4 th Workshop – Microphysics of ice clouds Vienna - Austria 16 th and 17 th of April 2016

The occurrence of ice-nucleating particles (INPs) in our atmosphere has a profound impact on the properties and lifetime of supercooled clouds. However, the identities, sources and abundances of airborne particles capable of efficiently nucleating ice at relatively low supercoolings (T > -15 °C) remain enigmatic. Recently, several studies have suggested that unidentified biogenic residues in soil dusts are likely to be an important source of these efficient atmospheric INPs. While it has been shown that cell-free proteins produced by common soil-borne fungi are exceptional INPs, whether these fungi are a source of icenucleating biogenic residues in soils has yet to be shown. In particular, it is unclear whether upon adsorption to soil mineral particles, the activity of fungal ice-nucleating proteins is retained or is reduced, as commonly observed for other soil enzymes. Here we show that proteins from a common soil fungus (Fusarium avenaceum) do in fact preferentially bind to and impart their ice-nucleating properties to the common clay mineral kaolinite. The icenucleating activity of the proteinaceous INPs is found to be unaffected by adsorption to the clay, and once bound the proteins do not readily desorb, retaining much of their activity even after multiple washings with pure water. The atmospheric implications of the finding that nanoscale fungal INPs can effectively determine the nucleating abilities of lofted soil dusts are discussed. Ice nucleating particles on the High Altitude Research Station Jungfraujoch: 5 years of measurements Larissa Lacher, Ulrike Lohmann and Zamin A. Kanji Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland Clouds containing ice play an important role in the Earth’s system, but fundamental knowledge on their formation and further development is still missing. Only a tiny fraction of ambient aerosol particles are ice nucleating particles (INP) and continuous measurements in an environment relevant for ice and mixed-phase clouds are rare. We perform INP measurements on a regular basis at the High Altitude Research Station Jungfraujoch. The site provides free tropospheric conditions with seasonal influence from boundary layer air and is regularly affected by Sahara dust events. By measuring INP concentrations during different seasons of the year we address the question of an annual cycle of INP concentrations in the free troposphere and quantify the influence of Sahara dust events and convectively lifted air. The measurements are performed with the Horizontal Ice Nucleation Chamber (HINC) at 242K in the watersuband supersaturated regime, which is relevant for the formation of ice and mixed-phase clouds. Results from field campaigns in winter, spring and summer are compared and show that INP concentrations range from almost zero to several hundred per liter. Below water saturation, INP concentrations are generally an order of magnitude lower than above water saturation, and don’t show a seasonal variation. At water-supersaturated conditions, seasonal occurrence of Sahara dust and boundary layer injections influence the INP concentrations and lead on average to higher values. Variations in free tropospheric conditions are mainly due to transported Sahara dust, leading to peak INP concentrations during Sahara dust events. Furthermore, a significant increase in INP concentrations was measured during an event with influence from marine air. It also appeals that when ambient temperatures at Jungfraujoch are low, INP concentrations are also lower suggesting scavenging of INPs being transported to the measurement site. With the continuation of these measurements we have a longer time series and therefore a better understanding of the evolution of INP concentrations on Jungfraujoch. Utilizing radar data in support of the hail mitigation operations in Styria, Austria Satyanarayana Tani, Helmut Paulitsch, Reinhard Teschl, Barbara Süsser-Rechberger Graz University of Technology, Institute of Microwave and Photonic Engineering, Graz, Austria ([email protected]) Hail storm damage is a major concern to the farmers in the province of Styria, Austria. Each year severe hail storms are causing damages to crops, resulting in losses of millions of euros. Hail mitigation operations are carried out in the province of Styria, to reduce the risk of hail damage. Radar serve for the identification of hail clouds and the aircraft’s injected the seeding material into the clouds. Radar data offer high spatial (1 km x 1 km x 1 km) and temporal resolutions (5 minutes), resulting in very promising option for hail mitigation operations and evaluation. In order to provide the guidelines to the pilots, here we offer a portable radar PC, which integrates the air craft’s location, cloud movement and radar reflectivity data. For the evaluation purpose, the HAILSYS software tool was developed by integrating single polarization C-band weather radar data, aircraft trajectory, radiosonde freezing level data, hail events and crop damage information from the ground. The radar data