Transparent exopolymer particles (TEPs) exhibit the properties of gels and
are ubiquitously found in the world oceans. TEPs may enter the
atmosphere as part of sea-spray aerosol. Here, we report number concentrations of TEPs with a diameter
Based on
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 TEPs likely possess good properties to act as INPs, in future experiments it is worth studying if a certain part of TEPs contributes a fraction of the biogenic INP population.
In marine ecosystems, polymer gels and gel-like material play an important role in the biochemical cycling of organic matter (OM) (Passow, 2000, 2002b). One type of gel-like particles, transparent exopolymer particles (TEPs), have increasingly received attention. TEPs exist as individual particles rather than diffuse exopolymeric organic material and are operationally defined as particles that are stained on 0.2- or 0.4-
In contrast to solid particles, TEPs have properties of gels; with similar constituents (carrageenans, alginic acid, and xanthan) to those that form gels, spontaneously forming from dissolved fibrillar colloids, and they can be broken up by calcium chelators such as EDTA. However, because TEPs have not yet been seen to undergo phase transition they can officially only be classified as gel-like particles (Verdugo et al., 2004). Regardless though, TEPs have been shown to be highly important in sedimentation processes and carbon cycling in the sea (Mari et al., 2017), as well as highly prevalent in the sea surface microlayer (SML) (Robinson et al., 2019a) with a potentially significant effect on air-sea release of marine aerosols.
Generally, TEPs can be formed via two pathways. First, the biotic pathway happens via a breakdown and secretion of precursor material or via a direct release as particles from aquatic organisms, e.g. as metabolic-excess waste products when nutrients are limited (Decho and Gutierrez, 2017; Engel et al., 2002, 2004). High TEP concentrations are usually associated with phytoplankton blooms, with the majority of precursor material being released by diatoms and to a lesser extent other plankton species. However, bacteria are also associated with TEP production, although their exact role is still not resolved (Passow, 2002a). Second, TEPs form through abiotic pathways. These could be formed spontaneously from dissolved organic precursors (e.g. dissolved polysaccharides) that are released by aquatic organisms. The abiotic formation is enhanced by turbulent or laminar shear (Engel et al., 2002; Passow, 2000). Recent studies confirmed that higher wind speeds, forming breaking waves, could be an effective transport and formation mechanism for TEPs to the ocean surface (Robinson et al., 2019b).
Transparent exopolymer particles are highly sticky and provide surfaces for other molecules and bacterial colonisation (Passow, 2002b), with between 0.5 % and 25 % (on average 3 %) of marine bacteria being attached to TEPs (Busch et al., 2017). TEPs naturally aggregate to other particles or highly dense matter and can sink in the ocean to contribute to downward carbon fluxes (Logan et al., 1995; Mari et al., 2017). However, TEPs that are not attached to sufficiently dense material will have a resulting low density and rise to the surface to form or stabilise the SML, which links the oceans to the atmosphere (Wurl and Holmes, 2008; Wurl et al., 2011).
From the ocean surface, TEPs have the potential to be transferred to the air.
Owing to wind and breaking waves, sea-spray aerosol particles are formed (de Leeuw et al., 2011; Lewandowska and Falkowska, 2013; Liss and Johnson,
2014) that could be a transfer mechanism for TEPs from the ocean to the atmosphere. Recently, high TEP mass concentrations of 1.4
Ocean-derived OM, of which TEPs are a part, has been reported to be enriched
and selectively transferred (compared with sea salt) to the atmosphere (Facchini et al., 2008; Keene et al., 2007; van Pinxteren et al., 2017).
Compared with seawater concentrations, OM in submicron aerosol particles is strongly enriched by factors of 10
In addition to an oceanic transfer, atmospheric in-situ formation might contribute to OM abundance in the atmosphere. Ervens and Amato (2020) provided a framework to estimate the production of secondary biological aerosol mass in clouds by microbial cell growth and multiplication. It was recently shown that this pathway might represent a significant source of biological aerosol material (Ervens and Amato, 2020; Khaled et al., 2021; Zhang et al., 2021). In another recent study, cloud water in-situ formation of amino acids resulting from biotic and abiotic processes has been measured and modelled (Jaber et al., 2021). Moreover, a higher microbial enzymatic activity on the aerosol particles compared with seawater was observed and it was hypothesised that after ejection from the ocean, active enzymes can dynamically influence the OM concentration and composition of marine aerosol particles (Malfatti et al., 2019). Still, the atmospheric in-situ formation of important OM compounds and its importance has not been well investigated to date and no studies exist on atmospheric in-situ TEP formation.
