ARTICLE IN PRESS
Atmospheric Environment 39 (2005) 7557–7569
www.elsevier.com/locate/atmosenv
Methyl and total mercury in precipitation in the
Great Lakes region
B.D. Halla,, H. Manolopoulosa, J.P. Hurleya,b, J.J. Schauera, V.L. St. Louisc,
D. Kenskid, J. Graydonb, C.L. Babiarza, L.B. Clecknera, G.J. Keelere
a
Environmental Chemistry and Technology Program, University of Wisconsin-Madison, 660 North Park Street,
Madison, WI 53706, USA
b
Aquatic Sciences Center, University of Wisconsin-Madison, Madison, WI 53706, USA
c
Department of Biological Sciences, University of Alberta, Edmonton, AB, Canada T6G 2E9
d
Lake Michigan Air Directors Consortium, 2250 E. Devon Ave, Suite 250, Des Plaines, IL 60018, USA
e
Department of Atmospheric, Oceanic and Space Sciences and Department of Environmental Health Sciences,
University of Michigan, Ann Arbor, MI 48109, USA
Received 1 November 2004; accepted 21 April 2005
Abstract
Methylmercury (MeHg) and total mercury (THg) concentrations were measured in precipitation collected from five
US sites in the Great Lakes region: three sites on the southern shore of Lake Superior (Brule River, WI, Eagle Harbor,
MI, and Tahquamenon Falls, MI), one at Isle Royale National Park (MI), and one in southern Wisconsin (Devil’s
Lake), between May 1997 and December 2003. MeHg and THg concentrations at these sites were compared to MeHg
and THg concentrations in precipitation collected at the Experimental Lakes Area (ELA) in north-western Ontario,
Canada. Detectable MeHg concentrations (40.01 ng L1) were found in the majority of rain and snow samples
collected from all sites (range ¼ 0.01–0.85 ng L1). In general, the lowest MeHg concentrations were observed in
samples taken at Tahquamenon Falls and the ELA, and the highest MeHg concentrations in precipitation were
observed in samples collected from Brule River and Eagle Harbor. Total Hg concentrations in precipitation were
generally between 10 and 60 ng L1, exceeding 60 ng L1 in one precipitation event sampled from each of Brule River,
Isle Royale, Tahquamenon Falls, and Devil’s Lake. The proportion of THg that was MeHg (%MeHg), was less than
6% at all sites, with the exception of seven events at Tahquamenon Falls and two events at the ELA that were between
6% and 18% MeHg. Generally, the highest MeHg concentrations were found in low-volume precipitation events
(o100 mL). At Tahquamenon Falls, meteorological analysis indicated that events with higher MeHg concentrations
and %MeHg exceeding 6% were generally associated with lake effect precipitation and weak local winds.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Methylmercury; Precipitation; Great Lakes; Meteorological analysis; Wisconsin; Michigan; Ontario; Lake Superior
1. Introduction
Corresponding author.
E-mail address: [email protected] (B.D. Hall).
Mercury (Hg) is a highly volatile metal that is easily
transported from anthropogenic sources (mainly coalfire electricity generation and waste combustion) to
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.atmosenv.2005.04.042
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B.D. Hall et al. / Atmospheric Environment 39 (2005) 7557–7569
remote ecosystems (Mason Sheu, 2002). The majority of
Hg in the atmosphere is gaseous and elemental Hg0
(Mason et al., 1994; Fitzgerald et al., 1998) which can be
oxidized to reactive Hg (Hg[II]) in photochemical
reactions (Schroeder and Munthe, 1998). Hg(II) is
water-soluble and either enters the terrestrial ecosystem
directly through precipitation or indirectly as dry
deposition to the forest canopy (Rea et al., 2000), which
is then deposited on the forest floor as throughfall from
the canopy during subsequent rain events or as litterfall
(Rea et al., 2001; St. Louis et al., 2001). Hg(II) enters
regions of open water as direct deposition or transported
through runoff, possibly after retention in the watershed
(Hurley et al., 1995; Rudd, 1995; Landis and Keeler,
2002). Once in anaerobic regions, such as those
commonly found in wetlands and lake sediments, Hg(II)
can be converted to methylmercury (MeHg) a neurotoxin that bioaccumulates through aquatic food webs
(Wiener et al., 2003).
