Impacts of lindane usage in the Canadian Prairies on the Great Lakes ecosystem. 2. Modeled fluxes and loadings to the Great Lakes

Atmospheric loadings of gamma-hexachlorocyclohexane (gamma-HCH) from May 1, 1998, to April 30, 1999, to the Great Lakes simulated by a coupled soil-air and water-air atmospheric transport model are presented. Modeling results on an annual basis indicate that Lake Superior received the highest dry deposition load of 2.17 kg yr(-1). Dry deposition to the lower Great Lakes (Lakes Erie and Ontario) was 2-6 times lower as compared to the upper Great Lakes. Greater deposition in the upper Great Lakes is due mainly to their larger surface area and greater proximity to sources of gamma-HCH. Dry deposition fluxes (pg m(-2)) to lake surfaces were much lower than to land as a result of lower deposition velocities and lower air concentrations of gamma-HCH over the lakes. The highest gamma-HCH loading (kg yr(-1), in 1998-99) due to wet deposition occurred for Lake Ontario. This was mainly attributed to greater annual rainfall over Lake Ontario. An investigation of average seasonal fluxes predicted by the model shows that deposition fluxes to the Great Lakes are considerably higher in the summer than that in the autumn and winter seasons. The net direction of gas exchange also exhibits a seasonal dependence. Lakes Michigan, Huron, and Ontario show net absorption in the summer 1998 whereas at all other times net outgassing occurred at all of the lakes. Overall, gas exchange was the dominant process affecting loadings to the Great Lakes. Model-derived loadings and total deposition flows across the Great Lakes basin due to dry and wet depositions and net gas exchange agree reasonably well with the summer estimates compiled by the Integrated Atmospheric Deposition Network whereas autumn values show greater discrepancies. Better agreement was also observed for dry deposition as compared to wet deposition. Specifically, to improve short-term loading estimates (e.g., over days to months), the model results indicate the need for better spatially and temporally resolved information on concentrations in air and surface water and better estimates of precipitation and deposition velocities over the lakes.     

Effects of wind and air trajectory directions on atmospheric concentrations of persistent organic pollutants near the Great Lakes

Three different regression models involving air temperature, time, and either wind direction or parametric or nonparametric air trajectory direction were used with concentrations of four representative persistent organic pollutants to quantitate the atmospheric transport of these compounds to the Great Lakes. The local wind and parametric trajectory models predicted an optimal source direction for each compound, whereas the nonparametric trajectory model was based on a hypothesized source region. All three regressions were used to calculate the factor by which the partial pressures of each compound measured at five sampling sites increased when the air came from a particular source direction. Dieldrin, chlordane, polychlorinated biphenyl, and polycyclic aromatic hydrocarbon partial pressures were used with each of these regressions, and the correlation coefficients (r2) were evaluated for each model, for each compound, and for each regression term. In general, with the exception of polycyclic aromatic hydrocarbons at some sites, the explanatory powers of the regressions were not improved by the inclusion of any of these directional terms.     

Temporal and spatial trends of atmospheric polychlorinated biphenyl concentrations near the Great Lakes

Polychlorinated biphenyl (PCB) concentrations were measured in the atmosphere at six regionally representative sites near the five Great Lakes from 1990 to 2003 as part of the Integrated Atmospheric Deposition Network (IADN). Concentration data for several individual PCB congeners and for total PCBs were analyzed for temporal and spatial trends after correcting for the temperature dependency of the partial pressures. Atmospheric PCB concentrations are decreasing relatively slowly for tetra- and pentachlorinated congeners, an observation that is in agreement with primary emissions modeling. Relatively rapid decreases in PCB concentrations at the sites near Lakes Michigan and Ontario may reflect successful reduction efforts in Chicago and Toronto, respectively. Atmospheric PCB concentrations near Lakes Superior and Huron are now so low that the air and water concentrations may be close to equilibrium. Atmospheric PCB concentrations at sites near Lakes Michigan, Erie, and Ontario are relatively higher than those measured at sites near Lakes Superior and Huron. The highest PCB level was observed at the site near Lake Erie, most likely due to nearby urban activity. However, this relatively higher concentration is still 6-10 times lower than that previously reported at the Chicago site. A correlation between average gas-phase PCB concentration with local population indicates a strong urban source of PCBs. The temperature dependence of gas-phase PCB concentrations is similar at most sites except at Burnt Island on Lake Huron, where very low concentrations, approaching virtual elimination, prevent reliable temperature correlation calculations.     

