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Mercury deposition in the Adirondacks: A comparison between precipitation and throughfall
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Atmospheric Environment 42 (2008) 1818–1827 www.elsevier.com/locate/atmosenv
Mercury deposition in the Adirondacks: A comparison between precipitation and throughfall Hyun-Deok Choi, Timothy J. Sharac1, Thomas M. Holsen Department of Civil and Environmental Engineering, Clarkson University, P.O. Box 5710, Potsdam, NY 13699, USA Received 26 June 2007; received in revised form 10 November 2007; accepted 13 November 2007
Abstract The volume-weighted mean (VWM) Hg concentrations and the total cumulative fluxes in deciduous throughfall (6.6 ng L1 and 12.0 mg m2, respectively) were statistically higher than in precipitation (4.9 ng L1 and 11.6 mg m2, respectively) during 2 years of sampling at the Huntington Wildlife Forest in Newcomb, NY. Seasonally, the VWM Hg concentrations and the Hg fluxes in both precipitation and throughfall were lowest in winter and highest in summer. Due to the wash-off of dry deposition and foliar leaching, concentrations in throughfall were almost 50% higher than those in precipitation during the leaf-on period, while concentrations in throughfall were slightly higher than (or similar to) those in precipitation during the leaf-off period. During the 2 years of sampling, the total deposited cumulative flux in precipitation and deciduous throughfall were very similar (11.6 mg Hg m2 and 12.0 mg Hg m2, respectively), because the higher concentrations in throughfall were offset by smaller throughfall depths. Meteorological analysis indicated that the precipitation events resulting in the highest Hg fluxes were associated with trajectories that passed through regions of Midwest where major Hg sources including coal/oil-fired power plants and waste incinerators are located. r 2007 Elsevier Ltd. All rights reserved. Keywords: Total mercury deposition; Throughfall; Precipitation; Meteorological analysis
1. Introduction Atmospheric deposition is a significant source of mercury (Hg) to the forest ecosystem (Fitzgerald et al., 1998; Lindberg et al., 1998; Miller et al., 2005). However, this input is poorly understood because of the complex interactions between atmospheric Hg and the canopy including (1) heterogeneous oxidaCorresponding author. Tel.: +1 315 268 3851; fax: +1 315 268 7985. E-mail address: [email protected] (T.M. Holsen). 1 Current address: Clean Air Markets Division, US EPA, Washington, DC 20460, USA.
1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.11.036
tion reactions on humid leaf surfaces (Iverfeldt, 1991), (2) deposition of particulate Hg (HgP) on the leaf surfaces (Lovett and Lindberg, 1984; Iverfeldt, 1991; St. Louis et al., 2001), (3) uptake and emission of elemental Hg (Hg0) by stomata (Lovett and Lindberg, 1984; Iverfeldt, 1991; Lindberg et al., 1991; St. Louis et al., 2001), and (4) uptake and deposition of gaseous divalent Hg (RGM) by and on the leaves (Lovett and Lindberg, 1984; Iverfeldt, 1991; St. Louis et al., 2001). Atmospherically deposited mercury is generally divided via deposition mechanisms into two broad categories: wet deposition and dry deposition. Wet
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deposition occurs when mercury associated with moisture (e.g. rain, snow, dew, clouds, etc.), deposits on a surface—often the ground. Throughfall is identical to wet deposition with respect to environmental partitioning (i.e. within rain, snow, dew, clouds, etc.), however, the forest canopy temporarily intercepts this precipitation before it deposits on the ground (Lindberg et al., 1994; Draaijers et al., 1997; Guentzel et al., 1998; Biester et al., 2002; Devlaeminck et al., 2005; Deguchi et al., 2006). In the absence of precipitation, mercury that is deposited onto surfaces is known as dry deposition, although this process can be further subdivided into dry particle deposition and air–surface exchange (Lindberg et al., 1994, 1998; Guentzel et al., 1998; Fang et al., 2001; Grigal, 2002). The forest canopy plays an important role in capturing Hg from the atmosphere which is subsequently deposited in the forest ecosystem as throughfall and litterfall (Zillioux et al., 1993; Hultberg et al., 1995; Rea et al., 1996). Typically, throughfall has higher Hg concentrations than precipitation (from 1.5 up to 1.8 times) because of wash-off of previously deposited Hg (Iverfeldt, 1991; Munthe et al., 1995; Kolka et al., 1999; Grigal et al., 2000; Schwesig and Matzner, 2000). This study was conducted as a part of the investigation of the mercury input, output, and cycling in a deciduous forest located in the Huntington Forest in the Adirondacks. Total Hg concentrations in, and fluxes from, precipitation and throughfall were measured from December 2004 to December 2006. The data was divided into leaf-on and leaf-off periods to investigate the importance of dry deposition and foliar leaching processes on total Hg concentrations in throughfall. Meteorological analysis was used to investigate the history of the major precipitation events which contributed the most Hg deposition at the sampling site. 2. Sampling and analyzing methods 2.1. Sampling site Precipitation and deciduous throughfall samples were collected at the Huntington Wildlife Forest in Newcomb, NY (43.971N, 74.221W) from December 2004 to December 2006. The precipitation site was located in a large open clearing used by the Mercury Deposition Network (MDN) (NY20) and US EPA CASTNET (HWF187). The deciduous
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throughfall site (Arbutus site) is located southeast of the 3.52 km2 Arbutus Lake (43.981N, 74.231W) and is covered by mixed hardwood forests having a stand age of 100 years and dominated by a mostly deciduous forest, largely dominated by American beech (Fagus sylvatica), also mixed with sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) (Sharac, 2006; Bushey et al., 2007). The throughfall precipitation collector was placed about 1 km from the forest edge. Site elevation was 530 m and soils, typically o1 m thick, are dominated by coarse, loamy, mixed, frigid, Typic Haplorthods in the Becket-Mundal series. The watershed has a mean slope of 11% and a total relief of 225 m (Johnson and Lindberg, 1992; McHale et al., 2004). 2.2. Sampling methods Weekly precipitation and throughfall samples were collected using a modified MIC-B (Meteorological Instruments of Canada, Thornhill, Ont.) automatic precipitation collector. The MIC-B has a heated precipitation sensor which allows for the automatic removal of the hood during precipitation exposing the sampling trains only during wet deposition events. The samplers were modified to hold two acid-cleaned 1-L Teflon bottles to capture mercury in precipitation (Landis and Keeler, 1997). The Hg sampling train, consisting of a borosilicate glass funnel and a glass vapor lock to minimize evaporative losses, has been validated by Landis and Keeler (1997). 2.3. Sample train cleaning procedure The sample train acid-cleaning protocol was developed following the EPA Method 1631 version E (US EPA, 2002). The borosilicate glass funnels were pre-rinsed with deionized water and immersed for 24 h (48 h for new funnels) in a polypropylene tank containing 4 N HCl heated to 60 1C. Next, the funnels were rinsed three times with 18.2 MO cm Milli-Q water (Millipore Corporation, Billerica, MA), placed in a laminar flow hood in a class 100 clean room, and allowed to dry. Dry funnels were triple bagged using polyethylene ziplock bags. Sample bottles, vapor locks, and connecting sheaths were pre-rinsed with deionized water, soaked in a polypropylene container containing 1% HCl solution prepared from 18.2 MO cm Milli-Q water and then heated for 12 h in an oven at 60 1C. This
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equipment was rinsed three times with 18.2 MO cm Milli-Q water and then placed into a polypropylene container containing 0.5% HCl for a minimum of 12 h. These components were then rinsed three times with 18.2 MO cm Milli-Q water and then allowed to dry in a laminar flow hood in a class 100 clean room. The bottles, vapor locks, and connecting sheaths were all triple bagged in polyethylene ziplock bags when dry until prior to installation into the MIC-B sampling equipment. Before field deployment, Teflon bottles were filled with 20 mL of 0.08 M HCl (to minimize Hg loss in precipitation or throughfall), capped, and then weighed (Landis and Keeler, 1997; Lai et al., 2007).
argon. The gaseous Hg0 was captured by amalgamation onto a gold trap. The Hg was thermally desorbed from this trap, and then measured with a cold-vapor atomic fluorescence spectrometer (CVAFS). Precipitation depths were measured at the Huntington Wildlife Forest in Newcomb, Essex County, NY (station ID: NY20) by the National Trends Network (NTN) rain gage using a precipitation hydrograph (Belfort Instrument Company, Baltimore, MD). This rain gage detects precipitation events as small as 0.025 cm and measures precipitation depth, duration, and intensity of each precipitation event.
2.4. Spatial variation of throughfall depth
2.6. Meteorological trajectory analysis
The spatial variation in deciduous throughfall depth was measured throughout the measurement site during spring (before leaf-out) and early summer (after leaf-out) 2006 to assess whether the location of the deciduous throughfall collector was capturing a typical amount of precipitation with respect to other sites under this forest canopy. One hundred 473-mL cups were spaced 4.6 m apart running perpendicular to the slope of the hill, and 3.4 m apart along the length of each transect (10 cups per transect). The spatial variation of throughfall precipitation events was measured with a graduated cylinder as close as possible to the end of precipitation events to minimize evaporative losses.
The transport path of air parcels to the sampling site in the Adirondacks was estimated using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) Model Version 4.8 (Draxler and Hess, 2005). The 48-h back trajectories were calculated with Eta Data Assimilation System (EDAS) (40-km grid) obtained from Gridded Meteorological Data Archives by National Oceanic and Atmospheric Administration—Air Resources Laboratory (NOAA—ARL). Hourly back trajectories during precipitation were calculated. The arrival height of the back trajectories was 500 m.
