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.

Minqiang Zhou; 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 MaziÞre, Martine

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 1.37ppb (-1.1-0.5%) with the standard deviations between 1.69 and 5.01ppb (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 (0.9-1.0ppb/year). The XN2O trends from the TCCON measurements are slightly lower (0.8-0.9ppb/year) due to the underestimation of the trend in TCCON a priori. The XN2O trends from the GEOS-Chem a priori simulation are about 1.25ppb/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, 17-50km), 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.

NITROUS oxide; NITRIC oxide

Atmospheric Measurement Techniques Discussions, 2018, p1

1867-8610

Academic Journal

10.5194/amt-2018-340