Tracing Dust Sources Using Stable Lead and Strontium Isotopes in

Chapter 4

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Tracing Dust Sources Using Stable Lead and Strontium Isotopes in Central Asia Nitika Dewan,1 Brian J. Majestic,*,1 Michael E. Ketterer,2 Justin P. Miller-Schulze,3 Martin M. Shafer,4,5 James J. Schauer,4,5 Paul A. Solomon,6 Maria Artamonova,7 Boris B. Chen,8 Sanjar A. Imashev,8 and Gregory R. Carmichael9 1Department

of Chemistry and Biochemistry, University of Denver, Denver, Colorado 80208, U.S.A. 2Department of Chemistry, Metropolitan State University of Denver, 1201 5th Street, Denver, Colorado 80204, U.S.A. 3Department of Chemistry, 6000 J Street, California State University, Sacramento, California 95819, U.S.A. 4Wisconsin State Laboratory of Hygiene, 2601 Agriculture Drive, Madison, Wisconsin 53718, U.S.A. 5Environmental Chemistry and Technology Program, 660 North Park Street, University of Wisconsin, Madison, Wisconsin 53706, U.S.A. 6U.S. EPA, Office of Research and Development, Las Vegas, Nevada 89193, U.S.A. 7Institute of Atmospheric Physics, 109017 Moscow, Russia 8Kyrgyz-Russian Slavic University, 44 Kievskaya Street, Bishkek 720000, Kyrgyzstan 9Department of Chemical and Biochemical Engineering, The University of Iowa, Iowa City, Iowa 52242, U.S.A. *E-mail: [email protected].

From 1960 to 2014, the Aral Sea’s surface area has receded about 90% in size from 68,000 km2 to 8,444 km2. Consequently, newly exposed sediments are resuspended by the wind and are now a source of atmospheric particulate matter in Central Asia, which may have an impact on human health and climate. In this study, strontium (Sr) and lead (Pb) stable isotopic ratios, along with other elemental compositions, are used to determine if the Aral Sea sediments are an important source of © 2015 American Chemical Society Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


air pollution to Central Asia. Ambient particulate matter with aerodynamic diameter < 10 µm (PM10) samples were collected every other day and included dust and non-dust events at the Bishkek and LIDAR (in Teploklyuchenka) in Kyrgyzstan. Soil samples also were collected in the vicinity of the air sampling sites, resuspended, and sized as PM10 for chemical analysis. The average 87Sr/86Sr ratio for the Aral Sea sediments was 0.70992 (range, 0.70951 - 0.71064), which is less radiogenic than the surface soils in Kyrgyzstan showing an average ratio of 0.71579 (range, 0.71448 - 0.71739). In contrast, the airborne PM10 collected in Kyrgyzstan had an average 87Sr/86Sr ratio of 0.71177 (range, 0.70946 - 0.71335), which is between the two ratios, indicating a possible mixture of sources. However, no differences in Sr ratios were observed between dust and non-dust events, which implies that the impact of Aral Sea sediments on the sampling sites is minimal. The element enrichment factors and stable Pb isotope ratios are employed to further understand the source of PM10. Airborne PM10 are characterized by enrichments in elements like As, Cd, Cu, Mo, Pb, and Zn at both sampling sites. The mean (K/Pb) ratio for aerosols is ~ 45 and for soils is ~800, which suggest that the aerosols contain a significant fraction of anthropogenic source of airborne Pb in Kyrgyzstan. 208Pb/206Pb ratios are higher for aerosols compared to the Kyrgyzstan soils and Aral Sea sediments also suggesting that Pb is most likely present due to anthropogenic sources.

Introduction Atmospheric particulate matter (PM) is a mixture of solid particles and liquid droplets suspended in the air. This includes organic carbon, elemental carbon, anions, cations, bulk metals, and trace elements. These particles exist in different shapes, concentrations, and are of varying chemical composition depending upon origin, source of emissions, atmospheric processing, and season. The United States Environmental Protection Agency regulates PM in two categories based on sizes e.g., fine particles (PM2.5) are less than 2.5 µm in diameter and coarse particles (PM10) which are less than 10 µm in diameter. PM in the atmosphere is a serious issue because of its impact on climate, human health, visibility, and atmospheric reactivity (1, 2). PM can travel up to thousands of km and their effects can be observed downwind, not just at the site of the emission (3). For example, dust from the Gobi Desert in Western China travels more than 10,000 km, affecting the air quality on the western coast of the United States (4). As particles can be transported, it is essential to determine the source of their emission. Recently, studies have shown that stable isotope ratios (e.g., Sr and Pb) are potent tools for source origin (5, 6) and tracing dust sources (7, 8). The power 80 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