have been examined, the evaluation of hail mitigation operation will be presented with the help of some case studies. Ice nucleating particle properties in the Saharan air layer Yvonne Boose1,*, Fabian Mahrt1, M. Isabel Garcia2,3, Sergio Rodríguez2, Andrés Alastuey4, Claudia Linke5, Martin Schnaiter5, Ulrike Lohmann1, Zamin A. Kanji1, Berko Sierau1 1 ETH Zürich, Switzerland; 2Izaña Atmospheric Research Centre, AEMET, Spain; 3 University of La Laguna, Tenerife, Spain; 4 Institute of Environmental Assessment and Water Research, CSIC, Spain; 5Karlsruhe Institute of Technology, Germany; * [email protected] Mineral dust is amongst the most common ice nucleating particle (INP) types in the atmosphere. Nevertheless, no field study exists to date investigating the ice nucleation properties of atmospheric desert dust close to its globally largest emission source, the Sahara. The presented study fills this gap. In August 2013 and 2014 measurements of INP concentrations, aerosol particle size distributions, bulk and single particle chemistry and fluorescence were conducted at the Izaña Observatory located at 2373m above sea level on Tenerife, west of the African shore. During summer, the observatory is frequently exposed to high dust loads which are transported within the Saharan Air Layer. INP concentrations at temperatures between -40 and -15 °C and RHi = 100 to 150 % were measured. We investigated how changes in the chemical composition and biological material affect the INP properties of desert dust. Airborne dust samples were collected with a cyclone for additional offline analysis in the laboratory under similar conditions as in the field. Results from the field measurements show that the background aerosol at Izaña was dominated by carbonaceous particles, which were hardly ice-active under the investigated conditions. When Saharan dust was present, INP concentrations increased by up to two orders of magnitude. Long-range transported biomass burning aerosol was not ice-active under the investigated conditions. The autofluorescence analysis of the ice crystal residuals showed that up to 25 % of the INPs contained biological material during Saharan dust events, compared to only 5 % of the ambient particles. Furthermore, condensation mode INP concentrations showed a positive correlation with the ammonium sulfate to dust mass ratio below a certain threshold value. This indicates that atmospheric aging processes involving ammonium can increase the ice nucleation ability of mineral dust. The relationships between insoluble precipitation residues, clouds, and precipitation over California’s Sierra Nevada during winter storms Jessie M. Creamean, Allen B. White, Patrick Minnis, Rabindra Palikonda, Douglas A. Spangenberg, and Kimberly A. Prather CIRES, University of Colorado, Boulder, CO, NOAA ESRL, Physical Sciences Division, Boulder, CO, NASA Langley Research Center, Hampton, VA, Science Systems and Applications, Inc., Hampton, VA, Dept of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA Ice formation in orographic mixed-phase clouds can enhance precipitation and depends on the type of aerosols that serve as ice nucleating particles (INPs). The resulting precipitation from these clouds is a viable source of water, especially for regions such as the California Sierra Nevada. Thus, a better understanding of the sources of INPs that impact orographic clouds is important for assessing water availability in California. This study presents a multi-site, multiyear (2009 – 2012) analysis of single-particle insoluble residues in precipitation samples that likely influenced cloud ice and precipitation formation. Dust and biological particles represented the dominant fraction of the residues. Cloud glaciation, determined using GOES satellite observations, not only depended on high cloud tops and low temperatures, but also on the composition of the dust and biological residues. The greatest prevalence of ice-phase clouds and INP concentrations occurred in conjunction with biologically-rich residues and mineral dust rich in calcium, followed by iron and aluminosilicates. We show that the dust and biological particles served as efficient INPs, thus these residues are what likely influenced ice formation in clouds above the sites and subsequent precipitation quantities reaching the surface during events with similar meteorology. The goal of this study is to use precipitation chemistry and immersion INP information to gain a better understanding of the potential sources of INPs in the Sierra Nevada, where cloud-aerosol-precipitation interactions are poorly understood and where mixed-phase orographic clouds represent a key element in the generation of precipitation and thus the water supply in California. Measuring the effect of pollen grains and their ice active macro-molecules by observations of the INP concentration during the birch pollen season Monika Kohn1*, James D. Atkinson1, Kevin Kilchhofer1, Ulrike Lohmann1 and Zamin A. Kanji1 1Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland *Corresponding author: [email protected] Pollen are amongst the largest aerosol particles