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

Fate of Mercury in the Arctic 99
Similarly, the product cannot be Hg2Br2 since this would imply a tri-molecular reaction, which
statistically is highly unlikely to occur in the atmosphere due to the very low concentrations of
elemental mercury and bromine atoms. The combination of Hg and Nr atoms and the dimerization
of HgBr is discussed in Grieg et al., (1970).
Nor would the product be HgO, since this formation is similarly thermodynamically not
favourable.
The final product is the divalent gaseous mercury unknown, HgXY, proposed to be HgBr2.
By modelling the reaction of Hg and Br in the atmosphere, with current rate constant data, and
BrO measurements, i.e., assuming 20 ppt BrO and 2 ppt Br in the atmosphere during a depletion
event, it is calculated 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, and therefore the above reaction
mechanism is plausible as one of the limiting reactions for the oxidation of elemental mercury to
reactive gaseous mercury in the post polar sunrise atmosphere, though it is realized that given the
dynamics of atmospheric chemistry, that the above scheme is based on a simple model with limited
laboratory kinetics data to carry out the calculations and that HgBr may also be reacting with O2 or
many other elements or compounds in the Arctic atmosphere.
RGM concentration measurements
RGM measurements performed in parallel at Station Nord in 2002 show that the concentration
varies from between values below detection limit and up to 75 ng/m 3 , Figure 13., page 71. These
values are comparable to those found at Barrow before the start of AMDE’s and are comparable to
levels measured in the Dr. Lindbergs’ laboratory in Oak Ridge. These values were measured prior