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 developing and 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 and densely built-up area (Jussieu), an urban forest (Vincennes), and a heterogeneous area combining highly urbanised areas with areas of vegetation (Romainville). The observations from the urban sites were also integrated with the EC flux measurements conducted on the ICOS atmosphere tower of Saclay and the observations from the ecosystem sites of Fontainebleau (FR-FON, forest) and Grignon (FR-GRI, crop).

Long-term measurements of CO₂ fluxes (2 years for the sites of Romainville and Jussieu and 1 year for the site of Vincennes) showed seasonal dynamics that reflected their respective degrees of urbanisation. The Jussieu site, in the city center, was constantly dominated by anthropogenic CO2 emissions, with maximum emission (up to  15 µmol m^-2 s^-1) during the winter months (November-February) and low absorptions (up to  -2.5 µmol m^-2 s^-1) during the summer (July-August) in the central hours of the day. On the other hand, the mixed urban forest of Vincennes showed a strong biogenic signature, characterized by a predominant CO₂ absorption in the central hours of the day (up to -10 µmol m^-2 s^-1 in the months of May, June and July). The 100 m-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 of the City of Paris will be included in the presentation.