found in the atmosphere but are found in low concentrations in comparison to other aerosol species. Amongst other bioaerosol particles such as fungal spores and bacteria, pollen can nucleate ice at small supercooling and are therefore of interest for atmospheric ice formation, despite their low concentrations. Birch pollen can induce freezing at temperatures as warm as 264 K in the immersion mode [1]. Laboratory measurements have shown that ice active macro-molecules (INMs) can be easily washed off from pollen grains and can be released in high numbers from a single grain [2,3]. The release of INMs directly into the atmosphere may contribute significantly to ice formation processes as they can be distributed well in the atmosphere without pollen grains as a carrier due to their small size. In this study, we present immersion freezing ice nucleating particle (INP) concentrations from two drop-freezing techniques which were conducted during the birch pollen season in Zurich. Due to differences in the aerosol particle sizes sampled between the two methods, our measurements allow for deduction of the size of the aerosol particles that contributed to ice nucleation. Measurements of meteorological conditions, PM10 mass and physical aerosol properties further support our discussion. Over the course of 6 weeks, INP concentrations were observed in a temperature range between 256269 K. We observed that particles larger than 10 μm may have contributed to ice nucleation at the peak of the pollen season, which indicates pollen grains (typically much larger than 10 μm) acted as potential INPs. The presence of INMs in the atmosphere was tested by measuring pollen and INP concentrations in parallel. Due to a release of large numbers of INMs from a single pollen grain, a substantial increase in INP concentrations was expected. From measured INP concentrations in samples only containing aerosol particle sizes smaller than 10 μm, we did not find indications that INMs were present in the atmosphere during the pollen season or else, contributed significantly to the INP concentration measured. Instead, environmental conditions such as relative humidity were found to effect the INP concentration for some periods investigated and are discussed. References: [1] K. Diehl et al. (2002) Atmos. Res., 61, 125-133. [2] S. Augustin et al. (2012) Atmos. Chem. Phys. Discuss., 12, 32911-32943. [3] B. G. Pummer et al. (2012) Atmos. Chem. Phys., 12, 2541-2550. An unmanned surface vehicle (USV) to study the air/water interface in aquatic environments Craig W. Powers David G. Schmale III Department of Civil and Environmental Engineering, Virgin Tech, Blacksburg, VA 24061, USA; Department of Plant Pathology, Physiology, and Weed Science, Virginia Tech, Blacksburg, VA 24061, USA; *Presenting author: PH: (803) 712-3142, Email: [email protected] The air/water interface (AWI) is a biologically rich and active ecological zone in aquatic environments. Little is known about the mechanics of the AWI, and how the AWI contributes aerosols that could impact weather. We developed and implemented an unmanned surface vehicle (USV) to study the AWI. In one set of experiments on a small pond in Blacksburg, VA USA, the USV was equipped with a distributed temperature sensing (DTS) system to measure temperature profiles from about 1 m below the surface of the water to about 1 m above the surface. This integrated USV DTS system resolved a temperature differential of about 6C at the AWI spanning a distance of about 8 cm with a vertical resolution of 1 mm. In other experiments, the USV was equipped with impingers and real-time particle counters positioned at multiple heights above the water surface. A field campaign was conducted over a large freshwater lake in Baton Rouge, Louisiana, USA in October, 2015 to coordinate sampling of microbial ice nucleators in and near the water with the USV, and 60 m above the water with a small drone. The USV is currently being using to resolve aerosolization phenomena at the AWI in natural aquatic systems that have the potential to impact weather. Such technology is poised to unleash a burgeoning field of aero-aquatic-ecology. Exploring the sources of variability in ice nucleating particles of biological origin Emiliano Stopelli, Franz Conen, Cindy E. Morris, Christine Alewell : Environmental Geosciences, University of Basel, Basel, CH : INRA, UR0407 Plant Pathology Research Unit, F-84143 Montfavet Cedex, FR Ice nucleation is a key step for the formation of precipitation on Earth. Ice nucleating particles (INPs) of biological origin catalyse the freezing of supercooled cloud droplets at temperatures warmer than 12 °C. In order to understand the effective role of these INPs in conditioning precipitation, it is of primary importance to describe and predict their variability in the atmosphere. Variability in timeBetween 2012 and 2014, 14 sampling campaigns in precipitating clouds were conducted at the High Altitude Research Station Jungfraujoch, in the Swiss Alps, at 3580 m a.s.l. A total of 106 freshly fallen snow samples were analysed immediately on site for the concentration of INPs