Oxygen isotope fractionation in stratospheric CO 2

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 CO 2 samples collected between 26 and 35.5km show a mass‐independent enrichment in both 17 O and 18 O of about 11 per mil (‰) above tropospheric values. Ozone enrichment in its heavy isotopes was 9 to 16% in 50 O 3 and 8 to 11% in 49 O 3 , respectively [Schueler et al., 1990]. A mechanism to explain the isotope enrichment in CO 2 has been recently proposed by Yung et al. [1991]. Their model is based on the isotope exchange between CO 2 and O 3 via O( 1 D), resulting in a transfer of the ozone isotope enrichment to carbon dioxide. Predicted enrichments and measured values agree well.