Atmospheric mercury (Hg) is currently measured as gaseous elemental Hg (GEM) in dual Ag cartridges, as gaseous oxidized Hg (GOM) by KCL-coated quartz traps, and as particle bound Hg (PBM) on a column of quartz chips and a quartz filter. GEM is thought to constitute 90 to 99 % of the Hg in the atmosphere and to be relatively inert. Suggested forms of the reactive GOM include HgCl₂, HgBr₂, HgO, and HgSO₄. The mechanisms important for their formation are unclear. The understanding of PBM is poor. Once collected, Hg is thermally desorbed from each unit and quantified as GEM using Cold Vapor Atomic Fluorescence Spectroscopy. There are no standard reference materials for GOM or PBM, and interferences have not been adequately explored.

Mae Sexauer Gustin, University of Nevada, Reno, USA, and colleagues compared the standard method to a new method. Here air samples are collected and the total amount of mercury as well as levels of elemental mercury are measured. The reactive mercury (RM) levels can be determined by subtracting the elemental numbers from the total numbers.

It was found that the standard method underestimates the levels of RM by two to three times. The KCl-coated denuder used in the standard method does not efficiently collect HgBr₂ and other RM compounds. The data point toward different forms of RM being present in the local atmosphere and being produced in-the-manifold. A potential mechanism for this could be heterogeneous reactions associated with aerosols containing nitrogen compounds and semivolatile organic compounds or homogeneous reactions influenced by ambient atmospheric chemistry.
Without understanding the chemistry of RM compounds, and the homogeneous and heterogeneous processes association with formation and destruction, the limitations of the instruments cannot adequately be assessed.

• Do We Understand What the Mercury Speciation Instruments Are Actually Measuring? Results of RAMIX,
  Mae Sexauer Gustin, Jiaoyan Huang, Matthieu B. Miller, Christianna Peterson, Daniel A. Jaffe, Jesse Ambrose, Brandon D. Finley, Seth N. Lyman, Kevin Call, Robert Talbot, Dara Feddersen, Huiting Mao, Steven E. Lindberg,
  Environ. Sci. Technol. 2013.
  DOI: 10.1021/es3039104