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Source and evolution of the “perfect Asian dust storm” in early April 2001: Implications of the Sr–Nd isotope ratios
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Atmospheric Environment 39 (2005) 5568–5575 www.elsevier.com/locate/atmosenv
Source and evolution of the ‘‘perfect Asian dust storm’’ in early April 2001: Implications of the Sr–Nd isotope ratios Takanori Nakanoa,, Masataka Nishikawab, Ikuko Morib, Kicheol Shinc, Takahiro Hosonoa, Yoriko Yokood a
Research Institute for Humanity and Nature, 335 Takashima-cho, Marutamachi-dori, Kawaramachi nishi-iru, Kamigyo-ku, Kyoto 602-0878, Japan b National Institute for Environmental Studies, Tsukuba, Ibaraki 305-0053, Japan c Institute of Geoscience, University of Tsukuba 305-8572, Japan d Faculty of Engineering, Doshisha University, 1-3 Kyotanabe, Kyoto 610-0394, Japan Received 23 September 2004; received in revised form 14 January 2005; accepted 30 May 2005
Abstract The ‘‘perfect Asian dust storm,’’ so-called from the huge, clear picture obtained of it by earth-orbiting satellites, occurred over a vast area of northern China and Mongolia and moved eastward across the northern Pacific in early April 2001. We determined the Sr–Nd isotopic ratios of acid-resistant minerals and the Sr isotopic ratios of weak-acidsoluble minerals in the dust from this storm deposited at nine sites from northern China to Japan and compared these data with those ratios of surface arid soils in northern China. The isotopic compositions of the dust minerals resembled those from soils of the Badain Juran, Tengger, and Ulan Buh deserts and the area to their north, which on meteorological grounds are considered to be the emission area of the dust plume, but they varied regionally, reflecting the heterogeneity of the source soils. Our results and those of other meteorological and modeling studies suggest that this variation was caused by mixing with local soils uplifted into the lower part of the dust plume, but further downwind the dust was less mixed with local soils and was derived mainly from the upper dust plume. Mineral isotope, mineralogical, and elemental data on Asian dusts and soils in northern China and Mongolia provide invaluable information on physical and chemical processes of dust storms and on dust source areas. r 2005 Elsevier Ltd. All rights reserved. Keywords: Asian dust; Perfect dust storm; Sr–Nd isotopes; Dust source; Soil-mixing
1. Introduction Mineral dust aerosol particles, which originate mainly in the desert regions of northern China and Mongolia, influence the radiative property and chemical composition of the atmosphere (Husar et al., 2001; Trochkine et al., 2003; Alfaro et al., 2003) and the biogeochemical Corresponding author. Tel.: +81 (0)75 229 6183; fax: +81 (0)75 229 6150. E-mail address: [email protected] (T. Nakano).
system over a wide area downwind of the northwestern Pacific region (Duce, 1995; Chadwick et al., 1999), western America (McKendry et al., 2001; Wilkening et al., 2000), Greenland (Bory et al., 2002) to Europe (Grousset et al., 2003). Recent increases in the amount of Asian dust have caused serious social and economic problems, particularly in part of China, Korea, and Japan which are near arid regions (Chun et al., 2001; Guo and Jiang, 2002; Sun et al., 2000; Kurosaki and Mikami, 2003). In spring 2001, the Asian Pacific Regional Aerosol Characterization Experiment
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(ACE-Asia) yielded a number of new findings and invaluable information about Asian dust (Huebert et al., 2003). During this experiment, from 6 to 9 April, a tremendously strong dust storm, which caused serious social and economic damage to China, was generated in northern China and Mongolia. This dust storm was observed clearly by the Total Ozone Mapping Spectrometer and the Sea-viewing Wide Field-of-view Sensor on earth-orbiting satellites and was named the ‘‘perfect Asian dust storm’’ (Huebert et al., 2003). According to satellite images, the perfect dust moved across the Pacific Ocean in the upper westerlies, arrived in North America on 12–13 April, and reached the Atlantic Ocean on 19–20 April (Liu et al., 2003). The perfect dust storm is ideal for the study of Asian dust because much is known about its characteristics, structure, generation, and movement (Takemura et al., 2002; Liu et al., 2003). However, the sources and degree of heterogeneity of the dust minerals in the storm have yet to be described. Stable isotopes of Sr and Nd can serve as powerful source-area fingerprints of atmospheric dust (Biscaye et al., 1997; Asahara, 1999; Chadwick et al., 1999; Bory et al., 2002; Grousset et al., 2003). Bory et al. (2003) showed that northern China and Mongolia can be divided into two regions on the basis of the Sr–Nd isotopic ratios of the surface arid soils. We previously reported that desert sand and loess in northern China are composed of two groups of minerals, water- and weak-acid-soluble minerals (halides, gypsum, carbonates) and acid-resistant minerals (dominantly silicate minerals), and are classified into five regions depending on their Sr isotopic ratios (water- and weak-acid-soluble group) and their Sr–Nd isotopic ratios (acid-resistant group) (Nakano et al., 2004). Although the grain-size of Asian dust decreases with distance from the dust emission area, the grain-size distribution of Asian dust in China is similar to that of Chinese loess deposits (Liu et al., 1981; Zhang et al., 1999). Moreover, the Sr–Nd isotopic ratios of these two groups of minerals in the Loess Plateau depend mostly on their provenance (northern to northwestern deserts), rather than on grain size (Yokoo et al., 2004). Hence, isotopic information on soil minerals from China and Mongolia can be used to determine the source of Asian dust in China and other parts of Far East Asia. We present here the Sr–Nd isotopic ratios of these two groups of minerals in dust from the perfect dust storm and compare our findings with isotopic ratios of minerals in the surface soils of northern China in order to discuss the source and evolution of soil minerals entrained into the storm.
2. Samples and experiment To collect Asian dust samples regionally, we set up sampling sites on the roofs of buildings (410 m above
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the ground) at eight sites in China and at Iki Island in southwestern Japan which is close to Korea (Fig. 1). For the collection of dust samples, we used a high-volume sampler with a 100-mm-diameter quartz-fiber filter (2500QAT-UP, Pallflex) and a flow rate of 1000 l min1 (Mori et al., 2003). The perfect Asian dust storm was sampled only during dust-fall events, on 6–9 April 2001 at the Chinese sites and on 11–12 April 2001 at the Iki Island site (Table 1, Fig. 1). At Ejin-Qi (site 1 in Fig. 1) in the southwestern Gobi Desert, the westernmost sampling site, we also collected two dust samples on 25 and 29 April, since this site (1011E, 421N) is considered to be in the source area of the perfect dust storm (Liu et al., 2003). Sample amounts on the filter ranged from 150 mg (Iki) to 500 mg (Ejin-Qi). We used a microspatula to pick up the dust samples (200 mg) off the filter for use in the analysis. Minerals (mainly carbonates) extracted by 5% hydroacetic acid (HOAc) solution from Chinese arid soils have almost the same 87Sr/ 86Sr ratio as those (halite and gypsum) extracted by ultrapure water (Yokoo et al., 2004; Nakano et al., 2004). Therefore, we extracted dust samples of about 1 g with 5% HOAc solution. Residue splits after extraction with HOAc were digested with a HF–HClO4–HNO3 solution. The Sr and Nd isotope ratios were determined with a Finnigan MAT262 mass spectrometer, installed at the Institute of Geoscience at the University of Tsukuba, Japan, following the analytical procedure of Na et al. (1995). The measured 87 Sr/ 86Sr values were normalized to a 86Sr/ 88Sr value of 0.1194, and those of 143Nd/ 144Nd to a 146Nd/ 144Nd ratio of 0.7219. The 87Sr/ 86Sr ratio of NIST.SRM987 and the 143Nd/ 144Nd ratio of the La Jolla standard determined during this study were 0.71024370.000018 (2smean, n ¼ 9) and 0.51184670.000011 (2smean, n ¼ 4), respectively. All 87Sr/ 86Sr data were normalized to the NIST.SRM987 value of 0.710250. Analytical precisions for samples were better than 70.000013 (2smean) for 87 Sr/ 88Sr and 70.000010 (2smean) for 143Nd/ 144Nd. Analytical results are given in Table 1.
