How Many Observation Sites Does it Take to Constrain the North American Carbon Flux?

The objective of this study is to characterize North American carbon flux at regional spatial resolution and synoptic-to-seasonal temporal resolution to inform our mechanistic understanding of the underlying carbon cycle dynamics. Some success has been achieved at identifying local carbon cycle mechanisms and dynamics, but the heterogeneity of the landscape confounds bottom-up attempts to aggregate site observations to larger spatial scales. On the other hand, global inversions of atmospheric observations can resolve continent- and ocean basin-scale fluxes on monthly scales, but fail at higher resolutions (both spatially and temporally) without the use of additional assumptions and constraints. This study addresses the constraint problem by complementing the traditional, primarily marine boundary layer, global carbon dioxide measurement network with high-precision, well-calibrated carbon dioxide mixing ratio observations at North American flux towers. Mid-day observations (when the atmospheric boundary layer is likely to be well-mixed at these continental sites) and virtual tall tower estimates of mid-day mid-mixed layer mixing ratios are tested. A global Bayesian synthesis inversion (a TransCom-like solution) for the year 2002 is constructed to estimate carbon fluxes for 10 regions in North America. Basis functions are from forward runs of monthly tracers using the NASA PCTM tracer transport model with Goddard Modeling and Assimilation Office GEOS-4 meteorological fields appropriate to the inversion time period. Several versions of the inversion are designed to explore the impact of different temporal resolutions and the inclusion of subsets of the flux tower network: flux towers with well-calibrated carbon dioxide observations in 2002, modeled data from flux tower sites active or planned post-2002, and modeled data from other sites in the North American flux tower network. Since the equipment costs required to achieve the necessary precision and calibration in flux tower carbon dioxide measurements is relatively modest (~ $10K), an expansion of the number of sites collecting such data is quite feasible.