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

by J. Aschmann, B. M. Sinnhuber
Atmospheric Chemistry and Physics ()

Abstract

Very short-lived substances (VSLS) still repre- sent a major factor of uncertainty in the quantification of stratospheric bromine loading. One of the major obstacles for short-lived source gases in contributing to the stratosphere is generally thought to be loss of inorganic bromine (Bry) in the tropical tropopause layer (TTL) due to dehydration. We use sensitivity calculations with a three-dimensional chemistry transport model comprising a consistent parametrization of convective transport and a comprehensive chemistry scheme to investigate the associated processes. The model considers the two most important bromine VSLS, bromoform (CHBr3) and dibromomethane (CH2Br2). The organic bromine source gases as well as the resulting profile of inorganic bromine in the model are consistent with available observations. In contrast to its organic precursors, Bry is assumed to have a significant sorption capacity regarding sedimenting liquid or frozen particles thus the fraction of intact source gases during their ascent through the TTL is a critical factor.We find that source gas injection is the dominant pathway into the strato- sphere, about 50% of CHBr3 and 94% of CH2Br2 is able to overcome the cold point tropopause at approximately 17km altitude, modulated by the interannual variability of the ver- tical transport efficiency. In fact, our sensitivity calculations indicate that the extent of source gas injection of CHBr3 is highly sensitive to the strength of convection and large-scale ascent; in contrast, modifying the photolysis or the destruc- tion via OH yields a significantly smaller response. In prin- ciple, the same applies as well to CH2Br2, though it is con- siderably less responsive due to its longer lifetime. The next important aspect we identified is that the partitioning of availble Bry from short-lived sources is clearly shifted away from HBr, according to our current state of knowledge the only member of the Bry family which is efficiently adsorbed on Earth System ice particles. This effect is caused by very efficient hetero- Dynamics geneous reactions on ice surfaces which reduce the HBr/Bry Open Access fraction below 15% at the tropical tropopause. Under these circumstances there is no significant loss of Bry due to dehy- dration in the model, VSLS contribute fully to stratospheric Geoscientific bromine. In addition, we conduct several sensitivity calcu- Instrumentation lations to test the robustness of this result. If heterogeneous Methods and chemistry is ignored, the HBr/Bry fraction exceeds 50%and Data Systems Open Access about 10% of bromine from VSLS is scavenged. Dehydra- tion plays a minor role for Bry removal under the assumption that HOBr is efficiently adsorbed on ice as well since the het- erogeneous reactions alter the partitioning equilibrium of Bry Geoscientific in favor of HOBr. In this case, up to 12% of bromine from Model Development VSLS is removed. Even in the extreme and unrealistic case Open Access that adsorbed species on ice particles are instantaneously re- moved the maximum loss of bromine does not exceed 25 %. Assuming 6 parts per trillion by volume (pptv) of bromine Hydrology and short-lived source gases in convective updrafts, a value that Earth System is supported by observational data, we find a most likely con- tribution of VSLS to stratospheric bromine in the range of Sciences Open Access 4.5–6 pptv.

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