Non-Equilibrium Plasma Oxidation of Elemental Hg

Abstract

Atmospheric mercury, specifically oxidized mercury (HgII), is the largest anthropogenic pool of mercury in the atmosphere. This type of mercury can accumulate in the food chain after undergoing methylation and entering ecosystems through wet and dry deposition. It is important to evaluate the level of wet and dry deposition, but measurements of atmospheric mercury have been biased due to insufficient consideration of measurement uncertainty and metrological traceability, especially for low ambient concentrations of gaseous HgII species.To address this issue, a calibration method for HgII species has been presented based on atmospheric pressure non-equilibrium plasma oxidation of Hg0 to HgII. Hg0 is generated by reducing HgII in aqueous solution by SnCl2 and aeration, then oxidized to different HgII species using He plasma with traces of different reactive gases. The non-equilibrium plasma oxidation efficiencies, with corresponding expanded standard uncertainty values, were evaluated using a highly sensitive radiotracer. The efficiencies were found to be 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).The results demonstrate the potential of non-equilibrium plasma oxidation for reliable calibration of ambient air measurement instrumentation, leading to a better understanding of the sources, transport, and fate of this toxic pollutant in the environment. Knowing the chemistry and composition of atmospheric HgII species is crucial for assessing the extent of dry and wet deposition and mitigating the negative impacts of mercury pollution on human health and the environment.