Wet deposition loadings of organic contaminants to Lake Ontario: Assessing the influence of precipitation from urban and rural sites

Atmospheric Environment 45 (2011) 5042e5049

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Wet deposition loadings of organic contaminants to Lake Ontario: Assessing the influence of precipitation from urban and rural sites Lisa Melymuk a, Matthew Robson b, Miriam L. Diamond a, b, Lisa E. Bradley c, Sean Backus c, * a

Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada Department of Geography and Program in Planning, University of Toronto, 100 St. George Street, Toronto, Ontario M5S 3G3, Canada c Ontario Fresh Water Quality Monitoring, Environment Canada, Canada Centre for Inland Waters, 867 Lakeshore Road, Burlington, Ontario L7R 4A6, Canada b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 July 2010 Received in revised form 31 January 2011 Accepted 2 February 2011

Wet deposition to Lake Ontario has been examined through a comparison of concentrations of polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides (OCPs), and brominated flame retardants (BFRs) in precipitation at three sites on the north shore of Lake Ontario: one rural, one suburban, and one urban site. Concentrations of SPAHs, BFRs, SPCBs, Schlordanes and g-HCH in precipitation are highest at the urban site, while concentrations of other OCPs were similar across all three sites. Loadings via wet deposition range from 0.42 kg year
1 for Schlordanes to 1900 kg year
1 for SPAHs. The distribution of concentrations reflects the use/emission pattern of the persistent organic pollutants (POPs), and indicates that concentrations in precipitation are predominantly the result of local sources rather than long-range transport from other regions. While elevated urban concentrations increase wet deposition in the urban region itself, this influence decreases rapidly downwind of the urban area. Chemical loads in precipitation from the highly urbanized regions bordering the Great Lakes are estimated to increase wet deposition loadings to lake areas adjacent to the urban areas. Estimates of annual wet deposition loadings of POPs to Lake Ontario indicate that when considering the influence of elevated loadings from Toronto, loadings via precipitation are 2.5%e42% higher depending on the compound, with the greatest relative increase in loadings resulting from PCBs and Schlordanes. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.

Keywords: Precipitation Persistent organic pollutants Urban areas Great Lakes

1. Introduction Wet deposition is a significant pathway for loading of persistent organic pollutants (POPs) to the Great Lakes. For Lake Ontario, wet deposition contributed between 6% (for phenanthrene) and 27% (SPCBs) of total atmospheric loadings in 2004e2005 (Blanchard et al., 2008). Furthermore, precipitation scavenges both gas-phase and particle-associated POPs from the atmosphere, and concentrations in wet deposition reflect those in air (Poster and Baker, 1996). The Great Lakes basin has a long record of environmental research and monitoring, particularly for POPs. Since 1990, the Integrated Atmospheric Deposition Network (IADN) program has been monitoring sites spanning the whole of the Great Lakes basin for selected contaminants in air and precipitation, with the goal of determining

* Corresponding author. Tel.: þ1 905 336 4646. E-mail addresses: [email protected] (L. Melymuk), matthewrobson@ hotmail.com (M. Robson), [email protected] (M.L. Diamond), lisa. [email protected] (L.E. Bradley), [email protected] (S. Backus).

the input of pollutants to the Great Lakes via atmospheric deposition. This is accomplished through the monitoring of air and precipitation at five IADN master stations, located in rural areas adjacent to each of the five Great Lakes (Hoff et al., 1996; Environment Canada, 2003). However, none of these sites capture the influence of cities on Great Lakes atmospheric loadings. Cities hold major stocks of many contaminants and consequently are significant sources to the surrounding environment, notably for polychlorinated biphenyls (PCBs) (Diamond et al., 2010; Robson et al., 2010), polybrominated diphenyl ethers (PBDEs) (Harrad and Hunter, 2006) and other brominated flame retardants (BFRs) such as hexabromocyclododecane (HBCD) (Hale et al., 2006), polycyclic aromatic hydrocarbons (PAHs) (Van Metre et al., 2000) and selected organochlorine pesticides (OCPs) (Sun et al., 2006b). This paper identifies the influence of urban areas on concentrations of POPs in precipitation and subsequent loadings via wet deposition to Lake Ontario through a comparison of three IADN sites representing urban, suburban and rural locations. The compounds examined in this study are PAHs, PCBs, BFRs (PBDEs and HBCD), and OCPs.

