Oxidation of organic monolayer films on atmospheric aerosol trapped in a laser tweezer traps: hygroscopic and kinetic studies

政府间气候变化专门委员会（IPCC）认为，目前对云形成的气候影响的认识“非常低”，显然需要对这些过程进行更好的研究。云滴在大气气溶胶中形成，气溶胶在大气上升气流中吸收水蒸气。气溶胶的化学成分决定了它是否会激活成为云滴。云的形成在气候上具有重要意义，因为云将阳光反射回太空，从而使大气层冷却。云的化学成分也决定了云的含水量，从而控制了云是否会下雨。大气中的气溶胶通常被包裹在一层分子厚的有机薄膜中。这样的单分子层有能力降低液滴的表面张力并增强云的形成（通过降低与科勒理论一致的临界过饱和度）。所有释放到大气中的有机化学物质都可能被氧化。这些有机膜的氧化降解可以增加有机膜的表面张力（和临界过饱和度），从而防止云形成。这项拟议中的研究将确定大气氧化的速度是否足以与气溶胶或云滴的寿命竞争。我们将提供新的实验证据，确定空气-水界面的化学物质是否有助于或阻碍云的形成。与两位导师密切合作，博士生将开发一种新的分析方法，对被困在激光束焦点的气溶胶液滴进行米氏分析。米氏分析将允许在真实的时间内快速探测液滴的表面化学，以研究液滴周围的一个分子厚的膜的氧化。初步实验表明这是可能的。实验装置的优点是，液滴的流体动力学特性可以在真实的时间内进行，同时保持液滴的形态。学生将量化三种主要大气氧化剂的氧化化学，OH，NO3和O3与水滴（1-20微米）上的单层有机膜反应。有机薄膜将是硬脂酸、油酸和亚油酸，选择它们是因为它们的相态和溶解性行为得到了充分研究，并代表了大气气溶胶中发现的分子类型。这些颗粒将被捕获在激光的焦点中，保持大气液滴的正确形态，并允许用Mie仪器真实的时间跟踪流体动力学和化学动力学性质。米氏仪器将允许跟踪由于化学氧化而引起的云滴激活和形成，即由于吸水而引起的粒子增长。云的化学氧化和吸湿特性都是云模式所必需的。拟议的研究是低风险，高回报和成本效益和皇家霍洛威和卢瑟福阿普尔顿实验室之间的伙伴关系是一个很好的培训机会，博士生。申请人已经在初步研究中证明，所有实验都是可行的，并且在使用化学系统和必要的激光实验方面具有丰富的经验。该项目非常令人兴奋和高度相关，因为它将提高对气溶胶对辐射过程和云形成的影响的认识，气专委认为这是评估颗粒物对气候变化影响的最大不确定性。所要求的财政资源是最低限度的，因为申请人将使用卢瑟福阿普尔顿实验室的STFC设备。对绝大多数实验的支持已经得到保证，PI和他的合作者在STFC资助方面有着非常成功的记录。学生将受益于从移动到新的研究复合体在哈威尔，并在一个肥沃的环境进行研究。

The current understanding of climatic influences of cloud formation has been described by the intergovernmental panel on climate change, IPCC, to be 'very low' and a better examination of these processes is clearly required. A cloud droplet forms on atmospheric aerosol that takes up water vapour in atmospheric updrafts. The chemical composition of the aerosol determines whether it will activate to become a cloud droplet or not. Cloud formation is of climatic importance since clouds reflect sunlight back into space and are thus responsible for cooling of the atmosphere. Cloud chemistry also determines the cloud water content and thus controls if a cloud will rain. Atmospheric aerosol is often coated in organic films that may be one molecule thick. Such monolayers have the ability to lower the surface tension of the droplet and enhance cloud formation (by lowering the critical supersaturation in line with Köhler theory). All organic chemicals released into the atmosphere may undergo oxidation. Oxidative degradation of these organic films may increase the surface tension of the organic film (and the critical supersaturation) and thus prevent cloud formation. The proposed research will determine whether the atmospheric oxidation is fast enough to compete with the lifetime of aerosol or cloud droplets. We will provide new experimental evidence establishing whether chemistry at the air-water interface will help or hinder cloud formation. Working closely with both supervisors the PhD student will develop a new analysis method on aerosol droplets trapped in the focus of a laser beam using a Mie analysis. The Mie analysis will allow the surface chemistry of the droplet to be probed fast in real time to study the oxidation of one molecule thick films around a droplet. Preliminary experiments have shown this to be possible. The advantage of the experimental set-up is that the hygrodynamic properties of the droplet can be followed in real time whilst maintaining the morphology of the droplets. The student will quantify the oxidation chemistry of the three main atmospheric oxidants, OH, NO3 and O3 reacting with monolayer organic films on water droplets (1-20 microns). The organic films will be stearic acid, oleic acid and and linoleic acid chosen for their well-studied phase and miscibility behavior and to represent the types of molecule found in atmospheric aerosol. The particles will be trapped in the focus of a laser maintaining the correct morphology for an atmospheric droplet and allowing hygrodynamic as well as chemical kinetic properties to be followed in real time with the Mie instrument. The Mie instrument will allow cloud droplet activation and formation to be followed owing to chemical oxidation i.e. particle growth owing to water uptake. The chemical oxidation and hygroscopic properties are both needed for cloud models. The proposed research is low risk, highly rewarding and cost effective and the partnership between Royal Holloway and the Rutherford Appleton Laboratory is an excellent training opportunity for a PhD student. The applicants have demonstrated in preliminary studies that all experiments are feasible, and have extensive experience in working with the chemical systems and necessary laser experiments. The project is very exciting and highly relevant since it will improve the understanding of the effect of aerosol on radiative processes and cloud formation which was identified by the IPCC to be the largest uncertainty in assessing the impact of particulate matter on climate change. The financial resources requested are minimal, since the applicants will use STFC equipment at the Rutherford Appleton Laboratory. The support for the large majority of experiments has already been secured and the PI and his collaborators have very successful track-records for STFC funding. The student will benifit from the move to the new Research Complex at Harwell and be in a fertile environment for research.