A review of current knowledge concerning dry deposition of atmospheric mercury

The status of the current knowledge concerning the dry deposition of atmospheric mercury, including elemental gaseous mercury (Hg0), reactive gaseous mercury (RGM), and particulate mercury (Hgp), is reviewed. The air–surface exchange of Hg0 is commonly bi-directional, with daytime emission and nighttime deposition over non-vegetated surfaces and vegetated surfaces with small leaf area indices under low ambient Hg0 conditions. However, daytime deposition has also been observed, especially when the ambient Hg0 is high. Typical dry deposition velocities (Vd) for Hg0 are in the range of 0.1–0.4 cm s−1 over vegetated surfaces and wetlands, but substantially smaller over non-vegetated surfaces and soils below canopies. Meteorological, biological, and soil conditions, as well as the ambient Hg0 concentrations all play important roles in the diurnal and seasonal variations of Hg0 air–surface exchange processes. Measurements of RGM deposition are limited and are known to have large uncertainties. Nevertheless, all of the measurements suggest that RGM can deposit very quickly onto any type of surface, with its Vd ranging from 0.5 to 6 cm s−1. The very limited data for Hgp suggest that its Vd values are in the range of 0.02–2 cm s−1.A resistance approach is commonly used in mercury transport models to estimate Vd for RGM and Hgp; however, there is a wide range of complexities in the dry deposition scheme of Hg0. Although resistance-approach based dry deposition schemes seem to be able to produce the typical Vd values for RGM and Hg0 over different surface types, more sophisticated air–surface exchange models have been developed to handle the bi-directional exchange processes. Both existing and newly developed dry deposition schemes need further evaluation using field measurements and intercomparisons within different modelling frameworks.