Nordstream pipelines CH4 leak estimates and transport uncertainty using ICOS data and the FLEXPART Lagrangian particle dispersion model

Following the sabotage of the Nord Stream 1 and 2 subsea pipelines on 26 September 2022, natural gas leaks resulted in unprecedented emissions of methane that were detected by several ICOS stations. As the plume traveled North, the detections occurred mainly in Scandinavia. NILU’s initial modeling activities provided a preliminary estimate of 155 KtonCH₄ for the leaks that was made public as a press release. A recent collaborative effortorganized by the United Nations Environment Programme’s International Methane Emissions Observatory (UNEP’s IMEO) provided new model-based pipeline rupture outflow rates. In combination with updated ICOS CH₄ time series we updated the estimated release values produced. We discuss the uncertainties associated with the atmospheric modelling for this updated analysis with emphasis on the Lagrangian transport aspects of the problem and the associated uncertainties.