Displays

We present our laboratory and field test results of a newly developed commercial ice nucleation chamber, the so-called PINE, for its semi-autonomous measurements of atmospheric ice-nucleating particles (INPs). The PINE instrument is developed based on the design of the AIDA cloud chamber (Möhler et al., 2003) to promote long-term ambient INP measurements even at a remote location. Unique features of the PINE instrument include its plug-and-play feature (so it runs on a standard power outlet), susceptivity to the INP detection for 0.2 – 50K L^-1 STP in the ~0.7 – 220 mm size range (256 channels) with ~8 min time resolution, cryo-cooler-based automatic ramping-temperature operation, capability of quantifying INPs in different IN modes (e.g., immersion freezing and deposition mode at >-60 °C), and small particle loss through the system (~5% for <3 mm diameter particles). Our laboratory test results show that ammonium sulfate homogeneously freezes at -33 °C in PINE, which is comparable to the previous homogeneous freezing AIDA result (Hiranuma et al., 2016). Further, we observe immersion freezing of Snomax and illite NX at approx. -7 °C and -20 °C in PINE as seen by other online INP instruments (Wex et al., 2015; Hiranuma et al., 2015). These results validate the PINE’s capability to detect INPs in a wide temperature range, where “clear and significant research issues remain” (DeMott et al., 2011). Next, as for the first field test, we have performed a ground-based INP measurement with PINE at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) observatory, where long-term measurements provide statistical context (Marinescu et al., 2019). Briefly, we have successfully operated PINE via network for INP concentration measurements on a 24/7 basis for 45 consecutive days. Other findings from our lab characterization of PINE and first field deployment in the Southern Great Plains (e.g., comparison to other INP techniques) will be presented.

Acknowledgement: This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research (DE-SC0018979) – work packages 1-2 of Implications of Aerosol Physicochemical Properties Including Ice Nucleation at ARM Mega Sites for Improved Understanding of Microphysical Atmospheric Cloud Processes.

References:

• DeMott, P. J. et al.: Bull. Amer. Meteorol. Soc. 92, 1623, 2011.
• Hiranuma, N. et al.: Atmos. Chem. Phys., 15, 2489–2518, 2015.
• Hiranuma, N. et al.: Atmos. Meas. Tech., 9, 3817–3836, 2016.
• Marinescu, P. J., et al.: Atmos. Chem. Phys., 19, 11985–12006, 2019.
• Möhler, O. et al.: Atmos. Chem. Phys. 3, 211–223, 2003.
• Wex, H. et al.: Atmos. Chem. Phys., 15, 1463–1485, 2015.

How to cite: Hiranuma, N., Vepuri, H. S., Lacher, L., Nadolny, J., and Möhler, O.: The Portable Ice Nucleation Experiment chamber (PINE): laboratory characterization and field test for its semi-automated ice-nucleating particle measurements in the Southern Great Plains, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12385, https://doi.org/10.5194/egusphere-egu2020-12385, 2020.

Supercooled drizzle droplets may produce multiple ice particles upon freezing. This mechanism could potentially explain the high ice number concentrations outside of temperature range where the well-known Hallett-Mossop mechanism of ice multiplication would take place. Limited experimental methods in the past prevented direct observations of the shattering droplets, resulting in a wide range of experimental results, unsuitable for the development of a sophisticated cloud model parameterization. Recently, we have revived experiments on secondary ice production by levitating individual drizzle droplets in electrodynamic balance (EDB) and observing the freeze-shattering with high-speed video microscopy and high-resolution infrared thermal measuring system. In this way we have been able to identify three additional SIP mechanisms (cracking, jetting and bubble bursts) associated with the freezing of drizzle droplets (Lauber et al., 2018).
Additionally, we have extended the range of experimental conditions to mimick the freezing of continental (pure water) and maritime (aqueous solution of analogue sea salt) drizzle droplets suspended in the updraft of cold moist air. We report a strong enhancement of shattering probability as compared to the previous studies conducted under stagnant air conditions. The high-definition video records of shattering events reveal the coupling between various microphysical processes caused by ice propagation inside the freezing drop and reveal striking difference between freezing of pure water and SSA solution droplets. Application of high-resolution infrared microscopy allowed us to record the evolution of the droplet temperature under realistic flow conditions and thus constrain the thermodynamic parameters controlling the pressure build-up inside the droplet. Based on these new observation data and theoretical model of freezing droplet, we discuss the physical mechanism behind the shattering of drizzle droplets and its implication for mixed-phase cloud modeling.

Lauber, A., A. Kiselev, T. Pander, P. Handmann, and T Leisner (2018). “Secondary Ice Formation during Freezing of Levitated Droplets”, Journal of the Atmospheric Sciences 75, pp. 2815–2826.

How to cite: Keinert, A., Kleinheins, J., Spannagel, D., Kiselev, A., and Leisner, T.: Laboratory Experiments on the Droplet Shattering Secondary Ice Production Mechanism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7609, https://doi.org/10.5194/egusphere-egu2020-7609, 2020.

Cloud glaciation is an atmospheric process with important implications for climate and weather. Indeed, clouds made of liquid water and of ice crystals impact the global radiative balance of the atmosphere by reflecting incoming solar radiation and by absorbing outgoing terrestrial radiation. The relevance of ice nucleating particles (INPs) to the atmosphere depends on three main factors, namely on (1) their atmospheric concentration, (2) their freezing temperature and relative humidity, and (3) their freezing mechanism (Cziczo et al., 2013). Research on characterizing ice nucleating organic matter often takes a “top-down” approach where a whole sample of a complex mixture of organic, often biological, macromolecules is subjected to separation techniques and heat treatments to identify IN active sub-components. Studies have used this approach for characterizing bulk soil organic matter, volcanic ash and biological macromolecules from pollen, fungi, and bacteria.

We and others have recently found that dissolved organic matter collected from rivers and swamps surprisingly contain active INP (Borduas-Dedekind et al., 2019; Knackstedt et al., 2018; Moffett et al., 2018). Yet, all three studies state that it is unclear which sub-component of the dissolved organic matter is responsible for the ice nucleating ability. There are clear challenges in attributing the ice nucleating ability when starting with a complex mixture of organic and/or biological material, including matrix effects, impurities accumulated through the separation and/or heating process and lack of molecule identity.

