Abstract
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 enhancements of more than 10% above background levels) in Hamburg and 81 LIs in Utrecht. 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±70tyr-1 from all sources in Hamburg and 150±50tyr-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 be detected and/or confirmed as pipeline leaks. This issue requires further investigation.
| Original language | English |
|---|---|
| Article number | acp-20-14717-2020 |
| Pages (from-to) | 14717-14740 |
| Number of pages | 24 |
| Journal | Atmospheric Chemistry and Physics |
| Volume | 20 |
| Issue number | 23 |
| DOIs | |
| Publication status | Published - 7 Dec 2020 |
Funding
Acknowledgements. This work was supported by the Climate and Clean Air Coalition (CCAC) Oil and Gas Methane Science Studies (MMS) hosted by the United Nations Environment Programme. Funding was provided by the Environmental Defense Fund, the Oil and Gas Climate Initiative, the European Commission, and CCAC. This project received further support from the H2020 Marie Skłodowska-Curie project Methane goes Mobile – Measurements and Modelling (MEMO2; https://h2020-memo2.eu/, last access: 30 November 2020), grant number 722479. Daniel Zavala-Araiza and Stefan Schwietzke were funded by the Robertson Foundation. We thank Rebecca Fisher, who supervised the RHUL contribution to the isotopic analysis for the Hamburg campaign. Special thanks to Stefan Bühler from the Meteorological Institute of Hamburg University and Stefan Kinne from the Max Planck Institute for Meteo- rology for hosting our team during the Hamburg city measurement surveys. We would like to extend our appreciation to the anonymous referees for the insightful comments, which led to improvements of the paper. We appreciate continuous efforts from the executive and management board of GasNetz Hamburg along with Luise Westphal, Michael Dammann, Ralf Luy, and Christian Feick-ert, who facilitated productive communications, provided information on the gas infrastructure in Hamburg, and organized leak repairs with their teams in the study area in Hamburg. We also thank the asset manager of STEDIN Utrecht, Ricardo Verhoeve, who provided information and planned leak repairs by STEDIN in Utrecht. We thank Charlotte Große from DBI Gas and Environmental Technologies GmbH Leipzig (DBI GUT Leipzig), who helped with clarifying information on reported emission factors provided in national inventory reports. We thank the former MSc students of Utrecht University, Laurens Stoop and Tim van den Akker, who helped with the measurements in the Utrecht study area. Financial support. This research has been supported by the Environmental Defense Fund, the Oil and Gas Climate Initiative, the European Commission, CCAC (grant no. 1043-000011-11070-100-00), and the H2020 Marie Skłodowska-Curie project Methane goes Mobile – Measurements and Modelling (grant no. 722479).