Atmospheric Regimes Create Structural Uncertainty in Offshore Methane Emission Estimates

Offshore methane emissions are increasingly quantified using a range of observational approaches, yet reported estimates often show substantial variability between methods. This study examines whether disagreement can arise from atmospheric transport processes alone by investigating how the marine boundary layer influences the relationship between measured concentrations and inferred emission rates. An idealised dispersion framework was used to compare three commonly applied approaches, Gaussian plume inversion, aircraft mass balance, and satellite-based methods, across representative offshore atmospheric regimes. By holding emissions constant and varying only atmospheric structure, the framework isolates the influence of transport processes on inferred emissions. The results show that each method exhibits regime-dependent bias arising from different physical mechanisms. Gaussian methods are highly sensitive to plume alignment and may fail under lateral displacement. Aircraft mass balance remains robust when the plume is fully sampled but becomes unreliable when the sampling volume does not intercept the plume. Satellite methods consistently detect the plume but exhibit systematic bias when plume transport is decoupled from assumed wind speeds. These results demonstrate that disagreement between offshore methane emission estimates can arise solely from changes in atmospheric regime under the conditions represented by the model. Under conditions where standard transport assumptions are not satisfied, differences between estimates reflect the influence of atmospheric structure on plume behaviour and sampling, rather than measurement error alone. Emission estimates are therefore only reliable where the underlying transport assumptions are satisfied, and convergence between methods cannot be universally assumed under realistic offshore conditions.