Can we obtain consistent emissions in Europe from three different CH4 TROPOMI products?

Satellite observations of total column methane atmospheric mixing ratios (XCH₄) combined with atmospheric transport inverse modeling offer enhanced capabilities to monitor the methane (CH₄) emissions at regional scale.

The TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor (S5P) satellite provides XCH₄ with global daily coverage and a relatively high (5.5×7 km²) horizontal resolution since 2017. Widely used for the hotspot detection and quantification, TROPOMI-CH₄ data is also exploited in regional and global CH₄ flux inversions.

Three retrieval products of XCH₄are produced and routinely updated from TROPOMI: the SRON official product, the WFMD product by the University of Bremen and the BLENDED product by the University of Harvard. The official dataset (v2.04) relies on the RemoTeC full-physics algorithm, which retrieves atmospheric methane concentration alongside atmospheric scattering properties. The WFMD scientific product is based on the University of Bremen’s Weighting Function Modified Differential Optical Absorption Spectroscopy (WFM-DOAS) and a machine learning classifier for quality filtering. The BLENDED product combines S5P-TROPOMI and GOSAT-TANSO retrievals. It is a post-processed of the official TROPOMI product, correcting biases using a machine learning model trained on collocated observations from both instruments. Despite recent advances in retrieval techniques, inter-product comparisons reveal notable differences in quality filtering, observed XCH₄ values, and associated uncertainties. It leads to discrepancies in flux estimates derived from inversions, particularly at local and country scales.

We assimilate these three TROPOMI XCH₄products into regional atmospheric inversions of CH₄ emissions over Europe at a 0.5°×0.5° resolution, for the year 2019. The inversions are conducted using the CHIMERE transport model within the inverse modeling platform Community Inversion Framework (CIF). In situ surface measurements are used for validation. We investigate the primary factors contributing to the inter-product differences, including albedo, aerosols and striping patterns. We also perform Observing System Simulation Experiments (OSSE) with synthetic pseudo-observations and perturbed prior fluxes to assess the sensitivity of the system to observations and isolate the causes of the differences in inversion results. We inquire into the impact of observation density, retrieval errors and inter-product biases on the posterior fluxes. The resulting methane emissions budgets are compared at pixel, country and regional scales, providing insights into the consistency of TROPOMI-based regional inversions.