ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-9-16811-2009 Bromoform and dibromomethane in the tropics: a 3-D model study of chemistry and transport Hossaini R. 1 Chipperfield M. P. 1 Monge-Sanz B. M. 1 Richards N. A. D. 1 Atlas E. 2 Blake D. R. 3 Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK Rosenstiel School of Marine and Atmospheric Science, University of Miami, USA Department of Chemistry, University of California, Irvine, USA 07 08 2009 9 4 16811 16851 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/9/16811/2009/acpd-9-16811-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/9/16811/2009/acpd-9-16811-2009.pdf

We have developed a detailed chemical scheme for the degradation of the short-lived source gases bromoform (CHBr<sub>3</sub>) and dibromomethane (CH<sub>2</sub>Br<sub>2</sub>) 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 CBr<sub>2</sub>O and CHBrO, but their peak tropospheric abundance relative to the surface 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. <br><br> 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 <i>p</i> levels and vertical winds from analysed divergence overestimates the abundance of CH<sub>2</sub>Br<sub>2</sub>, and to a lesser extent CHBr<sub>3</sub>, although the data is sparse and comparisons are not conclusive. Better agreement in the TTL is obtained in the run using θ levels and vertical motion from diabatic heating rates. Trajectory estimates of residence times in the two model versions confirm the more realistic transport in the θ-level version. In the <i>p</i>-level model even when we switch off convection we still find significant amounts of the SGs considered may reach the cold point; the stratospheric source gas injection is only reduced by ~16% for CHBr<sub>3</sub> and ~2% for CH<sub>2</sub>Br<sub>2</sub> without convection. <br><br> Overall, the relative importance of the SG pathway and the PG pathway for transport of bromine to the stratosphere has been assessed. Assuming a 10-day washout lifetime of Br<sub>y</sub> we find the delivery of total Br from CHBr<sub>3</sub> to be 0.72 pptv with ~53% of this coming from SGI. Similary, for CH<sub>2</sub>Br<sub>2</sub> 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.

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