1352-2310/$ e see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.02.007

L. Melymuk et al. / Atmospheric Environment 45 (2011) 5042e5049

2. Experimental section

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months (January 2007 to February 2009). The Toronto site was sampled for 19 months (August 2007 to February 2009).

2.1. Sites 2.3. Extraction and instrumental analysis Precipitation samples were collected at three sites adjacent to Lake Ontario (Fig. 1). The sites spanned a 230 km stretch of the north shore of the lake, from Burlington in the west to Point Petre in the east. The Burlington sample collection site is an IADN satellite station, and is located in a peri-urban area approximately 8 km from the Burlington city centre, with a population density in the surrounding area of 26 person/km2. The Toronto site is a recently established IADN precipitation monitoring site located at the University of Toronto in downtown Toronto (population of 2.7 million, population density of 6856 persons/km2). The Point Petre site is the Lake Ontario IADN master sampling station, located in a rural area on the eastern shoreline of Lake Ontario (population density of 5 persons/km2). 2.2. Sample collection Monthly integrated precipitation samples were collected using a MIC-B wet-only precipitation automated sampler (Meteorological Instruments of Canada, Thornhill, Ontario). The sampler comprised a stainless steel funnel with an area of 0.212 m2 connected to an amber glass sampling bottle containing 200 mL of dichloromethane (DCM) enclosed in a weatherproof housing. The roof of the sampler opened automatically whenever it rained or snowed (with the aid of a moisture sensor) and closed as soon as the precipitation stopped, thus ensuring only wet deposition was collected. A small heater was added inside the sampler housing to prevent samples from freezing during winter. The average duration of each sampling period was 30 days. After each sampling period, the stainless steel funnel was rinsed with DCM, and the DCM was then added to the sample. The Burlington and Point Petre sites were sampled for 26

Each month samples were collected from the sites, returned to the National Laboratory for Environmental Testing (NLET) in Burlington, Ontario, and stored at 4  C until analyzed. The aqueous phase was separated from the DCM in a separatory funnel, and the volume of precipitation was measured. The aqueous phase was then extracted twice with fresh DCM. The extracts were combined and concentrated by rotary evaporation and solvent exchanged to isooctane. The extract was then fractionated on 3% (w/w) waterdeactivated silica gel. Hexane was first used to elute PCBs, p,p-DDE, p,p-DDT and o,p-DDT and some PAHs and PBDEs. The remained OCPs, PAHs, PBDEs and HBCD were then eluted with 1:1 hexane:DCM. The extracts were concentrated by nitrogen evaporation to 1.0 mL. The two fractions were analyzed for OCPs by dual-capillary gas chromatography (GC) with electron capture detection on an Agilent 6890 GC. A 30 m HP-5 MS column was used as the primary column, and detections were verified on a 30 m HP1-MS column. PCBs were quantified using the HP-5 MS column only. For PAH analysis, equal amounts of the two fractions were combined and spiked with d10anthracene, d12-perylene, and d12-benz-[a]-anthracene as internal standards. The PAHs, PBDEs, and HBCD were quantified on an Agilent 6890 GC with a 5973 MSD using a 25 m HP-5-MS column. The PAHs were analysed in EI mode and the PBDEs and HBCDs were analysed in NICI mode. Analysis of BDE-209 was performed with the same instrument conditions as for the other BFRs using a 15 m HP-1MS column. Samples were analyzed for 10 groups of compounds: SPCBs, SPAHs, SPBDEs, HBCD, a-HCH, g-HCH, a- and g chlordane, a- and b-endosulfans, p,p’-DDT, o,p’-DDT, p,p’-DDE, and p,p’-DDD, and dieldrin. SPCBs is the sum of 77 PCB congeners,

Fig. 1. Precipitation Collection Site Locations.

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SPAHs is the sum of 19 PAH compounds, SPBDEs is the sum of 15 PBDE congeners. The complete list of compounds is given in Table S1 of the Supplementary Information. 2.4. Quality control and quality assurance Data were collected and analysed following the procedures of the IADN program. Quality assurance procedures are described in detail by Wu et al. (2009). Field blanks were collected every 12 samples at each site. As no seasonal or site-based variation was evident in the blanks, samples were blank corrected by subtracting an average blank from each sample. The field blank mass was on average 4% of PAH sample mass, 15% of PCB mass, 5% of BFR mass, and 1% of OCP mass. Surrogate recovery standards were added to the samples (PCBs-30 and -204, d8-naphthalene, d10-fluorene, d10pyrene, d12-benzo(a)pyrene, endo-ketone, d-HCH, BDE-71, 13CBDE-209, and d16-g-HBCD). Average recoveries were 80% for PAHs, 90% for PCBs, 82% for BFRs and 89% for OCPs. Method detection limit are listed in Table S1. 2.5. Meteorology Climate data (temperature, wind speed and direction, precipitation, and humidity) from the meteorological station nearest to each sample collection site were obtained from the Environment Canada National Climate Data and Information Archive (Environment

