A multi-year global methane data set obtained by merging observations from TROPOMI and IASI

Abstract. Data products of atmospheric methane (CH4) 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 CH4 data product derived from the synergetic combination of level 2 (L2) data from TROPOMI (Tropospheric Monitoring Instrument) and IASI (Infrared Atmospheric Sounding Interferometer), specifically CH4 total column and CH4 profiles, respectively. IASI enables high-quality observation of CH4 mixing ratios in the upper troposphere and lower stratosphere, and TROPOMI observations excel in providing sensitivity to the total column-averaged mixing ratio of CH4. By combining the IASI and TROPOMI L2 products synergetically, we can detect tropospheric CH4 (mixing ratios averaged over a layer from the surface up to 450 hPa) that is not significantly affected by the strong CH4 variations around the tropopause. This is not achievable by using IASI or TROPOMI data alone. For the synergetic L2 data combination, we use the method as presented in detail in Schneider et al. (2022b), and apply it to combine about 444 million individual and high-quality TROPOMI observations with about 805 million individual and high-quality IASI observations made globally over 42 months (from January 2018 to June 2021). The combination method is fast; it uses a tool designed for efficient geo-matching between large data sets and a computationally cheap Kalman filter for calculations and for merging the data sets. We show that the combined data set has a good global coverage. Moreover, we document that the sensitivity (response of the combined data product to real atmospheric CH4 variations) is extremely satisfactory throughout the globe, and the uncertainties are generally below 12–15 ppbv. Furthermore, we demonstrate the increased scientific value of the combined data product when compared to the two individual data products. The data set of the combined product consists of about 289 million individual data points, and it is provided as NetCDF files. One file has a typical size of 280 MB and contains all data for observations made in one day (the universal time of the TROPOMI observations are taken as the reference time). For review, the data are accessible at https://radar.kit.edu/radar/en/dataset/wq583rnzpmd83m5g?token=UEuECSWHlGgWBdoPVsvI (Shahzadi et al., 2025) and made freely available at https://www.imk-asf.kit.edu/english/CH4-synergy-IASI-TROPOMI_RemoTeC.php.