The importance of utilizing multiple methane source tracers when measuring European cities.

Abstract:

Recent research, dominated by studies in the United States, has shown that CH₄ emissions from urban areas depend on the design, age, and maintenance record of the natural gas distribution networks (NGDN). Compared to U.S cities, European cities are typically older, denser, utilize different hydrocarbon reservoirs for heating and transport, and have a wide range of cultural and behavioral styles, all of which affect emission patterns. Municipal policies are dictated by local social and economic priorities, with widely variable maintenance practices for the underlying infrastructure such as gas and sewage pipelines. These practices impact the extent of CH₄ emissions and the degree of mixing of source types.

Here, we compare δ¹³C, δ²H, and C₂:C₁ source signatures between an Eastern European city (Bucharest, Romania) and a Western European city (London, U.K). Stable isotopes (δ¹³C-CH₄) and ethane:methane (C₂:C₁) ratios can be used to verify whether emissions are associated with leaks from the NGDN. During extensive mobile surveys conducted between 2018 and 2019, to evaluate CH₄ emissions, air samples were collected and analyzed using continuous flow gas chromatograph isotope ratio mass spectrometers for δ¹³C at Royal Holloway, University of London and δ²H at Utrecht University, The Netherlands. An LGR Ultraportable CH₄/C₂H₆ Analyzer and a Picarro G4302 were used for C₂:C₁ ratios.

The NGDN in Bucharest had a δ¹³C of -49‰, δ²H of -175‰, and 0.02 for C₂:C₁; and London had a δ¹³C of -40‰, δ²H of -130‰, and 0.02 to 0.06 for C₂:C₁. Bucharest δ¹³C signatures for CH₄ enhancements ranged from -64 to -36‰ and showed a normal population distribution, reflecting a dominance of mixed sources. However, δ²H ranged from -388 to -157‰ and has a bimodal distribution, indicating a clear contribution of 2 source types. For London, δ¹³C sources ranged from -60 to -35‰ and are positively skewed, highlighting the dominance of gas leak sources over the few waste sources. For Bucharest, the δ²H signature is a more significant tracer since δ¹³C of gas is isotopically light, and because gas & sewage source signatures overlap. Measurements of N₂O, NH₃, & H₂S, can also be used for separation of sewage emissions. As Europe seeks to cut urban emissions, multiple source tracers will help identify causes of emissions from street level sources.