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

Abstract

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 free‐tropospheric 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 with an aerosol black‐carbon content of up to 20%. The Ångström exponent (for short wavelengths 355/400/532 nm) ranged mostly from 1 to 1.6 for Southeast Asian particles, from 0.8 to 1.4 for North Indian pollution, and from 0.6 to 1 for South Indian air masses. The comparably low Indian Ångström exponents (indicating a comparably large mean particle radius) are possibly caused by the high contribution of biomass combustion to the aerosol formation in India. Results of a correlation analysis based on the lidar ratio, extinction coefficient, Ångström exponents, and relative humidity are presented. In most cases only a weak relationship or no relationship was found between the different parameters.