A COSPECTRAL CORRECTION MODEL FOR MEASUREMENT OF TURBULENT NO2 FLUX

A correction model for eddy correlation flux measurements is developed and applied to nitrogen dioxide flux measurements obtained from a SOLENT sonic anemometer and a Scintrex Luminox LMA-3 analyser for NO2. Four field campaigns were carried out near the village of Merenschwand in Central Switzerland from which two were selected for further analysis in this paper. The need for the correction of measured eddy covariance fluxes arises due to the damping loss of the NO2 analyser at high frequencies. This damping loss is described by an analogy to inductance in an electronical alternating current circuit. The independent variables in the correction model are: 2. (measuring height above zero-plane displacement), ($) over bar u (mean horizontal wind speed), zeta (Monin-Obukhov stability parameter), f (natural frequency) and inductance L. The value for inductance L can be derived from spectral and cospectral analysis. The theoretical cospectrum of an ideal measurement is taken from Kaimal et al. (1972) and extended with a damping term in order to describe the real measurements of the cospectrum. The inductance L of the LMA-3 with a 0.6 cm teflon aspiration tube of 5 m length lies in the order of 0.30 to 0.35 for the dataset from Merenschwand. With this inductance, a correction factor of 1.17 in August/September 1992 and of 1.18 in May 1993 was determined for the NO2 flux maxima during daytime. The range of the correction factor is 1.05 to 1.31 for the mean daily cycles of both datasets.