苯系物二次有机气溶胶与氨反应形成棕色碳的光学特性和吸光组分研究

Secondary organic aerosol (SOA) particles formed from photooxidation of aromatic hydrocarbons which emission from motor vehicle exhaust can undergo continuous aged with gaseous ammonia resulted in production of brown carbon (BrC) products. The BrC compounds have greater toxicity and proved to be light absorbing in UV-Visible region, which are believed to affect radiative forcing and visibility. The optical property and light absorbing compositions of BrC have been the forefront and emphases in the field of atmospheric research. In view of the characteristics of the cross contamination of motor vehicle exhaust , atmospheric fine inorganic particles, and frequent haze of China's urban atmosphere, this project selects NH3-aged SOA particles formed from OH-initiated oxidation of aromatic hydrocarbons, utilizes photochemical smog chamber combined with a cavity enhanced aerosol albedometer and aerosol laser time-of-flight mass spectrometer coupled with coupled with Fuzzy C-Means clustering algorithm, focuses on the optical property and light absorbing compositions of BrC formed via reactions of aromatic secondary organic aerosol with ammonia.The scattering, absorption contribution ratio of organic amine, imidazole compounds and other main light absorbing components of NH3-aged aromatic SOA were investigated. Also, the relationship between the optical parameters of brown carbon of NH3-aged aromatic SOA and the component，concentration, humidity，acidity of inorganic fine particles would be determined. These works would provide the experimental basis for the sources identification of secondary BrC in the atmosphere, and contribute to accurately assess the effects of radiation forcing of carbonaceous aerosol and its role in the regional haze formation. At the same time, the reaction mechanisms of light absorbing components of NH3-aged aromatic SOA were also discussed using Gaussian software. These reaction mechanisms would provide the scientific basis for controlling for the urban atmospheric haze pollution.

来源于汽车尾气排放的苯系物大气光氧化反应形成的二次有机气溶胶(SOA)会与氨气发生老化反应，产生在紫外-可见光区能进行光吸收的棕色碳。棕色碳能够影响辐射强迫, 降低大气能见度，其光学特征和化学组分已成为大气化学研究的热点与重点。本项目针对城市大气受汽车尾气和无机细粒子复合污染、灰霾频发的特点，选择苯系物SOA氨老化棕色碳为研究体系，利用气溶胶单次散射反照率测量仪和气溶胶激光飞行时间质谱联用的烟雾腔系统，开展苯系物SOA氨老化棕色碳的光学特性和吸光组分研究；考察有机胺、咪唑化合物等组分对苯系物SOA氨老化棕色碳的散射、吸收贡献比例；确定苯系物SOA氨老化棕色碳光学参数与无机细粒子的类型、浓度、湿度和酸度的关系。这将为二次源棕色碳来源指认提供实验依据，有助于准确评估含碳气溶胶的辐射强迫效应及其对区域灰霾贡献。利用Gaussian 软件进行吸光组分形成机理理论研究，为控制灰霾提供科学依据。

本项目的主要研究目标是针对我国城市大气受汽车尾气和无机细粒子复合污染、灰霾频发的特点，选择苯系物二次有机气溶胶（SOA）氨老化棕色碳为研究体系，利用气溶胶单次散射反照率测量仪和气溶胶激光飞行时间质谱联用的烟雾腔系统，开展苯系物SOA氨老化棕色碳的光学特性和吸光组分研究；考察羧酸铵、咪唑化合物等组分对苯系物SOA氨老化棕色碳的散射、吸收贡献比例；确定苯系物SOA氨老化棕色碳光学参数与无机细粒子的类型、浓度、湿度和酸度的关系。项目组通过四年的努力工作，完成了本项目的主要研究任务。成功搭建用于SOA形成和老化研究的光化学烟雾腔系统与气溶胶单次散射反照率测量仪和气溶胶激光飞行时间质谱仪联用装置。在该联用装置内用OH自由基启动苯系物光氧化生成SOA并与氨发生老化反应，利用气溶胶单次散射反照率测量仪在线测得氨老化苯系物SOA粒子的单次散射反照率（SSA）在0.70-0.80之间，吸收系数k在0.035-0.055之间。使用ALTOFMS实时、快速检测氨老化产物的化学组分，聚类分析得到羧酸铵和咪唑类含氮有机物等吸光标记产物；结合量子化学计算，分析研究了咪唑类吸光组分产物的形成机理。测得甲酸铵、草酸铵、苯甲酸铵等羧酸铵粒子的SSA在0.86-0.91之间，k在0.008-0.015之间，属于散射型粒子；1H-咪唑、4-甲基-1H-咪唑、1H-咪唑-2-甲酸等咪唑类化合物粒子的SSA在0.60-0.71之间，k在0.052-0.091之间，是氨老化SOA粒子吸收的主要贡献者。无机细粒子能够促进含氮有机物的形成，使苯系物SOA氨老化粒子SSA减小，吸收系数k增大，促进的强弱顺序为：氯化钙>硅酸钠>硫酸铵>氯化钠>硝酸铵。氨老化粒子的SSA随着氯化钙细粒子浓度和酸度的增加而逐渐减小，随着相对湿度的增加而增大。这些工作为大气二次源棕色碳形成机理研究和来源指认提供实验依据，同时有助于评估含碳气溶胶的辐射强迫效应及其对区域灰霾贡献，更好的为生态文明建设服务。