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Volume 18, issue 9 | Copyright
Atmos. Chem. Phys., 18, 6625-6635, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 May 2018

Research article | 09 May 2018

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

Frank Keppler1,2,3, Enno Bahlmann4,5, Markus Greule1,3, Heinz Friedrich Schöler1, Julian Wittmer6,7, and Cornelius Zetzsch3,6 Frank Keppler et al.
  • 1Institute of Earth Sciences, Heidelberg University, Im Neuenheimer Feld 234–236, 69120 Heidelberg, Germany
  • 2Heidelberg Center for the Environment (HCE), Heidelberg University, 69120 Heidelberg, Germany
  • 3Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
  • 4Leibniz Centre for Tropical Marine Research, Fahrenheitstraße 6, 28359 Bremen, Germany
  • 5Institute of Geology, University Hamburg, Bundesstraße 55, 20146 Hamburg, Germany
  • 6Atmospheric Chemistry Research Unit, BayCEER, University of Bayreuth, Dr Hans-Frisch Strasse 1–3, 95448 Bayreuth, Germany
  • 7Agilent Technologies Sales & Services GmbH & Co. KG, Hewlett-Packard-Str. 8, 76337 Waldbronn, Germany

Abstract. Chloromethane (CH3Cl) 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 CH3Cl flux estimates. The largest degree of isotope fractionation is expected to occur for deuterium in CH3Cl 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 CH3Cl remaining after reaction with hydroxyl and chlorine radicals in a 3.5m3 Teflon smog chamber at 293±1K. We measured the stable hydrogen isotope values of the unreacted CH3Cl using compound-specific thermal conversion isotope ratio mass spectrometry. The isotope fractionations of CH3Cl for the reactions with hydroxyl and chlorine radicals were found to be −264±45 and −280±11‰, respectively. For comparison, we performed similar experiments using methane (CH4) as the target compound with OH and obtained a fractionation constant of −205±6‰ which is in good agreement with values previously reported. The observed large kinetic isotope effects are helpful when employing isotopic analyses of CH3Cl in the atmosphere to improve our knowledge of its atmospheric budget.

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Short summary
Chloromethane is involved in stratospheric ozone depletion, but detailed knowledge of its global budget is missing. In this study stable hydrogen isotope analyses were performed to investigate the dominant loss process for atmospheric chloromethane with photochemically produced hydroxyl radicals. The findings might have significant implications for the use of stable isotope signatures in elucidation of global chloromethane cycling.
Chloromethane is involved in stratospheric ozone depletion, but detailed knowledge of its global...