Uncertainties of Gaseous Oxidized Mercury Measurements: How to Move Forward?
Abstract:The atmosphere is a primary pathway by which mercury (Hg) enters ecosystems. To adequately assess the risk to humans and wildlife the chemistry of atmospheric Hg must be understood, along with the processes controlling deposition and assimilation into ecosystems. Given current high uncertainties with the Tekran analytical system used to measure gaseous oxidized Hg (GOM) and particulate bound Hg (PBM) recent work has combined these for data analyses as Reactive Hg (RM). GOM measurements using this system are biased low due to lack of efficient collection of different compounds and interferences with water vapor and ozone. Because of this, estimation of GOM dry deposition to ecosystems using concentrations derived from this instrument in models are biased low, and dry deposition measured with surrogate surfaces are typically higher than model values. It is likely that different compounds of GOM are present in the atmosphere. These will have different chemical and physical properties that will influence atmospheric behavior. Field and laboratory results have shown that a new active Hg collection system using membranes collected higher GOM concentrations than the Tekran KCl-coated denuder. Field data from 5 locations showed that different GOM compounds were present across space and time.
More work is needed to calibrate existing methods and develop methods to measure GOM. New approaches include collection of GOM on a membrane to determine concentrations; couple this with a ramp heating thermo-desorption through a pyrolyzer into a Tekran 2537, or GC-MS, to understand the chemistry of GOM compounds; and passive samplers for long-term global Hg monitoring of GOM. Lastly, during the NOMADSS/SAS experiment 2 Hg analyzers were used, with one having a cation exchange membrane and the other a pyrolyzer in-line to get GOM by difference. In all cases, calibration sources must be used to understand the GOM collection and recovery efficiency.