Regarding the properties of ocean-derived OM in the atmosphere, its ability to act as cloud condensation nuclei (CCNs) (Orellana et al., 2011; Sellegri et al., 2021) or ice nucleating particles (INPs) (Burrows et al., 2013; Gong et al., 2020a; McCluskey et al., 2018a, b) is not well understood at present. Bigg and Leck (2008) and Leck and Bigg (2005b) demonstrated, based on morphology and chemical properties, that the biogenic particles collected in air and in the SML could be consistent with polymer gels. For regions that generally show a low total particle number concentration and low CCNs (such as the high Arctic), it was suggested that microgels are CCNs (Leck and Bigg, 2005a, b; Orellana et al., 2011), owing to their hydrated and hygroscopic nature and because of the absence of other significant aerosol particle sources.
In addition, oceanic biogenic INP sources have been discussed (Creamean et al., 2019; Hartmann et al., 2020; Wilson et al., 2015; Zeppenfeld et al., 2019). In regions, however, where other sources dominate, oceanic sources might not suffice to explain the INP population, and non-marine sources most likely contribute significantly to the local INP concentration (Gong et al., 2020a). According to their structure, biopolymers consisting of proteins, lipids, and higher saccharides have been shown to play a role in the ice-nucleating activity (Pummer et al., 2015). In this context, TEPs may provide excellent functionalities to act as INPs, as they form a 3D network where water molecules can attach, providing a structured surface for ice formation. A direct link between TEPs and INPs, however, has not yet been experimentally shown in field studies.
Within the present study, the number concentrations and size distributions of TEPs in the ambient atmosphere in the tropical Atlantic Ocean were elucidated. We aimed to investigate the TEP number concentrations in the ambient aerosol particles and cloud water and to derive connections to oceanic transfer and potential in-situ formation mechanisms. Finally, we compared the TEP number concentrations with recently published atmospheric INP number concentrations at the same location (Gong et al., 2020a) and analyse possible interconnections. To our knowledge, this is the first study with detailed measurements of TEP number size distribution in different atmospheric marine compartments within the tropical Atlantic environment.
Samples were taken during the MarParCloud (“Marine biological production, organic aerosol particles and marine clouds: a process chain”) campaign that took place from 13 September to 13 October 2017 at the Cabo Verde archipelago island Sao Vicente located in the Eastern Tropical North Atlantic (ETNA). A detailed overview of the campaign, background, goals, and first results are available in van Pinxteren et al. (2020). Measurements were performed at the Cape Verde Atmospheric Observatory (CVAO) as described in more detail elsewhere (Triesch et al., 2021a, b; van Pinxteren et al., 2020). The CVAO is located directly at the shoreline at the northeastern tip of the São Vicente island at 10 m above seal level (a.s.l.) (Carpenter et al., 2010; Fomba et al., 2014). Owing to the trade winds, this site is free from local island pollution and provides reference conditions for studies of ocean–atmosphere interactions as there is a constant north-westerly wind from the open ocean towards the observatory. However, it also lies within the Saharan dust outflow corridor, and mainly in the winter months (January and February), dust outbreaks frequently occur.
