Investigations of marine aerosols over the tropical Indian Ocean

Results of the studies on the characteristics of marine aerosol optical depths made in remote regions of the tropical Indian Ocean using a multiwavelength solar radiometer, taken on a scientific cruise, are presented. The dependence of the aerosol optical depths on prevailing winds in marine boundary layer and on deck level relative humidity (RH) are examined. Aerosol optical depths (τp) in far oceanic regions with marine air mass prevailing, in general, are found to increase nearly exponentially with average wind speed (Ua) as τp=τo exp(a Ua), where a = 0.16 ± 0.04. No association is seen with deck level relative humidity, as the effects of wind appear to offset the effects due to RH. The columnar size distributions retrieved from aerosol spectral optical depths reveal two modes, one at ∼0.04 μm and a secondary mode at ∼0.8 μm. The columnar mass loading estimated from the size distributions increases exponentially with wind speed and has a wind independent value of ∼28 mg m−2.