3. Results and discussion 3.1. Emission area of the perfect dust storm Fig. 1 shows the five regions of arid soils in northern China based on Sr–Nd isotopic ratios of acid-resistant minerals and Sr isotopic ratios of water- and acetic-acidsoluble minerals: northern north China (NNC), at latitudes north of about 42 1N; the Taklamakan Desert and vicinity (TKM); the southwestern Gobi Desert and Loess Plateau (SG-LP); Beijing and areas immediately to its northwest (BJ); and the western region, the area between about 200 and 1000 km west of Beijing (WBJ) (Nakano et al., 2004). They analyzed the o100-mm
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Fig. 1. Map showing the sampling sites of the Asian dust samples studied and five arid areas in China and Mongolia based on Sr–Nd isotope ratios of surface soil minerals (Nakano et al., 2004). The four regions are dust emission areas in spring 2001 (Gong et al., 2003). Numbers of sampling sites are the same as in Table 1. The red line is the air-mass back trajectory at Yulin on 7 April 2001, taken from Fig. 5 in Zhang et al. (2003).
Table 1 Sr isotopic ratios of HOAc-soluble minerals and Sr-Nd isotopic ratios of HOAc-insoluble minerals in the Asian dust of spring 2001 Cite number
143
Nd/
1 1
144
Nd
Sampling date
Sampling duration (min)
Sample site
Area
(mg m3)
HOAc Leachate 87 Sr/ 86Sr
HOAc Residue 87 Sr/ 86Sr
HOAc Residue
HOAc Residue
Ejin Qi Ejin Qi
NNC NNC
2.200 1.900
0.711712 0.710955
0.722106 0.720174
0.512124 0.512221
10.02 8.14
Ejin Qi Yinchuan Baotou Datong Erenhot Jining Zhangjiakou Beijing Iki
NNC SG-LP WBJ WBJ NNC NNC BJ BJ Japan
1.700 7.900 9.200 2.100 1.900 2.600 2.100 0.758 0.180
0.709714 0.710889 0.711205 0.711646 0.710498 0.710607 0.711256 0.710283 0.710200
0.717903 0.719621 0.718394 0.719830 0.717360 0.718195 0.720062 0.714494 0.716612
0.512269 0.512150 0.511463 0.512081 0.512125 0.512223 0.512147 n.d 0.512195
7.21 9.51 22.91 10.87 10.00 8.09 9.58 n.d 8.63
eNd 25 March 29 April
300 510
Perfect dust storm 1 6 April 420 2 6 April 368 3 6 April 300 4 7 April 480 5 6 April 240 6 6–7 April 565 7 7 April 464 8 9 April 480 9 11–12 April 1482
n.d: not determined.
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fraction of desert sand and loess samples to obtain these isotopic ratios. The SG-LP is the largest region and includes the Badain Juran, Tengger, Ulan Buh, and Mu Us deserts. The WBJ includes the Hobq Desert. The soils of the NNC are distinguished from those of other regions by high 143Nd/ 144Nd ( ¼ eNd) values. According to Bory et al. (2003), soils in Mongolia also have high eNd values and are indistinguishable from soils of the NNC; therefore, they are grouped here with those of the NNC-MG. Perfect storm dust collection sites of dusts in China were in four regions: Ejin-Qi, Erenhot, and Jining in NNC-MG, Yichuan in SG-LP, Baotou and Datong in WBJ, and Zhangjiakou and Beijing in BJ. Liu et al. (1981) reported that the mean grain size of loess at 33 sites in China ranged from 19 to 46 mm; of these, they fell in a range of 20–40 mm at 29 sites. According to Liu and Chang (1962), the Asian dust sampled at Beijing on 18 April 1980 was dominated by coarse particles (10–50 mm) rather than by small particles (5–10 mm), and the grain-size distribution was similar to that of the Malan loess in the SG-LP region. Zhang et al. (1999) also reported that Asian dust from the Loess Plateau at the dust storm was dominated by coarse particles: 22% (420 mm), 55% (2–20 mm), and 23% (o2 mm). The volume or mass of particle is assumed roughly to be proportional to the cube of the grain size. For example, if a sample is composed of two cubic particles of 10-mm and 4-mm in diameter and with the same proportion, then 94% in the volume of the sample is calculated to be composed of the coarse particle. Accordingly, most elements including Sr and Nd in loess and Asian dust in China are contained mainly in coarse-grained particles (410 mm) rather than in fine particles (o5 mm), such as is the case for Asiandust derived particles deposited in remote area locations such as in the Greenland ice cores (Biscaye et al., 1997) and deep-sea sediments of the North Pacific (Asahara, 1999). The 87Sr/ 86Sr ratio of silicate minerals in Chinese loess increases with a decrease of grain size, but the difference in the 87Sr/ 86Sr ratio of the 2–20-mm fraction and the 420-mm fraction of bulk samples is small (o0.003) (Asahara et al., 1995; Asahara, 1999). According to Yokoo et al. (2004), silicates in the finegrained fraction (avg. 15.7 mm) of the desert sand of central northern China and those in loess derived from desert sand have similar 87Sr/ 86Sr ratios. This result also suggests that the 87Sr/ 86Sr ratio of whole silicates in Asian dust on China and other Far East Asia is not affected substantially by that of fine silicate particles whose grain-size is less than a few micrometers. Moreover, the Nd isotopic ratios of acid-resistant minerals and the Sr isotopic ratios of water- and weak-acidsoluble minerals are likely to be less dependent on grain size. Hence, the Sr–Nd isotopic compositions of these two groups of minerals in surface soils of northern
