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Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940

Abstract

The atmospheric nitrous oxide mixing ratio has increased by 20% since 1750 (ref. 1). Given that nitrous oxide is both a long-lived greenhouse gas2 and a stratospheric ozone-depleting substance3, this increase is of global concern. However, the magnitude and geographic distribution of nitrous oxide sources, and how they have changed over time, is uncertain4,5. A key unknown is the influence of the stratospheric circulation4,5, which brings air depleted in nitrous oxide to the surface. Here, we report the oxygen and intramolecular nitrogen isotopic compositions of nitrous oxide in firn air samples from Antarctica and archived air samples from Cape Grim, Tasmania, spanning 1940–2005. We detect seasonal cycles in the isotopic composition of nitrous oxide at Cape Grim. The phases and amplitudes of these seasonal cycles allow us to distinguish between the influence of the stratospheric sink and the oceanic source at this site, demonstrating that isotope measurements can help in the attribution and quantification of surface sources in general. Large interannual variations and long-term decreasing trends in isotope composition are also apparent. These long-term trends allow us to distinguish between natural and anthropogenic sources of nitrous oxide, and confirm that the rise in atmospheric nitrous oxide levels is largely the result of an increased reliance on nitrogen-based fertilizers.

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Figure 1: Changes in N2O since 1940.
Figure 2: Mean seasonal cycles for 1978–2005.
Figure 3: Large interannual variations in the mean seasonal cycles for 1978–2005.

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Acknowledgements

This work was supported by the UC Berkeley Atmospheric Sciences Center, the NASA Upper Atmosphere Research Program (NNX09AJ95G) and the Camille Dreyfus Teacher-Scholar Award (KAB) through UC Berkeley; the Brain Korea 21 Program and Korea NRF grant 2011-0013756 (S.P.) through the School of Earth and Environmental Sciences at Seoul National University; and the Australia Government’s Cooperative Research Centres Programme (T.D.v.O.). Long-term support by the Australian Government Bureau of Meteorology for the Cape Grim Air Archive Program is also gratefully acknowledged, and we are indebted to many Cape Grim staff for their diligent efforts in collecting the air archive samples. We thank S. Wofsy for comments on the analyses and manuscript and K-Y. Kim for additional insights from a cyclostationary empirical orthogonal function analysis of the data.

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S.P., P.C., and K.A.B. carried out the isotope measurements, analysed the results, interpreted the data and wrote the manuscript; P.C. developed and ran the box model, S.P. carried out the red-noise spectral calculations and both contributed equally to this work; R.L.L. and P.B.K. carried out the Cape Grim Air Archive time-series analyses; R.L.L., L.P.S., P.B.K. and P.J.F. provided air samples from the Cape Grim Air Archive, N2O mixing ratio measurements and manuscript comments. P.J.F., KRK, R.L.L. and L.P.S. proposed the study, gave conceptual advice and manuscript comments; D.M.E., D.F., C.M.T. and T.D.v.O. provided firn air samples and dating of those samples. C.M.T. modelled air movement in the firn and made the gravitational and diffusion corrections.

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Correspondence to K. A. Boering.

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Park, S., Croteau, P., Boering, K. et al. Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940. Nature Geosci 5, 261–265 (2012). https://doi.org/10.1038/ngeo1421

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