Experimental investigation of oxygen isotope exchange between CO₂ and O(¹D) and its relevance to the stratosphere

Abstract

Stratospheric CO₂ is found to be enriched in heavy oxygen isotopes relative to tropospheric CO₂ values caused by photo-induced isotopic exchange with stratospheric ozone, a gas highly enriched in heavy isotopes relative to the ambient oxygen. The transfer of isotopic enrichment from the ozone pool to the CO₂ pool takes place through CO∗₃ formed by reaction of CO₂ with O(¹D) derived from O₃ photolysis. Recent stratospheric measurements indicate that the transfer process results in CO₂ getting more enriched in ¹⁷O compared to ¹⁸O. Experiments, reported here, investigate this exchange and determine the effects of the initial isotopic composition of CO₂ and O₃ on the final isotopic composition of the exchanged CO₂. It is seen that the slope relating ¹⁷O and ¹⁸O enrichments in CO₂ depends on the initial isotopic composition of the two gases. Interestingly, with CO₂ of tropospheric composition and O₃ of stratospheric composition the observed slope (1.82 ± 0.04) is close to the value (1.71) found for stratospheric CO₂. The apparent extra ¹⁷O enrichment in CO₂ resulting in slope higher than that of O₃ can be explained by simple mixing if one assumes that O(¹D) originates mainly from the asymmetric ozone species and if additional fractionation occurs because of differences in collision frequencies of isotopic species and possibly also during dissociation of CO∗₃ to CO₂ and O atom.