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痕量元素是指地球系统中以超低浓度水平存在的元素,痕量元素作为微量营养盐和生命必要元素,能维持各生态系统正常运行,而某些痕量元素通过富集作用,也可成为有毒元素,危害生态系统安全[1]。长期的污染排放不仅会造成局部的环境污染(大气、土壤和水),同时也会通过远距离的运输和扩散对远离城市的高海拔地区甚至极地的冰雪产生不利影响。降雪是湿沉降的一种重要类型,尤其是在高海拔和高纬度地区[2],积雪作为气候环境变化信息的载体,可以很好地记录大气元素的含量水平和历史变化[3],因此雪中的痕量元素(trace elements)是评估雪质的关键参数。
青藏高原面积达250万 km2,平均海拔超过4000 m,是全球海拔最高的一个独特的地理单元,被誉为“第三极”[4],同时青藏高原冰雪融水也为亚洲许多大江大河的发源地,如长江、黄河、雅鲁藏布江、恒河和印度河等,因此也被称为“亚洲水塔”[5]。青藏高原的环境极其脆弱,对全球变化和人类活动的响应高度敏感[6],针对青藏高原雪冰中痕量元素研究已成为研究的重点。早在1978年,章申等[7]对珠穆朗玛峰高海拔地区冰雪中痕量元素的研究发现,该区域痕量元素受到了人类活动的影响;李月芳[8-10]、Cong等[11]研究发现,青藏高原可可西里、古里雅冰芯、哈日钦冰川、煤矿冰川、冬克玛底冰川和喜马拉雅山等大气中的Cd和Pb等痕量元素受到了人为因素的影响;Kang等[12]通过青藏高原冰芯-湖芯重建过去500年气候变化,发现青藏高原Hg元素受到人类活动排放污染物的影响;Chen等[13]还发现,牦牛粪便的燃烧也会对青藏高原大气环境中痕量元素造成污染,这些结果都说明青藏高原大气环境已经受到人类活动的影响。
青藏高原是我国主要的积雪区之一,稳定积雪区面积占230万平方公里,对青藏高原积雪中痕量元素的研究主要集中在南部、北部、西部以及中部[8, 11-19],缺乏东部积雪痕量元素污染的数据,本研究位于青藏高原东部,青海省东南部,为稳定积雪区,通过对青藏高原东部积雪进行采样分析来研究痕量元素的时空分布特征以及来源分析,不仅为青藏高原东部积雪痕量元素研究提供了基础数据,还揭示了人类活动对痕量元素的贡献,对大气生态环境评估具有重要意义。
不同研究者在样品的前处理方法上有所不同,例如酸化强度的不同[6, 18],有研究者对样品进行消解处理,测定样品的消解浓度[2, 14-15, 20],甚至还有学者对未融化的积雪中痕量元素浓度直接测定[21]。基于不同的样品处理方法会导致同一样品中痕量元素的测定结果存在差异[22],因此不同地区样品中痕量元素研究结果就没有可比性[15]。
本研究选择在青藏高原东部地区采集样品并进行酸化处理,测定了样品中13种痕量元素(Li、Al、Cr、Mn、Fe、Co、Ni、Cu、Zn、Se、Cd、Ba、Pb)的酸化浓度,分析探究了上述元素的含量在青藏高原东部的时空变化特点和富集系数变化,以及利用后向轨迹模型探讨该区域痕量元素的潜在来源。
青藏高原东部积雪中痕量元素的时空分布及污染评估
Spatial and temporal distribution and pollution evaluation of trace elements in snow in the Eastern Qinghai-Tibet Plateau
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摘要: 青藏高原的环境极其脆弱,对全球变化和人类活动的响应高度敏感,为了解青藏高原东部地区积雪中痕量元素的时空变化以及污染程度,本研究对2018年积累期和稳定期及2019年积累期、稳定期和消融期5个时期积雪样品进行酸化处理,利用电感耦合等离子体质谱仪(ICP-MS)测定了样品中的13种痕量元素(Li、Al、Cr、Mn、Fe、Co、Ni、Cu、Zn、Se、Cd、Ba、Pb)的酸化浓度。研究结果表明,痕量元素的浓度在时间上呈现积累期>稳定期>消融期的特征;空间上呈现为南部区域痕量元素浓度明显高于其余区域;北部区域除了Li、Se、Pb浓度相对较高,其余元素浓度均处于较低水平;东部和西部区域积雪痕量元素浓度相对较低。计算富集系数(EF)得知Al、Cr、Co和Ba主要来自于地壳粉尘(EF<2),其余各元素都受到了人类活动的影响(EF≥2),其中Cd和Se具有着明显的污染(EF≥5),受人为排放活动影响较大,结合后向气团轨迹分析,研究区主要受到偏西和偏西南气流的影响,这表明污染物主要来自上风方向的南亚、西亚和中亚地区。Abstract: The Qinghai-Tibet Plateau has an extremely fragile environment, which is highly sensitive to global changes and human activities. In order to investigate the spatial distribution, temporal changes and the degree of pollution of trace elements in the snow from the eastern part of the Qinghai-Tibet Plateau, the acid leachable concentrations of 13 trace elements (Li, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, Se, Cd, Ba, Pb) in snow samples in the five periods during 2018 to 2019 has been determined by inductively coupled plasma mass spectrometry (ICP-MS). Comparing the concentrations of trace elements, the results indicate that temporally the concentrations of trace element present the characteristics of accumulation period> stable period> ablation period, and spatially the concentrations