Understanding and monitoring emissions of methane (CH₄), the second-most important greenhouse gas for global warming, is a key target of the European Green Deal. Unlike carbon dioxide, which results mainly from fossil fuel burning, 95 % of European CH₄ emissions come from agriculture, waste, and energy production – sources whose emissions are poorly characterised. One method to trace CH₄ is measuring the ratio of isotopes in atmospheric samples, a value characteristic of different formation and transportation processes. While these measurements can be made through isotope ratio mass spectrometry (IRMS) or optical isotope ratio spectroscopy (OIRS) – which is required to analyse rare but useful ‘doubly-substituted’ molecules (e.g., ¹³CH₃D) – many source ratios have not been characterised, limiting the interpretation of measurements. Additionally, these systems do not have formal protocols for measurements or calibrations, and are not integrated into existing atmospheric monitoring networks, limiting their impact. The EMPIR project STELLAR improved measurement techniques to determine the isotopic composition of methane and carbon dioxide in order to differentiate between anthropogenic and natural sources of greenhouse gases.

This project will create validated IRMS and OIRS methodologies, creating isotope ratio datasets and developing procedures for comparing in-situ measurements. It will also develop a metrological infrastructure for measuring double-substituted molecules using OIRS. Project datasets will also be used to create models for estimating CH₄ emissions from different sources, which will allow in-situ sampling to be optimised. The data and measurement infrastructure created by the project will be shared with global monitoring networks and will encourage uptake of IRMS and OIRS. It will also allow understudied emission types to be characterised, giving a more complete picture of CH₄ sources and their impacts at European and global scales.