The Marginal Ice Zone as a dominant source region of atmospheric mercury during central Arctic summertime

Domain: earth_science
Published: 2023-08-14
Editorial outcome: accept

Abstract
Atmospheric gaseous elemental mercury (GEM) concentrations in the Arctic exhibit a clear summertime maximum, while the origin of this peak is still a matter of debate in the community. Based on summertime observations during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition and a modeling approach, we further investigate the sources of atmospheric Hg in the central Arctic. Simulations with a generalized additive model (GAM) show that long-range transport of anthropogenic and terrestrial Hg from lower latitudes is a minor contribution (~2%), and more than 50% of the explained GEM variability is caused by oceanic evasion. A potential source contribution function (PSCF) analysis further shows that oceanic evasion is not significant throughout the ice-covered central Arctic Ocean but mainly occurs in the Marginal Ice Zone (MIZ) due to the specific environmental conditions in that region. Our results suggest that this regional process could be the leading contributor to the observed summertime GEM maximum. In the context of rapid Arctic warming and the observed increase in width of the MIZ, oceanic Hg evasion may become more significant and strengthen the role of the central Arctic Ocean as a summertime source of atmospheric Hg.

Introduction
Mercury (Hg) is one of the most toxic heavy metals, and its long-distance transport in the air makes it a global pollutant1. Hg in the atmosphere exists in three forms based on its physical and chemical properties: gaseous elemental mercury (GEM or Hg(0)), and gaseous or particulated ivalent Hg (Hg(II) and Hg(p))2,3. GEM has a long residence time in the air (0.5–1 year) due to its high volatility, low water solubility and low chemical reactivity and accounts for more than 90% of the total content of Hg in the surface air4–6. Gaseous and particulate divalent Hg, which can be formed by GEM oxidation, have a shorter lifetime (hours to weeks) and can be effectively eliminated from the air via dry and wet deposition2,4,7.

The Arctic is an important component of the Northern Hemisphere Hg cycle and a sensitive area for environmental Hg exposure8. The latest Arctic Mercury Assessment Report (AMAP, 2021) and refs.9,10 show that people living near the Arctic Circle have some of the highest Hg levels in the world due to their traditional diet. In addition, relatively long Arctic food chains can also result in Arctic wildlife being at high risk of Hg exposure. Long-range transport of air, riverine input and coastal erosion from circumpolar land can be primary Hg sources to the Arctic11,12. A fraction of these historical primary emissions can deposit and accumulate in seawater, plants, glaciers and ice sheets in the Arctic and later be re-emitted as secondary emissions of atmospheric Hg13–15. Atmospheric Hg transport, transformation, and deposition pathways thus play important roles in the Arctic Hg cycle10.

Since Schroeder et al.16 first reported the occurrence of springtime atmospheric Hg depletion events (AMDEs) in Alert (Nunavut, Canada), a phenomenon during which GEM is near-quantitatively oxidized to Hg(II) leading to Hg deposition onto snow/sea-ice, many long-term observation studies on atmospheric Hg have been conducted at several Arctic coastal monitoring stations (e.g., Alert, Zeppelin in Svalbard, and Villum Research Station in Northern Greenland)17–20. Thanks to multi-year observations at these coastal monitoring stations, the unique seasonality of atmospheric Hg in the Arctic has been revealed. It is characterized by a springtime minimum, driven by AMDEs, followed by a summertime GEM maximum when mean concentrations typically exceed northern hemispheric background levels. However, the origin of the Arctic summertime GEM maximum is still a matter of debate in the community. A modeling study by ref.21 suggested that long-range transport of Asian air is the most important primary source of atmospheric Hg to the Arctic in all seasons. Sommar et al.22 and Fisher et al.12 later attributed enhanced summertime GEM concentrations to oceanic Hg evasion in the Arctic Ocean fed by terrestrial Hg inputs (rivers and coastal erosion) based on observations and several sensitivity runs using the GEOS-Chem model. Recent isotopic work suggests, however, the dominant role of re-emissions from the Arctic cryosphere, while the role of terrestrial Hg from rivers and coastal erosion was found to be minor23. Hg re-emissions from the cryosphere could be fueled by AMDEs deposited Hg that remains in the snowpack until the melt season. That fraction of Hg retained in the snowpack from spring to summer is, however, still highly uncertain. Using stable isotopes, Douglas and Blum13 found that 76 to 91% of AMDEs deposited Hg is re-emitted prior to snowmelt in Alaskan snow. However, Zheng et al.14 estimated generally less photoreduction loss (0 to ~60%) from snow at Alert, with an average of 20 ± 31%. These differences may be due to the complex factors that can affect the amount of Hg emitted from the snowpack/sea-ice, such as solar radiation, Hg speciation in snowpack/sea-ice, halide and particulate matter concentrations in the snowpack, snow temperature, snowfall frequency, and upward latent heat flux24–26.

Our current understanding of the Arctic Hg cycle mostly stems from coastal observations due to the lack of data over the central Arctic Ocean. This may limit our understanding of the factors controlling the atmospheric Hg cycle over the whole Arctic Ocean. For example, regional differences are expected for sea-ice physical properties (e.g., dynamic deformation, thermal processes (melting and freezing)), and chemical composition. These factors are expected to have non-negligible impacts on key processes of the Hg cycle, such as redox reactions and snow re-emission of Hg, ultimately affecting air–sea exchange and atmospheric Hg concentrations in the central Arctic27–29. Therefore, relying only on long-term observations from coastal stations might provide an incomplete understanding of the atmospheric Hg cycle in the central Arctic and lead to uncertainties in the modeling of atmospheric Hg in that region8.

This paper presents an analysis of summertime observations (June 9–September 30, 2020) of GEM in the central Arctic Ocean during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) international expedition. We focus here on (1) GEM concentrations and their variability in the central Arctic Ocean with a comparison to observations at Arctic coastal stations and (2) the identification of sources contributing to GEM levels in the central Arctic Ocean.

Results
Characteristics of GEM concentrations during the whole summertime in the Central Arctic Ocean

The concentration of GEM during the whole observation period (June–September) in the central Arctic Ocean ranged from 1.02 to 2.99 ng/m3, with an average (± standard deviation) of 1.54 ± 0.27 ng/m3 (n = 31,176; number of 5-min data points) (see Fig. 1). The GEM average from June to August (1.60 ± 0.28 ng/m3, n = 22809; latitude range: 78.34°N to 90°N) is very similar to the observations at several nearby coastal Arctic stations during the same months in 2011–2015 (e.g., 1.63 ± 0.37 ng/m3 at Alert, 1.60 ± 0.23 ng/m3 at Zeppelin, and 1.63 ± 0.37 ng/m3 at Villum; t-test: p > 0.01)17, indicating that the summertime GEM observations at these Arctic stations are comparable to the GEM levels in the central Arctic Ocean. In addition, our observations (1.32 ± 0.07 ng/m3, n = 3332) are comparable to the observed GEM average (1.40 ± 0.61 ng/m3; t-test: p < 0.01) during the same period (August 25–September 4) in 2012 in the central Arctic Ocean (80°N–87.6°N, 9.2°E–168.9°W), while with apparently lower standard deviation than the latter28. This difference in standard deviation may to some extent be attributed to the significantly higher and less variable sea-ice fraction (0.92 ± 0.07) in this study, which inhibited the oceanic evasion of Hg and led to the apparently more stable variation of GEM concentration, compared with ref.15 (with average sea-ice fraction of 0.36 ± 0.32).

Monthly variations

Monthly averaged GEM concentrations reached a maximum in July: July (1.80 ± 0.32 ng/m3, n = 8264) > June (1.59 ± 0.17 ng/m3, n = 5908) > August (1.42 ± 0.13 ng/m3, n = 8639) > September (1.35 ± 0.097 ng/m3, n = 8368) (Fig. 2a). A similar feature is typically observed at coastal Arctic monitoring stations, such as Alert, Villum, and Zeppelin17,19, implying that summertime observations at high-latitude Arctic coastal stations can generally capture the GEM monthly variation pattern in the central Arctic Ocean22. In addition, the time series of GEM concentrations (Fig. 1b) shows that GEM variability was the largest in July, with the highest hourly averaged GEM concentration (2.99 ng/m3). The highest GEM concentration was accompanied by high solar radiation (RD), but low carbon monoxide (CO) and sulfur dioxide (SO2) levels, both typical proxies for anthropogenic emissions (Supplementary Fig. 1), indicating that this GEM peak was not associated with anthropogenic emissions but likely attributable to natural processes (e.g., photoreduction and oceanic re-emission). A similar low correlation between GEM and CO was also observed during previous summertime cruises in the Arctic22 and Antarctic Ocean23. Previous studies suggested that the increased atmospheric GEM level during summertime in the Arctic could be associated with the re-emission of Hg from the snowpack and ocean, facilitated by higher temperature and solar radiation and the melting of sea-ice15,17,18,23,30. After August, the GEM concentrations gradually decreased and leveled off. In this study, we also found that the monthly averaged concentration of GEM has no consistency with CO (e.g., the monthly averaged CO concentration is the lowest during the summer, which is opposite to GEM, Fig. 2b). This seems to suggest a minor role of anthropogenic emissions on the GEM monthly variability during the summertime in the Arctic Ocean. This will be further discussed in Section 2.3.1.

Previous studies have observed AMDEs at Arctic coastal monitoring stations in June18, but this phenomenon was not observed between June and September during the MOSAiC expedition. One potential reason could be the air temperature. Low air temperature favors the production of reactive bromine radicals31,32 and the oxidation of GEM (by stabilizing the Hg(I) intermediate)33 and therefore favors the occurrence of AMDEs. AMDEs were actually observed in June at Zeppelin station when the temperature was between −5 and −10 °C, which were the lowest temperatures recorded for that month18. The warmer air temperature (average: −0.86 ± 1.12 °C) in June during the MOSAiC expedition likely explains why no AMDEs were observed at this time. It should, however, be noted that AMDEs were ubiquitous during the MOSAiC springtime (March–May) as reported in ref.34.

Sources of GEM in the central Arctic in summer

The summertime GEM maximum that follows springtime depletions (AMDEs) is a unique Arctic feature. This Arctic GEM seasonality suggests a transition from a “mercury sink” in spring to a “mercury source” in summer24. Thus, understanding the origins and corresponding mechanisms of this GEM variability is of great importance for accurately assessing the regional mercury budget and associated ecological consequences. However, the origin of this summertime GEM peak is still not entirely understood. To further understand the sources of GEM in the central Arctic Ocean summertime, we used a GAM (generalized additive model) simulation to evaluate the relative contributions of (1) the long-range transport of anthropogenic emissions, (2) local oceanic emissions and (3) meteorological conditions. Since the F test results from GAM imply the variance contribution of each predictor to the response, the relative importance of each predictor can be determined by the result of the F value for each predictor divided by the sum of all F values35,36. The results are displayed in Table 1 and discussed below.

The GAM results show that anthropogenic emissions associated with CO contribute only ~2% of the GEM variation. Its partial response curve shows that with increasing CO, GEM does not increase (Fig. 3a). Furthermore, the GAM partial response curve of Traj48h, which represents the regional transport potential of GEM, indicates that a positive relationship between GEM and Traj48h occurs in the low Traj48h range (<500 km). However, as Traj48h continues to increase, GEM generally decreases or remains at the same level (Fig. 3b). As a result, the GAM simulations indicate that the long-range transport of GEM is less significant than the regional transport over the ocean (<500 km from the ship). This hypothesis is further supported by the statistical analysis of air-mass back trajectories: the fraction of the 168 h backward trajectory’s transport time in land area over the total transport time (Land fraction), which characterizes the potential contribution of land-based emissions to regional transport, was calculated every 2 h during the observation period. We find that the average GEM concentration slightly decreases as the land fraction increases (Fig. 3c), which to some extent suggests a minor contribution of land-based Hg emissions to GEM in the central Arctic Ocean during summertime. One potential explanation for the minor contribution of land-based Hg emissions to summertime GEM levels in the central Arctic is the position of the Arctic dome. The Arctic dome acts as a transport barrier for air masses and isolates the Arctic lower troposphere from lower latitudes, driven by thermal stratification of the lower atmosphere within the polar dome37. The Arctic dome can extend to approximately 40°N in wintertime, therefore favoring the transport of mid-latitude (i.e., North of 40°N) Hg emissions, and recede poleward to roughly north of 70°N in summertime, isolating the central Arctic from mid-latitudes38,39. More information on the seasonality of poleward pollution transport from mid-latitudes can be found in Bozem et al.40 or Boyer et al.41. This is also reflected in the 7-day backward trajectories, which show that air masses were mainly concentrated over the central Arctic Ocean during the observation period (Supplementary Fig. 2). Moreover, the most recent AMAP/UNEP global Hg emission inventory shows few anthropogenic Hg sources in land areas around the Arctic Ocean11, which support the low contribution of anthropogenic land-based Hg emissions to the central Arctic Ocean in this analysis. Similarly, Skov et al.19 reported a low contribution of anthropogenic emissions (14–17%) to the GEM concentrations at the Villum station in northern Greenland.

