Dry Deposition of Semivolatile Organic Compounds to Lake Michigan

Abstract

The Lake Michigan Urban Air Toxics Study (LMUATS) was carried out from July 8 to August 9, 1991, in the southern Lake Michigan basin. The investigation was the first aimed at assessing the impact of the Chicago/Gary urban plume on the deposition of hazardous air pollutants (HAPs) to Lake Michigan. The concentrations of a large number of semivolatile organic compounds (SOCs) were measured at four locations (Kankakee, IL; Chicago, IL; over Lake Michigan onboard the R/V Laurentian; and South Haven, MI). These sites were chosen as they are often linked by air mass transport with air masses reaching South Haven, after passing through Chicago, often having an over-water path to the downwind site. The University of Michigan R/V lawentianwas also deployed off-shore of Chicago to measure the concentrations of HAPs as they were transported out over the water. A hybrid-receptor deposition (HRD) modeling approach, which utilized measured atmospheric concentrations at a receptor site together with observed meteorological data in a lagrangian dispersion modeling framework, was used to estimate both gas exchange across the air-water interface and particle dry deposition of SOCs to Lake Michigan. The atmospheric concentrations of selected pesticides, PCBs, and PAHs measured during the LMUATS were used as input to the HRD model. The comparison between the measured and calculated ambient concentrations gives reasonable results with measured/calculated concentration ratios in the range of 0.3-1.9 for pesticides, 0.9-2.6 for PCBs, and 0.5-3.8 for PAHs. A comparison of deposition fluxes obtained in this study with those found in literature reveals large differences for some compounds, especially for those that are primarily in the particle phase. This finding suggests that assuming constant values of critical parameters controlling the transfer processes of gaseous and particulate SOCs from the atmosphere to the water surface may result in estimates with large uncertainties. MethodsSample Collection. Ambient air samples were collected in Chicago, IL, on the roof of Farr Hall, a 4-story building on the campus of Illinois Institute of Technology (IIT); in South Haven, MI, a rural location 3 km from the eastern side of Lake Michigan; and onboard the RIV Laurentian located

Mass transfer coefficients for polycyclic aromatic hydrocarbons (PAHs) to the water surface sampler:

“…Paired or unpaired air and water concentration measurements in conjunction with the two-"lm model (Schwarzenbach et al, 1993) have been commonly used to estimate atmospheric loading of SOCs to water bodies (Baker and Eisenreich, 1990;Hornbuckle et al, 1994;Ho! et al, 1996;Pirrone et al, 1995;Erickson, 1987;Wanninkhof, 1992;Gustafson and Dickhut, 1997;McConnell et al, 1997). The mass balance approach has also been used to make estimates of air}water gas exchange (McVeety and Hites, 1988).…”

“…According to the two-"lm model, mass transfer is limited by the rate of molecular di!usion through thin "lms of air and water on either side of the surface (Schwarzenbach et al, 1993). The net #ux (F , ngm\ d\ ) is driven by the fugacity di!erence between air and surface water.…”

“…Due to these factors, "eld and laboratory data often show large discrepancies. Generally, laboratory experiments tend to overestimate gas exchange rates that occur in natural environments (Schwarzenbach et al, 1993;Mackay and Yeun, 1983). It has been shown that natural organic materials can coat the air}water interface of natural waters and inhibit gas transfer.…”

“…Mass transfer coe$cients of water vapor, oxygen (O ) and carbon dioxide (CO ) have been related to wind speed by many researchers (Schwarzenbach et al, 1993;Wanninkhof, 1992;Mackay and Yeun, 1983;Thibodeaux, 1979). O and CO are commonly used as reference substances to measure k since their gas exchange depends only on liquid phase resistance.…”

“…Eqs. (6) (Atlas et al, 1982) and (7) (Schwarzenbach et al, 1993) have been recommended to adjust individual mass transfer coe$cients:…”