Canada, 2009). The sites demonstrate strong similarities in meteorological conditions over the sampling period. In a one-way ANOVA test of monthly average data for the three sites, no significant difference was found among sites in temperature (F ¼ 0.22, p ¼ 0.79, df ¼ 2,69) or precipitation (F ¼ 1.92, p ¼ 0.15, df ¼ 2,69). Details are given in Figures S1 and S2. 3. Results and discussion 3.1. Concentrations and volume weighted means Concentrations are presented in Fig. 2 and Table S2 for 10 analytes. Concentrations ranged from 0.02 ng L
1 of Schlordanes (Burlington) to 254 ng L
1 SPAHs (Toronto). As each sample represents the total precipitation for one month, the monthly measurements are equivalent to monthly volume weighted means (VWM). Annual volume weighted means, calculated as the sum of the mass of compound collected for the whole year divided by the volume of precipitation for the whole year, were calculated for 2007 and 2008 (Table 1). The standard error of the VWM was calculated according to the method of Hawley et al. (1988). 3.2. Spatial variations One-way ANOVA was used to determine which concentrations differed significantly among sites. Toronto precipitation concentration

Fig. 2. (aee): Average concentrations for each compound by site. The error bars denote one standard deviation. Figure (a) is banned pesticides, (b) is current/recent use pesticides, (c) is PAHs, (d) is PCBs and (e) is BFRs.

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Table 1 Volume weighted mean precipitation concentration  standard error. Concentration (ng/L)

Burlington

Year

2007

a-HCH g-HCH aþb endosulfan

0.36 0.21 4.0 0.66 0.05 0.31 120 1.3 0.59 2.0

SDDT aþg chlordane dieldrin SPAH SPCB HBCD SPBDE

         

Toronto 2008

0.11 0.06 1.3 0.24 0.01 0.07 16 0.30 0.08 0.23

0.26 0.18 2.4 0.26 0.03 0.15 110 0.70 1.2 1.1

         

2007 0.11 0.05 1.1 0.10 0.01 0.06 13 0.13 0.43 0.11

0.33 0.41 1.6 0.49 0.13 0.17 240 9.9 8.0 4.5

were significantly higher (p > 0.05) for PAHs, PCBs, PBDEs, HBCD, g-HCH, and Schlordanes. a-HCH, Sendosulfans, dieldrin and SDDT precipitation concentrations were statistically indistinguishable between sites (Table 2). Since all the sites had similar air mass back trajectories (details listed in SI) and meteorology, but significant differences in concentrations of certain compounds, differences between sites were likely due to local/regional sources within the lower Great Lakes region rather than long-range transport from other regions of North America. The prevailing west-to-east wind direction suggests that high concentrations in Toronto could influence wet deposition downwind at Point Petre, however the measured concentrations did not support this hypothesis: the direct influence of urban precipitation concentrations did not appear to extend to Point Petre, which lies 182 km downwind of Toronto. Despite elevated concentrations of PCBs, PBDEs, and PAHs in Toronto, concentrations in Point Petre precipitation were similar to those at other IADN master stations. This matches reports of the similarity of air concentrations at Point Petre to those at the remote IADN stations on the upper Great Lakes for PCBs (Sun et al., 2007), PAHs (Sun et al., 2006d), and OCPs (Sun et al., 2006c). We hypothesize that urban areas elevate precipitation concentration in their immediate vicinity, and then contribute to ubiquitous regional contaminant levels in the Great Lakes basin that exceed those in remote locations. For example, while rural air or precipitation concentrations of PCBs at Point Petre were lower than urban concentrations, they were still significantly higher than concentrations at remote locations such as the Canadian arctic (MacDonald et al., 2000; Blanchard et al., 2008). The urban plume may extend farther for gas-phase air concentrations than for either precipitation or particlephase air concentrations (inter alia Hodge and Diamond, 2010). 3.3. OCPs e spatial variations Pesticides can be grouped according to their regulatory status in North America. While DDT, a-HCH, dieldrin and chlordane were all

