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

Measurements made during the Indian Ocean Experiment (INDOEX) have shown th e presence of large aerosol loadings over the region of the northern Indian Ocean and Arabian Sea. In recent years there has been significant interann ual variability in the magnitude of this aerosol loading during the NE mons oon months of. January-April. Monitoring of the integrated atmospheric colu mn effective aerosol optical properties was initiated in early 1998 and con tinued in 2000 on the island of Kaashidhoo in the Republic of Maldives. An Aerosol Robotic Network Sun-sky radiometer at the Kaashidhoo Climate Observ atory made spectral measurements of the direct Sun and directional sky radi ances which were utilized to infer spectral aerosol optical depths tau (a), single scattering albedos, asymmetry factors, and aerosol size distributio ns. 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 investig ated, 1998-2000. Interannual variations in the monthly mean Angstrom wavele ngth 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 fin e mode aerosols dominated over the optical influence of coarse-mode aerosol s. Some differences in spectral single scattering albedo and asymmetry fact or were observed for 1999 versus 2000 in the infrared wavelengths, but with relatively little change in the visible wavelengths. The spectral variatio n in the retrieved single scattering albedo was large, with approximately l inear wavelength dependence averaging from 0.91 at 440 nm to 0.83 at 1020 n m for January-March 1999 for observations where tau (a) at 440 nm >0.4.