An Arctic Marine Source of Fluorescent Primary Biological Aerosol Particles During the Transition from Summer to Autumn at the North Pole

2.2 Bioaerosol measurements

Particles of biological origin were measured on a single-particle basis with an MBS that utilizes ultra-violet light induced fluorescence and light scattering. Incoming particles are aligned in single-file in a sample flow of 0.3 L min^–1, surrounded by a sheath flow of around 1.7 L min^–1. A low power laser beam with a wavelength of 635 nm shines through the aerosol sample column. When a particle passes through, the scattered light is collected by an assembly of lenses and used to define the detection of a particle, as well as the particle size. The detection of a particle triggers a high-power laser pulse at 637 nm to illuminate the particle, and the scattering pattern obtained gives information on the particle’s morphology. Finally, 10 µs after particle detection, a xenon flash lamp with a wavelength of 280 nm shines on the particle for about one µs. Using two mirrors, the resulting fluorescence is focused through a diffraction grating to a photodetector. The detector has eight, equally spaced, channels between 310–650 nm, thus a spectrum is recorded for each fluorescent particle. The instrument and measurement process are described in more detail in Ruske et al. (2017).

The shape and intensity of the fluorescence spectrum of each particle can be used to differentiate between fPBAP and other fluorescent particles. The classification used in this work follows Freitas et al. (2022) for marine aerosol. All detected particles are counted and will be referred to as coarse particles (CP). Those with a fluorescence, in any channel, higher than three standard deviations from the mean of the background (periodically measured without sample flow) in that channel are labeled fluorescent particles (FP). Those with fluorescence greater than nine standard deviations above the mean background are considered highly fluorescent particles (HFP), and those among them with the highest signal recorded in the 364 nm channel are classed as fPBAPs. This channel corresponds to the peak fluorescence of the amino acid tryptophan, typically found in microorganisms. We refer to the HFP that are not fPBAPs as other HFP (OHFP). See Figure S1 in the SI for an overview of the particle classes and their overall contribution throughout the expedition. The particle fluorescence declines with size; therefore, all particles with an optical diameter below 0.8 µm are discarded, as the distinction between CP and FP below this limit cannot be made reliably. The fPBAPs can be further classified into subtypes, based on which additional fluorescence channels surpass the 9-standard-deviation threshold. We have applied the same definition as Pereira Freitas et al. (2023): type I corresponds to the fPBAPs that only fluoresce beyond the threshold in the 364 nm channel, which is the second channel; type II also in the third channel; type III in the first three channels; type IV in the first four channels; and finally, type V are any other combinations of channels, as long as the highest signal is in the second channel (the criteria for fPBAP classification).

Some particles have a fluorescence that is strong enough to saturate the detector, and if this happens in multiple channels, it is impossible to determine which channel recorded the highest signal. OHFP that only saturate the detector in the 364 nm channel could theoretically still be correctly classified as fPBAPs, but in this data set only 14 such particles were detected, hence negligible. All saturating particles are therefore classified as OHFP, which means that some potential fPBAPs are not considered. However, since this data set is influenced by the pollution from the ship exhaust, which most likely produces very potent HFP such as polyaromatic hydrocarbons (Garra et al., 2015; Savage et al., 2017; Yu et al., 2016), we assume that most of the saturating particles are linked to pollution, and the risk of rejecting real fPBAP can be considered low. Additional uncertainties in the fPBAP measurements stem from undercounting, as any nonfluorescent fPBAP is excluded, and overcounting, as the method applied still allows some non-PBAPs, but with a similar fluorescent spectrum, to be classified as such.

2.3 Auxiliary measurements

INPs were sampled behind the whole-air inlet using polycarbonate filters and subsequently analyzed on board the ship using a droplet-freezing array setup. As the ambient INP concentration was highly variable throughout the course of the expedition, the sampling time was adjusted to acquire sufficient INPs for analysis on each filter. Therefore, the sampling time differed between filters (see Figure 2 and Porter et al. (2022)), and calculated INP concentrations have been normalized to the sampled volume. A full description of the method and data can be found in Porter et al. (2022).

The particle number concentration and particle size distributions were measured using a mixing condensation particle counter (MCPC; Brechtel Manufacturing Inc., USA, Model 1702) and a custom-built differential mobility particle sizer (DMPS), respectively. The DMPS consists of a Vienna-type differential mobility analyzer (DMA) and a second MCPC for counting the size-selected particles in the diameter range of 10–921 nm. More details about the aerosol sizing instrumentation can be found in Karlsson et al. (2022).

