Airborne observations of dry particle absorption and scattering properties over the northern Indian Ocean

[1] Airborne measurements of the absorption and scattering coefficients of dry aerosols, at 525 and 545 nm, respectively, were performed over the northern Indian Ocean during the Indian Ocean Experiment (INDOEX). In the marine boundary layer the dry absorption coefficient decreased from about 25 Mm(-1) close the Indian subcontinent to 2 Mm(-1) close to the Intertropical Convergence Zone (ITCZ), which reflects the removal and dilution of anthropogenically produced aerosols during their transport over the Indian Ocean. The dry scattering coefficient initially decreased with distance from the Indian subcontinent, from 70 Mm(-1) at 8degreesN to 10 Mm(-1) at 2degreesN, resulting in a dry single scattering albedo between 0.6 and 0.8 (0.7-0.9 at ambient relative humidity). At further distance from the Indian subcontinent the scattering coefficient and single scattering albedo increased, indicating the increasing importance of aerosols of natural origin. Using Mie theory, the refractive index of the anthropogenically produced aerosols has been estimated to be 1.50-0.06i.