Methane mapping and emission quantification and attribution in two European cities; Utrecht, NL and Hamburg, DE

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Methane escape from natural gas distribution systems results not only in global warming, but also in economic loss. An estimate from the Boston region implies a financial damage of $90 million as about 15 billion cubic feet of natural gas escape from the distribution system annually (McKain et al., 2015). Mobile real-time measurements of methane in ambient air offer a fast and effective method to identify and quantify local methane emissions in urban areas.

Between February and April, 2018 we carried out extensive campaigns to measure methane concentrations at the street level in Utrecht, Netherlands, using a G2301 Picarro cavity ring-down spectroscopy (CRDS) analyser mounted on a small van. In October and November 2018, we mapped CH¬4 concentrations in the city of Hamburg, Germany, using two CRDS instruments (Picarro G2301 and G4302). The G2301 instrument measures Methane (CH4), Carbon Dioxide (CO2) and Water Vapour (H2O) mole fractions while the G4302 measures atmospheric CH4, Ethane (C2H6) and H2O mole fractions. Basic meteorological data (wind speed, 2D wind direction and temperature) were also collected on the vehicle. The campaign logistics were based on stable weather conditions, avoiding traffic rush hours, and passing each street at least two times.

Methane elevations were categorised using the empirical equation from Von Fischer et al. (2017) to translate the concentration elevations to emissions. The online measurements of ethane allow to quickly discriminate thermogenic from biogenic methane enhancements. In addition, air samples were collected in large methane plumes to allow isotopic characterisation with high-precision isotope ratio mass spectrometry in order to distinguish different source categories.

The results highlight methane elevations that are related to losses from the gas distribution system, industrial activates, domestic waste-water treatment plant, agriculture and incomplete combustion of vehicle engines. The measurements in Hamburg allow investigation of upstream, mid-stream and down-stream fugitive emissions from the oil and gas industry.