Significance of Population Centers As Sources of Gaseous and Dissolved PAHs in the Lower Great Lakes

11 Polyethylene passive samplers (PEs) were used to measure concentrations of gaseous and 12 dissolved polycyclic aromatic hydrocarbons (PAHs) in the air and water throughout the lower 13 Great Lakes during summer and fall of 2011. Atmospheric Σ 15PAH concentrations ranged from 2.1 14 ng/m 3 in Cape Vincent (NY) to 76.4 ng/m 3 in downtown Cleveland (OH). Aqueous Σ 18PAH 15 concentrations ranged from 2.4 ng/L at an offshore Lake Erie site to 30.4 ng/L in Sheffield Lake 16 (OH). Gaseous PAH concentrations correlated strongly with population within 3-40 km of the 17 sampling site depending on the compound considered, suggesting that urban centers are a 18 primary source of gaseous PAHs (except retene) in the lower Great Lakes region. The 19 significance of distant population (within 20 km) versus local population (within 3 km) increased 20 with sub-cooled liquid vapor pressure. Most dissolved aqueous PAHs did not correlate 21 significantly with population, nor were they consistently related to river discharge, wastewater 22 effluents, or precipitation. Air-water exchange calculations implied that diffusive exchange was a 23 source of phenanthrene to surface waters, while acenaphthylene volatilized out of the lakes. Comparison of air-water fluxes with temperature suggested that the significance of urban centers as sources of dissolved PAHs via diffusive exchange may decrease in warmer months.


INTRODUCTION
Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants that originate from oil spills as well as anthropogenic and natural combustion processes.2][3][4][5] PAHs are often associated with densely populated areas, especially in industrialized countries. 3,4,6,7PAHs and their transformation products are a primary carcinogenic component of urban air pollution and health effects resulting from chronic exposure are a serious concern.

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Polyethylene passive samplers (PEs) are cost-effective, simple tools with lower detection limits than traditional active sampling techniques.Instead of pumping air or water through a filter, PEs accumulate hydrophobic organic contaminants (HOCs) over time via diffusion, accumulating only truly dissolved or gas-phase molecules. 10Concentrations of truly dissolved HOCs are of interest because this fraction is available for direct diffusive exchange between water and other reservoirs such as air, biota, or sediment.
The use of PEs facilitates simultaneous spatially resolved measurements and calculations of air-water diffusive exchange rates.For most HOCs, concentrations measured by PEs reflect a time-integrated concentration representative of the entire deployment period.For compounds that equilibrate during deployment, concentrations reflect the most recent concentration the sampler was exposed to.PEs have previously been used to measure HOCs in water and air 11-13     and to calculate air-water gradients of HOCs, but this method has not been applied to the lower Great Lakes.

14-16
Lake Erie and Lake Ontario are the smallest of the Great Lakes by volume and have estimated residence times of 2.7 and 7.5 years, respectively. 17About 80% of Lake Erie's water is supplied by the Detroit River, which is fed by Lake Huron via Lake St. Claire.Among the Great Lakes, Lake Erie is the shallowest (average depth 19 m), warmest, and most biologically productive. 18,19Lake Ontario is much deeper (average depth 86 m) and primarily receives water from Lake Erie via the Niagara River. 19Currents in the Great Lakes are weak (a few cm/s) with complex temporal variability that depends on recent atmospheric conditions.In the summertime, circulation is generally counterclockwise. 20The lakes are stratified from May through October and well-mixed for the remainder of the year.20     Heavy urbanization and valuable ecosystems often coincide along the shores of the lower Great Lakes.Atmospheric deposition from urban sources has been identified as a major source of gaseous and particle-bound HOCs to the region's aquatic environment. 2,7,21,22Concentrations of total atmospheric PAHs have been shown to correlate strongly with population in this region and urban centers have been linked to significantly increased loadings of contaminants to the lakes. 23,24In some cases, however, the lakes have been found to act as a source of HOCs via revolatilization. 25,26Much of the previous work describing sources of atmospheric pollution to the Great Lakes is based on a limited number of air monitoring sites as part of the Integrated Atmospheric Deposition Network (IADN).While this data is indispensible in determining baseline concentrations and temporal trends of POPs in the Great Lakes, more detailed knowledge of spatial trends is crucial to identify major sources and transport pathways.
The objectives of this study were to (1) provide baseline concentrations of gaseous and dissolved PAHs in Great Lakes air and water, (2) evaluate the importance of urban regions as sources of dissolved PAHs by investigating the relationship between population and PAH concentration, and (3) determine whether the lower Great Lakes are sources or sinks for dissolved PAHs.

