Characteristics of the aerosol optical depth under the classification of the surface visibility

The observation of direct solar radiation in urban district of Beijing was carried out with a PIS spectrometer from March 1993 to March 1995. The aerosol spectral optical depths are deduced from the radiation data. The results show that although seasonal mean aerosol optical depth is larger in spring and summer, smaller in autumn and winter, the aerosol spectral optical depth is closely related to surface visibility. For the same visibility level, it has very weak seasonal variation. This means the seasonal variation of the aerosol spectral optical depth is mainly determined by the variation of surface visibility. The yearly averaged aerosol optical depth spectra under the various visibility levels are given and the comparison with the calculation from an aerosol model of LOWTRAN 7 are made at the same visibility level.