Environmental Pollution 267 (2020) 115272
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Environmental Pollution
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New insights into particle-bound trace elements in surface snow, Eastern Tien Shan, China*
* Ju Huang a, b, Guangjian Wu a, c, , Xuelei Zhang d, Chenglong Zhang e a Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100101, China b University of Chinese Academy of Sciences, Beijing, 100049, China c CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing, 100101, China d Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun, 130102, China e Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China article info abstract
Article history: Trace elements (TEs) in the insoluble particles of surface snow are less affected by melting processes and Received 27 May 2020 can be used as environmental proxies to reveal natural and anthropogenic emissions. Here the first Received in revised form comprehensive study of the 16 TEs (Al, As, Ba, Bi, Cr, Cu, Fe, Mn, Ni, Pb, Sn, Sr, Ti, U, V, and Zn) in insoluble 20 July 2020 particles (>0.45 mm) from surface snow samples collected at Urumqi Glacier No. 1 (UG1), Eastern Tien Accepted 26 July 2020 Shan, China, from February 2008 to January 2010 were presented. Results show that concentrations of Available online 18 August 2020 most insoluble particulate TEs (TEs insol) in the snow were higher in summer while lower in winter, due to the increasing particle inputs and melting processes. The abundances of As, Cr, Cu, Ni, Pb, and Zn in Keywords: Elemental composition some samples were higher than those in surrounding urban soils, which might due to these TEs have Seasonal variation further anthropogenic input beyond the already contaminated re-suspended urban soil particles and TEs Pollution assessment were mainly enriched in particles with small grain size. Based on enrichment factor (EF) and principal Sources component analysis (PCA), our results suggest that eight TEs (Al, Fe, Ti, Ba, Mn, Sr, U, and V) mainly came Glacier from mineral dust, while the remaining eight TEs (As, Bi, Cr, Cu, Ni, Pb, Sn, and Zn) were affected by coal combustion, mining and smelting of non-ferrous metals, traffic emissions, and the steel industry. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model suggests that pollutants might originate from Xinjiang province, Kazakhstan, and Kyrgyzstan. Moreover, UG1 received more significant inputs of particle-bound pollutants in summer than in winter due to the stronger convection and the prevailing valley wind that transports pollutants from the city of Urumqi. © 2020 Elsevier Ltd. All rights reserved.
1. Introduction and Settle, 1987) and delivered to terrestrial or aquatic surfaces by dry/wet deposition. Trace elements (TEs) are ubiquitous throughout the environ- Asian emissions are now the largest anthropogenic sources of ment. Levels of TEs are mainly determined by the local geochem- atmospheric TEs and still show an increasing trend (Liu et al., 2011; istry and anthropogenic emissions, with implications for the Pacyna and Pacyna, 2001). Records of past atmospheric deposition ecological environment and living beings (Barbante et al., 2011). preserved in snow and ice from Asian glaciers could offer unique Some TEs, such as Pb, Cu, and Cd are non-degradable and bio- insights into long-term changes of the chemical composition of the accumulate, and can be toxic at high levels. Trace elements are atmosphere and the nature and intensity of the regional atmo- transported in the atmosphere mainly bound to aerosols (Patterson spheric circulation systems (Beaudon et al., 2017; Dong et al., 2015; Dong et al., 2018; Li et al., 2007). In particular, Tien Shan is located close to sites of human habitation and industrialised regions. At- mospheric TEs deposited on the glaciers of Tien Shan could provide * This paper has been recommended for acceptance by Pavlos Kassomenos. regional archives of anthropogenic activities especially for species * Corresponding author. Key Laboratory of Tibetan Environment Changes and with a short atmospheric residence time (Avak et al., 2018). Several Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100101, China. previous studies investigated the acid-leached concentrations of E-mail address: [email protected] (G. Wu). TEs in Tien Shan using the traditional acid-leaching approach to https://doi.org/10.1016/j.envpol.2020.115272 0269-7491/© 2020 Elsevier Ltd. All rights reserved. 