Atmospheric deposition represents the major route of
inorganic Hg input to ecosystems (Mason and Fitzgerald, 1996; Fitzgerald et al., 1998). However, the major
source of MeHg is assumed to be production via
microbial methylation (Gilmour and Riedel, 1995; Pak
and Bartha, 1998; Ullrich et al., 2001). Atmospheric
deposition of MeHg is generally thought to be a minor
contributor to MeHg contamination of fish, but
recently, Rolfhus et al. (2003) identified the atmosphere
as the main source of MeHg to offshore regions of Lake
Superior, and the most probable source of MeHg to
offshore aquatic organisms. To assess the regional
importance of atmospheric deposition, this study presents long-term datasets of MeHg and total Hg (THg: all
forms of Hg) concentrations in samples taken at six sites
in the upper midwest United States and north-western
Ontario, Canada. We present evidence that MeHg was
‘‘washed-out’’ of the atmosphere during early stages of a
precipitation event. We also used atmospheric back
trajectories to formulate hypotheses on possible sources
of MeHg to precipitation.
2. Methods
2.1. Site Descriptions and Collection Methods
2.1.1. Lake Superior and Devil’s Lake locations
Samples were collected at four sites on Lake Superior:
Eagle Harbor, Tahquamenon Falls, Isle Royale, MI,
and Brule River, WI (Fig. 1). Brule River is the
westernmost site, located 30 km east of the city of
Superior, WI. Eagle Harbor is located on the Keweenaw
Peninsula, 45 km northeast of Houghton, MI. The
easternmost site, Tahquamenon Falls, is located 65 km
west of Sault Ste. Marie, MI. The northern Lake
Superior sampling site is located on the southern edge
Fig. 1. Locations of sampling sites used for precipitation
collection. Brule River: 46.75N, 91.50W. Eagle Harbor: 47.61N,
88.15W. Tahquamenon Falls: 46.61N, 85.20W. Isle Royale:
48.05N, 88.63W. Devil’s Lake: 43.44N, 89.68W. Experimental
Lakes Area (ELA): 49.40N, 93.44W.
of Isle Royale National Park (Fig. 1). Samples were also
collected at Devil’s Lake State Park, WI, located 70 km
northwest of Madison, WI (Fig. 1). Devil’s Lake is a
Wisconsin Department of Natural Resources (WDNR)
air quality monitoring station and a Mercury Deposition Network (MDN; MDN Site WI31) site.
Lake Superior and Devil’s Lake samples were taken
using an automated precipitation sampler, as described
by Landis and Keeler (1997). The collection system was
designed to collect ultra-clean samples from individual
precipitation events. Two acid-washed borosilicate glass
collection funnels (181 cm2 collection area) were attached to separate 1-L Teflon sampling bottles (one for
THg analysis and the other for MeHg analysis) via a
Teflon adapter and glass vapor lock (Landis and Keeler,
1997). With the exception of Isle Royale (where samples
were composed of week-long composites), samples were
collected after each precipitation event throughout the
calendar year. Details of sampling dates, durations, and
analytical procedures are listed in Table 1.
Samples from all of the Lake Superior sites arrived at
the laboratory unpreserved. All samples were visually
inspected and large particles were removed using a clean
nitex filter (500 mm). Samples were preserved by adding
trace-metal grade HCl (to 1% of total sample volume).
For samples taken at Devil’s Lake, 2 mL of trace-metal
grade HCl (regardless of sample volume) were added to
the sample bottle prior to deployment to preserve
samples until collection. Devil’s Lake samples were not
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B.D. Hall et al. / Atmospheric Environment 39 (2005) 7557–7569
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Table 1
Detection limits and blank levels from laboratories analyzing total mercury (THg) and methylmercury (MeHg) in precipitation
MeHg
Brule River
Eagle Harbor
Tahquamenon
ELA
Isle Royale
Devil’s Lake
THg
Brule River
Eagle Harbor
Tahquamenon
ELA
Isle Royale
Devil’s Lake
Collection period
Collection duration
Laboratory
Detection limits (ng L1)
1998–1999
1997–2002
1997–1998
1997–1999
2000–2003
1997–1999
2002–2003
By event
By event
By event
By event
450 mmc
Weekly
By event
UW-Madisona
UW-Madison
UW-Madison
Flett Researchb
Trent Universityd
UW-Madison
UW-Madison
0.01–0.03
0.01–0.03
0.01–0.03
0.01–0.02
0.02
0.01–0.03
0.01–0.03
1998–1999
1997–1999f
1997–1998
1997–1999
2000–2003
1997–1999
2002–2003
By event
By event
By event
By event
450 mm
Weekly
By event
Michigane
Michigan
Michigan
Flett Research
Trent University
Michigan
UW-Madison
0.1
0.1
0.1
0.2–0.3
0.05
0.1
0.15–0.7
a
University of Wisconsin-Madison (Madison, WI).
