Reducing the uncertainty of North American carbon flux estimates using an extended atmospheric carbon dioxide measurement network

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 Parameterized Chemistry Tracer Model (PCTM) for atmospheric transport and a modified version of the inversion used by the Atmospheric Tracer Transport Model Intercomparison Project (TransCom) with sub-continental resolution and annual variability of transport. We sub-sample 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 15% to 0.4 Pg yr-1 compared to a network without the tower sites. North American flux is estimated to be a net sink of 1.3 Pg C yr-1, 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. Including the towers reduces covariances between regions within North America. We estimated potential future uncertainty reduction with simulated observations at North American sites that are now or planned to be instrumented for suitable carbon dioxide measurements.;We also tested a micrometeorological adjustment to surface carbon dioxide measurements to approximate mid-continental-boundary-layer measurements. This adjustment can be calculated during mid-day hours using atmospheric measurements commonly available at flux tower sites. The atmospheric transport models used in global atmospheric inversions often do not have sufficient spatial or temporal resolution to capture small-scale variability in the continental boundary layer. We find that using mid-day hours of observations at continental sites, either with or without the micrometeorological adjustment, allows inclusion of continental sites in global atmospheric inversions. Increased continental observation density is necessary for estimating carbon fluxes with finer resolution in space and time.