Comparison of the Chemical Composition of Gaseous Oxidized Mercury in the Atmosphere at Three Different Locations in Nevada

Abstract:

There are three forms of mercury (Hg) in the atmosphere, gaseous elemental Hg (GEM), gaseous oxidized Hg (GOM), and particle-bound Hg (PBM). All forms of Hg are deposited from the atmosphere; however, because oxidized Hg is more available for methylation in ecosystems it is of significant concern. Recent work has suggested that different chemical compounds of GOM exist in the atmosphere. Each has unique chemical and physical properties. Understanding the relative abundances and properties of GOM in the atmosphere is critical for assessing the potential impacts on human and ecological health. A newly developed active system that measures the chemical form and concentration of GOM was deployed with triplicate nylon and cation-exchange membranes, respectively. Membranes were deployed and collected every two weeks at highway, free-troposphere, and agricultural impacted sites. Nylon membranes are used to estimate potential GOM compounds using thermodesorption and cation-exchange membranes are assumed to collect all GOM. The objective of this study was to understand distribution and chemistry of GOM compounds in the atmosphere, at these three locations with different oxidants. Thermodesorption profiles were generated using a laboratory system consisting of a custom-built eight-port manifold system and compared to profiles developed from samples collected from the field. GOM profiles developed in the laboratory were created from permeation of GOM from solid compounds: HgCl₂, HgBr₂, HgO, Hg(NO₃)₂, and HgSO₄. Results indicate different forms of GOM across space and time. GOM concentrations were observed to be 33% higher at the free-troposphere site compared to the highway-impacted site. Data suggest that different Hg reactions are occurring at different sites.