Hg(0) measurements were performed using a Tekran 2537B (Tekran Inc., Toronto, Canada). The sampling resolution ranged from 10 to 15 min with a sampling flow rate of 1.0 L min-1. Concentrations are reported here as hourly averages and are expressed in nanograms per cubic meter at standard temperature and pressure (273.15 K, 1013.25 hPa). Setting a 0.2 µm PTFE filter and a 10 m-long unheated sampling line on the front of the analyzer inlet, we assume that mainly Hg(0) (instead of total gaseous mercury, defined as the sum of gaseous mercury species) was efficiently collected and subsequently analyzed by the instrument (Steffen et al., 2002; Temme et al., 2003; Steffen et al., 2008).

External calibrations were performed twice a year by manually injecting saturated mercury vapor taken from a temperature-controlled vessel, using a Tekran 2505 mercury vapor calibration unit and a Hamilton digital syringe, and following a strict procedure adapted from Dumarey et al. (1985). As described by Angot et al. (2014), fortnightly to monthly routine maintenance operations were performed. A software program was developed at the LGGE (Laboratoire de Glaciologie et Géophysique de l'Environnement) following quality control practice commonly applied in North American networks (Steffen et al., 2012). Based on various flagging criteria (Munthe et al., 2011; D'Amore et al., 2015), it enabled rapid data processing in order to produce clean time series of Hg(0). According to the instrument manual, the detection limit is 0.10 ng m-3 (Tekran, 2011).

Eleven surface snow samples (the upper 3 cm) were collected during a traverse between DC and DDU conducted in February 2009. As described by Dommergue et al. (2012), samples were collected using acid cleaned PTFE bottles and clean sampling procedures. After sampling, samples were stored in the dark at -20 ∘C. Field blanks were made by opening and closing a bottle containing mercury-free distilled water. Total mercury (Hgtot) in snow samples was analyzed using a Tekran Model 2600. Hgtot includes species such as HgCl2, Hg(OH)2, HgC2O4, stable complexes such as HgS and Hg(II) bound to sulfur in humic compounds, or some organomercuric species (Lindqvist and Rodhe, 1985). The instrument was calibrated with the NIST SRM-3133 mercury standard. Quality assurance and quality control included the analysis of analytical blanks, replicates, and internal standards (reference waters for mercury: HG102-2 at 22 ng L-1 from Environment Canada). The limit of quantification – calculated as 10 times the standard deviation of a set of 3 analytical blanks – was 0.3 ng L-1 and the relative accuracy ±8 %.

Surface snow samples collected during traverses may have limited spatial and temporal representativeness given the variability of chemical species deposition onto the snow surface, and the occurrence of either fresh snowfall or blowing snow. The (in)homogeneity of surface snow samples was investigated at MM by Brooks et al. (2008b). Surface (3–5 cm) snow samples were collected daily (n=14) at different snow patches. Hgtot concentrations averaged 67 ± 21 ng L-1. This result indicates that the spatial and temporal representativeness of surface snow samples collected in Antarctica can be satisfactory and gives us confidence that spatial differences in Hgtot concentrations reported in Sect. 3.2.2 are not due to samples inhomogeneity.

O3 was continuously monitored with a UV absorption monitor (Thermo Electron Corporation model 49I, Franklin, Massachusetts) (Legrand et al., 2009). Collected at 15-s intervals, the data are reported here as hourly averages.

Back trajectories were computed using the HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model (Draxler and Rolph, 2013). Meteorological data from Global Data Assimilation Process (available at ftp://arlftp.arlhq.noaa.gov/pub/archives/gdas1) were used as input, and the model was run every hour in backward mode for 5 days at 0, 200, and 500 m above the model ground level. Three typical situations prevail at DDU: strong katabatic winds flowing out from the Antarctic ice sheet situated south of the station, pure marine air masses, or continental/marine mixed air masses with easterly winds due to the arrival near the site of low-pressure systems (König-Langlo et al., 1998). Oceanic origin was attributed to air masses having traveled at least 1 day over the ocean and less than 3 days out of 5 over the high-altitude Antarctic plateau. Conversely, plateau origin refers to air masses having traveled at least 3 days over the high-altitude Antarctic plateau and less than 1 day out of 5 over the ocean. Finally, mixed origin refers to air masses having traveled less than 1 and 3 days out of 5 over the ocean and the high-altitude Antarctic plateau, respectively. It should be noted that uncertainties associated with calculated backward trajectories arise from possible errors in input meteorological fields and numerical methods (Yu et al., 2009), and increase with time along the way (Stohl, 1998). According to Jaffe et al. (2005), back trajectories only give a general indication of the source region. Despite these limitations, back trajectories remained very similar at the three levels of altitude arrival at the site and we only use here those arriving at the model ground level. This method also gave consistent results with respect to the origin of various chemical species including O3 (Legrand et al., 2009), HCHO (Preunkert et al., 2013), NO2 (Grilli et al., 2013), and sea-salt aerosol (Legrand et al., 2016a).

