Tracking Atmospheric Mercury Sources by Isotopic Fingerprinting

B.A. Bergquist, B. Sherwood Lollar, University of Toronto

1 PhD:

The long-term goal of the Bergquist research program is to use Hg isotopes in improve our understanding of the Hg biogeochemical cycle and to develop Hg isotopes as a proxy of past environmental changes. Mercury is a globally distributed metal that bioaccumulates in aquatic food webs leading to dangerous exposure to humans and wildlife. Mercury is often emitted the atmosphere in its reduced form, gaseous elemental Hg (GEM), by both primary natural (e.g. volcanic, hydrothermal) and anthropogenic sources (e.g. artisanal small scale gold mining, coal burning) and by secondary re-emissions from the ocean and soils. GEM is relatively stable and has a long residence time (~0.5 to 1 yr), which allows it to be distributed globally. Despite decades of research, many knowledge gaps hinder our understanding of both the modern and past Hg cycle and make it challenging to predict how changes in emissions and climate will affect the Hg cycle along with limiting our ability to utilize Hg and Hg isotopes as proxies of past environmental change. For example a recent assessment of GEM exchange to and from terrestrial surfaces highlighted that very large uncertainties still exist over the controls and fluxes of GEM especially in forested ecosystems to the point that hinders our ability to determine whether some ecosystems are net sources or sinks of Hg. Another area that is heatly debated is the relative contribution of different sources of Hg to the atmosphere such as coal combustion and artisanal and small scale gold mining at local, regional and global scales.

While mercury isotopic fingerprinting is increasingly recognized as a powerful tool for tracking sources of mercury, its application to atmospheric source tracking is hampered by the difficulty to collect sufficient amounts of mercury from air for reliable isotopic quantification and/or by the possibility of imposing inconsistent levels of fractionation during sampling. Recently, a new highly precise passive air sampler for GEM concentrations was developed (McLagan et al., 2016). This low-cost sampler can collect GEM from air for periods of at least one year, and can collect sufficient mercury for isotopic analysis even at typical background concentrations. The U of Toronto Trace Metal and Metal Isotope Laboratory is seeking a PhD to lay the groundwork for confidently applying the passive Hg sampling technique for atmospheric source identification.

1. Perform a number of laboratory and field experiments to confirm the extent and reproducibility of the mercury isotope composition and potential fractionation occurring during passive sampling.
2. Conduct a reconnaissance of the spatial and temporal variability of the isotopic signature of mercury in the atmosphere.
3. Deploy the passive sampler along transects of increasing distance from known sources of gaseous mercury to establish the extent to which a source’s possible unique isotopic fingerprint fades into the regional background signal by dilution.