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| Titel |
Global atmospheric model for mercury including oxidation by bromine atoms |
| VerfasserIn |
C. D. Holmes, D. J. Jacob, E. S. Corbitt, J. Mao, X. Yang, R. Talbot, F. Slemr |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 10, no. 24 ; Nr. 10, no. 24 (2010-12-17), S.12037-12057 |
| Datensatznummer |
250008968
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| Publikation (Nr.) |
 copernicus.org/acp-10-12037-2010.pdf |
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| Zusammenfassung |
| Global models of atmospheric mercury generally assume that gas-phase OH and ozone are the main oxidants converting
Hg0 to HgII and thus driving mercury deposition to ecosystems. However, thermodynamic considerations argue against
the importance of these reactions. We demonstrate here the viability of atomic bromine
(Br) as an alternative Hg0 oxidant. We conduct a global 3-D simulation with the
GEOS-Chem model assuming gas-phase Br to be the sole Hg0 oxidant (Hg + Br model)
and compare to the previous version of the model with OH and ozone as the sole
oxidants (Hg + OH/O3 model). We specify global 3-D Br concentration fields based
on our best understanding of tropospheric and stratospheric Br chemistry. In both
the Hg + Br and Hg + OH/O3 models, we add an aqueous photochemical reduction
of HgII in cloud to impose a tropospheric lifetime for mercury of 6.5 months
against deposition, as needed to reconcile observed total gaseous mercury (TGM)
concentrations with current estimates of anthropogenic emissions. This added
reduction would not be necessary in the Hg + Br model if we adjusted the Br
oxidation kinetics downward within their range of uncertainty. We find that
the Hg + Br and Hg + OH/O3 models are equally capable of reproducing the spatial
distribution of TGM and its seasonal cycle at northern mid-latitudes. The Hg + Br
model shows a steeper decline of TGM concentrations from the tropics to southern
mid-latitudes. Only the Hg + Br model can reproduce the springtime depletion and
summer rebound of TGM observed at polar sites; the snowpack component of
GEOS-Chem suggests that 40% of HgII deposited to snow in the Arctic
is transferred to the ocean and land reservoirs, amounting to a net deposition
flux to the Arctic of 60 Mg a−1. Summertime events of depleted Hg0 at
Antarctic sites due to subsidence are much better simulated by the Hg + Br model.
Model comparisons to observed wet deposition fluxes of mercury in the US and Europe
show general consistency. However the Hg + Br model does not capture the summer maximum
over the southeast US because of low subtropical Br concentrations while the Hg + OH/O3
model does. Vertical profiles measured from aircraft show a decline of Hg0 above the
tropopause that can be captured by both the Hg + Br and Hg + OH/O3 models, except in
Arctic spring where the observed decline is much steeper than simulated by either
model; we speculate that oxidation by Cl species might be responsible. The Hg + Br
and Hg + OH/O3 models yield similar global budgets for the cycling of mercury
between the atmosphere and surface reservoirs, but the Hg + Br model results in
a much larger fraction of mercury deposited to the Southern Hemisphere oceans. |
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