Radiocarbon and tracer for fossil CO2 Methods for fossil CO2 determination

The separation of fossil and non-fossil CO₂ sources is crucial to understanding local and global CO₂ concentration variations. While natural CO₂ fluxes from photosynthesis and respiration exceed fossil emissions many times over on a global scale, fossil sources usually dominate in urban areas. In Heidelberg, local CO₂ variations originate roughly equally from fossil and non-fossil processes.

The isotope ¹⁴CO₂ is considered the ‘gold standard’ for determining fossil CO₂ components, as fossil fuels are practically free of ¹⁴C. The combustion of such fuels lowers the ¹⁴C/C ratio in the atmosphere, which can be detected by high-precision measurements. A detailed introduction can be found in Maier et al. (2023).

However, ¹⁴CO₂ measurements are complex and expensive, which is why they are only carried out at a few stations. Alternatively, proxies such as CO and NOx are analysed at the ICOS CRL pilot station. These trace substances are emitted during the combustion of fossil fuels and can also be used to estimate fossil CO₂ shares. We use atmospheric ¹⁴CO₂ observations to calibrate the proxy-to-fossil-fuel CO₂ ratios. An example of how well the increases in proxy concentrations match those in fossil CO₂ concentrations in Heidelberg can be seen in the following figure.

The ¹⁴C-calibrated proxy-to-fossil-fuel CO₂ ratios are used to obtain a high-resolution temporal estimate of the variation in fossil CO₂ concentrations from continuous CO or NOx measurement series. Maier et al. (2024) describe the proxy-based ffCO₂ estimation in detail.