Preprints
https://doi.org/10.5194/acp-2021-606
https://doi.org/10.5194/acp-2021-606

  28 Sep 2021

28 Sep 2021

Review status: this preprint is currently under review for the journal ACP.

Continuous CH4 and δ13CH4 measurements in London demonstrate under-reported natural gas leakage

Eric Saboya1,2, Giulia Zazzeri1, Heather Graven1,2, Alistair J. Manning3, and Sylvia Englund Michel4 Eric Saboya et al.
  • 1Department of Physics, Imperial College London, London, UK
  • 2Grantham Institute – Climate Change and the Environment. Imperial College London, London, UK
  • 3UK Met Office, Exeter, EX1 3PB, UK
  • 4Institute of Artic and Alpine Research, University of Colorado, Boulder, CO, USA

Abstract. Assessment of bottom-up greenhouse gas emissions estimates through independent methods is needed to demonstrate whether reported values are accurate or if bottom-up methodologies need to be refined. We report atmospheric methane (CH4) mole fractions and δ13CH4 measurements from Imperial College London since early 2018 using a Picarro G2201-i analyser. Measurements from March 2018 to October 2020 were compared to simulations of CH4 mole fractions and δ13CH4 produced using the NAME dispersion model coupled with the UK National Atmospheric Emissions Inventory, UK NAEI, and the global inventory, EDGAR, with model spatial resolutions of ~2 km, ~10 km, and ~25 km. Observed mole fractions were underestimated by 30–35 % in the NAEI simulations. In contrast, a good correspondence between observations and EDGAR simulations was seen. There was no correlation between the measured and simulated δ13CH4 values for either NAEI or EDGAR, however, suggesting the inventories’ sectoral attributions are incorrect. On average, natural gas sources accounted for 20–28 % of the above background CH4 in the NAEI simulations, and only 6–9 % in the EDGAR simulations. In contrast, nearly 84 % of isotopic source values calculated by Keeling plot analysis (using measurement data from the afternoon) of individual pollution events were higher than −45 ‰, suggesting the primary CH4 sources in London are actually natural gas leaks. The simulation-observation comparison of CH4 mole fractions suggests that total emissions in London are much higher than the NAEI estimate (0.04 Tg CH4 yr−1) but close to, or slightly lower than the EDGAR estimate (0.10 Tg CH4 yr−1). However, the simulation-observation comparison of δ13CH4 and the Keeling plot results indicate that emissions due to natural gas leaks in London are being underestimated in both the UK NAEI and EDGAR.

Eric Saboya et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-606', Anonymous Referee #2, 08 Oct 2021
  • RC2: 'Comment on acp-2021-606', Anonymous Referee #3, 13 Oct 2021
  • RC3: 'Comment on acp-2021-606', Anonymous Referee #1, 22 Oct 2021

Eric Saboya et al.

Eric Saboya et al.

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Short summary
Is London under-reporting its climate change inducing fossil-fuel methane emissions? Methane emissions arise from a range of different sources that are often collocated in megacities like London. Measurements of the atmospheric concentration and isotopic composition of methane in central London between 2018–2020 suggest methane emissions from natural gas pipe leaks in London are too low in emission inventory estimates.
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