Influence of atmospheric processes and urban vegetation on local-scale CO₂ concentrations and fluxes: insights from the CiTRY field campaign

Nocturnal CO₂ accumulation near the surface is a widely reported phenomenon, as stable nocturnal stratification suppresses turbulent vertical mixing, favouring the build-up of CO₂ in the lowest atmospheric layers. During the morning transition, surface CO₂ concentration drops rapidly, a process known as CO₂ flushing, yet its underlying dynamical mechanisms remain poorly investigated. Dupont et al. (2024) identified two flushing mechanisms under low-wind conditions: one of convective origin, driven by thermal plumes, and one associated with the merging between the residual nocturnal layer and the growing convective boundary layer. Under high-wind conditions, strong mixing prevents accumulation and no flushing occurs. This thesis investigates CO₂ flushing at an urban site with significant tree cover in Nantes, France, using eddy covariance and Doppler LiDAR measurements collected during the CiTRY field campaign. Turbulence intensity is found to strongly control the CO₂ dynamics. Specifically, under low turbulence conditions, two flushing mechanisms act concurrently: one of convective origin, consistent with Dupont et al. (2024); one driven by Low-Level Jet (LLJ) activity, strong winds in the lower atmosphere characterised by a nose-shaped vertical profile, which generate shear instabilities aloft and intensify turbulence, inducing downward ventilation of CO₂-poor air into the canopy. The flushing induced by merging between the residual nocturnal layer and the growing convective boundary layer, as described by Dupont et al. (2024), is not observed. LLJ-driven CO₂ flushing has rarely been documented and never for urban sites; this thesis provides first observational evidence of this link. Future work should expand the statistical basis of these findings, and extend measurements to greater atmospheric heights to better constrain the vertical extent of the layer affected by CO₂ flushing.