Evaluation of Regional Dust Emission with Different Surface Conditions at Dunhuang, China

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1 沙漠研究 26-1, 1-7 (2016) Journal of Arid Land Studies - Original Article - Evaluation of Regional Dust Emission with Different Surface Conditions at Dunhuang, China Mingyuan DU *1), Wanfu WANG 2), Seiichiro YONEMURA 1), Yanbo SHEN 3), and Taichi MAKI 4) Abstract: Optical dust particle counters were used to measure dust concentration at three sites with different surface conditions in a small area at Dunhuang, China, from March 25 to April 15, Distances between the three sites was about 10km, respectively. The surface conditions of the three sites were Gobi desert (sand with stones), bare cropland and shrub with saline and alkaline land, respectively. Wind and other meteorological elements were also measured at the three sites. A convergence/divergence method was used for evaluating regional dust emission from the area by calculating horizontal dust transportation (horizontal dust flux) from the three sites. The results show that although the horizontal dust flux at the Gobi desert was the highest among the three sites due to strong wind there, dust concentration at the cropland was the most among the three sites and regional dust emission occurred only when the wind blew from the cropland to the Gobi desert. This suggests that agriculture activities play a very important role for regional dust emission at Dunhuang, China. Key Words: Agriculture activities, Convergence/Divergence, Dust emission, Dust Particle Counter, Dunhuang. 1. Introduction The need for determining dust emission amount is obvious both for soil conservation and atmospheric studies (Shao et al. 2000). Dust emission over a large area can be estimated using inverse techniques (e.g. D'Almeida, 1986). This type of study requires reliable three dimensional measurements of dust concentration and the flow field over the area concerned, both of which are usually difficult to obtain. In most studies, dust emission is modelled directly (e.g. Tegen and Fung, 1994; Shao and Leslie, 1997). However, it is difficult to determine dust emission over a small area in which different surface conditions existed both by observation and by a known model. On the other hand, the arid and semiarid regions in Northwest China are the important areas of the atmospheric dust aerosol. The sandy deserts and the Gobi deserts in Northwest China have been considered as the primary sources of atmospheric dust. However, recent studies reported that the dry cropland in oasis are also the important dust sources (Zhang et al., 2002). Shen et al. (2005) had analyzed wind erosion factors influencing dust emission by using the observed data from the Gobi and the oasis sites at Dunhuang of ADEC (Aeolian Dust Experiment on Climate impact) project (Mikami et al., 2006). Their results indicate that dust emission rate over the ploughed cropland is the largest at Dunhuang, which is hundreds times larger than that over the Gobi desert. Therefore, it is very important to evaluated the dust emission for the whole region at Dunhuang due to there are different surface conditions. In this paper, we try to evaluate dust emission rate over the small area at Dunhuang with different surface conditions by using measured dust concentration and wind data obtained by ADEC and a convergence/divergence method. 2. Data and methods 2.1. Observation site and data Observation site Figure 1 shows the locations and the view of the three observation sites of the small area at Dunhuang, China ( N, E). Three observation sites at Dunhuang, China, were settled at the flat parts of the Gobi desert (Point A, N, E) near the famous Mogao Grottoes and the sand dunes, the bare cropland in an oasis (Point B, N, E), and the shrub land (Point C, N, E), respectively. The land surface of the Gobi desert site was covered by gravel (about 2-10 mm in diameter) and removable sand and dust particles. In the cropland site, the ploughing work was done just during the observation period, including harrowing, leveling, plowing, fertilizing and seeding. The shrub land site was saline and alkaline land with hard land surface (hard crust) Observation data The observation system consists of automatic weather station (AWS) including soil water contents and temperature sensors at several depths, optical dust particle counter (OPC) and an automatic visibility observation device for measuring dust concentration. The OPC (type ADS, Yamanashi-Giko Co.) was specially designed for use in the desert. The length from the air inlet to the chamber is set to 10 cm in order to minimize the sampling loss for coarse particles. And the body of the OPC is protected by a triple air filter to avoid * Corresponding Author: dumy@affrc.go.jp Kannondai, Tsukuba, Ibaraki, Japan Received, May 21th, 2015; Accepted, April 7th, 2016 Tel: Fax.: ) Institute for Agro-Environmental Sciences, NARO, Japan 3) National Climate Centre (NCC) of China Meteorological Administration (CMA) 2) The Conservation Institute of Dunhuang Academy, China 4) Japan International Research Center for Agricultural Sciences

