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Ice nucleating particles (INPs) are a rare but important type of atmospheric aerosol particles, contributed mostly by mineral dust particles and by biogenic macromolecules expressed by a range of different microorganisms. INPs’ ability to nucleate ice in cloud droplets already far above their homogeneous freezing temperature (at ~ -38°C) influences for instance cloud radiative processes and precipitation formation.Despite a recent surge of research on INPs, neither are all important sources of atmospheric INP sufficiently known, nor their atmospheric abundance which varies with location and season.Here we examine the heat sensitivity of some types of INP of biogenic origin by exposing them to a range of different heating temperatures (60°C, 85°C, and 90°C) for one hour. The heating is expected to destroy proteinaceous ice active macromolecules. The ice activity of different samples was examined before and after heating.Examined samples included birch pollen (Betula pendula), fungi (Mortierella alpina, Fusarium acuminatum), the bacteria Pseudomonas syringae (from a commercially available SNOMAX sample) and aspen leaves (from Populus tremuloides) which had been sampled and freeze-dried decades ago. We compare their heat sensitivity to that of INPs from airborne aerosol samples collected on filters in summer months at Villum Research Station (VRS) in North Greenland, which were exposed to the same heating procedure.For samples from F. acuminatum and P. syringae, a continuing decrease in ice activity (expressed as INP per sample mass) was observed for each of the heating steps. The decrease was larger than one order of magnitude for each heating step across the examined temperature range (roughly -5°C to -25°C). For the B. pendula sample, highly ice active macromolecules inducing ice nucleation at > -10°C were already destroyed by heating to 60°C, while the signal below -15°C was changed much less by any of the heating steps. The M. alpina sample showed no change in ice activity after heating to 60°C, but a strong decrease across the examined temperature range after heating further to 85°C, and some additional decrease (roughly one order of magnitude) after heating to 90°C. The aspen leave samples showed no noticeable reaction to heating at freezing temperatures below -15°C, but behaved similar to the M. alpina sample at -10°C.Interestingly, for freezing temperatures > -10°C, INP concentrations of VRS summer samples also showed no or only a small decrease in ice activity upon heating to 60°C, similar as the M. alpina and aspen leave samples. Also similar to these two, VRS samples showed a very pronounced decrease upon heating to 85°C and some further decrease upon heating to 90°C. This is interesting in the light that recent research suggested that M. alpina, together with the bacteria species Pantoea ananatis, are likely sources of the INPs present in a aspen leave sample of the same batch as the one examined here. Combining these findings, we speculate that M. alpina may be of considerable importance as terrestrially sourced atmospheric INPs for regions even including the summer Arctic.

Pollen belong to a subset of atmospheric aerosol particles that enable the glaciation of supercooled liquid water, which is present in clouds down to an ambient temperature of -38°C. While the ice nucleating properties of pollen are widely researched in laboratory studies, it is challenging to evaluate their effect on clouds in observations at a large scale. Using a combination of ground-based measurements of pollen concentration and satellite observations of cloud properties during springtime, we show that the ice nucleating properties of pollen promote the glaciation of supercooled liquid water. We further establish the link between the pollen-induced increase in cloud ice to a higher precipitation frequency. In light of anthropogenic climate change, the extended and strengthened pollen season and alterations in biodiversity can, therefore, introduce a localized climate forcing and a modification of the precipitation frequency and intensity, especially during springtime.

Arctic clouds are poorly represented in climate models partly due to a lack of understanding of their source and nucleating capability of natural aerosol in the high Arctic. Recent field campaigns provided evidence of a source of sea salt aerosol (SSA) from blowing snow above sea ice, which can account for SSA winter/spring maxima observed in the polar regions. SSA emissions from sea ice sources can potentially influence regional climate via the indirect radiative effect, but contributions to cloud-forming particles, in particular, ice-nucleating particles (INP), are unknown. Here we report the first online spring-time observations of INPs in the Central Arctic. INP concentrations were on the order of a few tens [particle m-3] activating between -38 and -15°C and were often associated with high wind speeds. Initial offline droplet assay analysis of snow on sea ice indicates the presence of potential INPs in winter/spring activating at -29 to -25°C. This is evidence that snow on sea ice represents a viable reservoir of INPs, which can be physically released via the blowing snow mechanism to the air above. We discuss sea ice sources of coarse SSA and INPs and their role in the lower atmosphere with a focus on blowing snow. To do this, we consider the comprehensive set of MOSAiC observations, including aerosol size and composition, airborne snow particles, and chemical and physical properties of both aerosol and snow on sea ice.