The GAM analysis indicates that the open-water fraction (proxy for oceanic emissions) accounts for ~52% of the GEM variation during the observation period, highlighting the dominant role of oceanic emissions. This result is further supported by the statistical analysis of air-mass back trajectories: the fraction of the 168 h backward trajectory’s transport time in marine area over the total transport time (Sea fraction (Traj)) was larger than 0.9 during most of the observation period, including the periods when GEM peaked (Fig. 4a). A previous study based on GEM data collected at Zeppelin station combined with the Lagrangian particle dispersion model FLEXPART also found that the highest GEM concentrations in July and August were associated with air masses from the marine boundary layer, suggesting the likely contribution of oceanic Hg evasion15. It is generally assumed that the presence of sea-ice inhibits the release of GEM from the ocean to the air42. Here, the partial response curve of the open-water fraction shows two obvious GEM peaks associated with low open-water fractions (approximately 0.2 and 0.5, respectively, Fig. 4b), which may be due to transport from regions with a higher open-water fraction.

To further identify GEM source regions, we conducted a PSCF (potential source contribution function) analysis (Fig. 5a), with 2,244 trajectories for hourly averaged GEM data. The PSCF results combined with the spatial distribution of sea-ice (Fig. 5a, b and Supplementary Fig. 3) show a GEM source region in the low-latitude Marginal Ice Zone (MIZ) between Greenland and Svalbard (black box area in Fig. 5a, b), in line with the hypothesis of a regional transport contribution. Our results, therefore, suggest that oceanic Hg evasion is not significant everywhere in the central Arctic Ocean but mainly occurs in the MIZ. This is in line with previous studies highlighting low dimethyl sulfide (DMS) levels in the central Arctic and concentration spikes due to regional transport from the MIZ43–45 (Fig. 5c, d). In addition, multi-year observations of GEM at several Arctic coastal stations showed that the summertime GEM peak is relatively weaker at Zeppelin station than at Alert and Villum stations, likely due to the altitude (474 m a.s.l.) and relative remoteness of that station from the MIZ17. These observations support the hypothesis that the MIZ is an important source of GEM in summer. Below, we discuss three hypotheses for this oceanic Hg evasion hotspot in the MIZ:

(1) The load of Hg(II) substratum in the surface seawater, which determines the upper limit of Hg that can evade into the atmosphere;

(2) The reduction capacity of Hg(II) in the surface seawater, which determines how much GEM (or dissolved gaseous mercury; the main evading form of Hg) is available;

(3) The physical environment of the surface sea layer (e.g., presence of sea-ice, meltwater, and seawater) that influences air–sea gas exchange.

Firstly, the sea-ice-covered central Arctic Ocean is the location where springtime AMDEs occur; therefore, it receives and stores large amounts of Hg(II)8,33,46. This is supported by springtime GEM observations (March–May) during this expedition, which showed that AMDEs were ubiquitous over the central Arctic34. According to a modeling study, only 4% of deposited Hg(II) over the central Arctic Ocean is re-emitted before snowmelt due to the presence of halides that inhibit Hg(II) photoreduction34. In summer, this previously deposited Hg(II) is transferred to the surface ocean with snow and sea-ice melt47, resulting in a large Hg load in surface seawater of the MIZ. This hypothesis is supported by the latest Arctic Hg mass balance budget by ref.8: approximately 9 tons (4~15 tons) of Hg are stored in the Arctic Ocean sea-ice with sea-ice melt contributing 1.4 ± 0.4 tons of Hg to the Arctic Ocean.

Secondly, we observe a significant correlation between the daily GEM concentrations in ambient air and chlorophyll-a (Chla) concentrations in surface seawater, characterized by similar variation trends during the whole summer period, and a significant linear correlation between GEM and Chla (R2 = 0.43, P < 0.001, Fig. 6). The Chla measurements in July (when Polarstern was located in the MIZ) indicate that a phytoplankton bloom occurred in the MIZ during the sampling period. This is a common feature of the MIZ due to increased stratification and reduced light limitation caused by melting snow and ice under conditions when surface nutrients are not yet limiting48,49. Several previous laboratory studies have suggested that the presence of marine phytoplankton can facilitate Hg(II) reduction to dissolved gaseous Hg3,50,51. The observed significant positive correlation between GEM and Chla could support the hypothesis that biological activity, which is particularly high in the MIZ in summer, facilitates the production of GEM (or Hg(0)). Different potential mechanisms have been suggested in the past: (1) the excretion of photoreactive organic compounds by phytoplankton can facilitate the photoreduction of Hg(II) to Hg(0) via electron transfer3,50,51. This hypothesis may be supported by the occurrence of concurrent high solar radiation and high levels of particulate organic carbon (POC) observed in the surface seawater during the GEM and Chla peaks (Fig. 6 and Supplementary Fig. 4). (2) Phytoplankton can directly contribute to the reduction of Hg(II) to GEM through an enzymatic detoxification mechanism linked to their photosynthetic activity3,52. These mechanisms could explain why the MIZ has a high Hg(II) reduction capacity.

Thirdly, the melting of sea-ice in the MIZ removes the physical barrier to air–sea gas exchange40, facilitating the re-emission of GEM. Contrary to the MIZ where the action of waves efficiently mixes the upper ocean, a clear vertical stratification of the upper ocean and the presence of a meltwater layer (10 cm to 1 m; fed by the rapid melt of snow and sea-ice during summer) was observed throughout summer in the central Arctic53,54. Previous studies and data collected during MOSAiC suggest that the presence of this meltwater layer alters exchanges across the air–sea interface and limits water-to-air fluxes of key trace gases54. This vertical stratification along with a less biologically active central Arctic (i.e., lower reduction capacity; point (2)) could explain why ocean Hg evasion is limited north of the MIZ. GEM measurements performed during a poleward transect in early August during the MOSAiC expedition support this hypothesis, as GEM concentrations obviously decreased when Polarstern entered the pack ice with increased distance from the MIZ (Supplementary Fig. 5).

Overall, we suggest that oceanic evasion in the MIZ combined with regional transport is the dominant GEM source in the central Arctic—a phenomenon that likely explains the observed summertime GEM maximum. The main processes that drive significant oceanic Hg evasion in the Arctic MIZ are summarized in Fig. 7. A back-of-the-envelope calculation (see Supplementary Discussion) suggests a Hg evasion flux of 56 ng·m−2·day−1 in the MIZ. This flux is more than twice the flux measured in the Arctic open ocean (<24 ng·m−2·day−1)42,55, but lower than that measured in coastal areas of the Canadian Arctic Archipelago (~130 ng·m−2·day−1)56. It is also an order of magnitude higher than the Arctic Ocean evasion flux reported by ref.8 in their latest Arctic Hg mass balance budget (23–45 tons/year, i.e., 3.7–7.3 ng·m−2·day−1 assuming a surface area of 1.7 × 107 km2). Finally, this flux is also higher than the evasion flux over continental shelf areas driven by riverine inputs57. Further dedicated field and modeling studies are needed to further constrain that flux.

Impact of meteorological factors

Meteorological conditions (wind speed, atmospheric pressure and air temperature) can influence the re-emission, transport and dilution of atmospheric Hg. Here, we find that the contribution of meteorological predictors to GEM variation in the central Arctic Ocean is up to 37%. Among these, air temperature (Temp) is the major contributor (20%). The partial response curve of Temp shows an obvious upward trend of GEM in the Temp range of 260~275 K (Supplementary Fig. 6b). Higher temperature could not only promote the release of Hg from the surface ocean and snowpack17, but also inhibit the oxidation process of GEM by reactive halogen radicals through causing the thermal decomposition of Hg(I) intermediate33. Alternatively, this partial response curve may reflect the transport of warmer air masses from the MIZ, which is supported by the PSCF analysis of Temp: for Temp in the range of 260~275 K, PSCF values are highest for the MIZ (Supplementary Fig. 7a). However, for Temp higher than 275 K, the contribution from the MIZ decreases significantly, which corresponds to the decreasing trend of PSCF values when Temp is larger than 275 K (Supplementary Fig. 7b). These results indirectly support the important role of oceanic evasion and regional transport of Hg from the MIZ in the central Arctic.

Atmospheric pressure (P) contributes 9% to GEM variation. The partial response curve of GEM and P generally shows a negative relationship (Supplementary Fig. 6c). This may to some extent be due to the fact that that lower surface air pressure conditions tend to produce low-level convergence, which would gather the GEM through the low-level compensated air mass (e.g., Ekman pump) and result in a GEM increase (e.g., the highest hourly GEM in this study was observed near a low-pressure center, Supplementary Fig. 8)58,59.

Wind speed (WS) contributes 8% to GAM variance. In stagnant meteorological conditions characterized by a low WS range (e.g., <2.5 m/s in this study), the (re)-emitted Hg from the ocean might tend to accumulate, causing the positive correlation between GEM and WS36. As WS increases, this may enhance the atmospheric dilution of GEM, leading to the decreasing trend (Supplementary Fig. 6a).

