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| Titel |
Transport and Chemistry of Short-Lived Bromocarbons in the Tropics |
| VerfasserIn |
Ryan Hossaini, Martyn Chipperfield, Beatriz Monge-Sanz, Nigel Richards, Elliot Atlas, Donald Blake |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042892
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| Zusammenfassung |
| We have developed a detailed chemical scheme for the degradation of the short-lived source
gases bromoform (CHBr3) and dibromomethane (CH2Br2) and implemented it in the
TOMCAT/SLIMCAT three-dimensional (3D) chemical transport model (CTM). The CTM
has been used to predict the distribution of the two source gases (SGs) and 11 of their organic
product gases (PGs). These first global calculations of the organic PGs show that their
abundance is small. The longest lived organic PGs are CBr2O and CHBrO, but their peak
tropospheric abundance relative to the surface volume mixing ratio (vmr) of the SGs is less
than 5%. We calculate their mean local tropospheric lifetimes in the tropics to be ~7 and
~2 days (due to photolysis), respectively. Therefore, the assumption in previous
modelling studies that SG degradation leads immediately to inorganic bromine seems
reasonable.
We have compared observed tropical SG profiles from a number of aircraft campaigns
with various model experiments. In the tropical tropopause layer (TTL) we find that the
CTM run using p levels (TOMCAT) and vertical winds from analysed divergence
overestimates the abundance of CH2Br2, and to a lesser extent CHBr3, although the
data is sparse and comparisons are not conclusive. Better agreement in the TTL is
obtained in the sensitivity run using θ levels (SLIMCAT) and vertical motion from
diabatic heating rates. Trajectory estimates of residence times in the two model
versions show slower vertical transport in the SLIMCAT θ-level version. In the p-level
model even when we switch off convection we still find significant amounts of the
SGs considered may reach the cold point tropopause; the stratospheric source gas
injection (SGI) is only reduced by ~16% for CHBr3 and ~2% for CH2Br2 without
convection.
Overall, the relative importance of the SG pathway and the PG pathway for transport of
bromine to the stratospheric overworld (θ>380 K) has been assessed. Assuming a 10-day
washout lifetime of Bry in TOMCAT, we find the delivery of total Br from CHBr3 to be 0.72
pptv with ~53% of this coming from SGI. Similary, for CH2Br2 we find a total Br value of
1.69 pptv with ~94% coming from SGI. We infer that these species contribute ~2.4
pptv of inorganic bromine to the lower stratosphere with SGI being the dominant
pathway. Slower transport to and through the TTL would decrease this estimate. |
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