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

Chloromethane (CH3Cl) 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 (eCl) associated with the major abiotic and biotic CH3Cl sinks in the environment, namely CH3Cl degradation by hydroxyl (·OH) and chlorine (·Cl) radicals in the troposphere, and by reference bacteria Methylorubrum extorquens CM4 and Leisingera methylohalidivorans MB2 from terrestrial and marine environments, respectively. No chlorine isotope fractionation was detected for reaction of CH3Cl with ·OH and ·Cl radicals, whereas large chlorine isotope fractionation (eCl) of -10.9 ± 0.7‰ (n=3) and -9.4 ± 0.9 (n=3) was found for CH3Cl degradation by M. extorquens CM4 and L. methylohalidivorans MB2, respectively. The large difference in chlorine isotope fractionation observed between tropospheric and bacterial degradation of CH3Cl provides an effective isotopic tool to characterize and distinguish betwe...