用同步辐射技术研究海盐颗粒物吸湿特征及其表面异相反应机制

Atmospheric particulate matter pollution has become one of the major environmental problems faced by many large coastal cities in China. As sea salt particles (SSA) contain a variety of soluble salt, they can easily absorb or lose moisture and react with other atmospheric pollutants via heterogeneous chemical processes on the surface. The inland transport of SSA will result in the generation of a large amount of secondary particles, thereby increasing the atmospheric particulate matter pollution coastal cities. To evaluate the environmental impact of SSA need to understand its hygroscopic characteristics and to find out the mechanism of the interaction between the SSA and other pollutants. In this project, a method using in situ x-ray phase contrast imaging (PCI) based on synchrotron radiation (SR) to investigate the hygroscopic behaviors of inorganic salt in atmospheric environment will be established and used to study the deliquescence and crystallization of SSA. X-ray fluorescence (XRF)spectroscopy and X-ray absorption near edge structures (XANES) spectroscopy with high-resolution and high-sensitivity based on SR will be used to analyze the concentration, micro-distribution and species of major and trace elements both in bulk and single SSA during the surface heterogeneous reactions. It will provide a high-precision and non-destructive investigation of how the SSA involved in atmospheric chemistry processes at the molecular level. The implementation of this project can provide more scientific basis for the prevention and control of air pollution in coastal cities. In addition, more related methodologies base on SR will be developed and improved, which will further expand the applications of SR technology.

大气颗粒物污染已经成为我国许多沿海大型城市面临的主要环境问题之一。由于含有多种可溶性盐分，容易吸收或失去水分并与其它大气污染物发生表面异相化学反应，海盐颗粒物（SSA）的输入将导致大量二次颗粒物的生成，从而加重沿海城市大气颗粒物污染。评估其环境影响需要了解SSA吸湿特征并明晰SSA和其它污染物相互作用的机制。本项目基于同步辐射技术拟在上海光源建立大气环境下无机盐吸湿过程的实时原位相衬成像方法从而系统研究SSA的吸湿特征。同时利用高分辨率和高灵敏度的同步辐射荧光谱（XRF）和近边吸收结构谱（XANES）分析技术，综合使用整体和单颗粒样品分析法，对SSA表面反应过程中主量和微量元素的含量、微区分布及形态分布进行高精度原位无损分析，从分子水平上揭示SSA参与大气化学进程的机理。本项目的实施可为我国沿海城市大气污染的防治提供科学依据，同时能够发展完善相关谱学技术，进一步拓展同步辐射技术的应用领域。

本项目发展完善了海盐颗粒物（SSA）吸湿过程的同步辐射成像技术以及大气颗粒物同步辐射X射线谱学（包括XRF和XANES）分析方法，并在此基础上研究了SSA吸湿潮解特征及其表面异相反应机制。项目按既定计划实施，完成了多项既定目标，取得了一系列研究结果。研制了一套便于加载到同步辐射实验线站在线使用的小型数控式相对湿度（RH）控制装置，该装置能将RH 稳定控制在20%-94%，误差在±1.0%。SSA吸湿过程可分为表面潮解、表面吸水累积、主体溶解和整体溶解几个阶段，各阶段吸湿过程由不同盐分主导。潮湿大气环境中，SSA及其盐分颗粒物表面吸湿过程会发生异相化学反应，可将二氧化硫催化氧化生成硫酸盐。Fe(III)是这一异相反应的有效催化剂，可溶性Fe(III)可以增加催化反应效果。上海地区大气颗粒物中硫酸盐主要以硫酸铵和石膏的形式存在。石膏存在于所有粒径段颗粒物中，是粗颗粒中主要的硫酸盐成分。而高浓度的硫酸铵只在灰霾期间的细颗粒中出现。高RH环境能够程度上会加剧二次颗粒物的生成。海盐源的输入会促进二次颗粒物的生成，同时也有利于污染性酸性气体的清除。本项目研究成果可为全面评估SSA对沿海地区大气环境的影响以及大气颗粒物污染事件（例如灰霾）的产生机制提供相关理论和实验依据。