Methods

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "SUMMARY\nThis manuscript presents continuous measurements of summertime gaseous elemental\nmercury (GEM) in the central arctic, made during the MOSAiC expedition. The authors\nmodeled the importance of oceanic evasion as the dominant source of GEM to the Arctic\natmosphere. The data and modeling presented have important implications in the\ndistribution and deposition of Hg in Arctic environments, and provide further evidence in\nsupport of trends in dissolved gaseous mercury (DGM) and GEM from earlier cruises in\nthe Arctic.\nThe authors provide significant data analysis and interpretation, originating from sound\nanalytical methodology. Overall, I found the manuscript very well written, making an\nimportant contribution to the field stemming from a unique opportunity (MOSAiC). I only\nhave a few comments, with my main concern dealing with the explanation of high GEM\nlevels in the marginal ice zone (MIZ).\nTECHNICAL COMMENTS\nLine 189 – You state that transport of anthropogenic emissions to high latitudes is less\nlikely in the summertime, but also state that the Arctic dome, which acts as a transport\nbarrier, recedes in the summer. Wouldn’t this make transport northward more feasible,\nor am I misunderstanding your reasoning?\nSection 2.3.2 second point – Line 235-253 – I don’t disagree with/dispute your observed\ncorrelation between Chla and GEM, and you present multiple hypotheses to explain the\ntrend, I just might be a little more careful about overstating a necessity for correlation\nto imply causation. Is it necessary for the increased GEM measured in the MIZ to be the\nresult of DGM produced in the MIZ? Or might it have just been released once the sea ice\nwas no longer present, which also happens to be where phytoplankton bloom. Previous\nstudies measuring DGM in the Arctic show high concentrations under contiguous ice, not\njust in the MIZ, and I’m not sure you addressed/discussed this sufficiently in the\nmanuscript.\nEDITORIAL COMMENTS\nI think the abstract highlights the role of observations made in the conclusions reached,\nbut could do a better job highlighting the major role that the modeling tools (e.g.,\ngeneralized additive model (GAM), potential source contribution function analysis\n(PSCF), air-mass back trajectories) played in this manuscript.\nLine 91-96 – the sentence could be broken up into two separate sentences\nSome abbreviations need to be defined earlier – eg, GAM (line 158), PSCF (line 209). I\nalso believe the abbreviation RD is only defined in the supporting info.", "reviewer_2": "Levels of atmospheric gaseous elemental mercury (GEM) exhibit a summertime\nmaximum in the Arctic, however, the source of this peak is not well understood. The\nsource of the summertime GEM maximum is debated. This paper presents an analysis of\nGEM concentrations and their variability in the central Arctic Ocean based on data\ncollected during the MOSAiC international expedition, with a focus on identifying sources\ncontributing to the summertime GEM levels in the region. To identify the source(s), the\nstudy used a GAM simulation.\nThe study found that long-range transport of anthropogenic and terrestrial Hg from\nlower latitudes is a minor contribution (~2%), while the dominant source (>50%) is\noceanic evasion and regional transport. The study also suggests that oceanic evasion\nmainly occurs in the Marginal Ice Zone, which could be a significant cause of the\nsummertime GEM maximum in the Arctic.\nThe manuscript is easy to read and does a good job of discussing the possible origin of\nthe summertime GEM maximum by addressing the relative contributions from (1)\nthe long-range transport of anthropogenic emissions, (2) local oceanic emissions and\n(3) meteorological conditions. The measurements collected in this study are significant,\nlargely contributing the current lacking dataset of GEM in central Arctic Ocean. The\nauxiliary data is also considerable and helps support the authors interpretations.\nThe results of the study are mostly clear, however, the overarching conclusions could be\nbetter highlighted. The study would benefit of emphasizing more the implications of\ntheir results. For example, if ocean evasion and regional transport are the main source\nof summertime GEM maximum, and if ocean evasion mainly occurs in the Marginal Ice\nZone, what are the implications of ocean evasion being mainly in the MIZ relative to the\nopen ocean? Other studies could be referenced to support the findings reported here. I\nwould suggest describing how the results of this study compare (or don’t) with the\nreported observations from coastal stations. The authors reference a few studies, but\nthere have been more. I recommend the authors draw on the work done by others to\nhelp support their conclusions.\nThe authors mention that long-term observations from coastal stations may provide an\nincomplete understanding of the atmospheric Hg cycle in the central Arctic and lead to\nuncertainties in modeling atmospheric Hg in that region. This is important, how do the\nresults from this study fill that gap?\nFinally, the big picture implications of the results are somewhat lacking. The manuscript\nwould benefit from providing some discussion of the results in context of a changing\nclimate, particularly with respect to a warming Arctic.\nOverall, I think this manuscript will contribute to the Hg research community and\nhopefully at a larger scale.\nLine by line comments:\n• Line 67 – two references here but only refers to the AMAP report\n• Line 73 – May want to provide some context, definition to AMDEs, since the first time\nintroduced\n• Line 78 – Word choice \"unraveled\"...revealed? Suggests this has been solved, yet the\nsummertime maximum is still debated\n• Line 85 – There are other important isotope studies to consider here, Douglas and\nBlum (2019); Zheng et al. (2021)\n• Line 89 – May be good to provide reference here to the coastal studies\n• Line 135 – define RD, might be worth indicating that these are used as a proxy for\nanthropogenic emissions\n• Line 139 – other studies could be referenced here as well\n• Line 149 – check section reference (numbering is wrong?)\n• Line 151 – this is interesting. Could authors provide reasoning as to why? Could it\nhave occurred earlier in the year?\n• Line 156 - May be good place to note the significance of the summertime GEM\nmaximum and knowing its origin\n• Line 216 – additional references needed here\n• Line 231 - This is somewhat confusing because above the authors suggest AMDEs\nwere not observed.\n• Line 282 – somewhat vague statement. Also what about temperature on atmospheric\nchemistry (e.g., increasing temperature on emission of halogens/ oxidants, AMDEs,\ndeposition of oxidized Hg to snow, regionally, MIZ as major sinks, etc.). Authors could\ndraw on other studies here.\n• Line 285 to 289 – reference to help support explanation.\n• Line 294 – it is not clear why WS outside of the range (at lower and higher ends of the\nrange) would promote sea-air exchange and regional transport and why this specific\nrange implies dilution. May need more clarity.\nMethods: What a sensitivity analysis completed on the model used? This can be added to\nsupplemental information.\nFigures:\nSome figures are fuzzy and difficult to read. Suggest higher resolution images (e.g., 4a,\n4b, 5b)", "reviewer_3": "General comments:\nThe study presents unique atmospheric GEM concentration data from the Arctic Ocean.\nThe data were obtained using a Tekran 2537B system running on board of the research\nvessel Polarstern from June to September 2020. The study confirms an offshore\nsummertime GEM peak between Greenland and Svalbard, a GEM maximum that has been\nobserved at several coastal Arctic monitoring stations. The peak is not associated with\natmospheric long-range transport of Hg. Instead a GAM simulation indicates that open\nocean evasion of GEM, in particular from the low-latitude marginal ice zone between\nGreenland and Svalbard, is a significant source of GEM to the Arctic atmosphere in\nsummer. While I want to point out the originality of the GEM dataset, I have some\nconcerns regarding the identification of sources contributing to the GEM summertime\npeak.\nThe authors report that 63.3% of the summertime GEM variation can be largely\nexplained by open water re-emission and meteorological factors. Araujo et al., 2022,\nNat. Commun. evidenced that the GEM peak originates from re-emission of Hg deposited\nto snow and sea ice during AMDEs. They concluded that GEM re-emission takes place\ndirectly from snow and sea ice but also from regional open waters that receive\nmeltwater Hg inputs. How much of the GEM variation in the present study can be\nexplained by direct GEM re-emission from snow and sea ice? Is re-emission from snow\nand sea ice not contributing to the GEM summertime peak in the central Arctic Ocean at\nall? Please include a paragraph in the ms about how direct snow and sea ice re-emission\nimpacts GEM variation in summer.\nLong-range transport of Hg contributes only marginally to the GEM summertime peak.\nMeteorological factors, however, explain 37% of the identified GEM variation in the\ncentral Arctic Ocean. Advection of Hg from the mid-latitudes (i.e. long-range transport\nof anthropogenic emissions and terrestrial Hg) and Hg evading from the ocean can\nindeed be considered sources contributing to GEM levels in the Arctic atmosphere.\nHowever, please explain how meteorological factors can be a source of GEM.\nMeteorological factors such as wind speed or solar irradiation have been demonstrated\nto influence the rate of GEM evasion from the ocean but cannot be considered a source\nin my understanding.\nIn July 2020, when the GEM peak was detected, the Polarstern was cruising between\nGreenland and Svalbard, correct? I wonder whether the July GEM peak could have been\ndetected close to the North Pole as well? Did you find any indication for that? Please add\ndates and the respective location of the Polarstern in Fig. 1a.\nOverall, ocean evasion explains less than 50% of the GEM variation in summer (L404:\n36.7 % of the GEM variation cannot be explained). Still, there is strong indication for\nlarge GEM evasion from the MIZ between Greenland and Svalbard. It would strengthen\nthe manuscript, if the authors could include a statement about the GEM flux rate (ng m-\n2 day-1) that could be expected from the MIZ between Greenland and Svalbard. What\nflux rate would be required to increase the GEM concentration within the Arctic Ocean\nboundary layer from 1.35 ng m-3 (September, background) to 1.8 ng m-3 (July,\nsummertime peak). Under the assumption that GEM re-emission from the other parts of\nthe central Arctic Ocean remain small.\nSuch a statement could be integrated into a yet missing “conclusion” or “implication”\nparagraph. I strongly suggest to add conclusions and implications at the end of the main\ntext. Please discuss the significance of your results and how your findings fit with our\ncurrent understanding of the Arctic mercury cycle (cp. with Dastoor et al., 2022, Nature\nreviews, earth and environment and Araujo et al., 2022, Nat. Commun.).\nSpecific comments:\nLine 62: There are more recent studies reporting atmospheric lifetimes of GEM.\nConsider\nreferring to Saiz-Lopez et al., 2018, Nat. Commun. or Horrowitz et al., 2017, ACP.\nLine 67: The AMAP report is referenced with number 44 instead of 8. Is there a\nreason for that? Adjust reference numbering throughout the ms.\nLine 90: Delete “unfortunately”.\nLine 135: Write out “RD”. You define all abbreviations in the Method section. Please\ndo that in main text instead.\nLine 194: “Central Arctic Ocean” or “central Arctic Ocean”? Be consistent.\nLine 218: What would be an important factor? This sentence is hard to understand.\nPlease rephrase.\nLine 236: Write out “Chla”.\nLine 270: Write out as follows: Wind speed, atmospheric pressure and air temperature.\nStick to “re-emission”.\nL313: The flow rate of the Tekran 2537B was 0.7 L min-1 which is low compared to most\nTekran deployments. Is there a reason why the authors used such a low flow rate?\nAnalysis of GEM in 3.5 L of air is at the lower limit. Please explain briefly why an\nadditional Tekran analyzer was setup even though GEM was determined in the CU\ncontainer on a 15 min resolution? Please also add a comment about how you avoided to\nmeasure GEM that potentially originated from ship exhaust gas emissions.\nL 685, 697, 709: Add units to the axis text in Figs. 3a,b, 4a,b, 5b.\nSupporting information:\nL 38: Is that a table of content? Please adjust.\nL 45: Why is the section title number 3.2? Please adjust.\nL 70: How was this figure created? Using HYSPLIT? Information is missing in the\ncaption.\nL 95: Add units to the axis text in Fig S7.", "reviewer_4": "This study investigated key mechanisms for occurrence of the summer concentration\npeak of gaseous elemental mercury (GEM) in the Arctic based on observations during\nthe Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC)\nexpedition and a statistical modeling approach. Oceanic evasion in the Marginal Ice Zone\nwas found to be the dominance source for the summertime GEM maximum, and the\nphotoreduction of Hg(II) to GEM by phytoplankton was proposed to be the main\nmechanism. The observational data obtained in the study are valuable, and the finding\nreported in the study is interesting which could draw widespread attention. However,\nthe generalized additive model (GAM) used in this study needs more information on\nmodel validation, and more solid evidence needs to be provided for the mechanisms the\nauthors proposed. A comprehensive schematic diagram for what’s going on during the\nsummer GEM maximum is encouraged to be added to the manuscript for readers to get a\nwhole picture of the crucial mechanisms. Overall in my opinion, substantial\nimprovement is required before the manuscript is acceptable for publication on Nature\nCommunications. Here are some specific comments:\n1. Abstract: The most probable cause of the summer GEM peak, the photoreduction of\nHg(II) to GEM by phytoplankton (if I understand the Results and Discussion part\ncorrectly), is not reflected in the abstract. The authors only mentioned that oceanic\nevasion mainly occurs in the Marginal Ice Zone, which could mislead the readers to think\nthat the air–ice Hg exchange might be the dominant mechanism of the summer GEM\nmaximum.