Isotopic Composition of Gaseous Elemental Mercury in the Free Troposphere of the Pic du Midi Observatory, France

摘要

Understanding the sources and transformations of mercury (Hg) in the free troposphere is a critical aspect of global Hg research. Here we present one year of observations of atmospheric Hg speciation and gaseous elemental Hg (GEM) isotopic composition at the high-altitude Pic du Midi Observatory (2860 m above sea level) in France. Biweekly integrated GEM from February 2012 to January 2013 revealed significant variations in δ~(202)Hg_(GEM) (-0.04‰ to 0.52‰) but not in Δ~(199)Hg_(GEM) (-0.17‰ to -0.27‰) or Δ~(200)Hg_(GEM) (-0.10‰ to 0.05‰). δ~(202)Hg_(GEM) was negatively correlated with CO and reflected air mass origins from Europe (high CO, low δ~(202)Hg_(GEM)) and from the Atlantic Ocean (low CO, high δ~(202)Hg_(GEM)). We suggest that the δ~(202)Hg_(GEM) variations represent mixing of recent low δ~(202)Hg_(GEM) European anthropogenic emissions with high δ~(202)Hg_(GEM) northern hemispheric background GEM. In addition, Atlantic Ocean free troposphere air masses showed a positive correlation between δ~(202)Hg_(GEM) and gaseous oxidized Hg (GOM) concentrations, indicative of mass-dependent Hg isotope fractionation during GEM oxidation. On the basis of atmospheric δ~(202)Hg_(GEM) and speciated Hg observations, we suggest that the oceanic free troposphere is a reservoir within which GEM is readily oxidized to GOM.