Importance of dust particles in the free troposphere over the Taklamakan Desert: Electron microscopic experiments of particles collected with a balloonborne particle impactor at Dunhuang, China

Abstract

Measurements of aerosol morphology and chemical elements were made in August 2002 at Dunhuang (40°00′N, 94°30′E), China, on the basis of direct sampling of free tropospheric aerosols with a balloonborne particle impactor, to understand nature of atmospheric particles over the desert areas in the Asian continent. Electron microscopic experiments of the particles directly showed that mineral (dust) particles were major constituents of coarse mode particles in the free troposphere over the Taklamakan desert. Typical types of the particles, according to energy dispersive X‐ray (EDX) analysis, were Si‐rich and Ca‐rich particles in heights of about 3–5 km, and the ratio of those particle number to total particle number was about 0.71 in coarse mode range (diameter larger than 1.0 μm). The ammonium sulfate particles were major in fine mode range (diameter smaller than 1.0 μm). This result shows good correspondence with the lidar measurements, which were made in collaboration with this balloonborne measurements. The large depolarization ratio, according to lidar measurements, distributed from near the surface to about 6 km, suggesting that lots of particles having irregular shape (possibly dust particles) were in the free troposphere in summer over the Taklamakan desert. Trajectory analysis of air masses showed the possibility that westerly wind transported those dust particles (Kosa particles) to downwind areas even in summer season above about 5 km, which is interesting and useful information to give explanation on the aircraft measurements made at Japan, showing possible transport of dust particles in the middle and upper troposphere in summer season.