Stable chlorine isotope composition of chlorofluorocarbons and chloromethane in the troposphere

Abstract. Chloromethane (CH3Cl) and chlorofluorocarbons (CFCs) play an important role in stratospheric ozone loss. Whereas anthropogenic CFC production was banned, CH3Cl emissions originating primarily from natural sources cannot be regulated. Stable chlorine isotope analysis has the potential to provide valuable insights into the sources and fate of these compounds, but to date only one study has reported atmospheric δ(37Cl) measurements linked to the isotopic standard of chlorine (SMOC, standard mean ocean chloride). In the current study, we collected tropospheric air samples over the course of one year at an urban site (Leipzig, Germany) and extracted CH3Cl, CFC-12, CFC-11, CFC-113, and HCFC-22 for stable chlorine isotope analysis. CFCs showed minor δ(37Cl) variations throughout one year, consistent with long atmospheric lifetimes and ceased production. In contrast, CH3Cl displayed an annual cycle, with lower δ(37Cl) in spring and higher δ(37Cl) in late summer to early fall. This trend likely reflects seasonal variations: increased emission of 37Cl–depleted CH3Cl by oceans and vegetation in spring and enhanced degradation in summer and fall causing isotope fractionation und thus higher δ(37Cl). Back trajectory modeling suggested a vertical difference of δ(37Cl), conceivably due to mixing of freshly emitted and 37Cl–depleted CH3Cl in the boundary layer. Additionally, stratosphere–troposphere transport of partially degraded and 37Cl–enriched CH3Cl was hypothesized. While these interpretations remain preliminary until further confirmation, isotopic data reported here represents a crucial step toward establishing δ(37Cl) fingerprints, which are prerequisites for future source appointment and atmospheric modeling studies.