Dry Deposition Monitoring of SO 2 , NH 3 and NO 2 over a Coniferous Forest

Since November 1992 vertical concentration gradients of SO2, NO2 and NH3 as well as relevant meteorological parameters have been continuously measured at a Douglas fir forest site in the Netherlands. It was found that for SO2 and NH3 routine application of the gradient method is possible and annual average fluxes can be estimated. Routine application for NO2 is not yet possible because of frequent instrumental failure and because the influence of chemical reactions on the fluxes can not yet be quantified. Eddy correlation measurements of u* and H at 30 m height by TNO and those at 36 m height were compared to evaluate the constant flux layer assumption. It is obvious from these measurements that during this month the constant flux layer assumption for momentum and sensible heat fluxes was valid. Flux profile relationships for heat and ozone were derived from simultaneous eddy correlation and gradient measurements by TNO. Conventional relationships can be applied, provided a correction is introduced. From the results of the hourly average measurements, dry deposition parameters have been derived. Surface resistances for SO2 and NH3 show strong diurnal variations during dry conditions. Then lowest values occur during the daytime. When the leaf surface is wet, R c values are generally low and independent on the time of the day. An R c parameterization for SO2 derived from analogous measurements over a heathland was tested at the forest site. Good agreement between measured and modelled dry deposition velocities was obtained with no systematic deviations. A first order surface resistance parameterization for NH3 has been derived from the measurements at Speulder forest. When the NH3 flux is directed towards the vegetation, there is reasonable agreement between deposition velocities obtained from the parameterisation and from the measurements. However, when NH3 is emitted (~ 20 % of the time), modelled and ‘measured’ V d can differ considerably.