We present here a “bottom-up” approach to compliment the top-down approach for atmospheric ice nucleation research of macromolecules. Using our home-built drop Freezing Ice Nuclei Counter (FINC) with automated imaging, a range of macromolecules were investigated. Indeed, we have analysed a wide range of dissolved organic matter subcomponents including proteins and fulvic acids. We find a range of ice nucleating ability. We find that lignin, the second most abundant biopolymer in plants, is ice active with 50% frozen fraction temperatures (T₅₀) at –18 °C at a concentration of 100 mg C/L. Furthermore, we have investigated the ice nucleation ability of common diatom exudates and found that at atmospherically relevant concentration they are likely not ice active in immersion freezing within the detection of our FINC instrument. We are currently investigating the effect of atmospheric processing on these macromolecules with the goal of understanding how macromolecules’ ice activity evolves over their one-week lifetime in the atmosphere.

How to cite: Borduas-Dedekind, N., Miller, A., Bogler, S., and Went, J.: Chemical insights into the ice nucleating ability of macromolecules in immersion freezing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20623, https://doi.org/10.5194/egusphere-egu2020-20623, 2020.

Anthropogenic aerosols impact cirrus clouds through ice nucleation, thereby changing the Earth’s radiation budget. However, the magnitude and sign of anthropogenic forcing on cirrus clouds are still very uncertain depending on the treatments for ice nucleating particles (INPs) and the ice nucleation scheme. In this study, a new ice nucleation scheme (hereafter the HYBRID scheme) is developed to combine the best features of two previous ice nucleation schemes, so that the global model is able to calculate the ice number concentration in both the updrafts and downdrafts associated with gravity waves and has a robust sensitivity to the change of aerosol number. The ice number concentrations calculated using the HYBRID scheme are overestimated somewhat but are in reasonable agreement with an adiabatic parcel model and observations. The forcing and cloud changes associated with changes in aircraft soot, sulfur emission and all anthropogenic emissions between the preindustrial period (PI) and the present day (PD) are examined using a global model with the HYBRID scheme. Aircraft soot emissions decrease the global average ice number concentration (Ni) by -1.0±2.4×10⁷ m^-2 due to the inhibition of homogeneous nucleation and lead to a radiative forcing of -0.14±0.07 W m^-2, while the increase in the sulfur emissions increases the global average Ni by 7.3±2.9×10⁷ m^-2 due to the increase in homogeneous nucleation and leads to a radiative forcing of -0.02±0.06 W m^-2. The possible effects of aerosol and cloud feedbacks to the meteorological state in remote regions partly contribute to reduce the forcing and the change in Ni due to anthropogenic emissions. The radiative forcing due to all increased anthropogenic emissions from PI to PD is estimated to be -0.20±0.05 W m^-2. If newly formed secondary organic aerosols (SOA) acts an INP and inhibit homogeneous nucleation, the Ni formed from heterogeneous nucleation is increased. As a result, the inclusion of INPs from SOA increases the change in Ni to 12.0±2.3×10⁷ m^-2 and increases (makes less negative) the anthropogenic forcing on cirrus clouds to -0.04±0.08 W m^-2 from PI to PD.

How to cite: Zhu, J. and Penner, J. E.: Radiative forcing of anthropogenic aerosols on cirrus clouds using a hybrid ice nucleation scheme, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4158, https://doi.org/10.5194/egusphere-egu2020-4158, 2020.

By triggering the formation of cloud ice crystals in the atmosphere, ice-nucleating particles (INP) strongly influence cloud properties, cloud life cycle and precipitation. Describing the abundance of atmospheric INPs in weather forecasts and climate projections remains challenging, as the global distribution and variability of INPs depend on a variety of different aerosol types and sources. Although widespread field measurements have been conducted, neither short-term variability nor long-term seasonal cycles have yet been well characterized by continuous measurements. In 2018, the University of Helsinki and the Karlsruhe Institute of Technology (KIT) initiated a field campaign HyICE to perform comprehensive long-term INP measurements in the Finnish boreal forest. The campaign took place in Hyytiälä, Southern Finland at the University of Helsinki SMEARII research station (Hari and Kulmala, 2005). KIT provided the INSEKT (Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology) to analyse the INP content of ambient aerosols sampled on filters. INSEKT is able to measure INP concentrations in the immersion-freezing mode at temperatures between 273 K and 248 K. The measurements started in March 2018 and ended in May 2019, which provides a unique continuous long-term time series of INP concentrations for over more than one year with a time resolution of about one to three days. This long-term observation record is used to examine systematic seasonal trends in the INP concentrations and to find meteorological and aerosol related parameters to describe the observed trends and variabilities. These findings will enable to find new parameterizations of atmospheric INP concentrations, as current parameterizations do not reproduce the observed seasonal cycle yet. In addition to INP concentration measurements, heat treatment tests of the aerosol samples have been conducted providing additional indications about the INP types dominating the INP population in the boreal forest, also in dependence on the season. Finally, this contribution will summarize and discuss major findings and implications from the HyICE long-term INP observation.

Hari and Kulmala (2005), Boreal Environ Res. 14, 315-322.

How to cite: Schneider, J., Höhler, K., Heikkilä, P., Keskinen, J., Bertozzi, B., Schorr, T., Umo, N., Vogel, F., Brasseur, Z., Wu, Y., Hakala, S., Duplissy, J., Petäjä, T., Adams, M. P., Murray, B. J., Korhonen, K., Thomson, E. S., Castarède, D., Leisner, T., and Möhler, O.: The seasonal cycle of biogenic ice-nucleating particles in a boreal forest environment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16445, https://doi.org/10.5194/egusphere-egu2020-16445, 2020.

The hydrogen bonding structure of adsorbed water on a solid substrate may control deposition nucleation, which is a pathway of heterogeneous ice nucleation. Hydrogen bonding of water molecules is also controlling the interface between the solid and liquid water relevant for other heterogeneous freezing modes. The hydrogen bonding structure may be affected by short and long-range interactions between the substrate and the water molecules nearby. Electron yield near edge X-ray absorption fine structure (NEXAFS) spectroscopy at the oxygen K-edge is used to experimentally explore the difference between the hydrogen bonding structure of interfacial H₂O molecules under different conditions of temperature and water vapor pressure. Experiments reported in this work were performed at the in-situ electron spectroscopy endstation at the ISS beamline at the Swiss Light Source (PSI, SLS). We report electron yield oxygen K-edge NEXAFS spectra and X-ray photoelectron spectra from silver iodide (AgI) particles and milled feldspar samples exposed to water vapor at high relative humidity, but subsaturated with respect to ice. AgI serves as a well-studied reference case; and it contains no oxygen in its lattice, which simplifies the analysis of NEXAFS spectra at the O K-edge. The feldspar samples include a potassium containing microcline and a sodium-rich albite. The analysis of the NEXAFS spectra indicate rather tetrahedrally coordinated adsorbed water molecules on AgI particles. On the feldspars, the mobility of ions, as directly observed by the XPS spectra appears to have a strong impact on the hydrogen bonding structure, as apparent from substantial differences between samples previously immersed in pure water or as prepared. To sum up, we attempt to understand the behavior of the hydrogen bonding structure, which provides rich information about the arrangement of water molecules in the vicinity of a solid surface, that is linked to the ability of the solid to induce ice formation.