2.5. Analytical methods Precipitation and throughfall samples were collected each Tuesday and immediately prepared for laboratory analysis by adding concentrated BrCl (0.5%, v/v) to each sample bottle to oxidize and desorb mercury from the inside walls of the Teflon bottles. These samples were stored for at least 24 h in a 4 1C refrigerator prior to analysis. Total Hg was quantified by a dual amalgamation technique followed by CVAFS with Tekran 2600 (Tekran Corporation Inc., Toronto, Ont., Canada). The analytical method used was a slightly modified EPA Method 1631 version E (US EPA, 2002). Briefly, each sample was treated with hydroxylamine (NH2OH HCl) to remove the free halogens (e.g. BrCl), and was then treated with stannous chloride (SnCl2) to convert Hg2+ to Hg0, which was removed from solution by purging with
2.7. QA/QC The method detection limit (MDL), calculated as three times the standard deviation of seven sequential reagent blanks, averaged 0.04 ng for 1 L reagent blank, the same as the MDL reported in EPA Method 1631. The initial (IPR) and on-going (OPR) precision and recovery were measured at the start of the analysis and every 20 samples, and ranged between 93% and 111% and 91% and 107%, respectively. These values were within the quality control acceptance criteria for performance in the EPA Method 1631 (IPR: 79–121% and OPR: 77–123%). Duplicate samples were taken bi-weekly (n ¼ 48). The relative standard deviation of the duplicate analysis was 3.2%. Field blanks (n ¼ 24) were obtained by rinsing the whole sampling assembly deployed inside the MIC-B for the same period as was used for sampling to ensure that there was no Hg absorption by the HCl preservative or to the walls of the sampling trains during nonprecipitation conditions. Laboratory blanks
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(n ¼ 12, every 2 months) were filled fully with deionized water, and handled in the same manner as a precipitation sample. The average field blank concentrations in precipitation and throughfall were 0.55 and 0.72 ng L1, respectively. The average lab blank concentration was 0.32 ng L1. All samples were corrected by the associated monthly field blank. For comparison, the actual sample concentrations varied between 0.21 and 28.51 ng L1 (blank corrected). 3. Results and discussion 3.1. Precipitation depth The distribution of precipitation volumes throughout the deciduous canopy was measured with 100 cups before leaf-out (n ¼ 3) and after leafout (n ¼ 4). The average event cup volume before the leaf-out period was 135745.2 mL (average7S.D.) which was not different than the cup located right beside the deciduous throughfall collector (131 mL). The average event precipitation volume for each cup after the leaf-out period was 178721.9 mL which was not different than the cup located next to the throughfall collector (179 mL). This finding suggests that the deciduous throughfall sample collector received a representative amount of precipitation both before and after leaf-out. The cumulative depths of precipitation and throughfall throughout this study were 238.4 and 183.5 cm, respectively; approximately 60% occurred during leaf-on periods (Table 1). The precipitation and throughfall depths were highly correlated during both leaf-off and leaf-on periods (r2 ¼ 0.92 and 0.95 for leaf-off and leaf-on, respectively). (The leaf-on season in Huntington Forest is from the beginning of May to the end of October.) However, the throughfall depth was smaller than the precipitation depth during both leaf-off periods (slope ¼ 1.28), and leaf-on periods (slope ¼ 1.16).
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These results are similar to previous measurements in mixed deciduous forests where throughfall volumes were approximately 8–24% smaller during leaf-out than wet deposition volumes (Neal et al., 1993; Rea et al., 1996; Price and Carlyle-Moses, 2003; Keim et al., 2005; Deguchi et al., 2006). For the 18 months of overlap when MDN data are available, the total measured MDN precipitation depth (NY20) and Clarkson measured precipitation depth were not statistically different (187.3 and 169.7 cm, respectively) (p ¼ 0.202) (Fig. 1, top). (MDN reports the precipitation depth measured by CASTNET and not the depth collected by the wet deposition sampler.) The differences were most pronounced during snow events. In addition, some sample-to-sample differences were due to different operating procedures between the sites. MDN samples were collected every Tuesday, however, if it was raining on a Tuesday, Clarkson samples were not collected until after the precipitation stopped. The result of this difference can be seen for example in the month of August 2005 where the individual sample depths were very different but the cumulative depths were very similar (MDN ¼ 11.2 cm and Clarkson ¼ 11.3 cm). 3.2. Hg in precipitation During the 2 years of sampling from December 2004 to December 2006, concentrations of total Hg in precipitation ranged from 0.2 to 28.5 ng L1, and the volume-weighted mean (VWM) Hg concentration was 4.9 ng L1 (Table 1). Lower Hg concentrations and fluxes were measured in winter and the highest Hg concentrations were measured in spring and summer (Fig. 2a). This result may be partly explained by an increase in the oxidizing rate of atmospheric Hg0 during spring and summer seasons due to increased photochemical activity (Munthe et al., 1995). Additional explanations for these differences include different source regions (dis-
Table 1 Cumulative precipitation depths, VWM Hg concentrations, and cumulative Hg fluxes in precipitation and throughfall depending on leafon and leaf-off from December 2004 to December 2006
Precipitation Throughfall a
Cumulative precipitation depth (cm)
VWM Hg concentration (ng L1)
Cumulative Hg flux (mg Hg m2)a
Leaf-on
Leaf-off
Total
Leaf-on
Leaf-off
Total
Leaf-on
Leaf-off
Total
138.7 110.1
99.7 73.4
238.4 183.5
5.5 8.0
4.0 4.5
4.9 6.6
7.7 8.9
4.0 3.3
11.6 12.2
Hg flux is calculated as the product of Hg concentration in the sample and its corresponding sample depth.
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80 60
2000 1500 1000
40
500
20
0
0 25
12 MDN (NY20) Clarkson Univ. MDN (NY20) Clarkson Univ.
20 15
10 8 6
10
4
5
2
0 Jun-06
May-06
Apr-06
Mar-06
Feb-06
Jan-06
Dec-05
Nov-05
Oct-05
Sep-05
Aug-05
Jul-05
Jun-05
May-05
Apr-05
Mar-05
Feb-05
0
Cumulative Flux [µg m-2] Cumulative Depth [mm]
Jun-06
May-06
Apr-06
Mar-06
Feb-06
Jan-06
Dec-05
Nov-05
Oct-05
Sep-05
Aug-05
Jul-05
Jun-05
Apr-05
Mar-05
Feb-05
Jan-05
MDN (NY20) Clarkson Univ. MDN (NY20) Clarkson Univ.
100
Jan-05
Total Hg Conc. [ng L-1]
Precip. Depth [mm]
120
May-05
H.-D. Choi et al. / Atmospheric Environment 42 (2008) 1818–1827
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Fig. 1. Comparisons of precipitation depth and total Hg concentrations between MDN (NY20) and Clarkson from December 2004 to June 2006.
cussed in Section 3.5) and increased convection during warmer months which can increase the ability of air to transport Hg over longer distances, and leads to greater precipitation amounts (Keeler et al., 2005). In addition, rain has a greater capacity to scavenge Hg than snow (Lindberg et al., 1991; Sorensen et al., 1994; Glass and Sorensen, 1999; Keeler et al., 2005). In general, our measured concentrations and fluxes were similar to those measured by MDN for the 18 months of overlapping samples (Fig. 1, bottom). The total deposited cumulative fluxes from MDN (NY20) and Clarkson were 10.0 and 8.7 mg Hg m2, respectively. Using the CASTNET precipitation depths with the Clarkson concentrations, the total deposited cumulative Hg flux was found to be 9.1 mg Hg m2. As indicated in the earlier text, some of the short-term differences were due to different sampling times. For example, in the month of August 2005, the individual sample depths and concentrations were very different but the cumulative fluxes were identical (MDN ¼ 0.64 mg Hg m2 and Clarkson ¼ 0.62 mg Hg m2). Statistically, Hg deposition from MDN (NY20) and Clarkson were significantly correlated (r2 ¼ 0.61 and po0.001) and not statistically different (p ¼ 0.155). The Clarkson fluxes using CASTNET precipitation were also well correlated with MDN
(NY20) (r2 ¼ 0.61 and po0.001) and not statistically different (p ¼ 0.345). 3.3. Hg in throughfall Mercury concentrations in throughfall ranged from 0.9 to 28.2 ng L1; the VWM Hg concentration was 6.6 ng L1 (Table 1). Mercury concentrations in throughfall and precipitation were significantly different (po0.001). Higher Hg concentrations in throughfall were measured in spring, summer, and fall, whereas lower concentrations were measured in winter (Fig. 2a). Concentration differences between throughfall and precipitation ranged from 2% to 166%, and the mean concentration difference was 50.3%. Even through the differences in concentrations between throughfall and precipitation have been found to vary depending on sampling sites and/or species of trees, our results are similar to previous results (Table 2). During leaf-on periods, this increased mass of Hg in throughfall vs. precipitation may originate from (1) the wash-off of dry deposition of airborne Hg that occurs during dry periods (Munthe et al., 1995), (2) foliar leaching (Lovett and Lindberg, 1984; Rea et al., 2000, 2001), (3) the evaporation of precipitation in the canopy which may concentrate the Hg present in the throughfall (Rea et al., 1996), and
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200 Precipitation Throughfall Precipitation Depth in Precipitation Precipitation Depth in Throughfall
14 12
180 160 140 120
10
100
8
80
6
60
4
40
Precipitation Depth [mm]
Monthly VWM Hg Concentration [ng L-1]
16
20
2
0 Dec-06
Nov-06
Oct-06
Sep-06
Aug-06
Jul-06
Jun-06
May-06
Apr-06
Mar-06
Feb-06
Jan-06
Dec-05
Nov-05
Oct-05
Sep-05
Jul-05
Aug-05
Jun-05
May-05
Apr-05
Mar-05
Feb-05
Jan-05
Dec-04
0
1.6
14 Precipitation Throughfall Cumulative Flux in Precipitation Cumulative Flux in Throughfall
1.2
12 10
1.0
8
0.8
6
0.6
4
0.4
2
0.2
0
Cumulative Flux [µg m-2]
Monthly Cumulative Flux [µg m-2]
1.4
Dec-06
Nov-06
Oct-06
Sep-06
Aug-06
Jul-06
Jun-06
May-06
Apr-06
Mar-06
Feb-06
Jan-06
Dec-05
Nov-05
Oct-05
Sep-05
Jul-05
Aug-05
Jun-05
May-05
Apr-05
Mar-05
Feb-05
Jan-05
Dec-04
0.0
Fig. 2. The monthly VWM Hg concentrations and precipitation depths (a), and the monthly Hg fluxes and cumulative fluxes in rainfall and throughfall samples (b).
