Indo-Asian pollution during INDOEX: Microphysical particle properties and single-scattering albedo inferred from multiwavelength lidar observations

Abstract

We present for the first time a comprehensive data set of vertically resolved physical particle properties, and the single-scattering albedo at 532 nm, derived from multiwavelength lidar observations of pollution plumes advected from India and Southeast Asia out over the tropical Indian Ocean during the northeast monsoon. The parameters follow from the inversion of optical data that were obtained from six-wavelength aerosol lidar observations at Hulhule Island (4.1°N, 73.3°E), Maldives, in the framework of the Indian Ocean Experiment (INDOEX) (February/March 1999) and three follow-up campaigns in July and October 1999 and in March 2000. Effective particle radii were 0.20 ± 0.08 μm for pollution plumes above 1-km height, advected during the northeast monsoon. Volume concentrations ranged from 6 to 44 μm3 cm-3, and surface-area concentrations were 95-774 μm2 cm-3. The particles showed substantial absorption when advected from northern India. The imaginary part of the wavelength-independent complex refractive index was as large as 0.045i. The respective mean value was 0.022i ± 0.014i for observations during the northeast monsoon. The mean real part was 1.54 ± 0.11. The mean single-scattering albedo at 532 nm was 0.90 ± 0.06. Values were as low as 0.8 during advection of air from northern India. On average, less absorbing particles were advected from southern India and Southeast Asia. The numbers indicate that the major contributor to the observed pollution from northern India is absorbing soot-like material from, e.g., fossil fuel and firewood burning. The effective radius was well correlated with the Ångström exponents of the underlying extinction spectra. Strong correlation of the single-scattering albedo was found not only with respect to the imaginary part of the complex refractive index but also with respect to the particle extinction-to-backscatter ratio. A comparison to Ångström exponents and single-scattering albedo obtained from Sun photometer and satellite observations showed good representativity of the lidar-derived quantities. Comparison with results from airborne in situ observations showed substantial deviations of Ångström exponents and single-scattering albedo.