Long-term carbon dioxide fluxes from a very tall tower in a northern forest: Flux measurement methodology

Methodology for determining fluxes of CO2 and H2O vapor with the eddy-covar iance method using data from instruments on a 447-m tower in the forest of northern Wisconsin is addressed. The primary goal of this study is the vali dation of the methods used to determine the net ecosystem exchange of CO2. Two-day least squares fits coupled with 30-day running averages limit calib ration error of infrared gas analyzers for CO2 and H2O signals to approxima te to2%-3%. Sonic anemometers are aligned with local streamlines by fitting a sine function to tilt and wind direction averages, and fitting a third-o rder polynomial to the residual. Lag times are determined by selecting the peak in lagged covariance with an error of approximate to1.5%-2% for CO2 an d approximate to1% for H2O vapor. Theory and a spectral fit method allow de termination of the underestimation in CO2 flux (<5% daytime, <12% nighttime ) and H2O vapor flux (<21%), which is due to spectral degradation induced b y long air-sampling tubes. Scale analysis finds 0.5-h flux averaging period s are sufficient to measure all flux scales at 30-m height, but 1 h is nece ssary at higher levels, and random errors in the flux measurements due to l imited sampling of atmospheric turbulence are fairly large (<approximate to >15%-20% for CO2 and approximate to 20%-40% for H2O vapor at lower levels f or a 1-h period).