Causes of variability in pesticide and PCB concentrations in air near the Great Lakes

Data through 2001 from the Integrated Atmospheric Deposition Network (IADN) were used to investigate the causes of variability in gas-phase polychlorinated biphenyl (PCB) and pesticide concentrations measured near Lakes Michigan, Erie, and Superior. A multiple linear regression model that incorporates temperature and time was used explain the variability in the concentrations. Our approach used autocorrelation analyses of the residuals to help us determine the effectiveness of the regression. Autocorrelation plots forthe in-use pesticide lindane indicated that an agricultural application cycle was also present in the regression residual data at all sites. The addition of parameters for this effect to the regression equation accounted for, on average, 16% more of the variability in the data. Similar analyses forthe in-use pesticide endosulfan did not show an agricultural application effect. The banned compounds DDT and chlordane showed that temperature and time correctly accounted for the variability in the atmospheric concentrations of these compounds at all sites. In contrast to the other compounds, PCBs and hexachlorobenzene showed strong residual autocorrelation patterns near Lake Michigan of an unknown origin.     

Time trend analysis of atmospheric POPs concentrations in the Great Lakes region since 1990

Using a multiple linear regression model of the concentrations of several persistent organic pollutants in the atmospheric vapor and particle phases and in precipitation, we have analyzed a data set of about 700,000 values to determine the rate at which these concentrations are decreasing. These concentrations were measured as part of the Integrated Atmospheric Deposition Network (IADN), which has operated several sites near the North American Great Lakes since 1991. The pollutants measured include 83 polychlorinated biphenyl congeners, 17 polycyclic aromatic hydrocarbons, and 24 organochlorine pesticides. In the approach used here, for each of the three phases, the concentrations of a specific chemical at all the sites were combined and fitted with a regression incorporating the sine and cosine of the Julian Day (relative to 1 January 1990 and with a periodicity of one year) and the population living and working within a 25-km radius of the sampling site. Partial residuals were then calculated for each datum, all of the residuals for the three phases were combined, and an overall halving time was calculated from them. This relatively simple approach indicated that the concentrations of PCBs in air around the Great Lakes are decreasing with an overall halving time of 17 ± 2 years, which is slow for a substance that was banned about 35 years ago. Phenanthrene, chrysene, and endosulfan showed halving times on the order of 10 years. The concentrations of several organochlorine pesticides were decreasing more rapidly; for example α- and γ-HCH (lindane) have halving times of about 3.5 years.     

Modeling the atmospheric transport and deposition of PCDD/F to the Great Lakes

Atmospheric deposition is a significant loading pathway for polychlorinated dibenzo-p-dioxins and dibenzofurans (dioxin) to the Great Lakes. An innovative approach using NOAA's HYSPLIT atmospheric fate and transport model was developed to estimate the 1996 dioxin contribution to each lake from each of 5,700 point sources and 42,600 area sources in a U.S./Canadian air emissions inventory. These unusually detailed source-receptor modeling results show that deposition to each lake arises from a broad geographical region, with significant contributions from up to 2,000 km away. The source categories contributing most significantly to 1996 dioxin deposition appear to be municipal waste incineration, iron sintering, medical waste incineration, and cement kilns burning hazardous waste. Model-predicted air concentrations and deposition fluxes were consistent with ambient measurement data, within the uncertainties in each, but there may be a moderate tendency toward underestimation using midrange emissions estimates. The most likely reason for this tendency appears to be missing or underestimated emissions sources, but in-situ atmospheric formation of octachlorinated dibenzo-p-dioxin (OCDD) and heptachlorinated dibenzo-p-dioxin (HpCDD) may have also contributed. Despite uncertainties, the findings regarding the relative importance of different sources types and source regions appear to be relatively robust and may be useful in prioritizing pollution prevention efforts.     

