Continental outflow shapes the circum-Antarctic pattern of summertime atmospheric mercury depletion zones

Domain: earth_science
Published: 2025-12-26
Editorial outcome: major_revision

Abstract
Atmospheric mercury depletion events (AMDEs) are a unique phenomenon in polar mercury cycling, involving intense atmospheric mercury oxidation and deposition that amplify marine mercury sinks. The Southern Ocean, a critical hotspot for environmental mercury exposure, exhibits AMDEs whose characteristics and distribution patterns remain unclear. This study presents the observational dataset of atmospheric gaseous elemental mercury (GEM) across the circum-Antarctic Southern Ocean. We observed pronounced summertime AMDEs that drove high spatial heterogeneity in circumpolar GEM distribution. By combining Generalized Additive Models (GAM) simulations with multi-platform observational evidence, we demonstrate that the Antarctic continental outflow largely shapes the circumpolar distribution of summertime AMDEs, concentrating these events within the convergence zones of katabatic winds. This work highlights the critical role of land-sea coupling effects in Antarctic atmospheric chemistry, and underscores the need to systematically assess the implications of these processes for mercury bioavailability across Southern Ocean ecosystems.

Introduction
Mercury (Hg) is a highly neurotoxic heavy metal pollutant whose long-range atmospheric transport makes it a pervasive global contaminant1,2. Chronic environmental exposure to mercury can lead to bioaccumulation and toxicity, causing irreversible damage to the nervous, immune, cardiovascular systems, and even death. Polar regions are highly sensitive to environmental Hg exposure3,4. In Antarctica, the Southern Ocean serves as a critical habitat for diverse biota, where Hg entering the marine environment can be methylated by microorganisms, enhancing its bioavailability and leading to bioaccumulation and biomagnification across food webs5. Evidence of Hg bioaccumulation has been documented in a wide range of species, including penguins, seabirds, fish, krill and seals6–9. In addition, the Southern Ocean is also a major sink for global atmospheric Hg10. A recent study presented that Hg burial in the Antarctic Ross Embayment and open ocean increased three- and eightfold, respectively, over the past 12,000 years11.

Atmospheric Mercury Depletion Events (AMDEs) represent a unique phenomenon in polar atmospheric Hg cycling. These events predominantly occur in spring and are primarily driven by the heterogeneous photochemical reactions involving reactive halogen species (mainly bromine) produced at interfaces such as sea ice, snowpack, frost flowers, and sea salt aerosols, leading to the rapid oxidation of gaseous elemental mercury (GEM)12–16. This process results in a pronounced decline in GEM concentrations, enhances atmospheric mercury oxidation and subsequent deposition, and increases mercury input to terrestrial and marine surfaces, thereby attracting significant scientific attention. AMDEs were first discovered in 1995 at Alert Station in the Arctic and have since been widely observed across various Arctic sites3,12. In contrast, due to Antarctica’s remoteness, research on AMDEs in this region remains scarce. The first documented evidence of springtime AMDEs in Antarctica was reported by Ebinghaus et al. at Neumayer Station17.

Notably, recent observations have revealed that AMDEs can also occur in winter and in summer in Antarctica18,19. For instance, Sprovieri et al. and Temme et al. observed the summertime GEM depletion phenomena, along with the concurrent GOM elevations in the Antarctic Terra Nova Bay and Neumayer station, respectively20,21. Angot et al. observed multi-day summer AMDEs over the Antarctic Plateau18. Unlike springtime events, these cannot be explained solely by bromine-driven oxidation, and their underlying chemical mechanisms remain unclear18. The occurrence of AMDEs outside spring further enhances annual atmospheric Hg deposition, potentially increasing biological Hg exposure—particularly during the polar summer breeding season. However, summer AMDEs in Antarctica have so far been detected primarily inland, while observations over the Southern Ocean—the key breeding and foraging habitat for Antarctic wildlife—remain extremely limited. The characteristics and distribution of summer AMDEs in this critical region, therefore, remain unknown, hindering robust assessments of Antarctica’s Hg budget and associated ecological risks.

The Antarctic environment differs fundamentally from the Arctic, being centered on a vast, high-altitude continent with an average elevation exceeding 2300 m. This unique geography drives distinct atmospheric processes that influence Hg cycling. In summer, photochemical reactions on the Antarctic Plateau snowpack generate a highly oxidative atmosphere characterized by elevated concentrations of oxidants such as NO x, OH and RO 22–24. In addition, radiative cooling along the ice sheet slopes produces strong, cold downslope katabatic winds that transport air masses from the interior to the coast. These katabatic winds can extend horizontally for thousands of kilometers, facilitating strong land-sea interactions25,26. Previous observations at Antarctic land-based stations have shown that Antarctic katabatic wind transport can influence isotopic signals of gaseous and particulate Hg in the nearshore atmosphere27,28. However, the implications of these processes for Hg dynamics over the Southern Ocean remain unclear, particularly at the Antarctic-wide scale.

To address these questions, this study presents the circumpolar observational dataset of atmospheric GEM over the Southern Ocean, obtained during the 33rd Chinese National Antarctic Research Expedition. These data reveal the spatial characteristics of GEM distribution around Antarctica, filling a critical data gap in polar Hg research. Building on these measurements, we performed observational data analysis with generalized additive model simulations to elucidate (1) the circumpolar distribution patterns of summer AMDEs in the Southern Ocean, and (2) the role of Antarctica’s unique land-sea atmospheric coupling in driving this phenomenon.

Results
Circum-Antarctic distribution characteristics of GEM and AMDEs

Here, we present the circum-Antarctic observations of GEM at a 5-min time resolution over the Southern Ocean, conducted from 26 November 2016 to 10 March 2017. The spatial coverage included the Ross Sea (150°W–160°E), Amundsen Sea (90°W–150°W), Bellingshausen Sea (60°W–90°W), Weddell Sea (30°W–60°W), Riiser-Larsen Sea (0°E–30°E), Cosmonauts Sea (30°E–60°E), Davis Sea (75°E–105°E), and D’Urville Sea (120°E–150°E). GEM concentrations ranged from 0.20 to 1.80 ng/m3, with a mean value of 0.88±0.23 ng/m3 and a median value of 0.88 ng/m3. The distribution histogram shows that GEM concentrations were primarily within 0.80–0.90 ng/m3 (26.1%), followed by 0.90–1.00 ng/m3 (21.9%), 0.70–0.80 ng/m3 (16.3%) and 1.00–1.10 ng/m3 (11.5%) (Supplementary Fig. S1).

During this expedition, several summertime AMDE episodes were observed across the circumpolar Southern Ocean (Fig. 1a, b), based on the definition of Antarctic AMDEs as GEM concentrations below 0.60 ng/m3, following Pfaffhuber et al. and Angot et al.29,30. These summer AMDEs accounted for 7.6% of the total GEM observation period in this study. This proportion exceeds the previously reported 5% occurrence rate of spring AMDEs at the Antarctic Troll Station30, indicating that AMDEs represent a non-negligible phenomenon in the Southern Ocean during summertime.

The spatial distribution of GEM exhibited pronounced heterogeneity across the eight marine sectors surrounding Antarctica. The highest average GEM concentration (1.03±0.23 ng/m3) was recorded in the Cosmonauts Sea (30–60°E; Fig. 2), nearly 35% higher than the lowest average concentration (0.76±0.32 ng/m3, t-test: p<0.01) observed in the Ross Sea (Fig. 2). To further elucidate spatial distribution patterns and hotspot regions of summertime AMDEs over the Southern Ocean, we calculated the percentage of time AMDEs occurred across the eight circum-Antarctic marine sectors (Table 1). The results reveal pronounced regional contrasts in AMDEs frequency. The Ross Sea exhibited the highest proportion of summertime AMDEs (29%), corresponding to its lowest average GEM concentrations, followed by the Davis Sea (5%). In contrast, the Amundsen, Weddell, and D’Urville Seas all showed AMDEs occurrences below 1%, while no events were detected in the Cosmonauts, Bellingshausen, and Riiser-Larsen Seas. These findings highlight substantial spatial heterogeneity in the distribution of summertime AMDEs across the Southern Ocean, which underlies the marked spatial variability in atmospheric GEM concentrations during the summer months.

The role of land-sea interactions in the circum-Antarctic distribution of GEM

Why do summertime AMDEs exhibit pronounced spatial heterogeneity around Antarctica? Previous studies have identified polar sea-ice zones as the primary regions for springtime AMDEs, where photochemically produced reactive bromine species at sea-ice interfaces drive significant GEM oxidation and depletion. However, 72-h backward air-mass trajectories during our circumpolar GEM observations reveal that low GEM concentrations predominantly corresponded to air masses originating from the Antarctic continent rather than from marine regions (Supplementary Fig. S2). This finding is further supported by Concentration-Weighted Trajectory (CWT) analysis, which shows that GEM-depleted air masses (CWT values <0.6 ng/m³) mainly originated from the Antarctic Plateau and western highland areas (Fig. 3a). These CWT results are consistent with the statistical analysis of GEM concentrations: both mean and median GEM levels decrease progressively with increasing mean terrain altitude along the corresponding 72-h backward air-mass trajectories (TERRMSL, indicative of Antarctic Plateau-originating air masses, Fig. 3b). When TERRMSL exceeds 2500 m, both mean and median GEM concentrations drop below 0.6 ng/m3, confirming strong linkages between high-altitude inland air masses from the Antarctic continent and AMDEs. In contrast, no apparent relationship is observed between GEM concentrations and local sea-ice fraction at the cruise observation sites (Fig. 3c). Across the circum-Antarctic observation period, the corresponding 72-h backward trajectories spent on average up to 35.9% over land (Traj_land), with a mean TERRMSL value of 616 m. For the Ross Sea region—where frequent AMDEs were observed (Table 1)—the average Traj_land and TERRMSL values are even higher (51.9% and 817 m, respectively). Meanwhile, due to substantial summer sea-ice melt and the avoidance of dense sea-ice areas during the voyage, the mean sea-ice fraction is only 6.9% over the entire observation period. The results collectively suggest that Antarctic geomorphological differences play a key role in influencing the occurrence of summertime AMDEs.

To gain deeper insights into key factors governing the circum-Antarctic spatial distribution patterns of summer GEM concentrations and of AMDEs, we used a Generalized Additive Model (GAM) to quantitatively evaluate the relative contributions of three characteristic Antarctic geophysical surface types—sea-ice zones (Sea-ice), open water zones (Open-water), and the Antarctic continent (TERRMSL)—as well as meteorological parameters, to GEM concentration variability around Antarctica. The results show that TERRMSL and atmospheric mixing depth (MIXDEPTH) together account for over 40% of the variability in GEM concentrations (Table 2).

Regarding geophysical surface types, TERRMSL has the greatest contribution to GEM variability (21.5%), significantly higher than that of other geophysical surface types, including Open-water and Sea-ice. This underscores the crucial role of air masses from the Antarctic continent in driving GEM variations around Antarctica. This influence is supported by the relationship between GEM concentrations and the proportion of time the corresponding 72-h backward trajectories spent over land (Traj_land) during the cruise, which shows that Traj_land typically increased to above 0.8 when summer AMDEs occurred (Supplementary Fig S3). Moreover, the partial response curve for TERRMSL shows a sharp decline in GEM concentrations when TERRMSL exceeds 2000 m, suggesting that air masses predominantly originating from the Antarctic highlands via katabatic winds are closely linked to GEM depletion (Fig. 4a). This finding is also consistent with the results of CWT analysis and the observed relationship between GEM and TERRMSL (Fig. 3). It is noteworthy that while katabatic wind transport occurred widely across Antarctica during the cruise, the most significant GEM depletions were observed primarily in the coastal areas of the Antarctic Ross Sea and Prydz Bay (Fig. 3a). These regions are “confluence zones”, located near the broad upslope areas of the Ross Ice Shelf and Amery Ice Shelf at 70°E26. In these zones, katabatic winds are intensified as air masses converge while moving from the high interior of the ice sheet, with maximum mean annual wind speeds in the lowest 100 m exceeding 20 m/s26. These zones also represent areas of concentrated northward mass flux26. Such conditions help preserve the original chemical state (e.g., concentration-depleted conditions) of atmospheric Hg as it is transported from inland Antarctica to the Antarctic coastal areas, and even to the nearshore Southern Ocean, with an influence range of ~20–60 km from the continent, as revealed by the observed GEM variations during the cruise.

Regarding meteorological factors, atmospheric mixing depth (MIXDEPTH) emerges as a key contributor, accounting for 19.2% of the GEM variation—second only to TERRMSL among all variables. The partial response curve for MIXDEPTH reveals that at very low levels (<400 m), GEM concentrations decline sharply with decreasing MIXDEPTH (Fig. 4b). Observational data further indicate that summertime AMDEs episodes are consistently associated with simultaneous decreases in MIXDEPTH (Supplementary Fig. S4a). This suggests two possible mechanisms: low atmospheric mixing depths may either trap Antarctic inland GEM-depleted air masses within the boundary layer, or facilitate the accumulation of atmospheric oxidants, thereby enhancing GEM oxidation. Notably, TERRMSL exhibits a negative relationship with MIXDEPTH, with both average and median MIXDEPTH values remaining low (~200 m) when TERRMSL exceeds 500 m (Supplementary Fig. S4b). This observational aligns with previous studies showing that the Antarctica Plateau maintains a persistently low planetary boundary layer height during the summer. Although the wind shear associated with katabatic winds can generate mechanical turbulence, the inhibitory effect of thermal stability driven by the radiative cooling of the surface snowpack typically far outweighs the lifting effect of mechanical turbulence. As a result, mechanical turbulence is confined to a very thin layer near the surface, ultimately leading to a shallow and stable boundary layer31–33. Consequently, the observed decrease in MIXDEPTH, which is likely closely linked to the transport of air masses from the Antarctic Plateau via katabatic winds, can further facilitate summertime AMDEs.

