Comparison of inversions of global methane emissions using TM5-MP/4DVAR with TROPOMI measurements

Methane (CH₄) is an important greenhouse gas (GHG), contributing about ~23% (0.62 W m^-2) of the additional radiative forcing on the troposphere due to its increased concentrations compared to pre-industrial levels. Mainly, anthropogenic sources of CH₄ are agriculture, livestock farming, fossil fuels and biomass burning. Further, natural sources are primarily wetlands, while minor ones are oceans, termites, wild animals and permafrost. There are persistent uncertainties regarding the sources of CH₄ due to limited knowledge of the processes underlying them, leading to inconsistencies between top-down and bottom-up estimates. It is relevant to reduce these uncertainties and accurately assess CH₄ emissions as it is meaningful for atmospheric modelling, climate change estimations, and policy making. Higher resolution and lower uncertainty data can benefit assimilation systems, thus increasing confidence in estimates of emission sources.

This work aims to assimilate high-resolution data from satellite observations and measurements obtained from surface stations to optimize global methane emission fields. We evaluate differences resulting from the assimilation of satellite observations at varying resolutions. Additionally, we compare different setups of the assimilation framework, using varied combinations of satellite and surface measurements, to assess potential discrepancies in the resulting emissions. The modelling framework is based on the TM5-MP (massive parallel) atmospheric chemistry-transport model, utilizing its adjoint in a four-dimensional (4DVAR) data assimilation system. Our study aims to constrain global CH₄ emissions first at a spatial resolution of 3° × 2° (longitude × latitude) for 2018 and then increase the resolution to 1° × 1°. The tropospheric CH₄ mixing ratio product slated for assimilation is acquired by the TROPOMI instrument onboard the satellite Sentinel 5-P, and retrieved using the weighting function modified differential optical absorption spectroscopy (WFMD-IUP) algorithm. This product offers enhanced coverage, especially over higher latitudes, and reduced uncertainty compared to the operational product. Conversely, near-surface CH₄ measurements are obtained from stations within the global NOAA network. Preliminary results suggest a relevant role of both the resolution of the satellite instrument and the type of data assimilated regarding convergence and final results.