Longterm urban eddy covariance observations of methane and other trace gases reveal characteristic anthropogenic emission hotspots

Eddy covariance observations are particularly well suited to study emission processes at the ecosystem scale. Here we combine longterm observations of methane, carbon dioxide and nitrogen oxides, with campaign-based observations of NMVOC fluxes in an urban area. The complex dataset allows unravelling the fate of urban methane emissions for the city of Innsbruck. Our analysis shows that most of the methane in the urban area is emitted via pre-flush operation and partially burned methane from poorly maintained gas furnaces. Methane fluxes show a negative temperature dependence and are highly correlated with ethane fluxes. An average ethane to methane flux ratio of 5% is observed, consistent with the gas composition supplied to Western Austria/Southern Germany. The 20y GWP of the emitted methane in the residential, commercial and public sector can be as high as 20-30% relative to CO₂. This study shows that the conversion of gas furnaces to heat pumps can have an additional immediate benefit through the reduction unburned methane.