3D cloud radiative effects on clear sky reflectances and associated aerosol optical depth retrievals are quantified for a cumulus cloud field in a biomass burning region in Brazil through a Monte Carlo simulation. In this study the 1km MODerate-Resolution Imaging Spectroradiometer (MODIS) cloud optical depth and surface reflectance datasets are used to compute the 3D radiation fields with ambient aerosol optical thickness of 0.1 at a wavelength of 0.66 micron. The 3D radiative effects range from -0.015 to 0.018 with an average of 0.004 and standard deviation of 0.006. The 3D effects are most pronounced and variable for cloud neighboring pixels, where both large negative effects over shadows and positive effects near sunlit cloud edges are found. The clear next-to-cloud pixels, that contain ~83% of the clear pixel population, are affected in the most complex way and not reliable for aerosol retrieval. In the area 2 km away from clouds, the 3D effects enhance the reflectance in clear patches. The average and variability of enhancements gradually decrease as a function of the cloud-free distance, resulting in a systematically higher aerosol optical depth estimates for pixels closer to clouds in 1D retrieval. At a distance of 3 km away from clouds, the 3D effect is still appreciable with the average enhancement slightly less than 0.004. This enhancement will lead to an over estimate of aerosol optical thickness of ~0.04 in 1D retrieval, which is significant for an ambient atmosphere with aerosol optical thickness of 0.1.