Investigation of the Oxygen Isotope Exchange Between Carbon Dioxide and O3 via O(1D)
Carbon dioxide in the middle atmosphere shows an increasing enrichment in the heavy oxygen isotopes with altitude which preferential enrichment for 17O according to d17O =1.7 d18O. This is contrary to tropospheric CO2 where d17O ~ 0.5d18O. These isotope enrichments are transferred from O3 into CO2, but details of the mechanism are not understood yet. A systematic study was carried out using O2 and CO2 gases of different isotopic composition. Results show the existence of a photochemical isotope equilibrium between O2 and CO2, which is independent of the initial isotopic composition and shows equal enrichments for 17O and 18O. Additional experiments were conducted to investigate the effect of temperature, pressure and O2/CO2 ratio. Data revealed that the magnitude of enrichment at photochemical equilibrium depends on pressure and on O2/CO2 ratios with a decrease in enrichment at higher pressures. Also, the enrichment in CO2 showed a positive temperature dependence. The measurement of asymmetric O3 in one experiment yields additional insight into the isotope exchange mechanism and shows the absence of anomalous fractionation steps in the CO3* intermediate. The experimental data were modeled with the chemical kinetics software Facsimile. In addition, a large set of new measurements of the isotopic composition of stratospheric CO2 are presented, extending the earlier data down to the tropopause.
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