华北地区二氧化硫日变化特征及其与霾爆发性增长的关系研究

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.

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。

本项目拟研究SO2的午峰现象对霾爆发增长的潜在贡献。为此，本项目一方面利用生态环境部的多点观测数据证实了SO2的午峰现象在华北地区普遍存在，且与颗粒物质量浓度的增长密切关联，另一方面利用河北固城生态与农业气象试验站所观测的SO2、NH3、O3、NOx、CO、H2O2 、HONO等污染气体浓度、气溶胶数谱、黑碳气溶胶浓度、气溶胶散射系数、气溶胶化学组分、气溶胶光学吸湿增长因子和大气辐射等气象参数等对SO2如何引起霾爆发增长的机制进行了深入的探讨。研究发现，是上午NH3的增长促进了NO2的水解，产生大量的HONO和NO3-，进而HONO的光解产生大量的OH自由基，进而促进H2O2的生成和SO2的转化。而SO2于午间在NH3之后出现峰值，进一步促进了SO42-的生成。无论是NH3的上午峰，还是SO2的午峰都是华北地区频繁出现的现象。NH3的上午峰出现频率为40~85%，而SO2午峰的出现频率在50-89%之间，二者的配合导致36%的天数出现午间霾爆发性增长。本项目的研究结果对未来的光化学与霾污染控制具有重要的指示意义。当前的光化学和霾污染控制策略主要专注于SO2和NOx的减排，且已获得了较好的成效，特别在SO2的减排方面。但霾污染是和光化学污染共同存在且相互促进的，即使在冬季辐射较弱的时候亦是如此，而控制二者的关键均在于控制NH3的污染。然而，当前生态环境部还尚未将NH3的监测列为常规监测项目，且并没有政策针对NH3设置环境浓度控制标准。因此，应在未来的污染控制策略的制定中重点考虑如何减少NH3的排放。