AS3.16

The aerosols that can act as ice nucleating particles (INPs) are of a large variety in both size and chemical composition. This leads to different temperatures at which INPs can nucleate ice. In recent years, the type and properties of INP species that initiate freezing at certain conditions have been investigated in more and more detail. If this growing knowledge is to be used in models, it implies that the models hold very detailed information about the aerosol species abundant in clouds. Especially for climate models, this is a difficult challenge.

In this study, we approach the problem of parameterizing heterogeneous ice nucleation from another angle: assuming well-diluted background aerosol, the probability of a specific INP concentration at the current temperature follows a log-normal distribution. We derived relative frequency distribution functions (RFDs) from measurements in marine environments (Welti et al., 2018). The number of INPs for the current temperature is being drawn from this RFD following its weighting. Thus, one randomly selected INP concentration is used from the range of all possible INP concentrations derived from observations at the given temperature. The advantage of our freezing parameterization is that it does not need any information about the chemical composition or size of the aerosols present in the cloud. It is valid for remote locations that are not close to a source of INPs, e.g. maritime or polar sites.

We implemented this new parameterization into the large-eddy simulation model MIMICA (Savre et al., 2014) and evaluated it for a mixed-phase Arctic cloud observed during the ASCOS expedition (Tjernström et al., 2014). For the Arctic there is large uncertainty about the types of INPs as well as their concentration, which is a challenge for modelling mixed phase clouds in this region. In our talk, we show that our new scheme does work well for the simulated case. We will present the performance of this new framework, as well as its sensitivity to RFD distribution variables and the model domain resolution. Additionally, we compare the new parameterization to “classic” heterogeneous nucleation schemes, such as a simple active sites parameterization.

Savre, J., Ekman, A. M. L., and Svensson, G. (2014), Technical note: Introduction to MIMICA, a large-eddy simulation solver for cloudy planetary boundary layers, J. Adv. Model. Earth Syst., 6, 630–649, doi:10.1002/ 2013MS000292.
Tjernström, M.; Leck, C., et al. (2014), The Arctic Summer Cloud Ocean Study (ASCOS): overview and experimental design, Atmos. Chem. Phys., 14, 2823–2869.
Welti, A., Müller, K., Fleming, Z. L., and Stratmann, F. (2018), Concentration and variability of ice nuclei in the subtropical maritime boundary layer, Atmos. Chem. Phys., 18, 5307–5320.

How to cite: Frostenberg, H., Welti, A., Sjöstrand, M., and Ickes, L.: How can we make use of the observed variability of ice nucleating particle concentrations in models?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-140, https://doi.org/10.5194/egusphere-egu22-140, 2022.

Aircraft operate mainly in the upper troposphere/lower stratosphere — altitudes where the aerosol loading is rather low — emitting gases (mainly H₂O and CO₂) and soot particles (a result of the incomplete combustion of aviation fuel). At these altitudes, clouds composed of micrometric ice crystals (known as cirrus clouds) originate from the freezing of small liquid droplets and/or from the deposition of water vapor onto solid particles (called ice nucleating particles, INPs). Aircraft soot particles are thought to be efficient INPs for cirrus-cloud formation, therefore potentially disturbing the cirrus cloud coverage, resulting in a modified cloud radiative budget, hence affecting climate. To date, the ice-nucleating abilities (INAs) of aircraft soot have not been quantified partly because of the challenge to sample such particles behind a turbine engine.

