CO2 dispersion modelling over Paris region within the CO2-MEGAPARIS project.

Accurate simulation of the spatial and temporal variability of tracer mixing ratios over urban areas is challenging, but essential in order to utilize CO₂ measurements in an atmospheric inverse framework to better estimate regional CO₂ fluxes. This study investigates the ability of a high-resolution model to simulate meteorological and CO₂ fields around Paris agglomeration, during the March field campaign of the CO₂-MEGAPARIS project. The mesoscale atmospheric model Meso-NH, running at 2 km horizontal resolution, is coupled with the Town-Energy Balance (TEB) urban canopy scheme and with the Interactions between Soil, Biosphere and Atmosphere CO₂-reactive (ISBA-A-gs) surface scheme, allowing a full interaction of CO₂ between the surface and the atmosphere. Statistical scores show a good representation of the Urban Heat Island (UHI) and urban-rural contrasts. Boundary layer heights (BLH) at urban, sub-urban and rural sites are well captured, especially the onset time of the BLH increase and its growth rate in the morning, that are essential for tall tower CO₂ observatories. Only nocturnal BLH at sub-urban sites are slightly underestimated a few nights, with a bias less than 50m. At Eiffel tower, the observed spikes of CO₂ maxima occur every morning exactly at the time at which the Atmospheric Boundary Layer (ABL) growth reaches the measurement height. The timing of the CO₂ cycle is well captured by the model, with only small biases on CO₂ concentrations, mainly linked to the misrepresentation of anthropogenic emissions, as the Eiffel site is at the heart of traffic emission sources. At suburban ground stations, CO₂ measurements exhibit maxima at the beginning and at the end of each night, when the ABL is fully contracted, with a very strong spatio-emporal variability. The CO₂ cycle at these sites is generally well reproduced by the model, even if some biases on the nocturnal maxima appear in the Paris plume parly due to small errors on the vertical transport, or in the vicinity of airports due to small errors on the horizontal transport (wind direction). A sensitivity test without urban parameterisation removes UHI and underpredicts nighttime BLH over urban and sub-urban sites, leading to large overestimation of nocturnal CO₂ concentration at the sub-urban sites. The agreement of daytime and nighttime BLH and CO₂ predictions of the reference simulation over Paris agglomeration demonstrates the potential of using the meso-scale system on urban and sub-urban area in the context of inverse modelling. [ABSTRACT FROM AUTHOR]