Mercury Stable Isotope Signatures of World Coal Deposits and Historical Coal Combustion Emissions

Mercury (Hg) emissions from coal combustion contribute approximately half of anthropogenic Hg emissions to the atmosphere. With the implementation of the first legally binding UNEP treaty aimed at reducing anthropogenic Hg emissions, the identification and traceability of Hg emissions from different countries/regions are critically important. Here, we present a comprehensive world coal Hg stable isotope database including 108 new coal samples from major coal-producing deposits in South Africa, China, Europe, India, Indonesia, Mongolia, former USSR, and the U.S. A 4.7‰ range in δ²⁰²Hg (−3.9 to 0.8‰) and a 1‰ range in Δ¹⁹⁹Hg (−0.6 to 0.4‰) are observed. Fourteen (p < 0.05) to 17 (p < 0.1) of the 28 pairwise comparisons between eight global regions are statistically distinguishable on the basis of δ²⁰²Hg, Δ¹⁹⁹Hg or both, highlighting the potential application of Hg isotope signatures to coal Hg emissions tracing. A revised coal combustion Hg isotope fractionation model is presented, and suggests that gaseous elemental coal Hg emissions are enriched in the heavier Hg isotopes relative to oxidized forms of emitted Hg. The model explains to first order the published δ²⁰²Hg observations on near-field Hg deposition from a power plant and global scale atmospheric gaseous Hg. Yet, model uncertainties appear too large at present to permit straightforward Hg isotope source identification of atmospheric forms of Hg. Finally, global historical (1850–2008) coal Hg isotope emission curves were modeled and indicate modern-day mean δ²⁰²Hg and Δ¹⁹⁹Hg values for bulk coal emissions of −1.2 ± 0.5‰ (1SD) and 0.05 ± 0.06‰ (1SD).