2 J. Huang et al. / Environmental Pollution 267 (2020) 115272 reconstruct past anthropogenic perturbations (Li et al., 2007; Liu after melting, minimized the loss of the soluble parts. The objec- et al., 2011; Shi et al., 2011; Wei et al., 2019). For TEs in snow and tives of this study were to examine the concentration levels and ice samples, the enrichment factor (EF) is widely used as an index to seasonal variations of the particle-bound TEs in snow, to evaluate assess the relative contribution from natural and anthropogenic the contamination levels of the particle-bound TEs, and to inves- activities. Li et al. (2007) found that the Pb, Cd, and Zn in the snow tigate their potential source areas and transport routes. pits of Urumqi Glacier No. 1 (UG1) were affected by anthropogenic activities as the EF values were between 20 and 770, especially 2. Materials and methods during winter, when the EF values were greater than 200. Shi et al. (2011) reported higher EF values for Pb (EF ¼ 51) and Cd (EF ¼ 94) in 2.1. Sample collection the snow pits of UG1. However, it cannot be excluded that the seasonal signal of anthropogenic activities in snow pits and ice The sampling site (43 060N, 86 490E, 4130 m a.s.l.) is located in a cores of previous studies might be disturbed by melting processes, percolation zone of the east branch of UG1 in eastern Tien Shan such as the movement of soluble TEs (Li et al., 2007). Furthermore, (Fig. 1). The high-level westerly jet stream prevails across the high studies based on EF calculated using the acid-leached concentra- mountains throughout the year. From February 2008 to January tion might overestimate the contribution from anthropogenic ac- 2010, 50 surface snow samples were collected weekly or bi-weekly tivities (Li et al., 2017, 2020). by using acid-cleaned wide-mouth low-density polyethylene As the composition of particle-bound TEs is less affected by (LDPE) Nalgene bottles. A detailed sampling procedure was re- melting processes, this fraction still could be used as a reliable ported in Li et al. (2006). Snow samples were stored frozen until environmental proxy (Avak et al., 2019; Wong et al., 2013). Li et al. analysis. (2012) studied the geochemical properties of insoluble particles (>0.22 mm) from Tibetan Plateau and Tien Shan. However, this 2.2. Sample treatment and measurement study only investigated provenance regions of the snow-pit dust and these samples only covered the non-monsoon period (Octo- All the snow samples were melted at room temperature (about bereMay). Thus, the concentration levels and seasonal variations of 20 C) just before filtration and were filtered on LCR hydrophilic the particle-bound TEs are not yet well understood. polytetrafluoroethylene (PTFE) membrane filters with a diameter of This study was focused on the acid-digested concentration of 47 mm and a pore size of 0.45 mm (Millipore Corporation). The particle-bound TEs (>0.45 mm) in surface snow samples that had filtrations were conducted at the State Key Laboratory of Cryo- been continuously collected from UG1 from February 2008 to spheric Science (SKLCS), Northwest Institute of Eco-Environment January 2010. It should be noted that some parts of elements from and Resources (NIEER), Chinese Academy of Sciences (CAS). The pollution sources (e.g., combustion activities) are easily dissolved particles with a diameter larger than 0.45 mm were digested with into melted water, resulting in weak EF signals of particle-bound super-pure HNO3eHF at 150e190 C in polytetrafluoroethylene TE. In this study, all samples were kept frozen until they were screw-top bombs in three steps. A detailed description of the transported to the lab, and the samples were filtered immediately digestion procedure is mentioned in Wu et al. (2009).
