Diurnal air sampling campaigns at the Royal Holloway site on the western fringe of London, United Kingdom, have been used to: (1) test the validity of using carbon isotopes to identify local methane sources, (2) determine the isotopic signature of overnight build-up profiles, in order to estimate regional emissions, and (3) verify statistical estimates of emissions. For CH4 an overall London δ13C source mix of −48.7±0.3‰ has been calculated from gradual overnight methane buildup in air masses moving from the east. Isotopic characterization of specific methane peaks shows them to be derived either from natural gas leaks (δ13C −33‰ to −35‰) or waste treatment emissions (δ13C −51‰ to −53‰). While landfill/waste emissions dominate, gas distribution losses represent ∼20% of the bulk local source. Various estimates of total London methane emissions for 1996 were made, using diurnal excess, isotopic data and trajectory movement across London to the sampling station. The results are in the range 240–312 kt/yr, higher than the London estimate in current U.K. greenhouse gas inventory assessments of emissions but within error of earlier statistical estimates for 1996. The results show that it is possible to use atmospheric concentration, isotopic, and meteorological data together to verify statistical estimates testing them for internal consistency and using better constrained data to calibrate more poorly known source fluxes. Importantly, atmospheric data can place constraints on poorly constrained landfill emission estimates for the region. These quasi-independent methods for verification of greenhouse gas emissions will contribute in assessing compliance with the Kyoto agreement.