4.7 Article

OCS photolytic isotope effects from first principles: sulfur and carbon isotopes, temperature dependence and implications for the stratosphere

Journal

ATMOSPHERIC CHEMISTRY AND PHYSICS
Volume 13, Issue 3, Pages 1511-1520

Publisher

COPERNICUS GESELLSCHAFT MBH
DOI: 10.5194/acp-13-1511-2013

Keywords

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Funding

  1. European Community [237890]
  2. Ministry of Education, Culture, Sports, and Technology (MEXT), Japan [23224013]
  3. MEXT, Japan [22-7563]
  4. Grants-in-Aid for Scientific Research [10J07563, 23224013] Funding Source: KAKEN

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The isotopic fractionation in OCS photolysis is studied theoretically from first principles. UV absorption cross sections for OCS, (OCS)-S-33, (OCS)-S-34, (OCS)-S-36 and (OCS)-C-13 are calculated using the time-depedent quantum mechanical formalism and a recently developed ab-initio description of the photodissociation of OCS which takes into account the lowest four singlet and lowest four triplet electronic states. The calculated isotopic fractionations as a function of wavelength are in good agreement with recent measurements by Hattori et al. (2011) and indicate that photolysis leads to only a small enrichment of S-34 in the remaining OCS. The photodissociation dynamics provide strong evidence that the photolysis quantum yield is unity at all wavelengths for atmospheric UV excitation, for all isotopologues. A simple stratospheric model is constructed taking into account the main sink reactions of OCS and it is found that overall stratospheric removal slightly favors light OCS in constrast to the findings of Leung et al. (2002). These results show, based on isotopic considerations, that OCS is an acceptable source of background stratosperic sulfate aerosol in agreement with a recent model study of Bruhl et al. (2012). The C-13 isotopic fractionation due to photolysis of OCS in the upper stratosphere is significant and will leave a clear signal in the remaining OCS making it a candidate for tracing using the ACE-FTS and MIPAS data sets.

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