Passive Sampling Procedures.
A map of all air and water sites and a table outlining temporal coverage and meteorology are provided in the Supporting Information (Figure S1, Table S1 & S2) along with information on sampler preparation and deployment.PEs were spiked with performance reference compounds (PRCs) via a method adapted from Booij et al. 27 and sent to trained volunteers throughout the Great Lakes region with the aim of completing three 8-week deployments at each site.After deployment, volunteers returned samplers via overnight delivery.
Four sites formed an east-west transect along Lake Ontario's southern shore.The westernmost site, Grimsby (ON), was an offshore buoy monitored by Environment Canada.On Lake Erie, samplers were deployed at nine US shoreline sites and six offshore sites monitored by Environment Canada.Samplers were deployed at the offshore sites once, during late summer.
Samplers at Gibraltar Island (OH) and Toledo (OH) were deployed once during late spring/early summer.

Meteorological Information & Site Characteristics.
Monthly wind speed averages during the sampling campaign ranged from 3.8 m/s in July to 6.1 m/s in November, with the greatest average wind speeds offshore of Toledo.Average air temperatures ranged from 7.7°C in April to 24.3°C in July and the mean deployment temperature for all sampling periods was 18.6±1.8°C.
Surface water temperatures were generally very similar to air temperatures and ranged from 3.7˚C (Lake Ontario in May) to 25.1˚C (Lake Erie in July). 28There were westerly prevailing winds during the sampling campaign for most of the study region (Figure S2). 29Precipitation and river discharge were lowest during June and July while flows in late spring and early fall were similar. 30Locations near major rivers are listed in Table S4.PAH concentrations were corrected for internal standard recoveries (Table S5) and blanksubtracted using the field blank relevant to the sampling site.If no field blank for the site was available, the average concentration from all available field blanks was used.More information on quality assurance and quality control is in the Supporting Information.

Sample
Determination of Sampling Rate and Ambient Concentration.The uptake of HOCs by PEs is described in detail by Lohmann 31 and PE-air partitioning is detailed by Khairy et al. 11 To determine ambient PAH concentrations from concentrations in polyethylene, site-specific sampling rates were estimated via a method adapted from Booij et al. 32 Further details are provided in the Supporting Information.The average air sampling rate was 28±17 m 3 /day and the average aqueous sampling rate was 112±57 L/day (for more details, see SI and Tables S1 and    S2).
Physico-chemical Parameters.Sampler-matrix partition coefficients (T=25°C) used to calculate ambient concentrations for each PAH are listed in Table S7, along with other physico-chemical properties.Temperature-adjusted partition coefficients were obtained using mean temperature during the deployment period for the nearest meteorological buoy or weather station and the modified van't Hoff equation, as in Khairy et al. 11 The enthalpy of vaporization (∆ H vap ) was used to account for K PEA -temperature sensitivity and internal energy of dissolution (∆ U U w ) for K PEWtemperature sensitivity.
Population Analysis.Population data for each sampling site are presented in Table S8.Total population within a circular area with a 1-cell (about 1 km) radius was calculated using the Focal Statistics tool in ArcMap.The process was repeated for larger radii to create a dataset of the total population within 1, 2, 3, 5, 10, 15, 20, 25, 30, 40, and 50 km of each of the sampling locations.
More information about the population dataset used is available in the Supporting Information.
Air-Water Exchange Rates.The difference in equilibrium concentrations of an HOC in two PEs deployed in different matrices is proportional to the difference in the compound's chemical activity between those two matrices. 14,15Air-water exchange gradients can therefore be determined from the ratio of PAH concentrations in PEs deployed simultaneously in air and water, corrected to equilibrium concentrations using PRC loss data.Details of air-water exchange calculations are shown in the Supporting Information.S9.Gaseous PAHs were dominated by phenanthrene (28 -60%) and fluorene (6 -48%) (Figure S4).Typical concentrations ranged from below the detection limit to  The two sites in Cleveland consistently displayed the greatest concentrations of gaseous PAHs except retene throughout the deployment season.Retene is often considered to be indicative of wood smoke or pulp/paper mill effluent, as opposed to fossil fuel combustion.