of Pb and Sr isotopic system lies in the fact that biological processes and phase changes do not fractionate these isotopes; therefore, the measured ratio represents the exact source or mixture of sources (9). Both rocks and minerals possess unique 87Sr/86Sr ratios depending upon the geological origin (10). 87Sr is a radiogenic and non-radioactive isotope produced from the radioactive decay of 87Rb; therefore, increased 87Sr/86Sr in older rocks and sediments is observed (10). Stable Pb isotope ratios allow efficient tracing of the sources of Pb pollution in the atmosphere (11). Pb has three naturally occurring stable radiogenic isotopes; 206Pb, 207Pb, and 208Pb, produced from the radioactive decay chain of 238U, 235U, and 232Th respectively. The only stable non-radiogenic isotope is 204Pb. 207Pb/206Pb, 208Pb/204Pb, 206Pb/ 204Pb, and 207Pb/204Pb ratios are useful in differentiating anthropogenic sources from natural sources (11, 12). These ratios can vary considerably depending on their origin, rainfall, wind direction, and the presence or absence of anthropogenic sources (13, 14). The Aral Sea, once one of the largest lakes of the world, is located in Central Asia between Kazakhstan and Uzbekistan. The Aral Sea’s surface area has receded 90% in size due to removal of water for irrigation, thereby transforming the region into a salt desert (15). Several ecological problems like soil salinization, groundwater table reduction (16), and increase in intensity and frequency of dust storms have resulted due to the Aral Sea’s desiccation (17). The Aral Sea basin also serves as a trap for wind-blown dust originating from deserts on all sides: Circum-Aral Karakum to the north, Kyzylkum to the east, Karakum to the south, and Ustyurt to the west, thus dust storms originating in the Aral Sea likely contain a mixture of these deserts, in addition to the native sediments (18). These relatively recently exposed sediments, emitted into the air, may be a previously unidentified source of PM10 in Central Asia, which could have an impact on human health and climate (19). In this study, we aim to characterize sources of PM10 from two sites in Kyrgyzstan to advance our understanding of regional and long-range transport of aerosol PM. Herein, we expand upon data and conclusions previously published in Atmospheric Environment (Atmos. Environ.) (20). We focus on the Aral Sea region, which is now the third largest source of mineral dust in Asia (21). The primary goal of this study is to determine if the resuspended Aral Sea sediments are an important source of air pollution in Kyrgyzstan. This is achieved by examining differences between the metal content of PM10 from dust event and non-dust event periods at both sampling sites and whether the Sr and Pb isotopic composition of the Aral Sea sediments and local soils collected in Kyrgyzstan are distinguishable from each other and those of previously published Western and Central China data.

Experimental Sample Collection Sites Kyrgyzstan is a country located in Central Asia, which is bordered by Kazakhstan to the north, Tajikistan to the south, China to the east, and Uzbekistan to the west. Bishkek is the capital and the largest city of Kyrgyzstan with a 81 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


population of ~854,000 (20). Kyrgyzstan is a mountainous country dominated by the Tien-Shan mountain range encompassing the entire nation. The samples were collected at two urban sites in Kyrgyzstan: Bishkek and LIDAR. The Bishkek site (42° 40′ 47.80″ N, 74° 31′ 44.30" E, altitude 1,250 m) is located ~23 km south of the Bishkek city center. The LIDAR site (42° 27′ 49.30″ N, 78° 3123 km south of the Bishkek city center. The LIDAR site (42° 27′ 49.30″ N, 78° 49.30″ E, altitude 1920 m) is located in Teploklyuchenka, which is about 380 km east of the Bishkek site. Both sampling sites are located in the mountains and the distance directly between them is approximately 315 km. The Bishkek site is ~ 1,200 km and the LIDAR site is ~1,500 km east-southeast (ESE) from the Aral Sea. A map of the region and sampling sites in shown in Figure 1 (20).

Figure 1. Map of Central Asia and geographical location of the sampling sites (shown as stars) and surrounding deserts (1-Kyzylkum, 2-Aral Karakum, 3-Karakum, 4-Ustyurt and Mangyshlak, 5-Betpak Dala, 6-Saryesik-Atyrau Desert, 7-Taukum, 8-Qaratal and Lepsy, 9-Moinkum, 10-Aralkum) relative to Aral Sea location. Reproduced with permission from reference (20). Copyright (2015) Elsevier.

