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

The absolute rate coefficients for the tropospheric reactions of chlorine (Cl) atoms and hydroxyl (OH) radicals with CF3CH2CHO and CF3(CH2)(2)CHO were measured as a function of temperature (263-371 K) and pressure (50-215 Torr of He) by pulsed UV laser photolysis techniques. Vacuum UV resonance fluorescence was employed to detect and monitor the time evolution of Cl atoms. Laser induced fluorescence was used in this work for the detection of OH radicals as a function of reaction time. No pressure dependence of the bimolecular rate coefficients, k(Cl) and k(OH), was found at all temperatures. At room temperature kCl and kOH were (in 10(-11) cm(3) molecule(-1) s(-1)): kCl(CF3CH2CHO) = (1.55 +/- 0.53); kCl(CF3(CH2)(2)CHO) = (3.39 +/- 1.38); k(OH)(CF3CH2CHO) = (0.259 +/- 0.050); kOH(CF3(CH2)(2)CHO) = (1.28 +/- 0.24). A slightly positive temperature dependence of kCl was observed for CF3CH2CHO and CF3(CH2)(2)CHO, and kOH(CF3CH2CHO). In contrast, kOH(CF3(CH2)(2)CHO) did not exhibit a temperature dependence over the range investigated. Arrhenius expressions for these reactions were: k(Cl)(CF3CH2CHO) = (4.4 +/- 1.0)x10(-11) exp{-(316 +/- 68)/T} cm(3) molecule(-1) s(-1) k(Cl)(CF3(CH2)(2)CHO) = (2.9 +/- 0.7)x10(-10) exp{-(625 +/- 80)/T} cm(3) molecule(-1) s(-1) k(OH)(CF3CH2CHO) = (7.8 +/- 2.2)x10(-12) exp{-(314 +/- 90)/T} cm(3) molecule(-1) s(-1) The atmospheric impact of the homogeneous removal by OH radicals and Cl atoms of these fluorinated aldehydes is discussed in terms of the global atmospheric lifetimes, taking into account different degradation pathways. The calculated lifetimes show that atmospheric oxidation of CF3(CH2)(x)CHO are globally dominated by OH radicals, however reactions initiated by Cl atoms can act as a source of free radicals at dawn in the troposphere.