Coastal Lake Michigan

New Brunswick, New Jersey 08903-0231. Air concentrations of PCBs and PAHs were measured in the urban/industrial complex of Chicago, IL, over southern...
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Environ. Sci. Technol. 1997, 31, 2141-2147

Urban Contamination of the Chicago/Coastal Lake Michigan Atmosphere by PCBs and PAHs during AEOLOS MATT F. SIMCIK, HUIXIANG ZHANG, STEVEN J. EISENREICH,* AND THOMAS P. FRANZ Department of Environmental Sciences, RutgerssThe State University of New Jersey, College Farm Road, New Brunswick, New Jersey 08903-0231

Air concentrations of PCBs and PAHs were measured in the urban/industrial complex of Chicago, IL, over southern Lake Michigan, and in a non-urban area as part of the AEOLOS (Atmospheric Exchange Over Lakes and Oceans) Project. Air samples were collected simultaneously in intensive experiments during May and July 1994 and January 1995 in order to determine the influence of contaminated air masses from the Chicago urban/industrial complex on the southern Lake Michigan atmosphere. Gas phase ∑PAH concentrations over the lake ∼10-20 km offshore ranged from 0.8 to 70 ng/m3 while urban air concentrations were 27-430 ng/m3. Photooxidation of gas phase PAHs during the day resulted in daytime over-lake concentrations that were ∼75% less than corresponding nighttime concentrations. Gas phase ∑PCB concentrations ranged from 0.14 to 1.1 ng/m3 over the lake and from 0.27 to 14 ng/m3 in the urban area. ∑PCB concentrations varied seasonally as a result of higher volatilization during the summer. The highest concentrations occurred when the air flow was from the urban/industrial area encompassing Evanston, IL, to Gary, IN, toward the lake, and concentrations were near regional background when the wind was from any other direction. The urban air emissions increased the average coastal atmospheric concentrations above the continental background signal by factors of 12 and 4 for ∑PAHs and ∑PCBs, respectively. Because of photooxidation of gas phase PAHs during the day, the average daytime concentration was increased by only a factor of 5 while the average nighttime concentration was increased by a factor of 18.

Introduction Atmospheric deposition is an important and sometimes dominant contributor to the loading of semivolatile organic contaminants (SOCs) to the Great Lakes (1-5). The regional atmosphere in turn receives a large amount of its contaminants from urban/industrial areas. Therefore, large urban/ industrial areas are also major sources of contamination to adjacent Great Waters via the atmosphere (5-9). This work is part of the AEOLOS Project (Atmospheric Exchange Over Lakes and Oceans), whose hypothesis is that emissions of hazardous air pollutants (HAPs) into the coastal urban atmosphere enhances atmospheric depositional fluxes to * Author to whom correspondence should be addressed; e-mail: [email protected]; fax: (908) 932-8644.

S0013-936X(96)00976-5 CCC: $14.00

 1997 American Chemical Society

adjacent Great Waters such as Lake Michigan off Chicago, IL/Gary, IN, and Chesapeake Bay off Baltimore, MD. This paper focuses on the increased coastal atmospheric concentrations of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) in southern Lake Michigan as a result of urban and industrial emissions in the Chicago area. PAHs are two to eight ring compounds formed primarily during incomplete combustion of fossil fuels and wood. Major sources of PAHs to urban atmospheres can include automotive emissions, oil and gas used in home heating, municipal incinerators, and coal combustion used for power, heating, and the steel industry. The large metropolitan area surrounding Chicago, IL, is criss-crossed with interstate highways, and there is a large volume of automobile traffic associated with the metropolitan area of approximately 8 million people. There are also numerous steel and coke mills concentrated in an area extending from southeast Chicago, IL, to Gary, IN. High levels of PAHs (9-11) have been previously measured in these areas. Simcik et al. (5) argue that PAHs emitted into the local atmosphere is the major source of contamination to the sediments of Lake Michigan (5). The major source of PCBs to the atmosphere is volatilization from sites where they have been disposed or stored and incineration of PCB-containing materials. The Chicago atmosphere has been historically contaminated with PCBs (7, 9, 10, 12). When the prevailing winds are from the southwest, these contaminated air masses are transported out over the lake. This paper addresses the concentration gradients of SOCs from the urban to the coastal lake Michigan atmosphere whereas future papers address enhanced atmospheric deposition and sources.

