Abstract: Based on the direct radiation data and related meteorological and pollutant data from 1961-2012 in Urumqi and Kashgar,this study investigated the characteristics and the annual and the seasonal variations of the atmospheric transparencies in the two regions. The atmospheric transparencies in Urumqi and Kashgar presented decreasing trends in general in the last 52 years and accelerated in the both regions since 1990s. The analysis of the impact factors of the atmospheric transparency proved that there was a negative correlation between the atmospheric dust content and the atmospheric transparency. Compared with in a sunny day, the daily peak value of the atmospheric transparencies decreased by 35.0%, 61.6% and 96.9% in the dust, the blowing-sand and the sand-stormy days, respectively. Vapor pressure also had a negative correlation with the atmospheric transparency. The cloud amount had more effects on the atmospheric transparency in Kashgar than in Urumqi. The atmospheric transparency can decrease by 61.5% in cloudy days in Kashgar. The cloud height had a negative correlation with the atmospheric transparency. At the beginning of the 1990s, the atmospheric transparency in Urumqi sharply attenuated. The peak values of the atmospheric transparency appeared in autumn instead of in winter as used to be in both Urumqi and Karshgar which indicated that the air pollution made the atmospheric transparency getting worse under the process of urbanization. NO₂ and SO₂ significantly affected the atmospheric transparency while PM₁₀, PM_2.5, PM_1.0 and BC had certain effects on the atmospheric transparency. O₃ had no effect on the atmospheric transparency.

Key words: atmospheric transparency, dust, vapor pressure, cloud amount, atmospheric pollutants, Xinjiang