Environ. Sci. Technol. 1999, 33, 426-434
Measurement of Dry Deposition and Air-Water Exchange of Polycyclic Aromatic Hydrocarbons with the Water Surface Sampler MUSTAFA ODABASI,† AYSUN SOFUOGLU,‡ NEDIM VARDAR,‡ YUCEL TASDEMIR,§ AND T H O M A S M . H O L S E N * ,‡ Department of Chemical and Environmental Engineering, Illinois Institute of Technology, 10 West 33rd Street, Chicago, Illinois 60616
accepted that the impact of atmospheric deposition of semivolatile organic compounds (SOCs) to the Great Lakes is large (2). Historically, most efforts to determine the loadings of SOCs have monitored the concentrations of these compounds in precipitation (3). Mass balance approaches (4) and application of paired or unpaired air and water concentration measurements with the two-film model have also been used to estimate atmospheric loadings of SOCs to the lakes (2, 5-7). Current dry deposition estimation methods often use measured air concentrations and modeled dry deposition velocities. These models assume the dry deposition flux of particles (Fp) can be estimated by using an overall particle dry deposition velocity (Vp) and particle phase air concentration (Cp):
Fp ) Vp‚Cp
Recent work has shown that dry deposition and gas exchange of semivolatile organic compounds in the Great Lakes is important. However, there is no generally accepted method for collecting particulate dry deposition or gas exchange. In this work the Water Surface Sampler (WSS) was modified and used in combination with dry deposition plates to measure the particulate dry deposition and gas exchange of PAHs. Simultaneous dry deposition and ambient air samples were collected between June and October 1995 in Chicago, IL. Total (particulate + gas) fluxes were measured with the WSS, while particulate fluxes were measured with dry deposition plates. Average Σ14PAH and particulate Σ14-PAH fluxes were 360 ( 146 µg/ m2 d and 144 ( 60 µg/m2 d, respectively. Gas-phase fluxes, determined by subtracting the dry deposition plate fluxes (particulate) from WSS fluxes (particulate + gas), averaged 170 ( 72 µg/m2 d. The measured particulate dry deposition and gas exchange fluxes were substantially higher than those previously reported for the Great Lakes and nonurban areas, probably due to the significantly higher ambient PAH concentrations found in Chicago. Particulate phase overall dry deposition velocities calculated using the dry deposition fluxes and ambient concentrations averaged 6.7 ( 2.8 cm/s. This value is higher than values estimated using dry deposition models; however, it is in good agreement with values determined using similar techniques.
Introduction Dry deposition is the transfer of airborne gases and particles to the earth’s surface, including soil, water, and vegetation, where they are removed (1). The removal rate by dry deposition is a function of the physical and chemical properties of the pollutant, meteorological conditions (temperature, wind speed, atmospheric stability), and surface characteristics. Since the mid-1980s it has been commonly * Corresponding author phone: (315) 268-3851; fax: (315) 2687636; e-mail:
[email protected]. † Current address: Dokuz Eylul University, Faculty of Engineering, Department of Environmental Engineering, Buca, Izmir, Turkey. ‡ Current address: Department of Civil and Environmental Engineering, Box 5710, Clarkson University, Potsdam, NY 13699. § Current address: Uludag University, Faculty of Engineering and Architecture, Department of Environmental Engineering, 16059 Gorukle, Bursa, Turkey. 426
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(1)
To date there has been no consensus on the appropriate dry deposition velocity to use in these types of models. Estimated (2) and experimental (8) dry deposition velocities of SOCs range over an order of magnitude. One of the reasons for this discrepancy is that deposition velocity is a function of particle size. Gravitational settling has a significant effect on the deposition of coarse particles, while Brownian motion dominates the deposition of very fine particles (2.5 µm) and compounds associated with them are responsible for the majority of dry deposition (9-11). There have also been many efforts to measure dry deposition directly. However, there is no generally accepted method for collecting particulate dry deposition or gas exchange. Different kinds of surrogate surfaces including Teflon plates, Petri dishes, dry or diol-coated filters, buckets, pans filled with water, oil-coated glass plates, and greased strips have all been used to measure particle dry deposition (12). Recently, using a water surface sampler (WSS) in combination with a greased surrogate surface, Yi (13) was able to measure both the gas-phase SO2 and particulate sulfate flux. The objectives of this study were to measure the airwater exchange and particulate dry deposition of polycyclic aromatic hydrocarbons (PAHs) and characterize the collection properties of the WSS. In this paper, we report PAH fluxes measured at Chicago, IL in 1995 and compare the particle collection properties of the WSS and dry deposition plates.
