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

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 δ202HgGEM (−0.04‰ to 0.52‰) but not in Δ199HgGEM (−0.17‰ to −0.27‰) or Δ200HgGEM (−0.10‰ to 0.05‰). δ202HgGEM was negatively correlated with CO and reflected air mass origins from Europe (high CO, low δ202HgGEM) and from the Atlantic Ocean (low CO, high δ202HgGEM). We suggest that the δ202HgGEM variations represent mixing of recent low δ202HgGEM European anthropogenic emissions with high δ202HgGEM northern hemispheric background GEM. In addition, Atlantic Ocean free troposphere air masses showed a positive correlation between δ202HgGEM and gaseous oxidized Hg (GOM) concentrations, indicative of mass-dependent Hg isotope fractionation during GEM oxidation. On the basis of atmospheric δ202HgGEM and speciated Hg observations, we suggest that the oceanic free troposphere is a reservoir within which GEM is readily oxidized to GOM.