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| Titel |
Contribution of very short-lived substances to stratospheric bromine loading: uncertainties and constraints |
| VerfasserIn |
J. Aschmann, B.-M. Sinnhuber |
| 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 ; 13, no. 3 ; Nr. 13, no. 3 (2013-02-01), S.1203-1219 |
| Datensatznummer |
250017625
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| Publikation (Nr.) |
 copernicus.org/acp-13-1203-2013.pdf |
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| Zusammenfassung |
| Very short-lived substances (VSLS) still represent a major factor of
uncertainty in the quantification of stratospheric bromine loading. One of
the major obstacles for short-lived source gases in contributing to the
stratosphere is generally thought to be loss of inorganic bromine (Bry)
in the tropical tropopause layer (TTL) due to dehydration. We use
sensitivity calculations with a three-dimensional chemistry transport model
comprising a consistent parametrization of convective transport and a
comprehensive chemistry scheme to investigate the associated processes. The
model considers the two most important bromine VSLS, bromoform (CHBr3) and
dibromomethane (CH2Br2). The organic bromine source gases as well as
the resulting profile of inorganic bromine in the model are consistent with
available observations. In contrast to its organic precursors, Bry
is assumed to have a significant sorption capacity regarding sedimenting
liquid or frozen particles thus the fraction of intact source gases during
their ascent through the TTL is a critical factor. We find that source gas
injection is the dominant pathway into the stratosphere, about 50% of
CHBr3 and 94% of CH2Br2 is able to overcome the cold point
tropopause at approximately 17 km altitude, modulated by the interannual
variability of the vertical transport efficiency. In fact, our sensitivity
calculations indicate that the extent of source gas injection of CHBr3 is
highly sensitive to the strength of convection and large-scale ascent; in
contrast, modifying the photolysis or the destruction via OH yields a
significantly smaller response. In principle, the same applies as well to
CH2Br2, though it is considerably less responsive due to its longer
lifetime. The next important aspect we identified is that the partitioning of
available Bry from short-lived sources is clearly shifted away
from HBr, according to our current state of knowledge the only member of the
Bry family which is efficiently adsorbed on ice particles. This
effect is caused by very efficient heterogeneous reactions on ice surfaces
which reduce the HBr/Bry fraction below 15% at the tropical
tropopause. Under these circumstances there is no significant loss of
Bry due to dehydration in the model, VSLS contribute fully to
stratospheric bromine. In addition, we conduct several sensitivity
calculations to test the robustness of this result. If heterogeneous
chemistry is ignored, the HBr/Bry fraction exceeds 50% and
about 10% of bromine from VSLS is scavenged. Dehydration plays a minor
role for Bry removal under the assumption that HOBr is
efficiently adsorbed on ice as well since the heterogeneous reactions alter
the partitioning equilibrium of Bry in favor of HOBr. In
this case, up to 12% of bromine from VSLS is removed. Even in the extreme
and unrealistic case that adsorbed species on ice particles are
instantaneously removed the maximum loss of bromine does not exceed 25%.
Assuming 6 parts per trillion by volume (pptv) of bromine short-lived source
gases in convective updrafts, a value that is supported by observational
data, we find a most likely contribution of VSLS to stratospheric bromine in
the range of 4.5–6 pptv. |
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