Stratospheric carbon isotope fractionation and tropospheric histories of CFC-11, CFC-12, and CFC-113 isotopologues

We present novel measurements of the carbon isotope composition of CFC-11 (CCl 3 F), CFC-12 (CCl 2 F 2 ), and CFC-113 (CF 2 ClCFCl 2 ), three atmospheric trace gases that are important for both stratospheric ozone depletion and global warming. These measurements were carried out on air samples collected in the stratosphere – the main sink region for these gases – and on air extracted from deep polar firn snow. We quantify, for the first time, the apparent isotopic fractionation, ϵ app ( 13 C), for these gases as they are destroyed in the high- and mid-latitude stratosphere: ϵ app (CFC-12, high-latitude) =(-20.2±4.4) ‰, and ϵ app (CFC-113, high-latitude) =(-9.4±4.4) ‰, ϵ app (CFC-12, mid-latitude) =(-30.3±10.7) ‰, and ϵ app (CFC-113, mid-latitude) =(-34.4±9.8) ‰. Our CFC-11 measurements were not sufficient to calculate ϵ app (CFC-11), so we instead used previously reported photolytic fractionation for CFC-11 and CFC-12 to scale our ϵ app (CFC-12), resulting in ϵ app (CFC-11, high-latitude) =(-7.8±1.7) ‰ and ϵ app (CFC-11, mid-latitude) =(-11.7±4.2) ‰. Measurements of firn air were used to construct histories of the tropospheric isotopic composition, δ T ( 13 C), for CFC-11 (1950s to 2009), CFC-12 (1950s to 2009), and CFC-113 (1970s to 2009), with δ T ( 13 C) increasing for each gas. We used ϵ app (high-latitude), which was derived from more data, and a constant isotopic composition of emissions, δ E ( 13 C), to model δ T ( 13 C, CFC-11), δ T ( 13 C, CFC-12), and δ T ( 13 C, CFC-113). For CFC-11 and CFC-12, modelled δ T ( 13 C) was consistent with measured δ T ( 13 C) for the entire period covered by the measurements, suggesting that no dramatic change in δ E ( 13 C, CFC-11) or δ E ( 13 C, CFC-12) has occurred since the 1950s. For CFC-113, our modelled δ T ( 13 C, CFC-113) did not agree with our measurements earlier than 1980. This discrepancy may be indicative of a change in δ E ( 13 C, CFC-113). However, this conclusion is based largely on a single sample and only just significant outside the 95 % confidence interval. Therefore more work is needed to independently verify this temporal trend in the global tropospheric 13 C isotopic composition of CFC-113. Our modelling predicts increasing δ T ( 13 C, CFC-11), δ T ( 13 C, CFC-12), and δ T ( 13 C, CFC-113) into the future. We investigated the effect of recently reported new CFC-11 emissions on background δ T ( 13 C, CFC-11) by fixing model emissions after 2012 and comparing δ T ( 13 C, CFC-11) in this scenario to the model base case. The difference in δ T ( 13 C, CFC-11) between these scenarios was 1.4 ‰ in 2050. This difference is smaller than our model uncertainty envelope and would therefore require improved modelling and measurement precision as well as better quantified isotopic source compositions to detect.