Dean B. Atkinson, Jennifer L. Spillman and M. Hossein Bazargan

Department of Chemistry
Portland State University
Portland, OR 97207 USA

Email: AtkinsonD@pdx.edu

A kinetic reactor system is described which is intended for the investigation of gas phase reactions involving organic peroxy radicals of importance in atmospheric chemistry. The kinetic system uses the recently described continuous wave excitation cavity ring-down (CWCRD) spectroscopic technique to detect absorption by the peroxy radicals in the near-infrared (NIR) following laser photolytic production. Preliminary spectroscopic and reaction kinetics results are presented for the ethyl peroxy (C₂H₅O₂) and acetyl peroxy (CH₃C(O)O₂) radicals. Organic peroxy radicals are the secondary photochemical oxidation products from volatile organic compounds (VOCs) which are emitted into the atmosphere by both anthropogenic and biogenic sources. Since the reactivity of the peroxy radicals is much less than that of the nascently formed (generally through reaction with the strong oxidant OH) organic radicals, these species are key intermediates in the transformation of atmospheric VOCs to benign species or to compounds with shorter atmospheric lifetimes.

The reactor which is described is of the conventional slow flow design, able to access pressures between 5 and 1000 torr and temperatures between 275 and 400 K. Vacuum ultraviolet (193 nm) laser photolysis is used to generate ethyl or acetyl radicals from suitable precursors, which then react with excess oxygen to produce ethyl peroxy and acetyl peroxy radicals. The peroxy radical concentrations are selectively monitored via their structured NIR absorptions near 1.3 m m using CWCRD spectroscopy. A portion of the medium resolution spectrum of the ethyl peroxy radical is shown in Figure 1.

This spectrum is consistent in shape with that originally reported[1] and with a more recent report which utilized the pulsed cavity ring-down method.[2] The broadened spectrum is very conducive to kinetics experiments, but discrimination between peroxy radical species is trivial. Kinetic decays can be observed with and without addition of stable reactants. Preliminary kinetic results for the radical’s self-reactions and the oxidation reactions with nitric oxide will be presented.