TCCON and NDACC X CO measurements: difference, discussion and application

Zhou, Minqiang; Langerock, Bavo; Vigouroux, Corinne; Sha, Mahesh Kumar; Hermans, Christian; Metzger, Jean-Marc; Chen, Huilin; Ramonet, Michel; Kivi, Rigel; Heikkinen, Pauli; Smale, Dan; Pollard, David F.; Jones, Nicholas; Velazco, Voltaire A.; García, Omaira E.; Schneider, Matthias; Palm, Mathias; Warneke, Thorsten; De Mazière, Martine

Column-averaged dry-air mole fraction of CO (Xinline-formulaCO) measurements are obtained from two ground-based Fourier transform infrared (FTIR) spectrometer networks: the Total Carbon Column Observing Network (TCCON) and the Network for the Detection of Atmospheric Composition Change (NDACC). In this study, the differences between the TCCON and NDACC Xinline-formulaCO measurements are investigated and discussed based on six NDACC–TCCON sites using data over the period 2007–2017. A direct comparison shows that the NDACC Xinline-formulaCO measurements are about 5.5 % larger than the TCCON data at Ny-Ålesund, Bremen, and Izaña (Northern Hemisphere), and the absolute bias between the NDACC and TCCON data is within 2 % at Saint-Denis, Wollongong and Lauder (Southern Hemisphere). The hemispheric dependence of the bias is mainly attributed to their smoothing errors. The systematic smoothing error of the TCCON Xinline-formulaCO data varies in the range between 0.2 % (Bremen) and 7.9 % (Lauder), and the random smoothing error varies in the range between 2.0 % and 3.6 %. The systematic smoothing error of NDACC data is between 0.1 % and 0.8 %, and the random smoothing error of NDACC data is about 0.3 %. For TCCON data, the smoothing error is significant because it is higher than the reported uncertainty, particularly at Southern Hemisphere sites. To reduce the influence from the a priori profiles and different vertical sensitivities, the scaled NDACC a priori profiles are used as the common a priori profiles for comparing TCCON and NDACC retrievals. As a result, the biases between TCCON and NDACC Xinline-formulaCO measurements become more consistent (5.6 %–8.5 %) with a mean value of 6.8 % at these sites. To determine the sources of the remaining bias, regular AirCore measurements at Orléans and Sodankylä are compared to co-located TCCON measurements. It is found that TCCON Xinline-formulaCO measurements are 6.1 inline-formula± 1.6 % and 8.0 inline-formula± 3.2 % smaller than the AirCore measurements at Orléans and Sodankylä, respectively, indicating that the scaling factor of TCCON Xinline-formulaCO data should be around 1.0000 instead of 1.0672. Further investigations should be carried out in the TCCON community to determine the correct scaling factor to be applied to the TCCON Xinline-formulaCO data. This paper alsopage5980 demonstrates that the smoothing error must be taken into account when comparing FTIR Xinline-formulaCO data, and especially TCCON Xinline-formulaCO data, with model or satellite data.

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Zhou, Minqiang / Langerock, Bavo / Vigouroux, Corinne / et al: TCCON and NDACC XCO measurements: difference, discussion and application. 2019. Copernicus Publications.

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