Preprints
https://doi.org/10.5194/amt-2023-233
https://doi.org/10.5194/amt-2023-233
14 Nov 2023
 | 14 Nov 2023
Status: a revised version of this preprint was accepted for the journal AMT.

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

Matthieu Dogniaux and Cyril Crevoisier

Abstract. Satellites have been providing spaceborne observations of the total column of CO2 (noted XCO2) for over two decades now and, with the need for independent verification of Paris Agreement objectives, many new satellite concepts are currently planned or being studied to complement or extend the already existing instruments. Depending on whether they are targeting natural and/or anthropogenic fluxes of CO2, the design of these future concepts vary greatly. The characteristics of their shortwave infrared (SWIR) observations notably explore several orders of magnitude in spectral resolution (from λ/Δλ~400 for Carbon Mapper to λ/Δλ~25000 for MicroCarb) and include different selections of spectral bands (from one to four bands, among which the CO2-sensitive 1.6 µm and/or 2.05 µm bands). Besides, the very nature of the spaceborne measurements is also explored: for instance, the NanoCarb imaging concept proposes to measure CO2-sensitive truncated interferograms, instead of infrared spectra as other concepts, in order to significantly reduce the instrument size. This study synthetically explores the impact of three different design parameters on XCO2 retrieval performance, as obtained through Optimal Estimation: (1) the spectral resolution; (2) the signal-to-noise ratio (SNR) and (3) the spectral band selection. Similar performance assessments are completed for the exactly-defined MicroCarb, Copernicus CO2 Monitoring (CO2M) and NanoCarb concepts. We show that improving SNR is more efficient than improving spectral resolution to increase XCO2 precision when perturbating these parameters across two orders of magnitude, and that low-SNR and/or low spectral resolution yield XCO2 with vertical sensitivities giving more weight to atmospheric layers close to the surface. The exploration of various spectral band combinations illustrates, especially for lower spectral resolutions, how including an O2-sensitive band helps to increase optical path length information, and how the 2.05 µm CO2-sensitive band contains more geophysical information than the 1.6 µm band. With very different characteristics, MicroCarb shows a CO2 information content only slightly higher than CO2M, which translates into lower XCO2 random errors, by a factor ranging from 1.1 to 1.9 depending on the observational situation. The NanoCarb performance for a single pixel of its imager compares to concepts that measure spectra at low-SNR and low-spectral resolution but, as this novel concept would observe a given target several times during a single overpass, its performance improves when combining all the observations. Overall, the broad range of results obtained through this synthetic XCO2 performance mapping hints at the future intercomparison challenges that the wide variety of upcoming CO2-observing concepts will pose.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
Matthieu Dogniaux and Cyril Crevoisier

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2023-233', Anonymous Referee #1, 19 Dec 2023
    • AC1: 'Reply on RC1', Matthieu Dogniaux, 09 Jun 2024
  • RC2: 'Comment on amt-2023-233', Anonymous Referee #2, 21 Dec 2023
    • AC2: 'Reply on RC2', Matthieu Dogniaux, 09 Jun 2024
  • RC3: 'Comment on amt-2023-233', Anonymous Referee #3, 05 Jan 2024
    • AC3: 'Reply on RC3', Matthieu Dogniaux, 09 Jun 2024
Matthieu Dogniaux and Cyril Crevoisier
Matthieu Dogniaux and Cyril Crevoisier

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Short summary
Many CO2-observing satellite concepts, with very different design choices and trade-offs, are expected to be put into orbit during the upcoming decade. This work uses numerical simulations to explore the impact of critical design parameters on the performance of upcoming CO2-observing satellite concepts.