\n2. Line 82: The citation should be Durnford et al. according to the reference list.\n3. Lines 82–87: The summary of the GEM source apportionment results could cause\nmisinterpretation. Long-range transport of Asian air is a primary source. Legacy Hg\nemissions or re-emissions from terrestrial regions or ice zones are considered as\nsecondary sources. Due to the different methodologies adopted, a certain part of the\ndiscrepancy between these studies could be caused by the categorization of GEM\nsources. Some legacy Hg from historical Asian air could be accumulated in plants,\nsnowpack and ice in the Arctic area and reemitted as a secondary source of GEM. It\nwould be helpful for the readers to understand the Arctic Hg cycle if the authors could\nmake a more comprehensive introduction here, incorporating these literatures.\n4. Line 116: Misspelling of “coastal”.\n5. Section 2.1: The discussion in this section is limited. Significance of difference from\nprevious studies based on t-test should be given. The standard deviation of the central\nArctic GEM observations during August 25–September 8, 2012 (0.61) is much higher\nthan that in this study (0.07). What caused the large fluctuation in the study of Yu et al.\n(2014)?\n6. Lines 149–151: What could be the reason for the absence of this phenomenon in the\nMOSAiC expedition (especially in June)? Why did the authors present this comparison\nhere?\n7. Figure 4c: What is the significance of the relationship between land fraction and\nGEM? The negative correlation seems not quite robust to me.\n8. Lines 182–194: The reasoning is a bit farfetched. More solid evidence should be\nprovided to support the speculation.\n9. Figure 5b: What do these three peaks stand for in this influencing pattern? This is an\nimportant question to verify the reliability of the GAM model.\n10. Line 230: How much Hg(II) is stored yearly approximately? More quantitative\ncomparisons are required.\n11. Line 239: It should be Figure 7.\n12. Section 2.3.3: Solar radiation should be included as a meteorological predictor in the\nGAM model. In the previous section, the photoreduction of Hg(II) to GEM by\nphytoplankton is proposed to be a dominant mechanism. If so, solar radiation is\nsupposed to be a key factor for the summertime GEM maximum."}, "author_response": "Responses to Reviewers’ Comments\nDear Reviewers:\nThank you for your comments on our manuscript entitled “The Marginal Ice Zone\nas a dominant source region of atmospheric mercury during Arctic summertime”\n(manuscript ID: NCOMMS-23-05574) and for offering us an opportunity for revision.\nWe believe the constructive comments have helped us improve the overall quality of\nthe manuscript. Below are our detailed responses to reviewers’ questions. Revised texts\nof the manuscript are marked in red in the responses to reviewers’ comments.\nReviewers’ Comments and response\nReviewer #1:\nThis manuscript presents continuous measurements of summertime gaseous\nelemental mercury (GEM) in the central arctic, made during the MOSAiC expedition.\nThe authors modeled the importance of oceanic evasion as the dominant source of GEM\nto the Arctic atmosphere. The data and modeling presented have important implications\nin the distribution and deposition of Hg in Arctic environments, and provide further\nevidence in support of trends in dissolved gaseous mercury (DGM) and GEM from\nearlier cruises in the Arctic.\nThe authors provide significant data analysis and interpretation, originating from\nsound analytical methodology. Overall, I found the manuscript very well written,\nmaking an important contribution to the field stemming from a unique opportunity\n(MOSAiC). I only have a few comments, with my main concern dealing with the\nexplanation of high GEM levels in the marginal ice zone (MIZ).\nis less likely in the summertime, but also state that the Arctic dome, which acts as a\ntransport barrier, recedes in the summer. Wouldn’t this make transport northward more\nfeasible, or am I misunderstanding your reasoning?\nRe: Atmospheric conditions in winter and early spring are conducive to long-range\ntransport from regions bordered by the Arctic front (Willis et al., 2018). Minimum\nvalues of potential temperature that occur in the Arctic lower troposphere, which isolate\nthe near-surface Arctic from the rest of the atmosphere, can form irregular closed domes\nover Arctic regions with surfaces of constant potential temperature (isentropic surfaces)\n(Klonecki et al., 2003). In the absence of diabatic heating or cooling, air masses will\ntravel adiabatically along isentropic surfaces and be forced to ascend as they move\nnorthward (Klonecki et al., 2003; Law and Stohl, 2007). Therefore, pollution emitted\nat low potential temperatures is much more likely to reach the lower Arctic troposphere\nthan pollution emitted at higher potential temperatures on time scales up to several days\n(Iversen, 1984). The Arctic front can extend as far south as 40 °N in January (wintertime)\n(Willis et al., 2018). As a result, pollution emitted in cold regions north of the Arctic\nfront (north of 40 °N, e.g., northern Eurasia) can be transported directly to Arctic\nregions in the lower troposphere during winter. In contrast, warm and moist air masses\noriginating from North America and Asia are forced to ascend and can access the Arctic\nmiddle and upper troposphere rather than low troposphere (Law and Stohl, 2007; Willis\net al., 2018). However, in summertime, the Arctic dome recedes poleward from\napproximately 40 °N in the winter to roughly north of 70 °N, such that direct transport\nfrom sources south of the Arctic front (south of about 70 °N, such as northern Eurasia\nwith higher anthropogenic emissions) becomes much less important. Mean residence\ntimes within the Arctic increase during summer, particularly near the surface,\nillustrating the decreased tendency for long-range transport during summer compared\nto winter and spring. The seasonality of poleward pollution transport from mid-latitudes\nis well described in the literature, see for instance the recent publications by Bozem et\n2\nal. (2019) or Boyer et al. (2023). We have also added some corresponding explanation\nin the manuscript, which is also displayed below:\n“Line 217 (section 2.3.1): One potential explanation for the minor contribution of\nland-based anthropogenic/terrestrial Hg emissions to summertime GEM levels in the\ncentral Arctic is the position of the Arctic dome. The Arctic dome acts as a transport\nbarrier for air masses outside the Arctic dome and isolates the Arctic lower troposphere\nfrom lower latitudes, driven by thermal stratification of the lower atmosphere within\nthe polar dome (Klonecki et al., 2003). The Arctic dome can extend to approximately\n40\"N in wintertime, therefore favoring the transport of mid-latitude Hg emissions, and\nrecede poleward to roughly north of 70° N in summertime, isolating the central Arctic\nfrom mid-latitudes (Shaw, 1995; Willis et al., 2018). More information on the\nseasonality of poleward pollution transport from mid-latitudes can be found in Bozem\net al. (2019) or Boyer et al. (2023). This is also reflected in the 7-day backward\ntrajectories, which show that air masses were mainly concentrated over the Arctic\nOcean during the observation period (Figure S5). Moreover, the most recent\nAMAP/UNEP global Hg emission inventory shows few anthropogenic Hg sources in\nland areas around the Arctic Ocean (Dastoor et al., 2022b), which supports the low\ncontribution of anthropogenic land-based Hg emissions to the central Arctic Ocean in\nthis analysis.”\nQ2: Section 2.3.2 second point – Line 235-253 – I don’t disagree with/dispute\nyour observed correlation between Chla and GEM, and you present multiple hypotheses\nto explain the trend, I just might be a little more careful about overstating a necessity\nfor correlation to imply causation. Is it necessary for the increased GEM measured in\nthe MIZ to be the result of DGM produced in the MIZ? Or might it have just been\nreleased once the sea ice was no longer present, which also happens to be where\nphytoplankton bloom. Previous studies measuring DGM in the Arctic show high\nconcentrations under contiguous ice, not just in the MIZ, and I’m not sure you\naddressed/discussed this sufficiently in the manuscript.\nand then be released when sea-ice disappears, and the presence of contiguous ice can be a\nbarrier to the sea-air exchange of Hg and interfere with the evasion of the DGM, despite\nhigh DGM accumulated under contiguous ice (DiMento et al., 2019). Therefore, we\nthink that the increased GEM observed in the MIZ is not only due to the DGM produced\nthere, but also due to the environmental conditions, including sea-ice melt and the low\nupper ocean stratification in the MIZ, which, taken together, contribute to the evasion\nof DGM to air. We have also discussed this in the third point of Section 2.3.2. For the\nobserved significant correlation between Chla and GEM, we agree that this does not\nmean there is a causation. Previous lab studies have shown that phytoplankton\ncontributes to Hg(II) reduction. Our in situ observations support this hypothesis, which\nis worth reporting here. We have rephrased the statement here, which is also shown\nbelow. This does not change the main finding of the manuscript: ocean Hg evasion\nmainly occurs in the MIZ, compared with open sea and contiguous ice areas.\n“Line 292: Several previous laboratory studies have suggested that the presence of\nmarine phytoplankton can facilitate Hg(II) reduction to dissolved gaseous Hg (Ariya et\nal., 2015; Deng et al., 2009; Lanzillotta et al., 2004; Ben-Bassat et al., 1972). The\nobserved significant positive correlation between GEM and Chla could support the\nhypothesis that biological activity, which is particularly high in the MIZ in summer,\nfacilitates the production of GEM (or Hg(0)). Different potential mechanisms for such\nan interaction have been suggested in the past: (1) the excretion of photoreactive\norganic compounds by phytoplankton can facilitate the photoreduction of Hg(II) to\nHg(0) via electron transfer (Ariya et al., 2015; Deng et al., 2009; Lanzillotta et al., 2004).\nThis hypothesis may be supported by the occurrence of concurrent high solar radiation\nand high levels of particulate organic carbon (POC) observed in the surface seawater\nduring the GEM and Chla peaks (Figure 7 and Figure S6). (2) Phytoplankton can\ndirectly contribute to the reduction of Hg(II) to GEM through an enzymatic\ndetoxification mechanism linked to their photosynthetic activity (Ariya et al., 2015;\nBen-Bassat et al., 1972). These mechanisms could explain why the MIZ has a high\nHg(II) reduction capacity.”\nreached, but could do a better job highlighting the major role that the modeling tools\n(e.g., generalized additive model (GAM), potential source contribution function\nanalysis (PSCF), air-mass back trajectories) played in this manuscript.\nRe Thanks for your suggestion. We have revised the abstract as follows:\n“Abstract: Atmospheric gaseous elemental mercury (GEM) concentrations in the\nArctic exhibit a clear summertime maximum, while the origin of this peak is still a\nmatter of debate in the community. Based on summertime observations during the\nMultidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC)\nexpedition and a modeling approach, we further investigate the sources of Arctic\natmospheric Hg. Simulations with a generalized additive model (GAM) show that long-\nrange transport of anthropogenic and terrestrial Hg from lower latitudes is a minor\ncontribution (~2%). The dominant source (> 50%) is oceanic evasion. A potential\nsource contribution function (PSCF) analysis further shows that oceanic evasion is not\nsignificant throughout the Arctic Ocean but mainly occurs in the Marginal Ice Zone due\nto the specific environmental conditions in that region. Our results suggest that this\nregional process could be the leading contributor to the observed summertime GEM\nmaximum. In the context of rapid Arctic warming and the observed increase in width\nof the MIZ, oceanic Hg evasion may become more significant and strengthen the role\nof the Arctic Ocean as a summertime source of atmospheric Hg.”\nQ4 Line 91-96 – the sentence could be broken up into two separate sentences\nSome abbreviations need to be defined earlier – eg, GAM (line 158), PSCF (line 209).\nI also believe the abbreviation RD is only defined in the supporting info.\nRe Thanks for your comments. We have broken up the corresponding sentence into\ntwo separate sentences, which are also shown below:\n“Line 109: For example, regional differences are expected for sea ice physical\nproperties (e.g., dynamic deformation, thermal processes (melting and freezing)), and\nchemical composition. These factors are expected to have non-negligible impacts on\n5\nkey processes of the Hg cycle, such as redox reactions and snow re-emission of Hg,\nultimately affecting air-sea exchange and atmospheric Hg concentrations in the central\nArctic (Durnford and Dastoor, 2011; Moore et al., 2014; Yu et al., 2014)”\nWe have also added the corresponding detailed definitions of the abbreviations in\nmain text based on your comments.\nReviewer #2:\nLevels of atmospheric gaseous elemental mercury (GEM) exhibit a summertime\nmaximum in the Arctic, however, the source of this peak is not well understood. The\nsource of the summertime GEM maximum is debated. This paper presents an analysis\nof GEM concentrations and their variability in the central Arctic Ocean based on data\ncollected during the MOSAiC international expedition, with a focus on identifying\nsources contributing to the summertime GEM levels in the region. To identify the\nsource(s), the study used a GAM simulation.\nThe study found that long-range transport of anthropogenic and terrestrial Hg from\nlower latitudes is a minor contribution (~2%), while the dominant source (>50%) is\noceanic evasion and regional transport. The study also suggests that oceanic evasion\nmainly occurs in the Marginal Ice Zone, which could be a significant cause of the\nsummertime GEM maximum in the Arctic.\nThe manuscript is easy to read and does a good job of discussing the possible origin\nof the summertime GEM maximum by addressing the relative contributions from (1)\nthe long-range transport of anthropogenic emissions, (2) local oceanic emissions and\n(3) meteorological conditions. The measurements collected in this study are significant,\nlargely contributing the current lacking dataset of GEM in central Arctic Ocean. The\nauxiliary data is also considerable and helps support the author’s interpretations.