Evaluation of nine years of continuous δ13CO2 measurements in Heidelberg, Germany

Measurements of atmospheric CO₂ mole fraction in combination with δ¹³CO₂ contain additional information on the CO₂ source mixture at a measurement station. Instrumental developments, such as cavity ring-down spectroscopy (CRDS), have facilitated the conduction of continuous in-situ measurements of CO₂ mole fraction and δ¹³CO₂ with a high temporal resolution. This has enabled a robust and detailed local time series to be established at an urban station in Heidelberg in south-western Germany, where a CRDS G2201-i analyser has been used to measure the CO₂ mole fraction and ¹³C/¹²C ratio from 2014 to 2023. This nine year time series is analysed for seasonal variations and trends in regional and local CO₂ sources. We applied different approaches based on the Keeling/Miller-Tans method to identify δ¹³CO₂ source signatures within the Heidelberg catchment area. Doing this gave δ¹³CO₂ source values that were less depleted in the summer and more depleted in the winter, indicating a stronger biogenic effect in summer and stronger fossil fuel contributions in winter.