UC San Diego

Atmospheric aerosols are ubiquitous, and they have substantial impacts on climate and human health thus it is important to understand their physical and chemical transformation. However, physicochemical properties of aerosols are poorly understood due to the complexity of the system and difficulties in experiment. To accurately simulate the atmospheric aerosols in the laboratory environment, particle levitation and non-destructive probing are crucial to monitor the change in physicochemical properties in real-time without contact. An environment-controlled mobile electrodynamic balance (EDB) coupled with dual-beam laser spectroscopy setup was developed, achieving stable trapping of single droplet over a long period of time (~72 hours) and providing information on size, phase, and chemical composition of trapped droplet by Mie scattering imaging (MSI) and Raman spectroscopy.The implementation of the custom-built EDB apparatus revealed new findings about the physical and chemical evolution of aerosol droplets. First, kinetic limitation of water diffusion was observed by the measurements of water diffusion coefficients in aqueous sucrose and aqueous citric acid droplets. The rates of water diffusion in single levitated droplets over a wide range of relative humidity (RH) were directly measured by H2O/D2O isotopic exchange. Although sucrose droplets show slower rate of water diffusion, the incomplete exchange of H2O to D2O was only observed for citric acid droplets. Second, the evolution of hydrogen bond interactions in sucrose and citric acid droplets versus RH was thoroughly investigated by 2D correlation analysis of Raman spectra. It was found that the hydrophilic functional groups govern the structures of hydrogen bond interactions with water, especially at lower RH conditions where the droplets experience metastable states. Next, the accelerated keto-enol tautomerization kinetics of malonic acid were quantized by spectroscopically monitoring the rate of C-H to C-D exchange at α-carbon. Lastly, reactions of nitrate anion (NO3ˉ) by the 266 nm photoexcitation of sulfanilic acid were investigated in single acidic droplets. Absorption-emission analysis supported by computational calculations of excitation energies suggest that the triplet-triplet energy transfer from sulfanilic acid triggers the reactions of nitrate. The experimental results and analysis presented in this thesis can improve the fundamental understanding of physical and chemical transformations of atmospheric aerosols.