Evidence for a major missing source in the global chloromethane budget from stable carbon isotopes
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Published:2019-02-08
Issue:3
Volume:19
Page:1703-1719
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Bahlmann EnnoORCID, Keppler FrankORCID, Wittmer Julian, Greule MarkusORCID, Schöler Heinz Friedrich, Seifert Richard, Zetzsch Cornelius
Abstract
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 5000 Gg (1 Gg = 109 g). 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: the
reaction with OH and Cl in the troposphere. We conducted photochemical
CH3Cl degradation experiments in a 3500 dm3 smog chamber to
determine the carbon isotope effect (ε=k13C/k12C-1) for the reaction of CH3Cl with OH
and Cl. For the reaction of CH3Cl with OH, we determined an
ε value of (-11.2±0.8) ‰ (n=3) and for the
reaction with Cl we found an ε value of (-10.2±0.5) ‰ (n=1), which is 5 to 6 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 values for the carbon isotope effect to the global
CH3Cl budget using a simple two hemispheric box model, we derive a
tropical rainforest CH3Cl source of (670±200) Gg a−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 CH3Cl source of (1530±200) Gg a−1.
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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