Monitoring Persistent Methane Emissions from the Secunda CTL Synthetic Fuel Plant using Satellite Observations

Reducing global methane emissions is vital in combating climate change. Satellite-based instruments provide a way to independently monitor methane emissions from various sources at different scales, helping to assess the progress toward emission reduction targets. In this study, we apply several data-driven methods to estimate methane emissions from the Secunda CTL (coal-to-liquids) synthetic fuel plant in South Africa, utilizing satellite observations from the TROPOspheric Monitoring Instrument (TROPOMI) aboard the Sentinel-5 Precursor (S5P) satellite, and the GHGSat fleet of high-resolution commercial satellites. We find annual mean emissions of about 13-22 t/h based on S5P/TROPOMI observations. These results are consistent with estimates from an automated TROPOMI methane plume detection and quantification method. Estimates based on GHGSat observations from individual sources within the plant sum to about 6 t/h on average. For comparison, Sasol, the operator of the Secunda CTL facility, reported methane emissions of 11.5 t/h for the period July 2023-June 2024, a value that falls between the TROPOMI- and GHGSat-based estimates. Our results highlight the value of satellite observations as a useful audit complementing reported emissions and demonstrate the importance of combining coarse- and fine-resolution data to monitor methane emissions at plant and intra-facility level in complex sources. This research is part of the activities carried out at the Finnish Meteorological Institute for the development of the Emission Observatory platform (https://www.emissionobservatory.org) and within the METSA (METhane emissions from South Africa's Secunda synthetic fuel plant) ESA Third Party Mission project. Most of the results of the project are included in the journal article by Virta et al. (Environ. Sci. Technol. Lett., 2026).