Potential mechanisms leading to a strong association between continental outflow and circum-Antarctic distribution of AMDEs

Building on the findings above, we explored the mechanisms through which air masses from the Antarctic highlands contribute to the pronounced spatial heterogeneity of GEM in the circum-Antarctic region. We observed a pronounced increase in gaseous oxidized mercury (GOM) concentrations (peaking at 201 pg/m³) in the Ross Sea region, concurrent with a decline in GEM concentrations (Supplementary Fig. S5). Backward trajectory analysis indicates that the air masses primarily originated from the Antarctic Plateau34. These results indicate that significant oxidation of GEM likely occurs over the Antarctic continent in summer, and subsequent transport of GEM-depleted air masses to the marginal seas contributes to the observed summer AMDEs. To further assess this mechanism, we analyzed three representative summertime AMDEs (E1–E3, Fig. 5) and identified three complementary processes involving continental outflow that could lead to GEM depletions:

The first process is near-surface GEM oxidation over the Antarctic Plateau and subsequent transport via katabatic winds. During the AMDEs around 4 February 2017 (E2 in Fig. 5), 72-h backward trajectories indicate that air masses originated from the upper troposphere or lower stratosphere (>4000 m), intruded over the Antarctic Plateau, and were subsequently advected toward the Southern Ocean by near-surface katabatic winds (Fig. 6a). Simultaneous GEM depletions were observed both at Dome C (Concordia Station, 75°06’ S, 123°20’ E, 3220 m above sea level) and over the Ross Sea, reaching similar concentrations (0.2–0.3 ng/m3, Fig. 6c). The 72-h trajectories at Dome C during this period remained within the near-surface layer (Fig. 6b), suggesting that GEM oxidation predominantly occurred close to the surface rather than aloft. Angot et al. reported similar summer AMDEs at Dome C driven by local oxidation18, while Song et al. confirmed through modeling that bromine-mediated two-step oxidation dominates GEM depletion at this site35. Observations of BrO levels (2-3 pptv) at Dome C by Frey et al.24 exceed the model upper limit (1 pptv) used by Song et al.35, with even higher BrO in the free troposphere. Holmes et al. also presented that the summer Antarctic troposphere exhibits elevated concentrations of reactive bromine, and high annual mean Hg(0) oxidation rates by bromine radical using GEOS-Chem model simulation36. In addition, the summer lower troposphere over the Antarctic Plateau is highly oxidizing, driven by photochemically produced reactive NO x, HONO, OH, HO 2, RO 2, and O 323,37,38. Thus, bromine-initiated GEM oxidation in the near-surface atmosphere of the Plateau is both chemically and observationally plausible.

Supporting this interpretation, snow mass (total snow storage on land) data along 72-h backward trajectories from the cruise track show elevated values during summer AMDEs, suggesting a possible contribution from snowpack photochemistry (Fig. 5). The concurrent O 3 increase observed during this event—unlike the depletions seen in springtime AMDEs—can be attributed to persistent inputs of O 3-rich air from the upper troposphere/lower stratosphere and in situ photochemical O 3 production from NO x emissions over the Antarctic interior39,40.

The second process is GEM oxidation in the lower troposphere over coastal Antarctica and ice-shelf regions. During the AMDEs around 1 February 2017 (E1 in Fig. 5), air masses primarily traversed the Ross Ice Shelf and coastal Antarctica within the lower troposphere. Long-term BrO observations at coastal Antarctic stations show that the snowpack on ice shelves and coastal regions is a significant source of reactive bromine41. GOME-2 BrO vertical column density maps for this period show elevated BrO levels along these trajectories (Supplementary Fig. S6). Therefore, GEM oxidation driven by coastal and ice-shelf bromine emissions, followed by lower-tropospheric transport, is another plausible pathway for summer AMDEs. In this case, due to limited O 3-rich air intrusion from the upper troposphere or lower stratosphere, O 3 decreased synchronously with GEM, consistent with catalytic loss via halogen chemistry (E1 in Fig. 5).

The third process is direct transport of GEM-depleted air from the upper troposphere/stratosphere. During the AMDEs around 13 February 2017 (E3 in Fig. 5), trajectories indicate that air masses from the Antarctic upper troposphere/lower stratosphere were transported at high altitudes toward the coast and then to the observation site via katabatic winds (Supplementary Fig. S7). Brooks et al. reported elevated gaseous oxidized mercury (GOM) at the South Pole linked to vertical transport from the Antarctic Plateau42. Holmes et al. used the GEOS-Chem model to analyze summer GEM depletion events at the Antarctic Neumayer Station, which were accompanied by increases in GOM and O 3. They attributed these events to bromine oxidation of Hg(0) in the Antarctic troposphere and the subsequent subsidence of the produced GOM and O 336. In addition, Frey et al. also observed higher BrO in the free troposphere than near the surface24. Thus, GEM oxidation in the upper troposphere followed by large-scale transport of GEM-depleted air to the Southern Ocean is also likely. The lack of O 3 depletion during this event can again be explained by the continuous supply of O 3-rich air from the upper layers.

In summary, our combined observations and trajectory analyses suggest that summer GEM depletion over the Southern Ocean results from bromine-initiated oxidation occurring across multiple Antarctic environments: (1) the near-surface atmosphere of the Plateau, (2) coastal and ice-shelf regions, and (3) the upper troposphere/lower stratosphere. The depleted air masses are then advected to the Southern Ocean via upper-level intrusions and katabatic winds. Reactive bromine likely originates from both snowpack emissions and upper-atmospheric inputs. The variable O 3 behavior during AMDEs reflects the changing contributions from bromine-driven O 3 loss, and O 3 replenishment from O 3-rich air intrusions and in situ photochemical production. These processes collectively explain the observed summer AMDEs and the associated O 3 response in a consistent framework.

It is noteworthy that Shah et al. proposed a new chemical mechanism for atmospheric mercury cycling and incorporated it into the GEOS-Chem global atmospheric chemistry model for comparison with observational data43. Their study revealed that Br and OH contribute comparably to the global net oxidation of Hg(0) to Hg(II), while O 3 serves as the primary oxidant for Hg(I), facilitating efficient Hg(0) oxidation via OH43. However, Shah et al. did not perform region-specific simulations for Antarctica. Given the inflow of upper tropospheric air masses over the Antarctic continent, along with active photochemical processes occurring on the snowpack surface during summer—which can enhance O 3 and OH concentrations—the relevance of this chemical mechanism in the Antarctic region merits further investigation.

Implications for the Antarctic Hg cycle

As noted above, GEM on the Antarctic continent undergoes significant photochemical oxidation during the summer polar day, forming atmospheric oxidized Hg (Hg(II)), which has higher water solubility and a shorter atmospheric residence time. Once produced, Hg(II) can be transported to the surrounding Southern Ocean via katabatic winds, where high humidity and increased precipitation in the marine atmosphere promote its deposition. These processes suggest that the Antarctic continent acts as a significant source of atmospheric Hg(II) in summer, while the adjacent Southern Ocean serves as an important sink. Katabatic winds play a critical role as a transport pathway connecting these sources and sink regions.

It is noteworthy that summer coincides with the breeding season and that the Southern Ocean serves as a vital habitat for Antarctic species, including krill, penguins, whales, and seals. Key colonies of Adélie and emperor penguins, such as those in Victoria Land and Amanda Bay, are located near the Ross Sea—the world’s largest marine protected area—and Prydz Bay, areas identified in this study as typical confluence zones of katabatic winds and sites of significant GEM depletion26. Additionally, spatial analyses of Hg concentrations in Adélie penguin feathers have shown the highest Hg levels in the Ross Sea region44.

These findings suggest that the transport of Hg(II) from the Antarctic interior via katabatic winds may influence Hg exposure levels in high-latitude Southern Ocean ecosystems during critical breeding periods. Furthermore, based on the circumpolar GEM observations of this study, we emphasize the potential of Antarctic land-sea interactions to impact atmospheric chemistry in the nearshore Southern Ocean, warranting further investigation.

Methods
Study site

We conducted continuous circum-Antarctic observations of GEM over the Southern Ocean from November 26, 2016, to March 10, 2017, as part of the 33rd Chinese National Antarctic Research Expedition aboard the icebreaker R/V Xuelong. The study area encompassed most of the seas surrounding the Antarctic continent and was divided into eight regions: the Ross Sea (150°W–160°E), Amundsen Sea (90°W–150°W), Bellingshausen Sea (60°W–90°W), Weddell Sea (30°W–60°W), Riiser-Larsen Sea (0°E–30°E), Cosmonauts Sea (30°E–60°E), the Davis Sea (75°E–105°E), and D’Urville Sea (120°E–150°E). These regions span a latitude range exceeding 20 degrees (58°S–79°S). The observations encompassed diverse hydrological environments, including coastal Antarctic areas, sea-ice zones, and open waters. The cruise track is shown in Fig. 1.

Experimental methods

Atmospheric GEM was measured using a TekranTM 2537B Hg analyzer based on the principle of cold vapor atomic fluorescence spectroscopy (CVAFS). Detailed descriptions of the methodology can be found in Yue et al.45 and are only briefly presented here. The air inlet for the Hg analyzer was mounted at the front of the R/V Xuelong (~30 m above the sea surface) to minimize contamination from the ship’s exhaust plume. Atmospheric Hg was continuously collected on two gold cartridges operating alternately at a constant flow rate of 1 L·min−1, thermally desorbed at 550°C, and detected via CVAFS. The inlet system was equipped with two soda lime tubes and two 0.45-μm Teflon filters to remove moisture and coarse particles. Daily calibration of the analyzer was conducted using an internal Hg permeation source, while external calibrations were performed via manual injections of known GEM concentrations using a TekranTM 2505 unit before and after the cruise. Both external calibrations demonstrated accuracy exceeding 94%. The detection limit (DL) of the TekranTM 2537B instrument for GEM measurement is below 0.10 ng·m−3.

Continuous measurements of Gaseous Oxidized Mercury (GOM) were carried out during the subsequent 34rd Chinese National Antarctic Research Expedition using an automated mercury speciation system. The system comprised a TekranTM 2537B Hg analyzer coupled with a TekranTM 1130 GOM unit. Operationally, GOM was defined as the fraction of oxidized mercury captured by a KCl-coated annular quartz denuder. Measurements were taken at a 2-h temporal resolution. The sampling flow rate for GOM was maintained at 7 L·min⁻¹, and coarse particles larger than 2.5 μm were removed at the inlet using a heated impactor. The method detection limit (DL) for GOM was estimated to be below 0.89 pg·m⁻³ based on sampling volumes. Although no calibration standards are available for GOM, its 1σ precision is ~15%46. More details of this method can be found elsewhere46.

Laurier et al.47 and Beck et al.48 have shown that contamination from vessel emissions typically occurs when the apparent wind direction angle (AWD) falls within the range of 120°–240°. In this study, 91% of the AWD values were outside this range, and most instances of elevated GEM concentrations corresponded to sparse AWD points (Supplementary Fig. S8a). In addition, no correlation was observed between GEM and CO levels, with elevated CO generally corresponding to low GEM concentrations (Supplementary Fig. S8b). These findings suggest that the impact of vessel exhaust emissions on GEM measurements during this cruise was minimal.

Other ancillary data and analysis

Ozone (O 3) and carbon monoxide (CO) were continuously measured using a Thermo FisherTM 49i O 3 analyzer and a Thermo FisherTM 48i CO analyzer, respectively. Navigation parameters were obtained from onboard monitoring systems.

Hourly data for sea-ice fraction (the proportion of sea-ice coverage per grid cell, Sea-ice), the land‒water–ice index (LWI), solar radiation (SUNFLUX, W/m2), and snowmass (total snow storage on land, kg/m2) were sourced from the assimilated meteorological dataset of the Goddard Earth Observing System (GEOS)—the second Modern-Era Retrospective Analysis for Research and Applications (MERRA-2), with a horizontal resolution of 2° × 2.5°. The open-water fraction (Open-water), representing the proportion of sea area without sea-ice cover in the corresponding 2° × 2.5° grid during the cruise, was calculated as follows:

Open-water fraction = Ocean fraction − Sea-ice fraction (1)

The HYSPLIT transport and dispersion model from the NOAA-Air Resources Laboratory, driven by Global Data Assimilation System (GDAS) meteorological data (1°×1°), was used to generate 72-h backward trajectories of air masses for the entire cruise. Considering that the GEM observations in this study were conducted at ~35 m above sea level, the height of the backward trajectory was set to 50 m to approximate the observational altitude. A total of 1648 trajectories were generated, enabling detailed statistical analyses of the air mass characteristics. These included the average values of land surface altitude (TERRMSL, m), land coverage fraction (Traj_Land), relative humidity (RH), air temperature (Airtemp, K), atmospheric mixing depth (MIXDEPTH, m), wind speed (WS, m), atmospheric pressure (P, hPa), and snowmass (total snow storage on land, kg/m2) encountered along each trajectory arriving at the ship location.

Atmospheric GEM measurements obtained at Concordia Station on the Antarctic Plateau were used to support the hypothesis of GEM-depleted air masses originating from inland regions (Fig. 6c). As detailed in previous studies18,30, continuous monitoring was carried out using a Tekran 2537 automated analyzer following strict quality assurance and control procedures.

Evaluating the relative importance of influencing factors

To assess the relative importance of various factors influencing the circum-Antarctic distribution of GEM, we used a generalized additive model (GAM). This data-driven, semiparametric model accommodates both linear and nonlinear relationships, making it suitable for complex environmental datasets. GAM was implemented using the “mgcv package” in R and can be expressed as:

g(μ) = f 1(x 1) + f 2(x 2) + ... + f n(x n) + ε (2)

Here, x i (i=1, 2, 3, ..., n) are the predictors, f i represents the corresponding smooth function, μ is the expected dependent variable, ε is the residual, and g is the link function relating the nonlinear formulation to the dependent variable. Since the observed GEM concentrations followed a Gaussian distribution, an “identity link” function with Gaussian distribution was used. Detailed explanations of the method are provided in Zhang et al.49 and Yue et al.45.

To validate the effectiveness of the predictors in the GAM, we evaluated R2 values, F-values, and the Akaike Information Criterion (AIC). Higher R2 values, greater F-values, and lower AIC values indicate improved model fit, greater sensitivity of added predictors, and appropriate parameter selection, respectively.

This study focused on two categories of predictors to understand their impact on GEM distribution:

(1) Land surface types in the Southern Ocean: these predictors account for emissions and/or transport associated with distinct Antarctic geographical and hydrological environments, including the Antarctic Plateau (TERRMSL), open water areas (Open-water), and sea-ice areas (Sea-ice).

