We analyze the requirements for detecting changes in midlatitude land carbon sources or sinks from sampling the atmospheric CO2 concentration. Programs to sample the continental lower troposphere have only recently started, and it is not yet clear which atmospheric sampling strategy is most adequate. To shed some light on this question, we use simulations of atmospheric CO2 over Eurasia with two regional-scale atmospheric transport models. Our analysis focuses on the detection of the monthly mean CO2 signal caused by a perturbation of Eurasian summer biospheric fluxes by 20% (0.06 PgC/month). The main results are (1) that several measurements per day, preferably during the afternoon, are necessary to permit the detection of the additional land sink and (2) that the ratio between signal and background variation, corrected for autocorrelation in time, suggests no preferred level in the vertical for sampling. However, (3) the signals in the free troposphere are very small (0.2 ppm per 0.06 PgC/month) given the precision of atmospheric measurements. In contrast, signals in the planetary boundary layer (PBL) are on the order of 1 ppm per 0.06 PgC/month. This suggests that optimal sampling on continents should concentrate on the mixed portion of the PBL during afternoon. (4) Finally, the spatial correlation structure of the atmospheric CO2 concentrations suggests that a horizontal sampling density on the order of a few 100 km is needed.