Abstract: In this paper,aerosol optical depths (AODs) at four ngstrms of 440,675,870,1 020 nm and ngstrm exponent between 440 nm and 870 nm are retrieved based on the observations of CE318 sun photometer from May 2002 to March 2005 at Hetian station. After making cloud screening and data quality controlling of these retrieved optical parameters,the aerosol optical properties over the southwestern margin of Taklimakan Desert are analyzed. The results show as follows: the frequency distribution of AOD can be well fitted by a bi-mode normal distribution centered about 0.31 and 0.64 with the standard deviation of 0.007 and 0.11,respectively. The frequency distribution of ngstrm exponent can be well fitted (R²=0.99) by a bi-mode normal distribution centered about 0.12 and 0.33 with the standard deviation of 0.002 and 0.022,respectively. And more than half of the ngstrm exponent data are concentrated around the peak whose center is 0.12.There is obvious seasonal variation trend of aerosol optical properties at Hetian. The annual average AOD at 440 nm reaches about 0.61 with the standard deviation of 0.35. AODs are high in spring and summer but low in autumn and winter. ngstrm exponent are low in spring and summer but relatively high in autumn and winter. The maximum AOD occurs in May with a value of 0.81±0.48,and the minimum in November with a value of 0.31±0.12.The maximum and minimum ngstrm exponent occur in January with a value of 0.66±0.26 and May with a value of 0.06±0.06,respectively. The diurnal variation of AOD (ngstrm exponent) shows the characteristic of high (low) values in the morning and evening,and is relatively constant during the daytime. The annual averaged AOD and ngstrm exponent are relatively stable with the values around 0.57-0.60 and 0.22-0.23,respectively; however,there is no significant annual variability during 2002-2004. The relationship between AOD and ngstrm exponent shows obvious pattern of decreasing ngstrm exponent with increasing AOD, which indicates the dust aerosol of larger particle size are the major contributions to the high AOD. Compared with the existing study results,one can see that the aerosol optical properties at Hetian show the typical desert aerosol characteristics,while anthropogenic aerosols have little effect on the aerosol optical properties of this region.

Key words: atmospheric aerosol, AOD, ngstrm exponent, Taklimakan Desert