Total suspended aerosol particles (TSPs) for TEP analysis and PM
The TSPs were sampled with a filter sampler consisting of a filter holder equipped with a 0.2-
PM
Cloud water was sampled on Mt. Verde, which is the highest point of the São Vicente island (744 m), situated in the northeast of the island
(16
Three cloud water samples collected on 20 September 2017, 28 September 2017, and 4 October 2017 were analysed for the TEP number concentrations. They were filtered (150–200 mL) through 0.2-
To investigate a direct oceanic transfer of TEPs via bubble bursting, TSPs were sampled from a plunging waterfall tank experiment that is
described in detail in the MarParCloud overview paper (van Pinxteren et al.,
2020, Supplement section). The tank was designed to study the bubble-driven transfer of organic matter from the bulk water into the aerosol phase. It consists of a 1400 L basin with a 500 L aerosol chamber on top. The bubble-driven transport of organic matter was induced using a skimmer on a plunging waterfall. A stainless steel inlet was inserted into the headspace of the tank and connected with three filter holders for offline aerosol particle sampling without size segregation (TSPs). The filter system for TEP analysis was equipped with a 0.2-
The filters obtained from ambient and tank-generated TSP aerosol particle
sampling and cloud water filtrations were stained with 3 mL of an Alcian blue stock solution (0.02 g Alcian blue in 100 mL of acetic acid solution,
pH 2.5) for 5 s yielding an insoluble non-ionic pigment and afterward rinsed
with milliQ water. The dye Alcian blue consists of a macromolecule with a central copper phthalocyanine ring linked to four isothiouronium groups via
thioether bonds (Passow and Alldredge, 1995). The isothiouronium groups are strong bases and account for the cationic nature. The exact staining mechanism is not resolved but it is believed that the cationic isothiouronium groups bond via electrostatic linkages (ionic bonds) with the polyanionic molecules of the TEP molecule; hence, the carboxylic and sulfonic side groups are stained. Alcian Blue can also react with carbohydrate-conjuncted proteins at proteoglycans, but not with nucleic acids and neutral biopolymers (Villacorte et al., 2015). After staining, the filters were kept at
For microscopic analysis, the protocol following Engel (2009) was applied. In short, abundance, area, and size-frequency distribution of TEP were determined using a light microscope (Zeiss Axio Scope A.1) connected to a camera (ColorView III). Filters were screened at
Microscopic analysis of transparent exopolymer particles (TEPs) from the cloud water sample “WW5”
(sampling interval: 28 September 19:30 LT–29 September 07:30 LT). Blue particles are TEPs, stained with Alcian Blue solution; brownish particles in the right picture are assumed to be dust particles. The scale refers to 50
Blank filters were taken for aerosol sampling (inserting filters in the aerosol sampler without probing them) and cloud water (filtering reagent water over a pre-cleaned filter), stained and treated the same way as the microscopic analysis. Blank number concentrations were on average 6 % of the cloud water results and between 5 % and 20 % for aerosol results and the blank values were subtracted from the samples.
The analysis of inorganic ions from PM
Non-sea-salt calcium was calculated from the ion ratio of
Ice nucleating particle number concentrations (
All the samples of this study are summarised in Table 1. In addition to samples from the MarParCloud campaign, surface seawater samples obtained from the ETNA (Engel et al., 2020) were considered.
Overview of sampling locations, types, and measurements.
To determine enrichment or depletion of TEPs in the atmosphere (i.e. on the
aerosol particles and in the cloud water) in relation to the TEP concentration in the ocean water, the concept of the aerosol enrichment factor can be applied. To this end, the concentration of the compound of interest in each compartment is related to the respective sodium mass
concentration, as sodium is regarded as a conservative sea salt tracer
transferred to the atmosphere in the process of bubble bursting (Sander et al., 2003). This concept is usually applied to calculate the enrichment of a compound in the aerosol particles (EF
Transparent exopolymer particle (TEP) number concentrations in the aerosol particles (red bars) and in the three cloud water samples (black-red squares). TEP concentrations were below the limit of detection (LOD) on 26 September 2017. The backgrounds represent the dust classification according to the ambient dust concentrations (blue: dust <5
Within the 3-week sampling period, TEPs varied within one order of
magnitude between
Box and whisker plot of the transparent exopolymer particle (TEP) number concentrations
In addition, TEPs were measured in four aerosol particle samples from the
plunging waterfall tank and the concentrations varied between
In addition to the total number concentrations, TEP number size distribution was derived from all ambient aerosol particle samples and are shown in Fig. 4a–d in both, linear and logarithmic form. In addition, the TEP number size distribution of one cloud water sample is presented in Fig. 4e and f. All samples exhibited very similar trends in their size distribution, with higher number concentrations for smaller sizes.
