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

Zhou, Minqiang; 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 Mazière, Martine

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 M4inlinescrollmathml chem normal X normal N normal 2 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 M5inlinescrollmathml chem normal X normal N normal 2 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 M9inlinescrollmathml chem normal X normal N normal 2 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 M10inlinescrollmathml chem normal X normal N normal 2 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 M13inlinescrollmathml chem normal X normal N normal 2 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 M21inlinescrollmathml chem normal X normal N normal 2 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 M24inlinescrollmathml chem normal X normal N normal 2 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 M27inlinescrollmathml chem normal X normal N normal 2 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 M28inlinescrollmathml chem normal X normal N normal 2 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 M29inlinescrollmathml chem normal X normal N normal 2 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 M31inlinescrollmathml chem normal X normal N normal 2 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.

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Zhou, Minqiang / Langerock, Bavo / Wells, Kelley C. / et al: 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. 2019. Copernicus Publications.

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