explorative and innovative dynamic flux bag method development
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Atmospheric Environment 39 (2005) 7481–7493
www.elsevier.com/locate/atmosenv
Explorative and innovative dynamic flux bag methoddevelopment and testing for mercury air–vegetation
gas exchange fluxes
Hong H. Zhanga,b,c, Laurier Poissanta,b,�, Xiaohong Xuc, Martin Piloteb
aCollaborative Mercury Research Network (COMERN)bAtmospheric Toxic Processes, Meteorological Service of Canada, Environment Canada, 105 McGill (7th floor),
Montreal, Quebec, Canada H2Y 2E7cDepartment of Civil and Environmental Engineering, University of Windsor, Canada
Received 14 October 2004; received in revised form 20 July 2005; accepted 20 July 2005
Abstract
An intensive field study quantifying total gaseous mercury (TGM) and mercury speciation fluxes in a wetland
ecosystem (Bay St. Franc-ois wetlands, Quebec, Canada) was conducted in summer 2003. This study is one of the first
attempts to design and develop an innovative approach—dynamic flux bag (DFB) technique to measure in situ mercury
air–vegetation exchange with a monoculture of river bulrush (Scirpus fluviatilis). Air–vegetation flux measurements
were conducted under dry condition at site 1 and flood condition at site 2. TGM fluxes fluctuated from �0.91 to
0.64 ng/m2 (leaf area)/h with an average value of –0.2670.28 ng/m2 (leaf area)/h at site 1 and ranged from �0.98 to 0.08
ng/m2 (leaf area)/h with a mean flux of �0.3370.24 ng/m2 (leaf area)/h at site 2 (positive sign means volatilization, and
negative sign indicates deposition). The data indicated that TGM air–vegetation exchange is bidirectional. However,
the net flux is primarily featured by dry deposition of TGM from atmosphere to the vegetation. In mercury speciation
study using the DFB approach, particulate mercury (PM) and reactive gaseous mercury (RGM) represented less than
1% of total mercury. Ambient ozone concentrations had significant influences on RGM concentrations (r ¼ 0.54,
po0.05), implicating oxidation of gaseous elemental mercury (GEM) by ozone to form RGM. A discussion about the
similarities and discrepancies between the DFB and other approaches (dynamic flux chamber and modified Bowen
ratio) is presented. During the course of this study, some operational effects associated with the bag design, mainly the
emergence of condensation within the bag, were encountered. Several improvements relating to the DFB design were
recommended. Upon improvement, the DFB method could be one of the most promising techniques to study the role of
a single plant in air–vegetation exchange of mercury.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Air–vegetation exchange; Mercury speciation; Wetlands; Dynamic flux bag
e front matter r 2005 Elsevier Ltd. All rights reserve
mosenv.2005.07.068
ing author.
ess: [email protected] (L. Poissant).
1. Introduction
Mercury (Hg) is a well-known hazardous, bio-
accumulative global pollutant (Poissant et al., 2002).
Environmental contamination by Hg has gained much
attention in many regions recently. A number of studies
d.
ARTICLE IN PRESSH.H. Zhang et al. / Atmospheric Environment 39 (2005) 7481–74937482
have illustrated that natural surfaces (e.g. soil, water and
vegetation) contribute a considerable amount of Hg
cycling in the global mercury pool (e.g. Poissant and
Casimir, 1998; Gustin et al., 2000; Lindberg et al.,
2002a). Nearly 30% of the earth’s land surface area is
covered by vegetation (�4� 109 ha) (Lindberg et al.,
1998). Therefore, understanding the role of vegetation in
the atmospheric mercury cycling is critical in assessing
Hg fates on regional and global scales.
Knowledge of plants’ function for mercury transport
in the atmospheric Hg cycling has been explored but still
remains insufficient, including Hg speciation. Some
studies have reported Hg air–vegetation exchange rates
using micrometeorological measurements (Lee et al.,
2000; Lindberg and Meyers, 2001; Poissant et al., 2004a)
or enclosure approaches, such as dynamic flux chambers
(DFC) (Hanson et al., 1995; Poissant and Casimir, 1998;
Leonard et al., 1998; Frescholtz and Gustin, 2004;
Ericksen and Gustin, 2004). One desirable feature of the
micrometeorological approaches is that they impose
little disturbance over surface (Mayers et al., 1996).
However, this system requires sophisticated and expen-
sive instruments. In addition, the site requirement of
homogeneity is often not met under some circumstances.
The enclosure approach, on the other hand, is widely
used because of several merits including portability and
low price (Xiao et al., 1991; Poissant and Casimir, 1998;
Carpi and Lindberg, 1998). Nevertheless, this technique
is known to suffer from the alternation of microclimate
conditions during its application (Kim and Kim, 1999),
with the extent of alternation depending on the chamber
design. Therefore, it is vitally important that the design
of the enclosure system should maintain the internal
environment as close as possible to the external
conditions.
The enclosure method assumes well-mixed in-coming
and out-going air (Eklund, 1992). Several studies
emphasized the importance of the flushing flow rate on
this approach (Gustin et al., 1999; Wallschlager et al.,
1999; Zhang et al., 2002). Low flush rates could result in
underestimation of exchange rates due to internal
accumulation of Hg and high resistance of boundary
layer (Lindberg et al., 2002b; Zhang et al., 2002).
Therefore, a relatively high turnover rate is necessary for
the operation of the enclosure system to provide a well-
mixed condition within the chamber.
Many studies performed air–vegetation flux measure-
ments using small rigid chambers under controlled
environment. For example, Frescholtz and Gustin
(2004) utilized a 12.3 L glass vessel to measure Hg gas
exchanges with young plants (�2 month old) in a
glasshouse. However, the chamber technique is re-
stricted by its small size and rigid body structure when
measuring air–surface exchange with tall plants, thus
limits its field application. Others studies used large
open-flow gas exchange chambers, so-called EcoCELL
(e.g., Obrist et al., 2005) but this system requires massive
infrastructure and is costly for operation and main-
tenance. Consequently, there is a need to seek a new
technique for mercury air–vegetation exchange measure-
ment in the field with flexible form and easy closure.
An intensive study aimed to design and develop a
new approach, namely dynamic flux bag method (DFB)
(Fig. 1a and b) for mercury air–vegetation measure-
ments, was conducted in August 2003. This study is part
of an ongoing research relating to atmospheric Hg fate
at the Bay St-Franc-ois (BSF) wetlands (Quebec,
Canada) (Fig. 2) (Poissant et al., 2004a, b). After
satisfactory testing of the DFB in the Montreal’s
laboratory Qc, Canada, this device was used, as one of
the first in situ attempts, to investigate the role of a
monoculture of river bulrush (Scirpus fluviatilus) in Hg
air-vegetation exchange processes. This paper presents
Hg air–vegetation fluxes measured using the DFB
approach. Besides analyses and interpretation of the
Hg flux data itself, emphasis was also placed on the
design of the DFB and its methodological advances.
