An experimental study of the dry deposition mechanism for airborne dust

A study of dry deposition of the coarse mode of airborne dust on mirror surfaces has been performed at Sede Boqer, Israel, as a part of a research program to protect the reflective surfaces of solar concentrating collectors. In particular, the angular dependence of the deposition rate was measured in order to separate between its diffusive (isotropic) and vertical components. Dust was collected on a flat sampler with changeable slope, mounted on a wind vane. No deviation from a cosine-type angular dependence was found for dust particles larger than 4 μm. This result is interpreted as domination, in the vicinity of the collecting surface, of vertically directed particle flux over diffusive components. This conclusion was compared to the results of existing dry-deposition models. The size distribution of particles, collected on the flat horizontal surface, was converted to a volume-mass concentration using calculated sedimentation velocity as the only component of deposition velocity. The resulting value was confirmed by our control measurements of the volume-mass concentration. Our results are also compared with recent data on deposition velocities, published by Lin et al. (1994).