(2) Meteorological factors: these include wind speed (WS), relative humidity (RH), air temperature (Airtemp), atmospheric mixing depth (MIXDEPTH), and atmospheric pressure (P).

Prior research suggests that GAM performs robustly when the fitting R2 value exceeds 0.549,50. The constructed GAM model explained 53% of the variance in GEM concentration, achieving a fitting R2 of 0.526. The results effectively captured GEM variability throughout the cruise, indicating good model performance (Supplementary Fig. S9a). Both predictor categories satisfied the AIC (Supplementary Fig. S10d) and demonstrated signification contributions (p<0.001).

Model validation

The reliability of the GAM was systematically evaluated using multiple methods. First, a 5-fold cross-validation test was conducted to assess the model’s accuracy51. In this test, four subsets were used to fit the model, while the remaining subset was reserved for prediction. This process was repeated 5 times to ensure each subset was tested50. The results showed good agreement between the GAM and the cross-validated predictions (slope=0.97, R2=0.94), demonstrating the model’s robustness (Supplementary Fig. S9b)52. Additionally, the following diagnostic methods were applied to test key assumptions of the GAM-homogeneity, normality, and independence: (1) quantile‒quantile (Q‒Q) plots comparing sampling quantiles against theoretical quantiles, (2) scatterplots of residuals against linear predictors to assess the distribution and trend of residuals, and (3) histograms of residuals to evaluate the normality of residuals49,50.

The Q‒Q plot indicated that the GAM performed well around the average GEM concentration (Supplementary Fig. S10a). The histogram of residuals closely resembled a normal distribution, indicating that model errors were random (Supplementary Fig. S10b). Furthermore, the scatterplot of residuals vs. linear predictors showed that residuals were generally centered around zero, with a random distribution and no discernible trend, suggesting unbiased predictions (Supplementary Fig. S10c). Overall, these analyses confirmed the GAM’s reliability and the appropriateness of the selected predictor variables.

CWT analysis

Concentration-weighted trajectory (CWT) analysis was used to identify potential source regions and transport pathways of GEM in the Southern Ocean during the observation period. This method integrates hourly GEM concentrations with 72-h backward trajectories. The 72-h trajectory domain was divided into grid cells with a 0.5°×0.5° resolution. The CWT value for each ij th cell was calculated as follows:

CWTij = (Σ l=1 m C l τijl) / (Σ l=1 m τijl) (3)

where CWTij denotes the weighted concentration of GEM in the ij th cell, l represents the index of each trajectory, m is the total number of trajectories, C l denotes the corresponding GEM concentration observed when trajectory l arrived, τijl is the time spent by trajectory l in the ij th cell, and Wij is the empirical weight function to minimize the influence of small Nij (number of trajectory endpoints in the ij th cell)53. A higher CWTij value indicates that elevated GEM concentrations at the receptor sites are associated with air masses traversing the ij th grid cell. The weight function Wij, as outlined in Ahmad et al.54, was applied to reduce the bias caused by cells with a limited number of endpoints, which was also shown below:

Wij = 1.0 (Nij ≥ 3Nave)
Wij = 0.7 (3Nave > Nij ≥ 1.5Nave)
Wij = 0.4 (1.5Nave > Nij ≥ Nave)
Wij = 0.2 (Nave > Nij)

