We evaluate North American carbon fluxes using a monthly global Bayesian synthesis inversion that includes well-calibrated carbon dioxide concentrations measured at continental flux towers. We employ the NASA Parametrized Chemistry Tracer Model (PCTM) for atmospheric transport and a TransCom-style inversion with subcontinental resolution. We subsample carbon dioxide time series at four North American flux tower sites for mid-day hours to ensure sampling of a deep, well-mixed atmospheric boundary layer. The addition of these flux tower sites to a global network reduces North America mean annual flux uncertainty for 2001–2003 by 20% to 0.4 Pg C yr−1 compared to a network without the tower sites. North American flux is estimated to be a net sink of 1.2 ± 0.4 Pg C yr−1 which is within the uncertainty bounds of the result without the towers. Uncertainty reduction is found to be local to the regions within North America where the flux towers are located, and including the towers reduces covariances between regions within North America. Mid-day carbon dioxide observations from flux towers provide a viable means of increasing continental observation density and reducing the uncertainty of regional carbon flux estimates in atmospheric inversions.