2 N B C 5km A in this paper. Fig. 2. Image of mass balance of a area (box). A B 2.2. Methods Considering dust moving balance in a certain area, simplified dust balance equation can be written as follows (Baldocchi et al., 1988): C cropland shrub Gobi sand q F f t f f x y uq vq f x F ( x f y ) y (1) (2) mountain Fig. 1. Location and land cover of three observation sites (Point A: Gobi desert site, Point B: bare cropland site, Point C: shrub with saline and alkaline land site) at Dunhuang, China ( N, E) (Photo were taken on March 18, 2004). contamination to the chamber and has a radiation shield to avoid a temperature increase inside the body. For use in the field, electric power is supplied from a solar battery and/or car battery and the data is automatically collected in the data logging system with a compact flash memory. For the measurement of dust particle, an 8-channel size discriminator; 0.3 to 0.5, 0.5 to 0.7, 0.7 to 1, 1 to 2, 2 to 3, 3 to 5, 5 to 7, and >7 m, was used. We evaluate the dust flux in this paper only using dust particle data and fluxes of different particle sizes, respectively. The details of the observation systems have been introduced by Du et al. (2002a, 2003) and Mikami et al. (2005). Observations were done during March 25 to April 15 in Dust emission seemed to be occurred during April 8 to April 10. There were no dust emission during March 25 to April 7, 2004 and during April 11 to 15, Data with an interval of one second was sampled and every minutes data was recorded. Only dust particle counter and wind data at 2 m above the grand surface during April 8 to April 10 are used Where q and F are dust concentration and vertical dust flux (dust emission rate) in the area, respectively. f x and f y are two components of horizontal dust flux f. u and v are two components of wind speeds. f is convergence (divergence) of dust movement in the area. Equation (1) is a mass balance conception. Theoretically, it can be used for any mass transportation without consider fetch (or boundary). The basic conception is that the variation of the mass (mass change) within a area (box) (whatever its scale or position) is induced by mass transportation as shown in Figure 2. If the net horizontal transportation (f x in - f x out and f y in - f y out) is larger or less than the mass variation (mass change) in the box there will be net vertical transportation occurred. However, this vertical transportation also depend on how the horizontal transportation occurred. If the transportation is convergence, there will be upward transportation. Otherwise (divergence) there will be downward transportation. If the box is on the earth surface, upward transportation will be emission and the downward transportation will be deposition. If we have observation data of dust concentration and wind for more than three points within a area, vertical dust flux within the area can be evaluated by Equations (1) and (2) using finite- difference approximations. Due to dust flux depends on the particle sizes, we only calculate dust fluxes of different particle sizes separately. This convergence (divergence)

3 Fig. 3. Description of calculation area. G is point A, C is Point B and S is point C. The distances between the three points are CS=13.24 km, CG=10.14 km, and SG=10.87 km, respectively. The calculation area (the small area) is the triangle formed by the three sites ( GCS), and the size is about 54 km 2. method has been used frequently for evaluating water vapor transport in the atmosphere (e.g. Starr and Peixoto, 1958; Rao et al., 1996) for very large area and also used for sand transport (Howard et al., 1978) and soil erosion (Braun, et al., 2001; Dietrich, et al., 2003) in small area. For evaluating dust transport we need only considering dust movement above the ground surface just like sand transport and soil erosion. In this paper, we used simply finite-difference approximations for calculating the convergence (divergence) within the triangle formed by the three sites as shown in Figure 3. The details of the calculation were as follows. Firstly, we calculated the area mean variations of dust concentration within the triangle by using the three sites concentration data for every time step (one minute). Then, calculate the convergence (divergence) within the triangle by calculating net horizontal dust flux pass-through