Retrieval of atmospheric H15NO3/H14NO3 isotope ratio profile from MIPAS/ENVISAT limb-scanning measurements

We report the first measurement of the isotopic partitioning between stratospheric H14NO3 and H15NO3. The vertical distribution of the isotope ratio of the two isotopologues of nitric acid was derived using the ν5 band Q branch of H15NO3 that was recently identified in measurements from the MIPAS experiment using newly generated spectral line parameters for the minor isotopologue. Because of the very low intensity of the spectral features adopted for the retrieval, we established an “ad hoc” inversion procedure based on a regularized linear Kalman filter approach. This procedure permits the retrieval of the deviations from the natural H15NO3/H14NO3 isotope ratio profile, averaged over different latitude bands and time intervals, in the altitude range from 18 to 42 km. The resulting profiles show an overall substantial depletion with height of H15NO3, with respect to the natural partitioning, and no significant latitude and time dependence. The developed retrieval procedure can be readily extended to other atmospheric trace gas isotope ratios.