Methane retrieved from TROPOMI: improvement of the data product and validation of the first 2 years of measurements

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.

Meteorological Drivers of Permian Basin Methane Anomalies Derived from TROPOMI

“…In this study, temporal trends from TROPOMI measurements of NO 2 and CH 4 were not evaluated due to multiple uncertainties including algorithm changes, unknown background values, spatial and temporal gaps in valid satellite retrievals, and uncorrected altitude and surface albedo CH 4 retrievals [25,46]. After careful evaluation of data availability and any temporal gap errors, temporal trends over multiple years could be potentially be evaluated after correcting for albedo and altitude effects (CH 4 ), while new improved cloud pressure effects for NO 2 will also be available.…”

“…The TROPOMI CH 4 measurements have also been recently validated. Two years of validation of TROPOMI CH 4 measurements against the Total Carbon Column Observing Network (TCCON) showed good agreement between satellite and TCCON data (−3.4 mean bias and ±5.6 ppbv standard deviation) [46], while other CH 4 measurement validation studies also found generally good agreement between TROPOMI and surface-based measurements [47][48][49]. In the Uintah Basin ONG producing region, TROPOMI methane columns have been shown to be well correlated with in situ measurements, while the methane columns were also correlated with the Lamont TCONN site in Oklahoma [25].…”

“…The TROPOMI CH 4 retrievals have gone through a number of algorithm improvements, including a constant regularization algorithm scheme and higher resolution surface altitude database (Table S1). In addition, the TROPOMI measurements have been found to be highly sensitive for low and high land surface albedo, and a posteriori correction for low and high land surface albedo was implemented [46,47,59] and the corrected product made available (however, the corrected albedo product is not yet available in GEE at the time of this publication [56]).…”

“…Trends and seasonal variations in CH 4 and NO 2 were not calculated for this study due to multiple versions of the satellite products (which could introduce temporal changes resulting from the different product version) available in GEE, as well as not having access to the a posteriori albedo correction for CH 4 data in GEE [46]. In addition, analysis of the temporal availability (not shown) of the CH 4 retrievals indicated that in the cloudier, cooler season there was frequently fewer available data.…”

“…In the Uintah Basin ONG producing region, TROPOMI methane columns have been shown to be well correlated with in situ measurements, while the methane columns were also correlated with the Lamont TCONN site in Oklahoma [25]. The retrievals have, however, been found to be sensitive to surface albedo, and a recent correction has been implemented [46].…”