Validating an Evaporative Calibrator for Gaseous Oxidized Mercury

Abstract

Understanding atmospheric mercury chemistry is the key for explaining the biogeochemical cycle of mercury and for improving the predictive capability of computational models. Increased efforts are being made to ensure comparable Hg speciation measurements in the air through establishing metrological traceability. While traceability for elemental mercury has been recently set, this is by no means the case for gaseous oxidized mercury (GOM). Since a calibration unit suitable for traceable GOM calibrations based on evaporation of HgCl2 solution was recently developed, the purpose of our work was to extensively evaluate its performance. A highly specific and sensitive 197Hg radiotracer was used for validation over a wide range of concentrations. By comparing experimental and calculated values, we obtained recoveries for the calibration unit. The average recoveries ranged from 88.5% for 1178 ng m−3 HgCl2 gas concentration to 39.4% for 5.90 ng m−3 HgCl2 gas concentration. The losses were due to the adsorption of oxidized Hg on the inner walls of the calibrator and tubing. An adsorption isotherm was applied to estimate adsorption enthalpy (ΔHads); a ΔHads value of −12.33 kJ mol−1 was obtained, suggesting exothermal adsorption. The results of the calibrator performance evaluation suggest that a newly developed calibration unit is only suitable for concentrations of HgCl2 higher than 1 µg m−3. The concentration dependence of recoveries prevents the system from being used for calibration of instruments for ambient GOM measurements. Moreover, the previously assessed uncertainty of this unit at µg m−3 level (2.0%, k = 2) was re-evaluated by including uncertainty related to recovery and was found to be 4.1%, k = 2. Calibrator performance was also evaluated for HgBr2 gas calibration; the recoveries were much lower for HgBr2 gas than for HgCl2 gas even at a high HgBr2 gas concentration (>1 µg m−3). As HgBr2 is often used as a proxy for various atmospheric HgBr species, the suitability of the unit for such calibration must be further developed.

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

“…Additionally, the precision was unsatisfactory due to the time dependence of the calibrator output. 19 Therefore, the only calibration systems that can be compared to the NTP calibration system are those that were evaluated at ambient Hg II concentration levels. As such, Hg II permeation tubes are currently the only calibration system suitable for comparison.…”

“… 18 Liquid-evaporative calibrators perform well at flue gas Hg II concentrations while having a biased low output at ambient Hg II concentrations, mainly due to the adsorption and reactive nature of Hg II . 19 Permeation and liquid-evaporative calibrators may also be subject to Hg 0 impurities. 14 No calibration is currently available for PBM measurements.…”

“…The use of the 197 Hg radiotracer (half-life 2.671 days) enables validation at ambient concentrations due to its high specific activity and absence of blanks and contamination issues ( 197 Hg is not present in the nature). 19 , 25 , 26 …”

“…Currently, the available calibrations for oxidized mercury species are based on either permeation , or liquid evaporation of Hg II salts. , Although permeation calibrators are promising, they are still in the development stage, showing inconsistent results at permeation rates relevant to ambient Hg II concentrations . Liquid-evaporative calibrators perform well at flue gas Hg II concentrations while having a biased low output at ambient Hg II concentrations, mainly due to the adsorption and reactive nature of Hg II . Permeation and liquid-evaporative calibrators may also be subject to Hg 0 impurities .…”

Behavior of KCl sorbent traps and KCl trapping solutions used for atmospheric mercury speciation: stability and specificity

“…It is generally accepted that all forms of gaseous mercury are collected on gold traps and the measurement represents TGM (Dumarey, 1985;Shafawi et al, 1999). However, there is disagreement concerning whether the applied sampling protocols are optimal for GOM (Gustin and Jaffe, 2010).…”