Flett Research Ltd. (Winnipeg, MB).
c
Samples were collected following accumulation of 50 mm of precipitation.
d
Trent University (Peterborough, ON).
e
University of Michigan Air Quality Laboratory (Ann Arbor, MI).
f
Although samples were collected at Eagle Harbor until 2002, funding restrictions did not allow us to analyze post-1999 samples for
THg.
b
filtered prior to analysis, however large particles were
excluded by decanting the sample.
2.1.2. Experimental Lakes Area
Wet deposition samples were collected on a per event
basis at the ELA meteorological site, a pristine boreal
forest 450 km northwest of Thunder Bay, Ontario. In
1998 and 1999 samples were collected manually. Just
prior to a precipitation event, ultra-clean 250 mL wide
mouth Teflon jars were placed on acid washed plexiglass
trays secured to wooden posts (St. Louis et al., 1995; St.
Louis et al., 2001). Immediately after the event, samples
were poured into ultra-clean Teflon bottles. MeHg
samples were preserved frozen until analysis, whereas
THg water samples were preserved using trace metal
grade concentrated HCl to 0.2% of total sample volume.
From 2000 to 2003, wet deposition samples were
collected from an automated collector installed on a
cliff in the Lake 658 watershed 7 km from the ELA
meteorological site. One acid-washed glass collection
funnel (167 cm2 collection area) was attached via a
0.64 cm diameter piece length of Teflon tubing and
Teflon compression fitting to a closed 2-L Teflon
sampling bottle containing 1 mL ultra clean HCl. MeHg
and THg samples were collected after 50 mm of wet
deposition had fallen and further preserved using ultraclean HCl equal to 0.2% of the sample volume. Wet
precipitation samples were collected at the ELA during
the ice-free season only and were not filtered prior to Hg
analysis (large particles were removed prior to analysis).
Although we recognize that there may have been differences among various collection methods, we assumed
differences to be insignificant.
2.2. Analytical methods used to measure methylmercury
and total mercury
Although samples were analyzed at a number of
different laboratories (Table 1), the same methods were
used to determine MeHg and THg concentrations, with
the exception of the samples collected at the ELA
between 2000 and 2003. Furthermore, all laboratories
participated in inter-laboratory comparisons over the
course of the study. Samples for MeHg were distilled,
ethylated, and analyzed by cold-vapor atomic florescence spectrometry (CVAFS) (Bloom, 1989; Horvat
et al., 1993; Liang et al., 1994). THg analysis followed
EPA Method 1631 (United States Environmental Protection Agency (US EPA), 2002). Samples collected at the
ELA between 2000 and 2003 were prepared as the other
samples, but Hg was detected using inductively coupled
plasma mass spectrometry (ICP-MS) as described by
Hintelmann et al. (1995) and Hintelmann and Evans
(1997). Detection limits for MeHg and THg analysis at all
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B.D. Hall et al. / Atmospheric Environment 39 (2005) 7557–7569
labs were between 0.01 and 0.03 ng L1 and 0.05 and
0.7 ng L1, respectively (Table 1). Spike recoveries for
MeHg and THg were generally 480% and 490%,
respectively. Because of the possibility of bottle contamination of THg in samples with low volumes (Landis
and Keeler, 1997), THg concentrations in samples
taken at the Lake Superior and Devil’s Lake sites with
total volume less than 20 mL (precipitation depth less
than 0.1 cm) were discarded. We did include the low
volume events in the MeHg dataset because it is much
more difficult to contaminate samples with MeHg
than with THg. To test the significance of differences
in concentrations among sites we used ANOVA and
multiple comparison tests (SigmaStat 3.0), with a
po0:05.
3. Results and discussion
3.1. Methylmercury
Detectable MeHg concentrations were measured in
the majority (84%) of precipitation samples at all sites
and ranged from 0.01 to 0.85 ng L1 (Fig. 2). The lowest
MeHg concentrations were observed in samples collected at the ELA, where concentrations never exceeded
0.33 ng L1. When all concentrations from each site were
grouped together, the median and mean MeHg concentrations in samples collected at the ELA (0.10 and
0.11 ng L1, respectively) were lower than those collected at any other site (Fig. 3A), with the exception of
MeHg concentrations in precipitation collected at
Tahquamenon Falls which ranged from 0.02 to
0.37 ng L1 (median and mean equal to 0.08 and
0.10 ng L1, respectively). There were no statistically
significant differences between concentrations in samples collected at the ELA and Tahquamenon Falls
(Kruskal–Wallis test, p ¼ 0:706). Events with MeHg
concentrations that were non-detectable were most
frequent in samples collected at Tahquamenon Falls
and Devil’s Lake (31% and 35% of total events
for respective sites). Events that had concentrations
below 0.10 ng L1 (Fig. 3B) were also most frequent at
those sites.