2. Methodology
2.1. Site description
The field site is located within the Bay St. Franc-ois
wetlands in the Lake St. Pierre, Quebec, Canada
(461070N, 721550W) (Fig. 2). The Bay is located between
Yamaska River and St. Franc-ois River, which border
the west and the east sides of the Bay, respectively. The
Bay leads to the St. Lawrence River on its northern
front. Therefore water levels in the Bay are changing
with time and space throughout the season. BSF
wetlands consist of water, soil and different species of
vegetation, such as river bulrush (Scirpus fluviatilis:
51–75%), arrowhead (Sagitaria latifolia: 11–25%) and
flowering rush (Butomus umbellatus: 11–25%). The
height and leaf surface area of vegetation varied from
month to month, starting from no vegetation to little
vegetation in May and reaching the maximum leaf area
index (LAI) with a mean vegetation height of 1.7m in
August. More detailed descriptions of the landscape can
be found in Poissant et al. (2004b).
Two study sites were set up for technical and practical
purposes (Fig. 2). The site conditions and the detailed
experiment dates at both sites are summarized in
Table 1. Site 1 was the main research station, which
was built up on elevated platforms near the river
shore. TGM concentration was measured using an
automatic Hg analyzer (Tekrans 2537A) housed in an
air-conditioned trailer. TGM air-vegetation exchange
measurement at site 1 was conducted under dry wetland
conditions. Site 2 was a satellite base campaign center
and was approximately 2 km south away from the
ARTICLE IN PRESS
Fig. 1. Field set up of dynamic flux bag approach at the BSF
wetlands in summer 2003: (a) Experimental design of flux
measurement (figure not drawn to scale). This figure shows one
side configuration of the bag; the other side is exactly the same
as the front one; (b) photograph of the setup using the bag
approach.
H.H. Zhang et al. / Atmospheric Environment 39 (2005) 7481–7493 7483
site 1. The automated Hg analyzer was housed in a
commercial available tent equipped with fans for
ventilation. At site 2, water level was about 20 cm above
the soil/sediment; thus rooting zone of vegetation was
under water. Two sets of instruments (e.g. Tekrans
analyzer, flux bag), one for TGM flux measurement and
one for Hg speciation (gaseous elemental mercury
(GEM), reactive gaseous mercury (RGM) and particu-
late mercury (PM)) study were used under flooded
wetland conditions at site 2.
2.2. Instrumentation
2.2.1. Mercury analyzer
TGM concentration was measured using a Tekran
2537A automatic analyzer. More information regarding
to this instrument is available in Poissant (1997). The
concentration of mercury species was measured using a
set of mercury speciation analyzer system, namely, a
Tekrans 2537A analyzer coupled with a Tekrans
1130 and a Tekrans 1135. Those instruments were
used to concurrently measure GEM, RGM and PM
(0.1–2.5 mm), respectively (Tekran, 2001). The sampling
and analyzing cycle for GEM, RGM, and PM is 2-h.
The detailed working procedures of the Hg speciation
unit can be found in Poissant et al. (2004a).
Total atmospheric mercury concentration (TAM)
was determined by adding all the Hg speciation
concentrations, i.e. GEM, RGM and PM. The percen-
tage of each Hg species in total mercury is calculated by
dividing each species, such as GEM, RGM and PM
concentration by TAM concentration. The precision of
the Tekran 2537A is 2% (Tekran, 2001). Assuming an
average TGM concentration of 1.4 ng/m3 in the ambient
air, the detection limit was 0.06 ng/m3 for TGM
concentration. The detection limit of the Tekrans
1130 and 1135 analyzer was established to 0.6 pg/m3
based on three standard deviation of field blanks (3s)(Landis et al., 2002).
2.2.2. Dynamic flux bag (DFB)
The DFB approach (Fig. 1a, b) was one of the first in
situ attempts to measure Hg air-vegetation exchange
with a single plant. A custom-made Tedlars bag with a
dimension of 900mm� 1250mm and a maximum
volume of 200L was selected for the measurement of
air-vegetation exchange fluxes in this study.
Tedlars was chosen as a constructional material
because it is inert, inexpensive, durable, and with high
transmission of photosynthetic active radiation (light
transmissivity: �89%). Moreover, it is flexible and can
follow the form of the vegetation. In order to maintain
the shape of the bag and minimize the interference of the
interior bag with leaf surfaces, the bag was fixed by
clamps on a steel frame that was outside the bag to
prevent direct contact. During deployment, the actual
volume of the bag was approximately 120L.
Mercury concentration within the bag was measured
by connecting the outlets to the analyzer. Considering
the possibility of Hg gradients within the bag, four
outlet holes (linked into a manifold) distributed evenly
at the upper section of the bag were designed in this
study, providing a better representation of TGM
ARTICLE IN PRESS
Table 1
Dates and locations of DFB experiments conducted at the BSF wetlands in 2003
Location Study Starting date Ending date Sample number Site condition
Site 1 Total gaseous mercury flux August 23 August 30 154a dry
Site 2 Total gaseous mercury flux August 18 August 22 96a flooded
Site 2 Mercury speciation flux August 20 August 24 48b flooded
aSample number is based on hourly data.bSample number is based on 2-h data.
Fig. 2. Site map of the field study at the BSF wetlands (Quebec, Canada). Site 1 is the main research center. Hg air–vegetation using
the dynamic flux bag method under dry wetland conditions were conducted at site 1. Site 2 is designed for TGM fluxes and Hg
speciation study using dynamic flux bag technique under flooded wetland conditions.
H.H. Zhang et al. / Atmospheric Environment 39 (2005) 7481–74937484
concentration inside the bag. Besides, an external
peristaltic pump with high flushing flow rates was used
to make the internal Hg concentrations as homogenous
as possible. Three flow rates: Q ¼ 20, 40 and 48L/min
(the turnover was 10, 20, and 24 times per hour,
respectively) were used for flux measurement at site 2.
Due to the possibility of underestimating TGM fluxes
using low flow rate (Q ¼ 20L/min), TGM fluxes
measured using high flow rates (Q ¼ 40 and 48L/min)
were kept for final data analysis. Based on the field
experience at site 2, only Q ¼ 48 L/min was used at site
1. Similar to the outlets, four inlet holes located at the
lower portion of the bag were used to allow the ambient
air in (Fig. 1). After all the connections being set, the
above-ground plant was covered by the bag and the bag
was tightened using ties (Fig. 1). The monoculture
covered by the bag was river bulrush (S. fluvialis), which
was the dominant vegetation species (51–75%) at the
BSF wetlands.