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China can be used as the source index for the perfect dust in the present study. Desert regions in northern China and Mongolia are considered to be the major source of Asian dust, and the Loess Plateau is considered the major dust depositional area in China (Zhang et al., 1996; Merrill et al., 1994). Gong et al. (2003) identified four regions as dust emission areas in spring 2001 based on a model analysis (Fig. 1); region 1 is in TKM, region 2 in SG-LP and WBJ, region 3 in NNC-MG and BJ, and region 4 in another part of NNC-MG. According to them, heavy dust clouds were generated mainly from regions 2 and 4 on 6 April in conjunction with a Mongolian cyclone, and as they moved eastward they became mixed with dust from another storm that originated in the Onqin Daga and Horqin deserts in region 3. The surface wind speed and dust emission flux over eastern China decreased on 7 April, whereas the dust mass concentration increased with the eastward movement of the cyclone and dust clouds (Figs. 4b, 5b and 7 in Liu et al., 2003). Subsequently, more dust clouds were generated in western China on 8–9 April during a second Mongolian cyclone. These two cyclone systems together constituted the perfect dust storm: a spiral band of dust clouds over eastern Mongolia and China (Liu et al., 2003). The front of the dust storm reached Korea on 8 April and Japan on 9 April. These studies show that the samples collected for the present study were deposited mainly from the dust clouds generated on 6 April. The acid-insoluble minerals of the perfect dust had different 87Sr/ 86Sr ratios and eNd values from those of soils around the collection sites (Fig. 2). Except for dust sampled at Baotou, these dust minerals plot with soil minerals of SG-LP, and they show a negative relationship between 87Sr/ 86Sr and eNd values, which is a diagnostic feature of acid-insoluble soil minerals in SGLP. However, their eNd values are slightly high compared with silicate minerals in SG-LP soils, showing an involvement of NNC-MG soils. The relationship between the 87Sr/ 86Sr ratios of the HOAc-soluble minerals and those of the HOAc-insoluble minerals for the dust samples and Chinese soil samples is shown in Fig. 3. Soils in BJ and NNC-MG have lower 87Sr/ 86Sr values than those in SG-LP, and those in TKM and WBJ have intermediate 87Sr/ 86Sr ratios. Despite a slight overlap, the BJ and NNC-MG soils are distinguished from the SG-LP soils by an 87Sr/ 86Sr ratio of about 0.7105 in the HOAc-soluble minerals and of about 0.715 in the HOAc-insoluble minerals. Most dust samples plot in the SG-LP area, but the average 87 Sr/ 86Sr values of HOAc-soluble minerals and HOAcinsoluble minerals (0.71070 and 0.71805, respectively) in the dust are lower than those in the SG-LP soil (0.71143 and 0.72063), suggesting a contribution of the NNCMG derived particle. Although the contribution of Sr from particles less than a few micrometers in size can be
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0
-5
5 6
1(4/6) 1(3/29) 2 1(3/25)
-10 εNd
NNC-MG BJ (Beijing) BJ (vicinity of Beijing) WBJ SG-LP TKM 2001 dust
7
-15
4 -20
3 Erdos
-25
-30 0.710
0.715 0.720 0.725 0.730 0.735 87Sr/86Sr (HOAc-insoluble minerals)
0.740
Fig. 2. 87Sr/ 86Sr vs. eNd values of HOAc-insoluble minerals of spring 2001 dust samples and soils from northern China. Soils data are taken from Nakano et al. (2004). Data for fine-grained (o5 mm) minerals from the Gobi and TKM deserts (indicated by a cross over a circle or triangle) are from Biscaye et al. (1997) and Bory et al. (2002, 2003), respectively. Blue line connects the 2001 dust and the surface soil at each site.
(0.7363)
0.720
NNC-MG BJ (Beijing) BJ (vicinity of Beijing) WBJ SG-LP TKM
Erdos 1(3/25) 0
1(3/29)
2001 dust
2
1(4/6)
-5
9 -10
8
0.715
5
6
4 7
0.710 0.708
3
0.710 0.711 0.712 HOAc-soluble minerals
0.713
0.714
Fig. 3. Relationship between 87Sr/ 86Sr values in HOAcextracted minerals and HOAc-insoluble minerals for spring 2001 dust samples and soils from northern China. Blue line connects the 2001 dust and the surface soil at each site.
ignored in the present dust samples, some fine-grained silicates (o5 mm) from the TKM and SG-LP regions (Biscaye et al., 1997; Bory et al., 2002, 2003) have
6 1(3/25) 7 2 3// 1(3/29) 5 4
-15
-20 0.709
1(4/6) 9
εNd
HOAc-insoluble minerals
0.725
higher 87Sr/ 86Sr ratios than those of bulk silicates (Fig. 3). Therefore, the 87Sr/ 86Sr ratios of the HOAcinsoluble minerals in bulk arid soil samples from northern China cannot be used directly as a source index for Asian dust in areas remote from China. Fig. 4 shows the relationship between the 87Sr/ 86Sr ratios of the HOAc-soluble minerals and the Nd values of the HOAc-insoluble minerals; both these indexes are considered to be independent of particle size. The SG-LP and TKM regions cannot be clearly distinguished in this diagram, but the contribution of TKM-derived particles to the studied dusts is considered to be insignificant based on the meteorological information mentioned above. Similarly to Figs. 2 and 3, most perfect dust samples (2001 dust) are plotted in the NNC-MG and SG-LP regions in Fig. 4. These results support the view that the perfect dust contained soil minerals uplifted from SG-LP and NNC-MG soils. The three dust samples collected at Ejin-Qi show a negative relationship between the 87Sr/ 86Sr ratios and eNd values of HOAc-insoluble minerals (Fig. 2) and a positive relationship between the 87Sr/ 86Sr values of the two groups of minerals (Fig. 3). This result is consistent with the Sr–Nd isotopic characteristics of soils in the SG-LP region. It is notable that the three dust samples at Ejin-Qi have higher eNd values in the HOAc-insoluble minerals and lower 87Sr/ 86Sr in the HOAc-soluble minerals than the SG-LP soils and other perfect dust samples from China, showing that the Ejin-Qi dust is more enriched with NNC-MG soils than
-25
-30 0.708
NNC-MG BJ (Beijing) BJ (vicinity of Beijing) WBJ SG-LP TKM 2001 dust
0.709
3
Erdos
0.710 0.711 0.712 0.713 (HOAc-soluble minerals)
0.714
87Sr/86Sr
Fig. 4. Relationship between 87Sr/ 86Sr values in HOAcextracted minerals and eNd values in HOAc-insoluble minerals for spring 2001 dust samples and soils from northern China. Blue line connects the 2001 dust and the surface soil at each site.
