Dynamic Oxidation of Gaseous Mercury in the Arctic Troposphere at

Environ. Sci. Technol. 2002, 36, 1245-1256
Dynamic Oxidation of Gaseous
Mercury in the Arctic Troposphere
at Polar Sunrise
STEVE E. LINDBERG*
Environmental Sciences Division, Oak Ridge National
Laboratory, Oak Ridge, Tennessee 37831-6038
STEVE BROOKS
Oak Ridge Associated Universities, P.O. Box 117,
Oak Ridge, Tennessee 37831-0117
C.-J. LIN
Department of Civil Engineering, P. O. Box 10024,
Lamar University, Beaumont, Texas 77710
KAREN J. SCOTT
Department of Microbiology, University of Manitoba,
Winnipeg, Manitoba R3T 2N2, Canada
MATTHEW S. LANDIS
U.S. EPA, 79 TW Alexander Drive,
Human Exposure Analysis Branch, MD-56,
Research Triangle Park, North Carolina 27711
ROBERT K. STEVENS
Florida Department of Environmental Protection,
2600 Blair Stone Road, Tallahassee, Florida 32399
MIKE GOODSITE
National Environmental Research Institute of Denmark,
Copenhagen, Denmark
ANDREAS RICHTER
Institute of Environmental Physics, University of Bremen,
D-28359 Bremen, Germany
Gaseous elemental mercury (Hg0) is a globally distributed
air toxin with a long atmospheric residence time. Any
process that reduces its atmospheric lifetime increases
its potential accumulation in the biosphere. Our data from
Barrow, AK, at 71° N show that rapid, photochemically
driven oxidation of boundary-layer Hg0 after polar sunrise,
probably by reactive halogens, creates a rapidly depositing
species of oxidized gaseous mercury in the remote Arctic
troposphere at concentrations in excess of 900 pg m-3.
This mercury accumulates in the snowpack during polar
spring at an accelerated rate in a form that is bioavailable
to bacteria and is released with snowmelt during the
summer emergence of the Arctic ecosystem. Evidence
suggests that this is a recent phenomenon that may be
occurring throughout the earth’s polar regions.
Introduction
Mercury has been targeted for global concern as a highly
toxic contaminant. Exposures are thought to be increasing,
* Corresponding author phone: (865)574-7857; fax: (865)576-8646;
e-mail: [email protected].
10.1021/es0111941 CCC: $22.00
Published on Web 02/13/2002
 2002 American Chemical Society
especially among indigenous populations who consume fish
and piscivorus species contaminated with methylmercury
(1, 2), a neurotoxin that biomagnifies in aquatic food chains.
Environmental mercury levels are known to be elevated in
the Arctic, to generally increase with latitude, and to have
increased over time (3, 4). Extensive wetlands exist in the
Arctic (5), and such areas are now recognized as important
sources of methylmercury (6).
In addition to Hg0, somewhat lesser amounts of oxidized
reactive gaseous mercury (RGM) species are now known to
be emitted from industrial sources (7). RGM species are watersoluble, exhibit a much shorter atmospheric lifetime than
Hg0, and their potential contribution to atmospheric deposition is widely recognized (8, 9). Although RGM compounds
represent only a few percent of the overall gaseous mercury
in typical ambient air, their dry deposition velocities and
scavenging ratios exceed those of Hg0 by more than an order
of magnitude (8). Since industrial emissions of water-soluble
RGM compounds are controllable to some extent (7), the
demonstration of direct production of RGM in the atmosphere has profound implications on ecosystem and human
exposure.
The concept of global fugacity has been used to explain
the condensation and accumulation of organic toxins in Arctic
regions (10), but Hg0 does not effectively “condense out”
even at -50 °C. However, other forms of airborne Hg,
especially oxidized Hg, might strongly partition from gas to
solid phases at low temperatures (11). Recent reports of
ground-level ozone (O3) and Hg0 depletions at Alert in the
Canadian High Arctic (∼82° N) provided the first evidence
that conditions may exist in the upper Arctic following polar
sunrise that promote depletion of airborne Hg0 (12). Unlike
O3, which is chemically destroyed, Hg only changes its
oxidation state, and depletion from the airmass implies
accumulation elsewhere. While the original Alert study did
not include measurements of the fate of the depleted Hg, a
recent paper reports elevated levels of Hg in snow throughout
the Canadian Arctic (13). The characterization of the associated Hg species and their ultimate fate in the Arctic
ecosystem is critical to our understanding of mercury
depletion events (MDEs).
In 1998, we initiated Hg0 measurements as part of the
Barrow Arctic Mercury Study (BAMS; 14) to determine the
geographic extent and reaction mechanism of MDEs. In 1999,
we added the first semi-continuous measurements of RGM
in the Arctic and measured both species during and after
polar sunrise through June 2001. In an intensive campaign
during April-June 2001, we also collected particulate Hg (Hgp) samples at Barrow, aircraft measurements of RGM, and
aerodynamic measurements of RGM fluxes over snow. We
report here the first detailed analysis of the BAMS data and
demonstrate that Hg0 is being “depleted” as a result of in-air
oxidation reactions that produce some form of oxidized
gaseous mercury that is rapidly deposited to the snow surface
at Barrow.
Methods
Study Site. All of the data reported here were collected at the
NOAA Climate Monitoring and Diagnostic Laboratory (CMDL)
in Barrow, AK. There are no Hg sources within the CMDL,
no emission points on the roof where our intakes were
located, nor any known major Hg point sources in the town
of Barrow, which is located ∼10 km southwest of the CMDL.