MeHg concentrations in samples collected at Brule
River, Eagle Harbor, Isle Royale, and Devil’s Lake were
significantly higher than those collected at the ELA and
Tahquamenon Falls (Fig. 3A; Kruskal–Wallis test,
po0:001). The frequency of events with MeHg concentrations exceeding 0.30 ng L1 was higher at Brule River
(14%), Isle Royale (13%), Eagle Harbor (10%), and
Devil’s Lake (10%) than at the ELA and Tahquamenon
Falls (0% and 2%, respectively; Fig. 3B). Precipitation
events with MeHg concentrations exceeding 0.6 ng L1
were rare and only occurred at the Brule River (1 event),
Eagle Harbor (1 event), and Devil’s Lake (4 events).
The percentage of all events with MeHg concentrations that were either undetectable or less than
0.10 ng L1 was greatest at Tahquamenon Falls (75%)
and lowest at the Brule River (38%). Conversely, the
percentage of events with MeHg concentrations that
exceeded 0.30 ng L1 was highest in samples collected at
Brule River (14%) and lowest at Tahquamenon Falls
(2%). Generally, average MeHg concentrations within
each site did not differ among years within each site
(Table 2).
To examine possible trends in MeHg with season, we
compared MeHg concentrations in samples collected at
Eagle Harbor in the spring (March, April, May),
summer (June, July, August), autumn (September,
October, November), and winter (December, January,
February). We used data collected at Eagle Harbor
because it was the largest of our datasets. Average
MeHg concentrations in samples collected in the spring
and summer (0.19 and 0.18 ng L1, respectively) were
significantly higher than those sampled in the winter
(0.08 ng L1; Kruskal–Wallis test, po0:001). There was
no significant difference among MeHg concentrations in
samples collected in the autumn compared to other
seasons (Kruskal–Wallis test, p ¼ 0:258).
Despite the existence of a Hg deposition network of
sampling locations in both Canada and the United States
(the MDN run by the National Atmospheric Deposition
Program), there are relatively few published datasets of
MeHg concentration collected in North America. However, the following reported concentrations are similar to
our values: Glass and Sorensen (1999) reported an
annual average MeHg concentration of 0.18 ng L1 in
samples collected in North Dakota, Minnesota, and
Michigan from 1990 to 1995. Fitzgerald et al. (1991)
found MeHg concentrations as high as 0.22 ng L1 in
precipitation sampled in northern Wisconsin. Additionally, precipitation collected in the southeast and northwest United States was found to have MeHg
concentrations between 0.16 and 0.35 ng L1 (Bloom
and Watras, 1989; Allan and Heyes, 1998).
3.1.1. Total mercury
THg concentrations in all samples ranged fromo1 to
130 ng L1 (Fig. 4). Similar to MeHg concentrations, the
lowest THg concentrations were observed at the ELA
(range ¼ 0.7–25.6 ng L1, mean ¼ 6.2 ng L1, median ¼
4.4 ng L1), however, there were no relationships between THg and MeHg concentrations at any of our sites.
There were statistically significant differences between
average THg concentrations collected at the ELA and
the other five sites (Fig. 3C; Kruskal–Wallis test,
po0:001).
There were no statistical differences in THg concentrations among the Eagle Harbor, Brule River, and
Tahquamenon Falls sites. Mean concentrations at these
three sites ranged from 11.3 (Eagle Harbor) to
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0.8
7561
Brule River
0.6
0.4
0.2
0.0
0.8
Eagle Harbor
0.6
0.4
0.2
0.0
0.8
Tahquamenon River
MeHg Concentration (ng/L)
0.6
0.4
0.2
0.0
0.8
Isle Royale
0.6
0.4
0.2
0.0
0.8
ELA
0.6
0.4
0.2
0.0
0.8
Devil's Lake
0.6
0.4
0.2
0.0
1997
1998
1999
2000
2001
2002
2003
2004
Fig. 2. Methylmercury (MeHg) concentrations (ng L1) in precipitation samples. Dates of sampling periods are presented in Table 2.