In this work, we present a series of experiments conducted at the aircraft engine test cell of SR Technics at Zurich airport, aiming at quantifying the INAs of aircraft turbine soot particles. Exhaust from commercial turbofan engines was sampled using a traversable probe within 1.5 m downstream of the exhaust nozzle over a range over power levels from medium to high thrust. The exhaust sample was drawn through trace-heated lines and a series of driers into a stirred stainless steel tank, allowing the coagulation of the particles, similar to that thought to occur in the restricted volume between aircraft wingtip vortices. The stainless steel tank also acts as a reservoir for the rest of the ice nucleation experiment. The coagulated particles were then size-selected according to their electrical mobility diameters in all experiments and injected into a cloud chamber where they experienced cirrus-relevant temperature (T < -40 °C) and relative humidity (RH_ice > 100%) conditions, allowing them to form ice crystals. Together with the inline particle size and mass distribution measurements, the fraction of soot particles forming ice crystals at different RH_ice levels has been measured. A catalytic stripper operating at 350°C removing volatile material and sulfur was used upstream of the cloud chamber, helping to infer the effect of the mixing state on the soot INAs. In parallel, soot samples were collected for additional offline measurements. Microscopy and gas adsorption techniques were used to characterize the morphology of the soot particles (e.g., primary particle size, pore size distribution) as well as their surface properties (e.g., water affinity, organic/inorganic content) which are known to be critical parameters for the freezing mechanism of soot particles in the cirrus regime. Preliminary results show that samples after the conditioning with the catalytic stripper are more active INPs than the unstripped, suggesting that mixing state and organic/sulfur content could be important for determining the role of aircraft soot as INPs in the upper troposphere.

How to cite: Testa, B., Durdina, L., Edebeli, J., Spirig, C., Anet, J., and Kanji, Z.: Ice nucleating properties of aircraft turbine engine soot particles with respect to cirrus clouds formations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1647, https://doi.org/10.5194/egusphere-egu22-1647, 2022.

In mixed-phase clouds, primary ice formation occurs only in the presence of ice-nucleating particles (INPs) [Vali et al., 2015], which are a very rare subset of the aerosols in the atmosphere. INPs are an important part of the earth climate system, as they can initiate the formation of precipitation [Mülmenstädt et al., 2015] and have an influence on the cloud radiative properties [Murray et. al, 2021]. In the last decades, different INP measurements have been performed within the atmospheric boundary layer at mixed-phase clouds conditions [Kanji et. al, 2017]. INP measurements in the free troposphere are challenging, as they can only be conducted by aircraft-based measurements or at high altitude mountain stations. However, it is important to study long-term INP concentrations at different altitudes and geographical locations to get a better understanding of the presence of INPs in the atmosphere.

The Sonnblick Observatory (SBO) in the Austrian Alps is located at an altitude of 3106 m above sea level and is an ideal location to investigate the INP concentration in the free troposphere, as the measured INPs are directly relevant for ice formation. Since August 2019, we continuously measure the INP concentration at the SBO via filter collection and offline analysis with INSEKT (Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology) [Schneider et. al, 2020]. The analysis of the sampled aerosols gives us the temperature dependent number concentration of INPs at temperatures above -25°C with a time resolution of one week. In order to receive a better insight into short-term fluctuations, we performed additional measurements with the online INP measurement PINE (Portable Ice Nucleation Experiment) [Möhler et al., 2021], since the end of July 2021. PINE has a time resolution of 5 – 6 min and usually measures at a constant temperature of ~ -23°C. In addition, INP activity screenings in the range from -15°C to -30°C are performed in regular intervals and during interesting meteorological periods such as Saharan dust events. The overlap in temperature ranges of INSEKT and PINE enables a comparison between the two measuring instruments.

Our results show that there are significant seasonal variations in the INP concentration. Especially during the summer time, strong diurnal variations in the INP concentration were observed, which could be explained by the influence of convectively lifted air from the boundary layer during the day. Correlations of the measured INP concentrations to meteorological parameters, aerosol properties and boundary layer stability will be discussed. In addition, we will present a case study of a dust event, which shows a sudden, strong increase in the INP concentration.

How to cite: Bogert, P., Graf, J., Lacher, L., Höhler, K., Ludewig, E., and Möhler, O.: A two-year record of INP concentration measurements at the Sonnblick Observatory (3106 m): Insight into seasonal variability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1606, https://doi.org/10.5194/egusphere-egu22-1606, 2022.

The properties and formation of clouds are one of the largest sources of uncertainties in climate models. Hereby, ice nucleating particles (INPs) play a major role since they directly affect the ice formation in clouds. To better characterize the impact of INPs, measuring devices are necessary to reliably determine the freezing temperatures of various aerosols.

We have developed a new ice nucleation assay, AU-Micro-INC, to measure the freezing temperatures with high accuracy. 96-well and 384-well plates can be inserted into a gallium matrix which ensures good thermal contact to the underlying cooling system. A Peltier element in combination with a vapor chamber provide a homogeneous cooling of the system. The freezing temperatures are measured with an infrared thermal camera with high precision.

