CO₂ + O(¹D) isotopic exchange: Laboratory and modeling studies

Carbon dioxide in the middle atmosphere is mass independently enriched in the heavy oxygen isotopes relative to tropospheric values. That is, increasing with altitude, the ¹⁷O/¹⁶O ratio shows an additional enhancement over what is expected on the basis of the ¹⁸O/¹⁶O increase. As tropospheric CO₂ has a mass-dependent isotopic composition that varies by less than 3‰ in δ¹⁸O, isotopic measurements of middle atmospheric CO₂, combined with a quantitative understanding of the enrichment mechanism, could provide valuable information regarding processes such as stratosphere-troposphere exchange and the mean age of an air mass. It is known that the mass-independent enrichment in stratospheric CO₂ occurs when CO₂ quenches an O(¹D) atom formed by the photolysis of O₃, but the details of this process remain uncertain. Here a series of laboratory and numerical experiments are presented which have been performed to study the time evolution and final equilibrium values of the CO₂+ O(¹D) reaction in an effort to reach a better understanding of the CO₂ enrichment mechanism. Results show that while the isotopic composition of the CO₂ reservoir is qualitatively controlled by the isotopic composition of the O(¹D) reservoir, there are a number of complicating factors. The simple mixing model discussed here consistently overpredicts the measured isotopic enrichment, thus indicating the CO₂+O(¹D) isotopic exchange is more complicated than has generally been recognized.