London methane emissions: Use of diurnal changes in concentration and delta C-13 to identify urban sources and verify inventories

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 delta C-13 source mix of -48.7 +/-0.3 parts per thousand 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 (delta C-13-33 parts per thousand to -35 parts per thousand) or waste treatment emissions (delta C-13 -51 parts per thousand to -53 parts per thousand). While landfill/waste emissions dominate, gas distribution losses represent similar to 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 tendon 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.