Sampling and Site Characterization High-volume air samples were taken simultaneously at two sites during intensive experiments in May and July 1994 and January 1995. One site was located on the south side of Chicago on the roof of a building on the campus of Illinois Institute of Technology (IIT) approximately 5.6 km south of the center of Chicago and 1.6 km from the shore of Lake Michigan (41°50′04′′ N, 87°37′29′′ W). The other was aboard the EPA’s RV Lake Guardian at one of the following three stations: site 5, 30 km NE of IIT and 18 km from shore (42°00′00′′ N, 87°25′00′′ W); site 1, 30 km ESE of IIT and 10 km from shore (41°46′00′′ N, 87°20′00′′ W); or site 0, 33 km SE of IIT and 5 km from shore (41°40′00′′ N, 87°22′00′′ W). The ship was held on station by a single bow anchor to maintain the ship’s position into the wind. The samplers were positioned on a yardarm that extended out over the starboard side of the ship to obtain samples ∼10 m above the surface of the water. A third site located in South Haven, MI, was employed during the July intensive. The South Haven site was located on a farm 3.6 km from the eastern shore of Lake Michigan approximately 125 km northeast of Chicago, IL (Figure 1). Consecutive 12-h day and night samples were taken from ∼08:00 h to ∼20:00 h and from ∼20:00 h to ∼08:00 h during each intensive experiment. The samplers were operated at 0.5 to 0.8 m3/min and calibrated frequently to ensure constant flow rates over the sampling period. Samples were taken using Graseby GMW (Cleaves, OH) high-volume air samplers equipped with a glass fiber filter (GFF) to collect particulate PCBs and PAHs and a polyurethane foam (PUF) plug to collect the gas phase. Meteorological data (temperature, wind direction, and wind speed) were recorded every hour at the land-based sites and every 5 min on the ship. The 12-h averages of meteorlogical parameters for each sample taken are summarized in Table 1.

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FIGURE 1. Map of Lake Michigan indicating the urban sampling site (IIT), the three EPA Master stations (0, 1, and 5), and the rural site (South Haven).

TABLE 1. Summary of Meteorological Data during AEOLOS I-III urbana date

temp (°C)

5/17/94 5/17-18/94 5/18/94 5/18-19/94 7/17-18/94 7/18/94 7/18-19/94 7/19/94 7/19-20/94 7/20/94 7/20-21/94 7/21/94 7/21-22/94 7/22/94 7/22-23/94 7/24/94 7/24-25/94 7/25/94 7/25-26/94 7/26/94 7/26-27/94 7/27/94 7/27-28/94 7/28/94 1/16/95 1/17/95 1/17-18/95 1/18/95 1/18-19/95 1/19/95 1/19-20/95

10 9.1 11 13 24 29 17 25 16 29 22 26 20 25 23 26 23 27 21 22 20 21 19 22 1.3 4.3 4.4 1.9 1.7 3.7 1.9

a

Urban, Chicago, IL.

b

WDd

(deg)

38 54 34 48 282 266 202 185 211 225 220 231 242 272 274 86 247 278 289 85 323 37 23 32 65 168 244 180 46 61 326

over-lakeb WSe

(m/s)

4.1 2.2 2.8 1.9 1.8 3.5 2.3 3.3 3.8 3.8 2.4 3.5 2.5 4.1 2.1 2.2 1.9 5.5 2.0 2.4 2.0 3.5 2.3 2.9 2.3 4.3 3.0 2.7 4.9 4.6 5.2

site

temp (°C)

WD (deg)

WS (m/s)

5 5 5

5.3 5.3 5.9

16 6 7

3.8 2.6 2.7

5 5 5 5 5

22 23 23 29 22

284 146 190 167 196

4.3 5.2 6.5 4.9 5.1

1 1 1 1 1 5 5

23 22 23 25 23 21 24

210 219 247 251 280 124 266

4.7 2.3 1.6 1.8 2.4 5.0 2.4

5 5 1 0 0 0 5 5 5 5

19 19 20 19 20 21 0.8 2.7 2.8 0.6

284 244 351 25 22 22 100 177 257 7

2.6 4.2 5.3 6.0 7.6 6.0 3.7 9.0 7.2 3.8

Over-lake, aboard ship. c Rural, South Haven, MI.

