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

Carbon dioxide in the middle atmosphere is mass independently enriched in t he heavy oxygen isotopes relative to tropospheric values. That is, increasi ng with altitude, the O-17/O-16 ratio shows an additional enhancement over what is expected on the basis of the O-18/O-16 increase. As tropospheric CO 2 has a mass-dependent isotopic composition that varies by less than 3 part s per thousand in delta(18)O, isotopic measurements of middle atmospheric C O2, combined with a quantitative understanding of the enrichment mechanism, could provide valuable information regarding processes such as stratospher e-troposphere exchange and the mean age of an air mass. It is known that th e mass-independent enrichment in stratospheric CO2 occurs when CO2 quenches an O(D-1) atom formed by the photolysis of O-3, but the details of this pr ocess remain uncertain. Here a series of laboratory and numerical experimen ts are presented which have been performed to study the time evolution and final equilibrium values of the CO2 + O(D-1) 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 contro lled by the isotopic composition of the O(D-1) reservoir, there are a numbe r of complicating factors. The simple mixing model discussed here consisten tly overpredicts the measured isotopic enrichment, thus indicating the CO2 + O(D-1) isotopic exchange is more complicated than has generally been reco gnized.