Evidence for Altitude-dependent photolysis-induced 18O isotopic fractionation in stratospheric ozone

We present vertical profiles of 18O fractionations in ozone, measured by balloon-borne infrared remote sensing between 15 and 40 km. The magnitudes of the 16O16O18O (668O3) and 16O18O16O (686O3) fractionations are 13.5 +/- 2.7% and 7.7 +/- 2.2%, averaged over the 20-35 km altitude range, in good agreement with previous atmospheric measurements by mass spectrometry and both infrared and far infrared remote sensing spectroscopy. We use our fractionation profiles, together with known fractionation effects of the ozone formation reaction, to deduce fractionations attributable to photolysis. These photolytic fractionation profiles show significant increases with altitude (3.5 +/- 2.2% and 4.0 +/- 1.6% for 668O3 and 686O3 respectively over the 20-35 km altitude range), indicating that the ozone formation reaction alone does not account for the observed enrichments.