A gradual 20 % decrease in Hg(0) concentrations from 0.89 ± 0.09 in average in May to 0.72 ± 0.10 ng m-3 in August (Fig. 3a) was observed at DDU. Conversely, concentrations remained rather stable at NM and TR in winter with mean values of 1.15 ± 0.08 and 1.00 ± 0.07 ng m-3, respectively (Ebinghaus et al., 2002; Pfaffhuber et al., 2012). Pfaffhuber et al. (2012) suggested that this stability of Hg(0) concentrations at TR is related to a lack of oxidation processes during the polar night.

A local reactivity at DDU – absent at other coastal stations – seems unlikely. Angot et al. (2016) showed evidence of a gradual 30 % decrease of Hg(0) concentrations at DC at the same period of the year (Fig. 3a), probably due to a gas-phase oxidation, heterogeneous reactions, or dry deposition of Hg(0) onto the snowpack. Since the decreasing trend observed in winter is less pronounced at DDU than at DC, it most likely results from reactions occurring within the shallow boundary layer on the Antarctic plateau, subsequently transported toward the coastal margins by katabatic winds. This assumption is supported by the HYSPLIT model simulations showing prevalence in winter (62 ± 23 %) of air masses originating from the Antarctic plateau reaching DDU (Fig. 4). The export of inland air masses towards the coastal regions is not uniform across Antarctica and is concentrated in a few locations – “confluence zones” – such as the Amery Ice Shelf region, the area near Adélie Land at 142∘, the broad region upslope from the Ross Ice Shelf, and the eastern side of the Antarctic Peninsula at ∼ 60∘ W (Fig. 1) (Parish and Bromwich, 1987, 2007). Given its geographic location, DDU in Adélie Land lies close to a confluence zone explaining the extent of the transport of air masses from the Antarctic plateau. Conversely, several studies showed that stations such as NM and HA are not significantly impacted by air masses originating from the Antarctic plateau (Helmig et al., 2007; Legrand et al., 2016b), consistently explaining why Hg(0) concentrations did not decrease at NM and TR throughout winter (Ebinghaus et al., 2002; Pfaffhuber et al., 2012).

Despite the overall decreasing trend in winter, Hg(0) concentrations sporadically exhibited abrupt increases when warm air masses from lower latitudes reached DDU. As illustrated by Fig. 5, Hg(0) concentration for example increased from 0.72 (8 June 2012) to 1.10 ng m-3 (14 June 2012) with increasing temperature, and a significant positive correlation was found between the two parameters (r=0.88, p value < 0.0001, Spearman test). This result is supported by an enhanced fraction of oceanic air masses reaching DDU at that time according to the HYSPLIT model simulations (Fig. 5d). Consistently, aerosol data gained in the framework of the French environmental observation service CESOA (http://www-lgge.obs.ujf-grenoble.fr/CESOA/spip.php?rubrique3) dedicated to the study of the sulfur cycle at middle and high southern latitudes indicate a mean sodium concentration of 450 ng m-3 between 10 and 14 June 2012 (not shown) instead of 112 ± 62 ng m-3 over the other days of this month. It can be noted that the mean Hg(0) concentration in June 2012 was 0.95 ± 0.04 ng m-3 at TR (Slemr et al., 2015), and 1.02 ± 0.04 ng m-3 on Amsterdam Island (37∘48′ S, 77∘34′ E, Angot et al., 2014). These values are consistent with the increase seen at DDU in air masses arriving from lower latitudes.

