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Temporal characteristics of mineral dust particles in precipitation of Urumqi River Valley in Tian Shan, China: A comparison of alpine site and rural site
Atmospheric Research 101 (2011) 294–306
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Atmospheric Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a t m o s
Temporal characteristics of mineral dust particles in precipitation of Urumqi River Valley in Tian Shan, China: A comparison of alpine site and rural site Zhiwen Dong a,b,⁎, Zhongqin Li a, Ross Edwards c,d, Lihua Wu a,b, Ping Zhou a a State Key Laboratory of Cryospheric Sciences/Tianshan Glaciological Station, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China b Graduate University of Chinese Academy of Sciences, Beijing 100049, China c Desert Research Institute, Raggio Parkway, Reno, NV 89512, USA d Curtin University of Technology, Bentley WA 6102, Australia
a r t i c l e
i n f o
Article history: Received 30 August 2010 Received in revised form 21 February 2011 Accepted 8 March 2011
Keywords: Mineral dust particles Precipitation chemistry Urumqi River Valley Alpine site Rural site
a b s t r a c t Physico-chemical characteristics of mineral dust particles in the precipitation at both an alpine site and a rural site were determined in the Urumqi River Valley, Tian Shan, China. Results showed that the concentration, size distribution and chemical constitutes of dust particles in the precipitation varied seasonally at both two sites, though there existed obvious differences between them. Dust particle concentration in the precipitation increased during winter and springtime, but it decreased during summer and autumn, which had a positive correlation with strengthened wind speed and Asian dust activity, but a negative correlation with the seasonal variation of the precipitation amount, implying that aerosol dust in the precipitation of Urumqi River Valley in Tian Shan was very sensitive to the seasonal change of regional atmospheric environment in central Asia. Volume-size distribution of mineral dust particles showed larger modal size in Asian dust period of winter–spring, but relatively smaller modal size in non-dust period of summer–autumn. Moreover, the average modal size and mass concentration of dust particles at the rural site are larger than that of the alpine site. Chemical constitutes of dust indicated that the precipitation chemistry was mainly controlled by regional dust storms, local dust and anthropogenic activities in central Asia. HYSPLIT Model was also employed to examine the transport process of air mass and mineral dust particles in this region. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Mineral dust derived from the crustal surface is an important atmospheric component (Osada et al., 2004) affecting the Earth's radiation budget (Nakajima et al., 1989; Andreae, 1995; Tegen and Lacis, 1996). Dust is also an important indicator of changes in the atmosphere associated with changes in temperature, precipitation, and atmospheric circulation. Much research has measured dust in rainwater and snow to understand recent
⁎ Corresponding author at: State Key Laboratory of Cryospheric Sciences/ Tianshan Glaciological Station, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China. Tel.: +86 13919808103; fax: +86 931827 094. E-mail address: [email protected] (Z. Dong). 0169-8095/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2011.03.002
climate and atmospheric environment conditions (Zdanowicz et al., 1998; Osada et al., 2004). Atmospheric transport processes (Merrill et al., 1989; Uno et al., 2001) and transformation processes of the dust particles have also been studied to characterize the geochemical role of aeolian dust events in the Asian region. These dust events have been observed frequently in spring over the Asia-Pacific region (Koizumi, 1932; Arao et al., 2003) because of the strengthened wind speed in springtime. The Tian Shan, western China, is located in an arid and semiarid region of central Asia, the source region of Asian dust (Fig. 1). Dust storms are an important phenomenon in this region. Aerosol dust particles in the snow and rainfall on high mountains contain information on atmospheric environment at high elevation, and they are an important indicator of regional environment change. It is thus very important to research the
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Fig. 1. Location map of the sampling sites in Tian Shan, China, including the alpine station (AS) and the base (rural) station (BS) in Urumqi River Valley, which is surrounded by many large deserts in central Asia.
characteristics of dust in the precipitation of Tian Shan. Furthermore, the amount of dust particles close to the snowforming cloud altitude may provide a useful insight into the free-tropospheric fraction of dust over the central Asian region. Much work has been done on the chemistry of snow and ice at the Urumqi River source (Li et al., 2006a, 2006b; Dong et al., 2009, 2010) and other sites (Dong et al., 2009; Li et al., 2006a) in the eastern Tian Shan, China. However, little research work has been carried out on characteristics of mineral dust particles in the precipitation of different altitudes along Urumqi River Valley. In this study, the precipitation samples were collected at two stations in the Urumqi River Valley, the eastern Tian Shan. The base station (BS) (43°12′N, 87°07′E, 2100 m), lies within a rural site, and the alpine station (AS) (43°06′N, 86°50′ E, 3600 m), lies within an alpine site in the glacial region of Urumqi River source. We present and analyze the data of concentration, size distribution and ionic constituents of dust particles in the precipitation. Furthermore, backward trajectory of HYSPLIT Model was also employed to examine the transport process of air mass and dust particles in this region.
2. Study area The Urumqi River Valley is surrounded by vast desert areas (Fig. 1): the Taklimakan deserts to the south, Gurbantunggut
deserts to the north, and the Gobi desert to the east. Evergreen vegetation exists between 1600 and 3400 m in the mountains. The westerly circulation prevails across these high mountains throughout the year. Most precipitation in the Urumqi River Valley occurs during the summer months. There are several local industrial activities and population centers that impact the measurement sites. Urumqi city and Changji city, with more than three million inhabitants, are about 100 km to the northeast and northwest of alpine station and 65 km from base station. Urumqi has been ranked as one of the most severely polluted cities worldwide by several recent studies (Li et al., 2007; Mamtimin and Meixner, 2007). Houxia, near the base station, which has two coal-fired power generation plants, a coal powered cement factory and various other factories, lies 7 km from base station and 50 km from alpine station. Emissions are not controlled at the factories. Local inhabitants primarily use coal for cooking and heating.
3. Sampling and laboratory analysis During April 2003 to June 2004, precipitation samples were continuously collected at two sites with different altitude in Urumqi River Valley in Tian Shan: an alpine site (3600 m) and a rural site (2100 m). A total of 178 precipitation samples were collected (90 at base station and 88 at alpine station) during the
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investigation period. Rain samples were collected in polyethylene bottles through funnels (14 cm diameter), while snow samples were collected in polyethylene containers (50 mL). Samples collected were placed on the rooftop of the buildings (about 8−10 m from ground level and 1 m from the floor of the roof) away from the surface soil and any specific emission source. Before and after each sampling, the entire collection and storage equipment were thoroughly washed and rinsed several times with de-ionized water (18.0 MQ cm quality) and then dried in a class 100 clean bench. A sampling event is defined as the sample collected from the onset until the end of a given precipitation event. Electrical conductivity (EC) and pH were measured immediately after the rain event ended. Snow samples were melted at room temperature and EC and pH were measured as soon as possible. A pH meter (PHJS-4A) (Shanghai Precision & Scientific Instrument CO., LTD.) with a glass electrode was used for pH measurement by using standard buffer solutions with pH of 6.86 and 9.18, respectively. The EC measurement used a digital conductivity meter with temperature compensation (DDSJ308A) (Shanghai Precision & Scientific Instrument CO., LTD.). After that the samples were poured into small polyethylene bottles from the containers as soon as possible. All precipitation samples were shipped frozen from the sampling sites and stored at −18 °C until time for analysis. Samples were then melted and aliquots were collected for micro-particle and chemical analysis. Micro-particle concentrations and size distributions were measured on an Accusizer 780A counter, which uses the Single Particle Optical Sensing (SPOS) method, equipped with a 120 orifice (Zhu et al., 2006). Measurements were performed under class 100 conditions on sample aliquots diluted with a pre-filtered NaCl solution to give a 2 vol.% electrolyte concentration. The data were acquired for a size range of 0.57 to 100 μm (micro meters) equivalent spherical diameter (d). Routine analysis of filtered deionized water blanks showed background counts to be on average 10 times lower than in samples. And, background counts were subtracted from the sample data. All samples were analyzed in random order and in triplicate. Results were then averaged for individual samples, yielding an estimated error of 10% or less on particle concentrations. The mass and volume size distributions of micro-particles were calculated from the raw count data by assuming spherical particles of uniform density ρ = 2.6 g cm− 3, which is close to that of average crustal material. Mass was derived by integrating the mass size distribution over the measured diameter range and normalizing the result to the sample volume. We also computed the slope, β of the log-linear Junge distribution, dV V 1 lgr−lgμ 2 = pffiffiffiffiffiffi0ffi exp − dlgr 2 lgσ 2π lgσ fitted to particles with d less than 26 μm (Junge, 1963; Wake et al., 1994; Steffensen, 1997). In addition to dust particles, the concentrations of major + − + ions (Na+, Mg2+, Ca2+, Cl−, SO2− 4 , NH4 , K , and NO3 ) were measured at trace levels on a Dionex-600 and Dionex-320 ion chromatographs using the procedure described by Dong et al.
