We report the results of sun-photometric measurements of Aerosol Optical Thickness (AOT) in India’s Exclusive Economic Zone (EEZ) over the Arabian Sea along with synchronous Ocean Color Monitor (OCM-II) derived AOT estimates during December 12, 2009–January 10, 2010. Relatively higher values of Angstrom exponent (α) around 1.2 near coast and 0.2–0.8 in the India’s EEZ, observed during the cruise period, indicate the presence of smaller particles near the coast due to anthropogenic activities; and larger particles in the India’s EEZ due to advection of pollutants from Indian subcontinent via long-range transport. Results related to α and its derivative reveal four different aerosol types (urban-industrial, desert-dust, clean-marine, and mixed-type) with varying fraction during the study period. Surface radiative forcing due to aerosols is found to be 20 W/m2 over India’s EEZ. OCM-derived AOTs showed good corroboration with in situ measurements with a correlation coefficient of about 0.95. A reasonably good correlation was also observed between AOT and wind speed (R = 0.6); AOT and relative humidity (R = 0.58). The concurrent MODIS AOT data also agree well with those observed by the OCEANSAT (OCM-II) satellite during the campaign period. 1. Introduction Although aerosols are minute particles, their cumulative radiative effect (both direct and indirect) on the atmosphere is tremendous [1]. They can scatter away and absorb the incident solar radiation leading to cooling of the earth’s surface and simultaneous warming of the lower atmosphere up to about 6？km [2, 3]. Aerosols also pollute the atmosphere and reduce visibility [4]. Continental aerosols are mainly wind-blown mineral dust and carbonaceous and sulphate particles produced by forest fires, land-use, and industrial activities, while marine aerosols are mainly sea-salt particles are produced by wave-breaking, and sulphate particles produced by the oxidation of Dimethyl Sulphide (DMS), released by the phytoplankton [5]. As the oceans cover more than 70% of the earth’s surface, they are one of the largest sources of natural aerosols. Being hygroscopic, marine aerosols are crucial in cloud formation in the marine boundary layer and are also important in the radiative coupling between the ocean and the atmosphere. While continental aerosols can be both scattering and absorbing, marine aerosols are mostly of scattering type [6], thus becoming a decisive factor in the albedo of the earth [7–10]. Aerosols are poorly characterized in climate models due to the lack of global information on their
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