active at -8 °C (INPs-8) by immersion freezing. Values of INPs-8 ranged from 0.21 to 434·ml-1. Multiple linear regression models were built to describe and predict the variation in the abundance of INPs-8. These models indicate that a coincidence of strong atmospheric turbulence and little prior precipitation from a cloud coincides with large concentrations of INPs-8, a set of conditions which can be easily met for instance along the passage of a front. To obtain more information on the presence of INPs-8 of biological origin a subset of precipitation samples was progressively filtered and heated. The abundance of bacterial cells and the presence of culturable Pseudomonas syringae were studied as well. Whilst each sample presents a specific distribution of the sizes of INPs-8, almost all ice nucleating activity is lost after heating at 80 °C, and a significant part of INPs-8 is sensitive to warming at 40 °C. Just a minor fraction of the INPs-8 is potentially due to intact bacterial cells or culturable P. syringae, indicating that the majority of INPs-8 measured in environmental samples is made of heat-sensitive molecules of biological origin which can either be singularly airborne or aggregated on soil and mineral particles.Variability in spaceTo start studying the variability of INPs8 over a spatial scale and testing the results obtained at Jungfraujoch, in October 2015 the project NICE (Nucleators of Ice at monte CimonE) was carried out, in the frame of the European Union funding for the Project ACTRIS. Airborne particles were collected on quartz fibre PM10 filters over a week at Monte Cimone (IT, 2165 m a.s.l.) and the concentrations of INPs-8·m-3 calculated. These results are currently being compared with the analyses of the PM10 filters collected for the same week at Jungfraujoch and Puy de Dôme (FR, 1465 m a.s.l.). Results on this spatial variability of airborne INPs-8 will be presented and constitute a first attempt to set up a network of observatories where measurements on INPs-8 can be carried out regularly. Ice nucleating particles measured during the field intercomparison FIN-3 by the vacuum diffusion chamber FRIDGE Daniel Weber, Jann Schrod, Heinz Bingemer & Joachim Curtius Institut für Atmosphäre und Umwelt Fachbereich Geowissenschaften/Geographie, Johann Wolfgang Goethe-Universität, Frankfurt/Main The Frankfurt Ice Nucleation Deposition freezing Experiment (FRIDGE) is an offline INPcounter, which first collects aerosol particles via electrostatic precipitation on silicon-wafers and exposes them to typical cloud conditions in a vacuum diffusion chamber afterwards to detect nucleation and growth of ice on INP. FRIDGE was a part of Fin-3, a field campaign at the Desert Research Institutes Storm Peak Laboratory in Colorado, USA in September 2015. Storm Peak Laboratory (3210 m asl) is within the boundary layer in the daytime and within the free troposphere at night. For 44 aerosol samples INP-concentrations at different temperatures and humidity were determined. We observed different INP-concentrations in both atmospheric layers, as well as a high dayto-day variability. Furthermore, the effect of previous snowfall was studied. Modification of mineral dust during long-range transport and associated impact on predicted ice nuclei concentration insights from SALTRACE Weinzierl, B. (1, 2), D. Sauer (2), A. Walser (3,2), M. Dollner (1), F. Chouza (2), O. Reitebuch (2), S. Groß (2), V. Freudenthaler (3), K. Kandler (4), A. Ansmann (5) (1) University of Vienna, Aerosol Physics and Environmental Physics, Boltzmanngasse 5, Wien, Austria (2) Deutsches Zentrum für Luftund Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany (3) Meteorologisches Institut (MIM), Ludwig-Maximilians-Universität, München, Germany (4) Technische Universität Darmstadt, Institut für Angewandte Geowissenschaften, Darmstadt, Germany (5) Leibniz Institute for Tropospheric Research, Physics Department, Leipzig, Germany Every year huge amounts of mineral dust are transported from Africa over Atlantic Ocean into the Caribbean affecting air quality, health, weather and climate thousands of kilometers downwind of the dust source regions. Despite this importance, there has been little research on the long-range transport of mineral dust and many questions such as the change of the dust size distribution during transport, the role of wet and dry removal mechanisms, and the complex interaction between mineral dust and clouds are still open. To investigate the long-range transport of mineral dust from African into the Caribbean, the Saharan Aerosol Long-range Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE: http://www.pa.op.dlr.de/saltrace) was conducted in June/July 2013. SALTRACE was designed as a closure experiment combining ground-based lidar, in-situ and sun photometer instruments deployed on Cape Verde, Barbados and Puerto Rico, with airborne aerosol and wind lidar measurements of the DLR research aircraft Falcon, satellite observations and model simulations. The DLR research aircraft Falcon spent more than 110 flight hours studying mineral dust from several dust outbreaks under a variety