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 (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 (${\mathrm{X}}_{{\mathrm{N}}_{\mathrm{2}}\mathrm{O}}$) 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 ${\mathrm{X}}_{{\mathrm{N}}_{\mathrm{2}}\mathrm{O}}$ 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 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 ${\mathrm{X}}_{{\mathrm{N}}_{\mathrm{2}}\mathrm{O}}$ measurement is strongly affected by its a priori profile.

Trends and seasonal cycles of ${\mathrm{X}}_{{\mathrm{N}}_{\mathrm{2}}\mathrm{O}}$ 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 N₂O fluxes in the model are optimized based on surface N₂O measurements with a 4D-Var inversion method. The ${\mathrm{X}}_{{\mathrm{N}}_{\mathrm{2}}\mathrm{O}}$ 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 ${\mathrm{X}}_{{\mathrm{N}}_{\mathrm{2}}\mathrm{O}}$ 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 ${\mathrm{X}}_{{\mathrm{N}}_{\mathrm{2}}\mathrm{O}}$ 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 ${\mathrm{X}}_{{\mathrm{N}}_{\mathrm{2}}\mathrm{O}}$ 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 ${\mathrm{X}}_{{\mathrm{N}}_{\mathrm{2}}\mathrm{O}}$ values show a minimum in February–April while the FTIR ${\mathrm{X}}_{{\mathrm{N}}_{\mathrm{2}}\mathrm{O}}$ 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 ${\mathrm{X}}_{{\mathrm{N}}_{\mathrm{2}}\mathrm{O}}$ seasonal cycle between the model simulations and the FTIR measurements in the Southern Hemisphere is mainly due to their stratospheric differences.