Features of aerosol optical depth with visibility grade over Beijing

The observation of direct solar radiation in an 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 the seasonal mean aerosol optical depth is larger in spring and summer, and smaller in autumn and winter. It is found that the aerosol spectral optical depth is closely related to surface visibility. For the same visibility level, it has a 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 comparison with calculations from the LOWTRAN 7 aerosol model are made at the same visibility level. The particle-size distributions are retrieved from observed aerosol optical depth spectra.