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| Titel |
The potential importance of frost flowers, recycling on snow, and open leads for ozone depletion events |
| VerfasserIn |
M. Piot, R. Glasow |
| 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 ; 8, no. 9 ; Nr. 8, no. 9 (2008-05-07), S.2437-2467 |
| Datensatznummer |
250006101
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| Publikation (Nr.) |
 copernicus.org/acp-8-2437-2008.pdf |
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| Zusammenfassung |
| We present model studies with the one-dimensional model
MISTRA to investigate the potential role of frost flowers, recycling on snow, and open leads
in the depletion of
tropospheric ozone in the Arctic spring. In our model, we assumed frost flower
aerosols to be the major source of bromine. We show that a major ozone
depletion event can be satisfactorily reproduced only if the recycling on snow of
deposited bromine into gas phase bromine is assumed. In the model, this cycling is more
efficient than the bromine explosion process and
maintains sufficiently high levels of bromine to deplete ozone down to few
nmol mol−1 within four days. We assessed the influence of different
surface combinations (open lead/frost flowers) on the chemistry in the
model. Results showed noticeable
modifications affecting the composition of aerosols and the deposition
velocities. A model run with a series of coupled frost flower fields and open leads,
separated by large areas of snow, showed results comparable with field
observations. In addition, we studied the effects of modified temperature of either the
frost flower field or the ambient airmass. A warmer frost flower field increases the
relative humidity and the aerosol deposition rate. The deposition/re-emission
process gains in importance, inducing more reactive bromine in the gas phase, and a stronger ozone
depletion. A decrease of 1K in airmass temperature shows in our model that the aerosol
uptake capacities of all gas phase species substantially increases, leading to enhanced uptake of acids from
the gas phase. Consequently, the so-called bromine explosion accelerated and
O3 mixing ratios decreased.
In our model representation, variations in wind speed affected the aerosol
source function and influenced the amount of bromine in the atmosphere and
thus the ozone depletion strength.
Recent studies have suggested the important role of the
precipitation of calcium carbonate (CaCO3) out of the brine layer for the possible acidification of the liquid phase by acid uptake. Our investigation showed that this
precipitation is a crucial process for the timing of the bromine
explosion in aerosols. Nevertheless, model runs with either 50% precipitation
or complete precipitation displayed a relatively weak difference in
ozone mixing ratios after four simulated days. By considering conditions
typical for "Arctic Haze" pollution events at the start
of the run we obtained a low pH in frost flower aerosols due to a greater
mixing ratio of SO2, and a strong recycling efficiency via large aerosol number
concentration. The aerosol acidification during a haze event most likely
intensifies the ozone depletion strength and occurrence. The comparison
between our modeled deposition on snow and sampled snow at Barrow (Alaska) shows
that approximately 75% of deposited bromine may be re-emitted into
the gas phase as Br2/BrCl. Among several non-halogen fluxes from
the snow, model simulations showed that only HONO
affects the chemistry. Finally, we investigated the release of Br2 potentially produced by
heterogeneous reactions directly on frost flowers. In this case, we obtained
unrealistic results of aerosol compositions and deposition rates on snow
compared to observations in the Arctic. |
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