         

Point Petre 2008

0.23 0.21 0.33 0.21 0.06 0.03 56.1 4.4 4.8 0.74

0.38 0.34 2.01 0.22 0.09 0.17 220 13 1.5 2.4

         

2007 0.22 0.08 0.69 0.05 0.03 0.04 17 3.3 0.22 0.37

0.21 0.13 4.1 0.25 0.02 0.14 71 0.88 2.6 1.6

         

2008 0.06 0.03 2.66 0.16 0.01 0.03 7.9 0.24 8.2 34

0.31 0.23 1.8 0.09 0.01 0.10 77 1.3 0.47 1.1

         

0.07 0.05 0.80 0.02 0.004 0.02 6.4 0.42 0.07 0.14

used extensively in the past in the United States and Canada, their use has since been banned (DDT in 1974, a-HCH in 1985, dieldrin in 1984 and chlordane in 1995) (Environment Canada, 2005). Endosulfan is still currently used in Canada and the USA, although use has declined as residential and ornamental use was banned in 2004 (Tuduri et al., 2006). Endosulfan use is under review by both Health Canada and by the Stockholm Convention (United Nations Environment Program, 2008; Health Canada, 2009). g-HCH (lindane) was very recently banned in 2006 in Canada and 2007 in USA. The sampling period overlapped with the end of its use. Concentrations of DDT, dieldrin and a-HCH in precipitation were not significantly different among sites (Fig. 2a). This result is expected given that current environmental concentrations result from past use of the chemicals and their subsequent cycling within environmental compartments. Spatially uniform concentrations of these pesticides have also been observed in other studies of air and precipitation (Chan et al., 2003; Motelay-Massei et al., 2005; Sun et al., 2006b), including at the IADN stations spanning the Great Lakes basin (Sun et al., 2006b, 2006c). Despite ongoing North American use, endosulfan showed no significant difference in concentrations among sites (p ¼ 0.31) (Fig. 2b), which suggests that either the contributions from urban and rural areas are of comparable magnitude, or long-range transport of endosulfan is leading to equal wet deposition at all three sites. Support for the former explanation “comes” from evidence that endosulfan use and air concentrations have been highest in Southern Ontario relative to the rest of Canada (Chan et al., 2003; Yao et al., 2008), and the isomer ratios were consistent amongst the sites. Furthermore, Point Petre had elevated endosulfan concentrations in air and precipitation compared with other IADN master stations (Sun et al., 2006b, 2006c). Despite its ban in the late 1980s/early 1990s, chlordane concentrations in precipitation had significantly higher concentrations at the more urban sites (Fig. 2a), possibly resulting from the use of chlordane as a termiticide during home construction (Leone et al., 2000; Hafner and Hites, 2003). Other studies have identified

Table 2 Comparisons of 2007e2009 monthly precipitation concentrations by means of a 1-way ANOVA test. Concentrations that are significantly different from those at other sites are bolded. “NSD” indicates no statistically significant difference between sites using a 95% probability criterion.

a-HCH g-HCH aþb endosulfan SDDT aþg chlordane Dieldrin SPAHs SPCBs HBCD SPBDEs

Burlington

Toronto

Point Petre

ANOVA results

NSD NSD from Point NSD NSD ∼33 higher NSD ∼23 higher NSD from Point NSD from Point NSD from Point

NSD ∼23 higher NSD NSD ∼63 higher NSD ∼33 higher ∼133 higher ∼33 higher ∼33 higher

NSD NSD from NSD NSD Lowest NSD Lowest NSD from NSD from NSD from

F ¼ 0.14, p ¼ 0.871, df ¼ (2,68) F ¼ 4.51, p ¼ 0.014, df ¼ (2,68) F ¼ 1.18, p ¼ 0.314, df ¼ (2,68) F ¼ 1.84, p ¼ 0.166, df ¼ (2,68) F ¼ 33.28, p < 0.001, df ¼ (2,64) F ¼ 1.91, p ¼ 0.156, df ¼ (2,68) F ¼ 13.55, p < 0.001, df ¼ (2,68) F ¼ 53.96, p < 0.001, df ¼ (2,68) F ¼ 9.69, p < 0.001, df ¼ (2,66) F ¼ 9.61, p < 0.001, df ¼ (2,66)

