An evaluation of direct measurement techniques for mercury dry deposition

摘要

In this project, several surrogate surfaces designed to directly measure Hg dry deposition were investigated. Static water surrogate surfaces (SWSS) containing deionized (DI), acidified water, or salt solutions, and a knife-edge surrogate surface (KSS) using quartz fiber filters (QFF), KCl-coated QFF and gold-coated QFF were evaluated as a means to directly measure mercury (Hg) dry deposition. The SWSS was hypothesized to collect deposited elemental mercury (Hg°), reactive gaseous/oxidized mercury (RGM), and mercury associated with paniculate matter (Hg_((p))) while the QFF, KCl-coated QFF, and gold-coated QFF on the KSS were hypothesized to collect Hg_((p)). RGM + Hg_((p)), and Hg° + RGM + Hg_((p)), respectively. The Hg flux measured by the DI water was significantly smaller than that captured by the acidified water, probably because Hg° was oxidized to Hg~(2+) which stabilized the deposited Hg and decreased mass transfer resistance. Acidified BrCl, which efficiently oxidizes Hg°, captured significantly more Hg than other solutions. However, of all collection media, gold-coated QFFs captured 6 to 100 times greater Hg mass than the other surfaces, probably because there is no surface resistance for Hg° deposition to gold surfaces. In addition, the Hg~° concentration is usually 100-1000 times higher than RGM and Hg_((p)). For all other media, co-located samples were not significantly different, and the combination of daytime plus nighttime results were comparable to 24-h samples, implying that Hg°, RGM and Hg_((p)) were not released after they deposited nor did the surfaces reach equilibrium with the atmosphere. Based on measured Hg ambient air concentrations and fluxes, dry deposition velocities of RGM and Hg° to DI water and other surfaces were 5.6 ± 5.4 and 0.005-0.68cms~(-1) in this study, respectively. These results suggest surrogate surfaces can be used to measure Hg dry deposition; however, extrapolating the results to natural surface can be challenging.