in which Nave represents the average number of trajectory endpoints in each grid cell.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Atmospheric mercury depletion events (AMDEs) are important phenomenon in polar mercury cycling. This manuscript reported the first observational dataset of gaseous elemental mercury (GEM) across the circum-Antarctic Southern Ocean, and utilized the generalized additive model (GAM) to quantify the contributions of key factors to AMDEs. Katabatic winds were identified to play the key role in AMDEs. However, I still have several major concerns:\n1. The illustration for the mechanism of the major contributor is not quite consistent throughout the manuscript. In the abstract, the role of reactive halogen oxidation is claimed to be minor. In Lines 285–288, halogen seems to be one of the key oxidants (just not the only one), and vertical mixing of GEM depleted and O3 rich air masses from upper troposphere in Antarctica plateau are the primary drivers.\n2. The expression of the vertical movements of air mass is also confusing. Are the katabatic winds the same as the intrusion from the upper troposphere or lower stratosphere? In Lines 300–307, it seems to be the case. However, in Line 328, the katabatic winds seem to be connected with high humidity, which during the upper tropospheric or lower stratospheric intrusion, the humidity in air mass often gets lower.\n3. The GAM model is a useful tool. However, the predictor TERRMSL could be a proxy for other factors (not limited to one). More evidence should be provided to support the speculation the authors proposed.\n4. My speculation is that the AMDEs are still most impacted by active halogen from upper tropospheric or lower stratospheric air mass intrusion. Ozone is also rich in the intrusion, which offsets its consumption by active halogen. That’s why ozone concentration is not largely reduced or even increases. Whether my speculation is correct or not, the authors need to provide more solid and consistent illustration.\nTherefore, overall in my opinion, major revision is required before the manuscript is acceptable for publication on Nature Communications.\nHere are some specific comments:\n1. Line 25: It should be “involving” instead of “involve”.\n2. Line 42: It should be “pollutant” instead of “pollutants”.\n3. Lines 109–112: The description of the histogram is not very useful. Since the mean value and the median value are very close, I assume the GEM concentration fits the normal distribution. The normality test result might be more useful here to be connected with the configuration of the GAM model.\n4. Figure 1, Figure 2 and Table 1: The order of the regions had better be consistent.\n5. Lines 199–204: TERRMSL is not necessarily linked only to katabatic winds. It can also be a (partial) proxy for the concentration of oxidants, especially active halogen, which could be derived from upper tropospheric or lower stratospheric intrusion. Since there is no observational data for halogen, it is a bit impetuous to draw this conclusion.\n6. Lines 273–276: Another explanation could be that upper tropospheric or lower stratospheric ozone intrusion provides more ozone which offsets the consumption by active halogen.", "reviewer_2": "[Overview]\nUsing shipboard measurements of gaseous elementary mercury (GEM) in the surface air conducted across the Southern Ocean and coastal seas nearly circling around Antarctica combined with the generalized additive model (GAM) simulation, the authors demonstrate that atmospheric mercury depletion events (AMDEs) occur frequently during summer in the atmospheric boundary layer over coastal seawater around Antarctica and that these events are most likely the consequence of transport of air having been advected down via katabatic winds over the sloping Antarctic plateau surfaces. Springtime AMDEs have been reported extensively in the polar regions of both hemispheres, which have been attributed generally with high confidence to the photochemistry of bromine radicals sourced from frozen materials containing bromide of seawater origin. As the authors discussed in the present study, summertime AMDEs essentially do not take place in the Arctic, whereas their occurrences have been sometimes reported over inland and coastal Antarctic stations. The causes of summertime AMDEs are still under debate, but photochemical processes other than that of bromine sourced from surface frozen materials most likely play a role. Although this chemical process aspect remains speculative in the present study, its novelty lies in the demonstration of prevalence of summertime ADMEs over the coastal Southern Ocean, not within Antarctica itself, and their strong association with rapid air mass transport via katabatic winds, from the extensive spatial coverage of field data reported here. These AMDEs are indeed potentially important for mercury input to the aquatic ecosystem in the Southern Ocean. Their conclusion is well supported by the data analysis conducted by the authors. The quality of data acquisition and post analysis method applications appears to meet the expected standards in the field; here, I note significant overwraps in terms of data analysis methodology (GAM simulation and trajectory-based footprint analysis) with another article published recently in Nature Communications (Yue et al., 2023), meaning that their methodology has once passed scrutinization by editor and referees for the same journal. I would support the publication of the present study after clarifications and additions to the text as suggested below.\n[Major comments]\n1. For atmospheric mercury depletion events (AMDEs) to contribute to the deposition of atmospheric mercury onto the ocean, rapidly depositing oxidized mercury, which is indicated by the authors to have been generated somewhere over Antarctica, must be remaining in the near-surface air when it exits to the ocean without too much deposition onto the land/glacier surfaces. This point is not discussed clearly enough in the current version of the manuscript and should be stressed in discussion and conclusions of the main text. I note that data showing the abundance of atmospheric oxidized mercury in the gas phase (GOM) measured during the shipboard expedition are presented only in the supplement (Figure S4). In my opinion, this figure is as equally important as other figures currently included in the main text. Unless there is a space limitation that prevents it, I encourage the authors the relocation of Figure S4 to the main text. Talking about GOM, I think I do not see a method description of specification between GEM and GOM as to whether it was attempted in the sampling of atmospheric mercury in the first place and, if it was, how (e.g., lines 410-413). The authors should include this method description regardless of their decision for where to present their GOM data in the revised manuscript.\n2. It is fair to cite two earliest observational studies for summertime AMDEs (GEM depletions accompanied by GOM increases) at Antarctic coastal sites (Sprovieri et al., 2002; Temme et al., 2003); the present study by the authors obviously builds upon findings reported therein. The authors should also refer to a study by Holmes et al. (2010) (see their Section 3.3 and Figure 8) where they discussed their interpretation of model-simulated summertime variability of gaseous elemental and oxidized Hg at the Antarctic coastal site (Neumeyer) in relation to the Temme study. The Holmes study backs the association between the Hg oxidation via Br radical chemistry in the free troposphere and Antarctic coastal AMDEs during the summer, in accordance with one of the hypotheses mentioned in the present study.\n[Technical suggestions]\n1. Line 58: The authors should add ‘snowpacks’ to the list of interfaces that source reactive halogen species (e.g., Pratt et al., 2013; Toyota et al., 2014).\n2. Line 64: The authors should consider citing a review article on AMDEs including their observations by Steffen et al. (2008).\n3. Line 86: According to BrO measurements at Dome C from ref. 20, the BrO mixing ratios were higher in the free troposphere than in the surface air, suggesting that local snowpacks at Dome C were not a strong source of gaseous reactive bromine compounds. This nuance should be expressed in this sentence. Also, it probably aligns with the authors’ argument that snowpacks on the Antarctic plateau are unlikely to be a major supplier of gaseous bromine to the near-surface air carried down by katabatic winds.\n4. Line 108: Please specify the time resolution of GEM measurements here. Hourly?\n5. Line 115: Please rephrase “referring” to “according”.\n6. Line 196: Perhaps the authors should drop “(29%)” from this line and keep “(21.5%)” on the next line.\n7. Line 209-211: The sentence might read better by saying instead: “…, katabatic winds are intensified as air masses converge while moving from …”. I am also wondering if this intensification facilitates the efficient downward and northward air-mass transport without much vertical mixing not only from inland to coastal areas but also a little further from coastal areas off the coast towards the ocean; please add this nuance if it is rationalized from the literature.\n8. Line 243: Change the comma to the period.\n9. Line 261-262: I feel that the section title could be rephrased to something like: “Potential mechanisms leading to a strong association between continental outflow and circum-Antarctic distribution of AMDEs”.\n10. Line 342: As I mentioned in my major comments above, I believe that the transport of “oxidized” Hg (rather than GEM) is what matters for the Hg exposure in the Southern Ocean ecosystems implicated from data reported in the present study. The authors should be clear about this point.\n11. Line 359: A typo? The authors may want to change “30 degrees” to “20 degrees”.\n12. Lines 401-409: The description of backward trajectory calculations lacks information about an altitude from which the trajectories are calculated back in time. Please specify this altitude and give a rationale for the choice. Also, I would rephrase the last phrase of this paragraph from “during the cruise” to “arriving at the ship location”.\n13. I suggest the authors consider citing the methodological foundation of the PSCF analysis in particular on the empirical weight functon(e.g., Polissar et al., 1999), along with specific parameter choices employed in this study based upon Ahmad et al. (ref 41).", "reviewer_3": "Overall, this is good paper and warrants publication in this journal. The new analysis and discovery of the source of AMDEs to the southern oceans from catabatic winds is novel. This information will be very useful to the research community and will help to understand the impacts of mercury pollution in the vulnerable region of Antarctica. I would like to see the authors discuss the chemistry a bit more in the difference between the traditional AMDEs and the catabatic AMDEs. I tended to get a little lost in the figures and the explanation of them so I would clean them up a bit to be consistent and have a bit more information for the reader to follow this complex analysis. The snow concentrations are shown but not discussed, maybe a small discussion about these measurements is warranted. The work supports the conclusions, with the suggestions included in this review. The methods used are sound and are well known and accepted in the atmospheric mercury research community.\nFigures 1a and b – what is the sampling frequency of the data shown?\nFigure 1b. is there a way to show the time of the year when the samples were collected on the map. The reader doesn’t necessarily know which sea is which and the table is not as useful as an image.\nFigure 2 – same sort of comment as Figure 1b. What times of the year are these averages? Over what period are these averages and are they the same for each sea?\nLine 163 – one important feature is the stable boundary layer. Does this area have a stable boundary layer at this time when you see these AMDEs? I would imagine that strong catabatic winds are not conducive to a stable boundary layer, where the bromine explosion occurs.\nFigures – suggestion: please use the same colour scheme for the concentrations throughout the paper. It makes it far easier to follow.\nAntarctica is filled with Glaciers, what role if any do you think they play in these findings?\nPlease have someone go over the grammar, it’s okay but there are visible mistakes in the text.\nFigure 6a- what are the units? What blue boxes? I only see grey and pink.\nFigure S4 – can you please indicate which is land and which is ocean in the figure?\nLine 269 – The grey box seems a “typical” depletion event and the other 3 are meteorologically modulated from the continent to the monitoring location. Are the back trajectories showing this in Figure 6b and c? Can you specify the location where the trajectories start? To me this looks like the start is the continent and the back trajectories are over the ocean. So, I am a little confused.\nLine 302 – are you suggesting that the primary reaction for the oxidation of mercury from the inland air masses is from OH rather than Br? Can you please include a discussion of the chemistry differences here between these 2 processes. I think that would help the discussion explanation a lot.\nLine 331 – were any water samples collected? Would you see that in the water or maybe snow as is seen in the Arctic during the latter seasonal AMDEs?\nLine 410 – were they shown in this paper? I think it would be very helpful to see that relationship shown with these results.\nFigure S5 – the axes are too hard to read\nFigure S8 – I don’t understand what that is so I can’t comment on its scientific rigor.", "reviewer_4": "This paper presents a significant advancement in our understanding of atmospheric mercury cycling in Antarctica by providing the first circum-Antarctic dataset of gaseous elemental mercury (GEM) concentrations in the Southern Ocean. The study's most notable innovation lies in its demonstration that summertime low atmospheric mercury concentration during some observation period. Authors gave their explanation and hypothesis for the specific mechanisms that the distinct spatial patterns linked to continental outflow from the Antarctic Plateau via katabatic winds. Through sophisticated trajectory analysis and generalized additive modeling, the authors convincingly show that terrain altitude and atmospheric mixing depth are the dominant controls on GEM distribution, rather than local sea-ice processes that drive Arctic AMDEs. This represents an important paradigm shift in polar mercury research and highlights the unique atmospheric chemistry of Antarctica's ice sheet-dominated environment. From this poinit, I strongly support this study.\nAdditionally, I also have the feeling that the study could have the limitation, or something not good as the authors' hypothesis. First, from my knowledge, during AMDEs, we always or at least most time refer to low GEM but high GOM concentration. Or in the forest, the uptake of GEM by foliage leading to low GEM concentration in the air sometime. This means there must be some Hg biogeochemical pathways to deplete the GEM concentration. However, this study did not measure the GOM concentration, and we did not know how change of GOM-GEM concentration. This would lead to a situation that if the wind of inland of Antarctic Plateau bringing the low GEM concentration (maybe this low GEM concentration from very high airmass of the Antarctic Plateau) to mix with air near to the sea, we would also observe low GEM concentration. Under this situation, it likely reflects the mixing effects between “clean” high air mass and “dirty” near sea air mass. Thus, this would be not the “AMDEs” in my opinion.\nDespite what’s my concerned (it may be not right), the study at least makes a substantial contribution to polar science by revealing how Antarctica's extreme geography and meteorology shape mercury cycling in ways fundamentally different from the Arctic.\nHere are line comments\nLine 34-36. It is not easy to understand the term of “katabatic wind convergence zone” to shape the AMDEs in the Antarctic region.\nLine 65-69. Here points that evidence of springtime AMDEs in Antarctica were reported by several previous studies. In the abstract, authors stated that their study presents the first observational dataset of atmospheric gaseous elemental mercury (GEM). So, it seems that two statements are conflict.\nLine 107-108: The claim of \"first circum-Antarctic observations\" is significant but would benefit from clarifying how this dataset compares spatially/temporally to previous regional studies (e.g., Neumayer Station data referenced in Line 67).\nLine 116: The 0.60 ng/m³ AMDE threshold seems appropriate based on cited literature, but the rationale for not using a relative depletion percentage should be briefly explained.\nLine 130-146: The Ross Sea's exceptional AMDE frequency (28.9%) deserves more discussion, ice shelf extent, ocean currents play a role?\nLine 196-203: The GAM results showing TERRMSL's dominant influence (21.5%) are compelling, but the physical interpretation of why terrain altitude matters more than sea-ice (6.1%) could be expanded.\nLine 267-279: The ozone-GEM relationship inconsistencies are intriguing - could this reflect different oxidation pathways (halogen vs. NOx/OH) operating in different air masses?\nLine 510-513: Data accessibility is excellent, but including instrument detection limits and precision metrics in the methods (Line 365-378) would help others evaluate data quality."}, "author_response": "Responses to reviewers’ comments\nDear Reviewers:\nThank you for your positive comments on our manuscript entitled “Continental\noutflow shapes the circum-Antarctic pattern of summertime atmospheric mercury\ndepletion zones” (manuscript ID: NCOMMS-25-44802) and for offering us an\nopportunity for revision. We believe the constructive comments have helped us\nimprove the overall quality of the manuscript. Below are our detailed responses to\nreviewers’ questions. Revised texts of the manuscript are marked in red in the responses\nto comments.