Simulated dry deposition of nitric acid near forest edges

Dry deposition is simulated to understand and generalize observations of enhanced deposition of air pollution near forest edges. Nitric acid is taken as an example as its deposition velocity is often assumed to be determined by turbulent transport only. The simulations are based on the micro-meteorological model of Klaassen (1992). The multi-layer representation of vegetation accounts explicitly for inflow of air at wind exposed forest edges. Simulated dry deposition near a forest edge appears sensitive to the surface resistance. A small but non-zero surface resistance seems most realistic. Edges of coniferous forest may receive more deposition than deciduous forests due to the small resistance of needles. The enhancement of deposition is caused by advection and inflow. Advection influences deposition over large distances, whereas the local edge effect is mainly caused by inflow. Deposition at a forest edge increases with length of the upwind grassland. On the other hand, deposition is decreased behind a forest. The model is used to scale up local observations into landscape averages. The modelled landscape average deposition is used to evaluate three methods which could be applied as boundary condition in large-scale deposition models. Good results are found when average dry deposition is calculated from the average surface roughness.