One-year observations of particle lidar ratio over the tropical Indian Ocean with Raman lidar

Observations of the extinction-to-backscatter ratio (lidar ratio) of South and Southeast Asian aerosol particles are presented for the wavelength of 532 nm. Raman lidar measurements were performed in the Maldives (4.1 degrees N, 73.3 degrees E) in the framework of the Indian Ocean Experiment (INDOEX) in 1999/2000. These observations in the tropics axe an important contribution to a growing global lidar-ratio climatology which is needed for an improved determination of the particle optical depth with ground-based and spaceborne lidars. The lidar ratio was found to be a useful quantity to trace back different pollution sources and to identify less and considerably light-absorbing particles. During the winter/spring seasons heavily polluted air from India and Southeast Asia was advected to the lidar site. Under these conditions lidar ratios up to 110 sr were observed in the lofted pollution plumes above 1000 m height. According to backward trajectories the highest lidar ratios were found for airmasses which crossed the eastern and northeastern parts of India. Large lidar ratios > 70 sr indicate small, considerably absorbing aerosol particles. Below 1000 in height, the lidar ratio typically ranged from 30-60 sr. The marine boundary layer contained a mixture of marine and anthropogenic particles. Under clean, marine conditions in October 1999, lidar ratios <30 sr were found.