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140
120
80
0.4
0.2
60
(ng/L)
0.6
THg concentrations
40
20
0
West
40
North
ke
il'
s
ev
D
28.2
44.0
27.6
18.9
H
le
en
am
R
le
u
hq
Is
Ta
East
e
al
oy
North
ke
iv
West
on
ar
2.8
La
r
bo
g
Ea
26.8
2.7
R
ru
le
South
La
A
EL
26.2
22.6
er
ke
(D)
East
30.2
31.8
31.8
B
Ta
22.5
39.4
La
R
le
Is
s
am
u
hq
il'
H
le
g
Ea
e
al
oy
27.0
26.4
0
ev
er
iv
R
ru
le
B
(B)
on
en
EL
r
bo
ar
27.5
60
18.2
9.4
A
7.3
8.5
14.7
20
D
1.9
35.9
36.4
5.4`
2.7 3.1
80
34.9
16.1
14.1
s
50.0
10.4
il'
28.6
30.6
10-20 ng/L
5-10 ng/L
<5 ng/L
ev
44.1
36.5
0
iv
ru
le
B
22.2
53.6
20
R
le
Is
Ta
12.1
25.0
e
al
oy
3.0
100
9.5
4.8
12.5
39.0
am
u
hq
>40 ng/L
20-40 ng/L
30.3
40
on
en
D
3.6
1.8
28.1
25.0
H
le
g
Ea
0.10-0.20 ng/L
<0.10 ng/L
undetectable
23.1
60
R
La
s
ev
(C)
% of total events
19.8
13.8
80
% of totalevents
3.6 1.8
9.7
r
bo
ar
A
>0.30 ng/L
0.20-0.30 ng/L
ke
A
R
le
Is
Ta
13.5
oy
D
B
am
u
hq
100
e
al
EL
er
iv
R
ru
le
H
le
g
Ea
(A)
on
en
il'
r
bo
ar
er
0
EL
MeHg concentrations
(ng/L)
0.8
South
1
Fig. 3. (A) Average methylmercury (MeHg) and (B) Average total mercury (THg) concentrations (ng L ) in all precipitation samples
over time. (C) Percent of total events with varying MeHg and (D) THg concentrations. The boundary bottom of the box closest to zero
indicates the 25th percentile, a the solid line within the box marks the median, dashed lines represent the mean, and the boundary top
of the box farthest from zero indicates the 75th percentile. Whiskers above and below the box indicate the 90th and 10th percentiles.
13.3 ng L1 (Brule River). Average THg concentrations
at Isle Royale and Devil’s Lake were similar to each
other (mean THg concentrations equal to 19.0 and
18.6 ng L1, respectively) and were significantly greater
than those measured at the other sites (Fig. 3C). At least
50% of all events sampled at each of the ELA,
Tahquamenon Falls, Brule River, and Eagle Harbor
sites were below 10 ng L1, as opposed to less than 30%
of all events at Isle Royale and Devil’s Lake (Fig. 3D).
At least 5% of all events collected at Isle Royale and
Devil’s Lake had THg concentrations exceeding
40 ng L1, compared to 3% or less of events at the
other sites (Fig. 3D). There were three events with
concentrations that exceeded 80 ng L1: Isle Royal
(129.6 ng L1), Tahquamenon Falls (115.6 ng L1), and
Devil’s Lake (85.6 ng L1).
There did not appear to be a geographic trend with
THg concentrations, contradicting previously reported
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Table 2
Average methylmercury (MeHg) and total Hg (THg) concentrations (ng L1) in precipitation samples collected from 1997 to 2003
Total number of samples analyzed
Site ng MeHg L1
Brule River
Eagle Harbor
52
185
1997
1998
1999
2000
2001
2002
2003
ns
0.14
(32)
0.16
(34)
0.10
(38)
0.11
(9)
0.20
(7)
0.23
(20)
0.17
(47)
0.11
(22)
0.10
(9)
0.22
(20)
ns
ns
ns
ns
ns
0.15
(19)
ns
0.11
(31)
ns
0.18
(30)
ns
0.16
(16)
ns
0.09
(2)
ns
0.02
(7)
ns
0.09
(3)
ns
0.14
(8)
ns
ns
ns
0.13
(3)
0.19
(40)
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
4.1
(2)
ns
10.0
(8)
ns
2.5
(5)
ns
4.1
(8)
ns
ns
ns
24.0
(15)
18.4
(56)
0.15
(8)
0.09
(14)
ns
Tahquamenon Falls
74
ELA
38
Isle Royale
32
Devil’s Lake
43
0.21
(5)
ns
Site ng THg L1
Brule River
66
ns
Eagle Harbor
106
Tahquamenon Falls
149
ELA
43
Isle Royale
37
Devil’s Lake
72
13.5
(40)
16.3
(52)
ns
14.9
(14)
ns
14.1
(45)
9.5
(48)
11.2
(73)
8.6
(8)
22.6
(21)
ns
11.5
(21)
10.4
(18)
8.3
(24)
8.5
(12)
12.7
(2)
ns
Numbers in first column represent the total number of samples analyzed at that site. Values in parentheses are the total number of
samples analyzed in that time period. ns ¼ not sampled.
geographical trends in Hg deposition to soils (Nater and
Grigal, 1992) and lake sediment (Brezonik and Schumaker, 2004) in the Upper Midwest. A number of other
factors, such as organic matter content and dry
deposition, may affect soil Hg content. Lake-specific
characteristics may also affect concentrations in sediments. As well, soil and sediment cores tend to represent
long-term measures of Hg accumulation. Generally,
annual average THg concentrations measured at each
site did not differ significantly among years (Table 2).
