Optical properties of the Indo-Asian haze layer over the tropical Indian Ocean

[1] Multiwavelength backscatter and extinction profiling was performed with a unique aerosol Raman lidar at Hulhule (4� N, 73� E), Maldives, as part of the Indian Ocean Experiment (INDOEX) between February 1999 and March 2000. The Raman lidar allowed a direct determination of the volume extinction coefficient of the particles at 355 and 532 nm at ambient conditions. Heavily polluted air masses from the Asian continent passed over the Maldives during the northeast monsoon seasons. The mean 532-nm particle optical depth was about 0.3; maximum values of 0.7 were measured. Above the polluted marine boundary layer, lofted plumes were found up to 4000-m height. On average, the freetropospheric aerosol layers contributed 30–60% to the particle optical depth. The volume extinction coefficient at 532 nm typically ranged from 25 to 175 Mm � 1 in the elevated layers. The pollution plumes are characterized separately for the air masses from Southeast Asia, North India, and South India. The analysis includes backward trajectories and emission inventory data for India. The extinction-to-backscatter ratio (lidar ratio) at 532 nm was mostly between 30 and 100 sr, and accumulated at 50–80 sr for highly absorbing particles from northern India. The shift of the lidar-ratio distribution for northern Indian aerosols by about 20 sr toward larger values compared to European values is consistent