the triangle for every time step using finite-difference approximations. As shown in Figure 3, x are e, f and d, respectively. And y are a, b and c, respectively. u and v can be obtained by following. Let observation data V=wind speed, D=wind direction, then u=sin(d) v=cos(d) (3) V=SQRT(u*u+v*v) horizontal dust flux= SQRT(f x * f x + f y * f y ) 3. Results Figure 4(a) shows the variation of wind direction at the Gobi site (Dg) which was almost the same as other two sites and wind speeds (V) at the three sites during April 8 to 10 (98 to 100 days of the year, DOY). Figure 4(b) shows the dust concentrations (numbers of particle with diameter of 0.5 m-1.0 m per liter air) during April 8 to 10. Figure 4(c) shows the horizontal dust fluxes (numbers per square meter per Fig. 4. (a) Variation of wind direction (Dg) at Gobi desert site and wind speed (Vx) at the three sites, (b) dust concentration (Qx) at the three sites, (c) horizontal dust flux (Fx) at the three sites and (d) vertical dust flux of the area (mean dust emission rate, F) during April 8 to 10 (98 to 101 DOY). (The subscript x are g: Gobi desert, c: cropland, s: shrub land, respectively). second) at the three sites and Figure 4(d) shows the vertical second) at the three sites and Figure 4(d) shows the vertical dust fluxes (dust emission rate) of the area for the same period. dust fluxes (dust emission rate) of the area for the same period. Following results were obtained. Following results were obtained Wind conditions during April to 10 (98 to 100 days 3.1. Wind conditions during April 8 to 10 (98 to 100 days of the year, DOY) of the year, DOY) As shown in Figure 4(a), there were several strong wind As shown in Figure 4(a), there were several strong wind

4 (wind speed over 4 m/s) periods during the three days. Winds were stronger in the morning than in the afternoon for the tree sites. Wind was the strongest at Gobi site and there were strong winds in the afternoon and evening also at the Gobi site when there were no strong wind at the cropland site and the shrub land site. Although the strong wind occurred in all the three mornings, wind directions were different. Wind direction were northeast (around 70 degrees), northwest (around 290 degrees) and north (around 20 degrees), respectively in the three morning Differences of dust concentrations for different surface conditions When wind was strong enough (over 2.0 m/s), wind direction at the three sites were almost the same. Although there were 8 channels for measure the dust particles with diameters of 0.3 to 0.5, 0.5 to 0.7, 0.7 to 1, 1 to 2, 2 to 3, 3 to 5, 5 to 7, and >7 m, respectively, here we only show the result of particles, m in diameter because the other particles showed the same results. As shown in Figure 4(a), wind direction changed frequently during the observation period. There were about three periods that dust concentration shown a peak at the Gobi site and the cropland site as shown in Figure 4(b). First period was in the morning of April 8, 2004 (98 DOY), when the wind direction was northeast (NE). Second period was in the daytime of April 9 (99 DOY), when wind direction changed from northwest (NW) to south (S). Around the noon of April 9, dust concentrations at the three sites were the highest during the whole observation period when the wind speeds at the three sites became lower. Third period was in the early morning in April 10 (100 DOY), when wind direction changed from south (S) to north (N). It can be also seen that dust concentration at the cropland decreased when wind direction changed to S in the evening of April 9. However, dust concentration at the Gobi site did not show a lower peak from evening in April 9 to midnight of April 10 as that at the cropland site. At first, dust concentration at the cropland site was almost the same as that at the Gobi desert site and then it was much higher than that at the Gobi desert site and the shrub land site. Variation of dust concentration with time at the cropland site was greater than that at the Gobi desert site. However, dust concentration at the shrub land site was very low and did not change much during the period. Therefore, dust concentration at the cropland was the most among the three sites Horizontal dust flux Due to wind speeds and dust concentrations being different at the three sites, horizontal dust flux at the three sites differed greatly also. Time variation of horizontal dust flux at the three sites was almost the same as that of dust concentration as shown in Figures 4(b) and (c). There were three peaks of horizontal dust flux when dust concentration was higher. However, there was another peak of horizontal dust flux at Gobi desert from the evening of April 9 (about 9:00 BST) to midnight