First discovered in the Arctic in 1995 (Schroeder et al., 1998), atmospheric mercury depletion events (AMDEs) have been subsequently observed after polar sunrise (mainly from early September to the end of October) at coastal or near-coastal Antarctic stations at NM (Ebinghaus et al., 2002), TNB (Sprovieri et al., 2002), MM (Brooks et al., 2008b), and TR (Pfaffhuber et al., 2012). These events, characterized by abrupt decreases of Hg(0) concentrations below 1.00 ng m-3 in the Arctic and 0.60 ng m-3 in Antarctica (Pfaffhuber et al., 2012), result from the oxidation of Hg(0) by reactive bromine species (e.g., Schroeder et al., 1998; Lu et al., 2001; Brooks et al., 2006; Sommar et al., 2007). At DDU, Hg(0) data covering the spring time period are scarce (Fig. 2) and we can just emphasize that the absence of Hg(0) drops in October 2012 tends to suggest that AMDEs, if exist, are not very frequent at DDU. Ozone depletion events (ODEs) are found to be less frequent and far less pronounced at DDU compared to other coastal stations such as NM and HA (Legrand et al., 2009, 2016b). Based on the oxygen and nitrogen isotope composition of airborne nitrate at DDU, Savarino et al. (2007) concluded to an absence of significant implication of BrO in the formation of nitric acid at this site, contrarily to what is usually observed in the Arctic where high levels of BrO are measured at polar sunrise (Morin et al., 2008). All these observations are consistent with a less efficient bromine chemistry in East compared to West Antarctica due to a less sea-ice coverage, as also supported by GOME-2 satellite observations of the tropospheric BrO column (Theys et al., 2011; Legrand et al., 2016a). Additionally, air masses originating from the Antarctic plateau prevailed (62 ± 23 %, Fig. 4) in spring at DDU according to the HYSPLIT model simulations. This can also explain, to some extent, the lack of AMDE-observations at DDU.

Despite the absence of large AMDEs at DDU, springtime oceanic air masses were associated with low Hg(0) concentrations (0.71 ± 0.11 ng m-3, see Fig. 3b). A slight but significant negative correlation was found between Hg(0) concentrations in spring and the daily averaged percentage of oceanic air masses reaching DDU (r=-0.38, p value = 0.01, Spearman test) while a significant positive correlation was observed between springtime Hg(0) concentrations and O3 mixing ratios in these oceanic air masses (r up to 0.65, p value < 0.0001, Spearman test). Therefore, though being not as pronounced as AMDEs observed at other coastal stations, we cannot rule out that the rather low background Hg(0) levels observed in spring at DDU are due to a weak effect of the bromine chemistry.

Figure 7 displays the monthly mean diurnal cycle of Hg(0) concentrations at DDU. Undetected from March to October, a diurnal cycle characterized by a noon maximum was observed in summer (November to February). Interestingly, Pfaffhuber et al. (2012) did not observe any diurnal variation in Hg(0) concentrations at TR and there is no mention of a daily cycle at NM, TNB, and MM (Ebinghaus et al., 2002; Temme et al., 2003; Sprovieri et al., 2002; Brooks et al., 2008b).

Hg(0) concentrations at DDU were sorted according to wind speed and direction. With north at 0∘, oceanic winds ranged from 270 to 110∘ E, coastal winds from 110 to 130∘ E, katabatic winds from 160 to 180∘ E, and continental winds from 130 to 160∘ E and from 180 to 270∘ E. Summertime Hg(0) concentrations exhibited a diurnal cycle regardless of wind speed and direction (Fig. 8). This result indicates that the observed diurnal cycle involves a local source of Hg(0) around midday which is, moreover, specific to DDU since the diurnal cycle is not observed at other coastal stations.

Large colonies of Adélie penguins nest on islands around DDU from the end of October to late February, with a total population estimated at 60 000 individuals (Micol and Jouventin, 2001). Several studies highlighted that the presence of these large colonies at DDU in summer significantly disturbs the atmospheric cycle of several species including ammonium and oxalate (Legrand et al., 1998), carboxylic acids and other oxygenated volatile organic compounds (Legrand et al., 2012), and HCHO (Preunkert et al., 2013). In a study investigating sediment profiles excavated from ponds and catchments near penguin colonies in the Ross Sea region, Nie et al. (2012) measured high mercury content in penguin excreta (guano). Similarly, elevated total mercury concentrations were measured in ornithogenic soils (i.e., formed by accumulation of guano) of the Fildes and Ardley peninsulas of King George Island (De Andrade et al., 2012). When soil temperature rises above freezing in summer at DDU, oxalate is produced together with ammonium following the bacterial decomposition of uric acid in ornithogenic soils (Legrand et al., 1998 and references therein). Dicarboxylic acids such as oxalic acid were shown to promote the light-driven reduction of Hg(II) species in aqueous systems and ice (Gårdfeldt and Jonsson, 2003; Si and Ariya, 2008; Bartels-Rausch et al., 2011). Emissions of Hg(0) from snow-covered ornithogenic soils are expected to peak early and late summer – following the reduction of Hg(II) species in the upper layers of the snowpack –, as also seen in the oxalate concentrations at DDU (Legrand et al., 1998). Furthermore the rise of temperature at noon would strengthen Hg(0) emissions from ornithogenic soils, possibly contributing to the observed diurnal cycle from November to February.

