Dry deposition of aerosol particles within model spruce canopies

A wind tunnel experiment is reported in which the dry deposition of a 1-μm MMAD monodisperse silica aerosol to Norway spruce (Picea abies) was investigated. Model canopies were constructed from individual 2- to 3-year-old saplings so as to allow the full development of a turbulent boundary layer within the wind tunnel above the canopy. Deposition velocities (Vg) and interception fractions (f) are presented for both total canopy and for 14 sub-components of the canopy in four horizontal layers. Total canopy Vg was approximately seven times greater than that of a grass sward in the same wind tunnel. The total canopy f value was also higher than that of grass, with a value for spruce of 98%. Both Vg and f values were attenuated in an approximately exponential fashion with vertical penetration downwards into the canopy. A series of two-way analyses of variance revealed significant differences in deposition between needle and stem tissues, and also between vertically and horizontally distinct canopy components. Furthermore, significant interactions between deposition to needle and stem tissues at different heights within the canopy indicated a complex pattern of deposition. It is suggested that the initial capture of the bulk of aerosol deposits on the upper and outer surfaces of individual spruce trees provides the potential for a substantial prompt redistribution of the deposit by resuspension due to wind action.