Current Issue

The morphology of original desert sands collected from four different desert areas was displayed in Photo 1. Nearly all of sands have irregular shape with yellowish coloration, whereas some sands show peculiar colors such as bluish, blackish, and reddish. Minerals are colored because certain wavelengths of light are absorbed. It is suggested that this color dissimilarity is influenced largely by chemical composition and atomic structure of the mineral. Idiochromatic color in mineral can be caused by the presence of elements like Cu and Mn in the mineral. For example, Cu in azurite, Cu in malachite, and Mn in rhodonite show blue, green, and pink color, respectively (Bates and Jackson, 1987). Chromaphore color can be caused by the presence of a small concentration of elements like Fe, Mn, Cu, Cr, Co, Ni, and V. Green quartz, black calcite, and red color in many minerals can be caused by dispersions of chlorite, MnO₂, and hematite, respectively (Bates and Jackson, 1987).

Photo 1. 
Microscopic image of sands collected at four desert areas in China (×100).

Fig. 2 illustrates the size distribution of original sands collected at four desert areas in China. The size of individual sands was calculated with the mean of apse line and minor line of each sand by using digital microscope (KEYENCE, VH-7000). For each sample, 500 sands were selected randomly for their sizing. The shape of size distribution for sands collected at Yinchuan, Wuwei, and Dulan sites is monomodal to show the maximum level between 200 and 300 μm. On the other hand, the maximum level for Yanchi desert sands is displayed between 100 μm and 200 μm. The size of sands at Wuwei desert area is relatively wide spread in the range up to 600 μm. As shown in Photo 1, the size of original sands collected at Yanchi desert site is relatively smaller than those collected at other three sites. In addition, many broken pieces of sand at several micrometer size are observed in the field of Yanchi sands. Hence, the sands of Yanchi desert area are to be fragmented into tiny pieces by aeolian processes. This desert dust finally dissipates when the dust particles are removed from the atmosphere by dry and/or wet removal processes (Ma et al., 2004b). As mentioned earlier, gravitational settling of desert sands and large dust particles (>10 μm) occurs near the desert regions within the first day of transport (Husar et al., 2001).

Fig. 2. 
Relative size distribution of original sands collected at four desert areas in China.

Wet removal occurs sporadically throughout the 5-10 day lifetime of dust particles with the remaining smaller size (Husar et al., 2001). Desert dust interaction with clouds results in cloud scavenging, while clouds can be removed by precipitation (Desboeufs et al., 2001). Cloud and precipitation thus can contribute significantly to the removal of Asian dust storm particles from the atmosphere (Ma et al., 2004b; Ma et al., 2004c).

Fig. 3 displays PIXE spectra of both insoluble (a) and soluble fraction (b) of original bulk sands collected in four different desert areas. In like with general expectation, soil originated components (such as Si, K, Ca, and Fe) are resolved as the remarkably high counts with no relevance to water solubility. The minor trace elements (like Cr, Cu, Zn, and Sr) are also in compliance with each channel number as small peaks in insoluble fraction of dust sands. Several elements in insoluble portion, especially Ca, have a relatively severe site-to-site variation with the maximum level in the sands of Dulan site. Elsewhere, in comparison to insoluble portion, albeit negligible in peak size relative to the other elements, Cl is found at the spectrum of water soluble portion.

Fig. 3. 
PIXE spectra of insoluble (a) and soluble fraction (b) for original bulk sands collected in four different desert areas.

