Surface Exchange and Transport Processes Governing Atmospheric

Feb 27, 1997 - Short-Term Temperature-Dependent Air-Surface Exchange and Atmospheric Concentrations of Polychlorinated Naphthalenes and Organochlorine...
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Environ. Sci. Technol. 1997, 31, 842-852

Surface Exchange and Transport Processes Governing Atmospheric PCB Levels over Lake Superior RICHARD E. HONRATH* Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, Michigan 49931 CLYDE I. SWEET Illinois State Water Survey, Champaign, Illinois 61820 CHRISTOPHER J. PLOUFF Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, Michigan 49931

Measurements of gas phase PCB concentrations obtained at a site on the shore of Lake Superior are analyzed to determine the importance of regional air/surface exchange as a determinant of PCB concentration over the lake. During periods of onshore flow, results obtained in congenerspecific regression analyses of log concentration against reciprocal temperature are strongly dependent upon time of year. During months when the lake is colder than the overlying air (resulting in a stable atmosphere over the lake), regressions for onshore flow explain 48-63% of the observed variance in log C, and the regression slope and intercept are consistent with Henry’s law equilibrium between the lower atmosphere over the lake and aqueous concentrations. Results consistent with Henry’s law are obtained only when water temperature is assumed to be similar to air temperature measured at the site during flow off the lake; poor regression results (r 2 ) 0.00-0.06) are obtained when lake water temperature measured by data buoys is used. Based on consideration of the spatial variability of lake skin temperature during onshore flow periods, it is inferred that, under stable conditions, air/water equilibrium is reached rapidly and that atmospheric concentrations at the measurement site are primarily affected by air/water exchange within 10-30 km offshore. These results are consistent with consideration of potential limitations of regression analyses of log C versus reciprocal temperature. Furthermore, they indicate that the net exchange of PCBs during the stable season over large lakes (AprilAugust or September for Lake Superior) may be significantly less than previously estimated. Regression analyses based on samples obtained during onshore flow in the unstable season and during over-land flow exhibit increased scatter and are inconsistent with the hypothesis of upwind equilibrium controlled by either Henry’s law- or vapor pressure-mediated exchange at local temperatures. This result is consistent with rate limitations to atmosphere/ surface exchange over the lake and land surface and with the effect of atmospheric mixing processes. Implications of these results to the interpretation of concentration measurements at lakeside sites and the use of such

measurements in calculations of the air/water exchange flux of PCBs over large lakes are discussed.

Introduction Polychlorinated biphenyls (PCBs) are persistent in the environment and have been distributed widely. PCB emissions are located primarily in equatorial to mid-latitude regions (1), and there are no known significant natural sources of the industrially emitted PCB congeners (2). However, measurements have detected PCBs in the Arctic and other remote regions (e.g., refs 3 and 4). PCBs bioaccumulate, reaching levels in animal tissue at which teratogenic and carcinogenic properties are of concern for wildlife health (5) and leading to fish consumption advisories in the Great Lakes. The importance of atmospheric transport in the budget of PCBs in remote regions has been clearly demonstrated. For example, studies indicate that 85-90% of the gross PCB flux into Lake Superior results from atmospheric deposition (6, 7), and PCBs detected in remote regions are the apparent result of atmospheric transport and deposition (3, 4). The net flux of PCBs from the atmosphere to the land or water surface is determined in part by the extent of disequilibrium between the two reservoirs. In the past, atmospheric levels were elevated by emission sources resulting from PCB use, causing a net flux out of the atmosphere in remote regions. An understanding of atmospheric PCB levels is necessary to predict future trends in PCB levels in the environment. PCB production in the United States was banned in 1979, and current uses are restricted. The impact of these restrictions on levels in the fish and waters of lakes with primarily atmospheric PCB sources is dependent upon the resulting impact on atmospheric concentrations. Dissolved PCB levels in Lake Superior appear to be dropping (8), and this decline has been attributed to volatilization. However, decreases below the aqueous level that is at equilibrium with current airborne concentrations are dependent upon a corresponding drop in airborne levels. In addition, estimates of the current volatilization flux of PCBs from Lake Superior are critically dependent upon accurate estimation of airborne concentrations just above the lake surface (9). However, current information on atmospheric PCB concentrations in rural and remote regions, vertical gradients in those concentrations, and understanding of the processes that control those levels are poor. To provide an assessment of these issues, we have analyzed measurements of airborne PCB concentrations in the atmosphere over the region of Lake Superior, a lake that is remote from industrial regions, dominated by atmospheric exchange (6), ice-free nearly year-round (10), and has been studied sufficiently in the past to provide the necessary information for the analyses described below. Although we have analyzed PCB concentrations, individual congeners spanning a considerable range of physical properties are considered, and the results may thus provide a basis for studies of other semivolatile organic compounds (SOCs) as well. The present analysis makes use of atmospheric PCB concentrations measured at Eagle Harbor, MI, during 19901994, with a focus on differentiating between three types of processes that may affect the observed levels: (1) local temperature-controlled equilibrium between the atmosphere and the surface-adsorbed or dissolved reservoir; (2) nonequilibrium exchange between the air and water during periods of flow off the lake; and (3) long-range transport of PCBs from regions of enhanced emissions. In this paper, we focus on the first process, which we will demonstrate is of * Corresponding author e-mail: [email protected]; fax: (906) 487-3292.

