Assessment of the differences in European CH4 emission estimates from three TROPOMI products

Abstract

Satellite observations from the Sentinel-5P TROPOMI instrument, combined with inverse modeling, provide a valuable resource for quantifying regional methane (CH4) emissions. This study compares the 2019 European emissions estimated from variational inversions assimilating three TROPOMI products of dry-column methane mole fractions (XCH4). The SRON (v2.4, operational product), BLENDED (v1.0), and WFMD (v1.8) products are retrieved from distinct algorithms. These retrievals differ in coverage, error characterization, and XCH4 spatial distribution. Machine learning predictions of XCH4 differences point to aerosol scattering and albedo sensitivity as the largest contributors to the differences. The derived 2019 European CH4 emission budgets show an increase of +2 % for SRON, and a decrease of -1 %, -33 % and -9 % relative to the prior, respectively, for BLENDED, WFMD and surface-based inversions. The range of budget estimates reveals inconsistencies in the total emissions derived from the inversions. At the national and sub-national scale, spatial emission patterns are similar for the non-independent SRON and BLENDED but differ substantially from WFMD. No inversion provides a systematically closer match to the spatial distribution of emissions derived from independent surface observations. Evaluation at surface stations shows that the residual between the observations and the posterior concentrations is reduced for 37 %, 53 % and 47 % of the stations, respectively, for SRON, BLENDED and WFMD. Observing System Simulation Experiments (OSSEs) are used to disentangle the drivers of differences between the posterior emissions. Results show that aligning coverage and individual observation errors increases the consistency between emission estimates. Residual differences in the OSSE posterior emissions can be attributed to differences in averaging kernels and prior profiles.