Increased emissions of methane (CH4) have contributed to global temperature rise since pre-industrial times by 0.3-0.8C. Reducing these emissions is crucial to mitigate climate change. Various sources produce CH4 with distinguishable isotopic compositions, allowing CH4 source identification using CH4, δ13C and δ2H values. This study reports continuous measurements of CH4 mole fraction, δ13C and δ2H for eight months in Cluj-Napoca, Romania. An automated extraction and purification system, coupled to an isotope ratio mass spectrometer alternately measured δ13C and δ2H of CH4 with 20-minute time resolution at the campus of the Babe.s-Bolyai University. In addition, samples from the vicinity of the continuous measurement location were measured to isotopically characterise CH4 sources in the region.
 
The time series show a pronounced diurnal cycle, characterised by CH4 elevations during the night, occasionally superimposed on multi-day events. From these elevations, we identified three main CH4 emission categories: CH4 similar in isotopic composition to Transylvanian biogenic gas, biogenic emissions from rivers and wastewater, predominantly observed during the summer and a third source emitting δ13C-enriched CH4 in winter, likely of pyrogenic origin.
Simulations with the FLEXPART-COSMO model underestimated the CH4 mole fractions in winter and with eastern wind (city centre), indicating that the emission inventory is not granular enough to represent the city centre. The strong underestimation in winter suggests that the emission inventory did not include the pyrogenic winter source. Despite this, when the model accurately estimated the CH4 mole fraction, it also predicted the isotopic compositions well.