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| Titel |
“Chlorine explosion” from sea-salt aerosols in a polluted atmosphere |
| VerfasserIn |
Joelle Buxmann, Sergej Bleicher, Cornelius Zetzsch, Ulrich Platt |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250077048
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| Zusammenfassung |
| Bromine and chlorine ’explosions’ (BE and CE) refer to autocatalytic, heterogeneous releases
of reactive halogen species (RHS). ClO and BrO play a key role as RHS, as they influence the
tropospheric oxidation capacity through destruction of ozone and fast reactions with nitrogen
oxides. Besides OH radicals, Cl atoms react at a fast rate with the greenhouse gas methane,
but the global effect is not clear yet. From smog-chamber experiments under tropospheric
light conditions, ClO, OClO (from CE) and BrO (from BE) released from artificial sea salt
aerosols were detected using a White system in combination with Differential Optical
Absorption Spectroscopy (DOAS). Up to 17 ppb of ClO, 6 ppb of OClO and 1.6 ppb
of BrO were observed under the influence of high NO2 and O3 concentrations.
The RHS activation is triggered by NO2 reactions starting during the dark period.
Formation of ClNO2 and ClONO2 and acidification of the aerosol by HNO3 or
HONO play key roles. The lifetime of Cl2 of 645 s against photolysis in our smog
chamber was estimated to be longer than the uptake onto the aerosol surface with a
lifetime of 83s. The fact that Cl2 photolysis is slower compared to uptake, indicates
that Cl2 might not be sufficient as a precursor for the observed ClO and OClO
mixing rations during the chamber experiments at high NOx. Furthermore, OClO
(Â 40ppt/s) is formed at a faster rate than ClO (Â 15ppt/s) in our experiments. A
simple model, including the known gas phase reactions of halogen oxides, O3 and
NOx, predicts the maximum ClO concentration to occur before the maximum OClO
concentration. The measurement indicates the opposite. This suggests heterogeneous OClO
formation. The lifetime of OClO against photolysis is only 20s in our chamber. But an
actual heterogeneous release mechanism to form OClO has not been confirmed yet.
Nevertheless, these results suggest that OClO is important for the heterogeneous
release process. While BE has been demonstrated to occur in nature, the question
remains how important CE is in the troposphere. The chlorine mechanism might
become important in highly polluted marine areas, where high NO2 and O3 levels are
present. |
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