The remoteness of the Antarctic continent has important implications for the microphysical properties of clouds: In particular, the rare abundance of ice-nucleating particles (INP) limits the primary nucleation of ice crystals. Yet, persistent mixed-phase clouds with ice crystal number concentrations of 0.1-1l-1 are still observed in the Arctic and Antarctic. However, the ability of regional climate models to reproduce these mixed-phase clouds remains limited, much like the knowledge about their climatological effects. Thus, we added a module to the regional climate model COSMO-CLM² aimed at improving the parametrisation of the aerosol-cycle, which allows us to prescribe different concentrations of INPs. We examined the model response to different concentrations by running it in an area around the Belgian Princess Elisabeth Station in Dronning Maud Land for one month and with four different concentration settings: The first, corresponding to the low end of INP concentrations we observed at the station, the second, corresponding to the high end of INP concentrations we observed at the station, and the third and fourth, to the low and high end of continental observations. The performance was evaluated by comparing the simulation results with radar and ceilometer observations taken at the station. Finally, we analysed the differences between the four simulations to determine the overall sensitivity of the model to variability in INP concentrations, which allows us to draw conclusions about the importance of accurately simulating processes related to ice nucleation, and about the climatological implications that a change in aerosol concentrations would have.

Transparent exopolymer particles (TEP) exhibit the properties of gels and are ubiquitously found in the world oceans. Here we demonstrate that TEP may enter the atmosphere as part of sea spray aerosol and likely influence cloud properties. We show number concentrations of TEP with a diameter > 4.5 µm, hence covering a part of the supermicron particle range measured in ambient aerosol and cloud water samples from the tropical Atlantic Ocean. Furthermore, TEP were analysed in generated aerosol particles using a plunging waterfall tank that was filled with the ambient seawater.Based on Na+ concentrations in seawater and the atmosphere, the enrichment of TEP in the tank generated aerosol particles was well in-line with another study. The TEP enrichments in the ambient atmosphere were, however, up to two orders of magnitude higher compared to the tank study and such high values are thus far not reported for supermicron aerosol particles. We propose that the high enrichment of TEP in the particles and in cloud water result from a combination of enrichment during bubble-bursting transfer from the ocean and secondary in-situ atmospheric formation. We suggest that similar (biotic and abiotic) formation mechanism reported for TEP formation in the (sea)water might take place in the atmosphere as well, as the required conditions (e.g. high concentrations of dissolved TEP precursors such as polysaccharides, presence of bacteria in the cloud water) were given.TEP concentrations in the atmosphere were two orders of magnitude higher than INP concentrations in the aerosol particles and cloud water, respectively. However, only a part of the TEP population, assumingly the one colonized by bacteria, might contribute to INP population, and are worth further studies.The study contributes to the international SOLAS program.Ref. : van Pinxteren, M., Robinson, T.-B., Zeppenfeld, S., Gong, X., Bahlmann, E., Fomba, K. W., Triesch, N., Stratmann, F., Wurl, O., Engel, A., Wex, H., and Herrmann, H.: High number concentrations of transparent exopolymer particles in ambient aerosol particles and cloud water – a case study at the tropical Atlantic Ocean, Atmos. Chem. Phys., 22, 5725–5742, https://doi.org/10.5194/acp-22-5725-2022, 2022.

