Accurate estimates of oxidized mercury dry deposition can only be obtained if measurements of ambient oxidized mercury concentrations are accurate.  KCl-coated denuders have been used to measure oxidized mercury in the past, but these systems are biased low, and the amount of bias is dependent on location and atmospheric conditions.  New technology is critically needed to replace KCl denuder-based measurement systems.

We developed a dual channel atmospheric mercury measurement system that fills this gap.  The system consists of a Tekran 2537 analyzer that alternately samples air that passes through a pyrolyzer and air that passes through a series of cation-exchange membranes.  The pyrolyzer reduces all mercury compounds to elemental mercury, providing a measure of total mercury in the atmosphere.  The cation exchange membrane channel measures elemental mercury, and we calculate oxidized mercury as the difference between these two.  We developed software to reprocess the 2537 analyzer’s raw detector output in real time, allowing us to achieve much more stable measurements than the analyzer outputs natively.  With this system, we have been able to routinely achieve detection limits for oxidized mercury of 10-15 pg/m³ (hourly averages). 

We used a permeation tube-based automated calibration system to verify the performance of the dual channel measurement system in ambient air.  We found that the system recovers 100% of the oxidized mercury compounds (HgCl₂ and HgBr₂) injected into it as oxidized mercury.  In similar tests, KCl denuder-based systems have recovered only 30-50% of injected oxidized mercury compounds as oxidized mercury. In KCl denuder-based systems, the remainder is recovered as elemental mercury, since much of the oxidized mercury captured by KCl denuders is reduced to elemental mercury and lost from the denuder. 

In deployments in Colorado and Utah, the new dual channel system has measured oxidized mercury concentrations that are higher than those reported using KCl denuder-based measurement systems.  Even during a cold Utah winter characterized by persistent, multi-day inversion conditions, 24-hr average oxidized mercury concentrations were in the range of 20 pg/m³.  Daytime values during summer routinely exceeded 100 pg/m³.  These higher oxidized mercury values show that the impact of oxidized mercury to the environment via dry deposition is higher than has previously been assumed.