The meteorological data, as well as the ship’s position, were taken from Prytherch and Tjernström (2019). An overview of the general meteorological conditions can be found in Vüllers et al. (2021). The visibility was taken from a sensor that was part of a counterflow virtual impactor inlet (Karlsson et al., 2022).

The bulk chemical composition of submicron aerosols was measured, at one minute time resolution, with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS, Aerodyne Research Inc., USA). Technical descriptions of the instrument’s functioning can be found in DeCarlo et al. (2006) and Canagaratna et al. (2007), and readers are referred to Karlsson et al. (2022) for details regarding the AMS field deployment and data processing during MOCCHA. The AMS uses a PM₁ aerodynamic lens and measures the mass concentrations of non-refractory dried aerosols, which correspond to the species that are flash-vaporized at temperatures below that of the instrument’s heated tungsten surface (600°C). Such species include sulfate, nitrate, ammonium, chloride, and organics but exclude crustal materials, black carbon, or sea salt (Jimenez et al., 2003).

The mass concentrations of methanesulfonic acid (MSA), an oxidation product of marine-sourced dimethylsulfide (DMS), were calculated from the AMS signal of the ${\displaystyle {\mathrm{C}\mathrm{H}}_3{\mathrm{S}\mathrm{O}}_2^{+}}$ fragment at the mass-to-charge ratio (mz) of 79, following the approach from Hodshire et al. (2019). Based on a post-campaign calibration of the instrument with MSA, we found a calibration factor of 12.14, by which ${\displaystyle {\mathrm{C}\mathrm{H}}_3{\mathrm{S}\mathrm{O}}_2^{+}}$ mass concentrations should be multiplied to retrieve MSA mass concentrations. This calibration factor falls within the same range (6.1–19.7) as previously reported values (Hodshire et al., 2019).

The AMS was installed behind a switching valve that alternated between the whole-air inlet and an interstitial inlet, used to sample unactivated particles below one micrometer, every hour. In this work we have only included the AMS data sampled behind the interstitial inlet, corresponding to odd hours, as the MSA signal was consistently lower behind the whole-air inlet, most likely due to higher losses.

2.4 Exclusion of data influenced by pollution

To ensure undisturbed clean air sampling, I/B Oden was pointed into the main wind direction. However, the periods of clean air sampling were interrupted by the influence of the ship’s own emissions (e.g., from the ship stack or kitchen exhaust), typically when the wind speeds were low, when the wind was outside the clean sector (±80° relative wind direction), during helicopter operations, or during ice breaking.

For measurememts of particle number concentration (including coarse particles measured by the MBS) and size distribution, potential pollution periods were excluded using a pollution mask developed and described by Karlsson et al. (2022). The mask follows a similar principle as outlined by Beck et al. (2022) and is primarily based on the total particle count. It excludes any 30 second periods when total particle concentration range exceeded 50 cm^–3 and the six hour running mean, as well as periods when the total particle concentration reached above 10⁴ cm^–3. Periods marked in the logbook as potentially polluted are also excluded.

Pollution masks based on particle count are not necessarily suitable for measurements of chemical composition (Beck et al., 2022). For the AMS measurements, we have applied the pollution mask developed for this dataset where data is excluded when the chemical spectrum was similar to that of a known spectrum of fresh pollution, composed mainly of hydrocarbons fragments (Dada et al., 2022).

The fPBAP dataset has not been filtered for pollution other than the exclusion of saturating particles (which have an inconclusive spectrum shape and cannot be classified as fPBAPs), as ship stack emissions do not contain fPBAPs. The choice of not excluding fPBAP during potentially polluted periods is further motivated in section 3.1.

2.5 Model and reanalysis data

3.2.1 In-situ observations

Figure 4 illustrates that the measured concentration of fPBAPs was virtually zero during the two days preceding and the three days following the event. Therefore, these two time periods outside the event are used as a reference. By comparing auxiliary data between these time windows and the fPBAP event, we highlight the circumstances that caused the event.