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the other dust samples. This result is consistent with the location of Ejin-Qi in the NNC-MG region. Among the three dust samples from Ejin-Qi, the dust collected on 6 April had the highest eNd value and the lowest 87 Sr/ 86Sr, showing the largest contribution from the NNC-MG soils on that date. This result is consistent with the meteorological modeling (Gong et al., 2003; Liu et al., 2003), which showed that the surface soil-dust speed and dust fluxes on 6 April were very high in region 2 and region 4 (Fig. 1). 3.2. Regional Sr–Nd isotopic variation of dust minerals The Sr–Nd isotopic ratios of the perfect dust vary regionally and generally plot along lines between the surface soils at the individual sites and the dust at Ejin-Qi on 25 April or 29 April (Figs. 2–4). The total amount of Asian dust deposited generally decreases exponentially in the Pacific-Rim region with distance from the source (Mori et al., 2002). But the dust concentration at Baotou was the highest among the studied sites (Table 1). At Baotou, dust silicates had higher 87Sr/ 86Sr and lower eNd values than those in the soil minerals at the collection site (Fig. 2). This finding can be explained by the incorporation into the dust sample of surface soil from the nearby Erdos site (Fig. 1), which is characterized by very high 87Sr/ 86Sr and low eNd values (Figs. 2 and 3). Likewise, at Yinchuan, where the dust concentration was second highest, the Sr–Nd isotopic ratios of dust minerals were almost identical to those of soil minerals in the vicinity. Dust minerals at Erenhot and Jining (sites 5 and 6) in NNC also plot closer to the NNC-MG soils compared with those from three other sites (sites 3, 4, and 7) in the WBJ and BJ (Figs. 2–4). Values for dust minerals at Baotou and Datong in WBJ (sites 3, 4) are closer to those in soils from SG-LP, which is geographically adjacent to WBJ. Thus, the Sr–Nd isotopic compositions of the perfect dust minerals at individual sites are more or less correlated with those of the ambient soils. Alfaro et al. (2003) reported that Asian dust at Yulin (Fig. 1) in the eastern SG-LP area contained anthropogenic particles from the nearby area but showed no significant differences in the elemental compositions of aerosol minerals. The present Sr–Nd isotopic data, however, demonstrate that the perfect dust contained some soil minerals from around the depositional site. Dust clouds on 6 April were modeled to have formed under the high dust flux in Mongolia to Inner Mongolia during the generation of a Mongolian cyclone (Liu et al., 2003). Local dust production occurs when the surface wind speed at 10 m above the ground is faster than 8 m s1 (Uno et al., 2001) or 10 m s1 (Alfaro et al., 2003). The surface wind speed on 7 April has been reported to have been less than 8 m s1 in almost the
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entire desert and loess region of China (Liu et al., 2003), but Gong et al. (2003) estimated that it was higher than 10 m s1 in WBJ and the NNC areas to the northeast of WBJ on that date. Hence, it is likely that the areal variation in the Sr–Nd isotopic ratios of dust minerals is attributable to the heterogeneity of the source soils, picked up by the storm on 6 April, except for samples collected on 7 April, which were composed of both soils uplifted locally as the storm moved eastward and source soils. In the dust source area on 6 April, the dust mass concentration in the storm decreased with altitude and became constant above 2.5 km, whereas in the eastern downstream area, it increased from ground level to 2–3 km of altitude and then decreased with altitude to 7 km (Figs. 12 and 13 in Liu et al., 2003). At Beijing (site 8), the top of the dust plume was 2–3 km high on 6 and 7 April, and it descended within a few days to less than 1 km on 8 and 9 April, when the dust sample was collected. It is therefore likely that most dust sampled in China was derived from the lower part of the dust plume, from the planetary boundary layer to less than 3 km. It is noteworthy that the Sr–Nd isotopic ratios of the two groups of dust minerals at Iki resemble those at Ejin-Qi despite the long distance between the two sites. The dust concentration at Iki was the lowest among the studied sites, suggesting that large amounts of the dust carried by the perfect storm had already been deposited before its arrival at this site. The height of the high dust concentration layer at Tsukuba and Nagasaki in Japan (Fig. 1) on 11 and 12 April was 3–7 km or higher (Figs. 18 and 19 in Liu et al., 2003). This result shows that the dust at Iki was derived from the upper part of the dust plume, which was generated in the northwestern high dust desert (Badain Juran, Tengger, and Ulan Buh deserts) by Alfaro et al. (2003) and the area to its north, resulting in the Sr–Nd isotopic similarity with the Ejin-Qi dust. The present result suggests that the Asian dust minerals transported over a long distance originated from a relatively narrow area, where they were picked up by a strong vertical updraft, and that most of this dust was redeposited on the desert and loess areas of China and Mongolia. This suggestion is consistent with the general assumption that Asian dust is formed by the rapid vertical uplift of soil minerals from the ground into the free troposphere, followed by rapid transportation by strong westerlies (Uno et al., 2001). Moreover, 75% of dust ejected from desert and loess areas of China and Mongolia return to these areas (Liu et al., 2003). The Sr–Nd isotopic compositions of dust in the downstream area, outside of China and Mongolia, reflect the averaged values of soils in the dust emission area, and therefore can potentially be used as a source-area fingerprint.
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4. Conclusions Comparison of Sr–Nd isotopic ratios of HOAcsoluble and HOAc-insoluble minerals of the perfect dust storm of early April 2001 with those of minerals from soils of northern China showed that the major source of dust minerals is the northwestern high dust desert and the area to its north. The perfect dust composition also showed a regional variation dependent on the Sr–Nd isotopic ratios of soils from sites near the depositional sites, indicating injection of soil minerals from the local area into the dust plume. Our data also show that this injection and accompanying modification was small in the upper part of the dust cloud. More extensive and size-dependent data on Sr–Nd isotopes of dust minerals and surface arid soils in China and Mongolia would be useful for determining the source and soil-mixing dynamics of Asian dust storms as well as for evaluating their impact on terrestrial and marine ecosystems of the Pacific region.
Acknowledgements This study was supported by a grant from the National Environmental Studies in Japan. The authors wish to thank Dr. Hao Quan at the Sino-Japan Friendship Center for Environmental Protection, Dr. Dong Xuhui, and JICA team at the Sino-Japan Friendship Center for Environmental Protection for their cooperation and help with aerosol sampling.
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Bory, A.J., Biscaye, P.E., Grousset, F.E., 2003. Two distinct seasonal Asian source regions for mineral dust deposited in Greenland (NorthGRIP). Geophysical Research Letters 30, 16–1. Chadwick, O.A., Derry, L.A., Vitousek, P.M., Huebert, B.J., Hedin, L.O., 1999. Changing sources of nutrients during four million years of ecosystem development. Nature 397, 491–497. Chun, Y., Boo, K-O., Kim, J., Park, S-U., Lee, M., 2001. Synopsis, transport, and physical characteristics of Asian dust in Korea. Journal of Geophysical Research 106, 18461–18469. Duce, R.A., 1995. Sources, distribution, and fluxes of mineral aerosols and their relationship to climate. In: Charlson, R.J., Heintzenberg, J. (Eds.), Aerosol Forcing of Climate. Wiley, New York, pp. 43–72. Grousset, F., Ginoux, P., Bory, A., Biscaye, P.F., 2003. Case study of a Chinese dust plume reaching the French Alps. Geophysical Research Letters 30, 10–11. Gong, S.L., Zhang, X.Y., Zhao, T.L., McKendry, I.G., Jaffe, D.A., Lu, N.M., 2003. Characterization of soil dust aerosol in China and its transport and distribution during 2001 ACE-Asia: 2. Model simulation and validation. Journal of Geophysical Research 108 (D9), 4262. Guo, A., Jiang, Z., 2002, Airborne dust blankets city. China Daily, 21 March. Huebert, B.J., Bates, T., Russell, P.B., Shi, G., Kim, Y.J., Kawamura, K., Carmichael, G., Nakajima, T., 2003. An overview of ACE-Asia: Strategies for quantifying the relationships between Asian aerosols and their climatic impacts. Journal of Geophysical Research 108 (D23), 8633. Husar, R.B., Tratt, D.M., Schichtel, B.A., Falke, S.R., Li, F., Jaffe, D., Gasso, S., Gill, T., Laulainen, N.S., Lu, F., Reheis, M.C., Chun, Y., Westphal, D., Holben, B.N., Gueymard, C., McKendry, I., Kuring, N., Feldman, G.C., McClain, C., Frouin, R.J., Merrill, J., DuBois, D., Vignola, F., Murayama, T., Nickovic, S., Wilxon, W.E., Sassen, K., Sugimoto, N., 2001. The Asian dust events of April 1998. Journal of Geophysical Research 106, 18317–18330. Kurosaki, Y., Mikami, M., 2003. Recent frequent dust events and their relation to surface wind in East Asia. Geophysical Research Letters 30, 1–2. Liu, T., Chang, T., 1962. The Loess in China. Scientica XI, 687–706. Liu, T., Gu, X., An, Z., Fan, Y., 1981. Geological Society of America Special Paper 186, 149–157. Liu, M., Westphal, D.L., Wang, S., Shimizu, A., Sugimoto, N., Zhou, J., Chen, Y., 2003. A high-resolution numerical study of the Asian dust storms of April 2001. Journal of Geophysical Research 108 (D23), 8653. McKendry, I.G., Hacker, J.P., Stull, R., Sakiyama, S., Mignacca, D., Reid, K., 2001. Long-range transport of Asian dust to the Lower Fraser Valley, British Columbia, Canada. Journal of Geophysical Research 106 (D16), 18361–18370. Merrill, J.T.E., Amold, M., Leimen, M., Weaver, C., 1994. Mineralogy of aeolian dust reaching the North Pacific Ocean 2. Relationship of mineral assemblages to atmospheric transport patterns. Journal of Geophysical Research 99, 21025–21032.
