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

Abstract

Chloromethane (CH 3 Cl) 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, CH 3 Cl will largely control future levels of stratospheric chlorine. The tropical rainforest is commonly assumed to be the strongest single CH 3 Cl source, contributing over half of the global annual emissions of about 4000 to 5000 Gg (1 Gg Combining double low line 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 CH 3 Cl 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 CH 3 Cl degradation experiments in a 3500 dm 3 smog chamber to determine the carbon isotope effect (ϵ = k 13 C=k 12 C-1) for the reaction of CH 3 Cl with OH and Cl. For the reaction of CH 3 Cl with OH, we determined an value of (-11:2 ± 0:8)(n = 3) and for the reaction with Cl we found an 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 δ 13 C.CH 3 Cl/values with the OH-driven seasonal cycle in tropospheric mixing ratios. Applying these new values for the carbon isotope effect to the global CH 3 Cl budget using a simple two hemispheric box model, we derive a tropical rainforest CH 3 Cl 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 CH 3 Cl source of (1530 ± 200) Gg a -1