Abstract
A thorough understanding of methane sources is necessary to accomplish methane reduction targets. Urban environments, where a large variety of methane sources coexist, are one of the most complex areas to investigate. Methane sources are characterised by specific δ13C-CH4 signatures, so high precision stable isotope analysis of atmospheric methane can be used to give a better understanding of urban sources and their partition in a source mix. Diurnal measurements of methane and carbon dioxide mole fraction, and isotopic values at King’s College London, enabled assessment of the isotopic signal of the source mix in central London. Surveys with a mobile measurement system in the London region were also carried out for detection of methane plumes at near ground level, in order to evaluate the spatial allocation of sources suggested by the inventories. The measured isotopic signal in central London (−45.7 ±0.5‰) was more than 2‰ higher than the isotopic value calculated using emission inventories and updated δ13C-CH4 signatures. Besides, during the mobile surveys, many gas leaks were identified that are not included in the inventories. This suggests that a revision of the source distribution given by the emission inventories is needed.
Highlights
Accurate quantification of methane emissions both at global and regional scale and full understanding of methane sources are still unresolved issues
Tracer methods based on Fourier Transform Infrared (FTIR) absorption spectroscopy[8], flux chambers[9] and mobile ground surveys[10] are all used for the estimation of total emissions from a specific source area, but they do not discriminate between different methane production processes
Comparison between the weighted δ13C isotopic value calculated with emission inventories, using the typical isotopic signatures assigned to methane source categories, and the isotopic signal measured in central London by with the order of sampling. (c) Wind direction, wind speed and CH4 mole fractions
Summary
Accurate quantification of methane emissions both at global and regional scale and full understanding of methane sources are still unresolved issues. While many specific approaches have been developed for the estimate of total methane fluxes (e.g. refs 3–7), there is no established methodology that leads to a comprehensive partitioning of emissions by sources. Tracer methods based on Fourier Transform Infrared (FTIR) absorption spectroscopy[8], flux chambers[9] and mobile ground surveys[10] are all used for the estimation of total emissions from a specific source area (e.g. landfill sites), but they do not discriminate between different methane production processes. Emissions from source areas such as landfill sites and sewage works can be widely distributed, and their spatial allocation highly uncertain. UK Methane Sources Enteric fermentation (cows) Waste disposal and landfills Gas transmission and distribution Manure management Wastewater handling Coal mining Combustion (industrial and domestic) Road transport Biomass burning UK Total. UNECE Sector Agriculture Waste treatment and disposal Offshore Agriculture Waste treatment and disposal Offshore Combustion in energy production and transfer Road Transport Agricultural burning and wildfires
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