Aircraft and surface measurements of turbulent thermodynamic variables and carbon dioxide (CO₂) were taken above a grassland in a convective atmospheric boundary layer. The observations were analyzed to assess the importance of the entrainment process for the distribution and evolution of carbon dioxide in the boundary layer. From the observations we were able to estimate the vertical profiles of the fluxes, the correlation coefficients, and the skewness of the virtual potential temperature, the specific humidity, and the carbon dioxide. These profiles indicate that important entrainment events occurred during the observed period. The data were also used to estimate the budgets for heat, moisture, and carbon dioxide. By studying this observational data we find that the entrainment of air parcels containing lower concentrations of water vapor and carbon dioxide significantly dries and dilutes the concentration of these two constituents in the boundary layer. This process is particularly important in the morning hours which are characterized by a rapidly growing boundary layer. The observations show that the CO₂ concentration in the boundary layer is reduced much more effectively by the ventilation with entrained air than by CO₂ uptake by the vegetation. We quantify this effect by calculating the ratio of the entrainment flux of CO₂ to the surface flux of CO₂(β _c = −() _e /() _o ). A value of β _c equal to 2.9 is estimated at around 1300 UTC from the vertical profile of the carbon dioxide flux. We corroborate this observational evidence by reproducing the observed situation using a mixed layer model. The mixed layer model also yields the variation in time of β _c . During the morning the ventilation process is more important than the CO₂ uptake by the vegetation (β _c > 1), whereas in the afternoon the assimilation by grass at the surface becomes the dominant process (β _c < 1). This research points out the relevance of the entrainment process on the budget of carbon dioxide in the lower troposphere and the relevance of boundary layer dynamics in controlling the diurnal variation of carbon dioxide.