Sample Extraction and Analysis. The GFFs were precombusted at 550 °C for 12 h in loosely wrapped aluminum foil envelopes. The envelopes were sealed and stored at -20 °C until sampling. The PUFs were precleaned by washing with detergent and water and rinsing with warm water, Milli-Q water, and acetone. They were then extracted in a Soxhlet apparatus twice for 24 h with acetone, once for 24 h with petroleum ether, and once for 12 h with 4:1 petroleum ether: dichloromethane. They were then placed in desiccators under

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ruralc

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d

temp

WD (deg)

WS (m/s)

20 25 21 27 24 27 21 24 20 23 20 25 19 24 16 21 17 20 15 21

246 259 171 188 196 225 185 225 211 253 256 277 223 277 234 279 182 153 91 265

1.3 2.2 1.8 2.7 2.6 4.3 2.1 3.9 2.3 4.4 1.6 2.1 1.6 4.1 1.3 2.6 0.9 1.8 1.2 2.6

WD, wind direction. e WS, wind speed.

vacuum to dry and stored in sealed glass jars at 4 °C until sampling. After sampling, the GFFs and PUFs were stored separately wrapped in aluminum foil and stored in sealed plastic bags at -20 °C until analysis. Both GFFs and PUFs were spiked with PCB and PAH surrogate standards prior to extraction to determine analytical recovery. The surrogate standards were made up of 3,5-dichlorobiphenyl (PCB 14), 2,3,5,6-tetrachlorobiphenyl (PCB 65), 2,3,4,4′,5,6-hexachlorobiphenyl (PCB 166), and perdeuterated naphthalene, fluo-

TABLE 2. Summary of ∑PCB and ∑PAH Concentrations at Urban, Over-Lake, and Rural Sites during AEOLOS urban

over-lake ∑PAH (ng/m3)

∑PCB (pg/m3) date

PUFa

GFFb

PUF

5/17/94 5/17-18/94 5/18/94 5/18-19/94 7/17-18/94 7/18/94 7/18-19/94 7/19/94 7/19-20/94 7/20/94 7/20-21/94 7/21/94 7/21-22/94 7/22/94 7/22-23/94 7/24/94 7/24-25/94 7/25/94 7/25-26/94 7/26/94 7/26-27/94 7/27/94 7/27-28/94 7/28/94 1/16/95 1/17/95 1/17-18/95 1/18/95 1/18-19/95 1/19/95 1/19-20/95

510 800 560 810 4300 2500 3960 3190 2910 7600 3110 14200 3060 1510 1680 1260 1710 1270 3490 1320 2260 1090 1720 1020 270 720 710 460 340 650 800

90

120 98 78 170 170 240 430 180 140

130 460 33 11 100 120 10 11

150 120 110 48 160 150 190 140 140 85 86 36 66 45 27 82 48 56 64 99 120

4.6 11 4.3 8.4 12 11 18 79 20 20 12 19 6.7 3.2 24 120 24 37 7.5 4.9 5.2

a

45 81 31 140 100 44 450 37 590 58 60 84 58 180 7.2 43 56 33 190 68

PUF, polyurethane foam adsorbent.

b

GFF

rural

∑PCB (pg/m3)

∑PAH (ng/m3)

∑PCB (pg/m3)

∑PAH (ng/m3)

PUF

GFF

PUF

GFF

PUF

PUF

GFF

242 254 137

1.6 2.1 3.1

10 5.0 4.7

0.5 0.1 0.1

900 950 880 710 850

6.1 6.6 7.1 7.6 8.1

56 13 51 18 35

1.4 2.4 7.4 1.8 2.6

1120 780 980 880 1040 310 520 540

9.1 9.6 10 11 11 13 13 14

13 27

0.4 0.5

15 15 16 16 17 4.5 12 13

9.1 1.4 8.6 3.0 3.1 0.4 1.2 1.7 3.1 0.4 0.1 0.3

1.1 0.4 0.8 1.0 0.2 0.0 1.4 0.6 1.2 0.4

650 210 200 150 130 140 280 230 150

70 14 59 19 58 9.2 43 18 12 4.2 2.9 1.6 0.8 2.8 12 29 16 9.9

55 36 13 22 14 8.7 20 33 25 9.0

14 4.0 16 21

1.4 0.1 0.5 0.2

480 470 500 510 410 690 590 570 580 480 600 430 230 780 330 230 300 410

GFF

2.6 10 2.2 4.6

GFF, Glass fiber filter.

