Gas Phase Spectroscopy and Kinetics of Atmospheric Radicals

Abstract

An important goal for atmospheric and combustion chemists is continued improvement in our understanding of the gas phase reactivity of free radical intermediates formed during hydrocarbon oxidation. The primary focus of this thesis was to measure gas phase kinetics of prototypical free radicals relevant to atmospheric and combustion chemistry, a goal that requires spectroscopy, quantitative product detection, and computational chemistry in order to address these complex chemical systems. Near-infrared cavity ringdown spectroscopy was used to study the peroxy radicals (RO2) formed from chlorine-initiated oxidation of isoprene and other unsaturated hydrocarbons. Isoprene is one of the most important hydrocarbons in the atmosphere; detection of RO2 formed directly from isoprene oxidation will aid in understanding the initial steps of its fate in the atmosphere. As expected, the near-infrared chloro-isoprenyl peroxy radical spectrum has many features; each spectral feature corresponds to a different isomer and conformer, indicating that several RO2 structures are formed. In small RO2, it was possible to identify the molecular structure of the absorber by comparing the experimental spectrum with the vibrationally-resolved electronic spectrum generated by computational chemistry. Identification of each feature then enabled preliminary isomer-specific kinetics measurements. Photoionization mass spectrometry is another useful method for selective detection of radicals, with the added bonus of detecting many of the other species of interest, leading to a comprehensive understanding of the reaction mechanism. The yields of radical chain-propagating product channels of prototypical RO2 reactions (self- and cross-reactions) are important in understanding radical chemistry in gas phase hydrocarbon oxidation. We obtained branching ratio information for reactions of acetyl peroxy radicals with HO2, with particular focus on OH-regenerating reactions. Along the way, we observed unexpected product formation from low-pressure reactions of acetyl radicals and oxygen. Using the same techniques, we also looked at the self-reaction of ethyl peroxy radicals, confirming past measurements of the radical-propagating channel for this reaction, and investigated interesting product formation, like what may be the dialkyl peroxide. These studies were supported by measurements of VUV photoionization cross sections for several radical species. The utility of this instrumentation was also extended by the development of a low-temperature (200–300 K) flow reactor. Finally, using time-resolved broadband cavity-enhanced absorption spectroscopy, we measured the rate coefficient of reactions of the smallest Criegee intermediate, CH2OO with ozone. We observed that this reaction is rather fast, which could have significant implications for experimental ozonolysis studies that are carried out under high initial reactant concentrations.