Air-vegetation mercury exchange flux using the DFB
approach is computed using the following equation
(Hanson et al., 1995; Leonard et al., 1998):
F ¼½Hg�b � ½Hg�a
A�Q, (1)
where [Hg]b and [Hg]a are the mercury concentrations
inside the bag and in the ambient air (ng/m3),
respectively, Q is the flushing flow rate through the
bag (m3/h), and A is the projected surface area. In this
study, A is the total one-side leaf area (m2) covered by
ARTICLE IN PRESSH.H. Zhang et al. / Atmospheric Environment 39 (2005) 7481–7493 7485
the flux bag, and F is the mercury flux (ng/m2 leaf area/
h, positive value means volatilization from foliar to the
atmosphere, otherwise deposition is indicated).
Mercury speciation flux study was performed using
the DFB approach as well. The setup of the speciation
system was simpler than the TGM flux measurement.
Only one outlet and one inlet were used for Hg
speciation flux measurements. The outlet was located
at the top of the bag and the bottom of the bag was open
to serve as an inlet without holes on the bag as inlets. Hg
speciation concentrations within the bag and in the
ambient air were measured by manually connecting or
disconnecting the outlet of the bag to the speciation
analyzer system. Every 2-h the Hg speciation unit
alternately measured GEM, RGM and PM concentra-
tions within the bag and in the air. By knowing the
concentrations of GEM, RGM and PM at outlet and
inlet, respectively, the corresponding flux of each Hg
species can be calculated using Eq. (1).
2.2.3. Environmental parameters
Atmospheric parameters were measured at both sites.
At site 1, air temperature (Tair) and relative humidity
(RH) were measured using HMP35 humidity/tempera-
ture probes (Tair: accuracy70.2 1C, RH: accuracy70.04%, Campbell Scientific). Soil temperature (Tsoil) was
monitored using a 107B probe (accuracy70.2 1C,
Campbell Scientific). Net radiation (NR) for both
shortwave and longwave was recorded using a Q-7.1
net radiometer (accuracy74.3%, Campbell Scientific).
Wind speed (accuracy72%) and direction (accu-
racy75%) were measured using an R.M. Young
anemometer (Young Scientific) at 8m height. Ozone
(O3) was measured at 0.5m above the vegetation surface
using a TECO 49s analyzer (Poissant, 1997). At site 2,
meteorological parameters including air and soil tem-
perature, relative humidity, and net radiation were
measured using the same models of probes as those at
site 1. In addition, air temperature and relative humidity
within the bag were measured at the same height as the
probes in the ambient air at both sites.
2.3. Data analyses
During the field campaign, atmospheric parameters such
as net radiation, wind speed and direction, air temperature,
and ozone concentration were recorded at 5-min intervals.
TGM concentrations in the air and within the bag were
integrated every 5min, thus the time resolution for TGM
flux was 10min. In order to take into account different
sampling frequencies, the data-matrix was constructed
with 1-h median value, except for mercury speciation
which was 2-h as described above. Eastern Daylight
Time (EDT) was used for data recording and reporting,
unless otherwise specified. Pearson’s correlation analyses
were performed between mercury speciation fluxes and
concentrations to investigate transformation of mercury
using Excels. The influence of ozone on mercury specia-
tion was studied as well.
2.4. QA/QC protocol
The quality assurance and quality control protocol
was carried out throughout the field and laboratory
sampling and analysis processes, as can be found in
Poissant (1997, 2000). The TECO 49s analyzer used to
measure ozone concentration was calibrated with both
internal zero air and known amount of ozone concen-
tration (Poissant et al., 2004a).
Before flux measurements, bag blanks were performed
in the laboratory and in the field by measuring the inlet
and outlet mercury concentrations with no vegetation
being covered by the bag. The mean TGM concentra-
tion differences between the inlets and outlets were
0.0670.12 and 0.0670.10 ng/m3 at site 1 and site 2,
respectively. The detection limit of the TGM flux was
established to 0.09 ng/m2/h (Eq. (1) by knowing the
flushing flow rate (Q ¼ 40 l/min) and total internal
surface area of the bag (the actual area during the
blanks was 1.6m2). The low blanks (close to the
instrumental detection limit of 0.06 ng/m3) indicated
that the Tedlars bag was neither a significant source nor
a sink for the atmospheric mercury.
Meteorological parameters at both sites were mea-
sured using the same models of probes to minimize inter-
instrumental differences. All instruments were calibrated
in the laboratory before shipping to the field to maintain
good working conditions.
3. Results
3.1. Environmental parameters
Mercury air–vegetation flux measurements using the
DFB approach were conducted under dry wetland
conditions at site 1 and under flooded wetland condi-
tions at site 2. The main meteorological parameters
measured inside and outside the bag at both sites are
summarized in Table 2. As observed, environmental
parameters at site 2 were characterized by warmer air
and soil temperature (Tair: 22.973.7 vs. 16.373.6 1C,
po0.001; Tsoil: 22.571.8 vs. 1770.9 1C, po0.001),
stronger net radiation (1437214 vs. 907166W/m2,
po0.001), and similar ambient relative humidity
(0.7670.17 vs. 0.7870.20, p40.05) compared to those
environmental conditions at site 1.