of atmospheric conditions between Senegal, Cape Verde, the Caribbean, and Florida. During SALTRACE, a comprehensive data set on chemical, microphysical and optical properties of aged mineral dust was gathered. Although changes in the size distribution were observed during transport, one interesting finding during SALTRACE was the detection of large 10-20 μm particles in the Caribbean even after more than 4000 km of transport. Here we use data from airborne in-situ aerosol measurements with the DLR Falcon research aircraft during the Saharan Aerosol Long-range Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE: http://www.pa.op.dlr.de/saltrace) to study the modification of the mineral dust layers between Africa and the Caribbean. We will present vertical profiles of dust properties including cloud condensation nuclei number concentration. In addition, we will use a parameterization predict the number concentration of potential ice nuclei in the presence of mineral dust. A laboratory study of the nucleation kinetics of nitric acid hydrates under stratospheric conditions Alexander D. James, Benjamin J. Murray & John M. C. Plane Department of Chemistry, University of Leeds, Woodhouse Lane, LS2 9JT, UK School of Earth and Environment, University of Leeds, Woodhouse Lane, LS2 9JT, UK Measurements of the kinetics of crystallisation of ternary H2O-H2SO4-HNO3 mixtures to produce nitric acid hydrate phases, as occurs in the lower stratosphere, have been a longstanding challenge for investigators in the laboratory. Understanding polar stratospheric chlorine chemistry and thereby ozone depletion is increasingly limited by descriptions of nucleation processes. Meteoric smoke particles have been considered in the past as heterogeneous nuclei, however recent studies suggest that these particles will largely dissolve, leaving mainly silica and alumina as solid inclusions. In this study the nucleation kinetics of nitric acid hydrate phases have been measured in microliter droplets at polar stratospheric cloud (PSC) temperatures, using a droplet freezing assay. A clear heterogeneous effect was observed when silica particles were added. A parameterisation based on the number of droplets activated per nuclei surface area (ns) has been developed and compared to global model data. Nucleation experiments on identical droplets have been performed in an X-Ray Diffractometer (XRD) to determine the nature of the phase which formed. β-Nitric Acid Trihydrate (NAT) was observed alongside a mixture of Nitric Acid Dihydrate (NAD) phases. It is not possible to determine whether NAT nucleates directly or is formed by a phase transition from NAD (likely requiring the presence of a mediating liquid phase). Regardless, these results demonstrate the possibility of forming NAT on laboratory timescales. In the polar stratosphere, sulfuric acid (present at several weight percent of the liquid under equilibrium conditions) could provide such a liquid phase. This study therefor provides insight into previous discrepancies between phases formed in the laboratory and those observed in the atmosphere. It also provides a basis for future studies into atmospheric nucleation of solid PSCs. Metastable Nitric Acid Trihydrate in Ice Clouds Fabian Weiss, Frank Kubel, Óscar Gálvez, Markus Hoelzel, Stewart F. Parker, Philipp Baloh, Riccardo Iannarelli, Michel J. Rossi and Hinrich Grothe* Institut fuer Materialchemie, Technische Universitaet Wien (Austria) Institut fuer Chemische Technologie und Analytik, TU Wien (Austria) Instituto de Estructura de la Materia, IEM-CSIC, Madrid (Spain) Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Technische Universität München (Germany) ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX (UK) Dr. M. J.Rossi, Dr. R. Iannarelli, Paul-Scherrer Institute, 5232 Villigen (Switzerland) The composition of high altitude ice clouds is still a matter of intense discussion. The constituents in question are ice and nitric acid hydrates. The identification and formation mechanisms, however, are still unknown but are essential to understand atmospheric processing such as the seasonal ozone depletion in the lower polar stratosphere or the radiation balance of planet Earth. We found conclusive evidence for a long-predicted phase, which has been named alpha nitric acid trihydrate (alpha-NAT). This phase has been characterized by a combination of X-ray and neutron diffraction experiments allowing a convincing structure solution. Additionally, vibrational spectra (infrared and inelastic neutron scattering) were recorded and compared with theoretical calculations. A strong affinity between water ice and alpha-NAT has been found, which explains the experimental spectra and the phase transition kinetics essential for identification in the atmosphere. On the basis of our results, we propose a new three-step mechanism for NAT-formation in high altitude ice clouds. Analysis of isothermal and cooling-rate-dependent immersion freezing by a unifying stochastic ice nucleation model Peter A. Alpert and Daniel A. Knopf Institute for Terrestrial and Planetary