Composition and sources of polycyclic aromatic hydrocarbons in

Science of the Total Environment 574 (2017) 991–999

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Composition and sources of polycyclic aromatic hydrocarbons in cryoconites of the Tibetan Plateau glaciers Quanlian Li a,⁎, Shichang Kang a,b, Ninglian Wang c, Yang Li d, Xiaofei Li a, Zhiwen Dong a, Pengfei Chen a a

State Key Laboratory of Cryospheric Science, Northwest institute of Eco-Environment and Resources, CAS, Lanzhou 730000, China CAS, Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100085, China c School of Urban and Environmental Sciences, Northwestern University, Xi'an, 710127, China d Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China b

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• PAHs in cryoconite from seven glaciers of the Tibetan Plateau were presented. • The highest ΣPAHs contents were found in the southeastern Tibetan Plateau. • Total organic carbon and grain size were minor factors affecting PAHs values. • Low toxic equivalent quantity of PAHs in most of glaciers suggested a limited biological risk.

a r t i c l e

i n f o

Article history: Received 22 July 2016 Received in revised form 19 September 2016 Accepted 19 September 2016 Available online xxxx Editor: Jay Gan Keywords: Tibetan Plateau Glacier Cryoconite Polycyclic aromatic hydrocarbon

a b s t r a c t Dark-colored cryoconite can absorb substantial solar radiation, reduce the surface albedo of glaciers, and thus greatly accelerate glacier melting. Organic matters in cryoconites such as polycyclic aromatic hydrocarbons (PAHs) are kind of the light absorbing compositions. In this study, 15 PAHs containing 3–7 rings were identified in 61 cryoconites samples collected from seven glaciers over the Tibetan Plateau (TP). The average concentration of total PAHs in cryoconites samples was in the range of 6.67–3906.66 ng g−1 dry weight. The highest average total PAH concentration was found in the southeastern TP, followed by the northern TP. The central TP contained the lowest amount of PAHs. Moreover, correlation analysis showed that total organic carbon (TOC) and grain size were only a minor factor for the accumulation of PAHs in cryoconites of the TP. Factor analysis and diagnostic ratios indicated that the PAHs were produced mainly from the incomplete combustion of coal, fossil fuels and biomasses. The exhaust gas of locomotives also contributed to the accumulation of PAHs in the glaciers. The PAHs in these seven glaciers showed low toxic equivalent quantity (TEQ), and thus had low biological risk. Nevertheless, the pollution of PAHs in the southeastern TP needs to be addressed. © 2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

⁎ Corresponding author at: Donggang West Road No. 320, Lanzhou 730000, Gansu, China. E-mail address: [email protected] (Q. Li).

http://dx.doi.org/10.1016/j.scitotenv.2016.09.159 0048-9697/© 2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Q. Li et al. / Science of the Total Environment 574 (2017) 991–999

1. Introduction Cryoconite is a dark-colored spherical particulate transported onto a glacier surfaces via aeolian processes, landslides from valley walls, and supraglacial and englacial entrainment (Macdonell and Fitzsimons, 2008; Mcintyre, 1984). It is a mixture originating from the local, regional and remote sources through both natural and anthropogenic pathways (Tieber et al., 2009). Global warming has led to the retreat and thinning of many glaciers throughout the Tibetan Plateau (TP) over the past decades (Yao et al., 2012; Kang et al., 2015). Dark-colored cryoconite on the surfaces of glaciers can absorb substantial solar radiation, reduce the surface albedo of glaciers, and thus significantly accelerate glacier melting (Fujita, 2002; Takeuchi et al., 2005). Cryoconite usually resides in cryoconite holes and in the entire ablation areas of glaciers in high alpine environments. Previous studies have suggested that the dark coloration of humic substances in cryoconite is attributed to their highly conjugated molecular bonds including benzene rings and polyethylene chains, which can effectively absorb visible light over a wide range of wavelengths (Takeuchi, 2002). Polycyclic aromatic hydrocarbons (PAHs) are a class of organic contaminants composed of two or more fused benzene rings within a conjugated system. They are some of the most disturbing contaminants on both regional and global scales because of their persistence, transportation over long ranges, and potential carcinogensis and mutagenesis (Guo et al., 2010; Lang et al., 2008). PAHs are also considered as a typical indicator of human activity (Vehviläinen et al., 2001). The levels of PAHs are strongly correlated to regional energy consumption and degrees of industrialization and urbanization (Metre et al., 2000). The TP is sensitive to environmental changes and thus is an excellent area for studying the distribution of organic contaminants in remote regions. The glaciers on the TP currently serve as a “sink” of pollutants and can release accumulated organic contaminants as the temperature increase. This discharge of pollutants may become a threat to both human health and the ecosystem in the future. Cryoconite organic matter may also play an important role in accelerating the melting of glaciers because it can absorb visible light (Takeuchi, 2002; Xu et al., 2010). Numerous studies on the organic matters in cryoconite holes have been published in recent years (Xu et al., 2010; Pautler et al., 2013). However, the PAHs in the cryoconites on the TP glaciers have been rarely reported up to now. In the present study, cryoconite samples were collected from seven glaciers, from the south to the north of the TP. The contents and spatial distribution of PAHs in cryoconites of the TP glaciers were investigated and the possible sources of these organic contaminants were determined. This work can provide a basic information for studying the organic matters in cryoconite and further for assessing the effects of PAHs on alpine environments. 2. Materials and methods 2.1. Study area Yulong (YL) Snow Mountain(27°40′N, 100°10′E) is located in the southeastern TP as a part of the southern Hengduan Mountains (Fig.1). It is characterized by maximum annual precipitation of 3100 mm and relatively high temperatures, with average annual and summer temperature of − 6 °C and 1–5 °C at the equilibrium line (Zhang et al., 2012). Dongkemadi (DKMD) glacier (33°05′N, 92°04′E) is located on the northern slope of Tanggula Pass of the central TP (Fig. 1.). It is 2.8 km long and its equilibrium line altitude (ELA) lies at 5620 m a.s.l. (Xiao et al., 2002). Yuzhufeng (YZF) glacier (35°39′N, 94°14′E) is located at the summit of the eastern Kunlun Mountains in the northern TP (Fig. 1), and has a maximum highest elevation of 6178 m a.s.l. and a snowline at 5100–5300 m a.s.l. (Wu et al., 2001). Muztagh (MZTG) glacier(36°40′ N, 87°30′E) is situated at the ridge of the central Kunlun Mountains with

