S.1.1 Validation of NIR by TIRVIM
There are two CO bands within the TIRVIM spectral range: 2.3 µm (transition 2-0) and 4.7 µm (transition 1-0). The signal-to-noise ratio of TIRVIM is 90–100 for pure Sun in the 4.7-µm range to ~150 at 2.3 µm. An example of TIRVIM spectra recorded during one occultation are shown at Figure S1. Despite the 4.7 µm spectral range being characterized by lower SNR compared to 2.3 µm, the fundamental 1-0 band is much stronger than 2-0 overtone (Fig. S1). We therefore used it to retrieve the CO vertical profiles based on the TIRVIM data.