Impacts of the Canadian forest fires on atmospheric mercury and carbonaceous particles in Northern New York

The impact of Canadian forest fires in Quebec on May 31, 2010 on PM(2.5), carbonaceous species, and atmospheric mercury species was observed at three rural sites in northern New York. The results were compared with previous studies during a 2002 Quebec forest fire episode. MODIS satellite images showed transport of forest fire smoke from southern Quebec, Canada to northern New York on May 31, 2010. Back-trajectories were consistent with this regional transport. During the forest fire event, as much as an 18-fold increase in PM(2.5) concentration was observed. The concentrations of episode-related OC, EC, BC, UVBC, and their difference (Delta-C), reactive gaseous mercury (RGM), and particle-bound mercury (PBM) were also significantly higher than those under normal conditions, suggesting a high impact of Canadian forest fire emissions on air quality in northern New York. PBM, RGM, and Delta-C are all emitted from forest fires. The correlation coefficient between Delta-C and other carbonaceous species may serve as an indicator of forest fire smoke. Given the marked changes in PBM, it may serve as a more useful tracer of forest fires over distances of several hundred kilometers relative to GEM. However, the Delta-C concentration changes are more readily measured.     

Spatial and temporal trends of chiral organochlorine signatures in Great Lakes air using passive air samplers

Passive air samples (PAS) were collected and analyzed to assess the spatial and temporal trends of chiral organochlorine signatures in the Laurentian Great Lakes. Samples were collected from 15 sites and analyzed for the concentrations and enantiomer signature of chlordanes and alpha-hexachlorocyclohexane (alpha-HCH). Levels of the chlordanes were typically 4 times higher in urban areas than what were observed at rural and remote locations, exhibiting strong urban-rural gradients. Near racemic residues were seen for the chlordane enantiomers in samples collected from sites located in Toronto and Chicago, which can be attributed to continued emissions of historical use of the technical chlordane mixture, while the chiral signature observed at sites located in rural and remote locations was indicative of an aged source. Knowledge of the spatial and temporal distribution of the enantiomer signatures of chlordane and alpha-HCH in air is useful for distinguishing sources of these compounds to ambient air. Results suggest that potential sources, such as those associated with Toronto and Chicago, have limited influence over the levels at rural and remote sites within the Great Lakes. Sources that are relatively close to sample sites, however, have a strong influence on levels observed at those sites. For instance, results indicate that Lake Superior continues to act as a source of alpha-HCH to sites located on its shores. Generally, it appears that during the warmer months, local enhanced surface-air exchange influences air concentrations and that during the cooler periods of the year, levels in the atmosphere are more strongly influenced by advective transport from source regions.     

Polybrominated diphenyl ethers in the sediments of the Great Lakes. 1. Lake Superior

Sediment cores were taken in 2001 and 2002 in Lake Superior at six locations away from lakeshores and segmented at 0.5-5 cm intervals. The year of sediment deposition was estimated for each segment of four cores using the 210Pb dating technique. Samples were Soxhlet-extracted and cleaned up by silica gel fractionation, and the concentrations of 10 polybrominated diphenyl ethers (PBDEs) and 19 polychlorinated biphenyls (PCBs) were measured by GC-MS in SIM mode. In contrast to recent declining or level-off trends in PCB fluxes, the sedimentary records of PBDEs generally show a significant increase in recent years. The load of total PBDEs to Lake Superior was estimated to be 2-6 metric tons, and the current loading rate was about 80-160 kg yr(-1). With the exclusion of decabromodiphenyl ether (BDE209), the surficial concentration of sumPBDE (sum of 9 congeners) ranged from 0.5 to 3 ng g(-1), and the current sumPBDEs flux was 8-31 pg cm(-2) yr(-1). The concentrations of BDE209 were about an order of magnitude higher than the sum of other congeners, comprising 83-94% of the total PBDE inventory in the sediments. Among the other nine PBDEs detected, congeners 47 and 99 were the most abundant, and congeners 100, 153, 154, and 183 were also detected in all the cores. Congener analysis demonstrated that the pattern of PBDEs in Lake Superior sediments differs from those in air and fish.     

A regional atmospheric fate and transport model for atrazine. 1. Development and implementation

The Community Multiscale Air Quality (CMAQ) modeling system is adapted to simulate the regional transport and fate of atrazine, one of the most widely used herbicides in the United States. Model chemistry and deposition are modified, and a gas-to-particle partitioning algorithm is added to accommodate semivolatile behavior. The partitioning algorithm depends on humidity, temperature, and particulate matter concentration and composition. Results indicate that gaseous atrazine will usually dominate warm season atmospheric concentrations, but particulate form can surpass gas forms when atmospheric humidity is high (> 70%) and less-acidic (pH > 2.5) aqueous aerosol component is present. Implementation of the modified CMAQ for atrazine is illustrated, and, within the limits of our current understanding, preliminary transport and fate patterns appear to be reasonable. This research represents one of the first attempts to include a gas-to-particulate matter partitioning mechanism in an Eulerian grid-model.     