(2009) and Li et al. (2006a, 2006b). The mean blank value for the whirlpack bags for dust particles number is 444 mL− 1 in the laboratory measurements of this work, and the blank value for major ions is shown in Table 1. These blank values were subtracted from the sample data. 4. Results and discussion 4.1. Seasonal variation of dust particle concentration in precipitation Fig. 2 shows the temporal variation of the precipitation amount at both the alpine station and rural station in Urumqi River Valley of the Tian Shan. The precipitation showed an obvious seasonal change with high value in summer and autumn, but obviously low value in winter and spring at both two sites. Furthermore, the precipitation at the alpine site and rural site was very similar, as they are located in two zones with plentiful precipitation in the mountains (Li, 1991). The average annual precipitation amount for each station is around 500 mm, of which about 85% occurred in June–August. Moreover, as the alpine station is located within the river source of Urumqi River, the continuous observation could reflect the seasonal change of precipitation in a glacial region in the eastern Tian Shan. There existed good coincidence in the precipitation amount at the alpine station and the base station, and the same date of 14 July 2003 for maximum precipitation, which was 39.0 mm at the alpine station, and 40.2 mm at the base station (Fig. 2). Table 2 shows average number and mass concentration of mineral dust particles in the precipitation (snow and rain) at the alpine station and base station in Urumqi River Valley, and a comparison with the similar research in the snow at various sites in Tian Shan and also other Northern hemisphere sites. Dust concentration of this work was very similar to the results obtained from snow samples on the glaciers in Tian Shan, China (Dong et al., 2009) and Mt Tateyama, Japan (Osada et al., 2004), but was very different from the results obtained from remote Canadian Arctic (Zdanowicz et al., 1998), which may reflect the significant influence of a main dust source region in central Asia to the particle concentration in the precipitation in Tian Shan. Fig. 3 shows the temporal variation of number concentration of dust particles in the precipitation at the alpine station (Fig. 3a) and base station (Fig. 3b), which included the dust number of fine particles (d b 1 μm) and that of the total particles (0.57b db l00 μm) measured. We can find that there was evident seasonal change for dust concentration in the precipitation at both two sites, as the concentration was very high in winter and springtime but relatively low in summer and autumn, which was inferred to be caused by seasonal variation of dust storm activity in central Asia. The dust storms, often occurred in springtime, could bring plentiful dust particles from dust sources in central Asia, e.g., Taklimakan, Gurbantunggut deserts and Gobi desert (Okada and Kai, 2004; Wang et al., 2004; Wei et al., 2005; Sun et al., 2007; Zhu et al., 2008; Shen et al., 2009; Wu et al., 2010a), into the atmosphere and transported to the Tian Shan region (Dong et al., 2009; Wu et al., 2010b), then deposited with the precipitation incidents to the crustal surface. Thus dust concentration was very high in the precipitation of springtime. However, during summer and autumn period, with the rapid increase of precipitation incident occurrence, dust activity decreased and dust particles in the atmosphere were washed out, thus the number of dust
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Table 1 The blank value for major ions of the whirl pack bags in the lab analysis (μg/kg) and SD is standard deviation.
Blank value SD
Na+
Ca2+
Mg2+
Cl−
SO2− 4
NO− 3
NH+ 4
K+
0.5223 0.1408
1.6984 0.4578
0.0968 0.0261
1.5826 0.4265
0.8436 0.1765
0.0513 0. 5498
0.0258 0.0381
1.0823 0.3352
particles is very low in precipitation samples during these seasons. The temporal change of dust particles concentration had a very good negative correlation with the precipitation amount change (Fig. 2), which also reflected the great influence of atmospheric precipitation to the seasonal change of regional dust activities in the Tian Shan of central Asia. However, at the rural station, we should also notice that, during September to October 2003, the dust concentration (0.57 b d b 100 μm) showed a relatively higher value (Fig. 3b) in autumn, and fine particles (db 1 μm) also increased significantly during this period (Fig. 3b) at the rural station, which was inferred to be caused by anthropogenic activities as there were many industries around the base station. Houxia town, with two coal-fired power generation plants, a coal powered cement factory and various other factories, lies 7 km from the base station. Emissions are not controlled at the factories, and local inhabitants primarily use coal for cooking and heating. Moreover, pollution may also transport from factories in Urumqi city (Lee et al., 2003; Li et al., 2007; Mamtimin and Meixner, 2007; Wu et al., 2010b). The emission of the industries often contains many particles (especially fine particles (db 1 μm)), which may also cause the significant increase of dust concentration in the precipitation at a rural site in Tian Shan.
4.2. Size distribution and its seasonal variation of dust particles in precipitation Fig. 4 is a typical example of the volume-size distribution of mineral dust particles in the precipitation at the alpine station, in which the peak value of the curve is the modal size, μ of dust particles. Fig. 5 is a comparison of size distribution during different seasons at both of the rural site and alpine site. The volume median diameters of dust particles at the alpine site range from 3 to 21 μm. While at the base station the diameters range from 3 to 23 μm, slightly larger than that of the alpine station. Moreover, volume-size distribution of dust particles showed single-modal structures. Volume-size distribution of dust particles in precipitation is closely related with atmospheric environmental conditions when the precipitation incidents occurred. As shown in Fig. 5, obvious seasonality of dust size distribution was observed at both of the alpine station and the rural station, and there also existed large difference between two sites. It seems that, the modal size of dust particles in the precipitation is larger in winter–spring period, but relatively smaller in summer and autumn. The comparison of parameters for dust volume-size distribution is shown in Table 3. At the alpine station, the average modal size of dust
Fig. 2. Seasonal variation of the precipitation amount at two sites from April 2003 to June 2004 at the: (a) alpine station and (b) rural station.
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Table 2 Dust concentration in snow and rain at alpine station (AS) and base station (BS) in the Urumqi River Valley and at various sites in the Northern hemisphere. Study sites
AS, Urumqi river valley BS, Urumqi river valley Tuomuer glacier No.72 (Tian Shan, China) Urumqi glacier no. 1 (Tian Shan, China) Haxilegen glacier no. 51 (Tian Shan, China) Hami Miaoergou glacier (Tian Shan, China) Tateyama (Japan) Penny ice cap (Canadian Arctic)
Elevation (m)
Samples
Years
3600 2130 4600
Snow and rain Snow and rain Snow
2003–2004 2003–2004 2007–2008
4130
Snow
2002–2005
3900
Snow-pit
2002–2005
4510
Snow-pit
2002–2005
2450 1980
Snow Snow
1997–2002 1988–1994
particles was 15.2 μm in winter and spring, 12.4 μm in summer, and 11.5 μm in autumn, respectively. However, at the rural station, the average modal size was 16.5 μm in winter and spring, 13.2 μm in summer, and 10.8 μm in autumn, respectively. The corresponding mass concentration at the alpine station was 1860 μg/kg, 1368 μg/kg, and 1206 μg/kg, respectively; while at the rural station it was 2180 μg/kg, 1679 μg/kg, and 1724 μg/kg, respectively, which showed a high value in winter and spring obviously. Moreover, the average modal size and mass concentration at the rural station were higher than that of the alpine station in the precipitation during each season, which may indicate the influences of altitude change to dust transport and deposition in this region. In high altitude region, dust concentration in precipitation may reduce during longer transport process. Such strong seasonality for concen-
Concentration and flux
Ref. 3
Size range
Number (10 /mL)
Mass (μg/kg)
0.57–100 0.57–100 0.5726 1–26 0.57–26 1–26 0.57–26 1–26 0.57–26 1–26 0.1–600 0.65–12 1–12
377 427 706 384 242 100 166 74 222 94
1563 1962 3806 1678 1442 666 969 436 3690 1016
31.6 13.7
143 129
This work Dong et al.(2009) Dong et al.(2009) Dong et al.(2009) Dong et al.(2009) Osada et al.(2004) Zdanowicz et al.(1998)
tration and size distribution of dust particles in the precipitation may imply that, there were more large particles in the precipitation during Asian dust period, and aerosol dust in the precipitation of Urumqi River Valley was very sensitive to the seasonal change of regional and local atmospheric environments in central Asia. Fig. 6 shows the seasonal change of prevailing wind speed (NE and NNE) at the alpine station in the Urumqi River Valley during the investigation period, which reflected that, the wind was much stronger during springtime compared to other seasons. As the Urumqi River Valley is located in a main source region of Asian dust surrounded by many large deserts and Gobi desert, therefore, the wind speed change is closely related to occurrence of regional and local dust storm events, as strengthened wind speed can cause dust storms very easily in
Fig. 3. Seasonal variation of dust number concentration of the finer particles (d b 1 μm) and the total particles (0.57 b d b 100 μm) in precipitation at the: (a) alpine station and (b) rural station.