Prevailing winds are from the northeast across the Beaufort
Sea. The CMDL is located near the peninsula at Point Barrow,
∼2 km from the shoreline, and is surrounded primarily by
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FIGURE 1. Schematic diagram of the sample flow path in the automated Tekran model 2537A Hg0 analyzer with the 1130 gaseous speciation
denuder module and 1135 particulate Hg pyrolysis unit.
water to the north, east, and west. Barrow is geographically
the northern-most point in Alaska, located at 71°19′ N, 156°37′
W, and the CMDL is ∼9 m above mean sea level. In latitude,
Barrow is ∼1600 km south of Alert. We initiated atmospheric
Hg sampling at Barrow in September 1998 and began
additional parallel sampling of speciated Hg in air 1 yr later
using the methods described below. These measurements
will continue through at least 2003. We also collected fresh
surface snow and snow cores for Hg analysis from the tundra
in the CMDL clean air sector during 2000 and 2001. Particulate
Hg, eddy flux, and aircraft Hg measurements were added
during the intensive BAMS-2001 campaign in March-June
2001 at the Barrow lab. Ancillary data available at the CMDL
include routine meteorological data and trace gases such as
ozone (15).
Sampling and Analytical Approaches. Atmospheric Hg0
was determined using a Tekran 2537A vapor-phase mercury
analyzer. Descriptions of the Tekran and its operating
parameters have been published (16), and it has been the
subject of a number of intercomparison studies (17-19).
The 2537A instrument utilizes two parallel solid gold traps
to preconcentrate Hg0 that is subsequently thermally desorbed into a cold vapor atomic fluorescence spectrometer
(20). The instrument was configured to sample at a 5-min
time resolution using a heated Teflon inlet line mounted ∼5
m above the ground on a mast ∼1 m above the roof of the
NOAA CMDL building. The insulated sampling line was
maintained at 50 °C by a PID temperature controller.
Atmospheric Hg speciation was determined by integrating
a Tekran 1130 speciation unit with the Tekran 2537A. The
model 1130 speciation unit consists of a heated denuder
module, a pump module, and a controller module. The model
1130 controller module integrates the analytical capabilities
of the Tekran model 2537A unit with the 1130 speciation
module allowing for continuous measurement of both Hg0
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and RGM at pg m-3 concentrations. The 1130 speciation unit
was configured to collect 2-h RGM samples onto a KCl-coated
quartz annular denuder at a 10 L/min flow rate. During the
2-h sampling period, 5-min Hg0 samples were continuously
quantified by the 2537A analyzer. After the 2-h sampling
period, the 1130 system was flushed with Hg-free air, and
the annular denuder was heated to 500 °C. The RGM collected
on the denuder was thermally decomposed into a Hg-free
airstream and subsequently analyzed as Hg0. The denuders
collect oxidized reactive gaseous mercury compounds with
a diffusion coefficient >0.1 cm2/s that readily adhere to a
KCl coating at 50 °C (21). The most probable candidate
compounds are HgCl2 and HgBr2; HgO is less likely. In this
configuration, the fine particulate phase mercury (Hg-p; e2.5
µm) that passes the impactor inlet was collected on a
downstream quartz fiber filter but was not analyzed.
For 3 months during the BAMS-2001 spring intensive
campaign, our EPA colleagues deployed additional mercury
monitoring equipment to allow separate quantification of
Hg0, RGM, and also Hg-p. The equipment included a Tekran
2537A/1130 configuration described above with the addition
of a prototype Tekran 1135 particulate pyrolysis unit. One of
the greatest problems with conventional Hg-p measurement
methods is that RGM has been shown to adsorb to filter
media and previously collected aerosols. This can result in
large and variable measurement artifacts (21). The Tekran
1130/1135 speciation system avoids this problem by collecting RGM prior to collecting the Hg-p onto a quartz fiber
filter. The quartz filter and quartz denuder components were
sequentially desorbed during the analysis phase at 500 and
650 °C, respectively. During the filter heating step, the
pyrolyzer was maintained at 650 °C to ensure complete
decomposition of all Hg-p compounds evolved during the
filter temperature ramp-up to Hg0. Figure 1 shows a schematic
of the flow path for the overall 1130/1135 system as integrated
with the 2537A. During desorption, the sampling system was
flooded with zero air to eliminate background air and achieve
good analysis blanks. This zero air also acts as the carrier gas
during subsequent analysis steps. The pyrolyzer for the quartz
regenerable filter was preheated to convert to elemental form
any mercury compounds that are eluted during subsequent
steps. As the regenerable particulate trap was heated, the
particle-bound mercury captured on the trap was desorbed
and quantified by the Tekran 2537A. The heating process
also reconditioned the trap for subsequent cycles. After
desorption was complete, the entire sampling train was
cooled to 50 °C. After being cooled, the denuder and
particulate trap were ready for another measurement cycle.
Atmospheric Hg fluxes were quantified during the BAMS2001 intensive by deployment of a relaxed eddy accumulation
(REA) system developed for RGM. The Danish REA system
uses a METEK sonic anemometer coupled with three heated
manual RGM denuders and filter packs (22). The basis of the
REA logging and control system used is as described in the
literature (23). The system was operated over the snow during
April 2001 to collect 12 4-h samples at a 10 L/min flow rate
and a switching frequency of 1 Hz to quantify the RGM
concentrations in the upflow and downflow eddies and a
deadband (we realize that there is decreased resolution of
the turbulence at 1 Hz; however, we feel that the data
represent a general trend and are in good agreement with
fluxes calculated based on the constant sampling during the
same time periods). The vertical flux was calculated as
described in ref 24: F ) σW(C_up - C_down)β, where σW is
SD (vertical wind speed), β is a proportionality constant (both
measured in real time), and C_up - C_down is the direct
difference in concentration between the upflow and downflow denuders for each period. The denuders were analyzed
manually as described in refs 22 and 25, a method based on
the automated approach described above. Development
continues on the REA system to increase the sampling
frequency and maintain a constant laminar flow in the
denuders.
Surface snow samples (upper 10 cm) were collected in
acid-washed Teflon bottles and maintained frozen until
analysis. Subsurface samples were similarly collected from
the freshly exposed snowpack face. Snowmelt runoff was
collected in prefired Pyrex bottles with Teflon caps. Total
Hg and methylmercury analyses were performed at Flett
Research Ltd. (Winnipeg, MB) and at ORNL using cold vapor
atomic fluorescence spectrophotometry (26). Assays for
bioavailable mercury (bioHg) were performed on a number
of snow samples using mer-lux bioreporters, genetically
engineered bacteria that produce light when divalent inorganic mercury enters their cells (27). Samples were melted
in the dark and analyzed immediately. Sample preparation
was done in a Class 100 laminar flow hood in a HEPA-filtered
Hg clean lab at the Freshwater Institute (Winnipeg, MB).
