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

Abstract

We present novel measurements of the carbon isotope composition of CFC-11 (CCl3F), CFC-12 (CCl2F2), and CFC-113 (CF2ClCFCl2), 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, ?app(13C), for these gases as they are destroyed in the high- and mid-latitude stratosphere: ?app(CFC-12, high-latitude)?=(-20.2 4.4)? , and ?app(CFC-113, high-latitude)?=(-9.4 4.4)? , ?app(CFC-12, mid-latitude)?=(-30.3 10.7)? , and ?app(CFC-113, mid-latitude)?=(-34.4 9.8)? . Our CFC-11 measurements were not sufficient to calculate ?app(CFC-11), so we instead used previously reported photolytic fractionation for CFC-11 and CFC-12 to scale our ?app(CFC-12), resulting in ?app(CFC-11, high-latitude)?=(-7.8 1.7)? and ?app(CFC-11, mid-latitude)?=(-11.7 4.2)? . Measurements of firn air were used to construct histories of the tropospheric isotopic composition, dT(13C), for CFC-11 (1950s to 2009), CFC-12 (1950s to 2009), and CFC-113 (1970s to 2009), with dT(13C) increasing for each gas. We used ?app(high-latitude), which was derived from more data, and a constant isotopic composition of emissions, dE(13C), to model dT(13C, CFC-11), dT(13C, CFC-12), and dT(13C, CFC-113). For CFC-11 and CFC-12, modelled dT(13C) was consistent with measured dT(13C) for the entire period covered by the measurements, suggesting that no dramatic change in dE(13C, CFC-11) or dE(13C, CFC-12) has occurred since the 1950s. For CFC-113, our modelled dT(13C, CFC-113) did not agree with our measurements earlier than 1980. This discrepancy may be indicative of a change in dE(13C, 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 13C isotopic composition of CFC-113. Our modelling predicts increasing dT(13C, CFC-11), dT(13C, CFC-12), and dT(13C, CFC-113) into the future. We investigated the effect of recently reported new CFC-11 emissions on background dT(13C, CFC-11) by fixing model emissions after 2012 and comparing dT(13C, CFC-11) in this scenario to the model base case. The difference in dT(13C, 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.