HR: 09:15h
AN: B41G-06 INVITED    [Abstracts]
TI: Diurnal Studies of Methane in the London Region: the use of Mixing Ratio and Automated Stable Isotope Measurements From a Fixed Site to Evaluate Sources
AU: * Fisher, R
EM: r.fisher@gl.rhul.ac.uk
AF: Royal Holloway, University of London, Egham Hill, Egham, TW20 0EX, United Kingdom
AU: Lowry, D
EM: d.lowry@gl.rhul.ac.uk
AF: Royal Holloway, University of London, Egham Hill, Egham, TW20 0EX, United Kingdom
AU: Sriskantharajah, S
EM: srimathy@gl.rhul.ac.uk
AF: Royal Holloway, University of London, Egham Hill, Egham, TW20 0EX, United Kingdom
AU: Nisbet, E G
EM: e.nisbet@gl.rhul.ac.uk
AF: Royal Holloway, University of London, Egham Hill, Egham, TW20 0EX, United Kingdom
AB: Over the last decade methane mixing ratios in the London area have decreased, with a reduction in mean annual methane mixing ratio of 4% between 1996 (2056 ppb) and 2006 (1974 ppb). Stable isotope studies can be used to consider which sources of the gas are changing. A continuous-flow gas chromatography isotope ratio mass spectrometry system has been developed to carry out high precision isotopic analysis of methane at ambient atmospheric mixing ratios. The repeatability (1σ) obtainable with this system is ± 0.05‰ for δ¹³C of CH₄. An automated inlet system, connected to an outside air intake has been set up for diurnal studies of methane isotopes at Royal Holloway, 32 km WSW of the centre of London. Studies of known local sources of methane have also been carried out to identify source signatures of emissions. The mean methane δ¹³C source signature in air from the London sector for diurnal studies carried out at Royal Holloway in 2005 was 50.1 ± 0.7‰. This indicated a larger proportion of methane from biogenic sources, such as landfill sites, than was recognised in the London emissions inventories. The isotope studies carried out using the new system can be compared with large volume samples collected at the same site in the mid to late 1990s for which methane δ¹³C was analysed using a conventional off-line extraction line and dual inlet mass spectrometer. The methane source signature has decreased by 1.4 ± 1.0‰ over the last decade. Stable isotopic analysis provides a top-down approach of partitioning methane between its sources and is independent of statistical emissions inventories. The modified instrumentation has been shown to be suitable for measuring diurnal variations at urban sites and has great potential for measuring seasonal variations at background stations. The small sample volume, rapid analysis time and high precision enables a significant increase in the number of methane isotope measurements in air samples to be made, providing additional data for developing global models of isotopic change, an important step to understanding changing emissions.
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0365 Troposphere: composition and chemistry
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0452 Instruments and techniques
DE: 0454 Isotopic composition and chemistry (1041, 4870)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting