Abstract. Monitoring CO2 from space is essential to characterize the
spatiotemporal 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 from its absorption signature on the solar reflected spectra over
the O2 A band (e.g. Orbiting Carbon Observatory-2 (OCO-2), Thermal
And Near infrared Sensor for carbon Observation (TANSO)/Greenhouse
Gases Observing Satellite (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 closer 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 Total Carbon Column Observing Network (TCCON) implemented for the validation of
space-based greenhouse gas (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 to the absorption signal that is used. In the frame of the CNES (Centre National d'Études Spatiales – the French
National Centre for Space Studies) MicroCarb project, scientific studies have been
performed in 2016–2018 to explore the problems associated with this O2*
airglow contamination and methods to correct it. A theoretical synthetic
spectrum of the emission was derived from an approach based on A21
Einstein coefficient information contained in the line-by-line high-resolution
transmission molecular absorption (HITRAN) 2016
database. The shape of our synthetic spectrum is validated when compared to
O2* airglow spectra observed by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography
(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 emission rate
(VER) of the O2* airglow. The VER profiles and corresponding integrated
nadir intensities were both compared to a model of the emission based on the
Reactive Processes Ruling the Ozone Budget in the Stratosphere
(REPROBUS) chemical transport model. The airglow intensities depend mostly on
the solar zenith angle (both in model and data), and the model underestimates
the observed emission by ∼15 %. This is 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) and found that the nadir
SCIAMACHY intensities are mostly dictated by solar zenith angle (SZA) and are larger than the model
intensities by a factor of ∼1.13. At a fixed SZA, the model airglow
intensities show very little horizontal structure, in spite of ozone
variations. It is shown that with the MicroCarb spectral resolution power (25 000) and
signal-to-noise ratio (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. Indeed,
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 the
O2 1.27 µm band only as the air proxy (without the A 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 consistent with 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/European Space Agency (ESA) CO2-M missions, for a better GHG
retrieval.
The use of the 1.27 µm O<sub>2</sub> absorption band for greenhouse gas monitoring from space and application to MicroCarb
