A multiannual and global synergetic satellite product of tropospheric CH4

Data products of atmospheric methane with improved vertical sensitivity in the lower troposphere are crucial for gaining a more comprehensive understanding of the impact of anthropogenic emissions. This study presents a methane data product derived from the synergistic combination of TROPOMI (Tropospheric Monitoring Instrument) total column and IASI (Infrared Atmospheric Sounding Interferometer) profiles, utilizing level 2 data spanning the period from 2018 to 2021. IASI enables high-quality retrievals in the upper troposphere-lower stratosphere, while TROPOMI observations excel in providing sensitivity to total-column-averaged CH4. The combined product retains the information from individual datasets and therefore enhances sensitivity to lower tropospheric signals independently of the upper troposphere, which is not achievable by using IASI or TROPOMI data alone. We present a method for optimally combining the IASI and TROPOMI level 2 data. Firstly, the products from the individual satellites are collocated in time and space (geomatching). Subsequently, the collocated data are optimally merged by fully considering the individual data characteristics (uncertainties and sensitivities) by the application of a Kalman filter. We show that the procedure is robust and computationally cheap, which allows the efficient combination of billions of IASI and TROPOMI observations, and the combined product offers good global coverage. The combined product is validated by comparison to reference datasets such as 14 globally distributed TCCON (Total Carbon Column Observing Network) stations, CH4 profile measurements made by 36 individual AirCore soundings, and tropospheric CH4 data derived from continuous ground-based in situ observations made at two nearby Global Atmospheric Watch (GAW) mountain stations. These comparisons confirm the theoretically predicted quality of the combined data product, in particular the increased quality of the tropospheric CH4 data. Following the procedure outlined above, the final data product will consist of CH4 partial columns below and above 6000 m a.s.l. together with their respective averaging kernels and uncertainties and it will be made available as netCDF files that are compliant with version 1.7 of the CF metadata convention.