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. Recent studies have reported both mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg isotopes in the nature. This highlights the potential application of Hg isotopes in biochemistry and geochemistry. Though precipitation is the main pathway in the transfer of Hg from the atmosphere to the earth’s surface, little was known about Hg isotopic composition in rainwater.
Dr. CHEN Jiubin and his workmates from the State Key laboratory of Environmental Geochemistry (SKLEG), Institute of Geochemistry, CAS (IGCAS) measured Hg isotopic compositions for rain and snow samples collected for the entire year of 2010 in Peterborough, ON, Canada, using the pre-concentration method developed by CHEN et al. (2010). 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‰). They also reported, for the first time, a seasonal variation of MIF of even Hg isotopes (Δ200Hg) in wet precipitation (see attached figure). Their results 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 paper of these results was just published in Geochimica Cosmochimica Acta (CHEN et al., 2012, Unusual fractionation of both odd and even mercury isotopes in precipitation from Peterborough, ON., Canada).
Their results also suggest that 200Hg anomalies are likely related to stratosphere incursion, presence of aerosols, oxidant intensity, solar irradiation and air mass movement. As a result, MIF of 200Hg could thus provide additional information about atmospheric chemistry and meteorological changes. Though the exact mechanisms inducing MIF of even Hg isotopes remain unknown up to now, the seasonal (temperature) variation of Δ200Hg is striking and may be a useful tool to help monitor related climate changes.
Figure A seasonal variation of MIF of even Hg isotopes (Δ200Hg) in wet precipitation (Image by IGCAS)
(By CHEN Jiubin)
