Terrestrial and Marine Sources of Ice Nucleating Particles in the Eurasian Arctic

www.rsc.org/faraday_d Faraday Discussions Royal Society of Chemistry Faraday Discussions Accepted Manuscript This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. View Article Online View Journal This article can be cited before page numbers have been issued, to do this please use: G. Li, A. Welti, A. ROCCHI, G. P. Fogwill, M. Dall'Osto and Z. A. Kanji, Faraday Discuss., 2024, DOI: 10.1039/D4FD00160E. Terrestrial and Marine Sources of Ice Nucleating Particles in the Eurasian Arctic Guangyu Li1*, André Welti2, Arianna Rocchi3, Germán Pérez Fogwill2, Manuel Dall’Osto3, and Zamin A. Kanji1 1Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland 2Finnish Meteorological Institute, Helsinki, Finland 3Department of Marine Biology and Oceanography, Institute of Marine Sciences (ICM, CSIC), Barcelona, Spain *Now at: Department of Carbon Emissions and Environmental Evolution, Deqing Academy of Satellite Applications, Deqing, China Correspondence: Guangyu Li ([email protected]) and Zamin A. Kanji ([email protected]) Abstract. Ice nucleating particles (INPs) catalyze primary ice formation in Arctic low-level mixed-phase clouds, influencing their persistence and radiative properties. Knowledge of the abundance and sources of INP over the remote Arctic Ocean is scarce due to limited data coverage, particularly in the Eurasian Arctic. This study presents summertime measurements of INP concentrations in seawater, fog water and air from the ship-based Arctic Century Expedition, exploring the Barents, Kara, and Laptev5 Seas, and the adjacent high Arctic islands and archipelagos in August and September 2021. Heat sensitivity tests of ambient aerosols revealed that heat-liable, biogenic INPs make up the majority of Arctic INP populations at temperatures above -20 °C, and to a lesser extent down to -25°C. INP content in fog water is found to be similar to ambient aerosol, indicating that INP in marine air could also act as cloud condensation nuclei. Measurements of aerosolized INPs using an on-board sea-spray aerosol bubble tank generator exhibit a positive correlation with ambient INP concentrations, but not with INP abundance in10 seawater samples. INP concentrations in air derived from sea water samples (using a NaCl conversion factor representative for the Arctic) were significantly lower than those measured in ambient air or bubble tank experiments. INP concentrations in bubble tank experiments positively correlated with the phosphate and fluorescence signals in the water. This suggests an important role of the aerosolization mechanism for preferentially partitioning biogenic INPs to the atmosphere. 1 Introduction15 Ice nucleating particles (INPs) trigger primary ice formation in mixed-phase clouds (MPCs), which modulates the cloud radiative and microphysical properties (Korolev et al., 2017; Serreze and Barry, 2011). Accurate simulations of cloud properties in climate models require a realistic representation of INPs (Murray et al., 2021). The concentration of INPs influences the number, size, and phase partitioning of hydrometeors in clouds, which in turn affect the cloud albedo, lifetime, and might impact precipitation patterns. In the Arctic, cloud cover impacts the regional energy balance while the amount of snow-producing20 clouds affects the formation of sea ice. A scarcity of INP observations limits our understanding of the abundance and sources 1 Page 1 of 30 Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E of INPs in different environments. The Eurasian Arctic Ocean is of particular interest because it could be the source of highly active INPs (Porter et al., 2022). INPs can originate from terrestrial and marine sources. Mineral dust particles are a significant terrestrial source, capable of initiating ice nucleation (IN) at temperatures lower than approximately -15 °C (Hoose and Möhler, 2012). Known sources of25 mineral dust near the Arctic Ocean include the Arctic coasts of Greenland (Li et al., 2023), the Russia coast (Porter et al., 2022), glacial outwash plains in Svalbard (Tobo et al., 2019), and sandy deserts on Iceland (Sanchez-Marroquin et al., 2020). Marine INP sources in the Arctic Ocean are related to sea-spray aerosol (SSA), generated through wave breaking and bubble bursting, which represents another crucial source of INPs in the high-latitude Arctic Ocean (Leck and Bigg, 2005; Wilson et al., 2015; Creamean et al., 2018; Prather et al., 2008). The production rate of SSA varies with meteorological conditions (e.g., wind speed,30 and sea surface temperature Quinn et al., 2015; Grythe et al., 2014) and sea ice coverage (Quinn et al., 2015). Also both the abundance of different components in