rene, fluoranthene, and perylene. The samples were then extracted in Soxhlet apparatus for 24 h separately; the GFFs in dichloromethane, the PUFs in 4:1 petroleum ether: dichloromethane. Extracts were concentrated to approximately 1 mL in a rotary evaporator (Buchi Model RE 111); the solvents were switched to hexane; and the extract was split, ∼75% for PCB analysis and ∼25% for PAH analysis. The PCB fraction was fractionated on a alumina/silica column (10 g of alumina, 80-200 mesh, activated at 200 °C overnight, and 10% water deactivated; 3 g of silica gel, 60-200 mesh, activated at 300 °C overnight, and 6% water deactivated) to remove any polar organics that might interfere with the analysis. The column was eluted with 60 mL of hexane followed by 60 mL of 10% (v/v) diethyl ether in hexane. The first fraction contains all of the PCBs, and the second fraction contains some chlorinated pesticides. The samples were again concentrated to 4 mL in a rotary evaporator and spiked with an internal standard made up of 2,4,6-trichlorobiphenyl (PCB 30), and 2,2′,3,4,4′,5,6,6′-octachlorobiphenyl (PCB 204). The PCBs were analyzed on a Hewlett Packard 5890 gas chromatograph equipped with a 63Ni electron capture detector. The PAH fraction was cleaned up using a silicic acid microcolumn to remove polar compounds that may interfere with the analysis. The procedure is a modification of that used by Liu (11) using 1 g of Sigma Sil-A-200, 60-200 mesh, activated at 200 °C overnight, and then 5% water deactivated. The column was eluted with 10 mL of 10% (v/v) dichloromethane in hexane. The samples were then concentrated to approximately 1 mL under a gentle stream of prepurified nitrogen and spiked with an internal standard made up of perdeuterated acenaphthylene, phenanthrene, pyrene, benzo[e]pyrene, and benzo[g,h,i]perylene. The PAHs were analyzed using a Hewlett Packard 5890 gas chromatograph equipped with a 5970 mass selective detector.

The analytical quality of the data was determined using field blanks, backup filters, split PUF plugs, and recoveries of surrogate standards and a series of performance standard spikes. Field blanks were used to construct blank-based detection limits for each individual compound. A field blank was a GFF/PUF that was placed in the sampler and sufficient air flowed through to allow for calibration of the sampler flow rate (1-3 min). The detection limit was defined as three times the average mass measured in a given medium for a given site and a given sampling experiment. In some cases where no quantifiable amounts were found in the blanks, an instrument detection limit was used. Individual PAH detection limits for glass fiber filters ranged in mass from 0.002 to 59, from 0.002 to 190, and from 0.002 to 44 ng for IIT, Lake Michigan, and South Haven, respectively. Individual PAH detection limits for PUFs ranged in mass from 0.002 to 1460, from 0.002 to 54, and from 0.002 to 230 ng for IIT, Lake Michigan, and South Haven, respectively. Detection limits for individual PCBs ranged from 0.007 to 5.4 ng for all sites and media. In all cases, if the mass of the analyte in the sample was above the detection limit, the mass measured on the blank was subtracted from the sample. Backup filters and split PUFs were occasionally used to test for gas adsorption to the filter and gas breakthrough on the PUF. Adsorption of SOCs onto the secondary filter and breakthrough to the second half of the PUF were both low accounting on average for only 5 ( 8% (n ) 11) and 12 ( 5% (n ) 11) of the total mass in the sample, respectively. Recoveries of PAH surrogate standards were 78 ( 18% for fluorene-d10, 92 ( 15% for fluoranthene-d12, and 101 ( 24% for perylene-d12 (n ) 178). Matrix spikes were used to determine the surrogates that best represent the recovery of each individual PAH. The recovery of fluorene-d10 was used to correct the masses of acenaphthylene, acenaphthene, and

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FIGURE 2. Gas phase ∑PAH concentrations in ng/m3 at all three sites during three seasons. Asterisks indicate rain events in Chicago.

