* Corresponding author. Tel.: #30-31-998192; fax: #30-31- 283752. E-mail address: balis@ccf.auth.gr (D. Balis) Atmospheric Environment 34 (2000) 925}932 Tropospheric LIDAR aerosol measurements and sun photometric observations at Thessaloniki, Greece D. Balis*, A. Papayannis, E. Galani, F. Marenco, V. Santacesaria, E. Hamonou, P. Chazette, I. Ziomas, C. Zerefos Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, P.O. Box 149, 54006 Thessaloniki, Greece Physics Department, National Technical University of Athens, Greece Italian Space Agency, Italy C.N.R.-I.R.O.E., Italy Laboratoire des Sciences du Climat et de l'Environnement (LSCE), France Received 23 February 1999; received in revised form 22 June 1999; accepted 13 July 1999 Abstract We present measurements of the vertical structure of the aerosol backscattering coe$cient in the lower troposphere, which have been performed at the city of Thessaloniki in N. Greece, during the years 1996 and 1997. A ground-based backscatter lidar system operated throughout the year, mostly around local noon hours. The lidar measurements were accompanied by measurements of the aerosol optical depth in the visible spectral region, using a CIMEL sun-tracking photometer. The seasonal variation of the aerosol loading and its vertical distribution in the lower troposphere over the city of Thessaloniki is discussed. The maximum values of the aerosol optical depth are found during the spring season. Indication about the origin of these maxima is given by inspection of the various aerosol layers observed in the lidar pro"les. Most of the aerosol loading is present in the "rst 3 km height, and only in rare cases there are important aerosol layers detectable above 3 km, as in a case of Saharan dust transported over the city of Thessaloniki, in May 1997. Both instruments used in this study show similar seasonal variation of the aerosol load. It was found that almost 85% of the aerosol load is located in the layer below 3 km. There is a bias between the CIMEL and lidar derived optical depth at 532 nm, mainly attributed to the aerosols present between ground level and 600 m height, which represent up to 50% of the total aerosol optical depth.  2000 Elsevier Science Ltd. All rights reserved. Keywords: LIDAR; Aerosol, Optical depth 1. Introduction Atmospheric aerosols and clouds play a substantial role in the radiative forcing of the earth's climate, as they in#uence the radiation balance of the Earth, mostly through scattering and absorption processes (D'Almeida et al., 1991; Ackerman and Chung, 1992; Lenoble, 1993; Chazette et al., 1999). Aerosols are produced by a variety of processes, both natural (volcanic activity, desert dust storms) and anthropogenic ones (fuel combustion, bio- mass burning etc.) (Lenoble, 1993; Liousse et al., 1995; Tegen and Fung, 1995). Most aerosols of anthropogenic origin are found in the lower troposphere and contribute signi"cantly to the haze often visible during early morn- ing hours and near sunset near the earth's surface. These aerosols have residence times of few days, and thus are distributed inhomogeneously in the atmosphere, with maximum concentrations close to their source regions (Lenoble, 1993). Presently, there are still many uncertain- ties concerning the spatial distribution, the shape and chemical composition of the tropospheric aerosols (Hop- pel et al., 1985; Frejafon et al., 1998), and especially the chemical coupling between particulate matter and ozone. 1352-2310/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 3 1 7 - 9