TROPOMI/WFMD v2.0: Improved retrievals of XCH₄ and XCO with XGBoost-based quality filtering

Abstract. The TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel-5 Precursor satellite provides daily global observations of atmospheric methane (CH₄) and carbon monoxide (CO) at relatively high spatial resolution. The dense spatial and temporal coverage is achieved by the instrument’s wide swath, which permits detailed mapping of the worldwide distribution of these important atmospheric constituents. The adaptation and optimisation of the Weighting Function Modified Differential Optical Absorption Spectroscopy (WFMD) algorithm for the simultaneous retrieval of the column-averaged dry-air mole fractions XCH₄ and XCO from TROPOMI’s shortwave infrared (SWIR) radiance measurements has proven to be a valuable complement and alternative to the operational TROPOMI products.

The latest release of the TROPOMI/WFMD product (version 2.0) includes several improvements expanding its suitability for a wider range of scientific applications. Data yield at mid and high latitudes has increased, accompanied by improved accuracy and precision according to the validation with the ground-based Total Carbon Column Observing Network (TCCON). These advancements are primarily due to more refined quality filtering that has been accomplished by replacing the previous Random Forest Classifier with the more efficient and potentially higher performing Extreme Gradient Boosting (XGBoost) algorithm in conjunction with improved training data incorporating an updated cloud product from the Visible Infrared Imaging Radiometer Suite (VIIRS) and the TROPOMI Aerosol Index. This enhanced training data set enables more reliable identification of cloudy scenes and mitigates issues related to specific aerosol events over bright surfaces. Importantly, as with previous product versions, the actual quality classification does not depend on the real-time availability of these external data products, which are only required during the training phase.