Author Institution: Laboratoire de Physico-Chimie de l'Atmosphere, Universite du littoral Cote d'Opale, Dunkerque - France; Laboratoire de Physicochimie des Processus de Combustion et de l' Atmosphere, Universite des Sciences et Technologies de Lille; 59655 Villeneuve d'Ascq Cedex - FranceThe OA-ICOS cavity consisted of two 1{\textquotedbl} high reflectivity spherical mirrors with 1 m radius of curvature, separated by a 0.5-m long quartz coated stainless steel tube. The mirrors reflectivity was {\textgreater} 99.996\% at 1435 nm as specified by the manufacturer (Layertec, GmbH). The effective optical path length of the OA-ICOS approach was determined with direct absorption signal intensity of the pure H2O vapor line at 6965.80233 cm{}-1 and was found to be 1.263 km. \bigskip The OH radicals were generated with the help of a 2.45 GHz microwave discharge in water vapor flow under low pressure (1 mbar) to evaluate the developed OA-ICOS performance. The OH radical concentration of 7.28 e+13 OH/cm{}3 was determined using calibration with a close H2O absorption line at 6965.80 cm{}-1. The detection limit, deduced from the signal to noise ratio, was 3.86 e+11 OH/cm{}3. Experimental instrument detail and the preliminary measurement results will be presented and discussed