The general assay method employed was as described in ref
27 with the following modifications: Vibrio anguillarum was
the host species, not Escherichia coli; cells were grown in
Glucose Minimal Medium; the assay medium was modified
to include 5 mM glucose and 18 mM (NH4)2SO4; in addition,
3 mM sodium/potassium phosphate buffer (pH 6), used to
wash and resuspend the mer-lux cells, was added to the
sample in the final cell suspension. The specific growth
requirements and cell preparation of V. anguillarum are
described in ref 28. The primary Hg standard used was a
1 µg mL-1 Hg(NO3)2 solution prepared by Flett Research Ltd.
(Winnipeg, MB). Light production was measured with a Beckman LS 6500 scintillation counter in noncoincidental mode.
Maps of BrO distribution in the total column were
generated from GOME satellite data (Global Ozone Monitoring Experiment). GOME is a 4-channel UV/visible grating
spectrometer on board the ESA satellite ERS-2. GOME
radiances and irradiances were used with a published
algorithm (29) to derive total column BrO. GOME measurements do not yield profile information but reflect the BrO
distribution throughout the column. Since stratospheric BrO
(which mainly depends on photolysis of the reservoirs and
total stratospheric BrY) is relatively constant with time and
shows little spatial variation, as supported by model calculations and ground-based observations of stratospheric BrO
(29), the BrO plots show mainly the boundary-layer variations
plus a more or less constant offset. This idea is supported
by enhanced boundary-layer BrO observed independently
at other polar sites using ground-based instruments that
match with enhanced BrO values in the GOME data (30).
Quality Assurance Activities. At Barrow, the Tekran 2537A
routinely undergoes automated periodic recalibrations daily
using an internal permeation source. Two-point calibrations
(zero and span) are performed separately for each cartridge.
A Commercial permeation tube (VICI Metronics) provides
approximately 1 pg/s at +50.0 °C. Since it is not practical to
certify these low rates gravimetrically, manual injections were
used in our home lab before shipment to Barrow to initially
calibrate the tube against a saturated mercury vapor standard
(16, 31). Since then, this procedure has been repeated once
or twice annually. The adjustment for perm tube drift has
been on the order of 1-2% per year. During routine
operations at Barrow, the Tekran was also subject to periodic
zero checks and spikes of ambient air with a known amount
of Hg0. During 1999, we performed spikes and zero checks
every 25 h using an automated system that externally
controlled the perm source to deliver 16 pg of Hg0 into the
Teflon sampling line. Over the year, spike recoveries ranged
from 85 to 114% and averaged 102 ( 3% recovery. There was
no significant difference in recovery during the MDE period
as compared to the rest of the year. Zero checks were
consistently at the detection limit (0.09 ( 0.01 ng/m3).
Although the 1130 RGM speciation system does not yet
include a direct calibration method with known amounts of
RGM, all analyses are performed with the 2537A after thermal
desorption and conversion of the RGM to Hg0, a process
demonstrated to be quantitative with an efficiency of 100%
(21). Field intercomparisons of paired 1130 instruments
performed elsewhere over a range of RGM from ∼20 to 400
pg m-3 showed that the denuder method exhibits good
precision (( 15%) and that no significant RGM breakthrough
occurs for the 2-h denuder samples collected over the RGM
concentration range seen at Barrow (21). We also performed
an intercomparison for RGM in Barrow using the Tekran
1130 automated denuder operated adjacent to a manual
quartz annular denuder of similar design (21). During May
1-6, 2000, these systems were operated simultaneously on
the roof of the Barrow CMDL to collect six sets of replicate
samples. The Tekran 1130 denuder was analyzed as described
above, while the manual denuders were thermally desorbed
in a tube furnace, and the exhaust gas was directed into a
second 2537A for an independent analysis of desorbed Hg
(21, 25). Five of the six replicates gave excellent results; the
data were well correlated (r2 ) 0.91), and the means were
within ∼5% (Tekran ) 108 ( 23 pg/m3, manual ) 115 ( 24
pg/m3). One set differed by a factor of 2 (manual lower), but
RGM was increasing rapidly during this period, and the two
samples were slightly offset in time.
During the REA flux measurements, the total concentration of the three denuder tubes for each run compared
favorably with a collocated manual denuder sampling system
as a QA check. On the basis of parallel measurements in the
lab and field study, the manual denuders exhibit a precision
of 10%, which infers a sampling error of 40% (95% confidence
level) for the RGM flux determined by the REA system, given
an allowed 10% error in the micrometeorological determinations of β and σw (23). Flow in the denuders was set by a
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mass flow controller to 10 L/min and controlled prior to and
after each measurement. However, because of flow development during switching, a more cautious number for the REA
system would be a denuder precision of 20%, thus giving a
sampling error for flux as determined by the REA system of
∼60% (95% confidence level; 22).
Approximately half of the Barrow snow samples were split
and analyzed in duplicate by each analytical lab with good
agreement ((15%). Assays for bioavailable mercury (bioHg)
in snow samples were routinely run in replicate with a
precision on the order of 5-10%.
Results and Discussion
Dynamics and Chemistry of Depletion Events. The 1999
data (Figure 2) provide the first confirmation of MDEs at this
more southerly Arctic site (Barrow is ∼1600 km south of Alert),
and the 2000 data show that RGM is produced during MDEs
(Figure 3). The early 2001 data exhibit comparable trends.
This is the first evidence that the MDEs could be a widespread
phenomenon of Arctic dawn and that RGM only appears at
significant levels when Hg0 is being depleted. Although others
have suggested that the depleted Hg0 at Alert accumulates
in the aerosol-phase Hg (12), our data clearly indicate an
important change in gaseous speciation during MDEs,
producing levels of RGM unprecedented at remote and rural
sites (8, 18, 25).