Sample Collection Sediments collected at the Aral Sea and Kyrgyzstan soils collected at the Bishkek and LIDAR sites were resuspended in the laboratory and collected on filters as PM10 (particles < 10 µm in diameter) for subsequent analysis. In addition, atmospheric PM10 samples were collected every other day at the Kyrgyzstan sites from mid-July 2008 to mid-July 2009 during dust and non-dust events. The 82 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


objective of this study was to focus on the long-range and trans-boundary transport of pollution from outside and within Central Asia, respectively; therefore, to minimize the influence of local PM10 sources, these sites were selected based on their distance from more populated areas. The PM10 samples were collected on pre-cleaned 47 mm Teflon filters (Teflo, Pall-Gelman) using URG 3000 ABC samplers (URG Corporation, U.S.A.) at a flow rate of 8 L min–1 for 24 h at each sampling site. Filters and filter portions were composited into biweekly or monthly samples depending on the sample loading. Additional details regarding sampling sites and sample collection can be found elsewhere (19). Chemical Analysis Sample preparation was performed under positive pressure HEPA filtered air. Using a high-precision microbalance (MX5, Mettler-Toledo, U.S.A.), the mass of the collected PM10 was determined gravimetrically by weighing the filters that were equilibrated pre- and post-sampling at constant humidity (35 ± 3%) and temperature (21 ± 2 °C) for 24 h. The mass measurement had an uncertainty of < 7% or ± 4 µg. A Po-ionization source was used to remove the static charges on the filters before weighing. The airborne PM10 and resuspended PM10 Kyrgyzstan soils and Aral Sea sediments were digested in a 36-position Microwave Rotor (Milestone Ethos). The digestion matrix consisted of 1 mL nitric (16 M), 0.25 mL hydrochloric (12 M), and 0.1 mL hydrofluoric acid (28 M). The microwave digested samples were diluted to 15 mL with Millipore water (>18 MΩcm, MQ) and elemental concentrations were determined using high-resolution magnetic sector inductively coupled plasma mass spectrometry (HR-ICP-MS, Element 2, Thermo-Fisher). Laboratory blanks and field blanks were rigorously applied to account for potential contamination of filters and reagents. Uncertainty for each element was determined from error propagation analysis. Analysis of Sr Stable Isotope Ratios Prior to Sr isotope analysis, Rb was removed since 87Rb interferes with the Sr isotope measurement. In this multistep process, the samples were first evaporated in Teflon vials and diluted to 1 mL with 2 M optima grade nitric acid (Fisher, U.S.A.). In the digests, Rb content was about 4 - 180 ng and Sr content was about 18 - 680 ng. Several airborne PM10 samples were composited to ensure concentrations above the method detection limit of 18 ng for Sr. Sr was separated from Rb using Sr Spec resin (Eichrom, U.S.A.) (22). The Sr Spec resin slurry was prepared in 0.05 M HNO3 and loaded into modified 5 mL glass wool fitted plastic Pasteur pipettes. The Sr Spec column was cleaned by passing 3 × 5 mL Millipore water, 2 mL 0.05 M HNO3, 5 mL of Millipore water through the column. The column was then conditioned by passing 4 × 0.5 mL 2 M HNO3 through it. Following column conditioning, the sample in 2 M HNO3, was loaded in 2 × 0.5 mL aliquots. The sample loaded column was washed with 1 mL of 2 M HNO3, 8 × 0.5 mL of 7 M HNO3, and 1 mL of 3 M HNO3, while Sr was retained. Elution of Sr was accomplished by passing 6 × 0.5 mL of 0.05 M HNO3 through the column. 83 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Sr concentrations were measured using a quadrupole inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700). The Sr recoveries for the samples were between 90-110%. 87Sr/86Sr ratios of the Aral Sea sediments were measured by a Nu Plasma II multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) and the PM10 and Kyrgyzstan soils by a Thermo-Finnigan Neptune MC-ICP-MS. Stable Pb isotopic ratios were measured in extracts with no further purification by using high-resolution magnetic sector inductively coupled plasma mass spectrometer (Thermo-Finnigan Neptune Plus). The Sr and Pb chemical separation of the samples was validated by analyzing certified standard references, NBS 987 and NIST 981, respectively. The NBS 987 yielded 87Sr/86Sr = 0.71030 ± 0.00001 (n = 17) by Neptune and 87Sr/86Sr =0.71023 ± 0.00003 (n = 17) by Nu Plasma. The certified value of 87Sr/86Sr for NBS 987 is 0.71024 ± 0.00007. The NIST 981 yielded 208Pb/206Pb = 2.1656 ± 0.0014 (n = 16) and 207Pb/206Pb = 0.9144 ± 0.0001 (n = 16). The certified value of 208Pb/206Pb for NIST 981 is 2.1681 ± 0.0008 and 207Pb/206Pb is 0.9146 ± 0.0003.

