One-year eddy covariance CO2 fluxes at short and tall towers in the Paris area

Nowadays, around 50% of the global population lives in cities and this percentage is expected to increase to 70% by 2050 (UN-Habitat, Word City Report 2022). As a result, cities have become a major source of greenhouse gases, and were estimated to account for over 70% of global GHG energy-related emissions (IEA, 2008). A correct quantification of these emissions is crucial for developing climate action plans and monitoring their effectiveness. To this purpose, the PAUL project was designed to develop systematic observations of GHG fluxes in three pilot cities of different size: Zurich, Paris and Munich.In this framework, eddy covariance measurements of CO2 fluxes were started in February 2023 at two urban sites in the Paris area and are currently running to capture the seasonal and spatial variation of fluxes along a gradient of urbanisation. The two sites: a short tower on the rooftop of a university building in the city centre of Paris (Jussieu) and a tall tower (~100 m) on the NE periphery of the city (Romainville) are characterised by a different land cover composition within their footprint: in Jussieu, the smaller footprint spans a densely urbanised area, while in Romainville the wider footprint covers a less densely urbanised area with some vegetated patches.Overall, one-year measurements confirmed the city was a source of CO2, as both sites showed a net positive CO2 flux. However, daily flux patterns were different: While in the city centre (Jussieu) the CO2 emission was highest during the diurnal hours FCO2 ~ 5 mol m-2 s-1 and close to zero during the night, on the contrary, in the periphery (Romainville) positive fluxes with highest intensity were observed during the night FCO2 ~ 5 7 mol m-2 s-1, while a decrease of CO2 emission were measured in the middle of the day. Romainville was indeed closer to the diurnal net CO2 flux patterns observed at the ICOS ecosystem sites in the south and west of Paris (FR-FON forest, FR-GRI crop).Peoples habits were found to play an important role on the observed fluxes in Jussieu as distinct daily patterns were seen between weekdays and weekends. This was not the case for Romainville where uniform daily flux cycles were observed along the week.Visible seasonal differences in the monthly diurnal patterns evidenced the influence of multiple anthropogenic and biogenic drivers which played a key role in different periods of the year and the day.