Most atmospheric chemical measurements systems are routinely calibrated against traceable standards, and uncalibrated measurements are usually considered inferior, if not suspect. Atmospheric elemental mercury (Hg) measurements are typically calibrated by manual injection of Hg from a saturated vapor source, but manual injections are challenging, and competing vapor pressure equations differ by about 10%. Oxidized Hg measurements are almost never calibrated at all.

Efforts to develop Hg calibration methods have been undertaken for several decades, but no traceable, field-deployable method has yet been established other than elemental Hg manual injections. We have developed a permeation tube-based, field-ready calibration system that is able to output concentrations of elemental and oxidized Hg that are relevant to the ambient atmosphere (100 to 1,000 pg/m3 for oxidized Hg; 0.5 to 10 ng/m³ for elemental Hg) and demonstrated the functionality of this system in lab and field settings. The standard deviation of HgBr₂ output from the calibrator depends on the generated concentration, varying from 3% to 7% of the measured mean for concentrations from 310 to 550 pg/m³. Permeation rates of elemental mercury and mercury compounds used in the system are traceable to SI gravimetric standards. The system incorporates an inert dynamic dilution process that allows for rapid variation of the output concentration. Changing from one concentration to another requires an equilibration time of about 10 minutes, allowing for multi-point calibration of elemental and oxidized mercury. This automated system allows for routine, independent, SI-traceable checks on atmospheric Hg instrumentation in real field conditions.