Environ. Sci. Technol. 3993, 27,1327-1333
Dry Deposition and Particle Size Distributions Measured during the Lake Michigan Urban Air Toxics Study Thomas M. Holsen,. Kenneth E. Noli, Guor-Cheng Fang, WenJhy Lee, and Jul-Mln Lln
Pritzker Department of Environmental Engineering, Illinois Institute of Technology, Chicago, Illinols 606 16 Gerald J. Keeler
Department of Environmental and Industrial Health, The University of Michigan, Ann Arbor, Michigan 48109 The mass and elemental dry depositional flux was measured in Chicago, IL, in South Haven, MI, and over Lake Michigan onboard the R f V Laurentian during the Lake Michigan Urban Air Toxics Study. The average measured mass flux in Chicago (130 mg/m2d) was higher and more variable than at either South Haven (40 mg/m2d) or over Lake Michigan (27 mg/m2 d). The flux of the crustal elements (Al, Ca, Fe, Mg, Si, and Ti) in Chicago were 2-3 times higher than those in South Haven and 3-4 times higher than those measured over Lake Michigan. The flux of primarily anthropogenic metals (Cd, Cr, Cu, Mn, Pb, V, and Zn) was on average 1-2 orders of magnitude lower than the flux of primarily crustal elements at all three sites. A modeling procedure that used measured atmospheric particle size distributions and modeled deposition velocities was used to calculate the dry deposition flux for comparison to the measured flux data. The average ratio of calculatedlmeasured fluxes for mass and the 13 elements was 1.4. In general, the flux in Chicago was slightly underestimated, and the flux in South Haven was slightly overestimated. Modeling results indicate that the majority of the flux (>98%) was due to particles >6.5 pm in size. A comparison of simultaneously measured dry depositional flux and the concentration of airborne particulate matter 10 pm, to dry deposition (2-51, and as a consequence, many estimates of dry deposition may be underestimated. Direct measurements of the dry deposition flux with a surrogate surface is a technique which can be used to obtain 0013-936X/93/0927-1327$04.00/0
0 1993 Amerlcan Chemical Society
data to compare to indirect estimation techniques. Measurements made with a smooth surrogate surface with a sharp leading edge have been shown to agree well with results of modeled dry deposition in an urban area if complete atmospheric size distributions are measured (4). Since dry deposition into the Great Lakes is important, these types of measurements need to be taken directly over the lake because little is known about how differences in meteorology and deposition velocities over land and water bodies effect dry deposition. Some models attempt to account for a possible increase in particle deposition rates due to high relative humidities near the airfwater interface (6),and others account for the effects of spray formation and high wind speed (7). A more recent model combined these two processes and also included the variation of turbulent transfer with wind speed, airfwater temperature difference, and surface roughness (8). It is well-known that the atmospheric boundary layer over lakes and land can be very different because the lake surfaces heat and cool more slowly than the surrounding land surface. In spring and summer, the lakes are typically cooler than the land, and a very stable air layer over the lake develops limiting vertical movement. In autumn and winter, the opposite is common, and the lower air layer becomes unstable enhancing vertical mixing. In addition, wind speeds tend to be higher over water bodies decreasing transit times and decreasing total deposition to the lake (9).
This paper compares measurements of the dry deposition and airborne concentrations of 13trace elements in Chicago, IL, in South Haven, MI, and aboard a ship (the R1 V Laurentian) which was stationed on Lake Michigan for 2 weeks in 1991 during the Lake Michigan Urban Air Toxics Study. These measurements are compared to modeled dry deposition estimates which used atmospheric size distributions and modeled deposition velocities.
Experimental Section Description of Study Area. Chicago. Samples were taken on a 1.6-m-high platform on the roof of Farr Hall, a four-story (12 m height) building located in a mixed institutional, commercial, and residential area on the campus of the Illinois Institute of Technology (IIT), 5.6 km south of Chicago’s center and 1.6 km west of Lake Michigan (Figure 1). The IIT campus consists of predominantly low-rise buildings, landscaped areas, and asphalt parking lots. R1 V Laurentian. Samples were also taken aboard the University of Michigan research vessel Laurentian a t two main stations offshore of Chicago between 6 and 12 km from land. The samplers were placed on the bow of the vessel at a height of 1.5 m off the deck (approximately 5 Environ. Sci. Technol., Vol. 27, No. 7, 1993 1327
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1. Map of sampling locallons.
m above the water). The ship was anchored while on station and pointed into the prevailing wind at all times. South Haven, MI. A large platform 1.2 m high was built in a wide-open field on the Barden Farm, which is 3.6 km from Lake Michigan. The farm is surrounded by rolling fields and orchards. Dry Deposition. Thedrydepositionfluxwasmeasured in Chicago, IL, and South Haven, MI, and onboard the R/V Laurentian using a smooth surface with a sharp leading edge, mounted on a wind vane (IO). The plate used in this study was similar to those used in wind tunnel studies ( 1 1 ) . It was made of polyvinyl chloride (PVC) and is 21.5 cm long, 7.6 ern wide, and 0.65 cm thick with a sharp leading edge (10 (preseparator), 10-9,9-5.8,5.& 4.7,4.7-3.3,3.3-2.1,2.1-1.1,1.1-0.65,0.65-0.43, and
