The CarboEurope Regional Experiment Strategy (CERES) experiment took place in May and June 2005 in France and offers a comprehensive database on atmospheric CO<inf>2</inf> and boundary layer processes at the regional scale. One "golden" day of CERES is interpreted with the mesoscale atmospheric model Meso-NH coupled on-line with the Interactions between Soil, Biosphere and Atmosphere, CO<inf>2</inf>-reactive (ISBA-A-gs) surface scheme, allowing a full interaction of CO<inf>2</inf> between the surface and the atmosphere. The rapid diurnal cycle of carbon coupled with water and energy fluxes is parameterized including, e.g., plant assimilation, respiration, anthropogenic emissions, and sea fluxes. During the analyzed day, frequent vertical profiles and aircraft transects revealed high spatial and temporal variabilities of CO<inf>2</inf> concentrations within the boundary layer at the regional scale: a 10-ppm gradient of CO<inf>2</inf>-mixing ratio is observed during the day by the aircraft measurements. The Meso-NH model proved able to simulate very well the CO<inf>2</inf> concentration variability as well as the spatial and temporal evolution of the surface fluxes and the boundary layer in the domain. The model is used to explain the CO<inf>2</inf> variability as a result of two complementary processes: (1) the regional heterogeneity of CO<inf>2</inf> surface fluxes related to the land cover (e.g., winter crops versus a pine forest) and (2) the variability of mesoscale circulation across the boundary layer: development of the sea breeze in the western part of the domain and dominating wind flow in the eastern part of the domain. Copyright 2007 by the American Geologiclal Union.