Depletion events begin within a few days of polar sunrise
(late January) and persist until snowmelt (early June, Figure
2), suggesting a role of both sunlight and frozen surfaces.
During this period, Hg0 exhibits a strong correlation with O3
(e.g., r2 ) 0.76 for the period graphed) as also seen at Alert
(12). Surface O3 destruction is a common feature at Barrow
during Arctic spring (15). There is no correlation between O3
and Hg0 in the months before polar sunrise (r2 < 0.1).
Gaseous and aerosol Br also exhibit strong seasonal cycles
at Barrow and, like RGM, peak annually between January
and June (32). During this period, aerosol Br increases nearly
20-fold over typical concentrations and can exceed 100 ng
m-3. Hypotheses for the sources of this Br include aerosol
enrichment by bubble bursting from the sea-surface microlayer, gaseous reactions resulting from organic Br emissions from marine algae (e.g., bromoform is thought to be
emitted by ice algae), and/or other aerosol-related reactions.
The most probable mechanism involves heterogeneous
reactions at the interface of hygroscopic sea-salt aerosols
(15), many of which are initiated in the surface microlayer
of snowflakes or the snowpack (33). Several gaseous reactive
halogen species (e.g., BrO, see Figure 2) may result with the
potential to oxidize Hg0 to gaseous Hg(II) (RGM) compounds.
Many of these halogen compounds exhibit a strong diel
pattern (e.g., BrO, ClO, Br, and Cl), indicating the importance
of sunlight and photochemical reactions (34), as reflected in
the diel cycle of the airborne Hg species. Our data indicate
that peak RGM production and Hg0 depletion generally occur
at midday under maximum UV (Figure 4).
We hypothesize that RGM is formed through a rapid,
in-situ oxidation of Hg0 in the gaseous phase during MDEs.
Production of RGM may be attributed to the same photochemically active halogen species involved in surface O3
destruction (15), suggesting that the overall process is
heterogeneous. On the basis of reported halogen activation mechanisms in the remote marine boundary layer
(34-39), we propose the physicochemical pathways conceptualized in Figure 5 for our observation of RGM during
MDEs. In the reaction mechanism, bromine and chlorine
radicals are produced autocatalytically from a heterogeneous photochemical mechanism involving sea-salt aerosol.
The halogen radicals (Br/Cl) and halogen oxide radicals (BrO,
ClO) produced from the ozone destruction reaction (reaction
1) exhibit strong diurnal patterns with solar radiation (34,
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36) and serve as the primary oxidants that produce RGM
(Figure 5):
Br/Cl + O3 f ClO/BrO + O2
(1)
BrO/ClO + Hg0 f HgO + Br•/Cl•
(2)
Hg0 + 2Br•/Cl• f HgBr2/HgCl2
(3)
and/or
These proposed mechanisms are plausible for the excellent
correlation between Hg0 and O3 concentrations during MDEs.
There are other reactive halogen species present in the
Arctic that are thermodynamically favorable in oxidizing Hg0
to form RGM in the gaseous phase, including Cl2, Br2, HOCl,
HOBr, and BrCl. However, we feel that molecular Cl2, Br2,
and BrCl are not likely to cause the in-situ RGM formation
seen here as they can be rapidly photolyzed under sunlit
conditions (34-36). HOCl and HOBr are more resistant to
solar irradiation. However, since they do not exhibit a strong
diel cycle in the remote marine boundary layer (35-38), they
may not account for the observed RGM at Barrow. Figure 5
identifies two RGM species as proposed products: mercury
oxide (HgO) and mercury halides (HgBr2/HgCl2). Considering
the extremely low concentration of all reacting species (both
Hg0 and reactive halogen), the bimolecular oxidation of Hg0
by halogen oxide would be the favorable RGM formation
pathway, yielding HgO as the product. However, since the
denuders may preferentially collect HgBr2/HgCl2 over HgO
(21), there may also be a significant contribution from halogen
radicals in the trimolecular oxidation of Hg0. On the basis of
published studies of reactive halogens in the Arctic (15, 3240), HgBr2 should be favored. Research to determine the
predominant species of RGM at Barrow continues.
The reaction scheme in Figure 5 explains the strong Hg/
O3 correlation and also the extreme seasonality of MDEs:
the depletions of O3 and Hg0 require both sunlight and a
frozen aerosol or snow surface. The reactive halogens are
initiated by the production of HOBr (or HOCl) from hydroxyl
radical reactions with Br- (or Cl-), which are highly concentrated in the surface layer of a frozen water droplet or
snow crystal (40). The abrupt end of the MDEs precisely at
snowmelt suggests that these reactions do not proceed when
the droplets deliquesce, decreasing the surface Br- and Clconcentrations as the droplets become homogeneously
mixed (Figure 5).
Meteorological factors strongly influence the extreme
levels of RGM measured at Barrow. We developed a simple
predictive model for airborne Hg depletion and dry deposition to snowpack using local meteorological data and an
inverse boundary-layer approach (41). Taken together,
boundary-layer entrainment rates and deposition velocities
(both modeled as a function of wind speed, UV-B, and air
temperature) explained ∼70% and ∼80% of the measured
variance in airborne Hg0 and RGM, respectively. The highest
RGM concentrations consistently occurred during periods
of reduced wind speed and maximum UV-B. Elevated RGM
also coincided with periods characterized by increased levels
of BrO in the vicinity of Barrow (e.g., Figure 2). These
conditions often follow periods of elevated wave activity and
sea-salt aerosol generation. For example, an extensive area
of elevated BrO occurred on March 29, 2000, when the
Beaufort gyre had opened a series of leads north of Barrow.
Under these conditions, RGM reached 900-950 pg/m3 (e.g.,
Figure 3, days 91 and 138-148), levels that exceed those
measured near industrial point sources (8, 42). Even the
average daytime RGM of 180 pg m-3 at Barrow during MarchMay is several times higher than typical event levels at rural
sites (8, 25).
