Methane Emissions in the London Region: Deciphering Regional Sources with Mobile Measurements

Abstract:

Methane stable isotope analysis, coupled with mole fraction measurement, has been used to link isotopic signature to methane emissions from the leading methane sources in the London region, such as landfills and gas leaks. A mobile Picarro G2301 CRDS analyser was installed in a vehicle, together with an anemometer and a Hemisphere GPS receiver, to measure atmospheric methane mole fractions and their relative location. When methane plumes were located and intercepted, air samples were collected in Tedlar bags, for δ¹³C-CH₄ isotopic analysis by CF-GC-IRMS (Continous Flow-Gas Chromatography-Isotopic Ratio Mass Spectroscopy). This method provides high precision isotopic values, determining δ¹³C-CH₄ to ±0.05 per mil.

The bulk signature of the methane plume into the atmosphere from the whole source area was obtained by Keeling plot analysis, and a δ¹³C-CH₄ signature, with the relative uncertainty, allocated to each methane source investigated. The averaged δ¹³C-CH₄ signature for landfill sites around the London region is – 58 ± 3 ‰, whereas the δ¹³C-CH₄ signature for gas leaks is fairly constant at -36 ± 2 ‰, a value characteristic of North Sea supply.

The Picarro G2301 analyser was installed also on the roof of King’s College London, located in the centre of the city, and connected to an air inlet located 7 meters above roof height. An auto-sampler was connected to the same air inlet and launched remotely when a high nocturnal build up was expected, allowing up to twenty air bags to be collected for methane isotopic analysis over a 24 hour period. The main source contributing to overnight methane build up in central London is fugitive gas, in agreement with inventories.

From the isotopic characterisation of urban methane sources and the source mix in London, the contribution to the urban methane budget and the local distribution of the methane sources given in inventories can be validated.