an area of 71.70 km2. It belongs to the watershed between the Milan and Che'erchen rivers at an elevation of 6973 m a.s.l. (Fig. 1). Its inventory number is 5Y624E34 (Xing et al., 2016). Qiyi (QY) glacier (39°14′N, 97°46′E) and Laohugou (LHG) No.12 glacier(39°26′N, 96°33′E) are on the western Qilian Mountains with the Badain Jaran and Tengger Deserts in its north and the Qaidam basin in its south (Fig. 1). These two glaciers are surrounded by many large deserts and wastelands and have a typical continental climate (Dong et al., 2013). Tianshan (TS) is an active intracontinental mountain belt that extends over 2500 km in the east-west direction across central Asia. The eastern branch of the Urumqi glacier No. 1 (43°06′N, 86°48′E) riverhead is located in the eastern TS and is surrounded by the Taklimakan Desert to the south, the Gurbantungut Desert to the north, and the Gobi Desert to the east (Fig. 1). The glacier's ELA was approximately 4055 m a.s.l. in 1959–2003 (Li et al., 2001). The TP covers climatic regions from the monsoon region to the areas affected by westerly depressions, and the climate changes from warm and wet to cold and dry. Consequently, the vegetation coverage in the region gradually decreases from the south to the north over the plateau. 2.2. Sample collection Cryoconite samples were collected from seven glaciers of the TP in July and August 2014 (Fig. 1), which was during the summer ablation season. Specifically, samples were collected with a stainless steel scoop from the superimposed ice surface or the bottom of cryoconite holes in the ablation area of glaciers. The numbers of collected samples were 9, 5, 5, 3, 14, 11, and 14 from the Baishui No. 1 glacier on YL Snow Mountain, DKMD glacier, YZF glacier, MZTG glacier, LHG glacier, QY glaciers, and TS Urumqi No. 1 glacier, respectively. All samples were placed into previously cleaned glass bottles that were then immediately stored in Whirl-Pak® bags and frozen in a refrigerator. The samples were transported to the laboratory and were stored at −20 °C until further analysis. 2.3. Chemicals and reagents Hexamethylbenzene, perdeuterated PAHs surrogate standards (including phenanthrene-d10, chrysene-d12, and perylene-d12), and 15 PAHs containing 3–7 aromatic rings were purchased from Sigma-Aldrich. The 15 PAHs were includes: phenanthrene (Phe); anthracene (Ant); fluoranthene (Flu); pyrene (Pyr); retene (Ret); benzo[a]anthracene (BaA); chrysene (Chry); benzo[b]fluoranthene (BbF); benzo[k]fluoranthene (BkF); benzo[a]pyrene (BaP); perylene (Pery); indeno[1,2,3-cd]pyrene (InP); dibenzo[a,h]anthracene (DbA); benzo[g,h,i]perylene (BgP); and coronene (Cor). HPLC grade Hexane (HEX), dichloromethane (DCM) and methanol (MeOH) were used for all sample preparation and analyses. Silica gel (80– 100 mesh) and alumina (120–200 mesh) were activated in an oven at 150 °C and 180 °C, respectively, for 12 h. Deionized water was obtained from a Milli-Q system. 2.4. Extraction and analysis of PAHs The cryoconite samples were freeze-dried at − 70 °C and were ground into powder. About 10–20 g of the powder was extracted with 9:1(v/v) DCM: MeOH for 72 h in a Soxhlet apparatus and desulfurized with activated copper. The extract was concentrated up to 2–3 mL and was passed through a chromatographic column packed with pre-activated alumina and silica gel (3:1 w/w) for cleanup and fractionation. The top of the column was covered with 0.5 g anhydrous sodium sulfate to remove the residual water. The column was eluted with 45 mL DCM, the eluent was concentrated and solvent-exchanged into 10 mL n-hexane which further reduced to approximately 1–2 mL by rotary evaporation at 40 °C and was then evaporated to 500 μL in hexane under a


