Variation characteristics of atmospheric aerosol optical depths and visibility in North China during 1980-1994

Using a method developed by Qiu (Qiu, J., 1998. A method to determine atmos pheric aerosol optical depth using total direct solar radiation. J. Atmos. Sci. 55, 734-758), 0.75 mu m aerosol optical depths at five meteorological observatories in north China during 1980-1994 are retrieved from global dir ect solar radiation, and variation characteristics of the depths and visibi lity are analyzed. These observatories are located in the cities of Wulumuq i, Geermu, Harbin, Beijing and Zhengzhou. It is found that during 1980-1994 the aerosol optical depths show an increasing trend at all five sites. Dur ing winter the trend is stronger. In winter at Beijing and Wulumuqi, the de pth increased by a factor of about two in 15 years. Pollution caused due to the burning of fossil fuel is the main cause of the change. In spring at G eermu the depth is larger and its increase is the quickest among the four s easons, mainly due to desert dust events. The Pinatubo volcanic eruption in 1991 had a significant influence on the aerosol optical depth. The yearly averaged depths over five sites in 1992 after the eruption increased by 0.0 68 to 0.212, compared to those in 1990, while from 1992 to 1994 they genera lly show a decreasing trend. In some cities such as Zhengzhou and Geermu, b oth visibility and optical depth show an increasing trend during 1980-1994, a possible reason for this is that the aerosol particle vertical distribut ion shifts up in the troposphere. At Geermu, Harbin, Beijing and Zhengzhou, optical depths in summer are larger, which may be because of the growth of aerosol particles growing in the moist summer. Apart from Geermu, at the o ther four sites visibility in winter is smaller, especially at Wulumuqi and Harbin. At Harbin, visibility in summer is about twice larger than that in winter, but the difference between depths is small, implying the turbid lo wer troposphere in winter and the larger extinction coefficient in the uppe r troposphere during summer. (C) 1999 Elsevier Science Ltd. All rights rese rved.