Eddy covariance (EC) measurements of CO2 are widely used to assess the CO2 budget of ecosystems. However, important uncertainties exist about how to obtain flux values that are representative for the regional scale from local-scale EC measurements. This paper addresses the issues of nocturnal CO2 advection in non-flat topography, and the representativeness of the local clover-rich grass surface for the larger region, at the end of the growing season. Local EC measurements were compared with the fluxes obtained from a boundary-layer budget method (BLB) which employed tethered balloon measured vertical profiles of CO2 concentration in the lowest few hundred meters of the atmosphere. It was found that the diurnal change in CO2 concentration was restricted to the lowest ≈450 m of the atmosphere.