Atmospheric Environment Vol. 27A, No. 7, pp. 1131-1138, 1993. 0004~6981/93 $6.00+0.00 Printed in Great Britain. Pergamon Press Ltd 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 JuI-MIN LIN, GUOR-CHENG FANG, THOMAS M. HOLSEN and KENNETH E. NOLL* Pritzker Department of Environmental Engineering, Illinois Institute of Technology, 3201 South State Street, Chicago, IL 60616, U.S.A. (First received 16 July 1992 and in final form 1 October 1992) Abstract--For 11 sampling periods, atmospheric particle and elemental (Pb, Ca) mass size distributions (0.1-100 #m di~tmeter) 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 element, tended to be associated with the fine- particle mode ( < 2.5 #m diameter); Ca, a primarily crustal element, was associated with the coarse particle mode ( > 2.5 #m 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 #m 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. Key word index: Dry deposition, lead, calcium, size distribution, dry deposition velocity, deposition model. INTRODUCTION Dry deposition is an important pathway for the transfer of pollutants from the atmosphere to natural surfaces. An accurate measurement of this pathway is critical in understanding pollutant movement in the environment. However, there is no generally accept- able technology for sampling and analysing dry de- position flux (Sehmel, 1980; Davidson et al., 1985). The quantification of dry deposition flux is difficult be- cause of large spatial and temporal variations and because most measurement methods do not simulate deposition to natural surfaces. Historically, modeling studies to predict dry depos- ition have used estimated deposition velocities or the deposition velocity of the mass median diameter (MMD) of the collected particles in conjunction with ambient concentrations. Recent work (Dolske and Gatz, 1985; Arimoto and Duce, 1986; Dulac et al., 1989; Holsen and Noll, 1992) shows that particles associated with.both the fine (< 2.5 gm diameter) and coarse (> 2.5/~m diameter) modes are responsible for deposition and that calculated fluxes should be calcu- lated by summing the partial fluxes from several classes. In this study, measured atmospheric particle size distributions were used with a particle dry depos- * To whom correspondence should be addressed. ition flux calculation method to calculate particle dry deposition fluxes for comparison with measured fluxes to a greased, smooth surface. The use of this surface allowed comparisons to be made between modeled and measured data because it can be used to assess deposited material directly. MATERIALSAND METHODS Dry deposition The dry deposition plate used in this study (Noll et al., t988) is similar to those used in wind tunnel studies (McCready, 1986). It was made of polyvinyl chloride (PVC) and is 21.5 cm long, 7.6 cm wide and 0.65 cm thick, with a sharp leading edge ( < 10 ° angle) that is pointed into the wind by a wind vane. Each of three plates was covered on top with four mylar strips (7.6 x 2.5 cm) and was separated by 46 cm (horizontally) to prevent sample interaction. Each strip was coated with approximately 8 mg of Apezion L grease (thick- ness 8 #m) to collect impacted particles (123 cm 2 total ex- posed surface). In this study, 33 duplicate plates (with 132 mylar strips) that were exposed to the atmosphere during the 11 different sampling periods had statistically the same weight at the 95% confidence level. Ambient particle concentration and size distribution Atmospheric particle concentrations were measured with both a Noll Rotary Impactor (NRI; Noll and Fang, 1986) and Anderson 1 ACFM (actual cubic feet per minute) non-viable ambient particle sizing sampler (AAPSS), with preseparator, 1131