Intercomparison of Instruments for Tropospheric Measurements using Differential Optical Absorption Spectroscopy A contribution to subproject TOPAS C. Camy-Peyret 1 , M. Carleer 2 , C. Clerbaux 2 , R. Colin2, A.C. Vandaele3, C. Hermans3, P.C. Simon3, B. Bergquisk4, B. Galle4, C. Fayt 5 , F. Goutail 6 , M. Nunes-Pinharanda 6 , J.P. Pommereau6, M. Hausmann 7 , F. Heintz7, U. Platt7, I. Pundt 7 , T. Rudolph7, J. Planes and N. Smiths 1 Universite Pierre et Marie Curie, Paris, France 2 Universite Libre de Bruxelles, Brussels, Belgium 3 Institut d 'A eronomie Spatiale de Belgique, Brussels, Belgium 4 Swedish Environmental Research Institute, Goteborg, Sweden 5 Universite de Mons-Hainaut, Mons, Belgium 6 Service d 'Aeronomie CNRS, Verrieres-le-Buisson, France 7 Universitiit fiir Umweltphysik, Heidelberg, Germany 8 University of East Anglia, Norwich, England In order to assess the accuracy and the advantages of instruments using the Differential Optical Absorption Spectroscopy technique, is was decided to organize an international intercomparison campaign of instruments. This campaign took place in an urban site in Brussels in September 1992. Eight different instruments were present and measured simultaneously several atmospheric consti~uents, in the same portion of the troposphere. All measurements were made in the UV and visible region and were based on the DOAS technique. This technique is based on the recognition of the spectroscopic signatures of the absorbants and the fact that no reference spectrum, without any absorption features, is obtainable. Critical parameters in this method are the absorption cross sections of the molecules to be detected. It was decided that each group should use the same references [1-3]. A brief description of all participating instruments can be found in Table 1, where the type of instrument and detector, the absorption path denomination, the measured molecules and the recording time are indicated. All the instruments were installed in the same laboratory at an altitude of about 20 m from ground level and pointing in the NW direction towards a set of sources and retroreflectors placed at various distances. It was decided to limit the intercomparison to the species NO 2 , 0 3 and SO 2 , however most of the instruments present could detect more constituents. It can be seen from Table 1 that the instruments can be classified in three groups, depending on their recording time. It was decided to compare the instruments on three different time scales: 5, 20 and 45 minutes. Fig. 1 shows the evolution of the concentrations of NO 2 , 0 3 and SO 2 for the 24 hour period starting at 4.00 pm on the 14th of September, as measured by all the instruments. The measurements integrated over a period of 45 min are presented in The Proceedings of EUROTRAC Symposium '94 edited by P.M. Borrell et al., pp. 935-939 © 1994 SPB Academic Publishing bv, The Hague, The Netherlands