(4) an increase in the oxidizing rate of atmospheric Hg0 by the canopy which provides a large reactive surface (Munthe et al., 1995). Some of the VWM total Hg concentrations in throughfall were very similar to those in precipitation even during leaf-on periods (Fig. 2a). For example, in June 2005, throughfall concentrations (22.1, 16.3, 3.6 ng L1) were higher than those in precipitation (13.8, 15.7, 2.8 ng L1); however, the
precipitation depths were approximately three times higher for precipitation than for throughfall (0.5, 6.1, 4.8 cm vs. 0.2, 3.7, 3.9 cm, respectively) resulting in similar VWM concentrations (precipitation ¼ 10.2 ng L1 and throughfall ¼ 10.1 ng L1). The Hg deposition flux was calculated as the product of Hg concentration in the sample and its corresponding sample depth (sample volume divide by diameter of funnel). Regression analysis revealed
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Table 2 Summary of differences of mercury concentration and mercury deposition flux between precipitation and throughfall Locations
Differences (%) a
New York Vermont Minnesota Ontario Sweden Sweden Sweden Tennessee Maryland a
Reference b
Concentration
Flux
2–166 11–430 76–240 100 100 30–40 Up to 85 – –
14 20–82 70 – 40 – 30–70 93 61
This study Rea et al. (1996) Grigal et al. (2000) St. Louis et al. (2001) Munthe et al. (1995) Grennfelt et al. (1985) Iverfeldt (1991) Lindberg et al. (1994) Lawson and Mason (2001)
Whole year. Growing season only.
b
Fig. 3. Regression plots between total Hg concentration (ng L1) and the ratio of dry day and precipitation depth (h mm1) in throughfall, and net throughfall.
that Hg deposition fluxes were more positively correlated with precipitation depths (r2 ¼ 0.46, po0.001) than concentrations (r2 ¼ 0.02, p ¼ 0.126), which is similar to the findings of Sorensen et al. (1994) and Mason et al. (1997, 2000). Weekly fluxes of precipitation and throughfall were significantly different (po0.001). However, the cumulative Hg flux in precipitation was almost the same as the cumulative Hg flux in throughfall over the two full years of sampling (precipitation ¼ 11.6 mg Hg m2 and throughfall ¼ 12.0 mg Hg m2). For the first year of this study (2005), the cumulative Hg flux in throughfall (6.5 mg Hg m2 yr1) was 16% higher than in precipitation (5.6 mg Hg m2 yr1) and the cumulative Hg fluxes during the growing season in through-
fall (4.9 mg Hg m2) were 26% higher than in precipitation (3.9 mg Hg m2). As mentioned in the earlier text, calculating the Hg deposition flux using the precipitation depth from the rain gauge instead of sample depth results in a 41% higher Hg deposition in throughfall than in precipitation during growing season. This finding is similar to or less than other published results which found that the throughfall fluxes of Hg are significantly greater than precipitation fluxes, since Hg concentrations in throughfall are comparably higher than in precipitation throughout the growing season (Table 2). However, the cumulative flux in throughfall (5.5 mg m2 yr1) was lower than in precipitation (6.0 mg m2 yr1) in the second year (2006), since the Hg concentrations in throughfall and precipitation were more similar than in the first year even during the growing season, and precipitation depths in precipitation were much higher than those in throughfall in the second year. Generally, depth estimated from sample volume in throughfall is lower than that in precipitation, particularly during leaf-on periods, because some of precipitation is captured and evaporated by the canopy or can be in stem flow. Therefore, even though the Hg concentration is higher in throughfall, the Hg flux due to throughfall can be close to and/or lower than that of precipitation because the sample depth is lower. A similar result has been reported by Guentzel et al. (1998) measuring throughfall under 4–7-m-tall trees. Calculating the Hg deposition flux using precipitation depth from the rain gauge instead of sample depth results in 12% higher total Hg deposition in throughfall than that in precipitation in 2006 and 30% higher in throughfall during the whole study period. 3.4. Leaf-on vs. leaf-off Concentrations of Hg in throughfall (8.0 ng L1) during leaf-on periods were higher than those in precipitation (5.5 ng L1) and they were moderately correlated (r2 ¼ 0.42). However, concentrations in throughfall (4.5 ng L1) were only slightly higher than in precipitation (4.0 ng L1) during leaf-off periods; during this period, concentrations were well correlated (r2 ¼ 0.92) (Table 1). It is generally accepted that Hg concentrations in throughfall are positively correlated with antecedent dry period and negatively correlated with precipitation depth (Rea et al., 2000). For samples that
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contained only one event (i.e. weeks in which precipitation occurred only once) during leaf-on periods, Hg concentrations in throughfall and net throughfall are strongly associated with the length of the antecedent dry period as well as the amount of precipitation. Throughfall and net throughfall were linearly correlated with the ratio of the length of the antecedent dry period and precipitation depth (r2 ¼ 0.77 and 0.56, respectively) (Fig. 3), while Hg concentrations in precipitation were not correlated with this ratio (r2 ¼ 0.18). 3.5. Back trajectory analysis The weekly precipitation samples that had the three highest Hg fluxes (1.0, 0.5, 0.6 mg Hg m2 week1 in 7 June 2005, 30 August 2005, 27 June 2006 weeks, respectively) were used to initiate 48-h back trajectories for every hour during the precipitation events. The precipitation depths of the three highest fluxes (6.1, 10.4, 6.2 cm, respectively) were 3–5 times higher than average precipitation measured (2.3 cm) and the concentrations (15.7, 5.1, 10.5 ng L1, respectively) were up to 3 times higher than the VWM Hg concentration (4.9 ng L1). The cumulative flux of these three highest samples accounts for 18.3% of total cumulative Hg flux over the 2-year period, suggesting that Hg inputs may be dominated by a small number of large events. During these events, air parcels were transported mostly from the south (27 June 2005) or southeast (30 August 2005 and 27 June 2006) to the sampling site, so most of the trajectories passed over eastern Pennsylvania, West Virginia, and New York City where there are several major Hg sources including large coal-fired power plants, oil-fired power plants, and waste incinerators (Han et al., 2005). A few of the trajectories passed over Rochester, NY, or Toronto. The Russell electric generation station and medical and sludge waste incinerators are located around Rochester and waste incinerators are located in southern Ontario including Toronto (Han et al., 2005). These results indicate that the air masses with relatively high precipitation amounts that pass over these Hg sources may deposit elevated amounts of Hg wet deposition. 4. Conclusions This study investigated Hg wet deposition and deciduous throughfall in the Adirondacks for 2
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years. During this study, the VWM Hg concentration in throughfall (6.6 ng L1) was higher than in precipitation (4.9 ng L1), while the total cumulative Hg flux in throughfall (12.0 mg Hg m2) was very similar to precipitation (11.6 mg Hg m2) due to relatively lower precipitation depths in throughfall. The cumulative Hg flux in throughfall was 14% higher than that in precipitation during the first study year due to higher Hg concentrations in throughfall throughout the growing season. However, the cumulative Hg flux in throughfall was lower than in precipitation during the second year due to the similar Hg concentrations in throughfall and precipitation even during the growing season. Two-day back trajectory analysis with the three highest fluxes indicated that air parcels during this study passed through the major Hg sources in eastern Pennsylvania, Virginia, and New York City, or Rochester, NY, and Toronto areas. During these three events, the fluxes were 5–9 times higher than the average flux resulting in these events being responsible for 18.3% of the total Hg deposited during the study. These findings support several previous studies which suggest that Hg found in wet deposition in the northeastern United States is largely derived from coal-fired power plants in the Ohio River valley. Acknowledgments This research was supported in part by a National Science Foundation No. 02-167 Biocomplexity Grant (Charles T. Driscoll, PI). This is the Clarkson Center for the Environment publication no. 339. The research described herein has not been subjected to EPA peer and administrative review. Therefore, the conclusions and opinions drawn are solely those of the authors and should not be construed to reflect the views of EPA.
