DRY DEPOSITION VELOCITIES AS A FUNCTION OF PARTICLE-SIZE IN THE AMBIENT ATMOSPHERE

The atmospheric particle mass size distribution (0.1-100 mum) and dry deposition flux were measured simultaneously with a wide range aerosol classifier (WRAC) and a smooth greased surface. Microscopic techniques were used to size the particles collected on the deposition surface and generate mass size distributions of deposited particles. All the depositional mass size distributions peaked (interval with the largest mass) between 30 and 100 mum in diameter. Deposition velocities were calculated bvu dividing the size segregated particle flux by the airborne particle concentration for each of the 10 WRAC stage intervals. Experimentally determined dry deposition velocities for atmospheric particles in the size range of 5-80 mum in diameter were greater than predictions made with the Sehmel-Hodgson deposition velocity model developed from wind tunnel experiments, particularly at higher wind speeds. A multistep method was used to calculate total and cumulative deposition fluxes with the Sehmel-Hodgson model. Calculated and measured fluxes were within 10% for low wind speeds but differed by up to 50% for higher wind speeds. The results also show that particles > 10 mum in diameter dominate atmospheric dry deposition.