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2.5. MARHAL - MODELING OF MARINE AND HALOGEN CHEMISTRY 77
2.5.2 The Potential Importance of Frost Flowers for Ozone Depletion Events
- A Model Study
Matthias Piot, Roland von Glasow
Abstract For more than 20 years, events with almost complete loss of ozone have been observed
in the Arctic in spring. We performed model studies with the one-dimensional model MISTRA to
investigate the potential role of frost flowers (FF) in this depletion of tropospheric ozone.
Figure 2.40: Left: Schematic depiction of the most important processes included in the Arctic version
of MISTRA. The boundary layer is denoted as BL, open lead as OL, and the free troposphere as FT.
Aerosol and gas phase chemistry are calculated in all layers. (1): Deposition process; (2): Br2/BrCl
re-emission from the snowpack. Right: typical gas phase O3 during an ODE in the BL.
Background Reactive halogens play a major
role in polar ozone depletion events (ODE): the
reaction of bromine atoms with ozone, followed
by the self-reaction of bromine oxides (BrO) represents
a catalytic loss mechanism for ozone in the
polar boundary layer (PBL). However, the triggering
of the so-called ”bromine explosion” remains
unclear. We present a sensitivity study where meteorological
as well as chemical parameters are
assessed. This study helps us better understand
the conditions favorable for the development of an
Ozone Depletion Event (ODE).
Methods and results We used MISTRA in
a “Lagrangian mode” where a model column of
2000 m height moves across a pre-defined sequence
of surfaces: snow, FF, and open lead (see
Fig. 2.40-left), with FF aerosols being produced
during the FF section. We show that a major
ozone depletion event can be satisfactorily reproduced
if the recycling on snow of deposited halogens
into gas phase Br2/BrCl is considered (see
ozone mixing ratio in Fig. 2.40-right). This cycling
maintains sufficiently high levels of bromine
to deplete ozone down to few nmol mol −1 within
four days. We also assessed the influence of different
combinations of open lead/frost flowers on the
chemistry of the moving column. Results showed
noticeable modifications affecting the composition
of aerosols and the deposition velocities due to
variation of the humidity in the air. In addition,
we studied the effects of modified temperature of
either the frost flower field or the ambient airmass.
A warmer FF field increases the relative
humidity and the aerosol deposition rate. The
deposition/re-emission process is larger, inducing
more reactive bromine in the gas phase, and a
stronger ozone depletion. A decrease of 1 K in
airmass temperature shows that the aerosol uptake
capacity substantially increases, leading to
enhanced uptake of acids from the gas phase: the
bromine explosion accelerates and O3 mixing ratios
decreases. Recent studies have suggested the
important role of the precipitation of calcium carbonate
(CaCO3) out of the brine layer for the
possible acidification of the liquid phase by acid
uptake. Our investigation showed that this precipitation
is a crucial process for the timing of
the bromine explosion in aerosols. Finally, we investigated
the release of Br2 potentially produced
by heterogeneous reactions directly on frost flowers.
In this case, we obtained unlikely results for
aerosol compositions and deposition rates on snow
compared to observations made in the Arctic.
Funding DFG-Emmy Noether Junior Research
group Marhal GL 353/1-2.