Urban European Methane Emission Characteristics and Insight from CH⁴ Source Tracers: na

Mitigating greenhouse gas emissions is becoming more of a priority in order to slow and reduce positive anthropogenic feedback to climate warming. As a consequence, there has been an increasing interest in understanding urban methane (CH₄) emissions around the world. Past research has shown that urban CH₄ emissions are influenced by a city's natural gas distribution network (NGDN) and pipeline replacement plans. National inventory estimates for NGDNs are heavily based on population density and may not correctly represent the spatial distribution of high emission locations. Mobile measurement studies can help improve and validate emissions from the energy sector and serve as a tool for effective mitigation planning. Therefore, this research focuses on locating and quantifying urban CH₄ emissions of Greater London and Bucharest Romania in comparison with other European cities surveyed around similar times. Also, highlighted are the applications and reliability of three commonly used source tracers, δ¹³C_CH4, δ²H_CH4, and C₂H₆:CH₄, with an additional focus on the development of a new δ²H_CH4 measurement system.
Presented here are measurements and quantifications of urban CH₄ emissions from a western European city (London, UK) and an eastern European city (Bucharest, Romania), which differ culturally, governmentally, and economically. Extensive mobile surveys were conducted from 2018 to 2019, measuring street-level CH₄ mole fractions, ethane (C₂H₆), and analyzing δ¹³C_CH4 and δ²H_CH4 for source type attributions.
A total of 969 leak indicators were confirmed in Bucharest, and 925 for London. Compared to other recently measured cities, both Bucharest (1832 tons CH₄ yr^-1) and London (~2220 tons CH₄ yr^-1) had greater city-wide emission rates than Paris, France, and Hamburg, Germany. Isotopes (δ¹³C_CH4 & δ²H_CH4) and ethane-methane ratios (C₂:C₁) were used extensively in an attempt to identify emission sources of both cities. In Bucharest, C₂:C₁ and δ²H_CH4 tracers showed the greatest emission dominance from wastewater (58%-63%), while less than half (32%– 42%) were fossil fuel, and 0-5% were pyrogenic. London source tracers indicated a dominance of fossil fuel δ¹³C_CH4 signatures and C₂:C₁ ratios (86% to 91%), while 7%-14% was biogenic, and only 0% - 2% were pyrogenic. This research will help to guide local governments with emission compliance and to prioritize greenhouse gas mitigation strategies for urban areas. This includes the identification of emissions that can most readily be reduced, helping to achieve the Global Methane Pledge of reducing human-induced emissions 45% by 2030.