In this study, we examine seasonal and geographical variability of marine aerosol fine-mode fraction (f m ) and its impacts on deriving the anthropogenic component of aerosol optical depth (tau a ) and direct radiative forcing from multispectral satellite measurements. A proxy of f m , empirically derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 5 data, shows large seasonal and geographical variations that are consistent with the Goddard Chemistry Aerosol Radiation Transport (GOCART) and Global Modeling Initiative (GMI) model simulations. The so-derived seasonally and spatially varying f m is then implemented into a method of estimating tau a and direct radiative forcing from the MODIS measurements. It is found that the use of a constant value for f m as in previous studies would have overestimated tau a by about 20% over global ocean, with the overestimation up to ˜45% in some regions and seasons. The 7-year (2001-2007) global ocean average tau a is 0.035, with yearly average ranging from 0.031 to 0.039. Future improvement in measurements is needed to better separate anthropogenic aerosol from natural ones and to narrow down the wide range of aerosol direct radiative forcing.