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

Journal name:
Nature Geoscience
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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.

At a glance


  1. Changes in N2O since 1940.
    Figure 1: Changes in N2O since 1940.

    a, Measurements of N2O mixing ratio versus effective date for 11 firn air samples pumped from Law Dome, Antarctica (67°S, 113°E) in 1997 and 2004 (blue circles), or versus collection date for 50 archived air samples from Cape Grim (40.7°S, 144.8°E) (black squares) between 1978 and 2005; and corresponding measurements of b, δ15N (‰ versus air–N2); c, δ18O (‰ versus VSMOW); and d, δ15Nα (‰ versus air–N2) of N2O versus N2O mixing ratio. The Poinsett firn air measurements (green triangles) have not been corrected for gravitation and diffusion in the firn. The red diamonds (positioned arbitrarily at 1700 for display) denote the average isotopic composition of the preindustrial troposphere (δT,i) modelled here and given in Table 1. The error bars reflect the 1σ single measurement uncertainty (or the standard error of the mean for six samples that were run multiple times) and, for the firn air, a small contribution (±0.01‰) from the firn air correction uncertainties. δ15Nβ and site preference are not shown.

  2. Mean seasonal cycles for 1978-2005.
    Figure 2: Mean seasonal cycles for 1978–2005.

    Mean seasonal cycles from the time-series analysis of the Cape Grim archived air measurements for a, N2O (in ppb) and b, δ15N (‰ versus air–N2), cδ18O (‰ versus VSMOW) and d, δ15Nα (‰ versus air–N2) of N2O. e, The contribution and timing of the influences owing to STE (°), the ocean ventilation source (*) and ocean solubility (+) of N2O to the seasonal cycle in N2O mixing ratio at Cape Grim estimated by Nevison et al. 8 in 2005 using measurements of CFCs, Ar/N2 and O2/N2 at Cape Grim. The error bars in ad are the 1σ standard deviations of the monthly residuals in the time-series analysis calculated from the difference between the smoothed and trend curves, sampled at weekly intervals and binned by calendar month, as discussed in Methods and Supplementary Information.

  3. Large interannual variations in the mean seasonal cycles for 1978-2005.
    Figure 3: Large interannual variations in the mean seasonal cycles for 1978–2005.

    The values for a, the detrended N2O mixing ratio and b, detrended δ15Nα of N2O are shown (blue filled circle) for Cape Grim, along with the mean seasonal cycles from Fig. 2 (thick grey curves) and the seasonal cycles inferred for each year (blue lines); horizontal lines are the mean seasonal maximum +1σand the mean seasonal minimum −1σ (solid), and the mean seasonal maximum +2σand the mean seasonal minimum −2σ (dashed).


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Author information


  1. Departments of Earth & Planetary Science and of Chemistry, University of California, Berkeley, California 94720, USA

    • S. Park,
    • P. Croteau &
    • K. A. Boering
  2. Centre for Australian Weather and Climate Research/CSIRO Marine and Atmospheric Research, Private Bag 1, Aspendale, Victoria 3195, Australia

    • D. M. Etheridge,
    • P. J. Fraser,
    • P. B. Krummel,
    • R. L. Langenfelds,
    • L. P. Steele &
    • C. M. Trudinger
  3. Formerly of National Institute of Water and Atmospheric Research, Private Bag 14901, Wellington 6021, New Zealand

    • D. Ferretti
  4. School of Earth and Environmental Sciences/Research Institute of Oceanography, College of Natural Sciences, Seoul National University, Seoul 151-747, Republic of Korea

    • K-R. Kim
  5. Australian Antarctic Division, Channel Highway, Kingston, Tasmania 7050, Australia

    • T. D. van Ommen
  6. Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Private Bag 80, Hobart, Tasmania 7001, Australia

    • T. D. van Ommen
  7. Present address: School of Earth and Environmental Sciences (BK21), Seoul National University, Seoul 151-742, Republic of Korea

    • S. Park
  8. Present address: Aerodyne Research, Inc., 45 Manning Rd., Billerica, Massachusetts 01821, USA

    • P. Croteau


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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