Reducing methane (CH4) emissions has a great potential for climate change mitigation. However, uncertainties remain in the CH4 budget: the emissions calculated from atmospheric observations do not match the ones reported in inventories.
The stable isotopic composition of methane, which is the proportion of heavier molecules compare to the most abundant form, provide information on the formation pathway. Therefore, measurements data on two isotopes of CH4, carbon 13 (d13C-CH4) and deuterium (d2H-CH4), allowed us to evaluate the emission inventories and investigate spatial variations in Europe.
We performed continuous measurements of ambient air over several months at fixed locations, in order to evaluate the influence of specific methane sources over a region. The CH4 isotopic composition data from in the North coast of the Netherlands and in the city of Krakow, Poland, were dramatically different. Biogenic sources, mostly of anthropogenic origin, such as from ruminant farming or waste management, are prominant in the Netherlands, as suggested by the emissison inventories. In Krakow, we detected large emissions from coal mines, but also from the city. The latter are not always reported in inventories, and appear to come from the use of fossil fuels, for exemple the use of coal for residential heating. The uses of d2H-CH4 as a tracer was particularly useful in the case of Poland, as there is an overlap in the d13C-CH4 from biogenic and fossil fuel sources. We showed that time series of CH4 isotopic composition in ambient air can help to assess emission inventories.
A large measurement campaign was organised in Romania, focusing on methane emissions from the oil and gas industry. The exploitation of fossil fuels is causing direct emissions of CH4 through leaks. We took samples close to oil and gas installations, but also while flying a scientific aicraft over the regions of extraction. The isotopic composition of CH4 from Romania was hardly studied until now, and we created an unprecedented dataset from our samples. We attributed most pollution of the regions we surveyed to the oil and gas industry. We identified microbial pathways in the subsurface formations that would help interpreting atmospheric data on larger scales.
We gathered all measurements in a publicly available database, and combined with the previous literature. Over Europe, we found strong regional variations in the isotopic composition from fossil fuel sources but not from microbial sources. Our dataset brings new tools for the interpretation of methane stable isotope data, especially by providing a significant amount of new d2H-CH4 observations. Future studies can use it for source attribution, and as input to atmospheric models.