Diurnal variation of oxygen isotopic enrichment in asymmetric-18 ozone observed by the SMILES from space

Atomic oxygen is a key oxidant for greenhouse gases such as H2O, CH4, and N2O whose amounts are currently increasing in the stratosphere. Ozone photolysis is a major source of atomic oxygen generation. We found, for the first time, that the ozone photolysis of O-48(3) and (OOO)-O-18-O-16-O-16 generated a sharp diurnal variation of delta(OOO)-O-18 in the middle stratosphere (37 km), in contrast to the temporal variation of 48O3. The average isotopic enrichment delta(OOO)-O-18 derived from the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) observation was higher during daytime (16.2 +/- 3.3%) than at night (13.7 +/- 3.6%). Isotopic fractionation effects were simulated for both ozone formation reaction and ozone photolysis with the ozone photolysis contribution estimated to be 80% of the daytime delta(OOO)-O-18 enhancement. Significant solar radiation dependence was observed in the isotopic fractionation of ozone photolysis derived from the SMILES observation and theoretical calculations. These results show that delta O-18 can be a powerful tool to investigate oxidant chemistry.