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Fig. 1. Location of cryoconite sampling sites on the Tibetan Plateau Abbreviated form: Yulong (YL); Dongkemadi (DKMD); Yuzhufeng (YZF); Muztagh (MZTG); Laohugou (LHG); Qiyi (QY); Tianshan (TS).

gentle stream of nitrogen. Prior to the analysis, 100 ng of hexamethylbenzene was added as an internal standard for Gas Chromatography–Mass Spectrometry (GC–MS) analysis. All PAHs were determined by using an ITQ 1100 GC–MS system (Thermo Electron Corporation, USA) equipped with a DB-5MS capillary column 30 m × 0.25 mm in inner diameter and 0.25 μm film thickness with high purity helium as the carrier gas, a transfer line temperature of 280 °C, and an ion source temperature of 250 °C. The following temperature gradient was adopted: hold at 50 °C for 2 min, increase to 300 °C at 4 °C/min, and final hold at 300 °C for 30 min. The MS was operated in the electron impact (EI) mode at 70 eV at a mass scanning range of m/z = 50–450.

of the instrument during the entire analytical process. The PAH surrogate standards (phenanthrene-d10, chrysene-d12 and perylene-d12) were added to the sample before the extraction to monitor the procedures of sample extraction, cleanup, and analysis. For all cryoconites samples, the recoveries efficiency of phenanthrene-d10, chrysene-d12 and perylene-d12 were 67–92%, 68–101%, and 72.5–88%, respectively, with relative standard deviations ranging from 6% to 22%.

2.5. TOC and particle size analysis

The cryoconite grain sizes from the seven glaciers were classified into three fractions including clay (b 2 μm), silt (2–63 μm), and sand (N63 μm; Table 1). It has been reported that smaller particles usually contain a higher content of organic carbon (Froehner et al., 2009; Guo et al., 2009). The organic carbons in larger particles is composed mainly of charcoal and biomass, that in smaller particles is mainly black carbons and humics substances. Silt is the predominant fraction in all cryoconites of the TP. The cryoconite in the YL Snow Mountain contains the highest amount of clay with a content of 16.29% and an average diameter of 22.57 μm, its TOC% is 6.73%. The high TOC content and fine particles indicate that the cryoconites in the YL Snow Mountain might originate from vegetation emission and human activities. Sand is predominant fraction with an average content of 32.07% in the cryoconite on the central TP glacier, its average TOC% is 0.52%. Large masses and coarse particles are not readily transported over long distance. The large cryoconite grains and scarce vegetation in this region indicate that the cryoconites in the central TP resulted from the local circulation and were affected by the arid area. Silt is the highest fraction with an average content of 83.75% on the northern TP glaciers. The average TOC%

The freeze-dried and ground cryoconites samples were treated with 10% (v/v) aqueous HCl solution for 24 h to remove inorganic carbonates. The carbonate-free samples were rinsed with distilled water three times to remove acid residues and dried at 60 °C for 48 h. The measurements of total organic compound (TOC) were conducted on a PerkinElmer CHN 2400 elemental analyzer by a combustion methods (Sojinu et al., 2010). The sample was further treated stepwise with 30% H2O2 and 0.5 M NaOH to remove organic matter and biogenic silica, respectively, to yield a powder for the cryoconite grain size analysis on the laser particle size analyzer (Mastersizer 2000). The particle size analysis of each sample was conducted in triplicate with relative standard derivations b 3%. 2.6. Quality control and quality assurance The procedural blanks and spiked blanks were analyzed with every 10 samples to test for interference, cross-contamination, and stability

3. Results and discussion 3.1. Physicochemical characteristics of cryoconites

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Table 1 Physicochemical characteristics of the cryoconites in the TP glaciers. Samples

YL DKMD YZF MZTG Central TP QY LHG TS Northern TP

Grain composition (%)

Grain size parameters (μm)

Sand

Silt

Clay

Mean ± SD

7.25 30.02 27.56 39.67 32.07 8.75 9.96 4.55 7.75

76.46 62.04 64.82 52.73 59.86 82.73 82.99 85.53 83.75

16.29 8.07 7.62 7.60 7.77 8.52 7.48 9.92 8.64

22.57 ± 4.65 55.12 ± 14.94 54.61 ± 8.21 128.43 ± 36.18 78.39 ± 40.17 27.49 ± 8.69 26.90 ± 6.31 20.36 ± 6.83 34.53 ± 7.98

TOC (%)

6.73 1.06 0.37 0.12 0.52 0.88 0.63 1.44 0.98

(All abbreviations are as same as Fig. 1.)