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S0013-936X(96)00645-1 CCC: $14.00

 1997 American Chemical Society

FIGURE 1. Location of the measurement site at Eagle Harbor, MI. Periods when the wind blew from the region above the dashed lines (wind direction 260-130°) are included in the over-water flow analyses; periods when the wind blew from the region below the dashed lines are included in the over-land flow analyses. Triangles indicate the locations of weather buoys. primary importance at the site during the months of AprilAugust. The role that long-range transport plays in airborne PCB levels at Eagle Harbor will be described in a succeeding paper. Experimental and regression modeling techniques are described in the following section. We then review processes that are expected to alter the results of regression analyses of the type conducted here and present and analyze regression results obtained using the Eagle Harbor/Lake Superior data. We conclude with a brief discussion of the implications of this work for estimates of the PCB flux out of Lake Superior and other large bodies of water.

Methods PCB Measurements. Atmospheric samples of PCBs were collected at an air monitoring station located on the shore of Lake Superior at Eagle Harbor, MI, as part of the Integrated Atmospheric Deposition Network (IADN) (11). The site is located on the Keweenaw Peninsula and has the advantage of being surrounded on most sides by the waters of Lake Superior (Figure 1). Particulate and vapor phase PCBs were determined using a modified high-volume sampler equipped with a glass fiber filter and absorbent cartridge. Samples were collected over a 24-h period once each 12 days from November 1990 through August 1994, providing a total of 94 observations. Prior to April 1992, a PUF absorbent was used to collect vapor phase PCBs; after that time, XAD-2 resin, a more efficient absorbent for gas-phase semivolatile organics, was used to reduce potential sample breakthrough. Tests to compare the behavior of the two absorbents and to determine breakthrough fractions were conducted during summer 1991. These tests indicated e10% difference in total PCB mass determined using the two absorbents and demonstrated that, of the PCB congeners analyzed in this study (see below), breakthrough using PUF absorbent was 90% of the total atmospheric burden (e.g., refs 7 and 16). In this study, the fraction accounted for by the vapor phase was always greater than 80% even for the heavier congeners, with the exception of five winter samples in which the vapor fraction of the heaviest, heptachloro congeners 182+187 dropped to 60-70%. Restriction of this analysis to the vapor phase is consistent with our focus on atmospheric concentrations. However, in other situations such a restriction may not be appropriate, particularly when estimating deposition fluxes of heavier congeners in winter, when vapor phase concentrations are at a minimum. For example, particulate deposition may be the dominant component of the gross PCB flux into Lake Superior during mid-winter (17). Meteorological Data. Temperature, wind direction, and wind speed were averaged over each PCB sampling period. Average wind direction was determined using vector averaging. The primary meteorological data were obtained from a 10-m tower at the Eagle Harbor site. Missing data were filled in with readings from the Houghton County Airport (45 km to the southwest, 145 m above lake level) during two periods totaling approximately 8 months. Measurements of water temperature, air temperature, wind speed and wind direction on Lake Superior were obtained from three National Oceanic and Atmospheric Administration (NOAA) weather buoys (locations shown in Figure 1). Wind directions measured at the data buoys were well correlated with the Eagle Harbor observations (r 2 ) 0.7-0.88) (18), indicating that the Eagle Harbor observations used here are generally indicative of regional flow patterns. Regression Analysis. Under conditions in which local or regional exchange processes (rather than long-range transport) control atmospheric levels, the atmospheric concentration of a given SOC is expected to exhibit temperature dependence. Regression analysis of equations of the form

