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The competition between thermal decomposition (k_dis) and reaction with O₂ (k_O2) has been studied for the 2-butoxyl radical in a newly built 210 L photoreactor constructed of quartz. 2-Butoxyl radicals were generated by continuous 254 nm photolysis of 2-butoxyl iodide in the presence of O₂ and NO, using N₂ as a buffer gas. Reaction educts and products were analysed by long-path (29 m) IR absorption using an FTIR spectrometer. The ratio k_dis/k_O2 was derived from the product ratios of acetaldehyde and butanone, corrected for small amounts of side products. At 280, 298, and 313 K and a total pressure of 1 bar (M = O₂ + N₂), k_dis/k_O2 was determined at O₂ partial pressures between 100 and 1000 mbar. At all temperatures, there was a systematic increase of (k_dis/k_O2)_eff ≡ (Δ[CH₃CHO]_corr × [O₂]) / (2 × Δ[CH₃C(O)CH₂CH₃]) with the partial pressure of O₂ which possibly is the result of an additional O₂ independent source of acetaldehyde (≈8% of the 2-butoxyl radicals reacting by either of the two competing pathways at 298 K, 1 bar). Pressure-dependence studies between 100 and 1000 mbar support the hypothesis that the additional acetaldehyde originates from the formation of 6–10% chemically activated 2-butoxyl radicals in the temperature range 280–313 K. Correction of (k_dis/k_O2)_eff for the O₂ independent yield of acetaldehyde results in k_dis/k_O2 = (6.8 ± 1.4) × 1017, (2.3 ± 0.5) × 1018, and (5.5 ± 1.1) × 1018 molecule cm−3 at 279.8, 298.2, and 313.5 K, respectively, leading to the Arrhenius expression k_dis/k_O2 = (2.0 ± 0.5) × 1026exp(−45.4 kJ mol−1/RT) molecule cm−3 at a total pressure of 1 bar. This temperature dependence of k_dis/k_O2 implies that, depending on temperature, either thermal decomposition or reaction with O₂ is the major loss process of 2-butoxyl radicals under the conditions of the lower troposphere. Using literature values for k_O2, k_dis = 3.9 × 1012 exp(−47.1 kJ mol−1/RT) s−1 is derived for a total pressure of 1 bar (M = N₂ + O₂), which compares very favourably with a recent theoretical estimate (ab initio + RRKM) by Somnitz and Zellner (H. Somnitz and R. Zellner, Phys. Chem. Chem. Phys., 2000, 2, 1907).

The competition between thermal decomposition (k_dis) and O₂ addition (k_O2) of linear and branched C₄ and C₅ alkanoyl (R-C(•)O, R=alkyl) radicals has been studied in a photochemical reaction chamber made from stainless steel (v=12 L). RCO radicals were prepared by continuous photolysis of Br₂–RC(O)H–O₂–NO₂–N₂ mixtures at wavelengths 420 nm. The products CO and RC(O)O₂NO₂ were analyzed by long-path IR absorption using an FT-IR spectrometer. Rate constant ratios k_dis/k_O2 were determined at 317 K for n-butyryl, n-pentanoyl, 3-methylbutyryl, 2-methylpropionyl and 2-methylbutyryl and at 6 temperatures between 293 and 317 K for 2,2-dimethylpropionyl (=pivaloyl, t-butyl-CO) radicals. Total pressures were 1 bar (M=N₂+O₂). Adopting the literature value of k_O2 for acetyl, unimolecular decomposition rate constants k_dis were derived from the measured ratios k_dis/k_O2. k_dis at 298 K, 1 bar, M=O₂+N₂ increases by factors of 35, 54 and 24 for each H atom in CH₃CO which is consecutively replaced by a methyl group (corresponding to increasing branching of R). For the unimolecular decomposition of 2,2-dimethylpropionyl radicals, the Arrhenius expression k_dis(t-butyl-CO)=6.0×10¹² exp(-41.6 kJ mol^-1/RT) s^-1 (2σ) was derived for the temperature range 293–317 K and a total pressure of 1 bar (M=N₂+O₂). The results on k_dis/k_O2 show that even for the thermally most unstable of the carbonyl radicals studied in this work, i.e. 2,2-dimethylpropionyl, only 1.8% decompose rather than add O₂ at 298 K and 1 bar in dry air.