Abstract
Abstract. Our knowledge of the distribution of mercury concentrations in air of the Southern Hemisphere was until recently based mostly on intermittent measurements made during ship cruises. In the last few years continuous mercury monitoring has commenced at several sites in the Southern Hemisphere, providing new and more refined information. In this paper we compare mercury measurements at several remote sites in the Southern Hemisphere made over a period of at least 1 year at each location. Averages of monthly medians show similar although small seasonal variations at both Cape Point and Amsterdam Island. A pronounced seasonal variation at Troll research station in Antarctica is due to frequent mercury depletion events in the austral spring. Due to large scatter and large standard deviations of monthly average median mercury concentrations at Cape Grim, no systematic seasonal variation could be found there. Nevertheless, the annual average mercury concentrations at all sites during the 2007–2013 period varied only between 0.85 and 1.05 ng m−3. Part of this variability is likely due to systematic measurement uncertainties which we propose can be further reduced by improved calibration procedures. We conclude that mercury is much more uniformly distributed throughout the Southern Hemisphere than the distributions suggested by measurements made onboard ships. This finding implies that smaller trends can be detected in shorter time periods. We also report a change in the trend sign at Cape Point from decreasing mercury concentrations in 1996–2004 to increasing concentrations since 2007.
Highlights
Our knowledge of the distribution of mercury in air over the Southern Hemisphere is mostly based on measurements made during ship cruises
As the gaseous oxidised mercury (GOM) concentrations at Amsterdam Island in particular and in the marine boundary layer in general are below 10 pg m−3 (Soerensen et al, 2010; Angot et al, 2014), the difference between total gaseous mercury (TGM) and Gaseous elemental mercury (GEM) at Amsterdam Island, Cape Grim and Cape Point is usually less than 1 %, which is insignificant when compared with the uncertainties discussed above
This finding has implications for the data from Amsterdam Island: if the influence of continental air masses is unimportant at Cape Point located on the coast of South Africa and at Cape Grim near the Australian continent, even less influence can be expected at Amsterdam Island, an isolated island in the middle of the Indian Ocean
Summary
Our knowledge of the distribution of mercury in air over the Southern Hemisphere is mostly based on measurements made during ship cruises. According to the most comprehensive review of shipboard measurements made between 1990 and 2009 by Soerensen et al (2012) mercury concentrations varied between 0.72 ng m−3 reported by Kuss et al (2011) for the southern Atlantic Ocean and 2.20 ng m−3 observed by Xia et al (2010) over the southeastern Indian Ocean. These data were collected in different areas during different seasons, typically over a period of 1 or 2 months. Stationary sites with continuous and reproducible measurements of higher quality over longer periods allow for the detection of smaller trends in shorter time periods
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