Atmospheres, School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, USA now at Université Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, 2 avenue Albert Einstein, F-69626 Villeurbanne, France ([email protected]) Immersion freezing is an important ice nucleation pathway involved in the formation of cirrus and mixed-phase clouds. Since ice nucleation cannot be directly observed in the atmosphere, laboratory immersion freezing experiments are necessary to determine the range in temperature and relative humidity at which ice nucleation occurs. In some experimental studies, it is assumed that droplets containing ice nucleating particles (INPs) all have the same INP surface area (ISA), but is this assumption valid or does it affect experimental analysis and interpretation of the immersion freezing process? Descriptions of ice active sites and variability of contact angles have been successfully formulated to describe ice nucleation experimental data in previous research; however, we consider the ability of a stochastic freezing model founded on classical nucleation theory to reproduce previous results and to explain experimental uncertainties and data scatter considering variable ISA. Experimental results derived from a variety of approaches are reproduced by our model including droplets on a cold-stage exposed to air or surrounded by an oil matrix, wind and acoustically levitated droplets, droplets in a continuous-flow diffusion chamber (CFDC), the Leipzig aerosol cloud interaction simulator (LACIS) and the aerosol interaction and dynamics in the atmosphere (AIDA) cloud chamber. Observed time-dependent isothermal frozen fractions exhibiting nonexponential behavior can be readily explained by this model if varying ISA are used. An apparent cooling-rate dependence of the heterogeneous ice nucleation rate coefficient, Jhet, can be explained by assuming identical ISA in each droplet. When accounting for ISA variability, the cooling-rate dependence of Jhet vanishes as expected from classical nucleation theory. A quantitative uncertainty analysis due to measureable parameters, such as the number of droplets used and variable ISA, will be discussed with implications for future experimental design and analysis. Air-plane soot surrogates and their ice nucleation activity: investigations in deposition mode C. Pirim, A-R. Ikhenazene, Y. Carpentier, C. Focsa, B. Chazallon Laboratoire de Physique des Lasers, Atomes et Molécules, Université Lille 1, 59655 Villeneuve d'Ascq, France Emissions of solid-state particles (soot) from engine exhausts due to incomplete fuel combustion is considered to influence ice and liquid water cloud droplet activation [1]. The activity of these aerosols would originate from their ability to promote ice formation above pure water homogeneous freezing point (~235 K). When considering deposition mode, i.e., when an ice embryo forms directly by water vapor condensation on a surface, it has been suggested that no liquid water is involved in the process because the relative humidity at which ice nucleation is generally observed is below the water saturation line [2]. Further, the ice nuclei (aerosols) are then considered to have active sites that promote heterogeneous ice nucleation at relative humidity below the homogeneous ice nucleation line (i.e. at RHi (relative to ice) below ~ 140-150%). There is no clear evidence of heterogeneous ice nucleation on soot aerosols for T > 235 K, and soot particles are reported to be generally worse ice nuclei than mineral dust because they activate nucleation at higher ice-supersaturations for deposition nucleation or at lower temperatures for immersion freezing than what is usually expected for homogeneous nucleation [2]. In fact, there are still numerous opened questions as to whether and how soot surfaces’ physico-chemical properties (structure, morphology and chemical composition) can influence their nucleation ability. Therefore, systematic investigations of soot aerosol nucleation activity via one specific nucleation mode, here deposition nucleation, combined with thorough structural and compositional analyzes are needed in order to establish any association between the particles’ activity and their physico-chemical properties. In addition, since the morphology of the ice crystals can influence their radiative properties [3], it is of paramount importance to investigate their morphology as they grow over both soot or pristine substrates at different temperatures and humidity ratios. In the present work, a CAST (Combustion Aerosol STandart) burner supplied with propane is used to produce soot samples using various experimental conditions. The nucleation activity is studied in deposition mode and is monitored using a temperature-controlled reactor in which the sample’s relative humidity is precisely measured via a cryo-hygrometer. Formation of water/ice onto the particles is followed both optically and spectroscopically, using a microscope coupled to a Raman spectrometer. Vibrational signatures of hydroxyls (O-H) emerge when the particle becomes hydrated and are used to characterize ice crystals. Acknowledgement: F.X. Ouf (IRSN) for providing soot samples References: [1] Koop, T. Atmospheric water, Water: Fundamentals as the Basis for Understanding the Environment and Promoting Technology, 187, 45-75 (2015). [2] Hoose & Möhler. Atmospheric Chemistry and Physics. 