Urban Areas as a Hybrid System for Methane Emissions to the Atmosphere, Case Study: Cluj-Napoca (ROMANIA)

Methane (CH₄), a potent greenhouse gas (GHG) with a relatively short atmospheric lifetime, is important for mitigation actions to bring a near-term climate advantage.

Although urban areas cover <2% of the Earth’s surface, they host over half of the world’s population, and at the same time, they are associated with a high degree of uncertainty regarding CH₄ emissions.

Studies of methane emissions in urban areas usually estimate gas leaks from natural gas and oil production sites, domestic natural gas networks, combustion systems, and landfills. However, according to recently published studies, the sewer and urban aquatic systems have become the focus of CH₄ emission investigations, which show urban areas as a hybrid system for CH₄ emissions. Nevertheless, CH₄ sources in urban areas are still poorly understood and require further investigation. Moreover, the Romanian national GHG inventory did not report urban area emissions of CH₄.

Our study aims at the preliminary identification of urban sources for CH₄ emissions in Cluj-Napoca, the second-biggest city in Romania, based on a simple detection of gas leaks.

Tunable Diode Laser Absorption Spectroscopy (TDLAS), with a resolution of 0.1 ppmv, was used to determine the atmospheric concentrations of gas at different points of potential CH₄ sources around the city. Also, gas from water samples was analyzed via head-space extraction. In addition, isotopic analysis was performed on samples collected from different systems in the urban area.

This study has revealed that CH₄ emissions come from multiple sources, including traffic, leaks from the natural gas distribution network, and the sewer system. It has also pointed out that the aquatic ecosystem is oversaturated with CH₄, which represents a hotspot for water-atmospheric exchange.