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  • Atmos. Chem. Phys., 13, 27792791, 2013www.atmos-chem-phys.net/13/2779/2013/doi:10.5194/acp-13-2779-2013 Author(s) 2013. CC Attribution 3.0 License.

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    Observation-based assessment of stratospheric fractionalrelease, lifetimes, and ozone depletion potentials of tenimportant source gases

    J. C. Laube1, A. Keil2, H. Bonisch2, A. Engel2, T. Rockmann3, C. M. Volk4, and W. T. Sturges1

    1School of Environmental Sciences, University of East Anglia, Norwich, UK2Institute for Atmospheric and Environmental Sciences, University of Frankfurt, Frankfurt (Main), Germany3Institute for Marine and Atmospheric Research, Utrecht University, Utrecht, the Netherlands4Department of Physics, University of Wuppertal, Wuppertal, Germany

    Correspondence to: J. C. Laube (j.laube@uea.ac.uk)

    Received: 9 October 2012 Published in Atmos. Chem. Phys. Discuss.: 30 October 2012Revised: 14 February 2013 Accepted: 15 February 2013 Published: 8 March 2013

    Abstract. Estimates of the recovery time of stratosphericozone heavily rely on the exact knowledge of the processesthat lead to the decomposition of the relevant halogenatedsource gases. Crucial parameters in this context are frac-tional release factors (FRFs) as well as stratospheric lifetimesand ozone depletion potentials (ODPs). We here present datafrom the analysis of air samples collected between 2009 and2011 on board research aircraft flying in the mid- and high-latitude stratosphere and infer the above-mentioned param-eters for ten major source gases: CFCl3 (CFC-11), CF2Cl2(CFC-12), CF2ClCFCl2 (CFC-113), CCl4 (carbon tetra-chloride), CH3CCl3 (methyl chloroform), CHF2Cl (HCFC-22), CH3CFCl2 (HCFC-141b), CH3CF2Cl (HCFC-142b),CF2ClBr (H-1211), and CF3Br (H-1301). The inferred cor-relations of their FRFs with mean ages of air reveal less de-composition as compared to previous studies for most com-pounds. When using the calculated set of FRFs to infer equiv-alent stratospheric chlorine, we find a reduction of more than20 % as compared to the values inferred in the most recentScientific Assessment of Ozone Depletion by the World Me-teorological Organisation (WMO, 2011). We also note thatFRFs and their correlations with mean age are not gener-ally time-independent as often assumed. The stratosphericlifetimes were calculated relative to that of CFC-11. Withinour uncertainties the ratios between stratospheric lifetimesinferred here agree with the values in recent WMO reportsexcept for CFC-11, CFC-12 and CH3CCl3. Finally, we calcu-late lower ODPs than recommended by WMO for six out of

    ten compounds, with changes most pronounced for the threeHCFCs. Collectively these newly calculated values may haveimportant implications for the severity and recovery time ofstratospheric ozone loss.

    1 Introduction

    Large-scale ozone depletion in the stratosphere is still oc-curring on an annual basis in the higher latitudes of bothhemispheres, with full recovery being projected around themiddle of this century (WMO, 2011). A number of mainlyman-made chorine- and bromine-containing trace gases havelong been identified to be largely responsible for this phe-nomenon (WMO, 1990). The effects of these ozone depletingsubstances (ODSs) are often quantified via their respectivefractional release factors (FRFs). These were first defined bySolomon and Albritton (1992) as the fraction of the halo-carbon species x injected into the stratosphere that has beendissociated. Knowledge of these measures is of major im-portance for the quantification of the ability of ODSs to de-stroy ozone with larger FRFs, resulting in more ozone beingdestroyed. Earlier studies have derived FRFs relative to thatof CFC-11 (Solomon et al., 1992; Daniel et al., 1995; Schauf-fler et al., 1999, 2003), whereas more recent studies focusedon FRFmean-age correlations (Newman et al., 2006, 2007;Laube et al., 2010a).

    Published by Copernicus Publications on behalf of the European Geosciences Union.