Fig. 1. Map showing the study area and sampling site. J. Huang et al. / Environmental Pollution 267 (2020) 115272 3
Concentrations of 16 TEs (Al, As, Ba, Bi, Cr, Cu, Fe, Mn, Ni, Pb, Sn, Sr, respectively. TEs insol concentrations were largely controlled by the Ti, U, V, and Zn) in the digested samples were determined using an content of the insoluble particles which were 37.5 mgg1, inductively-coupled plasma mass spectrometer (ICP-MS, X-7, 93.3 mgg 1, 76.7 mgg 1, and 50.2 mgg 1 in spring, summer, autumn, Thermo-Elemental Corporation, quadrupole mass) at the Institute and winter, respectively. The observed increases in TEs insol and of Tibetan Plateau Research, CAS. Special attention was paid to insoluble particle concentrations might be affected by increased avoiding possible contamination during the sampling, sample inputs of impurities from the atmosphere and meltwater-related preparation, and analysis. The quality assurance and quality control post-depositional processes. In summer, frequent dust storms (QA/QC) can be found in Text S-1 and Table S-1 of the could bring impurities to the glacier surface (Li et al., 2014; Wu supplementary material. et al., 2010). Furthermore, with higher temperatures (Fig. S-1), melting could increase the insoluble particle concentrations (Wu et al., 2018) and consequently TEs insol concentrations by reducing 3. Results and discussion the amount of accumulated snow and accumulating particles in snow. However, during winter, fewer dust emissions occurred in 3.1. Concentration levels and seasonal variations of TEs insol in the surrounding areas (Li et al., 2014; Wu et al., 2010) and lower surface snow temperatures prevented melting. Thus, the TEs insol concentrations in winter could reflect the significant contribution from dry depo- To clearly illustrate the concentration levels and detailed sea- sition. However, the variations of TEs insol concentrations in spring sonal variations of TEs, TE abundances in particulate matter (PM) and autumn were complex due to the instability of temperatures were converted into concentrations (TEs insol) in the melted snow. and precipitation (Fig. S-1). The main factors affecting element In this study, the samples were grouped into four seasons: spring concentrations in snow and ice during spring and autumn need to (March to May), summer (June to August), autumn (September to be further analyzed in combination with more samples in the November), and winter (December to February) according to the future. meteorological conditions recorded at Daxigou meteorological station (Fig. S-1). As shown in Fig. S-1, during winter, the PM in 3.2. Elemental composition of the particulate matter surface snow of UG1 is mainly deposited by dry deposition due to less precipitation (Wu et al., 2010). Whereas in other seasons PM is The minimum, maximum, mean, median, standard deviation, deposited by wet deposition during precipitation events, particu- and coefficient of variation values (CV, the ratio of the standard larly in summer when there is frequent precipitation, and by dry deviation to mean) of TEs are given in Table 1, along with TE deposition on days when there is no precipitation (Li et al., 2006; abundances in the upper continental crust (UCC), the average Wu et al., 2010). background values of Xinjiang soils, and TE abundances in the ur- As shown in Fig. 2, the following characteristics were observed: ban soils of Xinjiang province. The data were not normally the average concentrations of TEs insol in all samples ranged from distributed (P value < 0.05) using the Kolmogorov-Smirnov test and 1 1 0.2 ng g (Bi) to 4475 ng g (Al), showing high inter-element the Shapiro-Wilkins test. Thus, the median values were chosen to variability in elemental composition of the PM (abundances). Bi, study the composition of TEs in the PM. 1 U, and Sn had lower concentrations which were below 1 ng g ; the As shown in Table 1, the median abundances of TEs carried 1 concentrations of As, Ni, Cu, Cr, V, and Pb ranged from 1 ng g to by > 0.45 mm particles in this study were slightly higher (1.1e1.7 1 10 ng g ; the concentrations of Zn, Sr, Ba, Mn, and Ti ranged from times) than those in >0.22 mm particles which were reported by Li 1 1 10 ng g to 300 ng g ; Fe and Al had higher concentrations which et al. (2012), as well as the TE abundances (except Cr and Sr) in Li 1 were above 2000 ng g . et al. (2012) were within the abundances range of TEs in our Concentrations of most TEs insol (except Cu) were higher in study. Generally, TEs were enriched in small grain size. However, summer and autumn while lower in winter and spring. The con- the effect of particle size on the elemental compositions could not e centrations of TEs insol (except Cu) in summer were 1.5 2.8 times be found here due to only three samples were reported in Li et al. e and 2.2 3.7 times that of the values in winter and spring, (2012) and the abundances of some TEs (e.g., As, Bi, Cr, Cu, Ni, Pb, Sn, and Zn) in our study had higher inter-samples variabilities (0.5 CV 1.4). Furthermore, the abundances of Al, Fe, Ti, Ba, Mn, Sr, U, and V in PM were comparable to the UCC (Taylor and Mclennan, 1995) and the corresponding background values in Xinjiang soils which were not or less affected by anthropogenic activities (China National Environmental Monitoring Centre, 1990). However, the abun- dances of As, Bi, Cr, Cu, Ni, Pb, Sn, and Zn in PM were generally greater than in the UCC (Taylor and Mclennan, 1995) and the cor- responding background values in Xinjiang soils (China National Environmental Monitoring Centre, 1990), indicating these ele- ments might be affected by anthropogenic sources. The abundance of As in PM was 16 times that of the value in the UCC (Taylor and Mclennan, 1995) and was 2.1 times that of the corresponding background values in Xinjiang soils (China National Environmental Monitoring Centre, 1990), which indicates that As is naturally abundant in the study region. To further compare the enrichment elements such as As, Cr, Cu,
Ni, Pb, and Zn in PM with the corresponding average values in Fig. 2. Average concentrations (in units of ng g 1 snow) of particle-bound TEs surrounding urban soils which might be more affected by anthro- (>0.45 mm) in all surface snow samples (n ¼ 50), as well as in samples collected at UG1 in spring (March to May), summer (June to August), autumn (September to November), pogenic activities (Table 1). In this study, Bi and Sn could not be and winter (December to February). compared as no data have been reported for these two elements. 4 J. Huang et al. / Environmental Pollution 267 (2020) 115272
Table 1 Descriptive statistics of trace element (TE) abundances (mgg 1 PM, n ¼ 50) and comparison with their abundances in the >0.22 mm fraction of insoluble particles at UG1, the upper continental crust (UCC), the average background values of Xinjiang soil, and urban soils from different cities in Xinjiang province.