Figure 4

During the event period, the concentrations of fPBAPs and the total particle concentration exhibited largely similar temporal variations (Figure 4a), indicating that both were likely influenced by the same source during this time. This suggests that information about the submicron aerosol population can reasonably represent fPBAPs during the event, despite fPBAPs being larger than 0.8 µm.

The fraction of fPBAP to coarse mode is as low as outside the event during the first fPBAP peak on August 19, 1–30% between the peaks, and reaches 55% during the second peak on August 22 and 23. Thus, there is an increasing enrichment in fPBAP to other coarse mode particles. This indicates that the two main peaks during the fPBAP event might be quite different in terms of the main source, or processing, of the aerosol. To take this into account, the event can be separated into three main parts (as indicated in Figure 4); a first distinct peak (peak 1), a period of lower but still elevated fPBAP concentrations (between peaks), and a second, final, most prominent peak (peak 2).

For the period included in Figure 4, the observed bimodal size distribution is typical for marine aerosol (Heintzenberg et al., 2000). The chemical composition is also compatible with marine aerosol, with high fractions of organics (consistently 50% or above) and sulfate, although high organic fractions can also indicate terrestrial sources (Hodzic et al., 2020). As mentioned in section 2.3, we are only showing the submicron aerosol chemical composition from behind the interstitial inlet, which had lower sulphate fractions in fog than the whole-air aerosol population consistently during AO18 (Karlsson et al., 2022). Therefore, the drop in the sulphate fraction from around 35% to 20% or lower just before the start of the event (Figure 4b), is expected due to the onset of fog (Figure 4c). The fractions of organics, sulphate, and nitrate are similar to the average fractions during the ice drift portion of the expedition, where most of the air parcel influence came from ocean and ice (Karlsson et al., 2022). This is further supported by the continuous presence of methanesulfonic-acid (MSA), which is an oxidation product of dimethylsulfide (DMS) emitted from the ocean. While the MSA fractions decrease after peak 1, it is important to note that mass concentrations during this time are so low that many compounds are close to the detection limit (Karlsson et al., 2022) and thus mass fractions become more unreliable. Despite the fact that the MSA fraction is lower during the event than right before, the presence of MSA is still indicative of a marine signal, and during the event it is slightly higher during the two peaks than between them. Peak 1 stands out in terms of chemical composition, as it is associated with a detectable black carbon fraction, a tracer of biomass burning or anthropogenic emissions. This is an indication that peak 1 of the event could be influenced by long-range transported air carrying aerosol of terrestrial origin. However, peak 1 also coincides with an increase in coarse particles, which is most likely sea spray particles, and the aerosol size distribution (Figure 4c) is consistently bimodal. However, any air parcels with terrestrial origin that reach the North Pole will have had to pass over open ocean or sea ice and could, therefore, also carry a marine signal (Heutte et al., 2024). Therefore, it is crucial to investigate the origin and trajectories of the air parcels during the event.

3.2.2 Source and process analysis

The 5-day air parcel back trajectories show clear differences in source region between the fPBAP event, and the periods before and after (Figure 5). Between the peaks, and during peak 2, the air parcels originated from or spent time over the Laptev and Kara seas, or the Siberian coastline, where surface chlorophyll-a concentrations were highest in the Arctic at the time. This is confirmed by the FLEXPART-WRF air parcel origin attribution during the event (Figure S6), showing a likely contribution from the Laptev and Kara seas during peak 2 and between peaks, but also from the marginal ice zone of the Barents and Greenland seas, and the northern coast of Svalbard, during peak 1. The back trajectories during peak 1 also indicate a source region further west than for the rest of the event (Figure 5). The chlorophyll-a concentration does not appear to be elevated around Svalbard and the northern Barents Sea in Figure 5, but our comparison with satellite products (see Figure S5 and discussion in the Supplementary Information) revealed that the ocean productivity may be underestimated in this region in the reanalysis product used. Outside the event, no clear region of origin emerges and air parcels could have come from almost anywhere else in the Arctic (Figure S6). The trajectories that reach the ship outside the event have mostly spent time over the pack ice or the Greenland Sea (Figure 5), where chlorophyll-a concentrations were around five times lower than in the Laptev Sea. All trajectories passed over the pack ice before reaching the ship, as they must. The ice areas covered during the fPBAP event differ spatially compared to outside the event, implying influence from a different section of the pack ice, but the melt pond fraction is essentially homogeneous in all directions (Figure 5). Therefore, given that the trajectory time spent over ice is clearly greater outside the event, it seems less likely that the observed fPBAPs during the event would originate from melt ponds, re-suspension of deposited particles on the snowpack, leads, or any other possible source associated with the pack ice. Porter et al. (2022) reached the same conclusion regarding the sources of INPs. This is expected, as there is no correlation between fPBAPs and local wind speed, but this also shows that potential non wind-driven emission sources within the pack ice are also less likely. This result stands in contrast to recent findings by Beck et al. (2024), who connect high concentrations of HFP to long air parcel residence times over sea ice, and consider several plausible sources at low wind speeds within the ice. For the case of the event we present, however, it is clear that what differentiates the fPBAP event from periods with low concentrations (outside the event), is actually less contribution from sea ice. To further investigate the sources of the observed fPBAPs, we look into the history of the height of the trajectory points, as there must have been an interaction between the air parcel and the surface for the fPBAPs to become airborne. We also consider potential wet removal along the trajectories.