Petre

Petre Petre Petre

Burlington

Burlington Burlington Burlington

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high urban levels of chlordane attributable to its past use in house foundations and on lawns and gardens (Offenberg et al., 2004; Yao et al., 2006; Gouin et al., 2007). g-HCH also had higher urban than rural concentrations (Fig. 2b). While the major use of g-HCH (as lindane) in North America has been as a seed treatment, lindane was also used as a wood preservative; in 1979, 23% of the lindane use in the United States was in the treatment of hardwood lumber (Herbst and Van Esch, 1991). Lindane has also been used use as an indoor pesticide and currently there is a small amount of use remaining in pharmaceuticals (United Nations Environment Program, 2006), which could lead to elevated urban levels. 3.4. PAHs e spatial variations PAHs were the only compounds that exhibited different concentrations at all three sites, with the highest concentrations at the urban site (254 ng L
1 at Toronto) and the lowest at the rural site (96.1 ng L
1 at Point Petre) (Fig. 2c). Individual PAHs had similar distributions amongst compounds at all three sites, and were dominated by phenanthrene (13% of total PAHs), fluoranthene (16%), and pyrene (9%), similar to the profile at Point Petre in bulk air (Sun et al., 2006d). It is interesting to note this similarity in PAH profiles across all three sites. While the differences in concentration among the sites indicate that the magnitude of sources differs, the PAH profiles indicate that the source types do not. The concentration of PAHs in Toronto (254 ng L
1) was about 10 lower than concentrations observed in larger urban areas, such as Chicago (2300 ng L
1) and Guangzhou, China (2540 ng L
1) (Sun et al., 2006a; Huang et al., 2009), but were in line with concentrations at other urban locations in North America such as ∼200 ng L
1 near Galveston Bay, Texas, as well as in Sacramento, California (Park et al., 2001; Kim and Young, 2009). These differences are consistent with the correlation between atmospheric PAH concentrations and population (Hafner et al., 2005; Sun et al., 2006d; Galarneau et al., 2007), since the populations of both Chicago and Guangzhou are almost double that of the Greater Toronto Area. 3.5. PCBs e spatial variations PCB concentrations in Toronto precipitation were 13 higher than the concentrations at Burlington or Point Petre (Fig. 2d). Although PCB manufacturing has been banned for several decades, in 2006 over 95000 tonnes of PCB-containing materials remained in use and storage in Ontario alone (Environment Canada, 2006), and PCB stocks associated with urban areas are particularly high (Diamond et al., 2010). Given the large urban stocks of PCBs, it is not surprising that concentrations in Toronto precipitation were elevated. PCBs in Toronto precipitation were dominated by pentaand hexa-CBs, whereas the PCB profiles from Burlington and Pt Petre were dominated by the tetra- and penta-CBs (Fig. 3) (F ¼ 15.3, p < 0.001). The dominance of the heavier congeners at the urban site is consistent with primary PCB sources (electrical equipment, joint sealants, storage sites) directly influencing urban precipitation. Conversely, the shift towards lighter congeners at the suburban/rural sites is consistent with longer transport distances of primarily gas-phase congeners, and volatilization and atmospheric transport from other sources in the Great Lakes region. 3.6. BFRs e spatial variations The concentration of HBCD was 4 higher in Toronto than at Point Petre or Burlington (Fig. 2e). The concentrations at all three sites were fairly low, ranging from 0.15 ng L
1 to 4.40 ng L
1, with the exception of September 2008 when 42.9 ng L
1 was measured at Toronto and

Fig. 3. Distribution of PCBs among homolog groups by site. Toronto has elevated concentrations of 6- to 8-CBs compared with Burlington and Pt Petre.

