¹⁴C-based separation of fossil and non-fossil CO₂ fluxes in cities using relaxed eddy accumulation: results from tall-tower measurements in Zurich, Paris, and Munich

Abstract. 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. In all three cities, winter CO₂ fluxes were predominantly fossil, with mean ffCO₂ contributions of about 80 %. Summer fluxes could be most clearly partitioned in Munich, where improvements in the REA setup, the ¹⁴CO₂ measurement precision, the sampling strategy, and the source strength increased the signal-to-noise ratios compared to Zurich and Paris. In Munich, the observed nfCO₂ fluxes were predominantly positive (∼50 % of net summer fluxes), demonstrating the major role of respiration, biofuels, and certain industrial processes. Particularly large nfCO₂ fluxes from the direction of a brewery suggest non-respiratory anthropogenic contributions and highlight the complexity of urban environments. Additionally, the absolute CO₂ and ¹⁴CO₂ concentrations of the REA samples were compared to clean background concentrations to estimate ffCO₂ excess concentrations. Across all cities, ffCO₂ contributions to regional excess concentrations were much lower (<65 % in winter and <30 % in summer) than to local eddy covariance CO₂ fluxes, 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.