FIGURE 2. Trends in Hg0 at Barrow during 1999, showing a strong correlation between Hg0 and O3 depletions, and development of a zone
of elevated BrO near Barrow during this period (mean vertical column BrO is expressed as molecule cm-2, derived from GOME satellite
data).
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FIGURE 3. Trends in Hg0 (upper plot) and reactive gaseous mercury (lower) at Barrow during 2000, showing parallel behavior in total
Hg in the surface snowpack (lower) and mean daily UV-B (lower).
Fate of RGM and Dynamics of Hg in Snow. RGM is formed
continuously as long as O3 and reactive Br are present during
polar spring, and the observed air concentrations reflect the
dynamic balance between formation and deposition. Integrating the data in Figure 4 suggests that ∼30-40% of the
depleted Hg0 appears as airborne RGM and that the
remainder is deposited to the snow surface directly as RGM
and/or is scavenged by fine aerosols. Our inverse boundarylayer model of the local sink strength (41) predicted Hg
deposition fluxes of ∼40 µg m-2 at Barrow during FebruaryMay 1999 and ∼55 µg m-2 during January-May 2000. These
estimated 5-month fluxes are much higher than annual wet
deposition rates measured in the northeastern United States
(∼5-15 µg m-2 yr-1; 9) and can be supported only by direct
deposition of RGM since submicron aerosols are subject to
far less local deposition than is RGM (8).
Our year 2000 snow chemistry data confirm these modeled
rates of Hg accumulation (Figure 3). The measured Hg
concentrations in the snowpack and calculated snowpack
depth (86 cm) prior to melt in 2000 (assuming a 70% water
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equivalency) yield an estimated average flux of ∼50-60 µg
m-2. This flux is reflected in total Hg in snow, which increased
steadily from <1 to >90 ng/L over this period (Figure 3).
Such dramatic increases have not been seen in snow from
lower latitudes (43), but 1998 data from a ship frozen in the
Beaufort Sea 550 km north of Barrow showed similar temporal
trends in surface snow (although at lower concentrations;
44), suggesting that this phenomenon should be investigated
at higher latitudes. These concentrations of Hg in the May
snowpack are not only unprecedented for non-Arctic remote
sites but also exceed typical levels near industrialized regions
(9, 43).
The fraction of the total Hg pool in snow that was
biologically available to bacteria (bioavailable mercury) was
determined using the mer-lux bioreporter V. anguillarum
pRB28 and its constitutive control V. anguillarum pRB27.
The mer-lux bioreporters are genetically engineered bacteria
that produce light when divalent inorganic mercury (Hg(II))
enters their cells (27), thereby distinguishing biologically labile
from biologically inert Hg(II) species that do not enter the
FIGURE 4. Typical diel cycle of tropospheric gaseous Hg species
and UV-B at Barrow (UV is measured in near-realtime, while Hg0
and RGM represent integrated samples of 5 min and 2 h, respectively,
as described in the text).
FIGURE 5. Conceptual diagram of proposed Hg0 oxidation reaction
sequences in the Arctic at Barrow. As explained in the text, there
are several possible pathways and products. This schematic presents
those that appear most favorable given the observations (dashed
lines represent inter-phase transport; solid lines are reaction
pathways).
bacterial cell. This measurement is of interest because
microorganisms play an important role in the transformation
of Hg(II) in the environment (6). In January (2000), prior to
polar sunrise, bioavailable Hg(II) was undetectable in Barrow
snow. It then increased from 0.22 ng/L (∼1% of total Hg) in
March to 8.8 ng/L (nearly 13% of the total Hg) in May. Prior
to this study, bioavailable Hg(II) had never been measured
in the Arctic. However, concentrations in snow and precipitation in a remote Boreal site in northern Canada were
on average <0.5 ng L-1 (45). During the early snowmelt period
in June, the concentration of bioavailable Hg(II) in melting
(slushy) snow decreased to 2.9 ng/L. Despite this decrease,
the fraction of the total Hg pool that was bioavailable in
snowmelt increased from 13% to 55%. This finding was
supported by our 2001 data (not shown) and could reflect
differences in the behavior of inorganic Hg species during
the spring melt period; namely, that bioavailable Hg(II) is
less readily photoreduced and evaded than other inorganic
Hg species within the total Hg pool. This finding has
interesting implications for the fate of bioavailable Hg
entering the biosphere, especially in snow, and studies are
underway to explore this observation. Another area that
warrants study is the effect of other metals (e.g., Cu and Zn)
in Arctic meltwaters and snow (60) on the uptake of Hg(II)
by bacteria (28), currently unknown.
Given these elevated Hg levels, snowmelt is particularly
important to the Arctic Hg cycle because it occurs when the
ecosystem is initiating seasonal biological activity. Once
underway, snowmelt at Barrow proceeds rapidly under the
24-h daylight, and mercury exhibits a surprisingly dynamic
behavior. During the 1999 snowmelt (June 5-12), MDEs
ended abruptly, and Hg0 spiked dramatically and remained
elevated for several weeks (Figure 2, JD 156-230). Since a
possible source of this airborne spike was Hg0 evaded from
the snowpack (12), we also tracked Hg in snow as well as in
air during the 2000 snowmelt (June 4-10; Figure 6). MDEs
again ceased and airborne Hg0 spiked, while total Hg
concentrations in snow decreased drastically, by 92%. As
noted above, bioavailable Hg(II) also decreased, but somewhat less (67%) so that the fraction of the total Hg pool that
was bioavailable in snow increased. In addition, airborne
RGM decreased to our detection limit (∼1 pg m-3) (see also
Figure 3), supporting the hypothesis that frozen surfaces are
required in the Arctic Hg oxidation cycle (Figure 5). Hg2+(aq)
is readily photoreduced to Hg0 [e.g., by sulfite or by highly
reducing organic free radicals produced through photolysis
of iron/organic acid coordination compounds (11, 46, 47)]
and evaded from surface waters (16); our data suggest this
mechanism is active during the Barrow snowmelt.
