Stratospheric carbon isotope fractionation and tropospheric histories of CFC-11, CFC-12 and CFC-113 isotopologues - Université Grenoble Alpes Accéder directement au contenu
Pré-Publication, Document De Travail Année : 2020

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

Résumé

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

Dates et versions

hal-03098002 , version 1 (05-01-2021)

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Max Thomas, Johannes Laube, Jan Kaiser, Samuel Allin, Patricia Martinerie, et al.. Stratospheric carbon isotope fractionation and tropospheric histories of CFC-11, CFC-12 and CFC-113 isotopologues. 2020. ⟨hal-03098002⟩
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