1 log C ) a + b T

(1)

have thus been used to investigate the seasonal variation of PCB concentrations (19-21). Temperature dependence of this form may result from equilibrium exchange processes (see below). However, some factors that determine the usefulness and validity of eq 1 are discussed in the Results and Analysis section. In the following sections, the relationships of atmospheric concentration to temperature expected under the assumption of equilibrium exchange with dissolved and with surfaceadsorbed PCBs are briefly developed. These relationships are used to derive expected values of the regression slope

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and intercept, which will be compared to regression parameters obtained with the Eagle Harbor data. Equilibrium with PCBs Dissolved in Lake Superior. The atmospheric concentration Ca of a given PCB congener in equilibrium with aqueous concentration Cw is given by Henry’s law:

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Ca ) CwH

(2)

where H is the nondimensional form of the Henry’s law constant. The dimensional form of the Henry’s law constant (Hd ) HRT) may be estimated as the ratio of a compound’s saturation vapor pressure and aqueous solubility (22). Both parameters are temperature dependent, with the result that the temperature dependence of Hd is somewhat less than that of the saturation vapor pressure. We have used homologspecific temperature dependencies derived from the work of Burkhard et al. (22) with congener-specific values of H at 298 K (23), based on the relationship of H with total surface area (TSA) of the PCB molecule, to derive H as a function of temperature (17):

(

log H ) log H298 +

∆Hw 2.303R(298)

)

-

∆Hw 1 2.303R T

(3)

Combining eqs 2 and 3 we obtain

log Ca ) log Cw + log H ) log Cw + aH -

(4) ∆Hw 1 2.303R T

(5)

where aH is the term within parentheses in eq 3. Expected values of the slope and intercept in regressions of eq 1 may be obtained from eq 5 and are given in Table 1. Aqueous concentrations are taken from the 1986 measurements of Baker and Eisenreich (24). There is evidence that aqueous levels in the lake have dropped since 1986 (8). However, use of reduced aqueous levels corresponding to 1992 measurements (8) would reduce the expected intercept values shown in Table 1 by e5% for each congener. Equilibrium with Surface-Adsorbed PCBs. Atmospheric partial pressures of PCBs and other SOCs have been shown to exhibit a seasonal variation consistent with equilibrium adsorption to sites on the earth’s surface or on particulates (19, 20, 25, 26). As discussed in a number of studies, this partitioning can be described by a linear Langmuir isotherm (20, 27, 28). Pankow (29) proposed a model of the partitioning of SOCs between the atmosphere and the earth’s surface based on the presence of a constant total mass of SOCs (over an annual time scale) undergoing reversible adsorption or absorption to surface sites and the existence of a partition coefficient for exchange between the gas phase and material on the surface. Bidleman and Foreman (20) present data indicating that the enthalpy of desorption is essentially equal to ∆Hv, the enthalpy of vaporization. Thus, the slope of the best-fit line in regression analysis of eq 1 should equal ∆Hv/2.303R if surface adsorption-desorption processes control atmospheric concentrations. Expected values of the slope are shown in Table 1, based on the work of Falconer and Bidleman (30). The intercept of eq 1 expected under conditions of surface adsorptive equilibrium is a function of the sorbed concentration at the earth’s surface and the partitioning coefficient. Neither of these values are known. For comparison with regression results presented below, intercepts calculated by Hoff et al. (19) in regressions of log Sa versus 1/T at a continental site are given in Table 1.

Results and Analysis In this section, we present results of regressions of log C versus 1/T for the entire data set and for subsets of the data. Before