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Fig. 4. A typical example of volume-size distribution of dust particles in precipitation obtained from the alpine site, dv is the mode of distribution; fold-line indicates the volume-size distribution and flat-line shows normal regression distribution.
dry season of winter and spring in this region. The aerosol particles are floating dust in the atmosphere and transported to the high mountains of the Tian Shan, then deposited in the precipitation with higher concentration and larger size distribution during springtime. Therefore, the particle concentration and size distribution have a large difference between dust period in winter and spring and non-dust period of summer and autumn. These characteristics of particles also demonstrated the significant influence of regional atmospheric environment close to a dust source region to dust deposition in the precipitation of Tian Shan.
4.3. Chemical constitutes of dust particles in the precipitation Table 4 shows the concentrations of major anions and cations and pH, EC in the precipitation at the alpine station and the base station of Urumqi River Valley in Tian Shan, and also a comparison with the data of similar research at various sites in the world. The pH level and ionic concentration in Urumqi River Valley obtained by our study were relatively higher than many of the industrial cities in acid precipitation regions of China (Xu et al., 2007; Tang et al., 2000; Zhang et al., 2003, 2002; Huang et al., 2008; Tu et al., 2005; Yang et al., 2004; Tanner and Wong, 2000). In comparison to other regions around the world such as, Northeast America and Central Europe, and the city of neighboring countries, our pH value and ionic concentration were also relatively higher (Le Bolloch and Guerzoni, 1995; Ito et al., 2002; Lee et al., 2000; Okuda et al., 2005; Mouli et al., 2005). We also find that, the Ca2+ concentration was dominant among the major cations and anions in our study sites in Tian Shan, and was also much higher than the results at various sites in the world, which may imply that the precipitation in Urumqi River valley was significantly affected by high alkaline Ca2+ aerosol of
central Asia close to a dust source region, because Ca2+ is a tracer of mineral dusts from desert and loess areas in the Asian continent (Ichikuni, 1978; Suzuki and Tsunogai, 1988). Fig. 7 is the seasonal variation of major ionic concentrations of dust particles in the precipitation at both two sites in Urumqi River Valley. There existed obvious seasonal variation at two stations, and such a change seemed more obvious at the rural station. Strong seasonality showed high ionic concentrations in springtime, but relatively low concentration in summer and concentrations autumn (Fig. 7). Particulate Ca2+ and SO2− 4 varied seasonally, as their concentration was high during dust period of February–June, but was low during non-dust period of July–December, which was highly coincident with the occurrence of dust storm activity in central Asia. Particulate Ca2+ mainly existed in the insoluble part of dust particles in central Asia, the concentration of which showed high correlation with dust concentration in the precipitation (in Fig. 3). The 2− concentration changes of particulate NH+ 4 and SO4 , as water-soluble constitutes, are different from each other, as NH+ 4 concentration is rarely changed comparing the Asian dust period with the non-dust period. Table 5 shows correlation coefficients of major ionic concentration, EC and pH and dust in the precipitation at two sites. We can find that, there were relatively high correlation between most of the chemical constitutes, e.g., major ions and dust particles. Research has indicated that the Ca2+ in precipitation is mostly present in the coarse-mode particles, with high concentration in spring in the atmosphere of Asian region, which is always caused by dust events. Sulfate is generated as H2SO4 particles by the homogeneous and heterogeneous (oxidation of SO2 aqueous phase in cloud processes) processes, followed by the transition to (NH4)2SO4 through a reaction with NH3. These processes produce finemode particles (e.g., Seinfeld and Pandis, 1998). The equivalent
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Fig. 5. Comparison of volume-size distribution and its seasonal change at two sites in Urumqi River Valley: (1) alpine station (2) rural station; and (a) winter– spring (b) summer and (c) autumn.
+ ratio of SO2− 4 /NH4 in the accumulation mode was about 1. Chemical composition of the soluble part in the finer particles might have been close to that of (NH4)2SO4. The correlation
Table 3 Parameters for concentration and volume-size distribution of dust particles in the precipitation shown in Fig. 5. Sites
Time period
n
M (μg/kg)
dv (μm)
σg (μm)
Alpine station
Winter–spring summer autumn Winter–spring summer autumn
23 54 13 32 50 6
1860 1368 1206 2180 1679 1724
15.2 12.4 11.5 16.5 13.2 10.8
9.8 10.6 8.6 8.9 10.1 9.2
Rural station
Note: In the table, n is the sample number, dv is the mode of volume size distribution, and σg is standard deviation.
+ coefficient between SO2− 4 and NH4 was relatively high at two sites of this work (Table 5). Sulfate and ammonium in rainfall always come from both particles involved and absorption of their gaseous precursors (SO2 and NH3). There are several local industrial activities and population centers near the measurement sites in this work, so we can infer that the precipitation chemistry is also affected by local anthropogenic activities. Particulate Na+ was similar to the Ca2+ (most Na+ existed in coarse particles), and there was significant difference between the Na+ concentration in January–May and that in other months (June–December). We also find that, during the temporal variation of 2003–2004, at the alpine station, the ionic concentration was higher in springtime of 2003; however, at the rural station, it was higher in spring of 2004 (Fig. 7). As the sampling stations are located at different altitudes of the mountains in Tian Shan, such a seasonal difference of dust and
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Fig. 6. Seasonal variation of the prevailing wind speed (NE and NNE) obtained at the alpine station in the Urumqi River source during the investigation period.
chemical constitute concentrations in the precipitation at two sites is inferred to be caused by the raise of altitude and the influence of local mountain environment around the sampling stations. Fig. 8 shows temporal variation of pH and EC in the precipitation during investigation period in 2003–2004 at the rural station and the alpine station. PH and EC could reflect the acidity/alkalescence of precipitation and the total major ions carried in the atmosphere, thus they could show the chemical characteristics of the precipitation and atmospheric environmental condition when the precipitation occurred. Result showed that the pH and EC levels also varied seasonally, which was coincident with the occurrence of dust storm in
central Asia, because the pH and EC all have high values in springtime during dust period at both two sites, but have low values in non-dust period of summer and autumn with increased rain and snowfall in the atmosphere (Fig. 8). The seasonal change was very similar between two stations; however, at the rural station, such a change was more obvious. Research has shown that the pH and EC are closely related to the precipitation chemistry. The precipitation chemistry in springtime is significantly influenced by dust events and high alkaline Ca2+ aerosol of central Asia, and the pH and EC also have a high correlation with dust concentration. Therefore, the seasonal change of pH and EC in precipitation was inferred to be caused by regional dust storms and local dust in central Asia, which also
Table 4 Concentrations of major anions and cations (μg/L) and pH, EC (μs/cm) in the precipitation at various sites in the world and SD is standard deviation. Chemicals
pH
EC
Cl−
NO3−
SO42−
NH4+
Ca2+
Mg2+
K+
Na+
Ref.
