Transport of small particles to vertical surfaces

Radioactive particles have been used to study deposition from airstreams on to vertical surfaces. Polystyrene spheres 5 μm in diameter incorporating 51 Cr, tri-cresyl-phosphate droplets 2·1, 1·1 and 0·65 μm in diameter with 32 P activity, and natural Aitken nuclei tagged with ThB ( 212 Pb) were used. The deposition on to smooth surfaces by impaction from turbulent flow was compared with theoretical predictions. Deposition by Brownian motion was also important for the smaller particles, and the results were in accordance with the empirical law relating transport to the two-thirds power of the diffusivity of the particle. When the deposition surface was covered with filter paper it remained aerodynamically smooth, but the deposition of particles was greatly increased since roughness elements too small to disturb the viscous boundary layer of the airflow are nevertheless effective in removing particles from it.