Abstract: The physical and optical properties of aerosols in Beijing were analyzed based on ground-based remote sensing data collected by CIMEL sun/sky radiometers in spring of 2001. Aerosol optical depth at 6 wavelengths from ultraviolet to infrared was obtained from direct solar radiance measurements. Combined solar direct and scattering radiances were utilized to invert aerosol size distribution, refractive index and single-scattering albedo, as well as integral quantities such as modal radius, etc. Dramatic increase of aerosol optical depth from ~0.1 to high values of ~4 at 440 nm was observed during dust periods in Beijing; simultaneously, Angstrom wavelength exponent (¦Á), a measure of aerosol size, decreased to nearly zero, showing aerosol neutral or anomalous extinction due to emission of abundant large dust aerosols into the atmosphere. Anthropogenic sources also contributed remarkably to high values of aerosol optical depth in BJ, but with combination with relatively large values of ¦Á. Aerosol refractive index and absorption in BJ demonstrated notable difference between dusty and non-dusty period due to large difference in these properties between dust aerosols and anthropogenic emissions. Dust activities may reduce atmospheric absorption due to input of dust aerosols with lower absorption than that of urban pollution. Hence, dust activities may influence aerosol direct radiative forcing over downwind regions via their effects on aerosol loading, but also through their effects on aerosol extinction properties. Taking the average values of aerosol optical depth, single-scattering albedo and Angstrom wavelength exponent corresponding to non-dusty and dusty period as the inputs to SBDART, the results showed notable differences in surface, TOA and atmospheric direct radiative forcing efficiency.