• 关键词：大气颗粒物  无机离子  粒径分布  形成机理  上海

• 基金项目：国家自然科学基金（No.41473088）

• 张胜华
• 上海大学环境与化学工程学院, 上海 200444

• 黄伊宁
• 上海大学环境与化学工程学院, 上海 200444

• 毛文文
• 上海大学环境与化学工程学院, 上海 200444

• 朱济奇
• 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：为深入理解上海大气颗粒物的污染特征和来源，于2016年8月—2017年4月使用微孔撞击式采样器（MOUDI）采集了上海市不同季节18 μm以下11个不同粒径段的大气颗粒物样品44套，采用双通道离子色谱对颗粒物中Cl^-、NO₃^-、SO₄^2-、Na⁺、NH₄⁺、K⁺、Ca²⁺、Mg²⁺等无机离子组分进行了定量分析，研究了主要离子的浓度、粒径分布及其季节变化特征.结果表明，<1.8 μm的细颗粒中离子总浓度平均值在春、夏、秋、冬季采样期间分别为13.46、4.97、6.72和16.54 μg·m^-3，存在显著的季节变化，>1.8 μm的粗颗粒中离子总浓度平均值分别为4.65、3.78、5.90和4.14 μg·m^-3，季节变化不明显.上海大气颗粒物中SO₄^2-和NH₄⁺呈单峰型粒径分布，峰值粒径由夏季的0.32~0.56 μm逐渐转变为冬季的0.56~1.0 μm，说明夏季时SO₄^2-的形成方式以气相/非均相反应为主，而冬季时以云过程为主；冬、春季时NO₃^-以细颗粒态为主，而夏、秋季时在3.2~5.6 μm的粗颗粒出现峰值，夏、秋季较高的大气温度使得硝酸铵的气-粒平衡更偏向于气态，细颗粒态硝酸铵的浓度较低，硝酸气体与碳酸盐或海盐反应生成的粗颗粒态硝酸盐的比例因而大幅增加；K⁺主要存在于细颗粒中，峰值粒径为0.32~0.56 μm，夏、秋季时3.2~5.6 μm的粗颗粒中有较高浓度；Cl^-以粗颗粒态为主，而冬、春季时在0.32~0.56 μm和0.56~1.0 μm的颗粒物中出现峰值，燃煤等人为过程为其主要来源.阴、阳离子平衡分析表明，除夏季外，上海的大气颗粒物呈现微弱的酸性，但0.056~0.32 μm细颗粒中阴离子有明显的缺失，未定量的有机酸及浓度过低时离子色谱响应的非线性可能是导致0.056~0.32 μm细颗粒中离子平衡发生显著改变的原因.研究结果可为上海大气颗粒物来源及形成机理研究提供重要的信息.

• Abstract：To get a better understanding of the chemical characteristics and the sources of the atmospheric particles in Shanghai, 44 sets of size-segregated particle samples of 11 different sizes (<18 μm) were collected in Shanghai during August 2016 and April 2017 using a Micro Orifice Uniform Deposit Impactor (MOUDI) sampler. Water-soluble inorganic ions including Cl^-, NO₃^-, SO₄^2-, Na⁺, NH₄⁺, K⁺, Ca²⁺, Mg²⁺ were analyzed by a dual-channel ion chromatography. Seasonal variations for the concentrations and size-distributions of the ions were investigated. Results showed that the average concentration of the total ions in fine particles (<1.8 μm) in Shanghai was 13.46, 4.97, 6.72 and 16.54 μg·m^-3 in the spring, summer, autumn and winter sampling periods respectively, showing obvious seasonal variation, while 4.65, 3.78, 5.90 and 4.14 μg·m^-3 respectively for that in the coarse particles (>1.8 μm). Unimodal size distributions were found for SO₄^2- and NH₄⁺ in the atmospheric particles in Shanghai, with the peak size changed gradually from 0.32~0.56 μm in summer to 0.56~1.0 μm in winter, suggesting that SO₄^2- in summer was formed mainly via gas phase or heterogeneous reactions, but mainly from in-cloud process in winter. NO₃^- was mainly in fine mode in spring and winter, while coarse mode peaking at 3.2~5.6 μm was found in summer and autumn. Shift of the thermodynamic equilibrium between the gas and particle phase to the gas phase under high temperature resulted in the low concentration of NH₄NO₃ in the fine particles, the contribution of coarse mode NO₃^-, which was mainly from the heterogeneous reactions between gaseous HNO₃ and carbonate or sea salt, was thus obviously higher in summer and autumn. The major part of K⁺ was in fine mode peaking at 0.32~0.56 μm, while significant contribution from coarse mode particles of 3.2~5.6 μm was found in summer and autumn. Cl^- was mainly in the coarse particles in summer and autumn, while much of the Cl^- was found in the fine mode particles in winter and spring, indicating the contribution from anthropogenic sources such as coal burning. Analysis of the equivalent balance between total cations and anions suggested that particles in Shanghai was slightly acidic except that in summer, but the concentrations of the measured anions were obviously less than that of cations in fine particles of 0.056~0.32 μm. Existence of organic acids, which were not quantified in this study, and the non-linear response of the ion chromatograph under extremely low concentration could be the reasons for the low anion to cation ratio in the particles with size of 0.056~0.32 μm. Our results provided important information for the study of the sources and formation mechanisms of the atmospheric particles in Shanghai.

• Key words：atmospheric particles  inorganic ions  size distribution  formation mechanism  Shanghai

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