Investigations of the photochemical isotope equilibrium between Ch, CO 2 and O3

Abstract

Contrary to tropospheric CO2 whose oxygen isotopic composition follows a standard mass dependent relationship, i.e. δ17O∼ 0.5δ18O, stratospheric CO2 is preferentially enriched in 17O, leading to a strikingly different relation: δ17O∼1.7δ18O. It has been shown repeatedly that the isotope anomaly is inherited from O3 via photolytically produced O(1 D) that undergoes isotope exchange with CO2 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 O2-CO2 and O 3-CO2 mixtures to quantify the transfer of the anomaly from O3 to CO2 at room temperature. By following the time evolution of the oxygen isotopic compositions of CO2 and O 2 under varying initial isotopic compositions of both, O 2/O3 and CO2, the isotope equilibria between the two reservoirs were determined. A very strong dependence of the isotope equilibrium on the O22/CO2-ratio was established. Equilibrium enrichments of 17O and 18O in CO2 relative to O2 diminish with increasing CO2 content, and this reduction in the equilibrium enrichments does not follow a standard mass dependent relation. When molecular oxygen exceeds the amount of CO2 by a factor of about 20, 17O and 18O in equilibrated CO2 are enriched by (142±4)‰ and (146±4)%c, respectively, at room temperature and at a pressure of 225 hPa, independent of the initial isotopic compositions of CO2 and O22 or O 3. From these findings we derive a simple and general relation between the starting isotopic compositions and amounts of O2 and CO2 and the observed slope in a three oxygen isotope diagram. Predictions from this relation are compared with published laboratory and atmospheric data.