We present measurements of the vertical structure of the aerosol backscattering coefficient 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 profiles. Most of the aerosol loading is present in the first 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.