An examination of methods for the measurements of reactive gaseous mercury in the atmosphere

Reactive gaseous Hg (RGHg), usually assumed to be HgCl₂, may dominate the total Hg depositional flux due to its higher surface reactivity and water solubility. Three methods are currently used for RGHg sampling: multi-stage filter packs, refluxing mist chambers, and KCl-coated denuders, but none of these methods are considered standard. Field comparisons were performed at Chesapeake Biological Laboratory (CBL) to test if these methods could give comparable results. Mist chambers and denuders were operated continuously for 24 h in some cases to observe the diurnal variation. All methods demonstrated the dynamic fluctuation of atmospheric RGHg, ranging from a few picograms per cubic meter to more than 500 pg/m³. These methods also reported similar temporal RGHg trends. At low RGHg levels, the denuder tended to report higher values of RGHg relative to the filter pack, while mist chamber values were generally in agreement with the filter pack. Discrepancy among methods was more significant under higher RGHg levels. Considering the uncertainties associated with these methods, our data suggest that these methods did produce comparable results. The 24-h continuous measurements showed that R6Hg was usually undetectable at night. However, our data also suggest factors in addition to photochemistry, such as movement and mixing of air masses, are influencing the distribution of RGHg at CBL.