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

Introduction
The perennial oxidation of mercury in the Arctic, which occurs simultaneously with the
post solar sunrise destruction of ozone (1), potentially doubles the loading of mercury to the Arctic
(2). These atmospheric mercury depletion episodes, AMDEs, were discovered in 1995 at Alert in
the Canadian Arctic (1), and have since been observed at circum-Arctic locations, the Antarctic,
and sub-polar locations near sea ice (3 and citations therein). Tarasick and Bottenheim (4) have
noted that the frequency of occurrence of boundary-layer ozone depletion episodes has increased
since the 1960’s, particularly at Resolute in the Canadian Arctic (the only site with a sufficiently
long record for proper trend analysis). Those authors postulate that this increase could have arisen
from of an increase in open leads in the Arctic ice cover, possibly because of climate change
induced by increasing levels of greenhouse gases. Furthermore, the increase in frequency of ozone
depletion events may explain the increase in mercury levels observed in Arctic biota over the last
few decades (4).
The current knowledge of AMDEs is summarised in (3). It is clearly important to
understand the detailed mechanism of mercury oxidation, so that transport and deposition models
can be properly parameterised. Mercury exists in the atmosphere primarily in gaseous elemental
form, Hg 0 , which has an atmospheric residence time of approximately 1 year, allowing it to be
globally transported (5,6,7). Hg 0 can thus be transported to the polar regions, where it is removed
from the boundary layer during an MADE with an e-folding lifetime of less than 10 hrs, being
converted to an inorganic oxidised gaseous mercury compound HgXY (2,3). This compound,
which is commonly referred to as reactive gaseous mercury, RGM, is operationally determined in
the Arctic. The common method for measuring RGM is by collection onto a KCl-coated annular
denuder, followed by the pyrolytic reduction of the captured RGM to Hg 0 (8). This technique
results in information about the composition of the HgXY family being lost. A number of
environmental conditions favourable for AMDEs at high latitudes have been identified. These
2