16 Sep 2022
16 Sep 2022
Status: this preprint is currently under review for the journal AMT.

Accounting for surface reflectance spectral features in TROPOMI methane retrievals

Alba Lorente, Tobias Borsdorff, Mari C. Martinez-Velarte, and Jochen Landgraf Alba Lorente et al.
  • Earth science group, SRON Netherlands Institute for Space Research, Leiden, the Netherlands

Abstract. Satellite remote sensing of methane (CH4) using the TROPOMI instrument is key to monitor and quantify emissions globally. In the past years, analysis of TROPOMI methane data has pointed to few false methane anomalies that can potentially be misinterpreted as enhancements due to strong emission sources. These artefacts are caused by spectral features of the underlying surfaces, which are not well represented in the forward model. Surface reflectance spectral dependence in the full-physics RemoTeC retrieval algorithm is modelled using a second order polynomial in wavelength. We show in this study that a third order polynomial better represents the surface reflectance dependency with wavelength of specific surface materials (e.g., rock), resulting in an improved characterization of the spectral features that caused the artificial localized XCH4 enhancements found in several locations like e.g., Siberia, Australia, and Algeria. The use of a third order polynomial removed these artificial XCH4 enhancements and significantly improved the fit over these specific features, while outside of these areas globally the fit did not improve in most cases. This reflects that a second order polynomial is optimal to capture the spectral dependencies of most surfaces given the characteristic of the TROPOMI instrument, but a third order polynomial is needed for the specific spectral characteristics of several surfaces. Furthermore, increasing the order of the polynomial to higher degrees did not further improve the retrieval. We also found that the known bias in retrieved methane for low albedo measurements slightly improves, but still a posterior correction needs to be applied, leaving open the question about the root cause of the albedo bias. After applying the third order polynomial globally, we perform the routine validation with TCCON and GOSAT. GOSAT comparison does not significantly improve, while TCCON validation results show an overall improvement of 2–4 ppb, reflecting that TCCON stations are not close to any of the corrected artefacts and are typically located around spectrally smooth surfaces.

Alba Lorente et al.

Status: open (until 22 Oct 2022)

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Alba Lorente et al.

Alba Lorente et al.


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Short summary
In the TROPOMI methane data, there are few false methane anomalies that can be misinterpreted as enhancements caused by strong emission sources. These artefacts are caused by features of the underlying surfaces, that are not well characterised in the retrieval algorithm. Here we improve the representation of the surface reflectance dependency with wavelength in the forward model, removing the artificial localized CH4 enhancements found in several locations like Siberia, Australia, and Algeria.