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Gaseous elemental mercury (GEM) and 222Rn, a radioactive gas of primarily terrestrial origin with a half-life of 3.8 days, have been measured simultaneously at Cape Point, South Africa, since March 2007. Between March 2007 and December 2009 altogether 59 events with high 222Rn concentrations were identified. GEM correlated with 222Rn in 41 of the events and was constant during the remaining events without significant correlation. The average GEM/222Rn emission ratio of all events was −0.0047 ± 0.0054 pg mBq−1, with ± 0.0054 being the standard error of the average. With an emission rate of 1.1 222Rn atoms cm−2 s−1 and a correction for the transport duration, this emission ratio corresponds to a radon calibrated flux of about −0.53 ± 0.62 ng m−2 h−1 which is statistically not distinguishable from zero. With wet deposition, which is not included in this estimate, the terrestrial surface of southern Africa appears to be a net mercury sink.

Gaseous elemental mercury (GEM) has been measured at the WMO Global Atmospheric Watch station at Cape Point, South Africa since September 1995. Two techniques were used: a low resolution manual technique till the end of 2004 and a high resolution automated technique since March 2007. The GEM measurements at Cape Point constitute only one component of the GAW monitoring program consisting of continuous measurements of CO, CH4, CO2, O3, N2O, and since March 1999 also of 222Rn. The seasonality and trend of GEM concentrations from the low resolution data was analyzed by Slemr et al. (2008) and the trend of the combined low and high resolution data until the end of 2009 by Slemr et al. (2011). In this paper we will present an updated analysis of the trend and seasonality of GEM data until the end of 2011 and compare these to measurements made at Troll, a Norwegian research station in Antarctica (Pfaffhuber et al., 2012).