A COMPARISON OF DRY DEPOSITION MODELED FROM SIZE DISTRIBUTION DATA AND MEASURED WITH A SMOOTH SURFACE FOR TOTAL PARTICLE MASS, LEAD AND CALCIUM IN CHICAGO

For 11 sampling periods, atmospheric particle and elemental (Pb, Ca) mass size distributions (0.1-100 mum diameter) were measured with a Noll Rotary Impactor (NRI) and cascade impactor in Chicago, Illinois. The NRI and cascade impactor measurements were continuous; there was no displacement in the bimodal size distributions. Lead, a primarily anthropogenic clement, tended to be associated with the fine-particle mode (< 2.5 mum diameter); Ca, a primarily crustal element, was associated with the coarse particle mode (> 2.5 mum diameter). Atmospheric dry deposition fluxes were simultaneously measured with a specially designed and constructed smooth surface pointed into the wind. A particle dry deposition velocity model was used in conjunction with the measured size distributions to calculate dry deposition fluxes, which were then compared to the measured fluxes. The method, which combined a 12-step flux calculation with the particle dry deposition velocity model, agreed with the measured flux data to within a factor of two. The modeled cumulative fluxes show that fine particles are responsible for only a small fraction of the dry deposition flux. The per cent of the modeled flux due to particles less than 2.5 mum was 0.06,0.5 and 0.06% for particle mass, Pb and Ca, respectively. The results indicate that atmospheric dry deposition is dominated by coarse particles due to their high deposition velocities.