Hg 0 + I 2 → HgI 2 Absolute N 2 , 1 atm 296 ± 1 < (1.27 ± 0.58) × 10 -19 Raofie et al. 2 M06-2X/aug-cc-pVTZ-PP High pressure limit 3.94 × 10 -14 T 1.06 e -159080/RT Auzmendi-Murua et al. 3 Hg 0 + I → HgI RRKM/B3LYP N 2 , 1 atm 180-400 4.0 × 10 -13 (T/298) -2.38 Goodsite et al. 4 Hg 0 + Br 2 → HgBr 2 Absolute Air, N 2 , 1 atm 298 ± 1 < (9 ± 2) × 10 -17 Ariya et al. 5 Absolute Air, 1 atm ∼298 No reaction detected Sumner et al. 6 Absolute Air, 1 atm 296 (6.0 ± 0.5) × 10 -17 Liu et al. 7 CCSD(T)/AVTZ 1 atm 298-2000 1.62 -9 e -110800/RT Wilcox and Okano 8 M06-2X/aug-cc-pVTZ-PP High pressure limit 4.70 × 10 -14 T 1.06 e -169190/RT Auzmendi-Murua et al. 3 Hg 0 + BrO → HgBrO Relative N 2 , 1 atm 298 10 -15 < k < 10 -13 Raofie and Ariya 9 Hg 0 + Br → HgBr Ab initio N/A, 1 atm 1.01 × 10 -12 e 1738/RT Khalizov et al. 10 RRKM/B3LYP N 2 , 1 atm 200-300 3.7 × 10 -13 (T/298) -2.76 Goodsite et al. 4 ; Goodsite et al. 11 Absolute N 2 , 0.26-0.79 atm 243-293 (1.46 ± 0.36) × 10 -32 [cm 6 molec -2 s -1 ] Donohoue et al. 12 (T/298) (-1.86±1.49) CCSD(T) Ar, 1 atm 260 1.2 × 10 -12 Shepler et al. 13 Relative Air, N 2 , 1 atm 298 ± 1 (3.2 ± 0.9) × 10 -12 Ariya et al. 5 Absolute CF 3 Br, 0.26 atm 397 ~3 × 10 -16 molec -1 s -1 Greig, G. et al. 14 CCSD(T)/AVTZ 1 atm 298-2000 6.64 × 10 -14 (T/298) -0.859 Wilcox and Okano HgBr + Br → HgBr 2 Absolute CF 3 Br, 0.26 atm 397 ~7 × 10 -14 Greig, G. et al. 14 RRKM/B3LYP N 2 , 1 atm 180-400 2.5 × 10 -10 (T/298) -0.57 Goodsite et al. 4 CCSD(T)/AVTZ 1 atm 298-2000 3.32 × 10 -12 (T/298) -9.18 Wilcox and Okano CCSD(T)/aVTZ 1 atm 298 6.33 × 10 -11 Dibble et al. 15 ; Wang et al.

In this review, we synthesize the current knowledge on mercury (Hg) content and sources in foliage and vegetated ecosystems and the importance of vegetation to global Hg cycling. By means of a global database of over 35,000 samples across 416 sites, we discuss global Hg concentrations in all major tissues, and mechanisms of vegetation Hg uptake. Hg in aboveground vegetation largely originates from uptake of atmospheric gaseous elemental Hg (Hg(0)), whereas Hg in roots originates from a combination of uptake from soil and foliage-to-root transport. Vegetation Hg uptake from the atmosphere and transfer to soils is the major Hg source in all biomes. Using model sensitivity analyses with and without global vegetation present, we show that vegetation Hg uptake modulates atmospheric Hg(0) seasonality in the northern hemisphere and interhemispheric gradient. We estimate that vegetation uptake the global Hg pool in the atmosphere by approximately 660 Mg and reduces the Hg deposition to global oceans, which in the absence of vegetation might receive an additional 960 Mg yr -1 . We discuss future research needs to better constrain vegetation uptake mechanisms and their controlling physiological and environmental variables, improve model processes and address effects of climate and land use changes.

Abstract. This study is the most extensive study to date on the transport of mercury to the Arctic. Moreover, it is the first such study to use a fully-coupled, online chemical transport model, Environment Canada's Global/Regional Atmospheric Heavy Metals model (GRAHM), where the meteorology and mercury processes are fully integrated. It is also the only study to date on the transport of mercury across Canada. We estimated source attribution from Asia, North America, Russia and Europe at six arctic verification stations, as well as three subarctic and eight midlatitude Canadian stations.We have found that Asia, despite having transport efficiencies that were almost always lower than those of North America and often lower than those of Russia, was the dominant source of gaseous atmospheric mercury at all verification stations: it contributed the most mercury (29-37% at all stations, seasons and levels considered), its concentrations frequently explained nearly 100% of the variability in the concentrations produced by the simulation performed with full global emissions, particularly in the absence of local sources, and it generated the most long range transport (LRT) events, causing 43%, 67% and 75% of the events at the arctic, subarctic and midlatitude stations, respectively. For the Arctic, Russian transport efficiencies tended to be the strongest, as expected, while European and Asian efficiencies were lower and higher, respectively, than those found in the literature. This disagreement is likely produced by mercury's long lifetime relative to that of other pollutants. The accepted springtime preference for the trans-Pacific transport of Asian pollution was evident only in the midlatitude group of stations, being masked in the arctic and subarctic groups by the occurrence of atmospheric mercury depletion events. SomeCorrespondence to: D. Durnford (dorothy.durnford@ec.gc.ca) neighbouring arctic stations recorded dissimilar numbers of LRT events; despite their proximity, the behaviour of mercury at these stations was governed by different dynamics and transport pathways. The column burden of GEM in the lowest 5 km of the Northern Hemisphere was largest in summer from Asia, North America and Russia, but in winter from Europe. In the vertical, transport of mercury from all source regions occurred principally in the mid-troposphere.