\nReviewer #1:\nAtmospheric mercury depletion events (AMDEs) are important phenomenon in polar\nmercury cycling. This manuscript reported the first observational dataset of gaseous\nelemental mercury (GEM) across the circum-Antarctic Southern Ocean, and utilized\nthe generalized additive model (GAM) to quantify the contributions of key factors to\nAMDEs. Katabatic winds were identified to play the key role in AMDEs. However,\nI still have several major concerns:\nQ1. The illustration for the mechanism of the major contributor is not quite consistent\nthroughout the manuscript. In the abstract, the role of reactive halogen oxidation is\nclaimed to be minor. In Lines 285–288, halogen seems to be one of the key oxidants\n(just not the only one), and vertical mixing of GEM depleted and O3 rich air masses\nfrom upper troposphere in Antarctica plateau are the primary drivers.\nRe. Thank you for your comment. We apologize for the unclear wording in our abstract\nthat led to this misunderstanding. Our intention was not to deny the importance of\nreactive halogen oxidation for summer AMDEs over the Southern Ocean. Rather, we\n1\naimed to convey that the conventional spring AMDE mechanism—GEM oxidation\ndriven by reactive bromine released from the sea-ice interface—plays only a minor role\nin the summer AMDEs observed in this study. These summer events are primarily\ndriven by bromine-initiated oxidation of GEM over inland Antarctica and its\nsubsequent transport, rather than processes occurring at the sea ice interface. To avoid\nambiguity, we have removed the original phrasing from the abstract in the revision. In\naddition, we have expanded the discussion around lines 284-388 to clarify the potential\nmechanisms driving the observed summer AMDEs over the Southern Ocean.\nSpecifically, the AMDEs observed in the marginal seas of Antarctica during summer\nare likely driven by significant bromine-initiated oxidation under low-temperature and\nhigh-oxidant (e.g., NO , ∙OH) conditions over inland Antarctica rather than over the\n2\nocean. Potential sites for this oxidation process include: (1) the near-surface atmosphere\nover the Antarctic Plateau, (2) coastal and ice-shelf regions of Antarctica, and (3) the\nupper troposphere/lower stratosphere above Antarctica. The resulting oxidized mercury\nspecies may then be transported to the surrounding Southern Ocean via air mass\nintrusions from the upper troposphere and through Antarctic katabatic winds (see\ndetailed analyses in the response to Q4). When air masses originate persistently from\nthe upper troposphere/lower stratosphere over inland Antarctica, the continuous influx\nof ozone-rich air can replenish ozone levels, preventing synchronous depletion of ozone.\nAccordingly, we have revised both the abstract and the discussion to ensure consistency\nwith the main viewpoints presented throughout the manuscript.\nQ2. The expression of the vertical movements of air mass is also confusing. Are the\nkatabatic winds the same as the intrusion from the upper troposphere or lower\nstratosphere? In Lines 300–307, it seems to be the case. However, in Line 328, the\nkatabatic winds seem to be connected with high humidity, which during the upper\ntropospheric or lower stratospheric intrusion, the humidity in air mass often gets lower.\ntroposphere or lower stratosphere and the transport by Antarctic katabatic winds during\nthe study period likely represent sequential processes. Specifically, the intrusion of air\nmasses from the upper troposphere / lower stratosphere primarily occurred over the\nAntarctic interior or coastal regions. Subsequently, the katabatic winds transported\nthese near-surface air masses from the plateau or coastal areas downslope toward the\nsurrounding Southern Ocean (see Figure 4a in this response document). Both the\nintruding air masses from the upper troposphere / lower stratosphere and those carried\nby katabatic winds were characterized by exceptionally low humidity. The high-\nhumidity environment mentioned in Line 328 does not refer to the air masses\ntransported by katabatic winds, but rather to the atmosphere over the Southern Ocean\nsurrounding Antarctica. In this context, the dry katabatic winds originating from inland\nAntarctic transport oxidized mercury into the more humid marine boundary layer over\nthe Southern Ocean, where the higher humidity favors enhanced mercury deposition.\nQ3. The GAM model is a useful tool. However, the predictor TERRMSL could be a\nproxy for other factors (not limited to one). More evidence should be provided to\nsupport the speculation the authors proposed.\nRe. Thank you for this constructive comment. Regarding TERRMSL, it represents the\nmean surface elevation along the 72-hour backward trajectory of the air masses. One\ndefining feature of Antarctic katabatic winds is their origin from the high-elevation\ninterior of the Antarctic plateau. This distinguishes them from air masses originating\nover the Southern Ocean (where surface elevation is 0), making TERRMSL a direct\nand effective indicator for identifying katabatic wind influence. To further support this\ninterpretation, we calculated the proportion of time that the 72-hour backward\ntrajectories spent over land (Traj_land) during the cruise period. The results show that\nduring summer AMDEs, Traj_land typically exceeded 0.8 (as shown by the yellow\nshaded areas in the Figure 1 below). This suggests that the observed summer AMDEs\nwere primarily influenced by air masses originating from the Antarctic interior. When\ncombined with the Concentration Weighted Trajectory (CWT) analysis presented in the\nmanuscript, these results provide additional evidence supporting our interpretation of\nthe dominant influence of inland Antarctic air masses. We have now incorporated this\ndiscussion into the revised manuscript, as shown below:\nLine 209-213: “This influence is supported by the relationship between GEM\nconcentrations and the proportion of time the corresponding 72-hour backward\ntrajectories spent over land (Traj_land) during the cruise, which shows that Traj_land\ntypically increased to above 0.8 when summer AMDEs occurred (Figure S3).”\nFigure 1. Time series of gaseous elemental mercury (GEM, ng/m3) and the proportion of time that\nthe corresponding 72-hour backward trajectories spent over land (Traj_land) during the summertime\natmospheric mercury depletion events (AMDEs) in this study.\nupper tropospheric or lower stratospheric air mass intrusion. Ozone is also rich in the\nintrusion, which offsets its consumption by active halogen. That’s why ozone\nconcentration is not largely reduced or even increases. Whether my speculation is\ncorrect or not, the authors need to provide more solid and consistent illustration.\nTherefore, overall in my opinion, major revision is required before the manuscript is\nacceptable for publication on Nature Communications.\nRe. Thank you very much for this insightful comment. We agree that some of the\nobserved AMDEs may indeed be most strongly influenced by active halogen chemistry\nassociated with air mass intrusions from the upper troposphere or lower stratosphere\nover the Antarctic interior. Regarding the role of reactive halogen oxidation in\nsummer AMDEs, previous studies support this mechanism. For example, Holmes et al.\n(2010) analyzed summer GEM depletion events accompanied by concurrent increases\nin GOM and O at the coastal Neumayer Station using the GEOS-Chem model and\n3\nfound that simulations incorporating bromine oxidation pathways best reproduced the\nobserved features, rather than those based on O and ∙OH oxidation pathways1.\ntwo-step GEM oxidation mechanism provided the best agreement with summer GEM\nvariations observed at the inland Concordia Station2. Chemical kinetics studies also\nsupport this mechanism: the rate constant for the Hg(0) + Br (3.2×10-12 cm3\nmolecule−1 s−1) is significantly higher than for Hg(0) + OH (3.2×10-13 × (T/298) -3.06\ncm3 molecule−1 s−1) or Hg(0) + O (1.7×10-18 cm3 molecule−1 s−1) at typical Antarctic\n3\nsummer temperatures3, 4. These findings indicate that bromine oxidation can be a\ndominant pathway under such conditions.\nFurthermore, we observed a pronounced increase in gaseous oxidized mercury\n(GOM) concentrations (peaking at 201 pg/m³) in the Ross Sea region, concurrent with\na decline in GEM concentrations. Backward trajectory analysis indicated that the air\n5\nmasses primarily originated from the Antarctic Plateau (Figure 2 in this response\ndocument)5. Taken together, these results indicate that bromine-initiated oxidation of\nGEM likely occurs over the Antarctic continent in summer, and subsequent transport of\nGEM-depleted air masses to the marginal seas contributes to the observed summer\nAMDEs.\nTo further assess this mechanism, we analyzed three representative summertime\nAMDEs (E1-E3, Figure 3 of this response document) and identified three\ncomplementary processes involving continental outflow that could lead to GEM\ndepletion:\nFigure 2. (a) Time series of gaseous oxidized mercury (GOM), gaseous elemental mercury (GEM)\nand O concentrations in Ross Sea in December, 2017, and (b) spatial distribution of GOM in Ross\n3\n(total snow storage on land, kg/m2), average land surface altitude (TERRMSL, m), and atmospheric\nmixing depth (MIXDEPTH, m) for the corresponding 72-hour backward trajectory during GEM\ndepletion events. The gray boxes represent three typical atmospheric mercury depletion events\n(AMDEs) periods (E1-E3) during the cruise.\n(1) Near-surface GEM oxidation over the Antarctic Plateau and subsequent\ntransport via katabatic winds: During the AMDEs around 4 February 2017 (E2 in\nFigure 4 of this response document), 72-hour backward trajectories indicated that air\nmasses originated from the upper troposphere or lower stratosphere (>4000 m),\nintruded over the Antarctic Plateau, and were subsequently advected toward the\nSouthern Ocean by near-surface katabatic winds (Figure 4a). Simultaneous GEM\ndepletions were observed both at Dome C (Concordia Station, 75˚06' S, 123˚20' E, 3220\nm above sea level) and over the Ross Sea, reaching similar concentrations (0.2 – 0.3\nng/m3, Figure 4c of this response document). The 72-hour trajectories at Dome C during\n7\nthis period remained within the near-surface layer (Figure 4b of this response\ndocument), suggesting that GEM oxidation predominantly occurred close to the surface\nrather than aloft. Angot et al. (2016) reported similar summer AMDEs at Dome C driven\nby local oxidation7, while Song et al. (2018) confirmed through modeling that bromine-\nmediated two-step oxidation dominates GEM loss at this site2. Observations of BrO∙\nlevels (2-3 pptv) at Dome C by Frey et al. (2015)8 exceed the model upper limit (1 pptv)\nused by Song et al. (2018)2, with even higher BrO∙ in the free troposphere. In addition,\nthe summer lower troposphere over the Antarctic Plateau is highly oxidizing, driven by\nphotochemically produced reactive NO , HONO, OH, HO , RO , and O 9, 10, 11. Thus,\nx 2 2 3\nbromine-initiated GEM oxidation in the near-surface atmosphere of the Plateau is both\nchemically and observationally plausible.\nSupporting this interpretation, snowmass (total snow storage on land) data along\nthe cruise track show elevated values (typically > 0) during summer AMDEs and further\nincreases during E2, suggesting a possible contribution from snowpack photochemistry\n(Figure 3 of this response document). The concurrent O increase observed during this\n3\nevent –unlike the depletions seen in springtime AMDEs – can be attributed to persistent\ninputs of O -rich air from the upper troposphere / lower stratosphere and in situ\n3\nphotochemical O production from NO emissions over the Antarctic interior12, 13.\n3 x\nAMDEs period; and (b) the 72-hour backward trajectories of air masses over the Dome C region\nduring the E2 AMDEs period. The colourful and black trajectory profiles depict the vertical\ntransport paths of air masses and the corresponding terrain heights, respectively. (c) Time series of\ngaseous elemental mercury (GEM) observed in the Ross Sea region during this study and at\nConcordia Station (DCC, 75˚060' S, 123˚200' E, 3220 m above sea level) on the Antarctic Plateau\nover the same period. The black line indicates the GEM depletion threshold (0.6 ng/m3). The yellow\nbox represents E2 GEM depletion event that occurred concurrently in both the Ross Sea region and\nDCC.\n(2) GEM oxidation in the lower troposphere over coastal Antarctica and ice-\nshelf regions: During the AMDE around 1 February 2017 (E1 in Figure 3 of this\nresponse document), air masses primarily traversed the Ross Ice Shelf and coastal\nAntarctic stations show that the snowpack on ice shelves and coastal regions is a\nsignificant source of reactive bromine14. GOME-2 BrO∙ vertical column density maps\nfor this period shows elevated BrO∙ levels along these trajectories (Figure 5 of this\nresponse document). Therefore, GEM oxidation driven by coastal and ice-shelf\nbromine emissions, followed by lower-tropospheric transport, is another plausible\npathway for summer AMDEs. In this case, due to limited O -rich air intrusion from the\n3\nupper troposphere or lower stratosphere, O decreased synchronously with GEM,\n3\nconsistent with halogen consumption (E1 in Figure 3 of this response document).\nFigure 5. The 72-hour backward trajectories during E1 GEM depletion event, and the GOME-2\nmaps of BrO∙ vertical column density distribution during E1 event. The red box marks the elevated\nBrO∙ vertical column concentration the corresponding 72-hour air mass backward trajectories\npassed through during E1 event.\n(3) Direct transport of GEM-depleted air from the upper\ntroposphere/stratosphere: During the AMDE around 13 February 2017 (E3 in Figure\n3 of this response document), trajectories indicate that air masses from the Antarctic\n10\nupper troposphere/lower stratosphere were transported at high altitudes toward the\ncoast and then to the observation site via katabatic winds (Figure 6 of this response\ndocument). Brooks et al. (2008) reported elevated gaseous oxidized mercury (GOM) at\nthe South Pole linked to vertical transport from the Antarctic Plateau6, while Frey et al.\n(2015) observed higher BrO∙ in the free troposphere than near the surface8. Thus, GEM\noxidation in the upper troposphere followed by large-scale transport of GEM-depleted\nair to the Southern Ocean is also likely. The lack of O depletion during this event can\n3\nagain be explained by the continuous supply of O -rich air from the upper layers.\nIn summary, our combined observations and trajectory analyses suggest that\nsummer GEM depletion over the Southern Ocean results from bromine-initiated\noxidation occurring across multiple Antarctic environments: (1) the near-surface\natmosphere of the Plateau, (2) coastal and ice-shelf regions, and (3) the upper\n11\ntroposphere/lower stratosphere. The depleted air masses are then advected to the\nSouthern Ocean via upper-level intrusions and katabatic winds. Reactive bromine likely\noriginates from both snowpack emissions and upper-atmospheric inputs. The variable\nO behavior during AMDEs reflects the relative contribution between bromine-driven\n3 3\nproduction. These processes collectively explain the observed summer AMDEs and the\nassociated O response in a consistent framework. We have revised and expanded the\n3\ncorresponding sections of the manuscript to integrate this new analysis and discussion\n(Line 286-388).\nQ5. Line 25: It should be “involving” instead of “involve”.\nRe. Done, thank you.\nQ6. Line 42: It should be “pollutant” instead of “pollutants”.\nRe. Done, thank you.\nQ7. Lines 109–112: The description of the histogram is not very useful. Since the mean\nvalue and the median value are very close, I assume the GEM concentration fits the\nnormal distribution. The normality test result might be more useful here to be connected\nwith the configuration of the GAM model.\nRe. Thank you for this helpful suggestion. Following your advice, we conducted a\nnormality test based on the GEM concentration data. The results confirm that the GEM\nconcentrations in this study follow a normal distribution, consistent with the close\nagreement between the mean and median values. This finding supports the\nappropriateness of the configuration used in our GAM model. We have also updated\nthe corresponding figure in the Supplementary Materials (Figure 7 of this response\n12\ndocument) to include the results of the normality test.\nFigure 7. Distribution histogram of gaseous elemental mercury (GEM) during the whole cruise of\nthis study.\nQ8. Figure 1, Figure 2 and Table 1: The order of the regions had better be consistent.\nRe: In response, we have reorganized the presentation of regional information to ensure\nconsistency. Specifically, we have moved the tabulated content originally included in\nFigures 1 and 2 to Table S1. Additionally, we have directly annotated key information\n–such as the study area and corresponding observation periods – on the map to provide\na clearer and more intuitive presentation.\nQ9. Lines 199–204: TERRMSL is not necessarily linked only to katabatic winds. It can\nalso be a (partial) proxy for the concentration of oxidants, especially active halogen,\nwhich could be derived from upper tropospheric or lower stratospheric intrusion. Since\nthere is no observational data for halogen, it is a bit impetuous to draw this conclusion.\nRe. Previous observational studies on reactive halogens in Antarctica have shown that\nto their proximity to halogen sources such as sea ice, compared to the higher-altitude\nAntarctic Plateau15. Since TERRMSL represents the average surface elevation along\nthe 72-hour backward trajectory of the air mass –not the altitude of the air mass itself –\na high TERRMSL does not necessarily indicate the input of high-concentration BrO∙\nfrom high altitudes. In fact, when air masses pass through halogen-emission areas in\ncoastal Antarctica, a lower TERRMSL may correspond to higher BrO∙ concentrations.\nTherefore, TERRMSL is not a specific indicator for reactive halogens.