An intercomparison of total column-averaged nitrous oxide between ground-based FTIR TCCON and NDACC measurements at seven sites and comparisons with the GEOS-Chem model
Authors: Zhou Minqiang , Langerock Bavo , Wells Kelley C. , Millet Dylan B. , Vigouroux Corinne , Sha Mahesh Kumar , Hermans Christian , Metzger Jean-Marc , Kivi Rigel , Heikkinen Pauli , Smale Dan , Pollard David F. , Jones Nicholas , Deutscher Nicholas M. , Blumenstock Thomas , Schneider Matthias , Palm Mathias , Notholt Justus , Hannigan James W. , De Mazière Martine 
Published: March 2019

Abstract. Nitrous oxide (N2O) is an important greenhouse gas and it can also
generate nitric oxide, which depletes ozone in the stratosphere. It is a
common target species of ground-based Fourier transform infrared (FTIR) near-infrared (TCCON) and
mid-infrared (NDACC) measurements. Both TCCON and NDACC networks provide a
long-term global distribution of atmospheric N2O mole fraction. In this
study, the dry-air column-averaged mole fractions of N2O (XN2O) from
the TCCON and NDACC measurements are compared against each other at seven
sites around the world (Ny-Ålesund, Sodankylä, Bremen, Izaña,
Réunion, Wollongong, Lauder) in the time period of 2007–2017. The mean
differences in XN2O between TCCON and NDACC (NDACC–TCCON) at these
sites are between −3.32 and 1.37 ppb (−1.1 %–0.5 %) with standard
deviations between 1.69 and 5.01 ppb (0.5 %–1.6 %), which are within the
uncertainties of the two datasets. The NDACC N2O retrieval has good
sensitivity throughout the troposphere and stratosphere, while the TCCON
retrieval underestimates a deviation from the a priori in the troposphere and
overestimates it in the stratosphere. As a result, the TCCON XN2O
measurement is strongly affected by its a priori profile. Trends and seasonal cycles of XN2O are derived from the TCCON and NDACC
measurements and the nearby surface flask sample measurements and compared
with the results from GEOS-Chem model a priori and a posteriori simulations.
The trends and seasonal cycles from FTIR measurement at Ny-Ålesund and
Sodankylä are strongly affected by the polar winter and the polar vortex.
The a posteriori N2O fluxes in the model are optimized based on surface
N2O measurements with a 4D-Var inversion method. The XN2O trends
from the GEOS-Chem a posteriori simulation (0.97±0.02 (1σ) ppb yr−1) are close to those from the NDACC (0.93±0.04 ppb yr−1) and
the surface flask sample measurements (0.93±0.02 ppb yr−1). The
XN2O trend from the TCCON measurements is slightly lower (0.81±0.04 ppb yr−1) due to the underestimation of the trend in TCCON a priori simulation. The
XN2O trends from the GEOS-Chem a priori simulation are about 1.25 ppb yr−1, and our study confirms that the
N2O fluxes from the a priori
inventories are overestimated. The seasonal cycles of XN2O from the
FTIR measurements and the model simulations are close to each other in the
Northern Hemisphere with a maximum in August–October and a minimum in
February–April. However, in the Southern Hemisphere, the modeled XN2O
values show a minimum in February–April while the FTIR XN2O retrievals show
different patterns. By comparing the partial column-averaged N2O from the
model and NDACC for three vertical ranges (surface–8, 8–17, 17–50 km), we
find that the discrepancy in the XN2O seasonal cycle between the model
simulations and the FTIR measurements in the Southern Hemisphere is mainly
due to their stratospheric differences.