Eddy Covariance Measurements with Closed-Path Optical Humidity Sensors: A Feasible Concept?

Humidity spectra obtained with a closed-path optical hygrometer (LI-6262) show an apparent low-pass characteristic with a cutoff frequency varying between 0.01 and 0.1 Hz. Laboratory measurements revealed that contamination of 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 function has a nonlinear component with the consequence that the cutoff frequency is a function of the humidity variance. The uncontaminated “lifetime” of the intake filter depends on the particular air quality and may be less than 24 h. The usefulness of closed-path hygrometers for long-term eddy covariance measurements of humidity is therefore questioned.