ARTICLE IN PRESS T. Nakano et al. / Atmospheric Environment 39 (2005) 5568–5575 Mori, I., Nishikawa, M., Quan, H., Morita, M., 2002. Estimation of the concentration and chemical composition of kosa aerosols at their origin. Atmospheric Environment 36, 4569–4575. Mori, I., Nishikawa, M., Tanimura, T., Quan, H., 2003. Change in size distribution and chemical composition of kosa (Asian dust) aerosol during long-range transport. Atmospheric Environment 37, 4253–4263. Na, C., Nakano, T., Tazawa, K., Sakagawa, M., Ito, T., 1995. A systematic and practical method of liquid chromatography for the determination of Sr and Nd isotopic ratios and REE concentrations in geological samples. Chemical Geology 123, 225–237. Nakano, T., Yokoo, Y., Nishikawa, M., Koyanagi, H., 2004. Regional Sr–Nd isotopic ratios of soil minerals in northern China as Asian dust fingerprints. Atmospheric Environment 38, 3061–3067. Sun, J., Liu, T., Lei, Z., 2000. Source of heavy dust fall in Beijing, China on April 16, 1998. Geophysical Research Letters 27, 2105–2108. Takemura, T., Uno, I., Nakajima, T., Higurashi, A., Sano, I., 2002. Modeling study of long-range transport of Asian dust and anthropogenic aerosols from East Asia. Geophysical Research Letters 29 (24), 2158. Trochkine, D., Iwasaka, Y., Matsuki, A., Yamada, M., Kim, Y.-S., Nagatani, T., Zhang, D., Shi, G.-Y., Shen, Z., 2003. Mineral aerosol particles collected in Dunhuang, China,
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Atmospheric Environment 39 (2005) 5568–5575 www.elsevier.com/locate/atmosenv
Source and evolution of the ‘‘perfect Asian dust storm’’ in early April 2001: Implications of the Sr–Nd isotope ratios Takanori Nakanoa,, Masataka Nishikawab, Ikuko Morib, Kicheol Shinc, Takahiro Hosonoa, Yoriko Yokood a
Research Institute for Humanity and Nature, 335 Takashima-cho, Marutamachi-dori, Kawaramachi nishi-iru, Kamigyo-ku, Kyoto 602-0878, Japan b National Institute for Environmental Studies, Tsukuba, Ibaraki 305-0053, Japan c Institute of Geoscience, University of Tsukuba 305-8572, Japan d Faculty of Engineering, Doshisha University, 1-3 Kyotanabe, Kyoto 610-0394, Japan Received 23 September 2004; received in revised form 14 January 2005; accepted 30 May 2005
Abstract The ‘‘perfect Asian dust storm,’’ so-called from the huge, clear picture obtained of it by earth-orbiting satellites, occurred over a vast area of northern China and Mongolia and moved eastward across the northern Pacific in early April 2001. We determined the Sr–Nd isotopic ratios of acid-resistant minerals and the Sr isotopic ratios of weak-acidsoluble minerals in the dust from this storm deposited at nine sites from northern China to Japan and compared these data with those ratios of surface arid soils in northern China. The isotopic compositions of the dust minerals resembled those from soils of the Badain Juran, Tengger, and Ulan Buh deserts and the area to their north, which on meteorological grounds are considered to be the emission area of the dust plume, but they varied regionally, reflecting the heterogeneity of the source soils. Our results and those of other meteorological and modeling studies suggest that this variation was caused by mixing with local soils uplifted into the lower part of the dust plume, but further downwind the dust was less mixed with local soils and was derived mainly from the upper dust plume. Mineral isotope, mineralogical, and elemental data on Asian dusts and soils in northern China and Mongolia provide invaluable information on physical and chemical processes of dust storms and on dust source areas. r 2005 Elsevier Ltd. All rights reserved. Keywords: Asian dust; Perfect dust storm; Sr–Nd isotopes; Dust source; Soil-mixing
1. Introduction Mineral dust aerosol particles, which originate mainly in the desert regions of northern China and Mongolia, influence the radiative property and chemical composition of the atmosphere (Husar et al., 2001; Trochkine et al., 2003; Alfaro et al., 2003) and the biogeochemical Corresponding author. Tel.: +81 (0)75 229 6183; fax: +81 (0)75 229 6150. E-mail address: [email protected] (T. Nakano).
system over a wide area downwind of the northwestern Pacific region (Duce, 1995; Chadwick et al., 1999), western America (McKendry et al., 2001; Wilkening et al., 2000), Greenland (Bory et al., 2002) to Europe (Grousset et al., 2003). Recent increases in the amount of Asian dust have caused serious social and economic problems, particularly in part of China, Korea, and Japan which are near arid regions (Chun et al., 2001; Guo and Jiang, 2002; Sun et al., 2000; Kurosaki and Mikami, 2003). In spring 2001, the Asian Pacific Regional Aerosol Characterization Experiment
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.05.050
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(ACE-Asia) yielded a number of new findings and invaluable information about Asian dust (Huebert et al., 2003). During this experiment, from 6 to 9 April, a tremendously strong dust storm, which caused serious social and economic damage to China, was generated in northern China and Mongolia. This dust storm was observed clearly by the Total Ozone Mapping Spectrometer and the Sea-viewing Wide Field-of-view Sensor on earth-orbiting satellites and was named the ‘‘perfect Asian dust storm’’ (Huebert et al., 2003). According to satellite images, the perfect dust moved across the Pacific Ocean in the upper westerlies, arrived in North America on 12–13 April, and reached the Atlantic Ocean on 19–20 April (Liu et al., 2003). The perfect dust storm is ideal for the study of Asian dust because much is known about its characteristics, structure, generation, and movement (Takemura et al., 2002; Liu et al., 2003). However, the sources and degree of heterogeneity of the dust minerals in the storm have yet to be described. Stable isotopes of Sr and Nd can serve as powerful source-area fingerprints of atmospheric dust (Biscaye et al., 1997; Asahara, 1999; Chadwick et al., 1999; Bory et al., 2002; Grousset et al., 2003). Bory et al. (2003) showed that northern China and Mongolia can be divided into two regions on the basis of the Sr–Nd isotopic ratios of the surface arid soils. We previously reported that desert sand and loess in northern China are composed of two groups of minerals, water- and weak-acid-soluble minerals (halides, gypsum, carbonates) and acid-resistant minerals (dominantly silicate minerals), and are classified into five regions depending on their Sr isotopic ratios (water- and weak-acid-soluble group) and their Sr–Nd isotopic ratios (acid-resistant group) (Nakano et al., 2004). Although the grain-size of Asian dust decreases with distance from the dust emission area, the grain-size distribution of Asian dust in China is similar to that of Chinese loess deposits (Liu et al., 1981; Zhang et al., 1999). Moreover, the Sr–Nd isotopic ratios of these two groups of minerals in the Loess Plateau depend mostly on their provenance (northern to northwestern deserts), rather than on grain size (Yokoo et al., 2004). Hence, isotopic information on soil minerals from China and Mongolia can be used to determine the source of Asian dust in China and other parts of Far East Asia. We present here the Sr–Nd isotopic ratios of these two groups of minerals in dust from the perfect dust storm and compare our findings with isotopic ratios of minerals in the surface soils of northern China in order to discuss the source and evolution of soil minerals entrained into the storm.