Average THg concentrations in the summer (June, July,
and August) did appear to be higher than those in other
seasons, however, there was not enough data to test for
statistical significance.
There are several studies that report THg concentrations collected in the upper Midwest. Concentrations reported in these studies are similar to ours,
however, the upper limits of previously reported ranges
(4.3–28.9 ng L1 in Glass and Sorensen (1999);
1.2–59.5 ng L1 in Hoyer et al. (1995); 2.7–20 ng L1 in
Lamborg et al. (2000); and 5–35 ng L1 in Watras et al.
(2000)) are lower than those reported here.
3.1.2. Is there a ‘‘washout’’ effect of MeHg during
precipitation events?
MeHg concentrations measured in samples of less
than 100 mL total collected volume were significantly
higher than those from samples with larger collected
volumes (Fig. 5A). The negative correlation observed
between volume and concentrations suggests that watersoluble reactive gaseous species (Hg[II]) and particlebound Hg were ‘‘washed-out’’ of the atmosphere during
the early part of the rain event. Speciation could
therefore play an important role in the atmospheric
cycling of Hg by affecting its susceptibility to removal
through wet and dry removal processes and subsequent
deposition to watersheds and other landscapes. In a
separate study of rainwater collected during a single
event at our sampling site at Devil’s Lake in July 2003, it
was determined that 20–30% of the measured MeHg,
was indeed associated with particles (40.45 mm) at
concentrations of 0.06 ng L1 (H. Manolopoulos, unpublished data). This is the first study to present data
supporting the wash-out effect for MeHg. Despite the
elevated concentrations of MeHg from low-volume
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7564
60
Brule River
40
20
0
60
Eagle Harbor
40
20
0
60
116
Tahquamenon
THg Concentration (ng/L)
40
20
0
60
Isle Royale
130
40
20
0
60
ELA
40
20
0
60
86
Devil's Lake
40
20
0
1997
1998
1999
1
2000
2001
2002
2003
2004
1
Fig. 4. Total mercury (THg) concentrations (ng L ) in precipitation samples. Concentrations greater than 80 ng L
as numerals. Dates of sampling periods are presented in Table 2.
are represented
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B.D. Hall et al. / Atmospheric Environment 39 (2005) 7557–7569
% of THg that is MeHg
1.0
MeHg Concentration (ng/L)
0.8
0.6
0.4
0.2
<
(< 50
0. m
26 L
cm
(0 50)
1
.2
6- 00
0. m
52 L
cm
1
)
(0 00.5 30
2- 0
1. m
57 L
cm
30
(1 0)
.5 50
7- 0
2. m
62 L
c
>5 m)
(> 00
2. mL
62
cm
)
0
(A)
0.30
MeHg Mass (ng)
All site except Tahquamenon and ELA
Tahquamenon
14
ELA
12
10
8
6
4
2
0
1997 1998 1999 2000 2001 2002 2003 2004
Fig. 6. The percent of total mercury (THg) present as
methylmercury (MeHg) in precipitation collected from all sites
compared to samples collected at Tahquamenon Falls over
time.
r=0.3560
n=326
0.25
0.20
0.15
0.10
0.05
0.00
0
(B)
16
7565
100 200 300 400 500 600 700 800 900 1000
Precipitation Volume (mL)
Fig. 5. (A) Methylmercury (MeHg) concentrations (ng L1) in
precipitation sampled from Brule River, Eagle Harbor, and
Tahquamenon Falls grouped together in sample volume
increments. Values in parentheses represent corresponding
precipitation depths. See Fig. 3 for legend. (B) MeHg mass
(ng) in precipitation from events sampled at Brule River, Eagle
Harbor, Tahquamenon Falls, and Devil’s Lake plotted against
volume collected at each event (mL).
events, the opposite trend was noted between the
amount of MeHg deposited on the landscape and event
volume. Although low-volume events have larger MeHg
concentrations, high-volume events generally deliver
greater MeHg mass to the landscape (Fig. 5B). Intensive
studies of individual rain events are required to further
explore mechanisms behind this observation.
