Satellite Remote Sensing of Methane: From GOSAT to the Sentinel-5 Precursor
Hasekamp, Otto1; Detmers, Rob1; Hu, Haili1; Landgraf, Jochen1; Schepers, Dinand1; Aben, Ilse1; Hahne, Philipp2; Butz, Andre2
1SRON Netherlands Institute for Space Research, NETHERLANDS; 2Karlsruhe Institute for Technology (KIT), GERMANY
The main challenge when retrieving the methane dry air mole fraction (XCH4) from satellite measurements in the Short Wave Infra Red (SWIR) spectral range with sufficient accuracy, is to properly account for light path modification by aerosols and thin cirrus clouds. The RemoTeC algorithm is being jointly developed by SRON Netherlands Institute for Space Research and the Karlsruhe Institute for Technology (KIT) to accurately retrieve XCH4 (and XCO2) by fitting aerosol properties (amount, size, height) simultaneously with trace gas concentrations. In this contribution we present RemoTeC XCH4 retrievals from the Japanese Greenhouse gas Observing Satellite (GOSAT), launched January 2009. GOSAT observes backscattered radiances at the O2 A-band around 750 nm, weak CH4 and CO2 absorption bands around 1600 nm, and a strong CO2 band around 2000 nm. We present a validation of retrieved XCH4 with ground based measurements of the Total Carbon Column Observing Network (TCCON) and investigate possible correlations of the XCH4 error with geophysical parameters. Based on validation with TCCON we demonstrate that the precision and accuracy of the retrieved XCH4 is respectively about 0.25% and 0.8%. The ESA Sentinel-5 Precursor (S5P), to be launched in 2015, will allow XCH4 retrievals using the O2 A-band and the 2300 nm SWIR band. The key improvement of TROPOMI methane measurements compared to earlier instruments is its very good spatial coverage (global coverage within one day). For S5P we are developing the retrieval algorithm to be implemented in the operational processor also using the RemoTeC approach. Based on XCH4 retrievals for a realistic ensemble of synthetic S5P measurements, we expect similar performance of S5P compared to GOSAT concerning XCH4 retrieval accuracy and precision.