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

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. 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.