Four years aerosol observations with a Raman lidar at Thessaloniki, Greece in the framework of EARLINET

(EARLINET). Profiles of the aerosol extinction coefficient, backscatter coefficient, and extinction-to-backscatter ratio (so-called ‘‘lidar ratio’’) were acquired under nighttime conditions and have been used for statistical investigations. The statistical analysis was made both for the planetary boundary layer and for the free troposphere. It was found that the 4-year mean boundary layer particle optical depth at 355 nm was 0.44 ± 0.18, and the total aerosol optical depth was 0.63 ± 0.27. Free tropospheric particles account on the average for 30 % of the total aerosol optical depth, ranging from 5 % (clean free troposphere conditions) to 55 % (mainly Saharan dust events). For the cases examined, the integral of the lidar-derived extinction coefficient was in good agreement with colocated aerosol optical depth measurements at 355 nm obtained with a Brewer spectroradiometer. The mean value of the lidar ratio at 355 nm derived, for the period of measurements at Thessaloniki, was 40 sr with a standard deviation of 21 sr. Mean height profiles of the particle lidar ratio, extinction, and backscatter coefficients are shown along with their seasonal dependence, showing a significant seasonal variability in the free troposphere. An analysis of the data using back trajectories showed also a dependenc