seawater and the aerosolization process at the ocean-atmosphere interface impact aerosol emission and partitioning (O’Dowd et al., 2015; Prather et al., 2008). In addition to inorganic sea salt and sulfate, SSA contains marine biogenic aerosol, which can be composed of particulate microbes, phytoplankton cell exudates (Creamean et al., 2019; Knopf et al., 2011), and dissolved organic macromolecules (McCluskey et al., 2018b) released during phytoplankton blooms.35 These components can be IN-active at temperatures as high as -5 °C (Murray et al., 2012). It is hypothesized that the emission of IN-active marine biogenic aerosol could coincide with marine phytoplankton blooms and co-emission of biogenic aerosol precursors, such as Dimethylsulfide (DMS) and chlorophyll-a(Gabric et al., 2018; Rinaldi et al., 2013; Becagli et al., 2011). As the Arctic warms and sea ice retreats, marine biological processes are likely to become more active which could change the marine INP emission. Uncovering the sources and understanding the mechanisms of marine biogenic aerosol release into40 the atmosphere is therefore needed for predicting the abundance of Arctic low-level MPCs in the future (Ickes et al., 2020; Si et al., 2019). In this work, we present ship-borne measurements of INP concentrations (NINP) sampled from different environments (seawater, fog water, marine air, and air from a bubble tank generator) and related aerosol properties during the Arctic Century Expedition in the summer of 2021 over the Barents, Kara and Laptev Seas and the adjacent high Arctic islands and archipelagos45 in the Eurasian Arctic, a region that has not been studied before. 2 Methods 2.1 Campaign overview 2 Page 2 of 30Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E Figure 1. Position of instrumentation on board of the RV Akademik Tryoshnikov (adapted from vessel plans by the Arctic and Antarctic Research Institute). Height is provided relative to the approximate water line in the front view (left panel), and distance from the ship’s bow in the top view (right panel). A SSA bubble tank generator was operated on the 1st deck, online measurement equipment for aerosol characterization was located on the 2nd deck, and aerosol sample collector and fog water sampler were installed on the 6th deck. The Arctic Century Expedition took place from 5th August to 6th September 2021, starting and ending at the port of Murmansk, Russia. Figure 1 shows the location of experiments, instruments and samplers on board the research vessel (RV)50 Akademik Tryoshnikov reported in this work. Detailed information on instrument configurations and their functions in each sector is given in Table 1. For measurements on the 1st deck, an aerosol generation bubble tank (BT) system for controlled SSA generation was set up. Inside the BT, aerosol is generated by simulating the bubble-bursting mechanisms using water jets to create bubbles. Water samples include 32 L seawater collected at 2 m depth with Niskin bottles using a Conductivity–Temperature–Depth (CTD) rosette sampling system or freshwater samples collected on land using sampling buckets (see55 sampled water locations in Fig. 2). The SSA generated in the BT was collected on filters for NINP measurements, and characterized for the size distribution by a scanning mobility particle sizer (SMPS) and an optical particle counter (OPC). Fresh and seawater samples used for BT experiments were also filtered for biochemical analysis (see Rocchi et al., 2024 for details on BT experiments and analysis of biological variables). On the 2nd deck, an aerosol container configured as a laboratory included interval sampling of ambient aerosol by a liquid impinger (for INP analysis), and continuous characterization of particle size60 distributions with an SMPS and an aerodynamic particle sizer (APS). On the 6th deck, several fog water collectors were installed next to a low volume PM10 filter sampler (LVS) (Table 1). The LVS sampled ambient aerosol continuously for 12 h per filter along the ship track during the campaign. The fog water and filters were analyzed for INPs in the laboratory after the 3 Page 3 of 30 Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E Table 1. Overview of instrumentation set-up on the research vessel RV Akademik Tryoshnikov during the Arctic Century Expedition (for the onboard location, see Fig. 1) On-board location Instrument Temporal resolution Flow rate Function 1st deck CTD-rosette Station-based - Seawater sampling Aerosol generator (BT) Station-based - Aerosol generation from sampled water SMPS (BT-aerosol) 4 min 0.6 L min-1 Online particle size distribution (< 0.6 µm) OPC (BT-aerosol) 5 s 2.83 L min-1 Online particle size distribution (> 0.3 µm) Water filter Station-based - Water