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Six rate coefficients of relative ozone formation contradict the role of molecular symmetry in the process that results in the enrichment of heavy ozone isotopomers. The results show that collisions between light atoms, such as 16O, and heavy molecules, such as 34O2 and 36O2, have a rate coefficient advantage of about 25 and 50 percent, respectively, over collisions involving heavy atoms and light molecules. These results suggest that the observed isotope effect for each isotopomer may be caused by the preponderance of a single reaction channel and not through molecular symmetry selection.

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[1] Heavy oxygen isotope enrichment in stratospheric CO 2 has been identified in the past as a direct result of the large O 3 isotope anomaly though no simultaneous measurements are known. Results from 8 stratospheric balloon flights at two latitudes are reported that provide now a complete oxygen isotope analysis of both gases.

Tropospheric ozone samples collected during a twelve‐month period in urban air show an enrichment of about 9% in the heavy isotope 50O3 consistent with predictions from laboratory measurements. The enhancement of about 7% observed in 49O3 is still within the uncertainty of the expected value. These measurements confirm that the isotope effect, repeatedly found in laboratory experiments, is also produced in the atmosphere during the ozone formation process.

Abstract. Ozone isotope data for 4903 and 500 3 are presented, that were obtained from 28 stratospheric samples collected over 10 years onboard balloon payloads. Enrichments for 4903 range from 7 to 9% in the middle stratosphere. This is in very good agreement with laboratoryderived isotope predictions. For 5ø03 most enrichments are between 7 and 11%, a few, however, are too high to be of atmospheric origin. Arguments are presented that stratospheric ozone isotope data are consistent with enrichments determined in laboratory studies when pressure and temperature dependence of the isotope effect is included. Results from recent remote sensing experiments support the values obtained during balloon-borne sample collections. Very high enrichments occasionally measured in the past should be disregarded.

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