A study of the temperature dependence of the rate constant for the reaction of O2 with surface alkoxy radicals
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A method for studying the reactions of surface alkoxy radicals with O2 at temperatures of 230 to 300 K is described. Alkoxy radicals were generated directly in the cavity of an EPR spectrometer. Surface organic radicals, prepared from paraffin wax ((CH3)2(CH2) n , n = 16–20), were applied to Aerosil particles from a solution in heptane. The Aerosil sample was placed in the cavity of the EPR spectrometer in a cylindrical cup with a central hole for pumping out gases and exposed to H atoms. In this way, it is possible observe a steady increase in the EPR signal from the surface radicals. To measure the rate constant at tropospheric temperatures, the reaction tube was placed in a Teflon jacket, through which cool nitrogen vapor was pumped. The temperature in the reactor was varied from 230 to 300 K. The recorded EPR spectra belong to the (RO) s radical. After obtaining a stable EPR signal from the surface radicals, treatment with H atoms was stopped, additional flow of O2 was introduced ([O2] = 1014–1016 cm−3), and the reaction of O2 with the surface organic radicals was studied by monitoring the EPR signal decay. The temperature dependence of the rate constant for the (RO) s + O2 → HO2 + ketone was obtained within T = 230–300 K. The extrapolation of the data to real tropospheric conditions ([O2] = 1018 cm−3) was performed.
Keywordsoxidation of organic aerosols surface organic radicals EPR spectroscopy
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