Sources of Methane Emissions in the Urban Atmosphere - Case Study: Cluj-Napoca City

Methane is considered to be the second largest contributor to the greenhouse effect after CO₂, with a higher increase of its atmospheric concentration over the industrial era compared to other greenhouse gases, as CO₂ or nitrous oxide. At least 50% of CH₄ emissions come from anthropogenic activities in urban and rural areas, due to the combustion of fossil fuels, waste and wastewater treatment, leaks from the natural gas distribution system, etc.

Although, CH₄ is more than 20 times more potent than CO₂ in producing the greenhouse effect, it has a short residence time in the atmosphere, which makes the mitigation measures more effective in minimizing these emissions and bringing a short-term advantage to the climate.

Methane concentrations were determined at street-level in the city center, commercial area, residential area, and green area of Cluj-Napoca, Romania. A portable West Systems fluxmeter was used. The instrument is based on Tunable Diode Laser Absorption Spectroscopy, that allows high precision measurements and very low detection limit, of 0.1 ppm.  By eliminating wind effects at street-level, the residential area recorded the least values of these emissions, around the CH₄ average atmospheric concentration. Similarly, the emissions in the commercial area were fluctuating around same atmospheric concentration average, but strongly depending on the traffic density. On the sidewalk, at about 1.3 m height, the recorded methane concentrations ranged between 1.9 and 6.0 ppm. The highest values occurred at the passage of heavy vehicles as trucks.

Concentrations of tens or even hundreds ppm CH₄ were measured close to the drains that collect water run-off from the streets, and even higher at manhole covers of the sanitary sewerage system.

However, the outcomes of this study indicate the need for further investigation of CH₄ emissions in the urban area and the importance of the isotopic characterization of these emissions in order to identify their sources for prioritizing the CH₄ mitigation measures.