34,35
Retene was greatest west of Cleveland in Sheffield Lake, but even here accounted for less than 0.7% of total gaseous PAHs.In contrast, Ruge found retene to be a significant component of gaseous PAH profiles at many sites on Lake Superior.
Principal component analysis (PCA) using the FactoMineR package 37 in the statistical programming language R 38 was employed to visualize similarities and differences between PAH profiles (Figure S5).Profiles were similar at all sites with the exception of Sheffield Lake, Rochester, and the Cleveland sites, which were clustered separately.The clustering of most sites in the same region of the plot suggests that sources of PAHs were similar across the study region.
Profiles in Cleveland may have been distinct due to nearby point sources.In addition to impacts from vehicular emissions associated with heavy traffic in downtown areas, these sites were within 5 km of a greater number of industrial point sources (primarily chemical manufacturing, petroleum industry, and metalworking facilities) when compared to the other sites using the EPA Toxic Release Inventory (TRI).PAH concentrations in this study were comparable to those measured by Ruge at urban locations along the shore of Lake Superior. 36 S8).Mean Σ 15PAH for each type of site are summarized in Table 1.For both lakes, the greatest concentrations of gaseous PAHs were observed at urban sites.However, Σ 15PAH was not significantly different based on site classification using a one-way ANOVA (p > 0.05).There were no obvious changes in PAH profile composition based on whether the site was urban, semi-urban, rural, or remote (Figure S4). 3) (Figure 2).Significant correlations (0.58 < r 2 < 0.77, p < 0.001) were observed for all measured PAHs at some radius, with retene exhibiting the weakest correlation (r 2 1 km = 0.30 at a radius of 1 km, p = 0.02, SE=0.02).This is most likely due to retene's association with wood smoke, as opposed to fossil fuel combustion.34, 35 Strong correlations suggest that urban centers are a primary source of gaseous PAHs (except retene) in the lower Great Lakes region.
For each PAH, the strength of the correlation between population and concentration varied as we changed the radius used to characterize population at the site (Figure 3).All compounds except retene displayed a bimodal relationship, with two radii of maximum correlation.This relationship was less pronounced for the low molecular weight (LMW) PAHs than HMW PAHs.Strong similarities between correlation profiles (e.g., the 5-ring PAHs) suggest similar sources and affinities for transport.
Hafner and Hites suggested that the significance of local sources in determining Great Lakes HOC concentrations varies based on a compound's atmospheric lifetime. 7 The atmospheric lifetimes of gaseous PAHs is determined primarily by susceptibility to hydroxyl degradation and gas-particle partitioning. 7Anthracene exhibited a distinctly shaped correlation curve with two maxima at radii 25 km (r PAHs. 44,45However this does not explain the comparable correlation at 25 km.Acenaphthylene is expected to have a similar lifetime to anthracene (1.6 hrs) 44 and exhibited stronger correlations with more local population than fluorene (Figure 3).
Fluorene is often observed to be more stable with respect to photochemical oxidation than similarly-sized PAHs (average lifetime 22 -26 hrs), 44,46 but more distant sources did not become more significant for this compound due to its longer lifetime.Fluorene correlated less strongly with population than acenaphthylene at all radii, but the divergence was largest at shorter distances.Gaseous HMW PAHs are expected to have short atmospheric residence times due to reaction with hydroxyl radicals, which may contribute to the increased relevance of local versus long-range sources that was observed for these compounds. 7These results suggest that reaction with hydroxyl radicals limited the importance of sources distant from sampling sites.
8][49] Sub-cooled liquid vapor pressures (p L /Pa) for all PAHs (except methylphenanthrenes and retene, for which data was not available) were determined for average deployment temperature (18.6°C) using empirical regressions from Paasivirta et al.Gustafson et al. 53 Due to their lower vapor pressure, gaseous HMW PAHs are more likely than 2-3-ring PAHs to partition into the particulate phase where they will not be measured by PEs and may be deposited more readily via wet or dry deposition.

