The use of the 1.27 µm O₂ absorption band for greenhouse gas monitoring from space and application to MicroCarb

Abstract

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.

Local Anomalies in the Column‐Averaged Dry Air Mole Fractions of Carbon Dioxide Across the Globe During the First Months of the Coronavirus Recession

“…To support the PA, priority should be given on the development of all-weather CO 2 monitoring systems, ideally from space but at least in situ (e.g., Gurney et al, 2017;Wu et al, 2016). From space, at this stage, it is not clear how much complementary measurements that help characterizing the light path in the presence of optically thin clouds and aerosols (e.g., Bertaux et al, 2020;Rusli et al, 2020) or variations in the observation local time can increase the data yield in key emission regions. In addition, the modest 10.1029/2020GL090244 XCO 2 signal suggested by our model simulation (usually smaller than the retrieval random errors) in most areas affected by the pandemic suggests the importance of demonstrating a sufficient metrological resolution for averages of retrievals, possibly with direct (in situ) measurements of XCO 2 (Karion et al, 2010), so that small variations of XCO 2 can be robustly characterized.…”

Validation of spectroscopic data in the 1.27 µm spectral region by comparisons with ground-based atmospheric measurements

“…O2 absorption plays an essential role in the determination of air-mass and remote sensing calibration (e.g. [1][2][3][4][5][6]) because the volume-mixing ratio of atmospheric oxygen is well known (0.2095) and uniformly distributed. When compared to the commonly-used A-band near 0.76 m, the 1.27 m band is closer to the CO2 and CH4 bands (near 1.61 m, 1.67 m and 2.06 m) which are used by various greenhouse gas remote sensing experiments (e.g.…”

Toward High Precision XCO2Retrievals From TanSat Observations: Retrieval Improvement and Validation Against TCCON Measurements

“…When validated against measurements from the Total Carbon Column Observing Network (TCCON) (Wunch, Toon, et al, 2011), XCO 2 derived from GOSAT and OCO‐2 has an accuracy of better than 2 part per million (ppm) (Butz et al, 2011; Buchwitz, Dils, et al, 2017; Cogan et al, 2012; Crisp et al, 2012; Kim et al, 2016; O’Dell et al, 2012; Oshchepkov et al, 2013; Wu et al, 2018; Yang et al, 2015; Yoshida et al, 2013), thanks to the high performance of these instruments and long‐term calibration efforts (Crisp et al, 2017; Frankenberg et al, 2015; Kuze et al, 2014; Rosenberg et al, 2016; Yoshida et al, 2013). Further advances are expected from recently launched missions such as GOSAT‐2 (Nakajima et al, 2019) and OCO‐3 (Eldering et al, 2019) launched in 2018 and 2019 respectively, and future missions including MicroCarb (Bertaux et al, 2020) and CO2M (Kuhlmann et al, 2019).…”