in this area is 0.98%, indicating that the cryoconites might be affected by the surrounding desert and human activities. 3.2. Concentrations and spatial distribution of PAHs in cryoconites Table 2 lists the average concentrations (mean ± SD) of the total 15 PAHs (ΣPAHs) in the cryoconites collected from the seven glaciers in the order of YL (1850.17 ± 1254.40 ng g−1) N TS (236.43 ± 186.88 ng g− 1) N LHG (121.66 ± 47.27 ng g− 1) N QY (105.87 ± 46.12 ng g− 1) N DKMD (74.11 ± 54.90 ng g− 1) N YZF (47.27 ± 32.58 ng g− 1) N MZTG (8.38 ± 2.02 ng g−1). The total 15 PAHs concentration in the cryoconite collected from the YL Snow Mountain was about 220.78 times higher than that from MZTG glacier. The total 15 PAHs concentrations in the northern TP glaciers, including TS, LHG, and QY glacier were 13, 15, and 30 times higher than that in MTZG glacier, respectively. The United States Environmental Protection Agency (EPA) has listed 16 priority PAHs, among which naphthalene, acenaphthene, acenaphthylene, and fluorene are relatively volatile. Therefore, their contents in the glaciers were not analyzed in the present work due to their low and irreproducible recoveries. The total concentrations of the other 12 EPA priority PAHs, including Phe, Ant, Flu, Pyr, BaA, Chry, BbF, BkF, BaP, InP, DbA and BgP in the seven glaciers, are listed in the EPA column of Table 2. It is clear that the total concentrations of the EPA's priority PAHs have similar distribution in the seven glaciers in the order of YL (1745.53 ± 1160.58 ng g− 1) N TS (166.47 ± 115.95 ng g−1) N LHG (100.81 ± 40.75 ng g− 1) N QY (94.75 ± 41.33 ng g− 1) N DKMD (70.11 ± 52.58 ng g− 1) N YZF (43.23 ± 30.31 ng g− 1) N MZTG (7.93 ± 1.95 ng g−1). The highest ΣPAHs and average concentrations of the EPA's priority PAHs were found in the southern TP, followed by

the northern TP, and the lowest concentrations were in the central TP. Previous studies have shown that the lowest average concentrations of EPA's priority PAHs in the surface soils of the Changwengluozha glacier and the central TP are 6.04 ng g−1 (Yuan et al., 2014) and 9.21 ng g− 1 (Yuan et al., 2015), respectively. The average concentration of PAHs in top soil across the TP from the south to the north is in the range of 51.8–59.9 ng g−1 (Tao et al., 2011; Wang et al., 2013; Wang et al., 2014) and that in the Lalu wetland of Lhasa city is 82.5 ng g−1 (Liu et al., 2003). These PAHs concentrations in the surface soil of the TP were mostly close between the range of that in MZTG glaciers and YL Snow Mountain. The average content of PAHs in the surface soil of the Indian Himalayan region was 458 ng g−1 (Ningombam et al., 2016), which is higher than the average PAHs in the cryoconites of the central and northern TP and lower than that of YL Snow Mountain. The YL Snow Mountain is a world-famous scenery owing to the modern temperate glaciers and other features. Thousands of tourists visit the area daily in many types of shuttle vehicles (Zhang et al., 2012), which has increased fossil fuel usage, biomass burning, and other anthropogenic activities, hence leading to the highest concentration of PAHs in the YL Snow Mountain. The DKMD, YZF and MZTG glacier in the central TP are relatively pristine region. They are surrounded by the Taklamakan Desert and Qaidam Basin, and thus have less vegetation coverage and traffic volumes and limited industrial activities. The DKMD glacier lies on the northern slope of Tanggula Pass and has a cold steppe climate. The YZF glacier is located at the eastern Kunlun Mountains with low forest coverage owing to its high altitude and cold and arid climate. The plant species significantly decline from the Tanggula Mountains to the eastern Kunlun Mountains. Moreover, the ecological environment worsens along the same direction. Therefore, the concentration of PAHs in the DKMD glacier is higher than that of the YZF glacier. The MZTG glacier is the largest valley glacier in the Qiangtang Plateau and has a cold and dry climate with scarce precipitation owing to constant westerly airflow and the lowest temperature of −30 °C. The vegetation in this region is composed of scattered cushion plants. Therefore, the MZTG glacier has the lowest concentration of PAHs. The cryoconites samples from the Qilian Mountains, LHG and QY glaciers, in the northern TP have similar grain sizes, TOC, and PAH concentrations, which indicates that they may share the same primary sources. The QY glacier is very close to the Jingtieshan Coal Mine, the Jiayuguan chemical plant, and many other heavy industry facilities. The emissions of vehicle exhaust around the glacier are also considerable. All of these factors likely contribute to the accumulation of PAHs on the QY glacier. The TS glacier No. 1 is known worldwide as the closest glacier to a metropolis. Abundant steel plants and cement plants are located in Houxia township along the river valley 50 km from the glacier (Li et

Table 2 Average concentration of PAHs in cryoconites on the Tibetan Plateau. Glaciers

YL DKMD YZF MZTG QY LHG TS Lalu Wetland Qomolangma area TPa surface soil (1) TPa surface soil (2) TPa surface soil (3) CWLZG Central TPa IHR a

TP: Tibetan Plateau.

