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

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.