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 48O3 and 18O16O16O generated a sharp diurnal variation of δ18OOO in the middle stratosphere (37 km), in contrast to the temporal variation of 48O3. The average isotopic enrichment δ18OOO 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 δ18OOO 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 δ18O can be a powerful tool to investigate oxidant chemistry.