Analysis of Cassini/CIRS limb spectra of Titan acquired during the nominal mission II: Aerosol extinction profiles in the 600-1420 cm-1 spectral range

We have analyzed the continuum emission of limb spectra acquired by the Cassini/CIRS infrared spectrometer in order to derive information on haze extinction in the 3-0.02 mbar range (similar to 150-350 km). We focused on the 600-1420 cm(-1) spectral range and studied nine different limb observations acquired during the Cassini nominal mission at 55 degrees S, 20 degrees S, 5 degrees N, 30 degrees N, 40 degrees N, 45 degrees N, 55 degrees N, 70 degrees N and 80 degrees N. By means of an inversion algorithm solving the radiative transfer equation, we derived the vertical profiles of haze extinction coefficients from 17 spectral ranges of 20-cm(-1) wide at each of the nine latitudes. At a given latitude, all extinction vertical profiles retrieved from various spectral intervals between 600 and 1120 cm(-1) display similar vertical slopes implying similar spectral characteristics of the material at all altitudes. We calculated a mean vertical extinction profile for each latitude and derived the ratio of the haze scale height (H-haze) to the pressure scale height (H-gas) as a function of altitude. We inferred H-haze/H-gas values varying from 0.8 to 2.4. The aerosol scale height varies with altitude and also with latitude. Overall, the haze extinction does not show strong latitudinal variations but, at 1 mbar, an increase by a factor of 1.5 is observed at the north pole compared to high southern latitudes. The vertical optical depths at 0.5 and 1.7 mbar increase from 55 degrees S to 5 degrees N, remain constant between 5 degrees N and 30 degrees N and display little variation at higher latitudes, except the presence of a slight local maximum at 45 degrees N. The spectral dependence of the haze vertical optical depth is uniform with latitude and displays three main spectral features centered at 630 cm(-1), 745 cm(-1) and 1390 cm(-1), the latter showing a wide tail extending down to similar to 1000 cm(-1). From 600 to 750 cm(-1), the optical depth increases by a factor of 3 in contrast with the absorbance of laboratory tholins, which is generally constant. We derived the mass mixing ratio profiles of haze at the nine latitudes. Below the 0.4-mbar level all mass mixing ratio profiles increase with height. Above this pressure level, the profiles at 40 degrees N, 45 degrees N, 55 degrees N, at the edge of the polar vortex, display a decrease-with-height whereas the other profiles increase. The global increase with height of the haze mass mixing ratio suggest a source at high altitudes and a sink at low altitudes. An enrichment of haze is observed at 0.1 mbar around the equator, which could be due to a more efficient photochemistry because of the strongest insolation there or an accumulation of haze due to a balance between sedimentation and upward vertical drag. (C) 2010 Elsevier Inc. All rights reserved.