An evaluation of direct measurement techniques for mercury dry deposition

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 (Hg0), reactive gaseous/oxidized mercury (RGM), and mercury associated with particulate 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 Hg0 + RGM + Hg(p), respectively. The Hg flux measured by the DI water was significantly smaller than that captured by the acidified water, probably because Hg0 was oxidized to Hg2+ which stabilized the deposited Hg and decreased mass transfer resistance. Acidified BrCl, which efficiently oxidizes Hg0, 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 Hg0 deposition to gold surfaces. In addition, the Hg0 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 Hg0, 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 Hg0 to DI water and other surfaces were 5.6 ± 5.4 and 0.005–0.68 cm s−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.

Research Highlights
► Knife-edge surrogate surface samplers measured mercury dry deposition.
► Gold-coated quartz filters (QFF) and acidified solutions were used as sampling media.
► Acidified solutions collected more Hg dry deposition than non-acidified solutions.
► Gold-coated QFFs had the highest Hg capture efficiency.
► Vds of RGM and Hg0 to DI water and other surfaces were 5.6 ± 5.4 and 0.005–0.68 cm s−1, respectively.