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| Titel |
SO2 photolysis as a source for sulfur mass-independent isotope signatures in stratospehric aerosols |
| VerfasserIn |
A. R. Whitehill, B. Jiang, H. Guo, S. Ono |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 4 ; Nr. 15, no. 4 (2015-02-20), S.1843-1864 |
| Datensatznummer |
250119448
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| Publikation (Nr.) |
 copernicus.org/acp-15-1843-2015.pdf |
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| Zusammenfassung |
| Signatures of sulfur isotope mass-independent fractionation (S-MIF) have been observed
in stratospheric sulfate aerosols deposited in polar ice. The S-MIF signatures are thought to be
associated with stratospheric photochemistry following stratospheric volcanic eruptions, but
the exact mechanism responsible for the production and preservation of these signatures is
debated. In order to identify the origin and the mechanism of preservation for these signatures,
a series of laboratory photochemical experiments were carried out to investigate the effect of
temperature and added O2 on the S-MIF produced by two absorption band systems of
SO2: photolysis in the 190 to 220 nm region and photoexcitation in the 250 to
350 nm region. The SO2 photolysis (SO2 + hν → SO + O)
experiments showed S-MIF signals with large 34S/34S fractionations, which
increases with decreasing temperature. The overall S-MIF pattern observed for photolysis experiments,
including high 34S/34S fractionations, positive mass-independent anomalies in
33S, and negative anomalies in 36S, is consistent with a major contribution
from optical isotopologue screening effects and data for stratospheric sulfate aerosols. In contrast,
SO2 photoexcitation produced products with positive S-MIF anomalies in both 33S and
36S, which is different from stratospheric sulfate aerosols. SO2 photolysis in the
presence of O2 produced SO3 with S-MIF signals, suggesting the transfer of the S-MIF
anomalies from SO to SO3 by the SO + O2 + M →
SO3 + M reaction. This is supported with energy calculations of stationary points on the
SO3 potential energy surfaces, which indicate that this reaction occurs slowly on a single adiabatic
surface, but that it can occur more rapidly through intersystem crossing. Based on our experimental results,
we estimate a termolecular rate constant on the order of 10−37 cm6 molecule−2 s−1.
This rate can explain the preservation of mass independent isotope signatures in stratospheric sulfate aerosols
and provides a minor, but important, oxidation pathway for stratospheric SO2. The production and
preservation of S-MIF signals requires a high SO2 column density to allow for optical isotopologue
screening effects to occur and to generate a large enough signature that it can be preserved. In addition, the
SO2 plume must reach an altitude of around 20 to 25 km, where SO2 photolysis
becomes a dominant process. These experiments are the first step towards understanding the origin of the
sulfur isotope anomalies in stratospheric sulfate aerosols. |
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