资源环境科技发展态势分析平台

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 ($\mathit{\epsilon }=k^{\mathrm{13}}\mathrm{C}/k^{\mathrm{12}}\mathrm{C}-\mathrm{1}$) for the reaction of CH₃Cl with OH and Cl. For the reaction of CH₃Cl with OH, we determined an ε value of ($-\mathrm{11.2}\pm \mathrm{0.8}$) ‰ (n=3) and for the reaction with Cl we found an ε value of ($-\mathrm{10.2}\pm \mathrm{0.5}$) ‰ (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.