3.2. TGM fluxes
Table 2 summarizes the measured TGM fluxes using
the DFB approach at both sites. At site 1, the measured
ARTICLE IN PRESS
Table 2
Selected meteorological data measurements inside and outside the bag, as well as TGM fluxes using the DFB approach at sites 1 and 2
Parameters Statistical index Site 1 (dry wetland condition) Site 2 (flooded wetland condition)
Temperature in the air (1C)Range 9.1–25.5 15.4–29
Mean 16.373.6 22.973.7
Median 16.3 22.4
N 154 97
Temperature in the bag (1C)Range 7.8–44.2 15–43
Mean 20.779.7 27.378.9
Median 17.1 23.2
N 154 97
Relative humidity in the air (fraction)Range 0.31–1 0.43–1
Mean 0.7870.20 0.7670.17
Median 0.84 0.80
N 154 97
Relative humidity in the bag (fraction)Range 0.26–1 0.29–0.99
Mean 0.7570.25 0.7470.24
Median 0.85 0.84
N 154 97
Water vapor pressure in the air (kpa)Range 0.79–2.3 1.51–2.74
Mean 1.4470.4 2.0770.29
Median 1.42 2.05
N 154 97
Water vapor pressure in the bag (kpa)Range 0.93–2.87 1.7–4
Mean 1.7170.43 2.570.45
Median 1.73 2.4
N 154 97
Net radiation (W/m2)Range �54–521 �53–588
Mean 907166 1437214
Median 0.75 21.8
N 137 97
Soil temperature (1C)
Range 15.3–18.8 19.4–25.8
Mean 1770.9 22.571.8
Median 16.9 22.4
N 154 97
TGM flux (ng/m2/h)Range �0.91–0.64 �0.98–0.08
Mean –0.2670.28 �0.3370.24
Median �0.25 �0.27
N 154 96
H.H. Zhang et al. / Atmospheric Environment 39 (2005) 7481–74937486
TGM fluxes fluctuated from –0.91 to 0.64 ng/m2 (leaf
area)/h with an average value of –0.2670.28 ng/m2
(leaf area)/h. At site 2, the mean TGM flux was
�0.3370.24 ng/m2 (leaf area)/h within the range be-
tween �0.98 and 0.08 ng/m2 (leaf area)/h. The time
series of TGM fluxes at site 2, as an example, is shown in
Fig. 3. As observed, 1 s (re: 67% of the observations)
of TGM fluxes varied from �0.2 to �0.9 ng/m2 (leaf
area)/h. The magnitude of these TGM exchange rates
was far greater than the blank fluxes (0.09 ng/m2/h). On
the other hand, the average TGM fluxes measured at
both sites were similar though; the measurements were
conducted under dry and flooded wetland conditions,
respectively. In this case, we speculated that the
exchanges were more air–foliar related and the roots
played little roles in the exchange process. However,
over the long run, the plant’s physiology might be
altered due to water level conditions (e.g., ecological
ARTICLE IN PRESS
0.4
Date and Time (EDT)
TGM flux Net radiation
0.2
-0.2
-0.4
-0.6
-0.8
-1
-1.28-18-03 0:00
8-19-03 0:00
8-20-03 0:00
8-21-03 0:00
8-22-03 0:00
8-22-03 0:00
8-18-03 12:00
8-20-03 12:00
8-21-03 12:00
8-19-03 12:00
0T
GM
flu
x (n
g/m
2 /h)
700
600
500
400
300
200
Net
rad
iatio
n (W
/m2 )
100
40
-100
Q=40 l/min Q=48 l/min
Fig. 3. Time series of TGM fluxes using DFB approach and net radiation at site 2 under flooded wetland conditions in summer 2003.
H.H. Zhang et al. / Atmospheric Environment 39 (2005) 7481–7493 7487
adaptation and plant succession) and then be critical for
mercury air-vegetation exchange of gaseous species.
Among the 4-day measurement of TGM fluxes and
net radiation at site 2 (Fig. 3), the pattern of TGM fluxes
on August 19 and 20 differed from those on the other
two days (August 18 and 21). On August 19 and 20,
daytime TGM fluxes exhibited larger deposition rates
than those at night. Night fluxes oscillated around zero
and some were close to the detection limit of the
instruments (0.09 ng/m2/h). We speculate that the
different behaviors of daytime/night time fluxes on
August 19 and 20 were probably related to the stomata
and the foliar boundary layer resistance. During day-
time stomata are open when the solar radiation strikes
the leaves, resulting in extensive TGM air–foliar
exchange. In contrast, few stomata are involved in the
exchange process at night. Therefore, daytime Hg gas
exchange is more active. On August 18 and 21, no
significant difference was found between daytime and
night time TGM fluxes. The reasons for the inactive
exchange during daytime on these two days are still not
clear.
3.3. Mercury speciation
3.3.1. Concentration and flux
The importance of quantifying Hg speciation is well
emphasized by many researchers (e.g. Lindberg and
Stratton, 1998; Poissant et al., 2004a). In this study,
mercury speciation experiment was conducted using the
DFB approach from August 20 to 24, 2003 at site 2
under flooded wetland conditions. The statistical sum-
mary for main meteorological parameters, all three
species of Hg concentrations and fluxes is shown in
Table 3. In general, the mean fluxes of Hg species were
negative, indicating the dominant process for all three
species was deposition. Mercury concentrations of
GEM, RGM and PM were calculated separately in
terms of sampling locations, namely in the air and inside
the bag. GEM, PM and RGM concentrations measured
in the air were consistent with the results in a previous
study conducted at the same site using the modified
Bowen ratio approach by Poissant et al. (2004a) in
summer 2002. For all three Hg species, the mean
concentrations inside the bag were lower than the ones
in the air, which again suggested dry deposition to plant
foliar.
The mean RGM concentration inside the bag was
approximately 48% (Table 3) of the level measured in
the ambient air (1.4871.94 pg/m3), suggesting its fast
deposition due to high solubility. On average, GEM and
PM concentrations inside the bag account for approxi-
mately 74% and 89% of the ones in the ambient air. PM
concentration greatly exceeded RGM concentration
both in the air and inide the bag. The high PM/RGM
ratio indicated RGM was scavenged faster than PM,
which agreed well with the results reported by Poissant
et al. (2004a). On average, the amount of PM and RGM
accounted for less than 1% of TAM both in the ambient
air and inside the bag. These observations were
consistent with the results by Lindberg and Stratton
(1998), who reported �3% RGM percentage in TAM in
a forest using mist chamber technique, and similar to a
study conducted in summer 2002 at the same site
(Poissant et al., 2004a). The latter reported less than 1%
of RGM and PM in TAM concentration in the air.
