Fugitive Methane Across the UK Gas Distribution Network from Terminals to Cities: Characterisation and Methodology Development

Fugitive emissions from gas distribution are a top target for reduction of CH4 emissions to atmosphere with the UN considering that emissions from fossil fuel activities can be reduced by 61% (UN, 2021). Once the emissions are identified there are mitigation solutions to stop the leaks far more easily than emissions from waste and agricultural sectors. The RHUL group has identified fugitive methane from UK sources using a mobile survey vehicle since 2013, initially to identify plumes and characterise the emissions by source category using isotopic signatures (δ13C) and ethane to methane ratios (C2:C1). More recent measurements have focussed on CH4 emissions from buried city gas pipelines, primarily London, and development of methodology for interpreting data from a range of different high-precision instruments. Much of the gas in the UK distribution system has very similar charactersistics once mixing downstream of terminals has taken place. This is typically characterised by δ13C signature of -39 ± 1 ‰ and C2:C1 of 0.055 ± 0.015, which make it distinct from agricultural, waste and combustion CH4 sources. The small proportion of gas coming from the Southern North Sea and Morecambe Bay fields (now <20%) is more enriched in 13C (-34 to -28 ‰) and terminals receiving gas from these locations have different emission signatures; that for the Bacton terminal can be traced downstream toward London. City measurements by Picarro 2301 and LGR UMEA of London and Birmingham pipeline gas leaks in 2019 have been used to quantify emissions using methodology developed by Weller et al. (2019) and refined by Maazallahi et al. (2020). A total estimated emission for the Greater London area of 2.2 kT (Fernandez et al., in prep.), is much lower than the inventory suggests and lower than estimates and from aircraft or fixed site measurements. While fugitive gas emissions from rural areas (pipelines and above-ground infrastructure) are much larger than the inventory suggest, lowering expected urban emissions, and small peaks of <200 ppb cannot be definitively characterised as gas leaks, leading to underestimation, the methodology for leak emissions estimation needs further refinement for dense urban environments. A range of instruments measuring at 0.3 to 10Hz and different emissions methodologies are currently being assessed through repeat surveys of some London boroughs. Maazallahi et al., 2020, Atmos. Chem. Phys., 20, 14717–14740, https://doi.org/10.5194/acp-20-14717-2020United Nations Environment Programme, 2021, Emissions Gap Report 2021: The Heat Is On – A World of Climate Promises Not Yet Delivered, NairobiWeller et al., 2019, PLoS One 14, e0212287, https://doi.org/10.1371/journal.pone.0212287

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