Under the framework of the Global Mercury Observation System (GMOS) project,
a 3.5-year record of atmospheric gaseous elemental mercury (Hg(0)) has been
gathered at Dumont d'Urville (DDU, 66
The Antarctic continent is one of the last near-pristine environments on Earth and still relatively unaffected by human activities. Except for pollutants released from Antarctic Research stations (e.g., Hale et al., 2008; Chen et al., 2015) and by marine and air-borne traffic (Shirsat and Graf, 2009), only the long-lived atmospheric contaminants reach this continent situated far from anthropogenic pollution sources. With an atmospheric lifetime on the order of 1 year (Lindberg et al., 2007), gaseous elemental mercury (Hg(0)) is efficiently transported worldwide. Hg(0) is the most abundant form of mercury in the atmosphere (Lindberg and Stratton, 1998). It can be oxidized into highly reactive and water-soluble gaseous divalent species (Hg(II)) – that can bind to existing particles and form particulate mercury (Hg(p)) – leading to the deposition of reactive mercury onto various environmental surfaces through wet and dry processes (Lindqvist and Rodhe, 1985; Lin and Pehkonen, 1999). Upon deposition, Hg(II) can be reduced and reemitted back to the atmosphere as Hg(0) (Schroeder and Munthe, 1998). Assessing mercury deposition and reemission pathways remains difficult due to an insufficient understanding of the involved physical–chemical processes.
Map of Antarctica showing surface elevation (meters above sea level,
m a.s.l.) and the position of various stations: Halley (HA), Neumayer (NM),
Troll (TR), Zhongshan Station (ZG), Dome A (DA), South Pole Station (SP),
Concordia Station (DC), Dumont d'Urville (DDU), McMurdo (MM), and Terra Nova
Bay (TNB). The black line delimits the high altitude plateau
(> 2500 m a.s.l.), and the red dotted line Adélie Land
(from 136 to 142
Only sparse measurements of atmospheric mercury have been performed in
Antarctica and there are still many gaps in our understanding of its cycle at
the scale of this vast continent (
From January 2012 to May 2015, Hg(0) measurements were performed at
DDU station located on a small island (Ile des Pétrels) about 1 km
offshore from the Antarctic mainland. A detailed description of the sampling
site (“Labo 3”) has been given by Preunkert et al. (2013) while the
climatology of this coastal station has been detailed by König-Langlo et
al. (1998). The average surface air temperature ranges from
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
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
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
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 (
O
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
Hourly averaged Hg(0) concentrations (ng m
Pollution plumes due to the station activities (e.g., combustion, vehicular
exhaust) occasionally reached the sampling site. Such local pollution events
can be easily identified for instance by the fast decrease of O
The record of atmospheric Hg(0) from January 2012 to May 2015 is displayed
in Fig. 2. Hourly averaged Hg(0) concentrations ranged from 0.10 to 3.61 ng m
A gradual 20 % decrease in Hg(0) concentrations from 0.89
Box and whisker plot presenting the monthly Hg(0) concentration
distribution
Mean percentage (%) of continental/oceanic mixed air masses (pink), and of air masses originating from the Antarctic plateau (green) or the ocean (blue) according to the HYSPLIT model simulations in winter (May–August), spring (September–October), summer (November–February), and fall (March–April).
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
June 2012 variation of
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
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
Despite the absence of large AMDEs at DDU, springtime oceanic air masses
were associated with low Hg(0) concentrations (0.71
Schematic diagram illustrating the processes that may govern the
mercury budget at DDU in summer. Katabatic winds transport inland air masses
enriched in oxidants and Hg(II) toward the coastal margins. Hg(II) species
deposit onto the snowpack by wet and dry processes leading to elevated
concentrations of total mercury in surface snow samples. A fraction of
deposited mercury can be reduced (the reducible pool, Hg
Hg(0) concentrations were highly variable during the sunlit period as compared to wintertime (Fig. 2). Figure 6 displays processes that may govern the atmospheric mercury budget at DDU in summer, as discussed in the following sections.
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
Monthly mean diurnal cycle of Hg(0) concentrations (in ng m
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
Summertime (November–February) mean diurnal cycle of Hg(0)
concentrations (in ng m
November 2014 variation of
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/NO
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, NO
In addition to oxidants, inland air masses may transport mercury species.
Low Hg(0) concentrations (0.76
The Hg
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 Hg
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
According to Fig. 3b, Hg(0) concentrations in oceanic air masses were
elevated from December to February (1.04
The reactivity of atmospheric mercury is unexpectedly significant in summer
on the Antarctic plateau as evidenced by elevated Hg(II) and low Hg(0)
concentrations (Brooks et al., 2008a; Dommergue et al., 2012; Angot et al.,
2016). This study shows that katabatic/continental winds can transport this
inland atmospheric reservoir toward the coastal margins where Hg(II) species
tend to deposit due to increasing wet deposition (Fig. 10). However, the
post-deposition dynamics of mercury and its ultimate fate in ecosystems
remain unknown. Bargagli et al. (1993, 2005) showed
evidence of enhanced bioaccumulation of mercury in soils, mosses, and
lichens collected in ice-free areas around the Nansen Ice Sheet (Victoria
Land, upslope from the Ross Ice Shelf), suggesting an enhanced deposition of
mercury species. Interestingly, four large glaciers join in the Nansen Ice
Sheet region and channel the downward flow of air masses from the Antarctic
plateau toward Terra Nova Bay, generating intense katabatic winds. The
monthly mean wind speed is about 16 m s
The influence of the Antarctic continent on the global geochemical cycle of mercury remains unclear (Dommergue et al., 2010). This study shows that the reactivity observed on the Antarctic plateau (Brooks et al., 2008a; Dommergue et al., 2012; Angot et al., 2016) influences the cycle of atmospheric mercury at a continental scale, especially downstream of the main topographic confluence zones. The question is whether the katabatic airflow propagation over the ocean is important. According to Mather and Miller (1967), the katabatic flow draining from the Antarctic plateau merges with the coastal polar easterlies under the action of the Coriolis force. The near-surface flow takes the form of an anticyclonic vortex (King and Turner, 1997), limiting the propagation of katabatic flows over the ocean.
We presented here a 3.5-year record of Hg(0) concentrations at DDU: the first
multi-year record on the East Antarctic coast. Our observations reveal a
number of differences with other costal or near coastal Antarctic records.
In winter, observations showed a gradual 20 % decrease in Hg(0)
concentrations from May to August, a trend never observed at other coastal
sites. This is interpreted as a result of reactions occurring within the
shallow boundary layer on the Antarctic plateau, subsequently efficiently
transported at that site by katabatic winds. In summer, the advection of
inland air masses enriched in oxidants and Hg(II) species likely results in
the build-up of an atmospheric reservoir of Hg(II) species at DDU, at least
partly explaining the elevated (up to 194.4 ng L
Mercury data reported in this paper are available upon request at
We thank the overwintering crew: S. Aguado, D. Buiron, N. Coillard, G. Dufresnes, J. Guilhermet, B. Jourdain, B. Laulier, S. Oros, and A. Thollot.
We also gratefully acknowledge M. Barret for the development of a QA/QC
software program, Météo France for the meteorological data, and
Susanne Preunkert who helped to validate contamination-free ozone data. This
work contributed to the EU-FP7 project Global Mercury Observation System
(GMOS –