filtration for biochemical analysis BT aerosol filter 6-20 h 10 L min-1 Aerosol collection on filters for offline INP analysis 2nd deck Impinger 3 h 300 L min-1 Aerosol collection in water for offline INP analysis SMPS 4 min 0.6 L min-1 Ambient particle size distribution (0.012 - 0.6 µm) APS 4 min 1 L min-1 Ambient particle size distribution (0.5 - 20 µm) 6th deck Fog water collector 6-12 h - Fog water collection for offline INP analysis LVS (PM10 inlet) 12 h 38.3 L min-1 Aerosol collection on filters for offline INP analysis campaign. Additionally, several near-shore surface microlayer (SML) and underlying subsurface seawater (SSW) samples were collected. The locations of fog water, seaand freshwater, SML and SSW samples are marked in Fig. 2. Detailed experimental65 setups for sample collections and measurements are described in the following section. 2.2 Sample collection 2.2.1 Ambient aerosol samples From the aerosol container laboratory on the 2nd deck, ambient aerosols were collected (3 times a day for 3 hours) each into 15 mL ultra-pure water (W4502-1L, Sigma-Aldrich) using a high flow-rate impinger (Coriolis®µ, Bertin Instruments, with a70 lower limit aerodynamic cut-off size of 0.5 µm) at a flow rate of 300 L min-1. On the 6th deck, aerosol particles were collected onto 47 mm polycarbonate membrane filters (Whatman, 0.4 µm pore size) using the LVS (Model DPA14, Digitel) with a PM10 inlet. In the following, these filter samples are referred to as LVS filters. The LVS inlet was approximately 25 m above sea level. The operating flow rate was maintained at 38.3 L min-1 for 12-hour sampling intervals. The sampling was temporarily halted when the wind direction was detected to originate from the ship’s chimney, in order to prevent contamination from exhaust75 emissions. The impinger and LVS filter samples were stored at -20 °C on board and for transport until they were used for the INP analysis after the campaign back at the laboratory at ETH Zurich. During the campaign, a total of 75 impinger and 50 LVS filter samples were collected. 4 Page 4 of 30Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E Figure 2. Map of water sampling locations during the Arctic Century Expedition. The gray asterisks represent the ship’s track (1 h interval) throughout the expedition. 2.2.2 Aerosol samples collected from BT The BT aerosol generation system (Medina-Pérez et al., 2020) was used to characterize aerosols produced from bubble bursting.80 The BT consists of a 60 L stainless-steel cylindrical tank. For operation, the BT was filled with 30 L of water collected either with the CTD-rosette (sea water) or on islands (island run-off or freshwater). Water in the BT was circulated to the top via a peristaltic pump and showered back to the water surface as plunging jets at a flow rate of 12 L min-1. The spray aerosol forms at the water surface via bubble bursting (O’Dowd et al., 2015). Particle-free air was supplied into the tank headspace at 60 L min-1 to prevent aerosol from concentrating (see Rocchi et al., 2024 for detailed experimental setup). Two frequently85 regenerated diffusion dryers were used to dry the water spray aerosol for particle size distribution measurements and filter collection. 47 mm polycarbonate membrane filters (Whatman, 0.4 µm pore size) were used to collect aerosols generated from the BT. The sample flow rate was approximately 10 L min-1 for a duration of 6 to 20 hours. BT filters were stored frozen at -20 °C until INP analysis was conducted back in the laboratory at ETH Zurich. The INP concentration measured from 5 Page 5 of 30 Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E ambient aerosols and aerosolization from BT were not corrected for freezing point depression, following the representation of90 atmospheric immersion freezing occurring in dilute cloud droplets with water activity ∼1. 2.2.3 Environmental water samples A total of 21 fog water samples were collected using the instruments shown in Fig. 3 (a). An array of 7 cactus-like collectors were fixed by the white clamps and mounted on the railing at the 6th deck on board. Once covered in fog, the fog water/frost deposits onto the sharp tips of the "cactus" and drips down/melts into a glass bottle attached at the bottom.95 Figure 3. (a) Fog water sample collector; (b) Collection of SML. To compare any enrichment of INPs between the sea surface microlayer (SML, a sub-millimeter layer located at the interface between the ocean and atmosphere) and subsurface water (SSW), seawater was sampled at the coastal regions (see Fig. 2) accessed by a helicopter. Four SML samples were collected following the procedure shown in 3 (b). A clean glass plate is repeatedly vertically submerged and withdrawn from the sea surface to