[1] Aerosol samples were collected in the urban atmosphere of Beijing, China, by deploying a tethered balloon. Coarse particles (d > 1 mm) were individually analyzed using electron microscopes, to investigate the extent of dust modification by acidic gases in the atmosphere. Based on the elemental composition, irregularly shaped mineral dust was separated into carbonate and silicate groups. Both sulfate and nitrate were found to accumulate on carbonate more readily than silicate particles. Interestingly, spherical particles resembling Ca-carbonate in composition were spotted frequently in the samples. These Ca-rich spherical particles were more abundant under humid conditions, suggesting that they are deliquesced carbonate particles that formed in the atmosphere following the uptake of acidic gases. Sulfate and nitrate were more frequently detected in the Ca-rich spherical particles than in carbonate in the original solid form, indicating that the gas uptake efficiency of carbonate is further enhanced after the phase transition.

[1] Continental China has been recognized as one of the most important sources of atmospheric mineral dust particles (called Kosa in Japan, which literally means yellow sand). Many investigators have pointed out the importance of study of the long-range transport of mineral dust particles and their modifications in this process even during the nondust storm periods. Because of these modifications, particles can change their radiative properties and their ability to be a condensation nucleus. Therefore it is important to examine the composition of individual mineral particles in their source region and compare these particles with those after long-range transport. A number of investigations have been carried out on the subject; however, the amount of data is still insufficient. Samples of aerosol particles were collected in Dunhuang, China, in different seasons in 2001 and 2002 during the ACE-Asia campaign. The collected particles were examined using a scanning electron microscope equipped with an energy dispersive X-ray analyzer. The particles in all the samples were mainly mineral particles. Similar types of mineral particles were found in the free troposphere over Japan. A number of differences were found between the particles collected in China and those collected over Japan, and these differences can be explained by chemical modifications that occurred in the particles during their transport from China to Japan. Approximately 40-45% of mineral particles mixed internally with sulphate during their transport in the troposphere. Also, the particles collected over Japan were found to be different from those obtained in ground-based measurements in Nagasaki, Nagoya, and Fukuoka, Japan (reported by other research groups). The portion of mineral particles that mixed internally with sea salt and sulphates was considerably smaller than for the samples obtained in Japan near the ground. It is important to take this fact into account while investigating the impact of mineral particles on the biogeochemical cycle and climate.

[1] Lidar measurements were made at Dunhuang (40°00 0 N, 94°30 0 E), China, to understand the vertical distribution of aerosols in the free troposphere over the Taklamakan desert in summer of 2002. The vertical distributions of the scattering ratio suggested that particulate matter distributed from near the ground to about 6 km in the range of values of about 2 to 5 and rapidly decreased to about 1 at about 6 km. The depolarization ratio indicated that dust particles distributed in a aerosol layer, and the dust particle layer distributed to about 6 km. A very clear boundary was also identified at 6 km in the distribution of the depolarization ratio. Particulate materials were directly collected with a balloonborne particle impactor in the free troposphere over the Taklamakan desert, and an electron microscopic experiment on the particles suggested that the large depolarization ratio was certainly due to irregularly shaped dust particles. Vertical profiles of the scattering ratio and depolarization ratio suggested that nonspherically shaped dust particles floated from near the surface to about 6 km and this corresponded well with the wind system suggested by Sun et al. [2001] and Sun [2002], who reported that the typical surface wind was easterly and/or northerly in the Tarimu Basin, and westerly wind dominated above about 5 km. The trajectory of the balloon also showed that westerly wind appeared at about 4 km, and the wind speed largely increased above about 5 km. This suggests the possible long-range transport of dust particles entrained at an elevation of >5 km.

[1] Measurements of aerosol morphology and chemical elements were made in August 2002 at Dunhuang (40°00 0 N, 94°30 0 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 mm). The ammonium sulfate particles were major in fine mode range (diameter smaller than 1.0 mm). 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.

This study focuses on providing a direct insight into the process by which sulfate is formed on mineral dust surface in the actual atmosphere. Six sets of aerosol measurements were conducted in the outskirts of Beijing, China, in 2002-2003 using a tethered balloon. The mineralogy of individual dust particles, as well as its influence on the S (sulfur) loadings was investigated by SEM-EDX analysis of the directly collected particles.The mixed layer in the urban atmosphere was found to be quite low (500-600 m), often appearing as a particle dense stagnant layer above the surface. It is suggested that mineral dust is a common and important fraction of the coarse particles in Beijing (35-68%), and that it is relatively enriched with Calcite (>28%).An exceptional amount of S was detected in the mineral particles, which can be explained neither by their original composition, nor by coagulation processes between the submicron sulfates and the dust. Heterogeneous uptake of gaseous SO 2 , and its subsequent oxidation on dust was suggested as the main pathway that has actually taken place in the ambient environment. The mineral class found with the largest number of particles containing S was Calcite, followed by Dolomite, Clay, Amphibole etc., Feldspar, and Quartz. Among them, Calcite and Dolomite showed distinctly higher efficiency in collecting sulfate than the other types.A positive correlation was found with the number of S containing particles and the relative humidity. Calcite in particular, since almost all of its particles was found to contain S above 60% r.h. On the other hand, the active uptake of SO 2 by the carbonates was not suggested in the free troposphere downwind, and all the mineral classes exhibited similar S content. Relative humidity in the free troposphere was suggested as the key factor controlling the SO 2 uptake among the mineral types. In terms of sulfate loadings, the relationship was not linear, but rather increased exponentially as a function of relative humidity. The humidity-dependent uptake capacity of mineral types altogether showed an intermediate value of 0.07 gSO 2− 4 g −1 mineral at 30% r.h. and 0.40 gSO 2− 4 g −1 mineral at 80%, which is fairly consistent with laboratory experiments.