Mass spectrometric measurement of hydrogen isotope fractionation for the reactions of chloromethane with OH and Cl

Chloromethane (CHinline-formula₃Cl) is an important provider of chlorine to the stratosphere but detailed knowledge of its budget is missing. Stable isotope analysis is a potentially powerful tool to constrain CHinline-formula₃Cl flux estimates. The largest degree of isotope fractionation is expected to occur for deuterium in CHinline-formula₃Cl in the hydrogen abstraction reactions with its main sink reactant tropospheric OH and its minor sink reactant Cl atoms. We determined the isotope fractionation by stable hydrogen isotope analysis of the fraction of CHinline-formula₃Cl remaining after reaction with hydroxyl and chlorine radicals in a 3.5 minline-formula³ Teflon smog chamber at 293 inline-formula± 1 K. We measured the stable hydrogen isotope values of the unreacted CHinline-formula₃Cl using compound-specific thermal conversion isotope ratio mass spectrometry. The isotope fractionations of CHinline-formula₃Cl for the reactions with hydroxyl and chlorine radicals were found to be inline-formula $M10inlinescrollmathml-\mathrm{normal\; 264}\pm \mathrm{normal\; 45}$ 52pt10ptsvg-formulamathimg43b54b1a4760b2ad7bc64caa4dcf2d19 acp-18-6625-2018-ie00001.svg52pt10ptacp-18-6625-2018-ie00001.png and inline-formula $M11inlinescrollmathml-\mathrm{normal\; 280}\pm \mathrm{normal\; 11}$ 52pt10ptsvg-formulamathimg62d70adff0e7afd0390fdc4d52c372b5 acp-18-6625-2018-ie00002.svg52pt10ptacp-18-6625-2018-ie00002.png  ‰, respectively. For comparison, we performed similar experiments using methane (CHinline-formula₄) as the target compound with OH and obtained a fractionation constant of inline-formula $M13inlinescrollmathml-\mathrm{normal\; 205}\pm \mathrm{normal\; 6}$ 46pt10ptsvg-formulamathimgc22901eed2f7ed27e67c7076a8f4bad7 acp-18-6625-2018-ie00003.svg46pt10ptacp-18-6625-2018-ie00003.png  ‰ which is in good agreement with values previously reported. The observed large kinetic isotope effects are helpful when employing isotopic analyses of CHinline-formula₃Cl in the atmosphere to improve our knowledge of its atmospheric budget.