Investigations of radical kinetics at high pressures and low temperatures: Chemistry of the troposphere and lower stratosphere

The gas phase chemistry of several free radical reactions important to the chemistry of the atmosphere is investigated using a combination of theory and experiment.; Experiments were conducted using the Harvard High Pressure Flow System, a system optimized for studies of gas phase radical reactions free from heterogeneous interferences. Combining independent control of temperature and pressure with Fourier transform infrared spectroscopy and axially and radially resolved laser-induced fluorescence, the HPFS is ideally suited to solve the many problems usually associated with radical reactions.; The data presented herein represent a significant reduction in the uncertainty in the rate constant for the radical recombination of OH + NO₂ to form nitric acid under conditions typical of the lower stratosphere, and significantly extend the low temperature range of the literature set. The measured rate constant is consistently lower than the value recommended by the scientific community at the time the experiments were conducted, and led to a downward revision in the recommendation.; Reaction Modulation Spectroscopy is used to quantify the product branching ratios of the same reaction, and the data show no evidence of the formation of any product other than nitric acid, HONO₂. This is in contrast to predictions made by electronic structure calculations and Dual-well Rice-Ramsperger-Kassel-Marcus theory, which suggest formation of detectable amounts of an intermediate species, peroxynitrous acid, HOONO. Possible explanations for the lack of observed HOONO are discussed.; Rate constants for the reaction of OH with a series of alkanes are measured over a temperature range of 230–370 K. This work represents the first measurement of many of these rates, despite the abundance of the alkanes in the urban troposphere. The experiment's wide temperature range allows for accurate modelling of the temperature dependence of the reactions, and thus for the use of the measured constants in furthering our understanding of tropospheric pollution.; Finally, future directions for research using the HPFS are discussed, with emphasis on improvements to the system and the solution of chemical and environmental problems best solved using the experimental techniques presented here.