2. Samples and experiment To collect Asian dust samples regionally, we set up sampling sites on the roofs of buildings (410 m above
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the ground) at eight sites in China and at Iki Island in southwestern Japan which is close to Korea (Fig. 1). For the collection of dust samples, we used a high-volume sampler with a 100-mm-diameter quartz-fiber filter (2500QAT-UP, Pallflex) and a flow rate of 1000 l min1 (Mori et al., 2003). The perfect Asian dust storm was sampled only during dust-fall events, on 6–9 April 2001 at the Chinese sites and on 11–12 April 2001 at the Iki Island site (Table 1, Fig. 1). At Ejin-Qi (site 1 in Fig. 1) in the southwestern Gobi Desert, the westernmost sampling site, we also collected two dust samples on 25 and 29 April, since this site (1011E, 421N) is considered to be in the source area of the perfect dust storm (Liu et al., 2003). Sample amounts on the filter ranged from 150 mg (Iki) to 500 mg (Ejin-Qi). We used a microspatula to pick up the dust samples (200 mg) off the filter for use in the analysis. Minerals (mainly carbonates) extracted by 5% hydroacetic acid (HOAc) solution from Chinese arid soils have almost the same 87Sr/ 86Sr ratio as those (halite and gypsum) extracted by ultrapure water (Yokoo et al., 2004; Nakano et al., 2004). Therefore, we extracted dust samples of about 1 g with 5% HOAc solution. Residue splits after extraction with HOAc were digested with a HF–HClO4–HNO3 solution. The Sr and Nd isotope ratios were determined with a Finnigan MAT262 mass spectrometer, installed at the Institute of Geoscience at the University of Tsukuba, Japan, following the analytical procedure of Na et al. (1995). The measured 87 Sr/ 86Sr values were normalized to a 86Sr/ 88Sr value of 0.1194, and those of 143Nd/ 144Nd to a 146Nd/ 144Nd ratio of 0.7219. The 87Sr/ 86Sr ratio of NIST.SRM987 and the 143Nd/ 144Nd ratio of the La Jolla standard determined during this study were 0.71024370.000018 (2smean, n ¼ 9) and 0.51184670.000011 (2smean, n ¼ 4), respectively. All 87Sr/ 86Sr data were normalized to the NIST.SRM987 value of 0.710250. Analytical precisions for samples were better than 70.000013 (2smean) for 87 Sr/ 88Sr and 70.000010 (2smean) for 143Nd/ 144Nd. Analytical results are given in Table 1.
3. Results and discussion 3.1. Emission area of the perfect dust storm Fig. 1 shows the five regions of arid soils in northern China based on Sr–Nd isotopic ratios of acid-resistant minerals and Sr isotopic ratios of water- and acetic-acidsoluble minerals: northern north China (NNC), at latitudes north of about 42 1N; the Taklamakan Desert and vicinity (TKM); the southwestern Gobi Desert and Loess Plateau (SG-LP); Beijing and areas immediately to its northwest (BJ); and the western region, the area between about 200 and 1000 km west of Beijing (WBJ) (Nakano et al., 2004). They analyzed the o100-mm
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Fig. 1. Map showing the sampling sites of the Asian dust samples studied and five arid areas in China and Mongolia based on Sr–Nd isotope ratios of surface soil minerals (Nakano et al., 2004). The four regions are dust emission areas in spring 2001 (Gong et al., 2003). Numbers of sampling sites are the same as in Table 1. The red line is the air-mass back trajectory at Yulin on 7 April 2001, taken from Fig. 5 in Zhang et al. (2003).
Table 1 Sr isotopic ratios of HOAc-soluble minerals and Sr-Nd isotopic ratios of HOAc-insoluble minerals in the Asian dust of spring 2001 Cite number
143
Nd/
1 1
144
Nd
Sampling date
Sampling duration (min)
Sample site
Area
(mg m3)
HOAc Leachate 87 Sr/ 86Sr
HOAc Residue 87 Sr/ 86Sr
HOAc Residue
HOAc Residue
Ejin Qi Ejin Qi
NNC NNC
2.200 1.900
0.711712 0.710955
0.722106 0.720174
0.512124 0.512221
10.02 8.14
Ejin Qi Yinchuan Baotou Datong Erenhot Jining Zhangjiakou Beijing Iki
NNC SG-LP WBJ WBJ NNC NNC BJ BJ Japan
1.700 7.900 9.200 2.100 1.900 2.600 2.100 0.758 0.180
0.709714 0.710889 0.711205 0.711646 0.710498 0.710607 0.711256 0.710283 0.710200
0.717903 0.719621 0.718394 0.719830 0.717360 0.718195 0.720062 0.714494 0.716612
0.512269 0.512150 0.511463 0.512081 0.512125 0.512223 0.512147 n.d 0.512195
7.21 9.51 22.91 10.87 10.00 8.09 9.58 n.d 8.63
eNd 25 March 29 April
300 510
Perfect dust storm 1 6 April 420 2 6 April 368 3 6 April 300 4 7 April 480 5 6 April 240 6 6–7 April 565 7 7 April 464 8 9 April 480 9 11–12 April 1482
n.d: not determined.