Figure 5

From Figure 6, it is clear that for all stages during the event, and outside the event (panels d and h), the last 20–30 hours before reaching the ship were spent over ice. Had the time windows outside the event had major precipitation, lower wind speeds over the ice than during the event, or less points near the surface, then the lack of fPBAP could have been explained by higher losses during this time, or low emissions. Instead, most trajectory time over ice, with an average wind speed of 7 m s^–1, is seen outside the event. The average accumulated precipitation before reaching the ship is 8 mm for trajectories during peak 2, and 3 mm outside the event (similar magnitude between peaks and for peak 1). Additionally, within the event, it is the period between the peaks that shows the most time spent over ice (Figures 5 and 6), and it is also associated with the lowest fPBAP concentrations during the event. We therefore conclude that it is highly unlikely that the observed fPBAPs during this period were sourced within the pack ice. This is also confirmed by FLEXPART-WRF source attribution results (Figure S6), showing a very low chance of origin in the pack ice during the event, while very high outside the event.

Figure 6

The fPBAP event stands out when it comes to trajectory points at near-surface pressures over open ocean, in particular with high chlorophyll-a concentration, compared to outside the event (Figure 6d compared to panels a–c). In fact, peak 2 (Figure 6b), when the highest fPBAP concentrations were recorded, shows the highest wind speeds (8 m s^–1 on average over ocean) over the most chlorophyll-a-rich ocean. During peak 1, and to some extent also between the peaks (panels (a) and (c)), the air also passed over an ocean surface with elevated chlorophyll-a, but the wind speed was slightly lower (5 and 6 m s^–1 on average, respectively) compared to during peak 2. Wave breaking, which is the most effective sea spray production, occurs at wind speeds around 5 m s^–1 and higher, with a steep wind dependence (Monahan and Muircheartaigh, 1980; Salter et al., 2015). This could explain why fPBAP concentrations were not as high, along with the fact that the chlorophyll-a concentrations were also lower.

Both throughout and outside the event, the trajectories passed over land sporadically (Figure 6). During the event, the residence times over land was only 0–6 hours per trajectory. Outside of the event (Figure 6D), residence times were similar but at higher wind speeds. Figure 5 shows that the land influencing the air parcels during the event would have been the Siberian coast, Svalbard, or other islands in the Arctic ocean, while outside the event, the trajectories passed over the coastline of Greenland. The influence from land is, therefore, not necessarily comparable, as the surface cover could have been different between these areas. However, both Svalbard and the coast of Greenland have been observed to be sources of biological INPs in the summertime Arctic (e.g. Pereira Freitas et al., 2023; Sze et al., 2023). Therefore, the fact that the land influence does not differentiate the event from the periods of low fPBAP concentration suggests that they were not of terrestrial origin, although we cannot completely exclude it. Since the points over land are so few along the air parcel trajectories during the event, especially in combination with higher wind speeds assumed to be needed for the emission process, we consider a marine source of fPBAPs during the event more likely than a terrestrial one. Furthermore, extending the back trajectories to 10 days, as illustrated in Figure S4, only a couple of the trajectories during the event extend over land and do not increase the fraction of time spent over land, as also shown by Karlsson et al. (2022).

