Quantifying emission dynamics of coal mine ventilation shafts with a stationary hyperspectral camera

The global warming potential of methane on a 20-year scale is 80 times larger than that of carbon dioxide. Therefore reducing anthropogenic methane emissions can mitigate greenhouse gas-induced atmospheric warming in the short term. Thus, source attribution and budgeting of methane emissions have received particular attention in recent years. Coal mining activities were found to be accountable for approximately one-tenth of anthropogenic methane emissions. Observations of point sources (like coal mine ventilation shafts) by plume imagery from aircraft of satellites are emerging as a powerful and reliable tool for emission estimates. Yet, while these measurements cover large areas in a short time, the instrument revisiting rates do not allow observation of temporal variability of sources.
We present the results of a case study on source dynamics of coal mine ventilation shafts conducted in the Upper Silesian Coal Basin (USCB), Poland, in June 2022. We deployed a HySpex SWIR-384 hyperspectral camera at 1 km distance to a coal mine ventilation shaft. The camera repeatedly observed blue-sky scattered sunlight above the shaft in the shortwave infrared spectral range, taking approximately 1 minute per image. We detect methane plumes reliably using an adapted matched filter algorithm in the 2.3 μm absorption band. Co-located wind-lidar measurements allow us to estimate source emissions rates by the integrated mass enhancement (IME) method. Thereby, we produce several hundred emission estimates per day based on plume imagery, with an average uncertainty below 300 kg/h for minutely estimates under favourable measurement conditions. Our case study covers four consecutive days and reveals substantial source dynamics on all observed time scales from minutes to days. A 10-minute running average of the emissions can be a factor of 2 smaller or larger than the daily mean and daily averaged emissions ranged from 1.39 tCH₄/h to 4.44 tCH₄/h.