Concentration (ng g−1)

Reference

Low

High

EPA

ΣPAHs

Ret

Pery

867.51 26.33 11.45 6.67 27.96 60.87 34.17

3906.66 167.44 81.38 9.85 161.09 196.59 395.20

1745.53 ± 1160.58 70.11 ± 52.58 43.23 ± 30.31 7.93 ± 1.95 94.75 ± 41.33 100.81 ± 40.75 166.47 ± 115.95 82.5

1850.17 ± 1254.40 74.11 ± 54.90 47.27 ± 32.58 8.38 ± 2.02 105.88 ± 49.09 121.66 ± 47.27 236.43 ± 186.88

38.08 ± 37.92 2.66 ± 1.56 3.13 ± 1.38 0.41 ± 0.06 9.61 ± 5.52 17.27 ± 4.84 68.03 ± 69.28

16.56 ± 14.10 0.04 ± 0.03 0.04 ± 0.01 0.04 ± 0.01 0.62 ± 0.58 0.47 ± 0.18 0.48 ± 0.39

168 19

595 66

5.54 1.45 0.43 15.3

389 12.7 26.66 4762

51.8 56.25 59.9 6.04 9.21 458

This study This study This study This study This study This study This study Liu et al. (2003) Wang et al. (2007) Tao et al. (2011) Wang et al. (2013) Wang et al. (2014) Yuan et al. (2014) Yuan et al. (2015) Ningombam et al. (2016)


al., 2001). Local circulation systems can carry air pollutants to the glacier from industrial sources, such as Urumqi city, from the edge of the basin, which may account for the high concentration of PAHs in this glacier after that of YL Snow Mountain. 3.3. Composition of PAHs The composition of PAHs can provide useful and important information for the determination of their emission sources. On the basis of these structures, the 15 PAHs found in the seven glaciers can be divided into five groups including tricyclic, tetracyclic, pentacyclic, hexacyclic, and heptacyclic. Among all 15 PAHs, Phe, Flu and BbF were predominant species in cryoconites samples of the seven glaciers. The composition profiles of PAHs by ring number are depicted in Fig. 2. The compositions of PAHs at the YL Snow Mountain are dominated by five-seven-ring PAHs (N50%), followed by four-ring PAHs (25.48%). The lower molecular weight three-ring PAHs (LMW PAHs) account for only 21.62% of the total PAHs. The higher molecular weight four-seven ring (HMW) PAHs appear only at particulate phases and tend to be deposited and remain near the source region (Han et al., 2015; Agarwal, 2009). The compositions of PAHs at the central TP are dominated by four-ring PAHs (47.26%), followed by five-seven-ring PAHs (31.71%), and the threering PAHs account for only 21.03% of the total PAHs. This indicates that some PAHs originated from long-distance atmospheric transport, and some are from local sources. In the northern TP, the PAHs are dominated by three-ring PAHs (40.38%), followed by four-ring PAHs (31.97%). The five-seven ring PAHs contributed only 27.66% to the total PAHs. The higher proportion of three-ring PAHs in the cryoconites of the northern TP indicates that their origin is possibly long-range atmospheric transport. In addition to the anthropogenic PAHs, other PAHs such as perylene and retene are partially of biological origin (Bandowe et al., 2014). As can be seen from Table 2, perylene has five rings and is produced mainly by the diagenesis of biogenic precursors and combustion of fossil fuels (Silliman et al., 1998). The average concentrations of perylene in the seven glaciers are in the order of YL (16.56 ± 14.10 ng g− 1) N QY (0.62 ± 0.58 ng g− 1) N TS (0.48 ± 0.39 ng g− 1) N LHG (0.47 ± 0.18 ng g−1) N DKMD (0.04 ± 0.03 ng g− 1) = YZF (0.04 ± 0.01 ng g−1) = MZTG (0.04 ± 0.01 ng g−1). The mass ratio of perylene to the total penta-aromatic isomers including benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(e)pyrene, benzo(a)pyrene and perylene can be used to determine the possible origin of perylene. It has been reported that a ratio higher than 10% indicates diagenetic source and a ratio b 10% indicates pyrolytic source (Fang et al., 2007). The mass ratios of perylene to the total penta-aromatic isomers in the cryoconite samples from the seven glaciers are 0.37–2.29%, which suggests that the perylene were mainly derived from pyrolysis. Retene is produced by the decarboxylation and aromatization of abietic acid (Otto et al., 2006), a natural product found mainly in coniferous trees, during coniferous wood combustion or by slower, low-temperature degradation (Sun et al., 2006). The retene concentrations in the

Fig. 2. Percentage of different rings PAHs in cryoconites from the Tibetan Plateau glacier (n = 61).

