A one dimensional model study of the mechanism of halogen liberation and vertical transport in the polar troposphere E. Lehrer1, G. Hönninger1,*, and U. Platt1 1Institut für Umweltphysik, Universität Heidelberg, Heidelberg, Germany *now at: Meteorological Service of Canada, Toronto, Canada
Abstract. Sudden depletions of tropospheric ozone during spring were reported from the
Arctic and also from Antarctic coastal sites. Field studies showed that
those depletion events are caused by reactive halogen species, especially
bromine compounds. However the source and seasonal variation of reactive
halogen species is still not completely understood. There are several
indications that the halogen mobilisation from the sea ice surface of the
polar oceans may be the most important source for the necessary halogens.
Here we present a one dimensional model study aimed at determining the
primary source of reactive halogens. The model includes gas phase and
heterogeneous bromine and chlorine chemistry as well as vertical transport
between the surface and the top of the boundary layer. The autocatalytic Br
release by photochemical processes (bromine explosion) and subsequent rapid
bromine catalysed ozone depletion is well reproduced in the model and the
major source of reactive bromine appears to be the sea ice surface. The sea
salt aerosol alone is not sufficient to yield the high levels of reactive
bromine in the gas phase necessary for fast ozone depletion. However, the
aerosol efficiently "recycles" less reactive bromine species (e.g. HBr) and
feeds them back into the ozone destruction cycle. Isolation of the boundary
layer air from the free troposphere by a strong temperature inversion was
found to be critical for boundary layer ozone depletion to happen. The
combination of strong surface inversions and presence of sunlight occurs
only during polar spring.
Citation: Lehrer, E., Hönninger, G., and Platt, U.: A one dimensional model study of the mechanism of halogen liberation and vertical transport in the polar troposphere, Atmos. Chem. Phys., 4, 2427-2440, doi:10.5194/acp-4-2427-2004, 2004.