Use of the 1.27 μm O2 absorption band for CO2 and methane mixing ratio estimates in nadir viewing from space: Potential and application to Microcarb

Monitoring CO2 from space is essential to characterize the spatio/temporal distribution of this major greenhouse gas, and quantify its sources and sinks. The mixing ratio of CO2 to dry air can be derived from the CO2/O2 column ratio. The O2 column is usually derived form its absorption signature on the solar reflected spectra over the O2 A-band (OCO-2, Tanso/Gosat). As a result of atmospheric scattering, the atmospheric path length varies with the aerosols load, their vertical distribution, and their optical properties. The spectral distance between the O2 A-band (0.76 μm) and the CO2 absorption band (1.6 μm) results in significant uncertainties due to the varying spectral properties of the aerosols over the globe. There is another O2 absorption band at 1.27 μm with weaker lines than in the A-band. As the wavelength is much nearer to the CO2 and CH4 bands, there is less uncertainty when using it as a proxy of the atmospheric path length to the CO2 and CH4 bands. This O2 band is used by the TCCON network implemented for the validation of space-based GHG observations. However, this absorption band is contaminated by the spontaneous emission of the excited molecule O2*, which is produced by the photo-dissociation of O3 molecules in the stratosphere and mesosphere. From a satellite looking nadir, this emission has a similar magnitude as the absorption signal that is used. In the frame of the CNES Microcarb project, scientific studies have been performed in 2016-2017 to explore the problems associated to this airglow emission and methods to correct it. The intensities observed by SCIAMACHY/ENVISAT in limb viewing have been compared to a model of the emission based on the chemical-transport model Reprobus. The airglow intensities depend mostly on the Solar Zenith Angle and the agreement data/model is quite good. It was shown that, provided the spectra is acquired with a sufficient spectral resolution and SNR, the contribution of the O2* emission at 1.27 μm to the observed spectral radiance may be disentangled from the lower atmosphere/ground absorption signature. The CO2 mixing ratio may be retrieved with the accuracy required for quantifying the CO2 sources (pressure level error < 1 hPa, mixing ratio error < 0.4 ppmv). As a result of these studies, it was decided to include such a band in the Microcarb design, although keeping the O2 A band for reference. Some detailed results of these O2* studies and their 2018 update will be presented. We advocate for the inclusion of such a band in other GHG monitoring future space missions, such as GOSAT-2 and EU/ESA CO2M missions, for a better GHG retrieval.