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seven glaciers are in the order of TS (68.03 ± 60.28 ng g−1) N YL (38.08 ± 37.92 ng g− 1) N LHG (17.27 ± 4.84 ng g− 1) N QY (9.61 ± 5.52 ng g− 1) N YZF (3.13 ± 1.38 ng g− 1) N DKMD (2.66 ± 1.56 ng g−1) N MZTG (0.41 ± 0.06 ng g−1), which is different from the spatial distribution of total PAHs. This can be explained by the different vegetation coverage on these glaciers. The YL Snow Mountain has the highest forest coverage, which may contribute to the relatively high retene concentration at this location. However, the highest retene concentration at the TS glacier No.1 may have been caused by natural forest fires or by anthropogenic combustion of coniferous woods and the low-temperature transformation of organic precursor compounds from woods (Ramdahl, 1983). The TS glacier No. 1 is surrounded by the Gobi and other deserts and has very low vegetation coverage. Its retene mainly originated from biomass burning of Urumqi city and Houxia township. 3.4. Relationship between physicochemical characteristics of cryoconites and PAH distribution The processes controlling the level of PAHs in the cryoconites are complex. The distributions of PAHs in the cryoconites might be affected by the contents of TOC% and grain size. In this study, regression analysis was used to investigate the relationship between the concentration of ΣPAHs, TOC% and grain size. As shown in Fig. 3, the concentrations of ΣPAHs were not significantly correlated with TOC (r2 b 0.32, p = 0.05) or grain size (r2 b 0.10, p = 0.05), which suggests that the distribution and concentration of PAHs in cryoconites resulted more from direct input rather than generation from the cryoconites itself. The TOC and grain size might be of comparatively minor importance in controlling PAH concentrations of cryoconites. A poor correlation between PAHs and TOC has also been reported in previous studies (Jiang et al., 2009). 3.5. Principal component analysis (PCA) In the present study, principal component factor analysis was performed to investigate emission/transport patterns that affect the PAHs levels of cryoconites on the TP. Results obtained by varimax rotated factor analysis are given in Table 3. The factor analysis for the YL Snow Mountain reveals two factors that have an eigen value N1. Factor 1 contributes 90.96% of the total variance and contains Phe, Ant, Flu, Pyr, Ret, BaA, Chry, BbF, BkF, BaP, Pery, DbA, InP, BgP and Cor. Among these compounds, Phe, Ant, Flu, and Pyr are mainly emitted from coal combustion (Simcik et al., 1999; Harrison et al., 1996; Mastral et al., 1996). BaA and Chry often result from the combustion of both diesel and natural gas (Rogge et al., 1993b; Khalili et al., 1995). Five-ring PAH compounds (BbF, BkF and BaP) are prevalent in the particulate phases and are mainly producted by diesel engine vehicles (Park et al., 2002). InP and BgP are emitted from diesel and gasoline vehicles (Simcik et al., 1999; Harrison et al., 1996). DbA and Cor are the indicators of petroleum combustion exhausts and traffic emission (Harrison et al., 1996). Retene is a typical compound emitted from brushwood and vegetable burning (Ramdahl, 1983). Pery is thought to be from combustion processes (including fossil fuels and biomass) (Reddy et al., 2002) and diagenesis of organic matter. Therefore, factor 1 was identified as mixed combustion sources (fossil fuels and biomass combustion sources). Factor 2 contributes 7.19% of the total variance, and contains DbA and Cor. Therefore, factor 2 included the exhausts of petroleum combustion exhausts and traffic emission. In central TP, three factors are identified and they account for 94.83% of the total variance. Factor 1 accounts for 70.26% and contains Phe, Ant, Flu, Pyr, Ret, BaA, Chry, BbF, BkF, BaP, DbA, InP, BgP and Cor. Therefore, factor 1 also represented mixed combustion sources (fossil fuels and biomass combustion sources). Factor 2 accounts for about 16.04% of the total variances and consists mainly of pery. Therefore, factor 2 shows the character of pyrogenic source. Factor 3 accounts for about 8.53% of the total variances and consists mainly of Pyr and DbA. Therefore, factor 3 indicates the combustion of fossil fuels sources.

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Fig. 3. Correlations efficient between concentrations of ΣPAHs, TOC% and Diameter. (A) and (B) are Southeastern TP; (C) and (D) are Central TP; (E) and (F) are Northern TP.

In northern TP, three factors are found and they account for 86.63% of the total variance. Factor 1 accounts for 59.66% of the total variances and consists of Phe, Ant, Flu, Pyr, Ret, BaA, Chry, BbF, BkF, BaP, InP and BgP. Thus, factor 1 represented the contribution of biomass and fossil

Table 3 Factor analysis of PAHs in cryoconites on the TP glacier.