collect SML samples (van Pinxteren et al., 2017). The thin microlayer film attached to the plate is then transferred into sample bottles using a Teflon scraper to wipe down the glass100 plate. Four SSW bulk seawater samples were sampled by directly submerging sample bottles to about 50 cm below the ocean surface in parallel to the SML sampling. For BT experiments, 30 L (of the sampled 32 L) of either sea or freshwater was used for aerosol generation. The 13 seawater samples were collected from a depth of 2∼4 m using the uppermost Niskin bottle from a SEA-BIRD CTD rosette sampling system. The 3 freshwater samples were collected using a plastic bucket from an island river, island lake, and island runoff (at105 6 Page 6 of 30Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E the shore of Pioneer Island, October Revolution Island, and a Lake on Cape Baranov, respectively, see Fig. 2). 2 L of each water sample was filtered to analyze biogeochemical variables (Rocchi et al., 2024). All samples (fog water, SML and SSW, water filters, water filtrates, and unfiltered water) were stored frozen at -20 °C until analyses. Conductivity was measured prior to analyzing the INP concentrations for seawater samples to account for the freezing point depression from dissolved sea salt. The corrected freezing temperature were calculated according to Koop and Zobrist (2009),110 and following the procedure of Wilson et al. (2015). 2.3 INP analysis with DRoplet Ice Nuclei Counter Zurich (DRINCZ) Aqueous particle suspensions were used for immersion-mode INP analysis using DRINCZ (David et al., 2019). After frozen storage, impinger samples were allowed to melt at 4 °C overnight before analysis. Membrane filter samples were immersed in 10 to 15 mL ultra-pure water and agitated using a sonicator for 30 min to resuspend the particles from filters into the water. Each115 liquid sample was pipetted into a Polymerase Chain Reaction (PCR) tray with 96 aliquots of 50 µL and cooled in an ethanol bath at 1 °C min-1. Freezing events were detected optically from the change in transparency of an aliquot upon freezing. INP concentration (NINP) was derived from the frozen fraction of aliquots at each integer temperature following Vali (1971). For aerosol samples collected by impinger and LVS filters, NINP is calculated as per volume of sampled air: NINP, air(T) = − ln1−Nfrz(T) Ntot  Valiquot ·Vwater Vflow (1)120 Where NINP(T) is the INP concentration at temperature T,Nfrz(T) is the number of frozen aliquots at temperature T,Ntot is the total number of aliquots (Ntot = 96), Valiquot is the aliquot volume (Valiquot = 50 µL). Vwater is the total water volume of the impinger sample or the volume of water used to suspend LVS filters, and Vflow is the sampled air volume, which is the product of the sampling flow rate and sampling time. For the fog, fresh and seawater samples, NINP is quantified as per volume of sampled water:125 NINP, water(T) = − ln1−Nfrz(T) Ntot  Valiquot (2) The NINP in fog water was converted to NINP in air: NINP, air(T) = NINP, fog water(T)·Ffog, air (3) Where Ffog, air is the liquid water content (LWC) of fog water in g m-3 converted to a volume of water in the air. For simplification, Ffog, air was taken from fog layer measurements of Costabloz et al. (2024) who reported a LWC of 0.1 g m-3air 130 for fog up to 50 m above ground (Costabloz et al., 2024). The conversion factor from per volume of liquid to per volume of air is Ffog, air = 1 ×10-7 Lfog water L-1air. 7 Page 7 of 30 Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E Field blanks undergoing the same procedures as the samples were collected every three days during the campaign. The NINP were corrected for the background of the blanks according to Vali (2019), by subtracting the differential INP spectrum of the blanks from the samples. Based on the limit of detection (LOD) from the collected sample volume and the purity of135 the nano-pure water, the highest temperature for NINP detection was approximately -5 °C (above which sampled air volumes are deemed too low to detect any INPs), and the lowest temperature at which NINP can be reliably reported is -25 °C (below which nano-pure water initiates freezing). The overall uncertainty of the reported freezing temperatures is ±0.9 °C (David et al., 2019). 2.4 Heat treatment of INP samples140 Macromolecules originating from biological species (e.g., bacteria and phytoplankton), typically comprised of proteins can effectively catalyze ice nucleation (Hill et al., 2016) and are heat labile. Heating effectively unfolds the proteinaceous structure, degrading their IN ability (Hill et al., 2016; Pummer et al., 2015). In the post-campaign heat treatment, liquid samples extracted from the 50 LVS filters were subjected to 95 °C for 20 min. After cooling to room temperature, the samples were redistributed to PCR trays for INP analysis using DRINCZ. By comparing the IN activity before and after heating, it is possible to assess145 the contribution of heat-labile species to the INP population, which is a proxy to indicate the presence of biological INPs (Hill et al., 2016). 