Trace element Al (%) Fe (%) Ti (%) As Ba Bi Cr Cu Mn Ni Pb Sn Sr U V Zn References
UG1 Median 7.6 4.8 0.4 24 645 2.5 112 49 765 50 110 5.1 171 4.0 117 173 This study Minimum 5.2 3.2 0.3 11 462 0.6 79 28 460 30 32 2.6 122 2.7 82 86 This study Maximum 9.0 7.6 0.6 108 1204 17.8 477 687 1042 206 817 28.5 553 5.6 148 669 This study Average 7.5 4.8 0.4 26 653 3.6 134 70 763 59 148 6.2 177 4.1 118 201 This study SD 0.8 0.7 0.1 14 108 3.7 65 96 122 29 130 3.8 58 0.6 14 100 This study CV 0.1 0.2 0.1 0.5 0.2 1.0 0.5 1.4 0.2 0.5 0.9 0.6 0.3 0.1 0.1 0.5 This study >0.22 mm fraction 6.1 3.3 0.4 17 469 2.2 67 34 693 33 91 4.4 116 3.2 87 147 Li et al. (2012) Background value UCC 8.0 3.5 0.3 1.5 550 0.1 35 25 600 20 20 5.5 350 2.8 60 71 Taylor and Mclennan (1995) Xinjiang province 5.4 2.8 0.3 11.2 467 0.3 49 27 688 27 19 2.2 333 2.8 75 69 China National Environmental Monitoring Centre (1990) Urban area Urumqi eeeeee110 44 ee55 eeee169 Liu et al. (2006) Changji eee11 ee46 31 e 29 18 eeee72 Wang (2011) Shihezi eee13 ee80 31 e 30 23 eeee78 Liu and Ge (2011) Kuytun eee20 ee77 27 e 39 36 eeeeeLuo et al. (2012) Yining eeeeee33 31 e 35 61 eeee61 Wu et al. (2019) Karamay eee21 ee118 64 e 43 33 eeee123 Wang et al. (2016) Aksu eeeeee26 97 e 876eeee120 Keram and Halik (2018) Kashgar eeeeee61 ee 29 17 eeeeeMamut et al. (2018)
Results show the median abundances of As, Cr, Cu, Ni, and Zn in PM and severe environmental conditions limit the metabolic activities were generally higher (>1.2 times) than those in the urban soils of of flora and fauna. Thus, the TEs in these snow samples mainly Changji (Wang, 2011), Shihezi (Liu and Ge, 2011), Kuytun (Luo et al., derive from mineral dust and anthropogenic activities. 2012), Yining (Wu et al., 2019), Aksu (Keram and Halik, 2018), and The EF has been widely applied for evaluating the contamina- Karamay (Wang et al., 2016) while comparable (0.8e1.2 times) to tion status of TEs. The definition can be found in Text S-2 of the those in Urumqi (Liu et al., 2006) and Karamay (Wang et al., 2016). supplementary material. The EF was calculated using both the TE The median abundance of Pb in PM was higher than (>1.2 times) mean composition of the UCC (Taylor and Mclennan, 1995) and the that of soils of these cities. There were greater differences in TE average background values of soil elements in Xinjiang province abundances among the urban soils due to the variable amount of (China National Environmental Monitoring Centre, 1990), referred pollutant inputs from anthropogenic sources. to as EFc and EFs respectively in this study. As shown in Fig. S-2, However, As of PM in some (46% of the total) UG1 snow samples there was no significant difference between EFc and EFs for Fe, Ti, exhibited higher abundance (>1.2 times) than that of the already Ba, Mn, Pb, Sn, Sr, U, and V. However, the EFc of As, Bi, Cr, Cu, Ni, and contaminated urban soils. Other TEs, such as Cr (28% to the total), Zn were 11.2, 3.2, 4.1, 2.0, 3.8, and 2.3 times that of the corre- Cu (4%), Ni (42%), Pb (66%), and Zn (34%), also showed such a sponding EFs, respectively. The difference between EFc and EFs for characteristic. This might due to the PM might be affected by the As was greater due to As already being enriched in Xinjiang soil, as additional anthropogenic input and the effect of particle size on the shown in Table 1. To reduce the uncertainty in these calculations elemental compositions. Generally, TEs were mainly enriched in due to the differences between the chemical composition of local particles with small grain size (Li et al., 2009), and the grain size of soil and reference crustal composition, EFs values of TEs were used particles in the snow was finer than that of urban soils. Further- to estimate the contributions from mineral