Research Long-Term Elemental Dry Deposition Fluxes Measured around Lake Michigan with an Automated Dry Deposition Sampler USAMA SHAHIN,† SEUNG-MUK YI,‡ RAJENDRA D. PAODE,§ AND T H O M A S M . H O L S E N * ,† Department of Civil and Environmental Engineering, Clarkson University, Potsdam, New York 13699, Department of Environmental Science and Engineering, Ewha Women’s University, 11-1, Daehyon, Seodaemun, Seoul 120-750, Korea, and Arizona Department of Environmental Quality, 3033 North Central Avenue, Phoenix, Arizona 85012
Long-term measurements of mass and elemental dry deposition (Mg, Al, V, Cr, Mn, Ni, Co, Cu, Zn, As, Sr, Mo, Cd, Sb, Ba, and Pb) were made with an automated dry deposition sampler (Eagle II) containing knife-edge surrogate surfaces during the Lake Michigan Mass Balance/Mass Budget Study. Measurements were made over a roughly 700day period in Chicago, IL; in South Haven and Sleeping Bear Dunes, MI; and over Lake Michigan on the 68th Street drinking water intake cribs from December 1993 to October 1995. Average mass fluxes in Chicago, South Haven, Sleeping Bear Dunes, and the 68th Street crib were 65, 10, 3.6, and 12 mg m-2 day-1, respectively. Primarily crustal elemental fluxes were significantly smaller than the mass fluxes but higher than primarily anthropogenic elemental fluxes. For example, the average elemental flux of Al in Chicago, South Haven, Sleeping Bear Dunes, and the 68th Street crib were 1.0, 0.34, 0.074, and 0.34 mg m-2 day-1, respectively. The average Pb fluxes in Chicago, South Haven, Sleeping Bear Dunes, and the 68th Street crib were 0.038, 0.023, 0.035, and 0.032 mg m-2 day-1, respectively. The measured fluxes at the various sites were used to calculate the dry deposition loadings to the lake. These estimated fluxes were highest for Mg (3800 t/yr) and lowest for Cd (7 t/yr).
Introduction Dry deposition is recognized as a key process by which hazardous air pollutants are deposited to land and water. It may be particularly important near urban/industrial areas adjacent to water bodies where particle concentrations are high and winds can carry these particles, and pollutants associated with them, out over the water where they are deposited. There have been a variety of studies that have attempted to quantify dry deposition (1-4). These studies include both * Corresponding author phone: (315)268-3851; fax: (315)268-7636; e-mail:
[email protected]. † Clarkson University. ‡ Ewha Women’s University. § Arizona Department of Environmental Quality. 10.1021/es9907562 CCC: $19.00 Published on Web 04/04/2000
2000 American Chemical Society
direct measurements and modeled estimates (1, 2). Modeled estimates are typically made by multiplying the measured airborne concentrations by a modeled deposition velocity (3, 4). There are several problems with this approach. These problems occur because trace metals are associated with a wide range of particle sizes, each of which has a different characteristic deposition velocity, and the fact that deposition velocities vary with meteorological conditions and surface characteristics. Direct measurements of dry deposition can be made with aerodynamically designed surrogate surfaces that minimize air stream interruption and thus provide a measure of the minimum dry deposition flux. In addition, dry surrogate surfaces eliminate the effects of relative humidity, wave action, and bubble bursting that leads to enhanced deposition to water surfaces. Measurements made with surrogate surfaces have been shown to agree well with fluxes modeled using a multistep method that divides the aerosols into discrete size intervals and multiplies each size interval by an appropriate deposition velocity (1, 2). Surrogate surfaces can be used for extended periods of time at different locations to provide quantitative information on the temporal and spatial variation in dry deposition (3, 4). Many short-term dry deposition studies have been conducted over the past few years (3, 5-11). However, there have been no long-term direct dry deposition measurements because of the lack of an instrument that could automatically collect dry deposition samples. In this paper, deposition measurements of primarily crustal and anthropogenic metals made as a part of Lake Michigan Mass Budget/Mass Balance Study are presented. These measurements were taken simultaneously with PCB and PAH deposition measurements presented by Franz et al. (12). It is the first attempt to make long-term direct measurements of atmospheric dry deposition fluxes of crustal and anthropogenic metals. The objectives of this research were (i) to develop an automatic dry deposition sampler that could be used in remote areas for long-term sampling programs; (ii) to make long-term dry deposition measurements at several locations around Lake Michigan using the sampler; (iii) to examine the temporal and spatial variations of the deposition flux around Lake Michigan; and (iv) to compare the measured dry deposition flux to other measurements in the literature.
Materials and Methods Samples were collected at four locations around Lake Michigan: Sleeping Bear Dunes, MI (SBD), near South Haven, MI (SH), 68th Street Drinking Water Intake Crib offshore of Chicago, and at Illinois Institute of Technology in Chicago (IIT). Details about these locations are presented elsewhere (1, 2, 12). These samples were collected with Eagle II automatic dry deposition collectors. This sampler contains two deposition plates mounted on arms that are pointed into the wind with a wind vane. These plates are automatically covered when a moisture sensor detects rain or snow and exposed to the atmosphere during dry periods. The sampler is described in more detail elsewhere (12). Samples were collected on carefully cleaned Mylar strips covered with Apezion L grease. Details of sample preparation are described in Paode et al. (1) and in the Illinois Institute of Technology, Air Quality Laboratory Standard Operating Procedures (13). Sampling started in December 1993 and concluded in October 1995. Sampling times extend between VOL. 34, NO. 10, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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4 and 57 days depending on the sampling site and season with an average of 22 days. Analysis. Analytical methods are described elsewhere (1, 2). Briefly, the grease and associated particles from the deposition strips were washed into Teflon containers and digested with an ultrapure nitric acid solution in a microwave oven in a Class 100 clean room at the University of Michigan Air Quality Laboratories. Samples were subsequently analyzed with a Perkin-Elmer 6000 inductively coupled plasma-mass spectrometer (ICP-MS). Quality Assurance/Quality Control. The strict quality assurance and quality control guidelines developed as part of the LMMB are described in refs 1 and 2. These included development of method detection limits (MDLs), analysis of process and field blanks, and measurement of extraction efficiencies using Urban Particulate Matter (NIST 1643). Field blanks were prepared, taken to the field, loaded on the sampler, immediately removed, and returned to the lab where they were analyzed following the same procedures as the regular samples. Field blanks were occasionally slightly higher (
