A retrieval of xCO2 from ground-based mid-infrared NDACC solar absorption spectra from Ny-Ålesund and comparison to TCCON

Two global networks of ground-based Fourier transform spec- trometers are measuring abundances of atmospheric trace gases that absorb in the near and mid infrared, Network for the Detec- tion of Atmospheric Composition Change (NDACC) and Total Carbon Column Observing Network (TCCON). The first lacks a CO2 product, therefore this study focuses on developing a CO2 retrieval method for NDACC from a new spectral window in the 4800 cm−1 region with a focus in applying it to Ny- Ålesund historical spectra. This region is covered by both TC- CON and NDACC, which is an advantage for collocated com- parisons where available. The results are compared with collo- cated TCCON measurements of column-averaged dry-air mole fractions of CO2 (denoted by xCO2) in Ny-Ålesund, Svalbard and only TCCON in Burgos, Philippines. We found that it is pos- sible to retrieve xCO2 from NDACC spectra. The comparison between the new retrieval to TCCON showed that the sensitivity of the new retrieval is high in the troposphere and lower in the upper stratosphere, similar to TCCON, and that the seasonality is well captured. We worked on determining an optimal retrieval setup covered in section 7.