Mean Maximum Minimum SD Mean Maximum Minimum SD Urumqi, China Waligan, China Lhasa, China Germu and Xining, China Shenzhen, China Nanjing, China Beijing, china Hongkong, China Tokyo, Japan Seoul, South Korea Tirupati, India Adirondack, NY Ankara, Turkey Sardinia, Italy
6.99 7.55 6.75 6.24 7.27 8.09 7.15 7.64 6.86 6.38 7.5 6.86 5.02 5.15 6.01 4.55 4.52 4.70 6.80 4.50 6.30 5.18
26.98 32.46 15.98 17.86 35.53 24.43 133.69 43.28 91.04 14.58 25.6 – – – – – – – – – – –
20.95 266.73 1.52 38.7 15.89 75.68 75.68 15.47 106.48 6.1 9.7 48.77 37.9 142.6 31.5 37.6 55.2 18.2 33.9 2.14 20.4 322
14.11 80.75 nd 13.79 31.72 405.3 nd 58.63 26.29 8.3 6.9 48.08 22.1 39.6 84.1 18.9 30.5 29.9 40.8 22.6 29.2 29
75.46 413.6 3.21 70.51 106.86 457.76 9.0 97.22 297.92 24 5.2 84.01 74.3 241.8 248.9 48.6 50.2 70.9 128.0 36.9 48.0 90
28.77 120.89 nd 19.81 23.21 79.17 nd 18.63 67.78 45.5 14.3 160.61 35.2 193.2 234.0 – 40.4 66.4 20.4 10.5 86.4 25
220.31 1150.5 6.17 235.62 432.15 1624.74 43.18 380.82 239.5 34 197.4 314.31 77.7 295.4 191.2 15.3 24.9 34.9 150.7 3.59 71.4 70
25.97 131.65 1.12 28.54 56.07 226.89 4.39 46.86 49.17 12.1 10.9 37.9 9.7 31.7 33.8 7.8 11.5 6.9 55.5 0.99 9.3 77
4.62 26.94 0.39 3.89 8.24 40.27 0.92 7.8 18.97 3.8 5.14 69.17 7.2 12.1 12.0 2.2 2.9 3.5 33.9 0.33 9.8 17
28.89 261.5 1.47 43.03 16.3 54.5 0.92 14.44 45.65 8.7 11.2 96.56 40.3 23.0 16.3 31.8 37.0 10.5 33.1 1.61 15.6 252
This work (alpine station)
This work (base station)
Xu et al. (2007). Tang et al. (2000) Zhang et al. (2003) Zhang et al. (2002) Huang et al.(2008) Tu et al. (2005). Yang et al. (2004). Tanner and Wong (2000). Okuda et al. (2005). Lee et al. (2000). Mouli et al. (2005). Ito et al. (2002) Topcu et al.(2002) Le Bolloch and Guerzoni (1995)
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Fig. 7. Seasonal variation of major ionic concentration of dust particles in the precipitation at two sites of Urmqi River Valley: (a) alpine site and (b) rural site.
indicated that the regional atmospheric environment had significant influence to the dust particles and chemical characteristics of the mountain precipitation, even in different altitude areas in Tian Shan. Therefore, we can conclude from above that, chemical constitutes of mineral dust particles indicated that the precipitation chemistry was mainly controlled by regional dust storms, local dust and anthropogenic activities in central Asia.
Hysplit 4 model was also used to demonstrate the source of air mass (Fig. 9) and aerosol dust particles in the precipitation (Fig. 10) at two stations in the Tian Shan. Although there is large difference between the altitude of the two sampling stations, the backward trajectories of air mass to two station were very similar, especially the transport trajectory when the precipitation occurred (Fig. 9). However, as the base station is located within a rural site of Urumqi city, where there exists many
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Table 5 Correlation coefficient of concentrations between dust, pH and major ions at two sites: the upper triangle is the rural station (N = 90 and p b 0.05) and under triangle is the alpine station (N = 88 and p b 0.05).
−
Cl SO24 NO− 3 Na+ NH+ 4 K+ Mg2+ Ca2+ pH Dust
Cl−
SO2− 4
NO− 3
Na+
NH+ 4
K+
Mg2+
Ca2+
pH
Dust
– 0.643 0.417 0.891 0.519 0.277 0.745 0.738 0.439 0.16
0.388 – 0.53 0.775 0.73 0.462 0.594 0.645 0.445 0.64
0.264 0.477 – 0.432 0.323 0.215 0.451 0.417 0.263 0.23
0.942 0.536 0.397 – 0.51 0.338 0.716 0.697 0.434 0.78
0.266 0.683 −0.072 0.194 – 0.329 0.269 0.222 0.26 0.65
0.604 0.579 0.27 0.777 0.302 – 0.329 0.502 0.522 0.43
0.41 0.813 0.281 0.542 − 0.197 0.469 – 0.883 0.743 0.55
0.319 0.851 0.351 0.475 −0.20 0.518 0.892 – 0.803 0.86
0.406 0.709 0.116 0.522 − 0.108 0.545 0.825 0.893 – 0.52
0.16 0.64 0.23 0.78 0.65 0.43 0.55 0.86 0.52 –
anthropogenic activities, industries and contamination, the emission of anthropogenic activities may have a significant influence to dust particles and chemistry of atmospheric precipitation. Fig. 10 is derived from Hysplit dispersion model. In the particle dispersion model, a fixed number of
particles are advected about the model domain by the mean wind field and spread by a turbulent component (http://ready. arl.noaa.gov/HYSPLIT.php). The model's default configuration assumes a 3-dimensional particle distribution (horizontal and vertical). We use the dispersion model to analyze the source
Fig. 8. Temporal variation of pH and EC in the precipitation at two sampling sites: (a) alpine site and (b) rural site.
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Fig. 9. Backward trajectories of air mass to two study sites during precipitation in Urumqi River Valley.
and transport process of local dust particles in the precipitation at the base station. Result showed that, the rural station was largely influenced by a local particle source, 100 km to the northwest of sampling site, which may be caused by anthropogenic activities besides local dust. We could infer that the source of particles may be Changji city and Urumqi city in Singkang Province, China, as they are located about 100 km to the northeast and northwest of the rural station in Tian Shan, which are also the main base for petroleum industries in western China. The emission from those industries will bring many larger and finer particles to the atmosphere, and then deposition in rainwater with the occurrences of precipitation in the mountains. The results also indicated that the local particle sources of anthropogenic activities in central Asia had large influence to the precipitation chemistry at the rural station of the Tian Shan. 5. Conclusions Mineral dust derived from the crustal surface is an important atmospheric component affecting the Earth's radiation budget. Dust storms are an important phenomenon in the Tian Shan of central Asia. Physico-chemical characteristics of dust particles in
the precipitation at both alpine and rural sites were measured and analyzed in the Urumqi River Valley, eastern Tian Shan, China. Results showed that the concentration, size distribution and chemical constitutes of dust particles in the precipitation varied seasonally at both two sampling sites. Dust concentration increased significantly during winter and springtime, but decreased during summer and autumn, which had a positive correlation with strengthened wind speed and Asian dust activity, but had negative correlation with seasonal variation of precipitation amount, implying that aerosol dust particles in Tian Shan were very sensitive to the seasonal change of atmospheric environment in central Asia. Moreover, dust mass concentration in the precipitation was higher at the rural station than that of the alpine station in each season, which may indicate the obvious influence of altitude change to dust particle transport and deposition. Volume-size distribution of particles showed larger modal size in Asian dust period of winter–spring, but relatively smaller modal size in non-dust period of summer–autumn. The volume median diameters of the dust particles at the alpine station range from 3 to 21 μm; while at the rural station they range from 3 to 23 μm, slightly larger than that of the alpine station. Moreover, volume-size distribution of dust particles in the precipitation showed single-modal structures. Chemical
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Fig. 10. An example of backward dispersion analysis of dust particles from the rural site in Urumqi River Valley.
constitutes indicated that the precipitation chemistry was mainly controlled by regional dust storms, local dust and anthropogenic activities in central Asia. Hysplit model was also used to demonstrate the source of air mass and dust particles in the precipitation at the alpine station and at the rural station in Tian Shan. Although the altitude is largely different between the two sampling stations, the backward trajectories of air mass for two stations were very similar, especially the trajectory when the precipitation occurred. Particle dispersion model analysis showed that the rural station was also largely influenced by a local particle source caused by anthropogenic activities besides local dust, which may also imply that the local anthropogenic activities had large influence to the precipitation chemistry at a rural site of the Tian Shan. Acknowledgements We would like to thank the staff and the students of Tian Shan Glaciological Station of the Chinese Academy of Sciences for their valuable logistical supports of the field work. The study
is financially supported by the National Basic Research Program (973) of China (No. 2010CB951003), Knowledge Innovation Programs of Chinese Academy of Science (No. KZCXZ-YW-127), National Natural Science Foundation of China (Nos. 91025012, 40631001, 40701034, 40701035, and 1141001040). We also thank two anonymous reviewers for their helpful comments and suggestions that very much improved the manuscript.
