Retrieval of H15NO3/H14NO3 vertical distribution from MIPAS / ENVISAT limb-emission measurements.
Bianca Maria Dinelli(1), Gabriele Brizzi(2), Enrico Arnone(2), Massimo Carlotti(2), Enzo Papandrea(2), Marco Ridolfi(2), Jean-Marie Flaud(3) and Agnes Perrin(3)
(1) ISAC-CNR, Via Gobetti 101, 40129 Bologna, Italy
(2) DCFI- Universita' di Bologna, Viale Risorgimento, 2, 40136 Bologna, Italy
(3) LISA-CNRS, Universités Paris 12&7, 94010 Créteil Cedex, France
Because of its atmospheric interest, the HNO3 molecule has been the subject of a number of spectroscopic studies, particularly in the 11.2 µm spectral region which is widely used for the atmospheric retrievals of 14N nitric acid. However, only recently accurate spectral line parameters were generated for the isotopic species H15NO3. Thanks to these parameters, the Q-branch of the 5 band of H15NO3 was identified (around 871 cm-1) in the high signal-to-noise ratio atmospheric limb-emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on-board of ENVISAT . We have exploited this 5 Q-branch to retrieve for the first time of the isotopic partitioning between stratospheric H14NO3 and H15NO3. Due to the very low intensity of the considered spectral feature it was necessary to develop an ad-hoc inversion algorithm based on a regularized linear Kalman filter approach. This procedure enables the retrieval of H15NO3/ H14NO3 isotope ratio profiles, averaged over different latitude bands and time intervals, in the altitude range from 18 to 42 km. The resulting isotope ratio profiles show an overall significant substantial depletion with altitude of H15NO3 (with respect to the natural partitioning), and no significant latitude and time dependence.