The time series of GEM, RGM and PM fluxes using
the DFB approach is shown in Fig. 4. Measured mean
GEM flux was �1.0170.72 ng/m2/h with the maximum
deposition flux of 2.76 ng/m2/h and the highest emission
rate of 1.04 ng/m2/h. Much smaller RGM and PM fluxes
were measured compared to GEM fluxes. The average
RGM flux was �2.874.8 pg/m2/h (�19.2 to 2.8 pg/m2/
h) and mean PM flux was �3.3724.5 pg/m2/h (�94.8 to
57.1 pg/m2/h). The negative signs of Hg fluxes indicated
that Scirpus species had the ability to remove Hg from
ARTICLE IN PRESS
21.5
1
00.5
-0.5
TG
M &
GE
M f
lux
(ng/
m2 /h
)
RG
M &
PM
flu
x (p
g/m
2 /h)
-1.5
-2.5
-1
-2
-3-3.5
8-20-03 0:00
8-21-03 0:00
8-22-03 0:00
8-23-03 0:00
8-24-03 0:00
8-25-03 0:00
8-20-03 12:00
8-21-03 12:00
8-22-03 12:00
8-23-03 12:00
8-24-03 12:00
60
40
20
0
-20
-40
-60
-80
-100
Date and Time (EDT)
GEM flux TPM flux RGM flux
Fig. 4. Time series of GEM, RGM and PM fluxes measured using the DFB method
Table 3
Statistical summary of selected meteorological parameters, ozone concentrations, as well as measured fluxes and concentrations of
GEM, RGM and PM using DFB approach at site 2
Parameters Minimum Maximum Mean Std N
Environmental conditions Air temperature (oC) 8.75 29.72 20.31 20.75 49
Net radiation (W/m2) �76.1 611.6 134.3 26.7 49
Ozone (ppbv) 12.99 91.75 33.51 32.04 49
TAM (ng/m3) 0.82 1.7 1.18 0.21 24Hg concentration measured inside
the bagGEM (ng/m3) 0.82 1.67 1.17 0.21 24
RGM (pg/m3) 0 2.02 0.42 0.61 24
PM (pg/m3) 0.82 28.61 5.9 8.13 24
TAM (ng/m3) 1.32 2.16 1.61 0.16 25Hg concentration measured in the
airGEM (ng/m3) 1.31 2.15 1.6 0.16 25
RGM (pg/m3) 0 8.47 1.48 1.94 25
PM (pg/m3) 2.07 39.23 7.16 8.02 25
Ratio of Hg concentration in the
bag to the concentration in the airGEM (air)/GEM (bag) 0.46 1.09 0.74 0.16 24
RGM (air)/RGM (bag) 0 3.27 0.48 0.79 23
PM (air)/PM (bag) 0.12 5.03 0.89 1.07 24
Percentage of RGM and PM
inside the bag to TAMRGM/TAM (%) 0 0.17 0.04 0.05 24
PM/TAM (%) 0.07 1.88 0.46 0.56 24
Percentage of RGM and PM in
the air to TAMRGM/ TAM (%) 0 0.47 0.09 0.11 25
PM/ TAM (%) 0.13 2.5 0.44 0.49 25
Hg speciation fluxes TAM flux (ng/m2/h) �2.77 1.07 �1.01 0.73 48
GEM flux (ng/m2/h) �2.76 1.04 �1.01 0.72 48
RGM flux (pg/m2/h) �19.22 2.8 �2.82 4.77 48
PM flux (pg/m2/h) �94.75 57.11 �3.31 24.52 48
H.H. Zhang et al. / Atmospheric Environment 39 (2005) 7481–74937488
atmosphere as dry deposition to the foliar surfaces. On
August 21, an episode of PM emission was recorded;
though PM was supposed to readily deposit. This could
be explained by re-suspension (e.g. blown up of PM by
wind through the opening at the bottom of the bag) on
that day.
3.3.2. Hg transformation
In order to provide some insights into the transforma-
tion of atmospheric mercury among different species,
Pearson correlation analyses were performed between
mercury speciation fluxes and concentrations in the air
and inside the bag, respectively (Table 4). In the air,
ARTICLE IN PRESS
Table 4
Correlation coefficients between different Hg species of fluxes and concentrations as well as ozone (a) in the air (N ¼ 24) and (b) in the
bag (N ¼ 24)
Parameters
GEM flux
(ng/m2/h)
PM flux
(pg/m2/h)
RGM flux
(pg/m2/h)
GEM conc.
(ng/m3)
PM conc.
(pg/m3)
RGM conc.
(pg/m3)
Ozone
(ppvb)
(a)
GEM flux (ng/m2/h) 0.20 0.37 �0.42a �0.03 �0.36 �0.50a
PM flux (pg/m2/h) 0.15 0.32 �0.66b �0.11 �0.10RGM flux (pg/m2/h) �0.29 �0.11 �0.96b �0.44a
GEM conc. (ng/m3) 0.16 0.22 0.24PM conc. (pg/m3) 0.12 0.001RGM conc. (pg/m3) 0.54a
(b)
GEM flux (ng/m2/h) 0.38 0.33 0.81b 0.49a 0.02 �0.52a
PM flux (pg/m2/h) 0.27 0.53a 0.65b 0.16 �0.14RGM flux (pg/m2/h) 0.31 0.25 0.23 �3.8GEM conc. (ng/m3) 0.54a �0.02 �0.28PM conc. (pg/m3) 0.43a �0.14RGM conc. (pg/m3) 0.18
aCorrelation is significant at po 0.05 level (2-tailed).bCorrelation is significant at po 0.01 level (2-tailed).
H.H. Zhang et al. / Atmospheric Environment 39 (2005) 7481–7493 7489
significant negative correlations were found between the
same species of Hg fluxes and concentrations. For
example, GEM fluxes were moderately correlated with
GEM concentrations (r ¼ �0:42, po0.05) and the
correlation coefficients increased to r ¼ �0:66, (po0.01)
for PM fluxes and PM concentrations. RGM concentra-
tions exhibited the strongest negative correlation with
RGM fluxes (r ¼ �0:96, po0.01). Those negative
correlations suggested removal of atmospheric Hg by
dry deposition over a large area. Inside the bag, positive
correlations were found between the same species of Hg
concentrations and fluxes. Among them, GEM fluxes
with GEM concentrations (r ¼ 0:81, po0.01), and PM
concentrations with PM fluxes (r ¼ 0:65, po0.01) dis-
played significant positive correlations, which were
opposite to the negative relationships in the air.
Furthermore, the influence of ozone on the Hg
exchange was investigated. The mean ambient ozone
concentration was 33.5714.5 ppbv with a range from 13
to 92 ppbv. The results shown in Table 4 indicate that in
the atmosphere, RGM concentrations were significantly
correlated with ozone concentrations (r ¼ 0.54,
po0.05), suggesting oxidation of elemental Hg by ozone
to form RGM. On the other hand, weak correlation was
found between ambient ozone and RGM concentrations
within the bag (r ¼ 0.18, po0.05). However, ozone
concentrations within the bag might be different from
the ambient level. Thus, the correlation of RGM and
ozone concentrations within the small-scale ecosystem
might be dissimilar from that in the large-scale atmo-
spheric environment. Nevertheless, ozone concentration
inside the bag was unavailable during the study period.
Thus the correlation of ozone and RGM concentrations
within the bag remains unknown.
The time series of RGM concentrations and fluxes in the
air is shown in Fig. 5. Two peaks of RGM concentrations
concurrently occurred with the largest RGM deposition
around noontime of August 21 and on the late night of
August 22. The daytime peak of RGM concentration and
deposition might be explained by the oxidation of GEM
with ozone to form RGM which was then removed
quickly at the surface by dry deposition during daytime
(Poissant et al., 2004a). The night time peak of RGM
concentrations were likely due to the oxidation of GEM by
night time oxidant species (e.g., NO3 and N2O5) in gaseous
phase (Sommar et al., 1997). Then the RGM could be
transferred into the dew, resulting in the quick deposition
of RGM at night. It seems that the day and night time
peaks of RGM concentrations and fluxes observed in this
study were caused by different physical or chemical
processes. More studies are needed to investigate the
contribution of these processes.
