²²²Rn-calibrated mercury fluxes from terrestrial surface of southern Africa

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.