Investigations of the photochemical isotope equilibrium between O2, CO2 and O3

[摘要] Contrary to tropospheric CO₂ whose oxygen isotopic composition follows astandard mass dependent relationship, i.e. δ¹⁷O~0.5δ¹⁸O, stratospheric CO₂ is preferentially enriched in ¹⁷O,leading to a strikingly different relation: δ¹⁷O~1.7δ¹⁸O. It has been shown repeatedly that the isotope anomaly isinherited from O₃ via photolytically produced O(¹D) that undergoesisotope exchange with CO₂ and the anomaly may well serve as a tracer ofstratospheric chemistry if details of the exchange mechanism are understood.We have studied the photochemical isotope equilibrium in UV-irradiatedO₂-CO₂ and O₃-CO₂ mixtures to quantify the transfer ofthe anomaly from O₃ to CO₂ at room temperature. By following thetime evolution of the oxygen isotopic compositions of CO₂ and O₂ undervarying initial isotopic compositions of both, O₂/O₃ and CO₂, theisotope equilibria between the two reservoirs were determined. A very strongdependence of the isotope equilibrium on the O₂/CO₂-ratio wasestablished. Equilibrium enrichments of ¹⁷O and ¹⁸O in CO₂ relative toO₂ diminish with increasing CO₂ content, and this reduction in theequilibrium enrichments does not follow a standard mass dependent relation.When molecular oxygen exceeds the amount of CO₂ by a factor of about 20,¹⁷O and ¹⁸O in equilibrated CO₂ are enriched by (142±4)‰and (146±4)‰, respectively, at room temperature and at apressure of 225 hPa, independent of the initial isotopiccompositions of CO₂ and O₂ or O₃. From these findings we derivea simple and general relation between the starting isotopic compositions andamounts of O₂ and CO₂ and the observed slope in a three oxygenisotope diagram. Predictions from this relation are compared with publishedlaboratory and atmospheric data.

[学科分类] 大气科学