The use of O2 1.27 µm absorption band revisited for GHG monitoring from space and application to MicroCarb

Abstract. 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 (i.e. OCO-2, Tanso/Gosat, Tansat). 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 shape as the absorption signal that is used. In the frame of the CNES MicroCarb project, scientific studies have been performed in 2016–2018 to explore the problems associated to this O2* airglow contamination and methods to correct it. A theoretical synthetic spectrum of the emission was derived from a new approach, based on A21 Einstein coefficients information contained in the line-by-line HITRAN 2016 data base. The shape of our synthetic spectrum is fully validated when compared to O2* airglow spectra observed by SCIAMACHY/ENVISAT in limb viewing. We have designed an inversion scheme of SCIAMACHY limb viewing spectra, allowing to determine the vertical distribution of the Volume Emision Rate of the O2* airglow. The VER profiles and corresponding integrated nadir intensities were both compared to a model of the emission based on the chemical-transport model REPROBUS. The airglow intensities depend mostly on the Solar Zenith Angle (both in model and data) and the model underestimate the observed emission by ∼ 15 %. This is fully confirmed with SCIAMACHY nadir viewing measurements over the oceans: in such conditions, we have disentangled and retrieved the nadir O2* emission in spite of the moderate spectral resolving power (∼ 860). It is shown that with the MicroCarb spectral resolution power (25,000) and SNR, the contribution of the O2* emission at 1.27 µm to the observed spectral radiance in nadir viewing may be disentangled from the lower atmosphere/ground absorption signature with a great accuracy. Simulations with 4ARCTIC radiative transfer inversion tool have shown that the CO2 mixing ratio may be retrieved with the accuracy required for quantifying the CO2 natural sources and sinks (pressure level error ≤ 1 hPa, XCO2 accuracy better than 0.4 ppmv) with only the O2 1.27 µm band. As a result of these studies (at an intermediate phase), it was decided to include this band (B4) in the MicroCarb design, while keeping the O2 A band for reference (B1). Our approach is very similar (likely identical), to the approach of Sun et al. (2018) who also analysed the potential of the O2 1.27 µm band and concluded favourably for GHG monitoring from space. We advocate for the inclusion of this O2 band on other GHG monitoring future space missions, such as GOSAT-3 and EU/ESA CO2-M missions, for a better GHG retrieval.