42,54,55
Previous studies have reported that coastal areas receiving cleaner air from over water bodies exhibit lower atmospheric PAH concentrations than would be predicted based on surrounding population. 56Concentrations of total atmospheric PAHs were lower in Buffalo and Oswego than Cleveland or Rochester, though these sites were classified similarly in terms of population.One explanation is that prevailing westerly winds brought over-lake air towards Buffalo and Oswego, diluting the urban plume.Offshore measurements confirmed that air masses over Lake Erie had relatively lower PAH concentrations than shoreline sites (Table 1).
Likewise, the offshore site near Grimsby was closer to the shoreline and more likely to be impacted by westerly air masses arriving over land from Hamilton (ON).
To further explore this hypothesis, 6-hour HYSPLIT back trajectories were calculated every 24 hours during the entire deployment period at Cleveland, Buffalo, Rochester, and Oswego using EDAS 40km archived meteorology .The number of trajectories arriving from over water versus over land was counted (Table S3).This analysis supports the idea that Oswego's urban plume could be diluted by over-water air masses, but shows that Buffalo and Cleveland received similar amounts of air traveling from over water and over land.
Another explanation for lower concentrations at Oswego and Buffalo could be the amount or type of industry nearby.EPA TRI 39  PCA results for dissolved PAHs showed locations clustered differently than for gaseous PAH composition, suggesting that source profiles differed for atmospheric and aqueous PAHs.
This may be because in addition to atmospheric deposition, runoff and sediment-water exchange contributed to dissolved concentrations.The dissolved PAH profile was most distinct at Sheffield Lake, while Toledo and Buffalo, both expected to be impacted by river discharge, were clustered together (Figure S5).
Comparison with Literature Values.Dissolved PAH concentrations were similar to those reported by Ruge for heavily impacted sites on Lake Superior. 36Previous work in Lake Michigan reported average total dissolved aqueous PAH concentrations of 9 ng/L from shipboard measurements, which was similar to the mean dissolved Σ 18PAH concentration of all sites in this study (9.1 ng/L). 2 Concentrations reported here were generally greater than surface waters of Narragansett Bay (RI) 14 or the Patapsco River (MD), 57 though maximum concentrations measured on the Patapsco exceeded maximum concentrations measured here.Concentrations were lower than dissolved PAHs in a freshwater lake in China. 42PAH profiles were similar to those reported for Narragansett Bay.The lack of strong correlations also suggests that sources other than atmospheric deposition, such as river discharge and WWTP effluent, could have been significant in determining dissolved PAH concentrations in surface waters.In addition, longer-term reservoirs that are not representative of current emissions, such as PAHs from sediments or from deeper in the water column, could be contributing to surface concentrations so that aqueous concentrations reflect longer term deposition while atmospheric concentrations reflect recent emissions.
However, summertime stratification occurring throughout most of the sampling period is expected to reduce the importance of these contributions.
Concentrations at offshore Lake Erie sites were greatest in the western basin where the lake is shallowest and inputs from the Detroit and Maumee watersheds, both AOCs, were expected to be significant (Figure S3A).Due to the central Erie basin's counterclockwise circulation during the study season, 20 it is unlikely that elevated dissolved PAHs in Sheffield Lake resulted from aqueous transport from Cleveland.Black River, a historically polluted AOC, discharges 8 km west of the Sheffield Lake site and may have contributed to dissolved PAH concentrations there.More measurements over time are needed to determine whether elevated dissolved PAHs at Sheffield Lake were episodic or chronic.Unexpectedly, concentrations near Cleveland were lower than at Sheffield Lake.This may be because of sampler placement, as PEs at Cleveland were farther offshore where water was deeper and currents carrying more highly impacted water may have been entrained closer to the shore.
Besides Sheffield Lake, the greatest dissolved PAHs were measured in Toledo, Buffalo, and Erie.Average dissolved PAH concentrations in Erie sampled from early June to early September were greater (Σ 18PAH = 11.4 ng/L) and showed a lower percent contribution from LMW PAHs (Figure S5) than other rural sites, possibly due to contributions from contaminated sediments or WWTP effluent.The Erie site was within the recently delisted Presque Isle Bay AOC, which was dredged for the first time in 20 years during summer of 2011, possibly releasing elevated concentrations of PAHs into the water column. 15,58,59The greatest concentrations were seen during the second deployment, which took place in early fall (Σ 18PAH = 15.6 ng/L), perhaps due to the weakening of summertime stratification.The site was also within 5 km of a major (~ 150 million liters/day) WWTP (Figure S3A).
Air-Water Exchange.Mass transfer coefficients and flux gradients are listed in Tables S11 and    S12 and flux gradients for select PAHs are presented in Figure S9.Mass transfer velocity ranged from 0.2 cm/day to 73 cm/day and values decreased with decreasing volatility.Uncertainty in flux gradients was <30% for all compounds of lower molecular weight than benz(a)anthracene except retene.Flux gradients for HMW PAHs were not different from equilibrium within the 95% confidence level.
Net flux rates (ng/m 2 /day) are provided in Table S13.Patterns in flux direction were similar to those reported by Bamford et al. in that LMW PAHs were volatilizing and phenanthrene was being absorbed but less volatilization was seen here than in Patapsco River and depositional fluxes of phenanthrene in our study were greater on average. 57Fluxes for acenaphthylene, phenanthrene, methylphenanthrenes, and pyrene at each site were summarized in Figure 5 over three time periods: April -June, June -August, and August -November.and suggested that river input and runoff were more significant sources of dissolved PAHs than atmospheric deposition. 14Volatilization at Niagara may indicate that river discharge was a significant source of PAHs at these sites.
Air-water exchange is strongly influenced by air temperature, wind speed, and wind direction and large daily variations in fluxes have been observed. 57During deployments where mean temperature was greater than 19°C, phenanthrene and anthracene were the only PAHs

