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

• Main Authors: Zhou, Minqiang, Langerock, Bavo, Wells, Kelley, Millet, Dylan B, Vigouroux, C, Sha, Mahesh Kumar, Hermans, Christian, Metzger, J -M, Kivi, Rigel, Heikkinen, Pauli, Smale, D, Pollard, David F, Jones, Nicholas B, Deutscher, Nicholas M, Blumenstock, Thomas, Schneider, Matthias, Palm, Mathias, Notholt, Justus, Hannigan, James W, de Maziere, Martine
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Research Online 2019
• Subjects: Ny-Ålesund; Sodankylä; Ny Ålesund
• Online Access: https://ro.uow.edu.au/smhpapers1/600; https://ro.uow.edu.au/cgi/viewcontent.cgi?article=1614&context=smhpapers1

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.