References Biester, H., Muller, G., Scholer, H.F., 2002. Estimating distribution and retention of mercury in three different soils contaminated by emissions from chlor-alkali plants: part I. The Science of the Total Environment 284, 177–189. Bushey, J.T., Nallana, A.G., Montesdeoca, M.R., Driscoll, C.T., 2007. Enhancement of mercury deposition by the forest canopy within a northern forest landscape. Atmospheric Environment (submitted for publication). Deguchi, A., Hattori, S., Park, H.-T., 2006. The influence of seasonal changes in canopy structure on interception loss:
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application of the revised Gash model. Journal of Hydrology 318, 80–102. Devlaeminck, R., De Schrijver, A., Hermy, M., 2005. Variation in throughfall deposition across a deciduous beech (Fagus sylvatica L.) forest edge in Flanders. The Science of the Total Environment 337, 241–252. Draaijers, G.P.J., Erisman, J.W., Leeuwen, N.F.M.V., Romer, F.G., Winkel, B.H.T., Veltkamp, A.C., Vermeulen, A.T., Wyers, G.P., 1997. The impact of canopy exchange on differences observed between atmospheric deposition and throughfall fluxes. Atmospheric Environment 31, 387–397. Draxler, R.R., Hess, G.D., 2005. HYSPLIT 4 USER’s Guide. NOAA Technical Memorandum ERL ARL-230. Fang, F., Wang, Q., Li, J., 2001. Atmospheric particulate mercury concentration and its dry deposition flux in Changchun City, China. The Science of the Total Environment 281, 229–236. Fitzgerald, W.F., Engstrom, D.R., Mason, R.P., Nater, E.A., 1998. The case for atmospheric mercury contamination in remote areas. Environmental Science & Technology 32, 1–7. Glass, G.E., Sorensen, J.A., 1999. Six-year trend (1990–1995) of wet mercury deposition in the Upper Midwest, U.S.A. Environmental Science & Technology 33, 3303–3312. Grennfelt, P., Larsson, S., Leyton, P., Olsson, B., 1985. Atmospheric deposition in the Lake Gardsjon area, SW Sweden. Ecology Bulletin (Stockholm) 37, 101. Grigal, D.F., 2002. Inputs and outputs of mercury from terrestrial watersheds: a review. Environmental Reviews 10, 1–39. Grigal, D.F., Kolka, R.K., Fleck, J.A., Nater, E.A., 2000. Mercury budget of an upland-peatland watershed. Biogeochemistry 50, 95–109. Guentzel, J.L., Landing, W.M., Gill, G.A., Pollman, C.D., 1998. Mercury and major ions in rainfall, throughfall, and foliage from the Florida Everglades. The Science of the Total Environment 213, 43–51. Han, Y.J., Holsen, T.M., Hopke, P.K., Yi, S.M., 2005. Comparison between back-trajectory based modeling and Lagrangian backward dispersion modeling for locating sources of reactive gaseous mercury. Environmental Science & Technology 39, 1715–1723. Hultberg, H., Munthe, J., Iverfeldt, A˚., 1995. Cycling of methyl mercury and mercury—responses in the forest roof catchment to three years of decreased atmospheric deposition. Water, Air, & Soil Pollution 80, 415–424. Iverfeldt, A˚., 1991. Mercury in forest canopy throughfall water and its relation to atmospheric deposition. Water, Air, & Soil Pollution 56, 553–564. Johnson, D.W., Lindberg, S.E., 1992. Atmospheric Deposition and Forest Nutrient Cycling: A Synthesis of the Integrated Forest Study. Springer, New York. Keeler, G.J., Gratz, L.E., Al-Wali, K., 2005. Long-term atmospheric mercury wet deposition at Underhill, Vermont. Ecotoxicology 14, 71–83. Keim, R.F., Skaugset, A.E., Weiler, M., 2005. Temporal persistence of spatial patterns in throughfall. Journal of Hydrology 314, 263–274. Kolka, R.K., Nater, E.A., Grigal, D.F., Verry, E.S., 1999. Atmospheric inputs of mercury and organic carbon into a forested upland/bog watershed. Water, Air, & Soil Pollution 113, 273–294.
Lai, S.-O., Holsen, T.M., Hopke, P.K., Liu, P., 2007. Wet deposition of mercury at a New York state rural site: concentrations, fluxes, and source areas. Atmospheric Environment 41, 4337–4348. Landis, M.S., Keeler, G.J., 1997. Critical evaluation of a modified automatic wet-only precipitation collector for mercury and trace element determinations. Environmental Science & Technology 31, 2610–2615. Lawson, N.M., Mason, R.P., 2001. Concentration of mercury, methylmercury, cadmium, lead, arsenic, and selenium in the rain and stream water of two contrasting watersheds in Western Maryland. Water Research 35, 4039–4052. Lindberg, S.E., Turner, R.R., Meyers, T.P., Taylor Jr., G.E., Schroeder, W.H., 1991. Atmospheric concentrations and deposition of Hg to a deciduous forest at Walker Branch Watershed, Tennessee, USA. Water, Air, & Soil Pollution 56, 577–594. Lindberg, S.E., Owens, J.G., Stratton, W., 1994. Application of Throughfall Methods to Estimate Dry Deposition of Mercury. Lewis Publishers, Palo Alto. Lindberg, S.E., Hanson, P.J., Meyers, T.P., Kim, K.H., 1998. Air/surface exchange of mercury vapor over forests—the need for a reassessment of continental biogenic emissions. Atmospheric Environment 32, 895–908. Lovett, G.M., Lindberg, S.E., 1984. Dry deposition and canopy exchange in a mixed oak forest as determined by analysis of throughfall. The Journal of Applied Ecology 21, 1013–1027. Mason, R.P., Lawson, N.M., Sullivan, K.A., 1997. Atmospheric deposition to the Chesapeake Bay watershed—regional and local sources. Atmospheric Environment 31, 3531–3540. Mason, R.P., Lawson, N.M., Sheu, G.R., 2000. Annual and seasonal trends in mercury deposition in Maryland. Atmospheric Environment 34, 1691–1701. McHale, M.R., Cirmo, C.P., Mitchell, M.J., McDonnell, J.J., 2004. Wetland nitrogen dynamics in an Adirondack forested watershed. Hydrological Processes 18, 1853–1870. Miller, E., Vanarsdale, A., Keeler, G., Chalmers, A., Poissant, L., Kamman, N., Brulotte, R., 2005. Estimation and mapping of wet and dry mercury deposition across northeastern North America. Ecotoxicology 14, 53–70. Munthe, J., Hultberg, H., Iverfeldt, A˚., 1995. Mechanisms of deposition of methylmercury and mercury to coniferous forests. Water, Air, & Soil Pollution 80, 363–371. Neal, C., Robson, A.J., Bhardwaj, C.L., Conway, T., Jeffery, H.A., Neal, M., Ryland, G.P., Smith, C.J., Walls, J., 1993. Relationships between precipitation, stemflow and throughfall for a lowland beech plantation, Black Wood, Hampshire, southern England: findings on interception at a forest edge and the effects of storm damage. Journal of Hydrology 146, 221–233. Price, A.G., Carlyle-Moses, D.E., 2003. Measurement and modeling of growing-season canopy water fluxes in a mature mixed deciduous forest stand, southern Ontario, Canada. Agricultural and Forest Meteorology 119, 69–85. Rea, A.W., Keeler, G.J., Scherbatskoy, T., 1996. The deposition of mercury in throughfall and litterfall in the Lake Champlain watershed: a short-term study. Atmospheric Environment 30, 3257–3263. Rea, A.W., Lindberg, S.E., Keeler, G.J., 2000. Assessment of dry deposition and foliar leaching of mercury and selected trace elements based on washed foliar and surrogate surfaces. Environmental Science & Technology 34, 2418–2425. Rea, A.W., Lindberg, S.E., Keeler, G.J., 2001. Dry deposition and foliar leaching of mercury and selected trace elements in
ARTICLE IN PRESS H.-D. Choi et al. / Atmospheric Environment 42 (2008) 1818–1827 deciduous forest throughfall. Atmospheric Environment 35, 3453–3462. Schwesig, D., Matzner, E., 2000. Pools and fluxes of mercury and methylmercury in two forested catchments in Germany. The Science of the Total Environment 260, 213–223. Sharac, T.J., 2006. A Comparison of Mercury Wet Deposition and Throughfall Events at the Huntington Wildlife Forest of Newcomb, NY. Department of Civil and Environmental Engineering, Clarkson University. Sorensen, J.A., Glass, G.E., Schmidt, K.W., 1994. Regional patterns of wet mercury deposition. Environmental Science & Technology 28, 2025–2032.
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St. Louis, V.L., Rudd, J.W.M., Kelly, C.A., Hall, B.D., Rolfhus, K.R., Scott, K.J., Lindberg, S.E., Dong, W., 2001. Importance of the forest canopy to fluxes of methyl mercury and total mercury to boreal ecosystems. Environmental Science & Technology 35, 3089–3098. US EPA, 2002. Method 1631, Revision E: Mercury in Water by Oxidation, Purge and Trap, and Cold Vapor Atomic Fluorescence Spectrometry. US EPA. Zillioux, E.J., Porcella, D.B., Benoit, J.M., 1993. Annual review: mercury cycling and effects in freshwater wetland ecosystems. Environmental Toxicology and Chemistry 12, 2245–2264.
