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

Fate of Mercury in the Arctic 4
Abstract
Using a micrometeorological system, relaxed eddy accumulation, with a heated sampling
system specifically designed for Arctic use, the dry deposition of reactive gaseous mercury (RGM)
is quantified after polar sunrise, in Barrow, Alaska. KCl coated manually analysed annular denuders
were used as the accumulators. At 3 m above the snow pack significant RGM fluxes measured
during March 29 th – April 12 th 2000 were directed toward the snow surface. Overall mean flux was
found to be - 0.4 ± 0.2 pg m -2 s -1 ; N=9, ± SE, where the negative sign convention denotes
deposition. Using measured total RGM concentrations; depositional velocities were then calculated
and found on average to be 1 cm s -1 .
Examining the experimental data from this field campaign. as well as current published field
data and reaction kinetics data, a plausible mechanism for the oxidation of gaseous Hg (0) to the
divalent gaseous form is proposed. The mechanism is consistent with the kinetics, thermodynamics
and field observations, but the final end products are still not known, due to the measurement
method in the field, i.e., reactive gaseous mercury is operationally determined and defined. The
hypothesized mechanism is not the same as the mechanisms otherwise derived, where the depletion
of atmospheric boundary-layer mercury is said to be due to a reaction between gaseous elemental
mercury, GEM, and BrO free radicals or mechanisms resulting in the formulation of HgCl2.
The proposed reaction mechanism is that gaseous elemental mercury, Hg (0) combines with Br
atoms, called X, coming from the polar sunrise destruction of ozone, in a reversible reaction,
forming the energised HgBr*. Through a third body reaction, M, where M is N2 or O2, the HgBr
radical is formed. The HgBr radical can live long enough at the low temperatures of the Arctic to
combine with O2 forming the HgBrOO peroxy radical or can combine with Br forming HgBr2. It is
not likely to react with Cl, since this reaction would be endothermic. Similarly, the product cannot
be Hg2Br2 since this would imply a tri molecular reaction, which is highly unlikely to occur in the