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 a challenging and interesting task needed to be performed in order to utilise CO² measurements in an atmospheric inverse framework and 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 modelling between the surface and the atmosphere. Statistical scores show a good representation of the urban heat island (UHI) with stronger urban-rural contrasts on temperature at night than during the day by up to 7°C. Boundary layer heights (BLH) have been evaluated on urban, suburban and rural sites during the campaign, and also on a suburban site over 1 yr. The diurnal cycles of the BLH are well captured, especially the onset time of the BLH increase and its growth rate in the morning, which are essential for tall tower CO² observatories. The main discrepancy is a small negative bias over urban and suburban sites during nighttime (respectively 45m and 5 m), leading to a few overestimations of nocturnal CO² mixing ratios at suburban sites and a bias of +5 ppm. The diurnal CO² cycle is generally well captured for all the sites. At the 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. 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 strong spatio-temporal variability. A sensitivity test without urban parameterisation removes the UHI and underpredicts nighttime BLH over urban and suburban sites, leading to large overestimation of nocturnal CO² mixing ratio at the suburban sites (bias of +17 ppm). The agreement between observation and prediction for BLH and CO² concentrations and urban-rural increments, both day and night, demonstrates the potential of using the urban mesoscale system in the context of inverse modelling. [ABSTRACT FROM AUTHOR]