Time-resolved experiments on the atmospheric oxidation of C2H6 and some C2 hydrofluorocarbons

Abstract

We report new time-resolved measurements relevant to the atmospheric oxidation of C₂H₆, CF₃CF₂H (HFC-125), CF₃CH₃ (HFC-143a) and CF₃CFH₂ (HFC-134a). The radicals R = C₂H₅, CF₃CF₂, CF₃CH₂ and CF₃CFH were produced by photolysis of the corresponding iodides, RI, at 248 nm, using the output from a pulsed excimer laser, in the presence of a large excess of O₂ and with different concentrations of NO present. The loss of NO and the formation of the reaction products, CH₃C(O)H (from C₂H₅ radicals), C(O)F₂ and FNO (from CF₃CF₂), and C(O)F₂ and HFCO (from CF₃CFH) were followed, in real time, via absorption of infrared radiation provided by tuneable diode lasers. Our measurements essentially confirm previous conclusions, based mainly on continuous photolysis experiments, concerning the oxidation mechanisms for these compounds and also how they differ according to whether RO, the ethoxy or substituted ethoxy radical, is formed by mutual reaction of two alkyl peroxy radicals (RO₂ + RO₂) or by reaction of one of these radicals with NO (RO₂ + NO). In addition, we report rate constants for the important atmospheric reactions of ethyl peroxy with NO at 298 K, and of fluorinated ethyl peroxy radicals with NO, for temperatures between 198 and 298 K.