FIGURE 2. Seasonal cycles in monthly average air temperature, water temperature, and fractional ice cover of Lake Superior. Water temperature (solid circles) and over-water air temperatures (open circles) are from data buoy 45001 (48.0° N, 87.6° W) during 1979-1988 (45). (March water temperature is not available and has been estimated as the average of the February and April values.) Stable atmospheric conditions over the lake are indicated during April-August, when the water temperature at the buoy location is less than that of the overlying atmosphere. The atmosphere nearer the upwind shore is generally more stable than that over the open lake, as indicated by the larger difference between mean daily land temperatures (crosses) (32) and water temperatures. The bottom plot shows the monthly average ice-free fraction during 1960-1979 (10). presenting these results, however, we assess some factors that are expected to affect the validity of this type of regression analysis. Types of Exchange Processes Governing Atmospheric Levels. Although equilibrium exchange of airborne PCBs with either a surface-adsorbed or an aqueous reservoir is expected to lead to linear dependence of log C upon reciprocal temperature, the expected slopes are slightly different, and the expected intercepts are dependent on several parameters (Table 1). There is no reason to expect that identical relationships to temperature would result from equilibrium with the two different reservoirs. In the analysis below, we will consider separately the subsets of samples taken during over-water and during over-land flow periods. Atmospheric Stability. The extent to which mixing occurs between the boundary layer and air in the overlying free troposphere is highly variable and may alter the extent to which atmospheric SOC concentration are equilibrated with regional conditions. During daytime over land, surface heating induces buoyant mixing, while at night over land, radiative cooling results in inhibited mixing. As a result, air mixed above the boundary layer during the day is not subject

to exchange with the land surface during transport the following night. Nighttime transport of air above the boundary layer followed by mixing into the boundary layer the following morning (after breakup of the nighttime temperature inversion) may result in surface PCB concentrations that are not in equilibrium at the local surface conditions. Unfortunately, this effect cannot be eliminated or characterized with 24-h samples of the type used here. Vertical mixing over water is also highly variable, but changes in marine boundary layer stability occur over longer time scales and can largely be taken into account using 24-h average samples. During periods when the water surface is colder than the overlying air, a strong temperature inversion develops as air travels from land over the water (31). During such periods, vertical mixing within the temperature inversion region is extremely slow. In contrast, when the water surface is warmer than the atmosphere, vertical mixing is enhanced. As a result of slower seasonal temperature variations in water than in the overlying air, there is a stable “season” and an unstable season with respect to atmospheric transport over Lake Superior, as shown in Figure 2. Phillips (32) notes that neutral or stable conditions predominate from March

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through October. For the present discussion, the period of April-August, when lake water temperatures are colder on average than temperatures in the overlying atmosphere, will be termed the “stable” season, and the period OctoberMarch, when mean lake water temperatures are warmer than the atmosphere, will be termed the “unstable” season. Vertical temperature gradients in the lower atmosphere over large lakes can be quite extreme during the stable season. Lyons (31) reports a case in which an inversion of 10-26 °C existed between the lake surface and a height of 12 m at a distance only 8 km downwind from shore in Lake Michigan. The depth of the inversion increased as fetch across the lake increased, but remained less than 100 m. In such situations, warm air flowing from land over the lake may rise over the cooled over-lake air, and lake-air interaction is suppressed (31). Atmospheric mixing within over-lake inversion layers was also observed to be greatly suppressed, with the eddy diffusivity dropping by at least 2 orders of magnitude. Reduced atmospheric mixing during the stable season has a significant effect on pollutant transport and exchange over lakes. Pollutants emitted from near-lake sources may be transported over water for hundreds of kilometers with essentially no interaction with the water surface (33). For compounds such as PCBs, which are affected by air/water exchange, high atmospheric stability means that air/lake exchange affects only the region below the height reached by mechanically induced turbulence. Although this height is poorly characterized, it is low: diffusion experiments summarized by Lyons (31) found vertical dispersion parameters of 10°, it is likely that effects of these warmer regions were detected at the Eagle Harbor site on this day. This indicates that Eagle Harbor air temperature may provide a better approximation of nearshore water skin temperature than do the mid-lake buoys.

FIGURE 5. Dependence of regression parameters on data subset. Regression estimate of the slope and intercept are shown for over-water flow during the stable season (circles), over-water flow during the unstable and ice-free months (triangles), over-land flow (boxes), and over-water flow during the stable season using data buoy observations of temperature (diamonds). Slope and intercept values expected for Henry’s law equilibrium over Lake Superior and for equilibrium with surface-adsorbed levels (Table 1) are indicated by the dashed and dotted lines, respectively. Error bars represent (2 standard errors. (Daily satellite measurements would be the best data source, but frequent cloud cover at the site precludes their use in regression analyses of the type conducted here.) In summary, the following observations are noted for the over-water flow samples: (1) the regression results using Eagle Harbor air temperatures provided parameter estimates consistent with Henry’s law equilibrium at those temperatures, while regression analyses conducted using buoy observations were poor; (2) near-shore lake surface temperatures during onshore winds can be significantly warmer than temperatures at the buoy locations and are generally closer to air temperatures measured at the lakeside sampling site; and (3) lake/atmosphere exchange during periods of high atmospheric stability over the water may reach equilibrium rapidly, as discussed above. Based on these observations, we conclude that during periods of stable over-water flow, atmospheric PCB concentrations at Eagle Harbor reflect air/ water equilibrium achieved over the near-shore waters, at water surface temperatures similar to air temperature observed at the sampling site. The example considered here indicates that this region of influence extends ∼10-30 km into the lake (Figure 7). Over-Land Flow. Results for flow from the southern sector (130-260°) are shown in Table 2 and in the fourth column of Figure 4. The correlation coefficients in these regressions are notably lower than were those for over-water flow, and

