Reactions of gaseous mercury with atomic and molecular halogens: Kinetics, product studies, and atmospheric implications

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/X-2 where X = Cl, Br) at atmospheric pressure (750 +/- 1 Torr) and room temperature (298 +/- 1 K) in air and N-2. Kinetics of the reactions with X/X-2 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 Cl-2, Cl, Br-2, and Br were (2.6 +/- 0.2) x 10(-18), (1.0 +/- 0.2) x 10(-11), <(0.9 +/- 0.2) x 10(-16), and (3.2 +/- 0.3) x 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.