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

the literature of the reactions of Hg o with halogen atoms are also listed. All laboratory results are
obtained using relative rate conditions.
The reactions of halogens with Hg are independent of temperature (29,30) and therefore results of
the various studies should be directly comparable. On the other hand ozone reactions with halogen
atoms are temperature dependent and thus the rate constants obtained here of Hg are calculated at
233 K and 263 K representative for the conditions in Arctic during mercury depletion.
In general the half-life of GEM at Station Nord is 3 to 10 hours during an AMDE. Using rate
constants of the latest study (31) this lifetime corresponds to a concentration of Br or Cl at 1-3 pptv
and 0.1-0.3 pptv, respectively. Laboratories report Cl concentrations in the Arctic during AMDEs
from 0.001 to 0.004 ppt (24,32,33), at least a factor of 25 lower in concentration than needed for the
observed GEM depletion, whereas Br in the ppt level is reported by many authors (24,25,32,33).
This implies that Br most probably is the key species leading to mercury depletion.
Based on the results presented above the most important reactant for GEM removal is Br and the
results in the study indicate a second order reaction rate constant of about 1•10 -12 cm 3 molec -1 sec -1 .
The above result needs confirmation in the laboratory and though the result is a strong indication of
the removal channel for GEM other possibilities needs to be examined before a definite answer can
be given. In particular, it is important to clarify the role of heterogeneous chemistry during
AMDE’s. Furthermore, there is a factor 16 difference in the reaction rate constant of the reaction
between Br and Hg o reported (34,31) and in both cases determined by the relative rate technique.
The difference shows that secondary reactions in the laboratory systems clearly plays a role and
thus the reaction rate constant needs to be determined by an absolute method. To the knowledge of
the authors there is not any study of the reaction rates constant of the reactions between Hg and ClO
or BrO in the literature. Interesting enough there is good agreement between the theoretical
calculated rate constant (27) and the rate constant extracted from the field measurements presented
here. However, that might be a coincidence.
The reaction between Br and Hg o leads to the formation of a radical.
Hg + Br → HgBr •
(9).
The further fate of this radical is at the moment speculative but is the subject for a theoretical study
(27,35).
Modelling
The mercury model has been run for October 1998 to October 2002 and the model results for GEM
(Hg o ) are compared with measurements from Station Nord. The model does a good job reproducing
the occurrence and length of AMDE but the simple parameterisation does not and cannot describe
the fast variations in the period during the spring period, (Fig. 5). However, the main structure is
reproduced and the results present a large step forward in the understanding of the fate of mercury
in the Arctic atmosphere. The reproduction of the main structure is a strong indication for that the
limiting factor is the surface conditions, which has to be sea ice with surface temperature below –4
o C. A comparison of three model runs with observed GEM is shown in Fig. 5. The three model runs
are: 1) without depletion, 2) with depletion where lifetime during depletion is 10 hours, and 3) with
depletion and lifetime on 3 hours. The main difference between the two last runs with depletion is
only a slightly higher minimum level in the case of 10 hour life time. The variations and duration of
episodes are not changed.
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