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 covariance CO 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 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 ffCO 2 contributions (