Antarctic winter mercury and ozone depletion events over sea ice
Graphical abstract
Introduction
Mercury, a toxic pollutant, exists in the atmosphere predominantly as a stable monoatomic gas which can be transported to polar regions and can be oxidized and deposited, and thereby threaten polar ecosystems. The deposition of mercury is driven by chemical transformations between different mercury species with different physical and chemical characteristics. Models have shown that dry deposition of gaseous elemental mercury (GEM) is significant, especially onto plants (Zhang et al., 2012). Oxidized forms of mercury, however, is deposited faster onto nearby surfaces (Skov et al., 2004, Brooks et al., 2011). Oxidation of mercury to Hg(II) has been reported to occur at mid-latitudes due to reaction with halogen radicals formed from halogens released from sea surfaces (Obrist et al., 2011).
These reactions occur in large scale during springtime in polar regions, leading to large deposition of oxidized atmospheric mercury. Springtime atmospheric mercury depletion events (AMDEs) were first observed in the Arctic in 1995 (Schroeder et al., 1998) and since then have been regularly observed in both polar regions (Ebinghaus et al., 2002, Brooks et al., 2008, Steffen, 2008). AMDEs, oceanic transport, riverine inputs and dry and wet deposition contribute to the net accumulation of mercury in the Arctic marine and terrestrial ecosystems (Outridge et al., 2008). Considering only the Arctic, it is estimated that AMDEs alone are responsible for the deposition of up to 100 tons of mercury per year north of the polar circle (Durnford and Dastoor (2011). AMDEs have therefore been discussed as playing a significant role for the total deposition of mercury in polar regions (Steffen, 2008, Skov et al., 2004). Following the conversion into more toxic organo-metallic forms, mercury bioaccumulates and biomagnifies in polar marine food webs.
The corresponding lower tropospheric ozone depletion events (ODEs) were first described in 1988 and are connected to sunlight-induced production of bromine radicals reacting with ozone. These events occur within the surface boundary layer, but can reach up to an altitude of 2 km over sea ice-covered areas of the polar oceans during springtime (Barrie et al., 1988). It has been suggested that the frequency of ODEs has increased since the 1960s in the Arctic (Tarasick and Bottenheim, 2002, Shepler et al., 2005), but so far no significant trends have been observed for AMDEs, likely due to the short time scale of measurements (Berg et al., 2013).
ODEs and AMDEs have previously been observed during spring (from August to October) at several coastal stations around the Weddell Sea, but have so far not been recorded in July (Ebinghaus et al., 2002, Dommergue et al., 2010, Pfaffhuber et al., 2012, Jones et al., 2013, Temme et al., 2003). Mercury depletions have been detected at Cape Point, not only during spring but during all seasons, having no concurrent ozone depletions. The events occurred only during periods of low wind speeds (<5 m s−1) and are believed to have a small geographical scale (∼100 km), without being connected to polar spring halogen chemistry (Brunke et al., 2010).
Here we present measurements showing that mercury and ozone depletions occur over sea ice areas in Antarctica earlier than previously observed. These results will give new insights into the cycling and fate of mercury in polar regions.
Section snippets
Antarctic expeditions
The measurements were performed over the Weddell Sea, Antarctica onboard icebreaker R/V Polarstern during two scientific expeditions (ANTXXIX/6&7, Fig. 1). The winter expedition ANTXXIX/6 started in Cape Town on 8 June 2013, and ended in Punta Arenas on 12 August 2013. The spring expedition (ANTXXIX/7) lasted from 14 August to 16 October 2013, starting in Punta Arenas and ending in Cape Town. Snow samples were collected during both expeditions and analyzed for elemental and total mercury
Depletions over sea ice
Several depletion events were observed in winter (mid-July) and locations of detection are marked with white stars in Fig. 1. Red stars in Fig. 1 mark areas where spring depletions were observed. The results of the atmospheric measurements during both expeditions are presented in Fig. 2, with the light blue zones marking when R/V Polarstern operated within the marginal sea ice region (8 July – 23 July, 25 July – 9 August, 28 August – 5 October). GEM and ozone measurements show several distinct
Conclusion
Our results show that AMDEs and ODEs can be detected earlier over sea ice than previously measured on the Antarctic mainland (Ebinghaus et al., 2002, Pfaffhuber et al., 2012) and that winter depletions over sea ice lead to larger formation of atmospheric oxidized mercury associated with particles (HgP) than during the well-known spring depletions. Spring depletions over sea ice resulting in similar concentrations of HgP and GOM were previously observed at Barrow, Alaska suggesting that
Acknowledgements
We thank the Alfred Wegner Institute for Polar and Marine Research, which made our participation in the cruises possible. We thank the crew onboard R/V Polarstern for the support onboard. We also thank Nicola Pirrone and Francesca Sprovieri at CNR for kindly providing the mercury speciation unit. We gratefully acknowledge financial and logistical support by GMOS (Global Mercury Observation System), Svenska Polarforskningssekritariatet, CNRS-INSU through the programs LEFE and IPEV.
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