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

Fate of Mercury in the Arctic 23
The tropospheric depletion of ozone in the Arctic has been observed to be depleted during
spring since, at least, 1966 (Tarasick and Bottenheim, 2002). It is generally accepted that the
destruction of ozone is caused by catalytic reactions involving halogens, especially bromine, which
originate from the polar oceans (Barrie et al., 1988 and Barrie and Platt, 1997). Our measurements
suggest that the solar activity and present ice crystals influence the atmospheric transformation of
elemental gaseous mercury to divalent mercury, which is more rapidly deposited (Lindberg et al.
2002, paper 2, Appendix C). Satellite observations of BrO (as discussed in paper 2) suggest a
correlation between mercury depletion and the total amount of BrO in the atmosphere, though later
work shows the previously suggested mechanism to be more plausible, with the mechanisms
presented in Lindberg et al., 2002 being thermodynamically unfavourable i.e. it is not likely that
BrO/ClO + Hg 0 → HgO + Br/Cl radicals. Or that Hg 0 + 2Br/Cl radicals → HgBr2 or HgCl2.
The most likely source of the halogens is the sea ice. Mercury depletion events therefore appear
correlated with the presence of sea ice and are therefore probably found wherever there is sea ice
and the potential for open leads. Today, typical levels of gaseous elemental mercury in the remote
Northern hemisphere atmosphere and Arctic are about 1.5 ng m -3 (see paper 2). This level is a little
bit less in the Southern Hemisphere, since most of the anthropogenic sources of gaseous mercury,
are found in the Northern Hemisphere, and there is a lag time in transequatorial mixing.
How long have mercury depletion events been occurring? Tarasick and Bottenheim (2002)
have looked at surface ozone depletion events in the Arctic and Antarctic from historical ozone
sonde records, and have noted ozone depletions since 1966, with an increasing frequency through
2000 “explaining the apparent increase of Hg in Arctic biota in recent times”. They further note that
“If the key to GEM depletion is the presence of reactive bromine, which can only be sustained via
BrO recycling and hence ozone depletion, the increase of mercury in Arctic biota is a direct result
of the increase in occurrence of ozone depletion episodes. “ Work accomplished by the present