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Very short-lived substances (VSLS) still represent a major factor ofuncertainty in the quantification of stratospheric bromine loading. One ofthe major obstacles for short-lived source gases in contributing to thestratosphere is generally thought to be loss of inorganic bromine (Bry) in the tropical tropopause layer (TTL) due to dehydration. We usesensitivity calculations with a three-dimensional chemistry transport modelcomprising a consistent parametrization of convective transport and acomprehensive chemistry scheme to investigate the associated processes. Themodel considers the two most important bromine VSLS, bromoform (CHBr3) anddibromomethane (CH2Br2). The organic bromine source gases as well asthe resulting profile of inorganic bromine in the model are consistent withavailable observations. In contrast to its organic precursors, Bryis assumed to have a significant sorption capacity regarding sedimentingliquid or frozen particles thus the fraction of intact source gases duringtheir ascent through the TTL is a critical factor. We find that source gasinjection is the dominant pathway into the stratosphere, about 50% ofCHBr3 and 94% of CH2Br2 is able to overcome the cold pointtropopause at approximately 17 km altitude, modulated by the interannualvariability of the vertical transport efficiency. In fact, our sensitivitycalculations indicate that the extent of source gas injection of CHBr3 ishighly sensitive to the strength of convection and large-scale ascent; incontrast, modifying the photolysis or the destruction via OH yields asignificantly smaller response. In principle, the same applies as well toCH2Br2, though it is considerably less responsive due to its longerlifetime. The next important aspect we identified is that the partitioning ofavailable Bry from short-lived sources is clearly shifted awayfrom HBr, according to our current state of knowledge the only member of theBry family which is efficiently adsorbed on ice particles. Thiseffect is caused by very efficient heterogeneous reactions on ice surfaceswhich reduce the HBr/Bry fraction below 15% at the tropicaltropopause. Under these circumstances there is no significant loss ofBry due to dehydration in the model, VSLS contribute fully tostratospheric bromine. In addition, we conduct several sensitivitycalculations to test the robustness of this result. If heterogeneouschemistry is ignored, the HBr/Bry fraction exceeds 50% andabout 10% of bromine from VSLS is scavenged. Dehydration plays a minorrole for Bry removal under the assumption that HOBr isefficiently adsorbed on ice as well since the heterogeneous reactions alterthe partitioning equilibrium of Bry in favor of HOBr. Inthis case, up to 12% of bromine from VSLS is removed. Even in the extremeand unrealistic case that adsorbed species on ice particles areinstantaneously removed the maximum loss of bromine does not exceed 25%.Assuming 6 parts per trillion by volume (pptv) of bromine short-lived sourcegases in convective updrafts, a value that is supported by observationaldata, we find a most likely contribution of VSLS to stratospheric bromine inthe range of 4.5–6 pptv

Contribution of very short-lived substances to stratospheric bromine loading: uncertainties and constraints