In this paper, the heterogeneous reaction between oleic acid and ozone has been studied using infrared spectroscopy in two distinctly different experimental configurations. The effect of the experiment on the observed products and rates of reaction is compared in order to derive a better understanding of some of the variations in oleic acid reaction rates reported by a range of researchers. One set of measurements is made using thin films of oleic acid in an attenuated total internal reflection configuration, and it is shown that a treatment in which the ATR evanescent wave is convolved with a moving reaction front is essential for the extraction of reliable kinetic data. The results are compared to similar measurements in a recently developed aerosol flow tube equipped with a cross-beam infrared spectroscopic probe. Rates of reaction in the aerosol phase are observed to be approximately 10 times faster and possible reasons for this discussed.
A description of a new aerosol flow tube apparatus for measurements in situ under atmospherically relevant conditions is presented here. The system consists of a laboratory-made nebulizer generation system and a flow tube with a White cell-based Fourier transform IR for the detection system. An assessment of the White cell coupled to the flow tube was carried out by an extensive set of experiments to ensure the alignment of the infrared beam and optimize the performance of this system. The detection limit for CO was established as (1.0+/-0.3) ppm and 16 passes was chosen as the optimum number of passes to be used in flow tube experiments. Infrared spectroscopy was used to characterize dry aerosol particles in the flow tube. Pure particles composed of ammonium sulfate or sodium chloride ranging between 0.8 and 2.1 mum for size diameter and (0.8-4.9)x10(6) particles/cm(3) for density number were generated by nebulization of aqueous solutions. Direct measurements of the aerosol particle size agree with size spectra retrieved from inversion of the extinction measurements using Mie calculations, where the difference residual value is in the order of 0.2%. The infrared detection limit for ammonium sulfate aerosol particles was determined as d(p)=0.9 mum and N=5x10(3) particles/cm(3) with sigma=1.1 by Mie calculation. Alternatively, Mie calculations were performed to determine the flexibility in varying the optical length when aerosol particles are sent by the injector. The very good agreement between the values retrieved for aerosol particles injected through the flow tube or through the injector clearly validates the estimation of the effective optical path length for the injector. To determine the flexibility in varying the reaction zone length, analysis of the extinction spectra as function of the position of the injector was carried out by monitoring the integrated area of different absorption modes of the ammonium sulfate. We conclude that the aerosol loss in the flow tube reactor is negligible and that the aerosol particles remain on-axis for the length of the flow tube.
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