INFRARED SPECTROSCOPY OF THE DOWNWELLING ATMOSPHERIC EMISSION IN THE 5-100 MICRON RANGE

REFIR-PAD (Radiation Explorer in the Far-InfraRed - Prototype for Applications and Development) is a Fourier transform spectroradiometer capable to measure the atmospheric emitted radiance in the 100-1900 cm -1 spectral range, with a resolution up to 0.25 cm -1 . Starting from December 2011 the REFIR-PAD instrument is operating continuously in a ground based zenith-looking observation geometry, from Concordia Station, an Italian-French Antarctic base in the Dome C region of the Antarctic plateau. The instrument operates at a 0.4 cm -1 nominal resolution, with a 90% duty cycle and acquisition time of about 10 minutes. The REFIR-PAD dataset cover almost every kind of atmospheric state that can be encountered in the Dome C site, with total water vapor column amounts from <0.2 mm to about 1-2 mm, in clear sky conditions and in presence of different types of cloud cover, including thin cirrus clouds. Possible applications of such an exhaustive dataset are the characterization of atmospheric structure and composition, development and the validation of atmospheric spectroscopic models, and in particular the water vapor lines database and continuum parametrization. For this purpose, REFIR-PAD measured spectra were analyzed through a retrieval code based on the LBLRTM forward model and the MINUIT function minimization routines available from CERN. The atmospheric variables to be fitted have been optimized through the analysis of the Jacobian matrices of the simulated downwelling radiance spectra and include cloud optical depth, and 5 points at selected altitudes for the water vapor and temperature vertical profiles. Residuals from the fitting of a single spectrum are in good agreement with the measurement uncertainty, with a chi-square slightly lower than 1 due to the conservative choices made on error estimate. The averaging of fitting residuals on the timescale of one or more days, on the other side, shows interesting spectral features that are well above uncertainty. From these features we can obtain information useful to refine water vapor continuum models and water vapor spectroscopy, with a particular interest in the far-infrared region that, due to the lack of experimental atmospheric measurements can be still considered relatively unexplored.