1,2,6,1,7,7,5,4,3,8,9,1- 1Chair of Environmental Meteorology, Faculty of Environment and Natural Resources, University of Freiburg, Freiburg, Germany
- 2Institute of Environmental Physics, Heidelberg University, Heidelberg, Germany
- 3ICOS Central Radiocarbon Laboratory, Institute of Environmental Physics, Heidelberg University, Heidelberg, Germany
- 4ECOSYS, INRAE, AgroParisTech, Université Paris-Saclay, Palaiseau, France
- 5ICOS Flask and Calibration Laboratory, Max Planck Institute for Biogeochemistry, Jena, Germany
- 6Empa, Materials, Science and Technology, Dübendorf, Switzerland
- 7Institute of Meteorology and Climate Research, Atmospheric Environmental Research, Karlsruhe Institute of Technology, Garmisch-Partenkirchen, Germany
- 8Laboratoire des Sciences du Climat et de l'Environnement, CEA, CNRS, Université Paris-Saclay, Gif-sur-Yvette, France
- 9Department of Environmental Sciences, University of Basel, Basel, Switzerland
Cities are hotspots for fossil fuel CO2 (ffCO2) emissions, and independent top-down validation of ffCO2 emissions is essential to guide their reduction. However, the lack of direct ffCO2 flux observations and the heterogeneous distribution of ffCO2 and non-fossil CO2 (nfCO2) sources and sinks in urban areas impede the partitioning of net CO2 flux measurements. While radiocarbon (14CO2) is an ideal tracer for ffCO2, the logistics and costs associated with laboratory analysis of air samples prevent continuous measurements. Proxy-based ffCO2 estimates from continuous measurements of co-emitted species such as CO, on the other hand, are subject to larger uncertainties, as the proxy:ffCO2 emission ratio varies depending on the combustion process. Consequently, flux partitioning often relies on bottom-up information and/or results from inverse modeling.
Relaxed eddy accumulation (REA) measurements of 14CO2 and co-emitted species such as CO, conducted within the ICOS Cities project on urban tall-towers in the cities of Zurich, Paris and Munich, have enabled us to independently estimate ffCO2 fluxes and proxy:ffCO2 flux ratios for ~250 individual hours. The measurements show significant nfCO2 contributions as well as highly variable CO:ffCO2 flux ratios. We will analyze temporal and spatial patterns, examine the proxy:ffCO2 flux ratio of dominant sectors, and compare the mean atmospheric ratios between the three cities. Continuous ffCO2 estimates are obtained by combining the calibrated proxy:ffCO2 ratios with continuous proxy measurements. We will discuss the uncertainties of this approach with respect to the advantages of continuous CO2 estimates.
Although the combination of 14CO2 and co-emitted species observations with the REA method presents a sophisticated approach that challenges the limits of current analytical capabilities, it provides a unique opportunity to experimentally verify the CO2 flux composition for both bottom-up and inversion model-based urban CO2 emissions.
How to cite: Kunz, A.-K., Hammer, S., Bignotti, L., Borchardt, L., Della Coletta, J., Emmenegger, L., Eritt, M., Gutiérrez, X., Hilland, R., Holst, C., Jordan, A., Kneißl, R., Lan, C., Legendre, V., Loubet, B., Preunkert, S., Ramonet, M., Stagakis, S., and Christen, A.: Towards Estimating Continuous Fossil Fuel (ff) CO2 Emissions in Urban Areas: 14CO2-Calibrated Proxy:ffCO2 Flux Ratios From Relaxed Eddy Accumulation Measurements, 12th International Conference on Urban Climate, Rotterdam, The Netherlands, 7–11 Jul 2025, ICUC12-426, https://doi.org/10.5194/icuc12-426, 2025.
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