title: Large and unexpected enrichment in stratospheric 16O13C18O and its meridional variation.
abstract: The stratospheric CO(2) oxygen isotope budget is thought to be governed primarily by the O((1)D)+CO(2) isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare CO(2) isotopologue (16)O(13)C(18)O, in concert with (18)O and (17)O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric (16)O(13)C(18)O, observed as proportions in the polar vortex that are higher than in any naturally derived CO(2) sample to date. We show, through photochemical experiments, that lower (16)O(13)C(18)O proportions observed in the midlatitudes are determined primarily by the O((1)D)+CO(2) isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher (16)O(13)C(18)O proportions in the polar vortex show correlations with long-lived stratospheric tracer and bulk isotope abundances opposite to those observed at midlatitudes and, thus, opposite to those easily explained by O((1)D)+CO(2). We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO(2) or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric (16)O(13)C(18)O enrichments may impose additional isotopic constraints on biosphere-atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change.
authors: Divisions of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA. lyeung@caltech.edu
authors: Laurence Y
authors: LY
authors: Yeung
authors: Hagit P
authors: HP
authors: Affek
authors: Katherine J
authors: KJ
authors: Hoag
authors: Weifu
authors: W
authors: Guo
authors: Aaron A
authors: AA
authors: Wiegel
authors: Elliot L
authors: EL
authors: Atlas
authors: Sue M
authors: SM
authors: Schauffler
authors: Mitchio
authors: M
authors: Okumura
authors: Kristie A
authors: KA
authors: Boering
authors: John M
authors: JM
authors: Eiler
mesh_terms: D001272:Atmosphere / Q000032:analysis*; D002245:Carbon Dioxide / Q000032:analysis* / Q000737:chemistry; D002247:Carbon Isotopes / Q000032:analysis*; D017753:Ecosystem; D005843:Geography; D017752:Greenhouse Effect; D013058:Mass Spectrometry; D008956:Models, Chemical; D010103:Oxygen Isotopes / Q000032:analysis* / Q000737:chemistry
chemical_list: D002247:Carbon Isotopes; D010103:Oxygen Isotopes; D002245:Carbon Dioxide