How to cite: Ammann, M., Yang, H., Artiglia, L., and Boucly, A.: The hydrogen bonding structure of adsorbed water on silver iodide and Feldspar minerals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13167, https://doi.org/10.5194/egusphere-egu2020-13167, 2020.

Uncertainties in current predictions for the atmosphere’s radiative balance are dominated by the impact of clouds. Ice nucleating particles (INPs) play a dominant role in the formation of mixed-phase clouds, however there is still a lack of understanding of how INPs interact with water in the freezing process. Detailed elucidations of the organic aerosol chemical composition from IN active atmospheric samples are scarce which is due to the analytical challenge of resolving their high complexity. We chose to reduce sample complexity by investigating the IN activity of a specific sub-component of organic aerosols, the biopolymer lignin. This approach facilitates connecting ice nucleating abilities to molecular properties. Ice nucleation experiments were conducted in our home-built Freezing Ice Nuclei Counter (FINC) to measure freezing temperatures in the immersion freezing mode which is the dominant IN mechanism in mixed-phase clouds. We find that lignin acts as an INP at temperatures relevant for mixed-phase cloud processes (e.g. 50% activated fraction at – 20 °C concentrated 20 mg C/L). Photochemistry and ozonation experiments were subsequently conducted to test the effect of atmospheric processing on lignin’s IN activity. We discovered that this activity was not susceptible to change under environmentally relevant conditions even though structural changes were introduced by monitoring UV/Vis absorbance. Additionally to atmospheric processing, laboratory treatments including heating, sonication and oxidation with hydrogen peroxide were done, where only the heating experiments had a decreasing effect on lignin’s IN activity.  Based on these results, we present a thorough INP characterization of lignin, a specific organic matter subcomponent, and contribute to the understanding of how organic material present in the atmosphere can nucleate ice.

How to cite: Bogler, S. and Borduas-Dedekind, N.: Lignin's ability to nucleate ice via immersion freezing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-255, https://doi.org/10.5194/egusphere-egu2020-255, 2020.

Aerosol-cloud interactions are a source of high uncertainties in predicting future climate. One important aerosol-cloud interaction is ice nucleation of supercooled liquid water droplets caused by ice nucleating particles (INPs). Predicting the distribution and concentration of INPs is a challenge because of their spatial and temporal heterogeneity in source, number, and composition. Organic aerosols are particularly diverse and complex in chemical and physical composition and can be highly ice active to varying degrees. Here we present the development of our drop Freezing Ice Nucleation Chamber (FINC) for the quantification of INP concentration of aerosol in the immersion freezing mode. As part of the development and validation of FINC, we show results from an intercomparison using lignin as a comparison standard with three other drop-freezing instruments (ETH’s Drop Freezing Ice Nucleation counter Zurich (DRINCZ), University of Basel’s LED-based Ice Nucleation Detection Apparatus (LINDA), and Weizmann Institute’s Supercooled Droplet Observation of Microarray (WISDOM)). In addition, we present here preliminary findings of FINC’s application for determining predictors of the ice nucleating ability of organic matter, using several standards and field-collected samples of dissolved organic matter as a proxy for organic aerosol emitted from natural waters. These methods and results can aid in the community’s search for predictors and parameterizations of organic aerosol induced ice nucleation.

How to cite: Miller, A., Brennan, K., Wieder, J., Mignani, C., Zipori, A., and Borduas-Dedekind, N.: Development of drop freezing ice nucleation chamber (FINC), validation using lignin, and application to organic matter samples, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-630, https://doi.org/10.5194/egusphere-egu2020-630, 2020.

Primary biological aerosol particles (PBAP) are a significant fraction of total atmospheric aerosol burden and can exhibit unique properties in terms of ice nucleation. In current atmospheric models, it is usually assumed that the physicochemical properties of PBAP are constant during their atmospheric residence time. However, several experimental studies have shown that PBAP undergo microphysical, chemical, and biological ageing processes in the atmosphere. These processes include bacterial agglomeration, modification of protein surfaces by chemical reactions (e.g., nitration) and cellular responses to changing ambient conditions. In addition, possible biological ageing processes such as cell growth and multiplication may change cell size and number. Here, we explore by means of process models the modification of the ice nucleating, hygroscopic and optical (scattering/absorption) properties of PBAP by such ageing processes with an emphasis on biological ageing. We show that cell growth/multiplication of ice-nucleating bacteria could enhance IN activity. Besides, cell modficaiton by ageing processes might change bacteria scattering properties due to size and surface composition modfication. Modification of protein surfaces  decreases IN activity for certain types of PBAP over atmospeherically relevant time scales. We perform model sensitivity tests over wide ranges of chemcial and biological parameters to identify conditions, under which these and other ageing processes have a significant effect on physicochemical properties of aged PBAP. Based on this analysis, we develop parameterizations for PBAP ageing processes to be included in aerosol and cloud models of different scales.

How to cite: Zhang, M., Khaled, A., Amato, P., Delort, A.-M., and Ervens, B.: Surface modification of bioaerosol by physical, chemical, and biological ageing processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1518, https://doi.org/10.5194/egusphere-egu2020-1518, 2020.

The freezing of a supercooled water drop freely falling through a mixed-phase cloud is an ubiquitous natural process fundamental for the formation of precipitation in clouds. The freezing is known to proceed in two stages: first, a network of ice dendrites spreads across the volume of a supercooled droplet resulting in ultrafast release of latent heat and warming of the droplet up to the melting point of ice; during the second stage a solid ice shell grows from the outside into the droplet, leading to a pressure increase inside the liquid core. Once the pressure gets too high, either the shell cracks open or the droplet explodes. The resulting secondary ice fragments start growing in the water-saturated environment or cause the freezing of neighbouring droplets. This secondary ice production mechanism is important for the rapid glaciation of mixed-phase clouds, however, the details of the underlying mechanisms are poorly understood. To quantify this process of ice multiplication, the evolution of the droplet’s surface temperature during the second freezing stage was investigated with a high-resolution infrared thermography system (INFRATEC). Drops of about 300 µm diameter were levitated in an electrodynamic trap under controlled conditions with respect to temperature, humidity and ventilation. The surface temperature of the droplet was measured with the IR system while the freezing process and shattering of the freezing droplet was recorded by a high-speed video camera. Combining experimental results and comprehensive process modelling, we explore the thermodynamic conditions beneficial for secondary ice production upon freezing of freely falling drizzle droplets.