Polybrominated diphenyl ethers in the sediments of the Great Lakes. 3. Lakes Ontario and Erie

Sediment cores were taken in 2002 in Lakes Ontario and Erie at four locations. A total of 48 sediment samples were characterized, dated using 210Pb, and analyzed for 10 congeners of polybrominated diphenyl ethers (PBDEs) including BDE209 as well as 39 congeners of polychlorinated biphenyls (PCBs). The surficial concentrations of nine tri- through hepta-BDE congeners (sigma9PBDE) are 4.85 and 6.33 ng g(-1), at sampling sites ON40 and ON30 in Lake Ontario, and 1.83 and 1.95 ng g(-1) at ER37 and ER09 in Lake Erie, respectively, based on dry sediment weight. The surficial BDE209 concentrations are 242 and 211 ng g(-1) at ON40 and ON30 and 50 and 55 ng g(-1) at ER37 and ER09. The sigma(9-) PBDEs fluxes to the sediment around 2002 are 147 and 195 pg cm(-2) year(-1) at ON40 and ON30 and 136 and 314 pg cm(-2) year(-1) at ER37 and ER09, respectively. The fluxes of BDE209 are 6.5 and 7.3 ng cm(-2) year(-1) at ON30 and ON40 and 3.7 and 8.9 ng cm(-2) year(-1) at ER37 and ER09, respectively. Dramatic increases in PBDE concentrations and fluxes upward toward the sediment surface and the present time are evident at both locations in Lake Ontario, while PCBs concentrations peak in the middle of sediment cores around the dated time of 1970s and 1960s. For both locations of Lake Erie, the increasing trends of both PBDEs and PCBs from the bottom to the surficial segments were distorted by sediment mixing. BDE209 is the most abundant congener among PBDEs in the sediments, constituting about 96 and 91% of the total PBDEs on mass basis in Lakes Ontario and Erie, respectively.     

Polybrominated diphenyl ethers in the sediments of the Great Lakes. 2. Lakes Michigan and Huron

Sediment cores were taken in 2002 in Lakes Michigan and Huron at six locations. A total of 75 samples were characterized, dated using 210Pb, and analyzed for 10 congeners of polybromodiphenyl ether (PBDE) including BDE209, as well as 39 congeners of polychlorinated biphenyls (PCBs). The concentrations of nine tri- through hepta-BDE congeners (Sigma9PBDE) in the surficial sediments range from 1.7 to 4 ng g(-1) for Lake Michigan and from 1.0 to 1.9 ng g(-1) for Lake Huron, on the basis of the dry sediment weight. The Sigma9PBDEs fluxes to the sediment around the year 2002 are from 36 to 109 pg cm(-2) yr(-1) in Lake Michigan and from 30 to 73 pg cm(-2) yr(-1) in Lake Huron, with spatial variations in both lakes. The flux of BDE209 ranges from 0.64 to 2.04 ng cm(-2) yr(-1) and from 0.67 to 1.41 ng cm(-2) yr(-1) in Lake Michigan and Lake Huron, respectively. Dramatic increases in PBDE concentrations and fluxes upward toward the sediment surface and the present time are evident at all locations. The inventory of PBDEs in both lakes appears to be dependent upon latitude and the proximity to populated areas, implying that north-bound air plumes from urban areas are the major sources of PBDEs found in the lake sediments at locations away from the shores. Heavier congeners are more abundant in the sediments than in air and fish samples in the region. BDE209 is about 96% and 91% of the total PBDEs on a mass basis in Lake Michigan and Lake Huron, respectively; both are higher than the 89% found in Lake Superior, although a t test shows that the value for Lake Huron is not statistically different from that for Lake Superior at the 95% confidence level.     