Results and Discussion Site-Dependent Elemental Concentrations Trace and bulk elements were quantified in the resuspended Kyrgyzstan soils, resuspended Aral Sea sediments, and airborne PM10 collected at the two sampling sites during dust events and non-dust events (Tables 1 and 2). The major crustal element levels (µg g–1) of the LIDAR soils were Al (~67000 ± 6000), Fe (~50000 ± 3500), and Ca (~29000 ± 2500); and of Bishkek soils were Al (~63000 ± 6000), Fe (~40000 ± 3000), and Ca (~22000 ± 2000). This indicates that the elemental composition of the Kyrgyzstan soils at both sites were similar. In contrast, in the resuspended Aral Sea sediments, Ca (~50000 ± 3500) was the dominant element followed by Fe (~20000 ± 1500), and Al (~23000 ± 1500). Approximately, two or three fold differences also existed in the concentrations of Na, Cr, S, Ti, Cu, Ni, Zn, Rb, Sr, Mo, and Sb in Aral Sea sediments compared to the Kyrgyzstan soils (Table 1). The elements measured in PM10 showed significantly higher concentrations during dust events compared to non-dust events at the sampling sites (Table 2). For instance, concentrations (µg g–1) of the crustal related elements, such as Al, Ca, and Fe were ~11300, ~18000, and ~9800 respectively, during dust events and ~9900, ~13000, and ~9000 during non-dust events at the Bishkek site. Similarly, at the LIDAR site, Al, Ca, and Fe concentrations (µg g–1) were ~20000, ~26000, and ~19000, respectively during dust events and ~11000, ~15000, and ~12000 during non-dust events. At both sites, the dominant element in PM10 was Ca followed by Fe, K, and Al. Overall, the concentrations of most of the elements were higher at the LIDAR site relative to the Bishkek site. This may be because the LIDAR site is much closer to the Taklamakan Desert and thus more impacted by the dust transport from this desert (23). 84 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Table 1. Average Elemental Concentrations (µg g–1) of Resuspended Kyrgyzstan Soils and Resuspended Aral Sea Sediments. Reproduced with permission from reference (20). Copyright (2015) Elsevier. Element

Bishkek soils (µg g–1)

LIDAR soils (µg g–1)

Al

63000 ± 6000

67000 ± 6000

23000 ± 1500

K

19000 ± 6000

21000 ± 7000

16000 ± 3000

Ca

22000 ± 2000

29000 ± 2500

50000 ± 3500

Mg

10000 ± 1000

15000 ± 1100

6000 ± 500

Na

12000 ± 1100

16000 ± 1500

9500 ± 750

Cr

90 ± 6

100 ± 6

250 ± 40

P

600 ± 50

1100 ± 60

500 ± 30

Mn

800 ± 60

960 ± 60

650 ± 30

Fe

40000 ± 3000

50000 ± 3500

20000 ± 1500

Ni

42 ± 3

43 ± 4

60 ± 6

V

120 ± 7

130 ± 8

100 ± 5

Zn

56 ± 6

60 ± 7

73 ± 9

Co

16 ± 1

17 ± 1

11 ± 1

Cu

32 ± 2

34 ± 2

21 ± 1

Ti

4600 ± 360

5300 ± 400

1800 ± 120

As

11 ± 3

9±2

15 ± 3

Rb

60 ± 5

81 ± 6

46 ± 5

Sr

160 ± 15

250 ± 20

630 ± 50

Mo

1.0 ± 0.1

2.0 ± 0.2

8±1

S

370 ± 30

480 ± 40

11000 ± 1000

Sb

1.5 ± 0.1

1.2 ± 0.1

25 ± 2

Ba

500 ± 40

530 ± 40

400 ± 20

Pb

24 ± 2

25 ± 2

18 ± 2

La

20 ± 1

30 ± 2

15 ± 1

Th

8±1

9±2

5.0 ± 0.7

U

4.0 ± 0.3

9±1

3.0 ± 0.3

Aral Sea sediments (µg g–1)

Continued on next page.

