Unusual fractionation of both odd and even mercury isotopes in precipitation from Peterborough, ON, Canada
2012
Chen, Jiubin | Hintelmann, Holger | Feng, XinBin | Dimock, Brian
Once released into the atmosphere, mercury (Hg) is subject to long-range transport and a series of physico-chemical reactions before reentering terrestrial ecosystems. Though impressive progress has been made in understanding all aspects of Hg behavior in the atmosphere, many processes involved in the transformation and deposition of atmospheric Hg remain unidentified and source attribution is still an enormous challenge. Here, we examine the isotopic composition of Hg in precipitation collected during 2010 in Peterborough, ON, Canada and combine data on seasonal variations of mass-dependent (MDF) and mass-independent (MIF) fractionation with meteorological back-trajectory calculations to identify the Hg sources and to decipher Hg atmospheric transformation reactions. All precipitation samples displayed significant MDF (δ²⁰²Hg between −0.02‰ and −1.48‰) and MIF of odd isotopes (Δ¹⁹⁹Hg varying from −0.29‰ to 1.13‰). We also report for the first time a seasonal variation of MIF of even Hg isotopes (Δ²⁰⁰Hg) in wet precipitation. Our results may suggest that photoreduction in droplets or on the surface layer of snow crystals induces odd Hg isotope anomalies, while mass independent fractionation of ²⁰⁰Hg is probably triggered by photo-initiated oxidation occurring on aerosol or solid surfaces in the tropopause. The observed seasonal variation of even Hg isotope MIF (Δ²⁰⁰Hg decrease with ambient temperature) is possibly a powerful tool for meteorological research and may aid in monitoring related climate changes.
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