环境地球化学国家重点实验室知识库

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 (delta Hg-202 between -0.02 parts per thousand and -1.48 parts per thousand) and MIF of odd isotopes (Delta Hg-199 varying from -0.29 parts per thousand to 1.13 parts per thousand). We also report for the first time a seasonal variation of MIF of even Hg isotopes (Delta Hg-200) 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 Hg-200 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 (Delta Hg-200 decrease with ambient temperature) is possibly a powerful tool for meteorological research and may aid in monitoring related climate changes. (C) 2012 Elsevier Ltd. All rights reserved.