2.5 Particle size distribution In the aerosol container laboratory on the 2nd deck, the size distribution of submicron ambient aerosol was measured using an SMPS (Model 3938, consisting of a 3082 classifier, a 3081 long differential mobility analyzer, and a 3787 Condensation Particle150 Counter, TSI Inc.). The SMPS operated at a sampling flow rate of 0.6 L min-1 with a sheath-to-sample ratio of 10:1, covering a size range of approximately 12 to 600 nm in electrical mobility diameter. A charge correction was applied to account for the misclassification of larger particles carrying multiple charges. Concurrently, the size distribution of coarse-mode particles (ranging from approximately 0.5 to 20 µm in aerodynamic diameter) was monitored using an APS (Model 3321, TSI Inc.) at a flow rate of 1 L min-1. Both the SMPS and APS were synchronized to a time resolution of 4 minutes to ensure aligned155 size distributions. The electrical mobility diameters obtained from the SMPS and aerodynamic diameters from the APS were converted to volume-equivalent diameters, assuming an average particle density of 2 g cm-3 (Li et al., 2022; Tobo et al., 2019). To monitor the aerosol size properties generated from the BT experiments, particle size distribution measurements were conducted using a combination of an SMPS and an OPC (Model GT-526S, MetOne). Two diffusion dryers filled with silica gel were used to dry the particles before entering the SMPS and the OPC. The SMPS was configured the same as in the aerosol160 container. The OPC provided 6-binned particle count in optical diameter (i.e., > 0.3 µm, > 0.5 µm, > 1 µm, > 2.5 µm, > 5 µm and > 10 µm) to extend the range of size distributions. 8 Page 8 of 30Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E Figure 7. Cumulative INP spectra of sea surface microlayer (SML) and subsurface seawater (SSW) samples. Error bars show the 95 % confidence intervals. Literature data (Gong et al., 2020a; Irish et al., 2017; Wilson et al., 2015) of previous measurements of NINP in seawater at different locations are shown for comparison. The freezing temperatures for seawater NINP are corrected for freezing point depression. 3.2 Ocean-atmosphere partitioning of INPs275 15 Page 15 of 30 Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E Figure 8. Cumulative INP spectra for BT-generated particles, classified according to the sampling location. "Island water bodies" indicates water collected from island rivers and lakes, "MilliQ" denotes a reference experiment with pure water. All other categories are seawater samples. "close to island" indicated that the ship location was less than 15 km from the coastline. From the isolated experiments of different water samples in the BT, we distinguish the abundance of INPs generated by spray aerosolization of the water samples based on the sampling location. Figure 8 shows the INP spectra of aerosolized particles from the BT experiments (NINP, BT). Overall, freshwater samples that were collected from rivers and lakes on islands (labeled as "Island water bodies") and seawater samples collected close-to-islands produced higher aerosolized NINP compared to seawater samples farther away from land (see Fig. 8) supporting strong terrestrial INP sources. The "island water bodies" showing the280 lowest freezing temperature (Land_075BIS) was sampled on Pioneer Island in the bay in brackish waters as revealed by the intermediate salinity level (see Table ?? in the Appendix). The Land_80 sample was a seawater sample directly in contact with land. Given the highest INP concentrations were observed for samples with zero and high salinity (Land_078 and Land_80, respectively), the contact to land appears to control the INP concentrations observed rather than the salinity level. The influence of salinity is further discussed later sections. From the seawater samples collected at different locations, NINP, BT was highest285 for samples obtained close to islands. For seawater samples, reduced NINP, BT was associated with increased sea ice coverage, i.e., NINP, BT produced from samples collected in the open ocean contained more INPs than samples from within the MIZ/ice pack. 16 Page 16 of 30Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E In Figure 9 the NINP, BT, ambient NINP (from LVS filters) and NINP in air derived from the SML and SSW samples are shown. To convert the NINP in the SML and SSW