Atmospheric Environment 42 (2008) 1818–1827 www.elsevier.com/locate/atmosenv
Mercury deposition in the Adirondacks: A comparison between precipitation and throughfall Hyun-Deok Choi, Timothy J. Sharac1, Thomas M. Holsen Department of Civil and Environmental Engineering, Clarkson University, P.O. Box 5710, Potsdam, NY 13699, USA Received 26 June 2007; received in revised form 10 November 2007; accepted 13 November 2007
Abstract The volume-weighted mean (VWM) Hg concentrations and the total cumulative fluxes in deciduous throughfall (6.6 ng L1 and 12.0 mg m2, respectively) were statistically higher than in precipitation (4.9 ng L1 and 11.6 mg m2, respectively) during 2 years of sampling at the Huntington Wildlife Forest in Newcomb, NY. Seasonally, the VWM Hg concentrations and the Hg fluxes in both precipitation and throughfall were lowest in winter and highest in summer. Due to the wash-off of dry deposition and foliar leaching, concentrations in throughfall were almost 50% higher than those in precipitation during the leaf-on period, while concentrations in throughfall were slightly higher than (or similar to) those in precipitation during the leaf-off period. During the 2 years of sampling, the total deposited cumulative flux in precipitation and deciduous throughfall were very similar (11.6 mg Hg m2 and 12.0 mg Hg m2, respectively), because the higher concentrations in throughfall were offset by smaller throughfall depths. Meteorological analysis indicated that the precipitation events resulting in the highest Hg fluxes were associated with trajectories that passed through regions of Midwest where major Hg sources including coal/oil-fired power plants and waste incinerators are located. r 2007 Elsevier Ltd. All rights reserved. Keywords: Total mercury deposition; Throughfall; Precipitation; Meteorological analysis
1. Introduction Atmospheric deposition is a significant source of mercury (Hg) to the forest ecosystem (Fitzgerald et al., 1998; Lindberg et al., 1998; Miller et al., 2005). However, this input is poorly understood because of the complex interactions between atmospheric Hg and the canopy including (1) heterogeneous oxidaCorresponding author. Tel.: +1 315 268 3851; fax: +1 315 268 7985. E-mail address: [email protected] (T.M. Holsen). 1 Current address: Clean Air Markets Division, US EPA, Washington, DC 20460, USA.
1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.11.036
tion reactions on humid leaf surfaces (Iverfeldt, 1991), (2) deposition of particulate Hg (HgP) on the leaf surfaces (Lovett and Lindberg, 1984; Iverfeldt, 1991; St. Louis et al., 2001), (3) uptake and emission of elemental Hg (Hg0) by stomata (Lovett and Lindberg, 1984; Iverfeldt, 1991; Lindberg et al., 1991; St. Louis et al., 2001), and (4) uptake and deposition of gaseous divalent Hg (RGM) by and on the leaves (Lovett and Lindberg, 1984; Iverfeldt, 1991; St. Louis et al., 2001). Atmospherically deposited mercury is generally divided via deposition mechanisms into two broad categories: wet deposition and dry deposition. Wet
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deposition occurs when mercury associated with moisture (e.g. rain, snow, dew, clouds, etc.), deposits on a surface—often the ground. Throughfall is identical to wet deposition with respect to environmental partitioning (i.e. within rain, snow, dew, clouds, etc.), however, the forest canopy temporarily intercepts this precipitation before it deposits on the ground (Lindberg et al., 1994; Draaijers et al., 1997; Guentzel et al., 1998; Biester et al., 2002; Devlaeminck et al., 2005; Deguchi et al., 2006). In the absence of precipitation, mercury that is deposited onto surfaces is known as dry deposition, although this process can be further subdivided into dry particle deposition and air–surface exchange (Lindberg et al., 1994, 1998; Guentzel et al., 1998; Fang et al., 2001; Grigal, 2002). The forest canopy plays an important role in capturing Hg from the atmosphere which is subsequently deposited in the forest ecosystem as throughfall and litterfall (Zillioux et al., 1993; Hultberg et al., 1995; Rea et al., 1996). Typically, throughfall has higher Hg concentrations than precipitation (from 1.5 up to 1.8 times) because of wash-off of previously deposited Hg (Iverfeldt, 1991; Munthe et al., 1995; Kolka et al., 1999; Grigal et al., 2000; Schwesig and Matzner, 2000). This study was conducted as a part of the investigation of the mercury input, output, and cycling in a deciduous forest located in the Huntington Forest in the Adirondacks. Total Hg concentrations in, and fluxes from, precipitation and throughfall were measured from December 2004 to December 2006. The data was divided into leaf-on and leaf-off periods to investigate the importance of dry deposition and foliar leaching processes on total Hg concentrations in throughfall. Meteorological analysis was used to investigate the history of the major precipitation events which contributed the most Hg deposition at the sampling site. 2. Sampling and analyzing methods 2.1. Sampling site Precipitation and deciduous throughfall samples were collected at the Huntington Wildlife Forest in Newcomb, NY (43.971N, 74.221W) from December 2004 to December 2006. The precipitation site was located in a large open clearing used by the Mercury Deposition Network (MDN) (NY20) and US EPA CASTNET (HWF187). The deciduous
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throughfall site (Arbutus site) is located southeast of the 3.52 km2 Arbutus Lake (43.981N, 74.231W) and is covered by mixed hardwood forests having a stand age of 100 years and dominated by a mostly deciduous forest, largely dominated by American beech (Fagus sylvatica), also mixed with sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) (Sharac, 2006; Bushey et al., 2007). The throughfall precipitation collector was placed about 1 km from the forest edge. Site elevation was 530 m and soils, typically o1 m thick, are dominated by coarse, loamy, mixed, frigid, Typic Haplorthods in the Becket-Mundal series. The watershed has a mean slope of 11% and a total relief of 225 m (Johnson and Lindberg, 1992; McHale et al., 2004). 2.2. Sampling methods Weekly precipitation and throughfall samples were collected using a modified MIC-B (Meteorological Instruments of Canada, Thornhill, Ont.) automatic precipitation collector. The MIC-B has a heated precipitation sensor which allows for the automatic removal of the hood during precipitation exposing the sampling trains only during wet deposition events. The samplers were modified to hold two acid-cleaned 1-L Teflon bottles to capture mercury in precipitation (Landis and Keeler, 1997). The Hg sampling train, consisting of a borosilicate glass funnel and a glass vapor lock to minimize evaporative losses, has been validated by Landis and Keeler (1997). 2.3. Sample train cleaning procedure The sample train acid-cleaning protocol was developed following the EPA Method 1631 version E (US EPA, 2002). The borosilicate glass funnels were pre-rinsed with deionized water and immersed for 24 h (48 h for new funnels) in a polypropylene tank containing 4 N HCl heated to 60 1C. Next, the funnels were rinsed three times with 18.2 MO cm Milli-Q water (Millipore Corporation, Billerica, MA), placed in a laminar flow hood in a class 100 clean room, and allowed to dry. Dry funnels were triple bagged using polyethylene ziplock bags. Sample bottles, vapor locks, and connecting sheaths were pre-rinsed with deionized water, soaked in a polypropylene container containing 1% HCl solution prepared from 18.2 MO cm Milli-Q water and then heated for 12 h in an oven at 60 1C. This
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equipment was rinsed three times with 18.2 MO cm Milli-Q water and then placed into a polypropylene container containing 0.5% HCl for a minimum of 12 h. These components were then rinsed three times with 18.2 MO cm Milli-Q water and then allowed to dry in a laminar flow hood in a class 100 clean room. The bottles, vapor locks, and connecting sheaths were all triple bagged in polyethylene ziplock bags when dry until prior to installation into the MIC-B sampling equipment. Before field deployment, Teflon bottles were filled with 20 mL of 0.08 M HCl (to minimize Hg loss in precipitation or throughfall), capped, and then weighed (Landis and Keeler, 1997; Lai et al., 2007).
argon. The gaseous Hg0 was captured by amalgamation onto a gold trap. The Hg was thermally desorbed from this trap, and then measured with a cold-vapor atomic fluorescence spectrometer (CVAFS). Precipitation depths were measured at the Huntington Wildlife Forest in Newcomb, Essex County, NY (station ID: NY20) by the National Trends Network (NTN) rain gage using a precipitation hydrograph (Belfort Instrument Company, Baltimore, MD). This rain gage detects precipitation events as small as 0.025 cm and measures precipitation depth, duration, and intensity of each precipitation event.
2.4. Spatial variation of throughfall depth
2.6. Meteorological trajectory analysis
The spatial variation in deciduous throughfall depth was measured throughout the measurement site during spring (before leaf-out) and early summer (after leaf-out) 2006 to assess whether the location of the deciduous throughfall collector was capturing a typical amount of precipitation with respect to other sites under this forest canopy. One hundred 473-mL cups were spaced 4.6 m apart running perpendicular to the slope of the hill, and 3.4 m apart along the length of each transect (10 cups per transect). The spatial variation of throughfall precipitation events was measured with a graduated cylinder as close as possible to the end of precipitation events to minimize evaporative losses.
The transport path of air parcels to the sampling site in the Adirondacks was estimated using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) Model Version 4.8 (Draxler and Hess, 2005). The 48-h back trajectories were calculated with Eta Data Assimilation System (EDAS) (40-km grid) obtained from Gridded Meteorological Data Archives by National Oceanic and Atmospheric Administration—Air Resources Laboratory (NOAA—ARL). Hourly back trajectories during precipitation were calculated. The arrival height of the back trajectories was 500 m.