85 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Table 1. (Continued). Average Elemental Concentrations (µg g–1) of Resuspended Kyrgyzstan Soils and Resuspended Aral Sea Sediments. Element

Bishkek soils (µg g–1)

LIDAR soils (µg g–1)

Sn

4.5 ± 0.3

3.7 ± 0.3

1.2 ± 0.5

K/Pb

775

863

842

Aral Sea sediments (µg g–1)

Table 2. Average Elemental Concentrations (µg g–1) of Airborne PM10 Samples Collected at the Bishkek and LIDAR Sampling Sites Element

Bishkek dust events PM10 (µg g–1)

Bishkek non-dust events PM10 (µg g–1)

LIDAR dust events PM10 (µg g–1)

LIDAR non-dust events PM10 (µg g–1)

Al

11300 ± 1050

9900 ± 100

20000 ± 1800

11000 ± 1000

K

9000 ± 120

8000 ± 1000

16000 ± 3000

14000 ± 2500

Ca

18000 ± 1600

13000 ± 1200

26000 ± 2000

15000 ± 1200

Mg

3500 ± 250

3000 ± 300

7000 ± 500

4000 ± 300

Na

5000 ± 450

3500 ± 350

7000 ± 300

4500 ± 400

Cr

30 ± 5

37 ± 5

44 ± 2

66 ± 7

P

25 ± 2

10 ± 1

15 ± 1

10 ± 1

Mn

280 ± 18

260 ± 15

480 ± 30

350 ± 20

Fe

9800 ± 100

9000 ± 600

19000 ± 1200

12000 ± 1000

Ni

25 ± 8

25 ± 7

31 ± 5

30 ± 5

V

57 ± 3

65± 4

73 ± 5

65 ± 5

Zn

230 ± 30

300 ± 40

150 ± 30

330 ± 85

Co

50 ± 11

64 ± 15

350 ± 30

150 ± 15

Cu

45 ± 8

103 ± 12

320 ± 30

100 ± 10

Ti

1200 ± 100

1100 ± 100

2000 ± 150

1500 ± 120

As

24 ± 5

30 ± 5

21 ± 4

25 ± 5

Rb

25 ± 3

25 ± 2

55 ± 6

35 ± 7

Sr

180 ± 18

110 ± 10

200 ± 18

130 ± 12

Mo

2.5 ± 0.2

3.3 ± 0.4

2.6 ± 0.3

4±1

S

15000 ± 1500

17000 ± 1600

17000 ± 1500

10000 ± 1000

Continued on next page.



Table 2. (Continued). Average Elemental Concentrations (µg g–1) of Airborne PM10 Samples Collected at the Bishkek and LIDAR Sampling Sites Element

Bishkek dust events PM10 (µg g–1)

Bishkek non-dust events PM10 (µg g–1)

LIDAR dust events PM10 (µg g–1)

LIDAR non-dust events PM10 (µg g–1)

Sb

15 ± 1

15 ± 1

8±1

13 ± 2

Ba

150 ± 13

150 ± 10

270 ± 20

200 ± 15

Pb

180 ± 30

230 ± 40

160 ± 15

100 ± 12

La

5±1

5.5 ± 0.4

12 ± 3

7±1

Th

1.8 ± 0.1

2.0 ± 0.2

4±1

3±1

U

2.1 ± 0.2

1.9 ± 0.1

3.0 ± 0.5

2.3 ± 0.1

Sn

9±3

20 ± 3

5±3

10 ± 3

K/Pb

57

46

112

76

Enrichment Factor The enrichment factor (EF) of the elements was calculated by first normalizing the elemental concentrations in the sample with aluminum (Al) (24), and then dividing by the Upper Continental Crust (UCC) ratio (25). The EF is used to identify anthropogenic components of aerosols in the atmosphere. EF is close to unity for the elements related to the reference, Al (marker for crustal emissions). A high EF (>> 10) suggests an important anthropogenic source is associated with that pollutant (24). EF can be calculated using the following formula

