4.7 Article

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

Journal

ATMOSPHERIC CHEMISTRY AND PHYSICS
Volume 19, Issue 3, Pages 1703-1719

Publisher

COPERNICUS GESELLSCHAFT MBH
DOI: 10.5194/acp-19-1703-2019

Keywords

-

Funding

  1. German Federal Ministry of Education and Research (BMBF) [03F0611E, 03F0662E]
  2. DFG [KE 884/8-1, KE 884/8-2, KE 884/10-1]
  3. DFG research unit Natural Halogenation Processes in the Environment - Atmosphere and Soil [KE 884/7-1, SCHO 286/7-2, ZE 792/5-2]
  4. NASA (USA)
  5. DECC (UK)
  6. NOAA (USA)
  7. CSIRO (Australia)
  8. BoM (Australia)
  9. FOEN
  10. NILU (Norway)
  11. SNU (Korea)
  12. CMA (China)
  13. NIES (Japan)
  14. Urbino University (Italy)

Ask authors/readers for more resources

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 = 10(9) 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 dm(3) smog chamber to determine the carbon isotope effect (epsilon = k(13)C/k(12)C - 1) for the reaction of CH3Cl with OH and Cl. For the reaction of CH3Cl with OH, we determined an epsilon value of (- 11.2 +/- 0.8)parts per thousand (n = 3) and for the reaction with Cl we found an epsilon value of (-10.2 +/- 0.5)parts per thousand (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 delta C-13 (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).

Authors

I am an author on this paper
Click your name to claim this paper and add it to your profile.

Reviews

Primary Rating

4.7
Not enough ratings

Secondary Ratings

Novelty
-
Significance
-
Scientific rigor
-
Rate this paper

Recommended

No Data Available
No Data Available