Estimating the contribution of bromoform to stratospheric bromine and its relation to dehydration in the tropical tropopause layer B.-M. Sinnhuber1 and I. Folkins2 1Institute of Environmental Physics, University of Bremen, Bremen, Germany 2Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada
Abstract. The contribution of bromoform to the stratospheric bromine loading is estimated
using the one-dimensional tropical mean model of Folkins and Martin (2005), which is
constrained by observed mean profiles of temperature and humidity. In order to
reach the stratosphere, bromoform needs to be lifted by deep convection into
the tropical tropopause layer (TTL), above the level of zero radiative heating.
The contribution of bromoform to stratospheric bromine then depends critically
on the rate of removal of the degradation products of bromoform (collectively
called Bry here) from the TTL, which is believed to be due to scavenging
by falling ice. This relates the transport of short-lived bromine species into
the stratosphere to processes of dehydration in the TTL. In the extreme case of
dehydration occurring only through overshooting deep convection, the loss of
Bry from the TTL may be negligible and consequently bromoform will fully
contribute with its boundary layer mixing ratio to the stratospheric bromine
loading, i.e. with 3 pptv for an assumed 1 pptv of bromoform in the boundary
layer. For the other extreme that Bry is removed from the TTL almost
instantaneously, the model calculations predict a contribution of about
0.5 pptv for the assumed 1 pptv of boundary layer bromoform. While this gives
some constraints on the contribution of bromoform to stratospheric bromine,
a key uncertainty in estimating the contribution of short-lived bromine
source gases to the stratospheric bromine loading is the mechanism and
rate of removal of Bry within the TTL.
Citation: Sinnhuber, B.-M. and Folkins, I.: Estimating the contribution of bromoform to stratospheric bromine and its relation to dehydration in the tropical tropopause layer, Atmos. Chem. Phys., 6, 4755-4761, doi:10.5194/acp-6-4755-2006, 2006.