12, 9817-9854. (2012) [3] Schumann et al. Journal of Applied Meteorology and Climatology. 51, 1391-1406 (2012) Understanding Ice Nucleation by Alkali Feldspars Mark A. Holden, Thomas F. Whale, Benjamin J. Murray, Angela Bejarano-Villafuerte, Gavin Burnell, Fiona C. Meldrum and Hugo K. Christenson School of Earth and Environment, University of Leeds, Woodhouse Lane, LS2 9JT, UK The formation of ice in the atmosphere can affect cloud properties and have an impact on the Earth’s radiative balance. In mixed-phase clouds, freezing of super-cooled water typically occurs by heterogeneous ice nucleation. The ice nucleating particles (INP) that cause this come from a variety of sources, including atmospheric mineral dust. Amongst the minerals that form these dusts, alkali feldspars have been identified as being important INP. However, whilst feldspars are known to nucleate ice well, the mechanism by which this occurs is not understood. Using a range of analytical techniques, we have studied immersion mode ice nucleation for several alkali feldspars. By using thin sections, which are 30 μm thick polished sheets of natural feldspar with several centimetre diameters, we were able to characterise in detail the surfaces on which nucleation occurs. From this work, the microtexture of the feldspars studied has been seen to be an important factor in ice nucleating ability. This microtexture comprises exsolution lamellae, crystal twinning, grain boundaries and micropores. In the absence of microtexture, the ice nucleating ability of feldspars is substantially diminished. For samples with microtexture, a preference for nucleation on particular regions of the feldspar surface is seen. There are several types of active site on a feldspar surface, each with different activation temperatures. We conclude that the most active sites on the feldspar are rare, and that the microtexture of the feldspars is important for the ice nucleation observed at higher temperatures. This is important because it seems that ice nucleation by alkali feldspars is controlled by active sites which are related to specific features on a surface. This paves the way for understanding how these sites might be influenced by atmospheric processes, such as the action of acids, which are known to reduce the activity of feldspars. Understanding ice nucleation characteristics of selective mineral dusts suspended in solution Anand Kumar, Claudia Marcolli, Lukas Kaufmann, Ulrich Krieger and Thomas Peter1Institute for Atmospheric and Climate Sciences, ETH Zurich, Zurich, 8092, Switzerland2Marcolli Chemistry and Physics Consulting GmbH, Zurich, 8047, Switzerland Introduction & ObjectivesFreezing of liquid droplets and subsequent ice crystal growth affects optical properties of clouds andprecipitation. Field measurements show that ice formation in cumulus and stratiform clouds begins attemperatures much warmer than those associated with homogeneous ice nucleation in pure water,which is ascribed to heterogeneous ice nucleation occurring on the foreign surfaces of ice nuclei (IN).Various insoluble particles such as mineral dust, soot, metallic particles, volcanic ash, or primarybiological particles have been suggested as IN. Among these the suitability of mineral dusts is bestestablished. The ice nucleation ability of mineral dust particles may be modified when secondaryorganic or inorganic substances are accumulating on the dust during atmospheric transport. If thecoating is completely wetting the mineral dust particles, heterogeneous ice nucleation occurs inimmersion mode also below 100 % RH.A previous study by Kaufmann (PhD Thesis 2015, ETHZ) with Hoggar Mountain dust suspensions invarious solutes (ammonium sulfate, PEG, malonic acid and glucose) showed reduced ice nucleationefficiency (in immersion mode) of the particles. Though it is still quite unclear of how surfacemodifications and coatings influence the ice nucleation activity of the components present in naturaldust samples. In view of these results we run freezing experiments using a differential scanningcalorimeter (DSC) with the following mineral dust particles suspended in pure water and ammoniumsulfate solutions: Arizona Test Dust (ATD), microcline, and kaolinite (KGa-2, Clay Mineral Society).MethodologySuspensions of mineral dust samples (ATD: 2 weight%, microcline: 5% weight, KGa-2: 5% weight)are prepared in pure water with varying solute concentrations (ammonium sulfate: 0 – 10% weight).20 vol% of this suspension plus 80 vol% of a mixture of 95 wt% mineral oil (Aldrich Chemical) and 5wt% lanolin (Fluka Chemical) is emulsified with a rotor-stator homogenizer for 40 s at a rotationfrequency of 7000 rpm. 4 – 10 mg of this mixture is pipetted in an aluminum pan (closedhermetically), placed in the DSC and subjected to three freezing cycles. The first and the third freezingcycles are executed at a cooling rate of 10 K/min to control the