OCS photolytic isotope effects from first principles: sulfur and carbon isotopes, temperature dependence and implications for the stratosphere J. A. Schmidt1, M. S. Johnson1, S. Hattori2,3, N. Yoshida2,3, S. Nanbu4, and R. Schinke5 1Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen Ø, Denmark 2Department of Environmental Science and Technology, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama, 226-8502, Japan 3Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama, 226-8502, Japan 4Faculty of Science and Technology, Sophia University, 7-1 Kioi-Cho, Chiyoda-ku, Tokyo 102-8554, Japan 5Max-Planck-Institut für Dynamik und Selbstorganisation, 37073 Göttingen, Germany
Abstract. The isotopic fractionation in OCS photolysis is studied theoretically from
first principles. UV absorption cross sections for OCS, OC33S, OC34S,
OC36S and O13CS 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 34S 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 of Brühl et al. (2012). The 13C 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.
Citation: Schmidt, J. A., Johnson, M. S., Hattori, S., Yoshida, N., Nanbu, S., and Schinke, R.: OCS photolytic isotope effects from first principles: sulfur and carbon isotopes, temperature dependence and implications for the stratosphere, Atmos. Chem. Phys., 13, 1511-1520, doi:10.5194/acp-13-1511-2013, 2013.