Deriving daily carbon fluxes from hourly CO2 mixing ratios measured on the WLEF tall tower: An upscaling methodology

The temporal variation of the CO 2 mixing ratio in the atmosphere at a given height results from several processes, including photosynthesis and respiration of the underlying ecosystems, the vertical mixing of the atmosphere near the surface and in the planetary boundary layer (PBL), and entrainment of the air above the PBL. Theoretically, if all atmospheric processes are modeled accurately, we can estimate the magnitude of ecosystem photosynthesis and respiration from the variations in the measured CO 2 mixing ratio. Through analyzing the CO 2 concentration measured at several heights (30 m, 122 m, and 396 m) on the Wisconsin tall tower, we demonstrate that it is possible to derive the daily carbon flux resulting from CO 2 uptake from hourly CO 2 mixing ratio data. At 30 m, the concentration‐derived daily gross primary productivity (GPP) is well correlated with measured daily GPP derived from flux measurements (r 2 = 0.70), but the former was 20% larger than the latter. The correlation increased considerably for 10‐day averages (r 2 = 0.87). As the variations at lower heights have larger diurnal CO 2 amplitudes, the concentration‐derived GPP is more accurate at lower heights. The footprint distance of CO 2 concentration during the daytime under the influence of the mixed layer is estimated to be of the order of 10 3 km, or a footprint area of 10 3 –10 4 km 2 , which is much larger than that of CO 2 fluxes measured using eddy covariance methods (typically 1 km 2 ). The difference in these footprint areas may partly explain the differences between these two flux estimates at the Wisconsin tower. These differences also signify the importance of retrieving flux information from the mixing ratio as it provides a means to upscale from local sites to a region.