During the entire event (Figure 6a–c), trajectories passed, near the surface, over ocean areas with elevated surface chlorophyll-a between about 70 to 30 hours before reaching the ship. This is the most prominent common trait of the trajectories during the event. The source of fPBAPs seems unlikely to lie earlier along the trajectories, as they differed a lot within the event further back in time, spending time over ocean, ice, or above the boundary layer, with varying wind speeds (Figure 6a–c). All of this is also featured in the trajectories outside the event (Figure 6d) and therefore not characteristic of the unique fPBAP event.

Moreover, there is no common precipitation tendency along the trajectories launched during the fPBAP event (Figure 6e–g). During peak 1, the air parcels reaching the ship were only influenced by constant surface precipitation rates of 0.03 mmh^–1 or less, which can be considered negligible. During peak 2, the air parcels are, in general, associated with similarly low precipitation rates (with the exception of one trajectory about 120 hours before reaching the ship, before passing over ocean) until about 40 hours from the ship, when surface precipitation increased to rates about 10 times and higher, only decreasing slightly approaching the ship. Between the peaks, the onset of precipitation was similar along the trajectories, but the highest rates were reached slightly closer to the ship. The temperature along the trajectories in the last 40 hours before reaching the ship was close to 0°C for almost all trajectories, particularly during precipitation. The fact that the lack of precipitation leading up to peak 1 stands out from the rest of the event is another possible explanation as to why it also differs in terms of the chemical composition and fPBAP-to-coarse particle ratio. It could be that coarse particles other than FPBAPs were removed more efficiently by precipitation. If the fPBAPs were of marine origin, they would have been co-emitted with sea salt particles, which generally have a higher hygroscopic growth factor (Swietlicki et al., 2008; Zieger et al., 2017) than bioaerosols (Johnson, 1999; Lee et al., 2002) by a factor of two or more. This could have led to an earlier and more effective removal of the sea salt containing particles, which probably made up most of the coarse particles. On the other hand, INP of biological origin have been shown by Stopelli et al. (2015) to be removed by precipitation almost twice as effectively as other particles of the same size. However, their study was primarily conducted at below freezing temperatures. In our case, given that the precipitation seems to have occurred around 0°C (Figure 6, panels f-g), it was likely too warm for ice nucleation. Fog can also enhance concentrations of fPBAPs by strengthening their ability to survive air transport (Evans et al., 2019). This is a less likely explanation in our case, as it would not explain the decrease in other coarse mode particles, especially as fog was also present during peak 1, when coarse particle concentrations were still high. It also cannot be ruled out that the difference in fPBAP-to-coarse particle ratio between peak 1 and the rest of the event is due to different source regions during peak 1, indicated in Figure 5, but also by the presence of black carbon during this period (Figure 4). The fPBAPs detected during the rest of the event may have come from a different source that was more enriched in fPBAPs compared to the rest of the coarse mode already at the stage of emission. However, given the striking difference in precipitation along the trajectories during peak 1 and the rest of the event, we consider the second pathway in Figure 3 (i.e., selective wet removal of other coarse mode particles) the most plausible explanation. This hypothesis could potentially be further tested using modeling, which could advantageously also be applied to test the impact that fPBAP events like the one we observed here might have on clouds and radiation.

3.2.3 Large scale meteorological conditions

Although our analysis of air parcel origin during the event did not indicate long range transport, there may have been transport pathways above the boundary layer. In order to fully address the likelihood of the fPBAPs being transported from outside the Arctic, we have analyzed the large scale meteorological conditions in the Arctic during the fPBAP event (Figure 7). At this time, the dynamical meteorological conditions were affected by a dipole structure in the mean sea level pressure, a low-pressure system on the western side of a high-pressure area, which allowed relatively strong southerly transport of aerosols and moisture from the Arctic Ocean north of the Eurasian coast towards the pole (Figure 7). At the beginning of the event, the low-pressure system was located near Svalbard, and the high pressure over the Kara Sea. This kind of dynamical setting allows direct air parcel transport from the open ocean toward the pole within the sector 0–60°E in the lower troposphere. Between the peaks, both the low-pressure system, including the development of new cyclones, and the high-pressure area over the Kara Sea moved eastward. The dipole pattern became stronger over the course of the event, strengthening the northward geostrophic wind between the low and high-pressure areas (Figure 7). Before peak 2, the 10 m wind speed in the sector 120–150°E over the Laptev Sea exceeded 10 m s^–1, strengthening aerosol emissions from the sea surface. Emitted particles would have been transported towards the pole, with the strong northward directing winds.

