Atmospheric mercury depletion events at the Dead Sea: Spatial and temporal aspects

Abstract

Atmospheric cycling over the saline Dead Sea is highly dynamic due to high atmospheric BrOx (Br + BrO) concentrations. Elevated atmospheric BrOx concentrations cause large and frequent atmospheric mercury (Hg) depletion events (AMDEs), whereby the normally dominant gaseous Hg(0) drops well below global background levels and oxidized forms – particularly Hg(II)gaseous – increase dramatically. The high BrOx concentrations also cause ozone (O3) depletion events (ODEs), and the corresponding depletions of Hg(0) and O3 provide strong evidence that Dead Sea AMDEs are linked to active halogen chemistry. We conducted two measurement campaigns in the Dead Sea basin (summer 2009 and winter 2009/2010), and here provide: spatial and temporal data on atmospheric levels of Hg(0), Hg(II) (gaseous and particulate), and O3; Dead Sea water methyl-Hg and total-Hg concentrations; and an evaluation of the performance of current Hg measurement techniques under the particularly high atmospheric Hg(II) concentrations found at the Dead Sea. AMDEs (Hg(0) <1.0 ng m−3) occurred on 20 of 29 days in summer, of which eight events were very strong (<0.5 ng m−3); in winter, they occurred on eight of 20 days, of which four were very strong. Although all AMDEs occurred when BrO levels (measured by LP-DOAS) were enhanced, only four and three of the strong AMDEs (in summer and winter, respectively) showed corresponding ODEs, while other events showed no corresponding O3 concentration declines. This indicated that AMDEs can occur without detectable ODEs, even though BrOx chemistry is considered to drive AMDEs. We attribute these patterns to the fact that Hg concentrations seem more sensitive to active halogen chemistry than O3 which occurs at concentrations orders of magnitude higher and also shows photochemical daytime production. A second observation site some 400 m above the Dead Sea surface showed that AMDEs and ODEs occur throughout the Dead Sea basin and are not limited to the shore, although their frequency was lower (four events with levels <1.0 ng m−3 in winter). Total-Hg and methyl-Hg concentrations of the Dead Sea water were not enhanced in spite of the regular occurrence of AMDEs, with methyl-Hg concentrations below detection limits and total-Hg concentrations below 5 ppt. We further found that even at high Hg(II)gaseous concentrations, the commonly-used Tekran Model 2537 Hg vapor analyzer predominantly measures Hg(0) – as opposed to total gaseous Hg (TGM: GEM + Hg(IIgaseous); and that at high Hg(II)gaseous levels, the Tekran speciation unit experiences enhanced system blank levels because glassware possibly has difficulty fully retaining Hg(II).