17.8 ng L
1 at Point Petre. Additionally, “outliers” of HBCD that were on average 22 higher than site averages were measured at 2 other sites on 5 other occasions in monitoring data of precipitation across Ontario measured from 2004e2009, (Bradley, 2010, personal communication). Examination of average monthly air mass backtrajectories for months with outlier concentrations did not provide any insight into the source of the elevated concentrations. Few published results for HBCD in precipitation are available for comparison, however it is interesting to note that Peters et al. (2008) did not detect concentrations of HBCD in precipitation in the Netherlands above their method detection limit of 15 ng L
1 in 49 of 50 samples, yet reported one extremely high concentration of 1835 ng L
1. Event-specific precipitation sampling could help inform the basis of these outliers. Concentrations of PBDEs were 3 higher in Toronto than at Point Petre or Burlington (Fig. 2e). BDE-99 was on average 10% of total PBDEs and BDE-47 was on average 9% at all sites. Concentrations were dominated by BDE-209, which accounted for on average 78, 74 and 72% of the total mass of PBDEs at Burlington, Toronto and Point Petre, respectively. In comparison, Venier and Hites (2008b) reported that BDE-209 contributed less than 40% to total PBDEs measured in air at IADN sites from 2005e2006. Venier and Hites (2008a) also reported PBDEs in precipitation in the Great Lakes basin from 2005 to 2006. VWM concentrations of PBDEs in Chicago (94  19 ng L
1) were almost 50 higher than those measured for Toronto in the current study (2.4  0.37 ng L
1), however concentrations of 4.4  1.4 ng L
1 for Cleveland, Ohio, a second urban site, were similar to those in Toronto, and concentrations reported at the IADN master stations (0.65e1.00 ng L
1) were similar to those at Burlington and Point Petre (∼1 ng L
1) in the current study. As with HCBD, PBDE concentrations had outlier concentrations that were over 5 times higher than the site average, for which there is no obvious explanation. Presumably, more information could be obtained through event-specific sampling. Furthermore, the large difference between concentrations in Toronto and Chicago, observed for PAHs as well as PBDEs, highlights the importance of having multiple urban measurements to better characterize the influence of urban areas on atmospheric deposition. 3.7. Temporal trends The long record of monitoring of POPs in precipitation in the Great Lakes basin provides a context for the present-day values and

L. Melymuk et al. / Atmospheric Environment 45 (2011) 5042e5049

allows the determination of temporal trends. The longest term record is available for Point Petre, where PCBs and OCPs have been monitored since 1986. A detailed list of published values for POPs in precipitation at both Point Petre and Burlington is included in Table S3; selected Point Petre temporal trends is discussed. Concentrations of a- and g-HCH, dieldrin, SDDT, Sendosulfans, and SPCBs all declined over the past 20 years (Fig. 4). This trend is also seen in other media such as air (Cortes et al., 1998) and fish (Hickey et al., 2006). As an example, concentrations of a-HCH in precipitation in the eastern Lake Ontario region showed a consistent gradual decrease, from 5.8 ng L
1 measured at Wolfe Island (near Point Petre) in the late 1980s (Chan et al., 1994), to 2.5 ng L
1 at Point Petre in the early 1990s (Simcik et al., 2000), to 1.7 ng L
1 in the late 1990s (Chan et al., 2003), then 0.7 ng L
1 in the early 2000s (Sun et al., 2006b) and finally the concentration of 0.3 ng L
1 in the current study. Statistically significant decreases (p < 0.05) were also noted for g-HCH, dieldrin, SDDT and SPCBs. Concentrations of SPAHs and Sendosulfans did not change significantly compared with past studies. Endosulfan had declined 30% at Point Petre in this study compared with 1995e1999 values (Chan et al., 2003). While endosulfan remains in use, the volume of use has decreased, and the concentrations in precipitation have declined accordingly. SPAHs concentrations in rural Great Lakes region precipitation have been ∼70 ng L
1 for the past 12 years (Simcik et al., 2000; Sun et al., 2006a). 3.8. Seasonal trends Seasonal trends in monthly precipitation concentrations were observed for PAHs and endosulfan. Endosulfan concentrations peaked from May to August, with average May to August concentrations 5 higher than September to April (Figure S4). Carlson et al. (2004) identified late spring/early summer as the time of pesticide application to croplands in North America, which coincides with concentration peaks reported here. The spring/summer peak was significantly higher for 2007 (∼8) than for 2008 (∼2), perhaps reflecting the decline in endosulfan application as regulations on its use were tightened. Seasonal variation was observed for PAHs at Burlington and Point Petre (Figure S5). The concentrations follow a sinusoidal trend, reaching a maximum in the winter and minimum in the summer. This type of seasonal variation of PAHs in precipitation has

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been noted and modeled using a time-dependent sinusoidal function (Sun et al., 2006a). The winter peak in PAH concentrations has been attributed to increased fossil fuel combustion in winter, particularly in indoor heating (Galarneau et al., 2007), as well as decreased rates of degradation due to lower atmospheric hydroxyl radical concentrations (Bunce and Dryfhout, 1992). However, a shift in the proportion of the more reactive PAHs in winter is not evident, suggesting that the seasonal concentration changes are largely influenced by emission rates. 4. Loadings estimates Annual wet deposition loadings of organic contaminants to the lake were estimated using 2007e2008 average VWM concentrations and over-lake precipitation rates, obtained from Hunter and Croley (2009). Two loading methods are contrasted to assess the effect of the urban area on annual lake-wide loadings: (1) loadings based on background levels, where loadings to the whole lake were determined using the concentrations measured at Point Petre, and (2) loadings incorporating the influence of elevated Toronto concentrations. Loadings based on Point Petre concentrations were calculated on a monthly basis using the following equation,

