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

In the model the removal of GEM leads to build-up of RGM as observed in the field (6). The
calculated concentrations of RGM have been compared with measured integrated surface column of
BrO as obtained from the GOME satellite (36). In Fig. 6 the mean BrO column near the surface (in
the boundary layer) and RGM concentrations for each month are shown for the period of January to
June 2000. The figures show clearly that BrO and RGM have the same general temporal and
geographical variability and reach their maximum level and extension in April to May. This finding
supports that the conversion of GEM in fact is connected to sea-ice with temperatures below –4 o C
and to the chemistry of Br. Notwithstanding there is general good agreement between RGM and
BrO, some clear discrepancies can be observed. In May the largest BrO concentrations are found
along the coast of the Beaufort Sea (North of Canada and Alaska), whereas maximum RGM
concentrations are predicted North of Greenland. At present there is no explanation for this
observation but most probably it reflects the rough assumption that RGM is produced ubiquitously
above surfaces with temperatures below –4 o C. Bromine is most probably formed on the surface of
re-freezing leads (6). These leads form and disappear again more or less randomly around the Arctic
Ocean depending on the oceanic currents, wind, temperature and solar flux. Therefore large
variation in the concentrations of bromine is expected during spring and thus also in the removal of
GEM and build up of RGM concentration. This feature is in fact clearly seen in the measurements
of GEM, Fig. 2 and 5; and it explains the discrepancy between the model results giving a smooth
depletion event extending for the whole depletion period, whereas measurements show a long series
of shorter depletion episodes during the depletion period.
The deposition of atmospheric mercury calculated with DEHM is shown in Fig. 7 for some selected
locations in the Arctic: Station Nord (Greenland), Barrow (Alaska), Alert (Canada), Thule
(Greenland), Spitzbergen (Norway), in the sub-Arctic, Nuuk area (south Greenland), and on Faeroe
Islands and Denmark. The total deposition is divided into three components: the contribution from
deposition of; RGM, photo-chemically formed TPM, and directly emitted TPM. It is clearly seen
how important the depletion phenomenon is for the deposition of mercury in the high Arctic,
whereas it has practically no importance in the Faroe Islands. However, the Nuuk area appears to be
influenced by AMDEs and there are on going field activities to confirm experimentally if AMDEs
extends to this sub-arctic area. The importance of AMDE for the mercury load is seen for example
for the Thule area, where the total deposition of mercury is increased by a factor 3, while for the
Faeroe Islands the depletion phenomena only lead to a 10% increase of the mercury deposition. The
contribution from directly emitted particulate mercury is very small in the high Arctic. It is mainly
the large atmospheric reservoir of elemental mercury, which contributes through its chemical
conversion to RGM followed by fast deposition of RGM. For all places close to the sea the total
deposition is at the same levels. However, the large contribution of RGM deposition in the Arctic
occurs only over a 4 month period from March to June where the algae bloom occurs (9). This fact
might lead to a higher uptake of mercury in the food chain than would be expected if one simply
extrapolated data from mid latitudes to the Arctic.
The deposition of mercury for 1999 and 2000 is shown in Fig. 8 for the Northern Hemisphere with
and without AMDE’s. The largest depositions are found close to the sources in Asia, Europe and
North America mainly due to the deposition of primarily emitted RGM and TPM that is removed
fast mainly due to dry deposition and washout by rain. The calculations with and without AMDE’s
show again the importance of AMDE in the Arctic for the total deposition of mercury. Here the
photochemically formed RGM is removed mainly by dry deposition as the Arctic is characterized
by its very dry climate. The total annual deposition increases in the whole Arctic, and for the area
north of the Polar Circle the total deposition of mercury increases from 89 to 208 tons/year due to
the depletion.
7