3.1.3. Percent of total mercury present as methylmercury
The percentage of THg present as MeHg (%MeHg) in
the majority of events collected from all sites ranged
from 41% to 6% (Fig. 6), which falls within the range
of other published values (see review by Downs et al.,
1998). The majority of %MeHg at all sites were below
1% and these values were low compared to typical
values found in aquatic environments (such as lake
sediments, porewaters, and wetlands) that have high
potential for the production of MeHg (up to 40%, as
reviewed by Ullrich et al., 2001). The ELA, Devil’s
Lake, and Eagle Harbor sites did have a very small
number of events with %MeHg exceeding 6% (2, 1, and
1 events sampled for ELA, Devil’s Lake, and Eagles
Harbor, respectively). However, 12% of events sampled
at Tahquamenon Falls had %MeHg values that
exceeded 6%, despite the low average MeHg concentration at this site. The high %MeHg samples at
Tahquamenon Falls were consistently observed in
events that had above average MeHg concentrations
(0.10 ng L1) and below average THg concentrations
(13.5 ng L1).
3.1.4. What are the origins of MeHg and THg in
precipitation?
To investigate the sources of THg and MeHg at the
Tahquamenon Falls site a meteorological analysis was
performed. Trajectories were calculated using the
Hybrid Single Particle Lagrangian Integrated Trajectory
(HYSPLIT) program from the National Oceanic and
Atmospheric Association (NOAA) (Draxler and Hess,
1988; Cohen et al., 2004). Six 72-h back trajectories were
calculated for each day of precipitation collected at the
site. Each precipitation event was matched with the
trajectory arriving at the site during the hour of
maximum precipitation. In addition, surface and upper
air meteorological maps were utilized to determine the
appropriateness of the trajectories for the events and to
allow for a better understanding of the mesoscale
meteorological flows that are so important in the Great
Lakes region.
Precipitation events with above average THg concentrations were associated with air mass transport to
Tahquamenon Falls from the south. Industrial areas in
the southeastern United States have been implicated as
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B.D. Hall et al. / Atmospheric Environment 39 (2005) 7557–7569
7566
sources of THg to precipitation in other studies (Burke
et al., 1995; Hoyer et al., 1995; St. Louis et al., 1995;
Landis et al., 2002; Cohen et al., 2004). The regions to
the north and northwest of the Lake Superior watershed
have a lower density of industrial sources and urban
areas relative to the south and southeast regions of the
United States (Cohen et al., 2004). Synoptic scale
transport to Lake Superior from the Chicago/Gary
urban areas and the Detroit/Windsor areas were often
associated with elevated THg levels in event precipitation at the Lake Superior sites. However, the larger
proportion of high MeHg events at Tahquamenon Falls,
compared to the ELA (10.3% and 3.8% of total events
had %MeHg that exceed 6% at Tahquamenon Falls
and ELA, respectively), also suggest that perhaps
processes occurring near Lake Superior may contribute
to MeHg concentrations.
The source or sources of the MeHg measured in
precipitation are more difficult to diagnose but appear
to be more local in origin than the sources of the THg.
The majority of the events for which we obtained
trajectory data (Table 3) occurred in the colder months
when snow is the dominant form of precipitation in this
northern location. Mesoscale flow patterns and lake
effect snows were observed to have elevated MeHg
concentrations. As well, southerly transport to the
Tahquamenon Falls site ahead of a warm front followed
by over water flow from the north often brings higher
MeHg concentrations. A more detailed study of the
complex mesoscale meteorology for a large number of
events must be completed before a more definitive
picture of the MeHg sources can be provided.
However, we present three intriguing hypotheses on
the possible explanations for the elevated levels of MeHg
in precipitation in samples taken on the southern shores
of Lake Superior. One is the formation of MeHg in
‘‘lake-effect’’ cloud and fog. Clouds and fog may affect
Hg cycling by acting as a reaction vessel for aqueous
chemical reactions, which are the primary mechanisms
determining atmospheric Hg speciation (Pleijel and
Munthe, 1995; Malcolm et al., 2003). Secondly, elevated
levels of MeHg are also observed in throughfall under
forested canopies in the Lake Superior basin, due to the
dry deposition of Hg forms to the forested ecosystems
and therefore the source of the dry-deposited MeHg
may be emissions of the MeHg from the abundant
wetlands near the Lake Superior monitoring sites. The
final hypothesis invokes dimethylmercury (DMHg) as a
source of atmospheric MeHg (Prestbo and Bloom, 1995;
Bloom et al., 1996). DMHg is a dominant Hg species in
Table 3
Methylmercury (MeHg) concentrations, total mercury (THg) concentrations, the percent of THg that is MeHg (%MeHg), and
precipitation depth in events sampled at Tahquamenon Falls
Date of event
Precipitation depth
(cm)
MeHg concentration
(ng L1)
THg concentration
(ng L1)
%MeHg
06
12
27
05
12
03
10
28
01
19
22
26
06
14
26
28
30
16
16
28
02
20
0.14
0.19
0.99
1.26
0.17
0.11
0.34
0.02
0.70
0.18
0.38
1.08
0.35
1.35
0.65
0.91
1.12
1.73
1.53
1.15
0.41
0.35
nd
0.19
0.16
0.18
0.13
0.20
0.07
0.19
0.36
nd
0.06
0.03
0.21
0.08
0.07
0.04
0.11
0.37
0.04
0.17
0.27
0.05
115.6
32.1
4.3
1.1
8.9
3.8
22.5
16.4
11.5
46.7
0.5
3.3
1.9
4.3
1.2
4.3
1.3
5.4
8.0
1.6
3.1
9.7
nd
0.6
3.6
15.8
1.5
5.4
0.3
1.2
3.1
nd
11.6
0.9
11.2
1.9
5.3
1.0
8.5
6.6
0.4
10.2
8.7
0.5
June 1997
Aug 1997
Nov 1997
Dec 1997
Dec 1997
Mar 1998
Mar 1998
May 1998
June 1998
June 1998
Sept 1998
Sept 1998
Nov 1998
Nov 1998
Dec 1998
Dec 1998
Dec 1998
Jan 1999
Feb 1999
Feb 1999
Mar 1999
Mar 1999
Concentrations in bold represent above average values (average MeHg and THg concentrations ¼ 0.097 and 13.5 ng L1, respectively)
and %MeHg data in bold represent values greater than 6%.
