Oxygen isotope fractionation in stratospheric CO₂

A new cryogenic collection system has been flown on board a balloon gondola to obtain separate samples of ozone and carbon dioxide without entrapping major atmospheric gases. Precision laboratory isotopic analysis of CO₂ samples collected between 26 and 35.5km show a mass‐independent enrichment in both ¹⁷O and ¹⁸O of about 11 per mil (‰) above tropospheric values. Ozone enrichment in its heavy isotopes was 9 to 16% in ⁵⁰O₃ and 8 to 11% in ⁴⁹O₃, respectively [Schueler et al., 1990]. A mechanism to explain the isotope enrichment in CO₂ has been recently proposed by Yung et al. [1991]. Their model is based on the isotope exchange between CO₂ and O₃ via O(¹D), resulting in a transfer of the ozone isotope enrichment to carbon dioxide. Predicted enrichments and measured values agree well.