Annual variation of polycyclic aromatic hydrocarbon concentrations in precipitation collected near the Great Lakes

Polycyclic aromatic hydrocarbon (PAH) concentrations were measured in precipitation samples collected from 1997 to 2003 at seven sites near the Great Lakes as a part of the Integrated Atmospheric Deposition Network. The 28-day integrated concentrations of most PAHs showed significant seasonal trends with higher concentrations in the winter and lower concentrations in the summer. Long-term decreasing trends were observed for all PAHs measured in precipitation at Chicago. At the sites on Lakes Superior, Michigan,,and Erie, most PAHs did not show significant long-term trends. At the two Canadian sites on Lakes Huron and Ontario, lower molecular weight PAHs (e.g., fluorene to pyrene) showed long-term decreasing trends; however, no long-term trends were observed for higher molecular weight PAHs at these sites. Interestingly, retene, a marker for wood burning, showed increasing trends at the sites on Lakes Superior and Michigan. For all the other PAHs, precipitation collected at Chicago had by far the highest PAH concentrations followed by the site on Lake Erie. Generally, the Lake Superior sites had the lowest PAH concentrations. However, retene concentrations in precipitation collected at the Lake Superior site were higher compared to Lakes Michigan and Erie, which indicate more residential wood burning in the far north of the Great Lakes basin.     

Tracking toxaphene in the North American Great Lakes basin. 2. A strong episodic long-range transport event

In this paper we examine the modeled daily toxaphene air concentrations from September 9 to 13, 2000, during which air concentration levels were 2-3 orders of magnitude higher than those derived from in situ measurements around the Great Lakes during the same year and during the 1990s. Meteorological conditions revealed that a typical deformation flow system associated with a high-pressure system extending from the east coast of Canada to the southern United States was one of the critical elements that enabled the transport of toxaphene to the Great Lakes. Cloud bands seen on satellite imagery and the rain band predicted by an atmospheric forecast model indicate that the system also delivered warm and humid air from the Gulf of Mexico and the southern United States to the Great Lakes. This resulted in strong wet deposition of toxaphene to the lakes. Substantial increase in the air concentration of toxaphene over the Great Lakes in this short period contributed greatly to raising the annual average daily air concentration for all of 2000. The results suggest that such an episodic event could be a major pathway for atmospheric transport of toxaphene from the southern United States to the Great Lakes.     

Perfluoroalkyl contaminants in the Canadian Arctic: evidence of atmospheric transport and local contamination

Perfluorosulfonates (PFSAs) and perfluorocarboxylates (PFCAs) have been hypothesized to reach remote locations such as the Canadian Arctic either indirectly as volatile precursor chemicals that undergo atmospheric transport and subsequent degradation, or directly via oceanic and atmospheric transport of the PFSAs and PFCAs themselves. Water, sediment, and air samples were collected from three Arctic lakes (Amituk, Char, and Resolute) on Cornwallis Island, Nunavut, Canada. Samples were analyzed for PFSAs and PFCAs, precursor chemicals including the fluorotelomer alcohols (FTOHs) and polyfluorinated sulfonamides (FSAs), and precursor degradation products such as the fluorotelomer unsaturated carboxylates (FTUCAs). PFSAs and PFCAs were detected in water and sediment of all three Arctic lakes (concentrations ranged from nondetect to 69 ng/L and nondetect to 85 ng/g dry weight, respectively). FTOHs and FSAs were observed in air samples (mean concentrations ranged from 2.8 to 29 pg/m3), and confirm that volatile precursors are reaching Arctic latitudes. The observation of degradation products, including FTUCAs observed in sediment and atmospheric particles, and N-ethyl perfluorooctanesulfonamide (NEtFOSA) and perfluorooctanesulfonamide (PFOSA) in air samples, indicate that degradation of the FTOHs and FSAs is occurring in the Arctic environment. PFSAs and PFCAs were also observed on atmospheric particles (mean concentrations ranged from < 0.1 to 5.9 pg/m3). In addition, results of this study also indicate that local perfluoroalkyl contamination of Resolute Lake, which is located downstream of an airport wastewater input, has occurred.     

Concentrations and spatial variations of polybrominated diphenyl ethers and other organohalogen compounds in Great Lakes air

Air samples were analyzed from urban, rural, and remote sites near the Great Lakes to investigate the occurrence, concentrations, and spatial and temporal differences of polybrominated diphenyl ethers (PBDE) in air. The concentrations of PBDEs were compared to those of other organohalogen compounds such as PCBs and organochlorine pesticides. The samples were collected in 1997-1999 as part of the Integrated Atmospheric Deposition Network (IADN). To minimize the variability of the data, we selected only samples taken when the atmospheric temperature was 20 +/- 3 degrees C. PBDEs were found in all samples, indicating that these compounds are widely distributed and that they can be transported through the atmosphere to remote areas. The total concentrations of PBDEs were similar to some of the organochlorine pesticides such as sigmaDDT and ranged from 5 pg/m3 near Lake Superior to about 52 pg/m3 in Chicago. In fact, the spatial trend was well correlated to those of PCBs. Our results indicate a relatively constant level from mid-1997 to mid-1999. At 20 +/- 3 degrees C, about 80% of the tetrabromo homologues are in the gas phase and about 70% of the hexabromo homologues are associated with the particle phase. Thus, particle-to-gas partitioning in the atmosphere is an important process for these compounds.     