dust and anthropogenic more, due to the scarcity of human activities on the Tibetan Plateau activities. and consequently elements mainly come from parent rocks, the Five pollution levels were proposed by Sutherland (2000) and influence of particle size on elements could be studied. According the criteria can be found in Text S-1 of the supplementary material. to the data reported by Li et al. (2009), 16 TEs in our study were all As shown in Table S-2, the maximum values of the EFs of Fe, Ti, Ba, enriched in particles with small grain size. For example, median Mn, Sr, U, and V were less than 2, indicating these seven TEs mainly abundances of Al and As in the bulk samples were 1.3 and 1.5 times originated from mineral dust, known as the crustal fraction. How- that of the corresponding abundances in the <20 mm fractions of ever, about 18%, 34%, 3%, 2%, 2%, 54%, 28%, and 10% of the samples the topsoil which could be entrained and transported over long were “moderately polluted” with As, Bi, Cr, Cu, Ni, Pb, Sn, and Zn distances, respectively. Thus, although the particle size could affect (2
Fig. 3. Box plots for the EFs of trace elements in PM in spring (n ¼ 14), summer (n ¼ 9), autumn (n ¼ 14), and winter (n ¼ 13). 6 J. Huang et al. / Environmental Pollution 267 (2020) 115272 values higher than 5, while only 46% of the samples in winter were (EFs ¼ 2.3), and As (EFs ¼ 2.2) in the sample which was collected on higher than 5. For Pb, approximately 36%, 67%, 50% of the samples 19 October 2008. During winter (Fig. 4d), air mass trajectories in spring, summer, and autumn had EFs higher than 5 while only 8% showed more northwesterly and westerly flow and passed through of the samples in winter were higher than 5. These results indicated Dushanbe, Bishkek, Yining, Kuytun, Shihezi, Kashgar, and Novosi- UG1 received more significant inputs of particle-bound pollutants birsk, etc, causing the contamination of Cu (EFs ¼ 16.6), Sn in summer, followed by spring and autumn, but rarely in winter. (EFs ¼ 10.3), Pb (EFs ¼ 4.2), Zn (EFs ¼ 3.6) and Bi (EFs ¼ 2.5) in the Only three samples collected on 12 February 2008 (As, Bi, Pb, Sn, sample which was collected on 21 February 2009. and Zn), 7 February 2009 (Cu and Sn), and 21 February 2009 (Cu, Pb, Furthermore, such seasonal differences in seasonal enrichment Sn, and Zn) showed peak and higher EFs values, indicating that of contaminated TEs (Fig. 3) might be largely linked with seasonal these TEs were occasionally affected by the few extreme pollution changes in the vertical structure of the regional troposphere, events occurring in winter. assuming that UG1 has stable particle sources. In summer, there is The National Oceanic and Atmospheric Administration (NOAA) enhanced vertical transportation of pollutants in the troposphere, Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) which promotes the dispersion of pollutants from low altitude model (three-day back trajectory) was used to study the potential source areas to high-altitude areas (Schwikowski et al., 2004). As source areas for TEs in several typical pollution events during shown in Fig. S-3a, b, Tien Shan and the surrounding regions, different seasons (Fig. 4). These trajectories ended at the height of especially the northern regions of Tien Shan, were covered by a 500 m above the sampling site. thick polluted dust layer which could reach 5 km in altitude as well As shown in Fig. 4, most air masses passed through the eastern as the pollutants which tend to be transported to UG1 from the and southern Kazakhstan, Kyrgyzstan, and the northwestern re- surrounding regions. However, winter is indeed characterised by gions of Xinjiang province as well as some big cities. The pollutants frequent low altitude thermal inversions