Transparent exopolymer particle (TEP) number size distribution in the aerosol particles and cloud water in linear and logarithmic form; panels
From the observed size distributions, it can be assumed that the number
concentrations will continue to increase towards smaller sizes. A comparison of TEP number concentrations in the ambient aerosol particles or cloud water
with literature values is challenging owing to the availability of very few studies and different sample types and size ranges regarded in different
studies. However, the trend in the TEP number size distributions observed here is consistent with studies by Kuznetsova et al. (2005) showing increased TEP concentrations in simulated sea spray regarding particle sizes from 50
Regarding polymer gels in general, a strong increase with decreasing sizes
was observed in cloud water in the high Arctic (north of 80
These calculations show that the number of gel-like particles in the high
Arctic was still several orders of magnitude higher than TEP particles in the tropical Atlantic, e.g. 10
From a recent study of TEP number concentrations in different oceanic regions, TEP number concentrations in surface waters (10-m depth) of the East Tropical North Atlantic (ETNA) were obtained (Engel et al., 2020). ETNA is the region that geographically includes the Cabo Verde islands. The oceanic TEP number concentrations are shown in Fig. 5. The TEPs in the ocean showed a similar size distribution to the TEPs in the atmosphere (i.e. aerosol particles and cloud water, Fig. 4) with increasing TEP number concentrations towards smaller particle sizes (Table S5 and more details in Engel et al., 2020).
Transparent exopolymer particle (TEP) number size distributions in the ocean surface water (sampling
depth: 10 m) from the East Tropical North Atlantic (ETNA), averaged three stations from Engel et al. (2020). The data in this figure show the size distribution between
A detailed comparison of #TEPs in the ocean and in the atmosphere regarding the identical size bins showed that the #TEP distribution among the different size bins was much more balanced for seawater than for aerosol particles. In aerosol particles, on average 52 % of the #TEPs were located in the smallest size bin analysed (4.5–7
Relative contribution of the transparent exopolymer particle number concentrations in the aerosol particles (left) and in the ocean surface water (right) regarding the identical size bins.
To compare seawater and atmospheric TEP concentrations in terms of enrichment or depletion, the atmospheric enrichment factor EF
In order to compare the same TEP diameters in all compartments, the size
range between 5
It should be noted that the lower enrichment in the tank resulted from the lower TEP number concentrations in the generated aerosol particles, as the sodium concentrations in the tank aerosol were even higher than in the ambient particles (Table S3). This suggests that, although an artificial tank study cannot represent the ambient environment, the generation of sea-spray aerosol was in progress; however, TEP transfer seemed not to be pronounced.
In the following, the enrichment factors obtained here will be discussed in more detail considering studies available from the literature.
Atmospheric enrichment of ocean-derived OM has often been reported (e.g. Facchini et al., 2008; Keene et al., 2007; O'Dowd et al., 2004; Schmitt-Kopplin et al., 2012; Triesch et al., 2021a, b; van Pinxteren et al., 2017). Submicron particles are usually strongly enriched with OM with aerosol enrichment factors EF
The concept of the aerosol enrichment factor originates from
controlled tank experiments where a direct transfer of compounds from the
ocean via sea-spray aerosol formation occurs. Obviously, this does not
automatically correspond to the ambient environment as mixing processes,
ageing, and further transformation reactions are not accounted for. However,
the EF
The high abundance of TEPs in the aerosol particles and cloud water may
correspond to an oceanic transfer within the process of bubble bursting. To
investigate linkage to the bubble-bursting transfer, TEP concentrations were correlated with the wind speed, as well as with the sea-spray tracers sodium and magnesium. To account for biases due to a number-based (TEPs) and mass-based (sodium, magnesium) comparison, the particle volume of TEPs was calculated from the particle number concentrations (regarding the size range: 5–10
Reasonably good correlations of TEPs with sodium, sea-salt calcium (
Correlations of transparent exopolymer particle (TEP) volume concentrations (size range: 5–10
Despite the correlation of TEPs with sea-spray tracers, the high abundance and
