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

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 Hg 0 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