FIGURE 3. Gas phase ∑PCB concentrations in pg/m3 at all three sites during three seasons. Asterisks indicate rain events in Chicago. fluorene, and fluoranthene-d12 was used for all other PAHs. Recoveries of PCBs 14 and 65 were high due to chromatographic peak contamination. The recovery of PCB 166 was 106 ( 6% (n ) 119), and spike experiments of the performance standard containing all PCBs analyzed obtained essentially 100% recovery. Therefore, the decision was made to not correct PCB concentrations for surrogate recoveries.

Results and Discussion All concentrations of PAHs and PCBs are reported in this paper as sums, ∑PAHs and ∑PCBs. ∑PAHs corresponds to 19 parent compounds including acenaphthene, acenaphth-

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ylene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]fluorene, benzo[b]fluorene, benz[a]anthracene, chrysene, benzo[b+k]fluoranthene, benzo[e]pyrene, benzo[a]pyrene, perylene, indeno[cdi]pyrene, benzo[ghi]perylene, antanthrene, and coronene. ∑PCBs represents 55 chromatographic peaks corresponding to 87 PCB congeners. Gas and particulate phase ∑PAH and ∑PCB concentrations are summarized in Table 2 with ambient temperature, wind direction, and wind speed. Gas phase ∑PAH and ∑PCB concentrations at the three sites during the three intensive experiments (Figures 2 and 3) show a large concentration gradient from urban to over-water to rural sites. In all cases,

FIGURE 4. Total (vapor + particulate) ∑PAH in ng/m3 concentrations over Lake Michigan for three seasons indicating predominant wind directions.

FIGURE 5. Total (vapor + particulate) ∑PCB in pg/m3 concentrations over Lake Michigan for three seasons indicating predominant wind directions. the concentrations of ∑PAHs and ∑PCBs in the Chicago atmosphere were the highest. For most days the next highest concentrations were found over Lake Michigan, and the lowest were found in South Haven. The concentration of ∑PAHs and ∑PCBs in Chicago were approximately four times the concentration measured over the lake. The lower concentrations over Lake Michigan result from dispersion of the urban air mass acting to dilute the signal and losses due to dry gas and particle deposition to the lake and photochemical processes. The few instances where South Haven across the lake has a higher concentration than over Lake Michigan occur when the wind is out of the northwest, resulting in a low concentration over the lake. This suggests that South Haven is subject to local and regional sources of contamination either from the agricultural area surrounding the site or from longer range transport from a regional industry not associated with the Chicago urban/industrial complex. Gas phase ∑PAH concentrations (Figure 2) in Chicago range from 27 to 430 ng/m3 and account for ∼90% of the total atmospheric concentration. By comparison, Liu (11) reported a range of 20-217 ng/m3 in 1993-1994; Pirrone et al. (9) in 1993 measured an average of 150 ( 103 ng/m3 at the same site in Chicago; and Cotham and Bidleman (10) reported a

range of 75-1410 ng/m3 at another site in Chicago. The gas phase ∑PAHs are dominated by phenanthrene and fluorene while the particulate phase is dominated by the benzofluoranthenes, chrysene, fluoranthene, and pyrene. Total ∑PCB concentrations (Figure 3) in Chicago range from 270 to 14 200 pg/m3 and are highest during July. By comparison, Pirrone et al. (9) reported an average of 2140 ( 1200 pg/m3, Falconer et al. (12) reported 1900-3900 pg/m3, Holsen et al. (7) reported 7500-20 000 pg/m3, and Cotham and Bidleman (10) reported 300-9900 pg/m3. Gas phase ∑PCBs were dominated by the lower molecular weight species, which accounts for >90% of the total atmospheric concentration, while the particulate phase is dominated by the higher molecular weight species. Both ∑PAH and ∑PCB concentrations were highest during July and lowest in January. ∑PAH concentrations are frequently higher during the winter months due to greater fossil fuel combustion for heating (13-16). However measurements in Boston and Houston (17) and Chicago (11) report the opposite trend. The variation in ∑PCBs result from both variations in wind direction and temperature effects. Since PCBs are no longer produced, their source to the atmosphere is volatilization from areas where they were used, stored, or spilled. Volatilization, and therefore concentration, is ex-

