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

[摘要] The contribution of bromoform to the stratospheric bromine loading is estimatedusing the one-dimensional tropical mean model of Folkins and Martin (2005), which isconstrained by observed mean profiles of temperature and humidity. In order toreach the stratosphere, bromoform needs to be lifted by deep convection intothe tropical tropopause layer (TTL), above the level of zero radiative heating.The contribution of bromoform to stratospheric bromine then depends criticallyon the rate of removal of the degradation products of bromoform (collectivelycalled Br_y here) from the TTL, which is believed to be due to scavengingby falling ice. This relates the transport of short-lived bromine species intothe stratosphere to processes of dehydration in the TTL. In the extreme case ofdehydration occurring only through overshooting deep convection, the loss ofBr_y from the TTL may be negligible and consequently bromoform will fullycontribute with its boundary layer mixing ratio to the stratospheric bromineloading, i.e. with 3 pptv for an assumed 1 pptv of bromoform in the boundarylayer. For the other extreme that Br_y is removed from the TTL almostinstantaneously, the model calculations predict a contribution of about0.5 pptv for the assumed 1 pptv of boundary layer bromoform. While this givessome constraints on the contribution of bromoform to stratospheric bromine,a key uncertainty in estimating the contribution of short-lived brominesource gases to the stratospheric bromine loading is the mechanism andrate of removal of Br_y within the TTL.

[学科分类] 大气科学