How to cite: Kleinheins, J., Kiselev, A., Keinert, A., and Leisner, T.: Thermal imaging of a shattering freezing water droplet , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2889, https://doi.org/10.5194/egusphere-egu2020-2889, 2020.

Volcanic ash generated by explosive eruptions can act as ice-nucleating particles, promoting freezing of supercooled water droplets in the eruption plume and the ambient atmosphere, and so impacting processes such as plume electrification, ash aggregation, and cloud glaciation. Our initial study of a compositional range of milled ash and glass materials demonstrated that mineralogy is likely a key property influencing ice nucleation by ash.¹ However, the surface properties of ash are modified by interaction with magmatic gases in the hot core of the eruption plume, and it is not known how such in-plume interactions might affect the ice-nucleating activity (INA) of ash.

Here we investigated the influence of high temperature solid-gas interactions on the INA of three milled ash (Tungurahua, Astroni, Etna) and two milled mineral (K-feldspar, quartz) materials. Sub-samples of these materials were exposed to pure water vapour (H₂O) or mixtures of water vapour with HCl_(g) (H₂O-HCl) or SO_2(g) (H₂O-SO₂) under an 800 °C/400 °C heating sequence in the Advanced Gas-Ash Reactor.² The INA of the non-treated and treated samples was then assessed using a microlitre Nucleation by Immersed Particle Instrument.³ The H₂O treatment decreased the INA relative to that of the non-treated sample for all materials, and the H₂O-HCl treatment decreased the INA to the same extent or more. Conversely, the H₂O-SO₂ treatment increased the INA (Tungurahua ash, Etna ash), or decreased the INA 1) to a lesser extent than the other treatments (Astroni ash), 2) to the same extent as the other treatments (quartz), or 3) to a greater extent than the other treatments (K-feldspar).

The depression in INA induced in all cases by the H₂O treatment may relate to dehydroxylation of the silicate materials’ surfaces at high temperatures. On the other hand, differing effects on INA of the H₂O-HCl and H₂O-SO₂ treatments is inferred to relate to contrasting reactivities of these materials towards HCl_(g) and SO_2(g). Water leachates of the samples suggest that chloride and sulphate salts (e.g., NaCl, CaSO₄) formed on the H₂O-HCl- and H₂O-SO₂-treated ash surfaces, respectively, but not on the H₂O-HCl- and H₂O-SO₂-treated mineral surfaces. Additional tests suggest that the changes in INA observed for these treated ash samples do not reflect a ‘solute effect’⁴ imparted by the chloride or sulphate salts in water, implying that the ice-nucleating properties of the ash surfaces themselves are somehow changed by reaction with HCl_(g) and SO_2(g).

Surface-sensitive analyses could be useful to elucidate how sample surfaces have been modified by the different solid-gas interactions at the scale relevant for ice nucleation, and so potentially shed light on the cause of the depression and enhancement in INA observed here. The possibility that in-plume reaction with SO_2(g) can increase the INA of volcanic ash in particular merits further investigation, as a previous line of thought has been that exposure of silicate particles to this acidic gas decreases INA.

¹Maters et al. (2019) Atmos. Chem. Phys., 19, 5451–5465. doi:10.5194/acp-19-5451-2019

²Ayris et al. (2015) Bull. Volcanol., 77, 104. doi:10.1007/s00445-015-0990-3

³Whale et al. (2015) Atmos. Meas. Tech., 8, 2437-2447. doi:10.5194/amt-8-2437-2015

⁴Whale et al. (2018) Chem. Sci., 9, 4142-4151. doi:10.1039/C7SC05421A

How to cite: Maters, E., Casas, A., Cimarelli, C., Dingwell, D., and Murray, B.: Solid-gas interactions in the eruption plume can both depress and enhance volcanic ash ice-nucleating activity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5820, https://doi.org/10.5194/egusphere-egu2020-5820, 2020.

Biological materials are the most active ice nucleating particles (INPs), which can nucleate ice at relatively warm temperatures, affecting cloud properties and regional or even global climate. However, the understanding on the impact of biological INPs in urban areas is quite limited. Beijing is the biggest megacity in North China suffered from severe air pollution. Tianjin is the biggest coastal megacity in North China and influenced by both continental/anthropogenic pollution and marine air masses, especially in summer. In this study, we collected aerosol samples on the urban campuses of Tianjin University (39.11°N, 117.17°E) from 01 to 08 July 2019 and PeKing University (39.99°N, 116.31°E) from 11 to 18 August 2019 with SKC Biosamplers. The concentration of INPs in aerosols has been investigated using the PeKing University Ice Nucleation Array (PKU-INA). The abundance of total bacteria in aerosols was enumerated using the LIVE/DEAD bacterial viability assay and an epifluorescence microscope (DM2500, Leica, Germany). The average concentration of INPs in Beijing (18 ± 23 L^-1) is higher than in Tianjin (8 ± 18 L^-1) at -19 °C. Heat-sensitive INPs inactivated by heat treatment (inactivating ice nucleation protein, 95°C, 15 min) and lysozyme-sensitive INPs (digested by lysozyme) were inferred to biological INPs and bacterial INPs, respectively. The contribution of biological INPs in Beijing (86 ± 14%) was higher than in Tianjin (72 ± 26%), but the proportion of bacterial INPs in Beijing (57 ± 20%) was lower than in Tianjin (64 ± 22%). In addition, we measured the ice nucleation activity of ice nucleating macromolecules (INMs) in filtrate (0.22 µm) and after heat treatment. INMs can be found both in Tianjin and Beijing and the majority of them can be inactivated by heat treatment, indicating most of them were likely proteinaceous materials. Also, we found a significant increase in the concentration of INPs during a rain period with strong wind in Tianjin, which implies rainfall and wind speed may significantly influence the abundance of INPs in this region. Backward air masses trajectories indicated that continental air masses can bring high bacterial INPs in Tianjin and Beijing. Interestingly, the air masses in Tianjin with low bacterial INP concentration were mainly from marine areas. These results imply that biological sources including bacteria may contribute a large fraction of INPs above -19 °C in Tianjin and Beijing in the summer of 2019, and biological INPs potentially play an important role in cloud formation and precipitation in Chinese urban areas.