2.5. Analytical methods Precipitation and throughfall samples were collected each Tuesday and immediately prepared for laboratory analysis by adding concentrated BrCl (0.5%, v/v) to each sample bottle to oxidize and desorb mercury from the inside walls of the Teflon bottles. These samples were stored for at least 24 h in a 4 1C refrigerator prior to analysis. Total Hg was quantified by a dual amalgamation technique followed by CVAFS with Tekran 2600 (Tekran Corporation Inc., Toronto, Ont., Canada). The analytical method used was a slightly modified EPA Method 1631 version E (US EPA, 2002). Briefly, each sample was treated with hydroxylamine (NH2OH HCl) to remove the free halogens (e.g. BrCl), and was then treated with stannous chloride (SnCl2) to convert Hg2+ to Hg0, which was removed from solution by purging with
2.7. QA/QC The method detection limit (MDL), calculated as three times the standard deviation of seven sequential reagent blanks, averaged 0.04 ng for 1 L reagent blank, the same as the MDL reported in EPA Method 1631. The initial (IPR) and on-going (OPR) precision and recovery were measured at the start of the analysis and every 20 samples, and ranged between 93% and 111% and 91% and 107%, respectively. These values were within the quality control acceptance criteria for performance in the EPA Method 1631 (IPR: 79–121% and OPR: 77–123%). Duplicate samples were taken bi-weekly (n ¼ 48). The relative standard deviation of the duplicate analysis was 3.2%. Field blanks (n ¼ 24) were obtained by rinsing the whole sampling assembly deployed inside the MIC-B for the same period as was used for sampling to ensure that there was no Hg absorption by the HCl preservative or to the walls of the sampling trains during nonprecipitation conditions. Laboratory blanks
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(n ¼ 12, every 2 months) were filled fully with deionized water, and handled in the same manner as a precipitation sample. The average field blank concentrations in precipitation and throughfall were 0.55 and 0.72 ng L1, respectively. The average lab blank concentration was 0.32 ng L1. All samples were corrected by the associated monthly field blank. For comparison, the actual sample concentrations varied between 0.21 and 28.51 ng L1 (blank corrected). 3. Results and discussion 3.1. Precipitation depth The distribution of precipitation volumes throughout the deciduous canopy was measured with 100 cups before leaf-out (n ¼ 3) and after leafout (n ¼ 4). The average event cup volume before the leaf-out period was 135745.2 mL (average7S.D.) which was not different than the cup located right beside the deciduous throughfall collector (131 mL). The average event precipitation volume for each cup after the leaf-out period was 178721.9 mL which was not different than the cup located next to the throughfall collector (179 mL). This finding suggests that the deciduous throughfall sample collector received a representative amount of precipitation both before and after leaf-out. The cumulative depths of precipitation and throughfall throughout this study were 238.4 and 183.5 cm, respectively; approximately 60% occurred during leaf-on periods (Table 1). The precipitation and throughfall depths were highly correlated during both leaf-off and leaf-on periods (r2 ¼ 0.92 and 0.95 for leaf-off and leaf-on, respectively). (The leaf-on season in Huntington Forest is from the beginning of May to the end of October.) However, the throughfall depth was smaller than the precipitation depth during both leaf-off periods (slope ¼ 1.28), and leaf-on periods (slope ¼ 1.16).
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These results are similar to previous measurements in mixed deciduous forests where throughfall volumes were approximately 8–24% smaller during leaf-out than wet deposition volumes (Neal et al., 1993; Rea et al., 1996; Price and Carlyle-Moses, 2003; Keim et al., 2005; Deguchi et al., 2006). For the 18 months of overlap when MDN data are available, the total measured MDN precipitation depth (NY20) and Clarkson measured precipitation depth were not statistically different (187.3 and 169.7 cm, respectively) (p ¼ 0.202) (Fig. 1, top). (MDN reports the precipitation depth measured by CASTNET and not the depth collected by the wet deposition sampler.) The differences were most pronounced during snow events. In addition, some sample-to-sample differences were due to different operating procedures between the sites. MDN samples were collected every Tuesday, however, if it was raining on a Tuesday, Clarkson samples were not collected until after the precipitation stopped. The result of this difference can be seen for example in the month of August 2005 where the individual sample depths were very different but the cumulative depths were very similar (MDN ¼ 11.2 cm and Clarkson ¼ 11.3 cm). 3.2. Hg in precipitation During the 2 years of sampling from December 2004 to December 2006, concentrations of total Hg in precipitation ranged from 0.2 to 28.5 ng L1, and the volume-weighted mean (VWM) Hg concentration was 4.9 ng L1 (Table 1). Lower Hg concentrations and fluxes were measured in winter and the highest Hg concentrations were measured in spring and summer (Fig. 2a). This result may be partly explained by an increase in the oxidizing rate of atmospheric Hg0 during spring and summer seasons due to increased photochemical activity (Munthe et al., 1995). Additional explanations for these differences include different source regions (dis-
Table 1 Cumulative precipitation depths, VWM Hg concentrations, and cumulative Hg fluxes in precipitation and throughfall depending on leafon and leaf-off from December 2004 to December 2006
Precipitation Throughfall a
Cumulative precipitation depth (cm)
VWM Hg concentration (ng L1)
Cumulative Hg flux (mg Hg m2)a
Leaf-on
Leaf-off
Total
Leaf-on
Leaf-off
Total
Leaf-on
Leaf-off
Total
138.7 110.1
99.7 73.4
238.4 183.5
5.5 8.0
4.0 4.5
4.9 6.6
7.7 8.9
4.0 3.3
11.6 12.2
Hg flux is calculated as the product of Hg concentration in the sample and its corresponding sample depth.
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80 60
2000 1500 1000
40
500
20
0
0 25
12 MDN (NY20) Clarkson Univ. MDN (NY20) Clarkson Univ.
20 15
10 8 6
10
4
5
2
0 Jun-06
May-06
Apr-06
Mar-06
Feb-06
Jan-06
Dec-05
Nov-05
Oct-05
Sep-05
Aug-05
Jul-05
Jun-05
May-05
Apr-05
Mar-05
Feb-05
0
Cumulative Flux [µg m-2] Cumulative Depth [mm]
Jun-06
May-06
Apr-06
Mar-06
Feb-06
Jan-06
Dec-05
Nov-05
Oct-05
Sep-05
Aug-05
Jul-05
Jun-05
Apr-05
Mar-05
Feb-05
Jan-05
MDN (NY20) Clarkson Univ. MDN (NY20) Clarkson Univ.
100
Jan-05
Total Hg Conc. [ng L-1]
Precip. Depth [mm]
120
May-05
H.-D. Choi et al. / Atmospheric Environment 42 (2008) 1818–1827
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Fig. 1. Comparisons of precipitation depth and total Hg concentrations between MDN (NY20) and Clarkson from December 2004 to June 2006.
cussed in Section 3.5) and increased convection during warmer months which can increase the ability of air to transport Hg over longer distances, and leads to greater precipitation amounts (Keeler et al., 2005). In addition, rain has a greater capacity to scavenge Hg than snow (Lindberg et al., 1991; Sorensen et al., 1994; Glass and Sorensen, 1999; Keeler et al., 2005). In general, our measured concentrations and fluxes were similar to those measured by MDN for the 18 months of overlapping samples (Fig. 1, bottom). The total deposited cumulative fluxes from MDN (NY20) and Clarkson were 10.0 and 8.7 mg Hg m2, respectively. Using the CASTNET precipitation depths with the Clarkson concentrations, the total deposited cumulative Hg flux was found to be 9.1 mg Hg m2. As indicated in the earlier text, some of the short-term differences were due to different sampling times. For example, in the month of August 2005, the individual sample depths and concentrations were very different but the cumulative fluxes were identical (MDN ¼ 0.64 mg Hg m2 and Clarkson ¼ 0.62 mg Hg m2). Statistically, Hg deposition from MDN (NY20) and Clarkson were significantly correlated (r2 ¼ 0.61 and po0.001) and not statistically different (p ¼ 0.155). The Clarkson fluxes using CASTNET precipitation were also well correlated with MDN
(NY20) (r2 ¼ 0.61 and po0.001) and not statistically different (p ¼ 0.345). 3.3. Hg in throughfall Mercury concentrations in throughfall ranged from 0.9 to 28.2 ng L1; the VWM Hg concentration was 6.6 ng L1 (Table 1). Mercury concentrations in throughfall and precipitation were significantly different (po0.001). Higher Hg concentrations in throughfall were measured in spring, summer, and fall, whereas lower concentrations were measured in winter (Fig. 2a). Concentration differences between throughfall and precipitation ranged from 2% to 166%, and the mean concentration difference was 50.3%. Even through the differences in concentrations between throughfall and precipitation have been found to vary depending on sampling sites and/or species of trees, our results are similar to previous results (Table 2). During leaf-on periods, this increased mass of Hg in throughfall vs. precipitation may originate from (1) the wash-off of dry deposition of airborne Hg that occurs during dry periods (Munthe et al., 1995), (2) foliar leaching (Lovett and Lindberg, 1984; Rea et al., 2000, 2001), (3) the evaporation of precipitation in the canopy which may concentrate the Hg present in the throughfall (Rea et al., 1996), and
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200 Precipitation Throughfall Precipitation Depth in Precipitation Precipitation Depth in Throughfall
14 12
180 160 140 120
10
100
8
80
6
60
4
40
Precipitation Depth [mm]
Monthly VWM Hg Concentration [ng L-1]
16
20
2
0 Dec-06
Nov-06
Oct-06
Sep-06
Aug-06
Jul-06
Jun-06
May-06
Apr-06
Mar-06
Feb-06
Jan-06
Dec-05
Nov-05
Oct-05
Sep-05
Jul-05
Aug-05
Jun-05
May-05
Apr-05
Mar-05
Feb-05
Jan-05
Dec-04
0
1.6
14 Precipitation Throughfall Cumulative Flux in Precipitation Cumulative Flux in Throughfall
1.2
12 10
1.0
8
0.8
6
0.6
4
0.4
2
0.2
0
Cumulative Flux [µg m-2]
Monthly Cumulative Flux [µg m-2]
1.4
Dec-06
Nov-06
Oct-06
Sep-06
Aug-06
Jul-06
Jun-06
May-06
Apr-06
Mar-06
Feb-06
Jan-06
Dec-05
Nov-05
Oct-05
Sep-05
Jul-05
Aug-05
Jun-05
May-05
Apr-05
Mar-05
Feb-05
Jan-05
Dec-04
0.0
Fig. 2. The monthly VWM Hg concentrations and precipitation depths (a), and the monthly Hg fluxes and cumulative fluxes in rainfall and throughfall samples (b).
