Based on sensitivity of the direct solar short radiation on the aerosol property

a so called wide band extinction method is proposed to determine the 0 75μm aerosol opitical depth using th eradiation

and its accuracy is analyzed according to numerical simulations and comparative measurements As shown in numerical simulations using Junge and LOWTRAN aerosol size distributions

the depth solution error of <5% can generally be expected if a typical Junge distribution of v *=3 is taken instead of true distribution The method is tested in comparative observations with the result that aerosol optical depths measured by pyrheliometer conform well with those by sunphotometer As far as total 1267 sets of comparative experiments are concerned

the standard deviation is 10 5%

and the deviation of the mean optical depth only-0 7% Monthly mean or yearly mean result would have much better accuracy The present method is used in determining 0 75μm aerosol optical depths from the direct solat radiation detected by the phyrheliometer over 10 sites in China during 1980—1994 The 10 sites are meteorological observatories of Beijing

Shenyang

Wulumuqi

Germo

Keshi

Zhengzhou

Wuhan

Shanghai

Kungming and Guangzhou Some conclusions are: （1） Except Shenyang

aerosol optical depths over other 9 sites have an increasing trends with the annual increasing rates changing from 0 0008（Germo）to 0118（Beijing） （2） Pinatubo vocalnic eruption in 1991 had very great influence on aerosol optical depth Compared to that before 1990

the aerosol optical depths in 1992 over the ten sites increased obviously from 0 041 to 0 295 （3） In Wuhan

Beijing

Shenyang

Guangzhou and Zhenzhou cities

aerosol pollution was rather seriousq The total means of the aerosol optical depths during 1980—1994 were all more than 0 4

and especially in Wuhan

0 551 Germo and Kunming were relatively clear

and in particular in Germo it was only0 1687 （4） In the five sites of Kunming etc in the southern of Yellow River

the aerosol optical depths were relatively big in spring Except Wulumuqi

in the sites in the northern of Yellow River

they were relatively high in spring and summer and low in autumn and

inWulumuqi and Shenyang

they were comparatively high in winter