Stable carbon isotope fractionation in the UV photolysis of CFC-11 and CFC-12

Abstract

The chlorofluorocarbons CFC-11 (CFCl₃) and CFC-12 (CF₂Cl₂) are stable atmospheric compounds that are produced at the earth's surface, but removed only at high altitudes in the stratosphere by photolytic reactions. Their removal liberates atomic chlorine that then catalytically destroys stratospheric ozone. For such long-lived compounds, isotope effects in the stratospheric removal reactions have a large effect on their global isotope budgets. We have demonstrated a photolytic isotope fractionation for stable carbon isotopes of CFC-11 and CFC-12 in laboratory experiments using broadband UV-C (190–230 nm) light. ¹³C/¹²C isotope fractionations (ε) range from (−23.8±0.9) to (−17.7±0.4) ‰ for CFC-11 and (−66.2±3.1) to (−51.0±2.9) permil for CFC-12 between 203 and 288 K, a temperature range relevant to conditions in the troposphere and stratosphere. These results suggest that CFCs should become strongly enriched in ¹³C with decreasing mixing ratio in the stratosphere, similar to what has been recently observed for CFC chlorine isotopes. In conjunction with the strong variations in CFC emissions before and after the Montréal Protocol, the stratospheric enrichments should also lead to a significant temporal increase in the ¹³C content of the CFCs at the surface over the past decades, which should be recorded in atmospheric air archives such as firn air.