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Free tropospheric aerosol particles were collected using a balloon-borne particle impactor in August of 2002 and March of 2003 at Dunhuang in northwestern China (40°00′N, 94°30′E), and the morphology and elemental composition of the aerosol particles were analyzed in order to understand the mixing state of coarse dust particles (diameter >1 μm) over the desert areas in the Asian continent in spring. Electron microscopic experiments on the particles revealed that dust particles were major constituents of coarse mode particles in the free troposphere over the Taklamakan Desert in spring and summer. Si-rich or Ca-rich particles are major components of dust particles collected in the free troposphere over dunhuang and the values of [number of Ca-rich particles]/[number of Si-rich particles] differs markedly between spring and summer, being about 0.3 in the spring of 2003 and about 1.0 in the summer of 2002 at heights 3–5 km above sea level. It is likely that the condition of the ground surface and the strength of vertical mixing in source areas of Asian dust are potential factors causing the difference in the chemical types of dust particles. Comparison of the elemental compositions of these particles with those of particles collected over Japan strongly suggests that these particles were chemically modified during their long-range transport in the free troposphere. Analysis of wind systems shows that both the predominating westerly wind in the free troposphere and the surface wind strongly controlled by the geographical structure of the Tarim Basin are important in the long-range transport of KOSA particles originating in the Taklamakan Desert. [ABSTRACT FROM AUTHOR]

Measurements of aerosol were made in August and October 2001, and January 2002, at Dunhuang, China (40°00′N, 94°30′E), to understand the nature of atmospheric particles over the desert areas in the Asian continent. Balloon-borne measurements with an optical particle counter suggested that particle size and concentration had a noticeable peak in size range of super micron in not only the boundary mixing layer but also the free troposphere. Thickness of the boundary mixing layer, from distributions of particle concentration, was about 4 km in summer (17 August 2001), about 2.5 km in fall (17 October 2001), and about 3 km in winter (11 January 2002), which suggest active mixing of particles near the boundary in summer. Number-size distribution of particle showed a noticeable peak in the super micron particles size range in the mixing boundary layer: 0.4–2 particles cm-3 at diameter >1.2 μm in summer, 0.05–4 particles cm-3 at diameter >1.2 μm in fall, and 0.1–5 particles cm-3 at diameter >1.2 μm in winter. In winter strong inversion of atmospheric temperature was found in the height range from the boundary to about 3 km and vertical distribution of particle concentration well corresponded with the temperature distribution. Chemical elements of individual aerosols, which were collected in the boundary layer atmosphere at Dunhuang (18 October 2001) were analyzed with an electron microscope equipped with EDX. Those single particle analysis suggested that most of the particles with super micron size were soil particles, and those particles had little sulfate on its surface. This is a very important different point, comparing with the chemical state of soil particles, which were transported from the desert area of China to Japan, and showed frequently the existence of sulfate on the particle surface. Therefore, it is strongly suggested that dust particles can be chemically modified during their long-range transport from desert areas to Japan. [ABSTRACT FROM AUTHOR]

Vertical changes of aerosol concentration and size in the free troposphere over the Asian desert areas were firstly observed using a balloon-borne optical particle counter at DunHuang, China (40°00′N, 94°30′E) (17 August and 17 October 2001, and 11 January 2002). In the free troposphere highly concentrated aerosol layers were frequently observed, suggesting the importance of regional scale particle transportation over the Asian continent. Concentration of particles with a diameter larger than 0.15 μm was about 5–10 particles cm-3 in the free troposphere. Particle number-size distribution in the free troposphere shows important contribution of super micron particles. Regional scale transportation, in addition to diffusion of soil particles from the lower atmosphere to the free troposphere through local and small scale air motions, is suggested by backward trajectory analysis of air masses containing super micron particles. The importance of horizontal transport of coarse size particles in the free troposphere was strongly suggested. Thickness of the boundary mixing layer, from distributions of particle concentration, was about 4 km in summer (17 August 2001) and apparently higher than the height of layers in fall (17 October 2001) and in winter (11 January 2002), which suggest an active mixing of particles near the boundary in summer. In winter measurement (11 January 2002), strong inversion was found in the vertical profile of temperature, suggesting cold ground surface and vertically stable atmosphere near the ground. [ABSTRACT FROM AUTHOR]