Atmospheric photochemistry on the molecular level using nonlinear optical spectroscopy

• Applicant: Dr. Ahmed Abdelmonem, Ph.D.
• Subject Area: Atmospheric Science ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 450119152

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.

• DFG Programme: Research Grants