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.

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Coal‐fired power plants are one of the principal sources of mercury to the atmosphere. The form this mercury takes is the predominant factor determining its fate after emission. Recent ground‐level field and modeling studies suggest that oxidized mercury in stack emissions is converted into elemental mercury in the plume. We present here aircraft‐based plume mercury measurements taken by Environment Canada in 2000 at the Nanticoke Generating Station as part of the Health Canada Toxic Substances Research Initiative Metals in the Environment Research Network. Although the average mercury speciation observed in the Nanticoke plume (82% Hg0, 13% Hg(II)(g), 5% Hg(P), by mass) appears to be distinct from the average mercury speciation in the Nanticoke stacks (53% Hg0, 43% Hg(II)(g), 4% Hg(P)), we find that the in‐plume elemental mercury concentrations as a whole can be explained by plume dilution after emission. The discrepancy between in‐stack and in‐plume Hg(II) concentrations is statistically significant, yet is not associated with a transformation of Hg(II) to Hg0. Sampling biases associated with the differing techniques used to measure Hg(II) in‐stack and in‐plume may reconcile the concentration discrepancy without invoking novel chemical reactions or physical processes. Although the mercury speciation of the Nanticoke plume influences local mercury deposition, the majority of the mercury emitted is transported out of the surrounding area.