  • 2780 J. C. Laube et al.: Observation-based assessment

    The stratospheric lifetimes of these ODSs are equally im-portant in this context as the recovery time of the ozone layerwill depend on these quantities. This is especially true in thecase of the chlorofluorocarbons, where the stratospheric life-times are equivalent to the overall atmospheric lifetimes. Oneof the most important observation-based studies of strato-spheric lifetimes is that of Volk et al. (1997), who usedand further developed the theoretical framework of Plumband Ko (1992) and Plumb (1996) to infer steady-state life-times of six important ozone-depleting halocarbons: CFCl3(CFC-11), CF2Cl2 (CFC-12), CF2ClCFCl2 (CFC-113), CCl4(carbon tetrachloride), CH3CCl3 (methyl chloroform), andCF2ClBr (H-1211). For this purpose they utilised the com-pact correlations that long-lived trace gases form with eachother and also with the mean ages of air (i.e. the meanstratospheric transit times). We here use a similar methodand measurements on air samples collected in the mid- andhigh-latitude stratosphere to evaluate the stratospheric life-times of ten important ODSs: CFC-11, CFC-12, CFC-113,CCl4, CH3CCl3, CHF2Cl (HCFC-22), CH3CFCl2 (HCFC-141b), CH3CF2Cl (HCFC-142b), H-1211, and CF3Br (H-1301). Moreover, the calculation of the FRFs and their re-lation to mean ages of air enables us to derive observation-based semi-empirical ozone depletion potentials (ODPs) forthese compounds.

    2 Sample collection, measurements and data analysis

    The air samples were collected using whole-air samplers op-erated on board the M55 Geophysica high altitude aircraft(see Kaiser et al., 2006, and Laube et al., 2010a, for details onthe sampling systems and Cairo et al., 2010, for a descriptionof a typical Geophysica campaign and payload). Two Geo-physica flights departed from Oberpfaffenhofen, Germany,on 30 October and 4 November 2009 (covered altitude, lat-itude and longitude ranges: 1020 km, 4854 N, 712 E)and nine from Kiruna, Sweden, between 20 January and 2February 2010, and on 11 and 16 December 2011 (919 km,6277 N, 1 W29 E, RECONCILE and ESSENCE cam-paigns). The samples were then analysed via gas chromatog-raphy with mass spectrometric detection (GC-MS): 138 sam-ples at the University of East Anglia (UEA, see Laube etal., 2010b, for analysis details) and 72 at the University ofFrankfurt (UFra). All samples analysed in Frankfurt weremeasured using a system similar to the one at UEA exceptthat the detector is a quadrupole mass spectrometer (Agilent5975 MSD). In addition to the compounds mentioned above,we used measurements of SF6 to infer stratospheric meanages of air.

    All data underwent a thorough quality check as is ex-plained in the following. Some of the measurements car-ried out at UFra showed significantly worse analytical preci-sion than the UEA measurements. These data were excludedbased on the following procedure: first we calculated the av-

    erage 2 precision (in %) of all samples analysed at UEA. Inorder to represent the range of precisions observed at UEA,we then derived the 2 standard deviation of this averageprecision. UFra measurements with relative precisions largerthan this range were excluded (CFC-11: 6 samples, CFC-113: 13, HCFC-141b: 13, H-1301: all, H1211: 35, CH3CCl3:26) In addition, most samples from the 2010 and 2011 cam-paigns were contaminated with several parts per trillion (ppt)of HCFC-142b and were consequently excluded. For CCl4we excluded the 2010 data as these samples were analysedseveral months after collection and CCl4 is not stable in thestainless steel canisters over such extended periods (see alsoLaube et al., 2008). Finally, SF6 data from all UFra samplesand also from the 2011 Geophysica campaign (the latter wasanalysed exclusively at UEA) could not be used for the cal-culation of mean ages of air due to limited instrument preci-sions at the time.

    Table 1 shows the calibration scales used and the averagemeasurement precisions of the remaining data set. Also dis-played is a comparison of mixing ratios observed close to thetropopause with northern hemispheric (NH) monthly meanm