Six-year mercury dry deposition estimates to various land covers surrounding monitoring sites in North America
Dry deposition of atmospheric mercury (Hg) to various land covers surrounding 24 sites in North America was estimated for the years 2009 to 2014, making use of the monitored air concentrations of gaseous elemental Hg (GEM), gaseous oxidized Hg (GOM), and particulate-bound Hg (PBM). Depending on location, multi-year mean annual Hg dry deposition was estimated to range from 5.1 to 23.8 µg m-2 yr-1 to forested canopies, 2.6 to 20.8 µg m-2 to non-forest vegetated canopies, 2.4 to 11.2 µg m-2 yr-1 to urban and built up land covers, and 1.0 to 3.2 µg m-2 yr-1 to water surfaces. GEM was the main contributor to Hg dry deposition over vegetated surfaces, accounting for 31 to 98% among the sites and land covers, while GOM plus PBM became more important over non-vegetated surfaces. The dominant role of GEM in the Hg dry deposition budget over vegetated surfaces was due to the relatively low concentrations of oxidized Hg. Seasonally-averaged Hg deposition was the highest in the winter and the spring and the lowest in the summer over most land covers. Interannual variations in the estimated annual Hg dry deposition were in the range of a factor of 1.3 to 2.0 at the six sites that had six complete years of data. The estimated dry deposition was assessed using litterfall measurements and modeled Hg emissions at the regional scale, and was demonstrated to be reliable although with large uncertainties.
This study concludes that in the rural or remote environment in North America, annual mercury dry deposition to vegetated surfaces is dominated by leaf uptake of elemental gaseous mercury. This is contrary to what was commonly assumed in earlier studies which frequently omitted GEM dry deposition as an important process. The percentage contribution of GEM dry deposition in the total mercury dry deposition is expected to increase should mercury emissions in North America continue declining, considering that background GEM will not decrease as fast as anthropogenic emissions. Net dry deposition dominated the air-surface flux exchange in all of the seasons except in the summer months at half of the sites where net emissions from soil and leaf stomata dominated. Data for longer periods are needed to evaluate the effectiveness of Hg emission control policies and to assess the trends of atmospheric mercury dry deposition for subsequent ecosystem impact studies.
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