Abstract
Abstract. The TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel 5 Precursor (S5-P) satellite provides methane (CH4) measurements with high accuracy and exceptional temporal and spatial resolution and sampling. TROPOMI CH4 measurements are highly valuable to constrain emissions inventories and for trend analysis, with strict requirements on the data quality. This study describes the improvements that we have implemented to retrieve CH4 from TROPOMI using the RemoTeC full-physics algorithm. The updated retrieval algorithm features a constant regularization scheme of the inversion that stabilizes the retrieval and yields less scatter in the data and includes a higher resolution surface altitude database. We have tested the impact of three state-of-the-art molecular spectroscopic databases (HITRAN 2008, HITRAN 2016 and Scientific Exploitation of Operational Missions – Improved Atmospheric Spectroscopy Databases SEOM-IAS) and found that SEOM-IAS provides the best fitting results. The most relevant update in the TROPOMI XCH4 data product is the implementation of an a posteriori correction fully independent of any reference data that is more accurate and corrects for the underestimation at low surface albedo scenes and the overestimation at high surface albedo scenes. After applying the correction, the albedo dependence is removed to a large extent in the TROPOMI versus satellite (Greenhouse gases Observing SATellite – GOSAT) and TROPOMI versus ground-based observations (Total Carbon Column Observing Network – TCCON) comparison, which is an independent verification of the correction scheme. We validate 2 years of TROPOMI CH4 data that show the good agreement of the updated TROPOMI CH4 with TCCON (−3.4 ± 5.6 ppb) and GOSAT (−10.3 ± 16.8 ppb) (mean bias and standard deviation). Low- and high-albedo scenes as well as snow-covered scenes are the most challenging for the CH4 retrieval algorithm, and although the a posteriori correction accounts for most of the bias, there is a need to further investigate the underlying cause.
Highlights
Methane (CH4) is the second most important anthropogenic greenhouse gas after carbon dioxide (CO2)
It shows the shift to an overestimation at high latitudes where TROPOspheric Monitoring Instrument (TROPOMI) retrieves higher XCH4. This agrees with the conclusion that over snow TROPOMI XCH4 is too high, and this distribution resembles the latitudinal distribution of XCH4 shown in Fig. 1, it cannot be attributed to the selection of the spectroscopic database
That TROPOMI has been measuring for more than 2 years, the amount of data allows for the implementation of a series of updates that were not previously possible without the use of any reference data
Summary
Methane (CH4) is the second most important anthropogenic greenhouse gas after carbon dioxide (CO2). Satellite observations of CH4 are highly valuable to constrain emission inventories and for trend analysis, at global scale and at regional and local scales. CH4 measurements from satellite instruments like GOSAT (Greenhouse gases Observing SATellite) have been used to infer CH4 emissions from natural sources (e.g. tropical wetlands Lunt et al, 2019) and anthropogenic sources (e.g. coal mining in China Miller et al, 2019) and to map emissions and trends at global scale In this study we present the improvements that we have developed to retrieve CH4 from TROPOMI measurements using the full-physics approach, and we validate the TROPOMI CH4 product with satellite and ground-based measurements.
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