Investigations of the photochemical isotope equilibrium between O2, CO2 and O3 R. Shaheen1,2, C. Janssen1,3, and T. Röckmann1,4 1Max-Planck-Institute for Nuclear Physics, Division of Atmospheric Physics, 69117 Heidelberg, Germany 2University of California San Diego, Department of Chemistry, La Jolla, USA 3Laboratoire de Physique Moléculaire pour l'Atmosphère et l'Astrophysique, Université Pierre et Marie Curie/CNRS, 75252 Paris, France 4Institute for Marine and Atmospheric Research Utrecht, Utrecht University, The Netherlands
Abstract. Contrary to tropospheric CO2 whose oxygen isotopic composition follows a
standard mass dependent relationship, i.e. δ17O~0.5δ18O,
stratospheric CO2 is preferentially enriched in 17O,
leading to a strikingly different relation: δ17O~1.7δ18O. It has
been shown repeatedly that the isotope anomaly is
inherited from O3 via photolytically produced O(1D) that undergoes
isotope exchange with CO2 and the anomaly may well serve as a tracer of
stratospheric chemistry if details of the exchange mechanism are understood.
We have studied the photochemical isotope equilibrium in UV-irradiated
O2-CO2 and O3-CO2 mixtures to quantify the transfer of
the anomaly from O3 to CO2 at room temperature. By following the
time evolution of the oxygen isotopic compositions of CO2 and O2 under
varying initial isotopic compositions of both, O2/O3 and CO2, the
isotope equilibria between the two reservoirs were determined. A very strong
dependence of the isotope equilibrium on the O2/CO2-ratio was
established. Equilibrium enrichments of 17O and 18O in CO2 relative to
O2 diminish with increasing CO2 content, and this reduction in the
equilibrium enrichments does not follow a standard mass dependent relation.
When molecular oxygen exceeds the amount of CO2 by a factor of about 20,
17O and 18O in equilibrated CO2 are enriched by (142±4)‰
and (146±4)‰, respectively, at room temperature and at a
pressure of 225 hPa, independent of the initial isotopic
compositions of CO2 and O2 or O3. From these findings we derive
a simple and general relation between the starting isotopic compositions and
amounts of O2 and CO2 and the observed slope in a three oxygen
isotope diagram. Predictions from this relation are compared with published
laboratory and atmospheric data.
Citation: Shaheen, R., Janssen, C., and Röckmann, T.: Investigations of the photochemical isotope equilibrium between O2, CO2 and O3, Atmos. Chem. Phys., 7, 495-509, doi:10.5194/acp-7-495-2007, 2007.