Emissions of mercury in southern Africa derived from long-term observations at Cape Point, South Africa

Abstract. Mercury emissions in South Africa have so far been estimated only by a bottom-up approach from activities and emission factors for different processes. In this paper we derive GEM/CO (GEM being gaseous elemental mercury, Hg⁰), GEM/CO₂, GEM/CH₄, CO/CO₂, CH₄/CO₂, and CH₄/CO emission ratios from plumes observed during long-term monitoring of these species at Cape Point between March 2007 and December 2009. The average observed GEM/CO, GEM/CO₂, GEM/CH₄, CO/CO₂, CH₄/CO₂, and CH4/CO emission ratios were 2.40 ± 2.65 pg m⁻³ ppb⁻¹ (n = 47), 62.7 ± 80.2 pg m⁻³ ppm⁻¹ (n = 44), 3.61 ± 4.66 pg m⁻³ ppb⁻¹ (n = 46), 35.6 ± 25.4 ppb ppm⁻¹ (n = 52), 20.2 ± 15.5 ppb ppm⁻¹ (n = 48), and 0.876 ± 1.106 ppb ppb⁻¹ (n = 42), respectively. The observed CO/CO₂, CH₄/CO₂, and CH₄/CO emission ratios agree within the combined uncertainties of the observations and emissions with the ratios calculated from EDGAR (version 4.2) CO₂, CO, and CH₄ inventories for South Africa and southern Africa (South Africa, Lesotho, Swaziland, Namibia, Botswana, Zimbabwe, and Mozambique) in 2007 and 2008 (inventories for 2009 are not available yet). Total elemental mercury emission of 13.1, 15.2, and 16.1 t Hg yr⁻¹ are estimated independently using the GEM/CO, GEM/CO₂, and GEM/CH₄ emission ratios and the annual mean CO, CO₂, and CH₄ emissions, respectively, of South Africa in 2007 and 2008. The average of these independent estimates of 14.8 t GEM yr⁻¹ is much less than the total emission of 257 t Hg yr⁻¹ shown by older inventories which are now considered to be wrong. Considering the uncertainties of our emission estimate, of the emission inventories, and the fact that emission of GEM represents 50–78 % of all mercury emissions, our estimate is comparable to the currently cited GEM emissions in 2004 and somewhat smaller than emissions in 2006. A further increase of mercury emissions due to increasing electricity consumption will lead to a more pronounced difference. A quantitative assessment of the difference and its significance, however, will require emission inventories for the years of observations (2007–2009) as well as better data on the speciation of the total mercury emissions in South Africa.