Quantifying nitrous oxide emissions in the U.S. Midwest - A top-down
study
Abstract
Nitrous oxide (N2O), a potent greenhouse gas and ozone
depleting substance, plays a crucial role in the atmosphere.
Anthropogenic emissions from agriculture contribute to a rising trend in
global N2O emissions and atmospheric concentrations.
However, due to insufficient direct observations, regional
N2O emissions derived in bottom-up and top-down studies
are highly uncertain. The U.S. Midwest is one of the most intensive
agriculture areas worldwide and hence may contribute significantly to
the observed trend. Recent top-down studies suggest that bottom-up
estimates underestimate agricultural emissions in that area by up to an
order of magnitude. Here we quantify nitrous oxide emissions in the
Midwest in October 2017 and June-July 2019 with a top-down approach.
Unique continuous aircraft-based measurements of N2O
conducted during the ACT-America campaign together with forward WRF-Chem
model simulations are used to scale the EDGAR inventory thus quantifying
emissions. On average we had to upscale October 2017 and June-July 2019
agricultural EDGAR 4.3.2/5.0 emissions by a factor of 6.3/3.5 and
11.4/9.9, resulting in 0.42 nmol m-2
s-1 and 1.06 nmol m-2
s-1 emissions in the Midwest, respectively. Finally,
calculations of direct soil N2O emissions from the
DayCent biogeochemical model are compared to our estimates.