Constraining urban methane sources based on EC measurements

Methane (CH4) is an important greenhouse gases in the atmosphere. The methane emission budget has a sizeable anthropogenic contribution, which makes methane the most important non-CO2 greenhouse gas released from anthropogenic activities. Reduction of CH4 emissions can provide quick and cost effective cuts in global warming, but requires a sound understanding of the different anthropogenic sources. Although the total global CH4 budget is relatively well understood, the contributions of individual sources to CH4 emissions are poorly constrained. Recent data from London for example suggest significantly higher emissions. In this project (CH4IAO) we propose to study urban methane emissions based on direct eddy covariance measurements. Additionally, chemical tracers will be measured allowing to obtain chemical fingerprints of urban CH4 sources. These measurements will provide a top down assessment of urban methane fluxes in the urban core of Innsbruck, and will help to improve our understanding of urban methane emissions.

Methane is considered the second most important greenhouse gas in the atmosphere after carbon dioxide, with a radiative forcing of approx. 0.5 W/m2. Methane is emitted from natural and anthropogenic/agricultural sources, whereby the increase in concentration in the atmosphere is primarily attributed to the latter. However, as temperatures warm up in the future, an increase in natural sources can also be expected. Due to the strong radiative forcing, the reduction of methane emissions is being discussed as a potentially important measure to curb global warming. This requires quantitatively reliable data on emission levels from various anthropogenic sources. As part of the project "Constraining urban methane sources based on EC measurements", continuous methane emission flux measurements along with other trace gases were carried out in Innsbruck over a period of 3 years. Annual average emission values are around 20-30 nmol/m2/s. It was found that methane primarily escapes via 'methane slip' through the operation of gas boilers and furnaces. Emissions from leaks are found to be negligible. The project was able to show that this source of methane is significantly underestimated in assessments and can provide an additional contribution to the reduction of greenhouse gases (e.g. up to 30% for a period of 20 years) when switching to heat pumps. The existence of urban super emitters has also been shown. Results from the FWF project provide an initial indication of how methane emissions can be avoided in urban areas in Austria.