The interactions between aerosols and solar radiation are determined by a
combination of aerosol properties (i.e., types), surface properties (i.e., albedo) and clouds.
These determining factors vary for different regions. We examine how these differences
contribute to the impact of aerosols on the top-of-atmosphere (TOA) and surface
radiation. In this study, the AERONET (Aerosol Robotic Network) aerosol climatology is
used, in conjunction with surface albedo and cloud products from Moderate Resolution
Imaging Spectroradiometer (MODIS), to calculate the aerosol direct radiative effect
(ADRE) and its normalized form (NADRE). The NADRE is defined as the ADRE
normalized by optical depth at 550 nm and is mainly determined by internal aerosol
optical properties and geographical parameters. These terms are evaluated for cloud-free
and cloudy conditions and for all-mode and fine-mode aerosols. Single-scattering albedo
is an important variable determining ADRE of biomass burning. Because of stronger
absorption by the smoke over South Africa the average NADRE over South America is

35% larger at the TOA but
38% smaller at the surface than that over South Africa. The
surface albedo is another important factor in determining ADRE, especially for
mineral dust. As the surface albedo varies from
0.1 to
0.35, it is observed that the dust
NADRE ranges from 44 to 17 Wm2 t1 at the TOA and from 80 to 48 Wm2 t1
at the surface over the Saharan deserts, Arabian Peninsula, and their surrounding
oceans. We also find that the NADRE of fine-mode aerosol is larger at the TOA but
smaller at the surface in comparison to that of all-mode aerosol because of its larger singlescattering
albedo and smaller asymmetry factor. Cloudy-sky ADRE is usually not
negligible for the observed cloud optical thickness, but the TOA ADRE with clouds is
sensitive to the relative location of aerosols and cloud layer.