of April 10 due to strong wind there. Although dust concentration was much higher at the cropland than at the Gobi desert, horizontal dust flux at the Gobi desert was generally highest among the three sites, due to strong wind at the Gobi desert. Only in the morning of April 9, it was higher at the cropland than at the Gobi desert due to higher dust concentration. Horizontal dust flux was the greatest at the early morning at the Gobi while that at the cropland was at noon of April 9. Horizontal dust flux at the shrub land was smallest due to both lower dust concentration and lower wind speed there. Therefore, horizontal dust flux at the Gobi desert was the most among the three sites due to strong wind Vertical dust flux (Dust emission rate) By using the convergence/divergence method as Equations (1) and (2), vertical dust flux (dust emission rate) of the small area during the observation period was obtained as shown in Figure 4(d). Co pared with horizontal flux, vertical dust flux was much smaller. It was less than 1/10,000 of horizontal flux and had a very different variation pattern with time. There were three periods in which dust emission occurred (F>0) within the small area. The first period was from early morning of April 8 to early afternoon (about 2:00 BST to 14:00 BST), when wind directions were northeast (NE) or north (N). The second period was from the midnight of April 9 to the morning of April 9 (about 9:00 BST) when wind directions were N or NW. The third period was from the early morning to noon of April 10 (about 4:00 BST to 12:00 BST), when wind directions were NE or N. As shown in Figure 1 the oasis is located to the north of the Gobi desert. Therefore, NW, N, NE winds were blown from the oasis to the deserts. On the contrary, when wind directions were S or southwest (SW), winds were blown from the deserts to the oasis, when the vertical dust flux was minus (F<0), meaning there was dust deposition within the small area. The highest dust concentration appeared at about noon of April 9. However, due to wind direction change from west northwest (WNW) to SW at noon and decreased wind speed, horizontal dust flux became smaller at noon of April 9. However, these high dust concentrations and high horizontal dust flux resulted in no dust emission but dust deposition within the small area. It is thought that the great portion of

5 dust which transported from the Gobi deserts deposits to the oasis on the other hand. 4. Discussions 4.1. Application of the convergence/divergence method for dust emission Equation (1) is a mass balance conception. Theoretically, it can be used for any mass transportation. Therefore, the convergence/divergence method has been used widely. Although the convergence/divergence method has not been used for evaluating dust emission, our present study shows a reasonable example showing how the dust emitted from the cropland at Dunhuang, China. Shen et al. (2005) and Du et al. (2002b, 2005a) pointed out that, due to a lot of fine dust existed in the croplands in Dunhuang, China, dust concentration in the oasis is much more than that in Gobi deserts. It is too difficult for dust emission to occur when the cropland surface soil is hard with high water content. However, for the same croplands, when the surface is loosened and dried up by agricultural activities such as spring tillage (usually in April), dust emission becomes very easy and its amount would be several hundred times larger than that from the desert. Although the convergence (divergence) method has been used frequently in water vapor transport in the atmosphere (e.g. Starr and Peixoto, 1958, Rao et al., 1996) for very large area, it was also used for biologically related gas exchange (Baldocchi et al., 1988), sand transport (Howard et al., 1978) and soil erosion (Braun, et al., 2001, William, et al., 2003) in small area. Our present study by the convergence (divergence) method gives a similar result as Shen et al. (2005) and Du et al. (2002b, 2005a) which were used difference methods. This suggested that this method can also be used for dust emission when dust concentration and wind data for more than three sites are available. However, in our opinion, it is better to have more observation sites for measuring dust concentration and wind on different surface conditions. Only when there are very large differences of horizontal dust flux among the different observation sites can this method be used as shown in Figures 4 (b) and 2 (c) Quantitative evaluation of regional dust emission Our present work was only a qualitative evaluation of dust emission as shown in Figure 4(d), although the convergence (divergence) method was used. There are two indispensable conditions for quantitative evaluation of the regional dust emission by using the convergence (divergence) method. Firstly, dust mass concentration should be known. By using dust particle counter dust mass concentration (Q) can be obtained by follow equation. 