Tropospheric photooxidation of CF3CH2CHO and CF3(CH2)2CHO initiated by Cl atoms and OH radicals

摘要

The absolute rate coefficients for the tropospheric reactions of chlorine(Cl) atoms and hydroxyl (OH) radicals with CFCHCHO andCF(CH)CHO were measured as a function of temperature(263–371 K) and pressure (50–215 Torr of He) by pulsed UV laserphotolysis techniques. Vacuum UV resonance fluorescence was employed todetect and monitor the time evolution of Cl atoms. Laser inducedfluorescence was used in this work for the detection of OH radicals as afunction of reaction time. No pressure dependence of the bimolecular ratecoefficients, and , was found at all temperatures. At roomtemperature and were (in 10 cm molecule s):(CFCHCHO) = (1.55±0.53);(CF(CH)CHO) = (3.39±1.38);(CFCHCHO) = (0.259±0.050);(CF(CH)CHO) = (1.28±0.24). A slightlypositive temperature dependence of was observed forCFCHCHO and CF(CH)CHO, and(CFCHCHO). In contrast,(CF(CH)CHO) did not exhibit a temperaturedependence over the range investigated. Arrhenius expressions for thesereactions were:(CFCHCHO) = (4.4±1.0)×10exp{−(316±68)/T} cm molecule s(CF(CH)CHO) = (2.9±0.7)×10exp{−(625±80)/T} cm molecule s (CFCHCHO) = (7.8±2.2)×10exp{−(314±90)/T} cm molecule sThe atmospheric impact of the homogeneous removal by OH radicals and Clatoms of these fluorinated aldehydes is discussed in terms of the globalatmospheric lifetimes, taking into account different degradation pathways.The calculated lifetimes show that atmospheric oxidation ofCF(CH) CHO are globally dominated by OH radicals, howeverreactions initiated by Cl atoms can act as a source of free radicals at dawnin the troposphere.