title: Development of <i> in situ </i> Second-Order Nonlinear Optical Scatterings for Molecular Behaviors at Aerosol Surfaces
Creators: name: Qian, Yuqin
Publisher: name: Utah State University
Year: 2022

description: Aerosol particles are one of the most important components of the atmosphere. During the growth of aerosol particles, they directly or indirectly affect air quality, human health, and environmental chemistry. Therefore, understanding the chemical and physical properties of such particles is an important scientific, engineering, and medical issue. The growth of aerosol particles in the atmosphere is closely related to the chemical structure at its surface, as well as the heterogeneous reactions which take place at and below the particle’s surface. However, there is a lack of suitable surface-specific analytical techniques which directly measure the chemical structure of aerosol particle surfaces in situ under ambient conditions. The focus of this research is to study the fundamental nature of aerosol particle surfaces to better understand how interfaces play a role in the growth of aerosol particles.
Preliminary results in our early work demonstrated direct observations of molecules at the aerosol particle surface with the development of second harmonic scattering (SHS) spectroscopy. However, the sensitivity of the SHS system was insufficient to be an analytical tool for studying chemical compositions of aerosol surfaces. Initially, in the present work, we continued to optimize the SHS system for in situ chemical analysis of molecules at the aerosol particle surface. First, we found that femtosecond lasers with repetition rates closer to 5 MHz are more efficient for SHS. Next, we examined a more efficient detector, a charge-coupled device (CCD) detector, which greatly reduced the sampling time of the interface response. Then we combined the optimal laser system with a CCD detector, which greatly improved the detection sensitivity of interfacial molecules. These experimental results not only provided a comprehensive analysis of the SHS technique, but also laid a solid foundation for our subsequent developments of new electronic and vibrational sum frequency scattering (SFS) techniques.
Next, we used our SHS technique to examine interfacial behaviors of molecules at aerosol particle surfaces under different relative humidity (RH) and salt concentrations. Both relative humidity and salt concentration can change particle size and the overall phase of aerosols. RH not only varies the concentration of solutes inside aerosol particles, but also changes interfacial hydration in local regions. It was also found that the surface and bulk of organic molecules in aerosol particles exhibited different behaviors at different RH levels. Our quantitative analysis shows that surface adsorption free energy remains constant, while surface area increases with relative humidity. Furthermore, surface tension of aerosol particles decreases with increasing RH. Our experimental results underscore the importance of interfacial water behavior for aerosol particles in the atmosphere.
Later, we developed in situ surface-specific electronic sum frequency scattering (ESFS) spectroscopy to study the spectroscopic behaviors of molecules at aerosol particle surfaces. For example, we examined electronic spectra of malachite green (MG) at aerosol particle surfaces and found that the surfaces are less polar than the bulk. Our quantitative orientational analysis shows that MG is orientated with a polar angle of 25°-35° at the spherical particle surfaces of aerosols. Additionally, the adsorption free energy of MG at the aerosol surfaces was found to be much lower than that at the air/water interface. These results provide new insights into aerosol particle surfaces to further our understanding of the formation of secondary organic aerosols in the atmosphere.
Lastly, we developed in situ surface-specific vibrational sum frequency scattering (VSFS) spectroscopy to directly identify chemical structures of molecules at aerosol particle surfaces. This setup also enables simultaneous probing of in-particle phases through hyper-Raman scattering (HRS) spectroscopy. In this work, we examined polarized VSFS spectra of propionic acid on the surface and in the bulk of aerosol particles, proving the technique’s ability to characterize organic constituents of aerosol particles. We also quantitatively compared the curved gas/aerosol particle interface with the planar air/liquid interface. It was shown that the surface adsorption free energy of propionic acid onto aerosol particles was less negative than that at the air/water interface. These results challenge the long-standing hypothesis that molecular behaviors at the air/water interface are the same as those at aerosol particle surfaces. Our method opens a new avenue for uncovering surface composition and chemical reactions in secondary organic aerosol formation in the atmosphere and chemical analysis of viral aerosol particles.