In summer, the surface wind direction sometimes changes from 120–160∘ E to North as temperature rises over midday (Pettré et al., 1993; Gallée and Pettré, 1998), giving birth to an apparent sea breeze. This phenomenon usually lasts half a day or less and air masses cannot be referred to as oceanic (see Sect. 2.2.3). Legrand et al. (2001, 2016b) observed increasing atmospheric dimethylsulfide (DMS) and chloride concentrations, respectively, during sea breeze events. However, our results indicate that Hg(0) concentrations did not tend to increase systematically with the occurrence of a sea breeze (e.g., Fig. 9).

Angot et al. (2016) reported a daily cycle in summer at DC with maximal Hg(0) concentrations around midday. This daily cycle atop the East Antarctic ice sheet was attributed to: (i) an intense oxidation of Hg(0) in the atmospheric boundary layer due to the high level of oxidants present there (Davis et al., 2001; Grannas et al., 2007; Eisele et al., 2008; Kukui et al., 2014), (ii) Hg(II) dry deposition onto the snowpack, and (iii) increased emission of Hg(0) from the snowpack around midday as a response to daytime heating following photoreduction of Hg(II) in the upper layers of the snowpack. Even if DDU is located on snow free bedrock for most of the summer season, the same mechanism could apply since the station is surrounded by vast snow-covered areas. However, such a dynamic cycle of deposition/reemission at the air–snow interface requires the existence of a summertime atmospheric reservoir of Hg(II) species nearby DDU. This question is addressed in the following section.

Several previous studies pointed out that the major oxidants present in the summer atmospheric boundary layer at coastal Antarctic sites differ in nature from site to site: halogens chemistry prevails in the West, OH/NOx chemistry in the East (Legrand et al., 2009; Grilli et al., 2013). Measurements made at HA in summer indicate a BrO mixing ratio of 3 pptv (Saiz-Lopez et al., 2007), a NO2 mixing ratio of about 5 pptv (Bauguitte et al., 2012), and a 24 h average value of 3.9×105 radicals cm-3 for OH (Bloss et al., 2007). Conversely, BrO levels are at least lower by a factor of 2 at DDU (Legrand et al., 2016a) and Grilli et al. (2013) reported a daily mean of 20 pptv for NO2 in summer at DDU while Kukui et al. (2012) reported a 24 h average value of 2.1×106 radicals cm-3 for OH. Large OH/NOx concentrations at DDU compared to HA were attributed to the arrival of air masses originating from the Antarctic plateau where the OH/NOx chemistry is very efficient (Legrand et al., 2009; Kukui et al., 2012).

Goodsite et al. (2004) and Wang et al. (2014) suggested a two-step oxidation mechanism for Hg(0), favored at cold temperatures. The initial recombination of Hg(0) and Br is followed by the addition of a second radical (e.g., I, Cl, BrO, ClO, OH, NO2, or HO2) in competition with the thermal dissociation of the HgBr intermediate. Using the rate constants calculated by Wang et al. (2014) for the reactions of BrO, NO2, and OH with the HgBr intermediate, we found that BrO is the most efficient oxidant of HgBr at HA (lifetime of 1.9 against 2.2 min with NO2 and 11 days with OH). At DDU the situation is reversed with a lifetime of the HgBr intermediate of 0.5 min with NO2, 3.9 min with BrO (assuming the presence of 1.5 pptv of BrO in summer at DDU; Legrand et al., 2016a), and 2 h with OH. These results suggest that the formation of Hg(II) species at DDU could be promoted by oxidants transported from the Antarctic plateau towards the coast.

In addition to oxidants, inland air masses may transport mercury species. Low Hg(0) concentrations (0.76 ± 0.30 ng m-3) at DDU were associated with transport from the Antarctic plateau in summer (November to February, see Fig. 3b). A significant negative correlation was found in summer between Hg(0) concentrations and the daily averaged percentage of air masses originating from the Antarctic plateau (r=-0.49, p value < 0.0001, Spearman test). Brooks et al. (2008a) reported elevated concentrations of oxidized mercury species at SP in summer (0.10–1.00 ng m-3). Similarly, Angot et al. (2016) observed low Hg(0) concentrations at the same period of the year at DC (0.69 ± 0.35 ng m-3, i.e., ∼ 25 % lower than at NM, TNB and MM). Angot et al. (2016) also reported the occurrence of multi-day to week-long Hg(0) depletion events (mean Hg(0) concentration ∼ 0.40 ng m-3) likely due to a stagnation of air masses above the plateau triggering an accumulation of oxidants within the shallow boundary layer. These observations indicate that inland air masses reaching DDU in summer are depleted in Hg(0) and enriched in Hg(II).

