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Estimating the contribution of bromoform to stratospheric bromine and its relation to dehydration in the tropical tropopause layer

by B.-M. Sinnhuber, I. Folkins
Atmospheric Chemistry and Physics Discussions ()

Abstract

The contribution of bromoform to the stratospheric bromine loading\nis estimated using the one-dimensional tropical mean model of Folkins\nand Martin ( 2005), which is constrained by observed mean profiles\nof temperature and humidity. In order to reach the stratosphere,\nbromoform needs to be lifted by deep convection into the tropical\ntropopause layer (TTL), above the level of zero radiative heating.\nThe contribution of bromoform to stratospheric bromine then depends\ncritically on the rate of removal of the degradation products of\nbromoform ( collectively called Bry here) from the TTL, which is\nbelieved to be due to scavenging by falling ice. This relates the\ntransport of short-lived bromine species into the stratosphere to\nprocesses of dehydration in the TTL. In the extreme case of dehydration\noccurring only through overshooting deep convection, the loss of\nBry from the TTL may be negligible and consequently bromoform will\nfully contribute with its boundary layer mixing ratio to the stratospheric\nbromine loading, i.e. with 3 pptv for an assumed 1 pptv of bromoform\nin the boundary layer. For the other extreme that Bry is removed\nfrom the TTL almost instantaneously, the model calculations predict\na contribution of about 0.5 pptv for the assumed 1 pptv of boundary\nlayer bromoform. While this gives some constraints on the contribution\nof bromoform to stratospheric bromine, a key uncertainty in estimating\nthe contribution of short-lived bromine source gases to the stratospheric\nbromine loading is the mechanism and rate of removal of Bry within\nthe TTL.

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