The mass concentration (ng g^-1) for multielement can be quantitatively calculated by channel range and the characteristic X-ray count of each element. In order to conduct quantitative elemental analysis, the masses of deposited sand powders onto polycarbonate substrate film have to be precisely measured. However, due to the small amount of pulverized sands homogenously mounted onto polycarbonate substrate film, it is not possible to conduct mass measurement. Hence, the concentration ratios of Z/Si are listed for soluble and insoluble fractions of the four kinds of desert sand in Table 1. Here, the concentration ratios of Z/Si were calculated by the initial result (μg cm^-2) of PIXE analysis. As might be expected, there is a tendency towards higher Z/Si ratios for soil derived elements such as : Si>Fe>K>Ca at Yinchuan, Si>K>Fe>Ca at Wuwei, Si>Ca>Fe>K at Dulan, and Si>Ca>K>Fe at Yanchi. Unfortunately, Al concentration cannot be reliably determined by our PIXE system. Although it has negligible ratio, S was detected in insoluble fraction of Wuwei sample with 0.0003 S/Si ratio and soluble fraction of Dulan sample with 0.0282 S/Si ratio. In addition, Cl was contained in every desert sand with the exception of the insoluble portion of Yanchi area. As reported by Mason (1966), some crustal rocks (such as shale and limestone) contain S and Cl. Therefore one should give attention to data interpretation about the aging processes of dust particles with sea-salts and man-made S. The relative elemental ratios summarized in Table 1 can be a means to assess the transformation of dust particles during long-range transport and other aging processes.

An example of XRF elemental maps and microscopically imaged desert sands (top left) collected at Yinchuan desert area in China are presented in Fig. 4. It was possible to draw the distribution of components in and/or on desert sands. From XRF elemental maps, it can be visually confirmed that S, Cl, and Pb were present overall sand surface as the minor components. In contrast, Fe distribution is partially concentrated on sand surface. However, the elemental distribution on sand surface can be influenced by the degrees of an angle between Si(Li) detector recording the fluorescence X-rays and sample holder.

Fig. 4. 
The reconstructed XRF elemental maps corresponding to individual sands collected in at Yinchuan desert area.

Fig. 5 shows the XRF spectra for four different points on a desert-sand in Fig. 4. These XRF spectra can be drawn by counting XRF counts of each element during the point analysis. The XRF spectra for four points on a sand show that point numbers 1, 3, and 4 are Fe and S-rich, while point number 2 has enrichment for only Fe. It thus suggests that the chemical property of desert sands shows the point-to-point variation. The summary of elemental mass concentration (μg cm^-2), calculated by XRF microprobe analysis for individual sands collected in at each desert area, is tabulated in Table 2. The elemental mass concentration for multielement can be quantitatively calculated by X-ray energy and count of each element with very low level sensitivity.

Fig. 5. 
The XRF spectra for four points on a single sand collected in at Yinchuan desert area.

The result of point analysis by XRF microprobe with around 4 μm beam size shows the fluctuate variation of elemental mass concentration according to point. This dissimilar mass concentration depending on sand position indicates that the composition of individual desert sands is not homogeneous. The chemically inhomogeneous sands may be able to be chemically homogeneous fine sand pieces via aeolian processes and then they blown off as the dust particles. Since the number of point analysis is not enough, the chemical property of individual desert sands can be comparable to that of bulk sands. However, the mass concentration has still enrichment for crustal elements like Al, Si, K, Ca, and Fe. They were exhibited a predominant occurrence in point analysis of individual sands.

Though it can not fully estimate the property of whole desert sand since a part of desert sand was irradiated by XRF, the elemental compositions of individual sands detected in the present study are comparable to those of bulkily analyzed reference desert sands. Fig. 6 shows the relationship between Z/Si in four-kind local desert sands and Z/Si in the CJ-1. Here, the elemental mass concentrations determined by XRF analyses for the individual sands of four-kind local desert were used. The elemental mass concentration of CJ-1, which is the stand desert sample of China loess, was provided by the Sino-Japan Friendship Centre for Environmental Protection (SJC). The coefficients of determination (R2) of the ratio of each element to Si (Z/Si) between the individual sands collected from four local deserts and the bulk reference sands are varied from 0.70 to 0.95 (Fig. 6). Though some kinds of element are scattered from a one-to-one line, this result suggests that there is a relatively good accord between the individually estimated sand in this study and the bulk stand sand sample.

Fig. 6. 
Relationship between Z/Si in four types of local desert sands and Z/Si in the CJ-1.