Features of aerosol optical depths over the Bay of Bengal and the Arabian Sea during premonsoon season: Variabilities and anthropogenic influence

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Abstract
Spectral aerosol optical depths (AOD) measured on board a cruise over the Bay of Bengal and the Arabian Sea during March–May 2006 are analyzed. Mean 0.5 μm AOD over the Bay of Bengal is higher (0.36) than the Arabian Sea (0.25). AODs obtained from MODIS Terra and Aqua are found to track well the Sun photometer AODs. A comparison between Sun photometer and MODIS AODs yielded a correlation coefficient of 0.96. MODIS fine mode fraction over the Bay of Bengal is higher (0.71) when compared to the Arabian Sea value of 0.60. Ångström exponent (α) over the Bay of Bengal is higher at 1.12, indicating the dominance of smaller size aerosols in the aerosol columnar distribution than the Arabian Sea (0.73), while Ångström coefficient (β) is comparable at 0.15 and 0.16 for the Bay of Bengal and the Arabian Sea. Background AODs are higher over the Bay of Bengal and the Arabian Sea indicating a strong continental influence. The scaling distance is lower over the Bay of Bengal due to nearly similar AODs, while the scaling distance over the Arabian Sea is about 2000 km. The anthropogenic influence in AODs over the Bay of Bengal and the Arabian Sea estimated using wind speed dependent AODs and maritime clean AODs are found to agree within ±1σ of the anthropogenic fraction obtained from MODIS AODs and FMFs. The mean anthropogenic contribution to the AODs estimated from the three methods is higher over the Bay of Bengal (68–75%) than the Arabian Sea (51–65%).

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