Quantifying nitrous oxide emissions in the U.S. Midwest - A top-down
study using high resolution airborne in situ observations
Abstract
The U.S. Midwest, with its intensive agriculture, 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 increased agricultural
EDGAR version 4.3.2/5.0 emissions by a factor of 6.3±4.6/3.5±2.7,
resulting in Midwest N2O emissions of 0.42±0.28 nmol m-2 s-1. In
June/July 2019, a period when extreme flooding was occurring in the
Midwest, EDGAR was increased by a factor of 11.4±6.6/9.9±5.7, resulting
in N2O emissions of 1.06±0.57 nmol m-2 s-1. 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. Due to the complexity of N2O
emissions, further studies are necessary to fully characterize Midwest
emissions.