• 关键词：PM_2.5  水溶性离子  组成特征  NO₃^-/SO₄^2-比值  兰溪市

• 基金项目：兰溪市环保局PM_2.5源解析项目（No.dscg-lx2015095-129）；国家自然科学基金（No.41877373）

• 鲍宗炜
• 兰溪市环境保护监测站, 兰溪 321100

• 张宏
• 兰溪市环境保护监测站, 兰溪 321100

• 周志刚
• 兰溪市环境保护监测站, 兰溪 321100

• 张宁
• 上海大学环境与化学工程学院, 上海 200444

• 冯加良
• 上海大学环境与化学工程学院, 上海 200444

• 摘要：为探索浙江省中部地区大气细颗粒物（PM_2.5）中水溶性离子的组成特征及其季节变化，采集了兰溪市市区和近郊两个站点2016年4个季节的PM_2.5样品，利用双通道离子色谱对水溶性无机离子（Cl^-、NO₃^-、SO₄^2-、Na⁺、NH₄⁺、K⁺、Ca²⁺、Mg²⁺）进行了定量分析.结果表明，兰溪PM_2.5中离子总浓度存在明显的冬季高、夏季低的季节变化趋势，年均值为21.19 μg·m^-3，约占PM_2.5质量的45%；SO₄^2-、NO₃^-和NH₄⁺是水溶性离子中最主要的组分，年均浓度分别为8.11、5.92、3.87 μg·m^-3.Cl^-和NO₃^-浓度的季节变化最为显著，冬/夏浓度比接近10，其半挥发特性是导致兰溪PM_2.5中离子组成呈现季节变化的重要原因.兰溪PM_2.5中NO₃^-/SO₄^2-比值的冬季平均值为1.18，说明流动源对兰溪PM_2.5有很大贡献；夏季（以及春、秋季）时NO₃^-/SO₄^2-比值较低，且与PM_2.5浓度呈负相关，与矿物尘结合的硝酸根离子的较大贡献可能是导致夏季PM_2.5浓度较低时NO₃^-/SO₄^2-比值较高的主要原因.阴阳离子平衡、相关性及主成分分析（PCA）结果表明，矿物尘对兰溪市PM_2.5的酸度及离子赋存状态有较大影响；冬季及春、秋季兰溪的PM_2.5具有一定的酸性；NO₃^-和SO₄^2-主要与NH₄⁺结合，但部分可能与钙等其他组分结合；Cl^-和K⁺主要来源于生物质燃烧，但K⁺的年均浓度仅为0.31 μg·m^-3，说明生物质燃烧对兰溪PM_2.5的贡献不大.

• Abstract：In order to investigate the characteristics and seasonal variations of the water-soluble inorganic ions in the fine particles (PM_2.5) in the midlands of Zhejiang province. Seasonal PM_2.5 samples were collected at one urban and one suburban sites in Lanxi in 2016. The concentrations of the water-soluble inorganic ions including Cl^-, NO₃^-, SO₄^2-, Na⁺, NH₄⁺, K⁺, Ca²⁺, Mg²⁺ were quantified with a dual-channel ion chromatography. The results showed that the annual average concentration of the total ions in PM_2.5 in Lanxi was 21.19 μg·m^-3, accounting for about 45% of the PM_2.5 mass. The total ion concentration had a distinct seasonal trend of higher in winter and lower in summer. SO₄^2-, NO₃^-and NH₄⁺ were the dominant ions, with annual average concentrations of 8.11 μg·m^-3, 5.92 μg·m^-3 and 3.87 μg·m^-3, respectively. Cl^- and NO₃^- had the most profound seasonal variations, with the winter/summer mass ratio of about 10. The semi-volatility of nitrate and chloride should be the important cause of the seasonal changes in the composition of ions in PM_2.5 in Lanxi. The NO₃^-/SO₄^2- mass ratio of 1.18 in winter suggested that mobile sources had significant contribution to PM_2.5 in Lanxi. The NO₃^-/SO₄^2- mass ratio was low in summer (also in spring and autumn), and showed negative correlation with the PM_2.5 mass. The higher contribution of the non-volatile NO₃^- combined with mineral dust should be the cause of the higher NO₃^-/SO₄^2- mass ratio under lower PM_2.5 concentration in summer. Ionic balance, mutual correlation and principal component analysis (PCA) of the ions indicated that mineral dust had significant impact on the acidity of PM_2.5 in Lanxi, which should be acidic in winter, spring and autumn. NO₃^- and SO₄^2- in PM_2.5 in Lanxi were mainly neutralized by NH₄⁺, but part of them might be combined with other components, such as calcium ion. Cl^- and K⁺ were mainly from biomass burning, however, the annual average concentration of K⁺ was only 0.31 μg·m^-3, indicating that the contribution of biomass burning to PM_2.5 in Lanxi was relatively small.

• Key words：PM_2.5  water-soluble ions  compositional characteristics  NO₃^-/SO₄^2- ratio  Lanxi county

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