The rate of photosynthesis primarily depends on nutrients and photosynthetically available radiation (PAR) at sea surface. Several ship cruises were carried out to measure optical, biological, and atmospheric parameters in the Arabian Sea and their variability were studied. An analytical nonspectral photosynthesis-irradiance model was used to estimate euphotic primary production (EuPP) to study its variability during cruise periods. PAR was estimated using COART model using in situ measured aerosol optical depth (AOD) to compare with in situ measured PAR. In order to understand the variability of PAR under different types of aerosol and different aerosol loading, a simulation study was carried out using COART model. EuPP was estimated for various PAR values under different aerosol loading and cloud coverage conditions. Sensitivity analysis showed that for maritime, maritime polluted, and desert aerosols, the ratio PAR/PAR0AOD has attenuated to about 11–25%, whereas it has attenuated to 44% for urban aerosol type. PAR/PARclear？？sky was reduced by ~57% for high aerosol loading and for overcast sky. The decrease in EuPP under various aerosol loading and cloud coverage was observed to depend on the photoadaptation parameter. EuPP/EuPPclear？？sky was reduced by 38% for maximum maritime aerosol loading and for overcast sky. 1. Introduction Biological process in the ocean is mediated through the process of photosynthesis, where marine phytoplankton converts inorganic carbon to organic carbon and removes carbon dioxide from the atmosphere. The rate, at which photosynthesis occurs, also termed as primary production, primarily depends on nutrients and photosynthetically available radiation, or PAR, (~0.4–0.7？μm wavelengths) at sea surface. Productivity varies with the availability of light and takes place within the euphotic zone. This extends from the surface to a depth where there is 1% of the light intensity from the surface. The photosynthetic response of phytoplankton to available light is not linear. It is light dependent at the lower light intensities and becomes independent (saturated) at higher light intensities, producing a curve which is described by its slope ( ) and the maximum photosynthesis ( ) [1]. As solar radiation passes through the earth’s atmosphere, some of it gets absorbed or scattered by different atmospheric constituents like aerosols, cloud cover, ozone, water vapor, and various gasses. On a daily level, cloudiness and aerosols have a significant influence on the amount of radiation that reaches the earth surface [2–4]. Arabian Sea and
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