The setup is validated using well-studied samples such as Snomax^® and Illite NX. Further, the new setup is applied to sea water, sea surface microlayer, and sea ice samples previously collected in Kobbefjord and Nuup Kangerluaand in proximity of Nuuk, Greenland and preliminary data will be shown. 

How to cite: Wieber, C., Madsen, S. A., Jeppesen, M. R., Voldbirk, F., Melvad, P., Chuhutin, A., Vergeynst, L., Meire, L., Finster, K., Melvad, C., and Šantl-Temkiv, T.: The new ice nucleation assay AU-Micro-INC, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4323, https://doi.org/10.5194/egusphere-egu22-4323, 2022.

Precipitation is mostly formed via the ice phase in mixed phase clouds, and ice clouds are very relevant for Earths’ climate. Freezing or prevention of freezing is common to everyday life, e.g. for food and drug storage, icing and de-icing, etc. However, the ice nucleation process is not well understood, since it occurs on the size scale of clusters of molecules and time scales of molecular fluctuations. In this study, we have taken a step toward nanoscale observation of particles that nucleate ice by developing a new ice nucleation instrument, referred as the INXcell, which couples an ice nucleation environmental cell to the scanning transmission X-ray microscope (STXM) at the Swiss Light Source. We employ near-edge X-ray absorption fine-structure spectroscopy (NEXAFS) to map in situ chemical composition of ice nucleating particles with 35 × 35 nm² spatial resolution. The main technical challenge was control of temperature, T, and thus relative humidity, RH, while maintaining X-ray transparency. In the INXcell, X-rays are focused onto a sample through a temperature-controlled aperture, which was modified to host a jet of nitrogen cooled down to 170 K. The cold jet impinges on the back surface of a sample exposed to water vapor to control sample temperature and thus RH. We used our unique spectroscopic and ice nucleation capability and investigated the heterogeneous freezing ability of ferrihydrite particles with and without coatings of citric acid. Ferrihydrite is an amorphous or poorly crystalline iron oxyhydroxide abundant in mineral dust and is difficult to identify with conventional XRD analysis. We confirmed that ferrihydrite could nucleate ice via immersion freezing and deposition ice nucleation, depending on whether or not the particles first take up water, respectively. When coating ferrihydrite with citric acid, mimicking organic coatings that aerosol particles obtain throughout their atmospheric lifetime, we observed a reduction in the efficiency to nucleate ice following freezing point depression. Spectroscopic identification of the coated ferrihydrite structure emplyed the iron and carbon X-ray absorption L-edges and K-edge, respectively. We also investigated feldspar particles coated with xanthan gum, a surrogate for a highly ice active mineral with a highly viscous organic coating. We observed that deposition ice nucleation occurred only below the RH dependent glass transition of xanthan gum. Using a newly developed stochastic freezing model (SFM) based on solution water activity, we reproduced average conditions and data scatter of the RH and T at which ice formed. Additionally, we ran our model with atmospheric idealized air parcel trajectories and found overall that deposition ice nucleation was the dominant heterogeneous freezing mechanism. Homogeneous ice nucleation subsequent to water uptake out-performed immersion freezing.

How to cite: Alpert, P., Boucly, A., Shuo, Y., Yang, H., Kilchhofer, K., Luo, Z., Padeste, C., Finizio, S., Ammann, M., and Watts, B.: Ice Nucleation Imaged In Situ with X-ray Spectro-Microscopy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11105, https://doi.org/10.5194/egusphere-egu22-11105, 2022.

Within the last years pollen grains have gained increasing attention due to their cloud-forming potential. Especially the discovery that ice nucleating macromolecules (INMs) or subpollen particles (SPPs) obtained from pollen grains are able to initiate freezing has stirred up interest in pollen. INMs or SPPs are much smaller and potentially more numerous than pollen grains and could significantly affect cloud formation in the atmosphere.