How to cite: Hu, W., Huang, S., Chen, J., Chen, J., Pei, X., Wu, Z., and Fu, P.: Biological ice nucleation particles in the urban atmosphere of two megacities Beijing and Tianjin in North China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7064, https://doi.org/10.5194/egusphere-egu2020-7064, 2020.

Small particles of silver iodide (AgI) are known to have excellent ice nucleating capabilities and have been used in rain seeding applications. It is widely believed that the silver terminated (0001) surface of β-AgI acts as a template for the basal plane of hexagonal ice. However, the (0001) surface of ionic crystals with the wurtzite structure is polar and will therefore exhibit reconstructions and defects. Here, we use atomistic molecular dynamics simulations to study how the presence of defects on AgI(0001) affects the rates and mechanism of heterogeneous ice nucleation at moderate supercooling at -10 ºC. We first consider AgI(0001) surfaces exhibiting vacancies, step edges, terraces, and pits, and compare them to simulations of the corresponding ideal surface. We find that, while point defects have no significant effect on ice nucleation rates, step edges, terraces, and pits reduce both the nucleation and growth rates by up to an order of magnitude, which can be understood from the atomistic details extracted from the simulations. The reduction of the ice nucleation rate correlates well with the fraction of the surface area around the defects where perturbations of the hydration layer hinder the formation of a critical ice nucleus. Finally, we consider more realistic AgI(0001) surfaces with 5x5 surface reconstructions that cancel the surface dipole, and report on their ice nucleating abilities.

How to cite: Reischl, B., Roudsari, G., Turtola, S., Pakarinen, O., and Vehkamäki, H.: Towards understanding heterogeneous ice nucleation on realistic silver iodide surfaces from atomistic simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7758, https://doi.org/10.5194/egusphere-egu2020-7758, 2020.

Atmospheric mineral aerosols have direct and indirect influence on the climate system. So far, atmospheric interactions studies have mainly focused on pristine samples, despite the fact that aerosol particles may age under natural atmospheric conditions. For example, multiple freeze-melt or evaporation-condensation cycles of an aerosol-containing cloud droplet can change the surface chemistry of the aerosol particle, the droplet ionic strength and pH. These changes have a large impact on the ice nucleation ability of the aerosol particles. We probe the water structure and surface chemistry at water-mineral interface using surface spectroscopic techniques, particularly supercooled nonlinear spectroscopy [1-4]. We found that successive freeze-melt cycles disrupt the dissolution equilibrium, substantially changing the surface chemistry, giving rise to variations ice nucleation ability of the surface [4]. Along the aging process, the restructuring of the water molecules at the surface upon cooling changes. This was found to be correlated to the ice nucleation ability of the surface. We present here a spectroscopic overview on aging of selected mineral surfaces (Al₂O₃, Silica, Mica and PbO). We found that the pH, ionic strength, time in contact with water and number of freezing-melting events influence the aging dynamics and hence the ice nucleation ability.

1. Abdelmonem, A., Direct Molecular-Level Characterization of Different Heterogeneous Freezing Modes on Mica – Part 1. Atmos. Chem. Phys., 2017. 17(17): p. 10733-10741.
2. Abdelmonem, A., et al., Surface-Charge-Induced Orientation of Interfacial Water Suppresses Heterogeneous Ice Nucleation on α-Alumina (0001). Atmos. Chem. Phys., 2017. 17(12): p. 7827-7837.
3. Lützenkirchen, J., et al., A set-up for simultaneous measurement of second harmonic generation and streaming potential and some test applications. Journal of Colloid and Interface Science, 2018. 529: p. 294-305.
4. Abdelmonem, A., et al., Cloud history changes water-ice-surface interactions of oxide mineral aerosols (e.g. Silica). Atmos. Chem. Phys. Discuss., 2019. 2019: p. 1-17.

How to cite: Abdelmonem, A., Lützenkirchen, J., Ratnayake, S., and Hiranuma, N.: A spectroscopic view of mineral aerosol surface aging under atmospheric conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8279, https://doi.org/10.5194/egusphere-egu2020-8279, 2020.

Several biological particles are able to trigger heterogeneous ice nucleation at subzero temperatures above -38°C. Many plants species such as winter rye [1], certain berries [2], pines and birches [3, 4] are known to contain biological ice-nucleating particles (BINPs) or rather ice-nucleating macromolecules (INMs). However, the influence of these BINPs on atmospheric processes including cloud glaciation and precipitation formation, as well as transport mechanisms of BINPs from the land surface into the atmosphere remain uncertain. If those INMs are easily available on the surfaces of a plant, they could be washed down by heavy rain events and could add an important new source for BINPs in the atmosphere, which has not received enough attention in the past.

In this study, we have focused on alpine trees, which form INMs extractable from their surfaces. We examined ice nucleation activity of samples from different birches (Betula pendula) and pines (Pinus sylvestris) growing in the Alps in Austria, Europe. Filtered aqueous extracts of leaves, needles, bark and wood were analyzed in the laboratory in terms of heterogeneous ice nucleation using VODCA (Vienna Optical Droplet Crystallization Analyzer), a cryo-microscope  for  emulsion  samples.  All plant tissues contained INMs in the submicron size range. Furthermore, we conducted a field experiment, in which we investigated the possibility of INMs to be released from the surface of the trees into the atmosphere during rain showers.

[1] Brush, R.A., M. Griffith, and A. Mlynarz, Characterization and Quantification of Intrinsic Ice Nucleators in Winter Rye (Secale cereale) Leaves. Plant Physiol, 1994. 104(2): p. 725-735.

[2] Felgitsch, L., et al., Heterogeneous Freezing of Liquid Suspensions Including Juices and Extracts from Berries and Leaves from Perennial Plants. Atmosphere, 2019. 10(1): p. 1-22.

 [3] Pomeroy, M.K., D. Siminovitch, and F. Wightman, Seasonal biochemical changes in the living bark and needles of red pine (Pinus resinosa) in relation to adaptation to freezing. Canadian Journal of Botany, 1970. 48(5): p. 953-967.

[4] Felgitsch, L., et al., Birch leaves and branches as a source of ice-nucleating macromolecules. Atmospheric Chemistry and Physics, 2018. 18(21): p. 16063-16079.

