| Dust aerosol has tremendous impact on the global climate and environment change throgh influencing atmospheric radiation, atmospheric chemistry, cloud/ precipitation processes and changing the balance of the radiation energy in the land-hydrology-atmosphere system and the water cycle. Studying on the physical and chemical characteristics and the sources of the dust aerosol will be helpful for understanding the origin, the transport mechanism and the environmental effects of dust, and the prevention of modern dust storms. Golmud City is located in the northern Tibet Plateau and the southern Qaidam Basin, with typical plateau continental climate and frequent duststorms. It is of great importance to investigate the physical and chemical characteristics and the source regions of dust in Golmud for better understanding the aeolian dust transport processes in the northern Tibet Plateau, and the impacts of the human activities on dust aerosol.In this study, the iron-oxide minerals, the main water-soluble ions, the major elements, the trace elements and the rare earth elements (REEs) of the dust aerosol were detailed analyzed to find out the characteristics under the dust/non-dust condition in different season. The suitable source indexes were selected to trace the source areas of the dust aerosol in Golmud City. Some meaningful results are obtained.(1)Our results show a high mean TSP and high frequency of the modern dust at Golmud. The highest concentration of TSP occurred in spring, while the lowest concentration of TSP occurred in winter during the observation period. By comparing the TSP concentrations with different meteorological factors, we find that the TSP concentrations have a significant correlation with the atmospheric visibility and the wind speed in spring and winter. While the correlation between the TSP concentration and wind speed is not obvious in summer due to the windy weather often accompanying with the rain at that time.(2) The analysis of the diffuse reflectance spectra (DRS) of dust at Golmud indicates that the first derivative curves (FSD) of the reflectance spectra are consistent with signals from hematite and goethite, at wavelengths of 560nm and 435nm, respectively. And these values varied with the iron concentrations in the dust samples. The first-derivative values of Hm560 and Gt435 were strongly and positively correlated with the iron contents. It has been shown that the iron-oxide minerals play important roles in determining the aerosol color and reflectance properties. The Hm, Gt and Hm/Gt ratios are distinctly different from those samples in other regions, showing that they may be used for tracing the provenances of aeolian dust.(3) The water-soluble ions analysis shows that the Na+ concentration and Cl-concentration have a strong correlation of 0.95 at Golmud, which indicates that the forms of ions in aerosol mainly are NaCl. During dust periods, the concentrations of the Ca2+, Mg2+, Cl-,Na+ and SO42- have a substantial correlation, which suggests that the main forms of water-soluble ions are NaCl, CaSO4, Na2SO4, CaCl2 and MgCl2.K+ and Na+ have a good and positive correlation during dust periods, which indicates K+ and Na+ have same sources. Considering that the salt lake sediments distributed widely in the Qaidam Basin, we infer that these water-soluble ions may mainly come from salt lake area under dust condition. During non-dust periods, the high concentrations of K+ in summer may be caused by biomass burning and straw burning. The source of Ca2+ mainly is crust materials. Further analysis on the SO42- and NO3-respectinely indicates that the value of SO42-/TSP was stabilized at around 4.01% during dust weather and the mass concentration of NO3- has a strong and positive correlation with TSP concentration. The results show that the SO42- and NO3- are mainly from desert soils during dust weather. Sulfate and nitrate can also be influenced by anthropogenic pollution during non-dust period.(4) The concentrations of the major elements of the dust aerosol are Al>Ca>Fe>Mg>K>Na in order at Golmud. The concentrations of Ca are much higher than that of the upper continental crust (UCC). The mean value of Ca/TSP is 7.08%, which is close to the surface material in the Qaidam Basin. The values of Al/TSP are higher under non-dust weather than that under the dust weather which similar to UCC. The systematic assessment on the element concentrations was made by using the enrichment factor (EF) and the principal component analysis (PCA). The results indicate that the major elements (such as Al, Ca, Fe) and most of the trace elements (such as Ga, Rb, Sr, Ti, V, Cr) are terrestrial crust origin, some pollution elements such as Pb, Zn, Ni, Cu also being enrich in aerosol which are mainly anthropogenic origin such as vehicle exhaust, industrial emissions and coal burning.(5) The Golmud dust samples have the REEs chondrite-normalized distribution patterns with relative enrichment in light REE(LREE) and right inclination. The distribution patterns are similar to those of UCC during dust period. The REEs patterns of dust samples show a small variation under dust weather, which can be attributed to the well-mixing during the long distance transportation of the dust. The provenace analysis by REEs proxies reveals that the dust aerosols at Golmud were similar to surface soils from the Qaidam Basin, the eastern Tibet Plateau and the Taklimakan Desert. It indicates that dust aerosol not only was affected by near source from the Qaidam Basin, the eastern Tibet Plateau, but also was affected by long distance transport from the Taklimakan Desert during dust period. While the REEs proxies are different from Badain Jaran Desert which indicates the Badain Jaran Desert has little influences on the Golmud dust. |
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