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

Nitrous oxide (N ₂ O) 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) nearinfrared (TCCON) and mid-infrared (NDACC) measurements. Both TCCON and NDACC networks provide a longterm global distribution of atmospheric N ₂ O mole fraction. In this study, the dry-air column-averaged mole fractions of N ₂ O (XN ₂ O) 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 XN ₂ O 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 N ₂ O 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 XN ₂ O measurement is strongly affected by its a priori profile. Trends and seasonal cycles of XN ₂ O 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 N ₂ O fluxes in the model are optimized based on surface N ₂ O measurements with a 4D-Var inversion method. The XN ₂ O trends from the GEOS-Chem a posteriori simulation (0:97-0:02 (1) 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 XN ₂ O 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 XN ₂ O trends from the GEOS-Chem a priori simulation are about 1.25 ppb yr-1, and our study confirms that the N ₂ O fluxes from the a priori inventories are overestimated. The seasonal cycles of XN ₂ O 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 XN ₂ O values show a minimum in February–April while the FTIR XN ₂ O retrievals show different patterns. By comparing the partial column-averaged N ₂ O from the model and NDACC for three vertical ranges (surface–8, 8–17, 17–50 km), we find that the discrepancy in the XN ₂ O seasonal cycle between the model simulations and the FTIR measurements in the Southern Hemisphere is mainly due to their stratospheric differences.