[5] Pummer, B.G., et al., Ice nucleation by water-soluble macromolecules. Atmospheric Chemistry and Physics, 2015. 15(8): p. 4077-4091.

How to cite: Grothe, H., Seifried, T. M., Bieber, P., and Felgitsch, L.: The Surface of Tree Tissues as Source of Extractable Ice Nucleating Macromolecules during Rainfall Events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11569, https://doi.org/10.5194/egusphere-egu2020-11569, 2020.

Terrestrial ecosystems can contribute various particles to the troposphere, some of which are known for their ice nucleation activity. Most of the land-surface in Europe is covered with forests and fields, representing potential sources of ice nucleation active bioaerosols in form of pollen grains, fungal spores and bacterial cells. The presence of biogenic ice-nucleating particles (INPs) in clouds leads to heterogeneous freezing events and therefore influences the hydrological cycle and the Earth’s climate. Many studies focus on measurements and characterizations of INPs in clouds using aircrafts or sample on ground with stationary devices. Less is known about the actual emission and transport to high tropospheric layers. We focused on the development of an efficient sampling device that can be attached to small scale drones, such as the DJI Phantom 4 model. The Drone-based Aerosol Particles Sampling Impinger/Impactor (DAPSI) system was developed to sample airborne INPs above emission sources. It includes a cascade impactor that collects particles with size resolution and a self-build impinging system that accumulates INPs in a sterile solution. Additionally, the system contains an electric sensor for environmental data records (temperature, relative humidity and air pressure) as well as an optical particle counter to monitor particular matter concentrations during flight times. This study leads through the building, characterization and test-campaign of DAPSI. We present a validation test, regarding the sampling effectivity to sample aerosols (polystyrene latex spheres and INPs) as well as results from the first field campaign which took place in a rural sampling site in the Austrian Alps. Fluorescence- and cryo-microscopic assays show auto-fluorescent particles and heterogeneous ice nucleation activity of DAPSI samples. We highlight the opportunity to use DAPSI with small un(wo)manned aerial vehicles during field campaigns to sample and identify biogenic INPs in vertical and spatial resolution above emission sources.

How to cite: Bieber, P., Seifried, T. M., Gratzl, J., Burkart, J., Kasper-Giebl, A., Schmale III, D. G., and Grothe, H.: Development, Characterization and Testing of a Drone-based Sampling Method for Investigations of Ice-Nucleating Particles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12099, https://doi.org/10.5194/egusphere-egu2020-12099, 2020.

Deposition ice nucleation (IN) is a heterogeneous pathway by which water vapor deposits directly onto a solid surface and forms ice. Deposition IN happens below water saturation. However, the pore condensation and freezing (PCF) mechanism offers another explanation to ice formation on porous particles at low ice supersaturation. A single black carbon (BC) aggregate consists of several primary particles, forming crevices between primary particles. Whether BC IN happens via deposition or PCF remains uncertain due to the fractal nature of BC particles.

We estimated aggregate surface area, morphology, and primary particle size distribution directly from scanning electron microscopy (SEM) images of size-selected (200 nm, 300 nm, and 400 nm) commercial BC particles. Correlations between surface area data obtained from SEM image estimation and traditional BET tests were explored. Several shape parameters were chosen to characterize particle morphology. The IN ability of aerosolized BC particles was determined with the Spectrometer for Ice Nuclei (SPIN) in the cirrus regime (-46 to -38°C). Particle number concentration and chemical composition were monitored online by a Condensation Particle Counter (CPC) and the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument, respectively.

Preliminary experimental results suggest that larger (400 nm) BC particles are more fractal and branching compared with smaller (200-300 nm) particles. Larger, more fractal BC particles are superior ice nucleating particles (INP) when compared with smaller, more spherical ones. The primary particle size distribution of all samples peaks around 30-45 nm. To understand the relevance of the PCF mechanism with our experimental IN results, we established Young-Laplace equations for the potential liquid-vapor interfaces within inter-primary particle crevices and pores and inter-aggregate pores. Solutions of the Young-Laplace equation on a saddle surface was deducted. Whether ice nucleation happens via PCF mechanism or deposition still requires further investigation, since particle surface chemistry can also affect both ice formation pathways.

How to cite: Zhang, C., Zhang, Y., Wolf, M., Chen, L., and Cziczo, D.: Ice nucleation by black carbon particles in the cirrus regime: dominated by pore condensation and freezing or deposition ice nucleation?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12357, https://doi.org/10.5194/egusphere-egu2020-12357, 2020.

The effect of aerosol particles on ice nucleation and, in turn, the formation of ice and mixed phase clouds is recognized as one of the largest sources of uncertainty in weather and climate prediction.  We utilize an improved sectional dust module in NASA GISS Earth System ModelE2.1, which distinguishes eight different mineral species and accretions between iron oxides and the other minerals.  Simulations over a period of 20 years have been carried out with this model, and the mineral fields and other model variables (temperature, relative humidity) are used to calculate the ice nucleating particle (INP) number concentration, applying time-independent and time-dependent INP parameterizations, such as active site parameterization and water activity based immersion freezing model (ABIFM).  We study how the dependence of the parameterizations on different model variables affects the mean INP number concentration.  The sensitivity of the INP number concentration to fundamental dust properties such as emitted mineral size distributions and mixing state between minerals is also investigated.  Results show that the sensitivity of the total INP number concentration to the emitted dust size distribution is rather small, but the sensitivity over the whole size range obscures offsetting differences in the magnitude and the sign of the sensitivity between smaller and larger particles.

How to cite: Perlwitz, J., Knopf, D., and Miller, R.: Comparison of INP Parameterizations for Dust Minerals in Climatological Simulations With a Global Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13437, https://doi.org/10.5194/egusphere-egu2020-13437, 2020.

