SO<sub>2</sub> photolysis as a source for sulfur mass-independent isotope signatures in stratospehric aerosols
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 mechani...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/15/1843/2015/acp-15-1843-2015.pdf |
| Summary: | 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 O<sub>2</sub> on the S-MIF produced by two absorption band systems of
SO<sub>2</sub>: photolysis in the 190 to 220 nm region and photoexcitation in the 250 to
350 nm region. The SO<sub>2</sub> photolysis (SO<sub>2</sub> + <i>hν</i> → SO + O)
experiments showed S-MIF signals with large <sup>34</sup>S/<sup>34</sup>S fractionations, which
increases with decreasing temperature. The overall S-MIF pattern observed for photolysis experiments,
including high <sup>34</sup>S/<sup>34</sup>S fractionations, positive mass-independent anomalies in
<sup>33</sup>S, and negative anomalies in <sup>36</sup>S, is consistent with a major contribution
from optical isotopologue screening effects and data for stratospheric sulfate aerosols. In contrast,
SO<sub>2</sub> photoexcitation produced products with positive S-MIF anomalies in both <sup>33</sup>S and
<sup>36</sup>S, which is different from stratospheric sulfate aerosols. SO<sub>2</sub> photolysis in the
presence of O<sub>2</sub> produced SO<sub>3</sub> with S-MIF signals, suggesting the transfer of the S-MIF
anomalies from SO to SO<sub>3</sub> by the SO + O<sub>2</sub> + M →
SO<sub>3</sub> + M reaction. This is supported with energy calculations of stationary points on the
SO<sub>3</sub> 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<sup>−37</sup> cm<sup>6</sup> molecule<sup>−2</sup> s<sup>−1</sup>.
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 SO<sub>2</sub>. The production and
preservation of S-MIF signals requires a high SO<sub>2</sub> 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
SO<sub>2</sub> plume must reach an altitude of around 20 to 25 km, where SO<sub>2</sub> 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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| ISSN: | 1680-7316 1680-7324 |