Calibration Approach for Gaseous Oxidized Mercury Based on Nonthermal Plasma Oxidation of Elemental Mercury

Atmospheric mercury measurements carried out in the recent decades have been a subject of bias largely due to insufficient consideration of metrological traceability and associated measurement uncertainty, which are ultimately needed for the demonstration of comparability of the measurement results. This is particularly challenging for gaseous Hg-II species, which are reactive and their ambient concentrations are very low, causing difficulties in proper sampling and calibration. Calibration for atmospheric Hg-II exists, but barriers to reliable calibration are most evident at ambient Hg-II concentration levels. We present a calibration of Hg-II species based on nonthermal plasma oxidation of Hg-0 to Hg-II. Hg-0 was produced by quantitative reduction of Hg-II in aqueous solution by SnCl2 and aeration. The generated Hg-0 in a stream of He and traces of reaction gas (O-2, Cl-2 , or Br-2) was then oxidized to different Hg-II species by nonthermal plasma. A highly sensitive Hg-197 radiotracer was used to evaluate the oxidation efficiency. Nonthermal plasma oxidation efficiencies with corresponding expanded standard uncertainty values were 100.5 +/- 4.7% (k = 2) for 100 pg of HgO, 96.8 +/- 7.3% (k = 2) for 250 pg of HgCl2, and 77.3 +/- 9.4% (k = 2) for 250 pg of HgBr2. The presence of HgO, HgCl2, and HgBr2 was confirmed by temperature-programmed desorption quadrupole mass spectrometry (TPD-QMS). This work demonstrates the potential for nonthermal plasma oxidation to generate reliable and repeatable amounts of Hg-II compounds for routine calibration of ambient air measurement instrumentation.