Tropospheric photooxidation of CF₃CH₂CHO and CF₃(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, k_Cl and k_OH, was found at all temperatures. At room temperature k_Cl and k_OH were (in 10⁻¹¹ cm³ molecule⁻¹ s⁻¹): k_Cl(CF₃CH₂CHO) = (1.55±0.53); k_Cl(CF₃(CH₂)₂CHO) = (3.39±1.38); k_Cl(CF₃CH₂CHO) = (0.259±0.050); k_Cl(CF₃(CH₂)₂CHO) = (1.28±0.24). A slightly positive temperature dependence of k_Cl was observed for CF₃CH₂CHO and CF₃(CH₂)₂CHO, and k_OH(CF₃CH₂CHO). In contrast, k_OH(CF₃(CH₂)₂CHO) did not exhibit a temperature dependence over the range investigated. Arrhenius expressions for these reactions were:

k_Cl(CF₃CH₂CHO) = (4.4±1.0)×10⁻¹¹ exp{−(316±68)/T} cm³ molecule⁻¹ s⁻¹

k_Cl(CF₃(CH₂)₂CHO) = (2.9±0.7)×10⁻¹⁰ exp{−(625±80)/T} cm³ molecule⁻¹ s⁻¹

k_OH(CF₃CH₂CHO) = (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.