Reactions of Gaseous Mercury with Atomic and Molecular Halogens:  Kinetics, Product Studies, and Atmospheric Implications

Abstract

Rapid depletions of atmospheric mercury (Hg 0 ) and ozone coinciding with high halogen concentrations have recently been observed in several high-Arctic and sub-Arctic regions. The lack of kinetic data on the halogen-initiated reactions of elemental mercury precluded the drawing of any conclusions on the chemistry of these mercury depletion events. We carried out extensive kinetic and product studies on the reactions of gaseous Hg 0 with molecular and atomic halogens (X/X2 where X = Cl, Br) at atmospheric pressure (750 ± 1 Torr) and room temperature (298 ± 1 K) in air and N2. Kinetics of the reactions with X/X2 was studied using relative and absolute techniques by cold vapor atomic absorption spectroscopy (CVAAS) and gas chromatography with mass spectroscopic detection (GC-MS). The measured rate constants for reactions of Hg 0 with Cl2, Cl, Br2, and Br were (2.6 ± 0.2) × 10 -18 , (1.0 ± 0.2) × 10 -11 , <(0.9 ± 0.2) × 10 -16 , and (3.2 ± 0.3) × 10 -12 cm 3 molecule -1 s -1 , respectively. The reaction products were analyzed in the gas phase, from the suspended aerosols, and from the wall of the reactor using MS, GC-MS, and inductively coupled plasma mass spectrometry (ICP-MS). The major products identified were HgCl2 and HgBr2 adsorbed on the wall. Suspended aerosols, collected on the micron filters, contributed to less than 0.5% of the reaction products under our experimental conditions. The importance of halogen reactions of mercury in the formation of particulate matter, which tends to be bioaccumulative, is discussed herein.

Recent Advances in Atmospheric Chemistry of Mercury

“…Recently, Sun et al [24] determined the rate coefficients using a relative rate technique with ethane and 2-chloro-propane as references. The rate coefficient for the Hg 0 + Cl• reaction was reported to be (1.8 ± 0.5) × 10 −11 cm 3 molecule −1 s −1 , at 100 kPa and 298 ± 3 K, which was in agreement with three earlier laboratory studies by Horne et al [27], Spicer et al [20] and Ariya et al [19] (1.0-6.4 × 10 −11 cm 3 molecule −1 s −1 ). However, the reported rate coefficient was about two orders of magnitude larger than that determined by Donohoue et al [28] (5.4 × 10 −13 cm 3 molecule −1 s −1 ) and one order of magnitude smaller than that determined by Byun et al [29] (1.2 × 10 −10 cm 3 molecule −1 s −1 ).…”

“…Measurements using a scanning mobility particle sizer by Sun et al [24] also showed that Hg products largely existed as wall deposits in agreement with the previous product analysis by Ariya et al [19]. It has been suggested that the presence of aerosols in the Aitken mode during the oxidation reaction may be initiated by the vapor nucleation of mercury-containing products.…”

“…Previous quantum calculations by Dibble et al [47] demonstrated that BrHg could react with the abundant radicals in the atmosphere to form stable BrHgY compounds where Y is NO 2 , HO 2 , ClO, or BrO. Previous laboratory studies reported apparent rate coefficients in the range of (3.6 ± 0.9) × 10 −13 to (3.2 ± 0.3) × 10 −12 cm 3 molec −1 s −1 , at 1 atm and 298 K [19,20,22]. Theoretical studies estimated the rate constants to be from 9.8 × 10 −13 to 2.1 × 10 −12 cm 3 molec −1 s −1 , at 1 atm and 298 K [3,21,23].…”

“…Similar to Hg 0 -Br oxidation, the oxidation of Hg 0 by atomic Cl was believed to occur via a two-step chlorination process with HgCl as the intermediate [19,24,28]. Theoretical calculations indicated that after the initializing step to form the HgCl intermediate, the secondary oxidation of HgCl could be carried out by NO 2 , HO 2 , ClO, or BrO [47].…”

Global lifetime of elemental mercury against oxidation by atomic bromine in the free troposphere

“…Other theoretical [Khalizov et al, 2003;Goodsite et al, 2004] and experimental [Ariya et al, 2002] estimates of k 1 are faster (see Table 1), although Donohoue et al [2006] argue that these values are less accurate. The fastest k 1 value with reported temperature dependence [Khalizov et al, 2003] implies t global = 160 days, after recalculating k 2 to keep the k 1 :k 2 balance [Goodsite et al, 2004].…”