TCCON and NDACC XCO measurements: difference, discussion and application

• Published in: Atmospheric Measurement Techniques
• Main Authors: 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, Mazière, Martine
• Format: Text
• Language: English
• Published: 2019
• Subjects: Ny-Ålesund; Sodankylä; Ny Ålesund
• Online Access: https://doi.org/10.5194/amt-12-5979-2019; https://amt.copernicus.org/articles/12/5979/2019/

Column-averaged dry-air mole fraction of CO (X CO ) 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 X CO 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 X CO 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 X CO 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 X CO 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 X CO measurements are 6.1 ± 1.6 % and 8.0 ± 3.2 % smaller than the AirCore measurements at Orléans and Sodankylä, respectively, indicating that the scaling factor of TCCON X CO 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 X CO data. This paper also demonstrates that the smoothing error must be taken into account when comparing FTIR X CO data, and especially TCCON X CO data, with model or satellite data.