Characteristics of aerosols over a remote island, Minicoy in the Arabian Sea: Optical properties and retrieved size characteristics

Changes in the characteristics of atmospheric aerosol over a marine environment are investigated by making regular spectral extinction measurements in the visible and near-infrared region from a tiny island location, Minicoy (8.3 degrees N, 73.04 degrees E), situated in the Arabian Sea about 400 km due west of the southern tip of the Indian peninsula. The role of seasonally changing air-mass type in causing a regular annual variation in the spectral optical depths is delineated. The association between aerosol optical depths, surface wind speed and rainfall is examined. An increase in wind speed causes an increase in optical depths, the effect is predominant when a marine air mass prevails. The impact of changes in wind speed on optical depths (due to sea-spray production over the sea) is parametrized in the case that the island is influenced by a marine air mass. Columnar size distributions, retrieved from the spectral optical depths, in general, show a bimodal lognormal distribution in the optically active size range. The accumulation mode is more sensitive to continental air-mass types, while the coarse mode is influenced by the marine conditions. The coarse mode is sharper but its position is variable. Increase in wind speed leads to a remarkable enhancement in the concentration and relative abundance of coarse particles, particularly during the monsoon season. The mass loading and effective radius are well associated and depend on wind speed histories. The findings are discussed.