Anomalous oxygen isotope enrichment in CO₂ produced from O+CO: estimates based on experimental results and model predictions

Abstract

The oxygen isotope fractionation associated with O+CO→CO₂ reaction was investigated experimentally where the oxygen atom was derived from ozone or oxygen photolysis. The isotopic composition of the product CO₂ was analyzed by mass spectrometry. A kinetic model was used to calculate the expected CO₂ composition based on available reaction rates and their modifications for isotopic variants of the participating molecules. A comparison of the two (experimental data and model predictions) shows that the product CO₂ is endowed with an anomalous enrichment of heavy oxygen isotopes. The enrichment is similar to that observed earlier in case of O₃ produced by O+O₂ reaction and varies from 70‰ to 136‰ for and 41‰ to 83‰ for ¹⁷O . Cross plot of δ¹⁷O and δ¹⁸O of CO₂ shows a linear relation with slope of ~ 0.90 for different experimental configurations. The enrichment observed in CO₂ does not depend on the isotopic composition of the O atom or the sources from which it is produced. A plot of Δ(δ¹⁷O) versus Δ(δ¹⁸O) (two enrichments) shows linear correlation with the best fit line having a slope of ~ 0.8. As in case of ozone, this anomalous enrichment can be explained by invoking the concept of differential randomization/stabilization time scale for two types of intermediate transition complex which forms symmetric (¹⁶O¹²C¹⁶O) molecule in one case and asymmetric (¹⁶O¹²C ¹⁸O and ¹⁶O ¹²C¹⁷O) molecules in the other. The δ¹³C value of CO₂ is also found to be different from that of the initial CO due to the mass dependent fractionation processes that occur in the O+CO→CO₂ reaction. Negative values of Δ(δ¹³C) (~12.1‰) occur due to the preference of in CO₂∗ formation and stabilization. By contrast, at lower pressures ( ~100 torr) surface induced deactivation makes Δ(δ¹³C) zero or slightly positive.