15 and 9 days, respectively. The model results were compared against satellite and ground-based cloud-ice-related measurements, provided by SEVIRI (Spinning Enhanced Visible and InfraRed Imager) and the CNR-IMAA Atmospheric Observatory (CIAO) in Potenza, southern Italy. The predicted ice nuclei concentration showed a reasonable level of agreement when compared against the observed spatial and temporal patterns of cloud ice water. The developed methodology permits the use of ice nuclei as input into the cloud microphysics schemes of atmospheric models, assuming that this approach could improve the predictions of cloud formation and associated precipitation.