(4) an increase in the oxidizing rate of atmospheric Hg0 by the canopy which provides a large reactive surface (Munthe et al., 1995). Some of the VWM total Hg concentrations in throughfall were very similar to those in precipitation even during leaf-on periods (Fig. 2a). For example, in June 2005, throughfall concentrations (22.1, 16.3, 3.6 ng L1) were higher than those in precipitation (13.8, 15.7, 2.8 ng L1); however, the
precipitation depths were approximately three times higher for precipitation than for throughfall (0.5, 6.1, 4.8 cm vs. 0.2, 3.7, 3.9 cm, respectively) resulting in similar VWM concentrations (precipitation ¼ 10.2 ng L1 and throughfall ¼ 10.1 ng L1). The Hg deposition flux was calculated as the product of Hg concentration in the sample and its corresponding sample depth (sample volume divide by diameter of funnel). Regression analysis revealed
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Table 2 Summary of differences of mercury concentration and mercury deposition flux between precipitation and throughfall Locations
Differences (%) a
New York Vermont Minnesota Ontario Sweden Sweden Sweden Tennessee Maryland a
Reference b
Concentration
Flux
2–166 11–430 76–240 100 100 30–40 Up to 85 – –
14 20–82 70 – 40 – 30–70 93 61
This study Rea et al. (1996) Grigal et al. (2000) St. Louis et al. (2001) Munthe et al. (1995) Grennfelt et al. (1985) Iverfeldt (1991) Lindberg et al. (1994) Lawson and Mason (2001)
Whole year. Growing season only.
b
Fig. 3. Regression plots between total Hg concentration (ng L1) and the ratio of dry day and precipitation depth (h mm1) in throughfall, and net throughfall.
that Hg deposition fluxes were more positively correlated with precipitation depths (r2 ¼ 0.46, po0.001) than concentrations (r2 ¼ 0.02, p ¼ 0.126), which is similar to the findings of Sorensen et al. (1994) and Mason et al. (1997, 2000). Weekly fluxes of precipitation and throughfall were significantly different (po0.001). However, the cumulative Hg flux in precipitation was almost the same as the cumulative Hg flux in throughfall over the two full years of sampling (precipitation ¼ 11.6 mg Hg m2 and throughfall ¼ 12.0 mg Hg m2). For the first year of this study (2005), the cumulative Hg flux in throughfall (6.5 mg Hg m2 yr1) was 16% higher than in precipitation (5.6 mg Hg m2 yr1) and the cumulative Hg fluxes during the growing season in through-
fall (4.9 mg Hg m2) were 26% higher than in precipitation (3.9 mg Hg m2). As mentioned in the earlier text, calculating the Hg deposition flux using the precipitation depth from the rain gauge instead of sample depth results in a 41% higher Hg deposition in throughfall than in precipitation during growing season. This finding is similar to or less than other published results which found that the throughfall fluxes of Hg are significantly greater than precipitation fluxes, since Hg concentrations in throughfall are comparably higher than in precipitation throughout the growing season (Table 2). However, the cumulative flux in throughfall (5.5 mg m2 yr1) was lower than in precipitation (6.0 mg m2 yr1) in the second year (2006), since the Hg concentrations in throughfall and precipitation were more similar than in the first year even during the growing season, and precipitation depths in precipitation were much higher than those in throughfall in the second year. Generally, depth estimated from sample volume in throughfall is lower than that in precipitation, particularly during leaf-on periods, because some of precipitation is captured and evaporated by the canopy or can be in stem flow. Therefore, even though the Hg concentration is higher in throughfall, the Hg flux due to throughfall can be close to and/or lower than that of precipitation because the sample depth is lower. A similar result has been reported by Guentzel et al. (1998) measuring throughfall under 4–7-m-tall trees. Calculating the Hg deposition flux using precipitation depth from the rain gauge instead of sample depth results in 12% higher total Hg deposition in throughfall than that in precipitation in 2006 and 30% higher in throughfall during the whole study period. 3.4. Leaf-on vs. leaf-off Concentrations of Hg in throughfall (8.0 ng L1) during leaf-on periods were higher than those in precipitation (5.5 ng L1) and they were moderately correlated (r2 ¼ 0.42). However, concentrations in throughfall (4.5 ng L1) were only slightly higher than in precipitation (4.0 ng L1) during leaf-off periods; during this period, concentrations were well correlated (r2 ¼ 0.92) (Table 1). It is generally accepted that Hg concentrations in throughfall are positively correlated with antecedent dry period and negatively correlated with precipitation depth (Rea et al., 2000). For samples that
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contained only one event (i.e. weeks in which precipitation occurred only once) during leaf-on periods, Hg concentrations in throughfall and net throughfall are strongly associated with the length of the antecedent dry period as well as the amount of precipitation. Throughfall and net throughfall were linearly correlated with the ratio of the length of the antecedent dry period and precipitation depth (r2 ¼ 0.77 and 0.56, respectively) (Fig. 3), while Hg concentrations in precipitation were not correlated with this ratio (r2 ¼ 0.18). 3.5. Back trajectory analysis The weekly precipitation samples that had the three highest Hg fluxes (1.0, 0.5, 0.6 mg Hg m2 week1 in 7 June 2005, 30 August 2005, 27 June 2006 weeks, respectively) were used to initiate 48-h back trajectories for every hour during the precipitation events. The precipitation depths of the three highest fluxes (6.1, 10.4, 6.2 cm, respectively) were 3–5 times higher than average precipitation measured (2.3 cm) and the concentrations (15.7, 5.1, 10.5 ng L1, respectively) were up to 3 times higher than the VWM Hg concentration (4.9 ng L1). The cumulative flux of these three highest samples accounts for 18.3% of total cumulative Hg flux over the 2-year period, suggesting that Hg inputs may be dominated by a small number of large events. During these events, air parcels were transported mostly from the south (27 June 2005) or southeast (30 August 2005 and 27 June 2006) to the sampling site, so most of the trajectories passed over eastern Pennsylvania, West Virginia, and New York City where there are several major Hg sources including large coal-fired power plants, oil-fired power plants, and waste incinerators (Han et al., 2005). A few of the trajectories passed over Rochester, NY, or Toronto. The Russell electric generation station and medical and sludge waste incinerators are located around Rochester and waste incinerators are located in southern Ontario including Toronto (Han et al., 2005). These results indicate that the air masses with relatively high precipitation amounts that pass over these Hg sources may deposit elevated amounts of Hg wet deposition. 4. Conclusions This study investigated Hg wet deposition and deciduous throughfall in the Adirondacks for 2
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years. During this study, the VWM Hg concentration in throughfall (6.6 ng L1) was higher than in precipitation (4.9 ng L1), while the total cumulative Hg flux in throughfall (12.0 mg Hg m2) was very similar to precipitation (11.6 mg Hg m2) due to relatively lower precipitation depths in throughfall. The cumulative Hg flux in throughfall was 14% higher than that in precipitation during the first study year due to higher Hg concentrations in throughfall throughout the growing season. However, the cumulative Hg flux in throughfall was lower than in precipitation during the second year due to the similar Hg concentrations in throughfall and precipitation even during the growing season. Two-day back trajectory analysis with the three highest fluxes indicated that air parcels during this study passed through the major Hg sources in eastern Pennsylvania, Virginia, and New York City, or Rochester, NY, and Toronto areas. During these three events, the fluxes were 5–9 times higher than the average flux resulting in these events being responsible for 18.3% of the total Hg deposited during the study. These findings support several previous studies which suggest that Hg found in wet deposition in the northeastern United States is largely derived from coal-fired power plants in the Ohio River valley. Acknowledgments This research was supported in part by a National Science Foundation No. 02-167 Biocomplexity Grant (Charles T. Driscoll, PI). This is the Clarkson Center for the Environment publication no. 339. The research described herein has not been subjected to EPA peer and administrative review. Therefore, the conclusions and opinions drawn are solely those of the authors and should not be construed to reflect the views of EPA.