samples to that in air, a factor of 10-10 was used that represents the ratio of NaCl290 in the atmosphere to that in seawater for the Arctic (Gong et al., 2020b). First, we note that the BT-generated INPs overlap strongly with the ambient measurements. This suggests that the ambient INP has strong local sources from the seawater. Due to increased mixing and dilution in the ambient, it is also expected that NINP, BT would be biased higher in the BT than in the ambient as seen in Fig. 9. Secondly, the NINP derived for SML and SSW are much lower than those measured in ambient air and in the BT. This strongly supports the fact that the aerosol production mechanisms that occur at the sea surface (and in the295 BT) are responsible for preferentially partitioning INPs to the airborne phase. Modeling the bubble or jet bursting process at the sea surface is crucial to capture the true flux of INPs from the ocean to the atmosphere since the enrichment of airborne INPs cannot simply be represented by the mass transfer of NaCl. Figure 9. Atmospheric NINP as a function of temperature measured from ambient air by LVS filter with the PM10 inlet (gray shaded area), BT-aerosol collected on filters (red), and NINP derived from SML (green) and SSW (blue) samples, respectively. The factor used to convert NINP in the SML and SSW samples to concentration in air is 10-10 (Gong et al., 2020a). 3.3 Correlations of NINP with SSA size and bio-chemical seawater properties Figure. 10 shows the correlation between a selection of variables measured from the BT experiments during the Arctic Century300 campaign. Spearman rank correlation coefficients were calculated due to the unknown distribution of each investigated parameter. Notably, NINP, BT negatively correlates with salinity, reflecting INP-rich island water bodies. In addition, weak to moderate 17 Page 17 of 30 Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E correlations were found between NINP, BT and seawater temperature (Tseawater). This could be a purely latitudinal or seasonal effect. However, higher water temperature is reported to boost marine biological productivity by lowering the activation energy of enzymes (Hall et al., 2008), thereby increasing the emission of IN-active marine biogenic aerosol.305 18 Page 18 of 30Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E Figure 10. Spearman rank correlation matrix of variables measured from the BT experiments during the Arctic Century campaign, with the coefficients shown in each box. The cross-correlated parameters are NINP, BT measured at three selected temperatures (T= -10, -15 and -20 °C); the salinity and temperature of water samples (Tseawater) used for BT experiments; and quantity of nutrients and biological and terrestrial source indicators measured from the water samples, including chlorophyll-aconcentration, fluorescence, concentrations of glucose, dissolved organic carbon (DOC), total dissolved nitrogen (TDN), nitrate (NO3–), phosphate (PO43– ) and silicate. Additionally, the number concentration of aerosolized particles with diameters > 0.5 µm measured from the OPC (n>0.5), n<0.3 and n<0.05 measured from SMPS were cross-compared. The asterisks represent results with statistical significance (p< 0.05). Smoothed time stamps (in minutes) were applied to different data sets for correlation analyses. 19 Page 19 of 30 Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E Concentrations of several components were measured for the water samples used in the BT experiments. Figure 10 shows some overall positive and significant correlations were found between NINP, BT and chl-a(except T= -15 °C), fluorescence and phosphate. Although neither of these components is reported to be IN-active, their abundance is a tracer of promoted marine biological activity, from which IN-active organics and biogenic exudates can originate. Recently it was shown that fluorescent particle concentrations are a good predictor for INPs of dust and biological origin (Gao et al., 2024). No clear correlation310 between NINP, BT and DOC was found in contrast to McCluskey et al. (2018b) who suggested that a positive correlation is indicative of sub-0.2 µm DOC components acting as marine INPs. Strong correlations were also not observed between NINP, BT and TDN and NO3–. A significant and moderate correlation was found between NINP, BT and silicate concentration. A biological origin of silicate in the ocean from marine diatoms and microalgae with siliceous cells has been reported (e.g., Thalassiosira pseudonana) to be IN-active (Knopf et al., 2011). Another possibility for the positive correlation with silicate is the presence315 of dust-type INPs. Cornwell et al. (2020) showed in a laboratory study that the seawater doped with