1Max-Planck-Institut für Chemie, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
2South African Weather Service c/o CSIR, P.O. Box 320, Stellenbosch 7599, South Africa
3ANSTO Environment, PMB 1, Menai, NSW 2234, Australia
4Helmholtz-Zentrum Geesthacht, Institute of Coastal Research, Max-Planck-Strasse, 21502 Geesthacht, Germany
Abstract. 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 2011, altogether 191 events with high 222Rn concentrations were identified. GEM correlated with 222Rn in 94 of the events and was constant during almost all the remaining events without significant correlation. The average GEM / 222Rn flux ratio of all events including the non-significant ones was −0.0001 with a standard error of ±0.0030 pg mBq−1. Weighted with the event duration, the average GEM / 222Rn flux ratio was −0.0048 ± 0.0011 pg mBq−1. With an emission rate of 1.1 222Rn atoms cm−2 s−1 and a correction for the transport time, this flux ratio corresponds to a radon-calibrated flux of about −0.54 ng GEM m−2 h−1 with a standard error of ±0.13 ng GEM m−2 h−1 (n = 191). With wet deposition, which is not included in this estimate, the terrestrial surface of southern Africa seems to be a net mercury sink of about −1.55 ng m−2 h−1. The additional contribution of an unknown but presumably significant deposition of reactive gaseous mercury would further increase this sink.