Abstract
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) ‰ 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.
Highlights
The chlorofluorocarbons CFC-11 (CFCl3) and CFC-12 (CF2Cl2) are the most abundant anthropogenic halocarbons in the atmosphere. Before their production was banned under the Montreal Protocol and its amendments, the usage of these compounds as refrigerants, cleansers, aerosol propellants and foam-blowing agents worldwide resulted in significant atmospheric loading; at their peaks in 1990 and 2003, respectively, mean mixing ratios in the troposphere were approximately 260 ppt for CFC11 and 550 ppt for CFC-12 (Forster et al, 2007; AGAGE, 2010)
In this study we report a carbon (13C/12C) isotope fractionation (ε) in CFC-11 and CFC-12 during laboratory broadband UV-C (190–230 nm) photolysis at atmospherically relevant temperatures
We have plotted the natural logarithm of fraction remaining, ln(F ), against photolysis time (Fig. 5a and b) and ln(δ13C +1) against ln(F ) (Fig. 5c and d) respectively for both compounds at all three temperatures, in order to obtain J and ε through the slope of linear fits to the data
Summary
The chlorofluorocarbons CFC-11 (CFCl3) and CFC-12 (CF2Cl2) are the most abundant anthropogenic halocarbons in the atmosphere. CFC-11 and CFC-12 account for approximately 55 % of the total, outweighing contributions by natural compounds such as methyl chloride, HCl and naturally emitted molecular chlorine (Butler et al, 1999; WMO 2010). Chlorofluorocarbons such as CFC-11 and CFC-12 are produced by anthropogenic processes at the earth’s surface, but have their only significant sinks in the stratosphere, at altitudes where UV-C radiation (λ < 220 m) is sufficiently abundant to dissociate the C-Cl bond: Reaction (R1). A minor sink (approximately 3–7 %) is due to Cl abstraction by O(1D), which is produced from the photolysis of ozone in the stratosphere: Reaction (R2) (Seinfeld and Pandis, 1998; Laube et al, 2010a)
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