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

Fate of Mercury in the Arctic 14
After polar sunrise, in Barrow, Alaska, KCl coated manual RGM denuders were used as the
accumulator with the REA system. 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
deposition was found to be - 0.4 +/- 0.2 pg m -2 s -1 ; N=9, +/1 SE and re-evasion was also observed.
Using measured total RGM concentrations; depositional velocities were then computed and found
to be on the order of 1 cm s -1 . Upon closer examination of field data, a relative rate study (paper 3,
Skov et al., submitted) and utilizing knowledge from recently published reaction kinetics (Ariya et
al., 2002), an updated mechanism is suggested (paper 4, Goodsite et al., submitted).
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
atmosphere given the very low concentrations of the reactants. Nor would the end product be HgO,
since this formation is similarly thermodynamically not favourable. The final product is the divalent
gaseous mercury unknown, HgXY. By modelling the reaction of Hg and Br in the atmosphere, with
current reaction constant data, and BrO and Br measurements, we find that assuming 20 ppt BrO
and 2 ppt in the atmosphere during a depletion event that the lifetime of Hg is 4.6 hrs against
forming HgBr, The lifetime of HgBr is 0.35 hrs. against forming HgBr2; comparing with the
lifetime of HgBr of 0.75 hrs means that 68% of the time HgBr will form HgBr2. Thus the overall
lifetime of removal of Hg to HgBr2 is 4.6 hrs. / .68 = 6.7 hrs. This is in good agreement with the
observed 10 hr lifetime of Hg under depletion.