Isotopic characterization of methane emissions in Malaga, Spain

The reduction of methane emissions from energy, agriculture, and waste sectors presents a significant opportunity for making near-term strides in climate mitigation. To accomplish this, there is a pressing need for methods that facilitate the quantification of emissions. In Malaga province, Spain, the main sources of methane emissions, as estimated through model-based calculations, are predominantly associated with the waste and agricultural sectors. Natural gas, albeit playing a minor role, is mainly used in the industrial sector. However, the absence of ground-based measurements, such as methane concentration, isotopic attribution, and emission rates measured, poses a significant challenge to the improvement of local and regional inventories. This study aims to characterize the isotopic signatures (δ13C-CH4 and δD-CH4) of diverse methane sources in the province of Malaga during two field campaigns in April and September 2023. Methane plumes emitted from anthropogenic sources, such as wastewater treatment plants (WWTP), landfills, piggeries, manure, biowaste, combustion and biogas plants, among others, were identified through mobile surveys using a Methane/Ethane Mid-Infrared Laser Absorption Spectroscopy analyzer. Collected air samples from the plumes underwent subsequent isotope analysis by gas chromatography isotope ratio mass spectrometry (GC-IRMS) to characterize δ13C-CH4 and δD-CH4. The isotopic signatures of the sources were determined using a Keeling plot approach. Emissions from the waste sector display a variety of microbial δ13C-CH4 and δD-CH4 signatures. For WWTPs and landfills, δ13C-CH4 signatures range between -58.9 and -48.2 ‰, and δD-CH4 between -324.1 and -257.4 ‰. There is notably more variability in the WWTP signals. Landfill CH4 exhibits lower δ13C-CH4 values and higher δD-CH4 values, in some cases showing microbial oxidation processes when comparing the active and closed sectors within the same landfill. Biowaste from olive mill and meat production yields CH4 with δ13C-CH4 between -49.3 and -47.2 ‰ and δD-CH4 between -315.8 and -286.2 ‰. Two agricultural sources were investigated: piggeries with δ13C-CH4 between -51.9 and -56.1 ‰ and δD-CH4 between -344.6 and -304.6 ‰; and their associated manure with δ13C-CH4 between -52.8 and -49.8 ‰ and δD-CH4 between -327.3 and -318.1 ‰. Furthermore, a biogas plant using pig liquid manure as feedstock (δ13C-CH4 between -55.9 and -44.3 ‰; δD-CH4 between -235.9 and -185.9 ‰), a combustion plant utilizing olive mill waste (δ13C-CH4 between -46.2 and -41.8 ‰; δD-CH4 between -273.9 and -148.4 ‰), and a pig slaughterhouse engaged in meat production (δ13C-CH4 and δD-CH4 signatures of -40.0 and -139.6 ‰, respectively) all provide evidence of fossil fuel usage in the operations of these facilities.