The dashed line (EF = 10) on the plots shown in Figure 2(a) and 2(b) represents the level above which the element is considered to be anthropogenically sourced (24). There is no difference in EF for crustal elements like Ca, Fe, K, Mg, Na, and Ti during dust and non-dust events at both the sampling sites. Also, these elements were associated with natural sources at the Bishkek and LIDAR sites. The toxic elements like As, Cd, Cr, Cu, Mo, Sb, Sn, Pb, and Zn were highly enriched at both sampling sites. In Figure 2(a), significant differences were observed between the dust and non-dust events, with EF higher during the non-dust events at the LIDAR sites. The implication here is that anthropogenic sources are dominant during the non-dust events. At the Bishkek site, however, there was very little difference in EF between dust and non-dust sampling periods, aside from Cu and Sn as shown in Figure 2(b). 87 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 2. Enrichment Factor (EF) for PM10 collected at 2(a) LIDAR site during dust and non-dust events and 2(b) Bishkek site during dust and non-dust events.



Figure 3. Correlation between elemental Pb (µg g–1) and elemental Th (µg g–1) for 3(a) Aral Sea sediments and Kyrgyzstan soils and 3(b) PM10 during dust and non-dust events at the two sampling sites. Reproduced with permission from reference (20). Copyright (2015) Elsevier.



208Pb

is the final step in the radioactive decay chain of 232Th (26). Anthropogenic Pb sources from Aral Sea sediments, local soils, and airborne PM10 can be differentiated from natural Pb sources by plotting concentrations (µg g–1) of elemental Pb against elemental Th (20, 27). In Figure 3(a), thorium (Th) correlates linearly with Pb for Aral Sea sediments and Kyrgyzstan soils, indicating that Pb in these samples is primarily from natural sources (27). This is also consistent with low Upper Continental Crust Enrichment Factor for Pb for both soils (EFPb = 1.54) and Aral Sea sediments (EFPb = 3.09), since EF is less than 10. The K/Pb ratio for the local soils and Aral Sea sediments is in between 775 - 863, as shown in Table 1, which also implies that the Pb in these samples is most likely of natural origin (28, 29). The lack of relationship [Figure 3(b)] between the elemental Th and elemental Pb in PM10 indicates that airborne Pb is dominated by anthropogenic sources and not dust sources, such as from Aral Sea or local soils. This observation was in agreement with high UCC EFs for PM10 (EFPb = 37) at Bishkek site and (EFPb = 74) at LIDAR site. Also, the range of K/Pb ratios is in between 46-112, as shown in Table 2, indicating an anthropogenic component of Pb in PM10 at both sampling sites.

Impact of the Aral Sea The 87Sr/86Sr ratios for Aral Sea sediments, Kyrgyzstan soils, and airborne PM10 collected at Bishkek and LIDAR sites during dust and non-dust events as presented in the Atmos. Environ. Paper (20). The 87Sr/86Sr ratios for the resuspended Aral Sea sediments are between the range 0.70951 - 0.71064, whereas the local soils ratio are between the range 0.71448 - 0.71739. The PM10 ratios are in the range of 0.70946 - 0.71335. The 87Sr/86Sr ratios of airborne PM10 mainly fall between these potential sources. To determine the dust sources in PM10, 206Pb/204Pb is plotted against 87Sr/86Sr in Figure 4 (30). In the 87Sr/86Sr domain, a mixture consisting of only two sources (e.g., Aral Sea sediments and Kyrgyzstan soils) will lie on straight line. If not, additional sources are likely present. The data can also be plotted using Sr isotopic ratio against 1/Sr concentration (µg g–1) (20). The Aral Sea sediments are encircled and Kyrgyzstan soils are inside the square. The average 87Sr/86Sr ratio for the Aral Sea sediments and surface soils in Kyrgyzstan were 0.70992 and 0.71579, respectively, allowing differentiation between these two source types. The average Sr ratio for soils at Bishkek and LIDAR were similar (p > 0.05) and statistically different than the soils collected near the sites (p < 0.05) (20). The average 87Sr/86Sr ratio for the atmospheric PM10 at Bishkek site was 0.71047 (range, 0.70946 - 0.71156) and 0.71104 (range, 0.71027 - 0.71218) during dust and non-dust events, respectively. Similarly, the average 87Sr/86Sr ratio at LIDAR site was 0.71240 (range, 0.71088 - 0.71335) and 0.71179 (range, 0.71100 - 0.71285) during dust events and non-dust events, respectively. Ratios for the dust events and non-dust events are not statistically different at either site (t-test, p > 0.05). Although the dust and non-dust ratios are not significantly different, Sr ratios observed in PM10 (particularly at Bishkek), are more similar to 90 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


