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Published January 2, 2007 | Published + Supplemental Material
Journal Article Open

Oxygen isotopic composition of carbon dioxide in the middle atmosphere


The isotopic composition of long-lived trace molecules provides a window into atmospheric transport and chemistry. Carbon dioxide is a particularly powerful tracer, because its abundance remains >100 parts per million by volume (ppmv) in the mesosphere. Here, we successfully reproduce the isotopic composition of CO2 in the middle atmosphere, which has not been previously reported. The mass-independent fractionation of oxygen in CO2 can be satisfactorily explained by the exchange reaction with O(1D). In the stratosphere, the major source of O(1D) is O3 photolysis. Higher in the mesosphere, we discover that the photolysis of 16O17O and 16O18O by solar Lyman-{alpha} radiation yields O(1D) 10–100 times more enriched in 17O and 18O than that from ozone photodissociation at lower altitudes. This latter source of heavy O(1D) has not been considered in atmospheric simulations, yet it may potentially affect the "anomalous" oxygen signature in tropospheric CO2 that should reflect the gross carbon fluxes between the atmosphere and terrestrial biosphere. Additional laboratory and atmospheric measurements are therefore proposed to test our model and validate the use of CO2 isotopic fractionation as a tracer of atmospheric chemical and dynamical processes.

Additional Information

© 2007 by The National Academy of Sciences of the USA. Freely available online through the PNAS open access option. Communicated by Inez Y. Fung, University of California, Berkeley, CA, November 10, 2006 (received for review January 31, 2006). Published online before print December 26, 2006, 10.1073/pnas.0610009104 We give special thanks to G.R. Gladstone for the solar Lyman-α flux and S.T. Gibson for providing his coupled-channel code. We also thank B.C. Hsieh, X. Jiang, and R.L. Shia for helping us with the model; M. Gerstell, H. Hartman, A. Ingersoll, J. Kaiser, C. Miller, H. Pickett, T. Röckmann, and all of the members in our group for their helpful comments; and Inez Fung for handling the reviewing process and two anonymous referees for their insightful comments to improve the manuscript. This work was supported by National Science Foundation Grant ATM-0529268. Author contributions: M.-C.L., G.A.B., and Y.L.Y. designed research; M.-C.L. and B.R.L. performed research; and M.-C.L., G.A.B., B.R.L., and Y.L.Y. wrote the paper. The authors declare no conflict of interest.

Attached Files

Published - LIApnas07a.pdf

Supplemental Material - LIApnas07afig1.jpg

Supplemental Material - LIApnas07afig2.jpeg

Supplemental Material - LIApnas07afig3.jpeg

Supplemental Material - LIApnas07afig4.jpeg

Supplemental Material - LIApnas07afig5.jpeg

Supplemental Material - LIApnas07afig6.jpeg


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August 22, 2023
October 16, 2023