Methane and global warming
Global warming is a pressing issue, largely driven by carbon dioxide (CO₂) emissions. However, methane (CH₄) also plays a significant role in global temperature increases. Because of its shorter lifetime in the atmosphere, CH₄ is an attractive target for greenhouse gas reduction efforts, especially in the short term. Methane is emitted from a variety of sources, both natural and human-caused. These range from wetlands, lakes, and rivers to human activities such as agriculture and fossil fuel use. We have a good understanding of qualitative methane emission sources. To understand their quantitative contribution to the overall CH₄ budget, including local and regional sources, we use approaches such field measurements and modeling, and combinations of them.
Urban methane emissions
In recent years, there has been a growing interest in methane emissions from urban areas. These emissions can be a mix of different sources. Particularly emissions from local natural gas distribution systems present a great potential for efficient and low-cost reduction measures — simply by repairing leakages. Most studies have focused on cities in the U.S., while fewer have explored European cities like London, Paris, Utrecht, Florence, or Hamburg. Eastern European cities, in particular, have been underrepresented in studies about urban methane emissions. Within the two research projects MEMO2 and ROMEO we began measurement campaigns in Eastern European cities, in for example Bucharest, Romania, or Krakow, Poland. We are building on this work within our new EU project, PARIS.
Our study
Our recent study within the PARIS project focuses on urban CH₄ emissions in Cluj-Napoca, Romania. In collaboration with researchers from Babeș-Bolyai University in Cluj-Napoca, we measured urban methane and its isotopic composition continuously for eight months, using an Isotope Ratio Mass Spectrometer (IRMS). This device measures the methane mole fraction in parts per million, which is quite little. In addition, it measured the isotopic ratios of methane. Isotopes are distinct species of the same chemical element. They can help identify sources since different sources produce different isotopes. So we get an idea from which source our methane is coming.
The system in the lab at the Babeș-Bolyai University
Alongside these continuous measurements, we did mobile campaigns. We walked and drove through the city with a small analyzer directly measuring the amount of methane and ethane in the air. Ethane, like methane, is a gas, and the ratio between these two gases provides additional information about potential sources, particularly distinguishing between fossil fuel and non-fossil fuel origins.
When we detected elevated methane emissions, we collected air samples in silver bags, as seen in the pictures. These bags were later analysed at Utrecht University on the isotopic composition and mole fraction.
Detection and sampling at several sources
First results
The timeseries showed peaks every night, with isotopic signatures revealing three methane sources: The gas network, biogenic emissions from waterbodies and a third source, which only occurred in winter. The isotopic source signatures indicated that this source is most likely a combustion source, such as fireplaces or vehicles.
Cluj-Napoca’s orography of the city limits the wind direction to the valleys. We only saw methane elevation when the wind came from the “city centre” or the “river basin”. The most prominent sources were in the city centre and when comparing this with the isotopes, it showed that these were mainly from gas leaks, mixed with microbial fermentation sources. It also showed that the presumable pyrogenic source only came from the “river basin”.
What’s Next?
These preliminary results were presented at the European Geosciences Union (EGU) conference and will be published as part of the ongoing PARIS project. We are now preparing a new continuous measurement campaign at the ICOS station Monte Cimone, Italy. Stay tuned for more updates as we further investigate urban methane emissions and their impact on climate change!
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