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B.D. Hall et al. / Atmospheric Environment 39 (2005) 7557–7569
deep regions of the north Atlantic ocean (Mason et al.,
1998). We hypothesize that upwelling of DMHg from
deep waters in Lake Superior (average depth ¼ 150 m;
maximum depth ¼ 406 m) occurs and is subsequently
fluxed to the atmosphere, where it can undergo
photochemical transformations to MeHg.
4. Conclusions
This report presents long-term MeHg and THg
concentrations in precipitation collected from five sites
in the midwest United States and from the Experimental
Lakes Area in Ontario, Canada. Detectable MeHg
concentrations were measured in samples collected at
all sites and ranged from 0.01 to 0.85 ng L1. At one of
our sites on the southern shores of Lake Superior (Eagle
Harbor), we examined seasonal trends in MeHg
concentrations and observed significant differences
among average MeHg concentrations measured in the
spring and summer compared to those collected in the
winter. The highest MeHg concentrations were measured in rain events with volumes of less than 100 mL
and this ‘‘wash-out effect’’ suggests that particle-bound
MeHg were present in the atmosphere prior to the rain
event. THg concentrations at all of our sites ranged from
10 to 60 ng L1, with four events exceeding 60 ng L1.
THg concentrations were similar to previously reported
values. Generally, %MeHg values were low at all sites,
with the exception of a number of events sampled at
Tahquamenon Falls, that were between 6% and 18%.
Trajectory analysis at the Tahquamenon site showed
that the high %MeHg events were associated with air
mass transfers over Lake Superior. This led us to
propose three hypotheses on the source of MeHg in
precipitation: 1. MeHg is formed in association with
‘‘lake-effect’’ clouds and fogs, 2. MeHg is emitted from
wetlands near Lake Superior, and 3. DMHg is fluxed
from deep regions of Lake Superior and is transformed
to MeHg which is removed from the atmosphere in
precipitation. Regardless of the source, it is clear that
atmospheric deposition is an important delivery mechanism for MeHg to sensitive watersheds and resource
managers would benefit from incorporating these
findings into predictive models.
Acknowledgments
Thanks to the many people who maintained field
equipment and collected precipitation samples at the
Lake Superior, Devil’s Lake, and ELA sites (Wisconsin
Department of Natural Resources, UW Environmental
Chemistry and Technology Program [EC&T], University of Michigan, and Department of Fisheries and
Oceans Canada). Thank you to the staff and students at
7567
UW EC&T, University of Michigan, Trent University,
and Flett Research Inc. laboratories who preformed
THg and MeHg analysis. Site work including sample
collection and total Hg analysis at the US sites, except
Devils Lake, was conducted by the University of
Michigan Air Quality Laboratory under funding from
the Lake Superior Basin Trust. Charlene Nielsen
produced Fig. 1. Although the research described in
this manuscript has been funded in part by US EPA
though US EPA Region 5, Air Pollution Control Grant
97578601-1, and US EPA-STAR grants 827629-01-0
and R 829789-010, it has not been subjected to the
Agency’s required peer and policy review. This work
therefore does not necessarily reflect the views of the
Agency and no official endorsement should be inferred.
This study was also funded by the Great Lakes Air
Deposition (GLAD) program, the National Research
Council of Canada (including the Collaborative Mercury Research Network, COMERN), Manitoba Hydro,
and the Canadian Circumpolar Institute.
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