Isotope Tracing of Atmospheric Mercury Sources in an Urban Area of Northeastern France

Mercury (Hg) isotope composition was investigated in lichens over a territory of 900 km² in the northeast of France over a period of nine years (2001−2009). The studied area was divided into four geographical areas: a rural area, a suburban area, an urban area, and an industrial area. In addition, lichens were sampled directly at the bottom of chimneys, within the industrial area. While mercury concentrations in lichens did not correlate with the sampling area, mercury isotope compositions revealed both mass dependent and mass independent fractionation globally characteristic of each geographical area. Odd isotope deficits measured in lichens were smallest in samples close to industries, with Δ¹⁹⁹Hg of −0.15 ± 0.03 ‰, where Hg is thought to originate mainly from direct anthropogenic inputs. Samples from the rural area displayed the largest anomalies with Δ¹⁹⁹Hg of −0.50 ± 0.03‰. Samples from the two other areas had intermediate Δ¹⁹⁹Hg values. Mercury isotopic anomalies in lichens were interpreted to result from mixing between the atmospheric reservoir and direct anthropogenic sources. Furthermore, the combination of mass-dependent and mass independent fractionation was used to characterize the different geographical areas and discriminate the end-members (industrial, urban, and local/regional atmospheric pool) involved in the mixing of mercury sources.