SO2是硫酸盐气溶胶的前体物，其分布与变化特征会影响其向硫酸盐气溶胶的氧化过程。而硫酸盐是气溶胶中最主要的吸湿与散射性成分，其变化特征与霾的生成和发展密切相关。SO2的日变化特征不同于其它一次污染物，它常在午间氧化条件较好的时候出现峰值。该现象从理论上会导致更多的SO2被转化为硫酸盐气溶胶，从而增加气溶胶浓度，增强气溶胶吸湿性与散射特性，导致能见度衰减，引起霾爆发性增长过程。本项目将对SO2午峰现象对霾过程可能存在的促进作用进行定量分析。项目中首先通过多站点长时间的观测数据确定SO2午峰现象在华北地区存在的空间范围与季节变化，探讨其产生机制及其与霾爆发性增长事件同时出现的频次。接着再利用历史观测数据对霾爆发性增长事件进行特征分析，最后通过加强观测结合模式模拟探讨有和无SO2午峰现象存在的情况下，霾中硫酸盐气溶胶生成过程的差异、气溶胶吸湿与消光特性的差异，从而探讨该现象对霾爆发性增长的贡献。

As the precursor of sulfate aerosols, the distribution and variation characteristics of SO2 has important impacts on its oxidation towards sulfate. Since sulfate is the most hygroscopic and scattering aerosol component, the variation characteristics of sulfate aerosols are closely related to the development haze. SO2 shows distinct diurnal variations from other primary gas pollutants, often reaching its peak value during noontime, when the atmosphere is most oxidative. Theoretically, this phenomenon would lead to increased sulfate production, hence increase not only the aerosol concentration, but also its hygroscopicity and scattering ability. The aim of this project is to quantify the effect of the SO2 noontime-peak phenomenon on the explosive growth of haze. In this project, the spatial distribution and seasonal variation of the SO2 noontime-peak phenomenon, as well as the frequency it appears together with severe haze events will be analyzed using longtime measurements from multiple stations in the North China Plain. The characteristics of the explosive growth of haze events will be studied using historic observations on haze. The difference in sulfate aerosol formation, aerosol hygroscopicity and scattering ablity between the days with and without the SO2 noontime-peak phenomenon will be studied with data from an intensified measurement campaign, to quantify the impact of said phenomenon on the formation and development of haze.