Tropospheric photooxidation of CF3CH2CHO and CF 3(CH2)2CHO initiated by Cl atoms and OH radicals

Abstract

The absolute rate coefficients for the tropospheric reactions of chlorine (Cl) atoms and hydroxyl (OH) radicals with CF3CH2CHO and CF3(CH2)2CHO 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, kCl and k OH3, was found at all temperatures. At room temperature k Cl and kOH were (in 10-11cm 3molecule-1s-1): kCl(CF 3CH2CHO) Combining double low line (1.55±0.53); kCl(CF3(CH22CHO) Combining double low line (3.39±1.38); kCl(CF3CH2 CHO) Combining double low line (0.259±0.050); kCl(CF3(CH2) 2CHO) Combining double low line (1.28±0.24). A slightly positive temperature dependence of kCl was observed for CF 3CH2CHO and CF3(CH2)2CHO, and kOH(CF3CH2CHO). In contrast, k OH(CF3(CH2)2CHO) did not exhibit a temperature dependence over the range investigated. Arrhenius expressions for these reactions were: kCl(CF3CH2CHO) Combining double low line (4.4±1.0)×10-11 exp{-(316±68)/T} cm3 molecule-1 s-1 kCl(CF3(CH2)2CHO) Combining double low line (2.9±0.7)×10-10 exp{-(625±80)/T} cm3 molecule-1 s-1 k OH(CF3CH2CHO) Combining double low line (7.8±2.2)×10-12 exp{-(314±90)/T} cm3 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.