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.