\nGiven that the Southern Ocean surrounding Antarctica has an elevation of 0,\nTERRMSL is most directly indicative of katabatic winds originating from the high-\nelevation Antarctic Plateau. To further support this, we calculated the proportion of time\nthat the 72-hour backward trajectories spent over land (Traj_land) during the cruise. We\nfound that during summer AMDEs, Traj_land typically exceeded 0.8 (see response to\nQ3). Combined with the CWT results presented in the manuscript, these findings\nindicate that air masses during summer AMDEs were predominantly influenced by the\nAntarctic inland. We have added the discussion regarding Traj_land to the revised\nmanuscript:\nLine 209-213: “This influence is supported by the relationship between GEM\nconcentrations and the proportion of time the corresponding 72-hour backward\ntrajectories spent over land (Traj_land) during the cruise, which shows that Traj_land\ntypically increased to above 0.8 when summer AMDEs occurred (Figure S3).”\nQ10. Lines 273–276: Another explanation could be that upper tropospheric or lower\nstratospheric ozone intrusion provides more ozone which offsets the consumption by\nactive halogen.\nRe. Thank you for this comment. We agree that upper tropospheric or lower\nstratospheric ozone intrusion can contribute additional ozone, offsetting its\n14\nconsumption by active halogens. For a detailed discussion, please refer to our response\nto Q4.\nReviewer #2:\n[Overview]\nUsing shipboard measurements of gaseous elementary mercury (GEM) in the surface\nair conducted across the Southern Ocean and coastal seas nearly circling around\nAntarctica combined with the generalized additive model (GAM) simulation,\nthe authors demonstrate that atmospheric mercury depletion events (AMDEs) occur\nfrequently during summer in the atmospheric boundary layer over coastal seawater\naround Antarctica and that these events are most likely the consequence of transport of\nair having been advected down via katabatic winds over the sloping Antarctic plateau\nsurfaces. Springtime AMDEs have been reported extensively in the polar regions of\nboth hemispheres, which have been attributed generally with high confidence to the\nphotochemistry of bromine radicals sourced from frozen materials containing bromide\nof seawater origin. As the authors discussed in the present study, summertime AMDEs\nessentially do not take place in the Arctic, whereas their occurrences have been\nsometimes reported over inland and coastal Antarctic stations. The causes of\nsummertime AMDEs are still under debate, but photochemical processes other than that\nof bromine sourced from surface frozen materials most likely play a role. Although this\nchemical process aspect remains speculative in the present study, its novelty lies in the\ndemonstration of prevalence of summertime ADMEs over the coastal Southern Ocean,\nnot within Antarctica itself, and their strong association with rapid air mass transport\nvia katabatic winds, from the extensive spatial coverage of field data reported here.\nThese AMDEs are indeed potentially important for mercury input to the aquatic\necosystem in the Southern Ocean. Their conclusion is well supported by the data\nanalysis conducted by the authors. The quality of data acquisition and post analysis\n15\nmethod applications appears to meet the expected standards in the field; here, I note\nsignificant overwraps in terms of data analysis methodology (GAM simulation and\ntrajectory-based footprint analysis) with another article published recently in Nature\nCommunications (Yue et al., 2023), meaning that their methodology has once passed\nscrutinization by editor and referees for the same journal. I would support the\npublication of the present study after clarifications and additions to the text as suggested\nbelow.\n[Major comments]\nQ11. For atmospheric mercury depletion events (AMDEs) to contribute to the\ndeposition of atmospheric mercury onto the ocean, rapidly depositing oxidized mercury,\nwhich is indicated by the authors to have been generated somewhere over Antarctica,\nmust be remaining in the near-surface air when it exits to the ocean without too much\ndeposition onto the land/glacier surfaces. This point is not discussed clearly enough in\nthe current version of the manuscript and should be stressed in discussion and\nconclusions of the main text. I note that data showing the abundance of atmospheric\noxidized mercury in the gas phase (GOM) measured during the shipboard expedition\nare presented only in the supplement (Figure S4). In my opinion, this figure is as equally\nimportant as other figures currently included in the main text. Unless there is a space\nlimitation that prevents it, I encourage the authors the relocation of Figure S4 to the\nmain text. Talking about GOM, I think I do not see a method description of specification\nbetween GEM and GOM as to whether it was attempted in the sampling of atmospheric\nmercury in the first place and, if it was, how (e.g., lines 410-413). The authors should\ninclude this method description regardless of their decision for where to present their\nGOM data in the revised manuscript.\nRe. Thanks you for your comment. Observations in early February 2017 showed\nconcurrent summer Atmospheric Mercury Depletion Events (AMDEs) at both the\nDome C region on the Antarctic Plateau (Concordia Station) and over the Ross Sea\n16\n(Figure 6 of the manuscript). The 72-hour backward trajectories indicate that these air\nmasses originated from the Antarctic inland, suggesting that GEM oxidation likely\noccurred inland before transport to the Southern Ocean via katabatic winds.\nAdditionally, during December 2017, synchronous observations of GEM and gaseous\noxidized mercury (GOM) concentrations in the coastal Ross Sea regions showed a\nsignificant increase in GOM concentrations (peaking at ~200 pg/m³) coinciding with\nGEM depletion5. The air masses predominantly originated from the Antarctic continent,\nsupporting the hypothesis that oxidized mercury generated inland can be efficiently\ntransported to the adjacent Southern Ocean. We further propose that low atmospheric\nhumidity and aerosol concentrations over the Antarctic continent reduce dry and wet\ndeposition of GOM, facilitating rapid transport to the Southern Ocean by strong\nkatabatic winds.\nWe have incorporated these analyses into the revised manuscript and added a method\ndescription of GOM measurements. While the GOM data are of high reference value,\nthey were obtained during a separate summer cruise later in 2017 and are therefore\nincluded in the supplementary materials to complement the circum-Antarctic GEM\nobservations discussed in this study.\nQ12. It is fair to cite two earliest observational studies for summertime AMDEs (GEM\ndepletions accompanied by GOM increases) at Antarctic coastal sites (Sprovieri et al.,\n2002; Temme et al., 2003); the present study by the authors obviously builds upon\nfindings reported therein. The authors should also refer to a study by Holmes et al.\n(2010) (see their Section 3.3 and Figure 8) where they discussed their interpretation of\nmodel-simulated summertime variability of gaseous elemental and oxidized Hg at the\nAntarctic coastal site (Neumeyer) in relation to the Temme study. The Holmes study\nbacks the association between the Hg oxidation via Br radical chemistry in the free\ntroposphere and Antarctic coastal AMDEs during the summer, in accordance with one\nof the hypotheses mentioned in the present study.\naforementioned literature, which has been highly beneficial for our research. In the\nrevised manuscript, we have discussed and cited the relevant findings and perspectives\nfrom these articles, which were also shown below:\nLine 67-71: “Notably, recent observations have revealed that AMDEs can also occur\nin winter and in summer in Antarctica19, 20. For instance, Sprovieri et al. (2002) and\nTemme et al. (2003) observed the summertime GEM depletion phenomena, along with\nthe concurrent GOM elevations in the Antarctic Terra Nova Bay and Neumayer station,\nrespectively16, 17.”\nLine 286-299: “Regarding the role of reactive halogen oxidation in summer AMDEs,\nprevious studies support this mechanism. For example, Holmes et al. (2010) analyzed\nsummer GEM depletion events accompanied by concurrent increases in GOM and O\n3\nat the coastal Neumayer Station using the GEOS-Chem model and found that\nsimulations incorporating bromine oxidation pathways best reproduced the observed\nfeatures, rather than those based on O and ∙OH oxidation pathways1. Similarly, box-\n3\nmodel simulations by Song et al. (2018) showed that a bromine-initiated two-step GEM\noxidation mechanism provided the best agreement with summer GEM variations\nobserved at the inland Concordia Station2. Chemical kinetics studies also support this\nmechanism: the rate constant for the Hg(0) + Br (3.2×10-12 cm3 molecule−1\ns−1) is significantly higher than for Hg(0) + OH (3.2×10-13 × (T/298) -3.06 cm3\nmolecule−1 s−1) or Hg(0) + O (1.7×10-18 cm3 molecule−1 s−1) at typical Antarctic\n3\nsummer temperatures3, 4. These findings indicate that bromine oxidation can be a\ndominant pathway under such conditions. ”\n[Technical suggestions]\nQ13. Line 58: The authors should add ‘snowpacks’ to the list of interfaces that source\nreactive halogen species (e.g., Pratt et al., 2013; Toyota et al., 2014).\nRe. Done, thank you for the suggestion.\ntheir observations by Steffen et al. (2008).\nRe. Done, thank you for the suggestion.\nQ15. Line 86: According to BrO measurements at Dome C from ref. 20, the BrO mixing\nratios were higher in the free troposphere than in the surface air, suggesting that local\nsnowpacks at Dome C were not a strong source of gaseous reactive bromine compounds.\nThis nuance should be expressed in this sentence. Also, it probably aligns with the\nauthors’ argument that snowpacks on the Antarctic plateau are unlikely to be a major\nsupplier of gaseous bromine to the near-surface air carried down by katabatic winds.\nRe. Thanks! This paper did not state or intend to convey the view that \"local snowpacks\nat the Antarctic Plateau were not a strong source of gaseous reactive bromine\ncompounds.\" We have further investigated relevant studies. Frey et al. (2015) (ref. 20),\nusing MAX-DOAS technology, observed near-surface BrO∙ concentrations of\napproximately 2–3 pptv in the summer at Dome C region8. Song et al. (2018) used a\nbox-model to study the chemical mechanisms of atmospheric mercury oxidation at\nDome C on the Antarctic Plateau during summer and found that simulation results\nemploying a bromine-initiated oxidation mechanism best matched the observed\nsummer GEM depletion2. Since the model in that study set an upper limit of 1 pptv for\nBrO∙ concentration, the near-surface BrO∙ levels on the Antarctic Plateau are indeed\nsufficient to drive the observed GEM oxidation depletion. As Frey et al. (2015) did not\nfurther discuss the sources of near-surface BrO∙ on the Antarctic Plateau, and despite\ntheir observation of higher BrO∙ concentrations in the free troposphere over Dome C8,\nwe cannot currently rule out the possibility that \"snowpack emissions are not a source\nof bromine\" and the potential contribution of such processes to GEM oxidation.\nMoreover, previous researches have shown that summer BrO∙ concentrations in the\natmosphere over the Antarctic Plateau are still relatively low compared to those in\n19\ncoastal Antarctic regions15. Therefore, it may not be appropriate to characterize the\nAntarctic Plateau interior as having \"enriched\" BrO∙ levels. We have removed the\nstatement regarding BrO∙ in this line to avoid any potential controversy.\nQ16. Line 108: Please specify the time resolution of GEM measurements here. Hourly?\nRe. The GEM measurements have a time resolution of 5 minutes. We have added this\ninformation in the revised manuscript.\nQ17. Line 115: Please rephrase “referring” to “according”.\nRe. Done, thank you.\nQ18. Line 196: Perhaps the authors should drop “(29%)” from this line and keep\n“(21.5%)” on the next line.\nRe. Done, thank you.\nQ19. Line 209-211: The sentence might read better by saying instead: “…, katabatic\nwinds are intensified as air masses converge while moving from …”. I am also\nwondering if this intensification facilitates the efficient downward and northward air-\nmass transport without much vertical mixing not only from inland to coastal areas but\nalso a little further from coastal areas off the coast towards the ocean; please add this\nnuance if it is rationalized from the literature.\nRe. Thank you for this suggestion. We have revised the sentence as recommended.\nObservations by Angot et al. 2016）at the coastal Dumont d'Urville (DDU) site in East\nAntarctica showed that low GEM levels in summer were closely associated with\ntransport via katabatic winds originating from the Antarctic Plateau18. Building on this,\nour study—based on GEM variations along the Antarctica marginal seas —indicates\nthat katabatic wind transport significantly influences the spatial pattern of summer\nAMDEs in the marine boundary layer atmosphere over the coastal Southern Ocean,\n20\nextending approximately 20–60 km from the Antarctic continent. We have incorporated\nthis nuance into the revised manuscript:\nLine 226-230: “Such conditions help preserve the original chemical state (e.g.,\nconcentration-depleted conditions) of atmospheric Hg as it is transported from inland\nAntarctica to the Antarctic coastal areas, and even the nearshore Southern Ocean with\nan influencing distance range of approximately 20–60 km from the Antarctic continent,\nas revealed by the observed GEM variations during the cruise in this study.”\nQ20. Line 243: Change the comma to the period.\nRe. Done, thank you.\nQ21. Line 261-262: I feel that the section title could be rephrased to something like:\n“Potential mechanisms leading to a strong association between continental outflow and\ncircum-Antarctic distribution of AMDEs”.\nRe. We agree with your suggestion and have revised this title accordingly.\nQ22. Line 342: As I mentioned in my major comments above, I believe that the\ntransport of “oxidized” Hg (rather than GEM) is what matters for the Hg exposure in\nthe Southern Ocean ecosystems implicated from data reported in the present study. The\nauthors should be clear about this point.\nRe. We agree with your perspective and have further emphasized this point in the\nrevised manuscript within the section \"Implications for the Antarctic Hg Cycle\"\n(e.g., lines 432–441 and line 450).\nQ23. Line 359: A typo? The authors may want to change “30 degrees” to “20 degrees”.\nRe. Indeed, thank you.\ninformation about an altitude from which the trajectories are calculated back in time.\nPlease specify this altitude and give a rationale for the choice. Also, I would\nrephrase the last phrase of this paragraph from “during the cruise” to “arriving at the ship\nlocation”.\nRe. Considering that the GEM observations in this study were conducted at\napproximately 35 meters above sea level, the starting height of the backward\ntrajectories was set to 50 meters to approximate the observation altitude. We have added\nthis information to the revised manuscript (Line 523-525). We have also rephrase the\nlast phrase of this paragraph based on your comments.\nQ25. I suggest the authors consider citing the methodological foundation of the PSCF\nanalysis in particular on the empirical weight functon(e.g., Polissar et al., 1999), along\nwith specific parameter choices employed in this study based upon Ahmad et al. (ref\n41).\nRe. Thank you for the suggestion. We have added the relevant methodological\nreferences for the PSCF analysis. Additionally, we have also included the specific\nparameter choices employed in this study based on Ahmad et al. (ref 41) to the Methods\nsection of the revised manuscript.\nReviewer #3:\nQ26. Overall, this is good paper and warrants publication in this journal. The new\nanalysis and discovery of the source of AMDEs to the southern oceans from catabatic\nwinds is novel. This information will be very useful to the research community and will\nhelp to understand the impacts of mercury pollution in the vulnerable region of\ndifference between the traditional AMDEs and the catabatic AMDEs. I tended to get a\nlittle lost in the figures and the explanation of them so I would clean them up a bit to be\nconsistent and have a bit more information for the reader to follow this complex\nanalysis. The snow concentrations are shown but not discussed, maybe a small\ndiscussion about these measurements is warranted. The work supports the conclusions,\nwith the suggestions included in this review. The methods used are sound and are well\nknown and accepted in the atmospheric mercury research community.\nRe. Thanks! We have further discussed the difference between the traditional AMDEs\nand the katabatic AMDEs in the revised manuscript (please see responses to Q4).\nRegarding the snowmass parameter, the terrestrial snowmass data from 2°×2.5°\nlatitude-longitude grid cells along the cruise track during the observation period of this\nstudy indicate that when summer AMDEs (Atmospheric Mercury Depletion Events)\noccur in the marginal seas of Antarctica, the snowmass is generally greater than 0.\nFurthermore, the corresponding snowmass values show a further increase compared to\nadjacent non-AMDE periods (Figure 6 in the revised manuscript), implying that\nphotochemical processes at the snowpack interface in inland Antarctica may closed\nassociated with the occurrence of summer AMDEs over the Southern Ocean. We have\nalso added this discussion to the revised manuscript, which was also shown below:\nLine 337-340: “Supporting this interpretation, snowmass (total snow storage on land)\ndata along the cruise track show elevated values (typically > 0) during summer AMDEs\nand further increases during E2, suggesting a possible contribution from snowpack\nphotochemistry (Figure 5). ”\nQ27. Figures 1a and b – what is the sampling frequency of the data shown?\nRe. The sampling frequency of the GEM data here is 5 min, and we have added this\ninformation to Line 108.\ncollected on the map. The reader doesn’t necessarily know which sea is which and the\ntable is not as useful as an image.\nRe. Thanks! We have revised Figure 1 based on your suggestions. We added the\nname and the corresponding observation time of each sea area to Figure 1b, and separated the\ninformation table from Figure 1b to Table S1.