Methane Sources in Cluj-Napoca, Romania: Insights From Isotopic Analysis

Increased emissions of methane (${\text{CH}}_4$) have contributed 0.3–0.8°C to global temperature rise since preindustrial times. Reducing these emissions is crucial to mitigate climate change. Measurements of the isotopic composition of ${\text{CH}}_4$ ($\delta$¹³C and $\delta$²H) can be used to distinguish various sources of ${\text{CH}}_4$. This study reports continuous measurements of ${\text{CH}}_4$, $\delta$¹³C and $\delta$²H for 8 months in Cluj-Napoca, Romania. An automated extraction and a purification system, coupled to an isotope ratio mass spectrometer alternately measured $\delta$¹³C and $\delta$²H of ${\text{CH}}_4$ with 20-min time resolution at the campus of the Babeş-Bolyai University. In addition, point source samples were measured to isotopically characterize ${\text{CH}}_4$ sources in the region. The time series show regular ${\text{CH}}_4$ elevations during the night, occasionally superimposed on multiday events. From these elevations, we identified three main ${\text{CH}}_4$ emission categories: Transylvanian biogenic gas (75%); biogenic emissions from rivers and wastewater (38%), predominantly observed during the summer; and a third source emitting ¹³C-enriched ${\text{CH}}_4$ in winter, likely of pyrogenic origin (5%). We simulated the ${\text{CH}}_4$ mole fraction at the measurement site using Lagrangian footprints generated from the FLEXPART-COSMO model convolved with emissions from the TNO-CoCO2 inventory. The simulations show that the emission inventory is not granular enough to represent the city center. The strong underestimation in winter suggests that the emission inventory did not include the pyrogenic winter source. When the model accurately estimated the ${\text{CH}}_4$ mole fraction, it also predicted the isotopic compositions well.