the Aral Sea sediments or perhaps other deserts in the region relative to the local soils suggesting that local soils may not be an important source of PM10 at these locations. Based on the back trajectory analysis using HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory), the Aral Sea may have had an impact on the dust events at the Bishkek and LIDAR sites. The impacted samples are indicated by arrows in Figure 4. The model also showed the impact of Algeria, Gobi Desert, Iran, Libya, and Mediterranean Sea on the dust events at the two sites. There is no statistical difference in the 87Sr/86Sr ratios between dust events that passed over Aral Sea and the ones that took different trajectory suggesting that the Aral Sea sediments, at best, have only a minor influence in Kyrgyzstan and dust from other regions as well as regional anthropogenic sources that may be impacting PM10 concentrations in Kyrgyzstan.

Figure 4. 206Pb/204Pb vs. 87Sr/86Sr for Aral Sea sediments (in circle), local soils (in square), and PM10. The arrows suggest the impact of Aral Sea on dust events at Bishkek and LIDAR sites based on HYSPLIT modelling.

Impact of Other Source Regions Since the results above suggest that the Aral Sea appears to have only a minimal effect (at best) on the air quality in Kyrgyzstan, we compared our isotopic results with those from other potential source regions. To evaluate this, 87Sr/86Sr is plotted against 87Rb/86Sr ratios for Aral Sea sediments, Kyrgyzstan soils, and airborne PM10, with those of sediments from Western China (31) and the Tien-Shan mountain range bordering Kyrgyzstan (Figure 5).20 The 87Sr/86Sr 91 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


ratios for PM10 are almost identical to the ratios observed for sediments from North Tien-Shan region, which is a large mountain range located in Central Asia (32). These results suggest that transport from neighboring regions to Kyrgyzstan is important source of dust. The Sr ratios for PM10 were also similar to some of the soils measured in Tarim Basin, a major source of dust in Western China (32). However, previous modelling studies in the region suggest that trajectories may have also originated due to long-range transport from Africa, or Middle East regions (23, 33, 34). Sr isotopic ratio from these regions would be needed, however, to evaluate the impact of long-range transport as a significant contributor of dust to Kyrgyzstan.

Figure 5. Comparison of isotopic composition of the Aral Sea sediments, local soils, and PM10 with those of soils from Western and Central China.

Pb Isotope Ratios Stable Pb isotope ratios (208Pb/204Pb, 206Pb/204Pb, 206Pb/207Pb, and for resuspended Aral Sea sediments and local soils, and airborne PM10 are presented elsewhere (20). Ratios of 208Pb/206Pb versus 206Pb/207Pb are presented in Figure 6 for the Aral Sea sediments (in circle), Kyrgyzstan soils (in dotted circle) and PM10 (in rectangle). The average 208Pb/206Pb ratios of the Aral Sea sediments and Kyrgyzstan soils are 2.095 (range, 2.090 - 2.098) and 2.069 (range, 1.994 - 2.071), respectively. These ratios also suggest that both sediments and soils have a natural Pb source (35), which is also in agreement with the low UCC EF (range, 1.54 – 3.09) and high K/Pb ratio (range, 775 – 863). The average 208Pb/206Pb ratio for PM10 at the Bishkek site was 2.107 (range, 2.101 - 2.116) and 2.104 (range, 2.102 - 2.107) during dust events and non-dust events, respectively. Similarly, the average 208Pb/206Pb ratio at the LIDAR site was 2.099 (range, 2.091 - 2.108) and 2.108 (range, 2.102 - 2.112) during dust events 208Pb/206Pb)



and non-dust events, respectively. The average ratio for PM10 is 2.104 (range, 2.091 - 2.112), which is higher than both soils and the Aral Sea sediments. It is important to note that dust and non-dust events are not statistically different, also suggesting that airborne PM10 most likely impacted by anthropogenic Pb sources, which is consistent with high UCC EF (range, 37 - 74) and low K/Pb ratios (range, 46 – 112). Further, Figure 6 indicates that PM10 is not, as originally hypothesized, mostly a mixture of local soils and Aral Sea sediments, which is in agreement with the Sr results, all suggesting that one or more other Pb sources impact PM10 concentrations in the area. The data are plotted using another isotopic ratio in the Atmos. Environ. paper showing similar conclusions and make the conclusions more robust (20).