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fraction of desert sand and loess samples to obtain these isotopic ratios. The SG-LP is the largest region and includes the Badain Juran, Tengger, Ulan Buh, and Mu Us deserts. The WBJ includes the Hobq Desert. The soils of the NNC are distinguished from those of other regions by high 143Nd/ 144Nd ( ¼ eNd) values. According to Bory et al. (2003), soils in Mongolia also have high eNd values and are indistinguishable from soils of the NNC; therefore, they are grouped here with those of the NNC-MG. Perfect storm dust collection sites of dusts in China were in four regions: Ejin-Qi, Erenhot, and Jining in NNC-MG, Yichuan in SG-LP, Baotou and Datong in WBJ, and Zhangjiakou and Beijing in BJ. Liu et al. (1981) reported that the mean grain size of loess at 33 sites in China ranged from 19 to 46 mm; of these, they fell in a range of 20–40 mm at 29 sites. According to Liu and Chang (1962), the Asian dust sampled at Beijing on 18 April 1980 was dominated by coarse particles (10–50 mm) rather than by small particles (5–10 mm), and the grain-size distribution was similar to that of the Malan loess in the SG-LP region. Zhang et al. (1999) also reported that Asian dust from the Loess Plateau at the dust storm was dominated by coarse particles: 22% (420 mm), 55% (2–20 mm), and 23% (o2 mm). The volume or mass of particle is assumed roughly to be proportional to the cube of the grain size. For example, if a sample is composed of two cubic particles of 10-mm and 4-mm in diameter and with the same proportion, then 94% in the volume of the sample is calculated to be composed of the coarse particle. Accordingly, most elements including Sr and Nd in loess and Asian dust in China are contained mainly in coarse-grained particles (410 mm) rather than in fine particles (o5 mm), such as is the case for Asiandust derived particles deposited in remote area locations such as in the Greenland ice cores (Biscaye et al., 1997) and deep-sea sediments of the North Pacific (Asahara, 1999). The 87Sr/ 86Sr ratio of silicate minerals in Chinese loess increases with a decrease of grain size, but the difference in the 87Sr/ 86Sr ratio of the 2–20-mm fraction and the 420-mm fraction of bulk samples is small (o0.003) (Asahara et al., 1995; Asahara, 1999). According to Yokoo et al. (2004), silicates in the finegrained fraction (avg. 15.7 mm) of the desert sand of central northern China and those in loess derived from desert sand have similar 87Sr/ 86Sr ratios. This result also suggests that the 87Sr/ 86Sr ratio of whole silicates in Asian dust on China and other Far East Asia is not affected substantially by that of fine silicate particles whose grain-size is less than a few micrometers. Moreover, the Nd isotopic ratios of acid-resistant minerals and the Sr isotopic ratios of water- and weak-acidsoluble minerals are likely to be less dependent on grain size. Hence, the Sr–Nd isotopic compositions of these two groups of minerals in surface soils of northern
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China can be used as the source index for the perfect dust in the present study. Desert regions in northern China and Mongolia are considered to be the major source of Asian dust, and the Loess Plateau is considered the major dust depositional area in China (Zhang et al., 1996; Merrill et al., 1994). Gong et al. (2003) identified four regions as dust emission areas in spring 2001 based on a model analysis (Fig. 1); region 1 is in TKM, region 2 in SG-LP and WBJ, region 3 in NNC-MG and BJ, and region 4 in another part of NNC-MG. According to them, heavy dust clouds were generated mainly from regions 2 and 4 on 6 April in conjunction with a Mongolian cyclone, and as they moved eastward they became mixed with dust from another storm that originated in the Onqin Daga and Horqin deserts in region 3. The surface wind speed and dust emission flux over eastern China decreased on 7 April, whereas the dust mass concentration increased with the eastward movement of the cyclone and dust clouds (Figs. 4b, 5b and 7 in Liu et al., 2003). Subsequently, more dust clouds were generated in western China on 8–9 April during a second Mongolian cyclone. These two cyclone systems together constituted the perfect dust storm: a spiral band of dust clouds over eastern Mongolia and China (Liu et al., 2003). The front of the dust storm reached Korea on 8 April and Japan on 9 April. These studies show that the samples collected for the present study were deposited mainly from the dust clouds generated on 6 April. The acid-insoluble minerals of the perfect dust had different 87Sr/ 86Sr ratios and eNd values from those of soils around the collection sites (Fig. 2). Except for dust sampled at Baotou, these dust minerals plot with soil minerals of SG-LP, and they show a negative relationship between 87Sr/ 86Sr and eNd values, which is a diagnostic feature of acid-insoluble soil minerals in SGLP. However, their eNd values are slightly high compared with silicate minerals in SG-LP soils, showing an involvement of NNC-MG soils. The relationship between the 87Sr/ 86Sr ratios of the HOAc-soluble minerals and those of the HOAc-insoluble minerals for the dust samples and Chinese soil samples is shown in Fig. 3. Soils in BJ and NNC-MG have lower 87Sr/ 86Sr values than those in SG-LP, and those in TKM and WBJ have intermediate 87Sr/ 86Sr ratios. Despite a slight overlap, the BJ and NNC-MG soils are distinguished from the SG-LP soils by an 87Sr/ 86Sr ratio of about 0.7105 in the HOAc-soluble minerals and of about 0.715 in the HOAc-insoluble minerals. Most dust samples plot in the SG-LP area, but the average 87 Sr/ 86Sr values of HOAc-soluble minerals and HOAcinsoluble minerals (0.71070 and 0.71805, respectively) in the dust are lower than those in the SG-LP soil (0.71143 and 0.72063), suggesting a contribution of the NNCMG derived particle. Although the contribution of Sr from particles less than a few micrometers in size can be
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0
-5
5 6
1(4/6) 1(3/29) 2 1(3/25)
-10 εNd
NNC-MG BJ (Beijing) BJ (vicinity of Beijing) WBJ SG-LP TKM 2001 dust
7
-15
4 -20
3 Erdos
-25
-30 0.710
0.715 0.720 0.725 0.730 0.735 87Sr/86Sr (HOAc-insoluble minerals)
0.740
Fig. 2. 87Sr/ 86Sr vs. eNd values of HOAc-insoluble minerals of spring 2001 dust samples and soils from northern China. Soils data are taken from Nakano et al. (2004). Data for fine-grained (o5 mm) minerals from the Gobi and TKM deserts (indicated by a cross over a circle or triangle) are from Biscaye et al. (1997) and Bory et al. (2002, 2003), respectively. Blue line connects the 2001 dust and the surface soil at each site.
(0.7363)
0.720
NNC-MG BJ (Beijing) BJ (vicinity of Beijing) WBJ SG-LP TKM
Erdos 1(3/25) 0
1(3/29)
2001 dust
2
1(4/6)
-5
9 -10
8
0.715
5
6
4 7
0.710 0.708
3
0.710 0.711 0.712 HOAc-soluble minerals
0.713
0.714
Fig. 3. Relationship between 87Sr/ 86Sr values in HOAcextracted minerals and HOAc-insoluble minerals for spring 2001 dust samples and soils from northern China. Blue line connects the 2001 dust and the surface soil at each site.
ignored in the present dust samples, some fine-grained silicates (o5 mm) from the TKM and SG-LP regions (Biscaye et al., 1997; Bory et al., 2002, 2003) have
6 1(3/25) 7 2 3// 1(3/29) 5 4
-15
-20 0.709
1(4/6) 9
εNd
HOAc-insoluble minerals
0.725
higher 87Sr/ 86Sr ratios than those of bulk silicates (Fig. 3). Therefore, the 87Sr/ 86Sr ratios of the HOAcinsoluble minerals in bulk arid soil samples from northern China cannot be used directly as a source index for Asian dust in areas remote from China. Fig. 4 shows the relationship between the 87Sr/ 86Sr ratios of the HOAc-soluble minerals and the Nd values of the HOAc-insoluble minerals; both these indexes are considered to be independent of particle size. The SG-LP and TKM regions cannot be clearly distinguished in this diagram, but the contribution of TKM-derived particles to the studied dusts is considered to be insignificant based on the meteorological information mentioned above. Similarly to Figs. 2 and 3, most perfect dust samples (2001 dust) are plotted in the NNC-MG and SG-LP regions in Fig. 4. These results support the view that the perfect dust contained soil minerals uplifted from SG-LP and NNC-MG soils. The three dust samples collected at Ejin-Qi show a negative relationship between the 87Sr/ 86Sr ratios and eNd values of HOAc-insoluble minerals (Fig. 2) and a positive relationship between the 87Sr/ 86Sr values of the two groups of minerals (Fig. 3). This result is consistent with the Sr–Nd isotopic characteristics of soils in the SG-LP region. It is notable that the three dust samples at Ejin-Qi have higher eNd values in the HOAc-insoluble minerals and lower 87Sr/ 86Sr in the HOAc-soluble minerals than the SG-LP soils and other perfect dust samples from China, showing that the Ejin-Qi dust is more enriched with NNC-MG soils than
-25
-30 0.708
NNC-MG BJ (Beijing) BJ (vicinity of Beijing) WBJ SG-LP TKM 2001 dust
0.709
3
Erdos
0.710 0.711 0.712 0.713 (HOAc-soluble minerals)
0.714
87Sr/86Sr
Fig. 4. Relationship between 87Sr/ 86Sr values in HOAcextracted minerals and eNd values in HOAc-insoluble minerals for spring 2001 dust samples and soils from northern China. Blue line connects the 2001 dust and the surface soil at each site.
