Matthieu Dogniaux, Joannes D. Maasakkers, Daniel J. Varon , Ilse Aben 

In late September 2022, explosions of the Nord Stream pipelines caused what could be the largest anthropogenic methane leak ever recorded. We report on Landsat 8 (L8) and Sentinel-2B (S-2B) observations of the sea foam patch produced by the Nord Stream 2 (NS2) leak located close to Bornholm Island, acquired on September 29 and 30, respectively. Usually, reflected sunlight over sea is insufficient for these Earth-imagers to observe any methane signal in nadir-vewing geometry. However, the NS2 foam patch observed here is bright enough to possibly allow the detection of methane above it. We apply the Multi-Band Single-Pass (MBSP) method to infer methane enhancement above the NS2 foam patch and then use the Integrated Mass Enhancement (IME) method in an ensemble approach to estimate methane leak rates and their uncertainties. This very specific NS2 observation case challenges some of MBSP and IME implicit hypotheses, and thus calls for customized calibrations: (1) for MBSP, we perform an empirical calibration of sea foam albedo spectral dependence by using sea foam observations in ship trails, and (2) for IME, we yield a tailored effective wind speed calibration that accounts for a partial plume observation, as methane enhancement may only be seen above the NS2 sea foam patch. Due to large uncertainties, no firm conclusion can be drawn from the single overpasses of L8 and S-2B. However, if we opportunistically assume that the L8 and S-2B methane leak rates are independent, we obtain a positive leak detection with a weak confidence, showing an averaged dual-overpass (L8 and S-2B combined) NS2 methane leak rate of 415 +/- 321 t/hr. Overall, our work illustrates how implicit method hypotheses need to be considered and compensated for in unusual observation cases such as this one.