Quantifying Nitrous Oxide Emissions in the U.S. Midwest: A Top‐Down Study Using High Resolution Airborne In‐Situ Observations

• Published in: Geophysical research letters 2021-03, Vol.48 (5), p.n/a
• Main Authors: Eckl, Maximilian, Roiger, Anke, Kostinek, Julian, Fiehn, Alina, Huntrieser, Heidi, Knote, Christoph, Barkley, Zachary R., Ogle, Stephen M., Baier, Bianca C., Sweeney, Colm, Davis, Kenneth J.
• Format: Article
• Language: English
• Subjects: agriculture; climate change; flux estimate; Midwest; nitrous oxide; top‐down
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL091266

The densely farmed U.S. Midwest is a prominent source of nitrous oxide (N2O) but top‐down and bottom‐up N2O emission estimates differ significantly. We quantify Midwest N2O emissions by combining observations from the Atmospheric Carbon and Transport‐America campaign with model simulations to scale the Emissions Database for Global Atmospheric Research (EDGAR). In October 2017, we scaled agricultural EDGAR v4.3.2 and v5.0 emissions by factors of 6.3 and 3.5, respectively, resulting in 0.42 nmol m−2 s−1 Midwest N2O emissions. In June/July 2019, a period when extreme flooding was occurring in the Midwest, agricultural scaling factors were 11.4 (v4.3.2) and 9.9 (v5.0), resulting in 1.06 nmol m−2 s−1 Midwest emissions. Uncertainties are on the order of 50 %. Agricultural emissions estimated with the process‐based model DayCent (Daily version of the CENTURY ecosystem model) were larger than in EDGAR but still substantially smaller than our estimates. The complexity of N2O emissions demands further studies to fully characterize Midwest emissions. Plain Language Summary Nitrous oxide (N2O) is the third most important anthropogenic greenhouse gas contributing to the warming of the planet and the dominant man‐made ozone‐depleting substance in the stratosphere. Its atmospheric concentrations have been rising since industrialization mainly due to an increase in anthropogenic sources, with agriculture being the dominant source. The densely farmed U.S. Midwest plays an important role in the global N2O budget. However, previous studies that have collected observations of N2O indicate that estimates of surface emissions in the Midwest are substantially underestimating the truth. In this study, we combine unique aircraft‐based N2O measurements and model simulations to quantify Midwest emissions in October 2017 and June/July 2019. Agricultural inventory estimates had to be increased by factors up to 20 to match observations, revealing a large underestimation in current inventories. An extreme flooding event in 2019 when the summer observations occurred may be responsible for some of this discrepancy. Estimations of soil N2O emissions calculated with a state‐of‐the‐art biogeochemical model show less underestimation but are still too low compared to the fluxes derived from the aircraft observational data. Key Points Within the ACT‐America project, we gathered a unique airborne in‐situ N2O data set over the U.S. Midwest with enhancements up to 9  ppb N2O emissions in the U.S.