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the other dust samples. This result is consistent with the location of Ejin-Qi in the NNC-MG region. Among the three dust samples from Ejin-Qi, the dust collected on 6 April had the highest eNd value and the lowest 87 Sr/ 86Sr, showing the largest contribution from the NNC-MG soils on that date. This result is consistent with the meteorological modeling (Gong et al., 2003; Liu et al., 2003), which showed that the surface soil-dust speed and dust fluxes on 6 April were very high in region 2 and region 4 (Fig. 1). 3.2. Regional Sr–Nd isotopic variation of dust minerals The Sr–Nd isotopic ratios of the perfect dust vary regionally and generally plot along lines between the surface soils at the individual sites and the dust at Ejin-Qi on 25 April or 29 April (Figs. 2–4). The total amount of Asian dust deposited generally decreases exponentially in the Pacific-Rim region with distance from the source (Mori et al., 2002). But the dust concentration at Baotou was the highest among the studied sites (Table 1). At Baotou, dust silicates had higher 87Sr/ 86Sr and lower eNd values than those in the soil minerals at the collection site (Fig. 2). This finding can be explained by the incorporation into the dust sample of surface soil from the nearby Erdos site (Fig. 1), which is characterized by very high 87Sr/ 86Sr and low eNd values (Figs. 2 and 3). Likewise, at Yinchuan, where the dust concentration was second highest, the Sr–Nd isotopic ratios of dust minerals were almost identical to those of soil minerals in the vicinity. Dust minerals at Erenhot and Jining (sites 5 and 6) in NNC also plot closer to the NNC-MG soils compared with those from three other sites (sites 3, 4, and 7) in the WBJ and BJ (Figs. 2–4). Values for dust minerals at Baotou and Datong in WBJ (sites 3, 4) are closer to those in soils from SG-LP, which is geographically adjacent to WBJ. Thus, the Sr–Nd isotopic compositions of the perfect dust minerals at individual sites are more or less correlated with those of the ambient soils. Alfaro et al. (2003) reported that Asian dust at Yulin (Fig. 1) in the eastern SG-LP area contained anthropogenic particles from the nearby area but showed no significant differences in the elemental compositions of aerosol minerals. The present Sr–Nd isotopic data, however, demonstrate that the perfect dust contained some soil minerals from around the depositional site. Dust clouds on 6 April were modeled to have formed under the high dust flux in Mongolia to Inner Mongolia during the generation of a Mongolian cyclone (Liu et al., 2003). Local dust production occurs when the surface wind speed at 10 m above the ground is faster than 8 m s1 (Uno et al., 2001) or 10 m s1 (Alfaro et al., 2003). The surface wind speed on 7 April has been reported to have been less than 8 m s1 in almost the
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entire desert and loess region of China (Liu et al., 2003), but Gong et al. (2003) estimated that it was higher than 10 m s1 in WBJ and the NNC areas to the northeast of WBJ on that date. Hence, it is likely that the areal variation in the Sr–Nd isotopic ratios of dust minerals is attributable to the heterogeneity of the source soils, picked up by the storm on 6 April, except for samples collected on 7 April, which were composed of both soils uplifted locally as the storm moved eastward and source soils. In the dust source area on 6 April, the dust mass concentration in the storm decreased with altitude and became constant above 2.5 km, whereas in the eastern downstream area, it increased from ground level to 2–3 km of altitude and then decreased with altitude to 7 km (Figs. 12 and 13 in Liu et al., 2003). At Beijing (site 8), the top of the dust plume was 2–3 km high on 6 and 7 April, and it descended within a few days to less than 1 km on 8 and 9 April, when the dust sample was collected. It is therefore likely that most dust sampled in China was derived from the lower part of the dust plume, from the planetary boundary layer to less than 3 km. It is noteworthy that the Sr–Nd isotopic ratios of the two groups of dust minerals at Iki resemble those at Ejin-Qi despite the long distance between the two sites. The dust concentration at Iki was the lowest among the studied sites, suggesting that large amounts of the dust carried by the perfect storm had already been deposited before its arrival at this site. The height of the high dust concentration layer at Tsukuba and Nagasaki in Japan (Fig. 1) on 11 and 12 April was 3–7 km or higher (Figs. 18 and 19 in Liu et al., 2003). This result shows that the dust at Iki was derived from the upper part of the dust plume, which was generated in the northwestern high dust desert (Badain Juran, Tengger, and Ulan Buh deserts) by Alfaro et al. (2003) and the area to its north, resulting in the Sr–Nd isotopic similarity with the Ejin-Qi dust. The present result suggests that the Asian dust minerals transported over a long distance originated from a relatively narrow area, where they were picked up by a strong vertical updraft, and that most of this dust was redeposited on the desert and loess areas of China and Mongolia. This suggestion is consistent with the general assumption that Asian dust is formed by the rapid vertical uplift of soil minerals from the ground into the free troposphere, followed by rapid transportation by strong westerlies (Uno et al., 2001). Moreover, 75% of dust ejected from desert and loess areas of China and Mongolia return to these areas (Liu et al., 2003). The Sr–Nd isotopic compositions of dust in the downstream area, outside of China and Mongolia, reflect the averaged values of soils in the dust emission area, and therefore can potentially be used as a source-area fingerprint.
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4. Conclusions Comparison of Sr–Nd isotopic ratios of HOAcsoluble and HOAc-insoluble minerals of the perfect dust storm of early April 2001 with those of minerals from soils of northern China showed that the major source of dust minerals is the northwestern high dust desert and the area to its north. The perfect dust composition also showed a regional variation dependent on the Sr–Nd isotopic ratios of soils from sites near the depositional sites, indicating injection of soil minerals from the local area into the dust plume. Our data also show that this injection and accompanying modification was small in the upper part of the dust cloud. More extensive and size-dependent data on Sr–Nd isotopes of dust minerals and surface arid soils in China and Mongolia would be useful for determining the source and soil-mixing dynamics of Asian dust storms as well as for evaluating their impact on terrestrial and marine ecosystems of the Pacific region.
Acknowledgements This study was supported by a grant from the National Environmental Studies in Japan. The authors wish to thank Dr. Hao Quan at the Sino-Japan Friendship Center for Environmental Protection, Dr. Dong Xuhui, and JICA team at the Sino-Japan Friendship Center for Environmental Protection for their cooperation and help with aerosol sampling.
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