Cavity ring-down spectroscopic study of gas-phase photolysis of unsaturated dicarbonyls and aromatic aldehydes, and surface photolysis of acetaldehyde

Photochemical smog is a phenomenon associated with pollution in urban atmospheres arising from the photochemical oxidation of volatile organic compounds (VOCs) in the presence of nitrogen oxides. The oxidation process can lead to ozone and secondary air pollutants including particle formation which reduce visibility. The term "photochemical smog" is used to describe this pollutant mixture. The further degradation of secondary air pollutants could also have an impact on the atmospheric oxidation capacity. However, the atmospheric degradation mechanisms of the secondary pollutants are still not fully understood. In this dissertation, cavity ring-down spectroscopy combined with laser photolysis and Fourier transform infrared spectroscopy have been employed to investigate gas-phase photolysis of unsaturated dicarbonyls and aromatic aldehydes, and surface photolysis of acetaldehyde. The unsaturated dicarbonyls studied included trans/cis-4-oxo-2-pentenal and E,E-2,4-hexadienedial, which have been observed as ring-openning products from the OH radical initiated atmospheric photo-oxidation of aromatic hydrocarbons (AHCs). The aromatic aldehydes studied included benzaldehyde and 2-nitrobenzaldehyde, which have previously been observed as ring-retaining products from atmospheric degradation of aromatic hydrocarbons. For the gas-phase photolysis studies of trans/cis-4-oxo-2-pentenal, E,E-2,4-hexadienedial and 2-nitrobenzaldehyde, absorption cross section values in the UV/visible region and radical product quantum yields at different photolysis wavelengths have been measured or estimated. Numerous photolysis end-products have also been identified and quantified. The large differences between our measured gas-phase absorption cross sections and previously determined aqueous phase cross sections indicates that only the gas-phase cross section values should be employed in the modeling of gas-phase atmospheric chemistry of these secondary pollutants. Atmospheric photolysis pathways for these compounds have been characterized, which included photoisomerization, photocyclization, radical formation and molecular elimination. Our estimations on the atmospheric photolysis rate constants indicated that photolysis competes or supersedes OH radical reaction in removing these compounds from the atmosphere. We have successfully employed laser photolysis combined with cavity ring-down spectroscopy to study the photolysis of acetaldehyde adsorbed on aluminum surfaces and ice films. The formyl radical (HCO) has been found to be produced from the surface photolysis and its quantum yield has been determined. Our results suggest that surface photolysis of acetaldehyde also contributes to free radical formation in the atmosphere