大气复合污染条件下气溶胶酸性特征及其与污染源相互影响

PM2.5 is of concern in many cities across the world. Numerous studies have found that PM2.5 can have substantial adverse effects, direct and indirect, on human health, atmospheric visibility, ecosystem health and global climate change. Acidity, i.e. pH, is one of the key characteristics of PM2.5 Acidity plays an important role in the physical and chemical behavior of PM2.5, and has important implications in human and ecosystem health, secondary aerosol formation, aerosol hygroscopic property, and acid rain. Despite its importance, the pH of PM is mostly unknown, as it cannot be directly measured. Studies often use pH proxies based on ion balances and molar ratios, which however can be subject to considerable uncertainty. Given this and the key role pH plays, it is important to characterize the emission sources, behavior, and formation mechanisms of constituents that regulate particle pH levels. Some studies report the sources (such as soil, coal or secondary sources) may link to atmospheric acidity change, though, most of the works were the laboratory studies, and between a single source and acidity. Source impact analysis of detailed, ambient observations is lacking. Water-soluble ions (WS-ions) are the key drivers of particle pH – as they regulate both the aerosol water uptake, as well as the balance of ions that exist in the aerosol aqueous phase. WS-ions are a major component of PM2.5 and often account for 30 percent or more of the PM2.5 mass. In this work, We plan to relate the influence of different sources on WS-ions and aerosol acidity, using observations from a detailed field campaign. High temporally resolved (1-h) online aerosol ions and gas concentrations were measured in Tianjin, China. The source apportionment (Multilinear Engine: ME2) and thermodynamic (ISORROPIA-II) modeling will be employed to link sources to the acidity properties of the aerosol. In this work, we focus on the characteristic of aerosol pH and its influence on PM sources, under complex pollution condition. The source-pH curves would be established and the function between pH and sources would be discovered. The founding of this work can provide useful information for secondary sources confirmation.

二次粒子是PM2.5重要来源，气溶胶酸性对SO42-、NO3-等二次粒子的形成产生重要的影响。气溶胶pH值作为气溶胶酸性的重要指标，是近两年来的国际研究热点。但是我国关于气溶胶pH值的研究还相对较少；尤其复合污染条件下污染物来源复杂，污染源排放特征与气溶胶pH之间的关系更不明朗，很多研究仅针对烟雾箱模拟实验，需要大量的实际观测实验提供科学依据。本研究拟利用在线观测仪器测量1小时时间分辨率的在线水溶性离子及气态前体物浓度；使用源解析受体模型分析一次源和二次源的贡献，利用热力学模型估算气溶胶pH值；利用统计学方法分析主要污染源对气溶胶pH的影响；揭示在不同污染过程中，气溶胶pH的变化趋势、及气溶胶pH与污染源之间的相关性及规律，最终建立气溶胶pH值与污染源相应关系曲线及函数，为二次粒子的形成机制、颗粒物的综合防治提供科学的依据，具有非常重要的理论与实际意义。

PM2.5是城市环境空气中的重要污染物，对人体健康、能见度造成严重的影响。当前我国处于大气复合型污染状态，颗粒物来源主要有一次排放和二次生成。以往研究表明，二次粒子的生成与气溶胶酸性之间具有密切的联系。但目前关于我国复合污染条件下污染源排放特征与气溶胶pH值之间的关系很不清晰。因此，掌握气溶胶pH值的特征及变化规律，能为二次粒子形成机理的研究提供重要基础信息。本项目基于天津市观测点位获得的在线、离线高时间分辨率在线数据、源解析受体模型、热力学等模型，利用描述统计学、多元线性回归以及机器学习等方法，对因素之间的相互关系以及其对颗粒物和二次无机气溶胶的影响效应进行了分析。发现气态污染物和相对湿度对PM2.5浓度增高均起到促进作用，而温度与气溶胶酸性大体上与其呈现负相关关系，表明较低温的条件与高气溶胶酸性可对PM2.5浓度起到促进作用；气溶胶pH值与不同污染源之间存在不同的相关性，其与燃煤源和扬尘源呈弱正相关关系，与二次硫酸盐呈弱负相关关系；二次硝酸盐在重污染时期与气溶胶pH值呈弱负相关关系，在轻污染时期与气溶胶pH值呈明显的正相关关系。这是由于污染源排放的前体物及颗粒物中载带阴阳离子对气溶胶pH值起到影响。统计学结果表明，不同污染程度下，气象条件、污染源等驱动因素的对灰霾和二次粒子的影响程度不同。在pH=5左右时，容易发生严重灰霾事件；在pH=2~4范围内，SOR较高；在pH=4左右时，NOR达到最高水平，可能与气固分配更趋向于分配到颗粒相有关，而在其他酸性或碱性条件下，NOR均较低。理论分析表明，硫酸盐的存在对实际环境中pH-硝酸盐的“S型”曲线关系起到了影响。基于本研究得到的源-pH-二次粒子的响应关系，本研究构建了“源—硝酸盐敏感图”很好的表明了不同时期气态前体物与二次硝酸盐形成的主导/协同效应，可以为不同季节硝酸盐气态前体物的优先防控提供科学依据。综上所述，本研究为复合污染特征下二次粒子的形成机制提供了基础信息，为一次源合理防控提供了科学依据。