Circumpolar measurements of speciated mercury, ozone and carbon monoxide in the boundary layer of the Arctic Ocean

Atmos. Chem. Phys., 10, 5031–5045, 2010 www.atmos-chem-phys.net/10/5031/2010/ doi:10.5194/acp-10-5031-2010 © Author(s) 2010. CC Attribution 3.0 License. Atmospheric Chemistry and Physics Circumpolar measurements of speciated mercury, ozone and carbon monoxide in the boundary layer of the Arctic Ocean J. Sommar1 , M. E. Andersson2 , and H.-W. Jacobi3,4 1 State key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China 2 Department of Chemistry, Göteborg University, 41296 Göteborg, Sweden 3 CNRS/Université Joseph Fourier – Grenoble 1, Laboratoire de Glaciologie et Géophysique de l’Environnement, 54 Rue Molière, 38400 St Martin d’Hères, France 4 Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany Received: 19 August 2009 – Published in Atmos. Chem. Phys. Discuss.: 5 October 2009 Revised: 10 May 2010 – Accepted: 10 May 2010 – Published: 1 June 2010 Abstract. Using the Swedish icebreaker Oden as a platform, continuous measurements of airborne mercury (gaseous elemental mercury (Hg0 ), divalent gaseous mercury species HgII X2 (g) (acronym RGM) and mercury attached to particles (PHg)) and some long-lived trace gases (carbon monoxide CO and ozone O3 ) were performed over the North Atlantic and the Arctic Ocean. The measurements were performed for nearly three months (July–September 2005) during the Beringia 2005 expedition (from Göteborg, Sweden via the proper Northwest Passage to the Beringia region Alaska – Chukchi Penninsula – Wrangel Island and in-turn via a northpolar transect to Longyearbyen, Spitsbergen). The Beringia 2005 expedition was the first time that these species have been measured during summer over the Arctic Ocean going from 60◦ to 90◦ N. During the North Atlantic transect, concentration levels of Hg0 , CO and O3 were measured comparable to typical levels for the ambient mid-hemispheric average. However, a rapid increase of Hg0 in air and surface water was observed when entering the ice-covered waters of the Canadian Arctic archipelago. Large parts of the measured waters were supersaturated with respect to Hg0 , reflecting a strong disequilibrium. Heading through the sea ice of the Arctic Ocean, a fraction of the strong Hg0 pulse in the water was transferred with some time-delay into the air samples collected ∼20 m above sea level. Several episodes of elevated Hg0 in air were encountered along the sea ice route with higher mean concentration (1.81±0.43 ng m−3 ) compared to the marine boundary layer over ice-free Arctic oceanic waters (1.55±0.21 ng m−3 ). In addition, the bulk of the variance in the temporal series of Hg0 concentrations was observed during July. The Oden Hg0 observations compare in this aspect very favourably with those at the coastal station Alert. Atmospheric boundary layer O3 mixing ratios decreased when initially sailing northward. In the Arctic, an O3 minimum around 15–20 ppbV was observed during summer (July–August). Alongside the polar transect during the beginning of autumn, a steady trend of increasing O3 mixing ratios was measured returning to initial levels of the expedition (>30 ppbV). Ambient CO was fairly stable (84±12 ppbV) during the expedition. However, from the Beaufort Sea and moving onwards steadily increasing CO mixing ratios were observed (0.3 ppbV day−1 ). On a comparison with coeval archived CO and O3 data from the Arctic coastal strip monitoring sites Barrow and Alert, the observations from Oden indicate these species to be homogeneously distributed over the Arctic Ocean. Neither correlated low ozone and Hg0 events nor elevated concentrations of RGM and PHg were at any extent sampled, suggesting that atmospheric mercury deposition to the Arctic basin is low during the Polar summer and autumn. Correspondence to: J. Sommar () Published by Copernicus Publications on behalf of the European Geosciences Union. 5032 1 J. Sommar et al.: Circumpolar measurements of speciated mercury, ozone and carbon monoxide Introduction The average residence time of gaseous elemental mercury (Hg0 ) and carbon monoxide (CO) in the lower troposphere is sufficient to make its distribution homogeneous over each hemisphere. While the seasonal cycle of CO exhibits a summertime minimum largely due to chemical oxidation initiated by the hydroxyl radical OH (e.g. Holloway et al., 2000), the seasonality of Hg0 is less obvious taking into consideration worldwide long-term observations (Kim et al., 2005). However, several background stations in the mid-latitudes north of 45◦ N (Kellerhals et al., 2003; Kim et al., 2005) report a wintertime Hg0 maximum. This has been attributed to seasonal trends in anthropogenic emissions and/or meteorological conditions, atmospheric oxidation processes, the height of the atmospheric mixing layer (Kock et al., 2005) and the terrestrial carbon pool (Obrist, 2007). Unlike Hg0 and CO, the mixing ratio of ozone (O3 ) in the lower troposphere reaches a maximum during summer. The Arctic atmospheric boundary layer Hg0 and O3 cycles derived from background observation sites in the European and American high Arctic (Schroeder et al., 1998; Berg et al., 2003; Helmig et al., 2007), however, exhibit large seasonal discrepancies from that of the mid-latitudes. It is very likely that high level of neurotoxic mercury in the Arctic ecosystems is partially linked to rapid, near-complete depletion of Hg0 (MDEs) in the atmospheric boundary layer occurring episodically during the polar spring (Steffen et al., 2008). Upon reaction with reactive bromine species such as bromine atoms (Donohoue et al., 2006) hundred(s) of tons of oxidised mercury (HgII ) are produced and perennially deposited into the Arctic environment (Ariya et al., 2004; Banic et al., 2003; Dastoor et al., 2008; Skov et al., 2004). The relative magnitude of this sink is profound, resembling almost half of the annual atmospheric input within a few weeks (Outridge et al., 2008). To a yet not well quantified, but significant degree, a back-reduction of HgII to Hg0 occurs resulting in a re-cycling of volatile mercury to the atmosphere (Schroeder et al., 2003). However, it cannot compensate for the total deposition, and a net assimilation into the food chain occurs. The Hg levels in the traditional food of indigenous people living in the Arctic pose a threat for human pre- and neonatal neurological development (Steffen et al., 2008). Arctic marine mammals such as beluga whales frequently contain total Hg levels well above Canadian Federal Consumption Guidelines (Lockhart et al., 2005). Again, the fate of surplus mercury deposited to the Arctic basin during polar spring is largely unknown with reference to transport and transformation. This issue has recently been discussed in modelling papers by Hedgecock et al. (2008) and Outridge et al. (2008) in favour of less net accumulation in the abiotic reservoirs. The Beringia 2005 expedition, taking plac (...truncated)