云雾过程对二次有机气溶胶形成机制的影响研究

Clouds are known to act as efficient sinks and reactors for water-soluble gaseous species and oxidants via aqueous-phase reaction, which has been advocated to be an important mechanism contributing to the formation of secondary organic aerosols (SOA). Laboratory studies using a bulk reactor equipped with a sunlight simulator demonstrated that low volatility products were formed in the aqueous phase and subsequently retained in the particles upon cloud evaporation, contributing to the particle growth. However, cloud droplet evaporation in response to the lowered relative humidity of the surrounding environment can result in the formation of highly concentrated and supersaturated particles, which may alter physicochemical processes inside the shrinking droplets. As a result, chemical processes in cloud processing cannot be simply treated as a homogeneous reaction; reactions may happen inside the droplets as well as during droplet evaporation processes and the contribution from these two processes on SOA formation have to be evaluated separately, to gain a better understanding on the formation mechanisms of SOA during cloud processing..We propose to simulate the aqueous phase reactions as well as the evaporation process using bulk reactor followed by a drying system. Molecular-level characterization of reaction products will be performed using various online and offline analytical techniques both before and after the evaporation processes. Besides, the effects of the aerosol compositions, acidity and ionic strength on both aqueous phase processing and evaporation processes will be evaluated systematically. The proposed study will provide new molecular and mechanistic information for the formation of SOA in aqueous phase reactions.

水溶半挥发性有机物在云滴中的氧化转化过程会显著促进二次有机气溶胶（SOA）形成。传统观点认为SOA的形成发生在云滴的水相中，由水溶半挥发性有机物氧化转化而成，整个过程中不考虑反应环境—即云滴的动态变化。然而，在实际大气环境中，云滴会不断地蒸发，在大气中停留时间仅为2-30分钟，至使有机物反应的环境条件，如酸碱度、相态、颗粒相组分及浓度等在短时间内发生显著变化。现有的研究忽略了云滴的动态变化，无法准确揭示SOA的形成机制。为了完善该机制，本项目拟同时模拟云滴中的液相过程及云滴本身的蒸发过程，分别对两个过程的产物进行定性定量的表征，评估不同的过程对SOA形成的贡献，以此为基础，明确SOA的形成机制及影响因素；在此基础上调整溶液氧化剂浓度及组成，研究不同的液相条件对液相反应动力学及化学过程的影响，进而探究蒸发前云滴理化性质的改变对蒸发过程中SOA生成机制的影响。

本项目开展了云雾过程中二次有机气溶胶（aqSOA）的生成与演化机制研究，以期为液相化学的模式模拟提供科学依据。首先，本项目采用更贴近真实大气环境的实验设计，对液相反应及液滴蒸发产物的化学（成分等）和物理（吸光性）特性做了大量表征和测试，发现液相反应与液滴蒸发过程会通过生成高氧化态的单体或者多聚体促进aqSOA的生成，而液滴蒸发过程较液相反应过程会加速促进低挥发性小分子有机物的蒸发从而呈现较低的氧化性。在此基础之上，改变液相反应条件，发现溶液中有硝酸盐存在时一方面硝酸盐会通过光解产生二次氧化剂，加速反应的进行，另一方面硝酸盐会促进液相样品与液滴蒸发样品中高分子量多聚体的生成，且随着硝酸盐浓度的升高，多聚体的生成越显著。研究进一步发现液相反应过程中生成的多聚体与硝化产物具有吸光性，是棕碳的潜在来源，且随着反应的进行，多聚体与硝化产物的生成量与吸光性出现先上升后下降的趋势，揭示了随着氧化反应的进行，aqSOA的生成由聚合反应主导到裂解反应主导的主要演化机制。此外，生物质燃烧会产生大量的aqSOA的前体物，本项目结合生物质燃烧的外场实验，开展了在aqSOA前体物较为丰富且湿度较高的真实大气环境中，aqSOA的生成与演化研究，阐明了液相化学对真实大气环境中二次有机物生成的贡献。通过以上研究，揭示了云雾过程中aqSOA生成机制、影响因素及云雾过程对SOA生成的贡献，相关研究成果在未来可以应用于多相化学机制的模拟研究，为模式模拟提供参数化方案与科学依据。