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Atmospheric Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a t m o s
Temporal characteristics of mineral dust particles in precipitation of Urumqi River Valley in Tian Shan, China: A comparison of alpine site and rural site Zhiwen Dong a,b,⁎, Zhongqin Li a, Ross Edwards c,d, Lihua Wu a,b, Ping Zhou a a State Key Laboratory of Cryospheric Sciences/Tianshan Glaciological Station, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China b Graduate University of Chinese Academy of Sciences, Beijing 100049, China c Desert Research Institute, Raggio Parkway, Reno, NV 89512, USA d Curtin University of Technology, Bentley WA 6102, Australia
a r t i c l e
i n f o
Article history: Received 30 August 2010 Received in revised form 21 February 2011 Accepted 8 March 2011
Keywords: Mineral dust particles Precipitation chemistry Urumqi River Valley Alpine site Rural site
a b s t r a c t Physico-chemical characteristics of mineral dust particles in the precipitation at both an alpine site and a rural site were determined in the Urumqi River Valley, Tian Shan, China. Results showed that the concentration, size distribution and chemical constitutes of dust particles in the precipitation varied seasonally at both two sites, though there existed obvious differences between them. Dust particle concentration in the precipitation increased during winter and springtime, but it decreased during summer and autumn, which had a positive correlation with strengthened wind speed and Asian dust activity, but a negative correlation with the seasonal variation of the precipitation amount, implying that aerosol dust in the precipitation of Urumqi River Valley in Tian Shan was very sensitive to the seasonal change of regional atmospheric environment in central Asia. Volume-size distribution of mineral dust particles showed larger modal size in Asian dust period of winter–spring, but relatively smaller modal size in non-dust period of summer–autumn. Moreover, the average modal size and mass concentration of dust particles at the rural site are larger than that of the alpine site. Chemical constitutes of dust indicated that the precipitation chemistry was mainly controlled by regional dust storms, local dust and anthropogenic activities in central Asia. HYSPLIT Model was also employed to examine the transport process of air mass and mineral dust particles in this region. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Mineral dust derived from the crustal surface is an important atmospheric component (Osada et al., 2004) affecting the Earth's radiation budget (Nakajima et al., 1989; Andreae, 1995; Tegen and Lacis, 1996). Dust is also an important indicator of changes in the atmosphere associated with changes in temperature, precipitation, and atmospheric circulation. Much research has measured dust in rainwater and snow to understand recent
⁎ Corresponding author at: State Key Laboratory of Cryospheric Sciences/ Tianshan Glaciological Station, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China. Tel.: +86 13919808103; fax: +86 931827 094. E-mail address: [email protected] (Z. Dong). 0169-8095/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2011.03.002
climate and atmospheric environment conditions (Zdanowicz et al., 1998; Osada et al., 2004). Atmospheric transport processes (Merrill et al., 1989; Uno et al., 2001) and transformation processes of the dust particles have also been studied to characterize the geochemical role of aeolian dust events in the Asian region. These dust events have been observed frequently in spring over the Asia-Pacific region (Koizumi, 1932; Arao et al., 2003) because of the strengthened wind speed in springtime. The Tian Shan, western China, is located in an arid and semiarid region of central Asia, the source region of Asian dust (Fig. 1). Dust storms are an important phenomenon in this region. Aerosol dust particles in the snow and rainfall on high mountains contain information on atmospheric environment at high elevation, and they are an important indicator of regional environment change. It is thus very important to research the
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Fig. 1. Location map of the sampling sites in Tian Shan, China, including the alpine station (AS) and the base (rural) station (BS) in Urumqi River Valley, which is surrounded by many large deserts in central Asia.
characteristics of dust in the precipitation of Tian Shan. Furthermore, the amount of dust particles close to the snowforming cloud altitude may provide a useful insight into the free-tropospheric fraction of dust over the central Asian region. Much work has been done on the chemistry of snow and ice at the Urumqi River source (Li et al., 2006a, 2006b; Dong et al., 2009, 2010) and other sites (Dong et al., 2009; Li et al., 2006a) in the eastern Tian Shan, China. However, little research work has been carried out on characteristics of mineral dust particles in the precipitation of different altitudes along Urumqi River Valley. In this study, the precipitation samples were collected at two stations in the Urumqi River Valley, the eastern Tian Shan. The base station (BS) (43°12′N, 87°07′E, 2100 m), lies within a rural site, and the alpine station (AS) (43°06′N, 86°50′ E, 3600 m), lies within an alpine site in the glacial region of Urumqi River source. We present and analyze the data of concentration, size distribution and ionic constituents of dust particles in the precipitation. Furthermore, backward trajectory of HYSPLIT Model was also employed to examine the transport process of air mass and dust particles in this region.
2. Study area The Urumqi River Valley is surrounded by vast desert areas (Fig. 1): the Taklimakan deserts to the south, Gurbantunggut
deserts to the north, and the Gobi desert to the east. Evergreen vegetation exists between 1600 and 3400 m in the mountains. The westerly circulation prevails across these high mountains throughout the year. Most precipitation in the Urumqi River Valley occurs during the summer months. There are several local industrial activities and population centers that impact the measurement sites. Urumqi city and Changji city, with more than three million inhabitants, are about 100 km to the northeast and northwest of alpine station and 65 km from base station. Urumqi has been ranked as one of the most severely polluted cities worldwide by several recent studies (Li et al., 2007; Mamtimin and Meixner, 2007). Houxia, near the base station, which has two coal-fired power generation plants, a coal powered cement factory and various other factories, lies 7 km from base station and 50 km from alpine station. Emissions are not controlled at the factories. Local inhabitants primarily use coal for cooking and heating.
3. Sampling and laboratory analysis During April 2003 to June 2004, precipitation samples were continuously collected at two sites with different altitude in Urumqi River Valley in Tian Shan: an alpine site (3600 m) and a rural site (2100 m). A total of 178 precipitation samples were collected (90 at base station and 88 at alpine station) during the
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investigation period. Rain samples were collected in polyethylene bottles through funnels (14 cm diameter), while snow samples were collected in polyethylene containers (50 mL). Samples collected were placed on the rooftop of the buildings (about 8−10 m from ground level and 1 m from the floor of the roof) away from the surface soil and any specific emission source. Before and after each sampling, the entire collection and storage equipment were thoroughly washed and rinsed several times with de-ionized water (18.0 MQ cm quality) and then dried in a class 100 clean bench. A sampling event is defined as the sample collected from the onset until the end of a given precipitation event. Electrical conductivity (EC) and pH were measured immediately after the rain event ended. Snow samples were melted at room temperature and EC and pH were measured as soon as possible. A pH meter (PHJS-4A) (Shanghai Precision & Scientific Instrument CO., LTD.) with a glass electrode was used for pH measurement by using standard buffer solutions with pH of 6.86 and 9.18, respectively. The EC measurement used a digital conductivity meter with temperature compensation (DDSJ308A) (Shanghai Precision & Scientific Instrument CO., LTD.). After that the samples were poured into small polyethylene bottles from the containers as soon as possible. All precipitation samples were shipped frozen from the sampling sites and stored at −18 °C until time for analysis. Samples were then melted and aliquots were collected for micro-particle and chemical analysis. Micro-particle concentrations and size distributions were measured on an Accusizer 780A counter, which uses the Single Particle Optical Sensing (SPOS) method, equipped with a 120 orifice (Zhu et al., 2006). Measurements were performed under class 100 conditions on sample aliquots diluted with a pre-filtered NaCl solution to give a 2 vol.% electrolyte concentration. The data were acquired for a size range of 0.57 to 100 μm (micro meters) equivalent spherical diameter (d). Routine analysis of filtered deionized water blanks showed background counts to be on average 10 times lower than in samples. And, background counts were subtracted from the sample data. All samples were analyzed in random order and in triplicate. Results were then averaged for individual samples, yielding an estimated error of 10% or less on particle concentrations. The mass and volume size distributions of micro-particles were calculated from the raw count data by assuming spherical particles of uniform density ρ = 2.6 g cm− 3, which is close to that of average crustal material. Mass was derived by integrating the mass size distribution over the measured diameter range and normalizing the result to the sample volume. We also computed the slope, β of the log-linear Junge distribution, dV V 1 lgr−lgμ 2 = pffiffiffiffiffiffi0ffi exp − dlgr 2 lgσ 2π lgσ fitted to particles with d less than 26 μm (Junge, 1963; Wake et al., 1994; Steffensen, 1997). In addition to dust particles, the concentrations of major + − + ions (Na+, Mg2+, Ca2+, Cl−, SO2− 4 , NH4 , K , and NO3 ) were measured at trace levels on a Dionex-600 and Dionex-320 ion chromatographs using the procedure described by Dong et al.