This study investigates to what degree the information about the Drop Size Distribution (DSD) of rain can be used to narrow down uncertainty associated with complex ice microphysics. For this purpose, measurements from vertically pointing multi-frequency Doppler radar are thoroughly analysed. Linear Depolarization Ratio information is used to unambiguously separate hydrometeor phases. Within radar volumes where pure rain is identified multi-frequency Doppler spectra are utilised to retrieve a binned DSD with a high degree of confidence (Tridon et al. 2017). By assuming no breakup and negligible interaction between melting particles (Szyrmer and Zawadzki 1999, Olson et al. 2001, Matrosov 2008) the rain drop size distribution closest to the melting region is used to predict the particle size distribution (PSD) in the overlying snow. With these assumptions the resulting shape of the ice PSD depends solely on the hydrodynamical properties of snow that are dictated by its microphysics.  Several ice models are considered in the analysis, ranging from aggregates of columns, dendrites, needles and plates to different stages of rimed snow. Their scattering properties are simulated with Self-Similar-Rayleigh-Gans approximation (Leinonen et al. 2018) whereas falling velocities are modelled after Khvorostyanov and Curry (2005). Doppler spectra are simulated for the predicted ice PSD and compared to the measurements above the melting region. Results suggest that, if appropriate snow model used, the predicted reflectivity differs by less than 3 dB from the measured values as has been tentatively suggested by Fabry and Zawadzki (1995).

Tridon, F., A. Battaglia, E. Luke, P. Kollias, 2017. Rain retrieval from dual-frequency radar Doppler spectra: validation and potential for a midlatitude precipitating case study. Q. J. Roy. Meteorol. Soc. 143, 1364-1380. DOI: 10.1002/qj.3010

Szyrmer, W. and I. Zawadzki, 1999: Modeling of the Melting Layer. Part I: Dynamics and Microphysics. J. Atmos. Sci., 56, 3573–3592, https://doi.org/10.1175/1520-0469(1999)056<3573:MOTMLP>2.0.CO;2

S. Olson, P. Bauer, N. F. Viltard, D. E. Johnson, W-K. Tao, R. Meneghini, and L. Liao, “A melting layer model for passive/active microwave remote sensing applications—Part I: Model formulation and comparisons with observations,” J. Appl. Meteorol., vol. 40, no. 7, pp. 1145–1163, Jul. 2001

Y. Matrosov, "Assessment of Radar Signal Attenuation Caused by the Melting Hydrometeor Layer," in IEEE Transactions on Geoscience and Remote Sensing, vol. 46, no. 4, pp. 1039-1047, April 2008. doi: 10.1109/TGRS.2008.915757

Fabry, F., and I. Zawadzki, 1995: Long-term radar observations of the melting layer of precipitation and their interpretation. J. Atmos. Sci., 52, 838–851.

Jussi, Leinonen, Kneifel, Stefan, Hogan, Robin J.. Evaluation of the Rayleigh–Gans approximation for microwave scattering by rimed snowflakes. Q J R Meteorol Soc 2018; 144 ( Suppl. 1): 77– 88. https://doi.org/10.1002/qj.3093

How to cite: Mroz, K., Battaglia, A., Kneifel, S., and Neto, J. D.: Do the rain microphysics provide information on the overlying ice cloud?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16462, https://doi.org/10.5194/egusphere-egu2020-16462, 2020.

Understanding the formation of ice is of great importance to many fields of science. Sufficiently pure water droplets can remain in the supercooled liquid phase to nearly -40 ºC. Crystallization of ice in the atmosphere therefore typically occurs in the presence of ice nucleating particles (INPs), such as mineral dust or organic particles. These can trigger heterogeneous ice nucleation at clearly higher temperatures. Therefore, a better understanding of how the various types of aerosol particles present in the atmosphere affect ice nucleation (IN) in clouds would be an important advance in the field of atmospheric science.

Experiments have shown in great detail what is the IN activity of different types of compounds, and recently also clarified the importance of small surface features such as surface defects, which function as active sites for ice nucleation. On most mineral dust particles, there may be only a few active sites for ice nucleation, typically around defects or pits (Holden et al., 2019). Simulations also showed enhanced ice nucleation efficiency in confined geometry such as wedges or pits (Bi, Cao and Li, 2017).

We systematically study the effect of water confining defects with different surface geometries; pyramidal pits, wedge-shaped cracks and slits with water confined between two parallel walls, using molecular dynamics simulations with both all-atom and monatomic water models, and show that that these defects enhance ice nucleation both at large supercooling and at very low supercooling.

Results of simulations on pyramidal pits on Si (100) surfaces, realizable experimentally, show a clear (∆T > 10 ºC) enhancement of ice nucleation compared to the very weakly IN active flat Si (100) or Si (111) surfaces. To show that water confinement can enhance IN also at very low supercooling, at temperatures above −10 ºC, we constructed wedge shaped structures with β-AgI (0001) surface as one of the two side walls, and slit systems by positioning two β-AgI (0001) slabs to mirror each other to cancel the dipole field from the polar surfaces. Depending on the wedge angle or the relation of the width of the gap between two slabs in the slit systems with the thickness of ice bilayers, ice nucleation can be clearly enhanced or hindered. We also clarify the different mechanisms behind IN enhancement at different geometries.

Understanding the enhanced activity at surface features may enable characterization of ice nucleation active sites on some atmospheric particles, creation of IN active sites at otherwise poorly active materials such as silicon, and also enable enhancing very active IN materials such as AgI, to nucleate ice at nearly zero supercooling.

This work was supported by the Academy of Finland Center of Excellence programme (grant no. 307331) and ARKTIKO project 285067 ICINA, by University of Helsinki, Faculty of Science ATMATH project, by the National Center for Meteorology (NCM), Abu Dhabi, UAE, under the UAE Research Program for Rain Enhancement Science, as well as ERC Grant 692891-DAMOCLES. Supercomputing resources were provided by CSC–IT Center for Science, Ltd, Finland.

How to cite: Pakarinen, O., Roudsari, G., Reischl, B., and Vehkamäki, H.: Effect of water confinement on heterogeneous ice nucleation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19008, https://doi.org/10.5194/egusphere-egu2020-19008, 2020.

The formation of ice in mixed phase clouds occurs in the presence of aerosol particles with the ability to nucleate ice on their surface. These ice-nucleating particles (INPs) represent usually a small fraction of particles in an atmospheric aerosol. One of the main particle types which act as INPs are mineral dust particles. Among other factors, the accumulation of semivolatile substances on the particle surface can alter the ice nucleation properties of such particles.

In recent immersion freezing experiments, we investigated the influence of organic acids, amino acids and polyols on the highly ice nucleation active K-feldspar microcline. Microcline dust was suspended in solutions of the above-mentioned substances and frozen in a differential scanning calorimeter (DSC). These experiments give us insight into the ice nucleation characteristics of the particles in the presence of the tested organic and biogenic substances. Our measurements show an overall decrease in ice nucleation activity of microcline in the presence of organic acids and amino acids.

How to cite: Klumpp, K., Marcolli, C., and Peter, T.: Influence of organic and biogenic substances on the ice nucleation properties of mineral dust particles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19183, https://doi.org/10.5194/egusphere-egu2020-19183, 2020.