However, INMs and SPPs are not clearly distinguished. This has motivated the present study, which focuses on birch pollen and investigates the relationship between pollen grains, INMs and SPPs. According to the usage of the term SPPs in the medical fields, we define SPPs as the starch granules contained in pollen grains. We show that these insoluble SPPs are only obtained when fresh pollen grains are used to generate aqueous extracts from pollen. Due to the limited seasonal availability of fresh pollen grains almost all studies have been conducted with commercial pollen grains. To enable the investigation of the SPPs we develop an alternative extraction method to generate large quantities of SPPs from commercial pollen grains. We show that INM are not bonded to SPPs (i.e. can be washed off with water). Further, we find that purified SPP are not ice nucleation active: after several times of washing SPPs with ultrapure water the ice nucleation activity completely disappears. To our knowledge, this is the first study to investigate the ice nucleation activity of isolated SPPs.

To study the chemical nature of the INMs, we use fluorescence spectroscopy. Fluorescence excitation-emission maps indicate a strong signal in the protein range (maximum around λ_ex = 280 nm and λ_em = 330 nm) with all ice nucleation active samples. In contrast, with purified SPP the protein signal is lost. We also quantify the protein concentration with the Bradford assay. The protein concentration ranges from 77.4 μg mL^-1 (Highly concentrated INM) to below 2.5 μg mL^-1 (purified SPP). Moreover, we investigate the connection between proteins and ice nucleation activity by treating the ice nucleation active samples with subtilisin A and urea to unfold and digest the proteins.  After this treatment the ice nucleation activity clearly diminished. The results indicate a linkage between ice nucleation activity and protein concentration. The missing piece of the puzzle could be a glycoprotein, which exhibits carboxylate functionalities, can bind water in tertiary structures and displays degeneration and unfolding of its secondary structure due to heat treatment or reaction with enzymes. Even though purified SPPs are not ice nucleation active they could act as carriers of INMs and distribute those in the atmosphere.

Reference of the study: Burkart, J., Gratzl, J., Seifried, T. M., Bieber, P., and Grothe, H.: Isolation of subpollen particles (SPPs) of birch: SPPs are potential carriers of ice nucleating macromolecules, Biogeosciences, 18, 5751–5765, https://doi.org/10.5194/bg-18-5751-2021, 2021. 

How to cite: Burkart, J., Gratzl, J., Seifried, T., Bieber, P., and Grothe, H.: Isolation of subpollen particles (SPPs) of birch: SPPs are potential carriers of ice nucleating macromolecules, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12404, https://doi.org/10.5194/egusphere-egu22-12404, 2022.

Certain biological macromolecules can play a unique role in heterogeneous ice nucleation, triggering freezing of atmospheric cloud droplets at high sub-zero temperatures (Pummer et al., 2012). Some ice nucleating proteins (INPs) of procaryotic organisms (e.g. Pseudomonas syringae) have been identified as highly efficient ice nuclei, but the isolation and identification of INPs from eucaryotic cells (e.g. pollen or fungal spores) remains challenging due to the increasing complexity of the samples’ matrices (Burkart et al., 2021, Seifried et al., 2020). To analyze INPs from birch pollen extracts, we applied a top-down workflow, including ice-shell purification, size exclusion chromatography and gel electrophoresis as separation techniques followed by fluorescence spectroscopy, infrared spectroscopy and mass spectrometry for characterization and the Vienna Optical Droplet Crystallization Analyzer (VODCA) for determining the ice nucleation activity (Felgitsch et al., 2018). We found several proteins as possible contributors to the freezing activity of birch pollen at around -16°C. However, the exact sequence of the INP and the molecular mechanism behind the ice nucleation remains elusive. To address this knowledge gap, we are currently focusing on a broader bottom-up approach which illuminates the ice nucleation mechanism of proteins in general. Specific peptides can be synthesized in-vitro and the ice nucleation activity of pure synthetic substances will be analyzed by using the drop Freezing Ice Nuclei Counter (FINC) (Miller et al., 2021). Exchanging or modifying single amino acids will allow to determine the mechanisms behind the nucleation and to draw a picture of sequences that indicate possible INPs in various organisms. Such a method can provide a basis for the investigations of INPs across the borders of genera and species and can help building fundamental understanding behind biologically induced ice nucleation at high sub-zero temperatures in the atmosphere.

