Continuous methane (CH4) isotope measurements in Lindenberg, Germany

Methane (CH4) plays a crucial role in the Earth’s radiative balance because it is a potent greenhouse gas with a shorter lifetime compared to CO2. Mitigating CH4 emissions can potentially mitigate climate change over a short period [1]. Mitigating CH4 requires a solid understanding of the emissions, in particular, which source emits the CH4. Isotopic analysis can aid in source partitioning, as different production processes produce CH4 with subtle but significant differences in isotopic composition [3], enabling the differentiation of multiple sources.CH4 isotopic source signatures are typically obtained through mobile where sources are sampled as close to the emission point as possible [2]. While these  campaigns are valuable, they only capture for a short duration and miss many smaller and unknown emissions. In contrast, continuous CH4 measurements cover longer periods and can detect inaccessible or unknown sources. However, the downside is that identifying the exact source can be more challenging as the source origin is not always known.Researchers at Utrecht University developed an isotope ratio mass spectrometer (IRMS) system that measures CH4 mole fraction, δD and δ13C at highprecision with a 40-minute resolution. This system was deployed from 15 April 2022 till 8 January 2023 at a tall tower in Lindenberg, Germany. Measurements were initialised at 40 m.a.g.l and later continued 98 m.a.g.l. The station is part of the Integrated Carbon Observation System (ICOS), providing mole fraction measurements of CO, CO2, and CH4. CH4 isotopic data were also compared with simulations from EMPA. These simulations include the CH4 emissions for each category, allowing us to assign an isotopic source signature to each emissions category, and thereby simulating a CH4 isotopic source signature. For the isotopic measurements, we observed 169 peaks shorter than 24 hours.This corresponds to 67% of the deployment days. Most source signatures indicate a microbial fermentation source (δ13C : [-55‰, -62‰], δD : [-260 ‰, -360 ‰]). Additionally, we identified 19 multi-day elevations, lasting up to 20 days. Eight of these multi-day elevations displayed isotopic signatures similar to those of the diurnal peaks, while the remaining multi-day peaks showed distinctly different source signatures from one another and the diurnal elevations.