Feedback mechanisms between snow and atmospheric mercury: Results and observations from field campaigns on the Antarctic plateau
Graphical abstract
Introduction
The Antarctic Plateau snowpack has been identified as a sink and source of atmospheric mercury.
A pioneering study, conducted at Dome C in 2013, highlighted the roles of photochemical reactions, the oxidative capacity of the boundary layer, and the dynamics of chemical exchanges of Hg at the air–snow interface (Angot et al., 2016c).
The Antarctic continent is considered a natural laboratory for environmental studies. Its distance from the main anthropogenic sources makes Antarctica one of the best locations for experiments to understand chemical and physical processes, especially those that define features of the biogeochemical cycles of anthropogenically and naturally emitted elements. Antarctica is isolated by the circumpolar vortex from the rest of the troposphere meaning that only long lived contaminants such as Hg(0) can reach the plateau (Dommergue et al., 2010). Understanding the role of Antarctica and the Antarctic Plateau (AP) (altitude > 2500 m a.s.l.) in the global mercury cycle is crucial since the Antarctic Plateau and its snowpack are considered both a sink and a source of atmospheric mercury (Angot et al., 2016b).
Mercury (Hg) is an element with a known toxicity and it is present in the environment in several different chemical forms. Mercury is reactive in the environment and undergoes photochemical reactions that change its speciation and chemical behaviour.
Continuous measurements of atmospheric Hg in Antarctica are scarce (Angot et al., 2016a). A few years ago, long-term atmospheric Hg measurements started at the near-coastal Norwegian Station Troll (TRS) (Pfaffhuber et al., 2012). In the framework of the Global Mercury Observation System (GMOS) project (Sprovieri et al., 2016, Sprovieri et al., 2017), Hg measurements in air and wet deposition were initiated in 2012, including in the 40 ground-based monitoring sites of this network, the French coastal station Dumont D'Urville (DDU) (Angot et al., 2016b) and at the Italo-French station Concordia at Dome C (DC) on the Antarctic Plateau (Angot et al., 2016c).
On the Plateau, where the snowpack is perennial and very distant from coastal influences; the local atmospheric conditions are very different from those observed at coastal sites where most of the measurements have been made up to now. Observations already made on the Antarctic Plateau suggest that deposition of mercury in the snowpack occurs in summer, resulting in levels of total Hg in the surface snow of up to 200 of pg g−1 (Han et al., 2014, Angot et al., 2016c). After the dark and more stable winter conditions, Hg(0) concentrations on the Antarctic Plateau become highly variable during the sunlit period due to photochemical processes. Occasionally, and less intensively than at coastal sites, Atmospheric Mercury Depletion Events (AMDEs) associated with coastal air masses enriched in halogen radicals can be observed in the spring (Angot et al., 2016c). Spring increases in gas phase Br have been detected at DC (Legrand et al., 2016b) suggesting that during this period, emissions from sea ice might reach the Antarctic plateau and interact with Hg. Mercury in the atmosphere is present in its gaseous elemental form (Hg(0), GEM) or as particulate (Hg(p))/gaseous (reactive gaseous mercury (RGM) or gaseous oxidized mercury (GOM)) divalent mercury (Hg(II)) that can undergo wet and dry deposition (Schroeder et al., 1998).
Mercury in its oxidized form can be deposited onto the snowpack, altering Hg concentrations in the upper snow strata. Once present in the snowpack, Hg is very labile, it can be reduced back to Hg(0) and can undergo dynamic exchange with the atmosphere above (Steffen et al., 2002). Photochemical reactions are important in altering the speciation of Hg in the snowpack and depend on environmental properties and snowpack chemistry. The role of the snowpack is crucial in the mercury cycle in Polar Regions since it acts as both a sink (deposition, accumulation) and a source (re-emission).
Several studies have already been carried out on the Antarctic continent with the aim of determining the extent of mercury recycling between the surface snow and the lower atmosphere over the plateau (Brooks et al., 2008, Dommergue et al., 2012, Han et al., 2014, Angot et al., 2016c, Wang et al., 2016).
Differences in concentration between the surface snow and deeper snow pit samples suggests a significant re-emission of Hg from surface snow to the atmosphere (Han et al., 2014). Combining the results from Han et al. (2014), with atmospheric measurements from the South Pole (Brooks et al., 2008) and Dome C, and total Hg measurements in the Dome Fuji snowpack, it can be inferred that the Antarctic Plateau snowpack might act as a temporary reservoir for Hg from mid-winter to mid-summer, but changes to a source of atmospheric Hg(0) during mid to late-summer. However, the overall understanding of the mechanism is still not fully clear.
