RESULTS OF ENVISAT VALIDATION MEASUREMENTS OBTAINED BY THE SAFIRE-A SPECTROMETER AT MID-LATITUDE AND IN THE ARCTIC REGION U. Cortesi (1) , G. Bianchini (1) , l.. Palchetti (1) , E. Castelli (2) , B.M. Dinelli (2) , C. Lee (3) , G. Redaelli (4) (1) Istituto di Fisica Applicata (IFAC - CNR), Via Panciatichi, 64, Firenze, Italy, u.cortesi@ifac.cnr.it (2) Istituto di Scienze dell'Atmosfera e del Clima (ISAC - CNR), Via Gobetti, 101, Bologna, Italy, b.dinelli@isac.cnr.it (3) University of Wales, The Parade, 5, CF243YB, Cardiff, United Kingdom, c.lee@astro.cf.ac.uk (3) Università dell'Aquila, Via Vetoio, 10, Coppito - L'Aquila, redaelli@aquila.infn.it ABSTRACT/RESUME The SAFIRE-A Fourier transform Far-infrared spectrometer has been involved in field campaigns carried out with the M-55 Geophysica stratospheric aircraft in 2002-2003, with the aim of contributing to the validation of level-2 products of the ENVISAT chemistry instruments. The limb sounding observations of volume mixing ratio vertical profiles of Ozone, Nitric Acid and Nitrous Oxide were especially focused on the validation of MIPAS data in the altitude range 10-20 km. Here we report the results of validation measurements performed at mid-latitude in October 2002 and in the Arctic region in March 2003. Comparison between O 3 , HNO 3 and N 2 O vertical distribution measured by SAFIRE and profiles of the same species obtained by MIPAS onboard the ENVISAT satellite are presented and discussed. - THE SAFIRE-A INSTRUMENT SAFIRE-A (Spectroscopy of the Atmosphere using Far InfraRed Emission-Airborne) is a high resolution Fourier Transform spectrometer for limb sounding measurements of the atmospheric emission in the far- infrared spectral region (10-200 cm -1 ). The instrument operates aboard the high flying research aircraft M-55 Geophysica and is capable of measuring the concentration of minor atmospheric constituents displaying detectable features within the spectral windows that can be selected by using narrow-band filters (typically 2 cm -1 wide) on each of the two detection channels. Vertical volume mixing ratio (VMR) profiles from the flight level (max. 20 km) down to tropopause can be retrieved for each of the observed species, along with their column content above the aircraft altitude. A summary of the main instrument characteristics is reported in Table 1. A full description of the spectrometer is given in [1]; details of most recent upgrading in the instrument configuration and an updated assessment of its spectroscopic performances and measurement capabilities can be found in [2]. Instrument type Polarising interferometer Observation geometry Limb sounding Overall dimensions 1800 x 880 x 650 mm Total weight 387 kg Spectral range 10 - 250 cm -1 Max. spectral resolution 0,004 cm -1 Number of detection channels 2 Filter bandwidth typically 2 cm -1 Instantaneous Field Of View 0,57° Vertical resolution 1.5 km Interferogram acquisition time 12, 24, 48, 96 sec Number of spectra in a sequence 11 Spectral Signal-to-Noise Ratio > 500 Table 1 SAFIRE-A Instrument Specifications - THE M-55 GEOPHYSICA MEASUREMENT CAMPAIGNS FOR ENVISAT VALIDATION Three ENVISAT validation campaigns with the stratospheric aircraft M-55 Geophysica have been conducted from Forlì, Italy (Lat. 44°N, Lon. 12°E) in July and October 2002 and from Kiruna, Sweden (Lat. 68°N, Lon. 20°E) in February-March 2003, as part of the ESABC (ENVISAT Stratospheric Aircraft and Balloon Campaigns) activities. The SAFIRE-A spectrometer was involved in both the mid-latitude and Arctic flights performed with the chemistry payload of the Geophysica aircraft. More detailed information about the scientific payload and the validation activities with the M-55 can be found on the web site of the Arctic campaign (http://ape.ifac.cnr.it/apeartic/). The configuration of SAFIRE-A detection unit adopted for the ENVISAT validation measurements included a "low frequency" channel, centred at 23 cm -1 and using a bolometric detector cooled at 0.3 K, and a "high frequency" channel, centred at 117 cm -1 or 124 cm -1 , with photo-conductive detectors operating at 4.2 K The frequency interval of each detection band is