Experimental and Computational Approaches for Probing Atmospherically Relevant Interfaces

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Small hydrocarbons and their oxygenated derivatives are critical to atmospheric organic aerosol formation and growth. Secondary organic aerosols (SOAs) constitute a significant portion of total atmospheric organic aerosols, and are often generated through the oxidation of biogenic volatile organic compounds (VOCs) such as isoprene, -pinene, and -caryophyllene. Interfaces of SOA particle surfaces and interacting gas phases play a crucial role in SOA climate-relevant and cloud-forming properties, yet SOA systems have mainly been analyzed by bulk phase methods like mass spectrometry. Determining molecular structures, orientations, and adsorption dynamics of SOA precursors and their oxidation products at interfaces are important for understanding surfaces of SOA derived from them. In this work, we sought to gain a molecular level understanding of SOA precursors, molecular constituents, and SOA material at interfaces. A combination of vibrational sum frequency generation (SFG) spectroscopy, density functional theory (DFT)-based SFG calculations, and isotope labeling was employed to probe small oxygenated molecules, including an oxidation product of isoprene, at vapor/solid interfaces. We calculated SFG spectra that agreed reasonably to experiment and were associated with molecular orientations that were experimentally validated by a phase-resolved SFG spectroscopic technique based on interfering the sample SFG response with that from crystalline quartz. We used a combination of experimental and computational probes to analyze -pinene, the most abundant monterpene in nature, adsorbed to fused silica. SFG calculations with Fermi resonances and isotope labeling experiments were used to help assign vibrational modes within the SFG spectrum. SFG and molecular dynamic (MD) simulations were used to not only determine molecular orientation distributions, but fully quantify adsorption thermodynamics. It was found that the adsorption of -pinene was partially reversible and that -pinene molecules, while rigid in structure, had fluxional trajectories on the silica surface. In addition to -pinene studies, oxidation products of -caryophyllene that are proposed to be in SOA particles were synthesized and analyzed by SFG spectroscopy. The molecular orientations of the oxidation products were derived through phase-resolved SFG measurements. We utilized our understanding of the SFG spectroscopy of -pinene, -caryophyllene, and their oxidation products to interpret the SFG spectra generated from the surfaces of synthetic SOA derived from them. We were able to identify possible functional group moieties and molecules that may reside on the surfaces of synthetic SOA material. SFG spectroscopy was used to analyze natural SOA samples obtained from Beijing and Brazil and observed notable spectral differences that may be attributed to organic complexity within regions dominated by anthropogenic emissions. We also carried out dynamic temperature and relative humidity experiments for synthetic -pinene-derived SOA material and identified spectral changes which may be attributed to unique disordering of the organic surface layer and other surface chemical processes.

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  • 01/11/2019
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