References

Burkart, J., Gratzl, J., Seifried, T. M., Bieber, P., and Grothe, H.: Isolation of subpollen particles (SPPs) of birch: SPPs are potential carriers of ice nucleating macromolecules, Biogeosciences, 18, 5751–5765, https://doi.org/10.5194/bg-18-5751-2021, 2021.

Felgitsch, L., Baloh, P., Burkart, J., Mayr, M., Momken, M. E., Seifried, T. M., Winkler, P., Schmale III, D. G., and Grothe, H.: Birch leaves and branches as a source of ice-nucleating macromolecules, Atmos. Chem. Phys., 18, 16063–16079, https://doi.org/10.5194/acp-18-16063-2018, 2018

Miller, A. J., Brennan, K. P., Mignani, C., Wieder, J., David, R. O., and Borduas-Dedekind, N.: Development of the drop Freezing Ice Nuclei Counter (FINC), intercomparison of droplet freezing techniques, and use of soluble lignin as an atmospheric ice nucleation standard, Atmos. Meas. Tech., 14, 3131–3151, https://doi.org/10.5194/amt-14-3131-2021, 2021.

Pummer, B. G., Bauer, H., Bernardi, J., Bleicher, S., and Grothe, H.: Suspendable macromolecules are responsible for ice nucleation activity of birch and conifer pollen, Atmos. Chem. Phys., 12, 2541–2550, https://doi.org/10.5194/acp-12-2541-2012, 2012.

Seifried, T. M., Bieber, P., Felgitsch, L., Vlasich, J., Reyzek, F., Schmale III, D. G., and Grothe, H.: Surfaces of silver birch (Betula pendula) are sources of biological ice nuclei: in vivo and in situ investigations, Biogeosciences, 17, 5655–5667, https://doi.org/10.5194/bg-17-5655-2020, 2020.

How to cite: Bieber, P., Seifried, T. M., Reyzek, F., Borduas-Dedekind, N., and Grothe, H.: The atmospheric ice nucleation behavior of biological macromolecules: how top-down and bottom-up approaches help disentangle the role of proteins, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13322, https://doi.org/10.5194/egusphere-egu22-13322, 2022.

The presence of ice in mixed-phase clouds has a vital impact on their radiative properties, lifetime, and  ability to precipitate. Ice crystal formation is initially induced by a rare subset of ambient aerosol  particles called ice-nucleating particles (INPs). Despite the importance of INPs on aerosol-cloud interactions and century-long research efforts, the knowledge about their nature and atmospheric  abundance still needs improvement. Recent instrument developments allow more automated and  continuous INP measurements with a high time resolution, to gain a better understanding of the natural variability of INPs in different locations, and to investigate the identity and source regions of them. 

Here we present long-term observations of INPs at Storm Peak Laboratory (SPL) located in the Rocky Mountains of Colorado. SPL is at an altitude of 3200 m a.s.l. within the lower free troposphere, and in winter it is a location where mixed-phase clouds frequently occur. Therefore, the present aerosol particles are directly relevant for ice formation in such clouds. The ongoing INP measurements started in October 2021, and are conducted with the Portable Ice Nucleation Experiment (PINE) at conditions resembling the formation of mixed-phase clouds at temperatures between -22°C and -32°C. Results on the short-term and inter-seasonal variability will be presented, with a focus on parallel measurements of aerosol particle properties and meteorology. During the winter months of January and February 2022, we characterized the size of the INPs by selecting ice crystal residuals downstream of PINE using a pumped-counterflow virtual impactor and a novel optical particle counter. As this setup operates continuously, it is capable to investigate INP properties for longer time periods and to improve sampling statistics. In the future, it will be used with other diagnostic instruments, such as a single particle mass spectrometer, giving insights into the size and chemical composition of INPs, and thus allows to have a direct measure of the nature of INPs in ambient air.

How to cite: Lacher, L., Froyd, K., Hallar, G., McCubbin, I., Jacquot, J., Dameto de Espana, C., Shen, X., Schill, G., Baker, D., Leisner, T., Möhler, O., and Cziczo, D.: Measurements of ice-nucleating particle concentration and size at Storm Peak Laboratory, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13565, https://doi.org/10.5194/egusphere-egu22-13565, 2022.