the regression slopes are significantly flatter than expected based on vapor pressure temperature dependence (see also Figure 5). The wind sector used here includes some directions with a moderate fetch over water. However, results are nearly identical when only those samples taken when average wind directions of 200-245° indicated air flowing on average directly over the Keweenaw Peninsula. The poor regression results for this wind sector are attributed to three sources: the effect of vertical mixing over land, which will lead to elevated concentrations at a given temperature as discussed above; rate limitations that prevent the achievement of equilibrium between advected air massesscooled during transport from the southsand regional surface-adsorbed PCB levels; and mixing or wind direction shifts that brought to the site air affected by both exchange with the land surface and transport over the lake. The first two of these processes indicate a significant role of transport from other regions in determining PCB concentrations at Eagle Harbor. We are currently completing an analysis of air-flow trajectories to test this hypothesis. Implications for Atmosphere/Lake Exchange. The results presented above have significant implications regarding the seasonal cycle of air/water fluxes of species volatilized from or deposited to the lake surface. These are discussed with respect to PCBs in the next section, followed by consideration of the appropriate use of lakeside or lake-wide measurements

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FIGURE 6. Lake surface temperature July 26, 1992, based on a NOAA Coastwatch satellite image. The average wind direction during the 24-h sample taken on this day is indicated by the solid line leading upwind to Isle Royale. Temperatures along this line, along the dotted line drawn 10° westward, and within the circled region are summarized in Figure 7. air/water exchange fluxes. The present results indicate that during the season of over-lake atmospheric stabilitys AprilsAugust, and perhaps Septembersequilibrium between aqueous PCB concentrations and concentrations in the nearlake atmosphere is achieved rapidly, apparently within transport distances of 10-30 km. As a result, atmospheric concentrations downwind of the lake are affected by air/ water exchange over this scale of fetch, and net air/water exchange over the remainder of the lake is expected to be of small magnitude. This observation is attributed to inhibition of over-lake mixing resulting from frequent temperature inversions above the lake during the April-August period.

FIGURE 7. Temperatures from Figure 6 as a function of distance from the sampling site. Temperatures along the solid line shown in Figure 6 are indicated by solid squares, those along the dotted line in Figure 6 are indicated by open squares, and those within the circled region of Figure 6 are indicated by solid circles. Also shown are average temperatures during the sampling period based on air temperatures from data buoy 45001 (dotted horizontal line), water temperatures from data buoy 45001 (dashed horizontal line), and air temperatures at the sampling site (solid horizontal line). Water skin temperatures 10-30 km upwind and within the circled region of Figure 6 are closer to the near-shore air temperature than to either air or water temperature reported by the data buoy. of atmospheric concentrations and temperature in calculations of air/water exchange fluxes. Seasonal Cycle of PCB Volatilization/Deposition Flux. The seasonal cycle of boundary layer stability over Lake Superior is expected to play an important role in determining

