鄱阳湖流域庐山酸雨区大气降水化学特征及其来源解析

Variations and source apportionment of precipitation ions in a typical acid deposition region,located in the Poyang Lake watershed:A case study of Mt.Lushan region

为了理解在酸沉降背景下长江中下游典型酸雨区大气降水化学组成与来源,于2018—2020年共收集209个鄱阳湖流域庐山地区大气降水样品,分析了降水pH、电导率及各离子浓度特征,并采用富集因子、PMF模型和相关分析等多元统计数理方法,揭示庐山地区降水水化学特征和控制机理。结果表明,庐山降水pH的加权平均值为6.1,分布范围为4.52~7.89;降水电导率的加权平均值为16.3μS/cm,分布范围为1.4~96.6μS/cm;降水中离子雨量加权平均当量浓度依次为:Ca^(2+)>NO_(3)^(-)>SO_(4)^(2-)>Cl^(-)>SO_(4)^(+)>K^(+)>Na^(+)>Mg^(2+)>F^(-),其中阴离子(SO_(4)^(2-)、NO_(3)^(-))和阳离子(Ca^(2+)、SO_(4)^(+))是庐山地区大气降水中主要的离子成分,Ca^(2+)和SO_(4)^(+)对降水酸度中和起主导作用,这是造成庐山降水pH偏大的主要原因之一。庐山降水中SO_(4)^(2-)/NO_(3)^(-)(S/N)当量浓度比呈逐渐减小的趋势,从1987年的6.2减小为2018—2020年的0.6,表明庐山降水中NO_(3)^(-)离子浓度不断增大,降水由早期硫酸型转变为硫酸型和硝酸型混合型,这归结于我国近年来能源结构和使用方式的改变,清洁能源有效地降低了燃煤过程中SO_(2)的排放量,使向大气释放的SO_(2)排放量不断减少。降水中K^(+)和Ca^(2+)的海洋富集因子EF_(R)较大,说明主要以陆源贡献为主,降水中Na^(+)的土壤富集因子EFS<1,说明其相对于土壤被稀释,主要来自海洋源贡献。降水中约90%以上SO_(4)^(2-)和NO_(3)^(-)由人为大量排放的SO_(2)和NO_(x)气体贡献,而降水中Cl^(-)主要以海源输入为主,贡献占比达60.2%。研究结果有利于酸雨区水质的保护,能够有效评估人类活动对酸雨区大气环境质量的影响。

To understand the precipitation chemical compositions and their sources in the typical acid region,located in the Poyang Lake watershed under the background of acid deposition,209 precipitation samples were collected in the Mt.Lushan during 2018-2020.The characteristics of pH,conductivity and ionic concentrations of precipitation were analyzed using the Pearson’s correlation,enrichment factor correlation,PMF model and statistical methods,to reveal the main sources and controlling factors on the ionic compositions in precipitation.Results showed that the pH value in precipitation ranged from 4.52-7.89,with a weighted average of 6.1.The conductivity varied from 1.4 to 96.6μs/cm,with a weighted average of 16.3μs/cm.The order of weighted average of specific ions in precipitation is:Ca^(2+)>NO_(3)^(-)>SO_(4)^(2-)>Cl^(-)>SO_(4)^(+)>K^(+)>Na^(+)>Mg^(2+)>F^(-).The SO_(4)^(2-),NO_(3)^(-),Ca^(2+)and SO_(4)^(+) are the main ions in precipitation.Ca^(2+)and NH4+play a key role in neutralizing the acidity of precipitation,resulting in the high mean precipitation pH value in Mt.Lushan.The SO_(4)^(2-)/NO_(3)^(-) ratio decreased from 6.2 in 1987 to 0.6 during the period between 2018 and 2020,indicating that the acid conditions in Mt.Lushan changed from early sulfuric acid type to mixed type of sulfuric acid type and nitric acid type,which indicated that the NO_(3)^(-) concentration gradually increased in precipitation.Due to the adjustment of energy structure and the use of clean energy in China,the emission of SO_(2) from coal burning was effectively controlled,resulting in the decreasing amount of SO_(2) released into the atmosphere.The enrichment factor(EF_(R))value of K^(+)and Ca^(2+)is higher in precipitation,indicating that precipitation is mainly contributed by landbased sources.The EFS value of Na^(+)in precipitation is less than 1,indicating that the precipitation is diluted relative to the soil,and it is mainly contributed by marine sources.More than 90%of precipitation SO_(4)^(2-) and NO_(3)^(-) were contributed by a large amount of man-made SO_(2) and NO_(x) gases,while precipitation Cl^(-)was mainly contributed by marine source with a contribution ratio of 60.2%.These findings can provide a scientific basis for water quality protection in the middle and lower reaches of the Yangtze River and for evaluating the impact of human activities on the atmospheric environment in this region.