4. Discussion
4.1. Comparison of DFB approach with other enclosure
studies
This study was established and developed based on
the previous work using the dynamic flux chamber
technique (Hanson et al., 1995; Poissant and Casimir,
1998; Leonard et al., 1998). As mentioned before,
previous enclosure studies were mostly conducted under
ARTICLE IN PRESS
10
5
0
-5
-10
-15
-20
RG
M f
lux
(pg/
m2 /h
)
25-08-24-08-24-08-23-08-23-08-22-08-22-08-21-08-21-08-20-08-20-08-03-0:0003-12:0003-0:0003-12:0003-0:0003-12:0003-0:0003-0:0003-0:00 03-12:0003-12:00
Date and Time (EDT)
-20
-15
-10
-5
0
5
10
RG
M c
once
ntra
tion
(pg/
m3 )
RGM fluxRGM concentration
Fig. 5. Time series of RGM concentrations within the bag and RGM fluxes using the DFB approach
H.H. Zhang et al. / Atmospheric Environment 39 (2005) 7481–74937490
controlled environment. Hg fluxes from those labora-
tory measurements displayed a large variability depend-
ing mainly on air and soil Hg concentrations (Hanson
et al., 1995; Leonard et al., 1998; Frescholtz and Gustin,
2004; Ericksen and Gustin, 2004). For example,
Leonard et al. (1998) reported large emission of
10–93 ng/m2/h during daytime and low exchange rates
of 0.5–12 ng/m2/h at night when measuring air–foliar
exchange at mean soil Hg concentrations of 24–54 mg/gand air Hg concentration of 0.5 ng/m3. Ericksen and
Gustin (2004) reported daytime deposition of 32 ng/m2/
h and night time deposition flux of 29 ng/m2/h at soil Hg
concentration of 12.3mg/g and air Hg concentration of
30 ng/m3. In the same study, these authors measured
much lower fluxes of �0.2 to 1.6 ng/m2/h with emission
during daytime and deposition at night at soil Hg
concentration of 0.03mg/g and air Hg concentration of
2.4 ng/m3. The present study, on the other hand, was
performed in the field at soil Hg concentration of
0.06mg/g and mean air Hg concentration of 1.4 ng/m3.
Measured TGM fluxes ranged from �0.98 to 0.64 ng/
m2/h with larger deposition during daytime and lower
exchange rates at night. As observed, TGM fluxes were
one or two orders of magnitude lower than the studies
conducted at Hg-enriched soil and high air Hg
concentrations. However, less difference was found
between this study and the one performed at similar
low soil and air Hg concentrations by Ericksen and
Gustin (2004). Since various plants displayed different
growth habits and physiological attributes (e.g. stomatal
conductance, leaf texture, and leaf density), the small
discrepancy of Hg fluxes from the study by Ericksen and
Gustin (2004) under clean background is expected.
4.2. Comparison of DFB approach with modified Bowen
ratio (MBR) studies
In this study, TGM fluxes measured using the DFB
approach were compared with the data using the MBR
technique conducted in parallel with the DFB approach.
TGM fluxes measured using the MBR approach ranged
from �61 to 63 ng/m2/h with a mean value of
�2.4721 ng/m2/h (Poissant, unpublished data). Both
DFB and MBR approaches measured the net deposition
fluxes, indicating deposition-dominant processes in the
air-vegetation exchange during the study period. How-
ever, the mean net flux measured using the MBR
approach (�2.4721 ng/m2/h) was about one or two
orders of magnitude larger than the one using the DFB
method (–0.2670.28 ng/m2 leaf area/h). We speculate
that the different exchange rates could be mainly
attributed to two different methodologies. The DFB
technique measures Hg air-vegetation exchange, in the
unit of per leaf area, with a monoculture of Scirpus,
which has its unique physical/biological properties. On
the other hand, the Bowen ratio approach is an open
system which integrates exchange rates over large
landscapes. The BSF site covers multi-species of
vegetation thus has different plant structure character-
istics. Besides, the MBR approach quantifies Hg fluxes
in the unit of per land area, which includes the air-
surface exchange with soil, water and vegetation. The
quantity of Hg fluxes measured using the MBR depends
on the coverage of landscape. Several studies (e.g.
Lindberg et al., 2002a, Poissant et al., 2004a) reported
emission fluxes over wetland vegetation using the MBR
approach. For example, Poissant et al. (2004a) reported
a net emission of 32756 ng/m2/h over vegetation
canopy using the MBR approach at the BSF wetlands
in summer 2002. The net emission fluxes reported from
some studies (Lindberg et al., 2002a, Poissant et al.,
2004a) could be a result of high emissions from soil and
water, as well simultaneous low deposition to vegeta-
tion. Ericksen et al. (2003) reported accumulation of Hg
in foliar over time, indicating that foliar is a sink for
airborne mercury. In this study, the deposition fluxes
quantified using the MBR method were opposite to the
emission fluxes in a previous study conducted at the
ARTICLE IN PRESSH.H. Zhang et al. / Atmospheric Environment 39 (2005) 7481–7493 7491
same wetlands by Poissant et al. (2004a). We speculate
that the discrepancy between two studies is due to
different landscape and environmental conditions dur-
ing sampling periods.
4.3. Operational effects associated with the bag design
and methodological advances
During the course of this study, some operational
effects relating to the bag design, mainly some dis-
crepancies between environmental conditions within the
bag and in the ambient air were encountered, especially
for RH. The time series of RH inside the bag and in the
air measured at site 2, as an example, is shown in Fig. 6.
As observed, the difference between RH in the ambient
air and within the bag ranged from �37% to 56%. Of
the 4-day RH measurement, night time RH in the
ambient air was 0.94, 0.66, 0.90, and 0.86, respectively.
The same parameter within the bag on this 4-day was
0.96, 0.87, 0.97 and 0.95, respectively. Night time RH
inside the bag was higher than that in the ambient
environment, with significant differences on the last 3
days (po0.01). Most of the time, RH within the bag
approached saturation at night. Consequently, the
excess water vapor formed condensation on the interior
of the bag. Throughout the field campaign, condensa-
tion inside the bag was observed mostly at night and
sometime at early morning hours as well. Samples of the
condensation were collected and analyzed as summar-
ized in Table 5. Hg concentrations in the condensation
8-20-038-19-038-19-038-18-038-18-030:00 0:00 0:0012:00 12:00
Date and T
0.25
0.45
0.65
0.85
1.05
Rel
ativ
e hu
mid
ity (
frac
tion)
Relative humidity in the bag
Fig. 6. Time series of relative humidity measured in the ambi
Table 5
Volume and Hg concentration in the condensate collected inside the
Site Date
Site 2 (Total gaseous Hg study) 8/19/2003
Site 2 (Hg speciation study) 8/22/2003
Site 1 (Hg speciation study) 8/24/2003
ranged from 3.6–11.8 ng/L, which were slightly higher
than the level measured in dew (2.6–5.2 ng/L) over a
Teflons plate during the same campaign period. While
the concentrations were lower than the ones from
Malcolm and Keeler (2002), who reported 1.0 to
22.6 ng/L of Hg concentration in dew in the Great
Lakes region and in the Florida Everglades.