(2009) and Li et al. (2006a, 2006b). The mean blank value for the whirlpack bags for dust particles number is 444 mL− 1 in the laboratory measurements of this work, and the blank value for major ions is shown in Table 1. These blank values were subtracted from the sample data. 4. Results and discussion 4.1. Seasonal variation of dust particle concentration in precipitation Fig. 2 shows the temporal variation of the precipitation amount at both the alpine station and rural station in Urumqi River Valley of the Tian Shan. The precipitation showed an obvious seasonal change with high value in summer and autumn, but obviously low value in winter and spring at both two sites. Furthermore, the precipitation at the alpine site and rural site was very similar, as they are located in two zones with plentiful precipitation in the mountains (Li, 1991). The average annual precipitation amount for each station is around 500 mm, of which about 85% occurred in June–August. Moreover, as the alpine station is located within the river source of Urumqi River, the continuous observation could reflect the seasonal change of precipitation in a glacial region in the eastern Tian Shan. There existed good coincidence in the precipitation amount at the alpine station and the base station, and the same date of 14 July 2003 for maximum precipitation, which was 39.0 mm at the alpine station, and 40.2 mm at the base station (Fig. 2). Table 2 shows average number and mass concentration of mineral dust particles in the precipitation (snow and rain) at the alpine station and base station in Urumqi River Valley, and a comparison with the similar research in the snow at various sites in Tian Shan and also other Northern hemisphere sites. Dust concentration of this work was very similar to the results obtained from snow samples on the glaciers in Tian Shan, China (Dong et al., 2009) and Mt Tateyama, Japan (Osada et al., 2004), but was very different from the results obtained from remote Canadian Arctic (Zdanowicz et al., 1998), which may reflect the significant influence of a main dust source region in central Asia to the particle concentration in the precipitation in Tian Shan. Fig. 3 shows the temporal variation of number concentration of dust particles in the precipitation at the alpine station (Fig. 3a) and base station (Fig. 3b), which included the dust number of fine particles (d b 1 μm) and that of the total particles (0.57b db l00 μm) measured. We can find that there was evident seasonal change for dust concentration in the precipitation at both two sites, as the concentration was very high in winter and springtime but relatively low in summer and autumn, which was inferred to be caused by seasonal variation of dust storm activity in central Asia. The dust storms, often occurred in springtime, could bring plentiful dust particles from dust sources in central Asia, e.g., Taklimakan, Gurbantunggut deserts and Gobi desert (Okada and Kai, 2004; Wang et al., 2004; Wei et al., 2005; Sun et al., 2007; Zhu et al., 2008; Shen et al., 2009; Wu et al., 2010a), into the atmosphere and transported to the Tian Shan region (Dong et al., 2009; Wu et al., 2010b), then deposited with the precipitation incidents to the crustal surface. Thus dust concentration was very high in the precipitation of springtime. However, during summer and autumn period, with the rapid increase of precipitation incident occurrence, dust activity decreased and dust particles in the atmosphere were washed out, thus the number of dust
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Table 1 The blank value for major ions of the whirl pack bags in the lab analysis (μg/kg) and SD is standard deviation.
Blank value SD
Na+
Ca2+
Mg2+
Cl−
SO2− 4
NO− 3
NH+ 4
K+
0.5223 0.1408
1.6984 0.4578
0.0968 0.0261
1.5826 0.4265
0.8436 0.1765
0.0513 0. 5498
0.0258 0.0381
1.0823 0.3352
particles is very low in precipitation samples during these seasons. The temporal change of dust particles concentration had a very good negative correlation with the precipitation amount change (Fig. 2), which also reflected the great influence of atmospheric precipitation to the seasonal change of regional dust activities in the Tian Shan of central Asia. However, at the rural station, we should also notice that, during September to October 2003, the dust concentration (0.57 b d b 100 μm) showed a relatively higher value (Fig. 3b) in autumn, and fine particles (db 1 μm) also increased significantly during this period (Fig. 3b) at the rural station, which was inferred to be caused by anthropogenic activities as there were many industries around the base station. Houxia town, with two coal-fired power generation plants, a coal powered cement factory and various other factories, lies 7 km from the base station. Emissions are not controlled at the factories, and local inhabitants primarily use coal for cooking and heating. Moreover, pollution may also transport from factories in Urumqi city (Lee et al., 2003; Li et al., 2007; Mamtimin and Meixner, 2007; Wu et al., 2010b). The emission of the industries often contains many particles (especially fine particles (db 1 μm)), which may also cause the significant increase of dust concentration in the precipitation at a rural site in Tian Shan.
4.2. Size distribution and its seasonal variation of dust particles in precipitation Fig. 4 is a typical example of the volume-size distribution of mineral dust particles in the precipitation at the alpine station, in which the peak value of the curve is the modal size, μ of dust particles. Fig. 5 is a comparison of size distribution during different seasons at both of the rural site and alpine site. The volume median diameters of dust particles at the alpine site range from 3 to 21 μm. While at the base station the diameters range from 3 to 23 μm, slightly larger than that of the alpine station. Moreover, volume-size distribution of dust particles showed single-modal structures. Volume-size distribution of dust particles in precipitation is closely related with atmospheric environmental conditions when the precipitation incidents occurred. As shown in Fig. 5, obvious seasonality of dust size distribution was observed at both of the alpine station and the rural station, and there also existed large difference between two sites. It seems that, the modal size of dust particles in the precipitation is larger in winter–spring period, but relatively smaller in summer and autumn. The comparison of parameters for dust volume-size distribution is shown in Table 3. At the alpine station, the average modal size of dust
Fig. 2. Seasonal variation of the precipitation amount at two sites from April 2003 to June 2004 at the: (a) alpine station and (b) rural station.
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Table 2 Dust concentration in snow and rain at alpine station (AS) and base station (BS) in the Urumqi River Valley and at various sites in the Northern hemisphere. Study sites
AS, Urumqi river valley BS, Urumqi river valley Tuomuer glacier No.72 (Tian Shan, China) Urumqi glacier no. 1 (Tian Shan, China) Haxilegen glacier no. 51 (Tian Shan, China) Hami Miaoergou glacier (Tian Shan, China) Tateyama (Japan) Penny ice cap (Canadian Arctic)
Elevation (m)
Samples
Years
3600 2130 4600
Snow and rain Snow and rain Snow
2003–2004 2003–2004 2007–2008
4130
Snow
2002–2005
3900
Snow-pit
2002–2005
4510
Snow-pit
2002–2005
2450 1980
Snow Snow
1997–2002 1988–1994
particles was 15.2 μm in winter and spring, 12.4 μm in summer, and 11.5 μm in autumn, respectively. However, at the rural station, the average modal size was 16.5 μm in winter and spring, 13.2 μm in summer, and 10.8 μm in autumn, respectively. The corresponding mass concentration at the alpine station was 1860 μg/kg, 1368 μg/kg, and 1206 μg/kg, respectively; while at the rural station it was 2180 μg/kg, 1679 μg/kg, and 1724 μg/kg, respectively, which showed a high value in winter and spring obviously. Moreover, the average modal size and mass concentration at the rural station were higher than that of the alpine station in the precipitation during each season, which may indicate the influences of altitude change to dust transport and deposition in this region. In high altitude region, dust concentration in precipitation may reduce during longer transport process. Such strong seasonality for concen-
Concentration and flux
Ref. 3
Size range
Number (10 /mL)
Mass (μg/kg)
0.57–100 0.57–100 0.5726 1–26 0.57–26 1–26 0.57–26 1–26 0.57–26 1–26 0.1–600 0.65–12 1–12
377 427 706 384 242 100 166 74 222 94
1563 1962 3806 1678 1442 666 969 436 3690 1016
31.6 13.7
143 129
This work Dong et al.(2009) Dong et al.(2009) Dong et al.(2009) Dong et al.(2009) Osada et al.(2004) Zdanowicz et al.(1998)
tration and size distribution of dust particles in the precipitation may imply that, there were more large particles in the precipitation during Asian dust period, and aerosol dust in the precipitation of Urumqi River Valley was very sensitive to the seasonal change of regional and local atmospheric environments in central Asia. Fig. 6 shows the seasonal change of prevailing wind speed (NE and NNE) at the alpine station in the Urumqi River Valley during the investigation period, which reflected that, the wind was much stronger during springtime compared to other seasons. As the Urumqi River Valley is located in a main source region of Asian dust surrounded by many large deserts and Gobi desert, therefore, the wind speed change is closely related to occurrence of regional and local dust storm events, as strengthened wind speed can cause dust storms very easily in
Fig. 3. Seasonal variation of dust number concentration of the finer particles (d b 1 μm) and the total particles (0.57 b d b 100 μm) in precipitation at the: (a) alpine station and (b) rural station.
