Atmospheric chemistry of CH₂Br₂: Rate coefficients for its reaction with Cl atoms and OH and the chemistry of the CHBr₂O radical

Rate coefficients for the reaction of OH radicals and Cl atoms with dibromomethane, OH + CH₂Br₂ → CHBr₂ + H₂O (1) and Cl + CH₂Br₂ → CHBr₂ + HCl (3), and for the reaction of Cl atoms with methyl bromide, Cl + CH₃Br → HCl + CH₂Br (4), have been obtained using relative rate techniques. At 298 K, the value of k₁, was determined to be (1.2 ± 0.3) × 10^-13 cm³ molec^-1 s^-1. The rate coefficients k₃ and k₄ were determined at a series of temperatures ranging from 228 K to 296 K and combined with the measurements of Gierczak et al., yielding the following Arrhenius expressions for k₃ and k₄; k₃ = (6.35 ± 0.6) × 10^-12 exp(-807 ± 50/T) cm³ molec^-1 s^-1 and k₄ = ( 1.49 ± 0.2) × 10^-11 exp(- 1056 ± 50/T) cm³ molec^-1 s^-1. In addition, the Cl atom initiated oxidation mechanism of CH₂Br₂ in air has been studied at 298 K. The products observed were HC(O)Br and small amounts of CO. The dominant atmospheric fate of the alkoxy radical, CHBr₂O, is elimination of a Br atom, which occurs at a rate estimated to be greater than 4 × 10⁶ s^-1 in 700 torr of O₂ at 298 K.