Experimental Section Sample Collection. Twelve dry deposition and ambient air samples were collected between June and October 1995. All samples were collected during the daytime when there was no rain. Average sampling time was 6 days (6 × 12 h) for dry deposition samples and 24 h (2 × 12 h) for air samples (3/dry deposition sample). The average sampling volume for air samples was about 140 m3. Samples were taken on a 1.6 m-high platform on the roof of a four-story building (12 m height) located on the campus 10.1021/es9801846 CCC: $18.00
1999 American Chemical Society Published on Web 12/15/1998
FIGURE 1. Water surface sampler (WSS1) and dry deposition plate used to measure total (particle + gas) and particle PAH dry deposition. of Illinois Institute of Technology, 5.6 km south of Chicago’s center and 1.6 km west of Lake Michigan. The sampling site is in a mixed institutional, residential, commercial area and is the urban research and monitoring site of the Integrated Atmospheric Deposition Network (IADN). There are landscaped areas, large parking lots, and low-rise buildings around the sampling site (14). Water Surface Sampler (WSS). Total (particulate + gas) PAH fluxes were measured using an aerodynamically smooth, circular, water collection surface similar to the one used by Yi (13); however, the WSSs used in this study contained a filter (WSS1 only) and a resin column in the water recirculation system and were made of aluminum and stainless steel. The water surface holder has an airfoil shape with a sharp leading edge to minimize airflow disruptions caused by collector geometry (Figure 1). The water surface plate (39.4 cm diameter, 0.65 cm depth) was made of aluminum. It is placed inside the holder at a height which allows the water on the plate to be at the same level as the top of the water surface holder. The WSS has a water replenishing system to maintain a constant water level and control the water retention time on the surface. Water enters the water surface plate from its center and overflows from the triangular weirs located at the sides. The retention time on the water surface is maintained as short as possible (2-4 min) in order to prevent losses of deposited PAHs. The recycled water goes through a filter which collects particles (WSS1 only) and a resin (XAD-2)
column which adsorbs dissolved PAHs before completing a cycle. All tubings and fittings were made of Teflon or stainless steel to minimize adsorption. A 4-L aspirator bottle was used as the water reservoir. The pump was a chemical-resistant (all wetted parts were made of Teflon) adjustable liquid pump. Dry Deposition Plate. The particulate phase dry deposition flux was measured using a smooth greased plate with a sharp leading edge (10 µm) which were not taken into consideration in the dry deposition velocity estimates by Hoff et al. (2) and Doskey and Andren (52). However, results of the study by Holsen et al. (8) indicated that the contribution of coarse particles to the dry deposition fluxes of PCBs was important. Lipiatou et al. (54) also reported that the dry deposition fluxes of PAHs were dominated by the coarse particles (about 50-90%) in the western Mediterranean Sea. The presence of PAHs on coarse particles has been previously reported (24, 33, 48, 49). However, the maximum cut diameter of the instruments used in those studies was 20 µm, and they did not provide information about the quantity of PAHs existing on larger particles (>20 µm). The PS-1 sampler collects particles with diameters smaller than approximately 25 µm (2), and the cut diameter varies depending on the orientation of the sampler with respect to the direction and speed of the wind (55). If there is a considerable amount of PAHs on the particles larger than 25 µm, the calculated deposition velocity will be inflated because the air concentration is underestimated. This inflation would be more important for lower molecular weight PAHs since they are associated with coarse particles to a greater extent than higher molecular weight compounds (24, 33, 48, 49).
Acknowledgments We would like to thank Warawut Suadee for his assistance in air sampling; Kenneth E. Noll and Nasrin R. Khalili of Illinois Institute of Technology for their support during the study.
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Received for review February 24, 1998. Revised manuscript received November 3, 1998. Accepted November 3, 1998. ES9801846
