Beijing experiences air pollution such that the sky overhead is gray much of the time even on cloudless days. In order to understand the cause of this problem, the aerosol light scattering coefficient &sgr;sp and absorption coefficient &sgr;ap were measured under dry conditions (instrumental relative humidity 1.0 μm), the submicron aerosol was responsible for ~80% of the light scattering at 530 nm. The largest contribution to the PM2.5 aerosol mass was due to organic compounds, which accounted for ~30% of the mass. The contributions of sulfate, ammonium, and nitrate to the PM2.5 mass concentration were ~15%, 5%, and 8%, respectively. Mineral aerosol contributed ~16% to the PM2.5 aerosol mass. These data show that combustion-related particles rather than wind-blown dust dominated the light extinction budget during June 1999. ¿ 2001 American Geophysical Union