dust can be re-aerosolized with retained IN activity and added to the atmospheric INP populations during bubble bursting. This pathway of INP transfer could be responsible for the relatively high NINP aerosolized from land-sourced freshwater or seawater samples collected approaching islands (see Fig. 8) that are often rich in suspended sediments due to the fluvial outwash. NINP, BT is negatively correlated with particle concentrations n>0.5 aerosolized from the water samples, indicating that the ab-320 solute loading of SSA (dominated by NaCl) does not determine the abundance of aerosolized INPs (supporting the conclusions from Figure 9) and emphasizes that NINP cannot always be predicted by aerosol number concentration (Li et al., 2022) like parameterizations for dusty or highly polluted atmospheres (DeMott et al., 2015; Niemand et al., 2012; DeMott et al., 2010). However, the negative correlation between NINP, BT and n<0.3 and n<0.05 do not support that finer aerosol mode could be used as predictors for INP concentrations. The absence of a positive correlation with aerosol number concentration of different size325 ranges strengthens our conclusion that NINP cannot always be predicted by aerosol number concentrations as has been observed elsewhere in the Arctic (Li et al., 2022; Paramonov et al., 2020; Lacher et al., 2018). Combining the correlation analyses of NINP, BT with biological indicators (i.e., Chl-a, fluorescence, PO43– ) and silicate, we infer that the aerosolization processes at the water-air interface preferentially partitions the IN-active species to the aerosol phase and that the composition (phosphate, silicate) and fluorescence information are crucial to predict INP concentrations (Gao et al., 2024).330 The relative abundance of INPs in different samples taken simultaneously including ambient air (impinger samples), seawater samples (CTD seawater), and BT-aerosolized SSA are shown in Fig. 11. The comparison reveals insignificant correlations between NINP in ambient air and the coincidentally sampled seawater (Fig. 11 a). In contrast, a statistically significant correlation was found between airborne (NINP, ambient) and BT-aerosolized (NINP, BT) INP concentrations at the selected freezing temperatures (Fig. 11 b). NINP, BT are present in higher concentrations compared to ambient INPs by about an order of magni-335 tude due to the absence of dilution in the BT-aerosolized air. The positive correlations suggest that the BT experiments simulate a realistic aerosolization process at the sea-air interface, and the seawater can be a considerable source of INPs in the Arctic Ocean. The data from Fig. 11 (b) suggest that SSA is a source of INPs, but the absence of a positive correlation with NINP derived from seawater in Fig. 11 (a) suggests otherwise. This additional support is that the missing factor for water samples is the aerosolization process that enriches INPs in the air, which cannot simply be accounted for by the flux of SSA from water to340 20 Page 20 of 30Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E Figure 11. Comparisons between ambient NINP measured from impinger samples and coincident NINP measured from (a) seawater (b) BTaerosol filters. The dashed lines indicate the linear regression in the logarithmic scale, and only the statistically significant (p< 0.05) rvalues (correlation coefficients) are shown in the figure. The correlation coefficients of all data are given in Tab. ??. air. The mechanisms of INP aerosolization at the ocean-air interface could be impacted by several compounding factors, e.g., an enrichment of INPs at the sea surface microlayer (Wilson et al., 2015), dissolved species and their abundance (van Pinxteren et al., 2017; Cochran et al., 2016), biological productivity (Creamean et al., 2019; Rinaldi et al., 2013), sea surface temperature (van Pinxteren et al., 2017; Ladino et al., 2016), wind speed (Lewis et al., 2004), and sea ice concentrations (Gabric et al., 2018).345 4 Conclusions This study presents summertime observations of NINP and aerosol related properties over the Barents, Kara, and Laptev Seas in the Eurasian Arctic from August to September 2021. Variability in NINP is highly influenced by local sources, and to some extent meteorological conditions, and potentially long-range transport. A series of online and offline measurements were applied to investigate the INP abundance in seawater, fog, and ambient air to understand the sources of the INP and production350 mechanisms. The majority of ambient aerosol samples collected on LVS filters were prone to degradation in INP concentration after heating to 95 °C for 20 min, indicating the presence of proteinaceous or biogenic INPs that are heat-labile. By comparing NINP in the ambient