\ncould be better highlighted. The study would benefit of emphasizing more the\nimplications of their results. For example, if ocean evasion and regional transport are\nthe main source of summertime GEM maximum, and if ocean evasion mainly occurs\nin the Marginal Ice Zone, what are the implications of ocean evasion being mainly in\nthe MIZ relative to the open ocean? Other studies could be referenced to support the\nfindings reported here. I would suggest describing how the results of this study compare\n(or don’t) with the reported observations from coastal stations. The authors reference a\nfew studies, but there have been more. I recommend the authors draw on the work done\nby others to help support their conclusions.\nRe Thank you for your suggestions. We have added a sub-section titled\n“Implications in the context of rapid Arctic warming” (see also Q3 of your comments)\nto discuss the implications. Briefly, and as stated in the revised abstract, in the context\nof rapid Arctic warming and the observed increase in width of the MIZ, oceanic Hg\nevasion may become more significant and strengthen the role of the Arctic Ocean as a\nsummertime source of atmospheric Hg.\nWe have also referred to more Arctic coastal studies:\n“Line 232: Skov et al. (2020) reported a low contribution of anthropogenic emissions\n(14%-17%) to the GEM concentrations at Villum station in northern Greenland.”\n“Line 243: A previous study based on GEM data collected at Zeppelin station\ncombined with the Lagrangian particle dispersion model FLEXPART also found that\nthe highest GEM concentrations in July and August were associated with air masses\nfrom the marine boundary layer, suggesting the likely contribution of oceanic Hg\nevasion (Hirdman et al., 2009).”\n“Line 261: In addition, multi-year observations of GEM at several Arctic coastal\nstations showed that the summertime GEM peak is relatively weaker at Zeppelin station\nthan at Alert and Villum stations, likely due to the altitude (474 m a.s.l.) and relative\nremoteness of that station from the MIZ (Angot et al., 2016). These observations\nsupport the hypothesis that the MIZ is an important source of GEM in summer.”\nprovide an incomplete understanding of the atmospheric Hg cycle in the central Arctic\nand lead to uncertainties in modeling atmospheric Hg in that region. This is important,\nhow do the results from this study fill that gap?\nOverall, I think this manuscript will contribute to the Hg research community and\nhopefully at a larger scale.\nRe Previous studies building on observations at coastal stations speculated that the\nsummertime GEM maximum is mostly driven by oceanic Hg evasion from the Arctic\nOcean (Angot et al., 2016; Hirdman et al., 2009). Based on the unique datasets collected\nin the central Arctic Ocean during the MOSAiC expedition, this study shows that\noceanic evasion is not significant throughout the Arctic Ocean but mainly occurs in the\nMarginal Ice Zone. Moreover, current model simulations can accurately simulate the\nseasonal cycle of atmospheric Hg species, but do underestimate the amplitude of the\nseasonal variation in the Arctic (Dastoor et al., 2022a; Dastoor and Durnford, 2014).\nOur findings may thus help better parameterize key processes of Hg evasion in the\nMarginal Ice Zone and improve the overall model performance.\nQ3 Finally, the big picture implications of the results are somewhat lacking. The\nmanuscript would benefit from providing some discussion of the results in context of a\nchanging climate, particularly with respect to a warming Arctic.\nRe Thank you for the great suggestion. We have added the following paragraph:\n“Line 372: The Arctic is warming faster than the rest of the planet and Arctic sea-ice\nis declining rapidly with potential consequences for the Hg cycle (Yadav et al., 2020).\nFirstly, projected future Arctic warming might favor the conditions that stimulate\nAMDEs-driven Hg deposition through enhancing reactive halogen sources (Dastoor et\nal., 2022a), as previous reported long-term (1996-2017) dataset of BrO· vertical column\ndensities (VCDs) displayed an increasing trend of about 1.5 % of the tropospheric\nBrO· VCDs per year during polar springs, which may be attributed to increase in first-\nyear ice coverage that has a higher salinity than multiyear ice and facilitates the\nproduction of BrO· (Bougoudis et al., 2020). This could enhance the deposition of Hg\nand thus the load of Hg in surface seawater during the melt season. Secondly, previous\nstudy found that the width of MIZ in the warm season (July–September) in Arctic\nOcean has increased by 13 km decade–1 from 1979 to 2011, driven by the decrease of\nArctic sea-ice extent and the replacement of thick, multi-year ice by thin, first-year ice\n(Strong and Rigor, 2013). In this context, it can be expected that the retreat of multi-\nyear ice and the decline in sea-ice extent will continue, causing further increase in width\nand range of MIZ. This would translate into a larger source area of GEM in summertime.\nThirdly, the decline in sea-ice extent and expanse in MIZ, can 1) transmit substantially\nmore light into the underlying water column and make a longer growing season, leading\nto the increase in phytoplankton production (Arrigo et al., 2008; Pabi et al., 2008;\nPayne et al., 2022), and 2) promote the CO uptake by open seawater and the biologic carbon\n2\npump process in MIZ (Bates et al., 2006; Qi et al., 2022), which would further increase\nthe photo- and/or biological reduction capacity of Hg(II) here. As a result, we expect\nthat in a context of Arctic warming, oceanic Hg evasion in the MIZ might become\nincreasingly significant, and strengthen the role of the Arctic Ocean as a summertime\nHg source. Given the relatively long atmospheric lifetime of GEM, this may have global\nconsequences on the Hg cycle.”\nLine by line comments:\nQ4. Line 67 – two references here but only refers to the AMAP report\nRe. Thanks. We have added another reference’s information (Chételat et al. (2022))\nin the manuscript.\nQ5. Line 73 – May want to provide some context, definition to AMDEs, since the\nfirst time introduced\nRe. Thanks. We have added the corresponding information in the text, which was\nalso showed below:\n“Line 77: Since Schroeder et al. (1998) first reported the occurrence of springtime\natmospheric Hg depletion events (AMDEs) in Alert (Nunavut, Canada), aphenomenon\n9\nduring which GEM is near-quantatively oxidized to Hg(II) leading to Hg deposition\nonto snow/sea-ice, many long-term observation studies on atmospheric Hg have been\nconducted at several Arctic coastal monitoring stations (e.g., Alert, Zeppelin in\nSvalbard, and Villum Research Station in Northern Greenland) (Angot et al., 2016;\nBerg et al., 2013; MacSween et al., 2022; Skov et al., 2020).”\nQ6. Line 78 – Word choice \"unraveled\"... revealed? Suggests this has been solved,\nyet the summertime maximum is still debated\nRe. Thanks for your suggestion. We have revised the word “unraveled” as “revealed”\nbased on your suggestion. The debate about the summertime maximum has been\nmentioned later (Line 83), so we would not put it here.\nQ7. Line 85 – There are other important isotope studies to consider here, Douglas\nand Blum (2019); Zheng et al. (2021)\nRe. Thanks. We have referred to and cited these two studies here, which is also\nshown below:\n“Line 96:Hg re-emissions from the cryosphere could be fueled by AMDE deposited\nHg that remains in the snowpack until the melt season. That fraction of Hg retained in\nthe snowpack from spring to summer is, however, still highly uncertain. Using stable\nisotopes, Douglas and Blum (2019) found that 76 to 91% of AMDE deposited Hg is re-\nemitted prior to snowmelt in Alaskan snow. However, Zheng et al., (2021) estimated\ngenerally less photoreduction loss (0 to ~60%) from snow at Alert, with an average of\n20 ± 31%. These differences may be due to the complex factors that can affect the\namount of Hg emitted from snowpack/sea-ice, such as solar radiation, Hg speciation in\nsnowpack/sea-ice, halide and particulate matter concentrations of snowpack, snow\ntemperature, snowfall frequency, and upward latent heat flux (Ferrari et al., 2005;\nKamp et al., 2018; Mann et al., 2015).”\nQ8. Line 89 – May be good to provide reference here to the coastal studies\nbased on your comments.\nQ9. Line 135 – define RD, might be worth indicating that these are used as a proxy\nfor anthropogenic emissions\nRe. Thanks. We have defined RD (solar radiation) in this section. We have also\nindicated CO and SO as typical proxies for anthropogenic emissions in the revised\n2\nmanuscript based on your comments.\nQ10. Line 139 – other studies could be referenced here as well\nRe. Thanks. We have added the related reference (Wang et al. (2017)) here based on\nyour comments.\nQ11. Line 149 – check section reference (numbering is wrong?)\nRe. Sorry for my editing error, and the section number here is “2.3.1”, not “3.3.1”.\nWe have revised it.\nQ12. Line 151 – this is interesting. Could authors provide reasoning as to why?\nCould it have occurred earlier in the year?\nRe. Thanks. We think that one reason would be the air temperature. Low air\ntemperature favors the production of reactive bromine radicals (Simpson et al., 2007)\nand the oxidation of GEM (by stabilizing the Hg(I) intermediate) (Steffen et al., 2008)\nand therefore favors the occurrence of AMDEs. AMDEs were actually observed in June\n;L :?II?ECG KL;LCHG OB?G LB? L?FI?J;LMJ? O;K <?LO??G S+ ;G> S'& U$ OBC=B O?J? LB?\nlowest temperatures recorded for that month (Berg et al., 2013). The warmer air\nL?FI?J;LMJ? \";N?J;A?. S&%-, R '%'( U# CG 5MG? >MJCGA LB? 678/C1 ?PI?>CLCHG ECD?EQ\nexplains why no AMDEs were observed at this time. It should, however, be noted that\nAMDEs were ubiquitous during the MOSAiC springtime (March – May) as reported\nin Ahmed et al. (2023).\nQ13. Line 156 - May be good place to note the significance of the summertime GEM\nmaximum and knowing its origin\nRe. Thanks for your suggestion. We have added the significance of the summertime\nGEM maximum here, which was also displayed below:\n“Line 187: The summertime GEM maximum that follows springtime depletions\n(AMDEs) is a unique Arctic feature. This Arctic GEM seasonality suggests a transition\nfrom a \"mercury sink\" in spring to a \"mercury source\" in summer (Araujo et al., 2022).\nThus, understanding the origins and corresponding mechanisms of this GEM variability\nis of great importance for accurately assessing the regional mercury budget and\nassociated ecological consequences.”\nQ14. Line 216 – additional references needed here\nRe. Thanks. We have cited the corresponding references ((Galí et al., 2021); Park et\nal. (2018)) here based on your comments.\nQ15. Line 231 - This is somewhat confusing because above the authors suggest\nAMDEs were not observed.\nRe. In fact, AMDEs were not observed in summer during the MOSAiC expedition\n(June-September), which was the observation period of this study, but they were\nobserved in spring of the MOSAiC expedition (March-May), which was reported in\nanother study (Ahmed et al., 2023). We have clarified this in the revised manuscript.\nQ16. Line 282–somewhat vague statement. Also what about temperature on\natmospheric chemistry (e.g., increasing temperature on emission of halogens/ oxidants,\nAMDEs, deposition of oxidized Hg to snow, regionally, MIZ as major sinks, etc.).\nAuthors could draw on other studies here.\n“Line 338: Higher temperature could not only promote the release of Hg from the\nsurface ocean and snowpack (Angot et al., 2016), but also inhibit the oxidation process\nof GEM by reactive halogen radicals through causing the thermal decomposition of the\nHg(I) intermediate (Steffen et al., 2008).”\nQ17. Line 285 to 289–reference to help support explanation.\nRe. Thanks. We have added the corresponding references ((Jin et al., 2021; Prijith et\nal., 2018)) here to support explanation.\nQ18. Line 294–it is not clear why WS outside of the range (at lower and higher\nends of the range) would promote sea-air exchange and regional transport and why this\nspecific range implies dilution. May need more clarity.\nRe. Thanks. We have revised and more clearly clarified this content, which is also\nshown below:\n“Line 356: Wind speed (WS) contributes 8% to GAM variance. In stagnant\nmeteorological condition characterized by low WS range (e.g. < 2.5 m/s in this study),\nthe (re)-emitted Hg from the ocean might tend to accumulate, causing the positive\ncorrelation between GEM and WS (Zhang et al., 2021). As WS increases, this may\nenhance the atmospheric dilution of GEM, leading to the decreasing trend(Figure S6a)”\nQ19. Methods: What a sensitivity analysis completed on the model used? This can\nbe added to supplemental information.\nRe. Thanks. For sensitivity analysis of model, we used GAM to fit the non-linear\nrelationship between the dependent variable (GEM) and independent variable (various\ninfluencing factors) to get a best fitting result of model, and subsequently evaluated the\nrelative importance of these various influencing factors based on their respective F\nvalues (see section 2.3 in the manuscript). The bigger of F value, the more sensitivity\n13\nand the relative importance the corresponding factor will have. In addition, the factors\nwith low F values and had insignificant contributions to the improvement of model\nfitting result (fitting R2) will not be used. We have further clarified this selecting process\nof various factors which considered their sensitivities in model in section 3.4 of the\nmanuscript, and added the information including F values of various factors to the\nsupplementary material.\nQ20. Figures: Some figures are fuzzy and difficult to read. Suggest higher\nresolution images (e.g., 4a, 4b, 5b)\nRe. Thanks for your suggestions, and we have improved the qualities of\ncorresponding images in the revised manuscript.