Vertical profiling of optical and physical particle properties over the tropical Indian Ocean with six‐wavelength lidar: 2. Case studies

Abstract

We present the seasonal cycle of optical and physical particle properties over the Indian Ocean based on case studies of six‐wavelength aerosol lidar observations performed in the framework of the Indian Ocean Experiment. From February 1999 to March 2000 the lidar system made routine measurements at the International Airport of the Maldives on Hulule island (4.1° N, 73.3° E). The measurement from February 18, 1999, during the northeast monsoon showed high optical depths of 0.27 at 532 nm. Lidar ratios of 40–45 sr at 532 nm indicated the long‐range transport of well‐aged polluted air from Southeast Asia. Effective radii of <0.14 μm, complex refractive indices >1.6–0.01i, and single‐scattering albedos of 0.86–0.93 at 532 nm were retrieved for the pollution layer above 1000 m height. Similar physical particle properties followed from the measurement on March 22, 2000. Optical depths were 0.4; lidar ratios of 60 sr at 1000 m height indicated anthropogenic pollution from India. Values of 45 sr at 3000 m height indicated a considerable influence by clean‐continental particles from Africa and Arabia. Accordingly, the single‐scattering albedo ranged from 0.85–0.9. During the southwest monsoon in July, air was advected from the western Indian Ocean, eastern Africa, and Arabia. Particles were transported into maximum heights of 5 km from these regions on July 12, 1999. Small effective radii of 0.18–0.3 μm, considerably lower mean complex refractive indices of <1.6–0.02i, and single‐scattering albedos >0.88 indicated the prevailing clean‐marine and clean‐continental conditions. The intermonsoon season was characterized by strong washout processes during the crossover of the Intertropical Convergence Zone. On October 16, 1999, low optical depths of 0.13 and lidar ratios from 20–35 sr indicated clean‐marine conditions. Accordingly, large effective radii of 0.2–0.35 μm, low complex refractive indices ∼1.45–0.005i, and single‐scattering albedos >0.95 were found.