Uptake of small particles by tree canopies

A study was undertaken to determine experimentally the deposition of particles on components of different trees and to extrapolate the experimental data so obtained to large-scale canopies by means of a model. The experiments were performed in a wind tunnel allowing canopy components to be exposed to a flow of suspended fluorescent particles of almost uniform size. Emphasis was put on particles in the 0.3- to 1.2-μm subrange, since most of the radioactive particles sampled at long distance from sources are in this size interval. The leaf deposition rates were determined for leaf-bearing twigs of several evergreen species, as a function of wind speed and particle size. The rates obtained were used as input to a model that describes the uptake of particles by a large-scale canopy under specified conditions of weather and canopy structure. Simulations carried out for a typical spruce canopy indicate that the deposition is limited by the strong decrease in wind speed with height in the canopy. The deposition rate to a forested horizontal surface (canopy deposition rate) is between 10−4 and 10−3 m·s−1 depending on wind friction velocity. This rate is practically independent of particle size in the size subrange of interest.