\nReviewer #3:\nGeneral comments:\nThe study presents unique atmospheric GEM concentration data from the Arctic\nOcean. The data were obtained using a Tekran 2537B system running on board of the\nresearch vessel Polarstern from June to September 2020. The study confirms an\noffshore summertime GEM peak between Greenland and Svalbard, a GEM maximum\nthat has been observed at several coastal Arctic monitoring stations. The peak is not\nassociated with atmospheric long-range transport of Hg. Instead a GAM simulation\nindicates that open ocean evasion of GEM, in particular from the low-latitude marginal\nice zone between Greenland and Svalbard, is a significant source of GEM to the Arctic\natmosphere in summer. While I want to point out the originality of the GEM dataset, I\nhave some concerns regarding the identification of sources contributing to the GEM\n14\nsummertime peak.\nQ1. The authors report that 63.3% of the summertime GEM variation can be largely\nexplained by open water re-emission and meteorological factors. Araujo et al., 2022,\nNat. Commun. evidenced that the GEM peak originates from re-emission of Hg\ndeposited to snow and sea ice during AMDEs. They concluded that GEM re-emission\ntakes place directly from snow and sea ice but also from regional open waters that\nreceive meltwater Hg inputs. How much of the GEM variation in the present study can\nbe explained by direct GEM re-emission from snow and sea ice? Is re-emission from\nsnow and sea ice not contributing to the GEM summertime peak in the central Arctic\nOcean at all? Please include a paragraph in the ms about how direct snow and sea ice\nre-emission impacts GEM variation in summer.\nRe. Indeed, GEM can be reemitted by both (1) snow and sea ice, and (2) oceanic\nevasion. We cannot quantify the contribution of direct GEM re-emission from snow\nand sea-ice due to the lack of relevant measurements and data in this study (e.g. Hg\nconcentration and isotopic data in snow and sea-ice). This, however, does not change\nthe main message of the manuscript that is re-emissions are mostly occurring in the\nMIZ, not throughout the Arctic Ocean. We speculate that the direct contribution of re-\nemission of Hg from snowpack/sea-ice to the summertime GEM peak may not be\ndominant, since the cruise observations of GEM and this study’s PSCF analysis showed\nthat the GEM concentrations and the regional contributions in the higher-latitude\ncontiguous ice area are low (Figure 1 and Figure 5a in the manuscript). In addition,\nGEM measurements performed during a poleward transect early August during the\nMOSAiC expedition support this hypothesis, as GEM concentrations obviously\ndecreased when Polarstern entered the pack ice with increased distance from the MIZ\n(please see the response of your Q3). More studies are needed to further evaluate the\nrole of direct Hg re-emission from snowpack and sea-ice in GEM variation during\nArctic summertime. We have also added some of these contents to Line 314\nsummertime peak. Meteorological factors, however, explain 37% of the identified\nGEM variation in the central Arctic Ocean. Advection of Hg from the mid-latitudes (i.e.\nlong-range transport of anthropogenic emissions and terrestrial Hg) and Hg evading\nfrom the ocean can indeed be considered sources contributing to GEM levels in the\nArctic atmosphere. However, please explain how meteorological factors can be a source\nof GEM. Meteorological factors such as wind speed or solar irradiation have been\ndemonstrated to influence the rate of GEM evasion from the ocean but cannot be\nconsidered a source in my understanding.\nRe. We agree with you that meteorological factors cannot be classified as a source\nbut are a potentially important influencing factor of GEM in the Arctic Ocean. As the\ninvestigation of the source of GEM in the Arctic Ocean is the focus of this paper, we\nemphasize it in the abstract and introduction. We also revised the original subtitle:\n“2.3.3 Meteorological factors” of section 2.3 as an independent title: “2.4 Impact of\nmeteorological factors”.\nQ3. In July 2020, when the GEM peak was detected, the Polarstern was cruising\nbetween Greenland and Svalbard, correct? I wonder whether the July GEM peak could\nhave been detected close to the North Pole as well ? Did you find any indication for\nthat? Please add dates and the respective location of the Polarstern in Fig. 1a.\nRe. Yes. When the summertime GEM peak was detected, the Polarstern was cruising\nbetween Greenland and Svalbard. Due to the geographical differences, we do not know\nwhether the July GEM peak could have been observed close to the North Pole. In early\nAugust, however, the ship transited northward from the MIZ to a new ice floe area\nlocated close to the North Pole, and a clear decline of GEM concentrations was\nobserved, with GEM decreasing to background concentrations when entering the pack\nice (Figure 1). Based on these results, we strongly believe that oceanic Hg evasion\nwould have been low close to the North Pole in July as well.\nship transited northward from the MIZ to the North Pole.\nTo more clearly show the corresponding information of locations and dates of the\nPolarstern, we have added their information in Figure S3 based on your comments,\nwhich is also shown below:\nArctic Ocean during the summer legs (June–September) of the MOSAiC expedition. The grey\nboxes mark the corresponding locations and date during the expedition\nQ4. Overall, ocean evasion explains less than 50% of the GEM variation in summer\n(L404: 36.7 % of the GEM variation cannot be explained). Still, there is strong\nindication for large GEM evasion from the MIZ between Greenland and Svalbard. It\nwould strengthen the manuscript, if the authors could include a statement about the\nGEM flux rate (ng m-2 day-1) that could be expected from the MIZ between Greenland\nand Svalbard. What flux rate would be required to increase the GEM concentration\nwithin the Arctic Ocean boundary layer from 1.35 ng m-3 (September, background)\nto\n18\n1.8 ng m-3 (July, summertime peak). Under the assumption that GEM re-emission from\nthe other parts of the central Arctic Ocean remain small.\nRe. Thanks for your comments. To estimate the flux rate of GEM from the MIZ\nbetween Greenland and Svalbard that is necessary to explain the increase in GEM\nconcentration within the Arctic Ocean boundary layer from 1.3 ng/m3 to 1.8 ng/m3,\nthe\nfollowing assumptions are made here:\n(1) Neglect other sources of GEM in the Arctic Ocean and the GEM exchange\nbetween the Arctic Ocean and other areas;\n(2) The GEM emitted from the MIZ will affect its nearby Arctic Ocean area that is\nnorth of 70°N, as the summertime GEM maximum phenomenon was mainly observed\nat high-latitude (north of 70°N) Arctic stations (e.g. Zeppelin, Alert, Villum stations),\nwhile low-latitude Arctic stations (e.g. Andøya) observed no apparent GEM maximum\nphenomenon (Angot et al., 2016). In this study, the speculated range of the Arctic\nOcean area that the GEM evasion from the MIZ between Greenland and Svalbard\nwould influence is shown below (Figure 3, the fan-shaped area with red border is the\nrange of influence, and the triangle area with yellow border is the MIZ that serves as\nthe GEM source):\nGreenland and Svalbard would influence. The fan-shaped area with red border is the range of\ninfluence, and the triangle area with yellow border is the MIZ that is served as GEM source)\nBy geometric calculation, the area of MIZ that emits GEM accounts for about 6.25%\nof its range of influence in the Arctic Ocean.\n(3) Based on the time series of GEM in this study, in which the GEM concentration\nshowed apparent increases during three periods in July (from about 2020/6/19 to\n2020/7/2, from 2020/7/4 to 2020/7/9, and from 2020/7/11 to 2020/7/18, Figure 4) and\nmainly occurred in the MIZ, we set the duration of GEM evasion in the MIZ as 20 days.\nexpedition, in which the period of summertime GEM peaks in July is marked.\nBased on the assumptions above, the GEM flux rate can be calculated using the\nfollowing equation, according to the principle that mercury evaded from the MIZ in\nsummer leads to the increase in observed GEM concentration in its range of influence\nin the Arctic Ocean:\nFlux × 0.0625S × 20 = (1.80 -1.30) × S × PBLH\nWhere Flux is GEM evasion flux rate (ng·m-2·day-1) from the MIZ between\nGreenland and Svalbard; S is the range of influence (km2) in the Arctic Ocean by GEM\nevasion from the MIZ; 1.80 (ng/m3) and 1.30 (ng/m3) are July peak concentration and\nSeptember background concentration of GEM in the Arctic respectively; 0.0625 is the\narea fraction of the MIZ to its range of influence in the Arctic Ocean; and PBLH is the\nplanetary boundary layer height (m) of the Arctic Ocean. Here we set this value as the\nJuly average (140m) during the MOSAiC cruise. The calculated flux is 56 ng·m-2·day-\n21\n1, which is more than twice the flux in the Arctic ice-free open ocean (< 24 ng·m-2·day-\n1) (Andersson et al., 2008; DiMento et al., 2019), while lower than that observed in\ncoastal areas in the Canadian Arctic Archipelago (~130 ng·m-2·day-1) (Kirk et al., 2008).\nWe also added this estimation to Section 2.3.2, which is also shown below:\n“Line 321: Overall, we suggest that oceanic evasion in the MIZ combined with\nregional transport is the dominant GEM source in the central Arctic – a phenomenon\nthat likely explains the observed summertime GEM maximum. A back-of-the-envelope\ncalculation (see Text S1) suggests a Hg evasion flux of 56 ng·m-2·day-1 in the MIZ.\nThis flux is more than twice the flux measured in the Arctic open ocean (< 24 ng·m-\n2·day-1) (Andersson et al., 2008; DiMento et al., 2019), but lower than that measured in\ncoastal areas of the Canadian Arctic Archipelago (~130 ng·m-2·day-1) (Kirk et al., 2008).\nIt is also an order of magnitude higher than the Arctic Ocean evasion flux reported by\nDastoor et al. (2022) in their latest Arctic Hg mass balance budget (23–45 tons/year,\ni.e., 3.7–7.3 ng·m-2·day-1 assuming a surface area of 1.7×107 km2). Further dedicated\nfield and modeling studies are needed to further constrain that flux.”\nQ5. Such a statement could be integrated into a yet missing “conclusion” or\n“implication” paragraph. I strongly suggest to add conclusions and implications at\nthe end of the main text. Please discuss the significance of your results and how your\nfindings fit with our current understanding of the Arctic mercury cycle (cp. with\nDastoor et al., 2022, Nature reviews, earth and environment and Araujo et al., 2022,\nNat. Commun.).\nRe. Thanks for your important comments. We have added the corresponding\nconclusions and implications in which we discuss how our findings fit with our current\nunderstanding of the Arctic mercury cycle and highlight the significance of our results.\nwarming Arctic, which are also shown below:\n“Line 363: Based on the unique whole summertime observation of GEM in the\ncentral Arctic Ocean during the MOSAiC expedition, we show that oceanic evasion is\nan important source of atmospheric Hg in summertime.. This study further verifies our\ncurrent understanding of the Arctic Hg cycle, specifically regarding the source of the\nsummertime GEM maximum and the general role of the Arctic Ocean as a Hg source\nduring the summertime (Araujo et al., 2022; Dastoor et al., 2022a). Furthermore, this\nstudy offers new insights by showing that oceanic evasion is not significant throughout\nthe Arctic Ocean but mainly occurs in the MIZ. Our estimate of the Hg evasion flux\nsuggests a higher magnitude than in the open ocean, causing a rapid increase of GEM\nconcentrations throughout the Arctic.\nThe Arctic is warming faster than the rest of the planet and Arctic sea-ice is declining\nrapidly with potential consequences for the Hg cycle (Yadav et al., 2020). Firstly,\nprojected future Arctic warming might favor the conditions that stimulate AMDEs-\ndriven Hg deposition through enhancing reactive halogen sources (Dastoor et\nal., 2022a), as previous reported long-term (1996-2017) dataset of BrO· vertical column\ndensities (VCDs) displayed an increasing trend of about 1.5 % of the tropospheric\nBrO· VCDs per year during polar springs, which may be attributed to increase in first-\nyear ice coverage that has a higher salinity than multiyear ice and facilitates the\nproduction of BrO· (Bougoudis et al., 2020). This could enhance the deposition of Hg\nand thus the load of Hg in surface seawater during the melt season. Secondly, previous\nstudy found that the width of MIZ in the warm season (July–September) in Arctic\nOcean has increased by 13 km decade–1 from 1979 to 2011, driven by the decrease of\nArctic sea-ice extent and the replacement of thick, multi-year ice by thin, first-year ice\n(Strong and Rigor, 2013). In this context, it can be expected that the retreat of multi-\nyear ice and the decline in sea-ice extent will continue, causing further increase in width\nand range of MIZ. This would translate into a larger source area of GEM in summertime.\nThirdly, the decline in sea-ice extent and expanse in MIZ, can 1) transmit substantially\nmore light into the underlying water column and make a longer growing season, leading\n23\nto the increase in phytoplankton production (Arrigo et al., 2008; Pabi et al., 2008;\nPayne et al., 2022), and 2) promote the CO uptake by open seawater and the biologic carbon\n2\npump process in MIZ (Bates et al., 2006; Qi et al., 2022), which would further increase\nthe photo- and/or biological reduction capacity of Hg(II) here. As a result, we expect\nthat in a context of Arctic warming, oceanic Hg evasion in the MIZ might become\nincreasingly significant, and strengthen the role of the Arctic Ocean as a summertime\nHg source. Given the relatively long atmospheric lifetime of GEM, this may have global\nconsequences on the Hg cycle.”\nSpecific comments:\nQ6. Line 62: There are more recent studies reporting atmospheric lifetimes of GEM.\nConsider referring to Saiz-Lopez et al., 2018, Nat. Commun. or Horrowitz et al., 2017,\nACP.\nRe. Thanks for your suggestion. We have referred to these references and cited them\nbased on your comment.\nQ7. Line 67: The AMAP report is referenced with number 44 instead of 8. Is there a\nreason for that? Adjust reference numbering throughout the ms.\nRe. Thanks for your suggestion. I have not concerned the order of references we\ncited before I have revised it and checked the corresponding reference numbering\nthroughout the manuscript.\nQ8. Line 90: Delete “unfortunately”.\nRe. Thanks. We have deleted it.\nQ9. Line 135: Write out “RD”. You define all abbreviations in the Method section.\nPlease do that in main text instead.\nof the abbreviations in main text based on your comment.\nQ10. Line 194: “Central Arctic Ocean” or “central Arctic Ocean”? Be consistent.