Uncertainty of continuous ∆CO-based ∆ffCO₂ estimates derived from ¹⁴C flask and bottom-up ∆CO / ∆ffCO₂ ratios

Abstract. Measuring the ¹⁴C / C depletion in atmospheric CO₂ compared to a clean-air reference is the most direct way to estimate the recently added CO₂ contribution from fossil fuel (ff) combustion (∆ffCO₂) in ambient air. However, since ¹⁴CO₂ measurements cannot be conducted continuously nor remotely, there are only very sparse ¹⁴C-based ∆ffCO₂ estimates available. Continuously measured tracers like carbon monoxide (CO), which are co-emitted with ffCO₂ can be used as additional alternative proxies for ∆ffCO₂, provided that the ∆CO / ∆ffCO₂ ratios can be determined correctly. Here, we use almost 350 ¹⁴CO₂ measurements from flask samples collected between 2019 and 2020 at the urban site Heidelberg in Germany, and corresponding analyses from more than 50 afternoon flasks collected between September 2020 and March 2021 at the rural ICOS site Observatoire pérenne de l'environnement (OPE) in France, to calculate average ∆CO / ∆ffCO₂ ratios for those sites. By dividing the hourly ∆CO excess observations by the averaged flask ratio, we construct continuous and bias-free ∆CO-based ∆ffCO₂ records. The comparison between ∆CO-based ∆ffCO₂ and ¹⁴C-based ∆ffCO₂ from the flasks yields a root-mean-square deviation (RMSD) of about 4 ppm for the urban site Heidelberg and of 1.5 ppm for the rural site OPE. While for OPE this uncertainty can be explained by observational uncertainties alone, for Heidelberg about half of the uncertainty is caused by the neglected variability of the ∆CO/∆ffCO₂ ratios. We further show that modelled ratios based on a bottom-up European emission inventory would lead to substantial biases in the ΔCO-based ∆ffCO₂ estimates for Heidelberg, and also for OPE. This highlights the need for an ongoing observational calibration/validation of inventory-based ratios, if those shall be applied for large-scale ΔCO-based ∆ffCO₂ estimates, e.g. from satellites.