IMPLICATIONS
Strong correlation with population suggests that urban centers played an important role in determining spatial distributions of gaseous PAHs.However, air-water fluxes and distributions of dissolved PAHs implied that additional sources beyond diffusive exchange influenced aqueous distributions, especially in urban areas.In some cases surface waters acted as a source of PAHs to the atmosphere.Enhanced spatial coverage near AOCs and major urban areas like Air.Average atmospheric Σ 15PAH ranged from 2.1 ng/m 3 in Cape Vincent (NY) to 76.4 ng/m 3 at George T. Craig air sampling station in downtown Cleveland (OH).The spatial distribution of Σ 15PAH is shown in Figure 1A.Concentrations of all PAHs during each deployment are detailed in Table

8
To explore relationships with population in more detail, population within discrete radii of 1 to 40 km from each site were compared to average atmospheric PAH concentrations to determine the importance of local versus distant contributions in determining PAH concentrations.Total gaseous PAHs correlated most strongly with population within a 20 km radius around each site (r 2 20 km = 0.73, p < 0.001, n = 17, SE=11. 0.77).Anthracene has a shorter lifetime (1.5 hrs) with respect to hydroxyl radical degradation relative to other PAHs, which may explain why stronger correlation is observed at short distances than for other 3-ring

2 20 km /r 2 3
/Pa) was plotted against the radius where maximum population-concentration correlation was seen for each compound in Figure S6.Excluding anthracene, PAHs with p L > 10 -4 Pa were most highly correlated with population within a 20 km radius, while PAHs with p L < 10 -4 Pa were most highly correlated with population within 3 km.Other studies have observed similar values for log(p L ) at which PAHs transition from being primarily gaseous to particle-bound.26,51,52 While Figure S6 highlights maximum correlation, many PAHs exhibited significant correlation with population at both 20 km and 3 km.As shown in Figure 4, the relative significance of correlation at 20 km versus 3 km (r km ) was significantly correlated with log(p L ) (r 2 = 0.62, p < 0.005, n = 13, STE = 0.1), suggesting the existence of two sources of varying importance depending on PAH volatility.The relatively greater importance of local sources in determining concentrations of gaseous HMW PAHs could be due to the partitioning of these compounds to relatively cleaner background aerosols at remote sites as described by

14 Potential
Sources of Dissolved PAHs.Linear correlation with population was not significant (r 2 < 0.3, p > 0.05) for dissolved PAHs, with the exception fluorene (59, p < 0.001).The explanation for correlations observed for these three compounds is unknown.One possible reason for the weak correlation for most aqueous PAHs is that the two most populated sites in downtown Cleveland were absent from the aqueous dataset.Aqueous sampling near Cleveland was not done at the same sites as air sampling, rather PEs were deployed further from shore.
being absorbed into surface waters, with the exception of measurements from Oswego (3 rd deployment) as well as Dunkirk.In Buffalo, most PAH fluxes changed from net deposition during the first deployment (mean temperature of 11 °C) to net volatilization during the second deployment (mean temperature 19 -20.5 °C).During the third deployment, most fluxes were not significantly different from equilibrium.In Oswego, the temperature dependency observed in Buffalo was not evident.

Figure 1 .
Figure 1.PAH Concentrations in Air (A) and Water (B).Average gaseous Σ 15PAH (A) and dissolved Σ 18PAH (B) in Lake Erie and Lake Ontario.Orange shading delineates population centers.

Figure 2 . 3 )Figure 3 .
Figure 2. Atmospheric Σ 15PAH and Population within 20 km.Average atmospheric concentrations of gaseous PAHs at each site correlated well with population within 20 km.The

Figure 4 . 2 20
Figure 4. Relative significance of population within 20 km and 3 km.The ratio of r 2 20 km to r 2 10

Figure 5 .
Figure 5. Net Air-Water Flux of Four PAHs.Air-water fluxes (ng/m 2 /day) for four PAHs Melymuk et al. measured a total gaseous PAH 43Gaseous PAHs and Population.Sampling sites were classified as urban, semi-urban, rural, or remote based on population within 3 km (Table