Heterogeneous Organic Oxidation Reactions of Atmospheric Importance Studied by Broadband Vibrational Sum Frequency and Second Harmonic Generation

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Professor Franz Geiger of Northwestern University is supported by the Atmospheric Chemistry program in the Division of Atmospheric Sciences and the Analytical and Surface Chemistry program in the Division of Chemistry to investigate the chemistry of organic compounds at the surface of atmospheric aerosols and their chemical reactions. Partially oxidized organic species present in the tropospheric gas phase can bind to aerosol surfaces through their polar ends, with their remaining double bonds exposed to the gas phase for reaction with atmospheric oxidants. In this laboratory study, heterogeneous carbon-carbon double bond oxidation reactions of cyclic and acyclic surface-bound olefins will be studied under conditions relevant to the atmosphere, specifically in the presence of ppb-levels of ozone and at a total pressure of 1 atm. Tailored cyclic and acyclic organic compounds will be tethered to solid substrates through organosilane chemistry, and then exposed to ozone. The rate of ozone uptake will be determined. Nonlinear optical spectroscopy methods that are sensitive to interfaces only, specifically second harmonic generation (SHG) and vibrational sum frequency generation (VSFG), will be used to probe the interaction of ozone with the surface-active organic compounds. This will allow identification of the surface-bound reactants and the reaction intermediates and products. Reaction products will also be studied using a chemical ionization-mass spectrometer (CIMS).This study will provide new insights into the mechanisms of heterogeneous organic oxidation reactions at solid/air interfaces and detailed, molecular-level information on chemical fate of organic compounds and ozone in the atmosphere. This project will support one graduate student and will be linked to outreach activities by the principal investigator aimed at K-12 students and the public.