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

Diurnal air sampling campaigns at the Royal Holloway site on the western fr inge 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 re gional emissions, and (3) verify statistical estimates of emissions. For CH 4 an overall London delta C-13 source mix of -48.7 +/-0.3 parts per thousan d has been calculated from gradual overnight methane buildup in air masses moving from the east. Isotopic characterization of specific methane peaks s hows 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/was te 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 move ment across London to the sampling station. The results are in the range 24 0-312 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 atmospheri c concentration, isotopic, and meteorological data together to verify stati stical estimates testing them for internal consistency and using better con strained data to calibrate more poorly known source fluxes. Importantly, at mospheric data can place constraints on poorly constrained landfill emissio n estimates for the region. These quasi-independent methods for verificatio n of greenhouse gas emissions will contribute in assessing compliance with the Kyoto agreement.