Atmospheric ice nucleating particle (INP) concentration data from the free troposphere are sparse, but urgently needed to understand vertical transport processes of INPs and their influence on cloud formation and properties. Here, we introduce the new High-volume flow aERosol particle filter sAmpler (HERA) which was specially developed for installation on research aircraft and subsequent offline INP analysis. HERA is a modular system constisting of a sampling unit and a powerful pump unit and has several features which were integrated specifically for INP sampling. Firstly, the pump unit enables sampling at flow rates exceeding 100 L min−1, which is well above typical flow rates of aircraft INP sampling systems described in the literature (~10 L min−1). Consequently, required sampling times to capture rare, high-temperature INPs (≥-15 °C) are reduced in comparison to other systems and potential source regions of INPs can be confined more precisely. Secondly, the sampling unit is designed as a seven-way valve, enabling switching between six filter holders and a bypass with one filter being sampled at a time. In contrast to other aircraft INP sampling systems, the valve position is controlled remotely via software so that manual filter changes in-flight are eliminated and the potential for sample contamination is decreased. This design is compatible with a high degree of automation, i.e., triggering filter changes depending on parameters like flight altitude, geographical location, temperature, or time. In addition to the design and principle of operation of HERA, this paper presents laboratory characterization experiments with size-selected test substances, i.e., SNOMAX® and Arizona Test Dust. The particles were sampled on filters with HERA, varying either particle diameter (300 nm to 800 nm) or flow rate (10 L min−1 to 100 L min−1) between experiments. The subsequent offline INP analysis showed good agreement with literature data and comparable sampling efficiencies for all investigated particle sizes and flow rates. Furthermore, the deposition efficiency of atmospheric INPs in HERA was compared to a straightforward filter sampler and good agreement was found. Finally, results from the first campaign of HERA on the High Altitude and LOng range research aircraft (HALO) demonstrate the functionality of the new system in the context of aircraft application.

Ice-nucleating particles (INPs) can initiate ice formation in clouds at temperatures above -38 °C through heterogeneous ice nucleation. As a result, INPs affect cloud microphysical and radiative properties, cloud lifetime, and precipitation behavior and thereby ultimately the Earth’s climate. Yet, little is known regarding the sources, abundance and properties of INPs, especially in remote regions such as the Arctic. In this study, 2-yearlong INP measurements (from July 2018 to September 2020) at Villum Research Station in northern Greenland are presented. A low-volume filter sampler was deployed to collect filter samples for offline INP analysis. An annual cycle of INP concentration (NINP) was observed, and the fraction of heat-labile INPs was found to be higher in months with low to no snow cover and lower in months when the surface was well covered in snow (> 0.8 m). Samples were categorized into three different types based only on the slope of their INP spectra, namely into summer, winter and mix type. For each of the types a temperature-dependent INP parameterization was derived, clearly different depending on the time of the year. Winter and summer types occurred only during their respective seasons and were seen 60 % of the time. The mixed type occurred in the remaining 40 % of the time throughout the year. April, May and November were found to be transition months. A case study comparing April 2019 and April 2020 was performed. The month of April was selected because a significant difference in NINP was observed during these two periods, with clearly higher NINP in April 2020. In parallel to the observed differences in NINP, also a higher cloud-ice fraction was observed in satellite data for April 2020, compared to April 2019. NINP in the case study period revealed no clear dependency on either meteorological parameters or different surface types which were passed by the collected air masses. Overall, the results suggest that the coastal regions of Greenland were the main sources of INPs in April 2019 and 2020, most likely including both local terrestrial and marine sources., Atmospheric Chemistry and Physics, 23 (8), ISSN:1680-7375, ISSN:1680-7367

Marine viruses have thoroughly been studied in seawater, yet their dispersal from neuston ecosystems at the air-sea interface towards the atmosphere remains a knowledge gap. Here, we show that 6.2 % of the studied virus population were shared between air-sea interface ecosystems and rainwater. Virus enrichment in the 1-mm thin surface microlayer and sea foams happened selectively, and variant analysis proved virus transfer to aerosols and rain. Viruses detected in rain and aerosols showed a significantly higher percent G/C base content compared to marine viruses, and a genetically distinct rain virome supports that those viruses could inhabit higher air masses. CRISPR spacer matches of marine prokaryotes to “foreign” viruses from rainwater prove regular virus-host encounters at the air-sea interface. Our findings on aerosolization and long-range atmospheric dispersal implicate virus-mediated carbon turnover in remote areas, viral dispersal mechanisms relevant to human health, and involvement of viruses in atmospheric processes like ice-nucleation.