L ¼ R$P$A

(1)

where L is the loading of a chemical to Lake Ontario (g/yr), R is the average concentration of that chemical in precipitation at Point Petre (g/L), P is the average precipitation rate (mm/yr), and A is the area of Lake Ontario (19430 km2). For compounds with higher concentrations in Toronto, an adjusted annual loading was calculated. The concentration in overlake precipitation was assumed to decrease with increasing distance from Toronto based on a first-order decay model,

C ¼ R þ ðU
RÞe
kd

(2)

where C is the concentration of the chemical (g/L) at distance d (km), U is the concentration of the chemical in Toronto precipitation (g/L), and k is a rate constant. The rate constant was determined by assuming that the concentration of the chemical decreased by 90% at a distance of 40 km from Toronto (Wethington and Hornbuckle, 2005). Using a GIS of Lake Ontario, this first-order

Fig. 4. Temporal trends in OCPs and PCBs at Point Petre. Concentrations compiled from Chan et al. (1994), Hoff et al. (1996), Simcik et al. (2000), Chan et al. (2003), Sun et al. (2006b), Blanchard et al. (2006), and this study.

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Table 3 Loadings (kg/year) via wet deposition to Lake Ontario, using Point Petre levels applied to the entire lake and the influence of the Toronto area over 19% of the lake. The range reflects the standard deviation in the average monthly concentrations. Compound

Loadings based on Point Petre concentrations

Range

Loadings based on Toronto & Point Petre concentrations

Range

PAH PCB PBDE HBCD a-HCH y-HCH Endosulfan DDT Chlordane Dieldrin

1700 24 40 25 5.4 4.0 44 3.2 0.35 2.5

1500 to 1900 17 to 31 25 to 55 5.0 to 45 2.4 to 8.5 2.0 to 6.0 30 to 57 2.0 to 4.6 0.32 to 0.38 2.2 to 2.9

1900 34 41 27 No change No change No change No change 0.42 No change

1700 to 2100 25 to 43 26 to 56 6.0 to 48

decay model was applied to the surface of Lake Ontario to obtain a 3-dimensional distribution over the lake with height representing chemical concentrations. The volume under this surface was scaled using the annual precipitation rate to obtain estimates of annual loadings. With this method 3780 km2 of lake area were influenced by contributions from the Toronto area, and the remaining 15650 km2 of the lake was assumed to receive wet deposition at Point Petre levels. These estimates only incorporated the area of the lake influenced by the city of Toronto. While Toronto is the largest urban area bordering Lake Ontario, it is not the only one. In fact, 16% of the Lake Ontario shoreline is considered “urban” based on the Statistics Canada definition of an urban area (population > 1000 people, population density > 400 people/km2), and with the 40 km buffer, 54% of lake area may be influenced by elevated urban POP concentrations. Table 3 summarizes the loadings based on Point Petre and Toronto. Loadings of PAHs are highest, at 1900 kg year
1, followed by PCBs, PBDEs and HBCD, at 34, 41 and 27 kg year
1 respectively. Loadings were generally low (<10 kg year
1) for each OCP, with the exception of endosulfan (44 kg year
1). For a-HCH, g-HCH, endosulfan, DDT, and dieldrin loadings to the lake did not change with the inclusion of the Toronto influence, either because there was no significant difference in concentrations measured at Toronto and Point Petre, or because the difference in concentration, while statistically significant, was so small that it did not result in a significant change in loadings. SPCBs and Schlordanes showed the greatest adjustment in loadings, reflecting the steep concentration gradient between urban and rural sites. With the incorporation of Toronto concentrations, loadings increased by 42% for PCBs and 20% for Schlordanes over loadings calculated using Point Petre concentrations alone. For PAHs, PBDEs, and HBCD the inclusion of Toronto resulted in 11%, 2.5%e7.4% higher loadings, respectively. The increase in loadings estimated with the inclusion of Toronto concentrations emphasizes the importance of incorporating urban influence in wet deposition loading estimates. 5. Conclusions Concentrations of PBDEs, HBCD, PCBs, PAHs, g-HCH and Schlordanes in precipitation were 2 to 13 higher in Toronto than at Point Petre or Burlington. In contrast, precipitation concentrations of a-HCH, endosulfan and DDT were not significantly different among sites. Concentrations of SPCBs, SPBDEs, and HBCD in urban precipitation were predominantly influenced by local sources, whereas compounds without dominant urban sources reflected the ubiquitous levels in the Great Lakes basin caused by multiple regional sources (e.g. endosulfan). Estimates of loadings to Lake Ontario via wet deposition to the lake were 2.5e44% higher for SPBDEs, SPCBs, SPAHs, Schlordanes

