Mass-dependent and mass-independent fractionation of mercury isotopes in precipitation from Guiyang, SW China

The isotopic composition of mercury (Hg) is increasingly used to constrain the sources and pathways of this metal in the atmosphere. Though China has the highest Hg production, consumption and emission in the world, Hg isotope ratios are rarely reported for Chinese wet deposition. In this study, we examined, for the first time outside North America, both mass-dependent fractionation (MDF, expressed as delta Hg-202) and mass-independent fractionation of odd (odd-MIF, Delta Hg-199) and even (even-MIF, Delta Hg-200) Hg isotopes in 15 precipitation samples collected from September 2012 to August 2013 in Guiyang (SW China). All samples displayed significant negative delta Hg-202 (-0.44 similar to-4.27 parts per thousand), positive Delta Hg-199 (+0.19 to +1.16 parts per thousand) and slightly positive Delta Hg-200 (-0.01 parts per thousand to +0.20 parts per thousand). Potential sources of Hg in precipitation were identified by coupling both MDF and MIF of Hg isotopes with a back-trajectory model. The results showed that local emission from coal-fired power plants and cement plants and western long-range transportation are two main contributing sources, while the contribution of Hg from south wind events would be very limited on an annual basis. The relatively lower Delta Hg-200 values in Guiyang precipitation may indicate a dilution effect by local sources and/or insignificant even-MIF in the tropopause contribution of this subtropical region. Our study demonstrates the usefulness of isotope fractionation, especially MIF for tracing sources and pathways of Hg in the atmosphere. (C) 2015 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.