Within the Southern Ocean, the greatest warming is occurring on the Antarctic Peninsula (AP) where clear cryospheric and biological consequences are being observed. Antarctic coastal systems harbour a high diversity of marine and terrestrial ecosystems heavily influenced by Antarctic seaweeds (benthonic macroalgae) and bird colonies (mainly penguins). Primary sea spray aerosols (SSA) formed by the outburst of bubbles via the sea-surface microlayer depend on the organic composition of the sea water surface. In order to gain insight into the influence of ocean biology and bio-geochemistry on atmospheric aerosol, we performed in situ laboratory aerosol bubble chamber experiments to study the effect of different leachates of biogenic material - obtained from common Antarctic seaweeds as well as penguin guano - on primary SSA. The addition of different leachate materials on a seawater sample showed a dichotomous effect depending on the leachate material added - either suppressing (up to 52%) or enhancing (22–88%) aerosol particle production. We found high ice nucleating particle number concentrations resulting from addition of guano leachate material. Given the evolution of upper marine polar coastal ecosystems in the AP, further studies on ocean-atmosphere coupling are needed in order to represent the currently poorly understood climate feedback processes., The study was further supported by the Spanish Ministry of Economy through project PI-ICE (no. CTM 2017–89117-R) and the Ramon y Cajal fellowship (no. RYC-2012-11922). This work acknowledges the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S). The National Centre for Atmospheric Science (NCAS) Birmingham group is funded by the UK Natural Environment Research Council. We gratefully acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation, Projektnummer 268020496–TRR 172) within the Transregional Collaborative Research Center “ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)3” in subproject B04.

By serving as condensation and ice nuclei, aerosols play a vital role in the formation of clouds. This has significant implications for the radiation balance and precipitation amounts over the Antarctic Ice Sheet, where type and amount of aerosols differ significantly from other places because of its remote location. However, that is also the reason observations are sparse, and consequently, few studies exist examining this effect. Recently, a module was added to the COSMO-CLM² regional climate model to account for the aerosol-cycle. The model was integrated for the region around the Princess Elisabeth Antarctic research station (PEA) in Dronning Maud Land for a period of 10 days in January 2016, of which the first 3 days were discarded. Varying cloud condensation and ice nuclei were prescribed to the model, based on observations from PEA. The model output was compared to observations of cloud structure and precipitation amounts taken at PEA, as well as the unmodified COSMO-CLM² model. The model integrations indicate that the number of ice nuclei has a significant impact on the microphysical composition of clouds, with higher numbers being associated with a lower amount of liquid water content of clouds and higher precipitation amounts. Additional runs are performed to confirm and extend these findings for an entire year. Recent measurements of ice nuclei particle concentrations obtained during two austral summers are also considered. Moreover, we analysed how atmospheric dynamics affect the cloud-aerosol interaction by analysing the model sensitivity for different weather regimes.

Ice nucleating particles (INPs) are rare but important atmospheric particles as they induce the formation of primary ice in mixed phase clouds and also in some cirrus clouds. A plethora of substances which can be found in atmospheric particles can induce ice nucleation. The most important ice active particle types in the atmosphere are assumed to be mineral dust and biological particles, which can originate from a large number of sources. It is hence not surprising that INP concentrations vary over several orders of magnitude at any ice nucleation temperature, with concentrations being typically larger in continental than in marine environments. Although research concerning INP and their global occurrence has seen a steep rise in the past years, global INP concentrations are still not well known, and not all important INP sources are clear, neither are there good parameterizations for describing INP concentrations in models.To increase the knowledge of typical global INP concentrations and to draw conclusions about INP sources, we examined INP concentrations in the Antarctic region, namely at the German research Station Neumayer III, located at shelf ice in close proximity (only some 10 km) to the ice edge, at the Belgian research Station Princess Elisabeth, located roughly 200 km inland and at an altitude of 1390 m, and during a campaign including ship- and land-based data at the Antarctic peninsula. We used our well-established methods of filter collection and off-line analysis with cold-stages to derive INP concentrations in these locations.For Neumayer, two years of data are available. INP concentrations there were generally lower than values derived, e.g., for northern mid latitudes, and they were similar to values published for the Southern Ocean in literature. No pronounced annual cycle was observed. Around and on the Antarctic peninsula, INP concentrations were roughly similar to those observed at Neumayer. However, the Princess Elisabeth station, for which only data obtained during two austral summers are available, showed the lowest values detected in this study. Our results suggest that sources of INP in the Antarctic region are rare, and particularly so on Antarctica itself.

