Measurements of CFC Isotope Changes in Firn, Stratospheric and Tropospheric Air

Thursday, 18 December 2014
Samuel Allin1, Johannes Laube2, Emmanuel Witrant3, Jan Kaiser2, Eimear McKenna2, Paul Dennis2, Robert Mulvaney4, Emilie Capron4, Patricia Martinerie5, Thomas Blunier6, Jakob Schwander7, Paul Fraser8 and William T Sturges2, (1)University of East Anglia, Norwich, NR4, United Kingdom, (2)University of East Anglia, Norwich, United Kingdom, (3)Univ. Grenoble Alpes/CNRS, Grenoble Image Parole Signal Automatique (GIPSA-lab), Grenoble, France, (4)British Antarctic Survey, Cambridge, United Kingdom, (5)LGGE/CNRS, St Martin D'Heres, France, (6)University of Copenhagen, Centre for Ice and Climate, Copenhagen, Denmark, (7)University of Bern, Physics Institute, Bern, Switzerland, (8)Commonwealth Scientific and Industrial Research Organisation, Centre for Australian Weather and Climate Research, Aspendale, Australia
The degradation of chlorofluorocarbons (CFCs) releases chlorine, which is a major contributor to the destruction of stratospheric ozone. Recent studies of CFC-12 (CCl2F2) have reported strong chlorine and carbon isotope fractionations in stratospheric and tropospheric samples, respectively. The δ(37Cl) variations were attributed to isotope dependent sink reactions, similar to effects seen in nitrous oxide (N2O), whereas adjustments to manufacturing processes were used to explain the δ(13C) changes. Using air archives to measure chlorine and carbon isotope ratios in CFCs could help to identify and quantify their sources and sinks. We analyse the three most abundant CFCs and show that CFC-11 (CCl3F) and CFC-113 (CClF2CCl2F) exhibit significant chlorine isotope fractionation in the stratosphere, in common with CFC-12. We then use a 2-box model to estimate the expected tropospheric isotope signature of these gases, based on their emissions and transport history, as well as their measured stratospheric isotope fractionation constants (εapp). We also present long-term δ(37Cl) and δ(13C) trends of all three CFCs, determined from background tropospheric samples from the Cape Grim air archive (1978 – 2010) and firn air samples from the Arctic (NEEM, Greenland) and Antarctica (Fletcher Promontory). These measurements are compared to our model trends, leading to an evaluation of long-term chlorine and carbon isotope changes. This study also extends the novel approach to measuring trace gas isotope ratios in small air volumes, using a single-detector gas chromatography-mass spectrometry system.