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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 2 | Copyright
Atmos. Chem. Phys., 18, 735-756, 2018
https://doi.org/10.5194/acp-18-735-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 22 Jan 2018

Research article | 22 Jan 2018

Top-down constraints on global N2O emissions at optimal resolution: application of a new dimension reduction technique

Kelley C. Wells1, Dylan B. Millet1, Nicolas Bousserez2, Daven K. Henze2, Timothy J. Griffis1, Sreelekha Chaliyakunnel1, Edward J. Dlugokencky3, Eri Saikawa4, Gao Xiang5, Ronald G. Prinn6, Simon O'Doherty7, Dickon Young7, Ray F. Weiss8, Geoff S. Dutton3,9, James W. Elkins3, Paul B. Krummel10, Ray Langenfelds10, and L. Paul Steele10 Kelley C. Wells et al.
  • 1Department of Soil, Water, and Climate, University of Minnesota, St. Paul, MN, USA
  • 2Department of Mechanical Engineering, University of Colorado at Boulder, Boulder, CO, USA
  • 3Earth System Research Laboratory, NOAA, Boulder, CO, USA
  • 4Department of Environmental Sciences, Emory University, Atlanta, GA, USA
  • 5Joint Program on the Science and Policy of Global Change, Massachusetts Institute of Technology, Cambridge, MA, USA
  • 6Center for Global Change Science, Massachusetts Institute of Technology, Cambridge, MA, USA
  • 7School of Chemistry, University of Bristol, Bristol, UK
  • 8Scripps Institute of Oceanography, University of California San Diego, La Jolla, CA, USA
  • 9CIRES, University of Colorado at Boulder, Boulder, CO, USA
  • 10Climate Science Centre, CSIRO Oceans and Atmosphere, Aspendale, Victoria, Australia

Abstract. We present top-down constraints on global monthly N2O emissions for 2011 from a multi-inversion approach and an ensemble of surface observations. The inversions employ the GEOS-Chem adjoint and an array of aggregation strategies to test how well current observations can constrain the spatial distribution of global N2O emissions. The strategies include (1) a standard 4D-Var inversion at native model resolution (4° × 5°), (2) an inversion for six continental and three ocean regions, and (3) a fast 4D-Var inversion based on a novel dimension reduction technique employing randomized singular value decomposition (SVD). The optimized global flux ranges from 15.9Tg N yr−1 (SVD-based inversion) to 17.5–17.7Tg N yr−1 (continental-scale, standard 4D-Var inversions), with the former better capturing the extratropical N2O background measured during the HIAPER Pole-to-Pole Observations (HIPPO) airborne campaigns. We find that the tropics provide a greater contribution to the global N2O flux than is predicted by the prior bottom-up inventories, likely due to underestimated agricultural and oceanic emissions. We infer an overestimate of natural soil emissions in the extratropics and find that predicted emissions are seasonally biased in northern midlatitudes. Here, optimized fluxes exhibit a springtime peak consistent with the timing of spring fertilizer and manure application, soil thawing, and elevated soil moisture. Finally, the inversions reveal a major emission underestimate in the US Corn Belt in the bottom-up inventory used here. We extensively test the impact of initial conditions on the analysis and recommend formally optimizing the initial N2O distribution to avoid biasing the inferred fluxes. We find that the SVD-based approach provides a powerful framework for deriving emission information from N2O observations: by defining the optimal resolution of the solution based on the information content of the inversion, it provides spatial information that is lost when aggregating to political or geographic regions, while also providing more temporal information than a standard 4D-Var inversion.

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This paper uses three different frameworks to derive nitrous oxide (N2O) emissions based on global surface observations. One of these frameworks employs a new approach that allows for fast computation and explores a larger solution space than other methods. Our results point to a few conclusions about the global N2O budget, including a larger contribution from tropical sources, an overestimate of natural soil emissions, and an underestimate of agricultural sources particularly in springtime.
This paper uses three different frameworks to derive nitrous oxide (N2O) emissions based on...
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