Development of an Understanding of Reactive Mercury in Ambient Air: A Review

Abstract

This review focuses on providing the history of measurement efforts to quantify and characterize the compounds of reactive mercury (RM), and the current status of measurement methods and knowledge. RM collectively represents gaseous oxidized mercury (GOM) and that bound to particles. The presence of RM was first recognized through measurement of coal-fired power plant emissions. Once discovered, researchers focused on developing methods for measuring RM in ambient air. First, tubular KCl-coated denuders were used for stack gas measurements, followed by mist chambers and annular denuders for ambient air measurements. For ~15 years, thermal desorption of an annular KCl denuder in the Tekran® speciation system was thought to be the gold standard for ambient GOM measurements. Research over the past ~10 years has shown that the KCl denuder does not collect GOM compounds with equal efficiency, and there are interferences with collection. Using a membrane-based system and an automated system—the Detector for Oxidized mercury System (DOHGS)—concentrations measured with the KCl denuder in the Tekran speciation system underestimate GOM concentrations by 1.3 to 13 times. Using nylon membranes it has been demonstrated that GOM/RM chemistry varies across space and time, and that this depends on the oxidant chemistry of the air. Future work should focus on development of better surfaces for collecting GOM/RM compounds, analytical methods to characterize GOM/RM chemistry, and high-resolution, calibrated measurement systems.

Improved Mechanistic Model of the Atmospheric Redox Chemistry of Mercury

“…Hg 0 is emitted to the atmosphere by mining, fuel combustion, and volcanism, and by volatilization of previously deposited Hg. , The Hg 0 oxidation pathways and the speciation of Hg II remain highly uncertain. − The Br atom is considered to be a major Hg 0 oxidant. − The oxidation of Hg 0 to Hg II by Br takes place in two steps, beginning with the formation of a BrHg I intermediate that then undergoes further oxidation to Hg II . − NO 2 and HO 2 have been thought to be the main BrHg I oxidants, but the BrHg II ONO and BrHg II OOH products are rapidly photolyzed . Preliminary theoretical calculations by Saiz-Lopez et al show that BrHg I may react rapidly with ozone to produce a BrHg II O radical, which can then be stabilized to nonradical Hg II forms by subsequent reactions. − , …”

Elevated Gaseous Oxidized Mercury Revealed by a Newly Developed Speciated Atmospheric Mercury Monitoring System

“…To achieve the best RM collecting method, the Reno Atmospheric Mercury Intercomparison eXperiment (RAMIX) project investigated different RM adsorption surfaces (e.g., nylon membrane, KCl-denuder, and cation exchange membrane (CEM)) and selected CEM as the best surface to collect RM in both laboratory and field experiments. , Based on the performance of the CEM, a Reactive Mercury Active System was developed to collect almost all RM, which was believed to greatly improve the accuracy of RM measurement. Collecting and analyzing Hg 2+ /RM on CEM requires at least one week of sampling to ensure a sufficient amount of Hg for analysis . Lyman et al incorporated CEM with an online pyrolizer-based method and achieved separated measurement of RM and Hg 0 in 20 min .…”

Spatial Distribution of Atmospheric Mercury Species in the Southern Ocean

“…We also evaluated our GOM observed results based on this method and found that the bias in our observations appeared to be negligible (see details in the supplementary information). In fact, Gustin's recent review paper also suggested that "We expect that KCl-denuder derived data is best for polar regions or high elevation locations in the free troposphere for these areas are dominated by halogenated compounds and dry air" in Section 6 of this paper (Gustin et al, 2021). In summary, we think that the uncertainty of GOM measurement from the Tekran system may not fundamentally change our main discussion and conclusions in this study based on our evaluations of GOM data, and we caution that the GOM results reported in this study should be considered as the lower limits of atmospheric GOM (Wang et al, 2014).…”