Snow cores collected in early June exhibited a uniformly
reduced total Hg concentration in the slushy surface layers
(21 ( 10 ng/L from 20 to 100 cm depth, temperatures from
-2 to 0 °C), but total Hg ranged from ∼50 to 90 ng/L in the
deeper, still frozen layers (∼140-150 cm, temperatures <-5
°C). The flux of snowpack Hg to air and water is clearly
influenced by the melting process. Total Hg in runoff (∼30
ng/L) was elevated over that in slushy snow (cf. Figure 6),
indicating that a portion of the Hg in the snowpack does
enter the local ecosystem during snowmelt. Some of the Hg
that runs off into the tundra continues to be photoreduced
and evaded to the atmosphere under the elevated 24-h UV-B
levels that persist through the summer until mid-July, leading
to the “hump” in Hg0 (days ∼160-210, Figure 2). The fate
of the Hg deposited to snow will be determined by the balance
between re-emission and runoff; quantifying both are
important objectives of future studies at several Arctic sites.
BAMS-2001Campaign: Particulate Hg, Aircraft, and
Micrometeorological Flux Measurements. Data just acquired from the BAMS-2001 spring campaign ending in June
2001 support our hypotheses and advance our understanding
of the processes involved. For 3 months, fine particulate Hg
(PM2.5) was collected immediately downstream of the RGM
denuder using a Tekran 1135 particulate Hg sampler as
described earlier. These first particulate Hg (Hg-p) samples
collected at Barrow (48) indicate that RGM is clearly the
primary species being formed during this MDE period and
that the two species behave differently. For example, from
March 19 to April 11 (a period with ∼3-5 h of darkness),
RGM averaged ∼70 ( 5 pg m-3 while the Hg-p concentration
averaged ∼10 ( 1 pg m-3 (N ) 166, (1 SE; median Hg-p/
RGM ratio ) 0.2; mean ) 0.4 ( 0.03). Particulate Hg and
RGM were anti-correlated during this period, with Hg-p
peaking just prior to sunrise when RGM was at its daily
minimum but decreasing rapidly upon sunrise (48). The
authors suggest that different reaction pathways or reactants
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FIGURE 6. Trends in several Hg species in the atmosphere and in the snowpack at Barrow around the period of annual snowmelt [during
the June 4-10, 2000, snowmelt (days 156-162), slushy snow was collected from atop the frozen snowpack for analysis]. Inset photographs
show the conditions of the snowpack at midday, sky conditions, and mean daily air temperature.
may be responsible for creating Hg-p as compared to RGM
and that the Hg-p produced at night is photosensitive. One
candidate reaction would involve aerosol-bound BrCl that
would readily oxidize any sorbed Hg0 but that is rapidly
decomposed under sunlight (36). However, upon the advent
of 24-h sunlight in mid-May, the Hg-p/RGM ratio dropped
to ∼0.1, and the two species were now positively correlated
(48). The authors speculate that the Hg-p detected after 24-h
sun reflects RGM sorbed onto the existing aerosol. These
observations may help explain why the air at Alert appears
to be characterized by a larger Hg-p/RGM ratio than at Barrow
(13). The surface reactivity of airborne RGM suggests that it
would readily partition to the aerosol phase upon formation.
Hence, Hg-p/RGM ratios may be useful indicators of the age
(time since oxidation of Hg0), and hence transport distance,
of depleted air masses. As noted here, our data suggest that
at least some RGM is being formed in situ at ground level
at the Barrow site, while Hg0 in the air sampled at Alert may
have undergone significant depletion/oxidation events over
the sea ice prior to being sampled at the Alert station.
To determine the vertical extent of RGM formation, upper
air RGM was sampled with heated RGM denuders attached
to the outer strut of a Cessna 207 and to a mass flow meter/
vacuum pump system. Two successive 1-h midday flights
were conducted on three separate days in late March and
early April at 1000 m (exterior to the boundary layer) and 100
m altitude (within the boundary layer) immediately northeast
of Point Barrow (48). The six aircraft surveys consistently
showed that RGM exists primarily in ground-level air below
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the marine boundary layer (concentrations decreased from
an average ∼70 to ∼20 to ∼2 pg m-3 from 5 to 100 to 1000
m), supporting the hypothesis that the Hg oxidation reactions
are occurring in the near-surface boundary layer driven by
halogen compounds derived from sea-salt aerosols.
We also performed the first RGM dry deposition measurements in the Arctic at 3 m above the snowpack using a
tower-based relaxed eddy accumulation (REA) approach (22,
24) with manual RGM denuders, as outlined above. Significant RGM fluxes measured during March 29-April 12 were
directed toward the snow surface (overall mean net deposition ) -0.4 ( 0.2 pg m-2 s-1; N ) 9, (1 SE). Computed dry
deposition velocities for RGM were high, on the order of 1
cm/s, and agree with those predicted by our inverse
boundary-layer model (41). Quantifying the extent of areas
of elevated Hg deposition is clearly an important research
need, as is understanding its fate.
Implications of the Barrow Study: Is This a Recent
Phenomenon and What Is Its Extent? We have convincing
evidence that tropospheric Hg0 from the global background
pool and long-range transport is being depleted from the air
and converted to a form of rapidly depositing reactive gaseous
mercury (RGM) following polar sunrise at Barrow, AK.
Tropospheric oxidation by sea-salt-derived reactive halogens
involved in O3 depletion (Br and Cl) generates peak levels of
RGM at this remote site not observed at more southerly
locations, including those near major point sources. This
reactive Hg species has a very short atmospheric lifetime
and accumulates in the Barrow snowpack in forms that
are bioavailable to bacteria. Mercury concentrations and
accumulation rates in snowpack prior to snowmelt greatly
exceed those in source regions such as eastern North America,
and some of this Hg reaches the Arctic tundra ecosystem at
the initiation of its annual growth cycle.