\nQ29. Figure 2 – same sort of comment as Figure 1b. What times of the year are these\naverages? Over what period are these averages and are they the same for each sea?\nRe. Thanks! We have also revised Figure 2 based on your suggestions. We added the\nname, the corresponding observation time and observed average GEM concentration\n(blue words) of each sea area to Figure 2, and separated the information table from\nFigure 2 to Table S1.\nQ30. Line 163 – one important feature is the stable boundary layer. Does this area have\na stable boundary layer at this time when you see these AMDEs? I would imagine that\nstrong catabatic winds are not conducive to a stable boundary layer, where the\nbromine explosion occurs.\nRe. Thanks! Essentially, Antarctic katabatic winds involve dense, extremely cold air\nmasses flowing down the slope of the ice sheet under the influence of gravity. This cold\nair accumulates near the surface, while the overlying air remains relatively warm,\nleading to the formation of a temperature inversion. Under strong inversion conditions,\nthe atmospheric stratification becomes highly stable, significantly suppressing\nturbulent exchange in the vertical direction19, 20. Although the wind shear associated\nwith katabatic winds can generate mechanical turbulence, in the extremely cold and\nstrongly stratified Antarctic environment, the inhibitory effect of thermal stability\ntypically far outweighs the lifting effect of mechanical turbulence. As a result,\n24\nmechanical turbulence is confined to a very thin layer near the surface, ultimately\nleading to a shallow and stable boundary layer21, 22. Our observational data corroborate\nthis: when summer AMDEs triggered by Antarctic katabatic winds occur, the mean\natmospheric mixed layer depth (MIXDEPTH) along the 72-hour backward trajectories\nof air masses shows a notable decrease (Figure 8a of this response document). This\nfacilitates the accumulation of oxidants—such as reactive halogens and NO released\nx\nfrom photochemical processes in the Antarctic snowpack—in the near-surface\natmosphere, thereby promoting the subsequent persistent oxidation and depletion of\nGEM (gaseous elemental mercury). In addition, TERRMSL exhibits a negative\nrelationship with MIXDEPTH, with both average and median MIXDEPTH values\nremaining low levels (approximately 200 m) when TERRMSL exceeds 500 m (Figure\n8b of this response document), indicating that the Antarctica Plateau maintains a\npersistently low planetary boundary layer height during the summer. We have also\nadded the corresponding discussion to the revised manuscript (line 255-260)\ncorresponding average atmospheric mixing depth (MIXDEPTH, m) traversed by 72-hour backward\ntrajectories during the cruise; and (b) relationship between TERRMSL and MIXDEPTH.\nQ31. Figures – suggestion: please use the same colour scheme for the concentrations\nthroughout the paper. It makes it far easier to follow.\nRe. Thanks! We have revised and used the same colour scheme for the concentrations\nthroughout the paper based on your comment.\nQ32. Antarctica is filled with Glaciers, what role if any do you think they play in these\nfindings?\nRe. Thanks! We presume that you are inquiring about the influence of the Antarctic ice\nsheet and its overlying snow cover on the phenomena observed in this study. Indeed,\n26\nthe ice sheet and snowpack over Antarctica significantly impact the observed summer\nAMDEs in this study, both physically and chemically, as elaborated below:\nChemical Aspects: The ice sheet and snowpack interfaces over the Antarctic\ncontinent undergo heterogeneous photochemical reactions under summer sunlight,\ngenerating key oxidants such as NO 8, 11. This, in turn, further promotes the two-step\n2\noxidation depletion of GEM in the low-temperature environment of the Antarctic\ncontinent7. Furthermore, sea-salt aerosols, blowing snow, and other halogen-containing\nsubstances deposited in the snowpack of Antarctic ice shelves and coastal land areas\nare significant sources of reactive bromine in the Antarctic atmosphere14, which can\nalso promote the oxidative depletion of atmospheric GEM. This study identifies the\noxidation of atmospheric GEM over the Antarctic Plateau, coastal land ice sheets, and\nthe Ross Ice Shelf region, followed by transport via katabatic winds, as important\npotential driving mechanisms for the observed AMDEs over the Southern Ocean in\nsummer (please see responses to Q4).\nPhysical Aspects: As indicated in the response to Q30, the radiative cooling effect\nof the Antarctic ice sheet and snow surface serves to lower the atmospheric boundary\nlayer height and enhance atmospheric stability20, 22. This promotes the accumulation of\natmospheric oxidants within the interior boundary layer and facilitates the continuous\noxidation of GEM7. On the other hand, radiative cooling promotes the katabatic flow\nof air masses along the ice sheet gradient, forming katabatic winds23. This subsequently\nleads to the transport of GEM-depleted air masses from the interior towards the coastal\nseas, contributing to the formation of summer AMDEs over the Southern Ocean.\nQ33. Please have someone go over the grammar, it’s okay but there are visible mistakes\nin the text.\nRe. Thanks! We have thoroughly checked the manuscript for grammatical issues and\nmade improvements to enhance its polish.\nRe. Thanks! We have revised (see Figure 6 in the revised version).\nQ35. Figure S4 – can you please indicate which is land and which is ocean in the figure?\nRe. Regarding Figure S4 in the original manuscript, the white areas represent the ocean,\nwhile the dark gray areas represent land. We have labeled this information in the Figure\nS5 of the revised manuscript.\nQ36. Line 269 – The grey box seems a “typical” depletion event and the other 3 are\nmeteorologically modulated from the continent to the monitoring location. Are the back\ntrajectories showing this in Figure 6b and c? Can you specify the location where the\ntrajectories start? To me this looks like the start is the continent and the back trajectories\nare over the ocean. So, I am a little confused.\nRe. Thanks! We agreed with you that the period marked by the grey box (i.e. E1 in\nFigure 6 in the revised manuscript) represents a \"typical\" GEM depletion event,\ncharacterized by the simultaneous depletion of both O and GEM.\nthe 72-hour backward trajectories of air masses for this event. The results indicate that,\nunlike other AMDEs period, the air masses during this time primarily originated from\nthe lower troposphere and passed through the region near the Ross Ice Shelf (see Figure\n9 of this response document). Previous long-term observational studies of atmospheric\nBrO∙ at inland Antarctic stations have shown that the snowpack on the coastal ice\nshelves is a significant source of reactive bromine in the atmosphere14. We further\nexamined the GOME-2 BrO∙ vertical column density maps corresponding to the period\nand region through which the air masses passed (highlighted by the red box in the\nFigure 9 of this response document). These maps revealed significantly elevated BrO∙\ncolumn concentrations in the corresponding area. Based on these observations, we infer\n28\nthat pronounced atmospheric reactive halogen oxidation reactions drove this GEM\ndepletion event. Furthermore, due to the lack of continuous replenishment from ozone-\nrich air masses from the upper troposphere/lower stratosphere, the reaction between O\n3\nand reactive halogens led to their concurrent depletion, resulting in a phenomenon\ndistinct from other GEM depletion events. We have also incorporated these analyses\ninto the revised manuscript (Line 354-364)\nFigure 9. The 72-hour backward trajectories during E1 GEM depletion event, and the GOME-2\nmaps of BrO∙ vertical column density distribution during E1 event. The red box marks the elevated\nBrO∙ vertical column concentration the corresponding 72-hour air mass backward trajectories\npassed through during E1 event.\nQ37. Line 302 – are you suggesting that the primary reaction for the oxidation of\nmercury from the inland air masses is from OH rather than Br? Can you please include\na discussion of the chemistry differences here between these 2 processes. I think that\nwould help the discussion explanation a lot.\nRe. Thanks! We have conducted further in-depth investigation and evaluation on this\nissue. Sprovieri et al. (2002) conducted atmospheric mercury speciation observations\nin Antarctica and, based on preliminary estimates, found that oxidation mechanisms\ninvolving ∙OH, H O , and O could not adequately explain the observed high\n2 2 3\n29\nconcentrations of gaseous oxidized mercury (GOM). They hypothesized that halogen\noxidation might be an important reaction pathway leading to the depletion of GEM\nduring summer. Holmes et al. (2010), in their GEOS-Chem modeling study, discussed\nthe simultaneous summer depletion of GEM alongside increases in O and GOM\n3\nobserved at Neumayer Station in Antarctica. Their results indicated that simulations\nbased on bromine (Br) oxidation pathways were most consistent with the observations1.\nAdditionally, the box-model simulation by Song et al. (2018) demonstrated that the\nbromine-initiated GEM oxidation mechanism provided the best simulation results for\nsummer GEM variations at Concordia Station in Antarctica2. In fact, previous chemical\nkinetics studies have shown that the rate constant for the first-order reaction Hg(0) +\nBr (3.2 × 10-12 cm3 molecule−1 s−1) is significantly higher than that for Hg(0) + OH\nunder Antarctic summer environmental conditions (3.2 × 10-13× (T/298)-3.06 cm3\nmolecule−1 s−1) 3, 4. In summary, we believe that bromine oxidation is likely still the key\ninitializing chemical process responsible for atmospheric mercury depletion over the\nAntarctic continent in summer, and its importance likely surpasses that of the ∙OH\noxidation pathway. We have incorporated the above discussions into the revised\nmanuscript (Line 286-299).\nQ38. Line 331 – were any water samples collected? Would you see that in the water or\nmaybe snow as is seen in the Arctic during the latter seasonal AMDEs?\nRe. Regarding this study, no water samples were collected nor were corresponding\nmercury observations conducted during the observation period. Previous research on\nspeciated mercury observations in Southern Ocean transect waters has demonstrated\nthat the concentration of total Hg (HgT) in surface waters increases poleward, with\napparently higher HgT levels (> 2 pmol/L) near the Antarctic continent24. This\nphenomenon was attributed to a net Hg input to the ocean surface near the Antarctic\ncontinent from an atmospheric source. The higher atmospheric Hg input has, to some\nextent, also facilitated further methylation of mercury in the water, leading to an overall\n30\nincrease in methylmercury concentrations throughout the water column near the\nAntarctic continent24. These observational findings also support the conclusions of the\npresent study.\nQ39. Line 410 – were they shown in this paper? I think it would be very helpful to see\nthat relationship shown with these results.\nRe. Thanks! Yes, the GEM measurements obtained at Concordia Station on the\nAntarctic Plateau have been discussed and analyzed in conjunction with our Southern\nOcean GEM observations in the \"Potential mechanisms leading to a strong association\nbetween continental outflow and circum-Antarctic distribution of AMDEs\" section of\nthe revised manuscript. These serve as observational evidence to support the\nconclusions of this paper, and the corresponding results can be found in Lines 323–328\nand Figure 6c.\nQ40. Figure S5 – the axes are too hard to read\nRe. Regarding further investigation and analysis based on Q37, we have removed the\ndiscussion on ∙OH content as well as Figure S5.\nQ41. Figure S8 – I don’t understand what that is so I can’t comment on its scientific\nrigor.\nRe. Regarding Figure S8, it presents a series of methods for evaluating the reliability\nof the Generalized Additive Models (GAM) used in this study. These methods have\nbeen widely employed for validating GAM approaches in environmental and\ngeoscientific research 25, 26. Detailed descriptions of these methods and their\nimplications are provided in the “Model Validation” section of the Methods chapter. We\nhave further correlated each method with the corresponding subplots in Figure S8;\nplease refer to Lines 542–548.\nThis paper presents a significant advancement in our understanding of atmospheric\nmercury cycling in Antarctica by providing the first circum-Antarctic dataset of gaseous\nelemental mercury (GEM) concentrations in the Southern Ocean. The study's most\nnotable innovation lies in its demonstration that summertime low atmospheric mercury\nconcentration during some observation period. Authors gave their explanation and\nhypothesis for the specific mechanisms that the distinct spatial patterns linked to\ncontinental outflow from the Antarctic Plateau via katabatic winds. Through\nsophisticated trajectory analysis and generalized additive modeling, the authors\nconvincingly show that terrain altitude and atmospheric mixing depth are the dominant\ncontrols on GEM distribution, rather than local sea-ice processes that drive Arctic\nAMDEs. This represents an important paradigm shift in polar mercury research and\nhighlights the unique atmospheric chemistry of Antarctica's ice sheet-dominated\nenvironment. From this point, I strongly support this study.\nQ42. Additionally, I also have the feeling that the study could have the limitation, or\nsomething not good as the authors' hypothesis. First, from my knowledge, during\nAMDEs, we always or at least most time refer to low GEM but high GOM\nconcentration. Or in the forest, the uptake of GEM by foliage leading to low GEM\nconcentration in the air sometime. This means there must be some Hg biogeochemical\npathways to deplete the GEM concentration. However, this study did not measure the\nGOM concentration, and we did not know how change of GOM-GEM concentration.\nThis would lead to a situation that if the wind of inland of Antarctic Plateau bringing\nthe low GEM concentration (maybe this low GEM concentration from very high\nairmass of the Antarctic Plateau) to mix with air near to the sea, we would also observe\nlow GEM concentration. Under this situation, it likely reflects the mixing effects\nbetween “clean” high air mass and “dirty” near sea air mass. Thus, this would be not\nthe “AMDEs” in my opinion. Despite what’s my concerned (it may be not right), the\nstudy at least makes a substantial contribution to polar science by revealing how\nfundamentally different from the Arctic.\nRe. Thanks! The key characteristic of AMDEs is the significant depletion of\natmospheric GEM concentration below a certain threshold driven by substantial\nchemical oxidation processes. We believe this process is not necessarily limited to in-\nsitu oxidation but could also occur elsewhere followed by transport of the affected air\nmasses. Multiple published studies reporting synchronous observations of GEM and\nGOM in the Antarctic continent have shown that summer GEM concentration depletion\n(below 0.6 ng/m³) typically corresponds with a synchronous increase in GOM\nconcentrations6, 16. Temme et al. (2003) observed GEM depletion accompanied by\nincreased O and a significant rise in GOM concentrations at the coastal Neumayer\n3\nstation in Antarctica, suggesting that GEM was likely oxidized in the highly oxidative\nenvironment of the Antarctic inland and subsequently transported to coastal\nAntarctica17. Angot et al. (2016) reported atmospheric mercury depletion events lasting\nfrom days to weeks during summer at the Concordia Station near Dome C on the\nAntarctic Plateau, attributing this phenomenon to significant in-situ oxidation in the\nnear-surface atmosphere7. Song et al. (2018), using box-model simulations, further\nconfirmed that bromine-initiated, two-step GEM oxidation reactions drive this\nphenomenon2. Combined with observations from coastal Antarctic stations, this process\ncan influence the coastal Antarctica region via Antarctic downslope winds18. We\nconducted another ship-based observational campaign in the same Ross Sea region in\nDecember 2017 and observed that GEM concentration depletion was accompanied by\na significant increase in GOM concentrations (peak values exceeding 200 pg/m³)5.\nDuring this period, the air masses consistently originated from the Antarctic continent\nfurther confirming that the summer GEM depletion observed in our study is primarily\ndriven by the chemical oxidation of GEM and its subsequent transport via Antarctic\ndownslope winds. Therefore, these events can indeed be termed as \"AMDEs\".\nzone” to shape the AMDEs in the Antarctic region.\nRe. We have revised this term to: \"convergence zone of katabatic wind\". Further\nelaboration on this terminology is provided in the main text, as detailed in Lines 222-\n230 of the revised manuscript.\nQ44. Line 65-69. Here points that evidence of springtime AMDEs in Antarctica were\nreported by several previous studies. In the abstract, authors stated that their study\npresents the first observational dataset of atmospheric gaseous elemental mercury\n(GEM). So, it seems that two statements are conflict.\nRe. Thanks! What the abstract of this study highlight is not the first Antarctic\nobservational dataset of gaseous elemental mercury (GEM) in the atmosphere, but\nrather the first circum-Antarctic observational dataset of GEM over the Southern Ocean.\nAlthough several previous studies have reported GEM observations from inland\nAntarctic sites and localized areas of the Southern Ocean, this study represents the first\ncontinuous GEM observation covering the entire circumpolar Southern Ocean around\nthe Antarctic continent.\nQ45. Line 107-108: The claim of \"first circum-Antarctic observations\" is significant\nbut would benefit from clarifying how this dataset compares spatially/temporally to\nprevious regional studies (e.g., Neumayer Station data referenced in Line 67).