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TABLE 3. Atmospheric Transport Times for PAH-Containing Air Masses Based on Wind Direction, Distance to Urban/ Industrial Complex, and Wind Speed for Six Daytime Samples

date

site

wind direction (deg)

7/18/94 7/19/94 7/21/94 7/22/94 7/25/94 7/26/94

5 5 1 1 5 5

146 167 219 251 284 244

distance (km)

wind speed (m/s)

time (h)

60 50 30 35 37 30

5.2 4.4 2.3 1.8 5.1 5.4

3.2 3.2 3.6 5.3 2.0 1.5

TABLE 4. Average, Standard Deviation, and Range of Environmental Rate Constants (day-1) Based on Transport Times and Loss of PAHs from Night to Day with Pseudo-First-Order Rate Constants from Literature Assuming a [OH•] Concentration ) 3 × 106 Molecules cm-3 a PAH

ke ave

SD

range

kOH literature

ref

Acy Ace Fluor MeFluor Phen Anth 2-MePhen 1-MePhen Flant Pyr

0.77 0.78 0.46 0.37 0.41 0.35 0.45 0.45 0.35 0.47

0.33 0.52 0.24 0.10 0.21 0.06 0.13 0.10 0.15 0.13

0.36-1.19 0.35-1.66 0.17-0.89 0.19-0.47 0.20-0.82 0.27-0.42 0.30-0.67 0.29-0.56 0.17-0.58 0.27-0.62

1.19 1.08 0.17 ( 0.05 0.47 ( 0.05 0.14 ( 0.02 0.14 ( 0.08 0.37 ( 0.02 0.37 ( 0.02 0.39 ( 0.02 0.94 ( 0.02

25 25 26 est. 26 27 27 est. 27 est. 27 est. 27 est. Phen

a Acy, acenaphthylene; Ace, acenaphthene; Fluor, fluorene; Phen, phenanthrene; MeFluor, methyl fluorene; Anth, anthracene; 2-MePHen, 2-methylphenanthrene; 1-MePhen, 1-methylphenanthrene; Flant, fluoranthene; Pyr, pyrene.

pected to be highest during July when the ambient temperature is highest (18-20). ∑PCB concentrations in Chicago were ∼5 times higher in July when temperatures ranged from 19 to 29 °C than in May and January when temperatures ranged from 0.3 to 5 °C. Both ∑PAH and ∑PCB concentrations vary by over an order of magnitude in Chicago during July as a result in variations in wind direction. During late July when the ∑PAH and ∑PCB concentrations were lowest, the wind was from the north to northwest, bringing clean air off the lake and diluting the urban signal. Day to day variability is also frequently impacted by several rain events that took place during July. Rain events occurred on the morning of July 19, the evening of July 20, and the evening of July 21 (indicated by asterisks in Figures 2 and 3). The air samples immediately following these rain events contained significantly less ∑PCBs and ∑PAHs, indicating significant washout of these atmospheric contaminants by rain. Offenberg and Baker (21), however, suggest that the rain is not an efficient scavenger of gas phase PCBs but does efficiently remove particulate-phase compounds. Over-lake ∑PAH concentrations (Figure 4) range from 0.8 to 79 ng/m3 and are highest during the JulyAEOLOS intensive experiment. By comparison, Andren and Strand (1) reported a range of 3-24 ng/m3 in the late 1970s, and Pirrone et al. (9) reported an average of 14.6 ( 12.1 ng/m3 around the southern and central basins of Lake Michiga in the early 1990s. Total ∑PCB concentrations range from 140 to 1130 pg/m3 (Figure 5). By comparison, Eisenreich and Strachan (3) estimated that the mean range of concentrations over Lake Michigan are 100-400 pg/m3, Hoff et al. (22) reported a concentration of 160 pg/m3 for northern Lake Michigan, and Pirrone et al. (9) and Hornbuckle et al. (23) reported concentrations of 808 ( 307 and 1540 pg/m3, respectively, for southern Lake Michigan. The temporal variations in the air concentrations over the lake in July result from temporal variations in wind direction.