stability of the sample. The secondfreezing cycle is executed at a 1 K/min cooling rate and is used for evaluation. Freezing temperaturesare obtained by evaluating the onset of the freezing signal in the DSC curve and plotted against wateractivity values corresponding to the solute concentration (obtained via Koop et al., (2000)).ObservationsA decrease in ice nucleation ability of the minerals (for immersion freezing) with increasing soluteconcentration (hence, decreasing water activity) was observed, similar as for homogeneous icenucleation. Though the decrease was more pronounced in case of microcline and ATD as compared tokaolinite. Therefore, there seem to be specific interactions which needs to be studied further to explainthe freezing behavior of minerals.The current study could be helpful in investigating the ice nucleation behavior of individual mineralswhen present in conjunction with a solute, viz. ammonium sulfate, which is of high atmosphericrelevance.ReferencesZobrist et al., (2008), doi: 10.1021/jp7112208.Koop et al., (2000), doi:10.1038/35020537.Kaufmann (PhD Thesis 2015, ETHZ). Cellulose and Their Characteristic Ice Nucleation ActivityFreezing on a Chip Häusler Thomas, Felgitsch Laura, Grothe Hinrich1 Vienna University of Technology, Institute of Materials Chemistry, Vienna, Austria The influence of clouds on the Earth’s climate system is well known (IPCC, 2013). Cloudmicrophysics determines for example cloud lifetime and precipitation properties. Clouds arecooling the climate system by reflecting incoming solar radiation and warm its surface bytrapping outgoing infrared radiation (Baker and Peter, 2008). In all these processes, aerosolparticles play a crucial role by acting as cloud condensation nuclei (CCN) for liquid dropletsand as an ice nucleation particle (INP) for the formation of ice particles.Freezing processes at higher temperatures than -38°C occur heterogeneously (Pruppacher andKlett 1997). Therefore aerosol particles act like a catalyst, which reduces the energy barrierfor nucleation. The nucleation mechanisms, especially the theory of functional sites are notentirely understood. It remains unclear which class of compound nucleates ice.Here we present a unique technique to perform dropfreezing experiments in a more efficientway. A self-made freezingchip will be presented. Measurements done to proof the efficiencyof our setup as well as advantages compared with other setups will be discussed.Furthermore we present a proxy for biological INPs, microcrystalline cellulose. Cellulose isthe main component of herbal cell walls (about 50 wt%). It is a polysaccharide consisting of alinear chain of several hundred to many thousands of β(1→4) linked D-glucose units.Cellulose can contribute to the diverse spectrum of ice nucleation particles. We present results of the nucleation activity measurements of MCCs as well as the influence of concentration,preparation or chemical modification. TINA: A New High-Performance Droplet Freezing Assay for the Analysis of Ice Nucleiwith Complex Composition Anna T. Kunert*, Jan F. Scheel*, Frank Helleis*, Thomas Klimach*, Ulrich Pöschl*, andJanine Fröhlich-Nowoisky* * Max Planck Institute for Chemistry, Mainz, Germany Freezing of water above homogeneous freezing is catalysed by ice nucleation active (INA)particles called ice nuclei (IN), which can be of various inorganic or biological origin. Thefreezing temperatures reach up to -1 °C for some biological samples and are dependent on thechemical composition of the IN. The standard method to analyse IN in solution is the dropletfreezing assay (DFA) established by Gabor Vali in 1970. Several modifications andimprovements were already made within the last decades, but they are still limited by eithersmall droplet numbers, large droplet volumes or inadequate separation of the single dropletsresulting in mutual interferences and therefore improper measurements. The probability that miscellaneous IN are concentrated together in one droplet increases withthe volume of the droplet, which can be described by the Poisson distribution. At a givenconcentration, the partition of a droplet into several smaller droplets leads to finely dispersedIN resulting in better statistics and therefore in a better resolution of the nucleation spectrum. We designed a new Twin Ice Nucleation Assay (TINA), which represents an upgrade of thepreviously existing DFAs in terms of temperature range and statistics. The necessity of observing freezing events at temperatures lower than homogeneous freezing due to freezingpoint depression requires high-performance thermostats combined with an optimal insulation. Furthermore, we developed a cooling setup, which allows both huge and tiny temperaturechanges within a very short period of time. Besides that, TINA provides the analysis of more than 750 droplets per run with a small droplet volume of 5 μL. This enables a fast and moreprecise analysis of biological samples with complex IN composition as well as better statistics for every sample at the same time. Immersion and contact freezing experiments utilizing contact-free droplet levitationtechniques