The occurrence of condensation within the bag from
late night to early morning might interfere with the
investigation of air-vegetation exchange. For instance,
condensation may drastically affect light transmission of
bag material, further altering photosynthesis of the
enclosed vegetation. Nevertheless, in this study con-
densation mostly formed during night time and evapo-
rated by 9:00–10:00. Thus, the alteration of
photosynthesis due to the changing light transmission
by condensation should be considered insignificant. On
the other hand, condensation within the bag might lead
to additional emission/deposition beyond the air–leaf
surface exchange processes. During the formation of the
condensation at night, gaseous and particulate mercury
maybe transferred into the water where they are more
favored to be absorbed or retained than under dry
conditions, resulting in enhanced deposition (Malcolm
and Keeler, 2002). During the evaporation of condensa-
tion after sunrise, dissolved mercury in the water will be
released along with water vapor, leading to increased
emission of TGM. In short, the night time condensation
on the internal wall of the bag should enhance the
deposition fluxes and the evaporation of water after
8-22-038-22-038-21-038-21-038-20-030:00 0:0012:00 12:00 12:00
ime (EDT)
Relative humidity in the air
ent air and within the bag at site 2 at the BSF wetlands
bags
Volume (mL) Hg concentration (ng/L)
35.9 3.6
125.5 11.2
71.3 16.8
ARTICLE IN PRESSH.H. Zhang et al. / Atmospheric Environment 39 (2005) 7481–74937492
sunrise should promote the emission of gaseous Hg
fluxes. The condensation process is more problematic to
RGM, which is water-soluble. Interestingly, we ob-
served large deposition during daytime and little at night
(Fig. 3), which were opposite to the condensation and
evaporation processes of water. Therefore, bag effect
blanks, both condensation and evaporation, should be
done and be used for field flux correction. However, no
bag effect blank was performed in this study. We suggest
improving the design by eliminating surface condensa-
tion within the bag.
Our recommendations to advance this device for
future studies include: (1) add a heating/cooling device
to maintain the temperature inside the bag as close as
possible to the ambient air condition; (2) employ an
internal electrical fan to further homogenize the air
inside the bag (Poissant and Casimir, 1998), providing a
better mixing and ventilation condition to disperse the
moisture within the bag; in addition, good ventilation
can cool the bag and replenish the carbon dioxide during
daytime in the process of photosynthesis, which is
critical to maintain normal biological activities of plants;
(3) apply a dynamic dryer to let the air streams in (with
Hg) but keep water vapor out through a membrane (e.g.,
Nafions); (4) measure the bioactivity indicators such as
CO2 concentration inside the bag to monitor the
photosynthesis and transpiration status of the plants.
The entire system can be automatically monitored and
controlled. Upon improvement, Hg concentration with-
in the bag should be well mixed. Thus, greatly eliminate
the need of bag effect blank tests and correction.
The above recommendations are largely applicable to
the Hg speciation design. Moreover, we suggest employ-
ing an external pump to increase high turnover rate and
adding more inlets and outlets to further improve
mixing within the bag.
In the original design, the placement of outlets at the
upper of the bag and the inlets near the bottom of the
bag was to skip any air bypass. During midday, there is
a possibility of a weak gradient between mercury
concentrations near the water surface and those near
the top of the bag due to the emission of Hg from water
surface. Nevertheless, the gradient should be small
because of well-mixed condition in the ambient air and
the short distance between these two heights (�45 cm,
Fig. 1). Upon improvement, the DFB approach could be
one of the most promising techniques to study the role
of a single plant in air-vegetation exchange of mercury.
5. Summary
A field study focusing on the design and development
of a new approach for air-vegetation exchange measure-
ment was performed in summer 2003 at the Bay St.
Franc-ois wetlands (Quebec, Canada). Measured TGM
fluxes ranged from �0.98 to 0.64 ng/m2 (leaf area)/h at
two study sites. Both emission and deposition fluxes
were observed using this new method, indicating that
foliar are dynamic exchange surfaces that can function
as a source or sink for atmospheric mercury. However,
the net TGM flux is primarily featured by deposition
of TGM from atmosphere to the vegetation. In the
mercury speciation study, measured concentrations of
GEM, RGM and PM were lower inside the bag than in
the ambient air, suggesting the dominant process for all
three species was deposition. Two peaks of ambient
RGM concentration and deposition fluxes, one in
daytime and one at night was observed, each likely
related to different physical and chemical processes.
During the field study, condensations on the interior of
the bag were observed. Further researches are in need to
improve the DFB method by minimizing the environ-
mental differences within the bag and in the ambient air.
More specifically, control measures should be taken to
eliminate the formation of condensation on the interior
wall of the bag. Upon improvement, this innovative
method could have wide applications on small scale
air-vegetation exchange measurements in the future.
Acknowledgements
This project is sponsored by the Natural Sciences and
Engineering Research Council of Canada, Collaborative
Mercury Research Network Initiative (Theme 2: The St.
Lawrence case study), and Environment Canada (EC). LP
would like to thank Dr. Steve Beauchamp (MSC-Atlantic
region) for discussion and helpful comments into the initial
elaboration of this project. Thanks also go to Drs. Brian
Fryer and Rajesh Seth at University of Windsor for their
comments and to Conrad Beauvais (EC), Abir Basu,
Hongyu You, and Ripon Banik at University of Windsor
for technical assistance. We also gratefully appreciate the
suggestions of the two anonymous reviewers.
References
Carpi, A., Lindberg, S.E., 1998. Application of a TeflonTM
dynamic flux chamber for quantifying soil mercury flux:
tests and results over background soil. Atmospheric
Environment 32, 873–882.
Eklund, B., 1992. Practical guidance for flux chamber measure-
ment of fugitive volatile organic emission rates. Journal of
Air and Waste Management Association 42, 1583–1591.
Ericksen, J.A., Gustin, M.S., Schorran, D.E., Johnson, D.W.,
Lindberg, S.E., Coleman, J.S., 2003. Accumulation of
atmospheric mercury in forest foliage. Atmospheric Envir-
onment 37, 1613–1622.
Ericksen, J.A., Gustin, M.S., 2004. Foliar exchange of mercury
as a function of soil and air mercury concentrations. Science
of the Total Environment 324, 271–279.