We performed laboratory experiments to examine the relationship between the technique of preparing a frozen sample; its morphology; and chemical speciation. The luminescence of aromatic compounds (naphthalene, methylnaphthalene, anthracene) was utilized to learn about the extent of the compounds’ aggregation.

In aqueous solutions, the cooling rate determines the morphology of the resulting ice samples (analyzed with an environmental scanning microscope), (Vetráková, Neděla et al. 2019) the extent of the solute crystallization, and the plasticity of the freeze-concentrated solution (FCS) glass formed in the veins in between the ice crystals. Faster cooling allows higher degree of hydration of the aromatic compounds in the FCS. Conversely, vapor deposition of naphthalene on ice at 253 K results in microscopic naphthalene crystals, suggesting molecular mobility on the surface layer of ice at atmospherically relevant temperatures. (Ondrušková, Krausko et al. 2018) Surface acidity was proved via sulfonephthalein dyes, finding strong dependence on the salts present.(Imrichova, Vesely et al. 2019)

Our results connect the freezing methods and sample history with the compounds’ chemical identity in ice.

Imrichova, K., L. Vesely, T. M. Gasser, T. Loerting, V. Nedela and D. Heger (2019). "Vitrification and increase of basicity in between ice Ih crystals in rapidly frozen dilute NaCl aqueous solutions." J Chem Phys 151(1): 014503.

Ondrušková, G., J. Krausko, J. N. Stern, A. Hauptmann, T. Loerting and D. Heger (2018). "Distinct Speciation of Naphthalene Vapor Deposited on Ice Surfaces at 253 or 77 K: Formation of Submicrometer-Sized Crystals or an Amorphous Layer." The Journal of Physical Chemistry C 122(22): 11945-11953.

Vetráková, Ľ., V. Neděla, J. Runštuk and D. Heger (2019). "The morphology of ice and liquid brine in an environmental scanning electron microscope: a study of the freezing methods." The Cryosphere 13(9): 2385-2405.

How to cite: Heger, D.: The genesis of an ice sample matters!, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20274, https://doi.org/10.5194/egusphere-egu2020-20274, 2020.

Ice nucleating particles (INPs) play an important role in the climate system by influencing cloud radiative properties, cloud lifetime and precipitation. An understanding of the interaction between INPs and clouds is needed in order to improve the accuracy of both climate projections and short term weather forecasts. In the high latitudes the influence of mid and low latitude sources of INPs (such as potassium feldspar from desert dust) is reduced and local sources could be important for ice nucleation. However, there is a scarcity of field observations and many dust sources which could be important sources of INPs have not been quantified. The south coast of Alaska, in particular the Copper River valley in the Valdez-Cordova region, is one such area where there are regular dust storms. These can clearly be seen from satellite imagery, which provides information on the frequency and extent of these outbreaks. In order to investigate the potential importance of the Copper River valley as a source of INPs we undertook a field campaign to collect samples in October 2019. During this campaign size segregated aerosol samples from the near surface (1.5  metre) were collected on to polycarbonate filter substrates using a multistage cascade impactor with 5 size categories in the range <0.25 μm to >2.5 μm. We collected samples during 7 dust emission events over a 10 day period. In addition, samples of dry sediment were collected from the surface. We used the University of Leeds Microlitre Nucleation by Immersed Particle Instrument (μL-NIPI) to quantify the ice nucleating ability of these samples. We also used laser diffraction particle size analysis to determine the surface area of particles to allow the subsequent calculation of ice active surface site density (n_s). In addition, surface samples were separated in order to isolate the atmospherically relevant fraction (<10 μm) and used to determine the chemical composition of the dust using x-ray diffraction. This, combined with further work such as heat testing, will be used to identify what controls the ice nucleating efficacy in this dust and if there is an active biological contribution.  We will present the results from the field campaign and subsequent analysis. These results show high ice nucleating activity of the samples, comparable to glacial dust from other regions, and highlight the importance of glacial dust as a source of INPs in the high latitudes.

How to cite: Barr, S., Wyld, B., Ratcliffe, N., McQuaid, J., and Murray, B.: The ice-nucleating efficacy of glacial dust from the Copper River, Alaska, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21557, https://doi.org/10.5194/egusphere-egu2020-21557, 2020.

Ice nucleating particles (INP) are a subset of atmospheric aerosols which strongly influence the radiative properties and precipitation onset in mixed phase clouds. Mineral dust and biogenic particles such as bacteria, pollen and fungal spores can act as INP and are present in the atmosphere as internal or external mixtures. However, the sources, abundance and distribution of INP are poorly understood. The current widely accepted method of determining the relative contributions of mineral and biogenic INP is to treat INP samples with heat. This is based on the hypothesis that proteinaceous biogenic INP will be deactivated and mineral INP will be unaffected. However the hypothesis that mineral INP are never deactivated by heating not been tested to date. Mineral surfaces may undergo a range of geochemical reactions when heated in air or water and the potential effects of this on their ice nucleating activity is not known. We therefore subjected a range of atmospherically relevant minerals and atmospheric dust analogues to heat treatments equivalent to those used in the past studies. The samples were heated both as aqueous suspensions (100°C for 30 minutes) and in air as dry powders (250°C for 4 hours) and their ice nucleating activity was tested before and after treatment with a microlitre droplet freezing assay. We found that silica based samples showed a significant response to aqueous heating (reduction of median freezing temperature of a 1% suspension of up to 5.4°C) but little response to dry heating. Similar responses were seen in Arizona Test Dust and calcite. In contrast, K-feldspar samples were largely unaffected by aqueous heating but some showed mild deactivations when dry heated. Notably, K-feldspar was sensitive to longer heat treatments. Overall this survey shows that the assumption that mineral INP are completely inert to heat should be reconsidered in the context of using heat to ‘detect’ biogenic INP. We conclude that while INP heat tests are an effective method for positive detection of biogenic INP in ambient air samples they have the potential to produce false positives. For example, in a silica-rich mineral dust a deactivation may be related to the mineral component. Nevertheless, in samples where the mineral ice nucleating activity is determined by K-feldspar the aqueous heat test provides a valid qualitative test for proteinaceous biological INP.

How to cite: Daily, M. I., Whale, T. F., and Murray, B. J.: The sensitivity of ice-nucleating minerals to heat and implications for the detection of biogenic ice-nucleating particles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22458, https://doi.org/10.5194/egusphere-egu2020-22458, 2020.