Estimates by Brooks et al. (2008) suggest that the Hg(II) deposited to the snowpack is subsequently photo-reduced and re-emitted as Hg(0) from the surface, and that only 10% of the deposited Hg is sequestered and deeply buried. In the Arctic snow it is estimated that 24 h after deposition, a fraction of Hg is already re-emitted as Hg(0) to the atmosphere (Lalonde et al., 2002, Dommergue et al., 2003). It has been shown that surface Arctic snow could lose up to 90% of its total Hg content within 48 h (Poulain et al., 2004). Similar re-emission/loss rates of Hg from surface (35–50%) and drifting snow (65–75%) over 10.5 h have been suggested by Sherman et al., (2010) (Sherman et al., 2010) in chamber experiments. Several studies have evaluated the loss rate of mercury in the Arctic and the general behaviour of this element, however there is still lack of knowledge regarding the Antarctic continent. Some studies evaluate the role of snow precipitation on the Antarctic coast (Douglas et al., 2005, Douglas et al., 2008, Domine et al., 2011) and only recently experiments trying to evaluate the mercury cycle in the Antarctic Plateau have been carried out (Dommergue et al., 2012, Angot et al., 2016c). No studies deeply investigate the processes at the snow atmosphere interface as well as the role of snow precipitation on the mercury cycle on the inner Antarctic plateau. To understand this it is crucial to understand the mechanisms and parameters governing/controlling mercury at the snow surface on the Plateau, and the roles of snowfall and diamond dust (DD), the main forms of precipitation, in non-coastal areas has yet to be determined. It has been shown that a significant part of the precipitation that falls on the Antarctic Plateau (Bromwich, 1988, Fujita and Abe, 2006, Stenni et al., 2016) is in the form of DD under clear-sky conditions. Additionally, bromine radicals are also thought to play a major part in Hg oxidation and therefore deposition (Goodsite et al., 2004, Holmes et al., 2010).
The results presented in this paper build on the work reported by Angot et al. (2016c), on weekly surface snow samples collected at DC in 2013. The snow sampling has been intensified and we focused on the summertime when Hg reactivity was at its highest. The results reported here cover three campaigns: 1st) November-December 2013, 2nd) December 2014-January 2015 and the 3rd) November-December 2015. The objective was to investigate the daily and hourly Hg variations in surface snow. Mercury concentrations in surface snow were combined with atmospheric Hg(0) measurements to evaluate the interchange of Hg at the air-snow interface. Additionally, halogens (I, Br) and Na were analysed in surface snow samples to investigate their potential interactions with Hg and their use as markers of marine air masses. Bromine (Br) concentrations in the snow could give indications on the presence/abundance of bromine radicals present at Dome C.
Section snippets
Dome C research station
Concordia station, located at Dome C (DC), is approximately 1200 km away from the coastline at an elevation of 3220 m above sea level. Temperature, relative humidity, wind speed and direction, and incoming solar radiation are routinely monitored (www.climantartide.it). DC is characterized by a high-pressure system favouring, for most of the year, clear sky conditions. However, cyclonic systems forming in coastal areas can occasionally reach the Antarctic Plateau causing snow deposition and
Spatial variability and influence of the station activities
The first objective of the XXIX campaign was to investigate whether station activities could influence concentrations in the surface snow samples. Five samples were collected consecutively on the same day at 150, 300, 450, 600 m from the station and within the clean area (approximately at 800 m from the station). The results show a possible contamination for Hg in the two surface snow samples collected closest to the station (Table 1). The mean (±standard deviation) Hg concentrations were
Snow and atmospheric mercury connection
The 3 campaigns had markedly different meteorological conditions and numbers of snowfall events (Fig. 4, Fig. 5 light green panels), which could explain the different patterns in total Hg surface snow concentrations. The total Hg concentration in the snow was stable during the XXIX and the XXX campaigns (Fig. 2, Fig. 4), which could be explained by a lack of meteorological events such as cyclonic intrusions from the coast and important oxidation events.
Total Hg concentrations slightly above the
Conclusions
The data extracted from the three campaigns provide a large dataset that produce a picture of the main environmental parameters involved in the mercury cycle in the surface snow on the Antarctic Plateau. The sampling strategy was modified year by year by to obtain the best operational condition for producing reliable results, and improving our understanding of the factors that control the presence of mercury in surface snow and their contribution to the total atmospheric Hg abundance. We have
Acknowledgements
This work was supported by the Programma Nazionale per la Ricerca in Antartide (PNRA, project number 2013/AC3.03 PEA 2013–2015). Analytical instrument resources for the elements determination in the snow were provided by the Institute for the Dynamic of Environmental Process (IDPA-CNR) of the National Research Council. Atmospheric Hg measurements were also supported by the FP7 (2010–2015) Global Mercury Observation System (GMOS) project. This work contributed to the EU-FP7 project Global
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Now at Institute for Data, Systems and Society. Massachusetts Institute of Technology, Cambridge, USA.