Phe Ant Flu Pyr Ret BaA Chry BbF BkF BaP Pery InP DbA BgP Cor Eigen values % of Variance Cumulative (%) a

3.6. Diagnostic ratios of PAHs

Southeastern TPa

Central TPa

Factor 1

Factor 2

Factor 1

Factor 2

Factor 3

Factor 1

Factor 2

Factor 3

0.98 0.96 0.99 0.99 0.95 0.99 0.99 0.97 0.99 0.98 0.93 0.99 0.82 0.97 0.74 13.64 90.96 90.96

0.12 –0.28 –0.07 –0.08 0.27 –0.11 –0.14 –0.22 –0.03 –0.20 –0.33 –0.01 0.53 0.17 0.60 1.08 7.19 98.15

0.81 0.77 0.93 0.76 0.92 0.91 0.90 0.97 0.87 0.95 0.45 0.97 0.73 0.97 0.39 10.54 70.26 70.26

0.50 0.40 0.18 0.11 0.05 0.31 –0.29 –0.14 –0.47 –0.28 0.75 –0.19 0.18 –0.16 –0.89 2.41 16.04 86.30

0.14 –0.45 –0.25 –0.59 –0.28 0.15 0.29 0.13 0.07 0.10 0.29 0.02 0.59 –0.08 –0.03 1.28 8.53 94.83

0.84 0.77 0.86 0.86 0.91 0.92 0.87 0.84 0.92 0.83 0.45 0.69 0.40 0.60 0.55 8.95 59.66 59.66

–0.40 –0.48 –0.37 –0.38 –0.29 –0.20 0.08 0.13 0.01 0.20 0.51 0.29 0.74 0.64 0.60 2.53 16.87 76.52

0.26 0.30 0.27 0.25 –0.06 –0.19 –0.35 –0.48 –0.22 –0.47 –0.27 0.49 0.34 0.31 0.19 1.52 10.11 86.63

TP: Tibetan Plateau.

fuels combustion sources. Factor 2 accounts for about 16.87% of the total variances and consists mainly of Pery, DbA, BgP and Cor. Therefore, factor 2 was identified as mixed sources of petroleum combustion exhausts and traffic emission. Factor 3 accounts for about 10.11% of the total variances and consists mainly of InP. Therefore, factor 3 represented the contribution of diesel and gasoline vehicles emission.

Northern TPa

The accumulation of PAHs in cryoconites is due to anthropogenic and natural emissions, such as forest fires, biomass burning, petroleum products, and short-term degradation of biogenic precursors. Among the anthropogenic factors, petrogenic and pyrolytic sources are the most important. The pyrolytic sources include combustion processes such as engine exhaust, industrial activities, natural gas, domestic heating systems and incinerators (Perra et al., 2011). These pollutants cannot be properly controlled unless their sources are accurately identified. Identifying the pollution source would facilitate pollutant control and management. The diagnostic ratio of PAHs is an excellent qualitative tool for this purpose. In the present work, the plots of Flu/Pyr versus Phe/Ant and BaA/(BaA + Chry) versus InP/(InP + BgP) were obtained to distinguish the sources of PAHs in cryoconites of the seven glaciers in the TP. Because the TP is at a high altitude and receives intensive solar radiation, atmospheric degradation may also affect the PAH level (Liu et al., 2003). Because Ant is more susceptible than Phe to photodegradation (Butler and Crossley, 1981), the Phe/Ant ratio was used to measure atmospheric degradation (Ding et al., 2007; Ohura et al., 2004). The cross-plot analysis shows that most cryoconites of YZF and MZTG glaciers fell within the biomass and coal combustion area


and that the PAHs in the cryoconites of YL Snow Mountain, DKMD, QY, LHG and TS glaciers have a mixture of pyrogenic and petrogenic sources (Fig. 4A). The average Phe/Ant ratios of the YL Snow Mountain, TS, DKMD, LHG, and QY glaciers were 22.16, 20.54, 44.91, 33.79, and 46.86, respectively, which is 20 times higher than all of the sites. It has been reported that the average Phe/Ant in the surface soil of the Qomolangma region is 36 (Wang et al., 2007), suggesting that PAHs in both cryoconites and soil underwent photodegradation before arriving at the TP region and that some PAHs originated from long-range atmospheric transport from urbanized regions. Flu is more thermodynamically stable than Pyr, the ratio of Flu to Pyr(Flu/Pyr) can be used to differentiate the pyrogenic(Flu/Pyr N1) and petrogenic (Flu/Pyr b1) sources of PAHs (Baumard et al., 1998). The Flu/Pyr ratios of all cryoconite samples tested in the present work were N1, indicating that the coal and biomass combustions were the major sources of PAHs. The plot of BaA/(BaA + Chry) versus InP/(InP + BgP) can also be used to identify the sources of PAHs (Fig. 4B). Yunker et al. (2002) reported that petrogenic and petroleum sources caused values of BaA/ (BaA + Chry) b 0.35 and InP/(InP + BgP) b 0.2. The combustion of petroleum including liquid fossil fuels, vehicles, and crude oil had values of 0.2 b InP/(InP + BgP) b 0.5 and 0.2 b BaA/(BaA + Chry) b 0.35. The biomass and coal combustion values were InP/(InP + BgP) N 0.5 and BaA/ (BaA + Chry) N 0.35. The BaA/(BaA + Chry) and InP/(InP + BgP) ratios of most of the cryoconite samples in the present work were b0.35 and N0.5, respectively, indicating that the sources of the PAHs were mainly the combustions of coal, liquid fossil fuels, vehicles, and biomass. 3.7. Cryoconites toxicity assessment Because the toxicity evaluation standards for PAHs are lacking in China, the toxic equivalency factor (TEF), the most popular and widely accepted standard for the evaluation of the toxicity of PAHs in soil, was adopted in the present work. Nisbet and Lagoy (1992) used the

