The operational methane retrieval algorithm for TROPOMI

Abstract

Abstract. This work presents the operational methane retrieval algorithm for the Sentinel 5 Precursor (S5P) satellite and its performance tested on realistic ensembles of simulated measurements. The target product is the columnaveraged dry air volume mixing ratio of methane (XCH 4 ), which will be retrieved simultaneously with scattering properties of the atmosphere. The algorithm attempts to fit spectra observed by the shortwave and near-infrared channels of the TROPOspheric Monitoring Instrument (TROPOMI) spectrometer aboard S5P.The sensitivity of the retrieval performance to atmospheric scattering properties, atmospheric input data and instrument calibration errors is evaluated. In addition, we investigate the effect of inhomogeneous slit illumination on the instrument spectral response function. Finally, we discuss the cloud filters to be used operationally and as backup.We show that the required accuracy and precision of < 1 % for the XCH 4 product are met for clear-sky measurements over land surfaces and after appropriate filtering of difficult scenes. The algorithm is very stable, having a convergence rate of 99 %. The forward model error is less than 1 % for about 95 % of the valid retrievals. Model errors in the input profile of water do not influence the retrieval outcome noticeably. The methane product is expected to meet the requirements if errors in input profiles of pressure and temperature remain below 0.3 % and 2 K, respectively. We further find that, of all instrument calibration errors investigated here, our retrievals are the most sensitive to an error in the instrument spectral response function of the shortwave infrared channel.

Monitoring Global Tropospheric OH Concentrations using Satellite Observations of Atmospheric Methane

“…The retrieval success rate (ratio between the number of successful retrievals and the number of attempted retrievals) is taken to be 5 3% for SWIR (Hu et al, 2016) and 60% for TIR (Xiong et al, 2008) because SWIR observations require cloud-free pixels whereas TIR has tolerance for fractional cloud cover. The retrievals are for the dry air column mixing ratio X [ppb] after applying typical averaging kernels to describe vertical sensitivity (Figure 3).…”

Reevaluating the Use of O2 a1Δg Band in Spaceborne Remote Sensing of Greenhouse Gases

“…To separate the sources/sinks of CO 2 and CH 4 from variations of surface pressure and specific humidity, the abundances of CO 2 and CH 4 are usually represented by the column‐averaged dry mole fractions ( and ). To sufficiently constrain their distributions and trends through inverse modeling, has to be retrieved to a precision of 0.25–0.5% (1–2 ppmv; Crisp, ; Frankenberg et al, ; Miller et al, ), and has to be retrieved to a precision of better than 1% (20 ppbv; Bergamaschi et al, , ; Hu et al, ) without significant geographically varying biases.…”

Determination of the TROPOMI-SWIR instrument spectral response function

“…Simulations have shown that in particular the methane retrieval is very sensitive to errors in the instrument spectral response function (ISRF or instrument line shape). As a result, the requirement on the ISRF is formulated that it should be known with an accuracy of 1 % of its maximum where the ISRF is greater than 1 % of its maximum (Hu et al, 2016). To reach the required accuracy, data have been measured with high spectral resolution using a scanning monochromatic light source covering the SWIR band.…”