The sources and abundance of ice-nucleating particles (INPs) that initiate cloud ice formation remain understudied, especially in the Southern Hemisphere. In this study, we present INP measurements taken close to Punta Arenas, Chile, at the southernmost tip of South America from May 2019 to March 2020, during the Dynamics, Aerosol, Cloud, And Precipitation Observations in the Pristine Environment of the Southern Ocean (DACAPO-PESO) campaign. The highest ice nucleation temperature was observed at −3◦C, and from this temperature down to ∼ −10◦C, a sharp increase of INP number concentration (NINP) was observed. Heating of the samples revealed that roughly 90 % and 80 % of INPs are proteinaceous-based biogenic particles at > −10 and −15◦C, respectively. The NINP at Punta Arenas is much higher than that in the Southern Ocean, but it is comparable with an agricultural area in Argentina and forestry environment in the US. Ice active surface site density (ns) is much higher than that for marine aerosol in the Southern Ocean, but comparable to English fertile soil dust. Parameterization based on particle number concentration in the size range larger than 500 nm (N>500 nm) from the global average (DeMott et al., 2010) overestimates the measured INP, but the parameterization representing biological particles from a forestry environment (Tobo et al., 2013) yields NINP comparable to this study. No clear seasonal variation of NINP was observed. High precipitation is one of the most important meteorological parameters to enhance the NINP in both cold and warm seasons. A comparison of data from in situ and lidar measurements showed good agreement for concentrations of large aerosol particles (> 500 nm) when assuming continental conditions for retrieval of the lidar data, suggesting that these particles were well mixed within the planetary boundary layer (PBL). This corroborates the continental origin of these particles, consistent with the results from our INP source analysis. Overall, we suggest that a high NINP of biogenic INPs originated from terrestrial sources and were added to the marine air masses during the overflow of a maximum of roughly 150 km of land before arriving at the measurement station.

Transparent exopolymer particles (TEP) exhibit the properties of gels and are ubiquitously found in the world oceans. Possibly, TEP may enter the atmosphere as part of sea spray aerosol. Here, we report number concentrations of TEP (diameter > 4.5 µm) in ambient aerosol and cloud water samples from the tropical Atlantic Ocean as well as in generated aerosol particles using a plunging waterfall tank that was filled with the ambient sea water. The ambient TEP concentrations ranged between 7 × 102 and 3 × 104 #TEP m−3 in supermicron aerosol particles and correlations to sodium (Na+) and calcium (Ca2+) (R2 = 0.5) suggested some contribution via bubble bursting. Cloud water TEP concentrations were between 4 × 106 and 9 × 106 #TEP L−1 corresponding to equivalent air concentrations of 2–4 × 103 #TEP m−3. The TEP concentrations in the tank-generated aerosol particles, produced from the same waters and sampled with an equivalent system, were significantly lower (4 × 102–2 × 103 #TEP m−3) compared to the ambient concentrations. Based on Na+ concentrations in seawater and in the atmosphere, the enrichment factor for TEP in the atmosphere was calculated. The tank-generated TEP were enriched by a factor of 50 compared to sea water and, therefore, in-line with published enrichment factors for supermicron organic matter in general and TEP specifically. TEP enrichment in the ambient atmosphere was on average 1 × 103 in cloud water and 9 × 103 in ambient aerosol particles and therefore about two orders of magnitude higher than the corresponding enrichment from the tank study. Such high enrichment of supermicron particulate organic constituents in the atmosphere is uncommon and we propose that atmospheric TEP concentrations resulted from a combination of enrichment during bubble bursting transfer from the ocean and TEP in-situ formation in atmospheric phases. Abiotic in-situ formation might have occurred from aqueous reactions of dissolved organic precursors that were present in particle and cloud water samples, while biotic formation involves bacteria, which were abundant in the cloud water samples. The ambient TEP number concentrations were two orders of magnitude higher than recently reported ice nucleating particle (INP) concentrations measured at the same location. As TEP likely possess good properties to act as INP, in future experiments it is worth studying if a certain part of TEP contributes a fraction of the biogenic INP population.