Investigations of the photochemical isotope equilibrium between O-2, CO2 and O-3

Contrary to tropospheric CO 2 whose oxygen isotopic composition follows a standard mass dependent relationship, i.e. δ 17 O~0.5δ 18 O, stratospheric CO 2 is preferentially enriched in 17 O, leading to a strikingly different relation: δ 17 O~1.7δ 18 O. It has been shown repeatedly that the isotope anomaly is inherited from O 3 via photolytically produced O( 1 D) that undergoes isotope exchange with CO 2 and the anomaly may well serve as a tracer of stratospheric chemistry if details of the exchange mechanism are understood. We have studied the photochemical isotope equilibrium in UV-irradiated O 2 -CO 2 and O 3 -CO 2 mixtures to quantify the transfer of the anomaly from O 3 to CO 2 at room temperature. By following the time evolution of the oxygen isotopic compositions of CO 2 and O 2 under varying initial isotopic compositions of both, O 2 /O 3 and CO 2 , the isotope equilibria between the two reservoirs were determined. A very strong dependence of the isotope equilibrium on the O 2 /CO 2 -ratio was established. Equilibrium enrichments of 17 O and 18 O in CO 2 relative to O 2 diminish with increasing CO 2 content, and this reduction in the equilibrium enrichments does not follow a standard mass dependent relation. When molecular oxygen exceeds the amount of CO 2 by a factor of about 20, 17 O and 18 O in equilibrated CO 2 are enriched by (142±4)‰ and (146±4)‰, respectively, at room temperature and at a pressure of 225 hPa, independent of the initial isotopic compositions of CO 2 and O 2 or O 3 . From these findings we derive a simple and general relation between the starting isotopic compositions and amounts of O 2 and CO 2 and the observed slope in a three oxygen isotope diagram. Predictions from this relation are compared with published laboratory and atmospheric data.