Using XCO<sub>2</sub> retrievals for assessing the long-term consistency of NDACC/FTIR data sets

Abstract

Abstract. Within the NDACC (Network for the Detection of Atmospheric Composition Change), more than 20 FTIR (Fourier-transform infrared) spectrometers, spread worldwide, provide long-term data records of many atmospheric trace gases. We present a method that uses measured and modelled XCO 2 for assessing the consistency of these NDACC data records. Our XCO 2 retrieval setup is kept simple so that it can easily be adopted for any NDACC/FTIR-like measurement made since the late 1950s. By a comparison to coincident TCCON (Total Carbon Column Observing Network) measurements, we empirically demonstrate the useful quality of this suggested NDACC XCO 2 product (empirically obtained scatter between TCCON and NDACC is about 4 ‰ for daily mean as well as monthly mean comparisons, and the bias is 25 ‰). Our XCO 2 model is a simple regression model fitted to CarbonTracker results and the Mauna Loa CO 2 in situ records. A comparison to TCCON data suggests an uncertainty of the model for monthly mean data of below 3 ‰. We apply the method to the NDACC/FTIR spectra that are used within the project MUSICA (multi-platform remote sensing of isotopologues for investigating the cycle of atmospheric water) and demonstrate that there is a good consistency for these globally representative set of spectra measured since 1996: the scatter between the modelled and measured XCO 2 on a yearly time scale is only 3 ‰.

Retrieval of xCO<sub>2</sub> from ground-based mid-infrared (NDACC) solar absorption spectra and comparison to TCCON

“…However, further analysis shows a strong sensitivity to the chosen a priori, as shown previously by Barthlott et al (2015).…”

“…The results show a non-negligible smoothing effect. Following the paper by Barthlott et al (2015), we further conclude that, under the assumption that the CarbonTracker model used is a good approximation for the true atmospheric profile, the smoothing correction is not able to compensate for the dampened seasonal cycle in the MIR xCO 2 retrievals.…”

“…Recently, Barthlott et al (2015) used MIR xCO 2 retrievals in combination with a simple model to investigate the longterm consistency of NDACC measurements. They conclude that the low sensitivity of the MIR averaging kernels in the lower troposphere is sufficient to perform comparisons to the model on timescales longer than monthly, and therefore xCO 2 retrievals in the MIR are appropriate for monitoring long-term instrument stability.…”

“…However, the annual increase of xCO 2 is already well understood; in order to gain new insights into carbon cycle processes, information on shorter timescales is needed to assess and complement model simulations. Additionally, the analysis of systematic differences in Barthlott et al (2015) is performed using monthly means, and the dampening of the seasonal cycle amplitude is proposed to be addressed using a dampening factor derived from the shape of the site's pressure profile. Although this is adequate for the paper's goal of monitoring instrument stability, a complete analysis of the different sensitivities of the MIR and NIR retrievals is necessary for the retrieval of information relevant to carbon cycle research.…”

Fourier transform spectrometer measurements of column CO<sub>2</sub> at Sodankylä, Finland

“…We established the FTS measurements at Sodankylä in early 2009. Since then, the XCO 2 retrievals have been used in several studies (e.g., Wunch et al, 2011b;Oshchepkov et al, 2012;Saito et al, 2012;Belikov et al, 2013;Guerlet et al, 2013;Yoshida et al, 2013;Agustí-Panareda, 2014;Deng et al, 2014;Reuter et al, 2014;Barthlott et al, 2015;Heymann et al, 2015;Lindqvist et al, 2015;Belikov et al, 2016;Feng et al, 2016;Inoue et al, 2016;Massart et al, 2016). This paper describes the instrumentation, measurement procedures and data processing at the Sodankylä FTS site, corresponding to the data re-trieval version GGG2014 .…”