High-precision measurements of mercury vapor in air: Design of a six-port-manifold mass flow controller system and evaluation of mass flow errors at atmospheric pressure

We constructed an atmospheric sampling system for Hg vapor that utilizes a single vacuum pump connected via a manifold to six separate mass flow controllers (MFC). The manifold system reduces the size and power requirements for collection of replicate samples, is ideally suited for use on meteorological towers, and achieves the precise control of air-sampling volumes required for computing air/surface exchange rates from concentration gradients of Hg vapor. In testing our air sampling systems, we found consistent calibration errors between the manufacturer's calibrations and a standard bubble flow meter. Errors as high as 30% decreased systematically with increasing flow rate to values of 3-5% at near-maximum flow. The relative error patterns established between adjacent MFC units in each system were found to be relatively stable over time. Using gold-coated sand amalgamation traps for Hg vapor and the flow correction factors computed from our calibrations, we routinely achieve precision for replicate measurements of Hg vapor in background air of 0.5-2% (expressed as relative standard errors of mean concentrations of 1.5-3.5 ng/m(3)). Application of the flow correction factors measurably decreases the level of bias between mean concentrations of Hg vapor measured with adjacent sampling systems and is necessary to reduce uncertainty associated with quantifying gradients in atmospheric concentrations.