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. Characterizing and attributing methane ( CH4 ) emissions across varying scales are important from environmental, safety, and economic perspectives and are essential for designing and evaluating effective mitigation strategies. Mobile real-time measurements of CH4  in ambient air offer a fast and effective method to identify and quantify local CH4  emissions in urban areas. We carried out extensive campaigns to measure CH4  mole fractions at the street level in Utrecht, the Netherlands (2018 and 2019), and Hamburg, Germany (2018). We detected 145 leak indications (LIs; i.e., CH4 enhancementsof more than 10 % above background levels) in Hamburg and 81 LIs inUtrecht. Measurements of the ethane-to-methane ratio ( C2:C1 ),methane-to-carbon dioxide ratio ( CH4:CO2 ), and CH4 isotope composition ( δ13C and  δD ) show that in Hamburg about  1∕3 of the LIs, and in Utrecht 2∕3  of the LIs (based on a limited set of C2:C1 measurements), were of fossil fuel origin. We find that in both cities the largest emission rates in the identified LI distribution are from fossil fuel sources. In Hamburg, the lower emission rates in the identified LI distribution are often associated with biogenic characteristics or (partly) combustion. Extrapolation of detected LI rates along the roads driven to the gas distribution pipes in the entire road network yields total emissions from sources that can be quantified in the street-level surveys of 440±70  t yr −1 from all sources in Hamburg and 150±50  t yr −1 for Utrecht. In Hamburg, C2:C1 , CH4:CO2 , and isotope-based source attributions show that 50 %–80 % of all emissions originate from the natural gas distribution network; in Utrecht more limited attribution indicates that 70 %–90 % of the emissions are of fossil origin. Our results confirm previous observations that a few large LIs, creating a heavy tail, are responsible for a significant proportion of fossil CH4  emissions. In Utrecht, 1∕3  of total emissions originated from one LI and in Hamburg > 1 / 4  from two LIs. The largest leaks were located and fixed quickly by GasNetz Hamburg once the LIs were shared, but 80 % of the (smaller) LIs attributed to the fossil category could not bedetected and/or confirmed as pipeline leaks. This issue requires further investigation.

Methane mapping, emission quantification, and attribution in two European cities: Utrecht (NL) and Hamburg (DE)