Quantifying Nitrous Oxide Emissions in the US Midwest: A Top-Down Study Using High Resolution Airborne In-Situ Observations

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.