Quantification of Urban Methane Emissions: A Combination of Stationary with Mobile Measurements

Abstract

The mixing ratios of methane (CH4) were recorded with high temporal and spatial resolution in Münster, Germany, to identify urban CH4 sources and to quantify the overall inner city CH4 emissions. Both mobile and stationary measurement techniques were employed. The background mixing ratios showed a diurnal cycle with higher values at night under stable stratification conditions. In the industrial park, periodic peaks were detected. Ten mappings of the urban CH4 mixing ratio were made with an instrumented cargo bicycle. Repeated local increases in mixing ratios were found at 13 individual locations. The emission rate was estimated to be 22.0 g h−1 km−1. A total of five leaks from the underground gas distribution network were identified. From the increase in background mixing ratio, the overall source strength of the study area was estimated to be 24.6 g m−2 a−1, which is approximately three times the total CH4 emissions from the city’s most recent emissions report. The contribution of point sources was 0.64 g m−2 a−1, suggesting that significant additional CH4 sources exist within the study area. In the interest of climate protection, there is an urgent need for further research on the urban CH4 sources and emission fluxes in detail.