Eddy covariance measurements with closed-path optical humidity sensors: A feasible concept?

Humidity spectra obtained with a closed-path optical hygrometer (LI-6262) s how an apparent low-pass characteristic with a cutoff frequency varying bet ween 0.01 and 0.1 Hz. Laboratory measurements revealed that contamination o f the intake filter is responsible for the damping of high frequencies. It was further found that the effect of the intake filter on the response func tion has a nonlinear component with the consequence that the cutoff frequen cy is a function of the humidity variance. The uncontaminated "lifetime'' o f the intake filter depends on the particular air quality and may be less t han 24 h. The usefulness of closed-path hygrometers for long-term eddy cova riance measurements of humidity is therefore questioned.