References Biester, H., Muller, G., Scholer, H.F., 2002. Estimating distribution and retention of mercury in three different soils contaminated by emissions from chlor-alkali plants: part I. The Science of the Total Environment 284, 177–189. Bushey, J.T., Nallana, A.G., Montesdeoca, M.R., Driscoll, C.T., 2007. Enhancement of mercury deposition by the forest canopy within a northern forest landscape. Atmospheric Environment (submitted for publication). Deguchi, A., Hattori, S., Park, H.-T., 2006. The influence of seasonal changes in canopy structure on interception loss:
ARTICLE IN PRESS 1826
H.-D. Choi et al. / Atmospheric Environment 42 (2008) 1818–1827
application of the revised Gash model. Journal of Hydrology 318, 80–102. Devlaeminck, R., De Schrijver, A., Hermy, M., 2005. Variation in throughfall deposition across a deciduous beech (Fagus sylvatica L.) forest edge in Flanders. The Science of the Total Environment 337, 241–252. Draaijers, G.P.J., Erisman, J.W., Leeuwen, N.F.M.V., Romer, F.G., Winkel, B.H.T., Veltkamp, A.C., Vermeulen, A.T., Wyers, G.P., 1997. The impact of canopy exchange on differences observed between atmospheric deposition and throughfall fluxes. Atmospheric Environment 31, 387–397. Draxler, R.R., Hess, G.D., 2005. HYSPLIT 4 USER’s Guide. NOAA Technical Memorandum ERL ARL-230. Fang, F., Wang, Q., Li, J., 2001. Atmospheric particulate mercury concentration and its dry deposition flux in Changchun City, China. The Science of the Total Environment 281, 229–236. Fitzgerald, W.F., Engstrom, D.R., Mason, R.P., Nater, E.A., 1998. The case for atmospheric mercury contamination in remote areas. Environmental Science & Technology 32, 1–7. Glass, G.E., Sorensen, J.A., 1999. Six-year trend (1990–1995) of wet mercury deposition in the Upper Midwest, U.S.A. Environmental Science & Technology 33, 3303–3312. Grennfelt, P., Larsson, S., Leyton, P., Olsson, B., 1985. Atmospheric deposition in the Lake Gardsjon area, SW Sweden. Ecology Bulletin (Stockholm) 37, 101. Grigal, D.F., 2002. Inputs and outputs of mercury from terrestrial watersheds: a review. Environmental Reviews 10, 1–39. Grigal, D.F., Kolka, R.K., Fleck, J.A., Nater, E.A., 2000. Mercury budget of an upland-peatland watershed. Biogeochemistry 50, 95–109. Guentzel, J.L., Landing, W.M., Gill, G.A., Pollman, C.D., 1998. Mercury and major ions in rainfall, throughfall, and foliage from the Florida Everglades. The Science of the Total Environment 213, 43–51. Han, Y.J., Holsen, T.M., Hopke, P.K., Yi, S.M., 2005. Comparison between back-trajectory based modeling and Lagrangian backward dispersion modeling for locating sources of reactive gaseous mercury. Environmental Science & Technology 39, 1715–1723. Hultberg, H., Munthe, J., Iverfeldt, A˚., 1995. Cycling of methyl mercury and mercury—responses in the forest roof catchment to three years of decreased atmospheric deposition. Water, Air, & Soil Pollution 80, 415–424. Iverfeldt, A˚., 1991. Mercury in forest canopy throughfall water and its relation to atmospheric deposition. Water, Air, & Soil Pollution 56, 553–564. Johnson, D.W., Lindberg, S.E., 1992. Atmospheric Deposition and Forest Nutrient Cycling: A Synthesis of the Integrated Forest Study. Springer, New York. Keeler, G.J., Gratz, L.E., Al-Wali, K., 2005. Long-term atmospheric mercury wet deposition at Underhill, Vermont. Ecotoxicology 14, 71–83. Keim, R.F., Skaugset, A.E., Weiler, M., 2005. Temporal persistence of spatial patterns in throughfall. Journal of Hydrology 314, 263–274. Kolka, R.K., Nater, E.A., Grigal, D.F., Verry, E.S., 1999. Atmospheric inputs of mercury and organic carbon into a forested upland/bog watershed. Water, Air, & Soil Pollution 113, 273–294.
Lai, S.-O., Holsen, T.M., Hopke, P.K., Liu, P., 2007. Wet deposition of mercury at a New York state rural site: concentrations, fluxes, and source areas. Atmospheric Environment 41, 4337–4348. Landis, M.S., Keeler, G.J., 1997. Critical evaluation of a modified automatic wet-only precipitation collector for mercury and trace element determinations. Environmental Science & Technology 31, 2610–2615. Lawson, N.M., Mason, R.P., 2001. Concentration of mercury, methylmercury, cadmium, lead, arsenic, and selenium in the rain and stream water of two contrasting watersheds in Western Maryland. Water Research 35, 4039–4052. Lindberg, S.E., Turner, R.R., Meyers, T.P., Taylor Jr., G.E., Schroeder, W.H., 1991. Atmospheric concentrations and deposition of Hg to a deciduous forest at Walker Branch Watershed, Tennessee, USA. Water, Air, & Soil Pollution 56, 577–594. Lindberg, S.E., Owens, J.G., Stratton, W., 1994. Application of Throughfall Methods to Estimate Dry Deposition of Mercury. Lewis Publishers, Palo Alto. Lindberg, S.E., Hanson, P.J., Meyers, T.P., Kim, K.H., 1998. Air/surface exchange of mercury vapor over forests—the need for a reassessment of continental biogenic emissions. Atmospheric Environment 32, 895–908. Lovett, G.M., Lindberg, S.E., 1984. Dry deposition and canopy exchange in a mixed oak forest as determined by analysis of throughfall. The Journal of Applied Ecology 21, 1013–1027. Mason, R.P., Lawson, N.M., Sullivan, K.A., 1997. Atmospheric deposition to the Chesapeake Bay watershed—regional and local sources. Atmospheric Environment 31, 3531–3540. Mason, R.P., Lawson, N.M., Sheu, G.R., 2000. Annual and seasonal trends in mercury deposition in Maryland. Atmospheric Environment 34, 1691–1701. McHale, M.R., Cirmo, C.P., Mitchell, M.J., McDonnell, J.J., 2004. Wetland nitrogen dynamics in an Adirondack forested watershed. Hydrological Processes 18, 1853–1870. Miller, E., Vanarsdale, A., Keeler, G., Chalmers, A., Poissant, L., Kamman, N., Brulotte, R., 2005. Estimation and mapping of wet and dry mercury deposition across northeastern North America. Ecotoxicology 14, 53–70. Munthe, J., Hultberg, H., Iverfeldt, A˚., 1995. Mechanisms of deposition of methylmercury and mercury to coniferous forests. Water, Air, & Soil Pollution 80, 363–371. Neal, C., Robson, A.J., Bhardwaj, C.L., Conway, T., Jeffery, H.A., Neal, M., Ryland, G.P., Smith, C.J., Walls, J., 1993. Relationships between precipitation, stemflow and throughfall for a lowland beech plantation, Black Wood, Hampshire, southern England: findings on interception at a forest edge and the effects of storm damage. Journal of Hydrology 146, 221–233. Price, A.G., Carlyle-Moses, D.E., 2003. Measurement and modeling of growing-season canopy water fluxes in a mature mixed deciduous forest stand, southern Ontario, Canada. Agricultural and Forest Meteorology 119, 69–85. Rea, A.W., Keeler, G.J., Scherbatskoy, T., 1996. The deposition of mercury in throughfall and litterfall in the Lake Champlain watershed: a short-term study. Atmospheric Environment 30, 3257–3263. Rea, A.W., Lindberg, S.E., Keeler, G.J., 2000. Assessment of dry deposition and foliar leaching of mercury and selected trace elements based on washed foliar and surrogate surfaces. Environmental Science & Technology 34, 2418–2425. Rea, A.W., Lindberg, S.E., Keeler, G.J., 2001. Dry deposition and foliar leaching of mercury and selected trace elements in
ARTICLE IN PRESS H.-D. Choi et al. / Atmospheric Environment 42 (2008) 1818–1827 deciduous forest throughfall. Atmospheric Environment 35, 3453–3462. Schwesig, D., Matzner, E., 2000. Pools and fluxes of mercury and methylmercury in two forested catchments in Germany. The Science of the Total Environment 260, 213–223. Sharac, T.J., 2006. A Comparison of Mercury Wet Deposition and Throughfall Events at the Huntington Wildlife Forest of Newcomb, NY. Department of Civil and Environmental Engineering, Clarkson University. Sorensen, J.A., Glass, G.E., Schmidt, K.W., 1994. Regional patterns of wet mercury deposition. Environmental Science & Technology 28, 2025–2032.
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St. Louis, V.L., Rudd, J.W.M., Kelly, C.A., Hall, B.D., Rolfhus, K.R., Scott, K.J., Lindberg, S.E., Dong, W., 2001. Importance of the forest canopy to fluxes of methyl mercury and total mercury to boreal ecosystems. Environmental Science & Technology 35, 3089–3098. US EPA, 2002. Method 1631, Revision E: Mercury in Water by Oxidation, Purge and Trap, and Cold Vapor Atomic Fluorescence Spectrometry. US EPA. Zillioux, E.J., Porcella, D.B., Benoit, J.M., 1993. Annual review: mercury cycling and effects in freshwater wetland ecosystems. Environmental Toxicology and Chemistry 12, 2245–2264.