Abstract
AbstractThe 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.
Highlights
Nitrous oxide (N2O) is the third most important anthropogenic greenhouse gas (GHG) in terms of longterm radiative forcing (Myhre et al, 2013) and is the dominant ozone-depleting substance in the stratosphere (Ravishankara et al, 2009)
The prior N2O emission estimates for the optimization were obtained from Emissions Database for Global Atmospheric Research (EDGAR) version 4.3.2 (EDGAR4.3.2, 2017; Janssens-Maenhout et al, 2019) and version 5.0 (Crippa et al, 2020;EDGAR5.0, 2019)
Compared to EDGAR4.3.2 no significant differences in the spatial distribution of emissions is seen, both versions just differ in the strength of the surface fluxes
Summary
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 Midwest were on average 0.42 ± 0.28 nmol m−2 s−1 in October 2017 and 1.06 ± 0.57 nmol m−2 s−1 in June to July 2019. Bottom-up estimates from EDGAR and DayCent underestimate U.S Midwest N2O emissions by factors up to 20. Quantifying Nitrous Oxide Emissions in the U.S Midwest: A Top-Down Study Using High Resolution Airborne In-Situ Observations. Maximilian Eckl , Anke Roiger, Julian Kostinek, Alina Fiehn , Heidi Huntrieser , Christoph Knote, Zachary R. Baier5,6 , Colm Sweeney , and Kenneth J.
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