Odd isotope deficits in atmospheric Hg measured in lichens

Redox reactions govern mercury (Hg) concentrations in the atmosphere because fluxes (emissions and deposition), and residence times, are largely controlled by Hg speciation. Recent work on aquatic Hg photoreduction suggested that this reaction produces non-mass dependent fractionation (NMF) and that residual aquatic Hg(II)is characterized by positive delta199Hg and delta201Hg anomalies. Here, we show that atmospheric Hg accumulated in lichens is characterized by NMF with negative delta199Hg and delta201Hg values (-0.3 to -1 per thousand), making the atmosphere and the aquatic environment complementary reservoirs regarding photoreduction and NMF of Hg isotopes. Because few other reactions than aquatic Hg photoreduction induce NMF, photochemical reduction appears to be a key pathway in the global Hg cycle. Based on a NMF isotope mass balance, direct anthropogenic emissions may account for only 50 +/- 10% of atmospheric Hg deposition in an urban area of NE France. Furthermore, isotopic anomalies found in several polluted soils and sediments strongly suggests that an important part of Hg in these samples was affected by photoreactions and has cycled through the atmosphere before being stored in the geological environment. Thus, mercury isotopic anomalies measured in environmental samples may be used to trace and quantify the contribution of source emissions.