of trace elements in the southern region are significantly higher than that in the other regions, and except Li, Se, and Pb, the other elements’ concentrations are lower in the northern region, the eastern and concentration of trace elements in the eastern and western regions are relatively low. The enrichment factor (EF) shows that Al, Cr, Co and Ba mainly come from crustal dust (EF<2) and all other elements are affected by human activities (EF≥2). Among which, pollution of Cd and Se are more serious (EF≥5), which might be greatly affected by anthropogenic emissions. The cluster analyses of backward air trajectories indicates that he study area is mainly affected by the westerly and southwesterly airflow. The pollutants mainly come from South Asia, West Asia and Central Asia in the upwind direction.
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表 1 研究区各采样点信息
Table 1. Information of sampling points in the study area
采样时间
Sampling time采样编号
Sample number坐标
Coordinate样品数目
Number of samples采样深度/cm
Sampling depth平均温度/°C
Average temperature2018年积累期
Accumulation period in 20181—7 97°59′17″—100°53′24″N;34°1′43″—35°13′27″E 15 5—17 −12 2018年稳定期
Stable period in 20188—32 97°8′52″—101°51′07″N;33°12′02″—35°8′26″E 55 5—48 −10.6 2019年积累期
Accumulation period in 201933—54 98°12′02″—100°52′54″N;33°54′14″—35°5′43″E 68 10—32 −12.1 2019年稳定期
Stable period in 201955—74 97°47′15″—100°5′32″N;33°57′57″—35°15′45″E 50 5—36 −10.5 2019年消融期
Ablation period in 201975—98 96°29′20″—98°39′05″N;32°11′58″—35°3′29″E 55 5—50 −7.9 表 2 研究区积雪痕量元素浓度(μg·L−1)
Table 2. The concentration of trace elements in snow in the study area(μg·L−1)
元素
Elements东部East 西部West 南部South 北部North 最大值
Max最小值
Min平均值
Mean最大值
Max最小值
Min平均值
Mean最大值
Max最小值
Min平均值
Mean最大值
Max最小值
Min平均值
MeanLi 13.93 4.40 7.41 16.17 0.80 5.78 31.64 1.73 10.80 30.56 0.88 9.71 Al 6571.67 2156.97 3666.94 6215.65 314.25 2205.73 19400.95 857.93 5158.95 10907.88 341.37 3399.63 Cr 11.13 3.48 6.30 11.03 0.78 4.31 26.29 1.62 8.29 20.34 0.71 6.61 Mn 265.50 83.14 149.38 337.52 18.44 112.65 1240.45 36.20 288.94 676.83 15.59 189.89 Fe 6983.87 2171.03 3952.34 7888.96 489.56 2723.78 24690.26 1242.55 6444.92 15352.55 530.86 4597.96 Co 4.96 1.50 2.74 5.06 0.29 1.84 15.73 0.71 4.23 10.24 0.27 3.04 Ni 13.39 4.03 7.30 13.66 0.84 5.11 38.94 1.86 11.12 26.61 0.75 8.34 Cu 9.89 3.05 5.48 9.49 0.66 3.66 31.85 1.78 8.73 21.24 0.60 6.43 Zn 35.11 12.17 23.32 42.34 3.64 15.62 154.20 9.98 44.16 72.75 4.91 24.49 Se 0.22 0.02 0.10 0.47 0.001 0.12 0.65 0.01 0.18 0.56 0.001 0.17 Cd 0.15 0.08 0.11 0.14 0.01 0.05 0.91 0.03 0.17 0.30 0.01 0.09 Ba 74.60 29.90 51.33 101.59 5.17 36.63 535.88 16.16 111.28 238.95 6.46 65.06 Pb 12.79 3.72 7.25 14.31 1.24 5.92 36.31 2.77 10.69 59.72 1.07 9.63 -
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