Investigating emissions of methane (CH4) to the atmosphere in Cluj-Napoca urban area

Methane is a primary greenhouse gas with a relatively short lifetime in the atmosphere (~ 12 y), making it a candidate for mitigation efforts aimed at improving the climate in the near future.

Urban areas account for ~2% of our planet’s surface, but they host more than half of the world’s population and a series of potential CH₄ source systems (e.g., natural gas distribution, landfills, sewages, and in some cases, natural or artificial wetlands). The role of urban areas in the global atmospheric CH₄ budget is, however, uncertain.

Our understanding of the exact CH₄ emission systems and related emission factors in urban areas is still limited (and unknown in Romania).

In Cluj-Napoca, the second-biggest city in Romania, our study aims at identifying potential urban sources for CH₄emissions via direct detection of CH₄ at potential emission sites (opposed to atmospheric monitoring, as generally done).

Our study aims to investigate potential sources in three major urban systems (natural gas distribution networks, sewage system, and aquatic systems).

We used a highly sensitive laser CH₄ sensor based on Tunable Diode Laser Absorption Spectrometry (TDLAS) with a resolution of ± 0.1 ppmv.

Preliminary data show that 86% of the natural gas end-use points (74 measured) release CH₄ into the atmosphere, 51% of the SEWAGE manholes (126 measured) are net sources of methane, and the aquatic systems (river and ponds, 50 sampling sites) are all oversaturated with dissolved CH₄ and represent potential sources.

Due to both natural and anthropogenic CH₄ sources, Cluj-Napoca can be considered a hybrid methane source. This approach helps in reducing the ambiguity in the attribution of CH₄ sources when only atmospheric CH₄ monitoring is performed.

This study can advance our understanding of CH₄ release in urban areas and serve as the first national systematic-based approach that can be replicated to effectively take advantage of all available resources. It will also serve as a reference for future research on CH₄ in the urban area.