$^{14}$C-based separation of fossil and non-fossil CO$_2$ fluxes in cities using relaxed eddy accumulation: results from tall-tower measurements in Zurich, Paris, and Munich

Relaxed eddy accumulation (REA) measurements for $^{14}$CO$_2$ enable the estimation of fossil fuel (ff) CO$_2$ fluxes in urban areas. This work is based on 252 REA ffCO$_2$ flux measurements conducted on tall towers in the cities of Zurich, Paris, and Munich. The ffCO$_2$ fluxes were compared to net eddy covarianceCO$_2$ fluxes to quantify the role of non-fossil (nf) CO$_2$ fluxes. While the measurements in Zurich and Paris were limited by small signal-to-noise ratios, improvements in the REA setup, the $^{14}$CO$_2$ measurement precision, the sampling strategy, and the source strength increased the significance of the results in Munich. Large nfCO$_2$ 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 nfCO$_2$ fluxes, the error-weighted average ffCO$_2$/CO$_2$ flux ratio in Munich was pproximately 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 ffCO$_2$ contributions (< 63 % in winter and < 28 % in summer, in all three cities), demonstrating fundamental differences between local and regional CO$_2$ fluxes. ... mehrThe combination of $^{14}$CO$_2$ observations and the REA method is a sophisticated approach that challenges the limits of current analytical capabilities, while providing unique opportunities for quantifying ffCO$_2$ and nfCO$_2$ fluxes.