衡山冻雨金属元素污染特征和来源解析

Chemical characteristics and source apportionment of trace elements in freezing rain at Mt.Hengshan

冻雨是冬春季节常见的灾害性天气,在我国主要分布在南方山区。冻雨在到达地表前以过冷水形式存在,是一种特殊的降水类型,其化学特性鲜见报道。2015年12月—2016年3月在南岳衡山气象站(海拔1265.9 m)收集了38个冻雨样品,使用电感耦合等离子体质谱分析了其中25种金属元素的浓度,并运用正定矩阵因子法受体模型解析了其来源。结果表明:冻雨中25种金属元素的浓度变化范围达7个数量级(2×10^(-4)—4×10^(3)μg·L^(-1)),且大部分元素的浓度随着冻雨温度和pH的降低而增加。26%的冻雨样本受东北气团的影响,地壳元素浓度较高;而来自西南气团的冻雨样本占38%,重金属污染较重;南部气团(36%)携带的元素浓度相对较低。与国内外其他高山站点观测结果相比,衡山冻雨中金属元素的浓度水平整体上高于雨水但低于云水。通过富集因子分析发现,冻雨中Sb、Se、Cd、As、Zn和Pb等重金属明显受到人为源的影响,呈严重富集特征。源解析结果表明燃煤对冻雨化学成分的贡献最大(占31%),二次源、扬尘、工业排放和生物质燃烧的贡献分别为30%、18%、15%和6%。本研究提供了第一手的冻雨化学观测数据,研究结果不仅有助于理解冻雨的形成过程,也为其生态环境风险评估提供了科学依据。

Background,aim,and scope Precipitation scavenging is one of the major pathways removing pollutants from the atmosphere.Freezing rain,a rare type of precipitation,has potentials in providing nutrients as well as harmful substances to alpine ecosystems.Freezing rain usually occurs in cold seasons,causing great impacts on traffic,power lines,communication lines,forestry and agriculture.Previous studies on freezing rain focused on its physical properties and formation mechanisms,with little attention paid to its chemical properties.The present study fills this knowledge gap through investigating chemical components contained in freezing rain collected during a cold season in southern China.Source apportionment analysis of the chemical components and potential impacts of freezing rain are also investigated.Materials and methods In this study,a total of 38 freezing rain samples were collected at the summit of Mt.Hengshan from Dec.2015 to Mar.2016.The concentrations of 25 trace elements in freezing rain samples were analyzed using Inductively Coupled Plasma-Mass Spectrometry.In addition,source apportionment of chemical components was performed using the Positive Matrix Factorization Receptor Model.Results The concentrations of 25 elements in freezing rain differ by up to 7 orders of magnitude,ranging from 2×10^(−4) to 4×10^(3)μg·L^(−1),with the lowest and highest concentration determined for U and Ca,respectively.Discussion Compared with observations in other alpine areas,the concentrations of trace elements in freezing rain in this study are overall higher than those in rainfall but lower than those in cloud water.The concentrations of most elements,in particular Na,K,Ni,Cu and Th,increased with decreasing temperature,indicating that these elements might play an important role in the formation of freezing rain or be scavenged faster by freezing rain due to their size distributions.In addition,the concentrations of Mg,Ca,Fe,As,Se,Cd and Pb dissolved in freezing rain were higher at lower pH,highlighting their potential ecological risks since dissolved metals are more toxic in the environment.Heavy metals such as Sb,Se,Cd,As,Zn and Pb were enriched in freezing rain relative to crustal sources(with Al as a reference).Conclusions Coal combustion emissions contributed 31%to the total chemical components in freezing rain,followed by secondary sources(30%),dust emissions(18%),industrial emissions(15%)and biomass burning emissions(6%).The main source areas of crustal elements and heavy metals in freezing rain were distributed in the northeast and southwest directions of Mt.Hengshan,respectively.Air masses from the south direction of Mt.Hengshan carried relatively low concentrations of elements.Recommendations and perspectives The findings in this study advanced our understanding in the chemical components of freezing rain.The dataset of elements obtained in this study provided information that is much needed in evaluating the ecological and environmental risks of freezing rain.