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| Titel |
What do we learn about bromoform transport and chemistry in deep convection from fine scale modelling? |
| VerfasserIn |
V. Marecal, M. Pirre, G. Krysztofiak, P. D. Hamer, B. Josse |
| 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 ; 12, no. 14 ; Nr. 12, no. 14 (2012-07-16), S.6073-6093 |
| Datensatznummer |
250011314
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| Publikation (Nr.) |
 copernicus.org/acp-12-6073-2012.pdf |
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| Zusammenfassung |
| Bromoform is one of the most abundant halogenated Very Short-Lived
Substances (VSLS) that possibly contributes, when degradated, to the
inorganic halogen loading in the stratosphere. In this paper we present a
detailed modelling study of the transport and the photochemical degradation
of bromoform and its product gases (PGs) in a tropical convective cloud. The
aim was to explore the transport and chemistry of bromoform under idealised
conditions at the cloud scale. We used a 3-D cloud-resolving model coupled
with a chemistry model including gaseous and aqueous chemistry. In
particular, our model features explicit partitioning of the PGs between the
gas phase and the aqueous phase based on newly calculated Henry's law
coefficients using theoretical methods. We ran idealised simulations for up
to 10 days that were initialised using a tropical radiosounding of
atmospheric conditions and using outputs from a global chemistry-transport
model for chemical species. Two simulations were run with stable atmospheric
conditions with a bromoform initial mixing ratio of 40 pptv (part per
trillion by volume) and 1.6 pptv up to 1 km altitude. The first simulation
corresponds to high bromoform mixing ratios that are representative of real
values found near strong localised sources (e.g. tropical coastal margins)
and the second to the global tropical mean mixing ratio from observations.
Both of these simulations show that the sum of bromoform and its PGs
significantly decreases with time because of dry deposition, and that PGs
are mainly in the form of HBr after 2 days of simulation. Two further
simulations are conducted; these are similar to the first two simulations
but include perturbations of temperature and moisture leading to the
development of a convective cloud reaching the tropical tropopause layer
(TTL). Results of these simulations show an efficient vertical transport of
the bromoform from the boundary layer to the upper troposphere and the TTL.
The bromoform mixing ratio in the TTL is up to 45% of the initial
boundary layer mixing ratio. The most abundant organic PGs, which are not
very soluble, are also uplifted efficiently in both simulations featuring
the convective perturbation. The inorganic PGs are more abundant than the
organic PGs, and their mixing ratios in the upper troposphere and in the TTL
depend on the partitioning between inorganic soluble and insoluble species
in the convective cloud. Important soluble species such as HBr and HOBr are
efficiently scavenged by rain. This removal of Bry by rain is reduced
by the release of Br2 (relatively insoluble) to the gas phase due to
aqueous chemistry processes in the cloud droplets. The formation of Br2
in the aqueous phase and its subsequent release to the gas phase makes a non
negligible contribution to the high altitude bromine budget in the case of
the large bromoform (40 pptv) initial mixing ratios. In this specific, yet
realistic case, this Br2 production process is important for the PG
budget in the upper troposphere and in the TTL above convective systems.
This process is favoured by acidic conditions in the cloud droplets, i.e.
polluted conditions. In the case of low bromoform initial mixing ratios,
which are more representative of the mean distribution in the tropics, this
Br2 production process is shown to be less important. These conclusions
could nevertheless be revisited if the knowledge of chlorine and bromine
chemistry in the cloud droplets was improved in the future. |
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