Eddy covariance measurements of CO2 fluxes along an urban-to-rural gradient in the Paris area

Cities are one of the main sources of greenhouse gases, accounting for over 70% of global CO₂ emissions. Accurate quantification of these emissions through direct observations is crucial for assessing the effectiveness of adopted mitigation strategies.

As part of the European project ICOS Cities (https://www.icos-cp.eu/projects/icos-cities), three eddy covariance towers were installed in the Paris area to capture the variability of surface-atmosphere CO₂ fluxes as a function of an urbanization gradient. Specifically, the selected sites were chosen to be representative of a highly urbanised area (Jussieu), an urban forest (Vincennes), and a heterogeneous area combining densely urbanised and vegetated zones (Romainville). The atmosphere ICOS Saclay tower was also equipped with an eddy covariance system to provide a sub-urban environment. Additionally, the observations from the urban sites were integrated with the EC flux measurements from the ecosystem sites of Fontainebleau (FR-FON, forest) and Grignon (FR-GRI, crop). The measurements span 2 years for Romainville and Jussieu and 1 year for Vincennes and Saclay.

CO₂ flux measurements showed seasonal dynamics that reflected the respective degrees of urbanisation of the sites and the presence of biogenic sinks. Jussieu, in the city centre, was dominated by anthropogenic CO₂ emissions, with maximum mean daily emission (up to 15 µmol m^-2s^-1) during winter months (November-February) and low daytime absorptions (down to -2.5 µmol m^-2s^-1) in summer (July-August). The mixed urban forest of Vincennes showed a diverse flux pattern with strong biogenic signature, characterized by a predominant CO₂ absorption in the central hours of the day (down to -10 µmol m^-2s^-1 in May, June and July). The 100m-tall tower of Romainville showed instead the coexistence of anthropogenic and biogenic fluxes, each contributing its own seasonal and daily variations to the measured flux. A comparison between our observations and the emissions estimates of the city of Paris will be included in the presentation.