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<p>We present novel measurements of the carbon isotope composition of CFC-11 (CCl<span class="inline-formula">₃</span>F), CFC-12 (CCl<span class="inline-formula">₂</span>F<span class="inline-formula">₂</span>), and CFC-113 (CF<span class="inline-formula">₂</span>ClCFCl<span class="inline-formula">₂</span>), 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, <span class="inline-formula"><i>ϵ</i>_app(¹³C)</span>, for these gases as they are destroyed in the high- and mid-latitude stratosphere: <span class="inline-formula"><i>ϵ</i>_app</span>(CFC-12, high-latitude) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mo>(</mo><mo>-</mo><mn mathvariant="normal">20.2</mn><mo>±</mo><mn mathvariant="normal">4.4</mn><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="75pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="278d2c7a915d921f7307dcd5d3ee2160"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-6857-2021-ie00001.svg" width="75pt" height="12pt" src="acp-21-6857-2021-ie00001.png"/></svg:svg></span></span> ‰, and <span class="inline-formula"><i>ϵ</i>_app</span>(CFC-113, high-latitude) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mo>(</mo><mo>-</mo><mn mathvariant="normal">9.4</mn><mo>±</mo><mn mathvariant="normal">4.4</mn><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="69pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="22e03b54614b411a6197f1a013d6bec5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-6857-2021-ie00002.svg" width="69pt" height="12pt" src="acp-21-6857-2021-ie00002.png"/></svg:svg></span></span> ‰, <span class="inline-formula"><i>ϵ</i>_app</span>(CFC-12, mid-latitude) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mo>(</mo><mo>-</mo><mn mathvariant="normal">30.3</mn><mo>±</mo><mn mathvariant="normal">10.7</mn><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="81pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="37eeaf0b2d646971dc8a25c42f1bfebd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-6857-2021-ie00003.svg" width="81pt" height="12pt" src="acp-21-6857-2021-ie00003.png"/></svg:svg></span></span> ‰, and <span class="inline-formula"><i>ϵ</i>_app</span>(CFC-113, mid-latitude) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M20" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mo>(</mo><mo>-</mo><mn mathvariant="normal">34.4</mn><mo>±</mo><mn mathvariant="normal">9.8</mn><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="75pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="97befda7017bffdd6ee8cd672c7efd74"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-6857-2021-ie00004.svg" width="75pt" height="12pt" src="acp-21-6857-2021-ie00004.png"/></svg:svg></span></span> ‰. Our CFC-11 measurements were not sufficient to calculate <span class="inline-formula"><i>ϵ</i>_app</span>(CFC-11), so we instead used previously reported photolytic fractionation for CFC-11 and CFC-12 to scale our <span class="inline-formula"><i>ϵ</i>_app</span>(CFC-12), resulting in <span class="inline-formula"><i>ϵ</i>_app</span>(CFC-11, high-latitude) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mo>(</mo><mo>-</mo><mn mathvariant="normal">7.8</mn><mo>±</mo><mn mathvariant="normal">1.7</mn><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="69pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="5fdc0dc1eb8fbb1cd8f4529133e34e8b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-6857-2021-ie00005.svg" width="69pt" height="12pt" src="acp-21-6857-2021-ie00005.png"/></svg:svg></span></span> ‰ and <span class="inline-formula"><i>ϵ</i>_app</span>(CFC-11, mid-latitude) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mo>(</mo><mo>-</mo><mn mathvariant="normal">11.7</mn><mo>±</mo><mn mathvariant="normal">4.2</mn><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="75pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="cf6d7d77e8e2252b7b6a4e35fd8c6874"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-6857-2021-ie00006.svg" width="75pt" height="12pt" src="acp-21-6857-2021-ie00006.png"/></svg:svg></span></span> ‰. Measurements of firn air were used to construct histories of the tropospheric isotopic composition, <span class="inline-formula"><i>δ</i>_T(¹³C)</span>, for CFC-11 (1950s to 2009), CFC-12 (1950s to 2009), and CFC-113 (1970s to 2009), with <span class="inline-formula"><i>δ</i>_T(¹³C)</span> increasing for each gas. We used <span class="inline-formula"><i>ϵ</i>_app</span>(high-latitude), which was derived from more data, and a constant isotopic composition of emissions, <span class="inline-formula"><i>δ</i>_E(¹³C)</span>, to model <span class="inline-formula"><i>δ</i>_T</span>(<span class="inline-formula">¹³C</span>, CFC-11), <span class="inline-formula"><i>δ</i>_T</span>(<span class="inline-formula">¹³C</span>, CFC-12), and <span class="inline-formula"><i>δ</i>_T</span>(<span class="inline-formula">¹³C</span>, CFC-113). For CFC-11 and CFC-12, modelled <span class="inline-formula"><i>δ</i>_T(¹³C)</span> was consistent with measured <span class="inline-formula"><i>δ</i>_T(¹³C)</span> for the entire period covered by the measurements, suggesting that no dramatic change in <span class="inline-formula"><i>δ</i>_E</span>(<span class="inline-formula">¹³C</span>, CFC-11) or <span class="inline-formula"><i>δ</i>_E</span>(<span class="inline-formula">¹³C</span>, CFC-12) has occurred since the 1950s. For CFC-113, our modelled <span class="inline-formula"><i>δ</i>_T</span>(<span class="inline-formula">¹³C</span>, CFC-113) did not agree with our measurements earlier than 1980. This discrepancy may be indicative of a change in <span class="inline-formula"><i>δ</i>_E</span>(<span class="inline-formula">¹³C</span>, 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 <span class="inline-formula">¹³</span>C isotopic composition of CFC-113. Our modelling predicts increasing <span class="inline-formula"><i>δ</i>_T</span>(<span class="inline-formula">¹³C</span>, CFC-11), <span class="inline-formula"><i>δ</i>_T</span>(<span class="inline-formula">¹³C</span>, CFC-12), and <span class="inline-formula"><i>δ</i>_T</span>(<span class="inline-formula">¹³C</span>, CFC-113) into the future. We investigated the effect of recently reported new CFC-11 emissions on background <span class="inline-formula"><i>δ</i>_T</span>(<span class="inline-formula">¹³C</span>, CFC-11) by fixing model emissions after 2012 and comparing <span class="inline-formula"><i>δ</i>_T</span>(<span class="inline-formula">¹³C</span>, CFC-11) in this scenario to the model base case. The difference in <span class="inline-formula"><i>δ</i>_T</span>(<span class="inline-formula">¹³C</span>, 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.</p>