European emissions of mercury derived from long-term observations at Mace Head, on the western Irish coast

Many emission inventories for mercury have been compiled but rarely constrained using observations of ambient air concentrations with a known quality. In this paper, we derive Hg/CO, Hg/halocarbon, and Hg/CH4 emission ratios from pollution episodes observed during the long-term mercury monitoring at the Mace Head Atmospheric Research Station in Ireland. The average Hg/CO emission ratio from 15 pollution episodes with air originating from the European continent observed between 1996 and 2003 was 0.0050±0.0021 ng m−3 ppbv−1, i.e. (5.5±2.3)×10−7 mol/mol, which is almost identical to the ratio reported recently for the continental plumes of eastern Asia. Mercury correlated also with CFCl3 (CFC-11), CF2Cl2 (CFC-12), CH3CCl3, CCl4, CCl2FCF2Cl (CFC-113), CHCl3, N2O, and CH4 during the pollution episodes. The mercury emissions calculated from the emission ratios and the European emissions of the above gases are in reasonable agreement with the estimated anthropogenic total mercury emissions of 250 t/yr in 1995. However, the measurements encompass almost exclusively elemental mercury whose anthropogenic emissions are estimated to be only 152 t yr−1. Several hypotheses are proposed to explain this discrepancy, such as natural sources, underestimation of the emissions of elementary mercury, and erroneous speciation of anthropogenic emissions.