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Fig. 4. A typical example of volume-size distribution of dust particles in precipitation obtained from the alpine site, dv is the mode of distribution; fold-line indicates the volume-size distribution and flat-line shows normal regression distribution.
dry season of winter and spring in this region. The aerosol particles are floating dust in the atmosphere and transported to the high mountains of the Tian Shan, then deposited in the precipitation with higher concentration and larger size distribution during springtime. Therefore, the particle concentration and size distribution have a large difference between dust period in winter and spring and non-dust period of summer and autumn. These characteristics of particles also demonstrated the significant influence of regional atmospheric environment close to a dust source region to dust deposition in the precipitation of Tian Shan.
4.3. Chemical constitutes of dust particles in the precipitation Table 4 shows the concentrations of major anions and cations and pH, EC in the precipitation at the alpine station and the base station of Urumqi River Valley in Tian Shan, and also a comparison with the data of similar research at various sites in the world. The pH level and ionic concentration in Urumqi River Valley obtained by our study were relatively higher than many of the industrial cities in acid precipitation regions of China (Xu et al., 2007; Tang et al., 2000; Zhang et al., 2003, 2002; Huang et al., 2008; Tu et al., 2005; Yang et al., 2004; Tanner and Wong, 2000). In comparison to other regions around the world such as, Northeast America and Central Europe, and the city of neighboring countries, our pH value and ionic concentration were also relatively higher (Le Bolloch and Guerzoni, 1995; Ito et al., 2002; Lee et al., 2000; Okuda et al., 2005; Mouli et al., 2005). We also find that, the Ca2+ concentration was dominant among the major cations and anions in our study sites in Tian Shan, and was also much higher than the results at various sites in the world, which may imply that the precipitation in Urumqi River valley was significantly affected by high alkaline Ca2+ aerosol of
central Asia close to a dust source region, because Ca2+ is a tracer of mineral dusts from desert and loess areas in the Asian continent (Ichikuni, 1978; Suzuki and Tsunogai, 1988). Fig. 7 is the seasonal variation of major ionic concentrations of dust particles in the precipitation at both two sites in Urumqi River Valley. There existed obvious seasonal variation at two stations, and such a change seemed more obvious at the rural station. Strong seasonality showed high ionic concentrations in springtime, but relatively low concentration in summer and concentrations autumn (Fig. 7). Particulate Ca2+ and SO2− 4 varied seasonally, as their concentration was high during dust period of February–June, but was low during non-dust period of July–December, which was highly coincident with the occurrence of dust storm activity in central Asia. Particulate Ca2+ mainly existed in the insoluble part of dust particles in central Asia, the concentration of which showed high correlation with dust concentration in the precipitation (in Fig. 3). The 2− concentration changes of particulate NH+ 4 and SO4 , as water-soluble constitutes, are different from each other, as NH+ 4 concentration is rarely changed comparing the Asian dust period with the non-dust period. Table 5 shows correlation coefficients of major ionic concentration, EC and pH and dust in the precipitation at two sites. We can find that, there were relatively high correlation between most of the chemical constitutes, e.g., major ions and dust particles. Research has indicated that the Ca2+ in precipitation is mostly present in the coarse-mode particles, with high concentration in spring in the atmosphere of Asian region, which is always caused by dust events. Sulfate is generated as H2SO4 particles by the homogeneous and heterogeneous (oxidation of SO2 aqueous phase in cloud processes) processes, followed by the transition to (NH4)2SO4 through a reaction with NH3. These processes produce finemode particles (e.g., Seinfeld and Pandis, 1998). The equivalent
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Fig. 5. Comparison of volume-size distribution and its seasonal change at two sites in Urumqi River Valley: (1) alpine station (2) rural station; and (a) winter– spring (b) summer and (c) autumn.
+ ratio of SO2− 4 /NH4 in the accumulation mode was about 1. Chemical composition of the soluble part in the finer particles might have been close to that of (NH4)2SO4. The correlation
Table 3 Parameters for concentration and volume-size distribution of dust particles in the precipitation shown in Fig. 5. Sites
Time period
n
M (μg/kg)
dv (μm)
σg (μm)
Alpine station
Winter–spring summer autumn Winter–spring summer autumn
23 54 13 32 50 6
1860 1368 1206 2180 1679 1724
15.2 12.4 11.5 16.5 13.2 10.8
9.8 10.6 8.6 8.9 10.1 9.2
Rural station
Note: In the table, n is the sample number, dv is the mode of volume size distribution, and σg is standard deviation.
+ coefficient between SO2− 4 and NH4 was relatively high at two sites of this work (Table 5). Sulfate and ammonium in rainfall always come from both particles involved and absorption of their gaseous precursors (SO2 and NH3). There are several local industrial activities and population centers near the measurement sites in this work, so we can infer that the precipitation chemistry is also affected by local anthropogenic activities. Particulate Na+ was similar to the Ca2+ (most Na+ existed in coarse particles), and there was significant difference between the Na+ concentration in January–May and that in other months (June–December). We also find that, during the temporal variation of 2003–2004, at the alpine station, the ionic concentration was higher in springtime of 2003; however, at the rural station, it was higher in spring of 2004 (Fig. 7). As the sampling stations are located at different altitudes of the mountains in Tian Shan, such a seasonal difference of dust and
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Fig. 6. Seasonal variation of the prevailing wind speed (NE and NNE) obtained at the alpine station in the Urumqi River source during the investigation period.
chemical constitute concentrations in the precipitation at two sites is inferred to be caused by the raise of altitude and the influence of local mountain environment around the sampling stations. Fig. 8 shows temporal variation of pH and EC in the precipitation during investigation period in 2003–2004 at the rural station and the alpine station. PH and EC could reflect the acidity/alkalescence of precipitation and the total major ions carried in the atmosphere, thus they could show the chemical characteristics of the precipitation and atmospheric environmental condition when the precipitation occurred. Result showed that the pH and EC levels also varied seasonally, which was coincident with the occurrence of dust storm in
central Asia, because the pH and EC all have high values in springtime during dust period at both two sites, but have low values in non-dust period of summer and autumn with increased rain and snowfall in the atmosphere (Fig. 8). The seasonal change was very similar between two stations; however, at the rural station, such a change was more obvious. Research has shown that the pH and EC are closely related to the precipitation chemistry. The precipitation chemistry in springtime is significantly influenced by dust events and high alkaline Ca2+ aerosol of central Asia, and the pH and EC also have a high correlation with dust concentration. Therefore, the seasonal change of pH and EC in precipitation was inferred to be caused by regional dust storms and local dust in central Asia, which also
Table 4 Concentrations of major anions and cations (μg/L) and pH, EC (μs/cm) in the precipitation at various sites in the world and SD is standard deviation. Chemicals
pH
EC
Cl−
NO3−
SO42−
NH4+
Ca2+
Mg2+
K+
Na+
Ref.
