The Space Carbon Observatory (SCARBO) concept: assessment of X_CO2 and X_CH4 retrieval performance

Abstract

Abstract. Several single-platform satellite missions have been designed during the past decades in order to retrieve the atmospheric concentrations of anthropogenic greenhouse gases (GHG), initiating worldwide efforts towards better monitoring of their sources and sinks. To set up a future operational system for anthropogenic GHG emission monitoring, both revisit frequency and spatial resolution need to be improved. The Space Carbon Observatory (SCARBO) project aims at significantly increasing the revisit frequency of spaceborne GHG measurements, while reaching state-of-the-art precision requirements, by implementing a concept of small satellite constellation. It would accommodate a miniaturised GHG sensor named NanoCarb coupled with an aerosol instrument, the multi-angle polarimeter SPEXone. More specifically, the NanoCarb sensor is a static Fabry–Pérot imaging interferometer with a 2.3×2.3 km2 spatial resolution and 200 km swath. It samples a truncated interferogram at optical path differences (OPDs) optimally sensitive to all the geophysical parameters necessary to retrieve column-averaged dry-air mole fractions of CO2 and CH4 (hereafter XCO2 and XCH4). In this work, we present the Level 2 performance assessment of the concept proposed in the SCARBO project. We perform inverse radiative transfer to retrieve XCO2 and XCH4 directly from synthetic NanoCarb truncated interferograms and provide their systematic and random errors, column vertical sensitivities, and degrees of freedom as a function of five scattering-error-critical atmospheric and observational parameters. We show that NanoCarb XCO2 and XCH4 systematic retrieval errors can be greatly reduced with SPEXone posterior outputs used as improved prior aerosol constraints. For two-thirds of the soundings, located at the centre of the 200 km NanoCarb swath, XCO2 and XCH4 random errors span 0.5–1 ppm and 4–6 ppb, respectively, compliant with their respective 1 ppm and 6 ppb precision objectives. Finally, these Level 2 performance results are parameterised as a function of the explored scattering-error-critical atmospheric and observational parameters in order to time-efficiently compute extensive L2 error maps for future CO2 and CH4 flux estimation performance studies.

Synthetic mapping of XCO2 retrieval performance from shortwave infrared measurements: impact of spectral resolution, signal-to-noise ratio and spectral band selection

“…Because of their very nature, these truncated interferograms are sensitive to the periodic signature of CO2 in the infrared spectrum. As in Dogniaux et al (2022), we use here the latest design of the NanoCarb instrument, which accounts for CO2 information entanglements with H2O and aerosols, while atmospheric temperature or albedo have been neglected. It measures truncated interferograms that are sensitive to four spectral bands, as shown in Fig.…”

“…retrievals that can be done for a same location on the ground are then combined to yield only one unique retrieval result, with reduced random error (assuming independent observations). Dogniaux et al (2022) details the NanoCarb concept performance results…”

“…Because an infrared detector has a limited number of pixels, future -planned or studied -CO2 imaging concepts explore various trade-offs between spatial and spectral resolution, spectral band selection, and even compromise with the very nature of the measurements made by the instrument, when other constraints such as size and thus costs are taken into consideration. These concepts -not exclusively -include the European Copernicus CO2 Monitoring (Meijer, 2020) mission, the Japanese Global Observing SATellite for Greenhouse gases and Water cycle (GOSAT-GW, Matsunaga and Tanimoto, 2022), the American non-profit Carbon Mapper initiative (https://carbonmapper.org/) based on the Next-Generation of NASA Airborne Visible/Infrared Imaging Spectrometer (AVIRIS-NG, Cusworth et al, 2021;Hamlin et al, 2011), the German CO2image concept (Strandgren et al, 2020;Wilzewski et al, 2020) or the European Space CARBon Observatory (SCARBO) H2020 concept, that does not measure spectra but only truncated interferograms (Brooker, 2018;Dogniaux et al, 2022;Gousset et al, 2019). Table 1 gathers the characteristics of upcoming or studied SWIR CO2 observing satellite concepts, provided either in scientific articles (in this case citations are provided), in conference presentations (just the conference name and dates are given), or websites (just the hyperlink is given), as some of these concepts are quite recent.…”

“…Table 1 gathers the characteristics of upcoming or studied SWIR CO2 observing satellite concepts, provided either in scientific articles (in this case citations are provided), in conference presentations (just the conference name and dates are given), or websites (just the hyperlink is given), as some of these concepts are quite recent. (Brooker, 2018;Dogniaux et al, 2022;Gousset et al, 2019) The characteristics of an observing concept (nature of measurement, spectral resolution, spectral band selection, signal-tonoise ratio) translate into an 𝑋 !" !…”

Impact of the selection of optical path differences for a spectral imager based on partially sampled interferograms

“…It is therefore necessary to transfer the optimal estimation algorithms into Fourier space: simulated interferograms are directly fit to the measured interferograms in order to retrieve total columns from its measurements. Within the framework of the European Space Carbon Observatory project (H2020 SCARBO), it was demonstrated that the relevant geophysical information (CO2 column for one camera and CH4 column for the other camera) is indeed present in the interferogram portions that were identified [2]. In parallel, two prototypes were built, one configured on the measurement of the CO2 column, the other on the measurement of the total CH4 column.…”