Analysis of Atmospheric Aerosol Particle Surfaces Using Sum Frequency Generation Spectroscopy

The scientific understanding regarding the mechanisms of aerosol particle formation and growth is limited. Aerosol particle surfaces are highly important for these processes. In fact, the interface between the gas and particle phases of an aerosol has emerged as a major topic of interest for environmental molecular chemistry, yet this interface is difficult to access experimentally. This thesis demonstrates the utility of vibrational sum frequency generation (SFG) spectroscopy for obtaining molecular level information about the organic functional groups present at aerosol particle surfaces.;Secondary organic material (SOM) formed from the ozonolysis of alpha-pinene, a dominant biogenic volatile precursor, in a flow tube at Harvard University is analyzed. Surfaces of aerosol particles prepared via two different growth modes, condensation and coagulation, appear spectrally similar in the C?H stretching region. The limit of detection for analyzing aerosol particle surfaces using SFG spectroscopy is established to be ∼100 particles per 30 mum laser spot.;alpha-Pinene-derived SOM surfaces are also probed under conditions of dynamically varying relative humidity (RH). The SFG signal intensity in the C?H stretching region decreases upon increasing the RH and increases with decreasing RH. This result indicates that the C?H oscillators of the particle surfaces become more disordered as the RH increases, possibly allowing for faster aerosol particle growth.;Since SOM is formed from the oxidation of organic compounds, the particle surfaces are also probed within the C=O stretching region. The SFG spectrum of alpha-pinene-derived SOM surfaces exhibits a characteristic C=O stretch around 1715 cm-1. The intensity of this peak is less than that of the dominant peak in the C--H stretching region. One possible explanation for this finding is that the C=O oscillators might be more disordered than the C?H oscillators at the particle surfaces. SFG spectra of oxidized pinene derivatives and oxygenated terpenes are reported as well in the C=O stretching region. Finally, to expand the scope of SFG spectroscopy to analyze aerosol particle surfaces, urban aerosol particles having diameters <2.5 micrometers collected in Sao Paulo, Brazil are also analyzed and found to be similar in their SFG responses, independent of time and location of particle collection.