997

TEF of each PAH and their data to calculate the toxic equivalent quantity (TEQ) of cryoconites by using the following formula: N

TEQ ¼ ∑ ci TEFi i¼1

where TEQ is the toxic equivalent concentration, and ci and TEFi are the concentration and relative toxic equivalency factor of PAH, respectively. In the present work, the TEQ of cryoconites was calculated by using 12 PAHs to assess the eco-toxicological risk of the PAH contamination in the glaciers (Table 4). The TEQ values of YL, DKMD, YZF, MZTG, QY, LHG, and TS glaciers were 281.32 ng g− 1, 5.47 ng g− 1, 3.56 ng g−1, 1.34 ng g−1, 14.34 ng g−1, 13.51 ng g−1, and 12.75 ng g−1, respectively. It is noteworthy that the TEQ value of the YL Snow Mountain is 20 times higher than those of TS, LHG, and QY glaciers, 50–70 times higher than those of DKMD and YZF glaciers, and 200 times higher than that of MZTG glacier. However, the TEQs of PAHs in all seven glaciers, except for the YL Snow Mountain, were lower than that of the unpolluted areas in Spain, at 25 ng g−1 (Nadal et al., 2004), and the soil protected natural areas in Italy, at 33 ng g−1 (Orecchio, 2010), as well as the soil in New Delhi suburb, at 48 ng g−1 (Agarwal, 2009). Although YL Snow Mountain is more polluted than other glaciers, its TEQ value, at 281.32 ng g− 1, is still lower than those of Agra, at 650 ng g− 1 (Masih et al., 2005), and Shanghai, at 892 ng g−1 (Jiang et al., 2009). Hence, the biological risk of PAHs at these glaciers could be generally minor, except that some actions may be indeed required to address the pollution at the YL Snow Mountain. 4. Conclusion The cryoconites on the TP glaciers were analyzed to determine their PAH compositions and TOC, as well as the source of the PAHs. The total contents of 12 EPA priority PAHs and 15 identified PAHs ranged from 7.93 to 1745.53 ng g−1 and from 8.38 to 1850.17 ng g−1, respectively. The YL Snow Mountain has the highest level of PAHs, and the lowest PAH levels was found in the MZTG glacier. The PAHs in YL Snow Mountain are dominated by five-seven-ring PAHs, which account for more than half of its total PAHs. BbF is the dominant component, indicating that the PAHs originated from the combustions of fossil fuels and overgrown vegetation emission. The four-ring PAHs contribute to 47.26% of the total PAHs in the central TP, in which Flu is the dominant component. The central TP is surrounded by the Taklamakan Desert and Qaidam Basin and has low vegetation coverage, rare traffic, and no industrial activities. Hence, the PAHs in the cryoconites on the central TP should be attributed to long-distance atmospheric transport. The three-ring PAHs account for 40.38% of the total PAHs in the northern TP in this region, Phe is the most abundant PAH, indicating that the PAHs in the northern TP may have come from both long distance atmospheric transport and regional sources. The specific isomer ratio and Table 4 The toxicity equivalency concentration (ng g−1) of PAHs in cryoconites.

Fig. 4. Cross plot of the diagnostic ratios for the cryoconite samples. (A) the plots of Flu/Pyr versus Phe/Ant, (B)BaA/(BaA + Chry) versus InP/(InP + BgP).

Phe Ant Flu Pyr BaA Chry BbF BkF BaP InP DbA BgP

TEF

YL

DKMD

YZF

MZTG

QY

LHG

TS

0.001 0.01 0.001 0.001 0.1 0.01 0.1 0.1 1 0.1 1 0.01 TEQ

83.78 3.78 216.98 138.12 71.76 197.94 323.33 132.48 178.69 230.28 22.94 145.46 281.32

30.04 0.67 10.89 5.51 1.19 5.90 5.62 1.42 3.71 2.46 0.56 2.13 5.47

9.32 1.12 10.28 7.99 0.73 3.08 3.83 0.97 2.65 1.60 0.12 1.54 3.56

0.37 0.20 1.72 1.08 0.34 1.31 1.44 0.26 0.99 0.09 0.12 0.01 1.34

19.17 0.41 14.11 7.85 3.78 11.59 19.18 2.83 11.01 2.46 0.33 2.05 14.34

21.88 0.65 15.45 8.47 3.04 10.49 14.34 3.69 9.05 7.05 1.44 5.27 13.51

45.83 2.23 37.13 21.68 7.80 12.86 14.51 4.88 8.39 7.10 0.64 3.42 12.75

998


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