Mean Maximum Minimum SD Mean Maximum Minimum SD Urumqi, China Waligan, China Lhasa, China Germu and Xining, China Shenzhen, China Nanjing, China Beijing, china Hongkong, China Tokyo, Japan Seoul, South Korea Tirupati, India Adirondack, NY Ankara, Turkey Sardinia, Italy
6.99 7.55 6.75 6.24 7.27 8.09 7.15 7.64 6.86 6.38 7.5 6.86 5.02 5.15 6.01 4.55 4.52 4.70 6.80 4.50 6.30 5.18
26.98 32.46 15.98 17.86 35.53 24.43 133.69 43.28 91.04 14.58 25.6 – – – – – – – – – – –
20.95 266.73 1.52 38.7 15.89 75.68 75.68 15.47 106.48 6.1 9.7 48.77 37.9 142.6 31.5 37.6 55.2 18.2 33.9 2.14 20.4 322
14.11 80.75 nd 13.79 31.72 405.3 nd 58.63 26.29 8.3 6.9 48.08 22.1 39.6 84.1 18.9 30.5 29.9 40.8 22.6 29.2 29
75.46 413.6 3.21 70.51 106.86 457.76 9.0 97.22 297.92 24 5.2 84.01 74.3 241.8 248.9 48.6 50.2 70.9 128.0 36.9 48.0 90
28.77 120.89 nd 19.81 23.21 79.17 nd 18.63 67.78 45.5 14.3 160.61 35.2 193.2 234.0 – 40.4 66.4 20.4 10.5 86.4 25
220.31 1150.5 6.17 235.62 432.15 1624.74 43.18 380.82 239.5 34 197.4 314.31 77.7 295.4 191.2 15.3 24.9 34.9 150.7 3.59 71.4 70
25.97 131.65 1.12 28.54 56.07 226.89 4.39 46.86 49.17 12.1 10.9 37.9 9.7 31.7 33.8 7.8 11.5 6.9 55.5 0.99 9.3 77
4.62 26.94 0.39 3.89 8.24 40.27 0.92 7.8 18.97 3.8 5.14 69.17 7.2 12.1 12.0 2.2 2.9 3.5 33.9 0.33 9.8 17
28.89 261.5 1.47 43.03 16.3 54.5 0.92 14.44 45.65 8.7 11.2 96.56 40.3 23.0 16.3 31.8 37.0 10.5 33.1 1.61 15.6 252
This work (alpine station)
This work (base station)
Xu et al. (2007). Tang et al. (2000) Zhang et al. (2003) Zhang et al. (2002) Huang et al.(2008) Tu et al. (2005). Yang et al. (2004). Tanner and Wong (2000). Okuda et al. (2005). Lee et al. (2000). Mouli et al. (2005). Ito et al. (2002) Topcu et al.(2002) Le Bolloch and Guerzoni (1995)
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Fig. 7. Seasonal variation of major ionic concentration of dust particles in the precipitation at two sites of Urmqi River Valley: (a) alpine site and (b) rural site.
indicated that the regional atmospheric environment had significant influence to the dust particles and chemical characteristics of the mountain precipitation, even in different altitude areas in Tian Shan. Therefore, we can conclude from above that, chemical constitutes of mineral dust particles indicated that the precipitation chemistry was mainly controlled by regional dust storms, local dust and anthropogenic activities in central Asia.
Hysplit 4 model was also used to demonstrate the source of air mass (Fig. 9) and aerosol dust particles in the precipitation (Fig. 10) at two stations in the Tian Shan. Although there is large difference between the altitude of the two sampling stations, the backward trajectories of air mass to two station were very similar, especially the transport trajectory when the precipitation occurred (Fig. 9). However, as the base station is located within a rural site of Urumqi city, where there exists many
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Table 5 Correlation coefficient of concentrations between dust, pH and major ions at two sites: the upper triangle is the rural station (N = 90 and p b 0.05) and under triangle is the alpine station (N = 88 and p b 0.05).
−
Cl SO24 NO− 3 Na+ NH+ 4 K+ Mg2+ Ca2+ pH Dust
Cl−
SO2− 4
NO− 3
Na+
NH+ 4
K+
Mg2+
Ca2+
pH
Dust
– 0.643 0.417 0.891 0.519 0.277 0.745 0.738 0.439 0.16
0.388 – 0.53 0.775 0.73 0.462 0.594 0.645 0.445 0.64
0.264 0.477 – 0.432 0.323 0.215 0.451 0.417 0.263 0.23
0.942 0.536 0.397 – 0.51 0.338 0.716 0.697 0.434 0.78
0.266 0.683 −0.072 0.194 – 0.329 0.269 0.222 0.26 0.65
0.604 0.579 0.27 0.777 0.302 – 0.329 0.502 0.522 0.43
0.41 0.813 0.281 0.542 − 0.197 0.469 – 0.883 0.743 0.55
0.319 0.851 0.351 0.475 −0.20 0.518 0.892 – 0.803 0.86
0.406 0.709 0.116 0.522 − 0.108 0.545 0.825 0.893 – 0.52
0.16 0.64 0.23 0.78 0.65 0.43 0.55 0.86 0.52 –
anthropogenic activities, industries and contamination, the emission of anthropogenic activities may have a significant influence to dust particles and chemistry of atmospheric precipitation. Fig. 10 is derived from Hysplit dispersion model. In the particle dispersion model, a fixed number of
particles are advected about the model domain by the mean wind field and spread by a turbulent component (http://ready. arl.noaa.gov/HYSPLIT.php). The model's default configuration assumes a 3-dimensional particle distribution (horizontal and vertical). We use the dispersion model to analyze the source
Fig. 8. Temporal variation of pH and EC in the precipitation at two sampling sites: (a) alpine site and (b) rural site.
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Fig. 9. Backward trajectories of air mass to two study sites during precipitation in Urumqi River Valley.
and transport process of local dust particles in the precipitation at the base station. Result showed that, the rural station was largely influenced by a local particle source, 100 km to the northwest of sampling site, which may be caused by anthropogenic activities besides local dust. We could infer that the source of particles may be Changji city and Urumqi city in Singkang Province, China, as they are located about 100 km to the northeast and northwest of the rural station in Tian Shan, which are also the main base for petroleum industries in western China. The emission from those industries will bring many larger and finer particles to the atmosphere, and then deposition in rainwater with the occurrences of precipitation in the mountains. The results also indicated that the local particle sources of anthropogenic activities in central Asia had large influence to the precipitation chemistry at the rural station of the Tian Shan. 5. Conclusions Mineral dust derived from the crustal surface is an important atmospheric component affecting the Earth's radiation budget. Dust storms are an important phenomenon in the Tian Shan of central Asia. Physico-chemical characteristics of dust particles in
the precipitation at both alpine and rural sites were measured and analyzed in the Urumqi River Valley, eastern Tian Shan, China. Results showed that the concentration, size distribution and chemical constitutes of dust particles in the precipitation varied seasonally at both two sampling sites. Dust concentration increased significantly during winter and springtime, but decreased during summer and autumn, which had a positive correlation with strengthened wind speed and Asian dust activity, but had negative correlation with seasonal variation of precipitation amount, implying that aerosol dust particles in Tian Shan were very sensitive to the seasonal change of atmospheric environment in central Asia. Moreover, dust mass concentration in the precipitation was higher at the rural station than that of the alpine station in each season, which may indicate the obvious influence of altitude change to dust particle transport and deposition. Volume-size distribution of particles showed larger modal size in Asian dust period of winter–spring, but relatively smaller modal size in non-dust period of summer–autumn. The volume median diameters of the dust particles at the alpine station range from 3 to 21 μm; while at the rural station they range from 3 to 23 μm, slightly larger than that of the alpine station. Moreover, volume-size distribution of dust particles in the precipitation showed single-modal structures. Chemical
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305
Fig. 10. An example of backward dispersion analysis of dust particles from the rural site in Urumqi River Valley.
constitutes indicated that the precipitation chemistry was mainly controlled by regional dust storms, local dust and anthropogenic activities in central Asia. Hysplit model was also used to demonstrate the source of air mass and dust particles in the precipitation at the alpine station and at the rural station in Tian Shan. Although the altitude is largely different between the two sampling stations, the backward trajectories of air mass for two stations were very similar, especially the trajectory when the precipitation occurred. Particle dispersion model analysis showed that the rural station was also largely influenced by a local particle source caused by anthropogenic activities besides local dust, which may also imply that the local anthropogenic activities had large influence to the precipitation chemistry at a rural site of the Tian Shan. Acknowledgements We would like to thank the staff and the students of Tian Shan Glaciological Station of the Chinese Academy of Sciences for their valuable logistical supports of the field work. The study
is financially supported by the National Basic Research Program (973) of China (No. 2010CB951003), Knowledge Innovation Programs of Chinese Academy of Science (No. KZCXZ-YW-127), National Natural Science Foundation of China (Nos. 91025012, 40631001, 40701034, 40701035, and 1141001040). We also thank two anonymous reviewers for their helpful comments and suggestions that very much improved the manuscript.
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