enrichment of #TEPs in the ambient aerosol particles compared with literature data and compared with the concentration and enrichment of the #TEPs from the plunging waterfall tank performed here, suggests that additional (secondary) TEP sources in the ambient atmosphere might exist from which TEPs are added to their primary transfer by bubble bursting from the oceans. At the Cabo Verde islands, besides the ocean, mineral dust is an important aerosol particle source (Fomba et al., 2014). TEPs are generally attributed to be ocean-derived compounds; however, dust has often been reported to transport attached biological particles (Maki et al., 2019; Marone et al., 2020). During the MarParCloud campaign, dust influences were low to moderate and the aerosol particle mass was found to be predominantly of marine origin (Fomba et al., 2014; van Pinxteren et al., 2020). Some dust influences were visible though, e.g. variations in the particle number concentrations, with elevated concentrations on (even low) dust-influenced air masses (Gong et al., 2020b). TEP number concentrations showed no clear connection to the ambient dust concentrations (Fig. 2). Within periods of moderate dust, TEP were partly below the detection limits (on 26 September 2017) and partly exhibited high concentrations (e.g. on 28 and 29 September 2017). No correlation between TEP and dust was found (
In aquatic environments, abiotic TEP formation has been reported to happen via several pathways, including spontaneous assembly from TEP precursors
(Passow, 2002b). The aerosol particle and cloud water samples from the MarParCloud campaign investigated here showed high mass concentrations of amino acids (up to 6.3 ng m
Another important parameter likely impacting TEP formation is the presence of mineral dust. As already discussed above, dust mass concentrations were low to moderate, though not negligible, during the MarParCloud campaign. In laboratory minicosm studies, the addition of dust to oceanic water resulted in an acceleration of the kinetics of TEP formation leading to the formation of fast sinking particles (Louis et al., 2017). This process likely happens because of particle aggregation, meaning that dissolved OM and dust aggregate to form TEPs (Louis et al., 2017). In addition, dust particles in cloud water may promote turbulence, which, in aquatic media, has been suggested to enhance abiotic TEP formation (Passow, 2002b). The dust deposition at Cabo Verde has been recognised as a potentially large contributing factor to the TEP enrichment in the SML (Robinson et al., 2019a). Here, we speculate that even low concentrations of mineral dust can influence TEP formation on the aerosol particles and in the cloud water. This is further supported by the microscopic detection of dust in the cloud water (Fig. 1), that likely enhance the possibility that particles in the cloud water collide and stick. Consequently, although dust did not seem to serve as a transport medium for TEPs (see Sect. 3.3.1), dust may contribute to in-situ TEP formation in cloud water due to abiotic particle aggregation.
From atmospheric studies, marine gel particles have been reported to undergo a volume phase transition in response to environmental stimuli, such as pH
and temperature, as well as cleavage of their polymers owing to ultraviolet (UV) radiation (Orellana et al., 2011). UV radiation can break down microgels in the ocean into a high number of smaller (nano-sized) particles (Orellana and Verdugo, 2003) – a mechanism that is expected to be highly relevant in the atmosphere where UV radiation is higher than in seawater. Furthermore, it has been shown that a lowering of the pH from neutral conditions (7 or 8) to 4.5 causes a sudden transition of gel particles in which the polymer network collapsed to a dense, non-porous array (Chin et al., 1998). As TEPs are reported to exhibit a gel-like character (Passow, 2002b), volume and number concentrations might be affected by the different factors such as pH, ion density, temperature, and pressure in the atmosphere. The measured cloud
water pH value of the samples analysed here was between 6.3 and 6.6, at which marine gels could split into smaller units (Chin et al., 1998). Hence, a part of the cloud water TEPs might be below the minimum detectable particle size of 4.5
Besides abiotic pathways, in aqueous media, TEPs can be directly released as
particulates from aquatic organisms involving phytoplankton and bacteria
(Passow, 2002a). Biotic TEP formation has by now been studied for seawater and lakes (Passow, 2002a); however, bacteria are also present in the atmosphere, are likely transferred from the ocean via sea spray (Rastelli et al., 2017), and can survive in cloud droplets (Deguillaume et al., 2020). The bacterial abundance in cloud water samples taken at Mt. Verde during the MarParCloud campaign ranged between 0.4 and
The presence of active enzymes on ambient aerosol particles (enriched compared with seawater), and therefore biogenic in-situ cycling of OM through enzymatic reactions in atmospheric particles, was recently suggested (Malfatti et al., 2019). This is in line with the findings that the aerosol particles and cloud water from the MarParCloud campaign contained high concentrations of OM (amino acids, lipids), assumed to be connected to a biogenic formation (Triesch et al., 2021a, b). A combined approach of laboratory experiments and modelling recently underlined the importance of biotic (and abiotic) formation processes of OM in clouds (Jaber et al., 2021).
