Atmospheric mercury depletion event study in Ny-Alesund (Svalbard) in spring 2005. Deposition and transformation of Hg in surface snow during springtime
Introduction
Mercury (Hg) is emitted to the atmosphere by both natural and anthropogenic sources (Munthe et al., 2001) and can then be largely dispersed in the atmospheric reservoir. Gaseous elemental mercury (GEM) is the main mercury species in the atmosphere, with an average concentration of about 1.7 ng/m3 (Slemr et al., 2003). Reactive gaseous mercury (RGM) and particulate mercury (PM) can be emitted directly by combustion sources (coal, waste incineration) or be formed through atmospheric GEM oxidation (Slemr et al., 1985, Lindberg and Stratton, 1998). This phenomenon, called Atmospheric Mercury Depletion Event (AMDE), has been observed mainly in polar atmospheres (Lu et al., 2001, Poissant and Pilote, 2001, Steffen et al., 2002, Lindberg et al., 2002, Ebinghaus et al., 2002, Temme et al., 2003, Berg et al., 2003, Skov et al., 2004, Sommar et al., 2004, Steffen et al., 2004, Gauchard et al., 2005). The oxidation chemistry is the result of halogen radicals and mainly bromine chemistry (Lindberg et al., 2002, Fain et al., 2006). GEM lifetime during a depletion event is about 4 h as compared to a normal lifetime of about one year. AMDEs can result from local, regional and/or long distance chemistry (Gauchard et al., 2005). The oxidation of GEM and subsequent formation of RGM and PM has, in some cases, led to elevated Hg deposition onto snow surfaces (Lu et al., 2001), while in other cases, no real increase in snow surface Hg levels was observed (Ferrari et al., 2005). This possible snow surface Hg deposition depends on the source of the AMDE, i.e. whether it is generated locally or due to already depleted air masses. Hg contained in snow can be reemitted back to the atmosphere via photochemical processes (Lalonde et al., 2002, Dommergue et al., 2003, Ferrari et al., 2005) or introduced into the ecosystem during snow melt. In this paper, we present the results of a springtime field campaign held in Ny-Ålesund (Svalbard) in 2005. The focal point of our study was to monitor atmospheric Hg speciation in order to determine the origin of the long AMDE recorded in April 2005. We also monitored snow surface Hg deposition during the event, as well as the stability of Hg complexes in surface snow during spring. Springtime Hg re-emission from the snow pack was also investigated so as to better understand the environmental parameters controlling this emission and the real impact of this process on the Arctic Hg snow budget.
Section snippets
Sampling locations
Two field sites were chosen around the Ny-Ålesund (78°54'N, 11°53'E) International Research and Monitoring Facility. Ny-Ålesund is a small settlement located on the west coast of Spitsbergen (Norway) which is the largest of the Svalbard islands (Fig. 1). The majority of the measurements took place in a small (~ 6 m²) electrically heated plywood shed located 300 m east of Ny-Ålesund, approximately 100 m from the seashore at 8 m.a.s.l. Data were also obtained from the Zeppelin Norway Research
Mercury and ozone
Fig. 2a and b shows the evolution of GEM and ozone concentrations from April 18 to May 6, 2005. An AMDE was recorded with a fast decrease in GEM and ozone. During the period from April 18 to April 24 (before 10:00), GEM and ozone concentrations were between 1.5 and 1.9 ng/m3 and between 80 and 100 ppbv, respectively. From April 24 at 10:00, ozone concentrations dropped to reach very low levels (around 5 ppbv) in less than 12 h. During the same time period, GEM concentrations decreased from ~
AMDE geographical and chemical origin
The ozone and GEM profiles and the air mass back trajectories show that there were three different periods during the AMDE: the beginning, the central period and the end, corresponding to three different air mass origins. Fig. 5 summarizes these changes in air mass origin and GEM and ozone patterns. Fig. 6a, b and c (April 21 to 23) shows that in Ny-Ålesund, BrO levels where low, corresponding to a period with no ozone and Hg depletion (see Fig. 2a and b).
Summary and conclusion
A field campaign was held in Ny-Ålesund, Svalbard (Norway) in order gain insights on Atmospheric Mercury Depletion Events (AMDE) in the Arctic. An AMDE, which lasted over 3 days and led to a net increase (2 fold) in Hg deposition onto snow surfaces, was observed. The origin of this AMDE seems to be linked to the transport of already depleted air masses as shown by both SCIAMACHY BrO data and Potential Frost Flowers (P.F.F.) maps. The increase in Hg deposition appears to be linked directly to a
Acknowledgements
This research was funded by the French polar Institute I.P.E.V. [Institut Paul-Emile Victor, program CHIMERPOL 399], the A.D.E.M.E. (Agence de l'Environnement et de la Maîtrise de l'Energie, Programme 0162020), the French Ministry of Environment and Sustainable Development, the French ministry of research (ACI 3012) and the CNRS [Centre National de la Recherche Scientifique]. Claude Boutron and Christophe Ferrari thank the Institut Universitaire de France (I.U.F.) for its financial help for
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