兰溪市PM_(2.5)中水溶性离子的组成特征及季节变化

Compositional characteristics and seasonal variations of the water-soluble ions in PM_(2.5) in Lanxi

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-、NO^(-)_(3)、SO^(2-)_(4)、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21.19μg·m^(-3),约占PM_(2.5)质量的45%;SO^(2-)_(4)、NO^(-)_(3)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、3.87μg·m^(-3).C1-和NO^(-)_(3)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NO^(-)_(3)/SO^(2-)_(4)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NO^(-)_(3)/SO^(2-)_(4)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NO^(-)_(3)/SO^(2-)_(4)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;NO^(-)_(3)和SO^(2-)_(4)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0.31μg·m^(-3),说明生物质燃烧对兰溪PM_(2.5)的贡献不大.

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-, NO^(-)_(3), SO^(2-)_(4), Na^(+), NH^(+)4, 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. SO^(2-)_(4), NO^(-)_(3)and NH^(+)4 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 NO^(-)_(3) 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 NO^(-)_(3)/SO^(2-)_(4) mass ratio of 1.18 in winter suggested that mobile sources had significant contribution to PM_(2.5) in Lanxi. The NO^(-)_(3)/SO^(2-)_(4) 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 NO^(-)_(3) combined with mineral dust should be the cause of the higher NO^(-)_(3)/SO^(2-)_(4) 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. NO^(-)_(3) and SO^(2-)_(4) in PM_(2.5) in Lanxi were mainly neutralized by NH^(+)4, 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.