Aerosol properties in a Chinese semiarid region

Aerosol optical properties and atmospheric water vapor content for 1999–2000 were obtained from ground-based solar radiometer measurements in Dunhuang, China. Seasonal changes of aerosol optical depth (AOD) and Angstrom wavelength parameter (ALPHA) were observed. Large values of AOD (at 500 nm) greater than 1.0 combined with low values of ALPHA less than 0.2 were mainly observed in spring and summer, which is consistent with the seasonal dust production from Gobi and Taklamaken deserts. The maximum monthly average of AOD was around 0.37 and occurred in the April of 1999, which was more than three times larger than the minimum monthly average in the November of 2000 (around 0.115). Thirty-three percent and 49% of observations of AOD in autumn and winter was larger than 0.2, respectively, whereas in spring, the percentage was approximately 63%. There was a large spread in ALPHAs in all seasons when AODs were less than 0.2. With the increase of AOD, ALPHAs decreased rapidly and were around zero finally. The large spread of ALPHA for low AODs and the domination of low turbidity in Dunhuang (nearly half of AODs less than 0.2) resulted in the negligible seasonal change of ALPHA. The domination of coarse particles over fine particles was also shown in aerosol size distribution retrievals, with the volume concentration ratio of coarse to fine particles being nearly 30. The volume mean radii and standard deviation of size distribution showed an insignificant correlation with AOD based on the analysis of all available retrievals, illustrating that it is suitable to use a fixed size model in the analysis of the dust climate forcing. During the dust outbreak episode, a rapid increase of AOD and a decrease of ALPHA were observed, ALPHA being usually less than 0.2 and even negative occasionally. The interesting result was that the volume mean radii of size distribution during the dust outbreak episode increased remarkably with the dust loading, which indicated that a dynamic size model should be used in the simulation of dust effects on meso-scale weather system in the semiarid region. The distinct seasonal change of water vapor content was observed, with high water vapor contents occurred in summer. The influence of water vapor on aerosol properties was absent due to the low humidification capability of dust aerosols and the different seasonal variation of aerosol properties and water vapor content.