Articles | Volume 8, issue 9
Research article
08 Sep 2015
Research article |  | 08 Sep 2015

Mapping spectroscopic uncertainties into prospective methane retrieval errors from Sentinel-5 and its precursor

R. Checa-Garcia, J. Landgraf, A. Galli, F. Hase, V. A. Velazco, H. Tran, V. Boudon, F. Alkemade, and A. Butz

Abstract. Sentinel-5 (S5) and its precursor (S5P) are future European satellite missions aiming at global monitoring of methane (CH4) column-average dry air mole fractions (XCH4). The spectrometers to be deployed onboard the satellites record spectra of sunlight backscattered from the Earth's surface and atmosphere. In particular, they exploit CH4 absorption in the shortwave infrared spectral range around 1.65 μm (S5 only) and 2.35 μm (both S5 and S5P) wavelength. Given an accuracy goal of better than 2 % for XCH4 to be delivered on regional scales, assessment and reduction of potential sources of systematic error such as spectroscopic uncertainties is crucial. Here, we investigate how spectroscopic errors propagate into retrieval errors on the global scale. To this end, absorption spectra of a ground-based Fourier transform spectrometer (FTS) operating at very high spectral resolution serve as estimate for the quality of the spectroscopic parameters. Feeding the FTS fitting residuals as a perturbation into a global ensemble of simulated S5- and S5P-like spectra at relatively low spectral resolution, XCH4 retrieval errors exceed 0.6 % in large parts of the world and show systematic correlations on regional scales, calling for improved spectroscopic parameters.

Short summary
The future satellite missions Sentinel-5 and its precursor will monitor methane column average dry air mole fractions. The ambitious accuracy required on regional scales demands a characterization of the systematic error sources in which spectroscopic uncertainties are crucial. This study investigates how methane and water vapour spectroscopic errors propagate into retrieval errors, showing that spectroscopy-induced errors exceed 0.6% in large parts of the world and are regionally correlated.