Limitations and challenges of using satellite remote sensing to estimate diffuse methane emissions at a national level in Denmark

Methane (CH₄) is the second-largest greenhouse gas contributing to climate change, and it is produced by both anthropogenic and biogenic sources. The TROPOspheric Monitoring Instrument (TROPOMI) on board ESA’s Sentinel 5 Precursor (S5-P) satellite provides daily total column-averaged methane mixing ratio values at high spatial resolution, allowing the monitoring and flux estimation of diverse methane sources.

In Denmark, CH₄ emissions are mainly related to the agricultural and waste sectors, mostly attributed to diffuse sources and point sources with low emission rates (below 100 kg/h). Emission estimates compiled in the national GHG emissions inventory are mostly based on emission factors derived from models, with few empirical measurements. The lack of measurements and spatial information of methane sources introduces uncertainty when projecting the inventory emission estimates into a spatial grid. Gridded emission estimates from emissions databases like EDGAR disagree with the inventory on both their spatial distribution and magnitude, raising the question as to how to correctly account for diffuse emissions and which sources to trust.
Understanding the distribution of diffuse anthropogenic methane fluxes and their quantification is crucial for nations to plan mitigation strategies and have an empirical knowledge of their inventories.

In this study, we use TROPOMI data to detect hot spots of diffuse methane sources and estimate fluxes attributed to different sectors and source types over Denmark. Focus is set on analyzing the limitations and challenges of pursuing these tasks, including flux detection thresholds, data availability, background estimation, and methods for flux estimation. A multi-year period ranging from 2019 to 2024 is chosen to both assess seasonal variability and enhance flux estimation through temporal averaging.