Conditions for Valid Offshore Methane Quantification

Methane emission estimates from offshore facilities are increasingly used for regulatory reporting and climate assessment, yet it remains unclear under what atmospheric conditions such estimates are physically meaningful. This study defines three necessary conditions for valid offshore methane quantification: plume detectability, adequate sampling (interception), and reliable inference. A simplified Monte Carlo modelling framework was used to examine how these conditions are affected by atmospheric regime. Results suggest that the ability to obtain a physically meaningful emission estimate is strongly regime dependent. Under well-mixed conditions, successful quantification is achieved in most simulations, with uncertainty dominated by limitations in the inversion method. Under shallow marine boundary layers, quantification becomes increasingly conditional, with success probabilities reduced to approximately 15-20% depending on sampling configuration. Under strongly stratified conditions, plume observability is limited and valid emission estimates are not obtained within the illustrative model framework. To place these findings in context, ERA5 reanalysis data were used to assess atmospheric regime occurrence at representative offshore locations. Well-mixed and neutral conditions occur approximately 70% of the time in the North Sea, whereas the Gulf of Mexico is dominated by shallow boundary layer conditions (~90%), with stratified conditions occurring more frequently (~5%). These results suggest that offshore methane quantification is not a universally achievable measurement capability, but a regime-dependent and probabilistic outcome controlled by atmospheric structure. Atmospheric conditions therefore determine when physically meaningful emission estimates can be obtained and when measurement results should be interpreted with caution.