Retrievals of heavy ozone with MIPAS

A method for retrieval of $^{18}$O-substituted isotopomers of O₃ in the stratosphere with the Michelson Interferometer
for Passive Atmospheric Sounding (MIPAS) is presented. Using a smoothing regularisation constraint, volume mixing ratio profiles are retrieved for the main isotopologue and the symmetric and asymmetric isotopomers of singly substituted O₃. For the retrieval of the heavy isotopologues, two microwindows in the MIPAS A band (685-970 cm$^{-1}$) and six in the AB band (1020-1170 cm$^{-1}$) are used. As the retrievals are performed as perturbations on the previously retrieved a priori profiles, the vertical resolution of the individual isotopomer profiles is very similar, which is important when calculating the ratio between two isotopomers. The performance of the method is evaluated using 1044 vertical profiles recorded with MIPAS on 1 July 2003.
The mean values are separated by latitude bands, along with estimates of their uncertainties. The asymmetric isotopomer
shows a mean enrichment of ~ 8 %, with a vertical profile that increases up to 33 km and decreases at higher altitudes.
This decrease with altitude is a robust result that does not depend on retrieval settings, and it has not been reported clearly
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in previously published datasets. The symmetric isotopomer is considerably less enriched, with mean values around 3%
and with a large spread. In individual retrievals the uncertainty of the enrichment is dominated by the measurement
noise (2-4 %), which can be reduced by averaging multiple retrievals; systematic uncertainties linked to the retrieval are
generally small at ~ 0.5 %, but this is likely underestimated because the uncertainties in key spectroscopic parameters are
unknown. The variabilities in the retrieval results are largest for the Southern Hemisphere.