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2.2. STRATOSPHERIC RESEARCH GROUP 39
2.2.1 Observational constraints on stratospheric ozone loss cycles
André Butz (Marcel Dorf, Sebastian Kreycy, Lena Kritten, Cristina Prados, Benjamin Simmes,
Frank Weidner, Klaus Pfeilsticker)
Abstract Balloon-borne observations of a comprehensive set of trace gases in the Arctic winter
stratosphere are used to constrain a photochemical model. Model-measurement comparisons aim
at testing the consistency of various recently suggested scenarios of the involved reaction kinetics.
Particular focus is put on the ClO-BrO and the ClO-ClO cycles, and on inferring implications for
ozone loss.
Figure 2.17: Measured (boxes) and modeled (lines) slant column densities (SCD) of OClO as a function
of tangent height. Various model runs are shown as indicated in the legend.
Background Several studies indicate that measured
ozone loss in the Arctic winter/spring
stratosphere is underestimated by current stratospheric
chemistry models when using standard
recommendations for the reaction kinetics. Erroneous
representation of the known catalytic ozone
loss cycles in the models could explain why modeled
ozone loss falls short when compared to observations.
Here, recently suggested updates of
the kinetics of the two most important cycles, the
ClO-ClO and ClO-BrO cycle, are tested for consistency
with the observations.
Methods and results We discuss a case study
of the Arctic stratosphere in February 1999 where
ozone loss occurred locally in a moderately activated
polar vortex. Simultaneous balloon-borne
solar occultation observations of HNO3, NO2, NO,
HCl, ClONO2, Cly, OClO, and BrO provide a
comprehensive view of the chemical species involved
in catalytic ozone loss. In particular, simultaneous
measurements of BrO and OClO can
be used to inspect the ClO + BrO reaction, the
branching ratio of which has been questioned recently.
Simultaneous observations of ClONO2 and
NO2 are particularly useful to investigate deactivation
of the short-lived chlorine species and
the speculative existence of unstable isomers of
ClONO2. A stratospheric chemistry model is constrained
to all observed species except for OClO.
Several recently suggested updates of the relevant
reaction kinetics are tested. Model-measurement
comparisons of OClO provide an estimate on how
well the ClO-BrO cycle and the ClO-ClO cycle
are represented by the proposed model scenarios.
Implications are discussed on the basis of the modeled
ozone loss rates.
The model-measurement comparisons show that
formation of an unstable isomer of ClONO2 cannot
be reconciled with our observations. Further
suggestions concerning the photolysis rate of the
ClO dimer, the equilibrium constant between the
ClO dimer and monomer, the rate of the ClO-
ClO association reaction, and the branching ratio
of the ClO-BrO reaction produce model output
consistent with observations of OClO. However,
ozone loss can be enhanced by 10%- 20% compared
to currently recommended kinetics.
Outlook/Future work Similar studies conducted
during night-time will allow us to further
discriminate between the various suggested
kinetic scenarios. Time-resolved observations of
the build-up of OClO during sunset could further
constrain the kinetics of the ClO-BrO cycle.
Funding The present work has been supported
by ESA, BMBF, DLR and the European Union.
Main publication Butz et al. [2006b]