air to other environments, similar NINP was observed in fog water compared to ambient air, indicating 21 Page 21 of 30 Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E INPs are not enriched in fog water and are likely cloud condensing nuclei. INPs in fog should be considered in studies of INP355 distribution and availability due to its role as an intermediate reservoir and for the formation of ice fogs in the atmospheric lifecycle of INPs. No clear trend was observed for INP enrichment in the SML compared to SSW, which is both in agreement with and in contrast to previous studies. The frequency of marine biological activity, such as phytoplankton blooms, fluvial input of sediment INPs, along with the variability in sea surface mixing and ocean state, could influence the abundance of INPs in different ocean layers.360 A comparison of NINP in the SML, SSW and CTD seawater samples to in situ aerosol measurements from a BT system and ambient INPs from the LVS filters reveals that the bubble-bursting mechanism is important in transferring INP from the ocean to the atmosphere. The abundance of INPs in the BT-sprayed aerosols showed a dependence on the composition of water components, with terrestrial freshwater possessing higher NINP compared to seawater samples. In accordance, a reduced NINP, BT was associated with a longer distance to land or an increased sea ice coverage where the subsurface seawater365 samples were collected. Based on the correlation analyses between NINP, BT and biological activity, this study supports the important role of marine biogenic aerosol as a source of INPs in the remote Arctic Ocean far from terrestrial sources. Our results highlight key relationships, such as the strong correlation between INP abundance and local biogenic factors (e.g., chlorophyll and fluorescence signals). These correlations suggest that integrating real-time biological activity data, especially during phytoplankton blooms, could improve model parameterizations of Arctic INP emissions. Lastly, other effects may be370 present in addition to the composition during the SSA generation and INP partitioning in the ocean and atmosphere, including the enrichment of IN-active materials in the sea surface microlayer, sea ice coverage, advection, and mixing in different ocean layers. Data availability. The data from this study are all incorporated into the main text and will be made available additionally through the ETH Library with a persistent DOI upon successful peer-review of the manuscript.375 Author contributions. GL performed sample processing and data analysis, produced figures, interpreted results, and co-wrote the manuscript. GL and AW participated in the campaign and conducted in situ sampling and measurements. AW contributed to the manuscript preparation and acquired funding. AR conducted the aerosol generation bubble tank experiment and collected samples for biochemical analysis. GPF performed INP analysis and heat treatment of filter samples, CTD seawater samples, and BT-aerosolized filter samples. IT provided the LAGRANTO backward trajectory data. MD provided the research idea on INP partitioning using the INP results from the bubble tank. ZAK380 supervised the project, obtained funding, and was involved in experiment and campaign planning, data interpretation, and manuscript writing. All authors reviewed the manuscript. Competing interests. The authors declare that no competing interests are present. 22 Page 22 of 30Faraday Discussions Faraday Discussions Accepted Manuscript Open Access Article. Published on 02 December 2024. Downloaded on 2/14/2025 10:37:33 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online DOI: 10.1039/D4FD00160E Acknowledgements. This research used samples and/or data provided by the Arctic Century Expedition, a joint initiative led by the Swiss Polar Institute (SPI), the Antarctic and Arctic Research Institute (AARI), and GEOMAR Helmholtz Centre for Ocean Research Kiel (GEO-385 MAR), and funded by the Swiss Polar Foundation. GL and ZAK acknowledge that this project has been made possible by a grant from the Swiss Polar Institute, Dr. Frederik Paulsen. GPF acknowledges funding from the Academy of Finland (grant no. 342227). ZAK acknowledges this project has received funding from the Horizon Europe programme under Grant Agreement No 101137680. We acknowledge all those involved in the fieldwork associated with the Arctic Century Expedition, including technical support from Dr. Michael Rösch. We acknowledge Dr. von Jackowski and Dr. Engel for sharing biogeochemical data of CTD water samples. We acknowledge Franziska Aemisegger390 and Iris Thurnherr calculated the backward trajectories used in this study. 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