Validating an Evaporative Calibrator for Gaseous Oxidized Mercury

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 HgCl(2) solution was recently developed, the purpose of our work was to extensively evaluate its performance. A highly specific and sensitive (197)Hg 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) HgCl(2) gas concentration to 39.4% for 5.90 ng m(−3) HgCl(2) 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 (ΔH(ads)); a ΔH(ads) 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 HgCl(2) 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 HgBr(2) gas calibration; the recoveries were much lower for HgBr(2) gas than for HgCl(2) gas even at a high HgBr(2) gas concentration (>1 µg m(−3)). As HgBr(2) is often used as a proxy for various atmospheric HgBr species, the suitability of the unit for such calibration must be further developed.