Atmospheric photochemistry on the molecular level using nonlinear optical spectroscopy

空气/水界面在环境中无处不在，通常暴露在大气和太阳光下。在水面上涂覆有机涂层可以提高空气/水界面上天然光敏剂的浓度。由此导致的这种界面上的光化学相互作用速度的增加影响了排放到大气中的挥发性有机化合物(VOC)的组成和浓度。这种相互作用可以发生在云滴、湖泊、河流、海洋和海洋的表面(例如，海面微层)。尽管有许多研究来阐明大气中气体和气溶胶的排放和吸收，但我们对表面实体的分子组成和结构及其在相互作用中的作用仍然缺乏基本的了解。因此，从分子水平上了解大气条件下的气液相互作用对于预测气溶胶的形成和老化、云的发生和性质以及最终地球气候系统的变化具有重要意义。拟议工作的总体目标是利用非线性光学(NLO)光谱探索空气/水界面的光化学，并关注水圈和大气中的有机物质。NLO光谱学在处理大气相互作用的基本过程方面的卓越能力已经在我以前的工作中得到了证明。本文提出的实验方案主要是在典型大气条件下(如温度和太阳辐射)探测空气/水界面的有机层。我将结合和频产生和二次谐波产生，作为表面光谱NLO技术，与整体技术(如吸收和质谱分析)相结合，研究在不同温度下暴露于空气和光化辐射下，吸附在空气/水界面上的有机化合物的界面层。为此，将设计和制造一种温控混合多相环境舱。在光敏剂(例如，4-苯甲酰苯甲酸作为天然光敏剂的模型)存在的情况下，将考察作为天然表面活性剂模型的壬酸。接下来，将对海洋中含量最丰富的有机硫化合物二甲基亚砜进行检测。这项研究将为更确定地描述影响VOCs排放的因素和SOA的成核和生长奠定基础，这是我们理解凝聚相自由基产生的一个主要尚未解决和迫切的问题，凝聚相自由基的产生对大气中的有机气溶胶和痕量气体收支非常重要。它将对我们对大气过程的理解产生重大影响，从而对气候系统产生重大影响。

The air / water interface is omnipresent in the environment and usually exposed to the atmosphere and solar light. Organic coatings on the water surface may enhance the concentration of natural photosensitizers at the air / water interface. The resulting increase in the rate of photochemical interactions at such an interface impacts the composition and concentration of volatile organic compounds (VOC) emitted to the atmosphere. Such interactions can take place at the surfaces of cloud droplets, lakes, rivers, seas and oceans (e.g. sea surface microlayer). Despite numerous investigations to elucidate the emission and uptake of gases and aerosols in the atmosphere, there is still a fundamental lack in our knowledge about the molecular composition and structure of surface entities and their role in the interaction. Therefore, molecular-level understanding of gas-liquid interactions under atmospheric conditions is of fundamental importance to the prediction of aerosol formation and aging, cloud occurrence and properties, and ultimately changes in the Earth’s climate system. The overall goal of the proposed work is to explore the photochemistry at air / water interface, using nonlinear optical (NLO) spectroscopy, and to pay attention to organic substances in the hydrosphere and atmosphere. The distinguished capability of NLO spectroscopy to address elementary processes of atmospheric interactions has been demonstrated in my previous work. The experimental plan proposed here is mainly based on probing organic layers at air / water interface under typical atmospheric conditions (e.g. temperature and solar irradiation). I will combine sum-frequency generation and second-harmonic generation, as surface spectroscopic NLO techniques, with bulk techniques (e.g. absorption and mass spectrometry) to study the interfacial layer of organic compounds adsorbed at air / water interface while exposed to air and actinic radiation at different temperatures. A temperature controlled hybrid multiphase environmental chamber will be designed and manufactured for this purpose. Nonanoic acid as a model for natural surfactants will be examined in the presence of photosensitizers (e.g. 4-benzoylbenzoic acid as a model for natural photosensitizers). Next, dimethyl sulfoxide, which is the most abundant organic sulfur compound in the oceans, will be examined. This study will lay the foundations for a more deterministic description of the factors influencing the emission of VOCs and the nucleation and growth of SOA, which is a major unsolved and pressing problem in our understanding of the generation of condensed phase radicals that are important for organic aerosol and trace gas budgets in the atmosphere. It will have a significant impact on our understanding of atmospheric processes and, hence, climate system.