基于颗粒物挥发特性对成都市灰霾过程大气PM2.5混合状态的研究

As an important characteristic of PM2.5, mixing state greatly affects its properties of optical, hygroscopicity and reactivity in atmosphere. The mixing state of PM2.5 can be deeply investigated through the study of the evaporation of particles. This study was carried out in a typical megacity of Sichuan basin—Chengdu, which is greatly influenced by the topography and climate of the basin. In this study, single particle aerosol mass spectrometry (SPAMS) and transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectrometry were combined to complement their advantages. Meanwhile, two sets of improved thermodenuder (TD) were installed in the front to build double TD-SPAMS/TEM system. This study is aiming to gain the detailed information of PM2.5 physicochemical characteristics during the haze processes in each season of Chengdu, through evaporating PM2.5 under different temperature conditions; focus on the integration of the obtained results with regard to mixing state, in order to clarify particle mixing state, morphology and structure characteristics of different particle types during haze processes; finally, further clarify the original sources and mixing mechanisms of different particles in each season after taken meteorological parameters and gaseous pollutants into account. This study is of great importance to expand our knowledge of the formation mechanism and environmental effects of PM2.5 pollution as well as the accurate modeling in Chengdu, and even the whole Sichuan basin.

混合状态作为PM2.5的重要特征强烈影响着其在大气中的光学、吸湿和反应性等特性，对颗粒物进行加热蒸发可实现其混合状态的深入剖析。本研究选取受四川盆地地形及气候影响显著的成都市为研究对象，将单颗粒气溶胶质谱仪(SPAMS)和带能谱的透射电子显微镜(TEM)方法联用，并将改良后的两套热扩散管(TD)前置，形成双TD-SPAMS/TEM分析系统，实现方法上优势互补。旨在通过对典型灰霾过程时大气PM2.5进行多温度梯度的加热蒸发，获得其理化特征的演变信息；重点整合混合状态观测结果，阐明各污染阶段不同类型颗粒物的混合情况及其形貌、结构等特征，揭示这些特征在污染过程中的演变规律；结合气象和气态污染物，厘清各季节不同类型颗粒物的初始来源及其在大气中的混合老化机制。该研究对成都市，甚至整个四川盆地大气灰霾的形成机制和环境效应研究以及模型模拟都具有重要意义。

PM2.5混合状态直接影响着其光学、吸湿性和反应性等特性，也是当前模型模拟不确定性的重要来源。为对成都大气PM2.5混合状态实现深入研究，本项目将单颗粒气溶胶质谱仪（SPAMS）和透射电子显微镜（TEM）方法联用，并将改良后的两套热扩散管（TD）前置，构建了双TD-SPAMS/TEM系统，利用该系统对成都各季节灰霾时段大气PM2.5进行了不同温度梯度的加热测定（室温、50℃、100℃、150℃、220℃及280℃）。结果发现：（1）各季节PM2.5颗粒主要以内混形式存在，大多数颗粒老化明显，就SPAMS质谱而言，多数颗粒物质谱中包含了丰富的硫酸盐和/或硝酸盐离子峰；就TEM-EDS结果而言，这些内混的颗粒通常以“哑铃状”、“半包含”或“全包含”形式存在，颗粒物核以烟尘和金属颗粒为主，壳以有机物和硫酸盐为主。根据质谱及混合状态，各季节颗粒物可分为6～10类，其呈现出了不同的化学组成、粒径分布及形貌特征。（2）随着温度升高，单颗粒质谱中硝酸盐和硫酸盐离子峰相对贡献分别在室温至100℃和100℃至280℃的温度范围出现了大幅降低；有机物离子峰相对贡献始终呈稳定降低；而元素碳、金属及矿尘等难挥发颗粒离子峰的相对贡献则随温度升高持续增加。（3）温度升高过程中，内混颗粒表面所包裹的壳逐步变薄，甚至消失，被包裹物掩盖的内核物质的源信息更加明确；同时，以外混形式存在的烟尘、矿尘和金属颗粒占比明显升高。（4）随着污染加重，颗粒物老化程度明显增强，质谱中含有更丰富的硫酸盐、硝酸盐和二次有机碳离子峰，同时，烟尘、矿尘和金属颗粒外部所包裹的物种种类增多且包裹层厚度明显变厚，证明了二次反应过程对当地污染形成的重要贡献。研究证明，SPAMS和TEM测定的颗粒物混合状态信息具有很强的互补性，通过TD加热，可以揭示更为深入且全面的颗粒物混合机制信息，这对污染形成机制研究和模型模拟具有重要价值。