Recent reports suggest this Hg oxidation phenomenon
may exist at many Arctic sites as well as in the Antarctic (12,
49-51) and could represent an important sink in the global
cycle of Hg0 (13). The implications of polar MDEs may be
assessed by addressing two frequently asked questions: Is
the phenomenon recent? and Are the polar regions an
important sink for Hg in the global cycle or likely to become
so? There are lines of evidence that suggest the answer to
both questions is yes.
Is This a Recent Phenomenon? Several data sets suggest
that there has been a recent increase in Hg levels in Arctic
biota despite a 20-yr decrease in global atmospheric Hg
emissions of ∼30% (52). Mercury levels in seabird populations
monitored within Arctic Canada have roughly doubled in
the last 20-30 years (53), while Hg accumulation in ringed
seals and beluga whales has also increased over the last two
decades (54, 55). Mercury emissions within the Arctic are
not thought to be increasing (52), and with global emissions
clearly decreasing, another explanation must be sought.
We suggest that Arctic MDEs are recent phenomena,
resulting from changes in Arctic climate that have increased
atmospheric transport of photooxidants and production of
reactive halogens (Br/Cl) in the Arctic. Observations show
that the Arctic region has undergone dramatic physical
changes in climate over the last 30-40 years, including a
decreasing trend in multi-year ice coverage, related increases
in annual ice coverage, later timing of snowfall and earlier
timing of snowmelt, increasing ocean temperature, and
increasing atmospheric circulation and temperature (56). The
changes related to ice formation can impact the dynamics
of MDEs. The GOME satellite data suggest that BrO enhancements are generally absent over multi-year ice (notably
within the Canadian basin) where ice thickness and windblown dust accumulation make sunlight conditions under
the ice insufficient for algal primary productivity (one source
of photolyzable Br). As multi-year ice is decreasing, annual
ice is increasing. The reactive Br surface source is this polar
annual sea ice region where ice thinness and optical
transparency support rich under-ice biotic communities.
Photolyzable bromine (a waste product of ice algae) builds
up under the ice and escapes through constantly changing
patterns of open leads and polynyas (open water in an actively
upwelling region). These dynamic open water areas are also
sources of sea-salt aerosols, water vapor, and heat from the
comparatively warm ocean waters. All these products remain
concentrated in the near surface air due to the lack of vertical
convection (caused by limited solar input, the high-albedo
snow/ice surfaces involved, and a positive temperature
inversion strength (57)), where they react with O3 and other
photooxidants, leading to oxidation of Hg0 as described
earlier.
Changes in the chemical climate of the Arctic may also
enhance Hg oxidation reactions. Satellite total ozone mapping
(TOMS) data indicate an ∼20% decrease in total column
ozone amounts over the Arctic since 1971, and decreased
ozone leads to increased surface UV-B exposure (58). The
link between Hg behavior and UV is clear from our data:
near-surface RGM during the March-April period at Barrow
is strongly correlated with a function of incident solar UV-B
(which controls production of BrO from photolyzable Br)
and wind speed (which controls the turbulent deposition
rate) (r2 ) 0.82; 41). Increased UV radiation reaching the
troposphere may also result in increased levels of the OH
radical through photolysis of tropospheric ozone (59). In the
Arctic atmosphere, increasing OH levels could lead to even
greater oxidation of Hg0 because of a positive feedback
between increasing OH and production of reactive halogens
(Figure 5). If MDE-enhanced mercury deposition in the Arctic
is a relatively recent phenomenon (as a result of increased
synoptic activity and increased annual ice area, for example),
this could explain the data sets showing a recent increase in
Hg accumulation in Arctic biota, despite the decrease in global
atmospheric emissions of Hg in recent decades.
Are the Polar Regions an Important Sink for Hg in the
Global Cycle? To address this question, one needs to assess
the evidence for the spatial extent of the MDE phenomena
and the extent to which deposited Hg is being re-emitted
back into the atmosphere during and after snowmelt.
Depletion events have now been recorded at five widely
dispersed, primarily coastal, polar sites (12, 14, 49-51). One
potential indicator of the overall spatial extent of these events
is illustrated in the monthly GOME maps of BrO distribution.
The average column BrO concentrations over the Arctic for
April 2000 are shown in Figure 7. These and related maps
(13) clearly suggest that MDEs and associated RGM production should be concentrated in coastal zones and in areas of
active open water and might not be expected in other
locations (e.g., continental Greenland). The bromine source
regions are concentrated in the dynamic areas of annual sea
ice, and emission products from these areas are advected
downwind where reactive halogen compounds form under
sunlight conditions (e.g., ref 36). The maps suggest that
horizontal advection of Br compounds to inland and iceshelf regions is controlled by prevailing winds and is
effectively dammed by topographic features such as the
Brooks, Anadyr, and Rocky Mountain ranges as well as by
the location of the polar front. The front tends to follow the
permafrost contours around the pole; the BrO map follows
roughly these same contours (Figure 7). Note that air over
the ice-covered Greenland and Ellesmere Islands is relatively
free of BrO enhancements because the predominating
katabatic (outward flowing) winds over the icecaps block
significant inland advection. Oxidation of Hg0 and enhanced
deposition of RGM would not be expected in these areas, a
hypothesis that could be tested by future snow surveys.
However, coastal locations, such as Nord and Alert, are
affected by the local marine environment and do experience
episodic BrO enhancements along with the associated
mercury depletion events and ozone losses (12, 49). We expect
that production of oxidized gaseous Hg species will also be
reported for these areas once new measurements are
underway in 2002.
Recent surveys of environmental Hg levels near Barrow
also indicate similarities in the spatial trends of enhanced
BrO and Hg accumulation, as would be expected if RGM
production is dependent on BrO. The concentrations of
marine-related reaction products taper off with distance from
the coastline, and Figure 7 illustrates a well-defined inland
gradient in BrO in Alaska. Mercury levels there are also
anticorrelated with distance from the coast: Landers et al.
