DRY DEPOSITION OF TRACE-ELEMENTS TO LAKE-MICHIGAN - A HYBRID-RECEPTOR DEPOSITION MODELING APPROACH

A hybrid-receptor deposition modeling approach was utilized to estimate the dry deposition flux of size segregated particles to Lake Michigan during the Lake Michigan Urban Air Toxics Study (LMUATS) from July 8 to August 9, 1991. The model estimated the flux of trace elements associated with particles in the fine (less than or equal to 2.5 mu m) and PM10-coarse (2.5-10 mu m) size range to Lake Michigan using ambient concentration measurements. The ensemble average deposition velocities for fine particles traversing Lake Michigan were found to vary considerably, by factors of 3-15. The average deposition velocities for PM10-coarse particles vary by factors of <1-3. The ratios of observed to estimated ambient concentrations ranged from 0.75 to 2 for fine fraction elements and from 0.8 to 3.6 for elements in the PM10-coarse fraction The ratios of the estimated deposition fluxes of Ca and Pb with those measured by surrogate surfaces over Lake Michigan during the same period ranged from 0.75 to 1 for the fine fraction and from 1.4 to 2.2 for the PM10-coarse fraction. Si (517 x 10(3) kg) had the highest deposition fur among trace elements primarily of soil or crustal origin, followed by Ca (250 x 10(3) kg), Fe (208 x 10(3) kg), Al (143 x 10(3) kg), and K (73 x 10(3) kg), Trace elements primarily of anthropogenic origin had the following deposition fluxes: S, 53 x 10(3) kg; Zn, 3.8 x 10(3) kg; Mn, 2.4 x 10(3) kg; Cu, 2.2 x 10(3) kg; Pb, 1.01 x 10(3) kg; Se, 1.0 x 10(3) kg; Ni, 0.86 x 10(3) kg; Cr, 0.5 x 10(3) kg; and Br, 0.35 x 10(3) kg. This paper discusses the effect of micrometeorological parameters, particle size distributions, water wave dynamics, and the type and location of measurement sites on the estimate of dry deposition fluxes.