© Author(s) 2007. This work is licensed under a Creative Commons License. Atmospheric Chemistry and Physics Investigations of the photochemical isotope equilibrium between O2,

Abstract. Contrary to tropospheric CO2 whose oxygen iso-topic composition follows a standard mass dependent rela-tionship, i.e. δ17O∼0.5 δ18O, stratospheric CO2 is preferen-tially enriched in 17O, leading to a strikingly different rela-tion: δ17O∼1.7 δ18O. It has been shown repeatedly that the isotope anomaly is inherited from O3 via photolytically pro-duced O(1 D) that undergoes isotope exchange with CO2 and the anomaly may well serve as a tracer of stratospheric chem-istry if details of the exchange mechanism are understood. We have studied the photochemical isotope equilibrium in UV-irradiated O2-CO2 and O3-CO2 mixtures to quantify the transfer of the anomaly from O3 to CO2 at room tempera-ture. By following the time evolution of the oxygen isotopic compositions of CO2 and O2 under varying initial isotopic compositions of both, O2/O3 and CO2, the isotope equilibri