Relaxed eddy accumulation (REA) measurements for ¹⁴CO₂ enable the estimation of fossil fuel (ff) CO₂ fluxes in urban areas. This work is based on 252 REA ffCO₂ flux measurements conducted on tall towers in the cities of Zurich, Paris, and Munich. The ffCO₂ fluxes were compared to net eddy covariance CO₂ fluxes to quantify the role of non-fossil (nf) CO₂ fluxes. While the measurements in Zurich and Paris were limited by small signal-to-noise ratios, improvements in the REA setup, the ¹⁴CO₂ measurement precision, the sampling strategy, and the source strength increased the significance of the results in Munich. Large nfCO₂ fluxes observed in Munich from the direction of a brewery demonstrate the efficacy of the partitioning approach and illustrate the complexity of urban atmospheric measurement data. Excluding these measurements potentially influenced by large anthropogenic nfCO₂ fluxes, the error-weighted average ffCO₂ / CO₂ flux ratio in Munich was approximately 47 % in summer and 76 % in winter, with the majority of measurements taken between 07:00 and 19:00 local time. Regional excess concentrations had much lower ffCO₂ contributions (<63 % in winter and <28 % in summer, in all three cities), demonstrating fundamental differences between local and regional CO₂ fluxes. The combination of ¹⁴CO₂ observations and the REA method is a sophisticated approach that challenges the limits of current analytical capabilities, while providing unique opportunities for quantifying ffCO₂ and nfCO₂ fluxes.