\nRe. Thanks. We have consisted this term as “central Arctic Ocean” throughout the\nmanuscript.\nQ11. Line 218: What would be an important factor? This sentence is hard to\nunderstand. Please rephrase.\nRe. Thanks. We have rephrased this sentence, which is also shown below: “These\nphenomena indicated that oceanic evasion of Hg and its regional transport from the\nMIZ would be an important factor that drives the summertime GEM maximum in the\nArctic Ocean, as we observed in this study.”\nQ12. Line 236: Write out“Chla”.\nRe. Thanks. We have written out “Chla” (chlorophyll-a) in the revised manuscript.\nQ13. Line 270: Write out as follows: Wind speed, atmospheric pressure and air\ntemperature. Stick to“re-emission”.\nRe. Thanks for your suggestion. We have revised them based on your comments,\nwhich are also shown below:\n“Line 361: Meteorological conditions (wind speed, atmospheric pressure and air\ntemperature) can influence the re-emission, transport and dilution process of\natmospheric Hg.”\nQ14. L313: The flow rate of the Tekran 2537B was 0.7 L min-1 which is low\ncompared to most Tekran deployments. Is there a reason why the authors used such a\nlow flow rate? Analysis of GEM in 3.5 L of air is at the lower limit. Please explain\nbriefly why an additional Tekran analyzer was setup even though GEM was determined\n25\nin the CU container on a 15 min resolution? Please also add a comment about how you\navoided to measure GEM that potentially originated from ship exhaust gas emissions.\nRe. 2MJCGAKMFF?JLCF? H<K?JN;LCHGK H@436$LOH&%*+TF 9?@EHG@CEL?J=ECIK O?J?\ninstalled to prevent oceanic sea-salt aerosols from entering the Tekran 2537X/BTM\ninstrument system. Considering that this may increase the air resistance of the sampling\nsystem during the measurement, we set the flow rate lower than 1.0 L/min. In fact, the\ninjection flow of the Tekran 2537X/BTM instrument is normal in the range of 0.5 L/min\n~1.5 L/min, and will not affect the GEM measurement during Arctic summertime in\nthis study. The sampling inlet was mounted at the front of the ship to minimize the\ninfluence of the ship exhaust and data were carefully screened for local contamination\nas discussed in Angot et al. (2022) and Beck et al. (2022). The comparisons between\nthe measured GEM and the typical exhaust components (including SO , particulate and\n4\n(Figure 5). We have also added this content to method section in the manuscript. Please\nalso note that it is quite common to have redundant measurements during such an\nexpedition to limit gaps in time-series and allow cross-evaluation. As discussed in\nAngot et al. (2022), redundant measurements were for example performed for carbon\ndioxide, methane, carbon monoxide, nitrous oxide, ozone or volatile organic\ncompounds.\n4\n27\n(marked by red box). It displays that the significant increases of exhaust components correspond to\nno apparent increase of GEM meanwhile (e.g. grey shaded area)\nQ15. L 685, 697, 709: Add units to the axis text in Figs. 3a, b, 4a, b, 5b.\nRe. Thanks, we have added units to the axis text based on your comments.\nSupporting information:\nQ16. L 38: Is that a table of content? Please adjust.\nRe. Thanks. We have adjusted it in form of a table.\nQ17. L 45: Why is the section title number 3.2? Please adjust.\nRe. Sorry of this editing error, this section title is redundant and we have removed it.\nQ18. L 70: How was this figure created? Using HYSPLIT? Information is missing\nin the caption.\nRe. Thanks. Yes, this figure was created using the HYSPLIT transport and dispersion\nmodel from NOAA Air Resources Laboratory (ARL). We have also added this\ninformation in the caption.\nQ19. L 95: Add units to the axis text in Fig S7.\nRe. Thanks. We have added them in the revised Figure S7.\nReviewer #4:\nThis study investigated key mechanisms for occurrence of the summer concentration\npeak of gaseous elemental mercury (GEM) in the Arctic based on observations during\nthe Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC)\nexpedition and a statistical modeling approach. Oceanic evasion in the Marginal Ice Zone\nwas found to be the dominance source for the summertime GEM maximum, and\nthe\nphotoreduction of Hg(II) to GEM by phytoplankton was proposed to be the main\nmechanism. The observational data obtained in the study are valuable, and the finding\nreported in the study is interesting which could draw widespread attention.\nQ1. However, the generalized additive model (GAM) used in this study needs more\ninformation on model validation, and more solid evidence needs to be provided for the\nmechanisms the authors proposed. A comprehensive schematic diagram for what’s\ngoing on during the summer GEM maximum is encouraged to be added to the\nmanuscript for readers to get a whole picture of the crucial mechanisms. Overall in my\nopinion, substantial improvement is required before the manuscript is acceptable for\npublication on Nature Communications.\nRe. Thanks for your comments. For the validation of GAM method, we have\nsubstantially referred to previous studies about environmental areas that used the GAM\nmethod (Gong et al., 2017; Shi et al., 2022; Wu et al., 2020; Zhang et al., 2021; Zhao\net al., 2022) and added section 3.5 and added more information in discussing the model\nvalidation, which is also shown below:\n“3.5 Model validation\nWe further systematically evaluated the quality of the GAM using different methods.\nWe used a 5-fold cross-validation test to assess the accuracy of GAM simulation. The\nprinciple of this method is randomly dividing the whole dataset into 5 subsets, and in\neach cross-validation round, four subsets are used to fit the model, and the remaining\nsubset is used to predict. This process was repeated 5 times to ensure that every subset\nwas tested (Shi et al., 2022; Wu et al., 2020; Zhang et al., 2021). The 5-fold cross-\nvalidation results displayed a good coincidence between the GAM and cross-validated\nresults (slope=1.00, R2=0.99), demonstrating good reliability of the model (Fushiki,\n2011)(Figure 3b). In order to test the underlying assumptions of homogeneity, normality,\n29\nand independence of GAMs to ensure the validity and accuracy of the model, we used\nthe following methods: (1) quantile-quantile (Q-Q) plot (sample quantiles against\ntheoretical quantiles), (2) scatterplots of residuals against linear predictors, and (3)\nhistograms of the residuals (Zhang et al., 2021). The Q-Q plot result showed that GAM\nproduced good results around the average concentration; the scatterplot of residuals vs.\nlinear predictor displayed that residuals were generally concentrated around 0 value,\nand presented a random distribution with no obvious trend, indicating unbiased\nsimulations of GEM. The histogram of residuals close to normal distribution, which\nsuggests that the error of model fitting is random and that the selection of predictor\nvariables is reasonable (Figure S1). These results suggest that the selected predictors\nwill be useful for identifying sources of Hg in the central Arctic atmosphere.”\nWe have also added a comprehensive schematic diagram for what’s going on during\nthe summer GEM maximum based on your comments, which is also shown below:\nMarginal Ice Zone, including (1) previously deposited Hg(II) during springtime AMDEs is\ntransferred to the surface ocean with melting ice/snow-water, resulting in a large Hg load in surface\nseawater in the MIZ; (2) high phytoplankton mass in MIZ may lead to a high reduction capacity for\nHg(II) in MIZ, as the excretion of photoreactive organic compounds by phytoplankton can facilitate\nthe photoreduction of Hg(II) to Hg(0) via electron transfer. In addition, phytoplankton can also\ndirectly contribute to the reduction of Hg(II) to Hg(0) through an enzymatic detoxification\nmechanism linked to their photosynthetic activity; and (3) the melting of sea-ice and the absence of\nupper ocean stratification in the MIZ (i.e., absence of a meltwater layer) facilitates air–sea gas\nexchange.\nHere are some specific comments:\nQ2. Abstract: The most probable cause of the summer GEM peak, the photoreduction\nof Hg(II) to GEM by phytoplankton (if I understand the Results and Discussion part\ncorrectly), is not reflected in the abstract. The authors only mentioned that oceanic\nevasion mainly occurs in the Marginal Ice Zone, which could mislead the readers to\nthink that the air–ice Hg exchange might be the dominant mechanism of the summer\nGEM maximum.\nRe. Thanks for your comments. Yes, the photoreduction of Hg(II) to GEM facilitated\nby high phytoplankton mass is likely to be an important contributor to GEM evasion in\nthe MIZ. Apart from that, the high Hg input from melting ice/snowpack in MIZ, and\nthe loss of sea-ice as a barrier of Hg sea-air exchange in the MIZ may also be important.\nWe have briefly added these factors in the abstract, which is also shown below:\n“Abstract: Atmospheric gaseous elemental mercury (GEM) concentrations in the\nArctic exhibit a clear summertime maximum, while the origin of this peak is still a\nmatter of debate in the community. Based on summertime observations during the\nMultidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC)\nexpedition and a modeling approach, we further investigate the sources of Arctic\natmospheric Hg. Simulations with a generalized additive model (GAM) show that long-\nrange transport of anthropogenic and terrestrial Hg from lower latitudes is a minor\ncontribution (~2%). The dominant source (> 50%) is oceanic evasion. A potential\nsource contribution function (PSCF) analysis further shows that oceanic evasion is not\nsignificant throughout the Arctic Ocean but mainly occurs in the Marginal Ice Zone\n(MIZ) due to the specific environmental conditions in that region. Our results suggest\nthat this regional process could be the leading contributor to the observed summertime\nGEM maximum. In the context of rapid Arctic warming and the observed increase in\nwidth of the MIZ, oceanic Hg evasion may become more significant and strengthen the\nrole of the Arctic Ocean as a summertime source of atmospheric Hg.”\nQ3. Line 82: The citation should be Durnford et al. according to the reference list.\nRe. Sorry for my editing error. Yes, the citation should be Durnford et al and we\nhave revised it.\nQ4. Lines 82–87: The summary of the GEM source apportionment results could\ncause misinterpretation. Long-range transport of Asian air is a primary source. Legacy\nHg emissions or re-emissions from terrestrial regions or ice zones are considered as\nsecondary sources. Due to the different methodologies adopted, a certain part of the\ndiscrepancy between these studies could be caused by the categorization of GEM\nsources. Some legacy Hg from historical Asian air could be accumulated in plants,\nsnowpack and ice in the Arctic area and reemitted as a secondary source of GEM. It\nwould be helpful for the readers to understand the Arctic Hg cycle if the authors could\nmake a more comprehensive introduction here, incorporating these literatures.\nRe. Thanks for your important comments. We have added more information about\nthe Hg cycle process in the Arctic region and revised the corresponding statements in\nthis section, which is also shown below:\n“Line 72: Long-range transport of air, riverine input and coastal erosion from\ncircumpolar land can be primary Hg sources to Arctic area (Dastoor et al., 2022b; Fisher\net al., 2012). A fraction of legacy Hg from these historical primary sources can\naccumulate in seawater, plants, glaciers and ice sheets in the Arctic and later be re-\nemitted as secondary emissions of atmospheric Hg (Douglas and Blum, 2019; Hirdman\net al., 2009; Zheng et al., 2021).\nLine 88: A modeling study by Durnford et al. (2010) suggested that long-range\ntransport of Asian air is the most important primary source of atmospheric Hg to the\nArctic in all seasons. Sommar et al. (2010) and Fisher et al. (2012) later attributed\nenhanced summertime GEM concentrations to ocean Hg evasion in the Arctic Ocean\nfed by terrestrial Hg inputs (rivers and coastal erosion) based on observations and\nseveral sensitivity runs using GEOS-Chem model. Recent isotopic work suggests,\nhowever, the dominant role of re-emissions from the Arctic cryosphere, while the role\nof terrestrial Hg from rivers and coastal erosion was found to be minor (Araujo et al.,\n2022).”\nQ5. Line 116: Misspelling of“coastal”.\nRe. Sorry for my editing error, and we have revised it.\nQ6. Section 2.1: The discussion in this section is limited. Significance of difference\nfrom previous studies based on t-test should be given. The standard deviation of the\ncentral Arctic GEM observations during August 25–September 8, 2012 (0.61) is much\nhigher than that in this study (0.07). What caused the large fluctuation in the study of\nYu et al. (2014)?\nRe. Thanks. We have added the t-test imformation to the manuscript. For the\ndifference in the standard deviation of data between this study and Yu et al., (2014),\nwe\nthink that an important reason might be the large differences in their corresponding\nobservation sites and the corresponding sea-ice cover. In fact, the research area that is\ndefined as “central Arctic Ocean” in Yu et al., (2014) has a large difference from this\nstudy (Yu et al., (2014): 80°N-87.6°N, 9.2°E-168.9°W; this study: 87.80°N-88.75°N,\n104.27°E-120.83°E), which can cause a large difference in sea-ice conditions (see\nFigure 7). The dense sea-ice cover (average sea-ice fraction: 0.92±0.07) in this study\nwould largely inhibit the air-sea exchange of Hg, which may cause the apparently more\nstable variation of GEM concentration. The oceanic evasion of Hg would be generally\nmore significant in the research area in Yu et al. (2014), in which the sea-ice fraction\nwas significantly lower (0.36±0.32) (Figure 7), and may lead to larger fluctuations of\nGEM compared with this study. We have clarified the corresponding statements as:\n“Line 137: In addition, our observed GEM average concentration (1.32 ± 0.07 ng/m3)\nare comparable to the reported GEM observation (1.40 ± 0.61 ng/m3; t-test: p<0.01)\nduring the same period (August 25-September 4) in 2012 in the central Arctic Ocean\n(80°N-87.6°N, 9.2°E-168.9°W) , while with apparently lower standard deviation than\nthe latter (Yu et al., 2014). This difference in standard deviation may to some extent be\nattributed to the significantly higher and less variable sea-ice fraction (0.92±0.07) in\nthis study, which inhibited the oceanic evasion of Hg and led to the apparently more\nstable variation of GEM concentration, compared with Yu et al., (2014) (with average\nsea-ice fraction of 0.36±0.32).”