Figure 6. 208Pb/206Pb and 206Pb/207Pb in resuspended Kyrgyzstan soils, resuspended Aral Sea sediments, and PM10 during dust events and non-dust events at the two sampling sites.

Comparing Pb Isotopic Ratios with Other Regional Measurements Figure 6 indicates that an unknown source of Pb is impacting ambient particle concentrations in the area. Therefore, similar to Sr system, we compare the Pb isotope results to results from other regional studies so that possible sources can be identified. Figure 7 shows 208Pb/204Pb plotted against 206Pb/204Pb ratios for the Aral Sea sediments, Kyrgyzstan soils and airborne PM10, with those of total suspended particles (TSP) and soils from Western China. The PM10 Pb ratios are similar to the Pb ratios observed in TSP (particles with aerodynamic diameter < 93 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


100 µm) collected on Teflon filters over the intervals of 1-7 days from May 2001 to September 2001 using a high volume bulk aerosol sampler operated at a flow rate of 16.7 L min–1 in Kosan and Dunhuang in South Korea (36) and in Western China (31). This implies a similar Pb source(s) between the sampling sites in this study, Dunhuang and Kosan, but the source has not been identified as yet. Back trajectory analysis of pollution to Dunhuang and Kosan is from Central Asia, suggesting that the Pb in PM10, is in fact, anthropogenic and likely includes local and regional sources and sources transported outside the region (Taklamakan Desert from Western China, Europe, North Africa, parts of Russia) (20). As noted above, this is further suggested by the low K/Pb ratio and high enrichment factors observed in this study. Results in Figure 7 also indicate that the Aral Sea sediments have a similar Pb source relative to other Asian soils, including those near the sites, N. Pacific Dust and Asian Dust (31, 36). Since composition data from most other deserts in Central Asia and the surrounding regions are not available, these results also suggest that Kyrgyzstan is likely impacted by dust from these other deserts supporting above results that the Aral Sea only has a minor impact at the sampling sites (20).

Figure 7. Comparison of isotopic composition of the Aral Sea sediments, local soils, and PM10 with those of soils from Western China.

Conclusions The initial hypothesis of this study was that newly exposed Aral Sea sediments might be a new and important source of dust in Kyrgyzstan. Based on elemental concentrations, the chemical composition of LIDAR and Bishkek soils were similar. The airborne PM10 collected at two sampling sites was also similar. The Aral Sea sediment’s elemental composition was different from both soils and PM10 for majority of the elements. Also, the composition of soils were different from PM10. This further supports that local soils had no impact and the 94 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Aral Sea had minimal impact on PM10 in Kyrgyzstan as shown in Table 1-2. Sr and Pb isotope analysis indicated that the impact of the Aral Sea is small and cannot be quantified based on the approach used in this study; although, other approaches arrived at the same conclusion (19, 23, 33, 34). Although previous studies do suggest long-range transport to be important (19, 34) our Sr isotope ratios suggest that dust from the more local Tien-Shan mountain range may also be significant. The Pb isotope ratios suggest that both Aral Sea sediments and Kyrgyzstan soils have natural source of Pb whereas airborne PM10 is dominated by unidentified combustion and/or anthropogenic sources. The similarities from these data compared to those collected in Dunhuang and Kosan hint at a similar source. We hypothesize that this source may be transport from industrial Europe, parts of Russia, or the Middle East (34). Both Sr and Pb isotope systems are consistent indicating little if any contribution from Aral Sea sediments and/or local soils to PM10 levels during dust events at Kyrgyzstan. Based on this and related work, Central Asia is impacted by yet unidentified regional and distance dust and anthropogenic sources that requires additional measurements of the comparison of desert sands in and around Central Asia as well as a network of PM10 and PM2.5 chemical speciation monitoring to understand both the sources to ambient PM and impact of transport of PM from this region.

Acknowledgments The authors thank Dr. Gwyneth Gordon and Dr. Rasmus Andreason for their help and guidance in Sr ratio measurements. The authors also gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for use of the HYSPLIT transport and dispersion model (www.ready.noaa.gov). We acknowledge the Association of Public Health Laboratories (APHL) for funding through their Environmental Health Fellows program. The US Environmental Protection Agency, through its Office of Research and Development, funded this study and collaborated in the research described here under Contract EP-D-06-001 to the University of Wisconsin-Madison as a component of the International Science & Technology Center (ISTC) project # 3715 (Transcontinental Transport of Air Pollution from Central Asia to the US). The isotope portion of this study was funded through a PROF Grant at the University of Denver.

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