Regarding time scales of biotic processing, Matulova et al. (2014) showed that the
Considering recent literature and the data reported here, we suggest that in-situ TEP formation related to biogenic processes and likely connected to bacteria, as reported for seawater, might be important in the marine atmosphere as well. Besides, although not measured here, microalgae and cyanobacteria, which are relevant for direct TEP formation in seawater, have been reported to occur in the atmosphere (e.g. Lewandowska et al., 2017; Sharma et al., 2007; Wiśniewska et al., 2019, 2022). It is worth studying whether these species and their metabolic degradation products contribute to atmospheric TEP processing.
Different kinds of ice-nucleating macromolecules have been found in a certain range of biological species and consist of a variety of chemical structures including proteins, polysaccharides (Pummer et al., 2015), and lipids (DeMott et al., 2018). TEPs, consisting of polysaccharidic chains bridged with divalent cations, may therefore possess good properties to act as INPs; however, such a link has not yet been shown in field experiments.
During the MarParCloud campaign INP number concentration (
In the present study, quantitative INP data (presented in Gong et al., 2020a)
and TEP data measured from the same campaign were compared. To this end, INP
concentrations achieved from PM
Transparent exopolymer particle number concentrations were on average between 10
The correlation between INP (active at
The INP functionalities of biomolecules are not straightforward and whether a macromolecule acts as an INP is dependent on many factors, such as its size, the proper position of functional groups, and their allocation (Pummer et al., 2015). Typically, not the entire surface of an INP but rather specific areas (active sites) participate in ice nucleation. This means that despite TEPs likely providing INP properties, only a fraction of TEPs, if any, may be able to act as INPs. This hypothesis is supported by the findings that marine gels exhibit hydrophobic and hydrophilic surface-active segments, strongly suggesting a dichotomous, non-uniform behaviour of polymer gels (Leck et al., 2013; Orellana et al., 2011; Ovadnevaite et al., 2011). As mentioned in the sections “Abiotic formation” and “Biotic formation”, TEPs are often attached to, or colonised with bacteria. Bacteria themselves have been shown to provide excellent INP functionalities (Pandey et al., 2016) and TEPs may act as a carrying medium for INPs, such as bacteria. Bacteria concentrations were higher than TEP concentrations and also higher than INP concentrations. However, only a fraction of all bacteria (0.5 %–25 %) is associated with TEPs and, vice versa, not all TEPs are colonised by bacteria (Passow, 2002b). There is an indication that especially in oligotrophic waters, as the Cabo Verde islands are, the fraction of bacteria attached to TEPs is comparably low (Schuster and Herndl, 1995). Hence, the concentration range of bacteria-colonised TEPs in relation to INPs is worth further consideration. This may help to unravel whether a functional relationship between bacteria-colonised TEPs and INPs exists and whether a certain number of TEPs contain fragments in the biological INP population that, beyond dust, play a role in the Cabo Verde atmosphere.
This study presented TEP number concentrations
The TEP data are accessible under the following link
The supplement related to this article is available online at:
MvP led the MarParCloud campaign and, together with the campaign participants KWF, XG, EB, NT, BR, FS and HW performed the aerosol particle and could water sampling at the Cabo Verde islands. HH contributed to the campaign conceptualisation, coordination towards the CVAO and ACD, and manuscript development and revision. EB designed and operated the plunging waterfall tank. BR performed the microscopic TEP measurements and XG performed the INP analysis. AE contributed the seawater TEP data. MvP performed the data interpretation with help from SZ and BR. MvP wrote the paper with contributions from all authors.
The contact author has declared that neither they nor their co-authors has any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This article is part of the special issue “Marine organic matter: from biological production in the ocean to organic aerosol particles and marine clouds (ACP/OS inter-journal SI)”. It is not associated with a conference.
We acknowledge the funding by the Leibniz Association SAW in the project
“Marine biological production, organic aerosol particles and marine clouds:
a Process Chain (MarParCloud)” (SAW-2016-TROPOS-2), the Research and
Innovation Staff Exchange EU project MARSU (69089) and 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)
This research has been supported by a Leibniz-Society SAW grant (MarParCloud) under SAW-2016-TROPOS-2. The publication of this article was funded by the Open Access Fund of the Leibniz Association.
This paper was edited by Kimitaka Kawamura and reviewed by three anonymous referees.