\nFigure 7. The spatial distribution of sea-ice during the same period (August 25-September 4) in Yu\net al., (2014) and in this study.\nQ7. Lines 149–151: What could be the reason for the absence of this phenomenon\nin the MOSAiC expedition (especially in June)? Why did the authors present this\ncomparison here?\nRe. In fact, AMDEs were just not observed in summer during the MOSAiC\nexpedition (June-September), which was the observation period of this study, but they\nwere observed in spring (March-May), as was reported in another study (Ahmed et al.,\n2023). We present this comparison here to further illustrate the conditions of the\noccurrence of AMDEs.\nWe think that one reason would be the air temperature. Low air temperature favors\nthe production of reactive bromine radicals (Simpson et al., 2007)\nand the oxidation of\nGEM (by stabilizing the Hg(I) intermediate) (Steffen et al., 2008)\nand therefore favors the occurrence of AMDEs. AMDEs were actually observed in June\nat Zeppelin station\nOB?GLB?L?FI?J;LMJ?O;K<?LO??GS+;G>S'&U$OBC=BO?J?LB?EHO?KLL?FI?J;LMJ?K\nJ?=HJ>?>@HJLB;LFHGLB\"0?JA?L;E%$(&')#%9B?O;JF?J;CJL?FI?J;LMJ?\";N?J;A?.S&%-,\nR'%'(U#CG5MG?>MJCGALB?678/C1?PI?>CLCHGECD?EQ?PIE;CGKOBQGH/623KO?J?\nobserved at this time. It should, however, be noted that AMDEs were ubiquitous during\nthe MOSAiC springtime (March – May) as reported in Ahmed et al. (2023).We have\nalso added this discussion the revised manuscript (Line 175).\nQ8. Figure 4c: What is the significance of the relationship between land fraction and\nGEM? The negative correlation seems not quite robust to me.\nRe. In this section we mainly investigated the variation of GEM average\nconcentrations with the increase of the corresponding land fraction, aimed at\nunderstanding whether the higher 168 h backward trajectory’s transport time over land\ncompared to the total transport time (land fraction) can cause the higher level of GEM.\nThe significance of their correlation was not our focus here. Based on your comments,\nwe explored the correlation between land fraction and average GEM here, and found\ntheir negative linear correlation (Figure 8). Although this linear correlation is\ninsignificant (p=0.056), it at least indicates that the average concentrations of GEM do\nnot increase with increased land fraction. Additionally, this analysis to some extent\nshows the minor contribution of terrestrial anthropogenic Hg emissions to GEM in the\nArctic Ocean during the summertime. We have also replaced the original Figure 4c with\nthe figure shown below, and revised the corresponding statement in the manuscript,\nwhich is also shown below:\n“Line 214: We find that the average GEM concentration slightly decreases as the\nland fraction increases (Figure 4c), which to some extent suggestsa minor contribution\nof land-based Hg emissions to GEM in the Arctic Ocean during the summertime.”\nFigure 8. Relationship between the fraction of the 168 h backward trajectory’s transport time over\nland area over the total transport time (Land fraction (Traj)) and its corresponding average GEM\nconcentration\nQ9. Lines 182–194: The reasoning is a bit farfetched. More solid evidence should\nbe provided to support the speculation.\nRe. The seasonality of poleward pollution transport from mid-latitudes and the\ninfluence of the Arctic dome is well documented in the literature (Boyer et al., 2023;\nBozem et al., 2019; Klonecki et al., 2003; Law and Stohl, 2007; Willis et al., 2018). A\nprevious model study that reported the simulation of atmospheric Hg global spatial\ndistribution also suggested that advection of mercury from mid-latitudes to the Arctic\nis insignificant in summer due to weak airflow and to a confined polar front(Dastoor\nand Larocque, 2004). This is also reflected in the 7-day backward trajectories during\nthe whole observation period (June-September) in this study, which show that air\nmasses were mainly concentrated over the Arctic Ocean, while the air masses from the\ncircumpolar land area were generally scarce (Figure S2 in the supporting information\nof the manuscript).\nQ10. Figure 5b: What do these three peaks stand for in this influencing pattern? This\nis an important question to verify the reliability of the GAM model.\nRe. Thanks for your comments. For the reliability of the GAM model, we have\nsystematically evaluated it using a series of widely used methods for the GAM model,\ndemonstrating good reliability of the model without overfitting (please see the newly\nadded section 3.5). In fact, these three peaks in the GAM simulation basically reflect\nthe complex sea-ice conditions in the lower-latitude marginal ice zone (MIZ), in which\nthe summertime GEM peaks characterized by elevated concentration levels and high\nfluctuations were also observed. This can be suggested by the observed relationship\nbetween sea-ice and GEM (Figure 9), which shows that the most apparent GEM\nfluctuations occur at sea-ice fractions of approximately 0.5 and 0.8, in accordance with\nthe two GEM peaks that occur at Open-water fractions of approximately 0.5 and 0.2 in\nFigure 5b, and the corresponding low latitudes (Figure 9) during GEM peaks indicate\nthe MIZ area. The spatial distributions of GEM and sea-ice also present these features,\nshowing that the GEM peaks occurred in the MIZ with complex sea-ice conditions\n(Figure 10).\nFigure 9. The relationship between GEM, sea-ice fraction and the corresponding latitude during the\nwhole observation period.\nFigure 10. The locations of summertime GEM peaks during the MOSAiC expedition.\nQ11. Line 230: How much Hg(II) is stored yearly approximately? More quantitative\ncomparisons are required.\nRe. Thanks. Dastoor et al. (2022) suggested that approximately 9 tons (4 ~ 15 tons)\nof Hg are stored in the Arctic Ocean sea-ice with sea-ice melt contributing 1.4 ± 0.4\ntons of Hg to the Arctic Ocean. We have also added this discussion in Line 281.\nQ12. Line 239: It should be Figure 7.\nRe. Sorry for my editing error, and we have revised it.\nQ13. Section 2.3.3: Solar radiation should be included as a meteorological predictor\nin the GAM model. In the previous section, the photoreduction of Hg(II) to GEM by\nphytoplankton is proposed to be a dominant mechanism. If so, solar radiation is\nsupposed to be a key factor for the summertime GEM maximum.\nRe. Thanks for your comments. We found that in the GAM model, the contribution\nof solar radiation to GEM was minor (1.3%), with respectively lower p-value (0.00049)\ncompared with other parameters. We speculated that the low contribution of solar\nradiation in the GAM model would be to some extent due to two factors: First, the\ndiurnal variation of hourly solar radiation data can influence the statistical results in\nGAM. As we used daily data of solar radiation instead to avoid the influence of its\ndiurnal variation, we found that solar radiation displayed moderate positive linear\ncorrelation with GEM (Figure 11), in accordance with concurrent elevated solar\nradiation during the GEM and Chla peaks (Figure 6 in the manuscript), which may\nsupport the photoreduction of Hg(II) to GEM. The second factor is the attenuation of\nsolar radiation caused by marine phytoplankton and sea-ice in the MIZ.\nFigure 11. 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Mercury stable isotopes reveal the sources and transformations of atmospheric Hg in the high\nArctic. Applied Geochemistry 131, 105002.\nhttps://doi.org/https://doi.org/10.1016/j.apgeochem.2021.105002\nThe authors have done a great job in addressing the reviewer comments. Still, there are two key\nstatements in the abstract (L45-47) “… shows that oceanic evasion is not significant throughout\nthe Arctic Ocean but mainly occurs in the Marginal Ice Zone (MIZ) due to the specific\nenvironmental conditions in that region.” and in the conclusions (L367-369) “this study offers new\ninsights by showing that oceanic evasion is not significant throughout the Arctic Ocean but mainly\noccurs in the MIZ” that are over-interpreting the results. How can the authors exclude that oceanic\nevasion of Hg0 can be significant from other parts of the Arctic Ocean during summer as well, i.e.\nthat Hg0 evasion mainly occurs from the MIZ only? Elevated oceanic evasion has been suggested\nfrom the Barents Sea or from the Beaufort Sea alike (Sonke et al., 2018, PNAS,\nhttps://doi.org/10.1073/pnas.181195711; Fig. 6a). I suggest to slightly reformulating these\nstatements. I think we needed Hg0 concentrations or direct Hg0 flux measurements not only along\nthe summertime leg of the Polarstern but from other parts of the Arctic Ocean as well to draw such\na conclusion.\nI’m unclear about the statement in the abstract in L44 “The dominant source (> 50%) is oceanic evasion”. In L404 of the previous version of the manuscript the authors have stated that 36.7 % of the GEM variation cannot be explained by the model (see comment Q4 by Reviewer 3). Wouldn’t it be correct to say that \"> 50% of the explained GEM variability is caused by oceanic evasion\"?\nThank you for clarifying this.\nThe manuscript reads very nicely!\nReviewer #4 (Remarks to the Author):\nThe manuscript has been substantially improved. Comments from all reviewers have been properly\naddressed. I think the manuscript is now acceptable for publication on Nature Communications.\nResponses to Reviewers’ Comments\nDear Reviewers:\nThank you for your comments on our manuscript entitled “The Marginal Ice Zone\nas a dominant source region of atmospheric mercury during Arctic summertime”\n(manuscript ID: NCOMMS-23-05574A) and for offering us an opportunity for revision.\nWe believe the comments have further helped us improve the overall quality of\nthe manuscript. Below are our detailed responses to reviewers’ questions. Revised texts\nof the manuscript are marked in red in the responses to reviewers’ comments.\nReviewers’ Comments and response\nReviewer #3 (Remarks to the Author):\nQ1. The authors have done a great job in addressing the reviewer comments. Still,\nthere are two key statements in the abstract (L45-47) “… shows that oceanic evasion is\nnot significant throughout the Arctic Ocean but mainly occurs in the Marginal Ice Zone\n(MIZ) due to the specific environmental conditions in that region.” and in the\nconclusions (L367-369) “this study offers new insights by showing that oceanic evasion\nis not significant throughout the Arctic Ocean but mainly occurs in the MIZ” that are\nover-interpreting the results. How can the authors exclude that oceanic evasion of Hg0\ncan be significant from other parts of the Arctic Ocean during summer as well, i.e.\nthat Hg0 evasion mainly occurs from the MIZ only? Elevated oceanic evasion has been\nsuggested from the Barents Sea or from the Beaufort Sea alike (Sonke et al., 2018,\nPNAS, https://doi.org/10.1073/pnas.181195711;Fig. 6a). I suggest to slightly\nreformulating these statements. I think we needed Hg0 concentrations or direct Hg0\nflux measurements not only along the summertime leg of the Polarstern but from other\n1\nparts of the Arctic Ocean as well to draw such a conclusion.\nRe. Thanks a lot for your comments! Sonke et al. (2018) indeed suggested a net air-\nair-\nsea Hg exchange over the continental shelf areas due to heat transfer from continental\nregions and turbulence from sea ice rafting. In this study we focus on the central Arctic,\ni.e., a region that excludes continental shelf areas (see for example map attached, Figure\n1). Our main point is that oceanic evasion is not prevalent throughout the ice-covered\ncentral Arctic but mostly happens in the MIZ. We have also revised the corresponding\nstatements in the abstract and conclusion in the manuscript respectively, which were\nalso shown below:\n“Line 43-46:A potential source contribution function (PSCF) analysis further shows\nthat oceanic evasion is not significant throughout the ice-covered central Arctic Ocean\nbut mainly occurs in the Marginal Ice Zone (MIZ) due to the specific environmental\nconditions in that region.”\n“Line 367-369: Furthermore, this study offers new insights by showing that oceanic\nevasion is not significant throughout the central Arctic Ocean but mainly occurs in the\nMIZ.”\nIn addition, the evasion flux calculated in the MIZ in this study is higher than the\nflux over continental shelf areas (include Sonke et al. 2018 in the list of references),\nand we also added this content to the manuscript (Line 327-328).\nSnoeijs-Leijonmalm et al., (2022) (Pauline Snoeijs-Leijonmalm et al. ,Unexpected fish and squid in\nthe central Arctic deep scattering layer.Sci. Adv.8,eabj7536(2022).DOI:10.1126/sciadv.abj7536)\nQ2. I’m unclear about the statement in the abstract in L44 “The dominant source (> 50%) is oceanic evasion”. In L404 of the previous version of the manuscript the authors have stated that 36.7 % of the GEM variation cannot be explained by the model (see comment Q4 by Reviewer 3). Wouldn’t it be correct to say that “> 50% of the explained GEM variability is caused by oceanic evasion”? Thank you for clarifying this.\nRe. Thanks for your suggestion. We agree with you and have revised this statement\nas “more than 50% of the explained GEM variability is caused by oceanic evasion”\nbased on your comment.\nQ3. Please add the dates and the respective position of the Polarstern to Figure 1a,\nin the same way as you did in Figure S9. It should immediately become clear in Fig. 1\n3\nwhether the research vessel was driving/drifting clockwise or counterclockwise.\nRe. Thanks for your suggestion. We have added the respective position and the\ncorresponding date information of the Polarstern to Figure 1a based on your comments,\nwhich was also displayed below:\nFigure 2. (a). Spatial distribution of observed gaseous elemental mercury (GEM) concentrations in\nthe Arctic Ocean during the summer legs (June–September) of the MOSAiC expedition. The color\nscale gives the GEM concentration. (b) The time series of GEM concentrations during the summer\nlegs (June–September) of the MOSAiC expedition.\nThe manuscript has been substantially improved. Comments from all reviewers have\nbeen properly addressed. I think the manuscript is now acceptable for publication on\nNature Communications.\nRe. Thanks for your positive comments!\n5"}]

Decision letter