Since 2000, regular lidar observations of the vertical aerosol distribution over Europe have been performed within the framework of EARLINET, the European Aerosol Lidar Research Network consisting of 22 lidar stations distributed over 12 European countries. Lidar observations are performed on a regular schedule of one daytime measurement per week, with a well developed boundary layer condition, and two night time measurements per week, with low background light, in order to perform Raman extinction measurements. In addition to the routine measurements, further observations are devoted to monitor special events such as Saharan dust outbreaks, forest fires, photochemical smog, volcano eruptions and the advection of North American air-pollution plumes. The data quality has been assured by performing intercomparisons at instrument level by using the available transportable systems. The quality assurance also included the intercomparison of the retrieval algorithms for both backscatter and Raman lidar data. Within EARLINET, a comprehensive, quantitative, and statistically significant data base for the aerosol distribution on a continental scale has been established. In particular, this database contains a large data set concerning the ratio of aerosol extinction to backscatter (lidar ratio) starting from both regular and special measurements and this is by far the largest data set of lidar ratio data on continental scale covering 3 years of systematic observations. Lidar ratio data are retrieved from simultaneous and independent lidar measurements of aerosol extinction and backscatter coefficients. These measurements, in conjunction with information on the air masses characteristics, can provide information on microphysical properties of the aerosol on a wide range of meteorological conditions on a continental scale. Within EARLINET, 10 lidar stations have the capability of measuring nitrogen Raman scattering in the UV simultaneously to the elastic backscatter; among these lidar stations, two have the capability to measure nitrogen Raman scattering also in the visible domain (Kühlungsborn and Leipzig). All the lidar ratio measurements have been collected and divided between regular measurements establishing the climatology, and special measurements (Saharan dust outbreaks, forest/industrial fires, photochemical smog episodes, volcanic eruptions etc.). A statistical analysis on climatological data has been performed: mean values of the lidar ratio data in the Planetary Boundary Layer have been calculated and for each station the seasonal variation and the frequency distribution have been studied.

ACKNOWLEDGEMENT The financial support of this work by the European Commission under grant EVR1-CT1999- 40003 is gratefully acknowledged. The authors also thank the German Weather Service for the air mass back-trajectory analysis.