An inter-comparison 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: December 2018

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 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 fraction 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, Reunion Island, Wollongong, Lauder) in the time period of 2007–2017. The mean differences in XN2O between the TCCON and NDACC (NDACC-TCCON) at these sites are between −3.32 and 1.37 ppb (−1.1–0.5 %) with the 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 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 are very close to those from the NDACC and the surface flask sample measurements (0.9–1.0 ppb/year). The XN2O trends from the TCCON measurements are slightly lower (0.8–0.9 ppb/year) due to the underestimation of the trend in TCCON a priori. The XN2O trends from the GEOS-Chem a priori simulation are about 1.25 ppb/year, 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 shows a minimum in February–April while the FTIR XN2O retrievals shows a minimum in August–October. 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.