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 (inline-formulaN₂O) 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 inline-formulaN₂O mole fraction. In this study, the dry-air column-averaged mole fractions of inline-formulaN₂O (inline-formula $M4inlinescrollmathmlchem{\mathrm{normal\; X}}_{{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}\mathrm{normal\; O}}$ 25pt14ptsvg-formulamathimg7083feeaa337c360bc1dec6cdd9e436c amt-12-1393-2019-ie00001.svg25pt14ptamt-12-1393-2019-ie00001.png ) 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 inline-formula $M5inlinescrollmathmlchem{\mathrm{normal\; X}}_{{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}\mathrm{normal\; O}}$ 25pt14ptsvg-formulamathimg5e6a681c49fd20b61f27782a4f0ae370 amt-12-1393-2019-ie00002.svg25pt14ptamt-12-1393-2019-ie00002.png between TCCON and NDACC (NDACC–TCCON) at these sites are between inline-formula−3.32 and 1.37 ppb (inline-formula−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 inline-formulaN₂O 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 inline-formula $M9inlinescrollmathmlchem{\mathrm{normal\; X}}_{{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}\mathrm{normal\; O}}$ 25pt14ptsvg-formulamathimg50b5fa68b9780aad29d3bc59a335671d amt-12-1393-2019-ie00003.svg25pt14ptamt-12-1393-2019-ie00003.png measurement is strongly affected by its a priori profile.

page1394Trends and seasonal cycles of inline-formula $M10inlinescrollmathmlchem{\mathrm{normal\; X}}_{{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}\mathrm{normal\; O}}$ 25pt14ptsvg-formulamathimg99677be2b065f598f9fe943d745811ab amt-12-1393-2019-ie00004.svg25pt14ptamt-12-1393-2019-ie00004.png 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 inline-formulaN₂O fluxes in the model are optimized based on surface inline-formulaN₂O measurements with a 4D-Var inversion method. The inline-formula $M13inlinescrollmathmlchem{\mathrm{normal\; X}}_{{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}\mathrm{normal\; O}}$ 25pt14ptsvg-formulamathimge10d4b76078a1e8806f098c6d853566d amt-12-1393-2019-ie00005.svg25pt14ptamt-12-1393-2019-ie00005.png trends from the GEOS-Chem a posteriori simulation (inline-formula0.97±0.02 (inline-formula1σ) ppb yrinline-formula⁻¹) are close to those from the NDACC (0inline-formula.93±0.04 ppb yrinline-formula⁻¹) and the surface flask sample measurements (inline-formula0.93±0.02 ppb yrinline-formula⁻¹). The inline-formula $M21inlinescrollmathmlchem{\mathrm{normal\; X}}_{{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}\mathrm{normal\; O}}$ 25pt14ptsvg-formulamathimg466b2088eb3ba38a6fcc0d0b8ea69279 amt-12-1393-2019-ie00006.svg25pt14ptamt-12-1393-2019-ie00006.png trend from the TCCON measurements is slightly lower (inline-formula0.81±0.04 ppb yrinline-formula⁻¹) due to the underestimation of the trend in TCCON a priori simulation. The inline-formula $M24inlinescrollmathmlchem{\mathrm{normal\; X}}_{{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}\mathrm{normal\; O}}$ 25pt14ptsvg-formulamathimg90450f00fd870e5f84133e6e1a36cf6c amt-12-1393-2019-ie00007.svg25pt14ptamt-12-1393-2019-ie00007.png trends from the GEOS-Chem a priori simulation are about 1.25 ppb yrinline-formula⁻¹, and our study confirms that the inline-formulaN₂O fluxes from the a priori inventories are overestimated. The seasonal cycles of inline-formula $M27inlinescrollmathmlchem{\mathrm{normal\; X}}_{{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}\mathrm{normal\; O}}$ 25pt14ptsvg-formulamathimgbf7b54d1602258a6bd4e0a2baf736945 amt-12-1393-2019-ie00008.svg25pt14ptamt-12-1393-2019-ie00008.png 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 inline-formula $M28inlinescrollmathmlchem{\mathrm{normal\; X}}_{{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}\mathrm{normal\; O}}$ 25pt14ptsvg-formulamathimgda6994d65f4a61e38d189bbe5fbdd62a amt-12-1393-2019-ie00009.svg25pt14ptamt-12-1393-2019-ie00009.png values show a minimum in February–April while the FTIR inline-formula $M29inlinescrollmathmlchem{\mathrm{normal\; X}}_{{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}\mathrm{normal\; O}}$ 25pt14ptsvg-formulamathimg8900c9f9c19d990507a65d899d2e82ec amt-12-1393-2019-ie00010.svg25pt14ptamt-12-1393-2019-ie00010.png retrievals show different patterns. By comparing the partial column-averaged inline-formulaN₂O from the model and NDACC for three vertical ranges (surface–8, 8–17, 17–50 km), we find that the discrepancy in the inline-formula $M31inlinescrollmathmlchem{\mathrm{normal\; X}}_{{\mathrm{normal\; N}}_{\mathrm{normal\; 2}}\mathrm{normal\; O}}$ 25pt14ptsvg-formulamathimg0d208196834a80a82d174963af43b993 amt-12-1393-2019-ie00011.svg25pt14ptamt-12-1393-2019-ie00011.png seasonal cycle between the model simulations and the FTIR measurements in the Southern Hemisphere is mainly due to their stratospheric differences.