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74 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING
in the marine troposphere and in seawater have shown that exchanges of climatically relevant gases
like dimethyl sulfide (DMS) and non-methane hydrocarbons occur which can have a significant impact
on regional and even global photochemistry and climate. Furthermore, sea salt aerosol particles
are produced that contain chlorine and bromine. Numerous aerosol samples have shown that marine
aerosol is depleted in bromine compared to sea water. Once released to the gas phase bromine participates
in ozone destruction cycles and also oxidizes dimethyl sulfide (DMS). Iodine is being released via
biological processes in dramatic amounts from several coastal regions but also from the open ocean.
Iodine is very efficient in destroying ozone and if concentrations of iodine oxides are high enough, new
particle formation can occur.
The role of halogens in tropospheric chemistry is not restricted to the marine environment. Very
high mixing ratios of the radical bromine oxide have been measured during strong surface Ozone
Depletion Events in polar regions in springtime and over salt lakes. In the Arctic BrO has also been
implicated in so-called Mercury Depletion Events with an increase in the deposition of bio-available,
toxic mercury. Various measurements indicate that there is a significant background of the bromine
oxide radical in the troposphere, most likely in the free troposphere, which would significantly impact
the background photochemistry in this sensitive environment [see von Glasow et al. , 2004]. An
overview of tropospheric halogen chemistry can be found in von Glasow & Crutzen [2006].
Although the mentioned “halogen domains” appear to be very different they have striking similarities
when halogen release and cycling are considered. This makes it feasible to address the associated
research questions in one research group to benefit from synergies. Figure 2.38 shows schematically
our research topics, making the interconnections more obvious.
Main methods
Our research tools are numerical models. The main model is the one-dimensional model of the
marine boundary layer MISTRA [von Glasow et al. , 2002a,b; von Glasow & Crutzen, 2004] that treats
chemical processes in the gas phase, in and on aerosol particles, and cloud droplets. The microphysical
module includes the growth of particles due to water vapor and uptake of other gases and a two-way
feedback with radiation. Aqueous phase chemistry is calculated in sulfate and sea salt aerosol and -
if a cloud is present - in cloud droplets that grew on each type of aerosol. The chemical mechanism
comprises about 170 reactions in the gas phase and 270 reactions in and on aqueous particles for
each of the four chemical bins. For some applications the model is run in box model mode where the
relevant meteorological parameters (temperature, relative humidity, particle radius and liquid water
content) are adjusted to the problem under investigation.
We are developing an improved microphysical/microchemical module that will allow us to make
more detailed studies of the microphysical (mainly growth) and chemical interactions of a particle
population.
Furthermore we have been using the global three-dimensional model MATCH-MPIC [von Glasow
et al. , 2004; Schofield et al. , 2006] for a first global assessment of tropospheric halogen chemistry.
Main recent activities
All group members actively take part in model development and application. Below we briefly
outline the single projects which are explained in more detail in the following sections.
Polar halogen chemistry is being investigated by Matthias Piot (see 2.5.2) focusing on the effects
of various chemical and meteorological parameters for the development of ozone depletion events. The
effects of organic surfactants on sea salt aerosol on particle growth and chemical processes like uptake
or scavenging of gas phase compounds is the subject of Linda Smoydzin’s work (see 2.5.3). Susanne
Pechtl (b. Marquart) has developed and applied a parameterization for the nucleation of new aerosol
particle from iodine vapors [Pechtl et al. , 2006b] and key modules for our improved microphysical
module. Another focus was the continuation of her work on iodine chemistry, esp. aqueous phase
reactions (see 2.5.1). Effects of surface reactions on sea salt aerosol had been proposed in the literature
to be of potentially great importance for the marine sulfur cycle, our model results showed, however,
that the effects had been overestimated and are unlikely to be of importance (see 2.5.4). We continued
our investigation of chemical processes in volcanic plumes in collaboration with Italian colleagues. One
focus was sulfur chemistry in volcanic plumes [Aiuppa et al. , 2006]. Furthermore, Thomas Kaschka
is developing an explicit model of the very-early plume chemical processes. We also contributed to
the Scientific Assessment of Stratospheric Ozone by the World Meteorological Organization [WMO,
2006].