The toxic chemical mercury is extremely mobile in its gaseous elemental form and is transported globally due to the long atmospheric residence time of Hg0. High concentrations of mercury have been found in the Arctic, but its accumulation at those latitudes cannot be explained by the same depositional processes used to account for the presence of organic toxins in the same areas, because Hg0 does not condense at the ambient atmospheric temperatures. Therefore, some other mechanism must be responsible.
Lindberg et al. measured concentrations of atmospheric mercury at Point Barrow, Alaska (71°N), from 1998 to 2001 and found that rapid photochemical oxidation of boundary-layer Hg0 after polar sunrise causes deposition of oxidized gaseous mercury. Depletion of atmospheric Hg0 began within a few days of polar sunrise and continued until snowmelt, and was correlated with variations in ozone concentration during these months. They attribute this behavior to rapid in-situ oxidation of gaseous Hg0 by the same photochemically active halogen species involved in surface O3 destruction: the halogen radicals Br and Cl and the halogen oxide radicals BrO and ClO. Studies of bioaccumulation of Hg in seabirds and marine mammals suggest that this depositional process has operated only for the past few decades and could be the result of changes in Arctic climate that have increased atmospheric transport of photo-oxidants and local production of reactive halogens. — HJS
Environ. Sci. Technol.36, 1245 (2002).