The Mid-Long Term Features of Atmospheric Aerosol Optical Depths over Beijing Area
北京地区气溶胶光学厚度中长期变化特征

The speCtral optical depths of atmospheric aerosols in the urban area of Beting are deduced from the direct solar radiation data from 1977 to 1985.In addition,the successive high speCtralresolution observations of the radiation with a PIS spectrometer are conducted from March 1993 toMarch 1995 under clear and partly cloudy conditions in the same area. The features of aerosol opticalproperties are described for an unprecedented long period. The results show that(1) aerosol opticaldepths (AOD) are thick in spring-summer, and thin in autumn-winter;(2) with the classification of thesurfaCe visibility the seasonal difference of the AOD is greatly reduced for the same kind of visibility,and the amplitude of the AOD is limited by the visibility; (3) there is a tendency of year by year increaseof the AOD, and on the yearly averaged basis the AOD in 1994 has increased by two thirds of that in1977.