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

• Published in: Nature geoscience Vol. 5; no. 4; pp. 261 - 265
• Main Authors: Park, S., Croteau, P., Boering, K. A., Etheridge, D. M., Ferretti, D., Fraser, P. J., Kim, K-R., Krummel, P. B., Langenfelds, R. L., van Ommen, T. D., Steele, L. P., Trudinger, C. M.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.04.2012; Nature Publishing Group
• Subjects: 704/106; 704/106/47; Air sampling; Anthropogenic factors; Earth and Environmental Science; Earth Sciences; Earth System Sciences; Firn; Geochemistry; Geographical distribution; Geology; Geophysics/Geodesy; Greenhouse gases; letter; Nitrogen; Nitrous oxide; Ozone depletion; PSE, Australia, Tasmania, Cape Grim; PS, Antarctica
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/ngeo1421

The atmospheric nitrous oxide concentration has increased by 20% since 1750. Analyses of Antarctic firn and archived air samples reveal seasonal cycles in the isotopic signature of nitrous oxide, which can help to disentangle the contribution of surface sources. The atmospheric nitrous oxide mixing ratio has increased by 20% since 1750 (ref.  1 ). Given that nitrous oxide is both a long-lived greenhouse gas 2 and a stratospheric ozone-depleting substance 3 , this increase is of global concern. However, the magnitude and geographic distribution of nitrous oxide sources, and how they have changed over time, is uncertain 4 , 5 . A key unknown is the influence of the stratospheric circulation 4 , 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.