Column-integrated aerosol optical properties over the Maldives during the northeast monsoon for 1998–2000

Abstract

Measurements made during the Indian Ocean Experiment (INDOEX) have shown the presence of large aerosol loadings over the region of the northern Indian Ocean and Arabian Sea. In recent years there has been significant interannual variability in the magnitude of this aerosol loading during the NE monsoon months of January-April. Monitoring of the integrated atmospheric column effective aerosol optical properties was initiated in early 1998 and continued in 2000 on the island of Kaashidhoo in the Republic of Maldives. An Aerosol Robotic Network Sun-sky radiometer at the Kaashidhoo Climate Observatory made spectral measurements of the direct Sun and directional sky radiances which were utilized to infer spectral aerosol optical depths τₐ, single scattering albedos, asymmetry factors, and aerosol size distributions. Monthly average aerosol optical depths at 500 nm varied by more than a factor of 2 during January through April for the 3 years that were investigated, 1998–2000. Interannual variations in the monthly mean Angstrom wavelength exponent were also observed, resulting from differences in the bimodal aerosol size distributions. Spectral variations in the Angstrom wavelength exponent were observed, especially at high aerosol optical depths when fine mode aerosols dominated over the optical influence of coarse-mode aerosols. Some differences in spectral single scattering albedo and asymmetry factor were observed for 1999 versus 2000 in the infrared wavelengths, but with relatively little change in the visible wavelengths. The spectral variation in the retrieved single scattering albedo was large, with approximately linear wavelength dependence averaging from 0.91 at 440 nm to 0.83 at 1020 nm for January–March 1999 for observations where τₐ at 440 nm >0.4.