Trichloroethene and tetrachloroethene: tropospheric probes for Cl- and Br-atom reactions during the polar sunrise

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Ariya, P. A. ; Catoire, V. ; Sander, R. ; Niki, H. ; Harris, G. W, (2011)

We report the results of laboratory and modeling investigations of the atmospheric fate of chlorinated ethenes and their rôle as indicators of halogen reactions in the springtime Arctic troposphere. The kinetics and mechanism of the gas-phase reactions of Cl- and Br-atoms with tetrachloroethene were studied using a Fourier transform infrared spectrometer (FTIR) in 93.3 kPa air and T = 296 ± 2 K. Along with our previous study on Cl and Br atom reactions of trichloroethene, using the known rate of the Cl + ethane reaction as reference, the values of 7.2 ± 0.2 × 10−11 and 3.8 ± 0.2 × 10−11 cm3 molecule−1 s−1 were obtained for the Cl-atom reaction rate constants of tri- and tetrachloroethene, respectively. For the Br-atom reactions, using ethene and propane as the reference molecules, we report the absolute values of 1.1 ± 0.1 × 10−13 and 9.0 ± 0.1 × 10−17 cm3 molecule−1 s−1 for the rates of Br attack on tri- and tetrachloroethene. The major products were XCl2C-C(O)Cl (X = H in trichloroethene and X = Cl in tetrachloroethene) and XBrClC-C(O)Cl in Cl-atom and Br-atom initiated reactions, respectively. We also observed phosgene and formyl chloride in the reactions of trichloroethene and phosgene in the tetrachloroethene reactions and report the branching ratios for these channels. Our observations show that the addition of Cl-atoms to the less substituted carbon is the major reaction pathway and that a Cl-atom is subsequently released after either Cl- or Br-atoms add to trichloroethene or tetrachloroethene, leading to the recycling of active halogen. We carried out a box modelling exercise to apply the kinetic and mechanistic information obtained in this work to the interpretation of measurements of C2HCl3, C2Cl4 and other hydrocarbons in the Arctic troposphere. Our results demonstrate that the presence of both Cl- and Br-atoms is required to explain the decreases in the concentrations of these species during ozone depletion events in the Arctic, that the amount of Cl- and Br-atoms required to account for the observed decreases of chlorinated ethenes is also sufficient to completely destroy ozone. However, at the low concentrations of chlorinated species observed, the cycling of Br to Cl through the studied reactions is not a significant effect in the atmosphere.DOI: 10.1034/j.1600-0889.49.issue5.12.x
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