FATE OF MERCURY IN THE ARCTIC Michael Evan ... - COGCI

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 Hg 0 (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 Hg 0 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) (r 2 ) 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 Hg 0 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 Hg 0 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 9 1253