The characteristics of atmospheric aerosol at Aksu, an Asian dust-source region of north-west China: A summary of observations over the three years from March 2001 to April 2004

Aerosol sampling and meteorological observations were carried out from March 2001 to April 2004 at Aksu, in the northern part of the Taklimakan Desert in north-west China. There were three intensive observation periods (IOP): 11-24 April 2002, 16-26 March 2003 and 10-17 March 2004. The observed maximum concentration of total suspended particles (TSP) was 5.2 mg m(-3) during the dust event of April 2003. Variations in the TSP and PM11 (particles smaller than 11 pm) show that high atmospheric concentrations were observed during dust events that occurred mainly in spring and summer. According to the meteorological observations, dust storms or blowing sand/dust occurred not only in spring, but also in summer and autumn, though floating dust occurred mainly in March and April. The frequency of dust events was consisted with the results of dry deposition. During spring and summer, the size-distribution of aerosols is unimodal with a maximum in the range 3.3-7.0 mu m, while in winter the size-distribution shows a bimodal pattern with two peaks; at 3.3-7.0 mu m for soil particles and 0.65-1.1 mu m for anthropogenic particles. From the results of size distribution of soluble ion concentration, anthropogenic particles consist mainly of ammonium sulfate and ammonium nitrate, which result from coal combustion for heating during winter. Concerning chemical composition, aerosols from Aksu contain a significant amount of evaporate minerals such as calcium carbonate (calcite), sodium chloride (halite), and calcium sulfate (gypsum and anhydrite ). These evaporates are common minerals in the saline soils which are widespread around observation site, and it is suggested that they are provided into atmosphere from the ground surface together with soil particles. Major element composition for aerosols in Aksu show a lower SiO2 content and higher FeO, K2O and MgO content compared with that for the saline soils ( phi < 125 mu m), the loess deposits in the Tarim Basin and Central Loess Plateau, and Taklimakan Desert sands (phi < 125 pm). This suggests that minerals rich in Fe, K and Mg, such as clay minerals and mica, come selectively from the soil surface into the atmosphere as mineral aerosols. This is because their particle size is generally smaller than that of quartz, the main constituent minerals in the soil.