Bromoform and dibromomethane in the tropics: a 3-D model study of chemistry and transport 1Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, UK
2Rosenstiel School of Marine and Atmospheric Science, University of Miami, USA
3Department of Chemistry, University of California, Irvine, USA
Received: 13 Jul 2009 – Published in Atmos. Chem. Phys. Discuss.: 07 Aug 2009Abstract. 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
(3-D) 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.
Revised: 19 Dec 2009 – Accepted: 30 Dec 2009 – Published: 25 Jan 2010
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.
Citation: Hossaini, R., Chipperfield, M. P., Monge-Sanz, B. M., Richards, N. A. D., Atlas, E., and Blake, D. R.: Bromoform and dibromomethane in the tropics: a 3-D model study of chemistry and transport, Atmos. Chem. Phys., 10, 719-735, doi:10.5194/acp-10-719-2010, 2010.