While wet deposition of mercury (Hg) is well quantified by Hg deposition networks, it remains challenging to determine the surface-atmosphere exchange of gaseous elemental Hg(GEM). This generates great uncertainty in recent estimates of global surface-atmosphere fluxes. Improving quantification of surface-atmosphere GEM exchange is of paramount importance to research and policy making because the GEM flux is a major control on how fast the environment will recover from anthropogenic Hg pollution. The first annual Hg budget based on continuous measurements of peatland-atmosphere Hg⁰ exchange using a relaxed eddy accumulation (REA) system revealed that the annual net GEM emission (9 µg m^-2) exceeded annual wet bulk deposition (4 µg m^-2) and annual discharge export (1 µg m-²). We hypothesize that the net loss of Hg from this peatland is explained by the halving of atmospheric Hg concentrations from peak levels in the 1970’s. These reductions could have turned the peatland from being a sink of Hg for millennia into a source of Hg emission. The Hg concentration gradients in the superficial peat from a chronosequence of nearby peatlands support this interpretation. We suggest that the strong Hg evasion demonstrated in this study means that open boreal peatlands and thus downstream ecosystems may recover more rapidly from past atmospheric Hg deposition than previously assumed.

Given the current international efforts to protect human health and the environment from the adverse effects of mercury in accordance with the United Nation’s 2013 Minamata Convention on Mercury, we believe our findings contribute to a better understanding of mercury cycling in northern peatlands and their role for the mercury status of fresh water fish in the Northern Hemisphere. Our findings also raise the question as to whether recent reductions of atmospheric mercury concentrations lead to changes in the sink/source strength of other ecosystems as well, such as forests and oceans. We believe that micrometeorological flux measurement techniques should be applied more widely to define the balance between new emissions and re-emission for different ecosystems. These findings also point to the need to develop direct eddy covariance as a “gold standard” for spatially and temporally representative measurements. Progress in this direction from months of measurements over the Baltic Sea and a temperate grasslands (with eddy covariance used on the latter site) show the promise for better quantification.