Contents
4. Dust Composition and Acid Neutralization
As we know, dust is one of the most immense and widespread commonplaces. The major sources of present-day dust emission are (1) the sub-tropical regions which occur in a board belt from west Africa to Central Asia; (2) semi-arid and sub-humid regions where dry, ploughed soils are exposed to severe winds at certain times of the year. The Sahara Desert is believed to be the world's most important source of dust. But, dust emission from other deserts have been less intensively studied, and no accurate estimates are available for the dust flux from the Gobi, Takelamagan and other deserts of central Asia.
Asian dust storms have been documented for thousands of years, and these records have climatic significance. Because the frequency and severity of dust storms delineate both ground characteristics and atmospheric conditions, which include soils particle size distribution and moisture content, air and surface temperatures, rainfall, and wind speed, dust storm records display temporal and spatial variations of weather.
Many great deserts and much semi-arid lands lie in China, where dust outbreaks are very common and severe dust storms occur frequently in the spring season. Furthermore, the human population in China is the largest in the world, and anthropogenic pollution maybe is the worst. Now, China is improving its economy and there are a lot of dust emissions and other harmful gases emissions from the industry cities. This background provides an ideal opportunity to study the natural dust emissions from China and their impact on the environment.
In arid and semi-arid areas, soil
erosion by wind is an impact process that affects both the surface features
and the biological potential of soils. In addition, emitted dust has a
significant residence time in the atmosphere and acts to modify the radiative
properties, mainly by back scattering the incoming solar radiation.
In order to identify the dust source in China, based on the FAO soil map, and simplified to get China FAO soil map. (See Figure 1).
Wind blown soil emissions can be calculated based on the work of Gillette and Passi, 1988. The basic equation is shown in Figure 2. In this equation the dust flux to the atmosphere happens when the friction velocity exceeds the threshold friction velocity (which is a function of soil and surface properties). An example of how this equation has been used in studies in China is shown in Figure 3 and Figure 4.
In order to calculate the dust emissions, it is necessary to estimate the threshold friction velocity (U*t ) for the soil types of China. The soil types were simplified to 5 main soil categories. Then according to these main soil categories, an aerodynamic roughness height, z0 was assigned, the U*t was then determined from Figure 5.
These soils were then assigned a surface roughness and threshold velocities estimated. The map of threshold velocities is presented in Figure 6. The actual emissions depend upon the state of the surface as well. The presence of standing vegetation as well as recent precipitation will reduce the potential for wind blown emissions. The standing vegetation can be estimated by the use of satellite greenness data. Greenness map for the month of May is shown in Figure 7. Combining the greenness maps with the threshold velocity maps, provides a seasonal picture of potential source areas as in Figure 8. If the surface is dry then this information can be used on-line with a meteorological model to derive emission rates.
Acid deposition has been recognized as an important and increasing environmental problem in China over the last decades. Rapid and increased energy demand have given severe air pollution problems in China, particularly in and near, the large cities.
Naturally, fresh rain water is slightly acidic (pH ~5.6) since some gases such as carbon dioxide and chlorine could dissolve in rain water to form acid.
CO2(g)+ H2O(aq) --> H+(aq) + HCO3-
Cl2(g)
+2H2O(aq) --> 4H+(aq)
+O2(g) + 4Cl-(aq)
The SO2 and NOx that emitted from anthropogenic and natural sources worsen the acidity of rain water. Even though, the major source of SO2 and NOx are anthropogenic (fossil fuel combustion). Natural sources such as sea salt, soil, vegetation and volcano show significant amounts of these gases. The meteorological processes transport these gases from sources. During several hour to day, these gases phase precursors are oxides with OH radical in atmosphere to form sulfuric and nitric acids. They can be deposited back to earth's surface by dry or wet process. The dry deposition contains NO2,HNO3,SO2,and H2SO4. Since, these acid could dissolved in clouds, fog, rain or snow, they returned to earth surface in wet deposition form. This acid deposition was known as " acid rain". The acid rain process was considered as long-range transport problem since pollutants could be transported and scavenged over ~100 km from sources. The picture of acid deposition processes is shown below.
4).
Dust Composition and Acid Neutralization
Emissions of alkaline substances can significantly influence precipitation acidity by neutralizing some fraction of the acids. We are concerned about the chemical composition of the soil types, especially about Ca2+, Mg 2+, Al3+, and pH of the different soil types. For the detail, see Table 1 . We have also prepared soil chemistry maps, for example, calcium (Figure 9), magnesium (Figure 10) and pH map (Figure 11). The weight percent calcium can be used along with the dust emissions equation.
As would be expected, fine particles in a moist atmosphere can act as condensation nuclei for cloud and rain formation. Dust particles can be collected by rain, hail or snow falling through a cloud of material. Fine dust particle can be captured by cloud droplets and falling through a rain. Dust deposition has a number of important geomorphological and geological implications, some of which, in turn, have economic and human significance.
The concept of acid neutralization process of aerosol in atmosphere is shown in below figure . The first step is emission of gases and particles to atmosphere. The precursor gases transform to acid gases during this step. The second step is absorbed of fine particle with humidity in atmosphere. The third step is the interaction between gases and mineral particles. The example of acid neutralization reactions are ;
Ca2++ 2NO3--->
Ca(NO3)2
Ca2++ SO42--->
CaSO4
The fourth step is the wet deposition process.
5).
Conclusion
The natural soil in the Northern
and North Eastern China originally has high pH and also has high percentage
of alkaline cations, which can been seen from pH
map, Ca
map and Mg
map.
The precipitation in the northern China is heavily affected by the natural soil dust from the north eastern and northern desert areas. Due to the neutralizing capacity of this dust, deposition of pH is high (~ 8) .
In the southern China, low pH in deposition has been recorded for the last 20 years and the area having acid rain has increased (Wang,w. et al, 1996).
The results which are from current
paper (Thojorn, Larssen, 1998) is in the Figure
12.
C. Gary-Lacaux and A. I. Modi. Precipitation Chemistry in Sahelian Savanna of Niger, Africa,Journal of Atmospheric Chemistry,Vol.30, Page 319-343,1998.
Dale A. Gillette, John Adams, Danel Muhs and Rolf Kihl, Threshold Friction Velocities and Rupture Moduli for Crusted Desert Soils for the Input of Soil Particles into the Air, Journal of Geographical Research, Vol. 87, No. C11, Page 9003-9005, Oct. 20, 1982.
Thojorn Larssen, Gregory Carmichael, Acid rain and acidification in China: The importance of base cation deposition.,1998
Beatrice Marticorena,Gilles Bergametti, Dale Gillette, Jayne Belnap, Factors controlling threshold friction velocity in semiarid and arid areas of the United States, Journal of Geographical Research, Vol. 102, No. D19, Page 23277-23287, Oct. 20, 1997.
Wang,w. and Wang, T.,1996. On
acid rain formation in China, Atmos. Environ., 30:4091-4093.
Last updated Dec. 12, 1998
Please Contact us if you are interested
or have any question. Email us at nthongbo@cgrer.uiowa.edu,jwang@icaen.uiowa.edu 
1) Introduction
2).
Dust Emission
3).
Acid Deposition
6). References
