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| Titel |
Autocatalytic halogen release from salt droplets and saltpans |
| VerfasserIn |
S. Bleicher, J. Buxmann, N. Balzer, T. Riedel, J. Thornton, U. Platt, C. Zetzsch |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250066830
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| Zusammenfassung |
| Reactive bromine and chlorine species are known to have an impact on the ozone
concentration, to change the chemical balance of nitrogen oxides and to have an influence on
human health: through their reactions with hydrocarbons (HCs) among other trace gases and
increased deposition of toxic compounds (like mercury). Especially the formation of higher
halogen oxides (OIO, OBrO) and the role of chlorine in tropospheric chemistry are not clear
yet.
We present experiments on halogen activation from salt mixtures of NaCl/NaBr and
salt droplet aerosols, simulating saltpans and sea spray in an illuminated Teflon
chamber. The starting conditions influence the total amount of released halogens, so we
varied the amounts of aerosol surface, NOx and HCs, monitoring the time profiles
of the HCs by gas chromatography in order to determine the time profiles of OH
and Cl. The chemical ionization mass spectrometer (CIMS) of the University of
Washington was used to observe stable halogen species in a short campaign, and a
multi-reflection cell (White-type) of the University of Heidelberg, coupled with Differential
Optical Absorption Spectroscopy (DOAS), was employed to measure BrO, ClO and
OClO.
We observe a qualitative difference in Br2 release between the saltpan and the sea salt
aerosol experiments: While BrO increases immediately to very high levels up to 6 ppb in the
saltpan experiments (Buxmann et al., 2011), formation of BrO is delayed in the aerosol
studies. This delay varies from about five to ten minutes after switching the solar simulator
on, depending on HCs and NOxconcentrations. Furthermore, we observe a “chlorine
explosion” with Cl2 mixing ratios of up to 12 ppb (by CIMS) as a function of the initial HC
and NOx conditions. More than 3 ppb ClO, 3.5 ppb OClO and 650 ppt BrO were observed at
initial levels of 150 ppb NO2 and of 770 ppb O3, leading to an ozone depletion rate of
0.6 ppb/s. The influence of NOx was tested on simulated sea spray alone. Due
to the intermediate formation of BrNO3 and its rapid uptake, releasing Br2, the
ozone depletion was a factor of 3-5 faster in that case. However, one mechanistic
explanation for the rapid BrO increase in saltpan experiments could be a photolabile
precursor, such as OBrO, having a large photolysis frequency of 0.2 s-1 in our chamber.
A release of OBrO could be possible in the dark, as known from the oscillating
Belousov-Zhabotinsky reaction. Remarkably high values of OClO were observed, while
the formation of OBrO is very likely. Such higher halogen oxides appear to be
important intermediates in the release process of reactive bromine and chlorine
species.
J. Buxmann, N. Balzer, S. Bleicher, U. Platt, C. Zetzsch (2011) Int. J. Chem. Kinetics (in
press) |
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