The Hgtot concentration of snow samples collected in summer 2009 between DC and DDU (see Sect. 2.2.2) ranged from 4.2 to 194.4 ng L-1 (Fig. 10). The closest sample from DC exhibited a Hgtot concentration of 60.3 ± 8.1 ng L-1 (n=3), in very good agreement with concentrations found in surface snow samples collected in summer at DC (up to 73.8 ± 0.9 ng L-1, Angot et al., 2016). As illustrated by Fig. 10, Hgtot concentrations increased between 600–800 and 1000–1100 km from DC in areas characterized by steeper slopes and higher snow accumulation values. Several studies reported a gradual increase in snow accumulation from DC toward the coast (Magand et al., 2007; Verfaillie et al., 2012; Favier et al., 2013), in good agreement with a gradual increase in humidity (Bromwich et al., 2004). These results suggest that the wet deposition of Hg(II) species was enhanced near the coast, resulting in elevated Hgtot concentrations in surface snow samples. Additionally, the presence of halides such as chloride in snow can reduce the reduction rate of deposited Hg(II) species by competing with the complexation of Hg(II) with dicarboxylic acids (Si and Ariya, 2008) resulting in higher Hgtot concentrations in coastal snowpacks (Steffen et al., 2014). It is worth noting that the Hgtot concentrations between DC and DDU were higher than the values measured in summer along other expedition routes in East Antarctica. Han et al. (2011) measured very low Hgtot concentrations (< 0.4–10.8 pg g-1) along a ∼ 1500 km transect in east Queen Maud Land, and Hgtot concentrations ranged from 0.2 to 8.3 ng L-1 along a transect from ZG to DA (Fig. 1) (Li et al., 2014). Unfortunately none of the samples collected during these two traverses were truly coastal – the most seaward samples were collected at altitudes of 948 and 622 m, respectively – preventing a direct comparison with the concentration measured near DDU. The mean Hgtot concentration of 67 ± 21 ng L-1 reported by Brooks et al. (2008b) at MM is the only truly coastal value available in Antarctica and is lower than the value reported here near DDU.

The advection of inland air masses enriched in both oxidants and Hg(II) likely results in the build-up of an atmospheric reservoir of Hg(II) species at DDU – as confirmed by elevated Hgtot concentrations in surface snow samples –, confirming the hypothesis of a dynamic cycle of deposition/reemission at the air–snow interface.

DDU is located on a small island with open ocean immediately around from December to February. It should be noted that during summers 2011/2012, 2012/2013, and 2013/2014, areas of open waters were observed but with a significant unusual large amount of sea ice. Sea ice maps can be obtained from http://www.iup.uni-bremen.de:8084/amsr2data/asi_daygrid_swath/s6250/ (Spreen et al., 2008).

According to Fig. 3b, Hg(0) concentrations in oceanic air masses were elevated from December to February (1.04 ± 0.29 ng m-3), and a significant positive correlation was found between Hg(0) concentrations and the daily averaged percentage of oceanic air masses in summer (r=0.50, p value < 0.0001, Spearman test). While in winter the ice cover limited mercury exchange at the air–sea interface (Andersson et al., 2008) leading to the build-up of mercury-enriched waters, large emissions of Hg(0) from the ocean likely occurred in summer. According to Cossa et al. (2011), total mercury concentrations can be one order of magnitude higher in under-ice seawater than those measured in open ocean waters. The authors attributed this build-up of mercury-enriched surface waters to the massive algal production at basal sea ice in spring/summer triggering a large production of Hg(0), and to the mercury enrichment in brine during the formation of sea ice. Elevated Hg(0) concentrations in oceanic air masses are consistent with observations in the Arctic where Hg(0) concentrations in ambient air peak in summer due to oceanic evasion and snowmelt re-volatilization (Dastoor and Durnford, 2014). Additionally, evasion from meltwater ponds formed on the remaining sea ice and observed around the station may contribute to the increase in Hg(0) concentrations (Aspmo et al., 2006; Durnford and Dastoor, 2011).