天津市大气降水化学组成特征及来源分析

为了解天津市大气降水的化学组成及其来源,2013年1—12月在天津市环境监测中心、天津市塘沽区环境保护监测站、天津市蓟县环境保护监测站采集降水样品,分析了pH、EC(电导率)和主要离子的分布特征. 结果表明:天津市降水的pH、EC雨量加权平均值分别为5.58、98 μS/cm. 降水中各离子雨量加权平均当量浓度(当量浓度＝质量浓度×离子价数/分子量)为Ca2+>SO₄2->NH₄+>NO₃->Cl->Mg2+>Na+>F->K+. 其中,Ca2+、SO₄2-、NH₄+、NO₃-和Cl-是降水中的主要离子,占总离子当量浓度的91.3%. 降水中SO₄2-/NO₃-(当量浓度比)为2.1,近10年来呈逐渐降低趋势,表明天津市大气污染向复合型转变趋势明显. 总离子当量浓度季节差异大,冬季高、夏季低；SO₄2-、NH₄+、NO₃-等当量浓度在冬季最高,Ca2+当量浓度在春冬季基本相当. 从空间分布上看,降水化学组成在天津市环境监测中心、天津市塘沽区环境保护监测站、天津市蓟县环境保护监测站分别表现为受综合源、工业源和海洋源、工业源和农业源的影响特征. 富集因子计算结果表明:降水中Ca2+和K+主要来自地壳；Mg2+主要来自地壳,同时也受海洋源的影响；Cl-的海洋源和人为源的贡献基本相当；但降水中的SO₄2-、NO₃-主要来自人为活动. 

Characteristics and Sources of Chemical Composition of Atmospheric Precipitation in Tianjin

To better understand the characteristics and sources of chemical composition of atmospheric precipitation in Tianjin, precipitation samples were collected at Tianjin Environmental Monitoring Center, Tanggu Environmental Monitoring Station and Jixian Environmental Monitoring Station from January to December 2013. The values of pH, electrical conductivity (EC) and main ions were analyzed. The volume-weighted means of pH and EC were 5.58 and 98 μS/cm, respectively. The volume-weighted mean equivalent concentrations of precipitation components showed a decreasing order Ca2+>SO₄2->NH₄+>NO₃->Cl->Mg2+>Na+>F->K+. Ca2+, SO₄2-, NH₄+, NO₃- and Cl-were the dominant compositions in precipitation, accounting for 91.3% of the total ions. The ratio of SO₄2-/NO₃- in precipitation was 2.1 in 2013, which represented a decreasing trend over the past decade. This shows that the type of air pollution in Tianjin clearly changed to be complex pollution. Significant seasonal variations in the total ionic equivalent concentrations existed; the total concentrations were high in winter and low in summer. SO₄2-, NH₄+ and NO₃- showed high values in winter, while Ca2+ varied slightly in spring and winter. The chemical compositions of precipitation at the studied sites were influenced by compositive sources, industrial and marine sources, and industrial and agricultural sources. The enrichment factor was used to assess sources of major ions. Ca2+ and K+were determined to be mainly originated from crust sources. Mg2+ was partly from marine sources, though the contribution from soil sources was also considerable. Cl-mainly came from both marine and anthropogenic sources, while SO₄2- and NO₃- were mostly attributable to anthropogenic sources.