Estimating daytime CO2 fluxes over a mixed forest from tall tower mixing ratio measurements

[1] Difficulties in estimating terrestrial ecosystem CO2 fluxes on regional scales have significantly limited our understanding of the global carbon cycle. This paper presents an effort to estimate daytime CO2 fluxes over a forested region on the scale of 50 km in northern Wisconsin, USA, using the tall-tower-based mixed layer (ML) budget method. Budget calculations were conducted for 2 years under fair-weather conditions as a case study. With long-term measurements of CO2 mixing ratio at a 447-m-tall tower, daytime regional CO2 fluxes were estimated on the seasonal scale, longer than in earlier studies. The flux derived from the budget method was intermediate among those derived from the eddy-covariance (EC) method at three towers in the region and overall closest to that derived from EC measurements at 396 m of the tall tower. The dormant season average daytime-integrated regional CO2 flux was about 0.35 +/- 0.18 gC m(-2). During the growing season, the monthly averaged daytime-integrated regional CO2 flux varied from -1.58 +/- 0.19 to -4.15 +/- 0.32 gC m(-2), suggesting that the region was a net sink of CO2 in the daytime. We also discussed the effects on theses estimates of neglecting horizontal advection, selecting for fair-weather conditions, and using single-location measurements. Daytime regional CO2 flux estimates from the ML budget method were comparable to those from three aggregation experiments. Differences in results from the different methods, however, suggest that more constraints are needed to estimate regional fluxes with more confidence. Despite uncertainties, our analyses indicate that it is feasible to estimate daytime regional CO2 fluxes on long timescales using tall tower measurements.