A bottom-up emission estimate for the 2022 Nord Stream gas leak: derivation, simulations, and evaluation

Abstract

A major release of methane from the Nord Stream pipelines occurred in the Baltic Sea on 26 September 2022. Elevated levels of methane were recorded at many observational sites in northern Europe. While it is relatively straightforward to estimate the total emitted amount from the incidents (around 330 kt of methane), the detailed vertical and temporal distributions of the releases are needed for numerical simulations of the incident. Based on information from public media and basic physical concepts, we reconstructed vertical profiles and temporal evolution of the methane releases from the broken pipes and simulated subsequent transport of the released methane in the atmosphere. The parameterization for the initial rise of the buoyant methane plume has been validated with a set of large-eddy simulations by means of the UCLALES model. The estimated emission source was used to simulate the dispersion of the gas plume with the SILAM chemistry transport model. The simulated fields of the excess methane led to a noticeable increase in concentrations at several carbon-monitoring stations in the Baltic Sea region. Comparison of the simulated and observed time series indicated an agreement within a couple of hours between the timing of the plume arrival/departure at the stations with observed methane peaks. Comparison of absolute levels was quite uncertain. At most of the stations the magnitude of the observed and modeled peaks was comparable with the natural variability of methane concentrations. The magnitude of peaks at a few stations close to the release was well above natural variability; however, the magnitude of the peaks was very sensitive to minor uncertainties in the emission vertical profile and in the meteorology used to drive SILAM. The obtained emission inventory and the simulation results can be used for further analysis of the incident and its climate impact. They can also be used as a test case for atmospheric dispersion models.