download pdf - Institut für Umweltphysik - Ruprecht-Karls-Universität ...

100 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING
2.6.2 Ozone Depletion Events in the Polar Boundary Layer in Spring: A
Model Study
Participating scientists Matthias Piot and Roland von Glasow
Abstract For more than 20 years, events with almost complete loss of ozone have been observed
in the Arctic in spring. The importance of several species and meteorological parameters for these
depletions have been investigated with sensitivity studies using a numerical box model.
Figure 2.56: Schematic depiction of the most important processes included in the MISTRA model, applied
for Arctic conditions. Fluxes over ice and sea salt production are switched ON/OFF, depending
on the air composition we study.
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. I present model results where prescribed
bromine or chlorine fluxes are used to reproduce
ODEs. I investigated the importance of several
compounds for the chemistry of the PBL as well as
meteorological parameters and focused on species
which influence the occurrence of an ODE. This
study allows us to better understand which species
are important in the process of depleting ozone.
Funding DFG: Emmy Noether Junior Research
Group MarHal GL 353/1-1
Methods and results I used the chemical and
microphysical model MISTRA in the lagrangian
box-model mode to study the mechanisms influencing
these observed depletions in the polar
boundary layer. My sensitivity studies consisted
of a set of four-day runs where I changed initial
mixing ratios or fluxes (or both) of 19 different
species (including halogens, NOx, NOy,
DMS, H2O2, HCHO...) and compared the results
with base runs. The influence of temperature
and humidity have also been examined. High
HCHO concentrations result via increases in HO2
in a redistribution of bromine species involving
the aerosol phase, slowing ODEs down. Elevated
DMS leads to an increase in HCHO, therefore also
slowing ODEs down. By a similar production of
HOx, hydrogen peroxyde (H2O2) also oxidizes Brx
compounds inducing an ODE slow down. The
presence of both Cl2 and Br2 in the air may lead
to an unexpected decrease in the destruction process,
depending on their relative mixing ratios. Increasing
the mixing ratio of ethane by a factor of
2 reduces the ozone mixing ratio from a partial
ODE to a major ODE. For cases with low Br,
ethene emphasizes the ozone loss. With high Br,
it reduces the destruction process. An increase in
HONO led to different eventual ozone mixing ratios,
depending on its use by Brx-Clx compounds.
A change in temperature or humidity modifies the
liquid water content of aerosols and therefore the
uptake coefficients. The recycling of halogens via
the liquid phase is modified.
Outlook/Future work This sensitivity study
will help us investigate the potential triggers like
frost flowers for the bromine explosion, using MIS-
TRA in the lagrangian 1D mode.
Main publication Piot and von Glasow, presentations
at EGU, Vienna, 2005 and OASIS
workshop, Toronto, 2005