\nRe: Thanks! We have further supplemented the circum-Antarctic observation data with\nadditional temporal and spatial information, as detailed below:\nLine 107-112: “Here, we present the first circum-Antarctic observations of GEM\nwith time resolution of 5 min in the Southern Ocean from 26 November, 2016 to 10\nMarch, 2017. The spatial range includes the Ross Sea (150°W–160°E), Amundsen Sea\n(90°W–150°W), Bellingshansen Sea (60°W–90°W), Weddell Sea (30°W–60°W), Riiser–\nand D'Urville Sea (120°E–150°E).”\nQ46. Line 116: The 0.60 ng/m³ AMDE threshold seems appropriate based on cited\nliterature, but the rationale for not using a relative depletion percentage should be\nbriefly explained.\nRe. Thanks! The AMDE threshold (<0.6 ng/m³) was determined based on the same\nrelative decrease in GEM concentrations observed during Arctic AMDEs, which are\noften operationally defined as episodes where GEM concentrations rapidly drop below\n1.0 ng/m³. This threshold was derived from multi-year (2007–2011) springtime\nobservations (August–October) of GEM at the Antarctic inland research station (Troll\nStation). During these events, GEM and O exhibited concurrent depletion with a strong\n3\npositive correlation (correlation coefficients r up to 0.79), indicating typical oxidation-\ndriven AMDEs27. Thus, this threshold, grounded in multi-year observational data, is\nmore representative and robust.\nQ47. Line 130-146: The Ross Sea's exceptional AMDE frequency (28.9%) deserves\nmore discussion, ice shelf extent, ocean currents play a role?\nRe. Thanks! The high frequency of AMDEs in the Ross Sea is closely linked to the\nregion's geographical and hydrological conditions:\nFirstly, the physiography of the Ross Sea generates strong katabatic winds that flow\nfrom land over the ocean. According to statistics, during the observation period of this\nstudy, air mass back-trajectories over 72 hours in the Ross Sea region spent an average\nof 51.9% of their time passing over the Antarctic continent, with a corresponding mean\nTERRMSL (Terrain Mean Sea Level) value of 816.56 m. This indicates a significant\ninfluence of inland katabatic wind transport during the observation period. The summer\nboundary layer atmosphere over the Antarctic Plateau exhibits high oxidative capacity7,\n35\n8, 11. Previous studies have shown that the elevated photochemical processes occurring\non the Antarctic snowpack contribute to the high oxidative capacity of the tropospheric\natmosphere over the Antarctic Plateau, leading to significant in-situ oxidation of GEM7.\nAdditionally, the summer upper tropospheric atmosphere over inland Antarctica also\npossesses high atmospheric mercury oxidation capacity, which can result in substantial\nGEM oxidation6. The persistent Antarctic katabatic winds, along with direct intrusions\nof air masses from the Antarctic upper troposphere, transport GEM-depleted air masses\nfrom the Antarctic Plateau to the Ross Sea region, thereby explaining the observed\nsummer AMDE phenomena.\nSecondly, the Ross Sea region features the extensive Ross Ice Shelf. Previous long-\nterm observations of atmospheric BrO∙ at coastal Antarctic stations have revealed that\nthe snowpack on the surface of coastal ice shelves is an important source of reactive\nbromine in the atmosphere14. In this study, the corresponding GOME-2 BrO∙ vertical\ncolumn density map for this period shows clearly elevated BrO∙ levels in the regions\ntraversed by the air masses (Figure 9 of this response document). Therefore, GEM\noxidation driven by reactive halogens emitted from coastal areas and the Ross Ice Shelf,\nfollowed by transport in the lower troposphere, would be also a potential mechanism\nfor summer AMDEs over the Southern Ocean. (please also refer to the responses to Q4\nand Q36 for additional details). We have also incorporated the above discussions to the\nrevised manuscript (Line 183-187 and Line 354-364).\nQ48. Line 196-203: The GAM results showing TERRMSL's dominant influence\n(21.5%) are compelling, but the physical interpretation of why terrain altitude matters\nmore than sea-ice (6.1%) could be expanded.\nRe. Thanks! Throughout the circum-Antarctic observation period of this study, the\ncorresponding 72-hour backward air mass trajectories spent an average of 35.9% of\ntheir time over land, with a mean TERRMSL value of about 616 m. These results\n36\nsuggest that during the observation period, the transport via katabatic winds from the\nAntarctic continent had a significant potential influence on the atmospheric physical\nand chemical properties over the marginal seas around Antarctica. This is further\nsupported by the analysis and discussion in the section of the revised manuscript titled\n“Potential mechanisms leading to a strong association between continental outflow and\ncircum-Antarctic distribution of AMDEs.” (please also refer to the response to Q4 for\nadditional details). In contrast, due to substantial summer sea ice melt and efforts to\navoid dense sea ice areas during the voyage, the mean sea-ice fraction was only 6.9%\nthroughout the circumpolar observation period. Additionally, the elevated summer\ntemperatures over the Southern Ocean (mean: -4.62°C) during this cruise likely\nsuppressed the release of reactive halogens from the sea ice interface and the two-step\noxidation of GEM4. Therefore, TERRMSL demonstrated a higher contribution to GEM\nvariations than sea ice in this study. We have also incorporated the above discussion\ninto the revised manuscript, which was also shown below:\nLine 181-187: “Across the circum-Antarctic observation period, the corresponding\n72-hour backward trajectories spent on average up to 35.9% over land (Traj_land),\nwith a mean TERRMSL value of 616 m. For the Ross Sea region –where frequent\nAMDEs were observed (Table 1) – the average Traj_land and TERRMSL values were\neven higher (51.9% and 817 m, respectively). Meanwhile, due to substantial summer\nsea-ice melt and the avoidance of dense sea-ice areas during the voyage, the mean sea-\nice fraction was only 6.9% over the entire observation period. ”\nQ49. Line 267-279: The ozone-GEM relationship inconsistencies are intriguing - could\nthis reflect different oxidation pathways (halogen vs. NOx/OH) operating in different\nair masses?\nRe. Thanks! Regarding this issue, Reviewer 1 provided valuable suggestions, as\ndetailed in Q4. Concerning the reason for the absence of a significant decrease or even\n37\nthe increase in ozone concentrations during summer AMDEs in this study, multiple\nprevious studies have indicated the photochemical ozone production processes from the\nAntarctic inland snowpack, as well as the intrusion of tropospheric/stratospheric air\nmasses over the Antarctic Plateau, can lead to elevated tropospheric ozone\nconcentrations in the Antarctic inland. These effects can extend over thousands of\nkilometers through air mass transport12, 13. We conducted a renewed in-depth analysis\nof typical summer AMDE events in the Ross Sea in this study. The results revealed that\nduring summer AMDEs, when ozone concentrations increased and exhibited an anti-\ncorrelation with GEM, the air masses typically originated from the ozone-rich upper\ntroposphere/low stratosphere over the Antarctic continent (with initial altitudes\nexceeding 4000 meters). These air masses were transported to the marginal seas of\nAntarctica via the katabatic winds along the Antarctic slope (Figure 4 and Figure 6 in\nthe responses to Q4). This transport process of these inland descending air masses\ntoward the ocean is persistent within the Antarctic \"Convergence Zone of katabatic\nwinds\"23. Therefore, we hypothesize that the continuous supply of high-concentration\nozone from the upper troposphere or from emissions by the Antarctic inland snowpack\nduring air mass transport, sustained by katabatic winds, prevented synchronous ozone\ndepletion in the observed marine area of this study (please also refer to the response to\nQ4 for additional details).\nQ50. Line 510-513: Data accessibility is excellent, but including instrument detection\nlimits and precision metrics in the methods (Line 365-378) would help others evaluate\ndata quality.\nRe. Thanks! We have further refined the metrics related to the instrument detection\nlimit and precision in the Methods section, as detailed in Lines 444-446.\n1. Holmes CD, et al. Global atmospheric model for mercury including oxidation by\nbromine atoms. Atmospheric Chemistry and Physics 10, 12037-12057 (2010).\n2. Song S, et al. Understanding mercury oxidation and air–snow exchange on the\nEast Antarctic Plateau: a modeling study. Atmos Chem Phys 18, 15825-15840\n(2018).\n3. Ariya PA, Khalizov A, Gidas A. Reactions of gaseous mercury with atomic and\nmolecular halogens: Kinetics, product studies, and atmospheric implications.\nJournal of Physical Chemistry A 106, 7310-7320 (2002).\n4. Goodsite ME, Plane JMC, Skov H. A theoretical study of the oxidation of\nHg<SUP>0</SUP> to HgBr<sub>2</sub> in the troposphere. Environmental\nScience & Technology 38, 1772-1776 (2004).\n5. Yue F, Xie Z, Yan J, Zhang Y, Jiang B. Spatial Distribution of Atmospheric\nMercury Species in the Southern Ocean. Journal of Geophysical Research:\nAtmospheres 126, e2021JD034651 (2021).\n6. Brooks S, Arimoto R, Lindberg S, Southworth G. Antarctic polar plateau snow\nsurface conversion of deposited oxidized mercury to gaseous elemental\nmercury with fractional long-term burial. Atmospheric Environment 42, 2877-\n2884 (2008).\n7. Angot H, et al. New insights into the atmospheric mercury cycling in central\nAntarctica and implications on a continental scale. Atmos Chem Phys 16, 8249-\n8264 (2016).\n8. Frey MM, et al. Atmospheric nitrogen oxides (NO and NO2) at Dome C, East\nAntarctica, during the OPALE campaign. Atmos Chem Phys 15, 7859-7875\n(2015).\n9. Legrand M, et al. Year-round record of surface ozone at coastal (Dumont\nd'Urville) and inland (Concordia) sites in East Antarctica. Journal of Geophysical\nResearch: Atmospheres 114, (2009).\n10. Bond AMH, Frey MM, Kaiser J, Kleffmann J, Jones AE, Squires FA. Snowpack\n39\nnitrate photolysis drives the summertime atmospheric nitrous acid (HONO)\nbudget in coastal Antarctica. Atmos Chem Phys 23, 5533-5550 (2023).\n11. Kukui A, et al. Measurements of OH and RO<sub>2</sub> radicals at Dome C,\nEast Antarctica. Atmos Chem Phys 14, 12373-12392 (2014).\n12. Cristofanelli P, et al. Analysis of multi-year near-surface ozone observations at\nthe WMO/GAW “Concordia” station (75°06′S, 123°20′E, 3280 m a.s.l. –\nAntarctica). Atmospheric Environment 177, 54-63 (2018).\n13. Girach IA, et al. Influences of downward transport and photochemistry on\nsurface ozone over East Antarctica during austral summer: in situ observations\nand model simulations. Atmos Chem Phys 24, 1979-1995 (2024).\n14. Friess U, et al. Source mechanisms and transport patterns of tropospheric\nbromine monoxide: findings from long-term multi-axis differential optical\nabsorption spectroscopy measurements at two Antarctic stations. Atmospheric\nChemistry and Physics 23, 3207-3232 (2023).\n15. Wagh SP, et al. Year-long ground-based observations of bromine oxide over\nBharati Station, Antarctica. Polar Science 38, (2023).\n16. Sprovieri F, Pirrone N, Hedgecock IM, Landis MS, Stevens RK. Intensive\natmospheric mercury measurements at Terra Nova Bay in Antarctica during\nNovember and December 2000. Journal of Geophysical Research-Atmospheres\n107, (2002).\n17. Temme C, Einax JW, Ebinghaus R, Schroeder WH. Measurements of atmospheric\nmercury species at a coastal site in the Antarctic and over the south Atlantic\nOcean during polar summer. Environmental Science & Technology 37, 22-31\n(2003).\n18. Angot H, Dion I, Vogel N, Legrand M, Magand O, Dommergue A. Multi-year\nrecord of atmospheric mercury at Dumont d'Urville, East Antarctic coast:\ncontinental outflow and oceanic influences. Atmos Chem Phys 16, 8265-8279\n(2016).\n19. Mahrt L. Stratified Atmospheric Boundary Layers. Boundary-Layer Meteorology\n40\n90, 375-396 (1999).\n20. King JC, Anderson PS. Heat and water vapour fluxes and scalar roughness\nlengths over an Antarctic ice shelf. Boundary-Layer Meteorology 69, 101-121\n(1994).\n21. Parish TR, Cassano JJ. The Role of Katabatic Winds on the Antarctic Surface\nWind Regime. Monthly Weather Review 131, 317-333 (2003).\n22. van den Broeke MR, van Lipzig NPM. Factors Controlling the Near-Surface Wind\nField in Antarctica. Monthly Weather Review 131, 733-743 (2003).\n23. Parish TR, Bromwich DH. Reexamination of the Near-Surface Airflow over the\nAntarctic Continent and Implications on Atmospheric Circulations at High\nSouthern Latitudes. Monthly Weather Review 135, 1961-1973 (2007).\n24. Cossa D, et al. Mercury in the Southern Ocean. Geochimica Et Cosmochimica\nActa 75, 4037-4052 (2011).\n25. Zhang L, et al. Quantifying the impacts of anthropogenic and natural\nperturbations on gaseous elemental mercury (GEM) at a suburban site in\neastern China using generalized additive models. Atmospheric Environment\n247, 118181 (2021).\n26. Wu Q, et al. Developing a statistical model to explain the observed decline of\natmospheric mercury. Atmospheric Environment 243, 117868 (2020).\n27. Pfaffhuber KA, Berg T, Hirdman D, Stohl A. Atmospheric mercury observations\nfrom Antarctica: seasonal variation and source and sink region calculations.\nAtmos Chem Phys 12, 3241-3251 (2012).\n41"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have improved the manuscript substantially based on the four reviewers' comments. I have no further comments and support its publication.", "reviewer_2": "I have a few concerns and questions in new statements incorporated into the revised manuscript. Please take them into account for the next manuscript revision.\n[Major comments]\nIn my opinion, the paragraph containing lines 285-300 needs major amendments.\nAt first, I want to raise a flag in the way the Holmes et al. (2010) study is referred to between Line 288-292. I would argue that the Holmes study assumed distinctly different processes between springtime AMDEs and summertime AMDEs. Yes, the Holmes study discussed the viability of Br-initiated oxidation of Hg in the global troposphere. However, when it comes to its relevance to summertime AMDEs observed at Antarctic coastal regions, we should note that their model did not consider any bromine source term from frozen surfaces in the austral summer. Here’s a quote from their Section 2.2: “Springtime photochemistry of sea salt on sea ice can produce unusually high Br concentrations in the polar boundary layer in spring", "reviewer_doi": "10.1021/acs.est.1c03160.\n[Minor comments]\nLines 334-336: This sentence is somewhat confusing because it states that lower tropospheric OH concentrations can also be high over the Antarctic plateau. Does it not fit better to the context in the next paragraph where an explanation for higher O3 concentrations in air masses depleted in Hg(0)? Recent studies such as Shah et al. (2022) indicate that a reaction with O3 is a dominant mechanism to stabilize initial products from Hg(0) + Br and Hg(0) + OH, so the presence of other secondary reactants such as NOx, OH, HO2, etc. does not necessarily make a difference for the completion of Hg(0) to Hg(II) oxidation initiated via reaction with Br-atom.\nLine 338-339, “snowmass (total snow storage on land) data along the cruise track”: Do the authors want to say “snowmass (total snow storage on land) data along 72-hour backward trajectories from the cruise track” here?\n[Technical suggestions]\nI also list some additional technical suggestions for the wording, some of which were previously unnoticed on my first read.\nLine 99: we combined observational analysis -> we performed observational data analysis\nLine 117: Notably -> During this expedition\nLine 120: Statistical analysis showed that summer AMDEs -> These summer AMDEs\nLine 189: play of key role -> play a key role\nLine 200: their associated AMDEs -> of AMDEs\nLine 202: land-surface -> geophysical surface\nLine 207", "reviewer_208": "land surface -> geophysical surface\nLine 365: halogen consumption -> catalytic loss via halogen chemistry\nLine 385-387: the relative contribution between A and B and C -> changing contributions from A, B, and C", "reviewer_3": "I have re-read the paper with the modifications from the suggestions of the 4 reviewers. I think the authors took much of the comments into consideration and the paper currently is excellent. My recommendation is to accept the paper as is.\nWith one little edit. Its the 33rd expedition and not the 33th.", "reviewer_4": "I appreciate the thoughtful and extensive responses to my review and the other reviewers. I have no further substantive comments."}, "author_response": "Responses to reviewers’ comments\nDear Editor and Reviewers:\nThank you for your positive comments on our manuscript entitled “Continental\noutflow shapes the circum-Antarctic pattern of summertime atmospheric mercury\ndepletion zones” (manuscript ID: NCOMMS-25-44802A) and for offering us an\nopportunity for revision. We believe the constructive comments have helped us\nimprove the overall quality of the manuscript. Reviewer 1, 3, 4 have no further\ncomments. Below are our detailed responses to comments from reviewer 2. Revised\ntexts of the manuscript are marked in red in the responses to comments.\nReviewer #2:\nI have a few concerns and questions in new statements incorporated into the revised\nmanuscript. Please take them into account for the next manuscript revision.\nQ1. In my opinion, the paragraph containing lines 285-300 needs major amendments.\nAt first, I want to raise a flag in the way the Holmes et al. (2010) study is referred to\nbetween Line 288-292. I would argue that the Holmes study assumed distinctly\ndifferent processes between springtime AMDEs and summertime AMDEs. Yes, the\nHolmes study discussed the viability of Br-initiated oxidation of Hg in the global\ntroposphere. However, when it comes to its relevance to summertime AMDEs observed\nat Antarctic coastal regions, we should note that their model did not consider any\nbromine source term from frozen surfaces in the austral summer. Here’s a quote from\ntheir Section 2.2: “Springtime photochemistry of sea salt on sea ice can produce\nunusually high Br concentrations in the polar boundary layer in spring"}]

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