Variability in aerosol optical and physical characteristics over the Bay of Bengal and the Arabian Sea deduced from Ångström exponents

Abstract

Spectral distribution of aerosol optical depths (AODs) measured in the 0.4–0.875 μm wavelength region using a Sun photometer over Bay of Bengal and Arabian Sea during the 2006 premonsoon season are analyzed to obtain more interesting information on the physical and optical characteristics of aerosols. Examination of spectral AODs measured over the Bay of Bengal and the Arabian Sea by deriving the Ångström exponent (α) for the entire spectral range (0.4–0.875 μm), α for different spectral ranges, and second derivative (α′) showed that the aerosol size distribution is of mixed type or bimodal with contributions from fine and coarse modes. The α‐AOD relationships in short (0.4–0.5 μm), long (0.65–0.9 μm), and full (0.4–0.9 μm) spectral ranges determined for various aerosol models (urban, maritime clean, maritime polluted, and desert) suggest that the α‐AOD relationship can vary depending on whether the size distribution is unimodal, mixed type, or bimodal, similar to the results obtained for measured AOD spectra. Significant curvature in the ln AOD versus ln λ is observed which causes spectral variation in α derived in different spectral ranges. Over the Bay of Bengal for 76% of AOD spectra, α2 − α1 is >1, suggesting the presence of fine‐mode aerosols from a wide variety of fine‐mode fractions or a mixture of modes, while over the Arabian Sea, α2 − α1 is <1 for 84% of AOD spectra, clearly indicating the dominance of coarse‐mode aerosols. These characteristics can be used in modeling the regional and seasonal aerosol radiative effects and in remote sensing.