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FIGURE 6. Concentration vs wind direction plots for a) ∑PAHs in ng/m3 and b) ∑PAHs in ng/m3. Note the different concentration scale for site 0. The over-lake atmospheric concentrations from July 17 to July 25 are on the order of four times higher than those from July 26 to July 28. The wind shifted from southwest to north between these two time periods. North to northwest wind is also responsible for the low ∑PCB and ∑PAH concentrations during May and the first and last samples in January. The low concentrations, measured in January when winds are from the southwest, result from the lower source strength in the urban area reflect lower temperatures. During the period between July 17 and 25, ∑PAH concentrations varied diurnally in the coastal atmosphere. The overnight samples are ∼3.5 times the concentration of the daytime samples. Assuming the source strength is constant from day to night for this time in July, losses occurring in the daytime occur during transport. This assumption seems valid since the concentration in Chicago does not vary diurnally. Environmental rate constants associated with the daytime losses are easily calculated given a specific transport time, estimated using wind speeds and travel distance. The latter is the distance between the over-lake sampling site and an arbitrarily selected point 5 km from shore along the wind trajectory as measured at the ship. Table 3 depicts the estimated transport times for six daytime samples. The calculated transport times range from 1.5 to 5.3 h. Firstorder rate constants necessary to account for gas phase loss of 10 PAHs from night to day were calculated, and the average,

standard deviation, and range are presented in Table 4. These 10 PAHs account for >90% of the total gas phase concentration. The dominant loss process for PAHs in the atmosphere is photooxidation involving the hydroxyl radical (24). Pseudofirst-order rate constants were obtained or estimated from several sources (25-27) for the photooxidative reaction with hydroxyl radical assuming a [OH•] concentration of 3.0 × 106 molecules m-3 (28). For all cases except phenanthrene and anthracene, the range of environmental rate constants encompasses the literature values, suggesting that the environmental loss of PAHs during the day is accounted for by the gas phase photooxidation of PAHs by hydroxyl radical. ∑PAH and ∑PCB concentrations over southern Lake Michigan are highly dependent on wind direction. Figure 6 is a plot of the over-lake air concentrations vs wind direction. The highest air concentrations occurred when the wind was from a direction of the urban/industrial area inclusive of the shoreline from Evanston, IL, to Gary, IN, and they were low when the wind was outside this area. The average nighttime ∑PAH concentrations with wind inside and outside of this area was 39 ( 23 and 3.2 ( 1.4 ng/m3, respectively. The latter is comparable to values measured over Lake Superior (2, 29) and represents the continental background concentration. Therefore, air moving from the direction of the urban/ industrial complex increased the average atmospheric concentration (both day and night) of ∑PAHs to a level 12 times the regional background concentration. The average nighttime ∑PAH concentration increased by 18 times while the average daytime ∑PAH concentration was only 5 times the regional background due to photooxidation of the PAHs during the day. During July, when ambient temperature is high, the average total ∑PCB concentrations are 830 ( 183 pg/m3 inside and 201 ( 70 pg/m3 outside the vector. This “north wind” concentration is less than or approximately equal to concentrations measured at land sites or over water in northern Lake Michigan (22, 23), Lake Superior (20, 29), or Bermuda (19) and also represents the continental background signal. Therefore, air moving from the direction of the urban/industrial complex increased the average atmospheric gas phase concentration of ∑PCBs by a factor of 4 over regional background.

Acknowledgments We would like to thank Captain Ronald Ingram and the crew of the RV Lake Guardian for their assistance in shipboard measurements; Tom Holsen of the Illinois Institute of Technology for the assistance in urban sampling; and Joel Baker, John Offenberg, John Ondov, and Jerry Keeler for helpful discussions during the development of this manuscript. The AEOLOS project was funded by the United States Environmental Protection Agency (Grant EPA CR 82204601-0; Alan Hoffman, NERL/RTP, Project Officer). We especially acknowledge the assistance of Jacki Bode and Angela Bandemehr of the Great Lakes National Program Office (U.S. EPA) for their support and assistance.

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Received for review November 20, 1996. Revised manuscript received March 13, 1997. Accepted March 25, 1997.X ES9609765 X

Abstract published in Advance ACS Abstracts, May 15, 1997.

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