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 ppb−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 ppm−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 ± 1.5 t GEM yr−1 is much less than the total emission of 257 t Hg yr−1 from older inventories. Considering that emission of GEM represents only 50–78% of all mercury emissions, our estimates come close to the total mercury emission estimates ranging between 40–50 t Hg yr−1 from more recent inventories.