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Impact of deep convection and dehydration on bromine loading in the upper troposphere and lower stratosphere

by J. Aschmann, B. M. Sinnhuber, M. P. Chipperfield, R. Hossaini
Atmospheric Chemistry and Physics ()
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Stratospheric bromine loading due to very short-lived substances is investigated with a three-dimensional chemical transport model over a period of 21 years us-ing meteorological input data from the European Centre for Medium-Range Weather Forecasts ERA-Interim reanal-ysis from 1989 to the end of 2009. Within this framework we analyze the impact of dehydration and deep convection on the amount of stratospheric bromine using an idealized and a detailed full chemistry approach. We model the two most important brominated short-lived substances, bromo-form (CHBr 3) and dibromomethane (CH 2 Br 2), assuming a uniform convective detrainment mixing ratio of 1 part per trillion by volume (pptv) for both species. The contribu-tion of very short-lived substances to stratospheric bromine varies drastically with the applied dehydration mechanism and the associated scavenging of soluble species ranging from 3.4 pptv in the idealized setup up to 5 pptv using the full chemistry scheme. In the latter case virtually the entire amount of bromine originating from very short-lived source gases is able to reach the stratosphere thus rendering the im-pact of dehydration and scavenging on inorganic bromine in the tropopause insignificant. Furthermore, our long-term cal-culations show that the mixing ratios of very short-lived sub-stances are strongly correlated to convective activity, i.e. in-tensified convection leads to higher amounts of very short-lived substances in the upper troposphere/lower stratosphere especially under extreme conditions like El Niño seasons. However, this does not apply to the inorganic brominated product gases whose concentrations are anti-correlated to convective activity mainly due to convective dilution and possible scavenging, depending on the applied approach.

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