ARTICLE IN PRESSH.H. Zhang et al. / Atmospheric Environment 39 (2005) 7481–7493 7493
Frescholtz, T.F., Gustin, M.S., 2004. Soil and foliar mercury
emission as a function of soil concentration. Water, Air, and
Soil Pollution 155, 223–237.
Gustin, M.S., Lindberg, S.E., Marsik, F., Casimir, A.,
Ebinghaus, R., Edwards, G., Hubble-Fitzgerald, C.,
Kemp, R., Kock, H., Leonard, T., London, J., Majewski,
M., Montecinos, C., Owens, J., Pilote, M., Poissant, L.,
Rasumssen, P., Schaedlich, F., Schneeberger, D., Schroeder,
W., Sommar, J., Turner, R., Vette, A., Wallchlaeger, D.,
Xiao, Z., Zhang, H., 1999. Nevada STROMS Project:
measurement of mercury emissions from naturally enriched
surfaces. Journal of Geophysical Research-Atmosphere 104
(D17), 21,831–21,844.
Gustin, M.S., Lindberg, S.E., Austin, K., Coolbaugh, M.,
Vette, A., Zhang, H., 2000. Assessing the contribution of
natural sources to regional atmospheric mercury budgets.
The Science of the Total Environment 259, 61–71.
Hanson, P.J., Lindberg, S.E., Tabberer, T.A., Owens, J.G.,
Kim, K.H., 1995. Foliar exchange of mercury vapor:
evidence for a compensation point. Water, Air and Soil
Pollution 80, 373–382.
Kim, K.-H., Kim, M.-Y., 1999. The exchange of gaseous
mercury across soil-air interface in a residential area of
Seoul, Korea. Atmospheric Environment 33, 3153–3165.
Landis, M.S., Stevens, R.K., Schaedlich, F., Prestbo, E., 2002.
Development and characterization of an annular denuder
methodology for the measurement of divalent inorganic
reactive gaseous mercury in ambient air. Environmental
Science and Technology 36, 3000–3009.
Leonard, T.D., Taylor Jr., G.E., Gustin, M.S., Fernandez, G.C.J.,
1998. Mercury and plants in contaminated soils: 1. uptake,
portioning, and emission to the atmosphere. Environmental
Toxicology and Chemistry 17 (10), 2063–2071.
Lee, X., Benoit, G., Hu, X., 2000. Total gaseous mercury and
concentration and flux over a coastal saltmarsh vegetation
in Connecticut, USA. Atmospheric Environment 34,
4205–4213.
Lindberg, S.E., Hanson, P.J., Meyers, T.P., Kim, K.H., 1998.
Air/surface exchange of mercury vapor over forests-The
need for reassessment of continental biogenic emissions.
Atmospheric Environment 32, 895–908.
Lindberg, S.E., Stratton, W.J., 1998. Atmospheric mercury
speciation: concentration and behavior of reactive gaseous
mercury in ambient air. Environmental Science and
Technology 32, 49–57.
Lindberg, S.E., Meyers, T.P., 2001. Development of an
automated micrometeorological method for measuring the
emission of mercury vapor from wetland vegetation. Wet-
lands Ecology and Management 9, 333–347.
Lindberg, S.E., Dong, W., Meyers, T., 2002a. Transpiration of
gaseous elemental mercury through vegetation in a sub-
tropical wetland in Florida. Atmospheric Environment 36,
5207–5219.
Lindberg, S.E., Zhang, H., Vette, A.F., Gustin, M.S., Barnett,
M.O., Kuiken, T., 2002b. Dynamic flux chamber measure-
ment of gaseous mercury emission fluxes over soils:
part 2—effect of flushing flow rate and verification of a
two-resistance exchange interface simulation model. Atmo-
spheric Environment 36, 847–859.
Malcolm, E.G., Keeler, G., 2002. Measurements of mercury in
dew: atmospheric removal of mercury species to a wetted
surface. Environmental Science and Technology 36,
2815–2821.
Mayers, T.P., Hall, M.E., Lindberg, S.E., Kim, K., 1996. Use of
the modified Bowen-ratio technique to measure fluxes of
trace gases. Atmospheric Environment 30, 3321–3329.
Obrist, D., Gustin, M.S., Arnone, J.A., Jhonson, D.W.,
Schorran, D.E., Verburg, P.J., 2005. Measurement of gaseous
elemental mercury fluxes over intact tallgrass prairie mono-
liths during one full year. Atmospheric Environment 39,
957–965.
Poissant, L., 1997. Field observations of total gaseous mercury
behaviour: interactions with ozone concentration and water
vapor mixing ratio in air at a rural site. Water, Air, and Soil
Pollution 97, 341–353.
Poissant, L., Casimir, A., 1998. Water-air and soil-air exchange
rate of total gaseous mercury measured at background sites.
Atmospheric Environment 32, 883–893.
Poissant, L., 2000. Total gaseous mercury in Quebec
(Canada) in 1998. The Science of the Total Environment
259, 191–201.
Poissant, L., Dommergue, A., Ferrari, C.P., 2002. Mercury as a
global pollutant. Journal de physique IV 12 (PR10), 143–160.
Poissant, L., Pilote, M., Xu, X., Zhang, H., Beauvais, C.,
2004a. Atmospheric mercury speciation and deposition in
the Bay St. Francois wetlands. Journal of Geophysical
Research 109, D11301.
Poissant, L., Pilote, M., Beauvais, C., Zhang, H., Xu, X.,
2004b. Mercury gas exchange over selected bare soil and
flooded sites in the Bay St. Francois Wetlands (Quebec,
Canada). Atmospheric Environment 38, 4205–4214.
Sommar, J., Hallquist, M., Ljungstrom, E., Lindqvist, O., 1997.
On the gaseous reaction between volatile mercury species
and the nitrate radical. Journal of Atmospheric Chemistry
27, 233–247.
Tekran, 2001. Tekran Model 1130 mercury speciation unit and
Model 1135-p particulate mercury unit User Manual.
Toronto, Canada.
Wallschlager, D., Tuner, R.R., London, J., Ebinghaus, R.,
Kock, H.H., Sommar, J., Xiao, Z., 1999. Factors affecting
the measurement of mercury emission from soils with
flux chambers. Journal of Geophysical Research 104,
21,859–21,871.
Zhang, H., Lindberg, S.E., Barnett, M.O., Vette, A.F., Gustin,
M.S., 2002. Dynamic flux chamber measurement of gaseous
mercury emission fluxes over soils. Part 1: simulation of
gaseous mercury emissions from soils using a two-resistance
exchange interface model. Atmospheric Environment 36,
835–846.
Xiao, Z.F., Munthe, J., Schroeder, W.H., Lindqvist, O., 1991.
Vertical fluxes of mercury over forest soil and lake surface in
Sweden. Tellus 43B, 267–279.