1. A new spectroscopic observatory in Créteil to measure atmospheric trace gases in solar occultation geometry C. Viatte, P. Chelin, M. Eremenko, C. Keim, J.-M. Flaud, J. Orphal, M. Ray Laboratoire Inter-Universitaire des Systèmes Atmosphériques (LISA), CNRS, Universités de Paris 12 (Paris-Est) et Paris 7, 61 Av. du Général de Gaulle, 94010 Créteil, France. ABSTRACT Ground-based Fourier Transform Infrared (FTIR) and Ultraviolet (UV) spectroscopy based on solar occultation is a powerful remote sensing technique to determine vertical distribution of various constituents in the atmosphere [1]. In this context, a new spectroscopic observatory (with motorised dome rotation) was installed on the roof of the University of Paris 12 in Créteil. It comprises a solar tracker (Bruker Ltd.) coupled with two spectrometers operating in different spectral regions, to obtain information on various atmospheric target species such as H 2 O, O 3, CO, CH 4, N 2 O, NO 2, HNO 3, H 2 CO, C 2 H 6, PAN etc. and the most abundant isotopic species. We have first characterized the ILS (Instrumental Line Shape) width of the FTIR at about 0.06cm -1 using CO absorption lines in a low pressure cell. The second step was to determine a set of micro-windows for O 3 in the infrared region that are appropriate for retrievals of vertical concentration profiles taking into account the limited spectral resolution of our instrument. The experimental data, in particular concerning the free troposphere, will be compared to predictions from an atmospheric chemistry model (CHIMERE) developed at LISA in order to improve its results, and also to satellite observations (IASI in particular) for validation. In addition, retrievals of the same trace gases combining data in different spectral regions will be attempted. Experimental setup Instrumental Line Shape set of micro-windows in IR for O 3 References : [1] C. Senten et al;Technical Note: New ground-Based FTIR measurements at Ile de La Réunion : observations, error analysis, and comparisons with independent data, Atmos. Chem. Phys. Discuss. 8, 827-891, 2008 [2] F. Hase et al: Intercomparison of retrieval codes used for the analysis of high-resolution, ground-based FTIR measurements, J. Quant. Spectrosc. Rad. Transf. 87, 25-52, 2004 IR : A Fourier Transform spectrometer (Bruker Vertex 80) is used for the infrared region 400  15000 cm -1 with a maximum spectral resolution of about 0.05 cm -1. UV-VIS : A grating spectrometer with a CDD array (Ocean Optics, HR 2000+) is used with 1.1 nm resolution (FWHM, sampling 0.035 nm) and covers the spectral range of 190  1100 nm. To select the IR MW’s of O 3 appropriate for the retrieval, we check the spectral region which has the larger variability according to a change of concentration (vmr) (from 10 to 30 %) in the troposphere and in the stratosphere using PROFFIT [2] (ground-based radiative transfer and retrieval model) First Results 1) 780-790 cm -1 2) 970-1150 cm -1 3) 2000-2140 cm -1 4) 2730-2820 cm -1 5) 3000-3100 cm -1 Conclusions FTIR Solar tracker Motorised dome rotationSet up  1) The new experimental set-up is now fully operational at LISA Créteil. 2) We will use H 2 O and O 3 to validate precision and accuracy of the new set-up. 3) Then we will validate satellite (IASI) and meteorological (ECMWF) H 2 O and O 3 data. 4) We aim to improve knowledge of free tropospheric composition above Paris (VOCs) FWHM = 0.06 cm -1 UV-VIS : main characteristics (07-28-08 at 11.00 am)  grating spectrometer with optical fiber  silicon CCD array 2048 pixels  Integration time 86 ms Band A of O 2 ATMOSPHERIC SPECTRA IR: main characteristics (07-22-08 at 16.08 pm)  OPD : 8.4 cm  1.5 mm aperture  DTGS detector  100 scans (32 min)  CaF 2 beamsplitter