from from from from

Point Point Point Point

Petre Petre Petre Petre

concentrations concentrations concentrations concentrations 0.40 to 0.44

from Point Petre concentrations

and HBCD if the influence of elevated precipitation concentrations from urban areas was included, while no statistically significant change was estimated for the other OCPs. This emphasizes the importance of including urban concentrations in loading estimates for compounds with high emissions associated with urban areas. Acknowledgments Funding was provided by Environment Canada, and by a Great Lakes Atmospheric Deposition grant to Diamond, Helm, Blanchard and Backus. We thank Brenda Treen at the National Laboratory for Environmental Testing, Environment Canada for data acquisition; and Orrin Carson, Bruce Harrison and Mary Lou Archer for managing the field collection at the sites, and Liisa Jantunen and Paul Helm for helpful suggestions during the course of this work. Appendix. Supplementary material Supplementary material associated with this paper can be found, in the online version, at doi:10.1016/j.atmosenv.2011.02.007. References Blanchard, P., Kallweit, D., Brice, K.A., Froude, F.A., Chan, C.H., Neilson, M., Holz, J., Millat, H., 2006. A comparison of European and North American atmospheric deposition networks: polycyclic aromatic hydrocarbons and lindane. J. Environ. Monit. 8, 465e471. Blanchard, P., Audette, C.V., Hulting, M.L., Basu, I., Brice, K.A., Backus, S.M., DryfhoutClark, H., Froude, F., Hites, R.A., Neilson, M., Wu, R., 2008. Atmospheric deposition of toxic substances to the great lakes: IADN results through 2005. Environment Canada, Toronto. 978-0-662-48287-1. Bunce, N.J., Dryfhout, H.G., 1992. Diurnal and seasonal modeling of the tropospheric half-lives of polycyclic aromatic hydrocarbons. Can. J. Chem.-Rev. Can. Chim. 70, 1966e1970. Carlson, D.L., Basu, I., Hites, R.A., 2004. Annual variations of pesticide concentrations in Great Lakes precipitation. Environ. Sci. Technol. 38, 5290e5296. Chan, C.H., Bruce, G., Harrison, B., 1994. Wet deposition of organochlorine pesticides and polychlorinated biphenyls to the great lakes. J. Gt. Lakes Res. 20, 546e560. Chan, C.H., Williams, D.J., Neilson, M.A., Harrison, B., Archer, M.L., 2003. Spatial and temporal trends in the concentrations of selected organochlorine pesticides (OCs) and polynuclear aromatic hydrocarbons (PAHs) in Great Lakes basin precipitation, 1986 to 1999. J. Gt. Lakes Res. 29, 448e459. Cortes, D.R., Basu, I., Sweet, C.W., Brice, K.A., Hoff, R.M., Hites, R.A., 1998. Temporal trends in gas-phase concentrations of chlorinated pesticides measured at the shores of the Great Lakes. Environ. Sci. Technol. 32, 1920e1927. Diamond, M.L., Melymuk, L., Csiszar, S.A., Robson, M., 2010. Estimation of PCB stocks, emissions, and urban fate: will our policies reduce concentrations and exposure? Environ. Sci. Technol. 44, 2777e2783. Environment Canada, 2003. Integrated Atmospheric Deposition Network - Background. Government of Canada. http://www.msc-smc.ec.gc.ca/iadn/overview/ background_e.html. Environment Canada, 2005. Descriptions of Some Toxic Contaminants Found in the Pacific and Yukon Region. Government of Canada. http://www.ecoinfo.ec.gc.ca/ env_ind/region/toxin_descript/toxin_description_e.cfm. Environment Canada, 2006. National Inventory of PCBs in Use and PCB Wastes in Storage in Canada – 2005 Annual Report. Environmental Stewardship Branch Government of Canada, Ottawa, Ontario.

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