MEASUREMENTS OF DRY DEPOSITION OF NO2 TO A DUTCH HEATHLAND USING THE EDDY-CORRELATION TECHNIQUE

Nitrogen dioxide (NO2) fluxes were determined over a dry heather moor by the eddy-correlation technique using a sonic anemometer and two Scintrex LMA-3 chemiluminescent analysers. Using power spectral analysis virtually all the fluctuations in NO2 concentration were of frequencies less than 1 Hz at the measuring height of 5.32 m. Even with the non-ideal response of the LMA-3 little information about NO2 concentration was lost. Co-spectral analysis showed there was no loss of flux at high frequencies arising from the slow response of the LMA-3. NO2 was deposited at a constant rate of 0.02-mu-g m-2 s-1 for nearly all of the measuring period. The deposition velocity maintained a mean value of 1 mm s-1 throughout with minimum values nearing zero and maximum values of 3.5 mm s-1. There was no diurnal variation in canopy resistances to NO2. Mean values were 548 s m-1 and 400 s m-1 with and without the inclusion of several brief periods when the resistance increased markedly. Except for a brief period on 1 May the deposition was to the leaf cuticle as temperature and water stress caused the stomata to be closed. During the afternoon of 1 May there was some stomatal activity when the canopy resistance to water vapour fell and NO2 flux increased. Finally a simulation of the cumulative sampling method was performed on the data set as described in Businger and Oncley (1990). They calculated a constant of proportionality, b, and showed it to have a value of 0.6 when considering heat and water-vapour fluxes. A value of b was calculated for NO2 flux using the data set presented here and was found to be 0.58.