Publication Date:
2017-06-18
Description:
Airborne measurements of a methane (CH 4 ) plume over the North Sea from August 2013 are analyzed. The plume was only observed downwind of circumnavigated gas fields, and three methods are used to determine its source. First, a mass balance calculation assuming a gas field source gives a CH 4 emission rate between 2.5±0.8x10 4 and 4.6±1.5x10 4 kg h −1 . This would be greater than the industry target of a 0.5% leak rate if it were emitting for more than half the time. Second, annual average UK CH 4 emissions are combined with an atmospheric dispersion model to create pseudo-observations. Clean air from the North Atlantic passed over mainland UK, picking up anthropogenic emissions. To best explain the observed plume using pseudo-observations, an additional North Sea source from the gas rigs area is added. Third, the δ 13 C-CH 4 from the plume is shown to be -53%0, which is lighter than fossil gas but heavier than the UK average emission. We conclude that either an additional small-area mainland source is needed, combined with temporal variability in emission or transport in small-scale meteorological features. Alternatively, a combination of additional sources that are at least 75% from the mainland (-58%0) and up to 25% from the North Sea gas rigs area (-32%0) would explain the measurements. Had the isotopic analysis not been performed, the likely conclusion would have been of a gas field source of CH 4 . This demonstrates the limitation of analysing mole fractions alone, as the simplest explanation is rejected based on analysis of isotopic data.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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