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https://doi.org/10.5194/acp-2018-855
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-855
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 11 Sep 2018

Research article | 11 Sep 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Evidence for a major missing source in the global chloromethane budget from stable carbon isotopes

Enno Bahlmann1,2, Frank Keppler3,4,5, Julian Wittmer6,7, Markus Greule3,4, Heinz Friedrich Schöler3, Richard Seifert1, and Cornelius Zetzsch5,6 Enno Bahlmann et al.
  • 1Institute of Geology, University Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany
  • 2Leibniz Centre for Tropical Marine Research, Fahrenheitstraße 6, 28359 Bremen, Germany
  • 3Institute of Earth Sciences, Heidelberg University, Im Neuenheimer Feld 234–236, 69120 Heidelberg, Germany
  • 4Heidelberg Center for the Environment (HCE), Heidelberg University, 69120 Heidelberg, Germany
  • 5Max-Planck-Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, 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 the most important natural input of reactive chlorine to the stratosphere, contributing about 16% to stratospheric ozone depletion. Due to the phase out of anthropogenic emissions of chlorofluorocarbons, CH3Cl will largely control future levels of stratospheric chlorine.

The tropical rainforest is commonly assumed to be the strongest single CH3Cl source, contributing over half of the global annual emissions of about 4000 to 5000Gg (1Gg=109g). This source shows a characteristic carbon isotope fingerprint, making isotopic investigations a promising tool for improving its atmospheric budget. Applying carbon isotopes to better constrain the atmospheric budget of CH3Cl requires sound information on the kinetic isotope effects for the main sink processes e.g. the reaction with OH and Cl in the troposphere. We conducted photochemical CH3Cl degradation experiments in a 3500L smog chamber to determine the carbon isotope fractionation (ε) for the reaction of CH3Cl with OH and Cl. For the reaction of CH3Cl with OH, we determined a ε of (−11.2±0.8)‰ (n=3) and for the reaction with Cl we found a ε of (−10.2±0.5)‰ (n=1) being five to six times smaller than previously reported. Our smaller isotope effects are strongly supported by the lack of any significant seasonal covariation in previously reported tropospheric δ13C(CH3Cl) values with the OH driven seasonal cycle in tropospheric mixing ratios.

Applying these new fractionation factors to the global CH3Cl budget using a simple two hemispheric box model, we derive a tropical rainforest CH3Cl source of (670±200)Gga−1, which is considerably smaller than previous estimates. A revision of previous bottom up estimates, using above ground biomass instead of rainforest area, strongly supports this lower estimate. Finally, our results suggest a large unknown tropical CH3Cl source of (1230±200)Gga−1.

Enno Bahlmann et al.
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Enno Bahlmann et al.
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Short summary
Chloromethane is the most important natural carrier of chlorine to the stratosphere. From a newly determined carbon isotope effect of −11.2 ‰ for the tropospheric loss of CH3Cl we derive a tropical rainforest CH3Cl source of 670 ± 200 Gg a−1, being t 60 % smaller than previous estimates. A revision of previous bottom up estimates, using above ground biomass instead of rainforest area, strongly supports this lower estimate. Finally, our results suggest a large unknown tropical of 1230 ± 200 Gg a−1.
Chloromethane is the most important natural carrier of chlorine to the stratosphere. From a...
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