We assess the uncertainty in continuous fossil fuel carbon dioxide (ffCO₂) estimates derived from carbon monoxide (CO) observations and radiocarbon (¹⁴CO₂) flask measurements from an urban and a rural site. This study provides the basis for using continuous CO-based ffCO₂ observations in atmospheric transport inversion frameworks to derive ffCO₂ emission estimates. We also compare the flask-based CO / ffCO₂ ratios with modeled ratios to validate an emission inventory for central Europe.