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102 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING
2.6.4 Impact of reactive bromine chemistry in the troposphere
Participating scientists Roland von Glasow, R. von Kuhlmann (MPI-C), M. G. Lawrence (MPI-
C), U. Platt, and P. J. Crutzen (MPI-C, SIO)
Abstract The potential impact of 0.5 - 2 pmol mol −1 BrO on the photochemistry in the free
troposphere was investigated with the help of a global three-dimensional transport model. Annual
zonal mean ozone mixing ratios are reduced by up to 20 % pointing to a potentially very significant
ozone loss process that has been ignored so far.
Figure 2.58: Annually and zonally averaged ratio of O3 in a model run with bromine chemistry to O3
in a run without bromine. The ordinate is the σ-level multiplied by 1000 which is approximately the
pressure in hPa and the abscissa is latitude in degrees.
Background Satellite observations suggest the
widespread presence of BrO in the troposphere
also outside the polar regions with global background
vertical columns of about 1-3 × 10 13
molec cm −2 , corresponding to BrO mixing ratios
of 0.5-2 pmol mol −1 if uniformly mixed in the
troposphere. The synopsis of the various measurmenets
indicates that the tropospheric BrO
was mainly located within the free troposphere
(FT). The sources for reactive bromine in the free
troposphere are thought to include the decomposition
of organic bromine compounds, release from
sea salt aerosol and upward transport, downward
transport from the stratosphere, upward transport
from ”ozone depletion events” in the polar
boundary layers during spring (”spillout”), and
release from slowly erupting volcanoes but a quantification
of these processes remains to be made.
Funding DFG: Emmy Noether Junior Research
Group MarHal Gl 353/1-1
Methods and results We used the global atmospheric
chemistry transport model MATCH-
MPIC and prescribed bromine sources to reproduce
the observed BrO vertical columns taking
inorganic gas phase chemistry and recycling on
sulfate aerosol surfaces into account. Our vertical
tropospheric columns are 0.3 - 2.4 x 10 13
molec cm −2 corresponding to BrO mixing ratios
of >0.1 to 2 pmol mol −1 . This amount of reactive
bromine lead to a reduction in the zonal annual
mean O3 mixing ratio of up to 18% in widespread
areas, and regionally up to 40% compared to a
model run without bromine chemistry. The lifetime
of inorganic bromine was increased due to the
recycling on sulfate aerosol and ranged from about
ten days in the lower troposphere to 20 days in the
upper troposphere. According to the model the
changes in ozone were not only due to photochemical
ozone destruction but also due to a reduction
of ozone production as evident by changes in the
HO2 : OH ratio that also affected the mixing ratios
of NOx and PAN. Our lower limit estimate
of the effect of BrO on dimethyl sulfide (DMS) in
the marine boundary layer showed that the effect
was even larger than on ozone, with up to 60%
reduction of DMS tropospheric column. This is
accompanied by dramatic changes in DMS oxidation
pathways, reducing sulphate production and
hence its cooling effect on climate. These results
highlight a significant missing component in the
tropospheric budgets of ozone and DMS.
Outlook/Future work We will use our experience
from the process modeling to developing
parameterizations for the sources of inorganic
bromine as well as a reduced reaction mechanism.
Main publication von Glasow et al. [2004]