(3) reported such trends for Hg levels in Arctic Alaskan
vegetation, and Snyder-Conn et al. (60) reported similar
trends in total mercury levels in Arctic Alaskan snow. More
recently, Garbarino et al. (61) showed that mercury concentrations in snow over sea ice were highest in the
predominately downwind direction of the open water leads
and polynyas surrounding Point Barrow (e.g., to the west),
an area that often shows enhanced BrO (e.g., Figure 2).
Comparable Data Exist for the Canadian Arctic. A recent
report shows that locations of high total mercury concentrations in snow are well correlated with areas of high
atmospheric BrO concentrations, especially in the Canadian
archipelago (13). Mercury levels in biotic surveys also follow
these trends; total mercury in Glaucous Gull eggs sampled
at four coastal locations in Canada are highest in the Canadian
VOL. 36, NO. 6, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Spatial patterns in monthly mean BrO over the Arctic for April 2000 showing locations of recorded mercury depletion events
at Barrow, AK; Alert, NWT; Ny-A° lesund, Spitzbergen; and Station Nord, Greenland (mean vertical column BrO is expressed as molecule
cm-2, derived from GOME satellite data). MDEs have also been recorded at Neumeyer station, Antarctica, in an area of elevated BrO (not
shown).
archipelago where BrO is enhanced (53). The Canadian
archipelago is dominated by annual ice and open water
polynyas and leads, and the extensive shorelines and ocean
currents between the islands create shear zones between
the “fast” ice grounded to shore and the pack ice moving
with the ocean currents. This interface area is dominated by
the open leads that are probable sources of bromine and
marine products to the near-surface air. A recent estimate
of the gross atmospheric Hg loading to northern waters in
this region was 50 T/yr (13). This estimate was based on Hg
levels in snowpack that were generally lower than those
reported near Barrow. Other estimates from models (49) and
our preliminary scaling from GOME BrO data such as Figure
7 (63) range from ∼150-300 T/yr, but all such estimates of
gross fluxes carry a high uncertainty.
To assess the overall net strength of the so-called missing
polar sink (14) using the GOME satellite BrO maps, we must
fully understand the importance of Hg re-emission during
snowmelt. Although re-emission is apparent (e.g., Figure 6,
also ref 12), we and the group working at Alert (62) are yet
unable to quantify its overall effect on the net accumulation
of Hg in the Arctic. A simple analysis based on the upslope
of the Hg0 concentration in air during Barrow snowmelt (as
an indicator of the re-emission signal, Figures 1, 2, and 6)
suggests that melt-related re-emission represents ∼10-20%
of the deposited Hg (63), and our measurements of Hg in
runoff indicate that Hg is being transported to the tundra
during snowmelt. Quantifying the net effect of re-emission
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in the Arctic is clearly an important goal in understanding
the fate of the deposited Hg.
Since several lines of evidence support the hypothesis
that elevated Hg levels exist in both abiotic and biotic pools
in areas that are characterized by enhanced levels of BrO, an
additional question arises: What will be the severity and extent
of mercury depletion/oxidation events in the future? It is
important to understand how the global mercury cycle will
be affected by changes within the Arctic, as well as changes
in atmospheric transport, and future and ongoing domestic/
worldwide Hg emission reductions. Since a recent modeling
study concluded that the concentrations of Hg in the Arctic
atmosphere were indistinguishable from the global background (50), changes in physical climate might actually have
a greater impact on the arctic Hg cycle than changes in global
emissions.
Multi-year ice thickness in the central arctic ocean, as
measured by U.S. Navy submarines over the last two decades,
has shown a remarkable 43% reduction in thickness (64). At
this rate, the Arctic Ocean may become seasonally free of sea
ice within 30-40 years. If this occurs, it will effectively double
annual ice coverage, thereby doubling the total area affected
by mercury depletion/oxidation and enhanced deposition.
One likely scenario is that climate-driven reductions in multiyear ice coverage in favor of increased annual ice coverage
throughout the Arctic will increase marine primary productivity (including ice algal communities). This scenario would
result in production and release of more photolyzable
bromine into the near-surface air. When these surface
emissions of photolyzable bromine encounter an airmass
containing Hg0 emissions from southern latitudes under
sunlight conditions, mercury depletion/deposition events
will occur. The springtime mercury deposition rates in the
Arctic could therefore be related (in the simplest sense) to
a function of the spatial coverage of annual sea ice, the airmass
transport of mercury emissions to this region, and local
airmass circulation. These phenomena are, in turn, controlled
by average spring and summertime temperatures (as a
surrogate for melting multi-year ice), by multi-year ice
coverage (which is decreasing; 65), by synoptic activity
(increasing), and by variations in the position of the polar
front (56, 66). There is a clear need for increased research on
all these phenomena, especially over the Arctic Ocean, to
determine if mercury depletion/oxidation events in the Arctic,
and possibly also in the free troposphere at mid-latitudes
(48, 67), could play an ever increasing role as an important
sink in the global Hg cycle (13, 14).
Acknowledgments
We thank the sponsors of this project for their continued
support [NOAA Office of Arctic Research, and the U.S. EPA
Offices of International Affairs (methods speciation work at
ORNL and Barrow) and Research and Development (BAMS2001 aircraft and Hg-p sampling)]; Dan Endres, Malcolm
Gaylord, and Glen McConville for local support at CMDL; F.
Schaedlich for extended help with Tekran equipment; S.
Oltmans for Barrow ozone data; and Lala Chambers, George
Southworth, and Mary Anna Bogle for snow analyses and
data management. GOME spectra have been supplied to A.R.
by the ESA through the German Aerospace Centre (DFD/
DLR Oberpfaffenhofen Germany). The REA system was
developed under the supervision of H. Skov of NERI-DK and
T. Meyers of NOAA. K.J.S. was supported by a scholarship
from the Arctic Institute of North America and by a fellowship
from the University of Manitoba. M.G. is supported by the
Danish Research Agency and the Department of Atmospheric
Environment, NERI-DK. This is Publication No. Hg-72 of
the ORNL Hg Group.
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Received for review August 15, 2001. Revised manuscript
received December 19, 2001. Accepted December 20, 2001.
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