Inter-city comparison of methane emissions measured using tall-tower eddy covariance and inventory analysis

Urban areas are important sources of methane (CH4) emissions, which need to be rapidly reduced to meet climate goals in the Paris Agreement and the Global Methane Pledge. To do this, CH4 emissions need to be attributed to sources and characterized over space and time. Here, we compare diurnal and seasonal variations in CH4 emissions measured using spatially-integrated tall-tower eddy covariance (EC) in three European cities as a part of the “ICOS Cities” project in Zurich (Switzerland), Paris (France), and Munich (Germany). In comparison to the inventories, our EC-based measurements consistently indicate higher CH4 emissions (relative to sum of CO2 and CH4 emissions). The tower footprints are unlikely to capture agricultural emissions, meaning that the gap is likely driven by Fugitives. We find substantial inter-city and temporal variations in CH4 emissions in the three tower footprints. When we compare the percentage of annual CH4 emissions (in relation to total CO2e emissions) to those reported in city-wide inventories, CH4 emissions in the tower footprint are higher by more than 50% in all three cities. Given that our EC measurements also include biogenic CO2 fluxes that are not fully included in inventories, this generally hints at possible unaccounted or underaccounted for CH4 sources in the inventories. Tall-tower EC data may help us understand potential drivers of urban CH4 emissions and may be used to design further measurement campaigns and develop strategies for reducing them.