Chlorine Isotope Fractionation of the Major Chloromethane Degradation Processes in the Environment.

Chloromethane (CHCl) is an important source of chlorine in the stratosphere, but detailed knowledge of the magnitude of its sources and sinks is missing. Here, we measured the stable chlorine isotope fractionation (ε) associated with the major abiotic and biotic CHCl sinks in the environment, namely, CHCl degradation by hydroxyl (OH) and chlorine (Cl) radicals in the troposphere and by reference bacteria CM4 and MB2 from terrestrial and marine environments, respectively. No chlorine isotope fractionation was detected for reaction of CHCl with OH and Cl radicals, whereas a large chlorine isotope fractionation (ε) of -10.9 ± 0.7‰ ( = 3) and -9.4 ± 0.9 ( = 3) was found for CHCl degradation by CM4 and MB2, respectively. The large difference in chlorine isotope fractionation observed between tropospheric and bacterial degradation of CHCl provides an effective isotopic tool to characterize and distinguish between major abiotic and biotic processes contributing to the CHCl sink in the environment. Our findings demonstrate the potential of emerging triple-element isotopic approaches including chlorine to carbon and hydrogen analysis for the assessment of global cycling of organochlorines.