Background ambient atmospheric mercury concentrations for the South African interior

Abstract

Atmospheric mercury (Hg) pollution is a global health concern. Once emitted into the atmosphere, gaseous elemental mercury (Hg0) can be converted to highly reactive gaseous oxidised mercury (Hg2+), which is deposited to the earth's surface. In aquatic environments Hg2+ is converted to methylmercury (MeHg) which is toxic, has the ability to bioaccumulate along the food chain and ultimately causes very serious medical conditions in humans. During this study continuous total gaseous mercury (TGM) measurements were conducted at Welgegund, a regional background site situated in the South African interior, from 17 September 2016 to 5 May 2017. The average TGM concentration measured over the study period was 1.68 ± 0.94 ng.m-3. This value compared well with data from most other measurement sites in South Africa, but was about twice as high as measurements from international, remote background sites. A clear diurnal trend was identified and attributed to the oxidation of Hg by ozone (O3) and the associated hydroxyl radical (OH), as well as the evolution of the planetary boundary layer (PBL). The diurnal profile also indicated transport of Hg from more distant sources to Welgegund. Furthermore, a seasonal trend could be recognised which was driven mostly by regional open biomass burning as an additional source in the dry season and enhanced wet deposition in the wet season. An overlay back trajectory map revealed three main air mass fetch regions for Welgegund during the study period, each with a unique mean hourly TGM concentration range associated to it. The TGM concentration range for each fetch region was dependent on the population density, point sources, ozone concentration, biomes and biomass burning events therein.