Vertical profile of δ 18OOO from the middle stratosphere to lower mesosphere from SMILES spectra
Ozone is known to have large oxygen isotopic enrichments of about 10% in the middle stratosphere; however, there have been no reports of ozone isotopic enrichments above the middle stratosphere. We derived an enrichment δ 18OOO in the stratosphere and the lower mesosphere from observations of the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) onboard the International Space Station (ISS) using a retrieval algorithm optimized for the isotopic ratio. The retrieval algorithm includes (i) an a priori covariance matrix constrained by oxygen isotopic ratios in ozone, (ii) an optimization of spectral windows for ozone isotopomers and isotopologues, and (iii) common tangent height information for all windows. The δ 18OOO by averaging the SMILES measurements at the latitude range of 20 to 40° N from February to March in 2010 with solar zenith angle < 80° was 13% (at 32 km) with the systematic error of about 5%. SMILES and past measurements were in good agreement, with δ 18OOO increasing with altitude between 30 and 40 km. The vertical profile of δ 18OOO obtained in this study showed an increase and a decrease with altitude in the stratosphere and mesosphere, respectively. The δ 18OOO peak, 18%, is found at the stratopause. The δ 18OOO has a positive correlation with temperature in the range of 220–255 K, indicating that temperature can be a dominant factor to control the vertical profile of δ 18OOO in the stratosphere and mesosphere. This is the first report of the observation of δ 18OOO over a wide altitude range extending from the stratosphere to the mesosphere (28–57 km).