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Abstract

It is important that accurate, reproducible values for the rates of chemical reactions occurring in the atmosphere be obtained. The value of accurate kinetics measurements is to improve the accuracy of mathematical models used to forecast the state of the atmosphere. We describe a versatile, robust apparatus for obtaining atmospheric reaction rate values and their temperature dependencies by a relative method. This relative method has the advantage over absolute methods in that errors due to impurities and adsorption on walls of the reaction apparatus are minimized. In addition, relative values serve as an important verification for absolute values. The apparatus serves to mix sample and reference gases, used in the studies, with helium, water vapor and oxygen and deliver them to a quartz reaction cell whose temperature can be controlled between of -50 C and +350 C. Reaction is carried out by use of a low pressure mercury vapor lamp radiating the sample for different periods of time. The mercury lamp produces hydroxyl radicals in the presence of the water vapor in the reaction mixture. The hydroxyl radicals abstract hydrogen atoms from the sample molecules. Oxygen is needed to mimic reaction conditions in the atmosphere which is rich in oxygen. Measurement of the concentrations of unreacted and reacted mixtures are obtained by injecting these samples into a gas chromatograph having a mass spectrometer detector, GC/MS. Analysis of the rates of reaction of cyclopropane and difluoromethoxydifluoromethane, CHF2OCHF2, (HFOC-134) with hydroxyl radicals yields the following results: For cyclopropane, a rate of 7.85x10-14:s-1 at 298.15 Kand an activation energy, E/R, of 1323 was obtained. For HFOC-134, a rate of 2.23x10-15 s-1 at 298.15 Kand an activation energy, E/R, of 1895 was obtained.

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