Figure 7

Air masses of warm continental origin did not reach the pole at the 850 hPa level or below it during the entire event. Instead, the cyclones carrying the warm air mass occluded before reaching the pole, and therefore the warm air mass could not be advected to the pole near the surface, preventing direct transport of continental aerosols to the pole. This is in support of our conclusion that the fPBAPs came from an Arctic marine source during the event. However, updrafts associated with fronts, which separate the warm air parcel from colder Arctic air parcels, may transport aerosol particles higher in the atmosphere, where they can act as condensation or INPs and fall with precipitation (e.g., Stopelli et al., 2015). During peak 1 of the fPBAP event, the warm air mass and precipitation associated with the frontal zones were located quite far from the pole, which further decreases the probability of a contribution of aerosol particles from continental sources. Between peaks and during peak 2, precipitation associated with the occlusion front of the warm air mass was located at or near the pole. Therefore, there is a possibility that the fPBAPs were transported above the boundary layer, from a mid latitude continental source. As fPBAPs can effectively act as INPs, they can fall from upper levels in the atmosphere to the surface with snowfall. Snow particles grow faster than or at the expense of water droplets (Wegener-Bergeron-Findeisen process), therefore, INPs can fall out from clouds before other aerosol particles. The surface temperature was most likely too warm for snow to form, but it could have happened above the boundary layer, allowing fPBAPs to precipitate down to the surface more effectively than the other coarse mode particles, contributing to the increased fPBAP to coarse particle fraction. This process corresponds to the first pathway illustrated in Figure 3, and was also hypothesized in Beck et al. (2024). For this pathway to be plausible, there would had to have been precipitation above the boundary layer during the event, and the snow particles must have sublimated near the surface, as the fPBAPs were sampled in the air. In our case, there was no precipitation at the surface during the event, and the air temperature was around 0°C, thus it is unlikely that there was frozen precipitation aloft that entirely sublimated in the boundary layer. In summary, based on our analysis of the large scale meteorological conditions during the fPBAP event, the only way the fPBAPs could have come from outside the Arctic would be if they were emitted from continental sources, lifted and transported above the Arctic boundary layer and precipitated out as ice, sublimating at the surface. We consider this scenario less likely than the second pathway in Figure 3, as argued in section 5. During peak 1, the meteorological conditions did not facilitate continental transport at all.

3.2.4 Ocean productivity in source regions

Based on the results of our analysis, we find it most likely that the fPBAPs during the event came primarily from an Arctic marine source, in the Laptev or Kara seas, or along the ice edge north of the Barents Sea. This conclusion is compatible with three of our initial hypotheses; the marginal ice zone (MIZ), an open ocean (ice-free) plankton bloom or river run-off influenced region (number 2–4 in Figure 3). An important factor in our analysis is the TOPAZ-ECOSMO Biogeochemistry Reanalysis data that we utilized to identify potential source regions of marine fPBAPs, assuming their connection to ocean productivity. To assess the spatial accuracy of the TOPAZ-ECOSMO product, we have also conducted a comparison using overlapping 7-day averages from three different satellite products (see Figure S5 and discussion in the Supplementary Information). Despite some uncertainties, there seems to be a clear indication of elevated surface chlorophyll-a concentration in the Laptev and northern Kara Sea, as well as along the Siberian coast during the time frame of the fPBAP event shown in Figure 5. This also aligns with earlier studies, indicating high primary production in this area (Anderson et al., 2011; Gibson et al., 2022; Terhaar et al., 2021). While the various products exhibit a reasonable level of agreement on a large scale, notable local discrepancies among the different datasets suggest that the data quality may not be robust enough to confidently compare smaller areas, such as the Siberian coastline, versus the open Laptev Sea. Therefore, we cannot confidently differentiate between the remaining plausible hypotheses. Neither the model or satellites are likely to be able to indicate whether phytoplankton levels in the MIZ are elevated, due to methodological limitations (see Supplementary Information). However, the comparison to satellite observations shows the TOPAZ-ECOSMO product effectively identifies regions of higher ocean productivity, adding robustness to our conclusion that the observed event of elevated concentrations of fPBAP at the North Pole resulted from transported sea-spray from ice-free Arctic Ocean regions with elevated biological productivity (section 3.2.2).