Advances in retrieving XCH 4 and XCO from Sentinel-5 Precursor: improvements in the scientific TROPOMI/WFMD algorithm

[摘要] The TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel-5 Precursor satellite enablesthe accurate determination of atmospheric methane ( CH 4 ) and carbon monoxide ( CO )abundances at high spatial resolution and global daily sampling. Due to its wide swath andsampling, the global distribution of both gases can be determined in unprecedented detail. Thescientific retrieval algorithm Weighting Function Modified Differential Optical AbsorptionSpectroscopy (WFMD) has proven valuable in simultaneously retrieving the atmosphericcolumn-averaged dry-air mole fractions XCH 4 and XCO from TROPOMI's radiancemeasurements in the shortwave infrared (SWIR) spectral range. Here we present recent improvements of the algorithm which have been incorporated into the currentversion (v1.8) of the TROPOMI/WFMD product. This includes processing adjustments such as increasingthe polynomial degree to 3 in the fitting procedure to better account for possible spectral albedovariations within the fitting window and updating the digital elevation model to minimise topography-related biases. In the post-processing, the machine-learning-based quality filter hasbeen refined using additional data when training the random forest classifier to further reducescenes with residual cloudiness that are incorrectly classified as good. In particular, the cloudfiltering over the Arctic ocean is considerably improved. Furthermore, the machine learningcalibration, addressing systematic errors due to simplifications in the forward model orinstrumental issues, has been optimised. By including an additional feature associated with thefitted polynomial when training the corresponding random forest regressor, spectral albedovariations are better accounted for. To remove vertical stripes in the XCH 4 and XCO data, an efficient orbit-wise destriping filter based on combined wavelet–Fourier filtering has beenimplemented, while optimally preserving the original spatial trace gas features. The temporalcoverage of the data records has been extended to the end of April 2022, covering a total length of 4.5 years since the start of the mission, and will be further extended in the future. Validation with the ground-based Total Carbon Column Observing Network (TCCON) demonstratesthat the implemented improvements reduce the pseudo-noise component of the products, resulting in animproved random error. The XCH 4 and XCO products have similar spatial coverage fromyear to year including high latitudes and the oceans. The analysis of annual growth rates revealsaccelerated growth of atmospheric methane during the covered period, in line with observationsat marine surface sites of the Global Monitoring Division of NOAA's Earth System ResearchLaboratory, which reported consecutive annual record increases over the past 2 years of 2020 and 2021.

[学科分类] 内科医学