Atmospheric sink of methyl chlorodifluoroacetate and ethyl chlorodifluoroacetate: temperature dependent rate coefficients, product distribution of their reactions with Cl atoms and CF₂ClC(O)OH formation

Rate coefficients as a function of temperature have been measured for the first time for the gas-phase reactions of chlorine atoms with methyl chlorodifluoracetate (k₁) and ethyl chlorodifluoroacetate (k₂) using the relative rate technique. The experiments were carried out in a 1080 L photoreactor over the temperature range 287–313 K at a total pressure of 1000 ± 10 mbar of synthetic air using in situ FTIR spectroscopy to monitor reactants and products. The following Arrhenius expressions were obtained: k₍MCDFA₊Cₗ₎ = (9.6 ± 5.1) × 10⁻¹² exp[−(1363 ± 79)/T] and k₍ECDFA₊Cₗ₎ = (64.4 ± 29.7) × 10⁻¹² exp[−(1110 ± 68)/T]. The kinetic results are compared with previous experimental and theoretical studies. In addition, a product study of the reactions of Cl with methyl chlorodifluoracetate and ethyl chlorodifluoroacetate is reported. The results indicate that in the absence of NOₓ the main fate of the alkoxy radicals formed after H-atom abstraction by Cl from the –CH₃ group in methyl chlorodifluoroacetate is reaction of the radical with O₂ to form the mixed anhydride CF₂ClC(O)OC(O)H. In the case of ethyl chlorodifluoroacetate the main fate of the alkoxy formed via H-atom abstraction by Cl from the –CH₂– entity in the ethyl group is α-ester rearrangement to produce chlorodifluoroacetic acid and the corresponding radical. The yields of chlorofluoracetic acid (CF₂ClC(O)OH) obtained were as follows: (34 ± 5)% and (86 ± 8)% for the reactions of Cl with CF₂ClC(O)OCH₃ and CF₂ClC(O)OCH₂CH₃, respectively. The measured yields are rationalized in terms of mechanisms consisting of competitive reaction channels for the formed products in the oxidation, i.e. reaction with O₂, α-ester rearrangement and a decomposition pathway. Atmospheric implications are discussed according to the rate coefficients obtained as a function of temperature and altitude, and regarding the formation of chlorofluorocarboxylic acid.