# 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 (CClinline-formula3F), CFC-12 (CClinline-formula2Finline-formula2), and CFC-113 (CFinline-formula2ClCFClinline-formula2), 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, inline-formulaϵapp(13C), for these gases as they are destroyed in the high- and mid-latitude stratosphere: inline-formulaϵapp(CFC-12, high-latitude) inline-formula $M14inlinescrollmathml=\left(-normal 20.2±normal 4.4\right)$ 75pt12ptsvg-formulamathimg278d2c7a915d921f7307dcd5d3ee2160 acp-21-6857-2021-ie00001.svg75pt12ptacp-21-6857-2021-ie00001.png  ‰, and inline-formulaϵapp(CFC-113, high-latitude) inline-formula $M16inlinescrollmathml=\left(-normal 9.4±normal 4.4\right)$ 69pt12ptsvg-formulamathimg22e03b54614b411a6197f1a013d6bec5 acp-21-6857-2021-ie00002.svg69pt12ptacp-21-6857-2021-ie00002.png  ‰, inline-formulaϵapp(CFC-12, mid-latitude) inline-formula $M18inlinescrollmathml=\left(-normal 30.3±normal 10.7\right)$ 81pt12ptsvg-formulamathimg37eeaf0b2d646971dc8a25c42f1bfebd acp-21-6857-2021-ie00003.svg81pt12ptacp-21-6857-2021-ie00003.png  ‰, and inline-formulaϵapp(CFC-113, mid-latitude) inline-formula $M20inlinescrollmathml=\left(-normal 34.4±normal 9.8\right)$ 75pt12ptsvg-formulamathimg97befda7017bffdd6ee8cd672c7efd74 acp-21-6857-2021-ie00004.svg75pt12ptacp-21-6857-2021-ie00004.png  ‰. Our CFC-11 measurements were not sufficient to calculate inline-formulaϵapp(CFC-11), so we instead used previously reported photolytic fractionation for CFC-11 and CFC-12 to scale our inline-formulaϵapp(CFC-12), resulting in inline-formulaϵapp(CFC-11, high-latitude) inline-formula $M24inlinescrollmathml=\left(-normal 7.8±normal 1.7\right)$ 69pt12ptsvg-formulamathimg5fdc0dc1eb8fbb1cd8f4529133e34e8b acp-21-6857-2021-ie00005.svg69pt12ptacp-21-6857-2021-ie00005.png  ‰ and inline-formulaϵapp(CFC-11, mid-latitude) inline-formula $M26inlinescrollmathml=\left(-normal 11.7±normal 4.2\right)$ 75pt12ptsvg-formulamathimgcf6d7d77e8e2252b7b6a4e35fd8c6874 acp-21-6857-2021-ie00006.svg75pt12ptacp-21-6857-2021-ie00006.png  ‰. Measurements of firn air were used to construct histories of the tropospheric isotopic composition, inline-formulaδT(13C), for CFC-11 (1950s to 2009), CFC-12 (1950s to 2009), and CFC-113 (1970s to 2009), with inline-formulaδT(13C) increasing for each gas. We used inline-formulaϵapp(high-latitude), which was derived from more data, and a constant isotopic composition of emissions, inline-formulaδE(13C), to model inline-formulaδT(inline-formula13C, CFC-11), inline-formulaδT(inline-formula13C, CFC-12), and inline-formulaδT(inline-formula13C, CFC-113). For CFC-11 and CFC-12, modelled inline-formulaδT(13C) was consistent with measured inline-formulaδT(13C) for the entire period covered by the measurements, suggesting that no dramatic change in inline-formulaδE(inline-formula13C, CFC-11) or inline-formulaδE(inline-formula13C, CFC-12) has occurred since the 1950s. For CFC-113, our modelled inline-formulaδT(inline-formula13C, CFC-113) did not agree with our measurements earlier than 1980. This discrepancy may be indicative of a change in inline-formulaδE(inline-formula13C, 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 inline-formula13C isotopic composition of CFC-113. Our modelling predicts increasing inline-formulaδT(inline-formula13C, CFC-11), inline-formulaδT(inline-formula13C, CFC-12), and inline-formulaδT(inline-formula13C, CFC-113) into the future. We investigated the effect of recently reported new CFC-11 emissions on background inline-formulaδT(inline-formula13C, CFC-11) by fixing model emissions after 2012 and comparing inline-formulaδT(inline-formula13C, CFC-11) in this scenario to the model base case. The difference in inline-formulaδT(inline-formula13C, 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.

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Thomas, Max / Laube, Johannes C. / Kaiser, Jan / et al: Stratospheric carbon isotope fractionation and tropospheric histories of CFC-11, CFC-12, and CFC-113 isotopologues. 2021. Copernicus Publications.

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