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A similar conclusion regarding the transfer of water vapor between the atmosphere and the lake has been reached previously, and calculations of evaporation/condensation fluxes have been modified to take this into account. Schertzer (38) used closure of the Lake Superior water budget to deduce that, during the months when water temperature was less than air temperature, condensation into the lake was significantly less than indicated by mass transfer calculations. It was concluded that the form of mass transfer technique used required modification for periods during which the atmospheric boundary layer is stable. Quinn (39) developed an improved mass transfer technique for evaporation calculations in which the transfer coefficient is dependent on measures of atmospheric stability, including the MoninObukhov length and vertical gradients in potential temperature and non-dimensional wind speed. The modified coefficients were applied to calculations of evaporation from Lake Superior by Derecki (40). Adjustments for atmospheric stability reduced the calculated condensation during MayAugust by 50%, while net evaporation fluxes during April and September were essentially unchanged. Indications that stability over water alters the response of atmospheric PCB concentrations to air/water exchange were observed in measurements over Green Bay in Lake Michigan by Hornbuckle et al. (14). Significant differences were found in atmospheric levels over Green Bay and over the surrounding land (14, 41, 42), and it was noted that the results implied a poorly mixed atmosphere and a vertical gradient in PCB concentration over water (14), consistent with the conclusions of the present study. However, many previous estimates of air/water exchange of PCBs over the Great Lakes have been

based on atmospheric concentrations measured at lakeside or on ship cruises that traversed a significant fraction of the lakes (e.g., refs 9, 24, 41, and 43). Such calculations have neglected the modification of near-lake atmospheric concentrations that occurs under stable conditions. This is equivalent to neglecting the role of resistance to air/water exchange within the atmospheric boundary layer over the lake. As a result of this resistance, air/lake exchange fluxes during the season of over-water stability (April-August or September for Lake Superior) are expected to be reduced significantly, and this may result in a significant reduction in calculated annual average exchange fluxes. For example, in recent calculations of the air/water exchange flux of PCBs in Lake Superior, all of the semimonthly periods of net deposition occurred during April-August, as did ∼30% of the annual net volatilization flux (9). Estimated volatilization during September, part of which probably corresponds to stable overlake conditions, contributed an additional 30% to the total net volatilization. If fluxes during periods of stable over-lake conditions are reduced due to poor mixing (by ∼50%, based on the water vapor calculations of Derecki (40) discussed above), the calculated annual-average volatilization flux of PCBs would be reduced significantly. While this effect is less than the total uncertainty in PCB flux calculations for Lake Superior (44), it is a source of potentially significant bias in such flux calculations. Use of Shoreline Concentration Measurements in Flux Calculations. Guidance on the appropriate use of shoreline measurements in air/water flux calculations has been given by Hornbuckle et al. (14). The present results provide an additional basis for the use of lakeside concentration measurements in a manner dependent on the over-lake stability conditions and wind direction regime under which they were obtained. During the stable season over large lakes, measurements obtained during onshore flow are indicative of near-surface concentrations over the water. However, they may apply only to the near-shore regionswithin 10-30 km in some cases, based on the example considered above. For use in computing the air/water exchange flux at a given location on a large lake, lake surface temperature in the region 10-30 km upwind should be used; the resulting fluxes are expected to be low relative to those during unstable periods. Measurements on land during periods of offshore flow during the stable season are characteristic of conditions over the upwind land surface. The concentration in the nearsurface atmosphere is expected to adjust rapidly toward equilibrium conditions during flow over the lake. Therefore, measurements during offshore flow are also not expected to be indicative of average over-lake concentrations during this season. During the season in which the over-water boundary layer is unstable (September or October-February for Lake Superior), the appropriate use of shoreside concentration measurements depends on estimates of the degree to which the atmosphere approaches equilibrium with aqueous levels during flow over the lake and the relative magnitudes of airborne concentration over land and that in equilibrium with the water surface. In the case of Lake Superior, air/ water equilibration is not expected during unstable periods, and concentrations in equilibrium with aqueous PCBs are lower than or comparable to over-land levels. Therefore, atmospheric concentrations are not expected to change significantly during flow over water. In such cases, shoreline concentration measurements may be used with lake-average values of the Henry’s law constant and exchange coefficient to calculate air/water exchange fluxes.

Acknowledgments This work was supported by the Michigan Great Lakes Protection Fund of the Michigan Department of Natural

Resources and by an American Meteorological Society/TRW Graduate Fellowship. Assistance was received from several people and is greatly appreciated. Ms. Shobha Subhash (MTU) assisted in data reduction, Ms. Karen S. Harlin and Ms. Ilora Basu (ISWS) conducted laboratory analyses, and Dr. Theodore Bornhorst (MTU) provided guidance in statistical procedures. Dr. Marty Auer (MTU) was the Eagle Harbor Site Coordinator.

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Received for review July 24, 1996. Revised manuscript received October 31, 1996. Accepted November 5, 1996.X ES960645S X

Abstract published in Advance ACS Abstracts, February 1, 1997.