limiting the trans- entrained into the atmosphere of these regions could be trans- portation of pollutants from low altitude source areas such as ported to UG1. During spring (Fig. 4a), air mass trajectories showed Urumqi to high altitude locations (Barbante et al., 2011; Mamtimin more northwesterly and westerly flow and passed through Kar- and Meixner, 2011). For example, Fig. S-3c, d showed that the aganda, Yining, Kuytun, Shihezi, and Changji, etc, causing the polluted dust and polluted smoke were mainly distributed at an contamination of Bi (EFs ¼ 25.6), Pb (EFs ¼ 9.7), Sn (EFs ¼ 2.9), and altitude of 1e2 km in the surrounding regions of Tien Shan on 9 As (EFs ¼ 2.3) in the sample which was collected on 28 May 2008. January 2010 during the winter. During summer (Fig. 4b), air mass trajectories showed more Wind direction and wind speed could affect the transport of westerly flow and passed through Tashkent, Bishkek, Almaty, Yin- pollutants. UG1 was generally dominated by the westerly flow at ing, and Kuytun, etc, causing the contamination of Bi (EFs ¼ 29.0), 500 hPa and the wind speeds in winter (12e15 m/s) were generally Pb (EFs ¼ 10.2), and Sn (EFs ¼ 2.6) in the sample which was greater than those in summer (7e10 m/s) over UG1 and the sur- collected on 28 June 2008. During autumn (Fig. 4c), air mass tra- rounding areas (Fig. S-4), indicating that once the central Asian jectories also showed more westerly flow and passed through pollutants were lifted to the height of the westerly flow, the Tashkent, Dushanbe, Bishkek, Almaty, Yining, and Kashgar, etc, westerlies could transport them to UG1. Furthermore, atmospheric causing the contamination of Bi (EFs ¼ 45.7), Pb (EFs ¼ 18.3), Sn pollutants from Urumqi, especially in Houxia which is a small town
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Variations of financial interests or personal relationships that could have trace elements and rare earth elements (REEs) treated by two different appeared to influence the work reported in this paper. methods for snow-pit samples on the Qinghai-Tibetan Plateau and their im- plications. Sci. Cold Arid Reg. 9 (6), 568e579. https://doi.org/10.3724/ SP.J.1226.2017.00568. Acknowledgments Li, Z.Q., Edwards, R., Mosley-Thompson, E., Wang, F.T., Dong, Z.B., You, X.N., Li, H.L., Li, C.J., Zhu, Y.M., 2006. Seasonal variability of ionic concentrations in surface snow and elution processes in snowefirn packs at the PGPI site on Urumqi This work was funded by the National Natural Science Foun- glacier No. 1, eastern Tien Shan, China. Ann. Glaciol. 43, 250e256. https:// dation of China [Grant No. 41725001], and the Strategic Priority doi.org/10.3189/172756406781812069. Research Program, Chinese Academy of Sciences [Grant No. Li, Z.Q., Li, C.J., Li, Y.F., Wang, F.T., Li, H.L., 2007. Preliminary results from measure- fi fi ments of selected trace metals in the snow- rn pack on Urumqi glacier No. 1, XDA20060201], and the Second Tibetan Plateau Scienti c Expedi- eastern Tien Shan, China. J. Glaciol. 53 (182), 368e373. https://doi.org/10.3189/ tion and Research Program (STEP) [Grant No. 2019QZKK0201]. 002214307783258486. Thanks are owed to the Tianshan Glaciological Station for their hard Liu, A.N., Ge, B.W., 2011. Assessment of heavy metal distribution in surface soils of fi different area in Shihezi, China. J. Anhui Agric. Sci. 39 (32), 19818e19821. work in the eld. https://doi.org/10.13989/j.cnki.0517-6611.2011.32.067 (in Chinese with English Abstract). Appendix A. Supplementary data Liu, Y.P., Hou, S.G., Hong, S.M., Do Hur, S., Lee, K., Wang, Y.T., 2011. High-resolution trace element records of an ice core from the eastern Tien Shan, central Asia, since 1953 AD. J. Geophys. 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