Kinetic and Spectroscopic Studies of Atmospheric Intermediates

Abstract

Atmospheric chemistry investigates the chemical transformations of atmospheric trace constituents through three complementary approaches: field observations, laboratory experiments, and computational modeling. This thesis used the laboratory experiment approach to explore persistent unknowns associated with free radical chemistry in the troposphere and stratosphere. Studies were conducted using two powerful techniques for the study of chemical kinetics: multiplexed synchrotron photoionization mass spectrometry and cavity ringdown spectroscopy. In the first part of this thesis, we describe experiments measuring of the absolute photoionization cross sections of chlorine monoxide (ClO) and chlorine dioxide (ClOOCl). The cross sections of ClO were found to be at least a factor of three greater than a prior determination and those of ClOOCl were measured for the first time. ClO and ClOOCl play important roles in the catalytic destruction of polar stratospheric ozone and yet values of the parameters controlling the rate of atmospheric ClOOCl photolysis are uncertain. Our results show that photoionization spectroscopy is highly sensitive to the ClO radical and may be ideally suited for future experiments constraining the quantum yields of ClOOCl photolysis at wavelengths of relevance to the polar stratosphere. We next discuss experiments investigating the kinetics of chlorine-substituted peroxy radicals (ClRO2). These species are formed in the troposphere upon oxidation of alkenes by chlorine atoms. We present rate constants for the formation and loss pathways of the simplest intermediate in this class: the β–chloroethyl peroxy radical. We also discuss measurements of the rate constants between NO and the ClRO2 formed upon oxidation of ethene, propene, 1-butene, 2-butene, 1,3-butadiene, and isoprene. Finally, we present results exploring the impact of temperature and humidity on the chemistry of hydroxyl-substituted peroxy radicals (HORO2). In particular, the self reaction of the β–hydroxyethyl peroxy radical was investigated and found to be significantly enhanced by water vapor at cold temperatures. The product branching ratio was also studied and found to shift in favor of radical suppression. Given the prevalence of water vapor throughout the troposphere, altered reactivity of HORO2–H2O complexes likely plays an important role in atmospheric oxidation mechanisms.