10 Q V ( d ) C( d ) ( d ) (4) 0 where d is particle size (diameter). V is the volume of the particle. C is the count numbers. is the density. However, the particle counter can get either all the different particles or their densities. On the other hand, it should have as many as possible dust mass concentration and wind data within the area to know the dust source and sink value distributions. That is it should be known the distributions of dust concentration and wind first and then to calculate the dust source and sink value distributions by the convergence (divergence) method. Du et al. (2005b) had estimated annual sand transportation amount on the Gobi desert in Dunhuang, China. Du et al. showed that, although there were many directions for sand transport in Dunhuang throughout a year, annual mean transportation direction was from NNE to SSW (from the oasis to the deserts as shown in Fig. 1). This result indicates that there should be a lot of annual dust emission during the sand transportation process from the croplands to the deserts in Dunhuang, China. However, as shown in Figure 4, there were no dust storm occurred during our observation period (no strong wind over 10 m/s). There were only some week sandstorms with dust emissions. Therefore, further studies for quantitative evaluation of regional dust emission are needed by more observations and by transfer the dust particle numbers data to dust mass data by Equation (3) Contributions of shrub land with saline and alkaline crust The values and variations of dust concentration and horizontal dust flux at shrub land, Point C were too small compared with the other two sites that it was difficult to be seen in Figures 4(b) and 4(c). However, dust concentration and horizontal dust flux were changed with wind direction and wind speed. Horizontal dust flux at shrub land were only about 1/10000 of that at other two sites. Time variation of horizontal dust flux at the shrub sites was almost the same as that at cropland. However, when wind speed was the same level, horizontal dust flux at the shrub sites was greater when west wind and south wind blowing than that when east wind and north wind blowing. This indicates that dust advection from cropland and Gobi desert were greater. We have calculated the emission rates of cropland, Gobi desert and shrub land at Dunhuang, China by using Shao's emission model (Shao, 2001). There were no dust emission at shrub land and dust emission rate at ploughed cropland was the largest, which was hundreds of times larger than that over the Gobi desert (Shen et al., 2005). Therefore, although, there

6 were some horizontal dust flux due to advection at the shrub land, shrub land at Dunhuang, China performs not only no dust emission but also fixing dust to ground surface due two both the vegetation and hard saline and alkaline crust covers of the ground surface. 5. Conclusion By measuring dust concentrations and wind at different sites with different surface conditions (Gobi desert, cropland and shrub land) and using a convergence/divergence method at Dunhuang, China in April 2004, we find that, for a small area with different surface conditions, regional dust emissions occurred only when the wind blew from the croplands to the Gobi deserts, although horizontal dust flux at the Gobi desert site was the largest among the observation sites. Our observation and analyzed results suggest that agriculture activities play an important role for regional dust emission at Dunhuang, China. We suggest that sand and dust are created by weathering in the mountain area to the south of Dunhuang and transported mainly by flood to the northern lower basin oasis areas. Fine dust was produced by agricultural activities in the cropland. Sand created in the mountain area and accumulated in the lower basin and oasis will then be transported and accumulated to the southern higher places creating sand dunes there. Fine dust formed in the oasis will be emitted to the atmosphere during its transport process form the northern lower basin to the southern higher sand dunes. Acknowledgement This study was a part of the ADEC (The Aeolian Dust Experiment on Climate Impact) experiment which was sponsored by the Ministry of Education, Culture, Sports, Science and Technology, the Japanese government and by the Chinese Academy of Sciences. 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7 沙漠研究 26-1, 1-7 (2016) Journal of Arid Land Studies - 原著論文 - *1) 2) 1) 3) 4) km / *Corresponding Author: dumy@affrc.go.jp Tel: fax.: ) 3) 2) 4)