Passive and active air samplers as complementary methods for investigating persistent organic pollutants in the Great Lakes Basin

Data obtained using passive air samplers (PAS) are compared to active high-volume air sampling data in order to assess the feasibility of the PAS as a method, complementary to active high-volume air sampling (AAS), for monitoring levels of organochlorine (OC) pesticides, polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PBDEs) in the Laurentian Great Lakes. PAS were deployed at 15 of the Integrated Atmospheric Deposition Network (IADN) sites on a quarterly basis between July 2002 and June 2003, and PAS and AAS results are compared. Levels for the OC pesticides are typically highest in agricultural areas, with endosulfan I dominating air concentrations with values ranging between 40 and 1090 pg x m(-3), dieldrin values between 15 and 165 pg x m(-3), and gamma-HCH values between 13 and 100 pg x m(-3). alpha-HCH was seen to be relatively uniform across the Great Lakes Basin with values ranging between 15 and 73 pg x m(-3). Large urban centers, such as Chicago and Toronto, have the highest levels of PCBs and PBDEs that range between 400 and 1200 pg x m(-3) and 10 and 70 pg x m(-3), respectively. Comparison of the AAS and the PAS data collected during this study shows good agreement, within a factor of 2 or 3, suggesting that the two sample methods produce comparable results. It is suggested that PAS networks, while providing data that are different in nature from AAS, can provide a cost-effective and complementary approach for monitoring the spatial and temporal trends of persistent organic pollutants.     

Snapping turtles (Chelydra serpentina) as bioindicators in Canadian areas of concern in the Great Lakes Basin. 1. Polybrominated diphenyl ethers, polychlorinated biphenyls, and organochlorine pesticides in eggs

We examined the concentrations and spatial patterns of congeners of PBDEs, PCBs, and organochlorine pesticides in snapping turtle (Chelydra serpentina) eggs from Areas of Concern (AOCs) on the Canadian shores of Lake Ontario, St. Lawrence River, and connecting channels. Eggs from Lyons Creek (Niagara River AOC) reflected a local PCB source over a range of 7.5 km (3.2-10.8) from the Welland Canal. PCB contamination in eggs declined with increasing distance from the Welland Canal, whereas the relative contribution of congeners associated with Aroclor 1248/1254 increased with sigma PCB concentrations. Compared to turtle eggs from other sites in Lake Erie and Lake Ontario, eggs from Lyons Creek and Snye Marsh had PCB congener patterns that reflected a strong contribution from Aroclor 1254. PCBs in the eggs were associated with industrial sources and reflected the composition of different Aroclor technical mixtures. Organochlorine pesticides in eggs tended to be highest at Hamilton Harbour and Bay of Quinte AOCs, and were dominated by DDE, sigma chlordane, and mirex. In contrast, PBDE congener patterns in turtle eggs resembled PentaBDE technical formulations regardless of absolute concentrations or location, and were largely associated with urban environments.     

Tracking toxaphene in the North American Great Lakes basin. 1. Impact of toxaphene residues in United States soils

A coupled atmospheric transport model was employed to study six scenarios to assess the contribution of reemission and long-range transport of toxaphene from different sources in the United States to its environmental fate in the Great Lakes ecosystem in the year 2000. Modeled air concentrations at the first model level (1.5 m) range from less than 5 pg m(-3) over the upper Great lakes (Lakes Superior and Huron) to several tens of picograms per cubic meter over the lower Great Lakes (Lakes Erie and Ontario) in the summer but drop off to the range from 0.05 to 2 pg m(-3) in the wintertime. The modeled toxaphene depositions to the lakes suggest a decreasing trend from the mid-1990s to 2000. Modeling results showed that, on an annual basis, for the Great Lakes basin as a whole, the southeast U.S. sources made the largest contribution to the toxaphene air concentrations and dry and wet depositions at 72%, 78%, and 88% respectively. The model results also showed that a significant proportion of these contributions occur during relatively short episodic events due primarily to the interseasonal changes in atmospheric circulation patterns.