Abstract
The fast response instrument designed to measure simultaneous fluctuations of carbon dioxide and water vapor is described. The measuring frequency was 30 Hz. The sensing path length of the instrument was 20 cm. This is compatible with the path length of the standard type of a sonic anemometer. The noise level was about 0.8ppm for carbon dioxide measurement and about 0.02g/kg for water vapor measurement. The cospectrum estimate of carbon dioxide and vertical wind velocity showed that the high frequency loss due to smoothing effect of sensing path length was about 2 % for wfc r measurement. This denotes that the present instrument is promising for flux measurements of carbon dioxide by the eddy correlation technique in conjunction with a sonic anemomet er. The carbon dioxide flux measured by the eddy correlation technique showed a well defined diurnal variation over a paddy field characterized by the negative values in the daylight hours and positive values in the nighttime. Examination of the stability dependency of eddy diffusivities for carbon dioxide (Xc), sensible heat (Kh), and momentum (Km) showed that the ratio Kh/Kc is unity, but the ratio Kc/Km increases with increasing instability of the air layer as a function of (1—16z/L)1/4.
