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

摘要

Nitrous oxide ( Nsub2/subO ) 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 Nsub2/subO mole fraction. In this study, the dry-air column-averaged mole fractions of Nsub2/subO ( X N 2 O ) from the TCCON and NDACC measurements are compared against each other at seven sites around the world (Ny-?lesund, Sodankyl?, Bremen, Iza?a, Réunion, Wollongong, Lauder) in the time period of 2007–2017. The mean differences in X N 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 Nsub2/subO 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 X N 2 O measurement is strongly affected by its a priori profile. Trends and seasonal cycles of X N 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-?lesund and Sodankyl? are strongly affected by the polar winter and the polar vortex. The a posteriori Nsub2/subO fluxes in the model are optimized based on surface Nsub2/subO measurements with a 4D-Var inversion method. The X N 2 O trends from the GEOS-Chem a posteriori simulation ( 0.97±0.02 ( 1σ )?ppb?yr sup?1/sup ) are close to those from the NDACC (0 .93±0.04 ppb?yr sup?1/sup ) and the surface flask sample measurements ( 0.93±0.02 ppb?yr sup?1/sup ). The X N 2 O trend from the TCCON measurements is slightly lower ( 0.81±0.04 ppb?yr sup?1/sup ) due to the underestimation of the trend in TCCON a priori simulation. The X N 2 O trends from the GEOS-Chem a priori simulation are about 1.25?ppb?yr sup?1/sup , and our study confirms that the Nsub2/subO fluxes from the a priori inventories are overestimated. The seasonal cycles of X N 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 X N 2 O values show a minimum in February–April while the FTIR X N 2 O retrievals show different patterns. By comparing the partial column-averaged Nsub2/subO from the model and NDACC for three vertical ranges (surface–8, 8–17, 17–50?km), we find that the discrepancy in the X N 2 O seasonal cycle between the model simulations and the FTIR measurements in the Southern Hemisphere is mainly due to their stratospheric differences.