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Large and unexpected enrichment in stratospheric 16 13 18 O C O and its...

Yeung, L. Y., H. P. Affek, K. J. Hoag, W. Guo, A. A. Wiegel, E. Atlas, S. Schauffler, M. Okumura, K. A. Boering, and J. M. Eiler (2009), Large and unexpected enrichment in stratospheric 16 13 18 O C O and its meridional variation, Proc. Natl. Acad. Sci., 106, 11496-11501, doi:10.1073/pnas.0902930106.

The stratospheric CO2 oxygen isotope budget is thought to be governed primarily by the O(1D)ⴙCO2 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 CO2 isotopologue 16O13C18O, in concert with 18O and 17O 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 16O13C18O, observed as proportions in the polar vortex that are higher than in any naturally derived CO2 sample to date. We show, through photochemical experiments, that lower 16O13C18O proportions observed in the midlatitudes are determined primarily by the O(1D)ⴙCO2 isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher 16O13C18O 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(1D)ⴙCO2. We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO2 or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric 16O13C18O enrichments may impose additional isotopic constraints on biosphere–atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change.

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Upper Atmosphere Research Program (UARP)
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